Results and Discussions It can be seen from the simulation results of two-stage SPGD and conventional single-stage SPGD algorithms under 64- and 100-beam conditions that the convergence speed of two-stage SPGD algorithm is significantly faster than that of conventional SPGD algorithm (Fig. 4). When the same standard deviation is reached, the number of iterations of the two-stage algorithm is reduced by 56.6% (64 beams) and 67.9% (100 beams) compared with single-stage SPGD (Table 1). Simulation results show that the number of iterations of single-stage SPGD algorithm increases rapidly with the increase in beam number and reaches 9522 for 1000 beams, while the number of iterations of two-stage SPGD for 1000 beams is 107.25 (Fig. 5). Using multi-stage SPGD algorithm, the number of iterations is reduced by 98.87%. The optimal number of stages for different number of beams is given (Fig. 8). In the experiment, an optical phased array system is built, and a two-stage SPGD algorithm is used to lock the phase of 16 beams. When the closed loop is on, the optical signals of the first and second stages increased rapidly to a stable state (Fig. 10), which proves that the multi-stage SPGD algorithm can realize the phase locking of beams. And the phase locking performances using single-stage SPGD and two-stage SPGD are compared under the conditions of 8/12/16/32 beams respectively. The results show that two algorithms have almost the same effect when the number of beams is small, but with the increase of the number of beams, the convergence speed of the two-stage SPGD algorithm becomes faster than that of the single-stage SPGD algorithm. For 32 beams, the multi-stage algorithm reduces the number of iterations to 688.7, which is 59.6% of that of the single-stage algorithm (Fig. 12).

In the process of space laser communication networking, optical phased array (OPA) can replace the traditional mechanical turntable to realize the lightweight and miniaturization of laser terminals, and can quickly switch links among different terminals. The space laser communication network based on optical phased array will be the inevitable trend of the future development. In OPA technology, it is necessary to ensure the same phase of all the beams split from the same laser source. Therefore, the monitoring part is set to obtain the phase of beams to do the phase locking. Considering a large number of beams, it is not practical to calculate the phase directly, so it is suitable to use iterative algorithm to calculate the compensation. At present, there are many algorithms in the field of multivariable control, and stochastic parallel gradient descent (SPGD) algorithm has more advantages in convergence speed and effect, but with the increase of the number of OPA elements, the convergence speed of SPGD becomes significantly slower. There exist some optimized SPGD algorithms now, such as AdmSPGD, AdaDelSPGD and other schemes, which have optimized the step size, the gain parameter and other coefficients of SPGD, and improved the speed to a certain extent. In the experiments using SPGD algorithm, the largest number of beams in the array reported is written by the 107-channel fiber laser coherent synthesis based on SPGD algorithm. As the number of beams increases, the structure becomes more complex and the performance of the algorithm must be higher. Nowadays, OPA has been widely used in laser communication and lidar where there is always a large optical aperture and a large number of array elements are required. When the number of array elements reaches thousands, only optimizing the parameters of SPGD algorithm does not change the essence of control of all array elements based on one single evaluation parameter, which has limited performance improvement. Therefore, it is necessary to optimize the SPGD algorithm in a deeper level for the situation of large-scale array.

In this study, based on the principle and convergence of conventional SPGD algorithm, a multi-stage SPGD algorithm for OPA with a large number of elements is proposed. Different from optimizing the parameters of SPGD algorithm, the beams of multiple array elements are divided into several stages, and there are groups in each stage. The perturbation phase of all the stages is added to the beams, then the phase locking of beams at each stage is made, and finally the phase of all the beams is locked. The corresponding phase adjustment is carried out by monitoring the change of the combined beam power of each stage and the change of the total combined beam power.

Based on the conventional SPGD algorithm, a multi-stage SPGD algorithm with better performance is proposed, which can achieve faster convergence under the condition of large-scale OPA. The core of the multi-stage SPGD algorithm is to group the phased array beams and add multi-stage perturbation, which can realize the global phase control as well as the local phase compensation. In this paper, the principle and flow of multi-stage algorithm are introduced, and simulation is carried out. The results show that the multi-stage SPGD algorithm is more advantageous than the conventional one, and the larger the number of beams is, the more obvious the effect is. When the beam number is 1000, the multi-stage SPGD algorithm reduces the number of iterations by 98.87%, and the optimal number of stages of the multi-stage SPGD algorithm for different beam numbers is given. Finally, an OPA experimental system is built to verify the feasibility of the multi-stage SPGD algorithm. The experimental results show that the multi-stage SPGD algorithm can realize phase locking in the case of 8/12/16/32 beams, and has fewer iterations compared with the conventional single-stage algorithm. Especially, the number of iterations can be reduced to 59.6% of that of the single-stage algorithm for 32 beams. In the actual application, the elements of OPA can be divided to several groups to connect different terminals at the same time, and the multi-stage SPGD algorithm fits well with the requirements, which can adjust the phase of global and local beams. The research is of great significance to the development of phase control technology of large-scale OPAs.

Results and Discussions It is found that the threshold pump power is 2.1 W, which is approximately 40% lower than that previously reported for a thulium-doped random fiber laser with the same pump wavelength and nearly the same lasing wavelength. At the threshold power, the laser output spectrum contains many stochastic narrowband wavelength components or spikes (Fig. 2). When the pump power slightly exceeds the threshold, a quasi-continuous wave is generated. Based on previous studies, the generation of stochastic spikes is related to the combined effects of distributed Rayleigh scattering and cascade-stimulated Brillouin scattering, while nonlinear spectral broadening arising from nonlinear interactions, such as frequency mixing and cross-phase modulation, can broaden and superimpose the spikes and further suppress the stimulated Brillouin scattering process. Mode competition and hopping are observed between two laser modes with different peak wavelengths of 1951.2 nm and 1951.4 nm when the pump power is approximately 3.5 W and 4.5 W, respectively (Fig. 3). The output power increases almost linearly with the pump power, with a slope efficiency of 3.9% (Fig. 4). When the pump power reaches 6.0 W, the laser output power is 142.9 mW, the optical-signal-to-noise ratio is up to 43 dB, and the wavelength and power fluctuations of the output laser within 1 h are less than 0.1 nm (Fig. 5) and 3.7 mW (Fig. 6), respectively, demonstrating good stability. The relatively low slope efficiency is predominantly caused by the high loss of the pump laser power from the output to the net injection into the thulium-doped fiber, of approximately 7.5 dB. The mismatching fiber connection introduces the high loss, and therefore, this problem may be resolved by customizing a WDM made of fibers whose parameters match those of fibers to be connected, i.e., the output fiber of the pump laser and the thulium-doped fiber. If the insertion loss is minimized, the slope efficiency of the laser may reach 20.6%, which is close to the slope efficiency of ordinary thulium-doped fiber lasers.

The 2 μm wavelength band is an eye-safe optical band that contains the absorption lines of water molecules and many atmospheric molecules as well as the spectrum windows for atmospheric communications. Therefore, fiber lasers in the 2-μm-band have been widely studied for their potential applications in fields such as lidar, laser ranging, and surgery. Currently, random fiber lasers (RFLs) have attracted extensive research attention owing to their simple structure and low spatial/temporal coherence properties. These fibers usually utilize back Rayleigh scattering as the random feedback mechanism, combined with the distributed Raman or Brillouin gain in the fiber, to generate random lasers in a long single-mode fiber. However, the Rayleigh scattering intensity of the single-mode fiber is inversely proportional to the fourth power of the wavelength, and the transmission loss is as high as 30 dB/km at 2 μm. Compared with the common RFLs operated at 1.0-1.5 μm, RFLs operated within the 2-μm-band are rarely reported, as they are highly difficult to realize. In this work, an RFL operated in a 2 μm region is demonstrated using a thulium-doped fiber as the gain medium and a random fiber grating for random distributed feedback with enhanced Rayleigh scattering efficiency. A laser output of wavelength 1951 nm is achieved with a relatively low pump threshold of 2.1 W.

The proposed RFL is formed using a 793 nm pump laser, 793 nm/2000 nm wavelength-division multiplexer, fiber loop mirror, 1.5-m-long thulium-doped fiber, and random fiber grating. The random fiber grating containing over six thousand index modulation points along a 10-cm-long single-mode fiber is inscribed using a femtosecond Ti:sapphire regenerative amplifier, which is operated at a wavelength of 800 nm with a repetition rate of 100 Hz and a pulse duration of 80 fs. The neighboring refractive index modulation points are spaced at random distances between 7.5 μm and 12.5 μm. The random fiber grating provides an enhanced backward Rayleigh scattering, equivalent to that achieved from a several-km-long optical fiber. The thulium-doped fiber provides strong amplification, and the fiber loop mirror helps to form a half-open laser cavity structure. Therefore, a low pump threshold is expected for the proposed RFL.

In this work, a thulium-doped fiber random laser operated at 1951 nm is demonstrated using a random fiber grating as the distributed random feedback medium. Owing to the enhanced Rayleigh scattering provided by the random fiber grating, a random fiber laser with a relatively low threshold power of 2.1 W is achieved, which is approximately 40% lower than that of the previously reported thulium-doped random fiber laser with the same pump wavelength. When the pump power reaches 6.0 W, the output power of the random laser reaches 142.9 mW, and the optical-signal-to-noise ratio is up to 43 dB. The stability of the laser output is relatively good, as the wavelength shift and power fluctuation are less than 0.1 nm and 3.7 mW, respectively, over the testing period of one hour.

Results and Discussions It is found that the threshold pump power is 2.1 W, which is approximately 40% lower than that previously reported for a thulium-doped random fiber laser with the same pump wavelength and nearly the same lasing wavelength. At the threshold power, the laser output spectrum contains many stochastic narrowband wavelength components or spikes (Fig. 2). When the pump power slightly exceeds the threshold, a quasi-continuous wave is generated. Based on previous studies, the generation of stochastic spikes is related to the combined effects of distributed Rayleigh scattering and cascade-stimulated Brillouin scattering, while nonlinear spectral broadening arising from nonlinear interactions, such as frequency mixing and cross-phase modulation, can broaden and superimpose the spikes and further suppress the stimulated Brillouin scattering process. Mode competition and hopping are observed between two laser modes with different peak wavelengths of 1951.2 nm and 1951.4 nm when the pump power is approximately 3.5 W and 4.5 W, respectively (Fig. 3). The output power increases almost linearly with the pump power, with a slope efficiency of 3.9% (Fig. 4). When the pump power reaches 6.0 W, the laser output power is 142.9 mW, the optical-signal-to-noise ratio is up to 43 dB, and the wavelength and power fluctuations of the output laser within 1 h are less than 0.1 nm (Fig. 5) and 3.7 mW (Fig. 6), respectively, demonstrating good stability. The relatively low slope efficiency is predominantly caused by the high loss of the pump laser power from the output to the net injection into the thulium-doped fiber, of approximately 7.5 dB. The mismatching fiber connection introduces the high loss, and therefore, this problem may be resolved by customizing a WDM made of fibers whose parameters match those of fibers to be connected, i.e., the output fiber of the pump laser and the thulium-doped fiber. If the insertion loss is minimized, the slope efficiency of the laser may reach 20.6%, which is close to the slope efficiency of ordinary thulium-doped fiber lasers.

