Nonlinear gain and the spectral output of short-external-cavity 1.3 μm InGaAsP semiconductor diode lasers

The effect of nonlinear gain on the steady-state spectral output of 1.3 μm InGaAsP semiconductor diode lasers was investigated by measuring the spectral output of lasers that were operated in a short external cavity (SXC). For the SXC lasers, an increase in the powers in both the long- and short-wavelength modes that are adjacent to the resonant mode (i.e., the mode that is resonantly enhanced by the SXC and hence lases strongly) was observed for output-power levels ⩾5 mW. These results suggest the presence of a symmetric-nonlinear-gain mechanism. Calculations that include a symmetric-nonlinear-gain mechanism correctly predict the observed trends in the evolution of the power in the longitudinal modes of an SXC laser with increasing output power. It is concluded therefore, that for strong single-mode oscillation and output powers above ≈5 mW such as found for an SXC laser operated well-above threshold, that the effects of a symmetric-nonlinear-gain mechanism are observable in the spectral output