JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 37, NO. 13, JULY 1, 2019 3351 Microcavity In-Line Mach–Zehnder Interferometers Fabricated in Single-Mode Fibers and Fiber Tapers for Visible (VIS) and Near-Infrared (NIR) Operation Tinko A. Eftimov , Monika Janik , Student Member, IEEE, and Wojtek J. Bock, Life Fellow, IEEE Abstract—In this paper, we analyze the performance of single- mode fiber (SMF) microcavity in-line Mach–Zehnder interfer- ometers (μIMZI) in the visible (VIS) and near-infrared (NIR) spectral ranges. Furthermore, we study the causes leading to a seeming paradox that NIR SMFs for the VIS range perform bet- ter in the NIR range and vice versa. We propose the use of short double tapers to improve the spectral range and visibility of the micro-interferometers. The sensitivities to surrounding refractive index obtained were as high as 11,400 nm/RIU. Index Terms—Fiber taper, in-line Mach–Zehnder interferome- ters, near-infrared, refractometry, visible. I. INTRODUCTION F IBER optic microcavity in-line Mach–Zehnder interferom- eters [1] have received a considerable interest and develop- ment in recent years for a number of evident reasons: extremely small cavity size of the order of 100 μm or less [2]–[4], high sensitivity to surrounding refractive index changes (of the or- der of 10 4 nm/RIU.) or high temperature sensitivity [5] which allows a number of sensor applications. The μIMZIs reported function in the infrared communication bands where most fibers and fiber components function. In this spectral range, however, the sources and the sensor interrogation units are costly which is a limitation to the development of low-cost sensors. On the other side Si-based photodetectors have a sensitivity in the 250 nm to 1100 nm with a maximum responsivity (A/W) around 900 nm and a spectral range of Δλ 0.5 = 450 nm (from 600 nm 1050 nm) at 50% relative responsivity and of Δλ 0.8 = 300 nm (from 720 nm to 1020 nm) at 80% relative responsivity. Photodetector arrays and spectrometers, as well as broadband sources in this spectral range, are considerably less expensive which defines our interest in the study and development of the μIMZI in the VIS/ NIR range. Manuscript received November 27, 2018; revised March 31, 2019 and April 19, 2019; accepted April 29, 2019. Date of publication May 10, 2019; date of current version June 10, 2019. This work was supported in part by the Natural Sciences and Engineering Research Council of Canada and in part by the Canada Research Chairs Program. (Corresponding author: Monika Janik.) The authors are with the Université du Québec en Outaouais, Gatineau, QC J8X3X7, Canada (e-mail: tinko.eftimov@uqo.ca; janm03@uqo.ca; Wojtek. Bock@uqo.ca). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2019.2915784 Fig. 1. Model of the μIMZI. II. THEORETICAL CONSIDERATIONS A. Basic μIMZI Construction The use of single-mode fibers in the visible (VIS) and near- infrared (NIR) for the fabrication of microcavity in-line Mach– Zehnder interferometers poses several challenges. They are as- sociated with the small size of the core and the variations of the fiber parameters such as cut-off wavelength and numerical aperture. These parameters are related to the V-parameter of the fiber and its critical value V c = 2.405. In this paper, we apply a simplified model of the in-fiber Mach–Zehnder interferometer, illustrated in Fig. 1. The fun- damental fiber mode with a given wavelength dependent modal radius w(λ) propagating along the core of effective refractive index n 0 couples a part of its power to the off-core region drilled across the cladding to a depth h over a length l and filled with a medium refractive index n. Because the mode propagation con- ditions change abruptly, losses occur, and ultimately the relative parts of the mode field that propagate along the core and in the micro-hole are η 0 and η 1 (η 0 + η 1 = 1). Evidently, the values of η 0 and η 1 will depend on how deep in the cladding the field of the fundamental mode penetrates. This, in turns, means that the higher the ratio mode radius/core w/a the higher will η 1 be. For 1.2 < V< 2.4 (λ c < λ< 2λ c ), the ratio of the fundamental mode to the fiber of core radius a, is given as: w(V ) a =0.65 + 1.619 V -3/2 +2.879V -6 (1a) 0733-8724 © 2019 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.