Optical switching using a dynamic local index gradient induced in an InGaAsP/InP Y-junction waveguide switch is investigated using optical, electrical, and thermal simulations, and demonstrated experimentally. Switching is accomplished by creating a steep index gradient in the optical path near the Y junction. Both simulations and experiments indicate that either carrier injection or the thermo-optic effect can be used as the active switching mechanism. For carrier injection the induced index change under the electrode, Deltan, is negative. The measured switch contrast ratio is about 20 dB, with a response time of approximately 5 ns. The thermo-optic effect gives a positive Deltan, and the observed contrast ratio is about 10 dB. The thermo-optic switch response time is several hundred nanoseconds, due to the highly localized nature of the thermal gradient. (C) 2004 American Vacuum Society.