Single Mode Photonic Crystal Vertical Cavity Surface Emitting Lasers

We review the design, fabrication, and performance of photonic crystal vertical cavity surface emitting lasers (VCSELs). Using a periodic pattern of etched holes in the top facet of the VCSEL, the optical cavity can be designed to support the fundamental mode only. The electrical confinement is independently defined by proton implantation or oxide confinement. By control of the refractive index and loss created by the photonic crystal, operation in the Gaussian mode can be insured, independent of the lasing wavelength. 1. Introduction Vertical cavity surface emitting lasers (VCSELs) have emerged as the commercial laser source of choice for short distance digital fiber optical interconnects and sensing applications. The principle advantage that VCSELs have in many of these applications are their low operating power requirements (only a few mW) as well as their low cost and large volume manufacturing. Compared to edge emitting semiconductor lasers, VCSELs also possess the benefits of a circular output beam, on wafer testing, and the ability to form 2-dimensional arrays. The most common emission wavelength is 850？nm, although wavelengths from 640 to 1300？nm have been demonstrated for VCSELs monolithically grown on GaAs substrates. Unlike an edge emitting laser, VCSELs have a single longitudinal mode but tend to operate in multiple transverse optical modes. This arises because the optical cavity of the VCSEL is short in the direction of light propagation (typically the cavity is 1 wavelength long, or approx. 265？nm for 850？nm emission), but the transverse cavity width defined by a selectively oxidized or ion implanted aperture [1] is much greater (typically a few to tens of microns in diameter). Edge emitting semiconductor lasers have much longer cavities (several hundreds of microns in length) supporting numerous longitudinal modes, but with a cavity cross-section that supports a few or a single transverse/lateral mode. The number of laser emission modes will influence the spectral width of the laser emission, while the near field and far field beam profile is determined by the transverse mode profiles. For a typical multimode 850？nm VCSEL, the emission bandwidth can be roughly 3？nm, while the far field is often a ring shape, due to the higher order mode operation. The multimode VCSEL bandwidth can be a limiting factor for high speed digital modulation through optical fiber due to spectral dispersion, while the smallest focused spot size will come from operation only in the fundamental Gaussian mode. Many approaches to achieve single fundamental mode