Mode Locking using the Half-Bandgap Nonlinearity in Passive AIGaAs Waveguides

Passive AlGaAs waveguides have been shown to be suitable for ultrafast all-optical switching when used at wavelengths in the vicinity of the half-bandgap (around 1.5-1.7 μm).1,2 Nonlinear phase shifts in excess of π radians have also been observed directly by monitoring self-phase modulation induced spectral broadening of pulses propagating through straight waveguides.3,4 These large phase shifts, caused by nonresonant nonlinear index changes, are possible due to the combination of low nonlinear absorption, coupled with a localised enhancement related to the two-photon band edge.5 In addition, semiconductor waveguides are, in general, polarisation preserving. Taken together, these attributes make passive AlGaAs waveguides attractive alternatives for optical fibres in coupled-cavity lasers operating at wavelengths around 1.5 μm. In this paper, we describe the application of passive AlGaAs waveguides as the nonlinear element in a coupled-cavity KCl:TI colour-centre laser to produce pulses of less than 170 fs in duration. The principal advantage of using semiconductor waveguides over silica-based optical fibres is their low power requirement which offers considerable potential for miniaturisation.