ObjectiveIn this study, technologies including beam combination structures, uncoupled two-dimensional adjustment, optical axis detection, and consistency control in fiber laser phased arrays are investigated. During the experiment implementation, seven fiber laser beams are placed in a regular hexagonal arrangement by adopting a “6 +1” array. Beam expansion is achieved by placing an expanding lens behind the fiber rod for each fiber beam, and two-dimensional adjustment is obtained by adopting high-precision uncoupled adaptive fiber optic collimator (AFOC) adjustment devices. Intelligent cameras are used to detect the reflected beam and control the optical axis in the closed loop, which provides an important guarantee of the coherent combination of the seven fiber lasers. The dynamic range of the AFOC is larger than ±230 μrad, the optical axis pointing accuracy is better than 1 μrad, the effective spot size of a single beam laser in the beam combination device is 85 mm, and the area duty cycle is 56.2%. The capability of beam combination, dynamic adjustment, and optical axis consistency control of the seven-beam laser combination device is fully verified by a number of full power experiments, and the average power-in-the-bucket after the phase close loop is 4.8 times that of the open loop.MethodsThe combination device adopts an optical fiber rod and a beam-expanding lens to collimate and expand a single laser beam. The reflective far-field detection system has three built-in charge coupled devices (CCDs) to detect the centroids of the seven fiber lasers. After calculating the optical axis shift, the control parameters are amplified by a proportional-integral-derivative (PID) control algorithm and loaded onto the AFOC for translation control to realize the optical axis pointing consistency control of multiple lasers, as shown in Figs. 2?4. By adopting large-thrust piezoelectric ceramics for high-precision adjustment, the AFOC can realize displacement amplification, and a two-dimensional uncoupled design can achieve high-precision control over a large dynamic range. The modal frequency and structural stability of the AFOC are improved through simulation optimization, as shown in Figs. 5?10. The beam combination device adopts a frame structure that integrates the AFOC, built-in CCD, beam expanding lens, weak mirror, and other components, and carries out an integrated lightweight design. Its layout is “6+1”, that is, a center light beam is surrounded by six light beams arranged in a regular hexagon. Invar rods are used for series-fixing and stiffening the plates to enhance their structural strength. The stability of the device is improved through simulations and design optimization. The structural diagram and simulation results are shown in Figs. 11?15.Results and DiscussionsThe effective output aperture of a single laser beam is 85 mm, the area duty cycle of seven lasers is 56.2% ( Fig. 15), and the dynamic range of the AFOC is greater than ±230 μrad (Figs. 20 and 21). Prior to the experiment, the zero point of the beam combination device is calibrated using a far-field sensing device. The calibrated device is used in multiple rounds of experiments. During the experiment, the AFOC is closed to ensure pointing consistency of the seven fiber lasers. After the closed loop, the optical axis pointing accuracy is better than 1 μrad. During coherent combining, the average power-in-the-bucket after the phase closed loop is 4.8 times that of the open loop (Fig. 23), which verifies the dynamic adjustment ability and optical axis pointing control ability of the beam combination device.ConclusionsA “6+1” layout is used to assemble seven fiber lasers with a phased array. A built-in CCD is used to detect the optical axis in different regions, and the consistency of optical axis pointing is adjusted in real time through a low-coupling and high-thrust AFOC. The performance test shows that the dynamic range of the AFOC is more than 230 μrad, and the resolution is better than 10 μrad. Finally, the experiment verifies the beam combination and optical axis consistency control abilities of the seven-fiber laser phased array combination device. The average power-in-the-bucket after the phase closed loop is 4.8 times that of the open loop.

ObjectiveA composite controller can achieve stable suppression of high-bandwidth beam jitter of large-aperture tilt mirrors. The composite control method comprises multiple control links in series or parallel, and the controller is large-scale. There are many jitter analysis results for a single high-frequency narrowband; however, the correction of complex-frequency beam jitter requires further consideration of the mutual amplification and traction between multiple control links in the composite controller. Therefore, parameter matching is a challenging problem in the design of composite controllers. Accordingly, in this study, a parameter optimization method is proposed based on stochastic parallel gradient descent assistance to achieve high-bandwidth and high-precision control of large-aperture tilt mirrors using a composite controller.MethodsTo minimize the beam jitter residual variance, the power spectrum characteristics of the open-loop beam jitter to be corrected are first analyzed, and a composite controller is designed accordingly, including a proportional integral (PI) controller and dual two-order filter. Subsequently, the residual variance of the beam jitter is selected as the cost function to optimize the parameters of the composite controller using the stochastic parallel gradient descent method. Finally, after several iterations, optimal adjustment of the control performance of the composite controller is achieved. After parameter optimization, the correction and residual variance suppression abilities of the composite controller are verified using the Bode diagram and power spectrum density.Results and DiscussionsFor the beam jitter signal of the multifrequency narrowband power spectral density function (Fig. 4), the design of multiple controller links includes a PI controller, 30-Hz, 70- Hz, 146-Hz dual two-order filters, 70-Hz advanced phase compensation, and resonance elimination model. The initial parameters of each link of the composite controller are designed accordingly (Table 1). The composite controller corresponding to the initial parameters suppresses the residual variance from 117.48 μrad2 to 84.58 μrad2, with a residual variance suppression ratio of only 28%, and has a significant amplification effect on the high-frequency jitter. After 1000 iterations using the stochastic gradient descent method, the transfer function of beam jitter suppression in multiple controller links with the optimized parameters is obtained (Table 2). The jitter residual variance of a single tilting mirror closed-loop beam is suppressed from 117.48 μrad2 to 44.35 μrad2, and the residual variance suppression ratio reaches 62.2%. Furthermore, jitter is significantly suppressed near the low-frequency broadband and 30 Hz, 70 Hz, and 146 Hz narrowbands (Fig. 9). An optical test platform is set up (Fig. 10) to verify the effectiveness of the optimization parameters. The beam jitter composite controller with optimized parameters can effectively suppress the jitter of 30 Hz and 70 Hz and low-frequency broadband. The jitter at 146 Hz is partially suppressed, and there is a certain amplification at high frequency. The magnification factor is small and does not exceed 6 dB. The beam jitter residual variance is suppressed from 200.05 μrad2 to 93.93 μrad2. Compared with the composite control algorithm with initial parameters, under the same correction conditions, the beam jitter suppression performance of a single large-aperture tilt mirror is improved by 34.2 percentage points after optimization by the proposed stochastic gradient parallel descent auxiliary method.ConclusionsThe experimental results show that the residual variance-suppression ratio of the high-bandwidth beam jitter of a single large-aperture tilt mirror is 53.1%, which is comparable to that of the traditional method based on two tilt mirrors. This method can effectively reduce the complexity of the system and contribute to the high-bandwidth and high-precision control of beam jitter by large-aperture tilt mirrors. Furthermore, it can contribute to an optimization and evaluation method for the design of beam jitter controllers.

ObjectiveA mode-division multiplexing system based on few-mode fibers (FMFs) can significantly enhance information capacity. With the increasing demand for expanded channels and extended transmission distances, fiber fusion splicing between FMFs and devices has become increasingly important. Unlike ordinary single-mode fibers, FMFs have larger core sizes and can support multiple modes. When fusion splicing is not properly performed, various problems, including mode coupling and mode dispersion, may occur. Mode coupling transfers the mode energy carrying the signal to other modes within the fiber, decreasing the purity of the original mode, increasing interchannel crosstalk, and greatly affecting the communication quality of the system. To analyze quantitatively specific parameters during the fusion splicing of FMFs, we utilized off-axis digital holography to measure the purity of multiple high-order modes in the fibers. We optimized the arc time and relative arc power during fusion between the FMFs based on mode purity measurements.MethodsThis study evaluated the fusion quality between FMFs with fusion loss and mode purity variation. We used different arc times and relative arc powers to splice the two FMFs. We also employed a mode-selective coupler to generate high-order modes. We built an off-axis digital holographic interference system to measure the mode-purity variation after each fusion. The light source used in this study was a Santec tunable laser (MLS-2100) with a working wavelength of 1550 nm. The erbium-doped fiber amplifier (EDFA) is a self-made erbium ytterbium co-doped double-clad fiber amplifier, where a working power of 20 mW was used in the experiment. Both fiber port couplers use a PAF2-2B produced by Thorlabs, and the charge coupled device (CCD) uses an LD-SW6401715-UC-G short-wave infrared camera produced by the Liding Optoelectronic Technology Company. The fusion splicer used was the 80C, produced by Fujikura Company.Results and DiscussionsFor the two types of FMFs with the same or approximately the same core radius within the error range, the effects of optimized arc time and relative arc power on the mode purity can be ignored. Suppose the standard arc power is Is. For the same two FMFs, we find that when the relative arc power is Is+25 bit and the arc time is 3100 ms, the purity variation of the high-order mode, excluding LP02, is the least. Figure 5 and Table 1 show the results. We also conducted comparative experiments using the fusion splicer auto-fusion program, where the results are shown in Figure 6. Following fusion splicing, the purity decreases by 12.3127 percentage points, indicating that the automatic fusion program cannot effectively maintain the purity of the transmission mode inside the fiber during fusion splicing between FMFs. For the two types of FMFs with approximately the same radius, we find that the best fusion parameters are a relative arc power of Is+32 bit and an arc time of 3000 ms, as shown in Figure 7 and Table 3. When significant differences are observed in the cores of the few-mode optical fibers, optimizing the arc time and intensity is insufficient to eliminate the effects of mode field mismatch. Particularly, when ring-core fibers are used to fuse with specific FMFs, the purity of the azimuthal modes is improved over that before fusion, with a maximum increase of 2.9137 percentage points.ConclusionsThis study evaluated the fusion quality of four types of FMFs under different fusion parameters based on mode purity measurements. We optimized the fusion parameters for the different FMFs with the same core radius. When the core radii of the FMFs are the same or approximately the same, using the optimized arc time and relative arc power can reduce the influence of the splicing on the mode purity. When appropriate fusion parameters are used, splicing ring-core fibers and FMFs can increase the purity of high-order modes, which may be due to the ring-core fibers filtering out the radial modes of the transmitted optical field in the FMFs. This method of optimizing FMF fusion parameters based on the proposed mode purity measurements is also applicable to the connection of FMF breakpoints, mode-generating devices, and transmission fibers in mode-division multiplexing systems under different fusion equipment.

