Gas sensing is essential for safety and maintenance operations in many industries, including power generation, petrochemical, capture and storage technologies, and the food-processing sector. The properties of fiber-optic sensors make them a superior choice for environmental monitoring applications, especially in extreme conditions, and particularly when compared against conventional electro-optical sensors. Their advantageous properties include immunity to electromagnetic radiation, high temperature durability, high sensitivity and the ability for high resolution detection, as well as multifunctional sensing capabilities such as temperature, humidity, pressure, strain, and corrosion. Among different types of interferometers, Mach-Zehnder Interferometers (MZI) have received significant attention because they are robust, compact, and have high levels of precision.

In this dissertation, we present an in-line and compact MZI point sensor designed for sensing refractive index. In comparison with various types of interferometers, fiber MZI based RI sensing was selected based on its enhanced sensitivity and fabrication simplicity. The MZI sensor is developed using photonic crystal fiber and demonstrated for high sensitivity detection and measurement of pure gases. The transmission spectrum of MZI sensors is formed by interference between the cladding and core modes. To construct the device, the sensing element fiber was placed and aligned between two single-mode fibers with air gaps at each side. Two linear-translation micro stages were used to accurately differ and adjust gap lengths from 0 to 5mm. Great measurement repeatability was shown in the cyclic test for the detection of various gases such as methane and helium. A high RI measurement resolution of 2.1 E-7 and a sensitivity of 4629 nm/RIU was achieved, which is among the highest reported. Results show that the sensitivity of the fabricated MZI increases from 3000 nm/RIU to 4600 nm/RIU when the length of the sensing element fiber decreases from 5 mm to 3.3 mm. Furthermore, the device was packaged to demonstrate the laboratory-scale monitoring, as well as leakage detection of different concentrations of CO₂ in both subsurface soil and aqueous environments. Two water resistant but gas permeable membranes were used to package the sensor, to achieve a good balance of CO₂ permeability and water resistance. The experimental study of this work reveals the great potential of the fiber-optic approach for environmental monitoring of CO₂.

This study also explores other potential applications. Three types of sensors were fabricated using the proposed configuration employing 4 mm stub of (i) solid core Photonic Crystal Fiber (PCF), (ii) 10 µm Hollow core PCF (HC-PCF), and (iii) 20 µm HC-PCF as the sensing elements. We compared the performance of these sensors for detecting and measuring the quantity of gas present. As the transmission signals correspond to the frequency components in the sensor’s Fast Fourier Transform (FFT) spectrum, the effect of gap distance on the number and amplitude distribution of the modes was examined in an effort to optimize the design elements. The MZI sensors are highly sensitive to low percentages of CH4 and CO₂, making them suitable for greenhouse gas measurement.