Influence of facet coating on the dual wavelength operation of asymmetric InGaAs-GaAs quantum-well lasers

A low-reflectivity SiN_yO_z film was deposited on one facet of asymmetric In _xGa_1-xAs-GaAs quantum-well (QW) laser diodes containing three different QW compositions with the indium fractions x=0.25,0.21, and 0.15, located, respectively, from the n-doped to p-doped sides of the device. Lasing is only observed on the two higher In-content QWs. The reduction of the reflectivity of one facet, with the other as-cleaved, leads to an increase in the transition cavity length, below which the diode initially lases on the shorter wavelength QW (x=0.21) at threshold and above which the diode lases on the longer wavelength QW (x=0.25). No lasing occurs on the shortest wavelength QW (x=0.15). With sufficient current injection simultaneous lasing on the two QWs is observed. When dual-wavelength emission occurs the facet reflectivity determines whether the short or long wavelength emitting QW lases first as the pump current is increased. A separation of up to 31 nm between the two wavelengths is observed under the dual wavelength emission conditions.