ObjectiveA high-power thulium-doped fiber laser with a wavelength in the range of 1900?2100 nm has broad application prospects in many fields, including absorption spectrum diagnosis, biomedical treatment, LIDAR, plastic processing, and mid-infrared laser pump source. Specifically, thulium-doped fiber laser systems have achieved a high power output in the kilowatt class using 793 nm laser diode (LD) pumps. However, the low optical conversion efficiency of 793 nm LD pumping mode causes problems such as serious thermal effects and refractive index distortion during high-power operation, which affect the beam quality of the output laser. Therefore, it is important to investigate the beam-quality variation characteristics of high-power thulium-doped fiber lasers under different conditions.MethodsIn our previous study, a beam quality prediction model was established based on the finite-difference beam propagation method. A complete simulation link, from solving the rate equation to predicting the output beam quality of a fiber laser, was realized. The model was applied to thulium-doped fiber laser systems with different power levels reported by other researchers and compared with the experimental measurement results; thus, the prediction accuracy of the model was verified. In this study, the model was used to further simulate and analyze the influence of different parameters, such as the input pump power, forward and backward pump ratio, active fiber absorption coefficient, and seed power, on the output beam quality of the laser.Results and Discussions The theoretical simulation and experimental results show thatthe beam quality of the laser deteriorates with an increase in input pump power; the backward and forward pump ratio of 1∶1 can effectively suppress the internal thermal effect of the laser and is conducive to obtaining the laser output with high beam quality (Fig. 4 and Fig. 5); the active fiber with low absorption coefficient can reduce the temperature inside the fiber and improve the beam quality of the output laser (Fig. 6 and Fig. 7); with the increase of seed power, the beam quality of laser output deteriorates slightly (Fig. 9); and compared with increasing the seed power, changing the pumping ratio has a more obvious effect on the refractive index, and thereby, on the beam quality (Fig. 5 and Fig. 9). The variation in the beam quality predicted by the theoretical model is in good agreement with the experimental result, and the average deviation is approximately 10%. Based on the aforementioned results, a 15 W seed and 1∶1 forward-to-backward pumping ratio in the amplification stage were employed to experimentally verify the beam quality characteristics at higher laser power levels. At a wavelength of 1940 nm, under an input pump power of 1131 W, a laser output power of 513 W with a beam quality factor M2 of 3.04 is experimentally achieved, and the deviation from the theoretically predicted M2 value 2.8 is approximately 8.6% (Fig. 12 and Fig. 14).ConclusionsThe characteristics of the laser power output, refractive index change, and beam quality change under different conditions are analyzed via theoretical simulation. Theoretical simulation results show that the beam quality of the laser deteriorates with an increase in the input pump power and exhibits a nearly linear growth trend. A backward-to-forward pump ratio of 1∶1 can better suppress the internal thermal effect of the laser and is conducive to obtaining a laser output with a high beam quality. Under the condition of the same total gain, an active fiber with a low absorption coefficient can reduce the temperature inside the fiber and improve the beam quality of the output laser. With an increase in the seed power, the laser beam quality deteriorates slightly. Compared with increasing the seed power, changing the pump ratio and using an active fiber with a low absorption coefficient have more obvious effects on the refractive index and thus on the beam quality. Based on a 200-W thulium-doped fiber laser system developed by us, the variation in beam quality with the aforementioned parameters is revealed by experiments. The experimental results are in good agreement with the predicted law of beam quality, with a deviation of approximately 10%. Based on the aforementioned findings, to obtain the output of high-power and high-beam-quality thulium-doped fiber lasers, the oscillator output with higher power should be preferred as the seed light, a 1∶1 forward-to-backward pump ratio should be adopted, and an active fiber with a low absorption coefficient should be selected.Based on the aforementioned theoretical simulations and experimental results, the beam-quality characteristics at higher laser power levels are verified. A 15 W seed light is selected, a 1∶1 forward-to-backward pump ratio is selected for the amplifier stage, and a pedestal thulium-doped fiber with a core ratio of 25/400 μm and an absorption coefficient of 4.2 dB/m is adopted. Under a total pump power of 1131 W, laser output with a wavelength of 1940 nm, average power of 513 W, and beam quality M2 of 3.04 is experimentally achieved. The measured M2 of 3.04 shows an approximate deviation of 8.6% from the theoretical prediction value of 2.8.

ObjectiveTransfer cavity locking is a high-performance, low-cost frequency stabilization method that is widely used in optical clocks and quantum computing with neutral atoms or ions. Typically, the transfer cavity-locked laser is not tunable, making it unsuitable for cold atom experiments. To address this problem, this study presents an automatic frequency-locking system for a cooling laser of mercury atoms. The wavelength of the cooling laser is 254 nm, which is quadrupled from a 1015-nm slave laser locked to the transfer cavity. To lock the deep-ultraviolet (DUV) laser to the atomic transition, we build an automatic frequency-locking system based on sideband PDH (Pound-Drever-Hall) technology. An analog-to-digital converter (ADC) is employed to acquire the saturated absorption spectra (SAS) and lock-in amplified signals, and an algorithm is developed to identify different atomic transitions. The processed signal is then fed back to the sideband frequency, realizing automatic frequency sweeping, peak searching, and frequency locking.MethodsTo derive the sideband PDH technique, sidebands are generated using two cascaded fiber electro-optical modulators (FEOMs) to modulate a 1062-nm master laser. Because the cavity is locked to one sideband, adjusting the sideband frequency can change the cavity length, which in turn changes the frequency of the 1015-nm slave laser. In addition, to achieve automatic frequency locking, we develop a program based on LabVIEW that integrates both algorithm and hardware communication functionalities. Before operation, we need only to set up an appropriate signal threshold, and the system automatically identifies the atomic transition based on this value. The program controls the ADC to collect and analyze the SAS and lock-in amplifier signals. The regulation signal is then fed into the signal generator to change the sideband frequency for scanning and locking. Consequently, when the 1015-nm slave laser is locked to the transfer cavity using the sideband PDH technique, the sideband frequency is regulated by a program that references the SAS signal and lock-in amplifier signal, and the laser frequency can be locked to the atomic transition.Results and DiscussionsTo demonstrate the excellent locking performance of the transfer cavity system, we first measure the frequency noise power spectrum and deep-ultraviolet laser linewidth during locking. The results show that transfer locking significantly reduces the low-frequency noise of the laser (Fig. 6). The linewidth of the two DUV lasers is 450 kHz after being locked to the slave laser by an optical phase-lock loop (Fig. 7), which can be used for laser cooling of mercury atoms. Using a wavelength meter, we also measure the frequency fluctuations of the slave laser after locking. The results show that the frequency of the slave laser has no significant drift within the accuracy of the wavelength meter (Fig. 9). Subsequently, the 254-nm DUV laser is locked to different atomic transitions. For 202Hg, only a single transition occurs in the broad Doppler profile. The system can normally lock the slave laser to this transition, and the SAS signal after locking exhibits excellent stability (Fig. 8). In addition, the transitions of 199Hg and 204Hg are very close to each other, and the system can identify them and lock the 254-nm DUV laser to them separately by inputting different signal thresholds (Fig. 10). Furthermore, the system offers high flexibility; after locking, detuning can be changed or switched to another atomic transition simply by changing the sideband frequency. Finally, we modulate the master laser in this system, effectively overcoming the issue of insufficient broadband FEOM at a certain wavelength. However, the slow frequency drift of the master laser can be directly compensated at the sideband frequency without introducing an additional FEOM.ConclusionsWe developed an automatic frequency stabilization system based on sideband PDH technology with a transfer cavity. By referencing the SAS signal and lock-in amplifier signal, we employ the sideband PDH technique to shift the frequency, allowing the 1015-nm slave laser to automatically lock to different transitions of the mercury atom. This method not only allows the slave laser to be locked to the atomic transition, but also achieves a narrow linewidth of the deep-ultraviolet laser. This system has a user-friendly interface that provides very simple operation for users. In short, this method combines the advantages of the SAS and transfer cavity, significantly improving frequency stability while ensuring that the laser is locked to the atomic transition. The method enables flexible detuning of the slave laser, making it widely applicable to various cold atom experiments.

ObjectiveIn this study, we explore the generation of high-order Laguerre?Gaussian (LG) mode vortex beams from a 914-nm Nd∶YVO4 laser using spherical aberration within the laser cavity. Traditional methods for generating high-order LG modes typically require complex setups involving custom-designed optical components such as phase plates, spatial light modulators, and amplitude masks. However, these approaches are often expensive and difficult to adapt to various wavelengths. In this study, we propose a simpler and more cost-effective solution that utilizes a single spherical lens inside the cavity to introduce spherical aberration, enabling the selective generation of high-order LG modes in low-gain wavelength region. A flexible, easily adaptable method to produce LG0,±m vortex beams with selective angular indices (m) over a large range is developed. We use only one lens in the cavity as opposed to two lenses in the former studies, which simplifies the cavity arrangement. A new model based on the cavity stability and pump overlap is developed to predict the relationship between the mode order and cavity parameters. This approach can be applied to a variety of scientific and industrial fields, such as optical manipulation, precision laser machining, and optical communications.MethodsThe experimental setup involves a 914-nm Nd∶YVO4 laser with a cavity that includes a single short-focus spherical lens (L1), with a nominal focal length of 25.3 mm, to introduce spherical aberration. Given that the mode sizes of the LG0,±m modes vary with their angular indices m, this aberration facilitates the mode discrimination and selection of high-order LG modes by affecting the focal lengths of the modes differently. The laser is end-pumped by a fiber-coupled 808-nm diode laser, and the cavity includes a concave mirror (M1), plane output mirror (M2), and spherical lens. Furthermore, the Nd∶YVO4 crystal, which serves as the gain medium, has dimensions of 3 mm×3 mm×5 mm and a doping mass fraction of 0.15%. Given that the effective focal length of lens L1 varies for different orders of modes, the specific distance between lens L1 and output coupler M2 ensures that only laser modes with ring sizes exceeding a certain threshold remain stable within the cavity. In the family of stable modes, the smallest mode exhibits the largest overlap with the pump beam and oscillates. Therefore, mode selection can be realized by adjusting the distance between the lens and output mirror.Results and DiscussionsThe experimental results show that the proposed method successfully generates LG modes with angular indices ranging from 8 to 34. At a pump power of 5.2 W, the laser initially produces a fundamental Gaussian mode. By gradually reducing the distance between the spherical lens and output mirror, the output transitions into a multimode. As the distance is further reduced, the output beam exhibits a hollow petal-like structure, indicating a single-mode high-order LG output. The highest mode order achieved is LG0,±34 at a pump power of 8.1 W. With increasing mode order, the effective focal length decreases, causing the spherical lens to focus more tightly, which leads to the selective excitation of higher-order modes. The relationship between the mode order and cavity parameters is consistent with the calculated relationship based on the spherical aberration, beam size, and cavity stability. The far-field beam patterns show good uniformity, even for the highest angular indices, owing to the self-healing properties of LG modes, which aid in maintaining beam quality during propagation. However, the output power for higher-order modes decreases because of the reduced overlap between the pump beam and higher-order modes, leading to a lower conversion efficiency. Nevertheless, the laser system demonstrates excellent stability in high-order LG modes with minimal transverse mode fluctuations.ConclusionsWe demonstrate that high-order LG mode vortex beams can be generated from a 914-nm Nd∶YVO4 laser using a single spherical lens to introduce spherical aberration in the cavity. This method provides a simple and cost-effective approach for generating high-order LG modes in flexible wavelength regions without the need for custom optical components. In the experiment, LG modes with angular indices, ranging from 8 to 34, are selectively generated by adjusting the position of the output mirror and focusing lens. The simplified cavity scheme and model facilitate the practical application of high-order LG lasers.

