Fig. 1. Schematic of optical homodyne receiver with Costas loop

Fig. 2. Principle for phase frequency detection based on delayed XOR in Costas loop

Fig. 3. PFD curves under different SNRs

Fig. 4. PFD curves under different delays

Fig. 5. Schematic for the composite loop based auxiliary control for frequency acquisition

Fig. 6. Basic structure diagram of loop filter

Fig. 7. Optical homodyne receiver prototype based on the Costas loop. (a) Schematic diagram; (b) physical diagram

Fig. 8. Setup for the autonomous locking capability test

Fig. 9. Results of the autonomous locking capability of the receiver. (a) Frequency of the signal light (F_l) is 12 GHz lower than the LO light (F_s); (b) frequency of the signal light is 12 GHz higher than the LO light

Fig. 10. Setup of the Doppler shift compensation test

Fig. 11. Results for the Doppler shift compensation test. (a) Frequency variation of signal light simulated for Doppler shift; (b) output voltage of LF and phase error with Doppler shift compensation; (c) output voltage of LF and phase error without Doppler shift compensation

Fig. 12. Setup of the sensitivity test

Fig. 13. Residual phase noise in closed-loop case

Fig. 14. BER curve of receiver, with an inset showing the eye diagram of the data after CDR regeneration at receive sensitivity of -39.5 dBm and BER of 1×10⁻⁹