Characterization of optical phase-locked two distributed-feedback fiber lasers for 87Rb atom interferometry

We report the optical phase-locking of two 1560 nm distributed-feedback (DFB) fiber lasers converted to 780 nm with a frequency difference of 6.834 GHz for 87Rb. The fiber laser exhibits a narrow linewidth and low-intensity noise, which are advantageous for a Raman pulse laser in an atomic gravimeter. However, the piezoelectric actuator of the DFB laser causes a low feedback rate in an optical phase-locking loop (OPLL). To overcome this limitation, we used an acousto-optic modulator as an additional actuator to increase the modulation bandwidth of the OPLL and achieved a residual phase noise of approximately -127 dBrad2/Hz in the offset frequency range from 10 Hz to 1 kHz. The sensitivity limit estimated using the sensitivity function and experimental parameters of 87Rb atomic gravimeter shows that our OPLL system can be used in state-of-the-art gravimeters.