Laser diodes include single heterojunction (SH), double heterojunction (DH) and quantum well (QW) laser diodes. Quantum well laser diode has the advantages of low threshold current and high output power. It is the mainstream product in the market at present. Compared with the laser, the laser diode has the advantages of high efficiency, small volume and long life, but its output power is small (generally less than 2mW). The linear difference and monochromatic color are not very good, so the application of the laser in the cable TV system is greatly limited, it can not transmit multi channel and the high character can simulate the signal. In the return module of bidirectional optical receiver, the quantum well laser diode is usually used as the light source for the uplink emission.