Fig. 1. Schematic of the FMS frequency offset lock. ECDL, external cavity diode laser; PBS, polarization beam splitter; PM fiber, polarization maintaining fiber; FEOM, fiber electro-optic modulator; PD, photodetector; λ/2, half-wave plate; λ/4, quarter-wave plate; M, mirror; DDS, direct digital synthesizer.

Fig. 2. Energy-level scheme for Rb85 and Rb87 D2 line.

Fig. 3. Comparison of (a) referenced SAS signal, (b) FMS signal with the frequency offset of 1 GHz, (c) FMS signal with the frequency offset of 1.2 GHz, (d) FMS signal with the frequency offset of 1.5 GHz, and (e) FMS signal with the frequency offset of 2 GHz.

Fig. 4. Error signals of the offset FMS obtained for different modulation frequencies from the 1st-order sideband with the frequency difference of −1.5  GHz to the Rb87 F=2→F′=2 CO 3 transition. The inset shows the peak-to-peak amplitudes of the error signals with varied DDS modulation frequencies.

Fig. 5. RF beat note power spectrum of two lasers at an offset central frequency of 1.367 GHz. An Agilent E4440A spectrum analyzer is used at a 10 MHz frequency span with a resolution bandwidth (RBW) of 1 kHz. The 3 dB linewidth of the beat note signal is 900 kHz.

Fig. 6. Allan variance of the beat frequency.

Fig. 7. Frequency fluctuations of the frequency offset locked laser for locking on and free running cases.