Abstract: An optoelectronic assembly obtains substantially improved coupling efficiency between small core optical fibers and small area detectors and emitters. The assembly includes a housing containing an active optoelectronic device and a light transmissive single crystal semiconductor window. The light transmissive single crystal semiconductor window seals the housing and physically protects the device from the environment. The single crystal semiconductor window has an index of refraction higher than conventional glass windows, preferably greater than 2, in order to reduce beam spreading between the optical fiber and small area detectors or emitters. The window material is preferably single crystal GaP.