Second harmonic generation in optical fibers: modal and nonmodal phase matching

Abstract

Ouelette et al.1 recently demonstrated the erasure, subsequent to the seeding with 532- and 1064-nm wavelength light, of the self-organized grating in an optical fiber by further illumination with 532-nm light alone. We have demonstrated second harmonic generation of 1064-nm radiation in modally phase matched fibers2 by prior treatment with argon laser radiation alone. In this paper we report experimental results on a modally phase matched and untreated (by argon laser radiation) fiber. First, it was seeded by injecting a small amount of 532-nm wavelength radiation simultaneously with 1064-nm wavelength radiation. Subsequently, transmission with 1064-nm wavelength radiation generated frequency doubled light in the seeding mode as reported in Ref. 1. This seeded mode interaction is generally believed to be phase matched by a self-organized χ(2) grating written into the fiber. Subsequent illumination with higher power 532-nm wavelength light and further probing with 1064-nm wavelength radiation alone showed the disappearance of the seeded frequency doubled mode and thus erasure of the grating, but also the appearance of the modally phase matched interaction. Our experiments highlight the differences in the nature of the phase matching processes in optical fibers. In Ref. 1 it is speculated that charge recombination occurs during the erasure process. Our experiments show that this is more likely not the case and that the model proposed by Payne,3 in which a uniform transverse internal field is created as a result of illumination with visible radiation, is partly responsible for second harmonic generation in modally phase matched fibers.