Atomic coherence-assisted wide tunable range laser frequency offset locking using four-wave mixing.

In this Letter, we present a widely tunable laser frequency offset locking technique based on four-wave mixing (FWM). The Raman-amplified probe light and newly generated conjugate light exhibit high signal amplitude and ultra-narrow spectral characteristics, which contribute to robust laser frequency stabilization. The laser frequency can be selectively locked to the Stokes or anti-Stokes frequency of the four-wave mixing spectrum, maintaining a fixed frequency difference relative to the pump light that corresponds to the atomic ground state hyperfine splitting. By adjusting the detuning of the pump light frequency, a wide tuning range of several GHz can be achieved. Compared to the frequency modulation spectroscopy method, the modulation transfer technique in a double-lambda atomic system provides an atomic coherence-enhanced error signal with a larger peak-to-peak amplitude and a steeper zero-crossing gradient, resulting in significantly improved laser frequency stabilization performance. This method can enhance the performance of atomic sensors, such as improving the stability of the Raman light for atomic interferometers and increasing the laser stability of highly sensitive atomic magnetometers.