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Applied Physics Letters

Symmetric autobalanced Ramsey interrogation for high-performance coherent-population-trapping vapor-cell atomic clock

Moustafa Abdel Hafiz, Grégoire Coget, M. Petersen, Claudio Eligio Calosso, Stéphane Guérandel, Emeric de Clercq, Rodolphe Boudot

Published Jun 11, 2018• 72 citations
Publisher

Abstract

We report a high-performance pulsed coherent population trapping (CPT) Cs cell atomic clock using the implementation of a symmetric auto-balanced Ramsey (SABR) interrogation sequence. The latter method is found to reduce the light-power induced frequency shift by an order of magnitude compared to a previous experiment using a simple auto-balanced Ramsey interrogation. The contribution of this shift to the clock frequency stability is now rejected in the 10−16 range at 104 s averaging time. Additional tricks, including a compensation method to reduce the laser amplitude noise contribution and the generation of novel error signals for local oscillator frequency and phase correction, have been implemented using a FPGA-based digital electronics board in order to improve the clock short-term stability by a of factor 2. The Allan deviation of the SABR-CPT clock, extracted from a selected 3 × 104 s-long dataset, is 2 × 10−13 τ−1∕2 and averages down to the level of 2.5 × 10−15 at 104 s. These results are encouraging to stimulate the development of hot cell CPT-based clocks for industrial, scientific, and instrumentation applications.

DOI
10.1063/1.5030009

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We report a high-performance pulsed coherent population trapping (CPT) Cs cell atomic clock using the implementation of a symmetric auto-balanced Ramsey (SABR) interrogation sequence. The latter method is found to reduce the light-power induced frequency shift by an order of magnitude compared to a previous experiment using a simple auto-balanced Ramsey interrogation. The contribution of this shift to the clock frequency stability is now rejected in the 10−16 range at 104 s averaging time. Additional tricks, including a compensation method to reduce the laser amplitude noise contribution and the generation of novel error signals for local oscillator frequency and phase correction, have been implemented using a FPGA-based digital electronics board in order to improve the clock short-term stability by a of factor 2. The Allan deviation of the SABR-CPT clock, extracted from a selected 3 × 104 s-long dataset, is 2 × 10−13 τ−1∕2 and averages down to the level of 2.5 × 10−15 at 104 s. These results are encouraging to stimulate the development of hot cell CPT-based clocks for industrial, scientific, and instrumentation applications.
The mid-term fractional frequency stability of miniaturized atomic clocks can be limited by light-shift effects. In this Letter, we demonstrate the implementation of a symmetric auto-balanced Ramsey (SABR) interrogation sequence in a microcell-based atomic clock based on coherent population trapping. Using this advanced protocol, the sensitivity of the clock frequency to laser power, microwave power, and laser frequency variations is reduced, at least by one order of magnitude, in comparison with continuous-wave or Ramsey interrogation schemes. Light-shift mitigation obtained with the SABR sequence benefits greatly to the clock Allan deviation for integration times between 102 and 105 s. These results demonstrate that such interrogation techniques are of interest to enhance the timekeeping performance of chip-scale atomic clocks.
We propose a preliminary theoretical investigation and demonstrate experimentally the application of the autobalanced Ramsey (ABR) interrogation protocol onto a pulsed hot vapor Cs cell microwave atomic clock based on coherent population trapping (CPT). This approach, based on the alternation of two successive Ramsey-CPT sequences with unequal free-evolution times and the subsequent management oftwo interconnected phase and frequency servo loops, is found to allow a reduction of the clock frequency sensitivity to laser power variations. This approach, combined with the implementation of advanced electronics laser power stabilization systems, yields the demonstration of a CPT-based Cs cell clock with a fractional frequency stability at the level of 3.1 × 10−13 τ−1/2 and averaging down to the level of 7 ×10−15 at 2000 s integration time.
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.
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 8 × 10 −10 at 1 s and averages down in the 10 −12 range at 1 day. These results might stimulate the development of chip-scale atomic clocks using Ramsey-based sequences, with reduced light-shift sensitivity, and enhanced long-term stability performances.
We report on the mid-term stability progress of a table-top coherent population trapping (CPT) microcell atomic clock, previously limited by light-shift effects and variations of the cell's inner atmosphere. The light-shift contribution is now mitigated through the use of a pulsed symmetric auto-balanced Ramsey (SABR) interrogation technique, combined with setup temperature, laser power, and microwave power stabilization. In addition, Ne buffer gas pressure variations in the cell are now greatly reduced through the use of a micro-fabricated cell built with low permeation alumino-silicate glass (ASG) windows. Combining these approaches, the clock Allan deviation is measured to be 1.4 × 10 −12 at 10 5 s. This stability level at one day is competitive with the best current microwave microcell-based atomic clocks.
We demonstrate a high-performance coherent-population-trapping (CPT) Cs vapor cell atomic clock using the push-pull optical pumping technique in the pulsed regime, allowing the detection of high-contrast and narrow Ramsey-CPT fringes. The impact of several experimental parameters onto the clock resonance and short-term fractional frequency stability, including the laser power, the cell temperature, and the Ramsey sequence parameters, has been investigated. We observe and explain the existence of a slight dependence on laser power of the central Ramsey-CPT fringe line-width in the pulsed regime. We report also that the central fringe line-width is commonly narrower than the expected Ramsey line-width given by 1/(2TR), with TR the free-evolution time, for short values of TR. The clock demonstrates a short-term fractional frequency stability at the level of 2.3×10−13 τ−1/2 up to 100 s averaging time, mainly limited by the laser amplitude modulation noise. Comparable performances are obtained in the conventional continuous wave regime, with the use of an additional laser power stabilization setup. The pulsed interaction allows to reduce significantly the clock frequency sensitivity to laser power variations, especially for high values of TR. This pulsed CPT clock, ranking among the best microwave vapor cell atomic frequency standards, could find applications in telecommunication, instrumentation, defense or satellite-based navigation systems.