The 2 μm wavelength band is an eye-safe optical band that contains the absorption lines of water molecules and many atmospheric molecules as well as the spectrum windows for atmospheric communications. Therefore, fiber lasers in the 2-μm-band have been widely studied for their potential applications in fields such as lidar, laser ranging, and surgery. Currently, random fiber lasers (RFLs) have attracted extensive research attention owing to their simple structure and low spatial/temporal coherence properties. These fibers usually utilize back Rayleigh scattering as the random feedback mechanism, combined with the distributed Raman or Brillouin gain in the fiber, to generate random lasers in a long single-mode fiber. However, the Rayleigh scattering intensity of the single-mode fiber is inversely proportional to the fourth power of the wavelength, and the transmission loss is as high as 30 dB/km at 2 μm. Compared with the common RFLs operated at 1.0-1.5 μm, RFLs operated within the 2-μm-band are rarely reported, as they are highly difficult to realize. In this work, an RFL operated in a 2 μm region is demonstrated using a thulium-doped fiber as the gain medium and a random fiber grating for random distributed feedback with enhanced Rayleigh scattering efficiency. A laser output of wavelength 1951 nm is achieved with a relatively low pump threshold of 2.1 W.

The proposed RFL is formed using a 793 nm pump laser, 793 nm/2000 nm wavelength-division multiplexer, fiber loop mirror, 1.5-m-long thulium-doped fiber, and random fiber grating. The random fiber grating containing over six thousand index modulation points along a 10-cm-long single-mode fiber is inscribed using a femtosecond Ti:sapphire regenerative amplifier, which is operated at a wavelength of 800 nm with a repetition rate of 100 Hz and a pulse duration of 80 fs. The neighboring refractive index modulation points are spaced at random distances between 7.5 μm and 12.5 μm. The random fiber grating provides an enhanced backward Rayleigh scattering, equivalent to that achieved from a several-km-long optical fiber. The thulium-doped fiber provides strong amplification, and the fiber loop mirror helps to form a half-open laser cavity structure. Therefore, a low pump threshold is expected for the proposed RFL.

In this work, a thulium-doped fiber random laser operated at 1951 nm is demonstrated using a random fiber grating as the distributed random feedback medium. Owing to the enhanced Rayleigh scattering provided by the random fiber grating, a random fiber laser with a relatively low threshold power of 2.1 W is achieved, which is approximately 40% lower than that of the previously reported thulium-doped random fiber laser with the same pump wavelength. When the pump power reaches 6.0 W, the output power of the random laser reaches 142.9 mW, and the optical-signal-to-noise ratio is up to 43 dB. The stability of the laser output is relatively good, as the wavelength shift and power fluctuation are less than 0.1 nm and 3.7 mW, respectively, over the testing period of one hour.

Results and Discussions The organic ligand dibenzoylmethane (DBM) exhibits a broad absorption band ranging from 285 nm to 450 nm; six narrow lines between 379 nm and 591 nm, belonging to the intrinsic absorption of Eu³⁺ ions from the ground states ⁷F₀ and ⁷F₁ to the excited states ⁵G₂, ⁵L₆, ⁵D₃, ⁵D₂, ⁵D₁,_{and ⁵D0, are observed for EuCl3. In the Eu(DBM)3Phen complex-doped PMMA film, the broad absorption of the organic ligands is significantly stronger than the intrinsic absorption of the Eu³⁺ ions (Fig. 1). A schematic of the intramolecular energy transfer and intrinsic absorption and emission of Eu³⁺ ions is presented (Fig. 2), based on the absorption and fluorescence emission of the doped film; the measured fluorescence lifetime of the ⁵D0 levelof Eu³⁺ ions in the PMMA host is 403 μs (Fig. 3). A ridge waveguide with a cross-section of 12 μm×5 μm can limit 93% of the signal laser and 95% of the pump light in the core layer. In the evanescent field waveguide with a cross-section of 4 μm×5 μm, the limitations in the core layer are 87% and 92% for the signal and pump light, respectively, owing to the smaller refractive index difference (Fig. 6). When pumping with the 405 nm LED, the relative gain in the ridge waveguide with a length of 1.5 cm increases from approximately 0.2 dB/cm to 1.9 dB/cm at 653 nm, as the pump power increases from 225 mW to 420 mW. For the evanescent field waveguide, a maximum gain of 1.5 dB/cm is obtained on a 2.0 cm-long waveguide under the excitation of the 420 mW 405 nm LED (Fig. 8); this demonstrates the possibility of the practical application of the evanescent-wave coupling method in PICs.}

Plastic optical fibers (POFs) have been widely used in Fiber to the Home (FTTH), automobile optical local area networks (LANs) and fiber-optic sensor fields owing to their large bandwidths, low prices, and easy coupling. POFs exhibit a low loss window in the red band around 650 nm; thus, it is considerably important to use optical waveguide amplifiers to compensate for the propagation loss at a wavelength of 650 nm. Furthermore, optical waveguide amplifiers can be integrated with optical switches, arrayed waveguide gratings, and optical sensors in photonic integrated circuits (PICs) to compensate for optical losses. Research on waveguide amplifiers has often utilized semiconductor lasers as pump sources to excite the intrinsic absorption bands of rare-earth ions. Consequently, the optical power density at the input side of the waveguide can reach approximately 10⁶ W/cm² with pumping power of 300 mW at a cross-section of 6 μm×5 μm for the waveguide, which leads to thermal damage in the waveguides and the up-conversion of rare-earth ions. Lanthanide ion complexes with organic ligands exhibit a continuous large absorption band in the blue-violet band, which is suitable for blue-violet light-emitting diode (LED) pumping. The energy absorbed by organic ligands can be effectively utilized to realize the radiative transition of rare-earth ions through intramolecular energy transfer. In addition, the LED pumping method can help improve the thermal stability of waveguides, which is expected to play an important role in optical integrated systems on chips.

The absorption spectra of organic ligands, EuCl₃ and Eu(DBM)₃Phen-doped polymethyl methacrylate (PMMA) films, are measured. The fluorescence emission and fluorescence lifetime of the Eu(DBM)₃Phen-doped PMMA film are characterized. Using an aluminum mask combined with inductively coupled plasma (ICP) etching and one-step photolithography, a ridge waveguide and an evanescent field waveguide are fabricated, respectively. Further, the film-forming properties of the doped film and the morphology of the waveguides are characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM), respectively. The optical field distribution of the signal laser in the waveguides is also simulated. Moreover, using a vertical top pumping mode with a 405 nm LED, the optical gains of the fabricated waveguides are measured at 653 nm.

In this study, the europium complex Eu(DBM)₃Phen is doped into a PMMA polymer as an active material to fabricate two types of polymer waveguide amplifiers—a ridge waveguide and an evanescent field waveguide—using an aluminum mask combined with ICP etching and one-step photolithography, respectively. Under the excitation of a 405 nm blue-violet LED, relative gains of 1.9 dB/cm and 1.5 dB/cm are obtained at 653 nm, respectively, for these waveguides. The UV absorption and fluorescence emission of the Eu(DBM)₃Phen-doped PMMA film are also characterized. The results show that the intramolecular energy transfer of organic ligands can realize the transition of Eu³⁺ ions from the ⁵D₀ energy level to the ⁷F₃ energy level under LED pumping. The relatively long fluorescence lifetime of the ⁵D₀ level_{of Eu³⁺ ions can facilitate high gains in optical amplifier systems.}

Results and Discussions The organic ligand dibenzoylmethane (DBM) exhibits a broad absorption band ranging from 285 nm to 450 nm; six narrow lines between 379 nm and 591 nm, belonging to the intrinsic absorption of Eu³⁺ ions from the ground states ⁷F₀ and ⁷F₁ to the excited states ⁵G₂, ⁵L₆, ⁵D₃, ⁵D₂, ⁵D₁,_{and ⁵D0, are observed for EuCl3. In the Eu(DBM)3Phen complex-doped PMMA film, the broad absorption of the organic ligands is significantly stronger than the intrinsic absorption of the Eu³⁺ ions (Fig. 1). A schematic of the intramolecular energy transfer and intrinsic absorption and emission of Eu³⁺ ions is presented (Fig. 2), based on the absorption and fluorescence emission of the doped film; the measured fluorescence lifetime of the ⁵D0 levelof Eu³⁺ ions in the PMMA host is 403 μs (Fig. 3). A ridge waveguide with a cross-section of 12 μm×5 μm can limit 93% of the signal laser and 95% of the pump light in the core layer. In the evanescent field waveguide with a cross-section of 4 μm×5 μm, the limitations in the core layer are 87% and 92% for the signal and pump light, respectively, owing to the smaller refractive index difference (Fig. 6). When pumping with the 405 nm LED, the relative gain in the ridge waveguide with a length of 1.5 cm increases from approximately 0.2 dB/cm to 1.9 dB/cm at 653 nm, as the pump power increases from 225 mW to 420 mW. For the evanescent field waveguide, a maximum gain of 1.5 dB/cm is obtained on a 2.0 cm-long waveguide under the excitation of the 420 mW 405 nm LED (Fig. 8); this demonstrates the possibility of the practical application of the evanescent-wave coupling method in PICs.}

Plastic optical fibers (POFs) have been widely used in Fiber to the Home (FTTH), automobile optical local area networks (LANs) and fiber-optic sensor fields owing to their large bandwidths, low prices, and easy coupling. POFs exhibit a low loss window in the red band around 650 nm; thus, it is considerably important to use optical waveguide amplifiers to compensate for the propagation loss at a wavelength of 650 nm. Furthermore, optical waveguide amplifiers can be integrated with optical switches, arrayed waveguide gratings, and optical sensors in photonic integrated circuits (PICs) to compensate for optical losses. Research on waveguide amplifiers has often utilized semiconductor lasers as pump sources to excite the intrinsic absorption bands of rare-earth ions. Consequently, the optical power density at the input side of the waveguide can reach approximately 10⁶ W/cm² with pumping power of 300 mW at a cross-section of 6 μm×5 μm for the waveguide, which leads to thermal damage in the waveguides and the up-conversion of rare-earth ions. Lanthanide ion complexes with organic ligands exhibit a continuous large absorption band in the blue-violet band, which is suitable for blue-violet light-emitting diode (LED) pumping. The energy absorbed by organic ligands can be effectively utilized to realize the radiative transition of rare-earth ions through intramolecular energy transfer. In addition, the LED pumping method can help improve the thermal stability of waveguides, which is expected to play an important role in optical integrated systems on chips.

