According to the transmission signal, it can be divided into digital transmission system and analog transmission system.
In an analog transmission system, the input signal becomes a continuous change in the amplitude (frequency or phase) of the transmission signal. Optical fiber analog transmission system simulates light intensity, and the modulation power of its light source varies with the amplitude of modulation signal. However, due to the serious nonlinearity of light source, its signal-to-noise ratio, transmission distance and transmission frequency are very limited.
The digital transmission system converts the input signal into pulse signals represented by "1" and "0" and uses them as transmission signals. At the receiving end, it is restored to the original information. In this way, the nonlinearity of light source has little influence on digital code stream, and digital communication can adopt some coding and error correction methods, and it is easy to realize multiplexing, so digital transmission system has great advantages and has been widely used in many places.