The function of the Fiber optic isolator is to let the forward transmission light pass and isolate the reverse transmission light, so as to prevent the reflected light from affecting the stability of the system, similar to the function of the diode in electronic devices.Fiber optic isolator are divided into two types according to polarization dependence: polarization dependent type and polarization independent type. The former is also called free space type (Freespace), because there is no optical fiber input and output at both ends. The latter is also known as in-Line, because of the optical fiber input and output at both ends. Free-space Fiber optic isolator are generally used in semiconductor lasers. Because the light emitted by semiconductor lasers has a high linearity, the polarization-dependent Fiber optic isolator can be used to enjoy the advantage of low cost. In communication lines or EDFA, linear optical isolators are generally used, because the polarization characteristics of light on the line are very unstable, requiring the device to have a small polarization related loss.