In optical fiber communications, a common assumption is that increasing the signal power will enhance performance. However, this isn’t always the case due to the phenomenon of non-linearity in optical fibers. Non-linear effects can degrade signal quality and cause unexpected issues, especially as power levels rise.
Non-Linearity in Optical Fibers
Non-linearity occurs when the optical power in a fiber becomes high enough that the fiber’s properties start to change in response to the light passing through it. This change is mainly due to the interaction between the light waves and the fiber material, leading to the generation of new frequencies and potential signal distortion.
Harmonics and Four-Wave Mixing
One of the primary non-linear effects is the creation of harmonics—new optical frequencies that weren’t present in the original signal. This happens through a process called Four-Wave Mixing (FWM). In FWM, different light wavelengths (λ) interact with each other inside the fiber, producing new wavelengths.
The relationship between these wavelengths can be mathematically described as:
or Here 𝜆1,𝜆2,𝜆3 are the input wavelengths, and 𝜆4 is the newly generated wavelength. This interaction leads to the creation of sidebands, which are additional frequencies that can interfere with the original signal.
How Does the Refractive Index Play a Role?
The refractive index of the fiber is a measure of how much the light slows down as it passes through the fiber. Normally, this refractive index is constant. However, when the optical power is high, the refractive index becomes dependent on the intensity of the light.This relationship is given by:
or 