ObjectiveOwing to their compact size, light weight, high reliability, and long lifespan, tunable semiconductor lasers have broad application prospects in fiber-optic communication, fiber-optic sensing, and spectral analysis. In particular, modulated grating Y-branch (MG-Y) lasers, which are a type of distributed Bragg reflection lasers, are characterized by a wide tuning range, high output power, high side-mode suppression ratio (SMSR), and high response speed. However, MG-Y lasers are adversely affected by wavelength jumping and power fluctuations when tuned based on a wavelength-current look-up table (LUT). These issues limit the practical applications of MG-Y lasers; therefore, the tuning methods for MG-Y lasers must be investigated and optimized.MethodsThe phase-region tuning characteristics of an MG-Y laser were investigated, which resulted in the proposal of a novel tuning method based on increasing and decreasing currents in the phase region. First, the currents in the left and right reflectors were scanned along an arc trajectory, where smooth wavelength-tuning paths were fitted rapidly within a tuning range of 40 nm. Next, the phase-region currents were loaded in increments of +0.2 mA and -0.2 mA, which revealed the existence of phase-jump windows. To address this issue, a new target-wavelength retrieval scheme was designed. The overlapping tuning segments of the phase region were obtained from the non-uniform feature points to splice the wavelength maxima and minima while ensuring sufficient current compensation. The remaining fine-wavelength data were filled with arithmetic progressions. Finally, a closed-loop calibration model was developed, through which the expected tuning performance of the MG-Y laser was realized.Results and DiscussionsThe stability of the proposed tuning method is attributed to two key aspects. The reflector current-tuning paths are located in the center of supermodes. Besides, the retrieved target wavelength is not only distant from the boundary of the overlapping tuning segments in the phase region but also exhibits monostable characteristics. An experimental system was used to evaluate the tuning performance of the MG-Y laser. First, an arbitrary wavelength-switching test was performed to verify the effectiveness and reliability of the optimized tuning method. The laser was tuned in increments of 0.1 nm from 1549.5 nm to 1550.5 nm and then back to 1549.5 nm to monitor wavelength switching at the same feature point, where 20 switches were regarded as one cycle. Subsequently, the tuning step was increased to 4 nm and the same operation was performed over the range of 1528?1568 nm to monitor wavelength switching between different feature points. The results show that the MG-Y laser not only effectively avoids the problem of wavelength jumps when switching any wavelength but also outputs lasers with stable wavelengths (1 pm drift in 12 h) and high SMSRs (>40 dB) for a long time. Second, to verify the effectiveness and practicality of the closed-loop calibration model, the MG-Y laser was tuned from 1528 nm to 1568 nm in increments of 4 pm. Because of the nonlinear tuning of the phase-region current and the periodic variations of the carrier absorption rate, the linear scanning test based on the uncalibrated LUT shows a maximum wavelength bias of 11.4 pm, in addition to sawtooth-shaped periodic power variations with a power drift of 2.31 mW (the corresponding absolute value of power is 0.895 dBm). By contrast, the wavelength and power are well controlled after calibration. The maximum tuning wavelength bias reduced to 1.1 pm with an accuracy of 0.243, whereas the maximum power drift is limited to 0.188 mW (the corresponding absolute value of power is 0.065 dBm) with a flatness of 1.49%, which verifies the feasibility of the MG-Y laser for practical engineering applications.ConclusionsIn this study, the phase-region tuning characteristics of an MG-Y laser were investigated, and a novel tuning method based on increasing and decreasing currents in the phase region was proposed. After the tuning paths were determined, the target wavelengths were retrieved from the overlapping tuning segments of the phase region to avoid wavelength jumps. Subsequently, the LUT was iteratively updated using a closed-loop calibration model, which improved the linear scanning performance of the laser. A corresponding experimental system was constructed to evaluate the tuning performance of the MG-Y laser. The results show the absence of wavelength jumps during arbitrary wavelength tuning based on the LUT, with SMSRs exceeding 40 dB and a wavelength drift of approximately 1 pm within 12 h. The tuning wavelength bias of the linear scanning across the entire wavelength range was less than ±1.1 pm, with an accuracy of 0.243 pm, whereas the output power drift was only 0.188 mW (the corresponding absolute value of power is 0.065 dBm), with a flatness of 1.49%. Thus, one can conclude that the optimized tuning method enables the MG-Y laser to achieve stable tuning over the 1528?1568 nm range at a wavelength spacing of integer multiples of 4 pm, thus demonstrating its practical application value.

ObjectiveSpectrum control of solid-state lasers, including wavelength tuning and linewidth control, is crucial for obtaining a specific wavelength and narrow linewidth laser. Narrow-linewidth solid-state lasers have the advantages of high spectral purity, long coherence length, high peak spectral density, and various wavelength options , leading to important application prospects in coherent optical communication, optical precision measurements, laser radar, and other fields. Therefore, investigating wavelength tuning and linewidth narrowing technology in solid-state lasers is essential. To realize the output of a narrow-linewidth laser in the ultraviolet (UV) band, an alexandrite continuous (CW) wave laser with a tunable wavelength and narrow linewidth, pumped by a 638-nm laser diode (LD), is reported in this study.MethodsTo obtain a narrow-bandwidth-tunable solid-state laser output, selecting a dispersive element that can perform both narrowband filtering and wavelength tuning is first required. Birefringent filters (BRFs) are widely used as optical elements in solid-state lasers. First, we simulate the control effect of BRFs inserted into the resonator at the Brewster and 0° angles on the spectrum and obtained the transmission curves under different conditions using MATLAB, which enables selection of the best scheme for the experiment (Figs. 2 and 3). The feasibility of intracavity frequency doubling is then analyzed. Next, a V-shaped folding cavity alexandrite laser is constructed, where the related parameters are obtained using reZonator software and according to specific experimental requirements (Fig. 5). The overall structure of the laser is shown in Fig. 4. The pump source used in the experiment is a coherent red light LD with a multi-mode fiber-coupled output, which can provide a laser output with a center wavelength of 638 nm and maximum power of 40 W. The pump light is passed through the collimating system as well as through the half-wave plate HWP1 and polarized beam-splitting prism (PBS) to obtain horizontally polarized light with a maximum power of approximately 30 W. The polarized light is then adjusted through half-wave plate HWP2 to make it parallel to the b axis of the alexandrite crystal to achieve the highest polarized light absorption efficiency. Finally, the light is converged in the emerald crystal through lens F1 at a focal length of 50 mm. The alexandrite crystal is 4 mm×4 mm×15 mm in size, and the mass fraction of the Cr3+ ions is 0.2%. To obtain a frequency-doubled UV laser, LiB3O5 (LBO, θLBO=90°, φLBO=37.6°) and β-BaB2O4 (BBO, θBBO=31.3°, φBBO=0°) are selected as type-I phase-matched nonlinear optical crystals for experiments. A tunable alexandrite CW laser with a narrow linewidth and tunable wavelength is obtained by adjusting the BRF and nonlinear crystals.Results and DiscussionsAfter a BRF is inserted into the cavity, a narrow-linewidth UV laser with a tuning width of approximately 15 nm is obtained using either of the nonlinear crystals. When LBO is used as a frequency-doubling crystal, the tuned wavelength range is 371.60?386.76 nm (Fig. 6). At this central wavelength, the peak power is 1.53 W (Fig. 8) at 379.49 nm with a linewidth of 7.6 pm (Fig. 9). When BBO is used as a frequency-doubling crystal, the tuned wavelength range is 370.95?386.88 nm (Fig. 10). At this central wavelength, the peak power is 1.98 W (Fig. 8) at 381.29 nm with a linewidth of 8.5 pm (Fig. 11). The conversion efficiencies of the horizontally polarized pump light to the frequency-doubled UV laser are 5.1% and 6.6%, respectively. The linewidth of the UV laser can be compressed to less than 10 pm within the entire tuning range. The output laser linewidth is significantly narrowed, which is important for the generation and corresponding application of tunable all-solid-state UV lasers with narrow linewidths. A self-designed water-cooled temperature control device is used in the experiment, and the temperature control effect is ideal; therefore, no saturation or decrease occurs in the output power due to the thermal focal length effect of the laser crystal. Thus, a 638-nm LD with a higher output power can be used as the pump source in subsequent experiments to obtain a high-power UV CW laser output and thereby meet the requirements of laser light sources in optical precision machining, coherent optical communication, and other fields.ConclusionsThis study conducts a theoretical design and experimental verification to realize an intracavity frequency-doubled UV CW alexandrite laser with a narrow linewidth and tunable wavelength. Based on the V-cavity alexandrite laser, a UV CW laser with a tuning width of approximately 15 nm and spectral linewidth of less than 10 pm is obtained using only a single BRF with a thickness of 2 mm. The results provide a foundation for obtaining the output of solid-state lasers with narrow linewidths and tunable wavelengths within a certain range of the UV band.

