高功率、双通泵浦的Nd:YVO4薄片激光器

在过去的几十年中,二极管泵浦的全固态激光器已经取得了显著的进展。一直以来,二极管泵浦的全固态激光器的发展面临的阻碍之一是:在高泵浦功率密度下所产生的严重的热梯度和像差对输出功率和光束质量的制约。采用薄的、片状的增益介质结合表面冷却和端面泵浦结构——碟片激光器,可以实现高输出功率、良好空间光束质量和高转换效率。因此,碟片激光器在材料加工等方面的广泛应用,已经受到了研究者们的普遍关注和认可。

由于Yb掺杂的激光增益介质具有相对较低的热负载率,高功率薄片激光器的研究最初主要集中在Yb:YAG。然而,Yb掺杂的激光晶体作为准三能级结构,阈值泵浦功率密度较高、对温度的波动非常敏感。相比而言,Nd掺杂晶体的一些光学性能优于Yb掺杂的晶体。其中,Nd:YVO4材料具有一些显著的优势:1064 nm下,受激发射截面大、吸收带宽大、可实现线偏振输出;808 nm下吸收截面大。这对于减少实现有效泵浦吸收的泵浦通数是非常有益的。为了实现有效的泵浦吸收,通常需要采用多通泵浦结构。然而,复杂而昂贵的多通泵浦结构对于实际应用是非常不利的。因此,研究结构简单的Nd:YVO4薄片激光器对于实际应用具有重要的意义。

中科院上海光机所强场激光物理国家重点实验室研制了一套简单、紧凑双通泵浦的Nd:YVO4薄片激光器系统,结合了带内泵浦结构、无掺杂的端面键合和SiC材料热沉来缓解晶体的热效应。研究成果发布在High Power Laser Science and Engineering 2020年第1期上(Wei Wang, Di Sun, Xiao Du, et al. High-power operation of double-pass pumped Nd:YVO4 thin disk laser[J]. High Power Laser Science and Engineering, 2020, 8(1): 01000e10)。

高功率、双通泵浦的Nd:YVO4薄片激光器系统

在双通泵浦的情况下,有效吸收长度为晶体厚度的两倍,由于晶体厚度较小,致使吸收效率较低。为了提高泵浦吸收效率,需要使用掺杂浓度较高或者较厚的晶体。除了有效的泵浦吸收之外,还必须考虑晶体的热效应。由于其高的吸收率和较低的热传导率,Nd:YVO4薄片激光器的最高输出功率、输出光束质量和转换效率等性能都会受到热效应的影响。热透镜是腔型设计的重要因素,热应力会导致晶体断裂,球面像差会导致输出光束质量退化和损耗。

在实验中,研究人员对比了两种不同掺杂浓度和厚度下晶体的输出性能。在采用0.5 at.%掺杂的情况下实现了最高17.7 W的功率输出,对应吸收的泵浦功率下,光-光效率为46%。与此同时,在考虑能量传输上的转换效率和与温度相关的热传导率的因素下,数值模拟了晶体中的温度分布和热透镜效应,并与实验结果进行对比。数值分析是基于有限元分析方法,利用COMSOL软件来完成的,数值模拟的结果与实验结果具有良好的匹配。通过进一步优化相关参数和散热结构,这种简单、紧凑双通泵浦的Nd:YVO4薄片激光器系统有望实现百瓦级高功率输出,可以用于工业加工和基于全固态激光器的可移动无线能量传输等。理论模型可以作为分散面冷却结构设计的有效工具。目前,基于Nd:YVO4薄片激光器的免调节、可移动激光谐振腔的研究正在进行之中。

High power operation of double-pass pumped Nd:YVO4 thin disk laser

Diode-pumped solid-state (DPSS) lasers have made great progress in the past decades. One of the enduring obstacles that DPSS have faced is the restriction from high thermal gradients and aberration under intense pumping conditions. However, with the face cooling configuration of a thin, disk-shaped active medium, the diode-pumped architecture allows building high output power solid-state lasers with excellent spatial beam quality and high conversion efficiency. Since then, thin disk lasers with high power have attracted attention because of their various applications in the material processing industry. Research conducted on high-power thin disk lasers has primarily focused on the Yb:YAG, because it exhibits considerably lower thermal loading factor and broad bandwidth for short pulse output. However, Yb:YAG require a high-pump density to reach the threshold, and are intrinsically sensitive to temperature due to their quasi-three-level nature. The spectroscopic parameters of the Nd-doped materials are superior to those of the Yb-doped materials. Among the Nd doped materials, Nd:YVO4 offers several advantages: a large stimulated emission cross section at 1064 nm, linearly polarized emissions, broad absorption bandwidth, and high absorption cross section at 808 nm pump wavelength, which is particularly advantageous for reducing passes of the pump radiation to achieve efficient absorption. Improvement of absorption efficiency is typically achieved by multi-pass pumping scheme. However, this complicated and expensive multi-pass pumping architecture is not desirable for many applications. Therefore, studying a simple structure of Nd:YVO4 thin disk laser is of great significance for practical applications.

For Nd:YVO4 thin disk laser, low absorption of the pump radiation occurs because the effective absorption path length is twice the thickness of the disk. Improvement of absorption efficiency implies the use of high doped or thick crystals to compensate for the lower absorption. In addition to effective pump absorption, the thermal effects of the crystal must also be considered. In practice, the performance of Nd:YVO4 thin disk laser in terms of maximum output power, beam quality, and efficiency, are often affected by thermal effects due to its strong absorption and moderate thermal conductivity. The thermal lens is a critical factor for resonator design. Moreover, the thermal stress will ultimately induce crystal fracture and the spherical aberration will induce degradation in beam quality and resonator losses.

The research group led by Prof. Xiaoyan Liang from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences demonstrated a simple, compact double-pass pumped Nd:YVO4 thin disk laser. The scheme of in-band pumping, undoped end face bonds, and SiC material heat sink were combined to alleviate the thermal effect. The influence of Nd doping concentration and thickness on the continuous-wave performance were experimentally investigated. The related research results are published on High Power Laser Science and Engineering Vol. 8, Issue 1, 2020 (Wei Wang, Di Sun, Xiao Du, et al. High-power operation of double-pass pumped Nd:YVO4 thin disk laser[J]. High Power Laser Science and Engineering, 2020, 8(1): 01000e10).

A simple, compact double-pass pumped Nd:YVO4 thin disk laser

The maximum output power of 17.7 W is achieved by employing a 0.5 at.% doped sample, corresponding to an optical-to-optical efficiency of 46% with respect to the absorbed pump power. At the same time, a numerical analysis of the temperature distribution and thermal lens effect in the laser considering the influence of energy transfer upconversion effect was presented and compared with the experimental results. The effect of the temperature dependence of the thermal conductivity tensor was also taken into account. The numerical simulations were based on the finite-element analysis methods and carried out using the COMSOL software, which matched well with the experimental results. Such a simple and compact dual-pass pumped Nd:YVO4 thin disk laser system can be used to achieve mobile wireless power transmission based on all-solid-state lasers. Consequently, the theoretical model can be employed as a useful tool for the design of the distributed face cooling scheme.

In order to achieve higher laser output power, a heat sink of diamond material can be used to further alleviate the thermal effect of the crystal. In addition, the research on the adjustment-free and movable laser resonator based on Nd:YVO4 thin disk laser is also under study.