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Reflective-refractive microlens for efficient LED-to-fiber coupling This publication appears in: Optical Engineering Authors: C. Rooman, M. Kuijk, R. Vounckx and P. Heremans Volume: 44 Issue: 9 Publication Date: Sep. 2005
Abstract: We propose a microlens for improving the coupling efficiency etween a Lambertian emitter and an optical fiber with small numerical aperture. The nonimaging tens 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 mu m 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 mu m, 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.
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