Fig. 1. Optical path diagram of laser heterodyne speech system

Fig. 2. Schematic diagram of arctangent demodulation

Fig. 3. Schematic diagram of differential cross multiplication demodulation

Fig. 4. Time domain diagram and spectrum of received signal. (a) Time domain diagram; (b) spectrum

Fig. 5. Time domain diagrams of signals filtered by bandpass filter. (a) Signal i1; (b) signal i2

Fig. 6. Demodulation results at different sampling rates. (a) 85 MHz; (b) 200 MHz

Fig. 7. Time domain diagrams of two signals after mixing and filtering. (a) Signal i1; (b) signal i2

Fig. 8. Demodulation results of proposed methods. (a) Arctangent; (b) differential cross multiplication

Fig. 9. Absolute error distribution after demodulation by different improved methods. (a) Arctangent; (b) differential cross multiplication

Fig. 10. Corresponding curve of analog speech signal SNR and demodulated signal SNR