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.     

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     

Dependence of curvature type of thermal lensing on number of bounces in a zigzag slab laser: numerical modeling

The curvature type of the thermal lens generated in a zigzag slab laser is numerically analysed. It is found that the curvature type of the thermal lens varies alternatively between the convex and the concave lenses with the number of bounces of light within the slab, which can be well explained by the trace of the zigzag propagation. In addition, we conclude that the beamlet with a larger number of bounces experiences weaker thermal lensing but more serious wavefront deformation due to the large side lobe portion in the curve of optical path difference.     

表面热透镜技术测量3.8μm和2.8μm激光薄膜的微弱吸收

 采用表面热透镜技术，对3.8μm和2.8μm激光辐照下镀制在Si基底上的单层ZnS，YbF₃和YBC薄膜及不同膜系的YbF₃/ ZnS多层分光膜和多层高反膜，以及镀制在CaF₂基底上的增透膜进行了吸收测量，并对3.8μm和2.8μm 激光的测量结果进行了比较分析。实验结果表明，2.8μm波长下的吸收比3.8μm的大得多，两者之间约相差一个量级，测得的多层高反膜YbF₃/ZnS薄膜在的3.8μm处的最低吸收为4.57×10^－4，测量系统的灵敏度约为10^-5。     

Experimental isolation and measurement of laser-induced effects: thermal lensing, turbidity and the optical Kerr effect

Optically induced effects such as thermal lensing, turbidity and the optical Kerr effect in liquids and reversed micellar solutions have been reported but not thoroughly compared, in terms of relative size and significance. Under the present experimental conditions, the largest effect is thermal lensing, which is about 86 and 43 times greater, on average, than the effect of turbidity, when polarized and nonpolarized laser pulses are applied, respectively, in microseconds, to certain liquids. The smallest effect is electric birefringence which is about 14% that of turbidity in a 0.1 M aerosol-OT/CCI₄ solution. A typical experimental set-up to distinguish the above effects is presented.     

Reduction of beam divergence of a thermal lensing insensitive Nd:YAG laser

In a previous paper we proposed an active resonator designed especially for Nd:YAG lasers. The beam generated by this resonator had a flat-top beam profile and a divergence that was insensitive to the thermal lensing effect. The beam generated by this resonator was, however, too divergent to be focused into a fibre. With the help of numerical simulations, we have found a way to reduce this divergence, and to render the far-field of the laser beam fine enough to be coupled into a fibre with 600 μm core and NA = 0.2.     

Study of thermal lensing effects in a copper vapor laser

The thermal lensing effect of the Copper Vapor Laser (CVL) was studied by considering both its windows and the active medium. The equivalent combined focal length of the active medium and windows was measured to be 52 mts under operating conditions for a 35W CVL. The variation of focal length with the operating parameters was studied.     

Population lensing effect in Cr:LiSAF probed by Z-scan technique

An experiment has been performed for measuring a nonlinear refractive index that is due to the polarizability difference Δ_P between excited (⁴T₂) and ground (⁴A₂) states in a Cr³⁺:LiSAF crystal at λ = 647 nm. The latter one is responsible for a population lensing effect which has been monitored by using the eclipsing Z-scan technique. We have performed a data analysis that allows to distinguish between thermal and population contributions to the lensing effect. We have found Δ_P = 4.6 × 10⁻²⁵ cm³ which is in a good agreement with our previous measurements with a different technique.     

Comparative study between interferometric and Z-scan techniques for thermal lensing characterization

This work presents an original, comparative study between two optical techniques for the analysis of thermal lensing induced by a low-power, cw laser beam focused onto a sample cell containing a weak absorbing medium. It deals with an interferometric technique and a Z-scan technique in real time. The interferometric method permits the determination of the spatial profile of the thermal lens. The development of the work puts in evidence the high sensitivity of both techniques for the detection and measurement of low absorption coefficients and refractive index changes in dye solutions at very low concentrations. Improvements in the sensitivity of both methods can make possible the measurement of very small phase shift distortions of the wavefront. One shows also the mutual complementary character of two techniques for the characterization and measurement of linear and nonlinear properties of materials.     

Estimation of thermal lensing effect in the high-power end-pumped direct-cut crystal lasers

At this working, analytical (with Gaussian pump beam profile) and numerical FEM (with super-Gaussian pump beam profile) methods were compared for estimation of thermal lensing effect in the diode-end-pumped solid-state laser rods. These results were applied to the design and realization of a direct-cut (i.e., cut in right-angle ) crystal Nd:YVO₄ laser. The results showed that for such crystal (direct-cut), two methods obtained almost the same results, but slightly different from experimental results. However, estimation of thermal lensing effect led to realization of a diode-end-pumped Nd:YVO₄ cw high-power laser with good optical quality.     

Thermal lensing measurements and thermal conductivity of Yb:YAB

By using holographic shear interferometry, we have measured the thermal lens in a diode-pumped Yb:YAB crystal, under lasing and non-lasing conditions, as a function of absorbed power. From these purely optical measurements we estimate the fractional thermal loading and the thermal conductivity of Yb:YAB. Knowledge of these thermal parameters is essential for the optimisation of Yb:YAB laser cavity designs. Received: 5 February 2002 / Final version: 22 November 2002 / Published online: 26 February 2003 RID="*" ID="*"Corresponding author. Fax +61-2/9850-8915, E-mail: Judith@ics.mq.edu.au     

