利用共振无源腔分析和抑制飞秒脉冲激光噪声的理论和实验研究

理论分析了共振无源腔对飞秒脉冲激光的强度和相位噪声的转化模型,分析表明,通过测量无源腔透射场或者反射场相对于输入场强度噪声的变化,可以间接得到输入场飞秒脉冲激光的相位噪声.在此基础上设计了精细度约为1500、自由光谱区为75 MHz的八镜环形共振无源腔,并测量了钛宝石锁模激光经过该共振无源腔后透射场和反射场强度噪声的变化.实验观察到,飞秒脉冲激光经过无源腔透射后,强度噪声特性得到较好改善,在探测频率2 MHz附近达到散粒噪声极限.同时,结合共振无源腔对激光强度和相位噪声的转化模型,间接给出了钛宝石锁模激光的相位噪声及无源腔对相位噪声的有效抑制作用.

A broadband passive cavity for analyzing and filtering the noise of a femtosecond laser

In this paper, the noise filtering effect on a femtosecond laser source via a broadband passive cavity is analyzed in detail. The results show that a passive optical cavity not only can be used as a low-pass noise filter, but also can inter-convert the phase and amplitude fluctuations of a light beam after transmission or reflection. Therefore, by measuring the intensity noise of the light field under test after transmission and reflection from a passive cavity, its phase noise properties can be explored. Based on this theoretical model, an eight-mirror ring passive cavity with a finesse of 1500 and a free spectral range of 75 MHz is designed and built. With a commercial Ti:sapphire femtosecond laser as a source, its intensity noises after transmission and reflection from the above cavity are measured with home-made self-homodyne detection setup. Furthermore, with the help of the noise conversion model of the passive cavity, the phase noise of the femtosecond laser as well as its evolution through the cavity transmission and reflection is indirectly derived. The result shows that after transmission through the passive cavity, both the amplitude and phase noise of the femtosecond laser source are evidently suppressed and reach the shot noise limit at the analyzing frequency of 2 MHz.