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.

在高能固体激光器中,通常每增加一片增益介质都要增加与其对应的一整套冷却系统,随着高能固体激光器功率的进一步提高,激光器系统体积越来越庞大,并且对激光器的热管理提出了越来越高的要求.直接液体冷却薄片固体激光器因其优越的热管理及非常小的体积输出功率比,近年来成为新型固体激光器研究的热点.本文介绍了直接液体冷却薄片固体激光器概念的提出,阐述了该类激光器的特性,并提出了该类激光器的分类.分析了两类直接液体冷却薄片固体激光器的研究进展以及在获取高光束质量方面的技术挑战.

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.

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.

The design of the direct-liquid-cooled thin-disk laser is detailedly discussed. It includes the choosing of crystal and coolant in the gain module as well as the design of the channel structure. Moreover, the advantages and disadvantages of two kinds of combination modes are analyzed. In the first mode, the incident angle of the beam and the cutting angle of the crystal should be controlled strictly. The theory calculations and analysis are given. In the second mode, the angles do not need to be specially selected, while the refractive index of the coolant should be close to that of the crystal. In the aspect of pump mode selection, the energy storage and aberration effects both in end-pumped and side-pumped lasers are analyzed. Theoretically, with the use of 10 pieces of Nd:YLF thin-disks as the gain media and the refractive index matched liquid as the coolant, an average output power larger than 1 kW is achieved at the pump power of 5 kW, corresponding to an optical-optical efficiency larger than 20%. The theoretical analysis is basically consistent with the experimental results.

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 direct-liquid-cooled Nd:YLF thin disk laser resonator is presented, which features the use of refractive index matching liquid (RIML) as coolant. Highly uniform pump intensity distribution with rectangular shape is realized by using metallic planar waveguides. Much attention has been paid on the design of the gain module, including how to achieve excellent cooling ability with multi-channel coolers and how to choose the doping levels of the crystals for realizing well-distributed pump absorption. The flow velocity of the coolant is found to be a key parameter for laser performance and optimized to keep it in laminar flow status for dissipating unwanted heat load. A single channel device is used to measure the convective heat transfer coefficient (CHTC) at different flow velocities. Accordingly, the thermal stress in the disk is analyzed numerically and the maximum permissible thermal load is estimated. Experimentally, with ten pieces of a-cut Nd:YLF thin disks of different doping levels, a linear polarized laser with an average output power of 1120 W is achieved at the pump power of 5202 W, corresponding to an optical-optical efficiency of 21.5%, and a slope efficiency of 30.8%. Furthermore, the wavefront aberration of the gain module is measured to be quite weak, with a peak to valley (PV) value of 4.0 μm when it is pumped at 5202 W, which enables the feasibility of its application in an unstable resonator. To the best of our knowledge, this is the first demonstration of kilowatt-level direct-'refractive index matching liquid'-cooled Nd:YLF thin disk laser resonator.

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.

A Nd:YAG thin disk is end-pumped by two high power laser diodes and the fluid flows in a narrow channel to cool it directly. The forced convection occurs between the fluid and disk. A system is designed to measure the convective heat transfer coefficient with different flow rate. With the measured coefficient, the temperature and thermal stress in the disk are numerically analyzed. The maximum permissible thermal load is calculated, which increases with the increasing flow rate. Furthermore, the optical path different distribution is numerically calculated by considering of the thermo-optical effect, and thermal expansion at the maximum permissible thermal load. These results are useful for design of a direct-liquid-cooled Nd:YAG thin disk laser.

A large-aperture Nd:YLF thin disk laser oscillator is demonstrated, in which the refractive index matching liquid is used as the coolant flowing in narrow channels to cool the multiple thin disks directly. A high uniformity of pump intensity distribution is realized by using waveguides. With the a-cut Nd:YLF thin disks at different doping levels, a linearly polarized laser with the maximum output energy of 346 mJ is achieved with the repetition of 350 Hz, corresponding to an optical-optical efficiency of 8.7%, and a slope efficiency of 10%. The beam quality β factor is estimated less than 8 in the horizontal direction due to the positive branch confocal unstable resonator. To the best of our knowledge, it is the first time that the direct-liquid-cooled Nd:YLF thin disk unstable resonator is reported.

建立了包括非热稳定热力学模型、光学传输模型、能量转换模型及非稳腔模型的综合理论分析平台.理论分析得到以下主要结论:谐振腔长度变化对光束质量及光光效率的影响不大,对于给定的增益,激光最佳效率和光束质量分别对应不同的非稳腔放大率.提出非稳腔腔内相位板补偿介质静态畸变提高光束质量的技术措施,并开展实验验证,光束质量由补偿前的5.48倍衍射极限提升到补偿后的2.46倍,输出功率为12.1 kW.

为了研究Yb:YAG薄片激光器的性能,采用16通抽运耦合、微通道冷却的方法,对薄片的优化厚度、热力学特性和激光性能等进行了理论分析,利用直径10mm、厚度为250μm、掺杂原子数分数为0.1的Yb:YAG薄片进行了实验验证,抽运耦合系统实现了对薄片的16通抽运,在抽运功率为81.9W时,获得了平均功率为24.4W的激光输出,光光转换效率达到了29.8%.结果表明,多通抽运微通道冷却Yb:YAG激光器可以获得较高的光光转换效率.

利用Nd:GGG晶体开展了非稳腔的设计研究,通过对抽运系统的优化设计,使抽运均匀性提高到94%;建立了热力学计算模型,计算得到了激光介质的温度分布,实验验证了计算模型的可靠性;利用非稳腔理论分析模型优化了非稳腔的设计参数,实现了输出功率大于10kW、平均光束质量为5.84倍衍射极限的激光输出。

We investigate heat generation in a Nd:YAG thin-disk laser composite with an undoped anti-amplified spontaneous emission (ASE) cap and a side ASE absorber under lasing and nonlasing conditions. The heat load unbalance in three different regions induces a large transverse temperature inhomogeneity under the nonlasing condition. The additional heat fraction generated by the concentration quenching is observed.

建立了侧面抽运板条激光器的热力学模型,对千瓦级侧面抽运Nd:YAG板条激光器的温度和应力进行了数值模拟,并对结果进行了分析。在抽运平均功率3890 W时,侧面抽运Nd:YAG板条激光器获得了1012 W平均功率输出,斜率效率约31%,光-光转换效率约26%。测量了板条侧面的温度,与数值模拟结果吻合较好。

均匀抽运及均匀冷却是激光二极管抽运激光器减小热致光学畸变以实现高光束质量输出的核心技术难题.通过开展键合薄片激光均匀抽运及均匀冷却技术的研究,采用透镜压缩结合波导匀化的设计思想,实现了95%以上的抽运均匀性.同时通过对薄片介质冷却通道的优化设计,实现了优于96%的温度分布均匀性.原理验证实验结果表明薄片介质的波前畸变得到了有效控制,当平均抽运功率密度为230 W/cm2时,忽略离焦效应后介质的反射波前畸变均方根值约为0.35μm.

Thin disk laser is the most successful design to overcome the degradation of the beam quality caused by the gain medium's thermal effects and has many advantages in beam quality keeping and power scalability over the traditional rod and slab laser. In this paper a different type thin disk laser with the large-aperture Nd:YAG disk face-pumped by 2D-stack diode arrays was presented. Over 3kW average power with the beam quality less than 10 times diffraction limitation was achieved by optimizing the pumping optics designs and improving the gain medium mounting technique.