Hi,
Frequency and phase modulation can still be used in optical communication systems in indirect ways to deal with noise and distortion.
For example, signals can be frequency- or phase-modulated onto a subcarrier before intensity modulating the optical carrier.
Noise performance of an analog signal will be enhanced in addition to the reduction of cross-modulation in frequency-division multiplexing strategies.
In optical fiber is signal distortion due to multiple light paths, or modes, having different distances over the length of the fiber. When light is emitted by a source, the photons (light particles) do not all travel the exact same path.
olarization mode dispersion (PMD) is caused by asymmetric distortions to the fiber from a perfect cylindrical geometry.
WM can be compared to the intermodulation distortion in standard electrical systems. When three wavelengths (λ1, λ 2, and λ 3) interact in a nonlinear medium, they give rise to a fourth wavelength (λ 4), which is formed by the scattering of the three incident photons, producing the fourth photon. This effect is known as four-wave mixing (FWM) and is a fiber-optic characteristic that affects WDM systems.
Thanks