Causes of signal degradation in optical fiber communication

There are two main reasons for signal degradation in optical communication systems ：

1. Interference caused by the noise of photoelectric devices ;
2. Caused by optical fiber transmission 1) Signal attenuation 2) Signal distortion .

1 Noise of photoelectric devices

Noise of photoelectric devices , Mainly from optical amplifier .

2 Signal attenuation

Loss is an important characteristic of optical fiber , It largely determines that there is no optical amplifier and optical repeater , The maximum distance that an optical communication system can transmit .
The transmission loss of optical fiber has reached the theoretical limit , As low as 0.15dB/km. But for long-distance transmission systems , Fiber loss is still the key factor limiting the growth of system capacity .EDFA The emergence of has completely solved the problem of optical fiber loss .

3 Signal distortion

3.1 Linear distortion

It refers to the optical pulse broadening caused by fiber dispersion , The optical pulse transmitted in the optical fiber widens with the increase of transmission distance , When the distance is long enough , Adjacent light pulses may overlap due to broadening , This leads to the wrong decision of the receiver .
generally speaking , In single-mode fiber , There are material dispersion and waveguide dispersion , Generally, it is mainly material dispersion ; There are mode dispersion and chromatic dispersion in multimode fiber , Mode dispersion predominates .

3.2 Nonlinear distortion

Nonlinear distortion refers to the signal distortion caused by the nonlinear effect of optical fiber .

With WDM Commercialization of Technology , Dispersion shifted fibers encounter serious nonlinear problems . The nonlinear effects of optical fibers can be divided into two kinds according to their causes ：1. Nonlinear inelastic scattering process , Stimulated Raman scattering (SRS) With stimulated Brillouin scattering (SBS);2. Intensity dependent refractive index changes in optical fibers , Including self phase modulation (SPM)、 Cross phase modulation (XPM)、 Four wave mixing (FWM), At present, the biggest influence is four wave mixing .