> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract—This paper presents a complete study on the spectral behavior of a multimode-coreless-multimode fiber-optic structure, as well as its application as a refractometer and liquid level sensor. The combination of two standard multimode fibers fused to a coreless fiber segment allows generating narrow interferometric bands in the optical spectrum, whose sensitivity can be improved by an adequate selection of the dimensions of the device (the coreless segment length and the diameter of the sensing area). A second way to improve the performance of the device is to deposit a thin-film of SnO2, which allows increasing the sensitivity up to 314 nm/RIU. This widens the number of applications where this structure can be used. As an example, a liquid level sensor with 0.73 nm/mm sensitivity is presented. Index Terms— Optical fiber sensor, refractive index, multimode-coreless-multimode (MCM), interferometer, refractometer, water level sensor. I. INTRODUCTION OWADAYS there is a trend towards monitoring the world around us. Consequently, sensors are becoming more and more important. More specifically, optical sensors offer high sensitivity, electromagnetic immunity, reduced weight, cost and size, and the option to work in a distributed sensing network. Although their commercial presence is not as widespread as it is the case of electronic sensors, the progress made in the field of optical sensors is opening the door to new opportunities and the use of this kind of sensors is more common over time. In this sense, fiber-optics has experienced an increasing interest and relevance as an adequate technology that combines the advantages of optical sensors in reduced- size and cost effective devices [1]. Currently there are many different types of fiber-optic sensing structures. Among them, fiber Bragg gratings (FBGs) [2], long-period fiber gratings (LPFGs) [3], resonance-based sensors [4-5] and interferometers. Regarding the latter group, interferometers, a wide variety of structures has been developed: single-mode – multimode – single-mode (SMS) both in transmission and reflection versions [6-8], capillary Manuscript received on Ago 8, 2018; This work has been supported by the Spanish Ministry of Economy and Competitiveness (TEC2016-78047-R). The authors are with the Institute of Smart Cities and with the Department of Electrical, Electronic and Communication Engineering, Public University of Navarre, 31006 Pamplona, Navarre, Spain (e-mail: * ab.socorro@unavarra.es, ignacio.delvillar@unavarra.es, silvia.diaz@unavarra.es, natxo@unavarra.es). and heterogeneous structures [9-10] or the structure studied in this paper: multimode – coreless – multimode (MCM) [11]. Most of these interferometric structures share a similar design and own the advantages of their easiness when being manufactured and the low cost of the structures. Additionally, they provide high-resolution measurements due to their capability of developing prominent attenuation/transmission bands. Moreover, although they are structures with a low sensitivity, it is possible to improve this magnitude by combining the reduction of the diameter with the deposition of a thin-film whose refractive index is higher than the refractive index of the optical fiber [6]. Among the interferometers mentioned, probably the MCM design is one of the simplest structures, since the fact of using standard fibers, as in this case, normally decreases the cost of the device. This supposes an advantage if compared to other structures such as certain complex Fabry-Perot-based interferometers [12], where it is necessary to create air bubbles and/or grooving one of the fibers to obtain deep and high resolution interferometric bands but a lack of sensitivity enhancement. In addition, the MCM structure is suitable to insert more light from the coreless segment to the collecting multimode fiber, since the diameter of a multimode fiber (MMF) is wider than the diameter of a single-mode fiber (SMF). However, though there exist interesting works on the MCM structure where the influence of the coreless segment on the performance of the device is analyzed [11], or where the coupling to localized surface plasmon resonances (LSPRs) is used to develop a biosensor in the visible wavelength range [13], a more detailed study is required that relates the attenuation and transmission bands in the optical spectrum to the self-image (SI) band, as it has been done with the SMS structure [11,14]. Moreover, in addition to the coreless segment length it is necessary to study the influence of the cladding diameter and the effect of depositing a nanocoating, which in [6] has led to an important sensitivity improvement. As a proof of concept for the performance of the device, the devices will be tested against refractive index, which is one of the most typical applications of fiber-optic sensors and it is typically used as an assessment tool for determining the further performance when used as chemical or biological sensor [15,16]. After that, and thanks to the conclusions extracted from the refractometric sensor, a more specific application consisting of an optimized liquid level sensor will Multimode – Coreless – Multimode Fiber-based Sensors: Theoretical and Experimental Study Noé San Fabián, Abián B. Socorro-Leránoz * , Ignacio Del Villar, Silvia Díaz, Ignacio R. Matías, Senior Member, IEEE N