ObjectiveAdaptive optics (AO) are commonly utilized for real-time wavefront detection and aberration correction. Nevertheless, a typical AO system often experiences a time delay equivalent to at least 2?3 sampling periods due to wavefront sensor readout data delay and control calculation delay. Hence, existing AO systems do not offer truly real-time corrections. When the frequency of turbulence alterations surpasses the closed-loop bandwidth of the AO system, this delay leads to the compensation wavefront on the deformable mirror (DM) trailing the changes in the distortion wavefront. This lag substantially hampers the correction efficacy of the AO technology. Consequently, predicting future atmospheric turbulence information in real-time bears significant research importance and practical value in AO technology.MethodsIn this paper, a spatiotemporal prediction network was proposed for AO wavefronts based on the attention mechanism. This network simultaneously considered the temporal and spatial characteristics of atmospheric turbulence and selected the slope data, which was easier to acquire in the AO system and had a lower dimension, as both the input and output of the network. With only six frames of previous wavefront slope information, the wavefront slope of the second frame at a subsequent moment was determined. The network initially employed the spatial attention mechanism to capture similar target features in each frame of the distorted wavefront. It then utilized the residual learning strategy to remove redundant information between the input wavefronts of two consecutive frames, producing refined target features. Additionally, recognizing that the degree of target feature information in each frame of the distorted wavefront varied, the channel attention mechanism was further employed. This mechanism emphasized distorted wavefronts containing a richer set of target feature values rather than evenly weighting the wavefront from each frame. Following these steps, the final prediction of the wavefront slope was realized.Results and DiscussionsThe generalizability of the distorted wavefront dataset is tested under different atmospheric turbulence intensities. As the turbulence intensity increases, the structural similarity (SSIM) between the predicted and true wavefronts stabilizes above 0.9300, and the ratio of the root-mean-square wavefront error (RMSe) between the predicted and true wavefronts to the true RMS is stable at approximately 5.00% (Table 2). In the two-frame delay system, when compared to the non-prediction method, the performance improvement of the proposed prediction method is stable at approximately 40% (Fig. 4). Extended tests are performed on the three-frame-delay AO system under the assumption that the RMSe increases when compared with the two-frame delay prediction; however, the performance of the proposed prediction method increases by more than 43% when compared with that of the non-predicted method (Fig. 5). Additionally, a set of ablation comparison experiments are conducted. Under the assumption of a two-frame delay system, compared to the non-prediction method, the RMSe based on the attention mechanism and residual learning prediction methods reduce by approximately 29% and 16%, respectively, while that based on the proposed method reduces by approximately 40% (Fig. 7). To further verify the performance of the proposed prediction network, we test the one-time open-loop data collected by an actual 1 km laser atmospheric transmission system (Table 3); the average ground truth RMS of the experimental data is 0.9488λ, the average prediction RMS is approximately 0.9557λ, the average RMSe is approximately 0.0675λ, and the RMSe is approximately 7.1% of the real true RMS (Fig. 9). Furthermore, to demonstrate the prediction capability of the network for the experimental data, we extend the simulation of the three-frame delay open-loop correction process again. Compared to the non-predicted method, in two-frame delay and three-frame delay systems, the average performance improvements of the proposed prediction method are 41.2% and 42.9%, respectively (Fig. 10). This reiterates the effectiveness of the proposed open-loop slope prediction network and the feasibility of applying this network model to open-loop correction systems.ConclusionsIn this study, an AO open-loop slope-prediction-network-based attention mechanism is proposed to achieve high-precision wavefront slope prediction. The effectiveness of the network is experimentally proven, which can effectively overcome the inherent delay problem of real AO systems. Additionally, the network uses only six consecutive frames of wavefront slope prior information to achieve a high-precision actual wavefront for correction. This input and output configuration reduces the hardware system's load capacity and enhances the network model's running speed, offering crucial guidance and application value for the subsequent deployment of an actual AO delay system.

ObjectiveWith the rapid development of laser technology, it has been widely applied in important fields such as medicine, biology, materials and national defense. The amplitude of a laser beam generally has a Gaussian distribution, and such an uneven energy limits its further application. Thus, beam shaping techniques have been proposed to transform Gaussian beams into flat top beams with a uniform energy distribution. Researchers have proposed various beam shaping methods, among which shaping using liquid crystal spatial light modulators has been widely investigated for its controllable transmittance function, good flexibility and real-time performance. Traditional phase distribution algorithms suffer from the problems of being easily trapped in local extrema, being sensitive to the initial value of the phase, and not being able to obtain high utilization of energy and high beam top uniformity at the same time. In this paper, the phase distribution function algorithm where beam is shaped using liquid crystal spatial light modulators is optimized by using the combination of lowliest place elimination (LPE), genetic algorithm (GA) and Gerchberg-Saxton (GS) algorithm. The hybrid method is called LPE-GSGA algorithm, which further improves the output beam top uniformity without sacrificing the utilization of energy, or even improving it. Meanwhile, it reduces the dependence of conventional algorithms on initial values to a certain extent and has important applications in flat top beam shaping with high utilization of energy and high beam top uniformity.MethodsThe LPE-GSGA algorithm designed in this paper uses the strong global search capability of the GA algorithm to help the GS algorithm to jump out of local extrema. Also, LPE is introduced to retain individuals with good phase points and accelerate convergence. Sum of squares for error ess and fitting coefficient η are used as evaluating indicators to describe the quality of output beams. The algorithm can be divided into two processes: the first is the iterations of all initial phase groups using GS algorithm, and the second is the calculation of the comprehensive evaluation index where some phase individuals with good indexes are selected to enter the next generation phase group directly and the remaining phase individuals experience selection, crossover (Fig. 1), mutation and LPE to enter the next generation phase population until the number of individuals in the phase population is 1. The flow chart of the process is shown in Fig. 2.Results and DiscussionsWe calculate the output beam's information use LPE-GSGA algorithm through simulation, show its iterative process (Figs. 3 and 4) and further compare it with those of the GS, generalized adaptive additive (GAA), weighted Gerchberg-Saxton (GSW) and GSGA algorithms under the same input and evaluation metrics (Table 1). The ess and η calculated by LPE-GSGA algorithm are superior to those obtained with other algorithms. Compared with GS algorithm, the LPE-GSGA algorithm shows great advantages with 10.1% reduction in ess and 0.85% improvement in fitting coefficient η. From the point of initial value dependence, the variances of ess and η of 50 sets of results figured by LPE-GSGA algorithm are much lower than those of the other algorithms, with the variance of ess being about 74% lower than that of the GS algorithm, and a nearly one order of magnitude reduction of variance of η. The role of each process is also discussed: process 1 makes use of the fast convergence ability of the GS algorithm to obtain the local extrema quickly, and process 2 uses the screening of the LPE and the global search ability of the GA algorithm to help the GS algorithm obtain better iterative initial phase values, reduce its dependence on the initial values, and thus obtain better phase distributions.ConclusionsThe LPE-GSGA phase distribution algorithm based on the LPE, GS algorithm and genetic algorithm is proposed in this paper. Based on the algorithm, we get the quality of the output beam by simulation which is superior to those of the GS, GAA, GSW and GSGA algorithms, and solve the problem of initial values dependence. Additionally, the improved algorithm diminishes the number of intensity abrupt change points on the top of output beam, the number of sidelobes, and the sidelobe amplitude. In a word, we demonstrate the effectiveness of the LPE-GSGA algorithm in improving the quality of the output flat top beam and getting a flat top beam with high utilization of energy and high beam top uniformity.

ObjectiveA phase-sensitive optical time-domain reflectometer (φ-OTDR) system is a front monitoring and early warning technology that can acquire the location of disturbances in space and phase information of disturbances in time. With the advantages of high resolution, wide monitoring range, and strong anti-interference capability, this technology has been widely used in pipeline safety maintenance, intrusion warning, and large-equipment monitoring. However, due to the complex diversity of the application environment, the system suffers from low recognition accuracy and insufficient stability in actual use, particularly when similar signals are recognized in the system application. To solve these problems, this study proposes a similar-signal recognition method based on multiscale feature fusion. This method can effectively improve the recognition accuracy of similar signals while maintaining the recognition accuracy of the base signal.MethodsThe original signal is first decomposed into sub-signals in different frequency ranges using empirical mode decomposition (EMD) and wavelet packet decomposition (WPD). The original signal and individual sub-signals are then subjected to time-frequency feature extraction and approximate entropy feature extraction. The time-frequency features are used to evaluate the details of the time and frequency variations of the signal, the approximate entropy features are used to evaluate the complexity and regularity of the signal, and the multiscale signal decomposition and multi-feature extraction are used to amplify the feature differences between similar signals. Because the multiscale and multi-feature approach increases the dimensionality of the data, the proposed method utilizes principal component analysis (PCA) to combine high-dimensional features and reduce the dimensionality of system features, thereby improving system efficiency. Finally, the fused features are passed into a lightweight back-propagation (BP) neural network as input variables for signal data processing. Compared to other traditional neural networks, BP neural networks have the advantages of lightweight structures and high speed, enabling them to process signal data quickly.Results and DiscussionsSub-signals decomposed by EMD and WPD have multiscale characteristics ranging from low to high frequencies. Each sub-signal contains a part of the signal domain within the main frequency-band range of the original data. Decomposition helps to amplify the feature gaps between different signals and facilitates subsequent multidimensional feature extraction (Fig. 10). Following feature extraction and fusion, the four signals show significant differences in the feature space. Thus, even with a simple classifier, signal classification and recognition can be achieved (Fig. 11).A comparison among extracting multi-features from original signal [Fig. 12(a)], the CNN model [Fig. 12(b)], and the multi-scale feature fusion[Fig. 12(c)] reveals that the multi-scale feature fusion has higher recognition accuracy, where knocking and shaking-signal recognition accuracies reach 100% and trolleying and walking-signal recognition accuracies reach 98.5% and 98.0%, respectively. A comprehensive analysis reveals that the comprehensive recognition accuracy of the proposed method is increased by 8.4 and 9.0 percentage points over extracting multi-features from original signal and CNN model, respectively, and the similar-signal recognition accuracy is increased by 13.5 and 12.4 percentage points (Fig. 13), respectively. These results verify that the method has high recognition accuracy.ConclusionsExperimental results show that the decomposition method using EMD combined with WPD can obtain sub-signals at different scales. The time-frequency domain and approximate entropy features can in turn be extracted from the original signal and sub-signal to enhance the differentiation of similar-signal features more effectively. The PCA algorithm can then reduce the dimensionality of high-dimensional data, thus effectively reducing the number of training features. A well-designed six-layer lightweight BP neural network model can also effectively identify different types of signals when identifying signal features with significant differentiation. Compared with the extraction of features directly from the original signal, the proposed method can improve the integrated and similar-signal recognition accuracies by 8.4 and 13.5 percentage points, respectively. Compared to those of the CNN method, the overall recognition accuracy is improved by 9.0 percentage points, and the similar-signal recognition accuracy is improved by 14.3 percentage points. This method effectively improves similar-signal recognition while maintaining the recognition accuracy of underlying signals, which is of great value for expanding the applications of φ-OTDR systems.

ObjectiveChaos laser has important applications in the fields of secure communication, key distribution, physical random number generation, and radar. In these applications, the key is a chaotic source, a common choice for which is a semiconductor laser with optical feedback because it is characterized by simple structure, easy integration, and complex dynamics. However, external-cavity resonance between the laser facet and the reflector in the conventional optical feedback structure gives the chaotic signal an obvious time-delay signature (TDS). This feature leaks the key parameter of the external-cavity length of the chaotic light source, which makes the system at a risk of being reconstructed and reduces the security of chaotic secure communication and key distribution. In addition, a TDS also introduces a weak periodicity to the chaotic signal, which limits the randomness of physical random numbers and the anti-jamming performance and resolution of radar. Therefore, the suppression of the TDS is an important prerequisite for the best use of chaos laser. The main methods of suppressing the TDS are increasing the complexity of the feedback cavity, introducing nonlinear feedback, and post-processing the chaotic signal. In this study, a TDS-free chaos laser generation scheme using inter-modal dispersion of a multimode fiber (MMF) is proposed. This work provides a basis for the application of TDS-free chaos laser in the fields of secure communication, key distribution, physical random number generation, and radar detection.MethodsThe light output from a semiconductor laser is divided into two paths by an optical coupler. One path is fed back to the laser via the MMF to perturb itself to generate chaos, and the other one is for detection. We utilize a variable optical attenuator and a polarization controller to adjust the strength and polarization state of the feedback light. The feedback strength is defined as the power ratio of the feedback light to the static output of the laser. An erbium-doped fiber amplifier is used to amplify the signal's optical power in the feedback and detection paths. When the bias current and operation temperature are set to 15 mA and 25 ℃, respectively, the static wavelength of the laser is stabilized at 1550.1 nm. Numerically, we employ the VPIphotonics design platform to construct the simulation system mentioned above. In the simulation, the bias current of laser is set to 20 mA, giving rise to a central wavelength of 1552.52 nm. Two typical MMFs with core diameters of 50 μm and 62.5 μm are used to analyze the effects of the core diameter (D), relative offset, and length of the MMF on the chaotic optical mode field. In addition, the evolution of the TDS as a function of the relative offset, feedback strength, and length of the MMF is explored. Note that, to quantitatively characterize the magnitude of the TDS, the autocorrelation function is used.Results and DiscussionsFirst, a TDS-free chaotic signal is obtained experimentally using an MMF with a length of 4.4 km and a core diameter of 62.5 μm while the optical feedback strength is fixed at 0.1 (Fig. 2). Next, we theoretically study the influences of the core diameter (Fig. 3) and relative offset (Figs. 4 and 5) of the MMF on the number of modes and the distribution of chaotic optical mode fields. As the core diameter and relative offset increase, the number of modes gradually increases and the mode field distributions become more complex. The effect of fiber length on mode separation is also investigated (Fig. 6). Note that the degree of mode separation (that is, the inter-modal dispersion) becomes larger as the fiber length increases. By comparing the typical chaotic characteristics of a laser subject to single-mode fiber and MMF feedback under the same parameter conditions, we find that the approach with MMF feedback can afford the elimination of the TDS, whereas that with single-mode fiber feedback cannot (Fig. 7). Furthermore, the effects of relative offset (Fig. 8), fiber length (Fig. 9), and feedback strength (Fig. 10) on the TDS are given. When the relative offset is 0 and the feedback strength is 0.1, the critical fiber length required to eliminate the TDS can be as short as 1 km.ConclusionsIn this study, we propose a scheme for TDS-free chaos laser generation using a semiconductor laser with MMF feedback. Chaos laser without the TDS is obtained experimentally. Theoretically, the effects of the core diameter, relative offset, and fiber length of MMFs with D=50 μm and D=62.5 μm on the chaotic optical mode field are analyzed. Furthermore, we explore the TDS evolution as a function of the relative offset, fiber length, and feedback strength. Finally, the parameter conditions required to suppress the TDS of chaotic signals are ascertained: When the relative offset is 0, the fiber length is greater than or equal to 1 km, and the feedback strength is greater than or equal to 0.1, chaotic signals without a TDS can be generated. This study underlies secure communication, key distribution, physical random number generation, and radar detection using TDS-free laser chaos.

