We study experimentally the role played by cavity boundary conditions in the onset of modulational instability in nonlinear and dispersive waves. This study is performed by means of an externally driven optical cavity made of a single-mode silica fiber. Spectral measurements of the intracavity field allow us to identify modulational instability in the presence of normal dispersion. This finding demonstrates that cavity boundary conditions fundamentally alter the physical mechanisms which are at the origin of modulational instability.

• Received 17 June 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.4139