Fig. 1. Schematic diagram of light field undergoing two phase modulations

Fig. 2. Saturated absorption signal and lock-in amplifier signal of ²⁰²Hg atom

Fig. 3. Schematic diagram of automatic frequency stabilization system

Fig. 4. Interactive interface of frequency stabilization software

Fig. 5. Flow chart of frequency stabilization software

Fig. 6. Power spectral densities of slave laser frequency noise in weak lock state and strong lock state

Fig. 7. Beat spectrum lines of two deep ultraviolet lasers when their frequencies are locked by OPLL technique

Fig. 8. Locking process of ²⁰²Hg. (a) Saturated absorption signal and lock-in amplifier signal; (b) sideband frequency

Fig. 9. Frequency fluctuations of 1015 nm slave laser and 1062 nm reference laser measured by wavelength meter

Fig. 10. Saturated absorption signals and lock-in amplifier signals of ¹⁹⁹Hg and ²⁰⁴Hg during locking. (a) ¹⁹⁹Hg; (b) ²⁰⁴Hg