Nonlinear gain dynamics in quantum-dot optical amplifiers and itsapplication 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 ~90 fs, ~260 fs, and ~3 ps, respectively, under a 50-mA bias condition. The dephasing time was evaluated to be ~85 fs. The third-order optical susceptibility (χ⁽³⁾) has been evaluated by means of both nonlinear transmission and four-wave mixing experiments. The results show that the nonlinearity expressed by χ⁽³⁾/g₀ 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