The absorption spectra of organic ligands, EuCl₃ and Eu(DBM)₃Phen-doped polymethyl methacrylate (PMMA) films, are measured. The fluorescence emission and fluorescence lifetime of the Eu(DBM)₃Phen-doped PMMA film are characterized. Using an aluminum mask combined with inductively coupled plasma (ICP) etching and one-step photolithography, a ridge waveguide and an evanescent field waveguide are fabricated, respectively. Further, the film-forming properties of the doped film and the morphology of the waveguides are characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM), respectively. The optical field distribution of the signal laser in the waveguides is also simulated. Moreover, using a vertical top pumping mode with a 405 nm LED, the optical gains of the fabricated waveguides are measured at 653 nm.

In this study, the europium complex Eu(DBM)₃Phen is doped into a PMMA polymer as an active material to fabricate two types of polymer waveguide amplifiers—a ridge waveguide and an evanescent field waveguide—using an aluminum mask combined with ICP etching and one-step photolithography, respectively. Under the excitation of a 405 nm blue-violet LED, relative gains of 1.9 dB/cm and 1.5 dB/cm are obtained at 653 nm, respectively, for these waveguides. The UV absorption and fluorescence emission of the Eu(DBM)₃Phen-doped PMMA film are also characterized. The results show that the intramolecular energy transfer of organic ligands can realize the transition of Eu³⁺ ions from the ⁵D₀ energy level to the ⁷F₃ energy level under LED pumping. The relatively long fluorescence lifetime of the ⁵D₀ level_{of Eu³⁺ ions can facilitate high gains in optical amplifier systems.}

Results and Discussions When T_OC=5%, the maximum continuous-wave output power reaches 20.4 W under an incident pump power of 70 W, resulting in optical-to-optical conversion efficiency of 29.1% and a slope efficiency of 32.5% (Fig. 2). Under the full output power, the beam quality factors are Mx2=1.65andMy2=1.81 (Fig. 3), and the power stability (root mean square) is 0.1% within 1 h. In addition, when T_OC =10%,the maximum output power reaches 19 W with an optical-to-optical efficiency of 27.2% and a slope efficiency of 32% (Fig. 2). After inserting an acousto-optic Q-switcher, when T_OC=5%, the average output power increases from 9.8 W at a pulse repetition frequency (PRF) of 1 kHz to 16.5 W at a PRF of 20 kHz, corresponding to a decrease in pulse energy from 9.8 mJ to 0.82 mJ (Fig. 6). The pulse duration increases from 119 ns at 1 kHz to 433 ns at 20 kHz, decreasing the peak power from 82.3 kW to 1.8 kW (Fig. 7). Under the full output power, the corresponding power stability (root mean square) within 1 h is 1.2% .

Lasers emitted in the 1.3 μm spectral region have received significant attention owing to increasing applications in remote sensing, timing systems, dermatologic procedures, and nonlinear frequency conversion. It is well known that Nd∶YLF is a promising material for generating high-energy 1.3 μm pulsed laser because of its extended upper-laser-level lifetime. However, the power scaling of 1.3 μm Nd∶YLF lasers is challenging because of their small stimulated emission cross-section and low thermal fracture limit. An end-pumped scheme with a broadband 880 nm laser diode (LD) is investigated to overcome these limitations. However, the power stability of the 1314 nm laser is reduced by the thermal wavelength shift and linewidth fluctuation of the broadband LD. When the broadband LD is used as the pump source, it is difficult to simultaneously improve the pump absorption efficiency, enhance the mode-to-pump overlap efficiency, and reduce the thermal stress of the laser crystal. Therefore, the high-power, high-efficiency laser output is greatly restricted. This paper introduces a wavelength-locked narrowband 880 nm LD as the pump source for generating a stable, efficient, and powerful 1314 nm laser.

Figure 1 shows the experimental setup. The pump source is a fiber Bragg grating (FBG) locked narrowband fiber-coupled LD with a numerical aperture of 0.22 μm and a core diameter of 200 μm. Its center wavelength is stabilized at 879.9 nm with a narrow spectral bandwidth of 0.2 nm. A pair of coupling lenses with 1:5 magnification is used to re-image the pump beam with a spot diameter of approximately 1 mm into the gain medium. An a-cut 1.0% (atomic fraction) Nd∶YLF crystal with a size of 3 mm×3 mm×30 mm is selected as the gain medium, which is coated for high transmission at 880 nm and 1047-1321 nm on the entrance surface and high transmission at 1047-1321 nm and partial reflectivity at 880 nm (reflectivity R≈60%) on the rear surface. Under non-lasing conditions, the pump absorption efficiency exceeds 90%. During the experiments, the gain medium is wrapped with indium foil and closely packed using a water-cooled copper holder at 16 °C. The Q-switched device is a 46-mm-long acousto-optic modulator plated with a 1314 nm antireflection coating on both surfaces and driven by a 27.12-MHz ultrasonic frequency generator operating at a 100 W radio frequency. The linear resonator is composed of a plano-concave mirror M1 with a radius of curvature of 500 mm and a plane output coupler M2. The input mirror M1 is coated for high transmission at 880 nm and 1047-1053 nm and high reflection at 1314-1321 nm, whereas the plane mirror M2 coated for partial reflectivity at 1314 nm (coupling output rate T_OC=5%, 10%) is employed as the output coupler. Considering the thermally induced diffraction loss and energy transfer upconversion (ETU) effect, the optimized mode-to-pump ratio is approximately 0.84. Consequently, the physical length of the resonator is set to approximately 250 mm based on the ABCD matrix theory.

A high-power end-pumped Nd∶YLF laser operating at 1314 nm is demonstrated using a wavelength-locked narrowband 880 nm laser diode. The optimized mode-to-pump ratio is approximately 0.84 considering the thermal and ETU effects. The Nd∶YLF laser delivers the maximum continuous-wave output power of 20.4 W with an optical-to-optical conversion efficiency of 29.1% and a slope efficiency of 32.5%. After Q-switching with an acousto-optic modulator, the laser system generates the maximum average output power of 16.5 W at 20 kHz and the maximum pulse energy of 9.8 mJ at 1 kHz. To the best of our knowledge, we demonstrate the highest average power and highest pulse energy from Q-switched end-pumped single-crystal 1.3 μm Nd∶YLF lasers. Future upgrades to achieve higher output power and pulse energy will involve a multisegment-doped or diffusion-bonded Nd∶YLF crystal and a double-end pumping scheme.

Results and Discussions When T_OC=5%, the maximum continuous-wave output power reaches 20.4 W under an incident pump power of 70 W, resulting in optical-to-optical conversion efficiency of 29.1% and a slope efficiency of 32.5% (Fig. 2). Under the full output power, the beam quality factors are Mx2=1.65andMy2=1.81 (Fig. 3), and the power stability (root mean square) is 0.1% within 1 h. In addition, when T_OC =10%,the maximum output power reaches 19 W with an optical-to-optical efficiency of 27.2% and a slope efficiency of 32% (Fig. 2). After inserting an acousto-optic Q-switcher, when T_OC=5%, the average output power increases from 9.8 W at a pulse repetition frequency (PRF) of 1 kHz to 16.5 W at a PRF of 20 kHz, corresponding to a decrease in pulse energy from 9.8 mJ to 0.82 mJ (Fig. 6). The pulse duration increases from 119 ns at 1 kHz to 433 ns at 20 kHz, decreasing the peak power from 82.3 kW to 1.8 kW (Fig. 7). Under the full output power, the corresponding power stability (root mean square) within 1 h is 1.2% .

Lasers emitted in the 1.3 μm spectral region have received significant attention owing to increasing applications in remote sensing, timing systems, dermatologic procedures, and nonlinear frequency conversion. It is well known that Nd∶YLF is a promising material for generating high-energy 1.3 μm pulsed laser because of its extended upper-laser-level lifetime. However, the power scaling of 1.3 μm Nd∶YLF lasers is challenging because of their small stimulated emission cross-section and low thermal fracture limit. An end-pumped scheme with a broadband 880 nm laser diode (LD) is investigated to overcome these limitations. However, the power stability of the 1314 nm laser is reduced by the thermal wavelength shift and linewidth fluctuation of the broadband LD. When the broadband LD is used as the pump source, it is difficult to simultaneously improve the pump absorption efficiency, enhance the mode-to-pump overlap efficiency, and reduce the thermal stress of the laser crystal. Therefore, the high-power, high-efficiency laser output is greatly restricted. This paper introduces a wavelength-locked narrowband 880 nm LD as the pump source for generating a stable, efficient, and powerful 1314 nm laser.

Figure 1 shows the experimental setup. The pump source is a fiber Bragg grating (FBG) locked narrowband fiber-coupled LD with a numerical aperture of 0.22 μm and a core diameter of 200 μm. Its center wavelength is stabilized at 879.9 nm with a narrow spectral bandwidth of 0.2 nm. A pair of coupling lenses with 1:5 magnification is used to re-image the pump beam with a spot diameter of approximately 1 mm into the gain medium. An a-cut 1.0% (atomic fraction) Nd∶YLF crystal with a size of 3 mm×3 mm×30 mm is selected as the gain medium, which is coated for high transmission at 880 nm and 1047-1321 nm on the entrance surface and high transmission at 1047-1321 nm and partial reflectivity at 880 nm (reflectivity R≈60%) on the rear surface. Under non-lasing conditions, the pump absorption efficiency exceeds 90%. During the experiments, the gain medium is wrapped with indium foil and closely packed using a water-cooled copper holder at 16 °C. The Q-switched device is a 46-mm-long acousto-optic modulator plated with a 1314 nm antireflection coating on both surfaces and driven by a 27.12-MHz ultrasonic frequency generator operating at a 100 W radio frequency. The linear resonator is composed of a plano-concave mirror M1 with a radius of curvature of 500 mm and a plane output coupler M2. The input mirror M1 is coated for high transmission at 880 nm and 1047-1053 nm and high reflection at 1314-1321 nm, whereas the plane mirror M2 coated for partial reflectivity at 1314 nm (coupling output rate T_OC=5%, 10%) is employed as the output coupler. Considering the thermally induced diffraction loss and energy transfer upconversion (ETU) effect, the optimized mode-to-pump ratio is approximately 0.84. Consequently, the physical length of the resonator is set to approximately 250 mm based on the ABCD matrix theory.