ObjectiveThe current information capacity of communication systems based on single-mode fibers (SMFs) is approaching its physical limits. To solve this problem, spatial-division multiplexing based on mode-division multiplexing (MDM) has been intensively investigated. Due to its orthogonal characteristics, MDM can help realize more multiplexed channels, and thus the capacity of existing optical fiber communications can be enhanced. Mode converters are critical devices in optical-fiber communication systems and are essential for improving the performance of future MDM systems applicable in long-distance and high-capacity optical-fiber communication. Mode-conversion efficiency is a major index of mode converters. Mode converters based on asymmetric Y junctions on polymer platforms offer the advantages of low cost, high fabrication tolerance, and wide bandwidth. Thus, the design and fabrication of mode converters with compact structures and high mode-conversion efficiencies based on asymmetric Y junctions on polymer platforms are essential to meet the increasing demands in data traffic.MethodsThe proposed mode converter consists of two identical inversely connected asymmetric Y junctions. The stem of the Y junction is a straight two-mode waveguide designed to support only the E11i and E21i modes (i=x or y, indicating the polarization direction), which correspond to the LP01 and LP11a modes (polarized in the i direction) of the optical fiber, respectively. This two-mode core gradually branches into two single-mode cosine S bends with different widths, forming an asymmetric Y junction. Based on the mode evolution principle of the asymmetric Y junction, the parameters of the proposed mode converter based on a cascaded asymmetric Y junction (Fig. 1) are optimized in this study. These parameters include the widths of the core (w1), arms A and D (w2), and arms B and C (w3), as well as the length of the arm (L), width of the Y-junction end (w4), and distance between the two Y-junction ends (w5). A three-dimensional finite-difference beam-propagation method (3DFD-BPM) is used to simulate the mode-conversion characteristics of the proposed mode converter. Under these device parameters, the proposed mode converter is fabricated with in-house microfabrication facilities. In addition, an experiment is conducted to characterize the mode-conversion performance of the proposed mode converter.Results and DiscussionsIn the proposed mode converter, the waveguide core height is fixed at 4 μm, and w1, w2, and w3 are set to 9.0, 6.3, and 2.7 μm, respectively. The refractive-index difference between the core and cladding is sufficient to achieve mode conversion. The mode-conversion efficiency between the LP01 and LP11a modes is optimal when the length of the arm , width of the Y-junction end, and distance between the two Y-junction ends are 1.5 μm, 3.1 μm, and 6.8 μm, respectively. The simulation results show that the mode-conversion efficiencies for the x polarized LP01-LP11a and LP11a-LP01 are 99.3% and 99.2%, respectively (Fig. 4). A experiment is conducted to characterize the mode-conversion performance, and the near-field spots detected by the infrared camera indicate that the device can implement mode conversion (Fig. 6). Over a wavelength of 1530?1600 nm, the insertion losses are between ~4.8 dB and ~5.8 dB and between ~3.5 dB and ~5.1 dB for the x and y polarizations, respectively (Fig. 7). To investigate further the mode-conversion and crosstalk characteristics at the asymmetric Y junction of the device, the device is cleaved at the middle position to obtain an asymmetric Y junction. The results show that, under the premise of neglecting the radiation losses of the asymmetric Y junction and propagation losses of the waveguide, the mode-conversion efficiencies are greater than ~98% and ~98.1% for the x and y polarizations over the C+L band, respectively, and the mode crosstalk is less than -17.5 dB (Fig. 9).ConclusionsWe propose and demonstrate a mode converter constructed using two identical asymmetric Y junctions connected inversely. Our proof-of-concept mode converter, designed for the conversion of the LP01 and LP11a modes and fabricated using an optical polymer material, has a miniature footprint of approximately 1.5 mm×14.0 μm. The results show that over the C+L band and for both polarizations, the mode-conversion efficiencies are greater than ~98%, the crosstalk is less than ~-17.5 dB, and the insertion loss is less than ~5.8 dB. Our proposed mode converter with polymeric materials is easy to fabricate and inexpensive. In particular, the same structure can be implemented with other high refractive index contrast material platforms such as lithium niobite on insulators, silicon nitride, and silicon on insulators to realize more advanced integrated photonic circuits.

ObjectiveDefects such as scratches and dust on the surfaces of optical components in interferometers can generate diffraction ring coherent noise in interferograms, which can significantly affect measurement accuracy. To address this issue, this study introduces an extended-light-source method in which the coupling angle of a multimode fiber is altered. By controlling the coupling angle of the multimode fiber after the parallel beam passes through the rotating frosted glass, the proposed method effectively minimizes the effects of scattering and coherent noise. This approach provides a valuable contribution to the theory of extended light sources based on multimode fibers. The derivation and validation of the range of fiber incident angles at the location exhibiting the strongest signal-to-noise ratio (SNR) of the interferometric signal offer important guidance for installing and calibrating interferometers and other optical instruments utilizing this light-source configuration.MethodsIn this study, the objectives of eliminating coherent noise and improving the SNR of interference fringes in a composite extended source based on multimode fibers are explained. With a change in the coupling angle in the multimode fiber, the shape and size of the light beam emitted from the fiber end change accordingly. When the coupling fiber angle is adjusted, the free degree of the composite-light-source speckle increases, leading to a decrease in system speckle contrast. However, the size of the extended source affects the contrast of the interference fringes. Based on the formula for obtaining the interferometric SNR, it is known that changes in the scattering contrast and interference fringe contrast affect the SNR of the entire extended light source. In this study, the critical incident angle at which the light beam emitted from the fiber end becomes a hollow beam is derived through calculations.The Zemax OpticStudio software is used to simulate the fiber-end optical field. In the non-sequential mode, a Gaussian light source is set to enter at different incident angles along the x axis. A detection viewer is used to observe changes in light-field distribution. The change in the output field is calculated at different incident angles based on the formula of the light beam emitted from the fiber end. Simulations and calculations are performed to investigate the effects of different fiber incident angles on the fiber output light-field distribution. The detector parameters are then adjusted, the light-field distribution emitted from the fiber end is recorded, and the number of speckle fields is characterized using the average value of the image gradient magnitude.To verify further the correctness of the theoretical and simulation results, an experiment is conducted on an interferometer with a diameter of 25.4 mm using a multimode fiber with a core diameter of 1 mm. The light-field distribution at the fiber output port is analyzed at different incident angles using a beam quality analyzer, and a wavelength phase-shifting measurement is performed on the measured mirror.Results and DiscussionsWhen light is coupled via a multimode fiber at different incident angles, the changing rules of the optical field distribution at the fiber output are the same in the simulation and experiment, as shown in Figs. 5 and 10, respectively. As the incident angle increases, the output light beam changes from a Gaussian distribution to a hollow disc. The interference SNR is maximized when the speckle contrast exhibits a minimum value, and the position of the minimum point is approximately midway between the calculated critical angle of the hollow beam and the normal incident angle. As Table 3 shows, when the angle of the incident multimode fiber is within the range of -3°?2° following a reduction in the temporal coherence of the beam (accomplished by rotating the ground glass), the SNR of the interferometric signal increases from 4.433 dB at normal incidence (0°) to 6.219 dB, which is an increase of 40.3%. In addition, the scattering contrast decreases from 0.333 (the highest level) to 0.204, and the coupling angle is -2° at the highest SNR. This coupling angle is also between the positive incident angle and the critical angle of emergence of the hollow beam, which is consistent with the calculation. It should be noted that the actual position of the hollow beam may deviate from the calculated value because of the fiber status. However, this does not affect the position of the maximum point of the speckle contrast. This position is used to approximate the position of the hollow beam, which in turn is used to determine the position of the minimum point.ConclusionsTheoretical derivation and simulation experiments prove that when the angle of parallel beam coupling in a multimode fiber is adjusted to the approximate middle position between the normal incidence and critical value, scatter and coherent noise can be suppressed and the SNR of the interferometric signal can be improved. This study also refines the theory of extended light sources based on multimode fibers. In addition, the derived calculation of the position range at the highest SNR provides a useful reference and guidance for mounting optical instrument systems such as interferometers.

ObjectiveWith the widespread deployment and large-scale application of 5G networks, global network data traffic is rapidly increasing. The rise of technologies such as artificial intelligence, autonomous driving, metaverse, and extended reality has presented higher requirements for the data transmission capabilities of existing fiber-optic communication networks, and upgrading them to meet future challenges is now urgent. Currently, erbium-doped fiber amplifiers (EDFAs) are widely used in fiber communication networks due to their excellent gain performance. However, the gain bandwidth of EDFAs covers only the conventional C band and a portion of the L band in the low-loss transmission band of silica fibers, which severely limits the further expansion of commercial communication bands. Utilizing communication bands other than the C+L bands is an effective means of improving the communication capacity of fiber-optic networks. Therefore, the development of high-Ge bismuth-doped fiber (BDF) amplifiers that can operate in the U band is of great significance. To date, only the Fiber Optic Research Center (FORC) of the Russian Academy of Sciences is capable of manufacturing high-Ge BDFs for U-band efficiency amplification. Therefore, developing high-gain and highly efficient BDFs for U-band amplification while overcoming foreign technological barriers and achieving the localization of related optical devices are all necessary.MethodsIn this study, a BDF is prepared using the modified chemical vapor deposition (MCVD) method combined with solution doping technology. The refractive index profile is measured using a preform analytical instrument. An optical microscope is used to observe the end face of the fiber. The mass fraction of Bi is measured using inductively coupled plasma mass spectrometry (ICP-MS) and is found to be approximately 0.02%. An electron probe micro-analyzer (EPMA) is used to test the mole fraction of the core GeO2, which is as high as 42%, and the radial distribution of the GeO2 in the core is measured using a line scan. The absorption spectrum of the BDF is measured using a fiber analyzer based on the truncation method. The total dispersion coefficient of the BDF in this study is 16.7?19.6 ps/(nm·km) in the range of 1600?1700 nm, as derived from a simulation conducted using COMSOL Multiphysics software. A single-stage amplification system is then constructed using a 205-m long BDF, and a 16-channel comb-shaped light source covering 1595?1670 nm with a spacing of approximately 5 nm is used to provide the input signal; the input signal power is then adjusted to -30 dBm. A 1550-nm laser diode (LD) is used to provide forward pumping, where the actual pump power that enters the BDF is ~800 mW.Results and DiscussionsFigure 4 shows the gain test results of the 205-m long BDF in a single-stage amplification system. Because the gain peak of a high-Ge BDF is typically located near 1700 nm and the long-wavelength range of the signal provided by the comb light source used in this study can reach only 1670 nm, the gain performance at 1670?1700 nm can not be directly characterized. To reasonably predict the gain performance of the BDF at 1670?1700 nm based on a test of its gain performance at 1595?1670 nm and an output spectrum at 1595?1700 nm, the input-signal and output spectra at 1590?1700 nm are shown in Fig.4(a). It shows that the amplified spontaneous emission (ASE) power increases with wavelength, and the gain is higher at wavelengths with higher ASE power in the range of 1590?1670 nm. In addition, Fig.4(b) shows that the gain increases with wavelength, and the noise figure (NF) decreases accordingly. Considering the growth trend of the ASE and gain, we can reasonably speculate that the gain between 1670 nm and 1700 nm will continue to increase, as shown by the dotted line in Fig.4(b). Figure 4(b) also shows that a measured gain of 26.3 dB is obtained at 1670 nm, and the predicted gain at 1700 nm exceeds 32 dB. The gain levels of the BDFs at different pump powers and pump conversion efficiencies (PCEs) under different input signal powers are tested, as shown in Figs.5(a) and 5(b), respectively. Figure 5(a) shows that the gain at 1670 nm increases and gradually saturates as the pump power gradually increases. A calculation of the ratio of the gain to pump power reveals that the maximum gain efficiency can reach 0.165 dB/mW.ConclusionsIn this study, a high-Ge BDF with a core GeO2 mole fraction of ~42% is prepared through the MCVD method combined with solution doping technology. In a single-stage amplification system under the 1550 nm forward pump light (pump power of approximately 800 mW) and input signal power of -30 dBm, the 205-m long BDF achieves a gain of 26.3 dB at 1670 nm with a gain efficiency of 0.165 dB/mW. In addition, based on the growth trends of ASE and gain with increasing wavelength, we predict that the BDF can achieve a gain of over 32 dB at 1700 nm.

ObjectiveSince its first report in 2010, transverse mode instability (TMI) remains one of the primary limiting factors in the power scaling of high-power fiber lasers. Fiber bending presents a straightforward and effective TMI suppression technique based on mode control. Typically, a negative correlation exists between the TMI threshold and fiber bending diameter. This implies that decreasing the bending diameter increases the TMI threshold. In near-single-mode fiber laser systems, the laser's output power usually increases by reducing the bending diameter to suppress TMI. However, to counteract nonlinear effects exemplified by the SRS and enhance the pumping capacity, fibers with larger mode fields and cladding diameters gain widespread use in high-power fiber lasers. Throughout this transition, an unusual TMI phenomenon is observed where the threshold power increases with the fiber bending diameter's increase. In this scenario, the beam quality of the output laser can be sacrificed to a certain extent in favor of power enhancement, thereby boosting brightness. In this study, an experimental investigation is conducted on the abnormal TMI effect in high-power fiber lasers with respect to varying fiber bending diameters. It is expected that these findings will pave new paths for the evolution of high-brightness fiber lasers.MethodsIn this study, we designed a high-power fiber laser amplifier based on a double cladding ytterbium doped fiber with a core/cladding diameter of 30/600 μm, combined with water-cooled columns with cylindrical fiber grooves of different diameters. The nominal absorption coefficient of this fiber was 0.40 dB/m@915 nm. In the experiment, we used water-cooled columns with diameters of 13, 14, 15, and 16 cm to compare the output characteristics of the lasers with different fiber bending diameters. First, under the condition of a fiber length of 20 m, the TMI threshold of the laser with different bending diameters was examined using a wavelength-stabilized 976 nm LDs as the pump source, and the abnormal TMI phenomenon was examined. Subsequently, the fiber length was increased to 45 m by combining the pump wavelength optimization and abnormal TMI suppression to realize a high-brightness fiber laser amplifier.Results and DiscussionsWhen the fiber length is 20 m and wavelength stabilized 976 nm LDs serve as the pump source, the TMI thresholds of the laser are 1650, 2839, 3182, and 3740 W as the fiber bending diameter gradually increases from 13 to 16 cm (Fig. 4). The corresponding calculated relative brightness values of the output laser are 519, 631, 751, and 970, respectively (Fig. 4). As the fiber bending diameter increases, even though this might lead to a slight degradation in the beam quality, the TMI threshold of the laser significantly rises, and the progressively increasing maximum output power results in an increase in the relative brightness of the output laser. After optimizing the fiber length and pump wavelength, as the bending diameter gradually increases from 13 to 15 cm, the TMI thresholds of the laser are 2825, 4020, and 6789 W (Fig. 6). When the bending diameter increases to 16 cm, the maximum output power reaches 7100 W without TMI and SRS. The beam quality (M2) at this power is about 2.17, and the relative brightness is 1293 (Fig. 6). As the fiber bending diameter increases, the relative brightness of the laser also gradually increases. At a bending diameter of 16 cm, the maximum output power obtains limited from the pump power, leading to no further advancements in the output power or brightness.ConclusionsIn this study, a design for a high-power fiber laser amplifier is presented based on a double cladding ytterbium doped fiber with a core/cladding diameter of 30/600 μm and water-cooled columns of different diameters. Based on this, experiments are conducted on abnormal TMI effects. The results indicate that when the fiber supports many modes, increasing the bending diameter can raise the TMI threshold of the laser and also enhance the brightness of the output laser. In the experiment, using a wavelength-stabilized 976 nm LDs, as the bending diameter increases from 13 cm to 16 cm, the TMI threshold of the laser rises from 1650 W to 3740 W, showing an increase of approximately 1.27 times. The relative brightness of the output laser rises from 519 to 970, marking an increment of 0.87 times. Although increasing the fiber bending diameter might slightly degrade the beam quality of the output laser, the notable rise in the TMI threshold significantly augments the laser's maximum output power, thereby elevating the brightness of the output laser. Ultimately, by expanding the fiber bending diameter and optimizing the pump wavelength, TMI is successfully suppressed, achieving a 7100 W high-brightness laser output with a relative brightness of 1293. This is 1.53 times the relative brightness of the output laser under identical conditions when the bending diameter is 13 cm. The findings from this study offer guidance for TMI suppression and enhancements in the output power and brightness of high-power fiber lasers.

