Downlink coupling of a novel POF directional coupler

Abstract

A new plastic optical fiber directional coupler has been developed. It is fabricated by cutting a shallow notch on the side of the 1 mm diameter cladding fiber, within which is placed a silica 125 μm cladding diameter optical fiber for uplink coupling. This novel directional coupler features high efficiency in the uplink and downlink directions because of its advanced geometrical structure and the proposed coupling technique. Compared with the physical size of the notch, the physical size of the silica fiber is relatively small, which in turn permits a very high uplink coupling efficiency. Furthermore the physical size of the plastic optical fiber is large relative to the notch which provides a high downlink coupling efficiency. In the uplink direction, we recently reported that with an optimized angle of coupling, the silica fiber can couple light into the plastic fiber with a measured efficiency up to 88% (equivalent to 0.56 dB insertion loss). In the downlink direction, because of the relatively small physical size of the notch, the loss of the power through the notch is low and is estimated to be less than 1.5 dB, given a notch width of 1 mm. In this paper, the variation in the downlink coupling losses with the notch dimensions are measured and compared to those from a theoretical model.