All-optical waveguide-type gain switch utilizing intersubband transition

All-optical switching performance of waveguide-type device utilizing intersubband transition is investigated by analyzing the position-dependent change of optical power densities and carrier densities in the waveguide. In the device, the three lowest energy-level subbands in the conduction band structure of asymmetric coupled quantum wells are used. The optical excitation pulse excites the electrons in the first subband to the third subband and the stimulated emission between the third subband and second subband amplifies the optical signal pulse. We find that the signal pulse is notably amplified when the excitation pulsewidth is less than the relaxation time of the third subband. The calculated results show that wavelength conversion from 1.35 to 1.6 μm with a gain of more than 10 dB is achieved by the excitation using an optical pulse with 1.5-ps width. This demonstrates the effectiveness of using a waveguide-type device structure with pulsed operation to make devices with a high-optical gain. We also derive a simple approximation for the optical gain. We show that the approximation gives results in good agreement with the results of the rigorous simulation using a finite difference method and the approximation is useful for designing devices