Piecewise parabolic phase modulation, in which the optical spectrum linewidth is equal to the product of its chirp rate and period, is believed to be promising in SBS suppression, as it can yield a nearly flat-top optical spectrum. However, no kilowatt-level laser output has been achieved with this scheme because of its high sampling rate dependency on arbitrary waveform generators. In this paper, its high dependency on high sampling rate is theoretically discussed based on fast Fourier transform. We propose and experimentally validate two approaches to mitigate the dependence on sampling rate: extrema clamping, in which the local maxima and minima of the waveform are reassigned, and increasing the modulation depth as a means of further relaxing the sampling rate requirement. We established an amplifier system which includes a piecewise parabola phase-modulated single-frequency fiber laser, a two-stage pre-amplifier, and a counter-pumping main stage, subsequently. Finally, a 7.56-GHz piecewise parabola phase-modulated, 1750-W MOPA fiber amplifier, is experimentally achieved.

Objective In the biomedical field, yellow-green light at 559 nm has demonstrated clear efficacy in treating ocular diseases, acne vulgaris, and other conditions. Meanwhile, as one of the wavelength bands to which the human eye is most sensitive for photopic vision, it also possesses unique application advantages in scenarios such as laser display and laser illumination. However, the conventional method for obtaining 559 nm yellow-green lasers via sum-frequency generation based on stimulated Raman scattering (SRS) technology has rarely been reported in engineering applications due to issues including low conversion efficiency, insufficient single-pulse energy, and poor stability. In this study, an external-cavity Raman laser technology was adopted. Based on a self-developed pump light source, a all-solid-state pulsed laser with high conversion efficiency, high repetition rate, high single-pulse energy, and excellent energy stability was developed through the design of a Raman resonant cavity. Furthermore, an engineering prototype was designed, which exhibits potential for engineering application. Methods This paper reports a 559 nm external-cavity Raman laser based on SRS effect, utilizing a 532 nm green laser as the pump source and selecting a SrWO4 crystal as the Raman gain medium. First, a semiconductor laser was employed to pump an Nd & ratio;YAG laser crystal. By combining intracavity frequency doubling and acoustic-optic Q-switching technology, a pump beam with high repetition rate, high output power, and excellent beam quality was obtained. To enhance the peak power density of the pump beam within the SrWO4 crystal, the pump light was collimated and reduced in beam diameter by a lens group before being focused onto the end region of the crystal. The Raman resonator adopted a plano-concave cavity structure, which offers higher stability and a larger mode volume compared to a plano-plano cavity. Through simulation and calculation of the theoretical transmission model for the external-cavity Raman laser, the optimal first-order Stokes reflectivity was determined. Furthermore, the Raman cavity mirrors were coated with selective films to suppress the oscillation of higher-order Stokes lines, thereby improving the Raman conversion efficiency. Finally, experimental testing of the external-cavity Raman laser was conducted. The measurement results indicate that the laser achieves high Raman conversion efficiency, high single-pulse energy, and excellent energy stability. Results and Discussions Numerical simulations were performed for concave mirrors with different radii of curvature. To achieve optimal mode matching, a concave mirror with a radius of curvature of R = 200 mm was selected as the most suitable. At a drive current of 4.8 A, the pump laser achieved a maximum output power of 2.55 W, with a corresponding output wavelength of 532.1 nm and a pulse width of 11.3 ns. After beam reduction, the spot diameter was 230 mu m. At the maximum output power of 2.55 W, the measured far-field divergence angles were theta(x )=5.24 mrad and theta(y )=5.16 mrad in the horizontal and vertical directions, respectively, with corresponding beam quality factors of M-x(2)=1.37 and M-y(2)=1.23.The oscillation threshold for the 559 nm Raman laser was determined to be 27.70 MW/cm(2). The maximum Raman conversion efficiency reached 51%, with a maximum output power of 1.22 W and a corresponding conversion efficiency of 47.8%. The energy stability was measured to have a root mean square (RMS) value of 3.3%. The central wavelength of the first-order Stokes light was measured at 559.5 nm. The generation of second Stokes light was also observed, which accounts for the decrease in conversion efficiency. The far-field divergence angles of the Raman output were theta(x)=4.77 mrad and theta(y)=4.57 mrad, with beam quality factors of M-x(2)=1.49 and M-y(2)=1.44.However, the beam quality of the generated yellow-green laser exhibited degradation compared to that of the pump source. This phenomenon may be attributed to several factors, including the thermal lensing effect in the crystal, inherent inhomogeneities in the crystal, and assembly-induced stress. The laser module has a dimension of 300 mm & times;115 mm & times;60 mm, demonstrating strong potential for engineering applications. Conclusions Based on external-cavity SRS technology, this study developed an all-solid-state pulsed laser operating at 559 nm with high repetition rate and high efficiency. In the experiment, intracavity frequency doubling technology was adopted, and the plano-convex cavity structure was utilized to optimize the intracavity thermal effect, thereby improving the output performance of the 532 nm laser. A green laser with an average power of 2.55 W and excellent beam quality was obtained as the pump source. The Raman resonant cavity employed a plane-concave cavity structure. A lens group was used to reduce the beam size of the pump light and improve the peak power density; meanwhile, selective coating of the Raman cavity mirrors was implemented to suppress the oscillation of higher-order Stokes light, further improving the Raman conversion efficiency. At a pulse repetition rate of 5 kHz, the experiment finally achieved 559 nm yellow-green light output, with an average power of 1.22 W (corresponding to a single-pulse energy of 242.5 mu J and a pulse width of 6.2 ns). The maximum Raman conversion efficiency reached 51%, and the beam quality factor M2 was better than 1.5. With excellent performance and engineering potential, this laser is expected to provide a high-quality light source for fields such as biomedicine and laser illumination.

