高亮度高增益全固态皮秒激光放大技术研究

发布时间：2018-02-01 07:29

  本文关键词： 皮秒激光器 MOPA 光束质量 增益 端面泵浦 球差补偿 侧面泵浦 掠入射结构 液态金属　出处：《浙江大学》2016年博士论文　论文类型：学位论文

【摘要】：脉冲重复频率小于1MHz的高亮度兆瓦级1064nm全固态皮秒激光器在工业微加工、非线性频率变换等众多领域具有广泛的应用。通常,此类激光器种子源输出功率极低,需要采用高增益放大器(Master Oscillator Power Amplifier, MOPA)来提升输出激光的功率及单脉冲能量,以满足应用需求。多级光纤放大器可以提供极高增益,但受限于光纤中的光学损伤及非线性效应,其难以实现高峰值功率激光输出；再生放大技术是实现高增益放大的有效途径,但其结构复杂、价格昂贵,并不实用。近年来,基于高增益固体介质直接放大低功率皮秒种子源技术日益得到关注。本论文对采用Nd:YVO_4晶体放大低功率的被动调Q微晶片皮秒激光器技术进行研究,以获得结构简单紧凑、成本较低的高亮度、高增益全固态皮秒激光放大器。论文主要研究内容分为两部分：基于端面泵浦Nd:YVO_4棒状结构高亮度高增益全固态皮秒激光放大技术研究；基于掠入射侧面泵浦Nd:YVO_4板条结构高亮度高增益全固态皮秒激光放大技术研究。端面泵浦Nd:YVO_4棒状结构散热效果较差。为了获得高增益及高功率激光输出,通常需要采用强泵浦以实现高反转粒子密度。这使增益介质内热效应更为严重,热畸变导致输出激光光束质量退化。理论分析附加增益介质热致球差后激光光束质量的演化(忽略增益饱和效应),得到：附加增益介质正球差的激光光束传播经过束腰后,波前球差会由正转负；若将此带负球差的激光光束通过具有相同热效应(热球差)的增益介质,便可实现波前球差补偿,使输出激光光束质量得到改善。指出：在端面泵浦Nd:YVO_4棒状高增益放大器中,增益对光束强度分布起优化作用,利于采用波前球差补偿实现光束质量改善。以此为基础,设计了基于波前球差补偿的高亮度、高增益三级全固态皮秒激光MOPA系统。预放大级采用特殊设计的双通放大结构,在实现增益介质波前球差自补偿的同时提高激光的提取效率,并有效抑制强泵浦导致的寄生激光,将脉冲重复频率100kHz、脉冲宽度～95ps、光束质量因子M~21.15的10mW低功率微晶片种子光放大到12W,光光效率20%,光束质量因子M~21.28；主放大级为两级单通波前球差补偿结构,采用较大的泵浦光与种子光半径,在实现增益介质波前球差互补偿的同时有效降低增益介质内峰值功率密度,最终获得了平均功率65W的激光输出,峰值功率～7MW,单脉冲能量650μJ,光束质量因子M~2≤1.30。与端面泵浦Nd:YV04棒状结构相比,掠入射侧面泵浦Nd:YV04板条结构在泵浦面附近形成一段长距离高反转粒子密度区,并具有极佳的散热能力,是实现高亮度、高增益激光放大的有效途径。对掠入射侧面泵浦Nd:YV04板条增益介质内热致折射率变化引起的热透镜及热致畸变效应进行分析,指出：垂直方向泵浦光为高斯分布时,垂直方向泵浦光斑尺寸的变化对板条热效应影响较小,采用较小垂直方向泵浦光斑尺寸实现高反转粒子密度并不会导致更为严重的热致畸变；在掠入射角度由小变大的过程中,放大激光水平方向的热致畸变呈先增加再减小的趋势,采用更小的掠入射角度可以实现更小的水平方向热致畸变及更高的增益(掠入射角度越小增益越高)。优化了种子光掠入射角度、种子光在泵浦面投射宽度及泵浦宽度等参数。采用新型的液态金属材料取代铟膜作为热传导材料,对其实用性进行实验验证,极大改善地改善了板条的热效应。采用连续激光进行Nd:YV04板条结构放大实验,对理论分析结果进行验证。以此为基础,设计了基于掠入射Nd:YV04板条结构的高亮度、高增益两级单通全固态皮秒激光MOPA系统。预放大级采用较小的泵浦光与种子光垂直半径,实现高增益,将脉冲重复频率100kHz、脉冲宽度-95ps、光束质量因子M~2≤1.15的10mW低功率种子光放大到12W,光光效率达到22%,光束质量M_x~2=1.23、M_y~2=1.20。主放大级采用较大的泵浦光与种子光垂直半径,有效降低增益介质内峰值功率密度,最终获得了平均功率32W的激光输出,峰值功率～3.4MW,单脉冲能量320μJ,光束质量因子M_x~2=1.25、M_y~2=1.20。
[Abstract]:The pulse repetition frequency is less than 1MHz high brightness megawatt 1064nm solid-state picosecond laser micro machining in industry, has been widely used in many fields of nonlinear frequency conversion. Usually, the seed laser output power is very low, the high gain amplifier (Master Oscillator Power Amplifier, MOPA) to improve the output power and the single pulse laser energy, to meet the application requirements. The multi-stage fiber amplifier can provide high gain, but limited by the optical damage and nonlinear effects in the fiber, it is difficult to achieve high peak power laser output; regenerative amplification technology is an effective way to achieve high gain amplification, but its structure is complex, expensive and not practical. In recent years, high gain solid media directly to enlarge the source of picosecond seed low power have been paid more and more attention. This paper based on the amplification of low power by using Nd:YVO_4 crystal Research on adjustable Q microchip picosecond laser technology, in order to obtain a simple and compact structure, low cost, high brightness, high gain solid-state picosecond laser amplifier. The main content of this thesis is divided into two parts: Based on Nd:YVO_4 end pumped rod structure with high brightness and high gain all solid state picosecond laser amplification technology research; Study on the amplification of grazing the incident side pumped Nd:YVO_4 slab with high brightness and high gain solid-state picosecond laser end pumped Nd:YVO_4 rod. Based on poor heat dissipation. In order to obtain high gain and high power laser