> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < Abstract —Optical fiber single mode – multimode – single mode (SMS) structures can be used as wavelength detection-based sensors. In this work, we focus on the performance at short wavelengths, where optical sources and detectors are less expensive. Here, a self-image band with a high transmission power is monitored in this short-wavelength range. In addition, the diameter and the length of the SMS structure have been optimized in order to improve the sensitivity of the device. In this sense, a maximum refractive index sensitivity of 305 nm/RIU was achieved by an etched SMS with a diameter of 34 μm. Furthermore, the obtained devices were used for testing the quality of automotive coolant and antifreeze liquid. Index Terms—optical fiber, multimodal interference, self-image, etching, coolant, antifreeze. I. INTRODUCTION Optical fiber sensors can be used in many applications due their interesting properties: small size, multiplexing capability and immunity to electromagnetic interferences [1-7]. Different fiber optic configurations have been studied for these purposes, such as those based on gratings (fiber Bragg gratings, long period gratings and tilted fiber Bragg gratings) or interferometers (Mach-Zender, Fabry- Pérot and Michelson) [1]. This manuscript was submitted on mm/ dd/ 2019. We appreciate the support from the Programa de Fortalecimiento de la Calidad Educativa (PFCE) and the Programa para el Desarrollo Profesional Docente (PRODEP), as well as by the Direction of strategic programs and projects of the Autonomus University of Tamaulipas (UAT) and the Direction of the Unit Academic Multidisciplinary Reynosa-Rodhe. To José Andrés Suárez Fernandez, Rector of the UAT for all the efforts made to obtain resources and continue supporting the research. Also, we appreciate the support from the Spanish Agencia Estatal de Investigación (AEI) and Fondo Europeo de Desarrollo Regional (FEDER) (TEC2016-78047-R), as well as by the Government of Navarra PC095-096 OPTISENS, PC081-082 BIOPTSENS AVANZA and Public University of Navarra PJUPNA26 research grants. One of the simplest interferometric structures is the single mode – multi mode – single mode fiber (SMS), is based on the multimodal interference (MMI) effect [2]. Interferometers based on the SMS structures allow to observe wavelength shifts as a function of the surrounding medium’s refractive index. However, most of the literature referencing these structures focus on its performance at telecommunications wavelengths (1100 to 1700 nm), requiring the utilization of expensive and complex experimental setups [3-7]. The performance of SMS structures at a shorter wavelength range (400-1000nm), permits the use of less expensive optical light sources and detectors [8,9]. However, the sensitivity at shorter wavelength range is reduced, hence it requires further optimization of these devices in order to overcome this limitation. The sensitivity improvement has been previously attained reducing the structure diameter by means of an etching process through immersing of the device in hydrofluoric acid (HF). Concerning SMS structures, the diameter reduction was initially applied in devices operating at telecommunications wavelengths [3,4,6]. Recently, the idea was also explored with low cutoff wavelength SMS structures [8,9] for operation at shorter wavelengths. Wenceslao Eduardo Rodríguez-Rodríguez, Adolfo Josué Rodríguez-Rodríguez and Manuel Zúñiga-Alanís are with the Computational Sciences and Technologies Department of the Autonomous University of Tamaulipas, CO 88690, Tamaulipas, Mexico (e-mails: wrodriguez@uat.edu.mx, arodriguez@uat.edu.mx, mazuniga@uat.edu.mx). Carlos R. Zamarreño and Ignacio Del Villar are with Electrical, Electronic and Communication Engineering Department of the Public University of Navarra, CO 31006, Navarra, Spain (e-mails: carlos.ruiz@navarra.es, ignacio.delvillar@unavarra.es). Low cutoff wavelength etched SMS structures towards verification of the quality of automotive antifreeze. Wenceslao Eduardo Rodríguez-Rodríguez, Adolfo Josué Rodríguez-Rodríguez, Carlos R. Zamarreño, Ignacio Del Villar, Manuel Zúñiga-Alanís. © 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other work.