To achieve uniform side-pumping of solar multi-disk laser and improve the output power, a solar concentrator system based on a parabolic mirror (f = 850 mm, sunlight-recieving area of 8 m2) and a circular compound parabolic concentrator (CPC) is proposed. Based on the compensation method, the uniform side-pumping of three Nd:YAG disks (Φ17 mm×2 mm) can be achieved. Ray tracing demonstrates that the absorbed pump power of the disk array is 1100 W, and the uniformity of the radial pump light distribution of each disk exceeds 92

Objective The high-brightness beam combining of high-power fiber lasers based on coherent combining and spectral combining technologies is a research hotspot in the international laser field. In these two high-brightness beam combining systems, there are higher requirements for increasing the power of narrow-spectrum (or single-frequency) high-power fiber lasers. Stimulated Brillouin scattering (SBS) is the first barrier that prevents power scaling in narrow-spectrum fiber lasers. Spectral line broadening through phase modulation technology is an important technical means to increase the threshold of the SBS effect and improve the power enhancement of single-channel narrow-spectrum fiber lasers. Among most modulation schemes, piecewise parabolic phase modulation has been numerically shown to excel in SBS suppression by Jeffery O. White and so on. It is shown in their simulations that with the same optical spectral linewidth, this scheme yields an SBS threshold power 33% higher than pseudo-random binary sequence (PRBS) phase modulation does. However, the parameters in this scheme have been barely optimized, and experiments have yet to be performed to verify the validity of these theoretical conclusions. Methods Both numerical simulations and experiments were performed in this research. First, numerical simulations were engaged to seek the optimal modulation parameters in a 10-meter-long passive fiber, and the Euler method was applied to simulate the SBS evolution. Second, in our experiments, a MOPA fiber laser was built and an arbitrary waveform generator (AWG) with a sampling rate up to 25 GSa/s was used to generate piecewise parabolic waveforms. Then, we experimentally measured the SBS threshold power achievable with this scheme, and we applied a method named "Assigning the Waveform" for increasing SBS threshold power. Finally, we also tested SBS threshold power provided by modulations that can provide the same optical spectral linewidth but have different periods to examine the correctness of our simulation results. Results and Discussions Simulation results show that there is an optimal value of modulation half-period, whereby half-periods either higher or lower than this value compromise SBS threshold power in the same linewidth (Fig. 10). The optimal value of half-period is independent of linewidth. For a 10-meter-long fiber, this optimal value is 12.2 ns, and this value increases for longer fibers. A high sampling rate of the AWG is required to experimentally realize this modulation scheme. Without sufficient sampling rate, a central spectral peak emerges in optical spectrum and compromises SBS suppression, even when we attempt to broaden the linewidth, the SBS threshold power still decreases. The method termed "Waveform Assignment" was experimentally verified to be capable of increasing the threshold power by approximately 160 W without requiring a higher sampling rate. Furthermore, the optimal range of the modulation half-period was experimentally demonstrated to be 5 ns to 20 ns. Conclusions We have theoretically discussed the sampling rate requirement of piecewise parabolic phase modulation, as well as the key parameters (chirp rate beta and modulation half-period tau). Experimentally, we have demonstrated that the half-period tau should be neither too high nor too low, but should be around 20 ns for a 20-meter-long fiber, which fits well with numerical simulations. To reach a nearly ideal top-hat optical spectrum, extremely high sampling rate of an AWG is required. Otherwise, a central peak appears in the spectrum and compromises SBS threshold enhancement. Two approaches can be adopted to suppress such a central spectral peak. One approach is to increase the modulation depth k(p) to 2 pi, 3 pi, etc. The other is to modify the folded piecewise parabolic waveform by setting every local maximum to kp and every local minimum to-k(p). The combination of these two approaches can effectively suppress the central spectral peak without requiring an extremely high sampling rate. However, the first approach is currently not feasible with our experimental equipment, and only the second approach is applied in our experiments. Using a high-finesse spectrometer, we verified that optical spectra generated by piecewise parabolic phase modulation with identical 219 tau values are nearly identical. Optical spectra measured in experiments still show high central peaks, and we attribute this to the insufficient waveform fidelity of the signal amplified by the RF driver. We further attribute the reduced SBS threshold enhancement factor (relative to the numerical simulation predictions) to this issue. Even though the threshold power is evidently lower than theoretical prediction, a phenomenon predicted by numerical simulations-that both high and low half-periods yield an SBS threshold lower than the "optimal" value-appears in our experiments. Finally, selecting tau =20 ns and 2 beta tau =6 GHz, we experimentally demonstrated a fiber amplifier that was phase-modulated by a piecewise parabolic signal, achieving an output power of 802 W with a FWHM spectral linewidth of 5.88 GHz.

