This paper presents a comparative investigation on laser-induced damage threshold (LIDT) of polycrystalline Fe:ZnSe pumped at 2.82 mu m and similar to 4.5 mu m with pulse widths of several nanoseconds. The corresponding pump sources were a KTA optical parametric oscillator and a room-temperature Fe:ZnSe laser. The single-pass LIDTs at 2.82 mu m (181 mJ, 8.0 ns) and similar to 4.5 mu m (53.4 mJ, 5.6 ns) were similar to 1J/cm(2) (130 MW/cm(2)) and similar to 0.9 J/cm(2) (160 MW/cm(2)), respectively. The LIDT decreased to an input peak power intensity of similar to 0.48 J/cm(2) (60 MW/cm(2)) at 2.82 mu m if the Fe:ZnSe crystal was implemented in an oscillator during the lasing operation. The main reason that the Fe:ZnSe crystal was damaged at lower incident pump intensity was the higher intra-cavity intensity of the resonated similar to 4.5 mu m laser with shorter pulse width. The enhancement of the pump absorption and laser re-absorption increased the damage risk of the Fe:ZnSe crystal during the lasing operation. The LIDT of the Fe:ZnSe crystal is one of the key parameters for high-energy and high-intensity mid-infrared Fe:ZnSe lasers because it is the main limit for energy scaling.

Nd:YAG solid-state lasers are considered a promising candidate to replace CO2 lasers as the driver laser for extreme ultraviolet (EUV) light sources. Laser-produced plasma (LPP) EUV driver lasers must simultaneously achieve higher than 10 kHz high repetition rate, hundred-millijoule pulse energy, ten-nanosecond pulse width, good beam quality, and long-term operational stability. Achieving these specifications imposes significant technical challenges, particularly in managing waste heat and suppressing thermally induced distortion at elevated temperatures. In response to the requirements of LPP EUV driver lasers, a master oscillator power amplifier (MOPA) architecture is adopted, employing an end-pumped Nd:YAG slab gain module with a zig-zag propagation path as the power amplifier. Short-pulse extraction is achieved through triple-pass amplification in a single slab combined with temporal pulse sequencing. Thermal distortion is mitigated using microchannel coolers laterally bonded to the slab surfaces. A 4f imaging relay system is implemented to image the beam profile, minimizing diffraction-induced intensity modulation during propagation. Based on a laser kinetic model, numerical simulations are carried out to compare the amplification behavior of ∼10 ns short pulses with that of long-pulse or continuous-wave (CW) signals. Output power at various repetition rates is also calculated. A laser system is subsequently constructed, producing laser pulses with a duration of 7.8 ns and an average output power exceeding 1500 W across repetition rates from 10 to 50 kHz. At 10 kHz, an average output power of 1507 W is achieved, corresponding to a peak power of 19.28 MW, and the beam quality factors in the slab thickness and width directions are 1.98 and 3.23, respectively. The single-slab configuration results in a compact overall laser structure, making it suitable as the main-pulse source for benchtop EUV systems. Using this laser as the driver to irradiate liquid metal droplets, the spreading and propulsion velocities of the droplets under various single-pulse energies are experimentally compared.

We demonstrated a high-efficiency mid-infrared (MIR) ZGP optical parametric oscillator (OPO) which was directly-pumped by a Tm:YLF laser. The overall efficiency from LD to MIR was improved by enhancing the cross relaxation in Tm:YLF as the pump laser and optimizing the cavity design of ZGP OPO. Three different types of OPO cavities were compared and investigated. The plano-concave cavity with double-pump-pass and doublyresonated-oscillation configuration achieved the highest output power of 6.3 W at similar to 3.82 mu m with an incident pump power of 9.2 W at 1.908 mu m and an absorbed LD power of 22.79 W at 0.793 mu m. The overall optical-tooptical efficiency (eta(o-o)) and photon efficiency (eta(o-o) lambda(MIR)/lambda(LD)) were 27.6 % and 133 %, respectively. In the linear increasing region above threshold, the slope efficiency from LD to MIR was 46.1 %, indicating that one absorbed 793 nm photon was able to generated more than two similar to 3.82 mu m photons. To the best of our knowledge, this is the highest overall LD-to-MIR efficiency ever reported for OPOs.

