Direct Evidence of the Leaky Emission in Oxide Confined Vertical Cavity Lasers
We report the observation of narrow tilted lobes in the far field emission pattern of leaky oxide confined vertical cavity surface emitting lasers (VCSELs) The VCSEL cavity is surrounded by two selectively oxidized aperture layers, which are intentionally designed to produce a high lateral leakage of the high order transverse modes of the vertical cavity. The device operates in the fundamental transverse mode at oxide aperture diameters below $5~\mu \text{m} and currents up to 4 and 5 mA. At higher currents or larger aperture diameters, an additional high order transverse optical mode evolves. This mode is revealed by the appearance of a shorter wavelength emission line in the electroluminescence spectra of the device, resulting in multi mode lasing and in the related changes in the near and far field patterns. In the far field pattern, the evolution of the high order mode is revealed by the evolution of the two overlapping emission lobes at moderate ~5deg) tilt angles with respect to the normal to the surface and by the appearance of a multi spot or a ring pattern in the CCD camera images. Most importantly, the appearance of this high order transverse mode is accompanied by an observation of much narrower lobes in the far field pattern observed at significantly larger tilt angles ~35deg) The tilt angle and the narrow angular width of these emission lobes in the far field spectrum are in agreement with those calculated in the 3 D cold cavity modeling of the optical modes of the device. These narrow tilted lobes revealed in this paper are the fingerprints of the leakage effect in specially designed oxide confined VCSELs with their intensity being proportional to the optical power leaving the aperture region in the direction parallel to the surface and propagating into the oxidized region. The effect can be applied for engineering of single mode VCSELs, coherently coupled 2 D VCSEL arrays, laterally integrated VCSEL photodetector chips, and VCSELs integrated to slow light waveguides, for coupled optical gates for optical computers and other types of photonic integrated circuits.
IEEE Journal of Quantum Electronics