Second harmonic generation in optical fibers.

Abstract

Discovery of photo-induced, self-phase-matched second-harmonic generation (SHG) in fused silica has been both exciting and intriguing. An understanding of the physical mechanisms, which destroy the centro-symmetry of fused silica and organize the nonlinearity for quasi-phasematching has remained a challenge. The purpose of this study was to investigate these mechanisms. In particular, spatially resolved Raman spectroscopy was applied to demonstrate a spatial periodicity, associated with the induced $\chi\sp{(2)}$, in the material properties of the fiber. Electro-optic sampling was used for the first time to measure volume polarizations. An internal field of 10$\sp4$ V/cm was measured. An internal field as large as this supports the view that the $\chi\sp{(2)}$ is induced by an internal field. A material modification which enhances the efficiency of SHG was achieved using gamma-ray irradiation. A microscopic picture based on oxygen vacancies has been proposed to explain the enhancement in efficiency. According to this picture, oxygen vacancies are a necessary ingredient for the self-organization process. A detailed study of the dynamics of $\chi\sp{(2)}$ gratings was also conducted. Results of this study show that the initial growth rate of $\chi\sp{(2)}$, which is linear in time, has the following dependence on the seed-second harmonic and IR fields:$$\rm {d \chi \sp{(2)} \over dt} \propto\left(\vert E\sbsp{cond}{2\omega} \vert\sp2\right)\sp{1.48 \pm 0.2} \left(\vert E\sbsp{cond}{\omega} \vert \sp2\right)\sp{0.9 \pm .2}.$$These results suggest that the driving mechanism for self-organization is an interference between a two-photon (2$\omega$, 2$\omega$) and a three-photon (2$\omega$, $\omega$, $\omega$) ionization processes. A model based on this type of interference has been explored. This model is in qualitative agreement with some of our experimental results.