Abstract

It is widely accepted that efficient second harmonic generation (SHG) in optical fibers results from self-organized χ⁽²⁾ gratings. In this paper, we report the first direct observation of a modulation in the material properties of a conditioned fiber as measured by its spatially resolved Raman spectrum. Light from an argon laser operating at 514.5 nm was coupled through the fiber and Raman spectra of scattered light emitted perpendicular to the fiber axis were recorded with a spatial resolution of 5 µm. A single run was comprised of 24 such spectra, each sequentially displaced from the other by 2-3 µm along the axis of the fiber. Fibers previously conditioned for SHG showed a periodic modulation with position of ±3% in the strength of the entire low frequency portion of the Raman spectrum. A four parameter least squares fit to a sine wave yielded a coherence length of 27 µm when the seeding SH was in the fundamental mode. Coherence lengths of 35 µm and 50 µm were measured when the seeding green was in LP₁₁ and LP₀₂ modes, respectively.

© 1989 Optical Society of America