Optocoupler has the advantages of unidirectional signal transmission, complete electrical isolation between input and output, no influence of output signal on input, strong anti-interference ability, stable operation, no contact, long service life and high transmission efficiency. Widely used in electrical insulation, level conversion, interstage coupling, driving circuit, switching circuit, chopper, multivibrator, signal isolation, interstage isolation, pulse amplification circuit, digital instrument, long-distance signal transmission, pulse amplification, solid state relay (SSR), instrument, communication equipment and microcomputer interface. There are two kinds of photoelectric couplers: one is nonlinear optical coupler, and the other is linear optical coupler.