We propose an optical coupling scheme that uses a fiber-end lens to achieve efficient coupling of the plastic optical fiber with a light source and with a detector. Transparent resin filled in the concave end of the fiber is fixed by surface tension to form a biconvex lens that is automatically positioned relative to the fiber core. A simple ray-matrix analysis shows that the lensed end yields a large coupling angle with a long range of working distance compared to a flat-end fiber. The proposed lensed-end fibers are demonstrated in both input and output proofs of the experiment.
Hajime Sakata and Aya Imada, "Lensed Plastic Optical Fiber Employing Concave End Filled With High-Index Resin," J. Lightwave Technol. 20, 638- (2002)