Cavity attenuated phase shift fiber optical sensor for humidity detection

Humidity sensing is crucial in many applications, such as food processing, chemical industry, and civil engineering work, to name a few. Conventional electronic humidity sensors are prone to electromagnetic interference and are less sensitive due to their inherent single-pass nature. However, the fiber optic relative humidity (RH) sensor using the cavity attenuated phase shift spectroscopy (CAPS) technique is immune to electromagnetic interference and provides high sensing sensitivity and accuracy because of its multi-pass nature. In CAPS, a sinusoidally amplitude-modulated light is injected into a cavity, and the output light phase shift is measured with respect to the input light. The phase shift measures a sensing event, specifically the absorption caused by the sensing event inside the cavity. Here, we propose a CAPS-based robust and inexpensive fiber optic sensor for humidity sensing applications. We prepare a tapered fiber of ~10 μm and coat the tapered fiber with polymethylmethacrylate (PMMA). The PMMA-coated tapered fiber is spliced between two fiber Bragg gratings to realize an optical cavity for conducting CAPS measurements. As a sensing head, we place the tapered fiber in a home-built humidity chamber. When the humidity increases in the chamber, the water vapors are absorbed by the tapered fiber’s PMMA coating, leading to a change in the effective index experienced by the cavity. Consequently, the cavity losses change, which induces the phase shift change in the CAPS signal. Our experimental results show that the sensor provides a linear relationship between 45% and 70% RH with a sensitivity of 0.246 degrees/%RH and a fast reaction time of around 1 s. We anticipate that the current work will lead to a viable relative humidity sensor for various agriculture and food safety applications.