Review of chip-scale atomic clocks based on coherent population trapping

Research on chip-scale atomic clocks （CSACs） based on coherent population trapping （CPT） is reviewed. The back- ground and the inspiration for the research are described, including the important schemes proposed to improve the CPT signal quality, the selection of atoms and buffer gases, and the development of micro-cell fabrication. With regard to the re- liability, stability, and service life of the CSACs, the research regarding the sensitivity of the CPT resonance to temperature and laser power changes is also reviewed, as well as the CPT resonance＇s collision and light of frequency shifts. The first generation CSACs have already been developed but its characters are still far from our expectations. Our conclusion is that miniaturization and power reduction are the most important aspects calling for further research.

Ramsey-coherent population trapping Cs atomic clock based on lin‖lin optical pumping with dispersion detection

A Cs vapor cell-based atomic clock that uses a lin‖lin pumping scheme with dispersion detection is reported. This atomic clock shows potential for high performance because of its high contrast pumping scheme, and for miniaturization because of its simple architecture. The experimental setup and optimal operating parameters for the clock are introduced.The current fractional frequency stability is measured to be 1.3 × 10^-12/■ up to 20s and reaches 3.1 × 10^-13 at 200 s. We have thoroughly investigated the related noise sources that affect clock frequency stability at the 1s and 100s levels. The investigation shows that the laser frequency noise limits the clock frequency stability significantly. The clock performance can be further improved by technically upgrading the laser frequency stabilization setup.

High-performance coherent population trapping clock based on laser-cooled atoms

We present a coherent population trapping clock system based on laser-cooled^(87)Rb atoms.The clock consists of a frequency-stabilized CPT interrogation laser and a cooling laser as well as a compact magneto-optical trap,a highperformance microwave synthesizer,and a signal detection system.The resonance signal in the continuous wave regime exhibits an absorption contrast of~50%.In the Ramsey interrogation method,the linewidth of the central fringe is31.25 Hz.The system achieves fractional frequency stability of 2.4×10^(-11)/(√τ),which goes down to 1.8×10^(-13)at 20000 s.The results validate that cold atom interrogation can improve the long-term frequency stability of coherent population trapping clocks and holds the potential for developing compact/miniature cold atoms clocks.

Dependence of the ^(85)Rb coherent population trapping resonance characteristic on the pressure of N_2 buffer gas

In order to exploit its potential applications, we experimentally study the dependence of ^85 Rb-based coherent population trapping （CPTi resonance on N2 buffer gas with 6 vapor cells filled with natural rubidium and N2. The experiments are carried out at different pressures and temperatures, and the results reveal that higher cell temperature makes the resonance more sensitive to N2 pressure. Thus, it is importmlt to choose a proper buffer gas pressure at a given cell temperature. This work provides valuable data for the application of 85Rb CPT resonance with a buffer gas of N2.

Characterizing passive coherent population trapping resonance in a cesium vapor cell filled with neon buffer gas

We present a pair of phase-locked lasers with a 9.2-GHz frequency difference through the injection locking of a master laser to the RF-modulation sideband of a slave diode laser. Using this laser system, a coherent population trapping （CPT） signal with a typical linewidth of ～ 182 Hz is obtained in a cesium vapor cell filled with 30 Torr （4kPa） of neon as the buffer gas. We investigate the influence of the partial pressure of the neon buffer gas on the CPT linewidth, amplitude, and frequency shift. The results may offer some references for CPT atomic clocks and CPT atomic magnetometers.

Theoretical analysis of suppressing Dick effect in Ramsey-CPT atomic clock by interleaving lock

For most pulsed atomic clocks, the Dick effect is one of the main limits to reach its frequency stability limitation due to quantum projection noise. In this paper, we measure the phase noise of the local oscillator in the Ramsey-CPT atomic clock and calculate the Dick effect induced Allan deviation based on a three-level atomic model, which is quite different from typical atomic clocks. We further present a detailed analysis of optimizing the sensitivity function and minimizing the Dick effect by interleaving lock. By optimizing the duty circle of laser pulses, average time during detection and optical intensity of laser beam, the Dick effect induced Allan deviation can be reduced to the level of 10 14.