A high-power end-pumped Nd∶YLF laser operating at 1314 nm is demonstrated using a wavelength-locked narrowband 880 nm laser diode. The optimized mode-to-pump ratio is approximately 0.84 considering the thermal and ETU effects. The Nd∶YLF laser delivers the maximum continuous-wave output power of 20.4 W with an optical-to-optical conversion efficiency of 29.1% and a slope efficiency of 32.5%. After Q-switching with an acousto-optic modulator, the laser system generates the maximum average output power of 16.5 W at 20 kHz and the maximum pulse energy of 9.8 mJ at 1 kHz. To the best of our knowledge, we demonstrate the highest average power and highest pulse energy from Q-switched end-pumped single-crystal 1.3 μm Nd∶YLF lasers. Future upgrades to achieve higher output power and pulse energy will involve a multisegment-doped or diffusion-bonded Nd∶YLF crystal and a double-end pumping scheme.

Results and Discussions In traditional tapered lasers, phenomena such as optical pumping, spatial hole burning, and beam filamentation cause the beam quality to deteriorate rapidly under high powers (Fig. 3). The residual reflected light on the front facet is transmitted in the tapered waveguide, and the optical field exhibits a significantly uneven distribution. With a decrease in the reflectivity of the front facet, the side lobes are suppressed. When the reflectivity of the front facet decreases below 1%, the light-field distribution tends to be stable. In particular, when the reflectivity of the front facet is 0.01%, the beam intensity in waveguide increases significantly (Fig. 4). The reflectivity of the rear facet directly induces an optical pumping effect. Reducing the reflectivity of the rear facet can weaken the optical pumping in the cavity (Fig. 5). Combined with the optimization of the front facet reflection, the mode characteristics are significantly improved, and an output close to the fundamental mode is obtained (Fig. 6). By optimizing the reflectivity of the facet, two optimization paths are proposed to achieve high beam quality (Figs. 7 and 8). Four optimized designs are selected to simulate the output characteristics. Compared with the traditional tapered lasers, the output characteristics are not significantly reduced, whereas the simulation of beam quality shows that the optimized design achieves a high beam quality with beam quality factor even lower than 2 (Fig. 9).

Tapered lasers undergo severe beam quality deterioration under high powers. Optical pumping, self-focusing, spatial hole burning, and beam filamentation are some phenomena that occur in traditional tapered lasers, severely degrading the beam quality. Facet reflection causes these problems, but the specific influence mechanism of facet reflection on beam quality is unclear. This study aims to clarify the effect of facet reflection on the beam quality of tapered lasers and proposes an optimization scheme to improve the beam quality.

Based on facet coating, this study establishes a physical model of a tapered laser without an antireflection structure. By varying the reflectivity of the front and rear facets and using the electro-optic model of the tapered lasers for simulations, the optical field distributions at different positions in the transmission direction in the facet are analyzed. In addition, the effect of facet reflection on the beam quality of the tapered laser is investigated. The output characteristics and beam quality factor of the optimized design are also verified using the simulation method.

Facet reflection is closely related to the beam quality. The residual reflection from the facets influences the field distribution in the cavity, causing the concentration of the local gain and refractive index, giving rise to nonlinear effects, and finally deteriorating the beam quality of the device. The reflection of the front facet plays a critical role in this process. The residual reflected light of the front facet is transmitted through a tapered waveguide owing to the unique structure of the tapered laser, and the optical field generates a significantly uneven distribution. In addition, the reflection of the rear facet directly causes optical pumping, which also significantly impacts the beam quality. By optimizing the reflectivity of the facet, two optimized paths are proposed to achieve high beam quality. Some optimal designs have their beam quality factor M2 less than 2 and maintain good output characteristics. Compared to previously designed schemes, there is no need to modify the overall structure. Adjusting only the reflectivity of the facet using the optical film can yield high beam quality. This study provides a reference for the design of a tapered laser.

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The experimental setup is shown in Fig. 1. Single-mode fiber-coupled LDs with center wavelengths of 976 nm and 979.5 nm are selected as the pump sources. To improve the mode matching, the pump light is collimated and focused through a coupling system with an imaging ratio of 1∶3, and the radius of the focused pump spot is 11 μm. A 2-mm long Yb∶KGW crystal coated with an anti-reflection film @ 980-1100 nm on both transparent surfaces is used as the gain medium. During the experiment, the crystal is wrapped with indium foil and mounted on a water-cooled copper heat sink block to ensure effective heat dissipation. The resonant cavity is a four-mirror ring cavity with a total cavity length of 274.5 mm, corresponding to a repetition rate of 1.093 GHz. M₁ and M₂ are concave mirrors with a radius of curvature (ROC) of 50 mm, and M₂ and M₃ are interferometer (GTI) mirrors providing -1600 fs² (1035-1055 nm) group delay dispersion (GDD) in total per round trip. An OC with a transmittance of 0.4% is selected to ensure a sufficiently high power in the cavity.

Optical frequency combs based on femtosecond lasers are powerful tools for the development of precision metrology. Femtosecond lasers with high repetition rates are particularly important when optical-frequency combs are applied to spectroscopy. However, it is difficult to realize high-repetition-rate Kerr-lens mode-locking because of the weak Kerr nonlinear effect. Due to the mode matching limitation of soft aperture Kerr-lens mode-locking, a single-mode fiber-coupled laser diode (LD) is usually used as the pump source, and an output coupling (OC) mirror with low transmittance is chosen to improve the intracavity peak power, resulting in a low output power. Output power is an important parameter when a high-repetition-rate laser is used for optical frequency comb generation. Therefore, it is essential to improve the output power of GHz-mode-locked lasers.

The Yb∶KGW crystal is an excellent candidate for Kerr-lens mode-locking because of its high nonlinear refractive index of 2×10^-15 cm²/W. It has a large absorption cross section (1.22×10^-19 cm²) at 980 nm, which additionally supports its high pumping efficiency. Moreover, the emission cross section at 1023 nm is as high as 2.8×10^-20 cm², meaning it has a high gain, which is expected to achieve a high power output. Based on the above properties, we demonstrate a high-power Kerr-lens mode-locked Yb∶KGW femtosecond laser with 1 GHz repetition rate.

First, using the 976 nm single-mode LD as the pump source, mode-locking with an average power output of 90 mW at a center wavelength of 1045 nm and a full width at half maximum (FWHM) of 4.5 nm is achieved [Fig. 2(a)], and the corresponding pulse duration is 265 fs [Fig. 2(b)]. To increase the output power of mode-locking, the pump source is replaced with a 979.5 nm single-mode LD, whose wavelength is closer to the main absorption peak wavelength of the crystal. In addition, the polarization of the pump light is adjusted using a half-wave plate to further improve the absorption efficiency of the crystal. Consequently, the average power of unidirectional mode-locking is increased to 151 mW. The FWHM of the optical spectrum is approximately 4.2 nm at a center wavelength of 1045 nm [Fig. 3(a)], and the corresponding pulse duration is 249 fs [Fig. 3(b)]. In the laboratory environment, mode-locking can be self-started only by increasing the pump power, and the root mean square (RMS) value of power fluctuations within 24 h is 0.76% [Fig. 4(b)]. The power jitter in the vicinity of 3 h and 6 h is because of environmental vibration and other factors that cause the laser to move out of the stable mode-locked state; however, it can restart in a short time. The signal-to-noise ratio of the 1.029 GHz fundamental frequency signal in radio frequency spectrum is 62 dB [Fig. 4(a)].

We report a 1 GHz Kerr-lens mode-locked Yb∶KGW laser pumped by a laser diode. A 976 nm single-mode fiber-coupled laser diode is used as the pump, and a stable mode-locked operation at a repetition rate of 1 GHz with an output power of 90 mW is obtained in a bow-tie ring cavity with a pulse duration of 265 fs. By using a 979.5 nm single-mode laser diode as the pump source and adjusting the polarization of the pump beam, the output power is increased to 151 mW at a center wavelength of 1045 nm, and the corresponding pulse duration is 249 fs.

Results and Discussions To evaluate the amplification ability of the amplification system under different seed laser repetition frequencies, the seed source is operated at repetition frequencies of 24.46 MHz, 4.89 MHz, 2.038 MHz, 1.019 MHz, 500 kHz, 200 kHz, 99.4 kHz, 49 kHz, and 25 kHz, and the output power is obtained as a function of the pump power (Fig. 7 and Table 1). Once the seed light passes through the isolator and the beam expansion system, the single pulse energy injected into the slab is 19.6 nJ. In the single-end pumped case, when the repetition frequency of the seed source is 24.46 MHz, the output power is 228 W, the single-pulse energy is 9.3 μJ, the optical-optical conversion efficiency is 20.2%, and the beam quality (M²) values in the guided and non-guided directions are 1.4 and 4.6, respectively. In order to measure the amplification system, we change the repetition frequency of the seed laser under the condition that the pulse width of the seed laser is 11.7 ps and the single pulse energy of the seed laser is fixed at 19.6 nJ.When the repetition frequency of the seed source is 49.8 kHz, a laser output power of 31.6 W is obtained with the single-pulse energy of 0.63 mJ. When the repetition frequency of the seed source is 25 kHz, a laser output power of 24.2 W is obtained with the single-pulse energy of 0.97 mJ and the peak power of 82.9 MW. The magnification factor is up to 4.9×10⁴. Experimental results show that this amplifier can suppress the amplification of the spontaneous emission (ASE) effect and can effectively increase the magnification, thereby improving the output power and conversion efficiency. Moreover, the method of improving the output and suppressing the ASE is further analyzed based on the theoretical calculations. Experimental results reveal that the system has a strong amplification ability.

High-energy, high-peak power, picosecond pulse lasers have broad application prospects in laser science research and industrial processing. The configuration of a master oscillator power amplifier (MOPA) is commonly used to obtain a high output power. In this case, a low-power oscillator with the desired characteristics is used as the seeder, whose radiation is injected into an amplifier to scale the output power or pulse energy, while other properties remain mostly unchanged. The planar waveguide can provide high seed light injection powers and high pumping power densities. Therefore, the planar waveguide lasers are a potential laser technology for obtaining picosecond pulse laser with high energy and high peak power.

