Coherent beams from high efficiency two-dimensional surface-emitting semiconductor laser arrays

We have fabricated and operated large two-dimensional (2D) arrays of phase-locked surface-emitting semiconductor lasers. The arrays were fabricated by reactive ion beam etching of epitaxial Fabry-Perot resonators comprising GaAs/AlGaAs quantum wells surrounded by AlAs-AlGaAs quarter-wave mirrors. Different arrays corresponding to different pixel size (2-5 μm) and spacing (1-2 μm) were produced to investigate evanescent coupling between pixels. The arrays were photopumped so that the array size could be conveniently varied from 1×1, 2×2,. up to 20×20. Except for the 1×1 which emits a circular pattern, all arrays exhibit a well-defined four-lobed far-field pattern in agreement with our theoretical analysis of the optical modes which predicts domination by the 2D out-of-phase eigenmode. As a consequence this pattern can be understood with simple Fraunhofer diffraction theory. The angular spread of the lobes, determined by the periodicity of the array elements, is 10°for the array with element size/spacing of 4/1 μm. The widths of the lobes are 6.7°for the 2×2 and narrow to 3.2°with increasing number of pixels in the array. The array exhibits a sharp onset for lasing, operation on a single longitudinal mode, and a linewidth which narrows to ∼1 Å with increasing array size. The differential power efficiency is as high as 70%. These observations provide further impetus and guidance for the development of 2D laser diode arrays.