A new thermal conductivity model for nanofluids

In a quiescent suspension, nanoparticles move randomly and thereby carry relatively large volumes of surrounding liquid with them. This micro-scale interaction may occur between hot and cold regions, resulting in a lower local temperature gradient for a given heat flux compared with the pure liquid case. Thus, as a result of Brownian motion, the effective thermal conductivity, k_eff, which is composed of the particles conventional static part and the Brownian motion part, increases to result in a lower temperature gradient for a given heat flux. To capture these transport phenomena, a new thermal conductivity model for nanofluids has been developed, which takes the effects of particle size, particle volume fraction and temperature dependence as well as properties of base liquid and particle phase into consideration by considering surrounding liquid traveling with randomly moving nanoparticles.The strong dependence of the effective thermal conductivity on temperature and material properties of both particle and carrier fluid was attributed to the long impact range of the interparticle potential, which influences the particle motion. In the new model, the impact of Brownian motion is more effective at higher temperatures, as also observed experimentally. Specifically, the new model was tested with simple thermal conduction cases, and demonstrated that for a given heat flux, the temperature gradient changes significantly due to a variable thermal conductivity which mainly depends on particle volume fraction, particle size, particle material and temperature. To improve the accuracy and versatility of the k_effmodel, more experimental data sets are needed.     

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.     

端面抽运Tm,Ho:YLF激光器热转换系数及热透镜效应的研究

根据激光二极管纵向抽运Tm,Ho:YLF激光器的能级跃迁和能量传递过程，在考虑能量传递上转换和基态重吸收的前提下，建立了准三能级速率方程理论模型.给出了Tm,Ho:YLF激光器的热转换系数的解析表达式，讨论了抽运光斑半径与激光束腰半径的比值对热转换系数的影响.实验上得到了热焦距随抽运功率的变化关系，并将其与理论结果进行了对比，发现符合较好. 关键词： Tm Ho:YLF晶体 能量传递上转换 速率方程 热转换系数     

Validation experiments for spatially resolved one-dimensional emission spectroscopy temperature measurements by dual-pump CARS in a sooting flame

In this work, an emission spectroscopic (ES) technique for the determination of soot temperatures in an axisymmetric flame is validated against coherent anti-Stokes Raman spectroscopy (CARS) as preparation for a zero gravity measurement campaign. In order to reduce valuable measurement times during the parabolic flights, a setup for spatially resolved one-dimensional measurements along a line has been built up using an imaging spectrometer and a CCD-camera for data acquisition.In order to assess the performance of the ES technique, accurate coherent anti-Stokes Raman scattering (CARS) measurements have been performed at the same conditions. Hereby, great care has to be taken to minimize the known interferences in sooting flames. The non-resonant signal contribution was suppressed by a polarization technique and the signal wavelength was shifted to an interference free region. These modifications are discussed in detail. The temperatures obtained by both methods are in good agreement in most regions of the flame. It turned out that the described approach for ES is an appropriate and robust alternative for measuring temperatures in sooting axisymmetric flames, if more accurate techniques cannot be applied, e.g., during investigations in zero-g parabolic flight experiments.     

一种测量二极管侧面抽运固体激光器热焦距的方法

提出了一种能够简单、准确测量二极管侧面抽运固体激光器热透镜焦距的方法,明确提出采用非对称平平腔测量时,激光晶体端面与腔镜的间距不需要为零,并从实验上证明了方法的可靠性. 应用该方法的测量结果可以优化复合折叠和频黄光激光器的结构,保证两束基频在腔内达到最佳的空间重合,提高腔内和频效率. 关键词： 全固态激光器 Nd∶YAG激光器 热焦距 腔内和频     

Strong gravitational lensing for photon coupled to Weyl tensor in Kiselev black hole

The objective of the present work is to highlight the phenomena of strong gravitational lensing and deflection angle for the photon coupling with the Weyl tensor in a Kiselev black hole. Here, we have extended the prior work of Chen and Jing (S. Chen and J. Jing, JCAP, 10: 002 (2015)) for a Schwarzschild black hole to a Kiselev black hole. For this purpose, the equation of motion for the photons coupled to the Weyl tensor, null geodesic, and equation of photon sphere in a Kiselev black hole spacetime have been formulated. It is found that the equation of motion of the photons depends not only on the coupling between the photons and the Weyl tensor, but also on the polarization direction of the photons. There is a critical value of the coupling parameter, \begin{document}$ \alpha$\end{document} , for the existence of the marginally circular photon orbit outside the event horizon, which depends on the parameters of the black hole and the polarization direction of the photons. Further, the polarization directions of the coupled photons and the coupling parameter, \begin{document}$ \alpha$\end{document}; both modify the features of the photon sphere, angle of deflection, and functions \begin{document}$ (\bar{a}$\end{document} and \begin{document}$ \bar{b})$\end{document} owing to the strong gravitational lensing in the Kiselev black hole spacetime. In addition to this, the observable gravitational lensing quantities and the shadows of the Kiselev black hole spacetime are presented in detail.     

热波法测良导体热导率实验中高频热波影响的分析与消除

在热波法测良导体热导率的实验中,由于高频热波的影响,使实验结果产生很大的误差.现提出一种可有效解决高频热波造成误差的方法,即利用计算机数字滤波滤去高频波,提取基波,从而提高了实验精度.     

激光窗口热透镜效应对光束质量的影响

分析了高功率激光通过窗口时,引起的窗口热透镜效应及其对激光光束质量的影响。用冲量定理方法,推导了二维热传导方程的广义傅里叶级数解,然后按照平面热应力问题,运用有限元方法数值计算热变形。考虑热变形、折射率随温度变化,忽略光弹性效应,解出出光面的波前相差分布,讨论了远场光束质量。以空心环形光束为例,计算了熔融石英(SiO2)和白宝石(Al2O3)两种常用窗口的温升分布、热变形、波前相差和光束质量,分析了冷却、辐照时间、光强分布的空间梯度和各种材料参数对光束质量的影响。