Spectral bandwidth analysis of high sensitivity refractive index sensor based on multimode interference fiber device Edwin G. P. Pachon a , Marcos A. R. Franco b,c and Cristiano M. B. Cordeiro a a Instituto de Física "Gleb Wataghin" – IFGW, UNICAMP, Campinas, SP, Brazil b Instituto de Estudos Avançados – IEAv, São José dos Campos, SP, Brazil c Instituto Tecnológico de Aeronáutica – ITA, São José dos Campos, SP, Brazil ABSTRACT Fiber optic structures based on multimode interference were investigated to the refractive index (RI) sensing. The proposed device is a singlemode-multimode-singlemode (SMS) structure, where the multimode section is a coreless fiber (MMF). The numerical analyses were carried out by beam propagation and modal expansion methods. Ultra-high sensitivity was obtained: 827 nm/RIU over a RI range of 1.30–1.44 and a maximum sensitivity of 3500 nm/RIU for RI∼1.43, considering Δ RI = 0.01. The dependence of spectral bandwidth was investigated taking into account the multimode fiber diameter and the coupling efficiency between modes at the input junction singlemode-multimode. Keywords: Multimode interference, Refractive Index Sensor, Refractive Index Measurement, Fiber Optic Sensors 1. INTRODUCTION Multimode interference (MMI) devices based on all-fiber have been used to build sensors for many physical parameters such as: temperature, pressure, strain, and refractive index [1]-[12]. Basically an all-fiber MMI device consists of a join of three optical segments: a step-index multimode fiber (MMF) spliced between two single-mode fibers (SMF), forming a SMS structure [1], [4], and [7]. The light coming from the input SMF excites several modes of the MMF section and, thus, causing interference among them along the fiber. The MMF length (L MMF ) is determined so that the interference pattern rebuilds the optical intensity and phase of the launch signal at start of the multimode region. The self-image occurs successive times along the MMF. If MMF section has these optimized lengths the self-image at output SMF provides a bandpass spectral peak. The lengths where the self-images occur depend on the: wavelength, MMF diameter, and the refractive index (RI) of the external medium at the MMF coreless region. In this work, we present a SMS structure based on a pure silica coreless MMF section, with reduced diameter to improve the sensitivity of a refractometric sensor. The designs were numerically investigated by vectorial wide-angle beam propagation (BeamProp - Rsoft) and full vectorial modal expansion (FIMMPROP - Photon Design) softwares. Three SMS structures were considered allowing reaching an ultra-high sensitivity to RI variation. The influence of dimensional parameters of the MMF section over the RI sensor sensitivity and over the spectral transmission bandwidth is studied taking into account the number of excited modes at MMF and the coupling efficiency between the fundamental mode at the SMF and the modes at the MMF. In both input and output tips it was used a standard singlemode optical fiber (SMF- 28) and between then a spliced coreless MMF section. Three MMF diameters were numerically considered: 125 µm, 78 µm, and 55 µm. The Figure 1(a) presents the typical geometric models of the SMS structures considering the three MMF diameters. 2. RESULTS Figures 2(a)-(c) present the interferometric pattern in the xz plane for the SMS structure with MMF diameters (φ MMF ) of 55 µm, 78 µm, and 125 µm, respectively. For an external medium with refractive index n liq =1.0 the length of first self- image is 11,373 µm, 22,801 µm, and 58,412 µm for φ MMF of 55 µm, 78 µm, and 125 µm, respectively. The Figure 2(d) presents the normalized spectral transmission to the most sensible SMS structure, with φ MMF = 55 µm, as function of external RI. Figure 2(e) present the wavelength shifts at the maximal transmission peak as a function of the RI. The curve to φ MMF =55 µm presents a better sensitivity with an average of 827 nm/RIU over a RI range of 1.30–1.44 [4]. To put this result in perspective we compare it with the result recently presented in [9], that uses a SMS structure with a tapered multimode section. The average sensitivity in our case is 1010 nm/RIU, two times higher than that obtained in [9] (∼487 nm/RIU) to RI range of 1.33 to 1.44. A maximum sensitivity of 3500 nm/RIU is achieved for RI ∼ 1.43, 80% OFS2012 22nd International Conference on Optical Fiber Sensors, edited by Yanbiao Liao, Wei Jin, David D. Sampson, Ryozo Yamauchi, Youngjoo Chung, Kentaro Nakamura, Yunjiang Rao, Proc. of SPIE Vol. 8421, 84217Q · © 2012 SPIE · CCC code: 0277-786/12/$18 · doi: 10.1117/12.969928 Proc. of SPIE Vol. 8421 84217Q-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/12/2015 Terms of Use: http://spiedl.org/terms brought to you by CORE View metadata, citation and similar papers at core.ac.uk provided by Repositorio da Producao Cientifica e Intelectual da Unicamp