基于分子吸收的脉冲激光锁频方法研究

Pulse Laser Frequency Locking Method Based on Molecular Absorption

为满足瑞利高光谱激光雷达对脉冲激光发射单元频率稳定性的需求,提出基于分子吸收的脉冲激光锁频方法。基于碘分子吸收光谱原理,采用GHz量级峰值保持电路、比例积分微分(PID)(proportional integration differential控制算法和精度为±0.02K的控温系统,构建一套脉冲激光锁频系统。首先,利用BBO(β-BaB2O4)晶体倍频的532nm连续激光测量精确温控的碘分子吸收池,获得其1109线在不同温度下的吸收光谱,进而确定了鉴频曲线;其次,利用鉴频曲线的拟合方程获得通过碘分子吸收池后脉冲激光能量变化与频移量之间的定量关系及测量灵敏度;最后,利用PID控制算法对比频率设定值与频移量之间的差异,将该差异以电压形式反馈于种子激光器,通过种子激光器的频率改变补偿脉冲激光的频率漂移,继而实现脉冲激光的动态锁频。实验结果表明:在25min内脉冲激光的频移小于±2.2MHz,瑞利高光谱激光雷达测风误差小于±0.6m/s,测温误差小于±0.5K。

Pulsed laser frequency locking method based on molecular absorption is proposed to meet the requirement of Rayleigh high spectral resolution lidar for the frequency stability of pulsed laser emission unit. We construct a pulse laser frequency locking system based on the principle of iodine molecule absorption spectrum, using the GHz magnitude peak holding circuit, proportional integration differential(PID) control algorithm and temperature control system with accuracy of ±0.02 K. First, the accurate temperature-controlled iodine molecular absorption pool is measured by using BBO(β-BaB2O4) crystal frequency-multiplied 532 nm continuous laser, and the absorption spectra of its 1109 line at different temperatures are obtained, so as to determine the frequency discrimination curve. Second, using the fitting equation of frequency discrimination curve, the quantitative relationship between the change of pulse laser energy and the frequency shift and the measurement sensitivity are obtained. Finally, the PID control algorithm is used to compare the difference between the set value of frequency and frequency shift, and the difference is fed back to the seed laser in the form of voltage. The frequency shift of the pulse laser is compensated by changing the frequency of the seed laser, and then the dynamic frequency locking of the pulse laser is realized. The experimental results show that the frequency shift of the pulsed laser is less than ±2.2 MHz within 25 min, which can make Rayleigh high spectral resolution lidar achieve wind measurement error less than ±0.6 m/s and temperature measurement error less than ±0.5 K.