In optical fiber long-distance transmission, repeaters are needed to amplify the weakened signals after long-distance transmission. The maximum relay distance of optical fiber transmission system is determined by four factors.

1. The transmitter outputs the average optical power coupled into the optical fiber. The greater the power coupled into the fiber, the longer the relay distance.

2. The dispersion of optical fiber. If the dispersion of optical fiber is large, the waveform distortion will be serious after a certain distance transmission. The longer the transmission distance, the more serious the waveform distortion. In digital communication system, waveform distortion will cause intersymbol interference, reduce the sensitivity of optical reception, and affect the relay distance of the system.

3. Loss of optical fiber. The loss of optical fiber line includes the loss of optical fiber movable connector and the loss of optical fiber fusion, of course, mainly the loss per kilometer of optical fiber. If the loss of optical fiber per kilometer is smaller, the loss of signal optical power on the optical fiber will be smaller, and the transmission distance of optical signal in the optical fiber will be longer.

4. Optical receiver sensitivity meeting certain bit error rate requirements. The higher the receiving sensitivity, that is, the smaller the minimum received optical power to meet the system bit error rate requirements, and the longer the relay distance.