冷原子光栅磁光阱的研制及CPT信号的探询

相干布居囚禁原子钟在小型化方面具备不可替代的优势。由于热原子气室内部高压缓冲气体的限制,导致其频率稳定度仍有进一步提升的空间。利用激光冷却原子技术作为替代,可以有效提升其中长期性能。然而,目前的冷原子物理系统仍然相对复杂,不利于原子钟整体系统的集成化和小型化。我们研制了高衍射效率光栅芯片、平面磁阱芯片以及微小型真空腔室,共同构建基于平面核心器件的磁光阱,利用单光束捕获冷原子2×106个。此外,为了简化CPT冷原子钟的激光系统,通过单激光结合时分复用系统的方式,仅用单一Rb D2线激光实现了原子冷却与CPT探询。以上的工作为将来实现微小型化高性能冷原子CPT钟的最终锁定和性能评估奠定了重要理论和技术基础。

微型CPT原子钟泵浦方法试验研究

CPT原子钟是一种基于相干布居囚禁(CPT)原理的小型原子钟,其不含微波谐振腔的特点有利于降低物理系统的体积与功耗。为探索适用于微型CPT原子钟的泵浦方法,在物理系统体积、功耗和复杂度的限制下,选出单一圆偏振光、平行线偏振光和椭圆偏振光三种方法。基于6 mm×6 mm×6 mm的微型87Rb原子气室,搭建桌面系统测得三种方法下CPT共振信号的线宽和信噪比数据。结果表明微型CPT原子钟更适合采用椭圆偏振光方法。

一种低功耗被动型CPT原子钟

介绍了一款新研制的被动型相干布局囚禁（CPT）原子钟样机及其主要性能。样机功耗约为4W，频率稳定度达到在τ〈8000s范围内优于6×10^-11τ^-1/2,10MHz输出频率在1Hz范围内的相位噪声达到-79．1dBc／Hz。所研制的CPT原子钟具有小型、低功耗的特点。

激光参量对CPT铷原子钟稳定度影响的数值分析

为具体分析激光参量对相干布居囚禁(CPT)原子钟的稳定度性能的影响,在Λ-型三能级原子系统模型的基础上,采用半经典密度矩阵方法,数值模拟计算了不同激光参量对CPT铷原子钟共振谱线信号的影响,并给出了比较分析。结果显示:两束激光强度的不对称对CPT钟信号强度的影响很大,而激光频率失谐对CPT钟信号影响较小;此外,当激光强度不对称与频率失谐同时存在时,荧光信号谱出现不对称,微波辐射信号发生频移。

基于CPT原子钟的VCSEL激光器控制系统设计

设计基于原子相干布居俘获原理(CPT)的原子钟系统中垂直腔面发射激光器(VCSEL)控制器,其包括20 mA量程、低噪声、精密直流稳流电源、介绍VCSEL控制系统组成和工作原理,并对VCSEL激光器系统进行了测试分析。结果表明,该控制系统可以实现单模态、窄线宽及低功耗稳定光源,对CPT原子钟VCSEL光源应用具有参考和使用价值。

从CPT原子钟到光钟

简要介绍了新型CPT(Coherent population trapping,相干布局囚禁)原子频标及光频标的基本原理和研究进展。被动型CPT铷原子钟物理部分的目前体积可控制在100cm^3以内,功耗1W左右,其稳定度为4×10^(-11)τ^(-1/2)(τ为测量取样的时间间隔)。CPT原理的铯原子频标的物理部分体积减小到1cm^3,功率减小到30mW,稳定度为6×10^(-10)τ^(-1/2),成为当今体积最小、功耗最低的原子钟。随着飞秒激光梳状发生器技术的发展,已将传统的谐波光频链的体积从几间实验室缩小到1.2×1.0m^2的光学平台上,它与光频测量技术的结合,使微波频标与光频标联系起来,建立了光钟,它的稳定性可以从现在的10^(-116)的水平提高到10^(-18)乃至10^(-22)水平,成为当前最精密的时间计量仪器。

国外CPT CSAC技术发展现状

概述了芯片级原子钟(CSAC)的特点和应用优势。介绍了美国和欧洲等发达国家启动的相干布局囚禁(CPT)CSAC系列研究项目。阐述了国外主要研究机构所取得的重大研究进展,如美国NIST机构和Symmetricom公司研制的CSAC样机和产品所达到的性能指标,并从物理系统、振荡器及频率合成器几个方面重点阐述国外的研究现状,其中重点阐述了物理系统部分的蒸气腔和垂直腔面发射激光器(VCSEL)的制作工艺、可实现的尺寸及密封性能等。列举了CSAC在卫星导航领域、用于时间保持和导航的水下系统以及个人手表系统中的应用现状。总结了CSAC在研制和应用中存在的问题,并提出了解决思路和发展方向。