Methods This study employs a research method that combines configuration proposal, parameter design, and experimental integration verification. A novel high-power direct-liquid-cooled distributed-reflective-type laser is designed, featuring a distributed gain system composed of tens of Nd:YAG disks densely stacked. A specialized laser cooling liquid flows through planar micro- channels between the gain media disks. Additionally, a Zig-Zag-like laser path is designed within the gain system to achieve high power output. This laser configuration merges the advantages of direct-liquid cooling and the Zig-Zag path. The laser's configuration is optimized. The key factors of the gain media disks, the laser gain of the laser system, temperature distribution, and wavefront aberration are simulated theoretically. Furthermore, an experimental verification platform based on the direct-liquid-cooled distributed- reflective-type MOPA laser was constructed. The laser characteristics, including output power, optical-optical (O-O) efficiency, and far-field distribution, have been obtained. Results and Discussions In the MOPA system, a QCW Nd:YAG rod oscillator was used as the seed, providing an average output power of 0.5 kW with a repetition frequency of 500 Hz and a pulse width of 220 mu s. The temporal profile of the output laser is shown in Fig. 16. As depicted in Fig. 15, a maximum average output power of 21.2 kW was obtained from the entire amplifier chain, corresponding to a peak power of 192.7 kW and a single-pulse energy of 42.4 J, achieved under an average pump power of 56 kW. An O-O conversion efficiency of 36.9 degrees o was achieved with an output of 21.2 kW. Attention is drawn to the extracted power and efficiency of the direct-liquid-cooled laser GMs, as shown in Fig. 17. Notably, the extracted power and O-O efficiency of GM1# were lower than those of GM2#. Specifically, GM1# achieved an extracted power of 9.3 kW with an O-O efficiency of 33.5 degrees o, while GM2# achieved an extracted power of 11.2 kW with an O-O efficiency of 40 degrees o. Two identical gain modules with opposite flow directions were placed in the MOPA to self-compensate for tilt aberration. Figure 18 shows the wavefront and far-field distribution of the amplified output beam. The beam quality, denoted by the diffraction limit multiplier, was measured using a beam analyzer. The analyzer images the beam into the far-field distribution, which is then compared to the ideal far-field distribution to determine the beam quality parameter. The peak-to-valley (PV) and root-mean-square (RMS) values of the output beam were 1.1 mu m and 0.23 mu m, respectively. After defocus and tilt aberration compensation, the wavefront consisted of high-order aberrations. The corresponding beam quality was measured at 4.8 times the diffraction limit. Conclusion A 20 kW-class direct-liquid-cooled MOPA for a direct-liquid-cooled distributed-reflective-type Nd:YAG disk array laser is designed, representing a new scheme with the potential for high laser performance. An average output power of 21.2 kW with an O-O efficiency of 36.9 degrees o is realized in the amplifier chain, with corresponding beam quality measured at 4.6 times the diffraction limit. Due to the high injected peak power density, the extraction efficiency of GM2# reached 40 degrees o. The experimental results demonstrate the validity and feasibility of this novel configuration for high-power operation, particularly in terms of distributed gain and distributed cooling. To our knowledge, the output power demonstrated in this study is the highest reported for a YAG direct- liquid-cooled multi-disk MOPA laser. Furthermore, the direct-liquid-cooled distributed-reflective-type laser shows potential for achieving high beam quality, high efficiency, and high power output in compact solid-state lasers. Objective Solid-state lasers represent a class of compact and efficient high-power laser sources, which are attractive for a broad range of medical, commercial, scientific, and military applications. However, due to the risk of serious thermal optical aberration and fracture of the gain medium, thermal effects become the primary limiting factors in further increasing the output power and beam quality of the laser. To meet the requirements of various practical applications, a compact high-power system with efficient thermal management needs to be developed. Direct-liquid-cooled configuration lasers (DLCLs) have become highly attractive in the high- power laser field due to their excellent heat dissipation capabilities. Multiple disk pieces, arranged as an array, are integrated into a single gain module (GM), leading to a low heat density in each gain disk by dispersing the heat across the entire gain disk array. The circulating liquid flows over the largest surface of the disk, efficiently carrying away the heat. Thanks to easier laser output, DLCL resonators have been extensively studied. However, due to the repetitive superposition of complex wavefront aberrations in DLCL resonators-caused by the coolant flow and gain medium-the laser beam quality is compromised. To overcome the challenge of achieving both high power and good beam quality, new DLCL configurations must be explored. This study demonstrates a new DLCL scheme with high performance (high power, high beam quality, and high efficiency) referred to as the thermal-dispersed reflectivity-type Nd:YAG disk array MOPA (master oscillator power-amplifier) laser.

