Effects of Two-Photon Absorption on Pump-Induced Refractive-Index Change in AlAsSb–InGaAs–AlAs Optical Waveguides

Injection of transverse-magnetic polarized 1.55- mum pump light pulses having an excessively high intensity into heavily-doped AlAsSb-InGaAs-AlAs coupled-double-quantum-well optical waveguides causes a large amount of electrons in the ground state of the subbands in the conduction band to transit to the excited state and results in bandgap shrinkage as well as a large amount of vacant sites in the ground state. Consequently, the simultaneously injected continuous-wave transverse-electric polarized 1.54- mum probe light is absorbed by the valence-band electrons through two-photon absorption (TPA) and transit to the vacant sites in the ground state of the subbands in the conduction band. As a result, the probe power is reduced instantaneously but recovers at a time constant in the order of 10
^-10
s. A theoretical evaluation of the effect of the above intersubband-pump-assisted TPA of the probe light shows that the pump-induced net refractive-index change in these devices is reduced by 50% when intersubband absorption and ISB-pump-assisted TPA of the probe light are of equal strength. Characterization of the TPA in these devices indicates that these devices have a TPA coefficient of 25 cm/GW.