Power scaling of high-power narrow-linewidth continuous-wave fiber lasers is limited by stimulated Brillouin scattering (SBS). While inversion probability-tunable (p-tunable) sequence modulation offers dynamic spectral control for enhanced SBS suppression, its practical implementation has been hindered by reliance on complex, expensive arbitrary waveform generators (AWG) or significant hardware complexity (particularly demanding logic resource requirements) of conventional field-programmable gate array (FPGA)-based implementations. This work introduces what we believe to be a novel, hardware-efficient approach to overcome these limitations. We present a hardware-efficient parallel linear feedback shift register (LFSR) architecture with comparator probability adjustment, enabling real-time, precise p-value control at 10 GHz clock rates, significantly reducing cost and complexity. Concurrently, we establish a quantitative acoustic-photonic dynamics model based on a time-dependent three-wave coupled equation, solving the nonlinear interactions among signal laser photons, phonons, and Stokes waves under p-tunable modulation. This model enables systematic optimization, theoretically predicting and experimentally confirming maximal SBS threshold enhancement at p = 0.58. Leveraging this optimized parameter, our system achieves 2.82 kW output power with a 10.34 GHz FWHM linewidth. Compared to systems using pseudo-random binary sequence (PRBS) modulation (achieving 2.46 kW at ∼10 GHz FWHM linewidth), this represents a 13% increase in output power. This integrated hardware-theory solution provides a practical and effective approach for SBS suppression in high-power narrow-linewidth fiber lasers, enabling higher power scaling and stable operation.

