Optimal design of a bifurcated refractive coupler for the HEDlight program

Abstract

In this contribution we consider the problem of designing coupling optics to optimally transfer light from a metal-halide arc lamp to a large core polymethyl methacrylate (PMMA) fiber optic cable. We investigate a refractive optical coupling concept comprising a non-axisymmetric bifurcated refractive glass TIR lens (TIRL) set. The design goal is to maximize the photometric flux incident onto two 19-mm-diameter apertures within an acceptance half angle of 38o. The light source is an 80-Watt metal halide arc lamp, characterized by means of photometric data measured by Radiant Imaging. The lenses each comprise a central refractive section combined with an annular TIR section. The exit pupil of each TIR lens is separated from the entrance aperture of the target by a short air gap. The refractive section of the TIR lens utilizes two aspheric surfaces to collect source flux over a central solid-angular region and redirect it into the target. The TIR section utilizes a total of three aspheric surfaces, two of which are refractive and one of which is a TIR surface. To account for right-left asymmetries in the optical source, the TIR lenses were independently globally optimized for both sides. Our TIRL design has the advantage of being able to collect and control radiation emanating at both large and small angles from the source, with little overall loss. The TIRL, with an ideal AR coating, has a predicted coupling efficiency of 89.6%. This was accomplished even though the target to source etendue ratio is only 63.7%. This is possible due to the ability of this design to preferentially transfer radiation from the higher radiance portions of the source phase space to the target. The work described above was funded by the Defense Advanced Research Projects Agency's High Efficiency Distributed Lighting Program known as HEDLight.