In fact, a balanced photodetector consists of two channels. Two PD tubes with completely similar characteristics are used as photoelectric conversion, one of which is added with a delay line, or the front end is adjusted with a Mach Zehnder interferometer to reverse the polarization of the other. The back-end uses a differential amplifier to amplify the differential mode signal and suppress the * * * mode signal. After adding two paths, the noise is completely cancelled and the output amplitude is greatly enlarged. Simply put, it is often used for weak signal detection and DPSK and differential detection. For detailed information, please go to Baidu Library to search for related papers.