Pulse characterisation of a quantum-well mode-locked laser diode by two-photon absorption frequency-resolved optical gating

The intensity and phase of ultrashort optical pulses from a 40-GHz monolithic InGaAsP quantum-well heterostructure laser diode are retrieved for the first time from two-photon absorption pulse spectrograms. The pulse traces showing the mode-locked pulses from the laser are distorted due to ultrafast absorption saturation associated with quantum-confined Stark excitons. It is suggested from comparison between experimental time-integrated spectrum and reconstructed pulse spectrum that the mode-locked pulses are formed only from a part of the longitudinal modes in the laser cavity.