A Nd∶YAG planar waveguide picosecond laser amplifier is designed. The seed is a fiber picosecond laser, which is amplified by a compactness amplifier with a Nd∶YAG planar waveguide as the gain medium to obtain a picosecond laser output. The small signal gain coefficient and power distributions at different positions within the planar waveguide are calculated using related theories. Finally, based on the results of theoretical calculations, a picosecond laser amplifier is designed and built, and the experiment is completed under different seed laser repetition frequencies.

In this study, a back end-pumped Nd∶YAG planar waveguide picosecond laser amplifier is designed. The fiber laser provides a picosecond seed source with a tunable repetition frequency, and the pumping source operates in the continuous mode. Based on the results, when the seed light repetition frequency is 24.46 MHz, the pulse width is 11.7 ps, the injection power is 0.48 W, the output power is 228 W, the single-pulse energy is 9.3 μJ, and the optical-optical conversion efficiency reaches 20.2%. Moreover, the beam quality values in the guided and non-guided directions are 1.4 and 4.6, respectively. In contrast, when the repetition frequency of the seed laser is reduced to 25 kHz, the average output power is 24.2 W, the single-pulse energy is increased to 0.97 mJ, and the single-pulse energy magnification factor is 4.9×10⁴, indicating that the amplification system has a strong amplifying ability. The methods to further improve the optical-optical conversion efficiency and suppress ASE are analyzed based on the theoretical calculations. To the best of our knowledge, this is the first report on a planar waveguide picosecond laser amplifier.

Results and Discussions The developed single-mode 980 nm semiconductor laser yields a high kink-free output power of 1650 mW with a maximum rollover power of 2.4 W. No failure is observed during the test involving a maximum driving current of 4 A. An accelerated long-life test is performed using 21 units of 5 mm long semiconductor lasers under an injection current of 2 A and a junction temperature of 137 ℃. The life test is conducted for more than 2000 h without failure. For the 14 pin butterfly module, wavelength locking over a wide range is accomplished under various power levels and temperatures. The kink-free fiber output power of the module exceeds 1300 mW. In the operating power range from 10 to 1300 mW, the wavelength is stable and locked at 974.5 nm±0.5 nm, with a side mode suppression ratio (SMSR) exceeding 30 dB, a spectral bandwidth (full width at halt-maximum) of less than 0.5 nm, and a power-in-band (PIB) exceeding 95%. The module performance fulfills the EDFA application requirements for telecommunication systems and satisfies the Telcordia GR-468-CORE standard.

For erbium-doped fiber amplifier (EDFA) pumping sources, 980 nm single-mode semiconductor lasers and modules are widely used. These modules, which are driven by dense wavelength division multiplexing (DWDM) in optical communication systems and yield more than 1 W of output power, are in high demand. However, achieving more than 1 W of output power is challenging owing to the restrictions caused by thermal rollover, catastrophic optical damage (COD), kinks in the light-current curve, reliable operation range, and wide-ranging stable wavelength locking requirements. This paper presents the development of high-power single-transverse-mode 980 nm semiconductor laser and modules. By adopting advanced chip designs, sophisticated device fabrication procedures, specific facet coating processes, and advanced module packaging, output power levels of 1650 and 1300 mW are achieved for the chip and module, respectively.

To fabricate the single-mode 980 nm semiconductor laser, an asymmetric epitaxial design was applied, and a material was grown via metal-organic chemical vapor deposition (MOCVD). Ridge waveguides were formed during device fabrication. A wafer was cleaved into bars with high- and low-reflection coatings on its two facets. Singulated chips were mounted p-side up on AlN submounts bonded by an AuSn alloy. A fiber Bragg grating and an optimized fiber lens were integrated to manufacture a standard 14 pin butterfly module with superior performance.

A single-mode 980 nm diode chip and module are investigated comprehensively. Both the chip and module achieve superior characteristics with kink-free single-mode power levels of 1650 and 1300 mW, respectively. Accelerated life testing shows the reliable operation of the chip and module, which satisfy the requirements of telecommunication and other applications.

Results and Discussions First, we evaluate the locking performance for the control of the phase difference between the two optical paths in the DD-MZM. Fig. 5(a) shows the DC output signal of photodetector 2 (PD 2) after measurement for half an hour. The figure shows that, without locking, the DC signal changes with a relative proportion of 8%, and this drift increases further as the observation time increases. However, with locking, the drift of the DC output signal is significantly suppressed, and the value is stabilized at the setpoint, which is 8.1 V in the current system. To further evaluate the locking bandwidth, we perform noise power spectral density analysis on the acquired data, and the results are shown in Fig. 5(b). It is clear that the locking bandwidth can reach 120 kHz, which is much larger than the linewidth of the fiber laser (1 kHz). Consequently, the relative phase noise of the two lasers can be greatly suppressed, and the coherence of the two lasers is improved.To evaluate the effect of phase locking on the ranging results, the distance of the target mirror is fixed. The distance results are continuously monitored for half an hour and are shown in Fig. 6. It is clear that when the phase difference of the two optical paths in the DD-MZM is unlocked, the obtained distance value drifts from 144.04 mm to 144.33 mm over a range of 0.27 mm. With locking, the drift of the phase measurement results is greatly suppressed from 0.004π to 0.0003π, and the suppression ratio reaches 11 dB. The remaining noise exhibits a white noise behavior. The retrieved distance with locking remains at approximately 144.32 mm.To analyze the stability of the ranging system and to assess its sensitivity, we perform Allan variance analysis based on the results shown in Fig. 6, and the obtained results are shown in Fig. 7. It can be seen that when the phase is unlocked, the uncertainty of ranging is 2.56 μm in the short term, and due to the influence of long-term drift, the Allan variance rapidly increases when the integration time exceeds 5 s. However, with locking, both the white noise and long-term drift of the system are suppressed, and the uncertainty of the distance measurement reaches 2.26 μm in the short term. When the integration time increases, the white noise of the system is further restrained. Further, the system achieves an optimal uncertainty of 0.4 μm when the integration time is 100 s. When repeating the experiments with different distances, the phase detection accuracy is found to be better than 0.00032π, and the ranging stability can be less than 0.42 μm when the measurement distance is 200-1800 mm.

Ranging technology has important applications in many fields. Lasers have excellent coherence and high brightness, and therefore, their employment in ranging has rapidly promoted the development of various absolute or relative distance measurement technologies. Among them, dual-frequency laser ranging technology, which combines multi-wavelength interferometry and the phase method, has been demonstrated to be a powerful ranging tool. The distance is deduced by the relative phase difference between two beat frequencies generated in the echo light and reference light, each of which is composed of two lasers with different frequencies. However, the application and generalization of the traditional dual-frequency laser ranging technology are impeded by its complexity, limited measuring range, low phase detection stability, large phase noise, and low sensitivity.

We propose a novel dual-frequency laser ranging technology based on a dual-drive Mach-Zehnder modulator (DD-MZM). Only the light in one of the two phase modulators inside the DD-MZM is modulated by a radio frequency and then is combined with that within the other phase modulator. Consequently, a light source analogous to a dual-frequency laser is generated in the output of the DD-MZM. The noise intensity induced by the phase difference is significantly suppressed due to the tiny difference in the phase shifts between the two light paths inside the DD-MZM. In addition, to further restrict the phase difference noise, a real-time phase shift control is implemented in one path of the DD-MZM using a feedback loop. The error signal of the feedback is generated from the direct current (DC) output of one detector monitoring the beat note relative to the phase noise. In this paper, the theoretical model for laser ranging is first presented using the electromagnetic transfer function. Subsequently, the principle of feedback control is analyzed. Based on the model, the experimental setup is established. The signal is acquired using frequency down-conversion and the distance information is retrieved using the phase method.

In this paper, we propose a novel laser ranging technology based on a DD-MZM. A dual-frequency laser for ranging is generated by modulating one light path inside the DD-MZM. To facilitate the detection system, the beat note is analyzed by the down-frequency method, and the ranging result is deduced using the phase method. Compared with the traditional method of generating dual-frequency lasers, which is based on the combination of fiber beam splitters, fiber beam combiners, and acousto-optic modulators or electro-optic modulators, the system has a more compact structure and a much smaller path difference between the two light paths. Therefore, its relative phase noise is smaller, and the ranging stability is better. In addition, to further suppress the relative phase noise of the two light paths, we use the DC output signal to generate a correction signal to perform real-time dynamic control of the DC input of one path of the DD-MZM . The results demonstrate that the long-term drift of the ranging results with locking is greatly suppressed with a suppression ratio of 11 dB, and the distance measurement stability can reach 0.4 μm. This novel scheme is conducive to realizing a long-term stable, miniaturized, and highly integrated laser ranging system.

Currently,the laser induced plasma(LPP)technology is the best method to obtain high quality extreme ultraviolet(EUV)light source,whereby a high-power,high-frequency,short-pulse CO₂ laser under the main oscillation power amplification technology is used to bombard a droplet tin target to obtain high-quality extreme ultraviolet signals.In an EUV lithography light source system,the laser beam direction is significantly affected by the cascade when the laser beam passes through the four-stage amplification system.During the amplification process,the optical components in the optical path between the amplifiers feature different thermal distortions under different laser powers;all the four-stage amplifiers used are high-power laser amplifiers,and the vibrations caused by the cooling device during operation are unavoidable.These factors cause the center of the laser beam to deviate from the optical axis and affect the EUV conversion efficiency.Therefore,for EUV lithography light source systems,the further research on beam pointing stability is necessary to achieve a certain EUV conversion efficiency.At present,four-quadrant detectors are widely used in high-precision laser measurements owing to their fast response,high position resolution,high measurement accuracy,and simple data processing.However,a high-precision spot location algorithm based on a four-quadrant detector is generally complex.Thus,during high-repetition-frequency pulse signal detection,the real-time requirements of spot location calculations cannot be met.Therefore,it is necessary to develop a new algorithm that considers the accuracy of spot location detection and its real-time performance.

In this study,we first obtain the four-quadrant detector output signal under the Gaussian distribution model for spot energy distribution and subsequently calculate the initial solution for the spot position under the influence of the detector radius and dead zone using the normalization and difference algorithm.The initial spot position solution is a transcendental equation,and its analytical solution cannot be derived using a mathematical method.The expression for the actual position of the spot centroid is then obtained using the approximate decomposition method,which compensates for the influence of the spot radius,detector radius,and dead zone width on the actual position of the spot centroid.Finally,to improve the solution accuracy,a correction factor is established,and the error characteristics of the correction factor are used to correct the actual position of the spot.This can help improve the spot position detection accuracy and detection range,without increasing the complexity of the algorithm.According to the detection principle of the four-quadrant detector,electrical signals from the four quadrants of the detector should be obtained simultaneously;if the acquisition of the electrical signals from each quadrant is not synchronized,the accuracy of the spot position solution is affected.Therefore,a multi-channel synchronous acquisition and processing circuit is designed for the acquisition of detector output signals,to ensure the accuracy and real-time acquisition of the signals.

