【作者】 欧群飞；

【导师】 陈建国；

【作者基本信息】 四川大学 ， 光学， 2005， 博士

【摘要】 随着大功率二极管(LD)及二极管阵列(LDA)生产工艺的成熟和制造成本的下降,二极管抽运固体激光器(DPSSL)的研究越来越受到人们的重视。由于LD发射波长与增益介质的吸收谱非常匹配,因此激光增益介质对抽运光的吸收效率较高。与传统的灯泵固体激光器比较而言,DPSSL具有效率高、热效应小、器件结构紧凑、输出功率高和光束质量好、寿命长等特点。体现了半导体激光器与固体激光器的双重优点。目前,中小功率的二极管抽运固体激光器已经走向实用化,在光通信、军事、医疗、航天航空、激光加工及科研等领域中逐渐出现大规模应用的趋势。而高功率、大能量的DPSSL也逐渐成为军事、国防、激光加工等领域的研究热点。也正是由于DPSSL系统能够产生高功率、大能量、高重复频率、高光束质量的激光,目前被认为是用于惯性约束聚变能(IFE)的最佳选择之一,因此高功率DPSSL在IFE领域中日益受到重视。 在所有的固体激光器系统中,激光介质吸收的抽运光中有一部分转变成热能沉积在介质内部,同时,激光介质受到外界的冷却,使得激光介质内部形成非均匀温度分布;温度分布的不均匀,在介质内形成热应力,导致介质的折射率发生变化,进而引起一系列的热效应问题,如热致双折射、热退偏、热致波面畸变、热应力破坏等。尽管DPSSL相对于传统闪光灯泵浦的固体激光器而言大大降低了热效应的影响,但是随着LDA抽运光功率的进一步提高,激光介质的热效应问题仍然是限制DPSSL向高功率高光束质量发展的一个重要制约因素,对整个激光装置的性能价格比的影响非常突出。因此研究DPSSL的热效应问题具有重要的实际意义。本文重点研究了不同工作参数条件下的高功率DPSSL的介质热效应,以及重复脉冲辐照下的光学元件的热效应问题。主要研究结果包更多还原

