Passive Optical Networks

each other and produce sum and difference frequency products. Nonlinea-

rities can be controlled by carefully introducing and balancing chromatic

dispersion, perhaps with fixed and/or tunable dispersion compensation. The

following nonlinear effects tend to significantly degrade the signal integrity:

(i) Self-phase modulation (SPM) occurs because the intensity profile of an

optical pulse on a single channel causes a time-varying index-of-refraction

profile and, thus, the higher intensity center of a pulse travels slower than the

lower-intensity pulse wings. (ii) When considering many WDM channels co-

propagating in a fiber, photons from channels 2 through N can distort the

index profile that is experienced by channel 1. This cross-phase modulation