Impurity-induced disordering in vertical-cavity surface-emitting lasers (VCSELs) has demonstrated enhanced performance such as higher modulation speeds, reduced series resistance, and higher-order mode suppression for singlemode operation. Initiated by the diffusion of Zn, impurity-induced disordering intermixes discrete AlGaAs-based distributed Bragg reflectors (DBR) pairs which leads to lower mirror power reflectivity and increased optical loss. When formed into an aperture where the center is non-disordered, suppression of higher-order transverse modes for high-power single-mode operation can be achieved. For maximal mode suppression, deep disordering apertures are desirable. However, due to the isotropic nature of diffusion, these apertures are limited to the lateral diffusion encroaching onto the fundamental mode. By tailoring the film stress of the SiNx diffusion mask, the capability to modify the diffusion front of the disordering aperture is demonstrated. Defined by their lateral-to-vertical (L/V) diffusion ratios, an L/V ratio of 3.7 to 0.90 is measured for corresponding SiNx diffusion mask strains ranging from a compressive -797 MPa to a tensile +347 MPa. This demonstrates that tensile strained diffusion masks limit the amount of lateral diffusion. To further reduce the lateral encroachment, increasingly tensile diffusion masks are deposited by modifying the SiH4/NH3 flow ratios. This diffusion mask is employed to fabricate high-power single-mode VCSELs designed for 850 nm emission. Compared to VCSELs fabricated with non-optimized disordering apertures, enhanced transverse-mode control is achieved and singlemode output power in excess of 3.8 mW with a side mode suppression ratio greater than 30 dB is measured.
A novel method for controlling the transverse lasing modes in both proton implanted and oxide-confined vertical- cavity surface-emitting lasers (VCSELs) with a multi-layer, patterned, dielectric anti-phase (DAP) filter is pre- sented. Using a simple photolithographic liftoff process, dielectric layers are deposited and patterned on individual VCSELs to modify (increase or decrease) the mirror reflectivity across the emission aperture via anti-phase reflections, creating spatially-dependent threshold material gain. The shape of the dielectric pattern can be tailored to overlap with specific transverse VCSEL modes or subsets of transverse modes to either facilitate or inhibit lasing by decreasing or increasing, respectively, the threshold modal gain. A silicon dioxide (SiO2) and titanium dioxide (TiO2) anti-phase filter is used to achieve a single-fundamental-mode, continuous-wave output power greater than 4.0 mW in an oxide-confined VCSEL at a lasing wavelength of 850 nm. A filter consisting of SiO2 and TiO2 is used to facilitate injection-current-insensitive fundamental mode and lower order mode lasing in proton implanted VCSELs at a lasing wavelength of 850 nm. Higher refractive index dielectric materials such as amorphous silicon (a-Si) can be used to increase the effectiveness of the anti-phase filter on proton implanted devices by reducing the threshold modal gain of any spatially overlapping modes. This additive, non-destructive method allows for mode selection at any lasing wavelength and for any VCSEL layer structure without the need for semiconductor etching or epitaxial regrowth. It also offers the capability of designing a filter based upon available optical coating materials.
Top emission 850-nm vertical-cavity surface-emitting lasers (VCSELs) demonstrating transverse mode selection via impurity-induced disordering (IID) are presented. The IID apertures are fabricated via closed ampoule zinc diffusion. A simple 1-D plane wave model based on the intermixing of Group III atoms during IID is presented to optimize the mirror loss of higher-order modes as a function of IID strength and depth. In addition, the impact of impurity diffusion into the cap layer of the lasers is shown to improve contact resistance. Further investigation of the mode-dependent characteristics of the device imply an increase in the thermal impedance associated with the fraction of IID contained within the oxide aperture. The optimization of the ratio of the IID aperture to oxide aperture is experimentally determined. Single fundamental mode output of 1.6 mW with 30 dBm side mode suppression ratio is achieved by a 3.0 μm oxide-confined device with an IID aperture of 1.3 μm indicating an optimal IID aperture size of 43% of the oxide aperture.