Objective Fiber lasers are widely used in industrial processing, defense, military, and numerous other fields due to their compact structure, high conversion efficiency, and excellent beam quality. However, the output power of a single fiber faces theoretical limitations imposed by mode instability and nonlinear effects. Spectral beam combination (SBC) currently serves as an effective approach to achieve breakthroughs in high power and high beam quality. In spectral beam combining systems, beam quality degradation of the combined beam can result from single-beam spectral linewidth, lens aberrations, grating thermal distortion, and fiber array pointing deviations. While prior research has provided thorough analyses of these factors, the impact of 3D pointing deviations, including those along the grating line direction, on combined beam quality remains unexplored. Considering pointing deviations of the combined beam along the grating line direction, this study establishes a theoretical beam propagation model for 3D spectral combining systems. It investigates the effects of displacement and pointing deviations in single-fiber and multi-fiber configurations on the beam quality of the combined laser. The results provide theoretical guidance for refining fiber array machining precision and alignment control requirements, thereby effectively guiding system design and mitigating the impact of random errors on system performance. Methods The theoretical beam propagation model integrates spatial propagation matrices and Fresnel diffraction theory to derive the optical field distribution at various stages of the combining process. By accounting for both displacement and angular deviations in single-fiber and multi-fiber configurations, the analysis comprehensively reveals how these errors propagate through the system and degrade final beam quality. The results demonstrate that even minor misalignments can lead to significant degradation in beam quality, highlighting the need for precise control over fiber array positioning and beam pointing. This study develops a theoretical model based on the single-grating spectral combining principle, incorporating 3D beam propagation dynamics. The optical field distribution at various stages of the combining system is derived using spatial propagation matrices and Fresnel diffraction theory. The combined optical field is then obtained through incoherent superposition of individual beams, allowing for detailed analysis of the irradiance distribution at different propagation distances. From the output optical field function, the beam radius evolution is determined, and the beam quality M2 factor is calculated using the second-moment curve fitting method applied to measured spot radii at multiple propagation planes. To investigate the statistical characteristics of beam array perturbations, the study employs a large-scale independent random distribution simulation approach. This statistical analysis reveals the relationship between beam quality degradation and both in-plane positional deviations and angular misalignments in the non-diffracting plane. The simulation framework enables a systematic investigation of how random errors in fiber positioning and beam pointing affect the overall combining efficiency and output beam quality. The results provide quantitative tolerances for fiber array alignment, offering practical guidance for system design and optimization. Results and Discussions This study investigates a 7-channel spectral beam combining system, comprehensively analyzing the effects of in-plane positional deviations and pointing angle errors on beam quality under both single-beam and multi-beam perturbation conditions. The key findings reveal: (1) When just only one single fiber exhibits deviations, the combined beam quality degrades significantly with millimeter-level positional offsets or milliradian-level angular misalignments, demonstrating the system's high sensitivity to minor alignment errors in individual fiber arrays. (2) For scenarios where all seven fibers experience random perturbations, maintaining a combined beam quality M2 factor below 1.2 requires stringent control measures: positional deviations along the grating line direction must be constrained within 0.18 mm per fiber, while angular deviations between actual and ideal propagation directions need to be limited to 50 mu rad per fiber. (3) A particularly noteworthy comparative analysis shows the system exhibits significantly greater sensitivity to angular deviations than to positional errors. This critical insight provides essential guidance for system design, indicating that in certain applications, prioritizing beam pointing accuracy over lateral position optimization proves more effective for enhancing system performance. Conclusions This study provides a detailed simulation-based analysis of how angular and positional errors along the grating line direction affect beam quality in spectral beam combining systems. By incorporating 3D pointing deviations, the proposed model offers a more realistic representation of alignment errors in practical spectral beam combining systems. The results highlight the necessity of precision engineering in fiber array fabrication and alignment to mitigate random errors and achieve high-quality laser output. The insights gained from this research serve as a theoretical foundation for optimizing spectral beam combining systems, ensuring robust performance in high-power laser applications. Future work may explore adaptive optics or real-time feedback control mechanisms to further compensate for alignment errors and enhance beam combining efficiency. By refining machining precision and alignment control based on these findings, engineers can develop more reliable and efficient spectral beam combining systems for advanced laser applications in industrial, scientific, and defense sectors. The continued advancement of this technology will play a crucial role in pushing the boundaries of high-power laser performance while maintaining excellent beam quality.

The spectral linewidth characteristics of BBO-optical parametric oscillator (OPO) with a volume Bragg grating (VBG) output coupler were investigated in theoretical simulation and experimental validation. Superior performance in narrowing the linewidth of OPO signal light could be obtained when lower reflectivity VBG was employed. In a concave-VBG cavity, pumped by a 532.23 nm single-longitudinal -mode (SLM) laser at 1 kHz repetition rate with maximum power of 6.3 W, 1.2 W power of 972.26 nm signal light with 3 pm linewidth was achieved when a VBG with 80% reflectivity was used. The pulse duration was around 2.2 ns, and the optical conversion efficiency of OPO signal light was up to 19%. Finally, 486.13 nm blue laser with 1.4 pm spectral linewidth was obtained by using one single LBO crystal as the second harmonic generator.

High-power narrow-linewidth fiber lasers combine compact structure, efficient thermal management, and high electro-optical conversion efficiency, making them highly attractive for applications such as coherent beam combining and nonlinear frequency conversion. However, further power scaling is severely constrained by nonlinear effects, particularly stimulated Brillouin scattering (SBS), during power amplification. In this work, we propose a high-order phase modulation scheme based on binary sequences with a controllable inversion probability to achieve tunable spectral broadening of the seed laser, thereby effectively suppressing the SBS effect and enhancing the amplifier output power. An inversion-probability-tunable sequence generation method based on a linear feedback shift register (LFSR) is developed and experimentally implemented. Using a three-stage fiber amplifier, a maximum output power of 3.0 kW is achieved with an 11.4 GHz FWHM linewidth. Under the same spectral broadening bandwidth, the output power is increased by 16% compared with pseudo-random binary sequence (PRBS) modulation and by 25% compared with conventional random-number-based p-tunable sequence modulation.

