The photorefractive effect in directional coupler and Mach-Zehnder LiNbO/sub 3/ optical modulators at a wavelength of 1.3 mu m

Previous work has shown that at a wavelength of 1.3 mu m and power levels on the order of a few milliwatts, directional coupler switches show negligible damage due to the photorefractive effect. The author's experiments at higher power levels (20 mW) show a significant reduction in performance for 1-cm-long directional coupler modulators with uniform electrodes. The reduction in modulation efficiency is in good agreement with a numerical simulation based on the coupled mode equations using index changes estimated from previous work. At similar power levels, Mach-Zehnder modulators showed little effect making them a good design for high power applications. Other electrode structures, such as the reverse Delta beta type, might help alleviate the effect in directional couplers. Modulators operating at 1.55 mu m should also show less susceptibility to the effect.>