ObjectiveSingle-frequency lasers with wavelengths above 2 μm have attracted significant interest because of their numerous applications in areas such as biomedicine, Doppler LiDAR, and space optical communication. Although the power of the fiber master oscillator power amplifier (MOPA) around 2 μm has already reached the kilowatt level, the output powers of single-longitudinal-mode (SLM) laser oscillators in this wavelength region are still limited to the hundred-milliwatt level due to the laser gain and frequency-selection approaches. When a piece of unpumped rear-earth doped fiber is found inside the cavity of a laser, the standing wave field inside it results in a dynamic grating because of the Kerr effect. Moreover, the absorption loss of the doped fiber is small under the oscillating longitudinal mode but considerably larger under the side modes. These processes may provide a strong frequency-selection effect, thus allow lasers with longer cavity lengths and resulting higher laser gain to operate in SLM. In this study, we demonstrate an efficient SLM fiber laser at 2050 nm. A piece of Tm3+/Ho3+-codoped fiber is used as gain fiber to provide a laser gain at over 2 μm wavelength, while a piece of Tm3+-doped fiber is inserted into the laser cavity as the saturable absorber (SA) for frequency selection. A linear cavity scheme is adopted, rather than the ring cavity usually used in SLM cavity lasers, based on the fiber SA approach to enhance longitudinal mode spacing for a higher power SLM output. A maximum SLM 2050-nm laser output power of 714 mW is obtained under the incident 1570-nm pump power of 3.5 W.MethodsA schematic of the SLM laser is given in Fig. 1. The gain fiber used is a piece of 9-μm/125-μm Tm3+/Ho3+-codoped fiber with a length of 4.6 m. The pump light launched from a 1570-nm fiber laser is coupled into the core of the active fiber via the filter wavelength division multiplexer 1 (FWDM1). Two fiber Bragg gratings (FBGs) are used to make the linear laser cavity. The reflectivity and 3-dB bandwidth of the partially-reflective FBG with its pigtail angle cleaved at 8° are 39.7% and 0.075 nm, respectively. The Tm3+-doped SA fiber used has a core absorption coefficient of 0.5 dB/m at 2050 nm. Another FWDM2 is used to couple the residual pump out of the oscillator.Results and DiscussionsIt is known that longer SA fibers have a stronger frequency-selection capability; however, their oscillation mode losses are greater, resulting in reduced laser power and efficiency. In the experiment, we use SA fibers with lengths of 1.5 m and 2.5 m. The laser wavelength determined by the FBGs is 2048.6 nm. Without the SA fiber in the cavity, the laser output power is 813 mW under the maximum 1570-nm pump power of 3.5 W, with a slope efficiency of 26.9%. When the 1.5-m and 2.5-m long SA fibers are added to the cavity, the laser output power under the same pump power decreases to 773 mW and 714 mW, with slope efficiencies of 26.1% and 25.1%, respectively, as shown in Fig. 2. The laser threshold also increases from 0.53 W without the SA to 0.62 W and 0.72 W when the two pieces of SA are used. With the 1.5-m long SA fiber, the laser is in SLM with a pump power of 2.8 W and lower (571-mW output power), which is confirmed using a scanning Fabry-Perot interferometer and the spectrum. For higher pump powers, the laser becomes multi-longitudinal-mode because of the insufficient frequency-selection capability. When the 2.5-m long SA fiber is used, the laser maintains stable SLM operation from the 0.72-W threshold pump power to the 3.5-W maximum pump power, with a maximum output power of 714 mW [Fig. 3(b)]. The optical spectrum recorded by the optical spectrum analyzer shows an optical signal-to-noise ratio (OSNR) of ~60 dB between the 2048.6-nm laser and the 1.9?2.0 μm amplifier spontaneous emission (ASE) peak, whereas the in-band OSNR is beyond 75 dB (Fig. 4). The 3-dB spectral linewidth of the SLM output is measured to be 17 kHz using the delayed self-heterodyne interferometer technique at the maximum power of 714 mW (Fig. 6).ConclusionsIn this study, we demonstrate a linear cavity SLM Tm3+/Ho3+-codoped fiber based on a Tm3+-doped SA fiber for frequency selection. The laser delivers a 714-mW SLM output at 2048.6 nm under a maximum 1570-nm pump power of 3.5 W, with a slope efficiency of 25.1% and an optical efficiency of 20.4%. The influence of the SA fiber length on the frequency-selection capability and the supported SLM output power is experimentally investigated. The results show that a linear cavity with an SA fiber inside can achieve a high-power SLM laser output.

ObjectiveHigh-power lasers within the 2-μm spectral range have been utilized in various applications in the fields of laser radar and satellite remote sensing, essentially serving as pump sources for optical parametric oscillators. Tm/Ho-doped fiber lasers are currently the primary methods for achieving high-power continuous-wave lasers or high-average-power pulsed lasers with high repetition rates owing to their homogeneous heat load distribution. However, photon dark effects, nonlinear effects, and transverse mode instability in fibers present as obstacles for further power/energy scaling, particularly for pico- or femtosecond-pulsed lasers. To tackle these challenges in the domain of high-power lasers, the introduction of gain media with novel structures is imperative and constitutes a popular area of research. Single-crystal fibers (SCFs) are long, thin crystalline rods, typically having a diameter of less than 1 mm and length of a few centimeters. Owing to their high thermal conductivities and low Brillouin gain coefficients, SCFs combine the advantages of both bulk crystals and glass fibers. Thus, they are promising candidates for high-power laser systems.SCFs have been showcased as promising candidates for high-average or peak-power laser oscillators and amplifiers within the 1 μm spectral region. Nevertheless, research on the laser performance of SCFs in the mid-infrared 2 μm spectral range, particularly when doped with Tm3+ or Ho3+ ions, is limited. Recently, the first Tm∶YAG SCF laser, utilizing a 783-nm pump, was reported. Owing to high quantum defects, the use of near-infrared pumping results in a significant thermal load, thereby limiting its potential for power scalability. However, by directly pumping Tm3+ ions into the upper laser level (3H6→3F4), the issue of thermal loading can be effectively addressed, thus paving the way for further power scaling and enhanced laser efficiency.MethodsThe experimental setup is shown in Fig. 1. The pump source is a fiber-coupled laser diode (LD) at 1719 nm, with a maximum output power of 25 W, numerical aperture of 0.22, beam quality factor (M2) of approximately 100, and core diameter of 400 μm. The fiber output is focused into the SCF with a variable magnification ratio depending on the telescope system, which consists of a collimating lens (L1) and focusing lens (L2). The focal lengths L1 and L2 are 25.4 mm /30.0 mm and 30.0 mm/25.4 mm for the two pump-coupling schemes. Therefore, the pump beam waist radius is either 236 μm or 170 μm within the SCF. The single crystal fiber used in this experiment is a Tm∶YAG SCF with a diameter of 1 mm and a length of 40 mm (the atomic fraction of doped Tm is 3.5%). For better thermal effect management, undoped YAG caps with bonding lengths of 5 mm at each end of the crystal are used. In order to avoid parasitic laser oscillation, both end caps are coated with 2 μm band anti-reflection films. The Tm∶YAG SCF is mounted on a custom-made aluminum module (Fig. 1), and both ends of the SCF are sealed with glue, leaving a small protrusion of approximately 1 mm outside the module. This arrangement allows the entire Tm-doped section of the SCF to be directly water-cooled to approximately 8 °C. A planoconcave mirror with a radius of curvature of -200 mm serves as the input mirror (IM). Plane-wedged mirrors with transmittance values of 3%, 5%, 10%, and 15% are used as the output coupler (OC). A dichroic mirror (DM) functions as the beam splitter for the pump and laser beams. The propagation mode and intensity distribution of the pump light within the SCF are simulated and analyzed using the ray tracing method [Figs. 2(a) and (b)].Results and DiscussionsInitially, the laser performance under different pump-guiding conditions is investigated. As shown in Fig. 2(c), with a 170-μm pump beam waist, the peak slope efficiency achieved is 46.3%. Next, the Tm∶YAG SCF laser performance with different OCs and a pump beam waist of 170 μm is examined in detail. An optimal OC with transmittance (TOC) of 5% yields a maximum output power of 7.85 W, correlating to slope efficiencies of 46.3% and 52.9% with respect to the incident pump power and absorbed pump power, respectively [Fig. 3(a)]. Figure 3(b) presents the optically measured spectra for different OCs, with the laser wavelength at TOC=5% situated at 2017.7 nm. A red-shift in the wavelength from 2012.8 nm to 2017.7 nm is observed with decreasing OC transmission. Figure 4(a) shows the M2 for various output powers. Within the measured range, the M2 value gradually increases from 1.2 to 1.9; a typical beam quality measurement in the latter case is exhibited in Fig. 4(b). The degradation in beam quality can be attributed to the excitation of higher-order transverse modes because no component for mode limitation is used in the cavity. Figure 5 represents the calculated thermal lens focal lengths along the x-axis for different incident pump powers. The focal length at the peak pump power of 25 W, corresponding to 7.85 W output laser power, is estimated to be62 mm.ConclusionsIn conclusion, a 1.7-μm LD is used as the pump source for resonant pumping, and the continuous laser operation of Tm∶YAG SCF is realized by integrating mode matching and pump guiding. This approach generates an output power of 7.85 W at approximately 2.02 μm, which corresponds to a slope efficiency of 46.3%. In addition to pump guidance, we find that mode matching also plays a crucial role in laser performance of such a 1-mm diameter SCF. Nevertheless, a theoretical analysis of the thermal lens suggests that a higher pump intensity in the front segment of the SCF may trigger instability in the laser cavity. Certain specific designs (e.g., undoped end caps) can further bolster the pump guidance and alleviate the thermal stress of the SCF.

ObjectiveThe vertical cavity surface-emitting laser (VCSEL) is a type of semiconductor laser that emits light perpendicular to the substrate surface. The VCSEL,as a chip-level atomic clock light source, must possess good single-mode characteristics. Thus the individual frequency required by the atomic clock can be precisely modulated to stimulate the atomic clock's operation, and the atomic clock is ensured not to absorb other signals during the modulation process. The practical design necessitates confining the electric and optical fields of the VCSEL to secure strong single-mode characteristics, while also optimizing the epitaxial structure of the device. This study aims to develop a 795 nm VCSEL device with excellent power characteristics, capable of functioning at high temperatures up to 380 K, and achieving a fundamental mode output in the desired wavelength range.MethodsFirst, the quantum well structure is designed using strain-compensated quantum well band theory and the Kronig-Penney model to determine the material composition and thickness parameters of the quantum well. This ensures that the quantum well material exhibits high gain, with a peak wavelength of 795 nm at the high temperature of 380 K. Next, the distributed Bragg reflector (DBR) is designed using the transfer matrix theory to determine the material compositions of the high and low refractive index layers. The logarithm of the power reflectance for the P-type DBR and N-type DBR is calculated. Following this, the oxide confinement layer is analyzed using the fiber waveguide theory and a thermoelectric coupling model to achieve good single-mode characteristics and thermal properties of the VCSEL. The oxide aperture for the VCSEL in the fundamental mode lasing is calculated. After simulating and calculating the parameters, the devices are fabricated. Different-sized mesa structures are designed in various regions of the layout, and four VCSEL devices with different oxide apertures are fabricated on a single chip. A comparative analysis is performed on these devices to draw conclusions.Results and DiscussionsThe epitaxial wafer results obtained in this study correspond well with the simulation results (Fig. 9). Wet oxidation serves to form the oxide confinement aperture under high-temperature conditions. The oxidation depth is controlled by adjusting the oxidation time, resulting in oxide apertures of 1.9, 3.8, 4.9, and 6.9 μm in a single-chip fabrication process (Fig. 12). Power-current measurements are performed on the fabricated devices. For the device with a 3.8 μm oxide aperture, the threshold current measures at 1 mA, the maximum output power is 2 mW, and the slope efficiency is 0.3 W/A (Fig. 14). When the oxide aperture is 1.9 μm, the device maintains single-mode output throughout the injection current range of 3?7 mA, with a side-mode suppression ratio exceeding 35 dB. When the oxide aperture is 3.8 μm, the side-mode suppression ratio surpasses 30 dB. The operating wavelength at room temperature hovers around 790 nm, meeting the requirements for applications (Fig.15).ConclusionsThis study concentrates on the design of a single-mode 795 nm VCSEL device structure and active region. The gain spectrum of In0.08Ga0.79Al0.13As strained quantum wells is simulated. At room temperature (300 K), the gain peak wavelength is 777 nm. At 380 K, the gain peak wavelength shifts to the desired 795 nm range, with a redshift rate of 0.238 nm/K. A single-mode device is achieved by employing an oxide confinement structure. The device structure and oxide aperture are optimized and designed. The optical and electrical limitations of the oxide aperture are simulated using the fiber waveguide theory and a thermoelectric coupling model, resulting in an oxide aperture of 3.72 μm for the VCSEL in single-mode operation. Moreover, VCSELs with oxide apertures of 1.9, 3.8, 4.9, and 6.9 μm are fabricated in a single-chip process. The fabricated devices are characterized by power-current characteristics and spectral properties. When the oxide aperture is 1.9 μm, the device maintains single-mode output throughout the injection current range of 3?7 mA, with a side-mode suppression ratio exceeding 35 dB. For the device with a 3.8 μm oxide aperture, it operates in single-mode, with a threshold current of 1 mA at room temperature, a maximum saturated output power of 2 mW, a slope efficiency of 0.3 W/A, and an emitted wavelength of 790 nm under 3 mA injection current, with a side-mode suppression ratio exceeding 30 dB. This study successfully obtains a robust single-mode output device with an emission wavelength of 790 nm, consistent with the design. Considering that atomic clocks need to operate at high temperatures and VCSEL emission wavelength redshifts with temperature, with a temperature coefficient of approximately 0.06 nm/K, the devices fabricated in this study have reserved a wavelength redshift of 5 nm. This ensures the achievement of 795 nm single-mode output under high-temperature conditions for atomic clock applications. It lays the foundation for subsequent high-temperature operation and polarization-selective VCSELs.

