sensors Article Monitoring of Carbon Dioxide Using Hollow-Core Photonic Crystal Fiber Mach–Zehnder Interferometer Farid Ahmed 1, * , Vahid Ahsani 2 , Kaveh Nazeri 2 , Ehsan Marzband 1 , Colin Bradley 2 , Ehsan Toyserkani 1 and Martin B. G. Jun 3 1 Multi-Scale Additive Manufacturing Laboratory, Department of Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, ON N2L3G1, Canada 2 Department of Mechanical Engineering, University of Victoria, Victoria, BC V8W2Y2, Canada 3 School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907, USA * Correspondence: farid.ahmed@uwaterloo.ca Received: 10 July 2019; Accepted: 29 July 2019; Published: 31 July 2019   Abstract: Monitoring of greenhouse gases is essential to understand the present state and predict the future behavior of greenhouse gas emissions. Carbon dioxide (CO 2 ) is the greenhouse gas of most immediate concern, because of its high atmospheric concentration and long lifetime. A fiber-optic Mach–Zehnder interferometer (MZI) is proposed and demonstrated for the laboratory-scale monitoring of carbon dioxide concentration. The interferometric sensor was constructed using a small stub of hollow-core photonic crystal fiber between a lead-in and lead-out standard single mode fiber, with air-gaps at both interfaces. At room temperature and atmospheric pressure, the sensor shows the sensitivity of 4.3 pm/% CO 2 . The device was packaged to demonstrate the laboratory-scale leakage detection and measurement of CO 2 concentration in both subsurface and aqueous environments. The experimental study of this work reveals the great potential of the fiber-optic approach for environmental monitoring of CO 2 . Keywords: carbon dioxide gas; fiber-optic sensor; Mach–Zehnder interferometer; photonic crystal fiber 1. Introduction Carbon dioxide (CO 2 ) has been identified as the primary heat-trapping gas to adversely affect our climate between 1750 and 2011 [1]. As our planet is likely to face greater future challenges, in recent years considerable carbon mitigation research has been undertaken in an effort to fight global warming caused by CO 2 . Deployment of carbon capture and storage (CCS) technologies have been proposed for a drastic reduction of CO 2 emission [2]. As of September 2012, 75 large-scale global CCS projects (at least 400,000 tons of CO 2 per year) and a number of projects under advanced stages of development have been identified by the Global CCS Institute [3]. While CCS has a vital role in controlling greenhouse gas emissions, CO 2 leakage at sequestration sites is the primary concern because of its adverse environmental impacts [4,5]. Research models predict that in an event of leakage, CO 2 escape rate above 0.1% may jeopardize the effectiveness of a geological storage site [6]. Hence, early detection of CO 2 leakage is essential to re-establish the efficiency of a CCS operation and minimize its ecological damages. Monitoring of CO 2 leakage has been investigated using diverse approaches including seismic [7], geoelectrical [8], geochemical [9], gravimetric [10], temperature logs [11], and soil gas composition [12]. There are many challenges inherent to monitoring a CCS project, most notably: prolonged periods (i.e., several decades), long transmission lengths, extreme physical and chemical conditions, and capital and operating costs. Most of the techniques mentioned above have a low sensing resolution or do not meet the challenges associated with monitoring CCS projects. Sensors 2019, 19, 3357; doi:10.3390/s19153357 www.mdpi.com/journal/sensors