Chip-level integrated diffractive optical microlenses for multimode vertical-cavity surface-emitting laser to fiber coupling

Abstract

The standard assembly technologies for vertical-cavity surface-emitting laser (VCSEL) to fiber coupling systems involve the integration of discrete elements with demanding requirements for alignment effort and time. We present a method for the monolithic integration of diffractive microlenses on the chip level. This process is based on a UV-casting replication technique using ORMOCER® (a registered trademark of Fraunhofer-Gesellschaft) materials [hybrid organic-inorganic polymers (Streppel et al. 2001)] and offers the capability to be extended to a wafer-scale process. A mathematical description for the propagation of the laser modes through the system and the resulting fiber coupling efficiency is presented. We use a model for the source characteristics of the VCSEL based on a step-index fiber model for the simulation of the mode-field propagation. A model for the estimation of the diffraction efficiency of the lens is developed. Finally the simulations are compared with first experimental results of single replicated elements. Experimental coupling efficiencies for a multimode fiber [50/125, numerical aperture (NA) = 0.20] better than 0.7 over the entire operation range of the VCSEL are achieved. Losses below 0.5 dB (10%) are observed within lateral fiber displacement tolerances of m10 μm.