Nonlinear gain dynamics in quantum-dot optical amplifiers and its application to optical communication devices

Ultrafast gain dynamics in quantum-dot (QD) optical amplifiers has been studied. It was found that there are at least three nonlinear processes, which are attributed to carrier relaxation to the ground states, phonon scattering, and carrier capture from the wetting layers into the QDs. The relevant time constants were evaluated to be similar to 90 fs, similar to 260 fs, and similar to3 ps, respectively, under a 50-mA bias condition. The dephasing time was evaluated to be similar to 85 fs. The third-order optical susceptibility (chi ((3))) has been evaluated by means of both nonlinear transmission and four-wave mixing experiments. The results show that the nonlinearity expressed by chi ((3))/g(0) is quite similar to that of bulk and quantum wells, which can be explained by similar relaxation times. Applications to optical communication devices are also discussed.