Complete optical field reconstruction and determination of linewidth enhancement factor in gain-switched semiconductor lasers using the Gerchberg–Saxton algorithm

We propose the use of the Gerchberg–Saxton algorithm to retrieve the time-resolved phase of optical pulses generated by gain-switched semiconductor lasers from the temporal and spectral intensities measured with standard instrumentation. We analyze numerically and experimentally the conditions for retrieving the optical phase and the corresponding time-resolved frequency chirp. The numerical analysis examines the performance of the algorithm under different gain switching conditions, with a particular emphasis on the number of points, the resolution of the input data and the choice of the initial phase seed. The results of this numerical analysis show that the algorithm converges effectively over a wide range of conditions, almost independently of the chosen phase seed and the number of points of the input data. We have experimentally retrieved the phase of optical pulses generated by a gain-switched 1550 nm laser diode. The retrieved frequency chirp was used for the experimental determination of the linewidth enhancement factor, resulting in a simple and robust technique for estimating this parameter.