Compact Doppler-free saturated absorption spectroscopy module for stable laser frequency

The compactness, stability, and autonomy of the optical frequency references are the essential prerequisites for the reliable operation of the spin-exchange relaxation-free magnetometer systems. We propose a saturated absorption spectroscopy-based atomic optical frequency device utilizing the D₂ line transition of rubidium at 780 nm. The optical system is integrated into a compact module and operates less than 2 mW of laser power. We analyzed the spectra from the aspects of optical power and cell temperature, obtaining well-defined spectral peaks. In the experiment, the spectral difference was processed to obtain the saturated absorption spectrum without Doppler background. The experimental comparison with the commercial CoSy system demonstrates that our frequency stabilization system achieves a smaller full width at half maximum in the spectral profile. The frequency stabilization performance was tested using a wavemeter. Over 8 hour period, the frequency fluctuation is approximately 1 MHz, the Allan deviation is 7.036 × 10⁻¹¹ at 20 s. Self-estimation laser frequency measurements yielded an Allan deviation of is 2.22392 × 10⁻¹¹ at 100s. Additionally, the circuit’s DC bias was adjusted to stabilize the frequency reference within a narrow range.