output, usually need to use strong pump to achieve high density inversion. This makes the gain medium thermal effect is more serious, resulting in degradation of the quality of the output of thermal distortion laser beam. The theoretical analysis of additional gain medium in the thermal evolution of laser beam quality caused by aberration (ignoring the effect of gain saturation), added: The gain medium positive spherical aberration laser beam propagation through the waist, wavefront aberration will turn from positive to negative; if the laser beam with negative spherical aberration by having the same thermal effect (hot spherical aberration) of the gain medium, we can realize the wavefront compensation of spherical aberration, the quality of the output laser beam is improved. It is pointed out that: in the end pumped Nd:YVO_4 rod high gain amplifier, gain of function optimization of beam intensity distribution, to improve the beam quality of the wavefront spherical aberration compensation. On this basis, the design of high brightness compensation based on wavefront aberration, high gain three level solid-state picosecond laser MOPA system. Pre amplifier uses a specially designed double pass structure in gain medium self compensation at the same time improve the wavefront aberration of laser extraction efficiency, and effectively suppress the parasitic laser caused by strong pumping, the pulse repetition frequency of 100kHz, pulse width 95ps, beam quality 10mW low power factor M~21.15 microchip to 12W seed optical amplification, 20% optical optical efficiency and beam quality factor M~21.28; the main amplification level two for single pass wavefront spherical aberration compensation structure, the larger the pump and seed light radius, while achieving the gain medium mutual compensation can effectively reduce the wavefront aberration of gain medium in the peak the power density, finally obtained the average laser output power of 65W, peak power 7MW, single pulse energy of 650 J, beam quality factor M~2 is less than or equal to 1.30. and end pumped Nd:YV04 rod structure compared to grazing side pumped Nd:YV04 slab to form a long distance high inversion density area near the surface of the pump, and heat capacity excellent, is to achieve high brightness, high gain laser amplification. The effective way of grazing side pumped Nd:YV04 slab gain medium heat induced thermal lens and thermal refractive index change caused by The distortion effect is analyzed, pointed out: the vertical pump for the Gauss distribution, influence the change of vertical pump spot size on slab thermal effect, with a smaller vertical pump spot size to achieve high density inversion does not lead to more serious heat distortion; the grazing angle changes from small to large in the process in the horizontal direction, amplified laser thermal induced distortion was first increased and then decreased, the grazing angle smaller can realize the horizontal thermal induced distortion smaller and higher gain (grazing incidence angle is small. The higher the gain) to optimize the seed light grazing angle, the seed width and surface projection in pump the pump width and other parameters. The liquid metal new material to replace indium film as thermal conductive material, the practicality of experimental verification, greatly improved to improve the thermal effect of the slab. By adopting continuous 婵，

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