Nearly transform limited pulses from a gain-switched InGaAsP distributed-feedback laser using polarization dispersion effect

Due to the large change of carrier density during the pulse emission, gain-switched distributed feedback semiconductor lasers suffer from a large frequency chirp that can extend over several nanometers. As the chirp is generally nonlinear, it is difficult to achieve transform limited pulses after a linear compression stage. We demonstrate here that polarization dispersion can be used to filter the residual nonlinear chirp after linear compression. Pulses as short as 5.5 ps with a time bandwidth of 0.38 were obtained, a value less than half that observed without polarization dispersion filtering.