Optical fiber tapers with a waist size larger than 1μm are commonplace in telecommunications and sensor applications. However the fabrication of low-loss optical fiber tapers with subwavelength diameters was previously thought to be impractical due to difficulties associated with control of the surface roughness and diameter uniformity. In this paper we show that very-long ultra-low-loss tapers can in fact be produced using a conventional fiber taper rig incorporating a simple burner configuration. For single-mode operation, the optical losses we achieve at 1.55μm are one order of magnitude lower than losses previously reported in the literature for tapers of a similar size. SEM images confirm excellent taper uniformity. We believe that these low-loss structures should pave the way to a whole range of fiber nanodevices.

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We propose and experimentally demonstrate an enhanced evanescent field fiber refractometer based on a tapered multimode fiber sandwiched between two single-mode fibers. Experiments show that this fiber sensor offers ultrahigh sensitivity [better than $1900\mathrm{nm}/\mathrm{RIU}$ at a refractive index (RI) of 1.44] for RI measurements within the range of $1.33–1.44$, in agreement with the theoretical predictions. This is the highest value reported to date (to our knowledge) in the literature.

We model and demonstrate a simple mode selective all-fiber coupler capable of exciting specific higher order modes in two- and few-mode fibres with high efficiency and purity. The coupler is based on inter-modally phase-matching the propagation constants in each arm of the asymmetric fused coupler, formed by dissimilar fibres. At a specific coupler diameter, the launched fundamental LP₀₁ mode is coupled into the higher order mode (LP₁₁, LP₂₁, LP₀₂) in the other arm, over a broadband wave-length range around 1550 nm. Unlike other techniques, the demonstrated coupler is composed of a multimode fiber that is weakly fused with a phase matched conventional single mode telecom fiber (SMF-28). The beating between the supermodes at the coupler waist produces a periodic power transfer between the two arms, and therefore, by monitoring the beating while tapering, it is possible to obtain optimum selection for the desired mode. High coupling efficiencies in excess of 90% for all the higher order modes were recorded over 100 nm spectral range, while insertion losses remain as low as 0.5 dB. Coupling efficiency can be further enhanced by performing slow tapering at high temperature, in order to precisely control the coupler cross-section geometry.

In this paper, the properties of nonsilica glasses and the related technology for microstructured fiber fabrication are reviewed. Numerical simulation results are shown using the properties of nonsilica microstructured fibers for mid-infrared (mid-IR) supercontinuum generation when seeding with near-IR, 200-fs pump pulses. In particular, bismuth glass small-core fibers that have two zero-dispersion wavelengths (ZDWs) are investigated, and efficient mid-IR generation is enabled by phase-matching of a 2.0-mum seed across the upper ZDW into the 3-4.5 mum wavelength range. Fiber lengths considered were 40 mm. Simulation results for a range of nonsilica large-mode fibers are also shown for comparison.