{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,10,28]],"date-time":"2025-10-28T10:08:13Z","timestamp":1761646093210,"version":"build-2065373602"},"reference-count":28,"publisher":"MDPI AG","issue":"4","license":[{"start":{"date-parts":[[2020,2,21]],"date-time":"2020-02-21T00:00:00Z","timestamp":1582243200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"Shantou University Start-up fund","award":["NTF18016"],"award-info":[{"award-number":["NTF18016"]}]},{"name":"Optics and Photoelectronics Project","award":["2018KCXTD011"],"award-info":[{"award-number":["2018KCXTD011"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>In this paper, we report a capillary-based Mach\u2013Zehnder (M\u2013Z) interferometer that could be used for precise detection of variations in refractive indices of gaseous samples. The sensing mechanism is quite straightforward. Cladding and core modes of a capillary are simultaneously excited by coupling coherent laser beams to the capillary cladding and core, respectively. An interferogram would be generated as the light transmitted from the core interferes with the light transmitted from the cladding. Variations in the refractive index of the air filling the core lead to variations in the phase difference between the core and cladding modes, thus shifting the interference fringes. Using a photodiode together with a narrow slit, we could interrogate the fringe shifts. The resolution of the sensor was found to be ~5.7 \u00d7 10\u22128 RIU (refractive index unit), which is comparable to the highest resolution obtained by other interferometric sensors reported in previous studies. Finally, we also analyze the temperature cross sensitivity of the sensor. The main goal of this paper is to demonstrate that the ultra-sensitive sensing of gas refractive index could be realized by simply using a single capillary fiber rather than some complex fiber-optic devices such as photonic crystal fibers or other fiber-optic devices fabricated via tricky fiber processing techniques. This capillary sensor, while featuring an ultrahigh resolution, has many other advantages such as simple structure, ease of fabrication, straightforward sensing principle, and low cost.<\/jats:p>","DOI":"10.3390\/s20041191","type":"journal-article","created":{"date-parts":[[2020,2,21]],"date-time":"2020-02-21T10:49:16Z","timestamp":1582282156000},"page":"1191","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":11,"title":["Ultrasensitive Gas Refractometer Using Capillary-Based Mach\u2013Zehnder Interferometer"],"prefix":"10.3390","volume":"20","author":[{"given":"Haijin","family":"Chen","sequence":"first","affiliation":[{"name":"Research Center for Advanced Optics and Photoelectronics, Department of Physics, College of Science, Shantou University, Shantou 515063, Guangdong, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0239-6144","authenticated-orcid":false,"given":"Xuehao","family":"Hu","sequence":"additional","affiliation":[{"name":"Research Center for Advanced Optics and Photoelectronics, Department of Physics, College of Science, Shantou University, Shantou 515063, Guangdong, China"},{"name":"Key Laboratory of Intelligent Manufacturing Technology of MOE, Shantou University, Shantou 515063, Guangdong, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Meifan","family":"He","sequence":"additional","affiliation":[{"name":"Research Center for Advanced Optics and Photoelectronics, Department of Physics, College of Science, Shantou University, Shantou 515063, Guangdong, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Pengfei","family":"Ren","sequence":"additional","affiliation":[{"name":"Research Center for Advanced Optics and Photoelectronics, Department of Physics, College of Science, Shantou University, Shantou 515063, Guangdong, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2346-6770","authenticated-orcid":false,"given":"Chao","family":"Zhang","sequence":"additional","affiliation":[{"name":"College of Mechanical Engineering, Yangzhou University, Yangzhou 225127, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Hang","family":"Qu","sequence":"additional","affiliation":[{"name":"Research Center for Advanced Optics and Photoelectronics, Department of Physics, College of Science, Shantou University, Shantou 515063, Guangdong, China"},{"name":"Key Laboratory of Intelligent Manufacturing Technology of MOE, Shantou University, Shantou 515063, Guangdong, China"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2020,2,21]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1900094","DOI":"10.1002\/lpor.201900094","article-title":"A Comprehensive Review of Optical Fiber Refractometers: Toward a Standard Comparative Criterion","volume":"13","author":"Urrutia","year":"2019","journal-title":"Laser Photonics Rev."