Experimental properties of injection lasers. IV. Modes of large cavity with sawed sides

Abstract

The article is concerned with determination of the type of modes in wide injection lasers and in particular with identification of the normal modes in the profile of the beam in the plane of the junction. In a cavity 70 μm wide × 120 μm long, with cleaved facets and sawed sides, the modes are found to be those of a rectangular parallelopiped of the same geometry. The smooth lateral profile of the radiated beam is broken into a series of sharp lobes by inserting a low‐resolution high‐intensity spectrometer between angle scanner and detector. The spectrometer resolution of a fraction of the separation of successive longitudinal mode groups rejects most of the off‐axis modes. Longitudinal mode numbers are unambiguously assigned to the lobes in the beam profile, out to 70° from the facet normal, and the dependence of lobe position on longitudinal number identifies the geometry of the modes. Consideration of the lobe widths and irregularities reveals that the coherence width of the laser modes is essentially its geometric width. The experimental value of the index of refraction of the propagating modes is found to be 3.54±0.07 and its normalized dispersion is 0.30±0.06.