【Abstract】 Along with the rapid development of the techniques and ever decrease of the cost of high power laser diode arrays (abbreviated to LDAs), diode pumped solid-state laser (abbreviated to DPSSL) is paid great attention all over the world. As the emission spectrum of a laser diode (abbreviated to LD) can be made to match the absorbing spectrum of laser gain medium, a high absorbing efficiency may be realized easily. Compared with traditional solid-state lasers pumped by flash lamps, DPSSL has a lot of advantages, such as high efficiency, low thermal effects, compact structure, high power output, excellent beam quality, long service time, and so on. By now, the DPSSLs with small output power have been commercially used in the fields of optical communication, military, medical treatment, spaceflight, laser machining and scientific research etc. And the high power and high energy DPSSLs are more and more paid attention in the field of military, national safety and laser machining etc. The DPSSL systems with high power, high energy, high beam quality output and high repeating frequency, are regarded as one of the best choice to realize the inertial fusion energy (abbreviated to IFE).As well known, in every solid-state laser, a portion of absorbedpumping energy will be transformed into heat and deposited in the active medium. In the meantime, the active medium is cooled by surrounding refrigerant, which can be gas or liquid. Under the effect of heat depositing and the effect of cooling, an inhomogeneous temperature distribution will arise in the active medium. The temperature variation and following thermal stress inevitably induced changes of the geometry size and refractive index of the host medium, which, in turn, cause a series of thermal related variations of the optical properties of the media, such as stress induced birefringence, thermally induced depolarization, thermally induced wave front distortion, stress induced destroy and so on. Although DPSSL may sustain a relatively higher thermal load than flash lamp pumped solid state laser, thermal effects still, to certain extent, restrict the DPSSL from giving out to a high power and high energy output as desired. The thermal effects seriously reduce the performance-to-price ratio of the DPSSL. Therefore, studies on the thermal effects in DPSSL systems are of current significance. In this dissertation, the thermal effects in different host media of high power DPSSL under different pumping geometries and different operation conditions have been analyzed theoretically, and the thermal distortions due to temperature rise of optics irradiated by periodically repeated short laser pulses have been analytically studied. The main results obtained in the dissertation can be summarized as follows:Firstly, a 3-dimensional temperature distribution inside the gain medium of a high power disk laser end pumped by the LDA and incurved wave front distortion of the light wave after passing through the gain medium have been studied by using finite element method. The temperatures and distortions for different host media with different geometry sizes have been calculated numerically, which indicates the thermal deterioration on the quality of laser beam amplified by the host media. The resultsshow that the thermally induced wave front distortion is relatively small in the end-pumped disk laser amplifier, because the heat flow is roughly coincident with the propagation direction of the amplified laser beam. Thus, the high power laser output and high beam quality can be realized under the end-pumped-disk configuration. However, if the medium disk is too thick, the temperature difference will be large and cause significant stress induced depolarization, which may lead to a problem that the laser beam cannot be dumped out of the laser system when the technology of multi-pass amplification with polarized laser beam is used. So, from the view of thermal distortion, thin disk medium is preferable to the end-pumped amplifier. But the thin medium cannot absorb the LDA pumping energy sufficiently. A stack of multiple disks, with an appropriate clearance between each disk for cooling, can be used to realize high pump absorbing efficiency and low thermal distortion as well.Secondly, the steady and transient thermal effects in ring-LDA continuously side-pumped laser (abbreviated to RCSPL) have been studied. Based on the intensity distribution of the pumping laser diode, the distribution of heat deposition inside the gain medium of a RCSPL is obtained using ray tracing method. The transient temperature rise and thermal stress distributions are calculated using finite element method. Also, the temperature rise and thermal stress are discussed under different input power and different rod radius. It is very satiable to analyze the thermal effect in the RCSPL by using the ray-tracing-based heat deposition model. The results show that temperature distributions are different under different pump structure parameters. And the rise time to steady state grows with the rod radius rather than the pump power. The temperature rises with the pump power and so do the thermal stress, thermally induced birefringence and thermally induced wave front distortion. The thermal stress maybe causes serious problems such asfracture around the center and surface of the laser rod. The thermally induced birefringence aggravates the depolarization when the laser beam is polarized, and the rings of the depolarized output pattern manifold with the pump power. The transient birefringence related optical path difference and the transient depolarized output pattern change with the transient temperature rise dynamically, and finally to the steady state.Thirdly, the transient thermal effects in ring-LDA pulse-side-pumped laser (abbreviated to RPSPL) have been studied in detail. By using the ray tracing method and finite element method, the temperature rises, the stress induced birefringence and the thermally induced wave front distortions have been numerically studied under different pulse repeating frequency and duty cycle. The results show that, at the beginning, the transient temperature rise, and the relative optical length difference (abbreviated to OPD) due to the thermal birefringence, increase with the number of pump pulses following a saw-toothed like curve, and finally after a number pulses, repeats itself at a period equal to that of the pump pulses, which is called as the periodical state in this work. The bigger the repeating frequencies or the duty cycles of the LD pulse are, the higher the temperature rise and the relative OPD are. The rise time to the periodical state and the aptitude of the periodical temperature increases with the repeating frequency and duty cycle. The deposited temperature rise in the laser rod will induce a wave front distortion to the next incoming laser beam, which may enlarge the beam size of the far field spot. The results show that the greater the pump power or the duty ratio is, the bigger the residual wave front distortion and the worse the laser beam quality will be.Fourthly, temperature variation and wave front distortions due to temperature rise and thermal stress, in rotationally symmetric andisotropic optical elements irradiated by periodically repeated short pulses, have been studied. The analytical expressions of the periodically varying temperature distributions have been obtained by making full use of the periodicity of the pulsed laser system. Especially, an analytical expression of the periodical temperature rise has been given for the case of ghost images generated in the center of the surface of optics. The results have shown that the polarization independent phase distributions due to thermal deformations of the optical element may be a problem of concern, especially if one considers the fact that the phase distortions of the light wave may be accumulated after passing through several optics. Some high power laser systems require a phase distortion induced by the Kerr effect be smaller than it or less, so the heat induced phase distortion should not be ignored. In other words, for a system, in which the self-focusing may eventually limit the output power or lead to a catastrophe, certain measures, such as a forced cooling to the optical plate, may be needed.Finally, a suit of computer numerical modeling software, which is named as Thermal Effect Analyst (TEA), has been developed to analyze numerically the thermal effects in ring-LDA side-pumped solid-state laser rod. By using the TEA software, a 3-dimensional transient and steady state temperature, thermal stress, thermal depolarization, thermally induced wave front distortion etc. can been calculated for different host media, and under different operating conditions, which can be used to optimize the design of high power ring-LDA side-pumped rod lasers or amplifiers.更多还原