Single transverse mode operation in a microlensed vertical-cavity surface-emitting laser

Summary form only given. We propose to integrate a partial concave mirror on top of an otherwise ordinary VCSEL device. In this scheme, a part of VCSEL laser output emitted from the VCSEL microcavity is reflected from the mirror and subsequently fed back into the microcavity. Upon re-entering, the light travels inside the cavity, with its path folded back and forth whenever encountering the high-reflection planar mirrors composing the microcavity. This allows multiple overlaps of the feedback light with the quantum well (QW) active layer. Along this passage of light, the spot size of the reflected laser beam gradually shrinks due to the focusing effect of the concave mirror until it reaches the minimum at the mirror focal length. Therefore the total field strength that the QW active layer effectively feels is much concentrated spatially toward the aperture center. This in turn results in locking laser oscillation to the fundamental transverse mode since it matches best with the accumulated reflected beam profile among the possible VCSEL modes. As a proof of concept we integrated a concave mirror, which rather looks like a microlens in shape from outside, on a GaAs/AlGaAs oxide-confined VCSEL device. Theoretical calculations were attempted based on the proposed mode stabilization mechanism. The results clearly showed the existence of mode discriminations. According to our simple calculations for the given microlensed device structure, the fundamental transverse mode (LP/sub 01/) dominates over the next higher-order mode (LP/sub 11/) by the factor of 4.