Research on the front-end system of high power solid-state laser driver

高功率固体激光驱动器的前端系统作为一个高质量的种子光源，其作用是为后级提供一个具有一定整形能力、一定能量、一定脉冲宽度、一定频谱宽度和一定信噪比的高稳定高光束质量的种子激光脉冲。它主要包括主振荡器、脉冲整形单元、再生放大器或多程放大器，分别承担激光光源、脉冲时间整形、脉冲能量放大等功能。论文的工作主要包括以下三个部分：发展稳定性好的LDA泵浦的主振荡器系统；改进和发展时一空变换激光脉冲整形系统；研制LDA泵浦的高增益、低噪声再生放大器。一、提出了两种稳定调Q激光器输出幅度的方法：等增益点调Q法和预激光调Q法，两种方法都成功实现了高稳定的、具有纳秒级精密同步能力的主振荡器。这种精密同步能力有利于实现多台激光器之间，以及主振荡器、脉冲整形器、能量放大器之间的时间精密同步。LDA克服了闪光灯的大部分不足之处，在未来的高功率固体激光驱动器中已成为代替闪光灯的候选者之一。我们研制的主振荡器和再生放大器都采用LDA泵浦的激光头。主振荡器均采用环形谐振腔，克服了空间烧孔效应，有利于单纵模稳定输出和提取能量。等增益点调Q法：驰豫振荡序列的第一个脉冲建立的时刻，谐振腔处于增益刚好超过损耗的状态。由于谐振腔的损耗相对稳定，所以每次第一个脉冲建立的时刻激光头的增益水平大致相同，我们称之为等增益点。于是用驰豫振荡的第一个脉冲作为Q开关的触发信号，可以使每次调Q时激光工作物质处于大致相同的增益水平，保证了输出调Q脉冲的幅度非常稳定。实验结果为输出调Q脉冲幅度起伏不超过2％。预激光调Q法：用输出激光强度来负反馈控制谐振腔的损耗，建立一个平坦的、准连续的～100μs宽预激光状态，然后在此预激光基础上调Q。预激光模式与谐振腔振荡模式一致并且具有一定的初始强度，为激光器提供了一个高质量的种子激光，所以调Q脉冲的建立时间和幅度都很稳定。实验结果为输出调Q脉冲幅度起伏不超过4.3％，建立时间抖动小于士4ns。光电反馈线路采用普通的砷化嫁光电导开关，具有结构简单、性能可靠的优点。二、提出了单偏转器时空变换脉冲整形新方法。通过改变调制狭缝的宽度和形状，获得了脉宽在74Ops～4.5ns范围内连续可变的整形激光脉冲，而且输出光束空间质量好。与通常的双偏转器时空变换方法相比，它少用了一个电光偏转器，降低了对关键元件电光偏转器在光学与电学性能匹配、时间精密同步方面的要求。降低了对脉冲高压驱动电源负载能力的要求，允许用雪崩管或MOS管代替极其昂贵的冷阴极管和稳定性差的高压火花隙，提高了可靠性和稳定性，简化了系统，增加了灵活性。此外，单偏转器方案允许通过晶体串联的方式来提高脉冲整形系统的时间分辨率。三、在国内首次研制成功一台LDA泵浦的Nd：YLF再生放大器。它采用折叠的线性驻波腔结构，实现了对纳秒级、纳焦耳能量激光脉冲的再生放大。再生放大器工作波长1053nm，重复工作频率1Hz，输出4ns宽基横模TEM_(00)，激光脉冲的能量为2mJ。总放大倍数达到1*10~7，输出稳定性为4.7％。As a source of high quality laser pulse, the front-end system of high power solid-state laser driver is to provide a highly stable shaped seeding pulse of proper energy, proper time width, proper spectrum width, proper signai-to-noise ratio and high beam quality. It includes main oscillator, pulse shaping unit, regenerative amplifier or multipass amplifier, which operate as laser source, time shaping unit and energy amplifier respectively. Our work mainly includes three aspects: developing highly stable diode-pumped main oscillator, developing and improving temporal-spatial transform pulse shaping system, developing diode-pumped regenerative amplifier of high stability and low noise. 1. Have put forward two methods to stabilize the output of Q-switched laser, one called equivalent gain Q-switching method and the other prelase Q-switching method. Both methods have realized highly stable main oscillator capable of precise synchronization at ns level. These precisely synchronized lasers are in favor of synchronization between two lasers, or between main oscillator, pulse shaping unit, and energy amplifier. Laser diode arrays overcome many disadvantages in flash lamp, and have gradually become a replacement in future solid-state laser driver. Our main oscillators and regenerative amplifier are both based on laser-diode-arrays pumping technique. The main oscillators use ring cavity to eliminate spatial burning effect, so is in favor of stable single longitudinal mode output and energy extraction. Equivalent gain Q-switching method: When the first pulse of relaxation oscillation is built, the optical gain slightly exceeds the cavity loss. Because the cavity loss is relatively fixed, the optical gain in laser head is of the same level each time the first relaxation oscillation pulse is built. We call such state an equivalent gain point. By using the first pulse signal to trigger Q-switch, the gain media is of the same gain at each Q-switching. That ensures a stable Q-switched output. Our experiments show a stability of 2%. Prelase Q-switching method: By using output laser intensity to negatively feedback control the cavity loss, a smooth and quasi-continuous prelase of ~100).is width is built, then the cavity is Q-switched during prelase. Because the prelase is well mode-matched to the cavity and has proper intensity, it provides a high quality seeding laser. That ensures stable building time and output intensity of Q-switched pulse. Our result is: intensity stability is 4.3% and building time fluctuation doesn't exceed ±4ns. The feedback circuit employs a GeaAs photoconductive switch, which has simple structure and reliable performance. 2. Have put forward a single-deflector temporal-spatial transform pulse shaping method. When changing the width and shape of filter, pulse duration ranging continuously from 740ps to 4.5ns is observed, and the mode quality of output beam is excellent. In comparison to conventional two-deflector temporal-spatial transform method, this novel method employs only one electrooptic deflector. That has relaxed requirements on electrooptic deflector's optical and electrical property matching and on precise synchronization between two electrooptic deflectors. It also requires less load capability of high-voltage pulse power supply, and allows using avalanche transistor or MOS transistor to replace expensive krytron and unstable spark gap. That has simplified the whole system and improved reliability, stability and flexibility. Furthermore, the time resolution of shaped pulse could be increased by using two crystals in series. 3. Have developed a diode-purnped Nd:YLF regenerative amplifier. It employs a folded linear stationary wave cavity and has realized amplification of ns, nJ laser pulse. The regenerative amplifier works at wavelength of 1053nm and repetition rate of 1Hz. It can produce laser pulses of 2mJ energy, 4ns width and base transverse mode TEM_(00). Total gain of 1*10~7 is reached, and pulse-to-pulse energy fluctuation doesn't exceed 4.7 %