Fig. 1. Structure of the laser system. Beams from two lasers with frequencies fRL and fSL are combined through two beam splitters (BS), and the beating signal, whose frequency is fb, is detected by a high-speed photo detector (HSPD). Afterwards, the beating signal is amplified and divided (division ratio is marked as D), and the divided beating signal (DBS) with a frequency fS=fb/D is generated. Taking a rubidium clock as the RF reference, the frequency counter measures the frequency of DBS and compares it with the expected frequency we set. Afterwards, a digital error signal is generated by the logic control and converted into analog by a digital-to-analog converter (DAC). Finally, this analog signal is applied to modulating the frequency of slave laser.

Fig. 2. Principle of the system. (a) Pulse-counting method used for frequency measurement. (b) Method we use to judge frequency polarity of the slave laser. (c) Working process of the entire system.

Fig. 3. Frequency distribution of the DBS when free running and locked (a) span = 100  kHz, (b) span = 1  kHz.

Fig. 4. Allan deviation of the beating signal for free running (blue dash line) and locked (red solid line) conditions.