We propose a kind of side pumping configuration with a three-stage solar concentrator to improve the output power of solar-pumped single crystal fiber (SCF) laser. The primary concentrator system consists of 10 off-axis parabolic mirrors, and the secondary and third-stage concentrators are 10 two-dimensional composite parabolic concentrators (2D-CPCs) and the 2V-shaped reflectors, respectively. Sunlight is reflected multiple times in the 2V-shaped reflector and is absorbed by a Nd : YAG SCF (∅1 mm × 80 mm) for laser oscillation. Ray tracing demonstrates that the solar power absorbed by the SCF reaches 132.7 W. We solve the rate equation and transmission equation, the obtained results show that the laser output power, slope efficiency, and solar-laser conversation efficiency are 46.3 W, 38.9

In this study, the optimization of welding parameters, pore defects, microstructure, and mechanical properties of Ti6Al4V welded joints subjected to ultrasonic-assisted laser welding was studied. The optimal parameter combination and the most significant factor influencing the tensile strength of the joint were determined by three-factor and three-level Taguchi experiments with range and variance analysis methods. A control experiment was set up under the optimal parameters to reveal the reason for ultrasonic influence on joint strength from the pore defects and microstructure. The control test showed that the cavitation effect and acoustic streaming effect of ultrasound could significantly improve the weld defects and microstructure. Compared with no ultrasound, the porosity of the welded joint decreased from 3.06 to 0.08

Objective Marine lidar is widely adopted for applications such as seawater optical-parameter profile detection, coastal-zone mapping, marine-resource surveys, and marine-environment monitoring. It offers high detection accuracy, wide measurement coverage, and high measurement efficiency and flexibility. The blue-green spectral band (420-580 nm) exhibits the lowest attenuation in seawater, thus establishing it as the optimal emission source for marine lidar systems. In the coastal seawater region, the optimal light-transmission wavelength of seawater is 520-580 nm. In clear oceanic water, the optimal wavelength for laser detection is 420-510 nm. Owing to advancements in light-source technology, the current wavelength of marine lidar light sources is typically 532 nm, which is not optimal for applications in ocean waters. Additionally, the Fraunhofer dark line (H-beta line) of the solar radiation spectrum is located in the seawater optical window, with a central wavelength of 486.13 nm and a spectral linewidth of approximately 0.14 nm. Therefore, the 486.13 nm wavelength of blue light is suitable for application in clear oceanic water. By restricting both the transmitting-laser linewidth and receiver optical-filter bandwidth to <= 0.1 nm (smaller than the H-beta dark line bandwidth), solar background noise can be effectively suppressed during signal acquisition, thereby enabling a high signal to noise ratio (SNR). Additionally, the 486.13 nm blue laser radar shows significant advantages in improving the depth (range) of ocean detection and in enhancing the SNR of echoes. Methods The 532 nm green laser radar enables hyperspectral resolution detection, with the frequency stability of its 1064 nm seed light source maintained below 10 MHz during long-term operation. To extend such hyperspectral detection capabilities to 486.13 nm blue laser lidar for oceanic applications, the laser source must exhibit a high peak power output, a single-frequency narrow linewidth, and high frequency stability. The most widely adopted approach to achieve these characteristics is optical parametric oscillation/ parametric amplification (OPO/OPA) techniques based on single-frequency seed injection, which has been verified to be applicable to oceanic detection lidar systems. This implies that the stabilization of the single-frequency seed laser is a critical performance metric for the overall laser system. Thus, this study proposes a compact frequency stabilization system for seed lasers using the Pound-Drever- Hall (PDH) technique. The system is designed to lock a 972.26 nm laser with high stability within a Fabry-Perot (F-P) cavity and operates reliably at room temperature. By employing this method, the system does not need to identify atomic transition spectral lines corresponding to the laser wavelength for stabilization. Consequently, a stable 486.13 nm blue laser can be achieved through frequency doubling, which fulfills the spectral linewidth and stability criteria for the hyperspectral detection marine-radar blue laser source. Results and Discussions The frequency stabilization system based on PDH technique proposed in this study utilizes the resonant frequency of the F-P cavity as the reference standard (Fig. 1). The distributed feedback single-frequency laser diode (DFB-LD) with an output wavelength of 972.26 nm is segregated into two beams using a fiber beam splitter. One beam serves as the seed light output, whereas the other undergoes phase modulation via an electro-optic modulator (EOM), thus yielding a symmetrical double-sideband spectrum (Fig. 2). The EOM is driven by a 20 MHz radio frequency (RF) signal from the circuit system to modulate the laser phase. The laser output from the modulator is injected into the resonant cavity through the coupling lens (lens 1) to achieve mode matching. The radius of curvature of the F--P cavity is R=50 mm, the free spectral range of the F--P cavity is 1.5 GHz, and the fineness is 1500. The laser oscillates repeatedly within the F--P cavity to form a stable resonance signal. The resonance signal emitted from the resonant cavity is detected by the high-speed photodetector (PD). Subsequently, the frequency error signal is acquired using heterodyne spectrum--detection technology. Using this error signal, the feedback control system swiftly adjusts the laser drive--current parameters and locks the output wavelength of the DFB-LD at the resonant frequency of the F--P cavity, which is precisely maintained at 972.26 nm. This procedure ensures the high--frequency stabilization of the laser system. Conclusions Based on the specific requirements of hyperspectral detection marine lidar systems for seed laser performance, this paper presents a highly robust frequency--stabilized and frequency--locked laser operating at 972.26 nm. This system utilizes PDH frequency--stabilization technology with a fiber connector to achieve precise control. A notable feature of its design is its compact architecture: while preserving critical components such as the optical fiber collimator, coupling lens, and resonator, all other components employ fiber--based input/output configurations. This design approach significantly simplifies the optical path, reduces system volume, and facilitates integration and alignment. The system utilizes the resonant frequency of the confocal cavity as the frequency reference standard. At room temperature, the root--mean--square value of the 30--min frequency jitter of the laser output is less than 25 MHz, and the corresponding Allen variance exceeds 4x10-8, thus fully satisfying the requirements of the seed light source for hyperspectral lidar. The frequency stability and frequency--locking output of a 486.13 nm blue laser can be achieved using a periodic polarization lithium niobate (PPLN) frequency--doubling crystal. Consequently, the frequency stability of the blue laser surpasses several tens of megahertz. The 972.26 nm frequency--stabilized and frequency--locked laser developed in this study establishes a robust technical foundation for the advancement of hyperspectral detection marine blue--laser radar.