ObjectiveBroad-area edge-emitting diode lasers have been widely applied in laser pumping, laser ranging, material processing, display technology, and medical fields owing to advantages such as high power, high efficiency, high brightness, compact size, and long lifetime. However, the broad transverse waveguide dimensions of these lasers typically result in significant variations in the optical field under high drive currents, which excites multiple transverse modes. Consequently, the “multilobe” phenomenon occurs in the near-field beam profile, thus severely deteriorating the transverse-beam quality. Additionally, it causes increased far-field divergence angles and reduced brightness, thus limiting their applicability in various fields. Therefore, achieving high power while maintaining high beam quality has become a priority.MethodsFirst, a 100 μm wide mesa is formed via photolithography and inductively coupled plasma (ICP) etching with an etching depth of 850 nm. Subsequently, side grating structures are etched on both sides of the ridge waveguide using photolithography and wet etching, with an etching depth of 150 nm. A 300-nm-thick SiO2 electrical insulation layer is deposited using plasma-enhanced chemical vapor deposition (PECVD). Subsequently, an 80 μm current injection window is fabricated via ICP etching. Next, a p-side metal contact is deposited, and the wafer substrate is thinned and polished. The n-side metal contact is subsequently deposited, followed by rapid thermal annealing to complete the alloying process. The distance from the etched grating grooves to the mesa is defined as x, and lasers with different values of x (0, 5, 10, 15, and 20 μm) are fabricated on the same wafer for comparison. After wafer fabrication, cleaving is performed using a dicing machine and a cleaver to separate the wafer into bar-shaped devices. The cavity surfaces are left uncoated and the bars are further cut into single-tube chips. The fabricated chips have a cavity length of 2 mm. Finally, the single-tube chips are bonded to the heat sink, with the p-side facing upward, using a chip mounter. The temperature is controlled using a thermoelectric cooler (TEC). The resolution of the spectrometer used for testing is 0.05 nm, and near-field testing of the device is performed using a 10× magnification objective lens.Results and DiscussionsAs shown in Fig. 4, the single-side output power and voltage characteristics of devices with different x under continuous wave operation at 25 ℃ are presented. First, the voltage performances of different devices are almost identical. However, under different x, the single-sided output powers of the devices exhibit significant differences. As the x increases, the output power of the devices increases as well. The laser spectra at a bias current of 2 A are shown in Fig. 5. When x=20 μm, the device has a 3 dB bandwidth of 2.223 nm at 2 A bias current, whereas for x=0 μm, the 3 dB bandwidth is only 1.882 nm, which is 15% lower. Figure 6 shows a comparison of the far-field divergence angles, which is defined by the 95% power, of the lasers at different x. At a current of 1 A, the slow-axis divergence angle is 17.2° when x=0 μm, and it increases to 19.9° when x=20 μm. Figure 7 shows the near-field distributions of lasers with different x at bias currents of 1 A and 2 A. An analysis of these near-field distribution curves reveals that when x=20 μm, the near field exhibits multiple peaks, thus indicating the presence of significant higher-order mode patterns, and the beam waist width is slightly broader. When x=0 μm, the near-field beam waist is slightly narrower and the near-field light field distribution is more concentrated.ConclusionsWe propose a lateral grating structure and demonstrate its effectiveness in controlling the transverse modes and spectrum of broad-area semiconductor lasers, where a reduced spectral bandwidth is achieved. This structure increases the loss of higher-order modes and enables injection-insensitive transverse divergence without significantly reducing the output power. Owing to its low cost and compatibility with the semiconductor laser manufacturing technology, this structure can be applied to the development of high-power broad-area semiconductor lasers with low divergence and high beam quality.

ObjectiveLaser-driven inertial confinement fusion ignition requires high-precision synchronization of multiple laser beams at the target. The common method of synchronization employs photodiodes and oscilloscopes to extract single-point temporal features of short pulse waveforms. However, the achievable time resolution is limited by the bandwidth of detection equipment, sampling rates of oscilloscopes, and signal jitter during acquisition and transmission. In addition, measurement efficiency is crucial for the maintenance of large-scale facilities with dozens to hundreds of laser beams. However, direct measurements using laser shots are limited by the thermal recovery time of the main amplifier and influence of residual lasers during harmonic conversion. This study proposes a synchronization measurement method based on a pulse-sequence alignment beam combined with a cross-correlation algorithm. This method reduces the uncertainties of single-point temporal feature extraction and bandwidth limitations by matching numerous temporal features using the cross-correlation algorithm. Moreover, the method is not affected by residual lasers and improves measurement efficiency as it does not require laser shots, which makes it particularly advantageous for achieving time synchronization of the laser beams split from the main amplifier output.MethodsIn this study, an alignment beam and a cross-correlation algorithm are employed. The alignment beam is coaxial with the laser output from the main amplifier and characterized by a nanosecond-scale pulse envelope consisting of picosecond-level sub-pulse sequences. These numerous sub-pulses provide sufficient temporal features for detailed data analysis. Photodiodes are installed at designated reference and target positions after beam splitting of the main amplifier output, and the temporal waveforms of the reference and target signals are recorded using a high-speed oscilloscope. The cross-correlation algorithm is then applied to extract the temporal intervals between the reference and each target signal. Since the reference and target signals originate from the alignment beam and have identical temporal features, the time relationships between each target signal can be obtained by comparing respective intervals with the reference signal. Finally, the method is successfully applied to precision time synchronization diagnostics in a high-power laser facility. Measurement efficiency and accuracy are verified through engineering practices.Results and DiscussionsThis study proposes a synchronized measurement method based on an alignment beam source and a cross-correlation algorithm. The experimental results demonstrate that the cross-correlation algorithm is effective in extracting the temporal interval between pulse-sequence signals (Fig. 3) and exhibits robust performance in analyzing complex waveforms. The method is successfully applied to precision time synchronization diagnostics in the SG II upgrade facility. Online tests show that the synchronization measurement accuracy (root-mean-square, RMS) reaches 3.27 ps, which approaches the temporal resolution limit of the oscilloscope (Fig. 4). Engineering practices indicate that this method improves measurement efficiency by 86.7%, as synchronization measurements can be completed within approximately 2 min for each beam, compared to the 15 min required between shots when using low-energy laser.ConclusionsThis study proposes a time synchronization measurement method based on a pulse-sequence alignment beam combined with a cross-correlation algorithm. The cross-correlation algorithm is suitable for time synchronization measurement applications as it matches numerous temporal features of the alignment beam waveform. The method is successfully applied to time synchronization measurements at the target in a high-power laser facility, achieving a measurement accuracy (RMS) of 3.27 ps and demonstrating superior stability compared with the approach of extracting single-point temporal features within a single pulse. In the online synchronization measurement of 16 ns laser beams in the SG II upgrade facility, the measurement efficiency is improved by 42.5% based on the pulse-sequence alignment beam combined with a cross-correlation algorithm. This method optimizes the measurement of the upper and lower hemispherical optical paths after beam splitting from the main amplifier output, with total time of about 138 min, whereas measurements using low-energy laser shots require estimated total time of 240 min. The alignment beam method significantly enhances the efficiency of iterative synchronization maintenance for the laser facility. Finally, a synchronization adjustment accuracy (RMS) of 8.63 ps is achieved for the 16 ns laser beams in the SG II upgrade facility by applying the alignment beam method. The alignment beam method offers a new approach for time synchronization measurements across all optical paths in high-power laser facilities without laser shots or single-pulse waveforms.

ObjectiveAnti-resonant hollow-core fibers (AR-HCFs) have tremendous potential and wide-ranging applications in the field of high-power laser transmission due to their low nonlinear effect, low latency characteristic, and high damage threshold, which serve as a bridge for the application of high-power lasers in more scenarios. However, the current coupling of high-power lasers with AR-HCFs is mostly spatially structured coupling, which is susceptible to interference from the external environment and has poor stability. Therefore, the realization of all-fiber high-power laser transmission through AR-HCFs holds significant importance for practical applications.MethodsThis study utilizes a commercial fusion splicer to perform precision fusion splicing between the coated solid-core fiber (SCF) and AR-HCF. The two fibers are carefully selected to ensure optimal mode-field matching. By optimizing splicing parameters, low-loss splicing is accomplished with a relatively low discharge current (16.7 mA) and short discharge time (600 ms), while the mechanical strength of the splice points is enhanced through the re-discharge technology. The anti-reflection coating deposited on the end-face of the SCF can effectively mitigate Fresnel reflection-induced losses. Furthermore, a cladding power stripper (CPS) is installed after the splicing point of the AR-HCF, which significantly reduces the operating temperature of the fiber coating during high-power laser transmission. Consequently, stable kilowatt-level laser transmission through an all-fiber AR-HCF system is successfully achieved. The splicing loss is less than 0.22 dB.Results and DiscussionsWe develop an all-fiber high-power laser transmission system based on AR-HCFs, with Fig.2 presenting both the experimental configuration and corresponding results. The laser source is a fiber oscillator, characterized by a wavelength of 1080 nm and a maximum output power of approximately 2700 W. Utilizing a 1.5 m long AR-HCF, a maximum output power of 2504 W is achieved, corresponding to a transmission efficiency of 92.2%. During a 15 min continuous monitoring, the output power remains highly stable when operating at maximum capacity, with power fluctuations kept well below 0.8%. Beam quality analysis reveals thebeam quality factor M2 of 1.3, with excellent beam profile characteristics at the focal point. In this study, due to the relatively high loss of AR-HCFs in the laser wavelength band, the transmission length is limited. Therefore, future work will focus on extending this technology to diverse application scenarios by employing high-power fiber lasers at different spectral bands and low-loss AR-HCFs, aiming to achieve all-fiber AR-HCF laser transmission across broader spectra, higher power levels, and longer distances.ConclusionsWe achieve the 2500 W all-fiber laser transmission by splicing a 1.5 m long AR-HCF with the coated SCF. The corresponding transmission efficiency is 92.2%.

ObjectiveThe development and maturation of laser technology have made laser inertial confinement an important method for controlled nuclear fusion. The final optics assembly (FOA), as a critical component of inertial confinement fusion (ICF) devices, primarily facilitates laser transmission and frequency conversion. Potassium dihydrogen phosphate (KDP) crystals are widely used in FOA due to their excellent nonlinear optical properties and resistance to laser damage. Ideal crystals are influenced by their own physical and chemical properties and are prone to deformation under external forces, leading to self-modulated wavefront distortions. Additionally, the focusing lens itself may introduce aberrations during the manufacturing process, causing the final output wavefront of the FOA system to distort and reduce energy conversion efficiency. Current measurement methods, such as laser interferometry, Hartmann detection, Shack-Hartmann wavefront sensors, Moiré deflection methods, and star point testing, have certain drawbacks in wavefront measurement applications, such as in performing in-situ measurements. As a significant indicator of the rationality of optical system design, wavefront aberration measurements can provide feedback for the design, processing, and calibration of optical systems. Therefore, it is especially important to adopt precise wavefront measurement methods to obtain in-situ wavefront aberration data for FOA systems.MethodsTo obtain the wavefront aberration of an FOA system, this paper proposes a transmitted wavefront aberration measurement method based on the transmission-phase measurement deflectometry for a simplified FOA system. The proposed method first employs a vector version Snell laws for ray-tracing through the frequency-conversion crystal and then utilizes a self-built backward ray-tracing model of the KDP crystal and focusing lens system to acquire the deflection angle of the output beam, from which the wavefront aberration is calculated. To validate the feasibility of the proposed method, MATLAB is used for numerical simulation of the forward and reverse system wavefront aberrations. Finally, a transmission deflectometry measurement system is constructed experimentally to measure the wavefront aberration of the o/e light from the KDP crystal and focusing lens system, demonstrating the feasibility of the proposed method for wavefront aberration system measurement including a KDP crystal and focusing lens system.Results and DiscussionsThe proposed transmitted wavefront measurement method based on transmission-phase deflectometry is well validated through numerical simulations and experiments. The parameters used in numerical simulations are consistent with experimental parameters, with simulation results shown in Fig. 5. For further verification, an additional surface is introduced, as illustrated in Figs. 6(a) and (b); the coordinate distribution of the crystal output surface is shown in Figs. 6(c) and (d). The simulated wavefront aberration of the system with the additional surface is depicted in Fig. 7, verifying the changes in system wavefront aberration due to the introduced additional surface while also indicating the consistency between forward and reverse wavefront aberrations. The experiments measure a pupil diameter of 58 mm, yielding the wavefront aberration results shown in Fig. 11. The wavefront aberration measurement results for the o light give root mean square (RMS) of 128.4 nm and peak to valley (PV) of 653.0 nm, while the measurement results for e light give RMS of 143.5 nm and PV of 697.3 nm. These results are consistent with numerical simulation results, indicating the feasibility of the proposed method for in-situ measurement of distortion wavefronts of the simplified FOA system for o/e light.ConclusionsThis paper presents a wavefront aberration measurement method based on transmission-phase measurement deflectometry for a simplified FOA system. It first introduces the principles of crystal birefringence ray-tracing and transmission-phase measurement deflectometry, followed by a detailed derivation of the calculations for crystal output surface coordinates and output beam deflection angles, ultimately reconstructing the system wavefront aberration. Additionally, numerical simulations of the measurement system are conducted to obtain the changes in system wavefront aberration before and after introducing the surface of the crystal, validating the feasibility of the proposed method. An experimental transmission deflection measurement system is constructed, whose results closely agree with those of numerical simulations, confirming the effectiveness of the proposed method.

