By introducing triangular holes into oxide confined 850nm monolithic vertical-cavity surface-emitting lasers (VCSELs), single-transverse-mode operation has been obtained for large oxide apertures of 14-15 microns. The two-dimensional triangular holes etched on the device surface were aligned circumferentially along the aperture perimeter, with their tips surrounding the device center. When the holes had a relatively large lateral penetration into the oxide aperture, the holey VCSEL lased with a single spot near field pattern with a high side-mode suppression ratio (SMSR) of 45-50dB, and an output power of 2mW. In this case, it is assumed that the triangular holes are acting as a highly mode selective loss mechanism. On the other hand, when the penetration of the holes was relatively small, an SMSR of 40dB was obtained from a large area "floral" near field pattern, with a record high single-mode output power of 7mW. The lasing spectrum and far field intensity profile of this "floral" type emission indicates that it is a somewhat deformed fundamental mode that is extending over the whole device, and oscillating in-phase. The ability of triangular holes to suppress high order modes in large area oxide confined VCSELs should be effective for systems with wavelengths other than 850nm as well.
We analyzed lateral mode characteristics in 850-nm GaAs-based VCSELs with holey structures by FDTD method. Full three-dimensional analysis, which requires huge computer resources, was carried out using cluster computer. As a result, we estimated the good mode selectivity in circular and triangular holey structures, which cannot be obtained in a simple oxide aperture structure. It is explained by the large ra-diation loss from the inter-hole spacing and scattering loss at the bottom of holes particularly for higher order modes. In addition, the experimental result reported by Furukawa, et al., which showed the record high power single mode operation by the floral fundamental mode in the triangular holey structure, was verified. For any changes of standard structural parameters, only the Gaussian-like fundamental mode was confirmed. The floral mode was observed only when we assumed a nonuniform refractive index around the holey structure. Such nonuniformity can be considered in actual devices because of the non-uniform distribution of carriers and temperature at high current injection level.