ObjectiveIn contrast to traditional lasers utilizing chirped pulse amplification (CPA), such as Ti∶sapphire, amplified signal pulses from optical parametric chirped pulse amplification (OPCPA) inherently experience excess spectral phase distortions during the parametric amplification process besides the linear phase accumulated from crystal dispersion. These excess spectral phase distortions, also known as optical parametric phases (OPP), represent a significant issue that impedes pulse compression in petawatt-level OPCPA laser systems. With this in mind, the present study seeks to examine the evolution of OPP in the SILEX-II all-OPCPA multi-PW laser facility, developed at the Laser Fusion Research Center of the China Academy of Engineering Physics (CAEP). Analytical and numerical calculations are carried out to determine the total group delay dispersion (GDD) and third-order dispersion (TOD) induced by the OPP, from the high-intensity picosecond pulse-pumped front-end to high-energy nanosecond pulse-pumped power amplifiers. These findings are expected to provide valuable insights for the temporal compression of the SILEX-II laser system and inform the design of high-peak-power laser systems (from 10 PW to 100 PW) utilizing OPCPA technology.MethodsThe OPCPA process is modeled using the classical coupled-wave equations [Eq.(3)], under the assumption of a slowly varying electric field envelope. This model is numerically solved using the split-step Fourier algorithm. The focus of this study is exclusively on the OPCPA process and evolution of OPP. Consequently, it is assumed that the initial pulse entering the OPCPA only carries a GDD, which stretches the signal in the time domain to match the pump pulse. The evolution of the OPP is deduced by subtracting the initial GDD and the material dispersion of the parametric crystal from the spectral phase of the amplified pulse. The numerical results are compared with the analytical ones, obtained using Eq.(1) for both high-intensity picosecond pulse-pumped front-end and high-energy nanosecond pulse-pumped power amplifiers. Table 1 details the parameters used in the pump simulation, Table 2 outlines those used for the signal, and Table 3 itemizes those used for the nonlinear crystals.Results and DiscussionsThe high-intensity picosecond pulse-pumped front end exhibits an OPP with a GDD of 71 fs2 and a TOD of 1092 fs3 [Fig.2(a)]. These values are obtained by fitting a third-order polynomial to the numerically calculated OPP within the wavelength range of 740-880 nm, which is the output spectrum range based on the numerical calculations [Fig.2(b)]. The GDD and TOD values obtained by fitting a polynomial to the analytically calculated OPP are 83 fs2 and 1370 fs3, respectively. Therefore, compared to the numerically calculated OPP, the main difference between the two lies in the TOD for the high-intensity picosecond pulse-pumped front end. For high-energy nanosecond pulse-pumped power amplifiers, including the preamplifier, booster amplifier, and main amplifier, the numerically calculated OPP is almost the same as the analytically calculated OPP [Figs.3(a) and 4(a)]. For the preamplifier, the GDD and TOD obtained from the OPP are 158 fs2 and 2398 fs3, respectively, whereas for the booster and main amplifier, the total GDD and TOD induced by the OPP are 302 fs2 and 2405 fs3, respectively. These results reveal that for the SILEX-II laser system, the OPP induces a GDD of 532 fs2 and a TOD of 5782 fs3 [Fig.5(a)], and the peak intensity of the compressed pulse is only 43% of that of the Fourier transform-limited pulse [Fig.5(b)]. By compensating for the GDD of the OPP, the peak intensity of the compressed pulse can be increased to 94% compared to that of the Fourier transform-limited pulse [Fig.5(b)].ConclusionsIn conclusion, a thorough study of the OPP evolution in the SILEX-II full OPCPA system at the China Academy of Engineering Physics is conducted. The OPP evolution across the entire SILEX-II laser system is obtained by numerically solving coupled wave equations combined with analytical formulas. The results reveal that the SILEX-II laser system accumulates a GDD of up to 532 fs2 and a TOD of up to 5782 fs3 due to the optical parametric amplification process. Consequently, the peak intensity of the compressed pulse is only 43% of that of the Fourier transform-limited pulse. Further calculations indicate that after compensating for the GDD induced by the OPP, the peak intensity of the compressed pulse increases to 94% of that of the Fourier transform-limited pulse. These findings offer invaluable theoretical guidance for the temporal compression of the SILEX-II laser system. In practical applications, the grating distance in the compressor can be precisely adjusted to offset the extra GDD. Additionally, this study paves the way for the design of future 10-100 PW peak-power lasers utilizing full OPCPA technology, suggesting that global OPP control should be taken into consideration during the design process.

ObjectiveSpace-based Doppler wind light detection and ranging technology is a highly competitive field among the world’s leading aerospace powers. As a key component of lidar, the performance of the single-frequency pulsed laser source determines the measurement accuracy and detection capability of the entire system. For coherent Doppler wind lidar, a laser pulse width exceeding 100 ns is required to ensure the accuracy of the wind measurement. Moreover, spaceborne lidar systems place stringent demands on laser sources in terms of reliability, volume, and weight. Due to its advanced maturity, efficiency, and reliability, the neodymium-doped 1.06-μm laser finds extensive applications in space. Thus, this study proposes a single-frequency, high-energy 1.06-μm laser with a pulse width of hundreds of nanoseconds, aiming to offer a technical approach for the space-borne coherent detection lidar laser source.MethodsA fiber-bulk hybrid amplification system is designed, consisting of a cascaded fiber pre-amplifier chain (Fig.1) and multi-stage solid-state amplifier chain (Fig.2). For the fiber pre-amplifier chain, a distributed feedback (DFB) semiconductor laser with a linewidth of approximately 2 MHz serves as a single-frequency continuous-wave (CW) seeder. A Lorentzian pulse waveform is adopted as the modulation signal for an acoustic-optical modulator (AOM) to chop and reshape the CW seeder into a Lorentzian pulse sequence at a repetition frequency of 60 Hz and pulse width of approximately 149.0 ns. The obtained pulsed seeder is then coupled to a Yb-doped single-mode fiber (YSF) amplifier to extract energy and is further amplified by a Yb-doped double-clad fiber amplifier (YDF). To enhance the signal-to-noise ratio, another AOM is utilized with a square modulation signal before the YDF amplifier. A fiber end cap is used at the output of the YDF amplifier to reduce the optical power density at the fiber facet, and the output is collimated using a collimator with a focal length of 4.6 mm, which enters the subsequent solid-state amplification system for further pulse energy scaling. The solid-state amplification system is developed using a fiber-coupled laser diode (LD) end-pumped Nd∶YVO4 crystal, acting as a high-gain double-pass preamplifier, followed by an LD array single-side-pumped Nd∶YAG slab preamplifier with a double-pass configuration. Finally, a two-stage LD array double-side-pumped Nd∶YAG slab serves as the power amplifier. For the Nd∶YVO4 crystal preamplifier, a double pass is achieved through angular displacement due to the polarization dependency of the vanadate crystal. After the first pass, the amplified beam is returned with approximately a 3° angular change of the beam direction via the dichromic mirror M2, which is coated with 0° anti-reflection (AR) films at 808 nm and high reflectivity (HR) films at 1064 nm. The Nd∶YAG preamplifier has a zigzag pass with Brewster angle faces, and a double pass is achieved by polarization rotation using a Porro prism and 0.57° plate. The two power amplifiers are single-pass and pumped onto the zigzag total internal reflection point. The Nd∶YAG slabs are conductively cooled from top to bottom by making contact with a conductively cooled Cu heat sink. The first slab power amplifier is cut at the Brewster angle, while the second is cut at an angle of 40°, and also has a near-normal incident.Results and DiscussionsThe modulation signals for AOM1 and AOM2 (Fig.3) are Lorentzian waveforms with a pulse width of approximately149.0 ns and rectangular waveforms with a pulse width of 2.4 μs, respectively. With pumping at a 1.2 ms pulse width and peak power of 580 mW for LD1 and 525 mW for LD2, the fiber amplifier produces 2.1 μJ pulse energy with a 216.7 ns pulse width (Fig.4). After collimation, the measured diameter of the near-field spot is approximately 0.6 mm, and the divergence is approximately 2.7 mrad (Fig.5). To further scale the pulse energy, the output of the fiber amplifier undergoes amplification using a multi-stage solid-state amplification system. The maximum pulse energy of 151.4 mJ is successfully achieved, with an optical-to-optical efficiency of approximately 7.3% relative to the total incident pump energy. The pulse width of the second slab amplifier output is approximately 267.8 ns with a rising edge of 191.7 ns and a falling edge of 161.4 ns (Fig.6). The measured laser beam quality factor (M2) is 1.39 and 1.60, respectively, in the x direction and y direction at a pulse energy of 151.5 mJ with a laser beam quality analyzer (Fig.7). The inset of Fig. 7 displays the near-field intensity distribution of the laser beam. Using a laser wavelength meter, the center wavelength of the pulse laser measures at 1064.49 nm, and the obtained linewidth of less than 500 fm is limited by the laser wavelength meter itself. To achieve an accurate linewidth, a self-built real-time monitoring system for the laser spectrum is employed. Based on optical heterodyne, the center frequency and linewidth of the laser pulse can be calculated according to the beat signal of the laser pulse and reference CW seeder. The linewidth stability of about 1.7×105 pulses is determined (Fig.8), and the mean value of the linewidth is approximately 14.2 MHz with a stability of about 0.25 MHz (root mean square). By adjusting the pulse width of the Lorentz modulation signal of the AOM1 (Table 1), the study on the amplified pulse waveform reveals that the laser output achieves a pulse width in the range of several hundred nanoseconds, thus meeting the specific pulse width requirement of coherent detection lidar.ConclusionsA hundred-nanosecond, single-frequency, high-energy 1064 nm laser based on fiber-bulk hybrid amplification undergoes experimental investigation as a laser source for space-based coherent detection wind lidar. A Lorentzian pulse waveform reshapes and chops the output of the CW DFB laser. A pulsed seeder with a pulse width of approximately 149.0 ns and a repetition rate of 60 Hz emerges. After the amplification of the cascaded fiber amplifier and multi-stage solid-state crystal amplifier, the system produces a laser output with a single pulse energy of about 151.4 mJ and a pulse width of about 267.8 ns. Utilizing the optical heterodyne method, the laser linewidth measures approximately 14.2 MHz. By altering the pulse waveform of the Lorentz modulation signal, the pulse width of the output laser can vary within several hundred nanoseconds. The study results offer a new technical route for employing a 1.06 μm laser source for space-based coherent detection wind lidars.

ObjectiveFiber lasers have been widely used in various fields including medicine, industrial processing, and national defense, because of their excellent beam quality, high conversion efficiency, straightforward heat management, and flexible operation. The pump-signal combiner occupies a crucial role in efficiently coupling the pump light to the double-cladding fiber, for signal light transmission, and is one of the most critical fiber laser components. Transverse mode instability (TMI) and nonlinear effects have been identified as bottlenecks for further improvement in fiber laser power. Counter-directional pumping and large-mode-area double-cladding fibers are helpful for suppressing nonlinear effects, which are beneficial for fiber laser power improvement. Most existing research on pump-signal combiners focuses on the few-mode signal fibers with core diameters smaller than 30 μm. In this study, the fabrication method for a counter-directional (6+1)×1 pump-signal combiner based on a large-core (50 μm) multimode signal fiber is introduced, with higher thresholds for nonlinear effects. The proposed pump-signal combiner achieves high pump-coupling efficiency alongside high-beam quality.MethodsBy conducting numerical simulations and experimental validations, the effects of the taper length and ratio, and refractive index of the glass tube on the coupling efficiency of the pump light were analyzed. The effect of the core axial offset on the transmission efficiency and beam quality of the signal light was also investigated. Consequently, the optimal parameters for fabricating the pump-signal combiner were obtained. During the pump-signal combiner fabrication process, signal fiber tapering was avoided by pre-tapering the pump fiber and signal fiber corrosion. While optimizing the cutting and fusion parameters, the tapered fused bundle (TFB) and output fiber were spliced using an inline feedback alignment. Subsequently, the pump and signal arm performances were tested using laser diodes (Reci, DAB 1200, 915 and 976 nm wavelengths) and a 3 kW fiber oscillator, respectively. Finally, an integrated device based on the proposed pump-signal combiner was fabricated and applied to a narrow-linewidth laser system, which included an end cap and a cladding light stripper.Results and DiscussionNumerical simulations and comparative experiments show that selecting an appropriate taper length, reducing the taper ratio of the pump fiber, and using a low-refractive-index glass tube can improve the pump-arm performance of the pump-signal combiner (Figs.3 and 4 and Table 1). To achieve this, a pump-signal combiner was fabricated using a semi-fluoride thin-walled glass tube. The proposed pump-signal combiner achieved a pump coupling efficiency of over 98.5% and a temperature rise coefficient below 10 ℃/kW without active cooling. It could be observed that the fiber core offset during multimode signal fiber fusion results in fundamental mode conversion to higher-order modes. Although this may not significantly impact the overall signal light passing rate, the M2 factor, which is more sensitive to the axial offset, was selected as the feedback alignment indicator (Fig.5). During the M2 feedback fusion process, fusion quality and strength are ensured by maintaining the angle of cleavage within 1° and controlling a slight collapse of the TFB (Fig.8). Consequently, the beam quality degradation ratio is only 3.4% (Fig.9). The integrated device based on the pump-signal combiner effectively reduces the number of splice points and the transmission fiber length, thus, enhancing fiber laser system compactness and stability. When applied in a narrow linewidth system based on a simple MOPA structure, at 4182 W output power, the beam quality is Mx2=1.48, My2=1.36, 3 dB linewidth is 0.44 nm, 20 dB linewidth is 2.14 nm, and the Raman suppression ratio is 40.5 dB (Fig.11).ConclusionsThe proposed pump-signal combiner fabrication method enables signal and pump fiber matching of any size without the necessity for signal fiber tapering. Hence, the beam quality of the pump-signal combiner can be effectively maintained. Through theoretical analysis and experimental verification, it is demonstrated that the use of a semi-doped fluorine thin-walled glass tube can improve pump arm performance. Additionally, the M2 factor is confirmed as a suitable indicator for aligning large-core multimode signal fibers. Consequently, development of a (6+1)×1 pump-signal combiner was achieved with a pump coupling efficiency of over 98.5% and beam quality degradation of only 3.4%. The temperature increase coefficient was maintained below 10 ℃/kW without active cooling. Based on the proposed pump-signal combiner, an integrated device without splice points was fabricated to reduce fusion loss and to ensure a more compact system. The proposed solution has broad application prospects for high-power, high-beam-quality fiber laser systems.

