Fig. 1. Relation curve of frequency-locked range, phase-locked time and splitting ratio

Fig. 2. Influence of splitting ratio on residual phase error of loop

Fig. 3. Influence of splitting ratio under different laser linewidths on loop residual phase error

Fig. 4. Bode diagram of loop open-loop transfer function when the k=0.02. (a) Amplitude frequency characteristic; (b) phase frequency characteristic

Fig. 5. Schematic diagram of the splitting ratio adaptive adjustment system

Fig. 6. Simulation model of BPSK homodyne coherent optical communication system

Fig. 7. Curve of splitting ratio with half-wave plate rotation angle

Fig. 8. System eye diagram and bit error rate under different splitting ratios. (a) System eye diagram when k=0.5; (b) system eye diagram when k=0.25; (c) system eye diagram when k=0.06; (d) system bit error rate for different splitting ratios

Fig. 9. Experimental structure diagram of bit error rate test under different splitting ratios

Fig. 10. Relationship between the bit error rate and splitting ratio

Fig. 11. Simulink simulation model of phase-locked loop

Fig. 12. Output voltage waveform of voltage controlled oscillator. (a) Output voltage waveform when the frequency difference is within the frequency-locked range; (b) output voltage waveform when the frequency difference is outside the frequency-locked range

Fig. 13. Loop frequency-locked range and frequency-locked time for different splitting ratios

Fig. 14. Control voltage waveform when the passive proportional-integral filter loop loses lock when k=0.01