1.Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China 2.China Aerodynamics Research and Development Center, Mianyang 621000, China
Fund Project:Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFB0701000, 2017YFB1104500).
Received Date:19 March 2019
Accepted Date:10 April 2019
Available Online:01 July 2019
Published Online:05 July 2019
Abstract:In recent years, high-energy single-axial-mode Q-switched lasers have been widely studied and applied because of their wide applications such as in nonlinear optics, laser spectroscopy and light detection and ranging (LIDAR). Many applications require a Q-switched pulse that has not only single axial mode but also can be synchronized with an external system. But two most commonly used methods (the build-up time reducing technique and ramp fire technique) are difficult to achieve single-axial mode operation. In this work, we apply the ramp-hold-fire technique to an injection-seeded Nd:YAG laser. The slave oscillator is a self-filtering unstable resonator (SFUR). The SFUR oscillator can achieve a smooth spatial profile TEM00 transverse mode. An RTP electro-optical crystal is adopted for intracavity phase modulator to modify the effective optical path length of the slave oscillator cavity. The seed-injection locking is realized by the ramp-hold-fire technique. The laser driver generates a pumping pulse. After a suitable time delay the driver is fired, a linear ramp voltage is applied to the RTP crystal. A photodiode detector monitors the interference signal. As soon as the interference peak is detected, the controlling electronics produces a stop signal. The ramp voltage on the RTP crystal is stopped and held at a fixed value. Then the Q-switch is fired at a set time, and finally single axial mode laser is demonstrated. Combining the advantages of intracavity phase modulation and Q-switch exact synchronization of the ramp hold fire technique, we obtain a narrow linewidth single-axial-mode laser pulse with precisely controllable output time. The laser is capable of generating 1064 nm pulse energy large than 50 mJ. The pulse build-up time is reduced by 31 ns to 48 ns. The pulse firing time is precisely controlled with jitter less than 1%. Then the frequency spectrum of the 1064 nm laser is measured with a commercial Fizeau wavemeter HighFinesse WS7. The multi-beam interference patterns of the pulse are shown to be smooth in the wavemeter. The wavelength is measured to be 1064.40416 nm and the linewidth is less than 0.5 pm which is limited by the instrument resolution. Meanwhile, the frequency stability is measured to be less than 0.1 pm (V-V) over 1700 pulses with a working frequency of 0.1 Hz. Keywords:single frequency/ seed injection/ phase modulation/ electro-optic
3.实验结果与讨论图2给出了扫描-保持-触发技术实际工作时的扫描电压信号和干涉信号, 由于电路系统的延时导致了从检测到峰值到晶体扫描电压保持时, 干涉信号从峰值下降了10%左右, 相当于约7 MHz的失谐量, 为了补偿这个失谐量, 可以旋转光电探测器前的偏振片一定角度进行光学补偿[20]. 其后干涉信号随扫描电压保持在了一个水平的位置. 实际工作时, 7 MHz的失谐量不会影响种子的成功注入. 图 2 扫描-保持-触发技术实际工作过程波形图 Figure2. Waveform of the working of the ramp-hold-fire technique.
图 1 种子注入单纵模Nd:YAG激光器
图 2 扫描-保持-触发技术实际工作过程波形图
图 3 激光脉冲输出能量随泵浦能量的关系曲线
图 4 种子注入前后脉冲波形及建立时间
图 5 同步触发100次脉冲叠加图
图 6 波长计WS7测量结果
图 7 脉冲波长稳定性