ObjectiveA 795 nm vertical cavity surface emitting laser (VCSEL) has the advantages of a low threshold current, single-mode operation, a low power consumption, and high temperature and reliability. It is an ideal light source for quantum precision measurement in devices such as atomic clocks, atomic magnetometers, and atomic gyroscopes. A VCSEL typically uses oxidation limiting structures for mode regulation to achieve electro-optic confinement. A VCSEL with conventional circular oxide apertures has an axisymmetric structure, making it difficult to achieve polarization control in two orthogonal directions and prone to polarization instability with increasing current. The application of a VCSEL in a device such as a chip-level atomic clock requires it to have a stable polarization direction and high polarization suppression ratio. Asymmetric oxidation apertures are introduced to improve the polarization stability of the 795 nm VCSEL used for rubidium atomic clocks. Controlling the oxidation conditions such as the temperature and gas pressure in the wet oxidation process makes it possible to fabricate VCSELs with different oxidation rates and ellipticity values for their apertures and analyze the effects of these on the polarization performance, which assists in achieving a low threshold and high polarization output.MethodsA two-dimensional cold-cavity simulation of a 795 nm VCSEL is conducted using the fluctuation optical frequency domain module in COMSOL Multiphysics, and the effects of different oxidation apertures on the resonant light intensity in the active region are simulated. The influence of the oxidation furnace temperature on the oxidation rate and ellipticity is studied using a real-time monitoring wet oxidation system and controlling oxidation parameters such as the temperature and gas pressure in the wet oxidation process. The ellipticity and aperture parameters are obtained by the ellipse fitting of the captured oxidation aperture images. Three types of VCSELs with different long-axis diameters and ellipticity values are prepared using a circular table. The power-current-voltage (P-I-V) curves, mode characteristics, line widths, polarization characteristics, and polarization angle rotation characteristics of the three devices are tested and analyzed.Results and DiscussionsThe influence of the oxidation furnace temperature on the oxidation rate and ellipticity is determined by using the real-time monitoring wet oxidation system and controlling oxidation parameters such as the temperature and gas pressure in the wet oxidation process. Three types of VCSELs with different long-axis diameters and ellipticity values are prepared using a circular tabletop, and their mode characteristics, polarization characteristics, and polarization angle rotation characteristics are analyzed and studied. The experimental results show that a VCSEL with elliptical oxidation holes with a long-axis diameter of 3.7 μm and ellipticity of 1.7 has the best performance (Fig. 7). At 85 ℃, with an injection current of 1.5 mA, the output power is 0.86 mW, laser wavelength is 795.4 nm, side mode suppression ratio (SMSR) is 43 dB, line width is 65 MHz (Fig. 13), and orthogonal polarization suppression ratio (OPSR) is 23.8 dB. The VCSEL remains unchanged in the main polarization direction within the range of 0.6‒2.7 mA. According to the test results, the three types of VCSELs with different major-axis diameters and ellipticity values show different unidirectional rotation values for the main polarization direction angle with an increase in current (Fig. 14).ConclusionsIn order to improve the polarization stability of the 795 nm VCSEL used for Rb atomic clocks, this study discusses the effects of different oxidation apertures and ellipticity values on the polarization performance of a VCSEL. The influence of different oxide apertures on the resonant light intensity in the active region is simulated using the fluctuation optical frequency domain module of COMSOL Multiphysics. The results show that the highest resonance intensity in the active region is obtained when the oxidation aperture is 3.5‒4.0 μm. The real-time monitoring wet oxidation system is used to study the effects of the oxidation furnace temperature on the oxidation rate and ellipticity. As the injection current increases, the three types of elliptical oxidation aperture VCSELs exhibit different mode characteristics, polarization characteristics, and polarization angle rotation values. The test results indicate that the VCSEL with an elliptical oxidation aperture with a long-axis diameter of 3.7 µm and an ellipticity of 1.7 has the best performance. At 85 ℃, with an injection current of 1.5 mA, the output power is 0.86 mW, laser wavelength is 795.4 nm, SMSR is 43 dB, line width is 65 MHz, and OPSR is 23.8 dB. The main polarization direction of the VCSEL remains unchanged within the range of 0.6‒2.7 mA.

ObjectiveThe collimation adjustment of an unstable resonator requires guiding light. Examination of the oscillation process and propagation characteristics of guiding light assists in the understanding and judgment of the resonator misalignment state. Off-axis unstable resonators are hybrid unstable resonators with high extraction efficiencies and beam quality values. The physical process of guiding light oscillation in an off-axis unstable resonator has rarely been investigated. Therefore, this study investigates the multi-beam interference process and far-field propagation characteristics of guiding light in an off-axis unstable resonator using theoretical calculations and experiments.MethodsA guiding light research device in an off-axis unstable resonator is constructed. The schematic of optical path of guiding light in the off-axis unstable resonator composed of four resonator mirrors is shown in Fig.1. The convex cylindrical mirror in the Y-direction and concave spherical mirror form a stable resonator in the X-direction and a positive-branch confocal unstable resonator in the Y-direction, respectively. Z-shaped folding in the X-direction is achieved by turning plane mirrors. The curvature radii of the concave spherical and convex cylindrical mirrors are 16 m and 14 m, respectively. The magnification in the direction of the unstable resonator is 1.14. The 632.8 nm He-Ne laser (guiding light) is injected through a small hole at the bottom of the concave spherical mirror, oscillates back and forth inside the resonator, and outputs off-axis in the Y-direction of the convex cylindrical mirror. The output guiding light is focused on the charge coupled device (CCD) target through a convex lens with 300 mm focal length to observe its far-field characteristics. In terms of numerical calculations, this study utilizes the diffraction theory of the plane wave angle spectrum to calculate the oscillation process of guiding light in a Z-shaped folded off-axis unstable resonator. In the calculations, the incident beam is a fundamental-mode Gaussian beam with a central wavelength of 632.8 nm. The number of samples in the calculation is 16384×16384. The flowchart of the optical field oscillation calculation is shown in Fig.2. The intracavity output loss, reflection loss and injected guiding light energy during the guiding light oscillation process reach a balanced condition within a limited number of oscillations. Throughout this process, the maximum number of guiding light oscillations is approximately 10.Results and DiscussionsThe calculated and experimental results for the near-field spot and far-field spot of the oscillating guiding light in the concave spherical mirror are shown in Figs.3 and 4, respectively. The concave spherical mirror acts as a convex lens in the X-direction to repeatedly focus and diverge the Gaussian guiding beam. In the Y-direction of a positive-branch confocal unstable resonator, for each round-trip oscillation in the resonator, the waist radius of the Gaussian guiding beam in the Y-direction increases M times (M refers to the confocal unstable resonator magnification), its divergence angle is reduced to 1/M, and its curvature radius increases M2 times. Therefore, during multiple oscillations, the Gaussian guiding beam tends to be a plane wave, eventually reaches above the convex cylindrical mirror and outputs. As shown in the calculation results in Fig.3(a), the interference pattern at the injection point of the small hole in the concave spherical mirror presents an elliptical water droplet shape, which differs from the circular interference patterns commonly observed in traditional positive/negative confocal unstable and stable resonators. The guiding light is output from an off-axis unstable resonator and passes through a focusing lens with 300 mm length to obtain a far-field spot pattern, as shown in Fig.3(b). Different order spot patterns are distributed along the Y-direction. As the number of oscillations increases, the divergence angle of the N-order Gaussian beam in the Y-direction in the resonator is reduced to 1/MN (N refers to the number of oscillations). The higher the order of the Gaussian beam, the closer the beam is to a plane wave. When the beam is a plane wave, the (X, Y) coordinate corresponding to the far-field focal spot is (0,0). The highest-order guiding light far-field spot can be used to indicate the far-field spot of an infrared laser. It should be noted that the number of guiding light oscillations in the resonator is limited because of the output loss and resonator mirror reflectivity loss. The highest-order guiding light far-field spot positions as shown in the calculations and experiments can only approximate infrared laser far-field spot positions.ConclusionsThe calculation and experimental results indicate that the interference pattern of an off-axis unstable resonator (with a flat concave stable resonator in the X-direction) differs from that of traditional confocal unstable or stable resonators. The guiding light in the resonator exhibits the interference pattern with an elliptical water droplet shape at the injection point of the small hole in the resonator mirror, which has strong brightness and can be used to determine the optical resonator collimation state. The far-field spot pattern of the output guiding light exhibits a series of bright spots along the Y-direction, and the higher-order bright spot can be used to indicate the far-field spot position of the infrared laser. Thus, the results of this study provide a reference for understanding the physical process of Gaussian beam oscillation in an optical resonator and for determining the collimation state of an off-axis unstable resonator.

ObjectiveThe tunable laser near the 2 μm wavelength has attracted significant attention due to its importance in gas detection and potential as a core component in high-capacity optical communication technologies. The scheme based on the integration of a silicon photonic chip with a III-V gain chip is gaining popularity in the research field of external cavity tunable lasers owing to its narrow linewidth and wide tuning range. While previous studies have primarily explored the ~1.55 μm wavelength range, limited research has been conducted on ~2 μm wavelength external cavity semiconductor lasers (ECDLs). In addition, in the fields of spatial signal transmission and gas detection, ECDLs must meet higher requirements for the output power, side mode suppression ratio (SMSR), and stability. Therefore, further optimization of the external cavity of the ~2 μm tunable laser is needed.MethodsBased on the 220 nm silicon-on-insulator (SOI) platform, we first used the Lumerical mode to simulate and analyze the mode loss characteristics of optical waveguides with different cross-sectional sizes and bending radii. We then studied the impact of the microring resonator Ggapmrr on the performance of a vernier filter (including the Q value, side mode suppression ratio, linewidth, and transmission loss). In addition, we analyzed the effect of waveguide termination reflectivity on the stability of the external cavity of a tunable laser and proposed waveguide termination based on the scattering and bending loss characteristics of multimode waveguides to terminate stray light in the waveguide. Finally, a thermal conduction analysis of the vernier filter was performed using Lumerical device, and the thermal analysis data were imported into Lumerical interconnect to study the thermal tuning performance of the tunable laser external cavity.ResultsBased on the above methodology, Si waveguide widths ranging from 0.28 μm to 0.63 μm facilitate quasi-single-mode transmission within the 1.95?2.05 μm wavelength range (Fig.2). Investigations of various Si waveguide thicknesses indicate a stable effective refractive-index difference between TE00 and TM00 when the waveguide thickness is below 0.24 μm (Fig.3). Optimizing the waveguide width enables low-loss TE00 mode transmission in straight waveguides (Fig.4), and a bending radius exceeding 5 μm tends to approach TE00 mode losses of approximately 0 dB/cm (Fig.5). By exploring the impact of Ggapmrr on the vernier filter performance (as depicted in Fig.8), it is found that increasing Ggapmrr enhances the SMSR. However, beyond 250 nm, the improvement stabilizes owing to the reduced coupling coefficients, elongating the effective length of the microring resonators. Consequently, the insertion loss increases with increasing Ggapmrr, affecting the efficiency of the filter. The linewidth decreases sharply and levels off, whereas the Q factor exhibits an inverse trend. Above 300 nm, the difference in radius between the microrings significantly influences the Q values and transmission losses. The transmission spectrum of the vernier filter (Fig.9) displays fine fringes and significant changes in the SMSR and full width at half maxima, owing to reflections from the bus waveguide terminator. To address this issue, the proposed multi-mode annular waveguide termination (Fig.10) effectively terminates stray light, with negligible reflected optical power in the waveguide on the order of 10-12 W. The vernier filter achieves wavelength tuning through nickel-chromium alloy microheaters atop the microrings, leveraging thermal-optic effects. Simulations in SOI waveguides reveal changes in the effective refractive index of the TE00 mode with temperature at wavelengths of 1.95, 2, and 2.05 μm (Fig.11). Utilizing Lumerical device for thermal conduction simulations, the temperature distribution in the circuit under an applied voltage indicates improved efficiency at 4 V, resulting in a temperature increase of 127 K (Fig.13). The study delves into the broad and fine-tuning of a silicon-based tunable laser's external cavity, showing both a wide tuning range of 66 nm (1967?2033 nm) at a 3.2 V bias when using a single microheater and a precise tuning with a 0.1 nm/K accuracy when using two microheaters simultaneously (Figs.14 and 15).ConclusionsResearch on silicon-based external cavity tunable lasers around the ~2 μm wavelength remains limited. Using a 220 nm SOI platform, we simulate and analyze the mode loss characteristics of optical waveguides of various sizes. A designed Si waveguide (600 nm×220 nm) ensures a low-loss TE00 single-mode transmission in a curved waveguide with a radius exceeding 5 μm. To investigate the impact of a single microring resonator on vernier filter performance, optimal coupling distances are discussed for applications with different requirements. We propose a highly process-compatible multi-mode annular waveguide termination method. Simulations demonstrate a wide 66 nm tuning range and a fine-tuning accuracy of 0.1 nm/K for the designed silicon-based tunable laser.

ObjectiveThe phenomenon of electromagnetically induced transparency (EIT) was first discovered in atomic systems with three-level distributions. It is a destructive interference phenomenon between strongly coupled light beams in different transmission paths, which makes initially opaque media transparent. However, the implementation of EIT in atomic systems requires extremely strict external conditions, such as ultralow temperatures and intense pumping light, which limit its practical application and development. In recent years, with vigorous research on metasurfaces, the EIT phenomenon has overcome traditional limitations and can be achieved via the coupling of the bright or dark modes of metasurfaces, thereby expanding its applications in molecular sensing, slow-light devices, and other fields. However, a large portion of the research on EIT metasurfaces selects nonadjustable metal structures in their design, which implies that the designed functionalities are limited to the characteristics of EIT. This considerably restricts the applicable scenarios and hinders further development of this research. This paper introduces adjustable materials into the design of EIT metasurfaces and proposes a multifunctional and polarization-independent metasurface based on EIT. By integrating multiple functions into a single structure, it achieves sensing and measurement of sucrose solvents, controllable slow-light effects, and a temperature-and-light dual-control switch. This significantly enhances the functionality of EIT metasurface devices and demonstrates the design concept of using adjustable materials to change the structural resonance and control the electromagnetic response, thereby providing valuable references for EIT metasurface research.MethodsIn this study, the EIT phenomenon was mainly achieved by coupling the bright modes. The bright-mode structure can couple with electromagnetic waves in a dipole resonance state with a low-quality factor. The primary metal square ring and cross structures were used as two modes that indirectly couple to form a transparency window, and the relevant mechanism is further explained by introducing the Lorentz resonance model. To expand the application functionality of EIT metasurfaces, we deposited photosensitive silicon on both ends of the cross-structure, which was excited by an 800 nm near-infrared laser pulse. Vanadium dioxide was embedded inside the square ring structure, which underwent a nonmetal to metal phase transition at 68 ℃. Using these two control methods, temperature and light, controllable slow-light effects, and dual-control dual-channel switches were achieved.Results and DiscussionsThe period of the metasurface structure designed in this study is illustrated in Fig. 1. When the two materials are in an unexcited state, the metasurface response exhibits an EIT phenomenon. We discuss the corresponding sensing characteristics when the ambient refractive index changes with a sensitivity of 306.49 GHz/RIU. To validate the potential application of this design in the field of molecular sensing, we introduced relevant data for sucrose molecules and achieved sensing measurements for sucrose molecules with a sensitivity of 97.6 GHz/(kg/m3). This also implies that the design can be extended to other molecular sensing and detection fields, such as tumor cells and hemoglobin. Regarding the slow-light effect, the group delay at the transparency window was 3.03 ps, and the group refractive index was 174.8. When both materials are in the excited state, the slow-light effect disappears, implying that the slow-light effect can be activated by controlling the excitation of the materials. For the temperature-light dual-control switch, when the photosensitive silicon is excited, the cross structure is connected, causing the original resonance of the cross structure to be destroyed and the resonance peak to disappear. The indirect coupling between the cross and square-ring structures weakens. Because the resonance frequency of the cross structure is located on the left side of the EIT response curve, it increases the amplitude of the front dip of the transmission curve. However, when vanadium dioxide is excited, the square ring structure is supplemented with a square structure, causing a change in the resonance position owing to structural alterations. The indirect coupling between the square-ring structure and cross structure is weakened, thus increasing the amplitude of the rear dip.ConclusionsThis paper proposes a versatile, polarization-independent metasurface structure that enables molecular sensing, controllable slow light, and dual-channel light-controlled switching. The basic structure of the metasurface is composed of a cross structure and four square ring structures. Photosensitive silicon and vanadium dioxide are introduced to achieve diverse and controllable functions. The formation mechanism of the EIT phenomenon is explained based on electric-field analysis and theoretical models, where the basic structures act as bright modes and undergo indirect coupling. Sensing measurements of sucrose solutions of different concentrations validate the potential of this design in the field of molecular detection. The slow-light effect of the metasurface is discussed, and its selectivity of the slow light effect is achieved using controllable materials, addressing the limitations of previous slow-light devices that cannot be turned off. Lastly, a controllable dual-channel switch based on controllable materials is realized, with bandwidths of 61.46 and 70.7 GHz, respectively. This provides a new design approach for EIT metasurfaces that disrupts the resonance of the original structure to obtain new electromagnetic responses, offering a reference for future research.

