Influence of the coherence length of a laser on the noise characteristics of frequency-modulated fiber-optic communication lines

The time dependences of the mode noise where determined for a fiber-optic communication line of length 20 km subjected to modulation at a subcarrier frequency of 70 MHz. The communication line consisted of multimode graded-index fiber waveguides. The spectral width of the emission line of the radiation source (λ = 1.3 μm) employed in this study was determined. It was found that a change in the frequency modulation depth of a signal by 2 dB could reduce the mode noise level by 10 dB.