{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,10]],"date-time":"2026-04-10T19:21:39Z","timestamp":1775848899747,"version":"3.50.1"},"reference-count":42,"publisher":"MDPI AG","issue":"16","license":[{"start":{"date-parts":[[2025,8,9]],"date-time":"2025-08-09T00:00:00Z","timestamp":1754697600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100006141","name":"Program of the Republic of Poland\u2014Research GrantMUT","doi-asserted-by":"publisher","award":["UGB 531-000031-W900-22"],"award-info":[{"award-number":["UGB 531-000031-W900-22"]}],"id":[{"id":"10.13039\/501100006141","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>This article describes an external refractive index (RI) sensor based on a spectral analysis of the light transmission through a long tapered side-hole optical fiber (S-H OF). A section of the S-H OF was fusion-spliced with SMFs at both ends and connected to a supercontinuum source at the input and an optical spectrum analyzer (OSA) at the output. To investigate the effect of the external RI on the spectral characteristics, immersion liquids with refractive indices in the ranges of 1.32\u20131.35 and 1.37\u20131.42, with increments of 0.01, were used. Power loss measurements were carried out for each liquid in two wavelength ranges: 600\u20131200 nm and 1200\u20131800 nm. The highest sensitivity obtained in this study was 622 \u00b1 11 nm\/RIU in the near-infrared region. This result highlights the suitability of longtapered S-H OFs for spectral-interrogation-based RI sensing, offering a promising yet simpler alternative to more complex interferometric or plasmonic configurations.<\/jats:p>","DOI":"10.3390\/s25164916","type":"journal-article","created":{"date-parts":[[2025,8,11]],"date-time":"2025-08-11T09:59:13Z","timestamp":1754906353000},"page":"4916","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":1,"title":["A Refractive Index Sensor Based on Spectral Interrogation of a Long Tapered Side-Hole Optical Fiber"],"prefix":"10.3390","volume":"25","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-7688-715X","authenticated-orcid":false,"given":"Rafa\u0142 A.","family":"Kosturek","sequence":"first","affiliation":[{"name":"Institute of Applied Physics, Military University of Technology, 2 gen. Sylwestra Kaliskiego St., 00-908 Warsaw, Poland"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7766-0969","authenticated-orcid":false,"given":"Micha\u0142","family":"Dudek","sequence":"additional","affiliation":[{"name":"Institute of Applied Physics, Military University of Technology, 2 gen. Sylwestra Kaliskiego St., 00-908 Warsaw, Poland"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8838-5846","authenticated-orcid":false,"given":"Leszek R.","family":"Jaroszewicz","sequence":"additional","affiliation":[{"name":"Institute of Applied Physics, Military University of Technology, 2 gen. 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