The appearance of optical fiber amplifier, especially EDFA, solves the limitation of signal transmission distance caused by the loss in optical fiber communication network, and makes long-distance optical fiber communication become a reality.

Optical Fiber Communication is an information transmission system using laser as carrier and Optical Fiber pulled by glass of high purity as transmission medium.Since its emergence, it has brought a revolution to the whole communication industry, and made the large-capacity, high-rate and long-distance communication possible. In the contemporary social and economic development, it bears the heavy responsibility of information transmission, and has become the main road of high-rate and long-distance large-capacity communication network.

Fiber optic communication system because of its low loss, transmission frequency band capacity, long distance relay, anti-electromagnetic interference, not easy crosstalk, good confidentiality, good stability, light weight, small size, low manufacturing cost and other advantages, since the birth of 40 years, has been applied to social life on a large scale.At the same time, with the explosive growth of data flow, people‘s demand for the total amount of information is getting higher and higher, and the consequent requirements for optical fiber communication system are also getting higher and higher. Therefore, it is necessary to expand the capacity of the existing communication lines without increasing the construction and operation costs as much as possible.Wavelength Division Multiplexing (WDM), especially Dense Wavelength Division Multiplexing (DWDM) technology, has emerged.According to the wavelength of each channel, WDM/DWDM technology divides the low loss window of optical fiber into several channels for optical signal transmission at the same time, which makes full use of the huge bandwidth resources brought by the low loss area of single-mode optical fiber, and has the characteristics of transparent transmission, diversified application and flexible networking.

Due to the wide application of WDM, especially DWDM technology, on the one hand, the transmission power in the optical fiber increases sharply, which puts forward the high power requirements for the relay amplifier in the optical fiber communication system;On the other hand, since the usual fiber amplifier operates at saturation, its total output power hardly changes with the number of input channels.Therefore, when the number of multiplexed channels in the system changes, the gain of the remaining channels also changes, which leads to the transient fluctuation of the power of each channel. When the output power exceeds a certain threshold, the nonlinear increase of the optical fiber will be caused, and the transmission performance of the whole communication system will be seriously affected.Therefore, the research of high power fiber amplifier with gain control function is the necessary condition for the development and popularization of DWDM technology.