ConclusionsBy embedding the PIN diodes in the metasurface and delicately designing the feed circuit, a cross-shaped switchable polarization conversion metasurface is proposed. When the diodes are in the on state, the metasurface works in the polarization conversion mode, the PCR exceeds 90% in the range of 4.037.71 GHz, and the corresponding relative bandwidth is 62.7%. However, when the diodes are in the off state, the polarization conversion is turned off, and the metasurface works as a reflector. The mechanism of the polarization conversion metasurface is explained by theoretical analysis and surface current distributions. The designed metasurface has good wide-angle incident characteristics, and the metasurface has good polarization performance within the incident angle of less than 45°. Furthermore, the measured and simulation results agree excellently, which confirms the feasibility of this work. The proposed metasurface is expected to be used in EM compatibility and polarization detection.

ConclusionsIn the space-borne lidar, the large-aperture dual-wavelength harmonic diffractive optical system is used to achieve light-weight, and the along-track direction multi-pulse sliding window processing is adopted to improve the detection performance of the system. The 12 sub-mirrors of the optical synthetic aperture can well replace the large-aperture main mirror (Fig. 3). A single-photon array detector with a scale of 10 (along-track direction)×1000 (cross-track direction) is used. When the laser emits a narrow pulse of 5 ns, the signal-to-noise ratio per pixel is about 0 dB. The sliding window processing realizes the incoherent accumulation of 7 pulses, which can increase the signal-to-noise ratio to about 4 dB, which can basically meet the requirements of 3D imaging. Using a laser local oscillator array detector, the signal-to-noise ratio per pixel of a single pulse is about 0 dB when the laser emits a wide pulse of 100 μs and adopts pulse compression technology, and the sliding window processing realizes the incoherent accumulation of 7 pulses, which can increase the signal-to-noise ratio to 8.4 dB.

ConclusionsIn this study, we design a theoretical model of glucose concentration sensor using the pole effect of the PT-symmetric structure. Unlike the traditional sensor, the designed model does not use spectrum positions as the sensing mechanism but uses transmittance detection as the sensing method. The new sensing mechanism has high precision and sensitivity. Although our model is for blood glucose sensing, it can be extended to general biomaterial sensing, including sensing various diseased cells. Therefore, this study provides a new universal method for biosensors.

ConclusionsIn this study, we verified the theory of multi-pulse coherent accumulation in the frequency domain based on FMCW coherent lidar through laboratory experiments. For echo signals with fixed pulse repetition rates, the RSN of beat frequency signals after M-cycle coherent accumulation in the frequency domain was 10lg M dB higher than the log RSN of the original signal, and the weak RSN of the echo signal increased exponentially. Under the same pulse accumulation period and target detection, for the frequency modulated continuous wave whose pulse repetition frequency meets m times, the RSN of the single-cycle high repetition frequency single-pulse echo signals was -10lg M dB higher than that of low-repetition frequency signals. With the amplitude and phase fluctuations of signals, the multi-pulse coherent accumulation RSN was lower than the ideal coherent accumulation RSN,and the pair mean values were 2.25×10-4 and 1.65×10-4 with relative amplitude fluctuations of 0.69 and 0.93, respectively, and the maximum error factors of the multi-pulse coherent accumulation RSN of the echo signal in the 50-pulse period were 0.57% and 0.3%, respectively, which are attributed to the amplitude fluctuation of the echo signal, linear frequency modulation power and fluctuations.

ConclusionsIn a two-dimensional plane, the characteristics of a light field near the focus of a 1D convergent GB are analyzed using a 1D diffraction integral formula. As the normalized inclination factor that is expressed by the square root of the cosine of the observation side inclination angle is engaged, the characteristics of the focal-plane light field of the 1D nonparaxial convergent GB that is expressed by a new diffraction integral formula conform to the law of conservation of radiation energy in traveling wave field, the characteristics of a light field near the focal line of the 1D nonparaxial convergent GB are reasonable. Then, the propagation characteristics of the GB are extended from the paraxial field to the nonparaxial field using the reasonable observation side inclination factor. Furthermore, the rationality of the observation side inclination factor expressed by the square root of the cosine of the observation side inclination angle is verified.

ConclusionsIn this paper, based on the contradictions among the scanning range, imaging speed of traditional photoacoustic microscope, and the prospect of miniaturization, a photoacoustic microscope based on a transparent transducer is proposed, and a series of works such as hardware design, software development and simulation experiment of the system are completed. On the basis of the traditional photoacoustic microscope, our laboratory independently develops a transparent ultrasonic transducer to optimize the optical path of the system. A set of photoacoustic microscope based on the transparent ultrasonic transducer is designed. The lateral resolution of the system is 18 μm and the signal-to-noise ratio is up to 38 dB, which both give a large imaging field of view and a fast imaging speed. A single imaging can achieve 16 mm×16 mm range. The laser repetition rate of the system is 5 kHz and the imaging speed of 100 Hz/mm can be achieved when the scanning step is 20 μm. At the same time, the vascular morphology in the biological tissue imaging experiment is consistent with that in the photos, suggesting that the system has the potential to be applied in the biomedical field. Above all, the photoacoustic microscope based on a transparent transducer can give consideration to both a large imaging field of view and a fast imaging speed, and has a good application prospect in system miniaturization and functional imaging of biological tissues.