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"012004","DOI":"10.1088\/0957-0233\/24\/1\/012004","article-title":"Optical gas sensing: A review","volume":"24","author":"Hodgkinson","year":"2012","journal-title":"Meas. Sci. Technol."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"177","DOI":"10.1016\/j.sna.2004.08.029","article-title":"Monitoring changes in the refractive index of gases by means of a fiber optic Fabry-Perot interferometer sensor","volume":"118","author":"Xiao","year":"2005","journal-title":"Sens. Actuators A Phys."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"22181","DOI":"10.1364\/OE.27.022181","article-title":"Antiresonant mechanism based self-temperature-calibrated fiber optic Fabry\u2013Perot gas pressure sensors","volume":"27","author":"Gao","year":"2019","journal-title":"Opt. Express"},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"27890","DOI":"10.1364\/OE.24.027890","article-title":"Antiresonant reflecting guidance mechanism in hollow-core fiber for gas pressure sensing","volume":"24","author":"Hou","year":"2016","journal-title":"Opt. Express"},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"3025","DOI":"10.1364\/OL.41.003025","article-title":"Hollow-core fiber Fabry\u2013Perot photothermal gas sensor","volume":"41","author":"Yang","year":"2016","journal-title":"Opt. Lett."},{"key":"ref_7","first-page":"1","article-title":"High-sensitivity gas pressure sensor based on Fabry\u2013Perot interferometer with a side-opened channel in hollow-core photonic bandgap fiber","volume":"7","author":"Tang","year":"2015","journal-title":"IEEE Photonics J."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"912","DOI":"10.1364\/JOSAB.29.000912","article-title":"High sensitivity gas refractometer based on all-fiber open-cavity Fabry\u2013Perot interferometer formed by large lateral offset splicing","volume":"29","author":"Duan","year":"2012","journal-title":"JOSA B"},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"2252","DOI":"10.1364\/OE.16.002252","article-title":"Laser-micromachined Fabry-Perot optical fiber tip sensor for high-resolution temperature-independent measurement of refractive index","volume":"16","author":"Ran","year":"2008","journal-title":"Opt. Express"},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"22813","DOI":"10.1364\/OE.20.022813","article-title":"Optical fiber Fabry-Perot interferometer cavity fabricated by femtosecond laser micromachining and fusion splicing for refractive index sensing","volume":"20","author":"Liao","year":"2012","journal-title":"Opt. Express"},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"7724","DOI":"10.1364\/AO.53.007724","article-title":"Hybrid optical fiber Fabry\u2013Perot interferometer for simultaneous measurement of gas refractive index and temperature","volume":"53","author":"Wang","year":"2014","journal-title":"Appl. Opt."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"4891","DOI":"10.1364\/OL.40.004891","article-title":"Ultra-high sensitivity Fabry\u2013Perot interferometer gas refractive index fiber sensor based on photonic crystal fiber and Vernier effect","volume":"40","author":"Quan","year":"2015","journal-title":"Opt. Lett."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"29649","DOI":"10.1364\/OE.27.029649","article-title":"Highly sensitive gas refractive index sensor based on hollow-core photonic bandgap fiber","volume":"27","author":"Zhang","year":"2019","journal-title":"Opt. Express"},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"1198","DOI":"10.1016\/j.snb.2011.09.048","article-title":"In-fiber Mach\u2013Zehnder interferometer formed by large lateral offset fusion splicing for gases refractive index measurement with high sensitivity","volume":"160","author":"Duan","year":"2011","journal-title":"Sens. Actuators B Chem."