The pump laser frequency stabilization method based on magnetic field signal in NMR magnetometer

Abstract

The nuclear magnetic resonance (NMR) magnetometer, leveraging spin-exchange optical pumping, has the advantage of small volumes and high accuracy. The pump laser frequency is a critical factor in the optical pumping process as it affects the magnetic field measurement. Traditional frequency stabilization methods rely on monitoring the transmitted laser power through the vapor cell to lock the laser to the absorption spectrum. However, for achieving the signal enhancement of NMR magnetometers, a reflective pump structure is applied; the transmitted beam is redirected by a retro-reflective mirror to repump alkali atoms, eliminating the conventional transmitted light path and thereby precluding transmitted laser power-based stabilization. Therefore, a pump laser frequency stabilization approach is proposed based on the magnetic field signal. Theoretical analysis establishes a deterministic correlation between the magnetic field amplitude and pump laser frequency detuning, identifying the frequency stabilization method utilizing the zero point of the magnetic field signal’s first-order derivative. Experimental validation demonstrates frequency stability with a maximum variance of 18.8 MHz (1σ: ±3.0 MHz) over 2-hour continuous operation, matching the performance of absorption-based methods (16.5 MHz, 1σ: ±4.5 MHz). Although short-term frequency noise increases, the baseline magnetic field sensitivity is maintained. This approach not only simplifies the system architecture but also ensures compatibility with reflective pump structures, advancing the NMR magnetometer with the reflective structure of pump light.