ConclusionsBased on the near-field optical fiber temperature probe, a nondestructive cell temperature measurement method is proposed. The probe has a temperature sensitivity of 0.16 nm/℃. To achieve nondestructive scanning, the distance between the probe and the sample surface is maintained constant during scanning via feedback control of the tuning fork. At the same time, the probe measures the sample temperature through the response of fluorescence collected by the probe to the temperature change. Both the fixed U87MG cell with gold nanoparticles and the living U87MG cell without gold nanoparticles are measured using the near-field optical fiber probe. A maximum temperature difference of 4 ℃ is measured on the fixed cell surface, and 0.5 ℃ is measured on the living cell surface. The cells have the same morphology before and after scanning, indicating that the probe has caused nondestructive damage. The temperature measurement method proposed in this study, when combined with the temperature sensitivity and liquid environment stability of quantum dots fluorescence, as well as the nondestructive property of near-field optical technology, presents a better method for the nondestructive study of living cells.

ConclusionsIn this study, we explored the high-power pico-second pulse propagation characteristics using a homemade low-loss NL-HCF. During the high-power propagation of NL-HCF, the fibre was chosen to meet the comprehensive requirements of confinement loss, bending performance, and laser damage threshold. The pulse width of the light source was 15 ps, and its repetition frequency was adjustable. Different lengths of NL-HCF were used for propagation tests. When the input laser was coupled to the NL-HCF with a 403 μJ pulse energy, 40.3 W average power, and 26.8 MW peak power, we obtained a 91.8% propagation efficiency with a maximum output pulse energy of 370 μJ. We analysed the spectral and temporal characteristics of ultra-short pulse passing through different NL-HCF lengths. When the NL-HCF length was 1 m, the laser pulse was propagated without distortion, and the spectrum was slightly deformed. Owing to the non-linear effect, the pulse width of the output laser was increased to 26 ps and the spectrum was expanded to 70 nm when the length of the NL-HCF was increased to 3.3 m. Experiments show that NL-HCF will play an important role in the propagation of ultra-short pulses.

ConclusionsIn order to reduce the problem of inter-mode crosstalk in the low-mode fiber, a new type of dual coupling ring assisted few-mode fiber structure design is proposed, the main influencing factors of crosstalk in the 6-LP mode are analyzed, and five main structural parameters of the new dual coupling ring structure are simulated and analyzed. Based on this designed structure, a 6-LP few-mode fiber is developed, which reduces the inter-mode crosstalk in the few-mode fiber. The results show that in the 6-LP few-mode fiber, the effective mode refractive index difference between the LP21 mode and the LP02 mode is the closest, and the difference is the main factor affecting crosstalk. The double coupling ring structure adjusts the effective mode refractive indices of the LP21 mode and the LP02 mode. In this structure, the minimum refractive index difference between LP21 and LP02 is 1.5×10-3, which is 1.88 times the minimum refractive index difference of the traditional step structure under the same conditions.

ConclusionsIn this paper, the influence of SRS crosstalk on the Super-PON system is investigated by simulation in which two cases are considered. The one case is using C-band for US and L-band for DS, and the other case is using C-band for DS and L-band for US. The simulation results show that when using C-band for DS and L-band for US, the power impairment of signals is the smallest, which is 0.43 dB (the single wavelength input optical powers of US and DS are 10 dBm and 13 dBm, respectively). Therefore, this combination mode is the best band-allocation scheme for the Super-PON system. Besides, we simulate the power impairment of signals caused by SRS when the Super-PON US and DS coexist with EPON US and 10G-EPON DS systems, and the results show that the SRS effect has little influence on them which can be basically ignored.

ConclusionsIn this paper, an all-fiber mode selective coupler based on vector modes is proposed, which consists of a single mode fiber and an air-core ring few mode fiber. Based on the mode-coupling theory, the coupling characteristics of this vector mode selective coupler are investigated. In addition, we explore the influences of core distance on the crosstalk between high order vector modes and the working bandwidth. The results show that this vector mode selective coupler can realize the coupling from the fundamental mode to the special high order mode with a high coupling efficiency in the wavelength range of 15431556 nm. The crosstalk and the working bandwidth can be regulated by changing the core distance. Moreover, the low crosstalk and the wide bandwidth mode coupling are achieved at a core distance of 11 μm. The results in this paper can find applications in fiber lasers, optical tweezers, and mode division multiplexing systems.