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"6673","DOI":"10.1364\/OE.23.006673","article-title":"Highly-sensitive gas pressure sensor using twin-core fiber based in-line Mach-Zehnder interferometer","volume":"23","author":"Li","year":"2015","journal-title":"Opt. Express"},{"key":"ref_16","doi-asserted-by":"crossref","unstructured":"Shavrin, I., Novotny, S., Shevchenko, A., and Ludvigsen, H. (2012). Gas refractometry using a hollow-core photonic bandgap fiber in a Mach-Zehnder-type interferometer. Appl. Phys. Lett., 100.","DOI":"10.1063\/1.3681171"},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"17591","DOI":"10.1364\/OE.19.017591","article-title":"Femtosecond laser fabricated all-optical fiber sensors with ultrahigh refractive index sensitivity: Modeling and experiment","volume":"19","author":"Jiang","year":"2011","journal-title":"Opt. Express"},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"44994","DOI":"10.1038\/srep44994","article-title":"An in-line Mach-Zehnder interferometer using thin-core fiber for ammonia gas sensing with high sensitivity","volume":"7","author":"Huang","year":"2017","journal-title":"Sci. Rep."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1038\/ncomms13371","article-title":"Ultrasensitive plasmonic sensing in air using optical fibre spectral combs","volume":"7","author":"Caucheteur","year":"2016","journal-title":"Nat. Commun."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"1071","DOI":"10.1007\/s11468-015-9897-2","article-title":"Gas-clad two-way fiber optic SPR sensor: A novel approach for refractive index sensing","volume":"10","author":"Mishra","year":"2015","journal-title":"Plasmonics"},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"094029","DOI":"10.1088\/0957-0233\/21\/9\/094029","article-title":"Low refractive index gas sensing using a surface plasmon resonance fibre device","volume":"21","author":"Allsop","year":"2010","journal-title":"Meas. Sci. Technol."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"20951","DOI":"10.1364\/OE.20.020951","article-title":"Long-period grating and its cascaded counterpart in photonic crystal fiber for gas phase measurement","volume":"20","author":"Tian","year":"2012","journal-title":"Opt. Express"},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"6639","DOI":"10.1364\/AO.55.006639","article-title":"Measurements of refractive indices and thermo-optical coefficients using a white-light Michelson interferometer","volume":"55","author":"Rocha","year":"2016","journal-title":"Appl. Opt."},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"51","DOI":"10.1364\/AO.6.000051","article-title":"Optical refractive index of air: Dependence on pressure, temperature and composition","volume":"6","author":"Owens","year":"1967","journal-title":"Appl. Opt."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"235","DOI":"10.1016\/j.snb.2011.10.025","article-title":"All photonic bandgap fiber spectroscopic system for detection of refractive index changes in aqueous analytes","volume":"161","author":"Qu","year":"2012","journal-title":"Sens. Actuators B Chem."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"4411","DOI":"10.3390\/ma7064411","article-title":"Materials development for next generation optical fiber","volume":"7","author":"Ballato","year":"2014","journal-title":"Materials"},{"key":"ref_27","doi-asserted-by":"crossref","unstructured":"Ma, Y., He, Y., Patimisco, P., Sampaolo, A., Qiao, S., Yu, X., Tittel, F.K., and Spagnolo, V. (2020). Ultra-high sensitive trace gas detection based on light-induced thermoelastic spectroscopy and a custom quartz tuning fork. Appl. Phys. Lett., 116.","DOI":"10.1063\/1.5129014"},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"32103","DOI":"10.1364\/OE.26.032103","article-title":"Quartz-tuning-fork enhanced photothermal spectroscopy for ultra-high sensitive trace gas detection","volume":"26","author":"Ma","year":"2018","journal-title":"Opt. Express"}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/20\/4\/1191\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T08:59:41Z","timestamp":1760173181000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/20\/4\/1191"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2020,2,21]]},"references-count":28,"journal-issue":{"issue":"4","published-online":{"date-parts":[[2020,2]]}},"alternative-id":["s20041191"],"URL":"https:\/\/doi.org\/10.3390\/s20041191","relation":{},"ISSN":["1424-8220"],"issn-type":[{"type":"electronic","value":"1424-8220"}],"subject":[],"published":{"date-parts":[[2020,2,21]]}}}