ObjectiveHigh-power laser systems are capable of outputting nanosecond pulses with power exceeding megajoule levels, allowing high-temperature and high-density conditions to be achieved in laboratory settings. As a result, they are widely employed in various fields such as high-energy-density physics experiments, inertial confinement fusion, astrophysics, and materials science. Solid-state laser systems based on neodymium glass with long optical path lengths are typically used in high energy laser system, which warrant precise control of beam quality and placement. Researchers have conducted extensive studies to maintain a large field-of-view while improving collimation accuracy. However, owing to the limited resolution and field-of-view of optical systems, it is generally not feasible to achieve high accuracy and a large field-of-view in the same collimation system. Therefore, a larger field-of-view is often compromised to ensure high automatic collimation accuracy. In such cases, precise mounting and a highly stable mechanical structure are required to ensure that the collimated beam enters the field-of-view. However, given that the limits of mounting accuracy and long-term stability of the mechanism can only reach a few hundred microradians, the outcomes of this approach are limited. In this study, we proposed a dual-trace far-field automatic alignment scheme using a bifocal lens that enables us to achieve microradian collimation accuracy and a field-of-view of several milliradians in the same optical system. This approach significantly reduces the need for precision mounting and long-term stability of the mechanical structure.MethodsBased on the ghost imaging theory of bifocal lenses, this study proposed a scheme for acquiring far-field feedback images with two different angular resolutions on a single camera. The core of the scheme is a bifocal-thick lens with different curvature radii on the left and right surfaces, coated with a 0° spectral dielectric film on both sides. For the off-axis incident beam, owing to the spectral effect of the dielectric film, a second-order ghost image is generated by the secondary reflection of the beam on the right inner surface of the lens; that is, a brighter main spot and a dimer ghost spot can be obtained simultaneously on the imaging plane. Owing to the multiple reflection and transmission processes, the angular deflection sensitivity of the ghost spot is much greater than that of the main spot; therefore, it exhibits a higher regulation accuracy in the feedback process. The low angular deflection sensitivity of the main spot ensures a field-of-view of several milliradians, whereas the high angular deflection sensitivity of the ghost spot ensures the collimation accuracy of microradians. A single imaging system that incorporates this bifocal lens is sufficient for far-field alignment. This study provided a design example of a bifocal lens and conducted a numerical analysis based on matrix optics theory and ZEMAX. The equivalent transfer matrix of the bifocal lens was used to demonstrate its focusing properties.Results and DiscussionsThe ray tracing results for the main spot, as obtained from the ZEMAX simulation, are shown in Fig. 4. The offset in the image plane is about 2.11 mm. The ray tracing results for a second-order ghost image in the system are shown in Fig. 5. The offset in the image plane is about 1.86 mm. For the same incident-angle offset, the angular magnification of the second-order ghost image is approximately 6.6 times that of the main spot under the given design parameter. To further verify the feasibility and effectiveness of the design, an experimental optical path was built based on the theoretical design. The experimental optical path is illustrated in Fig. 6. The spot offsets for the same step values obtained in the experiment are shown in Fig. 8. When determining the angular magnification of the spot at the same offset, the angular magnification of the ghost spot is observed to be approximately 6.9 times that of the main spot, which is a deviation of approximately 4.5% compared to the simulated result of 6.6 times.ConclusionsThis paper proposed a dual-trace far-field auto-alignment scheme using a bifocal lens and provided a design example to illustrate the implementation of the system. The theoretical results for the lens were analyzed through calculations and simulations, while its imaging performance was experimentally verified. Experimental verification confirms that the imaging performance is consistent with the design specifications. The proposed design leverages the ghost image of an optical lens, thereby enabling multiple imaging with a single bifocal lens in a lightweight and highly integrated manner. Combining this design with existing automatic alignment schemes is expected to reduce the requirements for mechanical mounting and long-term stability, minimize redundancy, and enable practical applications in high-power laser systems.

ObjectiveAn optical telescope is a crucial component of the space gravitational wave laser interferometer system, with its optical path length stability required to meet the picometer level. Therefore, it is significant to design a feasible measurement scheme for evaluating the optical path length stability of the telescope. Laser interferometry has become the predominant method for measuring optical path length stability owing to its high measurement precision and strong anti-interference capability. In this study, a measurement scheme for the optical path length stability of an off-axis four-reflection telescope is designed based on the Fabry-Perot interferometer. The noise analysis and measurement of each component in the scheme are performed to assess the feasibility of this scheme.MethodsA measurement scheme for the optical path length stability of the telescope is designed based on the Fabry-Perot interferometer. A resonant cavity is formed on the telescope by adding two high-reflectivity mirrors in a vacuum heat and vibration isolation system. The Pound-Drever-Hall (PDH) frequency-locking technique is utilized to lock the laser frequency to the resonant frequency of the telescope cavity, converting the optical path length variations within the telescope into laser frequency variations. The influence of absolute laser frequency and free spectral range on the measurement of optical path length stability is analyzed by utilizing their derivatives in the measurement principle formula. To assess the noise within the entire measurement loop, a low-precision cavity is employed as a substitute for the telescope cavity, and the measurement laser is locked to the low-precision cavity with a similarly low precision. Subsequently, the external noises in the scheme, including the reference system, beat frequency measurement, residual amplitude modulation, and electronic noise, are measured. The impact of these external noises on the measurement of optical path length stability is analyzed, and the primary noise source limiting the measurement accuracy is identified.Results and DiscussionsThe noise allocation requirements for each component in the measurement scheme within the frequency band of 1 mHz to 0.1 Hz are listed in Table 1. The influence of the absolute laser frequency and free spectral range on the measurement of optical path length stability is analyzed by utilizing their derivatives in the measurement principle formula. When the optical path noise and free spectral range change by 1 GHz and 0.25 MHz, respectively, they correspond to optical path length variations of 10 μm and 60 mm, respectively, both of which significantly exceed the optical path length stability measurement requirements. At these values, their impacts on optical path length stability are determined to be 3.5×10-6 pm/Hz1/2 and 5×10-3 pm/Hz1/2, respectively. The overall measurement of electronic noise introduced in the measurement scheme reveals that the electronic noise is 0.2 mV/Hz1/2 (Fig.5), which is below the specified requirement of 0.238 mV/Hz1/2. The frequency stability of the ultra-stable laser is shown in Fig.6. However, the optical path noise of 1 pm/Hz1/2 corresponds to a frequency noise of 94 Hz/Hz1/2. Therefore, the frequency noise of the ultra-stable laser is more than one order of magnitude smaller than the telescope requirement, making it suitable to be a frequency reference source. The beat frequency measurement device exhibits a noise level of 1 Hz/Hz1/2 (Fig.7), which is two orders of magnitude lower than the specified requirement, indicating that it will not be a limiting noise source for the measurements. The residual amplitude modulation (RAM) noise of the electro-optic phase modulator is determined to be 5 mV/Hz1/2 (Fig.8), which exceeds the requirement by one order of magnitude and becomes the primary noise source in the current scheme.In the future, the impact of RAM noise can be mitigated through methods such as active temperature control and active voltage bias on the crystal.ConclusionsIn this study, the off-axis four-reflection telescope is transformed into a resonant cavity by adding two highly reflective mirrors in the vacuum heat and vibration isolation system. A measurement scheme for assessing the optical path length stability of the telescope was designed based on the Fabry-Perot interferometer. The PDH frequency-locking technique is employed to measure the optical path length changes within the telescope cavity by converting them into laser frequency variations. The influence of the measurand changes and external noise on the measurement of optical path length stability is analyzed in the measurement scheme. Experimental results demonstrated that the absolute laser frequency and free spectral range variations had a negligible impact on the measurement of optical path length stability, both being less than 1 pm/Hz1/2. Within the frequency band of 1 mHz to 0.1 Hz, the electronic, beat frequency measurement, and residual amplitude modulation noises in the measurement scheme are equivalent to optical path noise of 0.14, 0.01, and 3.57 pm/Hz1/2, respectively. Therefore, the residual amplitude modulation noise emerged as one of the limiting factors in achieving picometer-level optical path length stability in telescope measurements, necessitating further studies on noise suppression techniques.

ObjectiveThe primary application of the host device involves research in high-energy density physics and inertial confinement fusion, handling energies up to 100000 joules. A significant challenge encountered during these experiments is the simultaneous detection of strong and weak signals in the far-field focal spot. Specifically, accurately measuring weak signals in the sidelobe area of the far-field focal spot has proven difficult. To address this, we introduce a peak parameter detection method for weak signal regions in the sidelobe, leveraging neighborhood vector principal component analysis (NVPCA) for image enhancement.MethodsOur optimization strategy includes several steps. First, we treat each pixel in the sidelobe image and its eight neighboring pixels as a column vector to construct a 9-dimensional data cube. The first dimension post-PCA transformation, the NVPCA image, is then selected. Next, we employ angle transformation to detect various peak parameters of the one-dimensional sidelobe curve in all directions, facilitating the quantification of energy distribution in the sidelobe’s weak signal area. Subsequently, we identify the maximum position points of each sidelobe peak in all directions, linking these to form a maximum ring for each peak and calculating the grayscale mean of these rings. The smallest grayscale mean exceeding the LCM target separation threshold is identified as the minimum measurable signal for the entire sidelobe beam.Results and Discussions1) We propose a sidelobe weak signal detection method using NVPCA image enhancement. This approach successfully isolates and extracts the minimum measurable signal from the 5th peak ring on the sidelobe image’s periphery, increasing the dynamic range ratio to 1.528 times. This method enhances the peak’s maximum value in any direction, ensuring the extraction of the minimum measurable signal from the peripheral 5th peak loop.2) The LCM target detection threshold formula is employed to segregate the minimum measurable signal. This formula, tailored to the characteristics of far-field focal lobe images, effectively separates background noise.3) We validate the one-dimensional curve peak parameters in various directions using a two-dimensional plane display method. Combining two-dimensional and one-dimensional displays, this method not only showcases the peak parameter distribution of one-dimensional sidelobe curves from multiple perspectives but also differentiates adjacent sampling angles’ peak positions. The validation using equations (11)?(13) yields rising edge, falling edge, and pulse width consistent with those in Table 5, confirming the two-dimensional display method’s efficacy in verifying one-dimensional curve peak parameters.ConclusionsAddressing the challenge of extracting the smallest measurable signal in the sidelobe image’s periphery for strong laser far-field focal spot measurements, we introduce a sidelobe weak signal region peak parameter detection method based on NVPCA image enhancement. Our findings demonstrate this method’s capability to isolate and extract the minimum measurable signal from sidelobe image peripheral peaks, increasing the dynamic range ratio to 1.528 times. This approach is crucial for accurately measuring weak signal areas in sidelobe beams, understanding their energy distribution, and laying the groundwork for future precise measurements of strong laser far-field focal spots in large-scale laser devices.

ObjectiveMicrolens array is one of the most important components in micro-optics, and it has been widely used in many fields. It is becoming more and more important to achieve higher measurement accuracy and faster measurement speed of the microlens array focal length. Traditional measurement methods such as interferometer measurement, microscope measurement, light intensity measurement, etc., are difficult to meet the requirements of high precision and rapid measurement simultaneously. Because the traditional scanning angle method needs to rotate the light tube or lenses, and the movement of the light spot cannot be determined in a single measurement, the measurement result is easy to be affected by the measuring device. Therefore, the scanning angle method based on multi-slit diffraction grating uses the multi-slit diffraction principle to determine the focal length by calculating the distance between adjacent orders, which improves the measurement efficiency. However, a single measurement can only obtain a single focal length value in this scheme, and it is still necessary to implement multiple measurements to eliminate various random errors. In addition, the non-negligible high diffraction orders of the traditional grating will introduce additional measurement noise, which will deteriorate the positioning accuracy of the centroids of those desired spots, resulting in the deterioration of the final accuracy. So, in order to eliminate random errors, multiple measurements have to be implemented in practice. To address the above problems, a fast focal length measurement scheme based on high-order-suppression Dammann gratings (HOSDGs) rather than traditional gratings is proposed in this paper.MethodsIn this study, a specially designed HOSDG is used to measure the focal length of the microlens. After the beam passes through the HOSDG, the diffractive light transmits through the microlens, and finally the camera receives the focused spot of each sublens on its focal plane. The distribution of the focus spots of each sublens is related to the focal length and the diffraction angle of the grating. After data processing, multiple distances among several desired orders are obtained, and then several values of the focal length for each sublens are calculated. In order to suppress the influence of high-order diffraction, the complex amplitude modulation combined with simulated annealing algorithm is used to optimize HOSDGs. In the experiment, this grating is fabricated by multistep overlapped lithography and wet etching technologies.Results and DiscussionsThe simulation results show that the high order sidelobe ratio is reduced from 11.13% to 5.3% (Fig. 4), and the experiment results indicate that the sidelobe ratio is reduced from 19.66% to 9.88%, which suggests that the high-order diffraction is effectively suppressed by this specially designed HOSDG. Due to its multiple equal-intensity orders (Fig. 5), HOSDG makes it possible to obtain multiple values of focal length through a single measurement after late-stage data processing (Fig. 7). It is shown that the single measurement error of the focal length of 11×7 microlenses is 3.5%, and the errors of the 15 repeated measurements are all within 4.5%.ConclusionIn this paper, a two-dimensional Dammann grating based on high-order diffraction suppression is proposed to measure the focal length of microlens array, which can effectively suppress the high-order diffraction energy and improve the measurement signal-to-noise ratio. In the proof-of-principle experiment, the designed five-beam HOSDG generates multiple focused light spots within each microlens aperture. The combination of multiple light spots to achieve a single acquisition is equivalent to 10 times of ordinary grating experiments, which effectively reduces the measurement random error, making the single measurement error less than 3.5% and repeated measurement error less than 4.5%. Therefore, this scheme can improve the measurement efficiency and reduce the measurement error in the high-precision measurement of the focal length of large-scale microlens array. This work will promote the fabrication, measurement and application of various microlenses.

