Optical mode lineshape and linewidth in passively mode-locked semiconductor laser

This paper analysis the lineshape and linewidth of individual optical modes in a passively mode-locked diode laser by mixing with a narrow linewidth tunable extended cavity diode laser. The device under study is a two section GaAs/InAs quantum-dot laser emitting at 1.3 mum as in, the length providing here a repetition rate of 16.1 GHz, and the absorber section representing 30% of the total length of the laser. The absorber bias voltage is -3.5 V and the gain current is 1.1 times the threshold. 3 ps wide pulses are produced. The spectral lineshape of each mode is found to be not a pure Lorentzian line but the superposition of two Lorentzian lines. The peaks central frequency difference ranges from -225 MHz to 150 MHz in a 5 nm span. The secondary modes are not equidistant and consequently not mode-locked. We think that they could correspond to the natural modes of the cavity which are not equidistant due to the dispersion of the cavity. We also noticed that these secondary peaks disappear at higher gain currents.