Laser-Frequency Stabilization Using Doppler-Free Spectrum of ⁸⁷Rb D1 Line Produced by Bichromatic Circularly Polarized Light

Coherent population trapping (CPT) magnetometers are important geomagnetic measurement tools. However, the shift and jitter of laser frequencies can seriously interfere with the measurement results of CPT magnetometers. A CPT magnetometer uses bichromatic light that reacts with alkali metal atoms to obtain magnetic field information. Therefore, traditional laser-frequency stabilization methods, which are primarily based on monochromatic light, are not suitable for CPT magnetometers. In this study, we propose a new laser-frequency stabilization method based on the Doppler-free spectrum of the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">87</sup>Rb D1 line produced by bichromatic circularly polarized light. First, the bichromatic Doppler-free spectrum at the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">87</sup>Rb D1 line was theoretically analyzed based on the energy level, which revealed that the Doppler-free spectrum produced by bichromatic circularly polarized light can exhibit a large amplitude in the geomagnetic field. Next, we utilized the bichromatic circularly polarized light to construct a self-assembly Dopplerfree spectroscopy of the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">87</sup>Rb D1 line to verify the proposed laser-frequency stabilization method. The experimental results demonstrate that the frequency stability within an integration period of 1s was 5.74 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−11</sup>. When the integration time was 100s, the frequency stability was 8.63 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−11</sup>. These results show that the proposed method is promising for ultra-highsensitivity CPT magnetometers, ensuring accurate measurements in practical applications.