Abstract

Intensive investigation has followed the surprising observation of self-organized second-harmonic generation (SHG) in optical fibers. It is now known that an alternating χ⁽²⁾ grating has formed in the fiber. The grating grows by mixing of the fundamental and harmonic waves so SHG gratings can be formed quickly by seeding with a coherent second-harmonic wave. The length of the grating is determined by the bandwidth of the input waves and the intensity-dependence of the refractive index. There are now attempts to incorporate saturation and bleaching into the macroscopic mixing model and to understand the growth of the grating and the effect of phase fluctuations on seeding. The microscopic models all require some sort of defect and involve multiphoton absorption. The defects themselves could directly be the source of the dipole-allowed susceptibility, or they could provide charges which are trapped elsewhere. Most models require a large internal field to orient defects or to induce SHG through the macroscopic χ⁽³⁾. There have been investigations using externally applied dc fields, and there are recent attempts to observe the internal field. While most effort is directed towards efficient doublers, SHG in fibers can provide new insights into structure and defects in glasses as well as self-organized systems in general.

© 1990 Optical Society of America