When the voltage is added to the electro-optic crystal, the refractive index and other optical properties of the crystal change, change the polarization state of the light wave, so that the circularly polarized light becomes elliptically polarized light, and then becomes linearly polarized light through the polarizer, and the light intensity is modulated. At this time, the light wave contains sound information and propagates in free space. The photodetector is used to receive the modulated optical signal at the receiving place, and then the circuit conversion is carried out to convert the optical signal into electrical signal. The sound signal is restored by demodulator, and finally the optical transmission of the sound signal is completed. The applied voltage is the transmitted sound signal, which can be the output of a radio recorder or a tape drive, and is actually a voltage signal that varies over time.