Fig. 1. The schematic of balance detector

Fig. 2. The curve of SNR with local oscillator optical power and nonlinear coefficient of photodetector

Fig. 3. Schematic of power spectrum of a narrowband signal

Fig. 4. Comparison of SNR calculated by cross-correlation method and spectrum method with theoretical SNR

Fig. 5. Schematic diagram of experimental setup

Fig. 6. Experimental results of three types of balance detectors