Optical Nonlinearities In Gaas-Gaalas Multiple Quantum-Well Hetero-Nipi Wave-Guides

This paper reports on measurements of large optical nonlinearities in GaAs-GaAlAs multiple quantum-well hetero-nipi (MQW-H nipi) waveguides using a Mach-Zehnder interferometer. A model shows how the distribution of the photogenerated carriers in the waveguide structure is affected by the finite sheet resistance of the neutral regions and the optical intensity. However, at normal operational powers and absorption levels the lateral spreading of the photogenerated carriers in the MQW-H nipi waveguide is so large that the nonlinearity must be defined from the ratio between the total optical power and the width of the MQW-H nipi region rather than from the optical intensity alone. The study concludes that the nonlinearity measured in a GaAs-GaAlAs MQW-H nipi waveguide at a wavelength far below the absorption edge, where the absorption coefficient is in the range 10-20 cm-1, is of the order of 10(-5) cm2/W. This value is 2-3 orders of magnitude larger than the optical nonlinearity measured in a typical MQW p-i-n diode waveguide at similar absorption, but the increased nonlinearity is obtained at the expense of speed. The time response was found to be in the microsecond regime. Good agreement for the measured turn-on time was obtained for a model with no adjustable parameters