摘要:
相干布居数拍频(coherent population beating, CPB)现象,产生于一个Λ型三能级原子系统中,当双色相干激光场的频率差和两基态能级频率间隔近失谐的时候,原子在激发态能级上的布居数会产生一个弛豫振荡,且振荡频率等于失谐量。当将此现象运用于原子标准频率的提取时, CPB频标的稳定度与CPB信号的幅度及信噪比直接相关。本文理论推导了描述CPB现象的表达式,数值模拟并实验研究了87 Rb基态超精细子能级的相干性对CPB信号的影响,通过控制与基态子能级共振相干激光场的抽运时间来控制能级的相干程度,观测不同相干程度对CPB信号质量的影响。研究结果表明CPB信号振荡的幅度与基态子能级相干程度成正比关系。要改善CPB信号信噪比、提高原子频标稳定度,建立、提高和保持基态超精细能级的相干性是关键。本文还讨论了CPB现象用于弱磁场测量及其他方面应用的可行性。
Abstract:
Coherent population beating (CPB) phenomenon occurs in a typical three-level Λ system. When the frequency difference between two coherent pumping laser fields has a certain detuning from the ground-state hyperfine splitting, the excited state population will experience a transient oscillation before reaching equilibrium, and the oscillation frequency is equal to the detuning. The CPB phenomenon enables us to directly obtain the beat frequency between the measured radio frequency (RF) signal and the atomic transition frequency. Then we can get the standard frequency by compensating the beat frequency to the RF. We propose a scheme to implement atomic clock based on the CPB phenomenon in 2009, and the scheme has been implemented. When this effect is used to achieve an atomic clock, the frequency stability is directly related to the amplitude and SNR (signal to noise ratio) of the CPB signal. Influence of the ground-state hyperfine sublevels’ coherence on CPB signal is theoretically simulated and experimentally investigated in this paper. A formula of the CPB signal is derived by using the semi-classical model of the interaction of atoms with light, and the theoretical simulation is done using the formula obtained. In the experiment two coherent pumping laser fields are used to interact with 87Rb atoms. A CPB process includes the coherence build-up and the CPB stimulation. The coherence of the ground-state hyperfine sublevels is achieved by controlling the pumping time of the coherent laser fields that are resonant to the ground-state hyperfine sublevels. With this method, the relationship between CPB signal and coherence of the ground-state hyperfine sublevels can be observed. Result shows that the amplitude of CPB signal is proportional to the ground-state hyperfine sublevels’ coherence. The hign quality CPB signal can be achieved when the CPB stimulation is started with a pure coherent population trapping (CPT) state. In the CPB process, the coherence build-up rate is approximately equal to the coherence decay rate. So a 50%duty cycle square wave can be used to modulate the RF, and the period of the square wave had better be twice of the decay time of the ground-state hyperfine sublevels’ coherence. To improve the SNR of CPB signal and the stability of atomic frequency standard, the ground-state hyperfine sublevels’ coherence must be built up, improved, and maintained before the CPB stimulation. The feasibility of applying CPB phenomenon to the weak magnetic field measurement and other applications is also discussed in this paper.
首页
登录
注册
下载: