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Samples of paracetamol liquid solutions with different concentrations, in the range from 52.61 to 201.33 g\/kg, were used as a case-study. The optical gain was provided by a commercial bidirectional Erbium-Doped Fiber Amplifier (EDFA) and the linear cavity was obtained using two commercial Fiber Bragg Gratings (FBGs). The main difference of each configuration was the coupling ratio of the optical coupler used to extract the system signal. The sensing head corresponded to a Single-Mode Fiber (SMF) tip that worked as an intensity sensor. The results reveal that, despite the optical coupler used (50:50, 60:40, 70:30 or 80:20), all the configurations reached the laser condition, however, the concentration sensing was only possible using a laser drive current near to the threshold value. The configurations using a 70:30 and an 80:20 optical coupler allowed paracetamol concentration measurements with a higher sensitivity of (\u22123.00\u00a0\u00b1\u00a00.24) pW\/(g\/kg) to be performed. In terms of resolution, the highest value obtained was 1.75 g\/kg, when it was extracted at 20% of the output power to the linear cavity fiber laser configuration.<\/jats:p>","DOI":"10.3390\/photonics11050393","type":"journal-article","created":{"date-parts":[[2024,4,24]],"date-time":"2024-04-24T07:38:51Z","timestamp":1713944331000},"page":"393","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Linear Fiber Laser Configurations for Optical Concentration Sensing in Liquid Solutions"],"prefix":"10.3390","volume":"11","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-1828-4633","authenticated-orcid":false,"given":"Liliana","family":"Soares","sequence":"first","affiliation":[{"name":"Institute for Systems and Computer Engineering, Technology and Science (INESC TEC), Rua do Campo Alegre, 687, 4150-179 Porto, Portugal"},{"name":"Department of Engineering Physics, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias s\/n, 4200-465 Porto, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6856-9143","authenticated-orcid":false,"given":"Rosa Ana","family":"Perez-Herrera","sequence":"additional","affiliation":[{"name":"Department of Electrical, Electronic and Communication Engineering, Public University of Navarra, 31006 Pamplona, Spain"},{"name":"Institute of Smart Cities (ISC), Public University of Navarra, 31006 Pamplona, Spain"}]},{"given":"Susana","family":"Novais","sequence":"additional","affiliation":[{"name":"Institute for Systems and Computer Engineering, Technology and Science (INESC TEC), Rua do Campo Alegre, 687, 4150-179 Porto, Portugal"}]},{"given":"Ant\u00f3nio","family":"Ferreira","sequence":"additional","affiliation":[{"name":"Laboratory for Process Engineering, Environment, Biotechnology and Energy (LEPABE), Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias s\/n, 4200-465 Porto, Portugal"},{"name":"Associate Laboratory in Chemical Engineering (ALiCE), Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7555-361X","authenticated-orcid":false,"given":"Susana","family":"Silva","sequence":"additional","affiliation":[{"name":"Institute for Systems and Computer Engineering, Technology and Science (INESC TEC), Rua do Campo Alegre, 687, 4150-179 Porto, Portugal"}]},{"given":"Orlando","family":"Fraz\u00e3o","sequence":"additional","affiliation":[{"name":"Institute for Systems and Computer Engineering, Technology and Science (INESC TEC), Rua do Campo Alegre, 687, 4150-179 Porto, Portugal"}]}],"member":"1968","published-online":{"date-parts":[[2024,4,24]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"493","DOI":"10.1038\/187493a0","article-title":"Stimulated Optical Radiation in Ruby","volume":"187","author":"Maiman","year":"1960","journal-title":"Nature"},{"key":"ref_2","unstructured":"Kissinger, C.D. (1967). Fiber Optic Proximity Probe. (3327584A), U.S. Patent."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"294","DOI":"10.1016\/j.ijleo.2017.10.032","article-title":"Temperature sensor and fiber laser based on optical microfiber knot resonator","volume":"154","author":"Ahmad","year":"2018","journal-title":"Optik"},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"97823","DOI":"10.1109\/ACCESS.2020.2997918","article-title":"Dual-polarization distributed feedback fiber laser sensor based on femtosecond laser-inscribed in-fiber stressors for simultaneous strain and temperature measurements","volume":"8","author":"Guo","year":"2020","journal-title":"IEEE Access"},{"key":"ref_5","doi-asserted-by":"crossref","unstructured":"Jauregui-Vazquez, D., Alvarez-Chavez, J.A., Lozano-Hernandez, T., Estudillo-Ayala, J.M., Sierra-Hernandez, J.M., and Offerhaus, H.L. (2023). Fiber Laser Sensor Configurations for Refractive Index, Temperature and Strain: A Review. Photonics, 10.","DOI":"10.3390\/photonics10050495"},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"888","DOI":"10.1364\/OL.12.000888","article-title":"High-gain erbium-doped traveling-wave fiber amplifier","volume":"12","author":"Desurvire","year":"1987","journal-title":"Opt. Lett."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"358","DOI":"10.1364\/OL.29.000358","article-title":"Concentration-induced nonuniform power in tunable erbium-doped fiber lasers","volume":"29","author":"Dong","year":"2004","journal-title":"Opt. Lett."},{"key":"ref_8","unstructured":"Correia, R., Sinha, R., Norris, A., Korposh, S., Talbot, S., Hernandez, F.U., and Morgan, S.P. (May, January 30). Optical fiber sensing at the interface between tissue and medical device. 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[5th ed.]."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"127939","DOI":"10.1016\/j.optcom.2022.127939","article-title":"The differentiation procedure between amplified spontaneous emission and lasing phenomena","volume":"510","author":"Ryglowski","year":"2022","journal-title":"Opt. Commun."},{"key":"ref_14","doi-asserted-by":"crossref","unstructured":"Soares, L., Novais, S., Ferreira, A., Fraz\u00e3o, O., and Silva, O.S. (2019). Detection of the Crystallization Process of Paracetamol with a Multi-Mode Optical Fiber in a Reflective Configuration. Sensors, 20.","DOI":"10.3390\/s20010087"}],"container-title":["Photonics"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2304-6732\/11\/5\/393\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T14:33:04Z","timestamp":1760106784000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2304-6732\/11\/5\/393"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2024,4,24]]},"references-count":14,"journal-issue":{"issue":"5","published-online":{"date-parts":[[2024,5]]}},"alternative-id":["photonics11050393"],"URL":"https:\/\/doi.org\/10.3390\/photonics11050393","relation":{},"ISSN":["2304-6732"],"issn-type":[{"type":"electronic","value":"2304-6732"}],"subject":[],"published":{"date-parts":[[2024,4,24]]}}}