A New Ultranarrow Bandwidth Atomic Optical Filter Applied in Faraday Active Optical Clock

The Faraday optical filter has a bandwidth of 25 MHz and a transmission of 18%. When applied in Faraday active optical clock, the filter together with 10 THz-spectral-width semiconductor fluorescence could produce stimulated emission light with a narrow linewidth of 300 Hz.

Faraday atomic optical filter is widely used in many fields, such as free space optical communication, lidar, laser frequency stabilization and atomic optical clock. Researchers aim to realize a filter with high transmission, narrow bandwidth and high noise rejection etc. Moreover, an atomic optical filter with both ultranarrow bandwidth and high transmission is a key component in Faraday active optical clock, which is a new kind of atomic optical clocks.

The research group, led by Prof. Jingbiao Chen, from Peking University, China, achieved the narrowest bandwidth of the filter at ⁸⁷Rb D₂ line reported by far via combining velocity-selective pumping and Faraday anomalous dispersion effect. It is reported in Chinese Optics Letters Vol. 12, No. 10, 2014 (/col/abstract.cfm?uri=col-12-10-101204).

The researchers realized an atomic optical filter at ⁸⁷Rb D₂ line with a bandwidth of 25 MHz and a transmission of 18% based on Faraday anomalous dispersion effect and velocity-selective pumping. The filter is composed of two permanent magnets producing the axial magnetic field, two polarization-orthogonal Glan-Tayor prisms with an extinction ratio 10⁵:1, and a frequency stabilized 780 nm diode laser.

The transmission spectrum of the atomic optical filter was studied with and without pumping laser and the dependence of the bandwidth on temperature of ⁸⁷Rb vapor cell, pumping intensity and axial magnetic field intensity was specially analyzed. Finally, an atomic optical filter working on optimal conditions for Faraday active optical clock was implemented.

The atomic optical filter with ultranarrow bandwidth from this research has already been applied in the Faraday active optical clock in Peking University.

"Faraday active optical clock is a new concept we introduced," Prof. Jingbiao Chen says. "We believe, Faraday magneto-optical effect discovered by Michael Faraday in 1845, will help us to build the modern optical atomic clock. Thus the achievement of the atomic optical filter with ultranarrow bandwidth from MHz to kHz is critical for the new-generation optical atomic clock". Moreover, the result will reduce out-band noises and improve signal-to-noise further in the fields of free space optical communication, lidar and so on.

Further works will be focused on integrating the atomic optical filter into the Faraday active optical clock, meanwhile utilizing the narrow optical clock transition lines of alkaline earth metal atoms, like the 689 nm transition with 7.6 kHz linewidth of strontium (Sr).

可用于法拉第主动光钟的超窄带宽原子光学滤波器

图片说明：法拉第滤光器的带宽25 MHz，透过率18%，应用到法拉第主动光钟系统中，再结合频谱宽度大于10 THz半导体荧光，可以实现线宽低至300 Hz受激发射输出光。

法拉第原子光学滤波器已广泛应用于自由空间光通信、激光雷达、激光稳频和原子光钟等系统中。高透过率，窄带宽，高噪声抑制等特性一直是原子光学滤波器研究的目标。特别地，在新型原子光钟-法拉第主动光钟系统中，作为频率参考的原子滤波器是其中最为关键的部件，必须同时具有超窄带宽和高透过率特性。

北京大学陈景标教授课题组通过将速度选择抽运技术与法拉第反常色散效应有效结合，获得了到目前为止报道的87Rb D2线原子光学滤光器的最窄带宽数据。相关研究成果发表在Chinese Optics Letters 2014年第10期上（http://www.opticsinfobase.org/col/abstract.cfm?uri=col-12-10-101204）。

该研究基于法拉第反常色散效应，由两块永磁体提供轴向磁场，采用两块偏振垂直的格兰泰勒棱镜降低通带外噪声，利用稳频激光对87Rb泡内原子进行速度选择抽运，实现了通带中心位于87Rb原子D2线，带宽达到25 MHz，透过率达到18%，满足法拉第主动光钟系统的要求。

实验重点研究了原子滤光器在抽运光开与关条件下的透过特性，滤光器透过率与抽运光强的关系曲线，特别研究了滤光器带宽随87Rb泡温度、抽运光强和轴向磁场等物理因素的变化曲线，得到法拉第原子光学滤波器的最佳工作条件。

该项研究得到的具有超窄带宽的原子光学滤波器在北京大学已经应用于法拉第主动光钟这一新型原子光钟系统中。

“法拉第主动光钟是个新概念，”陈景标教授说，“可以利用法拉第早在1845年发现的磁光效应，时隔170年后来实现一种崭新的现代版原子光钟，因此带宽从MHz到kHz的超窄带原子滤光器的研究成果将可能推动新一代原子光钟性能进一步提高。” 此外，该研究成果也可应用于太阳特征谱变化、自由空间光通信、激光雷达等系统中，有效降低带外噪声，进一步提高系统的信噪比。

接下来的工作将对该原子光学滤波器进行系统集成设计，同时可选用碱土金属元素的窄带光钟跃迁能级，拓宽其应用范围。