Wavelength dependence of gain recovery time in semiconductor optical amplifiers

The understanding of the phenomena related to the gain recovery of semiconductor optical amplifiers (SOAs) is necessary for the application of these devices as amplifiers or switching elements in future high speed networks. We have measured the gain recovery as a function of probe wavelength in SOAs of two different lengths but otherwise identical structure. The SOAs have InGaAs MQW active regions with peak gain in the 1550 nm window. Pump-probe measurements of recovery are made using a counter-propagating set-up with a gain-switched DFB laser as the pump and a tunable laser as the probe. Measured results for the recovery time in an SOA of length 2 mm show a strong dependence on probe wavelength with a pronounced minimum around 1580 nm, coincident with the peak of the gain spectrum of the device. Results for an SOA of length 1.2 mm indicate a rather shallow minimum around 1570 nm, which also is close to the peak gain wavelength of this device. Numerical simulation of SOA gain recovery is reported using a model that includes the material gain spectrum, saturation effects and the variation of optical intensities along the length of the device. Comparisons of simulated and measured results give a reasonable level of agreement.