We report the latest progress of gallium nitride-based Vertical-Cavity Surface-Emitting Lasers (VCSELs) containing a curved mirror. Highly uniform and efficient devices were developed. The average threshold current was 0.64 mA with a standard deviation of 0.043 mA. The peak wall-plug efficiency and output power were 13.4 % and 7.6 mW at operating currents of 5.2 mA and 12.8 mA, respectively. We obtained green VCSELs with milliwatt-class outputs and a wall-plug efficiency of 3.7%. We also report the progress of VCSELs with a single-cavity filtering mirror and a cavity length of approximately 25 μm showing highly varying reflectivity spectra, to demonstrate their single-longitudinal mode operation.
Many applications benefit from single mode operation of vertical-cavity surface-emitting lasers (VCSELs) due to the need for a consistent beam profile and wavelength. While long cavity VCSELs have demonstrated good results in recent years, with examples of sub 1mA thresholds for devices with blue and green emission, the mode has not been fully controlled. Traditionally, VCSELs, with their short cavities, inherently maintain single longitudinal mode operation while struggling with multi-lateral modes. However, the use of a long cavity device with a curved mirror provides the opposite challenges for mode control, as the lateral mode can be controlled with the lens curvature, but the long cavity length allows for multiple longitudinal modes. In this paper, we will present the use of a filtering mirror with a highly varying reflectivity spectrum for obtaining single longitudinal mode operation up to 2mW and 50 kA/cm2. The filtering mirror reflectivity leads to significant mirror loss for adjacent longitudinal modes. Thus, by engineering the beam profile with the lens and filtering the longitudinal modes with the mirror structure, we are able to control the output shape, divergence angle, and wavelength stability of the device. This mode control, along with the low thresholds and lifetimes greater than 2000 hours, shows the potential of the long cavity structure for a variety of applications.
The recent progress of GaN-based vertical-cavity surface-emitting lasers (VCSELs) with incorporated curved mirrors at one end of their cavities is reviewed. GaN-VCSELs consisting of 3 InGaN/GaN quantum wells, current apertures formed by boron ion implantation, and curved mirrors at one end of the cavities were fabricated. The near-field and far-field patterns exhibited Gaussian-like profiles, and their divergences agreed well with the theoretical values calculated from the radius of curvature of the curved mirror and the cavity length. The near-field beam waist for a GaNVCSEL with a 6-μm current aperture was as small as 1.4 μm (half width at 1/e2), which indicated that the light was laterally confined by the incorporated curved mirror and that the current aperture could be made smaller than 6 μm. For a GaN-VCSEL with a current aperture of 4μm, a threshold current of as low as 0.56 mA (Jth = 4.5 kA/cm2) was obtained at room temperature under CW operation at a wavelength of 451.8 nm. To the best of our knowledge, this is the lowest threshold current reported among all-dielectric DBR type GaN-VCSELs and is comparable with the lowest value reported for GaN-VCSELs that have a two-dimensional quantum well structure. The authors believe this result to be a milestone for the realization of GaN-VCSELs with extremely low power consumption.
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