According to the analysis of the simulation results,the root mean square error of the second-order error compensation algorithm is 0.0115;after improvement,the root mean square error is reduced to 0.003,which is 73.91% lower.The maximum error of the second-order error compensation algorithm is 0.0372 mm;after improvement,the maximum error is reduced to 0.0076 mm,which is 79.56% lower,and the absolute error is less than 0.005 mm.The detection range of the spot position is expanded from -0.12 mm≤x≤0.12 mm to -0.59 mm≤x≤0.59 mm,which is approximately five times larger. The average absolute error value in the absolute error range of less than 0.005 mm is reduced from 0.0025 mm to 0.0019 mm,which is approximately 24% lower(Fig.4). The spot position detection results are analyzed.It is shown that the root mean square error of the second-order error compensation algorithm is 0.0248, and the root mean square error of the second-order extended error compensation algorithm is 0.0042,i.e.,a reduction of 83.06%. The maximum absolute error of the second-order error compensation algorithm is 0.0625 mm,and the maximum absolute error of the second-order extended error compensation algorithm is 0.0092 mm,i.e.,a reduction of 85.28%. The average absolute error of the second-order error compensation algorithm is 0.0206 mm,and the average absolute error of the second-order extended error compensation algorithm is 0.0034 mm,a reduction of approximately 83.50%. Notably,the spot position detection accuracy is better than 19 μrad in the detection range of -0.5 mm≤x≤0.5 mm(Fig.8).

The simulation analysis and experimental results reveal that the detection range of the second-order extended error compensation algorithm is considerably larger than that of the second-order error compensation algorithm under the same detection accuracy.Compared with the traditional polynomial algorithm,the second-order extended error compensation algorithm offers clear advantages and practicability,significantly improving the detection accuracy of the spot position over a wide detection range.Based on the abovementioned discussion,the results of this work are expected to help realize the wide-range,real-time,and high-precision detection of the spot position for an ultraviolet lithography source driven by a high-repetition-frequency narrow-pulse CO₂ laser.

Ultrafast fiber lasers have many advantages, such as excellent heat removal, high single-pass gain, compactness. Moreover, it has numerous applications in fundamental research and industry, particularly in the biomedicine area. The wavelength tunability of ultrafast laser is one of the most demanding features for multiphoton microscopy imaging. For instance, in two-photon fluorescence microscopy, most fluorophores can be excited by femtosecond pulses in the 800-1300 nm wavelength range. Techniques such as optical parametric oscillator (OPO), supercontinuum generation (SCG), and self-phase modulation enabled spectral selection (SESS) are commonly used to generate such kinds of lasers. Although OPO can generate ultrafast pulses with broad spectral and high pulse energy, it is quite expensive and not user-friendly. SCG can generate an octave-spanning spectrum; however, the pulse energy is far lower than 1 nJ. SESS has been developed recently to generate tunable femtosecond pulses. The core of this scheme is to generate discrete spectral side-lobes enabled by the self-phase modulation-dominated nonlinear effect. A series of theoretical calculations and experiments have shown that this scheme can overcome the wavelength tuning restrictions such as fiber dispersion, high-order soliton fission, Raman effect, etc. The χ(3) nonlinear effect is generally understood to be connected not only to the fiber material but also to the polarization state of the incident pulse. When the B-integral is constant in the nonlinear transmission process, both theory and experiments suggest that circularly polarized pulse transmission can enhance the energy of pulses by approximately 1.5 times.

In this study, we fabricated an ultrafast fiber laser with tunable wavelength and scalable energy using circularly polarized pulses. The system consists of the front-end driven laser (fiber CPA laser) and the following spectral broadening and selection unit (SESS). The central wavelength of a home-built ultrafast-driven laser is 1030 nm. The maximum average power is 8 W, and the repetition rate is 55 MHz. The SESS unit comprises photonic crystal fiber, launching power adjustment setup, and spectral selection filter. The half-wave plate and PBS are used to modify the input power, while a 1/4-wave plate regulates the polarization state of the input pulse. The filter or dichroic mirror is responsible for filtering out the appropriate spectral components to produce output with wavelength adjustable.

The SESS and CPA systems are the major parts of the high-energy wavelength-tunable ultrafast fiber laser (Fig. 4). As the input power increases, the broadening of the output spectrum based on linearly and circularly polarized pulses displays a proportional increase. The spectral broadening of circularly polarized pulses is less than that of linearly polarized pulses when the input power is the same in two polarization states. The rightmost side-lobe of the linearly polarized pulse shifts to 1200 nm when the input power is increased to 3.45 W, and the power of the circularly polarized laser increases to 4.9 W to reach a similar spectral broadening. The power ratio of the two is 1.42, which is consistent with the results obtained via theoretical simulation. Furthermore, we discovered that the spectral structure of circularly polarized pulses is more distinct with the same degree of spectral broadening (Fig. 2 and Fig. 5). We compared their spectra to validate the input power ratio of the linear and circular polarization pulses under the same spectral broadening condition. When the spectrum s rightmost lobe is shifted to 1100 nm, the power in the linear polarization state is 1.5 W, while the power in the circularly polarized state will increase to 2.1 W, implying that the ratio is approximately 1.4. The linearly polarized pulse s input power is 2.55 W and 3.45 W, respectively, when switching to 1150 nm and 1200 nm, while the circularly polarized pulse s power is 3.55 W and 4.9 W, corresponding to power ratios of 1.39 and 1.42, respectively. The practical results of the three cases are consistent with our theoretical model, showing that using the circularly polarized pulse can boost the output power by around 1.4 times (Fig. 6). Furthermore, the output spectrum in a circularly polarized state has deeper modulation and a clearer lobe structure than the linearly polarized pulses, which benefits the subsequent filtering process and increases the stability of output pulse. Finally, a study on elliptically polarized pulses has discovered that the broadened spectrum shows broader but weak lobes due to the occurrence of cross-phase modulation exceptionally in elliptical polarization states (Fig. 3 and Fig. 7).

This study introduces a circularly polarized ultrafast fiber laser with tunable wavelength and scalable energy. The ultrafast input pulses spectrum is strongly broadened in PCF by nonlinear effects dominated by self-phase modulation. Furthermore, we filter out the desired spectral side-lobes with an optical filter, which will be a promising method for generating the light source with high energy and exotic wavelength. The theoretical and experimental studies show that, with the same degree of spectral broadening, the input power of circularly polarized pulses is about 1.4 times higher than that of linear polarized pulses, and the corresponding filtered out energy can be increased by about 1.4 times. Simultaneously, the study shows that circularly polarized pulses tend to have a larger wavelength tuning range by increasing the input power. Finally, this study presentes a high-energy femtosecond laser with a wavelength tuning range between 930 and 1200 nm by the self-phase modulation enabled spectral broadening method. This laser can be a promising alternative for driving multiphoton microscopy to enable high penetration depth imaging of biomedical tissue.

Results and Discussions The two flat-panel targets at distances of 102.56 m and 104.06 m were detected. The time of flight histogram (Fig. 9) shows two peaks with a time difference of 10 ns, and from the 3D image (Fig. 10) the points of the two targets can be clearly identified. The measured distance deviation of the two targets is consistent with the reference distance deviation. The standard deviations of points fit to plane of the measured data are 0.12 m and 0.11 m, respectively, and the results for the simulated data are 0.10 m and 0.10 m (Fig. 11). In dynamic imaging experiments, the point cloud results of the region near Baisha River Bridge, Qingdao, Shandong Province, were successfully captured at a platform velocity of 60 km/h. The resulting area coverage efficiency was 36 km²/h. The partial profiles of the Baisha River Bridge show detailed 3D information about the bridge, and the piers and the street lamps can be clearly identified in the 3D lidar image (Fig. 13). The high-resolution lidar image (Fig. 14) shows a 3D point cloud of the scenic spots along the river and a dam, which has a mean measurement density greater than 13000 points/m². The Google map photographs of the same area helped to identify the characteristics of these targets.

Geiger-mode avalanche photodiode (Gm-APD) arrays have single-photon sensitivity and each pixel can detect the echo photons independently. Lidar systems based on Gm-APD arrays have many advantages, including high imaging resolution, fast imaging rate and possibilities of using lower power laser as the transmitter hence reducing the overall system size, weight, and power (SWaP). These advantages make the Gm-APD array lidar system very suitable for applications in the fields of mobile platform terrain mapping, which have a strict restriction on the total SWaP of the payloads and require a fast imaging rate. In this study, we propose a miniaturized imaging lidar system based on a domestically developed InGaAs 64×64 Gm-APD array. This system uses a large-pixel-format detector array combined with a coaxial scanning mechanism to achieve fast terrain three-dimensional (3D) imaging on vehicle-mounted mobile platforms.

The system is composed of fiber laser module, detector array module, transceiver module, scanning module and system control module. The 1545 nm laser source can operate at a repetition rate of 25 kHz with a maximum pulse energy of 32 μJ, and the laser pulse width is 4 ns. In order to get a uniform illumination on the targets, the transmitting optics collimate and homogenize the laser pulses, so that the divergence angle of the emitted laser pulses is 8 mrad. The receiving optics collect the echo photons, and a 1-nm-bandwidth filter with a center wavelength of 1545 nm is used to reduce the solar background noise. The InGaAs 64×64 Gm-APD array with a detector efficiency of 20% at 1545 nm is adopted to detect the echo photons. Using a 64×64 detector array and a fast scanning unit, and with the help of a moving sensor platform, the system can achieve large-scale terrain mapping. A noise filtering method based on time-domain distribution characteristics of signal and noise is used to remove the noise points in the real-time data. Both static experiments and dynamic imaging experiments were conducted to verify the performance of the system. In static measurement conditions, two flat-panel targets were placed in front of the system at distances of 102.56 m and 104.06 m, respectively. Then the standard deviation of points to plane was evaluated for the two targets. In dynamic imaging experiment conditions, the lidar system, position and orientation system (POS), and panoramic camera were installed on a vehicle-mounted mobile platform with a velocity of 60 km/h to conduct large-scale 3D imaging of the test area. The 3D lidar images of the test area were compared with the Google map results, meanwhile, the area coverage rate and the average measuring point density were evaluated.

