Nonlinearities in glass for subpicosecond optical switches

Abstract

Ultrarapid (subpicosecond) optical switching and signal processing elements should find important applications in future communication and computing systems. Glasses with fast (< 100-fs) electronic third-order susceptibilities could be used to fabricate ultrarapid switches. These nonlinearities, described by the nonlinear index coefficient n2, are compared with slow thermal index changes for various glasses. A degenerate four-wave mixing (DFWM) technique using a 250-ns train of 80-ps pulses from a mode-locked Q-switched Nd:YAG laser was used to measure n2. Thermal index changes were obtained by comparison of the DFWM signal from initial and final pulses of the pulse train. Results for leaded glasses agree with previous reports, with n2 of SF-59 equal to 19% of that of CS2. Titanium glasses, e.g., Hoya FDS-90, have an n2 that is 7 % that of CS2, and n2 does not increase with linear index as for leaded glasses. The observed thermal index changes are small, as expected for very low absorption coefficients. We define a figure of merit to help compare nonlinear materials for use in optical switches. It is the ratio of the (fast) nonlinear index change necessary to produce switching to the accompanying (slow) thermal index change. We show that some optical glasses appear to have higher figures of merit than any other nonlinear optical material. We conclude with a discussion of some possible configurations for glass all-optical switches.