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Laboratory Science

Optimization and reserach of Rb vapor cell of CPT atomic clock

Guo Jia-tai

Published Jan 1, 2013• 0 citations
OpenAlex

Abstract

In order to meeting the high stability of the coherent population trapping(CPT) atomic clock,we studied the relationship between CPT resonance frequency shift with pressure ratio of the buffer gases(argon and neon) through the 300μm thickness rubidium cell.When the total pressure of the buffer gases was 10 kPa,We found that the frequency shift rate varied along a bell shaped curve,and when the pressure ratio of Ar and Ne was 1:1.2 or 1.2:1,CPT resonance frequency drift rate was zero.

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In order to meeting the high stability of the coherent population trapping(CPT) atomic clock,we studied the relationship between CPT resonance frequency shift with pressure ratio of the buffer gases(argon and neon) through the 300μm thickness rubidium cell.When the total pressure of the buffer gases was 10 kPa,We found that the frequency shift rate varied along a bell shaped curve,and when the pressure ratio of Ar and Ne was 1:1.2 or 1.2:1,CPT resonance frequency drift rate was zero.
We studied the relationship between pressure ratio of the buffer gases (argon and neon) and the rate of coherent population trapping resonance frequency shift with cell temperature in Rb85. We found that when the total pressure of the buffer gases varies within the range of 5–15kPa, the frequency shift rate varies along a bell shaped curve. Every curve crossed the horizontal axis at two points that are roughly symmetrical with respect to the midpoint at 1:1. This allows us to minimize the rate of frequency shift by adjusting the pressure ratio of the buffer gases to these two points.
The paper shows the presents results of studies of small-size rubidium cells with argon and neon buffer gases, produced by a patent pended technique of laser welding [Fishman et al. (2014)]. Cells were designed for miniature frequency standard. Temperature dependence of the frequency of the coherent population trapping (CPT) resonance was measured and used to optimize the ratio of partial pressures of buffer gases. The influence of duration and regime of annealing on the CPT-resonance frequency drift was investigated. The parameters of the FM modulation of laser current for two cases which correspond to the highest amplitude of CPT resonance and to the smallest light shifts of the resonance frequency were determined. The temperature dependences of the CPT resonance frequency were found to be surprisingly different in the two cases. A non-linear dependence of CPT resonance frequency on the temperature of the cell with the two extremes was revealed for one of these cases.
Controlling buffer-gas conditions in vapor cells is essential for frequency stabilization and atomic-clock performance. This study investigated the dependence of coherent population trapping (CPT) signal characteristics on nitrogen (N 2{}_{2}2​ ) buffer-gas pressure in micro-fabricated vapor cells containing natural-abundance rubidium (Rb) generated from Rb azide (RbN 3{}_{3}3​ ) for chip-scale atomic clocks. A wafer-level fabrication method enabling individual control of N2 pressure was developed by inkjet patterning of RbN3 and subsequent krypton fluoride (KrF) excimer laser decomposition. An increasing N2 pressure was found to reduce the CPT linewidth and cause a significant decrease in the signal amplitude, which resulted in monotonic degradation in the CPT figure of merit with higher pressures. A CPT-based laser frequency stabilization method that locked the laser wavelength to the CPT resonance peak yielded significantly larger signal amplitudes than a conventional absorption-based method in the low-pressure regime below 8 kPa. These results show that CPT performance in micro-fabricated Rb vapor cells is more restricted by the signal amplitude rather than the linewidth. They also offer practical buffer-gas design guidelines for enhancing short-term frequency stability in chip-scale atomic clocks.
This work presents a study of the short-term stability properties of Rb atomic clocks based on coherent population trapping (CPT) under dynamic excitation of a CPT resonance in cells without buffer gas. It is demonstrated that in an optical cell with anti-relaxation coating of its inner walls, the best stability is achieved at the scanning frequency of the frequency difference of the bichromatic pump field equal to 2 kHz at the resonance width of 450 Hz. In cells without a wall coating, the optimal scanning frequency range is found to be 1.2–2.8 kHz at the resonance width of 29 kHz. Examination of the slope of the stabilization system’s discriminant curve at zero error signal for buffer-gas-free cells uncovered an amount of correlation between the discriminant curve slope and atomic clock stability. It is demonstrated that the highest stability of atomic clocks in dynamic excitation mode is achieved at a ratio of scanning frequency and amplitude around 1.
The goal of this project is to optimize parameters of a prototype atomic clock to achieve the best Allan variance. We have established a prototype atomic clock using coherent population trapping (CPT) by creating a laser system to lock the laser’s frequency to an atomic resonance in rubidium and utilized radiofrequency (rf) modulation to create a sideband and carrier comb. With the system assembled, we have achieved our best Allan variance of 4 × 10−10 for a 15 torr Neon buffer gas cell, but have changed to a 5 torr Neon buffer gas cell to decrease the Allan variance.