Mass-Transport Fabrication of Refractive Gallium-Phosphide Micro-Optics

Gallium-phosphide GaP refractive micro-optical elements have been fabricated by mass-transport smoothing. An approximation to the desired optical surface the pre-form is fabricated in a GaP substrate using anisotropic etching techniques. The pre-form is then smoothed into the final surface by the mass-transport process, in which surface-energy minimization drives material diffusion at elevated temperature in a sealed ampoule. A favorable variation of smoothing with the spatial period of the surface-relief structure allows the quick smoothing of the sharp edges of the pre-form while preserving the overall shape of the optical element. Refractive GaP micro-optical elements are demonstrated using single- and multi-step pre-forms, including a Fresnel biprism and an off-axis aspheric collimating lens. New pre-form designs are also introduced which relax fabrication tolerances. A concave mirror is fabricated and used for spatial-mode control of a vertical-cavity surface-emitting laser VCSEL. The effect of the spacing of a sapphire cover-wafer above the GaP during mass-transport is investigated to test theoretical predictions of the relationship between surface and vapor diffusion. The surface roughness of a replicated GaP microlens is reduced by mass-transport. An array of off-axis GaP lenses is fabricated and integrated with an array of VCSELs to eliminate distortion in an optical interconnect system.