光梳主动滤波放大实现锶原子光钟二级冷却光源

Optical frequency comb active filtering and amplification for second cooling laser of strontium optical clock

提出一种结合注入锁定技术的主动滤波放大方法,将光梳直接注入锁定至光栅外腔半导体激光器,产生窄线宽激光光源,该光源可以用于锶原子光钟二级冷却.实验中,将中心波长为689 nm,带宽为10 nm的光梳种子光源注入689 nm光栅式外腔半导体激光器,通过半导体增益光谱与半导体光栅外腔,从飞秒光梳的多个纵模梳齿中挑选出一个纵模模式来进行增益放大,再通过模式竞争,实现单纵模连续光输出;同时,光梳的重复频率锁定在线宽为赫兹量级的698 nm超稳激光光源上,因此,注入锁定后输出的窄线宽激光也继承了超稳激光光源的光谱特性.利用得到的输出功率为12 mW的689 nm窄线宽激光光源实现了88Sr原子光钟的二级冷却过程,最终获得温度为3μK,原子数约为5×10~6的冷原子团.该方法可拓展至原子光钟其他光源的获得,从而实现原子光钟的集成化和小型化.

In this paper, we propose an optical frequency comb active filtering and amplification method combined with injection-locking technique to select and amplify a single mode from a femtosecond mode-locked laser. The key concept is to optically inject an optical frequency comb into a single mode grating external cavity semiconductor laser. The optical frequency comb based on a femtosecond mode-locked laser with a narrow mode spacing of 250 MHz is used as a master laser. The center wavelength of the optical frequency comb is 689 nm with a 10 nm spectral width. A single mode grating external cavity semiconductor laser with a grating of 1800 lines/mm is used as a slave laser, and the external-cavity length from the diode surface to the grating is approximately 50 mm. The master laser is injected into the slave laser, and in order to select a single comb mode, we adjust the power of the master laser to control the locking range of the slave laser whose linewidth is smaller than the optical frequency comb repetition rate（250 MHz）.While the operating current of the slave laser is set to be 55 mA and a seeding power is adopted to be 240 μW, a single longitudinal mode is selected and amplified from 2.5×10~4 longitudinal modes of the femtosecond optical comb despite the low power of the single mode. By tuning the optical frequency comb repetition frequency, the single longitudinal mode follows the teeth of the femtosecond optical comb, indicating the success in the optical frequency comb active filtering and amplification. The locking range is measured to be about 20 MHz. Meanwhile, the repetition frequency of the optical frequency comb is locked to a narrow linewidth 698 nm laser system（Hz level）, thus the slave laser inherits the spectral characteristics of the 698 nm laser system. The linewidth is measured to be 280 Hz which is limited by the test beating laser. Then a continuous-wave narrow linewidth 689 nm laser source with a power of 12 mW and a side-mode suppression ratio of 100 is achieved. This narrow linewidth laser is used as a second-stage cooling laser source in the^（88）Sr optical clock, the cold atoms with a temperature of 3 μK and a number of 5×10~6 are obtained. This method can also be used to obtain other laser sources for atomic optical clock, and thus enabling the integrating and miniaturizing of a clock system.