Abstract

We have observed an intensity threshold for Hill grating formation in single-mode polarization-preserving fibers from Andrews Corporation. A single-frequency argon-ion laser operating at either 488 nm or 514 nm is isolated from fiber reflection an acousto-optic modulator. The fiber rests in a loose coil on a copper plate that is temperature controlled to a few millidegrees Kelvin, cooled somewhat below room temperature. The fibers we report on here are "D" type and have core sizes 12 m and 1.52.5 m, although we have observed thresholds in other Andrews fibers as well. Grating formation commences at an intensity in the region of 130 mW, depending on the length of the fiber and on the initial intensity of the light reflected from the output end. The threshold behavior resembles that observed for self-seeded growth of second-harmonic light: Below threshold, no grating is written, above threshold, grating formation begins. However, reducing the light intensity below threshold stops further growth. The threshold may occur because of a balance between gain and loss for the growth process; it could also indicate the underlying physics and chemistry. Much, but not all, of our evidence seems to point to the former.

© 1990 Optical Society of America