A miniaturized imaging lidar system based on a domestically developed InGaAs 64×64 Gm-APD array is designed, which is capable of achieving fast terrain 3D imaging on a vehicle-mounted mobile platform. Both static experiments and dynamic imaging experiments were conducted to verify the performance of the system. In static measurement conditions, the standard deviation of points to plane for flat targets at a distance of 100 m was 0.12 m. In dynamic imaging experiment conditions, the 3D point cloud results of the measured area were successfully obtained when the system was mounted on a mobile platform with a velocity of 60 km/h. The mapping rate was about 36 km²/h and the average measuring point density was 13454 points/m². The results indicated that the lidar system based on a domestic Gm-APD array can realize topographical remote sensing detection on the mobile platform, providing a new technical means for high-resolution terrain mapping of the high-speed vehicle platform. The development of a smaller and more lightweight Gm-APD array lidar system, which can be mounted on small unmanned aerial vehicles (UAVs) to conduct complex terrain area mapping missions, will be explored in the future.

Results and Discussions The proposed registration method is used for vehicle-borne point cloud registration by using SSW vehicle-borne mobile measurement system to collect experimental data, including those obtained on urban roads and tens of kilometers of urban expressways and highways. After ground filtering (Fig. 6) and automatic matching (Fig. 7) of revisited point sets, the elevation registration results (Fig. 8) show that the registration method proposed in this paper can accurately register two ground point clouds with good coarse registration effect, providing robust initial pose for the plane registration. Subsequently, the improved ICP algorithm is used for plane fine registration (Fig. 9). Compared with mainstream algorithms such as RANSAC-ICP and GICP (Fig. 10), it is shown in Table 3 that even if the spatial distribution of the vehicle-borne point clouds in the large scenes is discrete and some ground objects are missing, the overall registration accuracy of the proposed algorithm is high, the calculation efficiency is increased by more than three times, and the high-efficiency and high-precision registration is realized. Compared with the traditional manual interactive registration results (Fig. 11), the translation deviations in the X and Y directions are 0.04 m, and that in the Z direction is 0.03 m. The root mean square error is about 0.03 m, which can meet the application requirements of point cloud registration.

Vehicle-borne mobile measurement system has been widely used in many industries and departments because of its high accuracy, fast speed and rich information. Vehicle-borne point cloud also plays an increasingly important role in the task of real scene three-dimensional reconstruction. In practical applications, due to the blocking of Global Navigation Satellite System (GNSS) signal by viaducts and high-rise buildings in urban areas, the calculated revisited road point clouds have problems of layering and offset, so that they cannot meet the needs of actual engineering projects. In order to improve the quality of vehicle-borne point cloud data, it is necessary to correct the position deviation of point cloud by registration technology. At present, the registration algorithms combining deep learning and feature extraction have been widely studied, but they mainly focus on the ground fixed stations, indoor and small-scale sample point clouds. There are relatively few studies on vehicle-borne point cloud registration. The traditional registration algorithms applied to large scene vehicle-borne point clouds still have the limitations of low accuracy and low efficiency. Aiming at the above problems, a point cloud registration method combining ground points and rod objects is proposed in this paper.

In the proposed method, firstly, the ground point cloud is extracted based on the gradient algorithm and the elevation density distribution function. Then, the mileage segmentation is used to segment the long route point cloud to calculate the overlapping area of two point clouds by using the extreme value range of the ground point. The elevation difference is constrained to automatically generate a stable matching relationship between the target point set and the point set to be registered. Secondly, aiming at the limitation of iterative closest point (ICP) algorithm with high requirements for initial position, the registration process is divided into two steps: the elevation registration based on ground points and the plane registration based on rod objects. The elevation registration uses voxel filter method to strengthen terrain features based on ground points, obtains accurate matching point sequence and calculates initial registration parameters by using distance constraints, so as to provide good pose information for the subsequent fine registration. The plane registration takes the rod objects as the registration primitive. The surface curvature feature is added on the basis of the pass-through filter to limit the cylindrical section of the rods, and the threshold is set to eliminate the wrong adjacent point pairs to improve the registration accuracy and speed. Finally, the point cloud smoothing of the long route is realized by linear interpolation.

Aiming at the problem of inconsistent position of multi-trip vehicle-borne laser point clouds on the revisited road section, we propose a fine registration method using the combination of ground points and rod objects. In this method, the rigid correspondence relationship between two point clouds is established by preprocessing such as ground point extraction, mileage segmentation and overlapping area calculation, and the registration process is divided into two stages: first elevation registration and then plane registration. Typical ground points and rod objects are used as the registration primitives. Combined with voxel filtering, spatial distance constraint and limited curvature threshold, ICP algorithm is improved to calculate the rotation matrix and translation vector. The results show that the method proposed in this paper can achieve automatic registration under the condition of complex point cloud objects, multiple noise points and no prior information, complete the high fusion of point clouds and improve the registration efficiency. Compared with the mainstream methods, facing the complex large scene urban environment, the robustness and universality of the improved ICP algorithm proposed in this paper are stronger, and the registration error is generally less than 0.04 m. In a word, this method is simple and accurate in practical applications.

Results and Discussions The calibration results of the sensor under different temperatures and different concentrations of standard gas are shown in Fig. 7. It can be observed that the temperature affects the peak-to-peak value difference d₁, which in turn affects the gas concentration value of the sensor. And it can be noted that the temperature affects the peak-to-peak value difference approximately linearly. By fitting the temperature and the peak-to-peak value difference d₁, the formula for calculating the compensated difference d can be obtained, as shown by Eq. (9). The relationship between the compensated difference d and standard gas concentration C can then be obtained, as shown in Fig. 8. The fitting curve of CO₂ concentration with temperature compensation is obtained by refitting the data. In order to verify the accuracy of the fitting curve, retest is carried out at different temperatures and different concentrations. The data are shown in Table 1. The retest results show that the sensor can measure the gas concentration of 0%-5% in the temperature range of 0-40 ℃, and the error is less than 0.1% for the concentration of 0%-2%, and less than 0.25% for the concentration of 2%-5%.

CO₂ gas sensor is a key device in industrial control, medicine and health protection. Gas sensor based on infrared absorption principle has the outstanding advantage of high selectivity, but it faces the problems of low integration, large size, and low precision. In recent years, with the efforts of major research institutions and universities, the design of infrared sensors is developing rapidly, but it is urgent to solve the limitations of poor sensitivity and small detection range of the sensors. In this paper, a miniaturized non-dispersive infrared CO₂ sensor with the design of dual-band single-gas-channel structure is proposed, which is based on the infrared pyroelectric effect. The temperature compensation technology of the sensor is explored by the standard gas calibration method. The output values of the detector at different concentrations and temperatures are measured, and the relationship among the temperature, CO₂ concentration and output value of the detector is established. Software algorithm is used to realize the temperature compensation function, so as to realize the accurate measurement of gas concentration in different environments.

The infrared CO₂ sensor is composed of four main parts: infrared light source, air chamber, dual-channel pyroelectric detector, and main circuit system, as shown in Fig. 2. In order to realize the miniaturization of the sensor while maintaining high performance, a C-type multi-reflection air chamber structure is designed to increase the optical path and ensure the length of the interaction between light and gas. Through modular integrated design, a miniaturized air chamber with a height of 8 mm and a diameter of 18 mm is obtained, which minimizes the volume of the sensor. Finally, a miniature infrared CO₂ gas sensor with a diameter of 23 mm and a height of 10 mm is realized. The single-optical-path dual-wavelength differential design concept can effectively eliminate the interference of the air chamber, the light source and impurities, and reduce the influences of environmental temperature, dust, moisture and other factors on the system, thereby reducing the measurement error and improving the measurement accuracy of the system. Both hardware circuits and software programs adopt modular design to reduce system coupling. In order to ensure the accuracy and detection range of the sensor, the method of calibrating the sensor by standard gas concentration is used to build a sensor calibration experimental platform, as shown in Fig. 6, which mainly consists of a high and low temperature humidity test chamber, a standard gas source, and a sensor fixture. During the calibration process, the temperature is first set at 0 ℃, and pure nitrogen is introduced after the sensor output is stable. It can be considered that the concentration of carbon dioxide in pure nitrogen is 0%, that is, the zero point. After 5 min of ventilation, the voltage values collected by the microcontroller are saved for data fitting. Then, carbon dioxide with concentrations of 0.2%, 0.5%, 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, and 5% is introduced in sequence. After the calibration is completed at 0 °C, the ambient temperature in the high and low temperature humidity test chamber is set to 5 ℃, 10 ℃, 15 ℃, 20 ℃, 25 ℃, 30 ℃, 35 ℃, and 40 ℃, respectively, and the calibration is completed in the same way. The peak-to-peak values, temperatures and concentrations of the two channels are recorded.

In this paper, a miniature non-dispersive infrared CO₂ sensor based on infrared pyroelectric effect is designed and implemented. The temperature compensation of the sensor is realized by calibrating the sensor with standard gas concentration, and the sensor can perform accurate measurement at different temperatures and different concentrations. The sensor achieves a miniaturized design with a diameter of 23 mm and a height of 10 mm, and can realize accurate measurement with the error less than 0.1% at 0%-2% concentration and less than 0.25% at 2%-5% concentration. It can provide core devices and technical support for CO₂ concentration monitoring in industrial manufacturing, production and living environments, and has important practical significance for ensuring safe production and human health.

Since its invention, the laser has developed tremendously; stimulated important breakthroughs in numerous related fields such as physics, chemistry, biology, and information science; and played crucial roles in fundamental science and technology application research. The National Science Foundation of China compiles statistics on the funding of Key Programs, Major Programs, Research Programs of National Major Research Instruments, General Programs, and Youth Science Foundation Programs. Based on the statistics compiled between 2017 and 2021, this study analyzes hot words in titles and keywords of previously funded projects to summarize the key developments and challenges of laser science and technology in China and propose topics requiring further research and discussion.

