Abstract

Recently, two photon absorption (TPA) in semiconductor materials has been found to impose a serious limitation on the amount of ultrafast optically induced phase modulation available for use in all-optical switching elements¹. The limit arises because nonlinear absorption reduces the effective available nonlinear length of an element so that it is not possible to obtain large, ultrafast, all-optical phase modulation at a given switching power by merely increasing the device length. To date the magnitude of ultrafast all-optical phase modulation in semiconductor materials operating relatively close to the bandgap has proved insufficient for their use in all all-optical switching structures^2,3, although it has been possible to obtain sufficient phase modulation by using the relatively slow (>50ps recovery time) refractive index change from photogenerated carriers⁴. In this paper we demonstrate that it is possible to work beyond the TPA limit by operating close to the half bandgap wavelength where TPA is minimal. We have observed over π radians ultrafast all-optical phase modulation in a GaAs/AlGaAs multi-quantum well (MQW) waveguide and have characterised the field and polarization dependences of the nonlinear absorption near half bandgap. These measurements show that GaAs/AIGaAs MQW waveguides can be successfully used for ultrafast all-optical switching in the 1.55μm optical fiber communications band.

© 1991 Optical Society of America