Abstract

Optical amplifiers permit the-extension of transmission distances in lightwave systems to several thousand kilometers before the accumulated noise causes unacceptable errors. When the signal power is maintained over these long distances, optical nonlinearities become a dominant concern. The nonlinearity of silica fibers can be expressed as an intensity-dependent refractive index, n = n₀ + n₂I, where n₂ = 3.2 × 10⁻¹⁶ cm²/W. There are three major manifestations of the nonlinear index: self-phase modulation, in which fluctuations in the signal power give rise to a modulation of the signal phase and lead to a broadening of the spectrum; cross-phase modulation, in which intensity fluctuations of a second channel propagating in the fiber modulate the phase of the first channel; and four-photon mixing,¹ in which the beating between two channels modulates the signal phase at their different frequencies, generating new tones as sidebands. In amplified systems, noise can act as a second channel for cross-phase modulation or four-photon mixing. The choices for modulation format fall into three categories: on-off keying, the standard format of today's systems; constant envelope modulation, such as phase-shift keying, which minimizes power fluctuation of the signal; and soliton transmission,² which uses pulses formed by balancing selfphase modulation and dispersion. We will describe various nonlinear effects and the constraints they impose on possible systems.

© 1990 Optical Society of America