This paper presents the first demonstration of a mid-infrared (MIR) Fe:ZnSe laser gain-switched by a non-critical phase-matched potassium titanyl arsenate optical parametric oscillator and amplifier at 3.47 mu m. A novel improvement in slope efficiency was achieved by this new pump source, which significantly promoted the quantum efficiency compared to the conventional pump wavelength near 2.9 mu m. The slope efficiency of 70.7% is a new record for Fe:ZnSe lasers with an output energy of 86 mJ and pulse width of 6.7 ns at 10 Hz. The output wavelength was tunable from 3.9 to 4.5 mu m by changing the crystal's temperature from 80 to 300 K. The influence of the pump beam size on transverse parasitic oscillation and crystal damage was investigated considering the dynamic absorption effect in Fe:ZnSe. This unique design provides an advancing and promising method of high-energy and short-pulse-width MIR lasers for extreme applications requiring both high-energy density and high-peak-power intensity.

We demonstrated high-energy and high-efficiency mid-infrared (MIR) optical parametric oscillator and amplifier (OPO and OPA) based on KTA crystals, which were pumped by 1.064 mu m Nd:YAG slab laser with square and nearly uniform profile. The KTA crystals were cut for non-critical phasing matching with its idler wavelength of 3.47 mu m. The highest MIR pulse energy from a single KTA OPO was 223 mJ when the pump energy was 1233 mJ at 10 Hz repetition rate and similar to 10 ns pulse width for the beam size of 7.5 mm x 7.5 mm. The conversion efficiency and quantum efficiency with smaller pump size of 5 mm x 5 mm were 21.2% and 69.3%, respectively. The MIR pulse energy was increased to 293 mJ by an additional KTA OPA stage. The highest MIR output energies ever reported in KTA OPO and OPA would provide useful laser sources for high-intense and high-peak-power MIR applications, and revealed the potential of several-nanosecond joule-level MIR pulse energy if the damage threshold of the coatings was improved.

An unstable resonator with seven large aperture ceramic disks and intra-cavity adaptive correction is presented.The composite ceramic disks with absorption rings were adopted to suppress amplified spontaneous emission.An intra-cavity aberration non-conjugate correction based on round-trip wavefront and relaxation iteration was applied in the resonator.After tilt and defocus were corrected in turn,an average output power of 4.5 kW was obtained.The corresponding beam quality factor β was 19.5.After tilt,defocus,and high order aberrations were corrected,the average output power was increased to 5.4 kW,and the beam quality factor β was improved to 6.8.

A high average power re-frequency operation Fe:ZnSe laser using laser diode side-pumped free-running Er:YAG lasers as activating sources is presented. Two pieces of subsurface layer doped Fe:ZnSe polycrystal are adoptive in a reflective resonator configuration and face-cooled by liquid nitrogen. A maximal Fe:ZnSe laser power of 105 W at a wavelength of 4.1 μm is achieved upon pumping by ten home-made Er:YAG lasers with fiber coupled output working at a frequency of 250 Hz and a pulse duration of ∼420 μs. Corresponding to the maximum Fe:ZnSe laser power, the optical–optical efficiency and slope efficiency with respect to the absorbed pump power are 43% and 44% respectively. The beam quality factor M 2 is measured to be 3.4. To the best of our knowledge, it is the highest output average power of an Fe:ZnSe laser reported.

高能激光广泛应用于材料加工、科学研究、空间碎片清除、军事应用等领域.二极管泵浦高能激光具有结构紧凑,系统简单、全电驱无限弹仓的特点,近年来,各类二极管泵浦高能激光围绕着同时实现高功率、高效率、高光束质量这一总目标发展迅速.详细综述了国内外高平均功率块状固体激光、高功率可见光波段激光、高峰值功率激光、高功率光纤激光、碱金属蒸气激光等二极管泵浦高能激光的研究进展,并对其发展趋势进行了展望.

