Photonic bandgap fiber tapers and in-fiber interferometric sensors

Nonadiabatic tapers in hollow-core air-silica photonic bandgap fibers (PBFs) were fabricated by the use of a fiber fusion splicer. In addition to the well-known scaling down of fiber dimensions, the innermost rings of air holes were found collapsed or significantly deformed, which results in almost doubling the diameter of the hollow core in the tapered region. The tapering of the PBF causes coupling of the fundamental core mode to a surface mode with ∼10% higher effective mode index. An in-fiber core-surface mode interferometer was constructed by cascading two such tapers along the PBF. The interferometer was experimentally demonstrated for strain and temperature measurement, and the sensitivities of the interferometric peak wavelength to strain and temperature are measured to be -0.64 pm/με and 495.6 pm/(°C m), respectively.