Reflective-refractive microlens for efficient light-emitting-diode-to-fiber coupling

Abstract

We propose a microlens for improving the coupling efficiency between a Lambertian emitter and an optical fiber with small numerical aperture. The nonimaging lens uses refraction in the central part for light rays emitted at small angles, and reflection in the peripheral part for rays emitted at large angles. In this way, the high numerical aperture of the source can efficiently be converted into the small numerical aperture of the fiber. Arrays of these lenses can be fabricated on a wafer scale using grayscale lithography. Simulation results for the coupling efficiency and alignment tolerances are presented for coupling to a standard multimode fiber with a core diameter of 62.5 µm and numerical aperture of 0.275. For small fiber core diameter to source diameter ratios, improvements in the coupling efficiency very close to the theoretical limit are predicted. For a light-emitting diode (LED) with a diameter of 25 µm, an improvement factor of 6 is observed in comparison to simple butt coupling, and a factor of more than 2 as compared to a standard spherical microlens.