Practical optical fiber digital communication systems use binary PCM signal to directly modulate the intensity of the light source. The intensity modulated optical pulse signal output by the optical transmitter is transmitted to the receiving end through optical fiber. Due to the influence and limitation of transmitting optical power, receiver sensitivity, optical fiber line loss and even dispersion, the maximum transmission distance between optical terminals is limited.
For example, in the 1.31 µ m working area, the maximum transmission distance of 34Mb / s optical transceiver is generally 50 ~ 70km, and the maximum transmission distance of 140MB / s optical transceiver is generally 40 ~ 60km. If this maximum transmission distance is exceeded, it is usually considered to add fiber optic repeaters to amplify and process attenuated and deformed optical pulses. The fiber optic repeaters often adopt photoelectric regenerative repeater, that is, optical—electric—fiber optic repeater, which is equivalent to the relay station of optical fiber transmission. In this way, the transmission distance can be greatly extended.
The cost of optical relay is too high. After the optical amplifier comes out, the optical amplifier is generally used to replace the optical relay. The commonly used amplifiers include erbium-doped fiber amplifier (EDFA) and Raman fiber amplifier, and there are also new dispersion compensation amplifiers to compensate dispersion.
The repeater in optical fiber communication is composed of optical detector and preamplifier, main amplifier, decision regeneration circuit, light source and drive circuit. Its basic functions are re-amplification, reshaping and retiming. Repeaters with these three functions are called 3R repeaters, while repeaters with only the first two functions are called 2R repeaters. The output pulse after regeneration completely eliminates the additional noise and distortion. Even in a system composed of multiple relay stations, noise and distortion will not accumulate, which is the most prominent advantage of digital communication for long-distance communication. As the optical amplifier has become mature, it can be used as a 1R repeater (only amplifying) instead of 3R or 2R repeaters to form an all-optical fiber communication system, or a hybrid relay mode with a 3R repeater, which can greatly simplify the structure of the system and is the direction of development.
The structure of repeaters in optical fiber communication equipment varies depending on the installation site. The rack-type fiber optic repeater is installed in the intermediate office computer room, and its structure should be consistent with the optical terminal, digital multiplexing equipment and its frame structure in the computer room. The repeater in optical fiber communication of each rack should include two receiving units and two transmitting units to form a two-way repeater. The power supply is supplied by the local station, and the box-type or tank-type fiber optic repeaters can be directly buried in the ground, manholes, or overhead on the line poles; the box must have good sealing, anti-corrosion and other properties; the power supply is supplied locally or remotely through copper wire.