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European Frequency and Time Forum

Laser Frequency Offset Locking with Dual-modulation

Wang Bisong, Peter Yun, Qiang Hao

Published Jun 25, 2024• 0 citations
Publisher

Abstract

We report a laser frequency offset locking scheme for coherent population trapping (CPT) atomic clock. Based on a usual half-wave modulation in CPT atomic clocks, we add a radio frequency signal to the laser driving current to form dual-modulation to compensate the frequency shift of optical transition due to buffer gas in the clock cell. The Doppler-free absorption enhanced spectrum produced by dual-modulation is observed and successfully applied to the laser frequency offset locking. In our scheme, a frequency shift device is no longer necessary, thus the clock's size, weight, power, complexity, environmental sensitivity, and cost are reduced, this is suitable for implementing compact and high-performance CPT and POP atomic clocks.

DOI
10.1109/eftf61992.2024.10722365

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Light shifts are known to be an important limitation to the mid- and long-term fractional frequency stability of different types of atomic clocks. In this article, we demonstrate the experimental implementation of an anti-light-shift interrogation protocol onto a continuous-wave (cw) microcell atomic clock based on coherent population trapping (CPT). The method, inspired by the autobalanced Ramsey spectroscopy technique demonstrated in pulsed atomic clocks, consists in the extraction of atomic based information from two successive light-shifted clock frequencies obtained at two different laser-power values. Two error signals, computed from the linear combination of signals acquired along a symmetric sequence, are managed in a dual-loop configuration to generate a clock frequency free from light shift. Using this method, the sensitivity of the clock frequency to both laser-power and microwave-power variations can be reduced by more than an order of magnitude compared to normal operation. In the present experiment, the consideration of the nonlinear light-shift dependence allows enhancement of the light-shift mitigation. The implemented technique allows an improvement of the clock Allan deviation for time scales higher than 1000 s. This method can be applied in various kinds of atomic clocks such as CPT-based atomic clocks, double-resonance Rb\mathrm{Rb}Rb clocks, or cell-stabilized lasers.
Abstract The coherent population trapping (CPT) atomic clock is very promising for use in next-generation spaceborne applications owing to its compactness and high performance. In this paper, we propose and implement a CPT atomic clock based on the direct modulation of a large-modulation-bandwidth and narrow-linewidth distributed Bragg reflector laser, which replaces the usually used external bulk modulator in high-performance CPT clocks. Our method retains the high performance while significantly reducing the clock size. Using this highly compact bichromatic light source and simplest CPT configuration, in which a circularly polarized bichromatic laser interrogates the 87 Rb atom system, a CPT signal of clock transition with a narrow linewidth and high contrast is observed. We then lock the local oscillator frequency to the CPT resonance and demonstrate an encouraging short-term frequency stability of 3.6 × 10 −13 τ −1/2 (4 s ⩽ τ ⩽ 200 s). We attribute it to the ultralow laser frequency and intensity noise as well as to the high-quality-factor CPT signal. This study can pave the way for the development of compact high-performance CPT clocks based on our scheme.
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