Fig. 1. Schematic diagram of active stabilization control principle for optical system

Fig. 2. Schematic diagram of PID controller principle

Fig. 3. Schematic diagram of PLI principle

Fig. 4. Schematic diagram of OPLL principle for laser frequency synchronization

Fig. 5. Schematic diagram of PDH phase-locked principle for optical resonator

Fig. 6. Performance assessments of phase-locked systems. (a) Comparison of error signals before and after phase-locked^[16]; (b) phase noise spectral density and integrated phase noise spectral density of the interferometer phase-locked system^[15]; (c) comparison of the beat frequency error signal spectral widths before and after the laser frequency locking^[25]

Fig. 7. Squeezed angle locking of vacuum squeezed state using frequency-shifted auxiliary light^[55]

Fig. 8. Squeezed angle locking principle of vacuum squeezed state by the balanced zero-beat detector noise^[57]

Fig. 9. Time controlling sequence of squeezed angle locking for vacuum squeezed state by chopped locking^[58]. (a) Timing sequence of light field; (b) timing sequence of trigger for phase locking; (c) timing sequence of trigger for measurement

Fig. 10. Coherent modulation locking of nonlinear SU(1,1) interferometer^[14]

Fig. 11. Arbitrary phase-locked scheme base on single photon detector^[61]. (a) Schematic diagram of MZI and reference light; (b) relationship between counting ratioof the output channels and phase difference of interferometer

Fig. 12. Phase recovery of TF-QKD without phase-locked^[68]