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IEEE International Instrumentation and Measurement Technology Conference

A subminiature adjustable and measurable atomic clock based on coherent population trapping

Xiaosong Zhu, Chao Guan, Qun Zhou

Published May 1, 2011• 1 citation
Publisher

Abstract

Based on the operating principle and the comprehensive analysis of the coherent population trapping (CPT) atomic clock system, the paper presents an appropriate physics package and design a small and digital servo circuit system, forming a subminiature adjustable and measurable CPT atomic clock system which uses vertical-cavity surface-emitting laser (VCSEL) as optical source, selects85Rb as operating atoms, and is in resonance with the D1transitions. The system can be used for experimental study of correlation of each parameter and study of the Ramsey-CPT atomic clock. The whole system, which adequately combines the advantages of the ordinary desktop CPT experimental system and the chip-scale atomic clock (CSAC), is compact, small, low-power and well-adapted, in line with the further requirements of commercial development.

DOI
10.1109/imtc.2011.5944021

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Based on the operating principle and the comprehensive analysis of the coherent population trapping (CPT) atomic clock system, the paper presents an appropriate physics package and design a small and digital servo circuit system, forming a subminiature adjustable and measurable CPT atomic clock system which uses vertical-cavity surface-emitting laser (VCSEL) as optical source, selects85Rb as operating atoms, and is in resonance with the D1transitions. The system can be used for experimental study of correlation of each parameter and study of the Ramsey-CPT atomic clock. The whole system, which adequately combines the advantages of the ordinary desktop CPT experimental system and the chip-scale atomic clock (CSAC), is compact, small, low-power and well-adapted, in line with the further requirements of commercial development.
Based on the model of the Λ-type three-level atomic system, the principle, the evolutionary equations and the resonance line characteristics of the coherent population trapping (CPT) are analyzed by means of ensemble-averaged density matrix approach. The paper presents a schematic illustration of practical realization and a block scheme of comprehensive analysis of the CPT system. Using vertical-cavity surface-emitting laser (VCSEL) as optical source, we propose an appropriate physics package and design a small and digital servo circuit system, forming a subminiature adjustable and measurable CPT atomic clock system. The whole system is compact, small, low-power and well-adapted, in line with the further requirements of commercial development.
The abstract presents a fully functional Ramsey-CPT atomic clock, which has the characteristics of compact structure, small volume, low cost, strong adaptability, high stability, convenient for adjustment, unique design, and easy-to-commercialization. The Ramsey-CPT atomic clock combines the method of separated oscillatory fields and the phenomenon of coherent population trapping, so as to obtain narrower line width and better signal-to-noise ratio of the resonance signal for improving the frequency stability. It not only keeps the advantages of miniaturization and low power of the CPT atomic clock, but also can obtain higher frequency stability than CPT atomic clock about one order of magnitude.
A Coherent Population Trapping(CPT) atomic clock servo control circuit is designed based on the CPT phenomenon of 85 Rb. Taking MSP430 chip as a platform, combining with the physical device made up of Vecsel laser, photoelectric detector, and small 85 Rb vapor cell, a miniature atomic clock with the high-performance, high stability, low power consumption and small size is implemented. The volume is 31 cm3 and the power consumption is 660 m W. The frequency stability of 10 MHz measured is 2×10-10/s, 4×10-11/1 000 s. Full-hyperfine-splitting frequency modulation is used to improve the frequency stability.
We propose a coherent population trapping (CPT) pulsed-excitation method based on direct drive-current modulation of a vertical-cavity surface-emitting LASER for miniature atomic clocks (MACs). Because our method needs only a simple device configuration without optical external modulators, it is suitable for MACs, including chip-scale atomic-clocks. Reducing the size, cost, and power consumption is also possible. Additionally, we report on experimental results for Ramsey-CPT fringes excited using the proposed method.
Microcell CPT atomic clock using laser current-actuated power modulation with 10−12^{-12}−12 range stability at 1 day
We present a coherent-population trapping (CPT) microcell atomic clock using symmetric auto-balanced Ramsey (SABR) spectroscopy. The pulsed SABR sequence is applied through direct current-based power modulation of the vertical-cavity surface-emitting laser, eliminating the need for an external optical shutter and enabling compatibility with fully-integrated clocks. The sequence is controlled by a single FPGA-based digital electronics board. A key aspect of proper clock operation was the implementation of a real-time tracking of the atomic signal detection window. The clock frequency dependence on laser power, microwave power, laser frequency, and timing of the detection window has been measured, obtaining sensitivity coefficients lower than those obtained with Ramsey-CPT spectroscopy. The Allan deviation of the SABR-CPT clock, based on a microfabricated cell with low-permeation glass windows, is 1.1~×\times×~10−9^{-9}−9 at 1~s and averages down to the low 10−12^{-12}−12 range at 1 day integration time. These results pave the way towards the development of Ramsey-CPT chip-scale atomic clocks with enhanced timekeeping performances.