To expand the application range of all-solid-state single-frequency lasers and achieve high-power single-frequency laser output, this study uses dual piezoelectric ceramics (PZT) resonant detection combined with a variable reflectivity unstable cavity and side pumping module composed of a laser diode (LD) array to develop an all-solid-state, electro-optical, Q-switched, and single-frequency pulse laser with a repetition frequency of 100 Hz. This study achieves a stable, high-beam quality, high-power, and narrow-linewidth laser output. The experimental results show that when the oscillator cavity length is 300 mm, the repetition frequency is 100 Hz, pump current is 160 A, and pump pulse width is 300 mu s. Thus, a stable, narrow-linewidth laser output with an average power of 10.06 W is achieved, corresponding to a pulse width of 6. 6 ns, linewidth of 93. 874 MHz, and beam quality factor of M-x(2)= 1. 510, M-y(2) = 1. 674. The laser can be used as a stable, narrow-linewidth pulse laser light source for detection equipment, such as Doppler wind radars.

A non-iterative pre-distortion algorithm is presented for linear frequency sweep in FMCW LiDAR using directly-modulated semiconductor lasers. This method achieves a residual frequency nonlinearity of ± 0.6 MHz and an accuracy of 18.7 cm for 300 m ranging distance.

Triangular stereo model and phase-height model are two commonly used methods in the field of 3D reconstruction. The former requires complex and time-consuming system calibration, and the projector’s nonlinear errors can significantly affect measurement accuracy. The latter requires strict geometric constraints or the use of a high-precision displacement platform to establish the relationship between phase and height for 3D reconstruction. This paper proposes a 3D reconstruction model based on a parallel-axis-display screen system, composed of a projector, camera, and display screen. The model uses the display screen as a reference plane and only requires that the optical axes of the projector and camera be parallel to each other and perpendicular to the display screen. This geometric condition is easy to achieve, and there is no need to establish the relationship between phase and height. Additionally, the model effectively addresses the projector’s nonlinear response errors. For any point in space, its coordinates can be calculated based on the geometric relationships in the system, without the need for complex system parameters. Experimental measurements of standard gauge blocks and sphere confirm the validity of the proposed parallel-axis-display screen system.

Significance The high-power fiber laser technology is currently one of the rapidly advancing laser technologies, primarily due to its outstanding performance in various fields, such as communication, research, industry, and defense. Progress Based on the development trend of the laser technology, the Shanghai Institute of Optics and Fine Mechanics (SIOM) took the lead in researching the fiber laser technology in China and decided that two key technologies are the high-power narrow linewidth fiber laser technology and high-power fiber laser beam combining technology. Based on the optical fiber-based laser properties, the narrow-linewidth fiber laser output power must be increased to suppress different nonlinear effects. This increase depends on the single frequency seed spectrum modulation combined with the multistage power amplifier technology, which can maintain the spectrum width and realize beam quality during the single-fiber laser power amplification process. For high-power fiber laser beam combining, SIOM first proposed and studied the all-fiber ring cavity coherent beam combining technology and spectral beam combining technology in China. This paper reviews the progress in recent years. SIOM has overcome a series of technical difficulties and achieved a series of achievements, which has driven the promotion and development of these technologies. Conclusions and Prospects In the future, the narrow-linewidth fiber laser technology can be used as an effective light source for the development of fiber lasers and beam combining technology. The main problem of nonlinear width fiber laser continues to be nonlinear effects: SBS and TMI. However, these may be improved via three aspects: using the new optical fiber material technology, new spectral modulation technology, and polarized high-power narrow-linewidth fiber laser technology. The future demand for higher power and excellent beam quality is expected to be the comprehensive application of different beam combining technologies, complementing each other. Of course, suppressing the nonlinear effect of the laser, developing high-performance devices, and combining cavity structure optimization and beam quality maintenance technology are the basis of the development of the high-energy fiber laser technology. At present, the narrow-linewidth fiber laser combined with the laser beam combining technology is the preferred solution of the high-energy light source.