ObjectiveMid-infrared (MIR) optical frequency combs exhibit potential advantages in many applications; however, owing to various limitations, it is difficult to directly generate optical combs in the MIR band. The near-infrared-band optical frequency comb, combined with nonlinear frequency transfer technology, can transfer the comb frequency to the mid-infrared band, which is widely used to generate mid-infrared-band optical frequency combs. Most nonlinear frequency transfer techniques only convert the operating wavelength of the pump comb source in the near-infrared wavelength band to a specific mid-infrared spectral window without broadening the optical comb bandwidth. This leads to local spectral expansion and noise accumulation in regions far away from the pump frequency, causing significant degradation of the comb structure. Thus, it is difficult to realize a coherent optical frequency comb with continuous coverage of the mid-infrared wavelength bandwidth. Nonlinear spectral broadening using nonlinear media, especially high nonlinear fiber (HNLF), is an effective method for improving the comb line; simultaneously, the comb spectrum can be continuously expanded to the mid-infrared band.MethodsIn this study, a highly nonlinear fiber based on highly doped germanium quartz is used as a nonlinear medium, relying on intra-pulse Raman scattering to induce frequency shifts. The output comb spectral coverage can be expanded from 1400 nm to 1700 nm. Experiments verify the importance of the dispersion characteristics of optical fibers and the importance of nonlinear effects occurring within different dispersion zones for high-quality broadening of comb spectra. The can be achieved in the near-zero-flat dispersion of HNLF. The Raman soliton frequency shift in HNLF with near-zero flat dispersion broadens the comb spectrum above 1700 nm. MIR band optical frequency combs have been hindered by the generation of broadband combs with high mutual coherence and wide bandwidth in this frequency region, which provides an effective technological approach for generating MIR band optical frequency combs with non-overlapping comb bottoms, spectrally continuous and wide bandwidth coverage, and high flatness in the MIR band.Results and DiscussionsIn this study, we continuously optimize the comb flatness by cascading electro-optical modulators and integrally varying parameters such as individual radio frequency (RF) power amplifier gains and intensity modulator bias. The comb spectrum is output after cascade modulation. Furthermore, 3-dB bandwidth of the comb spectrum is 1.74 nm, and the top flat part of the spectrum contains 13 comb lines with the same frequency spacing between the newly generated comb lines, all of which are 18 GHz, with a pulse width of 7.36 ps. Subsequently, the dispersion is compensated using a 1000-m long single-mode fiber, which is the closest to the limiting pulse width of the Fourier transform, and the negative dispersion of single-mode fibers provides a linear negative chirp for compensating the Gaussian pulse. The linear positive chirp at the center of the Gaussian pulse compresses the central part of the pulse, and the compressed pulse width is 3.34 ps. The average power of the compressed pulse is increased to 2 W using a single-mode preamplifier and two single-mode main amplifiers, which correspond to a peak power of approximately 30 W. The frequency components of the spectrum are still relatively clean, the comb flatness remains unchanged after power amplification, and the 3-dB bandwidth of the spectrum begins to exhibit a certain degree of broadening. Furthermore, the nonlinear optical loop mirror (NOLM) 1 is then injected into NOLM2 for time-domain pulse compression. The precisely designed NOLM transmits the high-power portion at the center of the pulse while reflecting the low-power portion of the pulse, including the parasitic parietal valve, which increases the region of the pulse containing the linear chirp and provides optimal conditions for time-to-frequency conversion. After shaping and compression using two nonlinear circular mirrors, a near-ideal Gaussian pulse with a width of 0.9 ps is obtained. The experimental setup is used as a seed source for subsequent comb spectral broadening in a nonlinear medium. The output of the seed source is passed through a fiber amplifier and injected directly into the fiber through an isolator to complete the nonlinear frequency conversion. Light pulses with different peak powers are incident on the highly nonlinear fiber to explore the evolution of the spectral broadening process, analyze and select the most suitable length of the HNLF, and expand the comb-tooth spectral coverage from 1400 nm to 1700 nm by relying on the frequency shift caused by intra-pulse Raman scattering.ConclusionsIn this study, a 100-femtosecond broadband optical frequency comb is generated using a cascaded electro-optic modulator, with two fiber-optic ring mirrors employed to filter and shape the comb via pulse regeneration. The resulting output realizes a repetition rate of 18 GHz, an average power of 0.75 W, a pulse width of 230 fs, and a peak power of 170 W. This serves as a seed source, producing a comb spectrum covering 1500?1600 nm with a top 10 dB bandwidth of 26.86 nm, encompassing 215 relatively flat comb lines. The experimental results demonstrate the critical role of fiber dispersion characteristics and nonlinear effects in different dispersion regions for realizing high-quality comb spectrum broadening. By utilizing Raman soliton frequency shifting in highly nonlinear fiber with near-zero flat dispersion, the comb spectrum is successfully extended beyond 1700 nm.

ObjectiveVarious methods have been proposed for generating optical frequency combs, including mode-locked lasers, electro-optic modulation combs, nonlinear supercontinuum combs, and nonlinear Kerr microresonator combs schemes. Compared with other methods, cascaded electro-optic modulator combs and cavity-less supercontinuum combs schemes can enable continuous tuning of comb spacing, providing greater flexibility for applications that require frequency spacing tuning. Optical frequency combs with good flatness are essential for many applications. By leveraging techniques such as pulse compression, self-phase modulation, and parametric mixing in near-zero normal dispersion highly nonlinear fibers (HNLFs) over hundreds of meters, a relatively flat broadband optical frequency comb can be achieved. However, during fiber fabrication, random fluctuations in the core radius cause irregular alternations between normal and anomalous dispersion, leading to non-uniformity and reduced bandwidth of the optical frequency comb. This issue can be mitigated by using small-core multicomponent glass fibers or integrated optical waveguides with high nonlinear refractive index coefficients while minimizing the nonlinear fiber/waveguide length.Compared with fibers, integrated optical waveguides offer a higher index contrast between the core and cladding, resulting in a high confinement factor and greater flexibility in dispersion engineering. Various integrated nonlinear optical materials have been explored for generating optical frequency combs. Silicon nitride (Si₃N₄) is widely used in nonlinear optical applications due to its broad transparency window and absence of two-photon absorption in the 1550 nm band. Additionally, Si3N4 waveguides can withstand continuous laser power of up to 10 W. Currently, a low-loss, high-confinement Si₃N₄ nonlinear integrated waveguide fabrication process with a thickness exceeding 600 nm is under development. Researchers have employed multimode-wide waveguides to reduce optical scattering loss at the core-cladding interface, achieving a waveguide loss of 1 dB/m.Regarding pumping light sources, some researchers have used electro-optically modulated pulses to pump Si3N4 optical waveguides for optical frequency comb generation. However, electro-optic modulated pulse source devices tend to be bulky, whereas phase-locked dual-frequency lasers offer a more compact alternative as wide-pulse sources and can be miniaturized using distributed feedback (DFB) lasers. Therefore, we focus on optimizing a phase-locked dual-frequency laser-pumped cavity-less Si₃N₄ nonlinear optical waveguide for optical frequency comb generation in the 1550 nm band. The resulting optical frequency combs have broad applications in optical communications, microwave photonics, and other related fields.MethodsAn optical frequency comb was generated using the TE0 fundamental mode. First, dispersion engineering of the TE0 mode in the Si3N4 optical waveguide at 1550 nm was performed to achieve suitable dispersion and nonlinear coefficient. The second-order and third-order dispersions (β2 and β3) at 1550 nm were determined to be 51.16 ps2/km and 0.16 ps3/km, respectively. The effective area Aeff was calculated as 1.61 μm², and the nonlinear coefficient (γ) as 0.63 W-1‧m-1. Since optical frequency comb generation via nonlinear effects requires high peak pulse power, a phase-locked dual-frequency laser was employed as a wide-pulse source. To enhance the peak pulse power, a two-stage pulse compression technique was used. This process involved chirped pulse compression in normal dispersion nonlinear fibers (HNLFs) and standard single-mode fibers (SSMFs), followed by pulse shaping using a programmable pulse shaper to enable spectrum broadening in the Si3N4 nonlinear optical waveguide. The optical frequency comb was generated through the combined effects of normal dispersion, self-phase modulation, and optical wave breaking in the Si3N4 optical waveguide. The time-frequency evolution of the pulse was simulated using the Generalized Nonlinear Schrödinger Equation (GNLSE) with a split-step Fourier algorithm. Additionally, the effects of power ratio and frequency spacing of the phase-locked dual-frequency laser on optical frequency comb generation were analyzed.Results and DiscussionsA Si3N4 optical waveguide structure with optimized normal dispersion and nonlinear coefficient was achieved through dispersion engineering (Fig. 1). A schematic diagram illustrating optical frequency comb generation using a phase-locked dual-frequency laser with two-stage pulse compression and a cavity-less normal-dispersion Si3N4 optical waveguide is shown in Fig. 2. A phase-locked dual-frequency laser with 1 W total power (power ratio x = 1) was initially used without pulse compression to directly pump a 3 m long cavity-less normal-dispersion Si3N4 optical waveguide with 1 dB/m waveguide loss. This resulted in a spectral bandwidth of approximately 10 nm (Fig. 3). In contrast, incorporating two-stage pulse compression significantly broadened the optical frequency comb spectrum due to the combined effects of normal dispersion, self-phase modulation, and optical wave breaking in the Si₃N₄ waveguide, achieving a 20 dB bandwidth of approximately 100 nm (Fig. 4). Considering a 3 dB/m Si3N4 optical waveguide loss, a slight reduction in both temporal pulse power and spectral bandwidth was observed. Additionally, when accounting for a 1 dB coupling loss between the fiber and the Si₃N₄ waveguide, the 20 dB bandwidth of the optical frequency comb decreased to approximately 80 nm. Due to the large pedestal and tail of the two-stage compressed pulse waveform, significant power fluctuations were observed in the central spectrum, which is unfavorable for practical applications such as multi-wavelength light sources in optical communications. To address this, a pulse shaper was used to enhance the spectral flatness. When the compressed pulse was shaped into a hyperbolic secant pulse, a 10 dB bandwidth of approximately 90 nm was achieved for a central flat optical frequency comb. When the pulse was shaped into a Gaussian profile, a 10 dB bandwidth of approximately 90 nm and a 3 dB bandwidth of approximately 57 nm were obtained. When the pulse was shaped into a third-order super-Gaussian pulse, a 3 dB bandwidth of approximately 90 nm was achieved for a central flat optical frequency comb (Fig. 5). The impact of an unequal power ratio in the phase-locked dual-frequency laser on optical frequency comb generation was also investigated. With a power ratio of x=0.1, the optical frequency comb exhibited a reduced 20 dB bandwidth of approximately 45 nm (Fig. 6). The power ratio of the dual-frequency laser significantly influenced the 3 dB bandwidth, with optimal optical frequency comb generation achieved at x=1 (Fig. 7). Finally, the effect of the frequency spacing (νm) of the phase-locked dual-frequency laser on the temporal and spectral characteristics of the optical frequency comb was examined. A frequency spacing of νm=200 GHz resulted in an optical frequency comb with a 20 dB bandwidth of approximately 100 nm (Fig. 8). However, changes in the frequency spacing had minimal impact on the 3 dB bandwidth of the optical frequency comb (Fig. 9).ConclusionsWe proposes a scheme for generating an optical frequency comb in cavity-less Si3N4 nonlinear optical waveguides pumped by a phase-locked dual-frequency laser. By employing two-stage pulse compression and leveraging the combined effects of normal dispersion, self-phase modulation, and optical wave breaking in a 3 m long normal-dispersion Si3N4 optical waveguide, an optical frequency comb with a 20 dB bandwidth of approximately 100 nm is achieved. When the compressed pulse is shaped into a Gaussian pulse, the central flatness of the optical frequency comb is improved, resulting in a 3 dB bandwidth of approximately 57 nm. Finally, the effects of power ratio and frequency spacing of the phase-locked dual-frequency laser on optical frequency comb generation are analyzed. This study demonstrates the feasibility of generating a broadband optical frequency comb in meter-length normal-dispersion Si3N4 integrated nonlinear optical waveguides, which will contribute to the advancement of cavity-less tunable broadband optical frequency comb research.

