Evanescent-wave fiber reflector

Abstract

A tunable optical fiber reflector is described, together with a method of making the same. A length of optical fiber has a core of a first light transmitting material, and a cladding of a second light transmitting material covering the core. The cladding is etched away to a predetermined thickness over a portion of the fiber. A layer of photoresist material is applied either to the etched away portion of the fiber or to a thin metal blade, i.e., mask, and then exposed to beams of light which optically interfere and generate a standing wave pattern in said material. The photoresist material is then developed to fix said wave pattern therein. An optical discontinuity is formed in one of the core and cladding by that fixed wave pattern, said discontinuity representing a quasi-periodical fluctuation in the refractive index and causing evanescent waves in the cladding to be reflected. Such discontinuity forms a distributed-feedback reflector. In one method, the fiber core in the etched portion is bombarded under a vacuum with a beam of ions passed through openings having said standing wave pattern, thus producing quasi-periodical fluctuations in the refractive index of the core. More preferably, the photoresist material is coated on the etched portion of the fiber, and counter-propagating light beams are coupled into opposite ends of the fiber. These beams expose the photoresist material and generate the standing wave pattern therein as residual quantities of the same. The etched portion of fiber is typically filled with reinforcing material such as an epoxy. Two such reflectors in an optical fiber make up a resonator, and several resonators can be used in a hydrophone line-array.