A high repetition rate, high peak power, narrow linewidth nanosecond 589 nm laser based on sum -frequency generation (SFG) of 1064 nm and 1319 nm laser is demonstrated. A 1064 nm single-frequency distributed-feedback laser diode (DFB-LD) externally modulated by an acousto-optic modulator (AOM) and amplified by a fiber-solid hybrid amplifier with three-stage fiber amplifier and six-stage solid-state end-pump Nd:YVO4/Nd:YAG amplifier, a high beam quality laser output with pulse energy of 20 mJ at repetition rate of 500 Hz is achieved, which pulse duration and waveform are adjustable and editable. The ramp-hold-fire detection and dual piezoelectric ceramic (PZT) feedback compensation are employed in the 1319 nm oscillator to achieve high stability and low jitter laser. Then the 1319 nm laser pulse is amplified to 12.2 mJ by three -stage end-pumped Nd:YAG amplifier, with pulse duration of 70 ns. By sum-frequency of the 1064 nm and 1319 nm lasers, finally, 589.1595 nm laser with pulse energy of 10.4 mJ and pulse duration of 62 ns is generated efficiently, the laser beam quality factor M2 is less than 1.3. With the ability of switching frequencies quickly and the characteristic of compatible frequency-stabilized, this laser is believed to be a very potential solution for sodium Doppler lidar.

A 1319 nm single frequency nanosecond pulsed laser based on injection seeded is demonstrated. The laser oscillator is injection seeded by a 1319 nm single-frequency narrow linewidth Nd:YAG nonplanar ring oscillator (NPRO), a laser pulse of repetition rate of 500 Hz, pulse energy of 2.3 mJ, pulse width of 70 ns, and jitter of <3 ns is obtained based on ramp-hold-fire resonance detection technology. Then, through a four-stage end pump laser amplifier, the pulse energy is amplified to 15.4 mJ, with a beam quality factor of M2<1.5.

The narrow linewidth nanosecond pulse 1319 nm laser used to generate 589 nm laser is the core technology of sodium Doppler lidar. Here, a high gain fiber-solid hybrid amplifier seeded by a 1319 nm distributed-feedback laser diode (DFB-LD) is proposed to achieve high-power amplification of small signal 1319 nm pulse laser. A pulsed 1319 nm laser with a repetition frequency of 500 Hz, a pulse duration of 500 ns, an energy of 10.3 mJ and a linewidth of 1.9 MHz was developed by utilizing a three-stage fiber coupled end-pumped amplifier and a two-stage Innoslab amplifier, and the beam quality factor M2 is less than 1.6, the gain of the solid amplification system exceeds 28 dB.

To satisfy the strict requirements of semiconductor lasers in terms of wavelength stability, phase noise, and other indicators in precision measurements and similar applications, an analysis method based on the combination of system transfer function theory derivation analysis and simulation verification is proposed. We designed a semiconductor laser diode drive circuit with a smaller wavelength drift and ultra-low noise. For an alternating current small-signal model and alternating current path, we theoretically analyzed the correlation between the circuit parameters and system stability. The circuit design was optimized by introducing a noisesuppression network. Using Tina-TI simulations, the drive circuit loop noise is effectively suppressed, and the system stability improves to a bandwidth below 2 MHz. Experimental results reveal that the effective value of alternating current noise is approximately 224 nA and 1. 9x10(-6) for the direct current ripple between 3 kHz and 2 MHz in 2. 5 h. When the 1 h optical power integration time is 1 s, the stability is 1. 177x10(-5). These results verify the theoretical model and simulation analysis. Notably, this approach provides a universal guideline for analyzing and designing ultra-low noise current drivers for semiconductor laser diodes.