ConclusionsThe two real water environments of the Bohai Sea and the South China Sea are simulated in this study by numerically setting the two parameters of salinity and turbidity, and experiments are conducted using this as the medium to obtain the original point cloud data, using the methods of threshold segmentation, refraction correction, and point cloud filtering for processing and reconstruction. In the follow-up error analysis, the k-d tree algorithm is used to calculate the error between the reconstructed point cloud and the standard point cloud, and the cause and impact are analyzed. The results show that: 1) the detection of underwater targets is unaffected by changes in salinity. The average error of the target changes within 1 mm in water environments of 30 PSU and 35 PSU. 2) Changes in water turbidity primarily affect light backscattering in the underwater transmission process, resulting in more noise points in the target point cloud and lowering imaging quality. 3) When the target’s detection distance is fixedly increased, the detection effect gradually deteriorates due to the large attenuation of the water body, and the target point contour of the cloud becomes blurred, The mean error effect increases as the number of effective point clouds decreases. In conclusion, the turbidity influence is dominant in point cloud reconstruction under different water environments, the regularity is not obvious, and the detection distance influence is controllable.

ConclusionsA novel method of stability analysis was proposed by threshold gain analysis based on a transfer matrix theory in this study. The results reveal that long coupling delays need weak coupling strength, whereas short coupling delays need strong coupling strength. To analyze the linewidth of mutually injection-locked semiconductor lasers under weak coupling, an analytical method based on noise correlation analysis is used. The results show that the coupling delay and coupling strength are the main factors affecting the linewidth; however, the phase difference has less influence on the linewidth, which can be used for fine adjustment.

ConclusionsThis study investigates the MTS frequency stabilisation method based on 1560 nm laser frequency doubling, optimises the analysis of the modulation transfer signal and finally locks the 1560 nm fiber laser frequency doubling to the 34 cross peak of the rubidium atom D2 line. After the laser is frequency-locked, it beats with an optical frequency comb and the standard deviation of the beat frequency value after locking for 1 h is 0.049. The Allan variance is used to characterise the performance of the system frequency locking, and the relative frequency stability after frequency locking reaches the minimum within the integration time of 10 s. The system results show that the 1560 nm fiber laser frequency locking system based on rubidium MTS can achieve excellent frequency stabilisation performance. Moreover, the frequency stabilisation system can output high-frequency stability at 1560 and 780 nm narrow-linewidth lasers simultaneously, which can be directly applied to fiber sensing, lidar and rubidium atoms as experimental media, quantum information, atomic and molecular physics and other fields.

ConclusionsThe paper introduces the fabrication, characterization and analysis of a novel graphene capacitor, which can be used as an effective saturable absorber modulator in a passive Q-switched system. The photoelectric interaction mechanism of the device is studied based on its electrical transmission and spectral absorption properties. Due to the change of Fermi level and carrier density, the absorption properties of graphene in the device can be adjusted by gate voltage without changing the properties of SA material. The output characteristics of all-solid-state pulse laser can be adjusted flexibly under ultra-low electric modulation power (~13 pA current and 1 nW power). It is applied to the Nd∶ YVO4 all solid state laser system, and the stable pulse output at 532.04 nm wavelength is achieved. The laser absorption pump power is kept constant, and the gate voltage is changed. The pulse duration of the Q-switched output can be compressed from 1.1 μs to 345 ns. This structure is expected to further promote the development of tunable pulse lasers from visible light to mid-infrared band. In particular, passive Q-switching/mode-locking devices based on graphene devices will be used in future applications such as infrared measurement, projection display, and optical modulation.

ConclusionsA compact all-solid-state DUV-laser source producing a short pulse at 228.5 nm is reported. The acoustically Q-switched quasi-three-level Nd∶YVO4 laser at 914 nm is first intra-cavity frequency doubled into a blue laser with an LBO nonlinear crystal, and then into a DUV output from externally frequency doubling of the blue laser with a BBO nonlinear crystal. The 228.5 nm DUV pulsed laser output power of 10 mW is obtained, with a stable output power and good output beam quality. The present laser system is simple, compact and portable compared with the Ti: sapphire UV Raman laser reported earlier. Our laser source can be favorably used in the spectroscopic measurement of explosives and biomolecules.

ConclusionsIn this paper, we demonstrate a high power fiber oscillator based on a pair of 20 μm /400 μm large mode area double cladding fiber Bragg gratings fabricated by a femtosecond laser together with a phase mask. The reflectivity of the high-reflection FBG is more than 99%, and that of the low-reflection FBG is about 10%. The slope efficiency of the fiber oscillator is 77.9%, and the maximum power output is more than 3.2 kW. To the best of our knowledge, the kW level class fiber oscillator using FBGs fabricated by a femtosecond laser has never been reported in our nation before, and our research works fill the vacancy in this field, which is meaningful in fabrication of high power FBGs.

