基于离子束流电路直接放大的铯原子钟锁频方法研究

Performance degradation of electron multiplier clearly restricts the lifetime of commercial small cesium atomic clocks. It can effectively prolong the lifetime of small cesium clock by the method of replacing electron multiplier with direct detection circuit to magnify the cesium ion beam flux. Moreover, there is no successful report that with the direct circuit detection method in cesium atomic clocks so far. We have acquired the Ramsey transition signal with the combination of electron multiplier and current amplifier to magnify the cesium ion beam and it is also explicit that signal-to-noise ratio (SNR) is the key question restricting the application of circuit detection method of ion flux in cesium atomic clock. The project brings forward and builds the frequency locking model of a cesium atomic clock based on direct circuit amplification of cesium ion flux and lock-in amplifier detection. With the model, the influences of shot noise and Johnson noise of the small cesium clock on the SNR and stability of output frequency would be simulated and analyzed. Then circuit parameters and digital lock-in amplifier are optimized to improve the SNR of error signal. The vacuum outgassing rate and noise characteristics of the sealed preamplification circuit on pA level signal is investigated. Moreover, according to narrow band characteristics of lock-in amplifier, a new type of slow square wave modulation mode and a detection algorithm of frequency difference are proposed with digital lock-in amplifier to realize close-loop frequency locking of the cesium clock. The goal of the research is to explore and clarify the frequency locking method of small type of the cesium clock with direct circuit amplification of cesium ion flux, and thus provide a new way for absolutely settling the lifetime of small cesium atomic clock.

电子倍增器的性能衰减严重制约着商品小铯钟的寿命，采用电路直接检测替代电子倍增器实现铯离子束流放大能有效提高小铯钟的寿命，且这项研究工作在国内外迄今为止尚无成功报道。本课题组已利用电子倍增器与电流放大器联合放大铯离子束流获得了Ramsey 跃迁信号，也明确了信噪比是制约离子束流电路检测方法在铯原子钟中应用的关键问题。 本项目提出并建立基于铯离子束流电路直接放大和锁定放大器检测的铯原子钟锁频模型，模拟分析散粒噪声和热噪声对小铯钟信噪比及其输出频率稳定度的影响，进而通过优化放大电路参数和数字锁定放大器改善误差信号信噪比。研究密封pA级信号前置放大电路的真空放气率和噪声特性，并根据锁定放大器的窄带特性提出一种慢方波微波调制新方式并获得利用数字锁定放大器得到频差的检测算法，实现铯原子钟的闭环锁频。本课题探索并阐明电路直接放大铯离子束流小铯钟的稳定锁频方法，为小铯钟完全突破寿命问题提供新途径。

电子倍增器的性能衰减严重制约着商品小铯钟的寿命，采用电路直接检测替代电子倍增器实现铯离子束流放大能有效提高小铯钟的寿命。本课题探索并阐明电路直接放大铯离子束流小铯钟的稳定锁频方法，为小铯钟完全突破寿命问题提供新途径。首先，提出了铯钟用铯离子流收集与传输装置、pA量级电流前置放大电路、带数字锁定放大器的四点方波调制新方法，突破了直接电路放大铯钟的核心问题。建立了直接电路放大铯钟模型用于研究电路参数、调制算法对信噪比和稳定度的影响。提出了一种磁选态铯束管束光学优化方法，建立了准直器效率模型改进准直器设计提高离子流强度，研究并建立了铯离子流强度测试系统和铯束管信噪比测试系统，开展了铯钟用铯离子流收集与传输装置真空放气量测试和电路放大方法噪声分析，解决了直接电路放大铯钟实现的关键技术问题。研究并实现了直接电路放大铯钟，经过3个月测试，稳定度达到1.95E-11/1s，6.58E-12/10s、1.84E-12/100s、5.60E-13/1000s、1.82E-13/10000s、5.39E-14/100000s和1.51E-14/1000000s，关键指标达到标准型铯钟水平。将电路放大方法与电子倍增器技术相结合实现该方法在铯钟上的快速应用，设计出nA量级放大电路通过降低放大增益和离子对打拿极表面的轰击解决了电子倍增器寿命问题，应用四点调制新算法，解决过渡时间增加对整钟伺服锁定的影响。该研究使本项目的铯钟电路放大方法在实际产品中获得应用。