Observation of interference in a fiber taper interferometer with a subwavelength tip and its sensing applications

We present an ultraminiature fiber-optic sensor based on a fiber taper interferometer with a subwavelength tip. The interferometer is fabricated on the end face of a single-mode fiber by wet etching using buffered hydrofluoric acid. The process creates a protruding core on the end face with a tapered shape. Strong interference is observed between the reflections from the fiber cladding and the tip of the taper even for tip size as small as lambda/4. A time-domain analysis of the sensor spectrum recorded during the etching process produces the temporal evolution of both reflections, indicating that the interference mechanism involves intermodal coupling. The interferometer's sensing application is demonstrated by the effective measurement of refractive index and temperature.