Frequency Modulation Spectroscopy and Modulation Transfer Spectroscopy for Laser Stabilization to Rubidium Atomic Resonances

We make a detailed comparison of frequency modulation spectroscopy (FMS) and modulation transfer spectroscopy (MTS) and of their applications in stabilizing laser frequency. In both FMS and MTS, intensity modulations of probe beam are observed in saturated absorption spectroscopy of 87Rb atoms. However, the two modulation profiles appear different due to their distinct origins. Demodulation gives rise to frequency discrimination spectra, which can serve as error signals for frequency locking. The optimal error signal of FMS has a larger linear slope but also a slanted baseline, in contrast to that of MTS. We apply FMS and MTS to stabilize two 780 nm external cavity diode lasers to D2 lines of 87Rb atoms. We measure laser linewidth and stability using beat notes of two independently locked lasers. A laser linewidth as narrow as 12.9 kHz is achieved in FMS locking, compared to 70.8 kHz in MTS locking. A laser stability at the level of 10−12 is reached in both schemes. Nevertheless, FMS locking offers a better short-term stability, whereas MTS locking yields a smaller long-term drift. Our investigation of FMS and MTS would be valuable for making a choice of these spectroscopy techniques for laser stabilization as well as other relevant applications.