title: Etched multimode microfiber knot-type loop interferometer refractive index sensor.
abstract: We propose a novel refractive index sensor based on multimode microfiber knot-type loop (NL) interferometer. The middle portion (~5 cm) of a 15 cm long multimode fiber is etched in 48% hydrofluoric acid to reduce its diameter to ~12 μm. A NL of diameter <1 mm is made from the etched fiber. The ends of etched fiber are spliced with single-mode fibers for launching and detecting light from the NL interferometer. The NL introduces path differences to produce interferometric spectra with free spectral range ~16 nm. The spectrum shifts as the surrounding refractive index of the loop is changed by adding chemicals. We observe the highest sensitivity of the NL interferometer ~172 nm/RIU (refractive index unit) at a refractive index value 1.370 as obtained experimentally using commonly available chemicals. The design could be used as simple, low cost, and highly sensitive biological and chemical sensor.
authors.affiliation: Central Scientific Instrument Organization (Council of Scientific and Industrial Research), Sector 30C, Chandigarh 160030, India.
authors.forename: Sudipta Sarkar
authors.initials: SS
authors.lastname: Pal
authors.forename: Samir K
authors.initials: SK
authors.lastname: Mondal
authors.forename: Umesh
authors.initials: U
authors.lastname: Tiwari
authors.forename: P Venu Gopala
authors.initials: PV
authors.lastname: Swamy
authors.forename: Manish
authors.initials: M
authors.lastname: Kumar
authors.forename: Nahar
authors.initials: N
authors.lastname: Singh
authors.forename: P P
authors.initials: PP
authors.lastname: Bajpai
authors.forename: Pawan
authors.initials: P
authors.lastname: Kapur