ConclusionsThe proposed algorithm and interferometric system can effectively detect the microvibration of the gas-solid interface excited via acoustic radiation. The improved PGC-DCM algorithm can accurately calculate the phase-modulation depth and carrier phase delay. Additionally, the orthogonal interference signal components can be completely normalized, thereby considerably reducing the nonlinear errors caused by the fluctuation of the phase-modulation depth and change in the carrier phase delay. The improved PGC-DCM algorithm eliminates the influence of interference signal visibility on the demodulation results. Furthermore, it realizes the complete normalization of orthogonal interference signal components of the sinusoidal phase modulating interferometer and improves the stability of the PGC-demodulation algorithm. The proposed method can detect the microvibration information of the solid surface. In the frequency range of 3 kHz, the average signal-to-noise distortion ratio of phase demodulation is 33.0956 dB and the dynamic range is 22.75 dB.

ConclusionsIn this paper, an SLR refractive index sensor based on a silver nanoring array is designed, and the optical and sensing performances of this structure are studied from two aspects of simulation and experiment. The optical simulation software FDTD is used to calculate the reflection spectra of the structure under different parameters, explore the optical performance of the structure, and optimize the structural parameters. The experimental part includes the initial preparation and the final test using methanol, ethanol, isopropanol, ethylene glycol, and glycerol. These five solutions with different refractive indexes are used as the test environment to explore the refractive index transmission of the structure. The simulation results show that the structure can be adjusted to the resonance wavelength and refractive index sensitivity. The experimental results of the optimized structure are in good agreement with the simulation data. The final experimental refractive index sensitivity is 663 nm/RIU and the quality factor is 9.2. The proposed structure exhibits potential application value in biosensing detection in terms of preparation and performance.

ConclusionsIn this paper, deep etched GaAs-based micro-nano grating structures are prepared using a combination of electron-beam lithography and ICP etching technology. Electron-beam exposure deforms and distorts the mask pattern for GaAs-based gratings with small periods, long lines, and deep etching depth due to the severe proximity effect. Using thin PMMA A4 resist and SiO2 film as multilayer resist, the spread range of electron scattering is effectively decreased and the proximity effect of an electron beam is reduced. A good photoresist mask pattern is obtained in this scheme, and the SiO2 film is used as a hard mask to achieve deep grating etching. Furthermore, the grass phenomenon during the ICP etching process is eliminated by optimizing the RF power and adjusting the physical etching mechanism. Under optimized process conditions, the grating structure with a period of 1.00 μm, a duty cycle of 0.45, and an etching depth of 1.02 μm is obtained, and the sidewall of the grating is steep and has good periodicity and uniformity. Simultaneously, the ICP etching process’s selection ratio to the SiO2 mask can reach 26.9∶1, allowing for the realization of a GaAs-based micro-nano grating structure with a high aspect ratio. This process provides a reference for the use of electron-beam lithography to prepare deeply etched, high-aspect-ratio GaAs-based micro-nano structures. Finally, the structure is applied to a DBR tapered semiconductor laser, and a laser output with a line width of 40 pm was obtained, indicating that the grating structure created by this process has good model selection performance for the semiconductor laser.

ConclusionsThis paper proposes a composite metamaterial structure based on vanadium dioxide that can dynamically manipulate the phases of THz waves. The simulation results show that when vanadium dioxide is tuned from its insulating state to the metallic state, the structure can achieve a phase shift of more than 140° in the frequency range of 1.151.65 THz. By the use of the calculation of the multipole scattering power and the electric field and surface current distributions at the resonance frequencies, the physical mechanism of the phase shift is investigated. The proposed structure has potential applications in the fields of THz wireless communications, sensing, and security imaging.

ConclusionsA single-/dual-band switchable terahertz absorber based on VO2-BDSs hybrid metamaterial is demonstrated through numerical simulations. When VO2 is in the insulating state and the Fermi level energy of BDSs is 160 meV, two absorption peaks appear at 0.97 THz and 3.152 THz with absorptivity of 99.3% and 99.7%, respectively. When VO2 is in the metallic state and the Fermi level energy remains unchanged, the absorber has an absorption peak at 4.246 THz with the value exceeding 98%. By using the phase change characteristics of VO2,the proposed hybrid metamaterial can switch the absorber from single band to dual band. Due to the existence of BDSs, it is found that the resonance frequency points show a blue shift as the Fermi level energy increases. In order to verify the potential of the proposed metasurface in practical applications, the sensitivity of incident polarization angle is also investigated. Numerical results show that the absorptivity of the proposed absorber remains 90% under large incident angles. The method proposed in this work has potential applications in switching, sensing, and modulation.