Photorefractive coupler for fault-tolerant coupling

We report the first experimental demonstration of a fault-tolerant laser-to-fiber coupler that is insensitive to both angular and lateral misalignments. Fault-tolerance is achieved by sending a laser beam into a photorefractive crystal that automatically senses and tracks the location of the fiber facet, and diffracts the input beam (via self-generated adaptive dynamic holograms) into the fiber. Using an argon laser (514.5 nm), a barium titanate crystal, and a multimode fiber (core diameter=400 μm, NA=0.16), we have achieved a coupling efficiency of 58% and a tolerance of more than 0.5 mm of lateral misalignment. Techniques to improve the coupling efficiency and the degree of fault tolerance, and to extend this method for the coupling of a laser diode output into a single-mode fiber are discussed.