Optimized wavelength-division-multiplexed lightwave communication system

DETAILED DESCRIPTION Lightwave signal transmission over very long distances is encumbered by the usual problems caused by loss and dispersion in the optical fiber.
In addition, such transmission is effected by a weak dependence of the fiber material refractive index on the intensity of each lightwave signal.
This dependence is nonlinear and can be expressed as n=n.
sub.
0 +n.
sub.
2 .
multidot.
.
vertline.
E.
vertline.
.
sup.
2, where n.
sub.
0 represents the linear part of the refractive index, n.
sub.
2 represents the nonlinear part of the refractive index, and E is the power density of the lightwave signal.
Dependence of refractive index on the intensity of the lightwave signal induces a change of the group velocity for that lightwave.
The refractive index change also leads to self phase modulation or self chirping of the lightwave signal which influences pulse shape via interaction of pulses from the lightwave signal with the fiber dispersion.
Self phase modulation and dispersion cancel completely during soliton formation.
When cancellation is incomplete, self phase modulation leads to spectral broadening and, as a result, additional pulse distortion.
In accordance with the principles of the present invention, the signal distorting influence of the weak refractive index nonlinearity in optical fibers is minimized for lightwave signals traveling distances comparable to present transoceanic spans.
This is achieved in a multi-channel wavelength division multiplexed (WDM) lightwave communication system by transmitting adjacent channels approximately 2 nm or more apart in the normal dispersion regime of the optical fiber and also by transmitting each channel more than 0.
4 nm below the zero dispersion wavelength, .
lambda.
.
sub.
0, of the optical fiber.
While the interchannel separation may be large, it has been determined from simulation results that an interchannel separation between 2 nm and 3 nm is desirable for a WDM lightwave system having at least two channels bearing ASK modulated information.
A large interchannel separation is permitted by employing fixed or adaptive equalization elements at the receive end of the communication system