Thermal effects are the main obstacle for side-pumped direct-liquid-cooled Nd: YAG thin-disk lasers (SDNTDLs) to achieve high-power near-diffraction-limited laser output. A multiphysics model is established and several basic understandings are obtained. The temperature distribution of the disk under lateral non-uniform pumping shows that the total reflection transmission of the pump lights in the disk does not result in severe temperature fluctuations. The thickness of the thermal boundary layer is much less than half the thickness of the fluid, indicating that there is no thermal interaction between two adjacent disks. The component analysis of the thermally-induced wavefront aberration (TIWA) reveals that the influence of the fluid’s thermo-optic effect (TOE) which is awful in liquid lasers is not necessarily dominant in SDNTDLS due to the thin thermal boundary layer. When cooled by D 2 O, the wavefront aberration induced by the disk is much larger than that induced by the fluid. As a result, the TIWA exhibits a distribution of high at both sides and low at the center along the transverse direction of the disk, which is consistent with the experimental results. The wavefront aberration induced by the fluid’s TOE and that induced by the disk’s both TOE and deformation compensate for each other, indicating that the beam quality can be significantly improved by means of design optimization.

对于直接液冷薄片激光器而言,实现功率的定标放大可以在保证泵浦光强恒定的情况下通过增大泵浦光斑的尺寸同时增加增益介质的尺寸或数量来实现.这种设计理念自提出以来就得到了学术界的青睐.本团队针对高功率直接液冷薄片激光器的实际工作条件,建立了单薄片双通道增益模块在对称情况下的模型,研究了微通道高度、薄片厚度、冷却液雷诺数等参数对激光器输出波前畸变的影响.结果表明:当其他条件一定时,随着微通道高度及薄片厚度的减小,激光器输出光束质量提高;随着冷却液雷诺数增大,激光器输出光束质量提高.经遗传算法优化后的设计参数产生的输出光束光程差均方根值为3.27 μm,峰谷值为6.11μm,相较于经典文献设计参数下的均方根值和峰谷值分别降低了 12.3％和15.6％.

The near field of the probe beam after passing through the gain module is measured. The "streamer effect" is first discovered in the experiment, which is characterized as continuity in the flow direction and non-uniformity in the lateral direction with high spatial frequency. To explain the "streamer effect", a fluid–solid coupling model and an unstable resonator oscillation model of the direct-liquid-cooled thin-disk laser are established. The simulation results show that the non-uniformity of the coolant along the width of the flow channel gives rise to the streamer effect. A feasible cooling channel to achieve a high uniformity coolant is proposed and experimentally verified. At CW pumping of 9.6 kW, the influence of coolant on the output beam quality can be controlled at β<2.

SummaryIn order to improve the quality of multi‐disk laser output beam quality, a theoretical method of astigmatism self‐compensation in multi‐disks resonator is developed based on which the maximization pre‐compensation of static aberration in the multi‐disk laser resonator is realized with the geometrical superposition of the astigmatism coefficient using the exhaustive method. In order to verify the theoretical method, a self‐compensating experiment has designed based on the theoretical method, which shows a clear improvement of the beam quality β.

The surface damage performance of fused silica under mono-wavelength configuration or simultaneous dual-wavelength exposures to 1053, 527, and 351 nm nanosecond-laser pulses is investigated in order to probe the mechanisms governing damage initiation during frequency conversion. Based on the absorption and heat transfer mechanism, coupled to laser damage statistics, a model considering the Gaussian distribution of irradiation intensity has been presented to obtain precise calculation result about laser damage probability. Laser damage probability curves are measured on the surface of fused silica with different wavelength configurations. By studying the influence of the laser wavelength configurations on the laser damage probability, we show with our methodology that the types of damage precursors under the irradiation of different wavelength configurations can be identified.

An average 9 kilowatt-level direct-D2O-cooled side-pumped Nd:YAG multi-disk laser resonator at QCW mode with a pulse width of 250μs is presented, in which the straight-through geometry is adopted the oscillating laser propagates through 40 Nd:YAG thin disks and multiple cooling D2O flow layers in the Brewster angle. Much attention has been paid on the design of the gain module, including an analysis of the loss of the laser resonator and the design of the Nd:YAG thin disk. Experimentally, laser output with the highest pulse energy of more than 20 J is obtained at a repetition frequency of 10 Hz. At high repetition frequency, the average output power 9.8 kW with ηo-o = 26% and 9.1 kW with ηo-o = 21.8% are achieved in the stable resonator and unstable resonator, respectively, and in the corresponding beam quality factor βstable= 14.7 and βunstable= 9.5 respectively. To the best of our knowledge, this is the first demonstration of a 9 kilowatt-level direct-liquid-cooled Nd:YAG thin disk laser resonator.

