端面抽运固体激光器热透镜效应的实验研究

提出一种易于实现、对激光器本身没有干扰、高空间精度、可实时测量介质热透镜的干涉测量方法. 并利用此方法全面研究了端面抽运Nd:YVO4激光器的热透镜效应，为认识激光介质的热效应和设计此类激光器提供了参考.  关键词： 热透镜 端面抽运 固体激光器     

The thermal effect in a grazing-incidence slab laser with the novel composite cooling method

The thermal properties of the grazing-incidence slab laser are investigated when cooling with a novel composite method, both conducted cooling and air cooling. The 3D temperature distribution and the correspondent thermal lensing effect were analytically modeled and numerically calculated. The absorption saturation and stress-optic effect were taken into account in the modeling. The influence of the parameters of the laser on the thermal lensing effect were investigated in detail. The results show a lower temperature and weaker thermal lensing effect for the composite cooling than for the traditional contaction cooling.     

Numerical modeling of the thermal lensing effect in a grazing-incidence laser

The numerical modeling of thermal lensing effect is investigated in a grazing-incidence laser. The deformation of the bounce face is introduced into the modeling for the first time, and the Gaussian distribution of the pump light and the anisotropic heat conduction are considered. The results indicate that the proportion of the deformation on the bounce face to the thermal lensing effect is as high as 80% for small grazing-incident angle of 5°. The thermal lensing effect sensitively depends on the pump power, grazing-incident angle and the pump distribution in a grazing-incidence bounce geometry laser.     

气体激光器中的类透镜效应分析

本文讨论了类负透镜介质一般曲面镜光学谐振腔,并用来分析气体激光器中的热透镜效应。对平凹腔连续波二氧化碳激光器进行了模式分析。结果表明,热透镜效应对输出功率和频率特性影响相当大,对基模输出激光器的影响最为突出。最后,简单地讨论了离子气体激光器和某些其它激光器中的可能热透镜效应。     

Experimental Investigation of Thermal Lensing in End-pumped Nd:YVO4 Crystal Using a New Simple Approach

The stability of linear planar resonators involved in solid-state lasers is entirely decided by the thermal lens of the laser crystal. There exists a critical point between stable state and unstable state at which the thermally focusing point falls on the surface of one end mirror. Based on this fact, a new method for measuring the thermal focal length has been presented, and applied to three different Nd3+-doped (2%, 0.8%, and 0.5% ) Nd:YVO4 crystals at various absorbed pump power levels up to 10 W, the results show a good agreement with those of theoretical calculation. It also indicates that the thermal lensing is strongly dependent on the Nd3+-doped concentration, and is affected by crystal surface temperature, especially in the situation of weak thermal lensing.     

反射式热透镜时变信号测量固体热扩散率的研究

介绍了采用反射式热透镜时变信号测量固体热扩散率的一种简便方法,该方法结合表面热透镜技术原理,采用表面透镜的动态变化和反射的探测光发散的幅度来确定固体的热扩散率,相对于传统的热扩散率测量方法要快捷简单。对较大范围的一系列物质的热学特性进行了实验研究,证明了其实用性。     

Temperature dependence of efficiency and thermal lensing of diode-laser-pumped Nd:YAG lasers

Efficiencies, lasing thresholds and thermal lensing of a Nd:YAG rod side-pumped with two 10 W diode-laser-arrays are measured for various rod temperatures between 80 K and 380 K. The threshold is reduced by a factor of 2.5–4 on cooling from room temperature to about 100 K, while the slope efficiency is increased by between 0% and 50% depending on the reflectivity of the outcoupling mirror. Extrapolation to a 240 W pumped laser gives an increase in optical efficiency of 25% to 30%. The thermal lensing measured with a beam deflection and an interferometric method is reduced by a factor of 10.     

