The effects of scattering and absorbing regions which exist along the stripe of 1.3 &mgr;m InGaAsP semiconductor diode lasers are examined. A model which accounts for scattering and absorbing nonuniformities is used to explain correlations between scattering, absorption, and the spectral properties of the lasers. Normally the dominant mode of the spectrum of a semiconductor laser shifts to longer wavelength as current is increased. Occasionally, the dominant mode will shift to shorter wavelength with increasing current for a limited current range. This negative tuning can be explained by considering the effect of multiple scatterers combined with a localized absorber.