Abstract

Observations of a coherent second harmonic signal evolving within certain microscopically disordered media such as optical fibres have recently been the subject of much speculation. The three-wave mixing process normally responsible for second harmonic generation is known to be rigorously forbidden to all orders of multipolar expansion when the conversion sites are randomly oriented. One mechanism for the production of a coherent second harmonic in such media entails effective removal by the incident laser beam of bulk isotropy, through orientational hole burning and the creation of a physically induced hologram. Such a process may in a sense be regarded as involving an overall six-wave conversion. In fact the direct six-wave mixing process ω + ω + ω + ω → 2ω + 2ω is invariably permitted, irrespective of local or bulk symmetry, without involvement of any light-induced anisotropy or of any memory effect associated with structural change.

© 1994 IEEE