ObjectiveOptical sparse aperture technology is applied to the design and optimization of metalenses to reduce the lens processing area, increase the numerical aperture, and improve the resolution. Currently, optical sparse aperture metalenses are limited to a single wavelength, and thus are difficult to use in broadband imaging fields. There are two primary types of metalens phase modulation: transmission and geometric. Transmission phase modulation is wavelength sensitive, making it difficult to focus beams of all wavelengths simultaneously; therefore, this type of structure is unsuitable for application in wavefront-encoding achromatic metalenses. Alternatively, during geometric phase modulation, the phase delay is wavelength independent, which makes the optical system insensitive to the position of the focal plane. As a result, geometric phase wavefront-encoding technology can achieve achromatism by increasing the focal depth of the system for all wavelengths. In this study, the cubic-phase wavefront encoding method is introduced for the design of an achromatic donut-like optical sparse aperture (DOSA) metalens. After cubic phase modulation, the optical transfer function of the proposed metalens hardly changes with the defocus. Furthermore, we hope that the proposed achromatic optical sparse aperture metalens can achieve a resolution consistent with that of an ideal lens in the visible light band (400?700 nm).MethodsAn achromatic optical sparse aperture metalens was designed based on the wavefront encoding method and optical sparse aperture technology. The designed metalens employed a donut-like optical sparse aperture. After passing through a fused quartz substrate, the wavefront reaches a donut-like medium layer, and the phase is modulated by periodically arranged nanopillar structures in the layer. An optimized geometric nanopillar structure was adopted to achieve greater polarization conversion efficiency at various wavelengths. In addition, cubic phase encoding based on Fourier optics was applied to preset the wavefront phase of an incoherent imaging system. When adjusting the cubic phase encoding factor, the optical transfer function (OTF) of the designed metalens barely changed with the defocus. The surface phase of the designed metalens was obtained by combining the focusing phase of the DOSA metalens with the cubic encoded phase. In addition, Wiener filtering was employed to restore the degraded images after passing through the metalens.Results and DiscussionsThe processing area of the designed metalens is reduced to 25% of the full-aperture transmittance metalens. Simultaneously, through the analysis of the modulation transfer functions (MTF) of the metalens at different wavelengths, the results show that the frequencies of the full-aperture and DOSA metalenses suffer severe losses at all wavelengths except 550 nm, leading to irreversible image blurring. Although the MTFs of the proposed achromatic DOSA metalens based on wavefront encoding decrease in the mid-frequency domain at all wavelengths, they are retained and can theoretically be restored (Fig. 3). To further verify the imaging quality, image formation simulations are conducted on three types of metalenses. The restored images of the full-aperture metalens at 400 nm, 470 nm, 625 nm, and 700 nm exhibit extreme minutia loss (Fig. 4). Similarly, the restored images of the DOSA metalens at 400 nm, 470 nm, and 700 nm profoundly lose minutia (Fig. 5). After wavefront encoding modulation, the quality of the restored image from the proposed metalens is basically the same at all wavelengths (Fig. 6). Consequently, this achromatic DOSA metalens not only achieves broadband achromatism, but also reduces the processing area of large-aperture metalenses.ConclusionsIn this study, an achromatic DOSA metalens based on wavefront encoding is realized by the superposition of the cubic encoded phase and DOSA focusing phase, realizing achromatic imaging in the visible light band. During the imaging processes of the full-aperture and DOSA metalenses, chromatic dispersion caused by changes in the incident wavelength leads to poor image quality. To obtain more minutiae and achieve a higher resolution in the visible light band instead of at a single wavelength, cubic wavefront phase encoding is introduced to achieve the phase modulation of all wavelengths. Based on the above theories, a cubic-phase wavefront encoding DOSA metalens is designed using gallium nitride with an effective half aperture of 0.35 mm, internal radius of 0.303 mm, focal length of 7 mm, and a cubic phase encoding factor of 20. Finally, simulations including MTF and image restoration of the full-aperture metalens, DOSA metalens, and achromatic DOSA metalens are conducted at 400 nm, 475 nm, 550 nm, 625 nm, and 700 nm. It is demonstrated that the DOSA metalens based on wavefront encoding can achieve achromatic imaging in the visible light band and theoretically achieve the same cut-off frequency as the ideal full-aperture metalens. Accordingly, it has excellent value in image acquisition owing to its low processing cost, large achromatic range, and high imaging quality.

ObjectiveThis study investigates the problems of the complex structure, large volume, and nonlinear projection of digital light processing (DLP) projectors, which are widely used in structured light 3D imaging systems. These problems restrict the application of structured light 3D imaging technology to small detection scenes. Therefore, this study proposes a structured light projection chip integrating a metasurface array with a micro-electromechanical system (MEMS) two-dimensional scanning platform. The metasurface array realizes the structured light stripe projection including the Gray code stripe and the phase-shifted stripe, and switching of the metasurface unit is achieved using a MEMS two-dimensional scanning platform. The experimental results demonstrate that the designed structured light projection chip exhibits superior characteristics in terms of precise detection accuracy, a rapid projection rate, and compactness, thereby satisfying the stringent detection requirements for small-scale application scenarios.MethodsThe metasurface array is initially investigated based on the geometric phase principle, and an analysis is conducted on the conversion efficiency and phase modulation capability of the nanopillars in each unit toward incident light. Subsequently, the GS algorithm is employed to determine the phases of mixed-code structured light stripes. The metasurface array is designed by obtaining the sizes of the nanopillars and phase information. For the MEMS two-dimensional scanning platform, the electrostatic comb driver design is primarily investigated, and the static, modal, and transient characteristics of the two-dimensional scanning platform are analyzed using ANSYS to ensure that its performance meets design requirements. Finally, this study investigates an integrated manufacturing process for metasurface arrays and MEMS two-dimensional scanning platform, providing a comprehensive manufacturing scheme for subsequent processing.Results and DiscussionsA chip model (Fig.1) that integrates a metasurface array and an MEMS two-dimensional platform is proposed. By designing the size of the nanopillar in the metasurface unit, a high conversion efficiency of 605 nm incident light is achieved, with the highest conversion efficiency being 88.61%. Additionally, the linear phase regulation ability of the nanopillar to the incident light is verified (Fig.6). The phase information required for constructing the metasurface unit is obtained by solving the mixed-code structured light stripes using the GS algorithm. After obtaining the optimal size of the nanopillars and the phase information, we establish a metasurface element model using FDTD and conduct simulations to evaluate its optical performance. The simulation results demonstrate that the fringes generated by the metasurface unit adhere to the characteristics of both the Gray code fringe (Fig.9) and the phase-shifted method fringe (Fig.10). The generated Gray code stripe exhibits distinct step distribution characteristics in terms of light intensity while maintaining a consistent width throughout. At a projection focal length of 50 mm, the stripe width is 3 mm. The generated phase-shifting stripe exhibits obvious sine distribution characteristics in terms of light intensity, and the stripe width is uniform. At a projection focal length of 50 mm, the stripe width is 2 mm. The generated stripe satisfies the high-detection accuracy requirements of the projection chip, and the static results (Fig.12) demonstrate that the MEMS two-dimensional scanning platform exhibits a low driving voltage. Specifically, the driving voltages for achieving an 80 μm displacement in the X and Y directions are measured to be 68.1 V and 57.6 V, respectively. The modal results (Fig.13) indicate that the two-dimensional scanning platform exhibits excellent anti-vibration and anti-interference characteristics, with a first-order modal frequency of 511.91 Hz and distinct boundaries for higher-order modal frequencies. The transient analysis results (Fig.14) demonstrate that the two-dimensional scanning platform exhibits a high response rate, with a response time of 2.4 ms obtained through the full transient method. Therefore, the structured light projection chip has a high projection rate, with a projection frame rate of 333 Hz. The chip size is 3 mm×3 mm, which has the advantage of miniaturization. The integrated manufacturing process flow of the metasurface array and MEMS two-dimensional platform is ultimately designed (Figs.17,18), encompassing various manufacturing processes for MEMS devices, such as oxidation, lift-off, BOE wet etching, DRIE dry etching, and bonding.ConclusionsThe results demonstrate that the designed metasurface exhibits excellent modulation performance for incident light while generating a striped pattern that satisfies the detection requirements of three-dimensional imaging technology. This effectively enhances the projection linearity of conventional projectors, thereby enabling submillimeter-level detection accuracy. The designed two-dimensional scanning platform exhibits dependable performance and a high projection rate. The integrated manufacturing process of the metasurface array and the MEMS two-dimensional platform provides a theoretical model and a system solution for designing a miniaturized projection device with high detection accuracy and projection rate.

ObjectiveWith the rapid developments in nanotechnology, cavity optomechanics has become an important research topic in quantum mechanics. Magnomechanical cavity systems have attracted considerable attention in recent years. Compared to the cavity optomechanical system, the cavity magnomechanical system exhibits many benefits such as high spin density, high cooperativity with microwave photons, and a very low damping rate. Therefore, this study provides a new platform for examining interactions between light and matter. This study analyzes the multi-transparent window phenomenon, fast- and slow-light effects, and precision measurements in a coupled cavity magnomechanical system. These results have potential applications in quantum information processing and high-precision measurements.MethodsIn this study, we commence with a coupled cavity magnomechanical system model. The hybrid cavity magnomechanical system consists of two yttrium iron garnet (YIG) balls located near the maximum magnetic field of the two resonant cavities, and a bias magnetic field is applied to the YIG balls in the z-direction of the two resonant cavities to excite the magnon mode and realize strong coupling with the cavity field. Mutual coupling exists between the optical fields of the two resonators, and the coupling strength is related to the distance between them. A weak probe laser beam εp with frequency ωp is applied to the optical cavity a1. The total Hamiltonian of the coupled cavity-magnetic field can be obtained in a frame rotating at the frequency of the driving field. Based on the Heisenberg equation and input-output relationship, we can obtain the output field (εout) expression and group delay (τ) expression of the system. Subsequently, the effects of various parameters on the optical response of the system are investigated.Results and DiscussionsWhen the coupling between two microwave cavities and magnon-phonon coupling are absent, there is only photon-magnon coupling between the cavity a1 and magnon m1. At this time, there is a magnon-induced transparency window in the absorption spectrum generated by the interaction between the magnon and optical cavity field. We introduce various coupling terms, and the absorption spectrum of the output field exhibits different numbers of transparent windows (Fig. 2). The dispersion spectrum of the output field is plotted under the same conditions (Fig. 3). Next, the influence of the coupling strength between the two microwave cavities on the transmission characteristics of the hybrid cavity magnomechanical system is examined. These results indicate that better transparency can be realized in the output field by adjusting the coupling strength using the coupling strength (Fig. 4). The absorption peak heights and widths of the detection field absorption spectra are plotted as a function of the coupling strength (Fig. 5). The results indicate that the absorption peak height of the detection field absorption spectrum is inversely proportional to the coupling strength (J) of the two microwave cavities, and the width is directly proportional to J. Therefore, the coupling strength J can be obtained by simply measuring the height and width of the absorption peak, which also indicates that the hybrid cavity magnomechanical system is an effective and accurate method for measuring the coupling strength J. We also investigate the effects of K1, K2, g2, and κa2 on the output field (Figs. 6 and 7). The results show that g2 and κa2 only cause a change in the position of the absorption peak of the absorption spectrum and do not affect the position of the transparent window. Finally, the functions of the group delay (τ) with the normalized detection field detuning (δ/ωb) are plotted for different J and κa2 (Fig. 6). There are upward peaks (slow-light effect) and downward valleys (fast-light effect) near δ=0.7ωb and δ=1.3ωb, and the peak value of the group delay τ decreases with an increase in cavity dissipation κa2 and increases with an increase in coupling strength J. Therefore, the conversion between fast- and slow-light effects can be realized by changing the dissipation and coupling strengths of the resonant cavity.ConclusionsBased on the coupled cavity magnomechanical system, the phenomenon of multiple transparent windows and effect of slow-fast light are investigated. The output characteristics of the system are discussed using quantum optics theory and standard input-output relations. The results show that different numbers of transparent windows can be obtained by adjusting the system parameters, and better transparency can be realized. Simultaneously, a method for precisely measuring the interaction strength between the two cavities is proposed by measuring the height and width of the absorption peaks. Additionally, fast-slow light conversion can be achieved by adjusting the system parameters. This scheme has important guiding significance for research on precision measurements and quantum information processing.

ObjectiveAs a breakthrough technology in recent years, super-resolution imaging has become an important research problem in computer vision and image processing and has wide practical applications in medical, biological, security, and other fields. However, classical imaging technology is limited by the diffraction resolution limit, and it is difficult to achieve resolution breakthroughs. Quantum entanglement can transcend diffraction resolution limits by sharpening spatial interference fringes based on quantum technology evolution . The entangled N00N state has been studied because it can exceed the standard quantum limit. The interference visibility of the three-photon N00N state is higher than the limit of classical spatial super-resolution, and the pattern of the N-photon entangled N00N state is N times finer than that of classical light. Thus, the N00N state can improve the resolution of the optical system by N times. However, the probability of all N photons arriving at the same location and the detection efficiency decreases exponentially with increasing N, making the advantages of the N00N state controversial. The optical centroid measurement (OCM) promotes the application of the N00N state in super-resolution imaging. This study further applies the advantages of N-photon entangled N00N state to super-resolution quantum imaging based on existing theories and technologies. This study further proposes a new quantum imaging system to improve the resolution of object imaging.MethodsThis study primarily adopts theoretical analysis and simulation methods. A simulation model based on the proposed quantum imaging system is created, and the resolution enhancement of our scheme is quantified by measuring the modulation transfer function (MTF). A photon source model is constructed to generate coherent photons that are irradiated onto the object and transmitted to the receiver. The centroid position of the photons is measured using the OCM method, and the point spread function (PSF) of the imaging system is calculated using the obtained simulation data. Finally, the MTF is obtained using the Fourier transform method. In addition to the theoretical analysis of the detection efficiency enhancement of N00N state by OCM, the advantages of OCM visibility are analyzed through simulation visibility. The data are obtained through model simulation, and the curve is fitted to the data point, following the visibility calculation and analysis using the fitted curve.Results and DiscussionsThe model simulation of the proposed imaging system shows that the MTF curve decreases with the increase of spatial frequency. However, the entangled two-photon curve changes more gently than the spatially uncorrelated two-photon curve, indicating that the resolution of entangled two-photon imaging is better than that of uncorrelated two-photon imaging. Similarly, the presence of more entangled photons changes the curve at a slower pace. The resolution of (16±2)% is enhanced in the two-photon N00N entangled state, and the resolutions of 4 and 8 photons are 30±3%, and 41±2%, respectively (Fig.2 and Table 1). The results verify the feasibility of the OCM imaging scheme for N-photon entangled N00N state super-resolution imaging. Moreover, the resolution can be enhanced by increasing the number of photons. The visibilities obtained by OCM for classical light and N00N entangled light are compared. The visibility decreases significantly as the number of photons of classical light increases from 2 to 4. The visibilities of 2, 3 and 4 photons are 45±5%,17±4%, and 12±2%, respectively, whereas the visibility obtained by OCM for N00N entangled light remains relatively constant. The obtained visibilities of 2, 3, and 4 photons are 50±4%, (44±2)%, and (42±4)%, respectively (Fig.3), achieving improved visibility.ConclusionsThe quantum imaging system scheme presented in this study improves the detection efficiency of N00N state by means of optical centroid measurement, and exploits the N-photon entanglement of N00N state to realize super-resolution imaging of objects. OCM does not require all photons to reach the same point in space as compared to the N-photon absorption scheme. The resolution of any number of photons can be improved by photon counting and proper post-processing, which significantly improves the detection efficiency of N00N entangled states. Moreover, the visibility of the OCM signal in N00N state is almost independent of the change in photon number N; therefore, the imaging system is suitable for higher photon numbers. The super-resolution quantum imaging system based on N-photon entanglement overcomes the problem in effectively detecting N-photon states, which improves quantum-enhanced measurement. Moreover, it is significant for Heisenberg finite phase detection and the development of super-resolution quantum imaging. Theoretically, the system can enhance N-1 times of image resolution. The prepared N00N state has high fidelity and stability. Thus, it is expected to be more commonly applied in research and promote new progress in the field of super-resolution quantum imaging.

