Etching of multimode optical glass fibers: A new method for shaping the measuring tip and immobilization of indicator dyes in recessed fiber-optic microprobes

We describe a new procedure for making recessed tips on multimode optical glass fibers. The method is based on etching fiber tips in 40% hydrofluoric acid for defined immersion times. As the etching velocity decreases radially from the core center in multimode graded index fibers, a recess can be formed in the tip of flat-cut tapered or untapered fibers. Etched fiber tips showed improved focussing of excitation light coupled into the fiber at the opposite end, and very efficient excitation of thin layers of optical indicators immobilized into the recess. The sensor chemistry is well protected when immobilized in recessed fiber tips and allows the construction of O2 microoptodes with improved mechanical stability that can measure repeatedly even in very cohesive biofilms, tissue and dry soil.