Objective Diode -pumped solid-state lasers (DPSSLs) are widely used in many applications owing to their high energy, high repetition rate, and high efficiency. The gain medium is one of the core components of the DPSSL system; however, when the gain medium is subjected to a high -power pumping source, an uneven heat source is formed in it, resulting in an uneven temperature distribution. Furthermore, the cooling device can only dissipate heat to its surface, which in turn generates temperature gradients in different directions. The thermal deformation and stress caused by the temperature gradient in the gain medium eventually degrade the laser output power and beam quality. In this study, the finite element analysis (FEA) is used to optimize the design of a microchannel heat sink for laser amplifier cooling, and the effects of parameters such as microchannel bottom -plate thickness, channel height, channel width, channel wall thickness, and inlet velocity on the maximum temperature of the gain medium surface are investigated. The results are expected to provide accurate guidance for practical experiments.Methods To study the ability of the microchannel heat sink to cool the laser amplifier, a full-size model containing a slab -type gain medium and a microchannel heat sink is established (Fig. 1). The uppermost layer is the gain medium, the middle layer is the microchannel heat sink, and the bottom layer is the cover plate. The flow and convection -diffusion phenomena occurring in the inhomogeneous heat and microchannel heat sink within the slab -type gain medium are then studied by a flow -heat -solid multiphysical field coupling analysis. Finally, the coupling of heat and fluid under full-size model conditions is directly simulated using the ANSYS FLUENT module, in which the heat source within the gain medium is loaded through the UDF command. The pressure -velocity coupling is achieved using the SIMPLE algorithm, the flow parameter interpolation method is second -order windward, the heat sink material is purple copper, and the cooling medium is deionized water.Results and Discussions Pulsed pumping can be approximated as continuous pumping when the repetition rate is high (Fig. 3). When cooling the gain medium using a microchannel heat sink, the entire system can be simulated using the ANSYS FLUENT module under the set initial parameters (Fig. 4), and the distribution of thermal deposition in the gain medium can also be determined at this time (Fig. 5). When the microchannel bottom -plate thickness increases from 1 mm to 5 mm, the maximum temperature of the gain medium surface also increases (Fig. 6). The bottom -plate thickness cannot be too small, given that microchannel thermal deposition will be deformed by heat and must withstand a certain water pressure. Therefore, the optimal value of the bottom -plate thickness is set as 2 mm. When the channel height increases from 2 mm to 4 mm, the maximum temperature decreases significantly; when the height increases from 4 mm, the maximum temperature decreases, but the decrease is not significant (Fig. 7). Therefore, a channel height of 4 mm is selected. In the process of increasing the width of the channel from 0.3 mm to 1.2 mm, the maximum temperature and thermal resistance increase gradually. The pressure loss decreases significantly when the channel width increases from 0.3 mm to 0.6 mm, and the pressure loss decreases slowly when the channel width continues to increase from 0.6 mm. However, the pressure loss increases sharply when the width of the microchannel is too small. Therefore, the channel width is set at 0.4 mm (Fig. 8). The lowest temperature is observed on the surface of the gain medium when the channel -wall thickness is 0.3 mm (Fig. 9). The inlet velocity also affects the temperature of the gain medium surface (Fig. 10). When the flow rate increases from 0.5 m/s to 3 m/s, the temperature of the gain medium surface decreases, but the pressure loss increases. Therefore, the flow rate of 2.5 m/s is optimal. The equivalent heat transfer coefficient of the microchannel system is also calculated under the premise that the temperature of the system is known (Fig. 11). This value can be used to measure the cooling capacity of the microchannel cooling system under different parameters (Fig. 12). The analysis shows that the equivalent heat transfer coefficient of the microchannel system is up to 5000 W/(m2 center dot K).Conclusions In this study, the temperature distribution characteristics of a high -repetition -rate and high-energy conduction cooling laser amplifier are numerically simulated by finite element analysis. The effect of each parameter within the heat sink of the microchannel on the maximum temperature of the gain medium surface is systematically discussed and analyzed, and the values of each parameter are optimized from the perspective of practical application and safety. The maximum temperature of the gain medium surface is the lowest when the bottom -plate thickness is 2 mm, channel height is 4 mm, channel width is 0.4 mm, and channel wall thickness is 0.3 mm. On this basis, the effect of the inlet velocity of the cooling fluid is further analyzed. The results show that the inlet velocity is not as high as possible, but its value is suitable; an extremely high inlet velocity is not conducive to a significant reduction in the surface temperature of the gain medium and will also cause a large pressure loss. Finally, the maximum temperature of the gain medium surface for specific microchannel parameters is determined. The equivalent heat transfer coefficient obtained can accurately and clearly reflect the cooling capacity of the microchannel system. The calculation results of this study can provide a strong numerical basis and theoretical foundation for practical fabrication and experiments on microchannel heat sink structures for slab laser amplifiers.