Experimental properties of injection lasers: modal distribution of laser power

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Abstract
The paper reports extensive measurements and analysis in terms of physical parameters of the frequency and spatial distribution of the radiation from GaAs injection lasers operated at room temperature. Covered are most of the internal configurations found in modern structures. All cavities have similar lengths and widths, cleaved uncoated facets, and sawed sidewalls. The power ranges from threshold to full drive as set by the limit for facet damage. Qualitatively the beam and spectral widths grow almost discontinuously with current near threshold and more slowly at higher drive, with up to a thousand cavity modes sharing the power at full output. Quantitative analysis is enabled by a new model of a laser cavity with sawed sidewalls and by a new equation for the modal power distribution function, which has the form of a sum over the normal modes of the cavity. Summing gives new predictive expressions for the beam shape and for the dependence of half‐width on current, both of which are found to be obeyed by all the lasers, and an expression in terms of observables for the critical power associated with the nonlinearity which spreads the coherent radiation among the normal modes. For all the lasers studied, the critical parameter is of the order of 10 mW per normal mode. It is speculated that the observed behavior is representative not only of GaAs injection lasers but of the entire class of semiconductors, that the critical parameter power per mode is a material constant associated with the onset of nonlinear absorption/emission, and that its low level is to be expected in semiconductors.