{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,2]],"date-time":"2026-04-02T03:22:47Z","timestamp":1775100167833,"version":"3.50.1"},"reference-count":29,"publisher":"MDPI AG","issue":"21","license":[{"start":{"date-parts":[[2021,10,26]],"date-time":"2021-10-26T00:00:00Z","timestamp":1635206400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>A special phenomenon of resonance mode separation is observed during the study of a high sensitivity folded-cavity enhanced absorption spectroscopy for the measurement of trace gases. The phenomenon affects the measurement of gas absorption spectrum in the cavity. This resonant mode separation phenomenon of the resonant cavity is different from the resonant modes previously observed in linear-cavity enhanced absorption spectroscopy systems. To explore the mechanism of this phenomenon, a series of hypotheses are proposed. The most likely reason among these hypotheses is based on the different reflectance properties of the plane mirror at the fold of the cavity for S-polarized light and P-polarized light. Based on the matrix calculation method, the different reflectance and phase shift of the plane mirror for S-polarized light and P-polarized light are analyzed theoretically, and the results are in better agreement with the phenomena observed in the experiment. Finally, in order to eliminate the resonant mode separation phenomenon, line polarizers were added. By improving the system, the cavity enhanced absorption spectrum of residual water vapor in the cavity was successfully measured, and a minimum detectable absorption coefficient of \u03b1min = 7.6 \u00d7 10\u22129 cm\u22121 can be obtained in a single laser scan of 10 s.<\/jats:p>","DOI":"10.3390\/s21217101","type":"journal-article","created":{"date-parts":[[2021,10,26]],"date-time":"2021-10-26T23:54:33Z","timestamp":1635292473000},"page":"7101","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":7,"title":["Study of a Mode Separation Due to Polarization Existing in a Cavity-Enhanced Absorption Spectroscopy"],"prefix":"10.3390","volume":"21","author":[{"given":"Shiyu","family":"Guan","sequence":"first","affiliation":[{"name":"College of Advance Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Dingbo","family":"Chen","sequence":"additional","affiliation":[{"name":"College of Advance Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Huilin","family":"Cao","sequence":"additional","affiliation":[{"name":"College of Advance Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Zhongqi","family":"Tan","sequence":"additional","affiliation":[{"name":"College of Advance Interdisciplinary Studies, National University of Defense Technology, Changsha 410073, China"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2021,10,26]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"9616","DOI":"10.3390\/s91209616","article-title":"Photoacoustic Techniques for Trace Gas Sensing Based on Semiconductor Laser Sources","volume":"9","author":"Elia","year":"2009","journal-title":"Sensors"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"38","DOI":"10.1126\/science.188.4183.38","article-title":"Photoacoustic Spectroscopy: New Uses for an Old Technique","volume":"188","author":"Maugh","year":"1975","journal-title":"Science"},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"12033","DOI":"10.1364\/OE.422926","article-title":"Room-temperature Fe:ZnSe laser tunable in the spectral range of 37\u201353 \u00b5m applied for intracavity absorption spectroscopy of CO2 isotopes, CO and N2O","volume":"29","author":"Fjodorow","year":"2021","journal-title":"Opt. 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