口径可定标放大的重频电光开关技术研究

An optical switch is a key device which is used to supress self-excitation and to control the number of passes that laser-pulses train make through the amplifier cavity. For repetition-rate and high energy laser system application, this device should be fast response, aperture scalable, and hold tens W/cm2 average power density laser irradiation. By now, the plasma-electrodes Pockels cell (PEPC) optical switch technology is the unique technology that meets the required aperture challenge. This PEPC switch can adopt thin crystal to mitigate thermo-effects by reducing absorption of laser. However, an optical switch is a thermo-sensitive device, a small quantity of thermo-deposition could lead to serius depolarization, wave-front distortion, and even stress fracture in the crystal. Based on this, the concept of reflective plasma-electrodes electro-optic switch is comed up with. Some key technoloyies, such as thermo-effects and its management, repetition-rate gas discharge in a large area, switch driving technology, and so on, will be studied. In addition, an experiment bench will be set up to examine switch performances under repetition-rate and high-energy laser load, and to correct the theory model as well as computing program. Consequently, both theroy and experiment groundworks are laid for this switch technology practical applications.

用来控制放大程数、抑制自激振荡的光开关是多程放大系统的核心器件。在高能重频运转条件下，开关不仅应有较快的上升沿、可定标到大口径，同时还需承受数十W/cm2的平均功率密度激光辐照。探索满足以上需求的开关技术基础将为高能重频激光器的发展提供有力的技术支撑。等离子体电极电光开关技术是目前唯一能够定标到大口径的开关技术，同时可采用薄晶体以减少对主激光吸收产生的热效应。但由于晶体两侧放电腔内气压低，晶体吸收激光产生的热量不能被有效地带走，而光开关为热敏感器件，较低的热沉积即可引起严重的退偏和波前畸变、甚至热应力导致晶体碎裂。基于此，提出了"反射式等离子体电极电光开关"概念。将对开关的热效应及其控制、介质阻挡大面积气体重频放电、开关单脉冲驱动等技术基础进行研究，并建立相应的实验研究平台，考核高能重频激光负载下开关性能，并校正所建立的理论模型和开发的计算程序。从而为该开关技术实际应用提供理论和实验依据。

高能、重频激光技术的发展产生了对口径可定标放大、可承受高平均功率密度、可重复频率运转的光开关技术。为满足高能重频应用，光开关除需要解决热效应的问题，还需要同时考虑大口径下低电压驱动、均匀电场加载、损伤阈值等问题，其关键在于如何同时解决光、热、电交叉约束难题。本项目创新地提出了反射式设计、纵向传导冷却TEC温控、混合电极纵向加载开关电压脉冲的重频光开关总体技术路线（入射侧等离子体、另一侧电极为反射镜背侧镀制的金膜），为突破光开关热效应、通光口径、损伤阈值等多重限制提供了新的思路。建立了热效应、低压气体辉光放电模型，并开发了相应计算程序，通过数值分析，掌握了光开关热效应和气体放电规律。提出了晶体和热沉低压气隙传导封接方法，数值分析了低压薄间隙He气热导率、热流密度随气压以及间隙厚度变化规律，在此基础上，优化了封接参数，并开发了封接工艺，实现了晶体和热沉之间无损伤、低应力传导封接。提出了分时分回路驱动方法，解决了反射镜小电容分压、电阻限流难题，实现了普克尔盒的低电压、快响应驱动。成功研制了30mm口径重频光开关实验样机，实验结果表明：静态消光比优于30dB、开关效率≥99.5%，驱动电压10.5 kV，上升时间为6.5ns，35W/cm2平均功率密度激光负载下，热退偏为0.95%、热致波前畸变小于0.25λ。另外，为10J@10Hz平台研制了40mm口径10Hz重频普克尔盒，较好地实现了放大器内自激振荡隔离，输出能量从先前的0.88J提升至1.5J。.混合电极普克尔盒换热效率、驱动电压、电场均匀性与通光口径无关，因此可将口径进一步定标放大。该开关技术的掌握为高能、重频激光技术的发展提供了有力的技术支撑。