Taper-based double Mach–Zehnder interferometers for temperature and humidity sensing

The fiber-optic sensor has been widely utilized in various environmental detections. In this work, an ultra-compact fiber-optic sensor is designed and fabricated for temperature and relative humidity (RH) sensing with high sensitivity. The two ends of a glue-coated microfiber segment are attached to another silica fiber taper for realization of the miniature Mach–Zehnder interferometer (MZI). Due to the reflection of the taper end face, reflected light passes through the two microfibers again to achieve the double MZIs. The mutual interaction of two MZIs promotes the mode interference and fiber-optic sensitivity. The UV glue on the surface of the microfiber segment exhibits a large thermo-optic coefficient and the hygroscopic expansion characteristics, further improving the temperature and humidity sensitivities of −109.9pm/ ∘ C and 25.6 pm/%RH, respectively. Comparatively, a fiber sensor with a single MZI is also measured with a similar temperature sensitivity and a humidity sensitivity of only 5.3 pm/%RH, showing the superiority of a double-MZI fiber sensor.