ObjectiveImaging light detection and ranging (LiDAR) utilizes the range values of target point clouds (or pixels) to estimate the three-dimensional (3D) shape of a target. However, ambiguity arises in determining which point clouds (or pixels) belong to the target when it is in cluttered or partially occluded environments, leading to uncertainties in reconstructing the 3D shape. Polarimetry, which examines the interaction between polarized light and materials, is a promising method for improving the determination of target pixels (or point clouds). The Mueller matrix, a comprehensive description of polarization properties, has been demonstrated as a powerful tool for characterizing and distinguishing targets with varying polarization characteristics. However, these demonstrations of Mueller matrix imaging have primarily focused on biomedical applications and have employed charge-coupled device time-sharing imaging. Additionally, the polarization diversity imaging technique has only been reported in synthetic aperture radar systems, based solely on the polarization imaging of target scattering matrices. We propose a multichannel polarization diversity imaging LiDAR that integrates Risley-prism multibeam scanning, frequency-modulated continuous wave (FMCW) coherent detection, and Mueller matrix polarimetry techniques for target classification. The proposed system eliminates ambiguity in determining which pixels (or point clouds) correspond to the target, a challenge faced by conventional imaging LiDAR systems.MethodsTo reduce artifacts caused by time-sharing polarization imaging, spatial parallelism and beam synchronization techniques are employed to simultaneously capture multipolarization point clouds using Risley-prism multibeam scanning. To achieve high spatial resolution in point clouds, the system incorporates the FMCW coherent detection, which offers high sensitivity, superior resolution, and a large dynamic range while simultaneously measuring distance and velocity. To address the issue of insufficient dimensionality for target recognition in unstructured environments with conventional imaging LiDAR, the Mueller matrix is introduced into LiDAR systems for target identification and classification. Different objects are distinguished and classified by leveraging variations in their polarization parameters.Results and DiscussionsA long-range 3D polarimetric imaging experiment was conducted using the proposed LiDAR architecture under outdoor conditions. The multidimensional information acquisition capability of the multichannel polarization diversity imaging LiDAR was exploited, and various filtering methods were applied to process the raw point clouds with a large field-of-view angle of ±30° (Fig. 3). To further demonstrate the LiDAR’s polarization imaging capability, small-scale buildings (A6 and A7) were selected as targets for recognition and classification (Fig. 4). In the cross-polarized point clouds [Fig. 4(b2)], buildings A6 and A7 were roughly divided into two regions: one with polarized reflectance intensities of 53?100 (colored in green) and the other with polarized reflectance intensities of 210?320 (colored in blue-violet). This indicates that the facades of buildings A6 and A7 are composed of two materials with distinct polarization properties. For more precise recognition and classification of building materials and structural details, Mueller matrix imaging was employed. The facades of buildings A6 and A7 exhibit a clear demarcation line (Fig. 5). Additionally, there is no ambiguity in determining which point clouds (or pixels) corresponded to the target. Different polarization parameters were utilized to distinguish and classify various targets, effectively enhancing the accuracy of target identification (Figs. 6 and 7).ConclusionsWe demonstrated both theoretically and experimentally a multichannel polarization diversity imaging LiDAR based on Risley-prism multibeam scanning, FMCW coherent detection, and Mueller matrix polarimetry techniques for building target recognition and classification. The multichannel polarization diversity and Mueller matrix characterization techniques were applied for the first time in imaging LiDAR. The proposed polarization diversity imaging LiDAR enables simultaneous multichannel distance, velocity, and intensity measurements, significantly reducing artifacts caused by time-shared polarization imaging. After noise filtering, building materials were initially distinguished from polarized reflection intensity point clouds for long-range targets. To achieve more accurate recognition and classification of building materials and structural details, the Mueller matrix and its associated parameters were utilized to characterize targets with different polarization properties. The proposed system resolved ambiguity in determining which point clouds (or pixels) belong to the target. Our architecture fully leverages the potential of 3D polarization imaging LiDAR for long-range target detection and classification in unstructured building environments, providing a promising approach for improving the accuracy of 3D perception in urban building target detection and classification.

ObjectiveBecause of the physical constraints of light traveling in straight lines, the field of view of a detector is limited by non-transparent obstacles and conventional imaging systems cannot detect target scenes outside the field of view. Regions outside the line of sight, such as those hidden around corners, are generally referred to as non-line-of-sight (NLOS) areas. NLOS imaging has garnered the attention of researchers in various fields, including machine vision, remote sensing, medical imaging, and autonomous driving. NLOS imaging based on a single-photon detector array offers the advantages of high detection efficiency and speed. However, NLOS imaging based on a single-photon detector array results in a non-confocal form and an efficient and high-quality image-recovery algorithm is not yet available, which limits its further development. This study aims to improve the image-recovery quality of hidden objects under the conditions of non-confocal measurements provided by a single-photon detector array.MethodsA technical approach for image recovery in NLOS imaging based on a single-photon detector array is presented, which involves converting the measurements under a single-photon detector array to confocal ones and then applying an image-restoration algorithm based on a convolutional approximation. The main concept of confocal measurement emulation involves selecting a new confocal point (the new illumination point overlaps with the new imaging point) outside the initial illumination and imaging points within the non-confocal detection model. For the non-confocal measurement in each time bin, an optimal spherical radius is selected such that a sphere centered at the new confocal point achieves the best fit with the ellipsoidal model for the initial non-confocal measurement. The confocalization process primarily comprises two aspects: rapid localization and ellipsoidal interpolation. Utilizing the recorded temporal range of several detection points, the envelope of the hidden object can be localized by ellipsoidal backprojection at high speed, which provides a specific area for the subsequent processing. Ellipsoidal interpolation exploits the advantage wherein the ellipsoidal section within a predetermined area can be refined to obtain more accurate timing information with respect to the new confocal point. Consequently, the emulated confocal measurements show better timing accuracy than the initial recording information determined by the detector. Subsequently, a convolutional approximation algorithm for image recovery is employed, which builds the convolutional relationship hidden inside the scene parameters between the backprojection result and the actual albedo. By implementing the deconvolution based on fast Fourier transforms, a closed-form solution for the recovered image can be obtained rapidly and accurately. Upon investigating the convolutional relationship, this approach not only reduces computational complexity but also allows for the incorporation of prior information, thus further enhancing the image quality. By combining the enhanced timing accuracy provided by the confocalization method with the high resolution provided by the convolutional approximation, high-quality recovery images of hidden objects can be achieved.Results and DiscussionsIn the simulation, two “E”-shaped objects with different sizes made of white cardboard were used as the NLOS hidden objects to evaluate the resolution achieved by the proposed approach and to compare the results obtained using different image-recovery algorithms. The illumination point was set at the center of the right edge of the field of view to avoid the pile-up effect of the single-photon detectors in the array. The root mean square errors (RMSEs) of the reconstructed images obtained using the proposed convolutional approximation algorithm combined with the confocalization of non-confocal measurements for objects of each size were lower than those obtained using filtered-backprojection-based (FBP-based) or light-cone-transform-based (LCT-based) algorithms. The advantages of the proposed approach are more pronounced for smaller hidden objects or a smaller field of view for detection. Furthermore, an experimental setup for NLOS imaging was designed and constructed. Initially, non-confocal measurements were performed using a pulsed laser and a single photon avalanche diode (SPAD) array. The original time resolution of the SPAD in the experiments is approximately 57 ps. With the measurements transformed into emulated confocal ones, a timing accuracy of approximately 30 ps is achieved, thus validating the improvement in time resolution beyond that of the original detector achieved via the confocalization process. Subsequently, the image of the hidden object was reconstructed using the FBP-based, LCT-based, and proposed algorithms on the confocalized data. For “E”- and “H”-shaped objects, the proposed convolutional-approximation algorithm achieved RMSEs of 0.3314 and 0.2730 with respect to the ground truth, respectively; the FBP-based algorithm achieved 0.3770 and 0.3060, respectively; and the LCT-based algorithm achieved 0.3566 and 0.2986, respectively. Hence, the image-restoration performance of the proposed algorithms is superior to those of existing ones.ConclusionsIn summary, the confocalization of non-confocal measurements combined with a convolutional approximation algorithm can provide a new feasible route for NLOS imaging based on a single-photon detector array. Simulation and experimental results show that the technique proposed herein offers more advantages than conventional algorithms based on FPB or LCT, with lower recovery errors. This study is significant for the advancement of NLOS imaging.

