主动光频标的研究进展

Research Progress of the Active Optical Frequency Standard

传统被动光钟激光锁频系统的频率稳定度最终受限于由布朗运动导致的腔长热噪声,是目前光频标发展面临的主要技术瓶颈之一。主动式新型光频标利用光学谐振腔的弱反馈在原子跃迁能级间形成多原子相干受激辐射,该受激辐射输出信号可直接作为钟跃迁信号使用,相比于被动光钟,主动光中原理上可实现更窄的量子极限线宽,并具有腔牵引抑制优势。分别介绍了国内外在主动光频标研究现状,主要介绍北京大学在该研究方向取得的理论及实验研究进展。为了实现窄线宽连续型主动光频标,提出双波长好坏腔方案减小剩余腔牵引效应对四能级主动光钟的影响,其中Nd:YAG 1064nm好腔激光和铯原子1470nm坏腔激光共腔输出,分别工作在好、坏腔区域,好腔激光通过Pound-Drever-Hall(PDH)稳频技术锁定主动光钟谐振腔的腔长,由于坏腔激光相比于好腔激光具有腔牵引抑制优势,因此坏腔激光线宽有望在腔长锁定之后的好腔激光的基础上进一步压窄。目前两套双波长系统的1470nm钟激光拍频线宽受限于两个谐振腔腔长的相对抖动,为了抑制共模噪声对钟激光拍频信号的影响,并验证采用PDH稳频技术实现腔长锁定的可行性,本文利用相位锁定技术同步两套双波长好坏腔系统的谐振腔腔长,从而消除腔长共模噪声对钟激光拍频线宽的影响,进而分析除剩余腔牵引效应以外的其他因素对1470nm主动光频标的影响。

The frequency stability of clock-transition laser in traditional passive optical clocks is ultimately limited by the Brownian noise,which is one of the main technical bottlenecks restricting the development of optical frequency standards.The active optical clock,utilizing the weak feedback of optical cavity to form coherent stimulated radiation between atomic-transition levels,is a novel laser.The atomic stimulated emission of active optical clock can be used as the clock-transition laser directly,therefore,compared with the passive part,the active optical clock can achieve narrower quantum limited linewidth and has the advantage of suppressed cavity-pulling effect.This paper reports the development of active optical frequency standards at home and abroad,and focus on its theoretical and experimental progress in Peking University.We proposed the dual-wavelength goodbad-cavity scheme to reduce the influence of the residual cavity-pulling effect on the Cesium(Cs)four-level active optical clock.The Nd:YAG 1064 nm and Cs 1470 nm lasers share one cavity and works in goodand bad-cavity regime,respectively.The good-cavity laser is frequency stabilized by the Pound-Drever-Hall(PDH)technique to stabilize the cavity length.Then the linewidth of the bad-cavity laser is expected to be further narrowed compared with the stabilized good-cavity laser,considering that the suppression of cavity-pulling effect in bad-cavity regime.Currently,the beating linewidth of 1470 nm lasers is limited by the asynchronous change of cavity lengths of two dual-wavelength systems.In order to suppress the effect of common-mode noise of cavity-length on 1470 nm beat-note signal,and verify the feasibility of PDH technique for cavity-length stabilization in dual-wavelength scheme,we put forward the phase locking technology to synchronize the cavity-length variation between two dual-wavelength systems.The influence of other factors besides residual cavity-pulling effect on 1470 nm active optical frequency standard is further analyzed.