Experimental Investigation of Thermal Lensing in End-pumped Nd:YVO4 Crystal Using a New Simple Approach

The stability of linear planar resonators involved in solid-state lasers is entirely decided by the thermal lens of the laser crystal. There exists a critical point between stable state and unstable state at which the thermally focusing point falls on the surface of one end mirror. Based on this fact, a new method for measuring the thermal focal length has been presented, and applied to three different Nd3+-doped (2%, 0.8%, and 0.5% ) Nd∶YVO4 crystals at various absorbed pump power levels up to 10 W, the results show a good agreement with those of theoretical calculation. It also indicates that the thermal lensing is strongly dependent on the Nd3+-doped concentration, and is affected by crystal surface temperature, especially in the situation of weak thermal lensing.     

固体激光器热透镜效应的调控

利用工作腔的热透镜模型, 分析了有激光发射下固体激光器的谐振腔配置对热透镜效应和稳定区的影响, 论述了通过改变谐振腔配置调控热效应的可行性.实验上, 使用一台Nd: YAG激光器对理论作了验证, 实现了对热透镜光焦度及稳定区分布的调控.     

Theoretical investigation on thermal lensing effects of Yb:KY(WO4)2 in diode-pumped lasers

Based on the features of laser diode end-pumped lasers, a thermal model of Yb:KY(WO₄)₂(Yb:KYW) with square cross-section was established. Considering the heat transfer on side faces, the anisotropic of thermal conductivity and the latest reports about thermo-optic coefficient, thermal expansion coefficient and thermal conductivity, temperature distribution, end-pumped face distortion and thermal lens focal length of Yb:KYW were more precisely obtained using finite difference method to solve Poisson equation in a rectangular Cartesian coordinate for the first time. At the pump power of 14 W, the highest temperature located at the center of end-pumped face was 243.8 °C, the highest distortion was 0.28 μm, and the thermal lens focal length was 5.4 cm along z-axis and −4.9 cm along x-axis. The results show that thermal lensing effects in the b-cut Yb:KYW were mainly determined by the anisotropic thermal expansion of Yb:KYW, and further present thermal lensing effects become weaker after considering the heat transfer. This work is significant for compensating the thermal lensing effect and improving the resonator stability of diode-pumped anisotropy crystal lasers.     

Influences of pump beam distribution on thermal lensing spherical aberration in an LD end-pumped Nd:YAG laser

A numerical investigation is made on the thermal lensing and spherical aberration effect in an LD end-pumped Nd:YAG laser. Based on the finite element method (FEM), the laser rod temperature distribution is calculated and the focal length of the thermal lens is deduced, the influences of pump beam on the thermal lensing spherical aberration are mainly studied. The results show the thermal lens which focal length varied with the radial coordinate r is not an ideal lens. Given the heat dissipation boundary conditions, the radial dependent focal length will be shortened when the pump power raised or the waist radius of the pump beam reduced, meanwhile the radial differences of the focal length will decrease when the pump power increased. For a Super-Gaussian profile pump beam, the higher the exponent number, the more similar to flat top the pump beam is, and the less the radial differences of the focal length are.     

Transient thermal lensing measurement in a laser diode pumped NdxY1−xAl3(BO3)4 laser using a holographic shearing interferometer

A new technique for measuring a thermal lens, using a holographic shearing interferometer is presented. This technique was used to measure transient thermal lensing in a laser diode pumped NYAB laser. The measured thermal lensing power was proportional to the pumping laser diode intensity, with a gradient of 1.1 × 10⁻¹ m⁻¹ mm²/W. The transient response time of the thermal lens was 1.5 s, this value being consistent with the temporal decline of the second harmonic power.     

Thermal lensing compensation principle for the ACIGA's High Optical Power Test Facility Test 1

Thermal lensing is becoming recognized as one of the dominant obstacles to the second generation of laser interferometric gravitational wave detectors. Very high optical power is required to circulate in the interferometer to reach the sensitivity goal, creating strong thermal induced wavefront distortion. These effects will be studied at the High Optical Power Test Facility in Gingin, Western Australia. In this paper, we present simulation results for the first test planned for the middle of 2004. This experiment will produce 5 kW of optical power circulating inside a Fabry–Perot cavity and will demonstrate large thermal lensing effects. Two compensation methods were investigated to offset the negative effect of thermal lensing on the cavity: a compensation plate within the arm cavity and adaptive laser mode matching. Advantages and disadvantages of both systems are discussed.     