ObjectiveLithium batteries, one of the most versatile energy storage technologies, play a pivotal role in the global transition from fossil fuels to renewable energy sources. With advantages such as high voltage, high energy density, and relatively low manufacturing costs, these batteries have found widespread application in fields such as aerospace, power grids, automotive, and robotics. With the rapid expansion of lithium battery applications, their reliability and longevity have become increasingly crucial. Consequently, the development of precise and effective battery management systems (BMSs) is an urgent concern. The state of charge (SOC) is a particularly important indicator monitored by the BMS because it reflects the available capacity within the battery and serves as a critical measure of its endurance and capability to sustain operation. Accurately estimating the SOC not only enhances operational efficiency but also plays a significant role in improving the safety and lifespan of lithium batteries. To ensure the safe operation of batteries, fiber Bragg grating (FBG) sensors have been gradually incorporated for the SOC estimation of lithium batteries to monitor their state variations during the charging and discharging processes. However, most studies primarily focus on introducing FBG and do not specifically determine the influence of strains at various spatial positions on SOC estimation.MethodsThis study investigates the effect of strain measurements at three different positions on the surface of an 18650 lithium battery on the accuracy of SOC estimation. First, a lithium battery strain-monitoring system based on FBG sensors is developed, wherein three FBG sensors are strategically placed near the negative terminal, positive terminal, and central region of the battery. These sensors continuously monitor the strain at different positions on the battery surface under different conditions. To estimate the SOC, the strains at different positions, currents, and voltages are used as state features. A convolutional neural network-gated recurrent unit (CNN-GRU) model is employed to estimate the SOC. By incorporating the strain data from various positions, we can compare the influence of different strain positions on the SOC estimation accuracy. Finally, both static and dynamic conditions are applied to validate the impact of strain at different positions as input features for SOC estimation, demonstrating that strain position can significantly affect SOC estimation for lithium batteries.Results and DiscussionsThe battery monitoring system based on FBG strain sensing collects data from different positions, as well as voltage and current measurements. Under static conditions, as shown in Fig. 4, the strains at all three positions initially increase, then decrease, and finally increase again. The most pronounced strain trend is observed at the central position, whereas the trends near the positive and negative terminals are less noticeable. Under dynamic conditions, as shown in Fig. 6, the strains at all three positions exhibit an upward trend, with the strain at the central position exhibiting the most significant changes, followed by the strain near the negative terminal, and finally, the strain near the positive terminal with the weakest trend. By combining different features and inputting them into the CNN-GRU model, the results indicate that the strain at the central position provides the strongest auxiliary effect for SOC estimation under static conditions, with a root mean square error (RMSE) and mean absolute error (MAE) of 0.59% and 0.46%, respectively. Under dynamic conditions, the central position strain achieves an RMSE and MAE of 1.17% and 0.81%, respectively.ConclusionsTo address the challenge of accurately estimating the SOC of lithium-ion batteries, this study proposes a battery-monitoring system based on FBG strain sensing. The FBG sensors are attached at different positions on the battery surface—near the negative terminal, positive terminal, and central position—to enable real-time strain monitoring. By inputting the strain from different positions, along with the voltage and current, into the CNN-GRU deep learning model, experimental data are collected, and SOC estimation under both static and dynamic operating conditions is performed. The experimental results indicate that during the charging and discharging processes, the strains at all three positions vary. Although the overall trends are similar, the strain at the central position exhibits the most pronounced variation, followed by the strain near the negative terminal, with the strain near the positive terminal exhibiting the least noticeable change. The consideration of the strain from each position as an auxiliary feature alongside electrical parameters results in a notable improvement in SOC estimation accuracy. However, the impact of the strain on the SOC estimation accuracy varies by position, with the strain at the central position contributing the most significant enhancement, whereas the strains near the positive and negative terminals have a weaker effect. This study demonstrates the importance of using FBG sensors for strain measurement on the battery surface and reveals the differential impact of strain at various positions on the accuracy of SOC estimation. This provides new insights and possible methodologies for the improved integration of FBG sensors into battery management systems. Based on the current findings, future research may further explore several directions, including the optimization of the FBG sensor placement, multimodal data fusion, and investigations of different battery models and types.

SignificanceThe research on photolithography technology and photoresist materials is crucial, acting as a foundation for the advancement of modern microelectronics, semiconductor manufacturing, and various high-tech industries. Photolithography, a process that transfers complex circuit designs onto silicon wafers or other substrates with high precision and resolution, has significantly enhanced the integration density and performance of integrated circuits (ICs). This technology has drastically reduced manufacturing costs and enabled the rapid development of information technology. With the escalating demand for high-performance chips, driven by the proliferation of the internet of things (IoT), artificial intelligence (AI), and quantum computing, advanced photolithography techniques are becoming crucial. For instance, IoT devices require low-power, high-integration sensors, and processors, whereas AI applications demand powerful computational capabilities and high-speed data transmission. The continuous breakthrough of the resolution limits in photolithography enables the fabrication of sophisticated chips, thereby supporting the evolution of these cutting-edge technologies. Moreover, the adaptability and accuracy of photolithography extend beyond semiconductor manufacturing, with extensive applications in microelectromechanical systems (MEMS), photonics, and biomedicine. In MEMS, photolithography facilitates the fabrication of intricate mechanical structures at a microscopic scale, which are essential for sensors, actuators, and miniaturized devices. In photonics, it is used to fabricate optical components and devices, such as waveguides and photonic crystals, which are crucial for optical communication and sensing. In biomedicine, photolithography is essential in developing microfluidic devices, biochips, and tissue engineering scaffolds, contributing to advancements in diagnostics, drug delivery, and regenerative medicine. The broad applicability and accuracy of photolithography render it an indispensable tool for innovation across these diverse fields.The development of novel photoresist materials is a key factor in the progress of photolithography, addressing the growing demands for higher resolution, sensitivity, and process compatibility. Traditional photoresists, such as positive and negative resists, have been extensively used for their reliability and ease of use. However, the increasing complexity and miniaturization of devices necessitate the exploration of specialized photoresists. These new materials, such as those used in nanoimprint lithography (NIL), laser direct writing (LDW), and scanning probe lithography, offer unique properties that enhance the performance of photolithographic processes. For example, NIL photoresists facilitate cost-effective and high-resolution pattern transfer, making them ideal for large-scale production. LDW photoresists provide unparalleled spatial resolution and flexibility, making them suitable for creating complex and precise structures. Continuous innovation in photoresist chemistry is essential for advancing photolithography and meeting the stringent requirements of next-generation devices. Furthermore, environmental sustainability is a growing concern in the semiconductor industry, and the development of eco-friendly photoresists is a crucial step toward mitigating the environmental impact of photolithography. Water- and bio-based photoresists signify important breakthroughs, providing biodegradability and reduced reliance on hazardous chemicals. These environmentally friendly alternatives enhance sustainable manufacturing practices and align with global initiatives to mitigate climate change and promote green technologies. By addressing existing challenges and exploring new frontiers, this research paves the way for more efficient, precise, and sustainable photolithographic processes, thereby advancing technological progress and environmental sustainability.ProgressPhotolithography and photoresist materials have witnessed significant advancements over the past few decades, driven by the relentless pursuit of higher resolution, increased efficiency, and broader application domains. The most significant development is the transition from conventional deep ultraviolet (UV) lithography (DUVL) to extreme UV lithography (EUVL). The EUVL, which uses a 13.5 nm wavelength, has revolutionized the semiconductor industry by facilitating the fabrication of features at 7 nm node and smaller. The introduction of the first commercial EUVL system by ASML in 2019 marked a significant milestone, as it significantly improved the resolution and production efficiency, making it possible to manufacture 5 nm and even smaller nodes (Fig. 5). This advancement has been crucial for the development of high-performance chips required by emerging technologies such as AI, the IoT, and quantum computing. The success of EUVL has prompted further research into optimizing EUV light sources, such as the development of new types of discharge-produced plasma sources based on liquid tin jet electrodes and the study of the temporal evolution parameters of the laser-produced tin plasma.Another significant advancement is the development of advanced photoresist materials tailored for high-resolution and high-sensitivity applications. Traditional photoresists have been the backbone of photolithography for many years. However, the increasing demands for finer feature sizes and better process control have prompted the investigation of novel photoresist formulations. For example, chemically amplified photoresists (CAPR) have become prevalent in EUVL, where the resist sensitivity and resolution are significantly enhanced through acid-catalyzed reactions. Furthermore, the development of multi-photon polymerization (MPP) photoresists has opened up new possibilities for three-dimensional (3D) and high-resolution patterning. MPP facilitates the fabrication of complex structures with sub-micron resolution, making it highly suitable for applications in MEMS and biomedicine. The integration of nanocomposites into photoresists has also been explored to improve their mechanical properties and functionality, such as the development of transparent magnesium aluminate spinel ceramics for additive manufacturing. These advancements in photoresist materials are crucial for pushing the boundaries of photolithography and facilitating the fabrication of more sophisticated and multifunctional devices.Conclusions and ProspectsResearch on photolithography technology and photoresist materials has significantly advanced, driving the development of modern microelectronics and semiconductor manufacturing. The transition from conventional DUVL to EUVL has facilitated the fabrication of features at the 7 nm node and smaller, markedly improving the performance and integration density of the ICs. The development of advanced photoresist materials, such as CAPR and MPP photoresists, has further pushed the boundaries of resolution and functionality, creating new opportunities for applications in MEMS, photonics, and biomedicine. Moreover, the integration of electron beam lithography with other nanofabrication techniques has improved the accuracy and adaptability of nanostructure fabrication, supporting the development of high-density, high-performance devices.The future of photolithography depends on interdisciplinary collaboration, intelligent automation, and multifunctional integration. Interdisciplinary efforts in materials science, chemistry, physics, and computer science will persist in advancing innovations in photoresist chemistry and lithography processes. The adoption of machine learning algorithms for real-time optimization and control of lithography parameters will improve process consistency and efficiency, thereby reducing production costs and waste. Moreover, the development of multifunctional photoresists with properties such as conductivity, magnetism, and biological activity will expand the applicability of photolithography, facilitating the creation of advanced devices for flexible electronics, data storage, and biomedical imaging. With the increasing demand for high-performance and sustainable technologies, the ongoing advancement of photolithography will be pivotal in influencing the future of microelectronics and other fields.

