Other Supporting Technologies for Coherent Optical Communication

 The performance of coherent optical communication is powerful, but the system is very complex and it’s hard to make the technology happen.

In order to realize the practical application of coherent optical communication, it is necessary to rely on the following technologies:

Polarization PreservingTechnology
Coherent detection requires that the polarization directions of the signal light and the local oscillator light are the same in coherent optical communication, that is, the electric vector directions of the two must be the same, so as to obtain the high sensitivity that coherent reception can provide.

Because, in this case, only the projection of the signal light electric vector in the direction of the local oscillator light electric vector can really contribute to the intermediate frequency signal current generated by the mixing. In order to ensure high sensitivity, it is necessary to take lightwave polarization stabilization measures. There are two main methods currently:

First, the “polarization-maintaining fiber” is used to keep the polarization state of the light wave unchanged during the transmission process. (Ordinary single-mode fiber will change the polarization state of the light wave due to factors such as mechanical vibration or temperature change of the fiber.)

Second, use ordinary single-mode fiber, but use polarization diversity technology at the receiving end.

Frequency Stabilization Technology
The frequency stability of semiconductor lasers is very important In coherent optical communication. The frequency of the laser is very sensitive to changes in operating temperature and current. If the frequency of the laser drifts with different operating conditions, it will affect the IF current, thereby increasing the bit error rate.

Spectrum Compression Technology
The spectral width of the light source also matters in coherent optical communication. Only by ensuring the narrow linewidth of the light wave, can the influence of the quantum amplitude modulation and frequency modulation noise of the semiconductor laser on the sensitivity of the receiver be overcome. Besides, the narrower the line width, the smaller the phase noise caused by the phase drift. In order to meet the requirements of coherent optical communication on the spectral width of the light source, spectral width compression technology is usually adopted.

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