Nonlinear absorption and refraction effect on thermal lensing of solid-state laserrods

In this work, the effect of nonlinear absorption and refraction on thermal lensing of an end-pumped solid-state laser rod is investigated. At high input pump powers and on the focused region of thermal lensing, nonlinear responses of the active medium will be important. By considering the nonlinear absorption and refraction, the intensity distribution inside the rod is obtained. A convergence lens models the intensity dependency of the refractive index. By modeling of the laser rod with some contributed effective lenses, the stability condition of a typical laser resonator is investigated.     

热透镜控制激光光束质量技术研究

在端面抽运固体激光器中,就如何改善在高抽运功率时输出激光的光束质量,提出一种激光器的设计方法。在激光器谐振腔中放入多根掺杂浓度不同的激光介质,利用激光介质内部产生的热透镜控制抽运光和基模振荡光的空间分布,并且最大限度地使抽运光的分布区和基模振荡光分布区重叠,实现抽运光与基模振荡光在空间上高度匹配,进而提高抽运光能量的利用效率和振荡光的光束质量。实验表明,在不同抽运功率下,抽运光和基模振荡光在晶体内部的光斑的空间分布可通过热透镜加以控制。在端面抽运功率200 W附近时,实现了抽运光与基模振荡光较高程度匹配,光束质量因子M2由14.7改善为4.1。     

棒状钕玻璃热容激光器热透镜自平衡补偿方法分析

针对热容激光器的热透镜效应,给出了棒状钕玻璃热容激光器的热透镜公式表达式。理论上分析了热透镜焦距自平衡补偿方法的基本原理。在考虑了棒状钕玻璃热容激光器的具体特性的基础上,设计了一种简单的热透镜焦距自平衡补偿方法,使光抽运钕玻璃热容激光器在热透镜焦距变化情况下,可以使激光输出光斑大小稳定。为了解棒状热容激光器热透镜焦距的影响,利用自研的光栅型曲率传感器测量了热容激光器总的热透镜等效焦距的大小,结果表明热传导在抽运光发射完20 s左右后才建立热平衡产生了明显的热透镜效应; 随着抽运脉冲个数的增加,热透镜效应随之变大。     

Experimental investigation of optical limiting and thermal lensing in toluene solutions of C70

Optical limiting and thermal lensing studies are carried out in C₇₀-toluene solutions. The measurements are performed using 9-ns pulses generated from a frequency-doubled Nd:YAG laser at 532 nm. Optical limiting studies in fullerene molecules lead to the conclusion that reverse saturable absorption is the major mechanism for limiting. Analysis of thermal lensing measurements showed a quadratic dependence of thermal lens signal on incident laser energy, which also supports the view that optical limiting in C₇₀ arises due to sequential two-photon absorption via excited triplet state (reverse saturable absorption).     

Review of the formula of thermal focal length in side-pumped laser rods

The expressions of focal length in side-pumped rod lasers are studied. Good agreement is obtained between simulation and experiments reported previously, using Koechner's theory. The results reveal that underestimation of thermal lens accrues from overestimating the material's thermal conduction and without considering the nonuniform heat deposit. In the case of Nd:YAG rods, temperature-dependent variation of the refractive index constitutes the major contribution of thermal lensing, whereas the temperature- and stress-dependent variations of the refractive index make the major contributions of thermal lensing for Nd:Glass rod lasers.     

表面热透镜技术探测光学薄膜的微弱吸收

对表面热透镜技术测量光学薄膜的微弱吸收进行了理论分析,并以此建立了薄膜微弱吸收测量实验装置,对几种典型薄膜的吸收进行了测试,证实了这种方法的可行性。