Dynamical optical nonlinearities in semiconductor optical amplifiers and quantum cascade lasers

Abstract

The ultrafast dynamics in semiconductor optical amplifiers (SOAs) relevant to all-optical switching have been studied by means of time-resolved spectroscopic experiments on a multiple quantum well InGaAs amplifier. The nonlinear light generation in Quantum Cascade (QC) lasers has also been a subject of research. The switching windows of a TOAD-like three-beam interferometric set-up, measured using 700 fs pulses in the 1.5 μm wavelength region, revealed an ultrafast feature (~ ps) dependent on the pulse energy of the pump beam. These dynamics, and the subsequent reshaping of the switching window edge, are attributed to refractive index changes in the SOA caused by ultrafast carrier heating, consistent with the predictions of a sliced propagation model based on rate equations. Counter-propagating sub-picosecond pulses were used to monitor gain saturation along the SOA waveguide at different wavelengths around the peak gain. The functional form of the spatial dependence of gain saturation is found to depend on pulse energy. These observations are interpreted by combining the optical nonlinearities associated with interband carrier dynamics and carrier heating together and their respective time constants. Aided by the rate-equation model, we show that the amplification of the pump as it propagates along the amplifier may lead to the saturation of a portion of the device and to an effective narrowing of the switching windows of a TOAD (even for pulse energies below 1 pJ). Particular attention is devoted to discussing the limit of full saturation across the entire SOA waveguide. Observation of stimulated electronic Anti-Stokes Raman emission in QC lasers is reported. We present four distinct designs of active regions with enhanced Anti-Stokes Raman nonlinearity. In all four, the pump laser is monolithically integrated with the non-linear region in a two-stack active core within the same waveguide. Stimulated electronic Raman emission was observed in samples of three of these designs. Additionally, for the designs with positive detuning, an incoherent up-conversion signal resulting from optical pumping is detected.