A novel high-power direct-liquid-cooled thin-disk solid-state laser is designed, in which the distributed gain system is composed of tens or hundreds transmission disks by intensive stacking. A special kind of laser cooling liquid flows in the planar micro-channels between gain media, and thus the direct cooling of disks is realized. The thermal stress, the reflective surface deformation and so on caused by the soldering between gain media and the heat-sink in the traditional high power solid-state laser arc successfully avoided. In addition, the parameters such as intra-cavity loss and aberration arc optimized. The key factors influencing the optical-to-optical conversion efficiency arc analyzed, and the methods for controlling laser beam quality arc introduced according to the thermal aberration characteristics. A gain module is composed of 20 disks by intensive stacking with a special angle. With these gain modules, a quasi-continuous-wave (QCW) polarized laser with an output power of larger than 9 kW is obtained in both stable and unstable cavities. Moreover, the whole volume of this laser source in laboratory is smaller than 0.1 m(3).

A direct-liquid-cooling side-pumped Nd:YAG multi-disk laser resonator works in quasi-continuous state is presented, in which twenty Nd:YAG thin disks side-pumped by laser diode arrays are directly cooled by flowing siloxane solution at the end surfaces, while oscillating laser propagates through multiple thin disks and cooling flow layers in Brewster angle. The laminar flow cooling flow field is designed to cool the thin disk. The dissipation capability of the inhomogeneity of the incoming flow is verified by numerical simulation. According to the experiment reported before, a numerical model based on laminar flow is built to measure the cooling ability of the flow field. The experimental result verifies the reliability of numerical model, the thermal safety of the thin disk in laser device is evaluated based on the model. The maximum pulse energy output of 15.7Jis obtained at the pump energy of 49.9J, corresponding to an optical-optical efficiency of 31.4％ and a slope efficiency of 39.2％. The average output power of 1440Wis achieved at the pump pulse width of 250μs, repetition frequency of 100Hz, and average pumping energy of 5kW.

A direct-liquid-cooled side-pumped Nd:YAG multi-disk QCW laser resonator is presented, in which the oscillating laser propagates through multiple thin disks and cooling flow layers in Brewster angle. Twenty Nd:YAG thin disks side-pumped by LD arrays are directly cooled by flowing deuteroxide at the end surfaces. A laser output with the highest pulse energy of 17.04 J is obtained at the pulse width of 250 μs and repetition rate of 25 Hz, corresponding to an optical-optical efficiency of 34.1% and a slope efficiency of 44.5%. The maximum average output power of 7.48 kW is achieved at the repetition rate of 500 Hz. Due to thermal effects, the corresponding optical-optical efficiency decreases to 30%. Under the 12.5 kW pumping condition while not oscillating, the wavefront of a He-Ne probe passing through the gain module is as low as 0.256 μm (RMS) with the defocus and tetrafoil subtracted.

A waveguide scheme is constructed by coating the matrix of randomly distributed ZnSe nanosheet structures with a layer of dye-doped polymer, which provides strong feedback or gain channels for the emission from the dye molecules and enables successful running of a random laser with FWHM of ~0.65 nm. The strong scattering by the nanostructures and the strong confinement provided by the active waveguide layer are the key essentials for the narrow-band and low-threshold operation of this random laser. The random laser scheme reveals an obvious two-threshold behavior, which is corresponding to the thresholds of TM and TE modes. The feedback mechanisms for laser action are investigated by power Fourier transforming of the spectra. This kind of active waveguide not only provides high quality confinement of the radiation for efficient amplification, but also enables possible directional output of this kind of random laser.

Thin films of Nd : YAG and Nd : Glass were prepared on Si (100) substrate by pulsed laser deposition technology. The morphology of film surface and cross section, composition, absorption spectrum and photoluminescence (PL) spectra of films were investigated by scanning electron microscope (SEM), energy disperse spectroscopy (EDS), Fourier transform infrared spectrometer(FTIR), optical parametric oscillator(OPO) and grating spectrometer. The results show that both Nd : YAG films and Nd : Glass films grown on the substrates at room temperature are amorphous. Nd : YAG films grown by PLD contain Nd element with 0. 15 at. % stoichiometric proportion. The absorption spectrum of bulk Nd : YAG target rather than deposited films exhibit two absorption peaks at 750 and 808 nm. There are no evident peaks in the photoluminescence spectra curve of Nd : YAG films. However, the photoluminescence spectra of Nd : Glass films with two sharp peaks at the wavelength of 877 and 1 064 nm are observed. It indicates that Nd is doped into glass host as optically active Nd3+ ions when Nd : Glass films grow at room temperature. But for Nd : YAG films, Nd don't incorporate into YAG host as Nd3+ ions.