Abstract

Standing waves produced by the forward and backward propagating beams in a laser medium cause spatial hole burning, resulting in multiple longitudinal mode laser oscillation. In order to eliminate tills standing wave eifert and to obtain single longitudinal mode operation, a ring geometry ensuring uni-directional operation is commonly used. Similarly, in a semiconductor laser array, the spatial in homogeneity in the transverse direction caused by transverse modes in the array is detrimental to obtaining single transverse mode operation. This spatial gain modulation can be minimized if the arrays are operated with beams making a small angle with the axis. Stable external cavity geometries utilizing off-axis array propagation, in the past have given single lobed output.¹,² We present lie re an alternative unstable external cavity geometry utilizing off-axis array propagation (Fig. 1). This geometry is simple and is extremely insensitive to misalignments of external mirror and collimating optics. In this geometry, a retro-reflecting mirror is placed at the image plane of the objective. This ensures insensitivity to misalignment. An output coupling mirror is placed at an angle at the focal plane of the objective lens. All the rays originating from the array in the downward direction miss the output coupling mirror and get reflected back by the external mirror. These rays enter the array propagating upward, get amplified in the array medium, and come out propagating upward. These rays in turn reach the output coupling mirror and couple out. This is thus an unstable resonator with three or four passes.

© 1993 Optical Society of America