LATERAL-MODE SELECTIVITY IN EXTERNAL-CAVITY DIODE-LASERS WITH RESIDUAL FACET REFLECTIVITY

The relatively flat lateral gain profile across a wide-stripe laser diode does not allow sufficient overlap with the Gaussian fundamental mode of an external resonator to achieve saturation of the gain in the wings of the mode profile. Thus, parasitic, free-running oscillation of the diode in an external-cavity laser is permitted by residual facet reflectivity of the antireflection-coated diode, and degrades the lateral-mode-selectivity of the external resonator at injection currents exceeding the threshold of the solitary diode. A relation between the maximum injection current at which the external-cavity laser will operate in a sigle mode and the diode's facet reflectivity is given. Evidence is also presented which indicates that the spectral perturbations of the compound cavity formed by the two facets of the diode and the external mirror may defeat the desired spatial-mode selectivity of the resonator. Despite these limitations, a 0.35 W single-frequency laser was made by displacing the feedback from the external mirror off the center of the diode and using an aperture outside of the cavity to mask the image of the diode facet.