Frequency locking of a 399 nm laser referenced to fluorescence spectrum of an ytterbium atomic beam

A digital feedback method is designed in the experiment. By fitting the signal, computer gives the feedback amplitude and direction, so that the laser frequency is locked. The overlaps of the fluorescence peaks among partial Bose and Fermi isotopes are eliminated by using an half-wave plate (HWP) to change the spatial observation angle between the polarization direction of excitation light and PMT detection.

Ytterbium (Yb) optical lattice clock is a very valuable research. In 2013, research group of NIST in America realizes a clock with milestone instability up to 1.6*10-18. Also in this year, the measured clock transition of 171Yb lattice clock is accepted as the secondary representation of the definition of the second. Research group led by Lingxiang He from Wuhan Institute of Physics and Mathematics, Chinese Academy of Science introduces a base skill toward Yb lattice clock to eliminate the fluorescence spectra overlap between the partial Boson and Fermion isotopes, when atomic beam is excited by 399 nm laser. It is reported in Chinese Optics Letters Vol. 12, No.2, 2014 (/col/abstract.cfm?uri=col-12-2-021401).

The research focuses on the difference of sensitivity of fluorescence intensity to the observation direction of Yb Boson and Fermion isotopes. The difference can be used to eliminate the spectra overlap, making the fluorescence spectra as reliable frequency reference and to be successfully used for frequency locking. Linearly polarized 399 nm laser is used to excite the 1S0-1P1 transition and fluorescence spectra are recorded with photomultiplier tube (PMT). Relative spectra intensity of Yb Boson and Fermion isotopes can be controlled by the angle between the linearly polarized excitation laser and the observation direction. When observation direction is parallel to the linear polarization vector, fluorescence spectra of Boson isotopes almost disappear completely. In the experiment, the researchers have succeeded in locking the 399 nm laser to a well resolved transition and obtained frequency stability around 304 kHz.

There exist partial fluorescence spectra overlaps of Yb Boson and Fermion isotopes when excited by 399 nm linearly polarized laser, which is bad for frequency locking based on these transition peaks. The feature of this work is to utilize the dependence of fluorescence spectra on observation direction to eliminate the spectra overlaps and well resolved transition peaks of Fermion isotopes can be used as the reliable frequency reference for required frequency locking. The method can serve high performance optical lattice clock.

Based on the method, precise and stable frequency locking of Yb Fermion isotopes is realized, enough atoms are loaded into 759 nm (magic wavelength) optical lattice after two-stage cooling. Finally, above 104 atoms are collected in optical lattice with temperature around 15°, which paves the way for the detection of the 578 nm clock transition.

基于镱原子束荧光谱的399 nm激光频率锁定

图片说明：实验设计了一套数字化的反馈方案。通过计算机拟合计算出反馈量的大小和方向，实现激光频率的锁定，并通过一个半波片（HWP）来改变399 nm激发光场的线偏振方向和探测方向的空间观测角，消除了荧光谱中部分玻色同位素和费米同位素峰的重叠。

镱原子光晶格钟(镱光钟)的研制是一个非常有意义的研究工作。最近，美国NIST将镱光钟频率稳定度推进到1.6×10-18；2013年6月，171Yb光钟的钟频测量值被国际度量衡委员会批准为二级秒定义。中国科学院武汉物理与数学研究所贺凌翔课题组提出了一种基于镱原子束荧光谱的激光稳频方法，其特色之一是可以消除镱原子399 nm激发荧光谱中部分玻色、费米同位素谱线重叠。该方法发表在Chinese Optics Letters 2014年第2期上（http://www.opticsinfobase.org/col/abstract.cfm?uri=col-12-2-021401）。

该课题组研究了镱原子玻色同位素和费米同位素的荧光谱强度对观测方向的敏感程度的差异，并利用这种差异消除了谱线重叠，使荧光谱线成为可靠的频率参考，进而成功地应用于激光的锁频。线偏振的399 nm激光激发1S0-1P1跃迁产生荧光谱。玻色同位素与费米同位素的相对谱线强度可以通过调节线偏振方向与观测方向的夹角来实现。当观测方向与激光偏振方向平行时，玻色同位素的荧光谱完全消失。将399 nm激光锁定到良好分辨的谱线上，获得了300 kHz的频率稳定度。

镱原子玻色同位素和费米同位素在399 nm激发光场下部分荧光谱线存在重叠，不利于基于这些谱线的激光锁频。该工作的特色在于，利用玻色同位素谱线对观测方向的依赖特性，消除了谱线重叠，从而得到了良好分辨的费米同位素谱线，使这些谱线成为激光锁频所需的可靠频率参考。这项技术可以应用于基于费米同位素的镱原子光钟。

在此工作的基础上，实现了镱原子费米同位素所对应跃迁频率的精确稳定的锁定，从而确保有足够多的原子经过两极冷却装载入759 nm的魔术波长光晶格，使一维光晶格中具有原子数达到104以上，温度达到15 μK左右的冷原子。该项研究为578 nm的钟跃迁探测提供了必要的技术准备。