Abstract

Active optical waveguides offer interesting possibilities for performing nonlinear functions in photonic circuits. By varying the injected carrier density one can change not only the linear components of gain and index of refraction but also the nonlinear ones. Some studies of ultrafast gain dynamics in AlGaAs⁽¹⁾ and InGaAsP⁽²⁾ diode laser amplifiers have been performed previously. Four-wave mixing experiments with large frequency detunings in diode lasers ^(3,4) have provided frequency domain measurements of the magnitude of the nonlinear susceptibility of χ⁽³⁾. The effect of dynamic index changes has also been revealed by the chirp and spectral changes experienced by ultrashort pulses traveling through amplifiers⁽⁵⁾. In this paper we discuss direct measurements of optically induced changes in index of refraction in active AlGaAs waveguides. With femtosecond pump-probe measurements sensitive, separately, to induced phase and amplitude changes, we have studied the relative amplitudes of these dynamics as functions of wavelength and injection current. We observe nonlinearity due to changes in carrier number, carrier heating and, for the first-time, a large above-bandgap ultrafast optical Kerr effect. An interesting aspect of this latter, instantaneous nonlinearity is that it may be utilized without change in carrier number, by operating near the wavelength of nonlinear transparency (where stimulated emission equals absorption).

© 1991 Optical Society of America