Experimental comparison of the effect of the structure on MZI fiber gas sensor performance

A compact refractive index (RI) sensor, based on Mach Zehnder Interferometer (MZI), was developed for high sensitive detection and measurement of gases using Single Mode Fiber (SMF), Photonic Crystal Fiber (PCF), and Hollow-Core Photonic Crystal Fibers (HC-PCF). The device was constructed by placing a short stub of sensing element fiber between a lead-in and a lead-out single mode fiber (SMF) with an air-gap at each interface. Three MZIs were fabricated using 4 mm stub of PCF, 10 μm HC-PCF and 20 μm HC-PCF as the sensing element, and performance of these sensors were compared. The gas sample was able to effectively fill the air gaps between fibers. Using PCF and HC-PCF as the sensing element provides a great opportunity for gas sensing because of the possibility of a stronger interaction between the light and gas molecules. Additionally, gas molecules penetrate the cavities within the air holes of photonic crystal fibers. The resulting fiber sensors were able to measure the RI of the gas filled cavity and showed excellent sensitivity to helium, methane, and carbon dioxide. The highest RI sensitivity of 3210 nm/RIU was demonstrated in the RI range of 1.0000347 - 1.000449 for a sensor with 4 mm length of 10 μm HC-PCF. The cyclic tests show great measurement repeatability of different gases. Response and recovery times were found for these MZIs. It was concluded that 20 μm HC-PCF sensor has the lowest and the PCF sensor has the highest response/recovery time. This study shows that sensors fabricated by the proposed method have good potential for detecting and quantifying pure gases. Additionally, they are highly sensitive to low percentages of methane and CO2 which is suitable for greenhouse gas measurement.