采用射频调制实现对单频激光器频率噪声的抑制

Frequency Noise Suppression of Single-Frequency Laser with Radio-Frequency Modulation

在使用铷原子饱和吸收谱线作为激光频率参考进行稳频的激光稳频系统中,环路带宽是影响激光输出频率噪声的重要因素之一。对激光稳频系统中限制环路带宽的主要因素进行分析,使用射频调制信号直接调制商用外腔半导体激光器的高速电流调制端来对激光稳频系统的环路带宽进行拓展。根据对稳频环路的分析,合理设置反馈电路,实现激光稳频。使用低频谱分析仪对稳频后的鉴频信号进行分析,发现带宽拓展后,在傅里叶频率为5kHz处对频率噪声的抑制度达到了20dB以上。通过将该稳频激光器输出的激光与锁定在极稳恒温晶振上的飞秒光学频率梳进行拍频,测量了该稳频激光相对光梳的频率噪声,测量结果与直接分析鉴频信号的结果吻合。经过测量,通过拓展带宽抑制频率噪声,稳频激光器的短期频率稳定度得到改善。最后,测量了稳频激光相对于锁定在恒温晶振上的飞秒光学频率梳的频率稳定度,Allan方差在平均时间1s时达到4.52×10^(-12),在平均时间20s时达到1.65×10^(-12)。

For the laser frequency stabilization system which uses rubidium saturated absorption spectroscopy as the laser frequency reference, the loop bandwidth is one of the important facts which influence the laser output frequency noise. The main factors which limit the loop bandwidth of the laser frequency stabilization system are analyzed. Then, the radio frequency modulation signal is used to directly modulate the high speed current modulation port of the commercial external cavity diode laser, and the loop bandwidth of the laser frequency stabilization system is expanded. According to the analysis of the frequency stabilization loop, the feedback circuit is set up to realize the laser frequency stabilization. After analyzing the error signal of stabilized laser by low frequency spectrum analyzer, it is found that the frequency noise suppression can be improved to be larger than 20 dB at the Fourier frequency of 5 kHz. The beat frequency is conduct between laser from stabilized frequency laser and femtosecond optical frequency comb locked on ultra-stable oven controlled crystal oscillator, and the frequency noise of stabilized laser is also measured. The results are consistent with the analyzing results of the error signal. The frequency noise is suppressed when we expand the bandwidth, and the short-term frequency stability of the stabilized frequency laser is improved. Finally, the frequency stability of stabilized frequency laser relative to OCXO referenced femtosecond optical frequency comb is measured. It can reach to 4.52×10-12 at averaging time of 1 s and reach to 1.65×10^-12 at averaging time of 20 s.