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在抑制了激光频率长期漂移的基础上,为了进一步控制激光频率的短期抖动,即±50 MHz范围内抖动,设计了基于碘分子吸收池的稳频系统,文中采用1111号吸收线进行频率稳定,采用的碘池长度为25 cm,温度控制在70 ℃。
基于碘分子吸收池稳频系统的原理就是将激光波长锁定在1111线边缘的半高宽位置,利用1111吸收谱线的陡峭性,当激光频率抖动时,透过碘吸收池的光强就会有明显的变化,根据光强的变化量就可以得知激光频率的变化量,随即反馈给计算机,通过调节种子激光器中PZT电压将激光频率调到1111线边缘的半高宽位置,即完成频率稳定。
稳频系统实验装置如图6所示,约有1%的发射光进入稳频单元。实验前首先要扫描碘分子1111吸收线的频谱曲线。通过等步长调节种子激光器的PZT电压来实现激光频率扫描,波长计可探测激光波长数值。经分束装置,一束光经过一25 cm长,温度控制在(70±0.03) ℃的碘蒸气池,被PMT1接收;另一束光直接被PMT2探测,由于两个探测器所探测为脉冲信号,低速采集卡很难采集到完整的脉冲波形,这就会导致测量误差,最终导致稳频效果不佳,因此文中实验采用采样率为1 GHz的高速采集卡对脉冲信号进行采集,并将脉冲信号点进行积分运算,得到准确的脉冲信号强度,提高了稳频精度。由PMT1和PMT2的信号比值可得到未经归一化的碘分子1111吸收谱线,如图7所示。
设定吸收线高频一侧两通道比值为0.48处为频率锁定点,当激光频率偏离零点时,该比值会偏离参考值,并且在一定频率范围内,与激光频率偏移量呈近似线性的变化关系。以该比值的偏移量作为误差信号:
$$e = R - {R_0} = a \cdot \Delta \nu $$ (1) 式中:R和R0分别为两通道比值的测量值和设定参考值;
$\Delta \nu $ 为频率偏移;a为对锁频斜边进行线性拟合得到的系数,为0.475。由于碘分子的热运动会对得到的谱线有一定影响。温度较低时,薄气室屮的分子密度比较小,和激光束作用的分子数较少,严重影响饱和吸收的信号强度;而温度过高时,又会增强由分子碰撞导致的压力增宽,使获得的信号线宽展宽,不利于获得理想的鉴频效果。因此,通过对碘分子池进行温度控制,降低分子无规则热运动的影响,进而有利于抑制压力增宽,同时也有利于获得高信噪比的饱和吸收信号。为了便于对吸收池进行均匀控温,利用空气升温对碘管进行加热,将碘管用夹具固定,悬浮在空中,减小接触,如图8所示为稳频系统实物图,其中碘分子吸收池采用半导体加热,通过智能PID精确控温,其控温精度可达±0.03 ℃,碘管两侧留有通光口,用窗片密封并镀有532 nm增透膜以保证控温精度。
激光器稳频实验中,通过调节种子激光器的PZT晶体的电压来改变输出光的频率。改变电压的优点在于调节的速度很快,而且电压相对于温度而言较为容易控制,只是通过调节电压的方法改变频率的范围比较小。由于前期已经通过“水浴”温控技术将出射激光的频率稳定在较小的变化范围内,因此仅仅采用PZT调节完全可以满足小范围频率的改变。
通过改变PZT晶体电压改变波长,需要知道波长(或者频率)与电压变化之间的关系,可以通过实验测量得到。在种子激光器的软件中输入不同的电压,改变激光器的出射光的波长,利用波长计测量出射光的波长,结果如图9所示。
图9中的横坐标为电压的调节值,纵坐标对应532 nm输出的激光频率的相对变化。直线为进行线性拟合的结果,可以看到电压的变化与频率相对改变基本成线性的关系,电压改变1 V对应大约180 MHz的频率相对变化。但是在实验中可以发现,这个关系并不是绝对准确,其误差较大,原因是激光出射的光的能量受到非常多的因素的制约,包括其自身的状态和环境的变化。因而对激光频率的稳定并不能完全按照这个比例进行控制,利用软件进行频率稳定的时候,需要引入PID算法,使控制更加精确。
激光器的结构和参数比较复杂,很难得到精确的数学模型。对于这类被控对象,在工程控制领域,广泛应用PID (比例—积分—微分)控制技术[10]。比例控制是最简单的控制方式,它输出一个与输入误差信号成比例的反馈信号,但当仅有比例控制时,会存在稳态偏差。引入积分控制可将误差信号随时间累积,产生积分项输出,从而减小甚至消除稳态偏差。而微分控制属于超前校正,一般应用于惯性较大的被控对象,因激光频率对PZT电压的调节很敏感,在此不采用。综上,采用基于计算机软件的数字PI(比例积分)控制方案来稳定激光频率:
$$u(t) = {K_p}e(t) + {K_i}\int_0^t {e(\tau )} {\rm d}\tau $$ (2) 式中:e(t)为输入的误差信号;Kp和Ki分别为比例系数和积分系数;u(t)为输出的反馈信号,用来调节种子激光器的PZT电压,即把反馈值与原来的PZT电压值相加,作为新的PZT电压值:
$$U = {U_0} + u$$ (3)
Laser frequency stabilization technology using temperature control and iodine absorption cell technology
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摘要: 多普勒测风激光雷达通过分析系统回波信号的多普勒频移反演出风速,为提高风场探测精度,从稳频技术方面展开研究。在稳频过程中,分别采取措施消除激光频率的长期漂移和短期抖动。针对激光频率的长期漂移,设计并研制了种子激光器温控箱,通过水浴的控温方式大大减小了激光频率的长期漂移,将激光频率稳定在±50 MHz以内;针对激光频率的短期抖动,采用以碘分子吸收池为核心器件的稳频系统,通过半导体控温方式对碘分子吸收池精确控温,控温精度达0.03 ℃,提高了稳频精度,将激光频率进一步稳定在±8 MHz以内,满足±10 MHz以内的设计精度要求。通过搭建多普勒测风激光雷达系统,对发射激光稳频装置进行系统验证,连续4组风场观测结果表明:系统探测高度为17 km,绝大部分方差在4 m/s以下,满足测风激光雷达测量指标的要求。Abstract: Doppler wind measurement lidar reverses the wind speed by the Doppler frequency shift of the echo signal of the system. In order to improve the detection accuracy of the wind field, the research was carried out from the aspect of frequency stabilization technology. During the frequency stabilization process, measures were taken to eliminate the long-term drift and short-term jitter of the laser frequency. For the long-term drift of the laser frequency, a temperature control box for the seed laser was designed and developed, which greatly reduced the long-term laser frequency shift by controlling the temperature of the water bath. The laser frequency was stabilized within ± 50 MHz. For short-term jitter of the laser frequency, a frequency stabilization system with an iodine molecular absorption cell as the core device was adopted to accurately control the temperature of the iodine molecular absorption cell through semiconductor temperature control, with a temperature control accuracy of 0.03 ℃, the frequency stabilization accuracy was improved, and the laser frequency was further stabilized within ± 8 MHz to meet the design accuracy requirements within ± 10 MHz. Through the establishment of Doppler wind measurement lidar system, the launching laser frequency stabilization device was verified. The observation results of 4 consecutive sets of wind field show that the detection height of the system was 17 km, and most of the variances were below 4 m/s. It meets the requirements of wind lidar measurement indicators.
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