Results and Discussions The classification model based on the first-order derivative combined with the AlexNet network can realize the fast and accurate classification and identification of this study s four aquatic plants. Compared with the VGG16 and CNN3 networks, this study s model has the highest test accuracy of 99.50%. The model s training and testing speeds are 13.56 s/epoch and 0.032 frame/s, respectively, which are 30.12 s/epoch and 0.016 frame/s lower than those of VGG16. Although the model s training speed is 8 s/epoch higher than that of CNN3 and the testing speed is 0.002 frame/s higher, the classification accuracy is 14.44 percentage points higher than that of the CNN3 model. To verify the model s classification accuracy under small samples, 40%, 60%, and 80% of the sample dataset were randomly selected as the training set. The lowest classification accuracy of the model was 99.15%, higher than the classification accuracy of the CNN3 and VGG16 models. The influences of spectral overlapping bands and background interference on the classification results were reduced using four spectral preprocessing methods to process the sample data, and the classification accuracy of the three models before and after preprocessing was compared. The first-order derivative method improved the classification accuracy. The first-order derivative combined with the AlexNet network has the highest classification accuracy of 99.50%. The Grad-CAM algorithm was used to visualize the established aquatic plant identification model, and the classification-sensitive bands of four aquatic plants were analyzed, including seven classification sensitive bands for Typha angustifolia L., two classification sensitive bands for Pontederia cordata L., eight classification sensitive bands for Hydrocotyle vulgaris, and five classification sensitive bands for Thalia dealbata.

Aquatic plants can purify pollutants and inhibit algae growth. Therefore, obtaining accurate information on the number and growth status of aquatic plant species helps monitor the aquatic ecological environment. Spectral analysis, as a vital method for aquatic plant identification, has the characteristics of noncontact, fast, and pollution-free. However, because they are affected by the surrounding water environment, the characteristic spectral peaks of green aquatic plants are more challenging to distinguish than terrestrial plants. The ground spectral data have high dimensions and numerous overlapping bands and background interferences, and the characteristic spectrum is not obvious. The data are more challenging, and a few ground spectral datasets are suitable for deep learning. Currently, conventional machine learning classification methods cannot accurately and comprehensively extract deep features on small samples, resulting in unsatisfactory final classification results. Therefore, the deep learning algorithm and hyperspectral data are used to classify aquatic plants for the problems of many overlapping spectral bands, background interference, inconspicuous characteristic peaks, and less self-built aquatic plant spectral sample data.

This study uses the first-order derivative method combined with the AlexNet network to classify and identify four nearshore aquatic plants. The classification accuracy and training speed of three convolutional neural networks (AlexNet, CNN3, and VGG16) were compared to verify the classification effect of our model on the nearshore aquatic plant spectrum and the AlexNet network was determined as the optimal network structure. Furthermore, the influence of the number of samples on different classification models was studied, and classification effect of three models under small samples was explored. The influence of spectral preprocessing on the model s classification effect was studied, and the sample data before and after preprocessing using four spectral preprocessing methods were compared. Finally, the Grad-CAM algorithm was used to study the classification model visually to extract the characteristic bands of four aquatic plants. The sensitive spectrum bands of nearshore aquatic plants were analyzed, extracted, and compared with the existing aquatic plant datasets. The results are compared to verify the effectiveness of the feature extraction of this study s model.

Typha angustifolia L., Pontederia cordata L., Hydrocotyle vulgaris, and Thalia dealbata. It provides an essential reference for classifying and identifying these four aquatic plants under hyperspectral remote sensing.

Bound states in the continuum (BIC) refers to the non-radiative state located in the radiative continuum. BIC provides a novel method for the research and development of functional devices with ultra-high quality factor (Q) in the terahertz band. It has the potential to be used in several applications, including narrow linewidth filtering, terahertz slow light devices, and the enhanced interaction between terahertz waves and matter. In this study, terahertz BIC metasurfaces composed of classical metallic split ring resonators (SRRs) are proposed and numerically studied based on the symmetry protection principle of the structure. The leakage of BIC to the far field can be observed in the spectrum by changing the gap width of SRR to form an observable quasi BIC (QBIC) mode. Moreover, the influence of ohmic loss on the Q of QBIC is systematically studied by applying the Drude model. The proposed BIC and QBIC also have unique responses to the incident angle. The BIC based on SRR metasurface proposed in this study not only provides a new framework with clear mechanism and easy implementation for the development of high-Q terahertz functional devices, but also provides research ideas for subsequent studies on the terahertz BIC metasurface from the aspects of loss and tilted incidence.

The metasurfaces are composed of different superlattices based on classical metallic SRRs. A single unit cell is composed of either 2 or 4 SRRs. For the superlattices with two SRRs in the lattice, two adjacent SRRs with different orientations are arranged vertically [superlattice ① in Fig. 1(b)] or horizontally [superlattice ② in Fig. 1(c)] to form two types of superlattices. For the superlattices composed of four lattices, each SRR orients in a clockwise direction (superlattice ③ in Fig. 4). All metasurfaces have 2-μm-thick high resistivity silicon wafer as substrate. The refractive index of silicon is set as 3.4 and the SRR is set as perfect electric conductor (PEC). The structure is simulated in CST microwave Studio.

First, the BICs in superlattices ①, ②, and ③ are numerically investigated using the eigen-mode solver. Subsequently, the frequency solver is applied to calculate the transmission of the corresponding QBIC metasurfaces by breaking the structural symmetry of the BICs. The field monitor is used to observe the field distribution to clarify the relationship between a BIC and its derivative QBIC. The evolution from BIC to QBIC is effectively presented by changing the gap widths of the SRRs, and the Fano coupling mode is used to calculate the Q of the QBICs. The influence of ohmic loss on the QBICs is investigated by applying the Drude model to the SRRs. Tilted incidence is realized by changing the input and output directions of the ports in the frequency solver, and the unique dependence of the QBICs in superlattices ① and ② is obtained.

Only one BIC exists in superlattices ① and ②. For superlattice ③, which is composed of 4 SRRs, there are two different BICs existing in the metasurface. QBICs with Fano line shape appear in the transmission spectra when the symmetry of the superlattices is broken. The Q of QBIC exhibits an inverse quadratic correlation with the asymmetric parameter. The residual ohmic loss in the SRRs deteriorates the Q of the QBICs, in which the Q of the metasurface calculated using the Drude model drops to half compared to the result with PEC. Regarding the incidence dependence, a tilted incidence with transverse electric (TE) polarization induces a leakage of the BICs in superlattices ① and ②, in which the linewidth of the derivative QBICs is proportional to the oblique angle. However, the tilted incidence with transverse magnetic (TM) polarization will not perturb the BICs in the superlattices.

In this study, we construct symmetry-protected BICs in three superlattices based on SRRs. Subsequently, the bound states at Γ point in these superlattices are investigated via numerical simulation. When the structural symmetry is broken, BICs are converted to the corresponding QBICs, and the Q of the QBICs decreases with the increase in structural asymmetry. The Q of the QBIC is also strongly correlated to the ohmic loss in the SRRs, which was generally neglected in previous studies related to terahertz metasurfaces developed by SRRs. In addition, the superlattices ① and ② have a certain pitch angle dependence. Oblique incidence of TE polarization with an electric vector parallel to the gap can lead to the leakage of BIC. The Q of the formed QBIC decreases with the increase in incident angle, while the TM wave does not have a similar effect. The metasurface designed in this study has a clear mechanism and is conveniently fabricated, which provides a novel direction for the design of high-Q terahertz devices.

The morphologies of the micro-defects are observed using focused ion beam scanning electron microscopy (FIB-SEM). The feature points are cut with an ion beam to obtain the cross-sectional morphology and structure, and the element composition in the designated area is analyzed using an energy dispersive spectrometer (EDS). Scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) images of the micro-defects are obtained using a 200 keV field emission transmission electron microscope (TEM). Three-dimensional reconstruction is conducted to analyze the submicron defects. Samples are prepared for the TEM observations using FIB-SEM. The LIDT is tested by 1-on-1. A 2ω Nd:YAG laser with a pulse width of 8 ns and a 3ω Nd:YAG laser with a pulse width of 8 ns are used for the 532 nm and 355 nm LIDT measurements, respectively. The effective beam sizes on the sample surface for the 532 nm and 355 nm LIDT measurements are approximately 0.32 mm² and 0.30 mm², respectively. Ten sites are tested for each energy step.

Results and Discussions This study predominantly analyzes the micro-defects with the micron and sub-micron (hundred nanometers) scale. The morphologies and structures of the micro-defects produced by coating (Figs.2 and 3) and substrate polishing (Figs.6-8) are characterized by means of precise cutting and three-dimensional reconstruction, and the element distributions before and after laser irradiation are analyzed using EDS analysis.The results demonstrate that the seeds of submicron nodule defects arise from the ejection of SiO₂ during the deposition process (Figs.4 and 5), whereby the seed diameter is 100 nm. The vacuum pumping and heating during the coating process cause the Na and K ions in the substrate to diffuse and concentrate in layers with a high refractive index (Fig.5). Further, the seeds of micron-scale nodule defects originate from the ejection of SiO₂ or HfO₂ during the deposition process, whereby damaged pits are formed after the nodule defect is irradiated by laser, the edge of pits becomes molten, and the HfO₂ layer near the edge of the damaged pits has a prominent porous columnar structure, of which the atomic fraction ratio of O and Hf is less than 2:1 (Fig.9). Impurity defects in the substrate produce plasma after being irradiated by laser, and the eruption temperature of the plasma is higher than the gasification temperature of HfO₂, which causes the gasified HfO₂ to enter the substrate crack (Fig.10).

Thin-film components of laser systems require excellent optical performance and a high laser-induced damage threshold (LIDT). The micro-defects in the components (such as coating material ejection defects and substrate defects) are the critical cause for the reduced LIDT. To control micro-defects, we must first detect the defects and trace the formation process of micro-defects. In this study, the morphologies and structures of the micro-defects produced by coating and substrate polishing are characterized by precise cutting and three-dimensional reconstruction. Additionally, the element distributions before and after laser irradiation are analyzed. The results clarify the direction of the increasing LIDT of thin-film components.

High-reflection films at 355 nm and 532 nm are deposited on BK7 substrates using the electron beam evaporation. The BK7 substrates are also used for surface topography measurements and additional measurements. Before deposition, the coating chamber is heated to 473 K and evacuated to a base pressure of 1×10^-3 Pa. The deposition rates of HfO₂ and SiO₂ in the multilayer coatings are 0.1 nm/s and 0.2 nm/s, respectively. H and L represent the HfO₂ and SiO₂ layers with a quarter-wavelength optical thickness (QWOT) at 355 nm or 532 nm, respectively (Fig.1).

The results provide detailed data and evidence for improving coating and substrate polishing processes. The analysis results show that to improve the LIDT, the SiO₂ pre-melting and deposition process should be further optimized to avoid 100-nm SiO₂ defects, high-purity quartz substrates should be used to avoid the diffusion of metal ions in substrates, and ion beam polishing technology can be utilized to remove the substrate surface defects caused by traditional polishing processes.