Abstract

Abstract
Optoelectronic Humidity and Temperature Sensor using Apodized Pi-Phase Shifted Fiber Bragg Grating
Navjot Kaur
Humidity control and moisture detection have a wide range of applications in major industries like food storage, chemical plants, electronic instruments, building/construction, medicine, museums and libraries. The most reliable humidity detection methods using capacitors and resistors fail under hazardous operational environments like transformers. Optical gratings based sensors are hence preferred in such conditions due to their properties of light weight, remote sensing and immunity to electro-magnetic interferences.
In this work, an optical humidity sensor based on a rarely investigated FBG is developed. It is called a π-phase shifted fiber Bragg grating (PSFBG). A sharp spectrum signal of PSFBG promises higher resolution in sensing applications. The surface area of the grating developed along an optical cable is coated by a hygroscopic polymer, Polyimide, that undergoes a reversible volume expansion on exposure to humidity. This leads to stress onto the grating and further a shift in the Bragg wavelength.
In order to investigate the electromagnetic wave propagation inside the core of fiber, a study is conducted in COMSOL multiphysics. The implementation of an optical fiber as humidity sensor is modeled. The mechanism of humidity absorption by the polymer layer is simulated and analyzed. Spectral improvements in the sensor signal in terms of interference are implemented using an optimized apodization function. The spectral signal of sensing element is simulated by transfer matrix solution of coupled mode theory in MATLAB. The results of modeling are used to fabricate a sensor containing two 24-mm PSFBGs separated by 12 mm. A distributed feedback laser scanner is used to precisely characterize and evaluate the device performance. The sensor response to changes in humidity and temperature of the environment is recorded. The experimental results are relatively in good agreement with those obtained theoretically.