IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 29, NO. 2, JANUARY 15, 2017 239 Multi-Parameter Interferometric Sensor Based on a Reduced Diameter Core Axial Offseted Fiber Carlos E. S. Castellani, Hozianna C. B. Ximenes, Rodolpho L. Silva, Anselmo Frizera-Neto, Moisés R. N. Ribeiro, and Maria J. Pontes Abstract— We report an optical sensor based on the interference pattern created by an all-fiber Mach–Zehnder setup. This pattern is produced by the combination of the core and cladding modes that are excited on a fiber with reduced diameter fusion spliced to a single-mode fiber with a 4-μm core axial offset. Discrete measurements of refractive index and temperature are achieved with sensitivities of 8.8 nm/refractive index units and 39.2 pm/°C, respectively, and also distributed measurements of liquid level up to 120 mm are made with a sensitivity of 6 pm/mm. Index Terms— Optical fiber interference, optical interferome- try, optical fiber applications, optical fiber devices. I. I NTRODUCTION O PTICAL fiber sensors have been over the last decade ubiquitously used in industry, due to its many advantages such as immunity to external electromagnetic interference, high sensitivity, simplicity and safe operation in harsh envi- ronments. Physical parameters such as liquid level [1]–[3], temperature [4], refractive index [2], [5], force [6], displace- ment [7] and humidity [8], are routinely measured by such technologies. Interferometric techniques [2], [4], [5] are par- ticularly interesting since they allow sensitivities much higher than what can be obtained in traditional sensors based on fiber Bragg gratings [9], and additionally, their spectrum-based interrogation system is often more stable and reliable than sensors which are power-interrogated [2]. A number of interferometric fiber sensors have already been published, mainly consisting on Mach-Zender [5], [7], [10] or Michelson interferometers [2], [4]. Although high sensitiv- ities in measuring temperature [4], displacement [7], refractive index [5] and liquid level [2] have already been obtained, the majority of such sensors were very short and therefore not able to measure liquid levels above 10, 20 or 30 mm [2], [5], [10], respectively. Liquid level measurement ranges can be increased by using long period [11] or titled fiber Bragg gratings [1], allowing sensing liquids up to 60 mm and 95 mm, respectively. Extended ranges can be obtained by interferometric techniques, however at the expense of extra fabrication complexity. For example, in [12] a range of Manuscript received July 13, 2016; revised November 4, 2016; accepted December 3, 2016. Date of publication December 9, 2016; date of current version January 17, 2017. This work was supported in part by the Coordenação de Aperfeiçoamento de Pessoal de Ensino Superior, in part by the Fundação de Amparo a Pesquisa do Espírito Santo, in part by the Conselho Nacional de Desenvolvimento Científico e Tecnológico, and in part by PETROBRAS. The authors are with the Electrical Engineering Department, Federal University of Espírito Santo, Vitória 29075-910, Brazil (e-mail: carlos.castellani@ufes.br). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2016.2637870 140 mm and in [13] a range of 200 mm were successfully obtained for liquid level measurements by using a combination of different fiber tapers. However, it is not clear how such complex arrangements respond to refractive index [12], [13] and temperature [12] variations. A similar situation can be found in [3] where a 120 mm sensor was built using a combination a SMF and a no-core fiber coated with a gold layer to act as a mirror, which besides the enhanced complexity presents no data regarding variations of temperature. Here we present a low-cost and easy to fabricate inter- ferometric fiber sensor capable of measuring temperature, refractive index and liquid levels with high sensitivities, allow- ing distributed level measurements of up to 120 mm to be realized. Our approach consists of combining an axial offset splice technique [7] with the use of two fibers with different diameters [2], [5] in order to generate an high contrast ratio interference pattern that can be reliably sensed over 120 mm of optical fiber. The combination of both techniques allows to obtain sensing results for longer lengths when comparing to previous results employing them separately. For instance, one order of magnitude increase on the level sensing range was achieved in comparison with [5], where an interference pattern is created by using fibers with different core diameters but that are not misaligned. This simple and compact Mach-Zehnder- like setup proposed here presents sensitivities of 8.8 nm/RIU, 39.2 pm/°C and 6 pm/mm, for refractive index, temperature and liquid level respectively. II. THEORY AND EXPERIMENT The working principle of the sensor consists of split- ting an optical beam into two paths with different phase velocities and then recombining them creating an all-fiber Mach-Zehnder. This is shown in Fig.1 where a standard single mode fiber (SMF) is spliced to a 120 mm reduced core fiber (RCF) with a core axial offset misalignment of 4 μm, allowing part of the initial power to propagate in the cladding of the RCF. The misalignment is easily created by using an default offset splicing function of a commercial fusion splicer (Fujikura 70 S), which is chosen to be 4 μm because it is the value that allowed the higher extinction rate to be obtained on the interference pattern, of about 45 dB. A 100 mm multi-mode fiber (MMF) is then spliced to the RCF with no misalignment in order to collect and recombine both core and cladding modes creating the interference pattern. The RCF is a commercial single mode fiber (OFS Raman fiber) that has a core diameter of 6 μm, differently from the SMF and MMF used which have a core diameter of 1041-1135 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.