ObjectiveHydrogen sulfide (H2S) is a toxic and flammable byproduct in many industrial processes. High concentrations of inhaled H2S can cause significant discomfort and pose serious health risks, even threatening human life. Photoacoustic spectroscopy is a key technology for trace gas detection due to its advantages of zero background noise and high detection sensitivity. This paper proposes a gas sensor utilizing a distributed feedback (DFB) laser with a wavelength of 1578 nm as a light source for detecting H2S. Additionally, the influence of varying temperatures on the resonance frequency of the photoacoustic cell and signal is investigated. We hope that the findings of the study will provide a significant reference for achieving high-precision photoacoustic spectroscopy detection under varying environmental temperatures.MethodsA DFB laser with a central wavelength of 1578.13 nm is selected as the excitation light source. The laser power is amplified to 150 mW using an L-band erbium-doped fiber amplifier (EDFA). A multipass cell structure is employed to achieve 46 reflections of the laser in the resonant cavity, further enhancing the effective laser power. A microphone with a sensitivity of 26 mV/Pa is installed at the center of the resonant cavity to detect photoacoustic signals. The detected signal is demodulated using a lock-in amplifier and subsequently collected via a data acquisition card for processing on a personal computer (PC). Vacuum pumps and mass flow meters are used to regulate the pressure and flow rate of gas in the photoacoustic cell. To mitigate the adsorption effects of H2S gas and prevent signal drift caused by environmental temperature fluctuations, the photoacoustic cell is heated and stabilized at 65 ℃.Results and DiscussionsAt a temperature of 65 ℃, the resonance frequency of the photoacoustic cell is measured to be 888 Hz, and the quality factor of the photoacoustic cell is 17.1 (Fig. 3). The resonance frequency of the photoacoustic cell increases with increasing temperature, in line with the sound speed. The photoacoustic signal decreases with increasing temperature due to the intensified thermal motion of molecules, which leads to a reduction in their absorption intensity (Fig. 4). The photoacoustic signal of H2S is measured in the volume fraction range of 100×10-6?1000×10-6[Fig. 6 (a)], confirming a linear relationship between the signal intensity and volume fraction , with a correlation coefficient of 0.9993 [Fig. 6 (b)]. Allan variance analysis shows that the sensor detection limit is 1.8×10-6 with an integration time of 278 s (Fig. 7).ConclusionsA gas sensor based on resonant photoacoustic spectroscopy is proposed for the detection of H2S gas. This study investigates the effects of ambient temperatures on the resonance frequency of the photoacoustic cell and signal. It is found that the resonance frequency of the photoacoustic cell is directly proportional to the ambient temperature, whereas the amplitude of the photoacoustic signal exhibits an inverse relationship with temperature. By optimizing key parameters, such as laser modulation depth and modulation frequency, a detection limit of 1.8×10-6 is achieved with integration time of 278 s. This provides a highly sensitive sensor under varying environmental temperature conditions.

ObjectiveMethane (CH4) and ethylene (C2H4) are important products of coal pyrolysis in coal chemical processes. Understanding their pyrolysis evolution mechanisms is crucial for comprehending the thermal decomposition of coal, optimizing fuel utilization efficiency, and reducing harmful gas emissions. This study develops an online monitoring device capable of simultaneously detecting CH4 and C2H4 volume fractions, which combines tunable diode laser absorption spectroscopy (TDLAS), wavelength modulation, and time-division multiplexing techniques. The system is successfully applied to gas analysis during coal pyrolysis.MethodsThe system utilizes distributed feedback (DFB) lasers as its light sources, with central wavelengths of 1653 nm and 1620 nm for CH4 and C2H4, respectively. The laser beams are directed into a Herriott-type high-temperature multipass gas cell with an effective optical path length of 15 m, which enhances the sensitivity of the detection system. Through the combination of hardware circuitry and specifically designed software programs, the system achieves precise modulation, demodulation, and volume fraction inversion of CH4 and C2H4 spectral signals. Calibration of the system is carried out using standard gases with varying volume fractions. The results demonstrate excellent linearity, with correlation coefficients of 0.999 and 0.998 for CH4 and C2H4, respectively, underscoring the reliability and accuracy of the device in measuring these gas volume fractions.Results and DiscussionsTo further evaluate the system performance, continuous measurements are conducted using standard gases with volume fractions of 200×10?? and 100×10?? for CH4 and C2H4, respectively. Over a duration of 1000 s, the maximum volume fraction fluctuations are found to be 2×10?? for CH4 and 6×10?? for C2H4. The standard deviations of the measurements are 0.7 and 2.1 for CH4 and C2H4, respectively, indicating the high stability of the system measurements. To further examine the stability and precision of the measurement system, Allan variance analysis is performed. The minimum detection limits derived from this analysis are 0.025×10?? for CH4 and 0.133×10?? for C2H4, demonstrating the system exceptional sensitivity for trace gas detection. The system dynamic response time is also evaluated through gas-switching experiments. By alternating between nitrogen and a standard CH4 gas with a volume fraction of 200×10??, the system dynamic response time is determined to be 17.8 s. This response time is considered suitable for real-time monitoring of gas volume fractions in coal pyrolysis processes, highlighting the system practical applicability in industrial scenarios. To validate the system performance under real-world conditions, it is employed to monitor gas emissions during the pyrolysis of Shandong bituminous coal. The device successfully measures the volume fraction release curves of CH4 and C2H4 over the course of the pyrolysis process. By altering experimental parameters such as heating rates, coal sample particle sizes, and oxygen volume fractions, the relationships between these experimental conditions and the release profiles of CH4 and C2H4 are systematically investigated. The findings reveal the release characteristics of both gases during coal pyrolysis, thus providing important insights into their evolution mechanisms. These results serve as a critical foundation for optimizing coal pyrolysis processes, improving fuel utilization efficiency, and controlling the emission of harmful gases in industrial applications.ConclusionsThis study successfully develops an advanced online monitoring system for the simultaneous detection of CH4 and C2H4 volume fractions using TDLAS. The system demonstrates high sensitivity, excellent linearity, and outstanding stability, making it a reliable tool for gas analysis in coal pyrolysis processes. Moreover, its application in real-world scenarios provides valuable data for understanding the release mechanisms of CH4 and C2H4 under different experimental conditions. These findings contribute significantly to the optimization of coal pyrolysis techniques and the development of more efficient and environmentally friendly fuel utilization strategies.

ObjectiveWith the rapid development of ultraintense and ultrafast lasers, strong terahertz (THz) sources and their applications are progressing significantly. THz electro-optic sampling (THz-EOS) is an effective means for THz coherent detection. However, for measuring irreversible or low-repetition processes, such as protein denaturation, material damage, structural phase transitions, and ultrafast dynamics, traditional THz-EOS methods face challenges owing to their reliance on multiple scanning. Unfortunately, strong THz sources are often driven by ultrafast laser systems with low repetition rates. Therefore, various THz single-shot detection methods have been developed. Among these detection methods, the spectral-coding method based on time-frequency mapping can realize the single-shot measurement of a THz waveform with a simple structure. However, this method is hindered by a low signal-to-noise ratio (SNR) and limited time resolution. The common-path self-reference spectral interference, which is based on spectral coding, can overcome the “over-rotation” problem that manifests in the traditional intensity-modulated spectral-coding method under strong THz fields. Simultaneously, two interfered pulses in the common-path scheme are always collinear transmissions and thereby effectively avoid stability issues caused by external factors. In the present study, we report a single-shot detection technique for THz time-domain spectroscopy that is based on two-dimensional spectral interference. By using two-dimensional spectral interference, this method significantly increases the information capacity of single-shot detection, which enables an improved SNR for THz pulse measurements and facilitates the detection of multiple THz-related ultrafast events. Therefore, the results of this study provide an accurate method with a high signal-to-noise ratio for single-shot measurements in strong-field THz time-domain spectroscopy.MethodsWe employ a Ti∶sapphire amplifier with an average power of up to 2.7 W as the light source. The output laser pulse is divided into two pulses by a beam splitter. One beam with 99% energy is incident into a LiNbO3 crystal to generate a vertically polarized THz field via the tilted-pulse-front technique. The other beam is stretched up to 12 ps by a pair of prisms as a probe pulse. The broadened probe pulse width is sufficient to cover the generated THz field. The probe pulse is horizontally polarized by a polarizer and then focused onto a 1 mm-thick (110) ZnTe crystal with the THz pulse. The crystal is an electro-optic crystal for electro-optical sampling, and its [0,0,1] axis is arranged horizontally. To introduce the appropriate spectral interference fringe density between the two orthogonal polarization components of the probe pulse, the beam passes through a 1 mm-thick α-crystal with its optical axis set at 45° to the horizontal direction. More importantly, we use an imaging spectrometer to achieve two-dimensional spectral interference. After passing through a second polarizer (P2), the probe pulse is directed into the imaging spectrometer, where the spectral interference fringes are recorded. The optical axis of P2 is aligned vertically to maximize fringe modulation.Results and DiscussionsBased on a theoretical analysis of the THz electro-optical effect, this study optimizes the probe pulse polarization direction and electro-optic (EO) crystal angle for spectral interference with common-path self-reference (Fig. 2). For the vertically polarized THz pulse, the optimization is achieved when the polarization direction of the probe pulse and EO crystal axis [0,0,1] are both horizontal, as is shown in Fig. 4 (b). Owing to the intrinsic inhomogeneity of the probe pulse intensity, the recorded interference spectrum exhibits a nonuniform intensity distribution (Fig. 6). This uneven intensity affects the signal-to-noise ratio of the measured THz time-domain waveforms, as weaker interference spectra result in lower SNR values under the same system noise conditions. Consequently, the SNRs of THz time-domain waveforms vary across different lines on the imaging spectrometer (Fig. 8). The imaging spectrometer provides a spatially resolved visualization of the THz waveform distribution (Fig. 7), thereby offering a novel method for the single-shot detection of THz time-domain spectra across multiple ultrafast events. By averaging the time-domain waveforms measured across all lines of the imaging spectrometer [Fig. 9 (a)], the SNR of the THz signals reaches 166.6∶1, which is 2.2 times the maximum SNR achieved by a single line on the charge coupled device (CCD) panel of an imaging spectrometer.ConclusionsIn this study, a single-shot detection for THz time-domain spectroscopy based on two-dimensional spectral interference is proposed. This technique further improves THz detection efficiency by optimizing the polarization direction of the probe pulse and rotation angle of the detection crystal. The optimized structure enhances THz detection efficiency by 13.6%, achieves a higher optical modulation depth, and simplifies the alignment process of the detection setup. Additionally, imaging spectrometers replace traditional one-dimensional spectrometers to enable two-dimensional spectral interference and significantly increase the information capacity of each measurement. The experimental results demonstrate that, compared with the spectral interference scheme with common-path self-reference, the introduction of two-dimensional spectral interference improves the detection SNR by nearly 2.2 times, resulting in a maximum SNR of 166.7∶1. This technique can overcome the “over-rotation” problem that manifests in traditional electro-optic sampling as well as offer a high SNR and optimal detection efficiency. This study provides an effective detection means for strong THz radiation sources with single-shot or low-repetition frequency. Moreover, two-dimensional spectral interference shows potential for developing multi-channel THz time-domain spectrometers.