ObjectiveBioaerosols are extremely easy to spread in the atmosphere and cause widespread disease infections, and a large number of bioaerosols are produced by natural human activities, environmental pollution, and sewage treatment processes. In addition, in the military field, bioaerosols are also the main combat form of biological agents, which have the characteristics of low cost and large destructive range and are used in battlefield or terrorist attacks with high possibility. Therefore, there is a need to develop technical means for real-time detection and early warning of bioaerosols. Lidar technology based on physical effects such as elastic and inelastic scattering between laser and bioaerosol particles, as well as laser-induced fluorescence (LIF), can provide information about the size, shape and composition of aerosol particles and has very fast response, and more importantly, long-range non-contact detection makes the safety significantly improved. So it has gained extensive research from scholars at home and abroad. LIF lidar is a broad-spectrum system, which is affected by atmospheric visibility and background radiation. It is obviously different from narrow-spectrum systems such as Mie scattering lidar. Different atmospheric conditions can affect the detection performance of LIF lidar, and we hope to quantify the influences.MethodsIn order to evaluate the detection performance of LIF lidar under different atmospheric conditions, a LIF lidar system is designed, and the formula of the system signal-to-noise ratio (SNR) is given. Then the simulation of atmospheric radiation transmission is started, and the broad-spectrum background radiation and atmospheric transmission transmittance on the horizontal path of several typical atmospheric visibility and background radiation (or operating hours) conditions are carried out using Modtran5. Finally, the parameters of LIF lidar are set, and then the signal-to-noise ratio of LIF lidar is simulated numerically according to the broad-spectrum background radiation and atmospheric transmission transmittance of different atmospheric conditions obtained in the previous step, and the variation function of signal-to-noise ratio with distance is obtained. The system works for detecting whether the aerosol contains biological substances and then for identifying biological aerosol species at two levels of detection needs. The simulation analysis is carried out separately.Results and DiscussionsThe fluorescence emission spectra of 355 nm light excitation of six biological substances are measured (Fig.1), which verify the basic principle of LIF lidar that spectral discrimination can be performed based on the characteristic fluorescence emission spectra of various biological substances. Then the real-world atmospheric background radiation is measured, and the comparison of the measured real-world atmospheric background radiation with the simulation results (Fig.3) fits well, proving that the simulation results of Modtran5 are credible. Subsequently, the simulated broad-spectrum background radiation and atmospheric transmission transmittance on the horizontal path under different atmospheric conditions (Figs.4, 5, 6, and 8) are obtained to provide sufficient atmospheric data for the following simulation of the system performance. Finally, the signal-to-noise ratio of the total fluorescence spectroscopy system (Fig.9) and the that of the 360‒370 nm channel system (Fig.10) are calculated. The results show that the atmospheric conditions largely affect the detection performance of the system: the higher the atmospheric visibility, the longer the effective detection distance of the system. Comparing the two atmospheric environments of visibility of 23 km and 3 km, the difference in the effective detection distance for different working hours is 2‒4 times. The working hours of the system also have a great impact on the detection performance. The detection performance of the system during daytime is poor and the difference between different time periods is small, while the effective detection distance of the system is generally increased (at least doubling) at night because the intensity of the atmospheric background radiation is greatly reduced, and the higher the atmospheric visibility, the larger the increase. Under the visibility of 23 km, the effective detection distance at night is 3.7 times that at the daytime. At the same time, the effective detection distance of the aerosol biological detection based on the total fluorescence intensity is higher than that of the aerosol biological component identification based on the spectrally distinguishable fluorescence intensity, and the former is more significantly affected by atmospheric conditions, i.e., good atmospheric conditions will bring higher improvement to the aerosol biological detection function.ConclusionsIn this paper, the detection performance of LIF lidar under different atmospheric conditions is evaluated through simulations. The LIF lidar is suitable for biofluorescence measurements at night because of the high atmospheric background radiation intensity in the fluorescence band. The decrease of atmospheric visibility leads to the decrease of background radiation intensity and also the decrease of transmittance of broad fluorescence spectrum, which in turn leads to the decrease of LIF lidar detection performance, but the influence is far less than the effect of changing operating hours between daytime and night. The most suitable working environment for LIF lidar is the night with high visibility.

ObjectiveWhen high repetition frequency integrated transceiver pulsed laser coherent method is utilized to measure static target distance, the maximum unambiguous distance caused by high repetition frequency is shortened on the one hand. On the other hand, reflected light is formed due to the reflection from the lens end face and the light leakage of the circulator at the time of emission. So the echo light will be submerged in the reflected light when it arrives during the pulse emission period. It is impossible to detect the echo light within the pulse emission time. Multiple blind range points and ambiguous distance problem appear. The traditional staggered pulse repetition frequency method requires frequency switching to obtain multiple sets of measurement data under different conditions. The control process is complicated and the echo light is still indistinguishable from the emitted light. A new scheme, high repetition frequency long-distance pulsed laser coherent ranging based on local oscillator modulation, is proposed in this paper. The maximum unambiguous distance determining factor is transferred from the pulse signal repetition frequency to the local oscillator modulation period. It expands the unambiguous range and solves the multiple blind distance points problem to make the measurement range continuous. There is no need to switch the repetition frequency, and the difficulty of system control is reduced. At the same time, requirements of the repetition frequency adaptability with the laser amplifier are reduced.MethodsIn pulsed laser coherent ranging, laser light is divided into two parts. One part is chopped and modulated to form pulsed light sent to the target, and the other is continuous local oscillator light. The proposed scheme mainly performs periodic chirp modulation on the continuous local oscillator light. And different pulse start frequencies are set according to the corresponding time relationship between the transmitted pulse signal and the local oscillator light. The modulation slopes of all pulse repetition frequencies are consistent. In order to make the measurement accuracy higher, the pulse compression method is used for data processing. So the slope of the optical pulse modulation is different from that of the local oscillator modulation. When the echo signal is coherent with the local oscillator signal, there will be two different effects. Within the local oscillator repetition period, one effect is that the pulse coherent signal with the same starting frequency is generated [Fig. 6(a)]. The other is that two different pulse coherent starting frequency signals are generated [Fig. 6(b)]. When the initial frequencies are different, the echo data are grouped according to the pulse repetition period that starts from the triggering time of the local oscillator modulation. The matching results of each group are analyzed. The number of pulses with the first initial frequency and the pulse compression results with the second initial frequency are recorded. Based on the recorded results, the target distance can be obtained by simple calculation.In experiments, three acousto-optic modulators (AOMs) are mainly used. The first AOM is used as a pulse signal optical modulator. Its design parameters include 200 MHz initial frequency, 2 μs pulse width, 64 μs repetition period and 5.03905 MHz/μs modulation slope. The second and the third AOMs are used simultaneously to modulate the local oscillator signal. The second modulation period is 512 μs and its modulation bandwidth is 170?190 MHz. The third one realizes a fixed -110 MHz frequency shift. Finally, the local oscillator optical modulation frequency is 60?80 MHz. The ranging experiment is conducted (Fig. 2). Dual lens, beam splitter and combiner are used to simulate the circulator light leakage in the laboratory. Different delay optical fibers of 7.96 km, 36.5 km, and 58.1 km lengths are respectively connected to the receiving lens to prolong the arrival time of echo pulses and simulate targets at different distances.Results and DiscussionsTwo measurement schemes with and without the local oscillator modulation are compared. Data acquisition and processing are performed. Relevant measurement results are obtained (Tables 3 and 4). With the local oscillator modulation high repetition frequency ranging scheme, the measured round-trip distances are 7.9608 km, 36.5156 km, and 58.1052 km, respectively. The corresponding accuracies are 2.7274 m, 5.1906 m, and 7.819 m. Since the 36.5 km delay line exceeds 19.2 km corresponding to a pulse repetition period, the measured distance based on the scheme without local oscillator modulation is 17.3 km (Table 3). An ambiguity phenomenon occurs and the target cannot be accurately measured. However, the measured distance 36.5 km can be obtained based on the local oscillator modulation, which matches the actual length of the optical fiber. Under the experimental condition of 58.1 km delay line, the data processing result is 0 based on the scheme without local oscillator modulation. The reason is that the echo light and the emitted light partially overlap in the time domain. The measurement scheme with local oscillator modulation shows advantage in frequency domain. Although these two pulsed lasers illuminate the detector surface simultaneously, their information in frequency domain is separated. The echo optical signal can be effectively distinguished through the matching filter algorithm. The specific distance value of 58.1052 km is obtained.ConclusionsSome problems exist in the high repetition frequency integrated transceiver pulsed laser coherent ranging, including that the unambiguous distance is short and the echo signal light cannot be received normally at the moment of emission. The local oscillator modulation pulsed laser coherent ranging based on local oscillator and signal double-chirp modulation proposed in this paper overcomes the short unambiguous distance and multiple blind distance points. The measurement results with 36.5 km and 58.1 km delay fibers are obtained with the conditions including 512 μs local oscillator repetition period, 20 MHz local oscillator frequency modulation bandwidth, 64 μs pulse signal light repetition period and 10.0781 MHz single pulse modulation bandwidth. Data processing is carried out on the situations where the echo time exceeds the light modulation period with 36.5 km fiber and the echo signal falls within the light emission time with 58.1 km fiber. The results prove the advantages of the proposed scheme in high repetition frequency continuous long distance measurement.

ObjectiveTunable diode laser absorption spectroscopy tomography (TDLAT) is an important optical noninvasive combustion detection technique. Two-line thermometry is widely used in TDLAT for temperature imaging, in which the absorbance density distributions for two spectral transitions with different temperature-dependent line strengths are individually reconstructed, and the temperature image is then retrieved from the ratio of the absorbances in each pixel of the region of interest. Owing to the limited number of available line-of-sight TDLAT measurements in practical applications, the inverse problem of reconstructing the absorbance density distribution is inherently ill-posed, leading to severe artifacts in the reconstructed temperature image. To alleviate this problem, iterative tomographic algorithms have been proposed by formulating an inverse problem with a heuristically determined prior, such as the smoothness of absorbance density distributions. These algorithms improve the quality of the reconstructed smooth characteristics in temperature images to some degree; however, the lack of detailed features in the reconstructed image is evident. To address this problem, a cartoon-texture model in the field of image processing is introduced into TDLAT, and the temperature reconstruction algorithm based on the cartoon-texture model (TRACT) is proposed.MethodsThe proposed TRACT individually reconstructs the cartoon and textural components of the absorbance density distribution with smoothness and sparsity priors, and retrieves the temperature image with two-line thermometry from the combination of the reconstructed cartoon and texture components. First, the cartoon component is reconstructed using the total variation (TV) regularized Landweber algorithm (Landweber-TV) to effectively retrieve the smooth characteristics and edge structure in the absorbance density distribution. Second, the texture component is reconstructed with a modified deep network unfolded using the iterative shrinkage-thresholding algorithm (ISTA-mNet) to supplement the detailed information in the absorbance density distribution. Third, the temperature image is reconstructed using two-line thermometry from the complementation of cartoon-component and texture-component reconstructions of the absorbance density distribution. With complementary reconstructions of the cartoon and texture components, the accuracy of the retrieved absorbance density distribution and the quality of the reconstructed temperature image are improved.Results and DiscussionsTo examine the performance of the proposed TRACT, it is compared with two state-of-the-art iterative tomographic algorithms and one pioneering data-driven tomographic algorithm for TDLAT temperature imaging. These are temperature imaging algorithms based on Landweber (referred to as Landweber), algebraic reconstruction techniques and TV regularization (referred to as ART-TV), and convolutional neural networks (referred to as HCNN). In addition, to verify the effectiveness of the cartoon-texture model, TRACT is compared to the temperature imaging algorithm based on Landweber-TV, that is, the cartoon-component reconstruction algorithm. In the simulations, the dataset is generated using Fire Dynamic Simulator (FDS). Tests are conducted in a practical signal-to-noise ratio (SNR) range of 25 dB?45 dB. The normalized mean square error (NMSE) is adopted to quantitatively evaluate the reconstruction accuracy. The simulation results show that the NMSE obtained by TRACT is always lower than those obtained by the other four algorithms (Fig. 5). Taking an SNR of 35 dB as an example, compared with the NMSEs obtained by Landweber, ART-TV, HCNN, and Landweber-TV, the NMSE obtained by TRACT decreases by 58.67%, 51.96%, 39.44%, and 35.38%, respectively. In terms of subjective quality, the temperature image reconstructed by TRACT is more consistent with the ground-truth phantom, and less information remained in the residual image than in the temperature images reconstructed by Landweber, ART-TV, HCNN, and Landweber-TV (Fig. 6). Laboratory-scale experiments are conducted to validate the performance of the proposed TRACT. In the temperature image reconstructed by TRACT from the actual TDLAT measurements, the location of the flame agrees better with the true combustion field, and fewer artifacts exist compared to the temperature images reconstructed by Landweber, ART-TV, HCNN, and Landweber-TV (Fig. 7). Moreover, the peak temperature value retrieved by TRACT is closer to the highest temperature value measured by the thermocouple than those retrieved by the other four algorithms.ConclusionsThe cartoon-texture model is introduced into the TDLAT, and a temperature reconstruction algorithm based on the cartoon-texture model (TRACT) is proposed. TRACT utilizes the Landweber-TV iterative tomographic algorithm and the ISTA-mNet network, designed with different priors of the image features, to achieve efficient reconstruction of the cartoon and detailed texture components in the absorbance density distribution, respectively. This improves the accuracy of the reconstructed absorbance density distributions and, in turn, the quality of the reconstructed temperature image. Simulations with the dataset generated from the fire dynamics simulator showed that, in comparison to Landweber, ART-TV, HCNN, and Landweber-TV, the normalized mean square errors obtained by TRACT decrease by 54.37%?58.67%, 45.93%?51.96%, 29.60%?39.44%, and 28.48%?35.38%, respectively, in the SNR range of 25 dB?45 dB. The temperature images reconstructed by TRACT have fewer artifacts and are closer to the ground-truth phantoms. Reconstructions with actual TDLAT measurements obtained from the lab-scale TDLAT system show that in comparison to Landweber, ART-TV, HCNN, and Landweber-TV, the performance of TRACT for reconstructing the temperature distribution in a real combustion field is higher, as evaluated quantitatively and visually.