Modulation instability that leads to breakup of intense cw radiation into a train of ultrashort pulses during propagation in optical fibers occurs only in the presence of anomalous group-velocity dispersion. It is shown that a new kind of modulation instability can occur even in the normal-dispersion regime when two copropagating optical fields interact with each other through cross-phase modulation initiated by the nonlinearity. The quantitative aspects of this cross-phase–modulation–induced modulation instability are discussed and illustrated by use of a realistic experimental example.

• Received 3 June 1987

DOI:https://doi.org/10.1103/PhysRevLett.59.880