Investigations of the Spectral Characteristics of 980-nm InGaAs–GaAs–AlGaAs Lasers

Semiconductor quantum-well (QW) lasers at 980 nm exhibit unique spontaneous emission spectra with a periodic envelope of approximately 2 3-nm wavelength. This phenomenon has been observed in both front facet and side spontaneous emission. The modulation is modeled in terms of coupling between the laser waveguide and the substrate waveguide which is transparent to 980-nm light. Modal gain spectra of the entire waveguide structure including substrate are calculated numerically by a transfer matrix method. The gain spectra in the active stripe and loss spectra in the unpumped QW exhibit modulation. This results in modulation of the emission spectra. An analytical approach based on coupled mode equations is developed to explain and clarify the results of the numerical modeling. The interesting case of a coupling length that is small by comparison with the gain/loss length is examined in detail. Front facet and side spontaneous emission spectra calculated using the modal gain spectra are in good agreement with the measured spectra. The results presented make it possible to interpret the unique modal characteristics of 980-nm lasers quantitatively and relate them to the physical structural parameters.