CPT铷原子钟微波信号源设计

CPT原子钟由于其体积小、重量轻、功耗低等优点,广泛应用于通信、导航及数据传输等领域。设计基于数字锁相倍频技术,采用锁相环芯片ADF4350,根据CPT铷原子钟的需求实现了一种中心频率为3 417 MHz的微波信号源。经测试,信号源电路尺寸为30 mm×30 mm,功耗小于150 m W,输出微波功率范围为(-20^-5)d Bm,输出信号相位噪声与理论分析相符,杂散抑制满足设计要求,可用于CPT铷原子钟。

一种微型CPT铯原子钟的设计

被动型CPT原子钟是一种可实现微型化和芯片化设计的原子钟。针对微小卫星和地面手持设备的应用需求,提出了一种微型CPT铯原子钟的设计方案。分析了基于^(133)Cs的CPT原子钟物理设计的优点。完成了原理样机的设计实现与测试,原理样机的体积小于100 ml、功耗小于2 W、稳定度指标优于6×10^(-11)τ^(-1/2)(1~100 s)。最后,对芯片化设计及微型CPT原子钟应用进行了展望。

微小型CPT原子磁力仪

针对同轴对射式相干布居俘获(CPT)原子磁力仪光纤位于探头两端,导致探头体积过大,不利于小型无人机(UAV)搭载应用的问题,设计了一种微小型CPT原子磁力仪,实现了入射与出射光纤同侧安装,解决了光纤弯折导致探头体积过大的问题。测试结果表明:微小型CPT原子磁力仪准确度优于0.24 nT,非线性度优于0.00024%,磁场跟踪能力优于375 nT/s,噪声优于15.34 pT/Hz^(1/2)@1~15 Hz,探头体积15.6 cm^(3),满足了小型无人机搭载使用需求。

基于相干极化调制的小型化CPT原子钟进展

基于相干布局囚禁(Coherent Population Trapping)的被动型气泡原子钟,因其优良的短稳2E-13@1s和中期稳定度2.5E-15@10^4s,而成为高性能原子钟的有力竞争者。本文基于一种新构型来探索实现高性能小型化CPT原子钟。我们通过3.4GHz微波直接调制分布式布拉格反射(DBR)激光器产生相干双色光,同时在微波和激光光束上分别施加同步的相位调制和偏振调制,实现相干极化调制,获得了较高对比度(14.7%)和较窄线宽(416Hz)的CPT共振信号。该方案采用直接调制技术使原子钟系统体积、复杂性和环境敏感性都得到减小,这使得高性能CPT原子钟的小型化成为可能。

芯片原子钟——CPT钟研究进展

介绍了基于激光与原子相干布局囚禁(CPT)理论的芯片原子钟(CSAC)钟。该原子钟代表了小型化原子钟未来发展方向,近年来在设计技术、制造工艺以及物理机制方面都取得了较大进展,并实现了芯片级的CPT钟原理样机。预计,在未来3～5年将会有芯片原子钟产品面世。

基于^(85)Rb的微型化CPT原子钟的设计和实现

介绍基于85Rb原子相干布居囚禁(CPT)现象的微型原子钟的设计与实现,系统以MSP430单片机作为主控芯片,实现电流源、TCXO和射频等功能模块,并与CPT原子钟物理部分实现联调与整机封装,实现了高稳定度、低功耗的小型CPT原子钟。整机体积只有31 cm3,功耗为660 m W,测得10 MHz输出信号稳定度约为2×10-10 s-1,4×10-11/1 000 s。系统采用全宽调制在85Rb的D1线实现CPT原子钟方案,可提升CPT共振谱线对比度,提高原子钟稳定度。

高性能被动型CPT原子钟发展趋势

运用单一圆偏光构型达成CPT态会产生极化暗态使得CPT信号对比度较低。因此,传统的CPT原子钟频率稳定度指标不高。为提高CPT信号对比度从而提高CPT原子钟性能,各种消除极化暗态的方案不断被提出并获得研究,这些方案可以根据所使用的激光偏振状态及其产生方式分为平行线偏振光构型、相位延迟的σ^(+)-σ^(-)构型、推挽光泵浦构型、垂直线偏振光构型、偏振调制构型。基于这些方案的原理和发展水平,对比分析各个方案的优缺点和改进方案,可探索不同需求条件下高性能原子钟新的发展方向。