Fiber optical interferometric sensors for water quality monitoring

Abstract

Optical fiber based interferometer sensors have earned interest over an extended period of time due to their unique characteristics such as high sensitivity, short response time, simple construction process, etc. Meanwhile, contaminated water demands to be monitored as it causes a hazardous risk to the environment. Thus, this dissertation addresses three unique interferometer based fiber sensors for water quality monitoring. We have presented a simple, reliable, and quick reactive Fabry-Pérot (FP) structure based fiber optic pH sensor. The pH-sensitive hydrogel and single-mode fiber (SMF) are placed inside a fused silica capillary to form the FP cavity. The gel thickness is characterized by the spin coating method with respect to different spin speeds. The proposed sensor shows a pH sensitivity of 0.30 nm/pH along with a fast response time of 15 s to 20 s for different pH solvents in the acidic range. Also, the temperature sensitivity of the FPI sensor is found to be -0.56 nm/˚C. Furthermore, a compact, reliable, and fast responsive PCF (photonic crystal fiber) based modal interferometric sensor for lead ion detection is proposed and experimentally demonstrated. The sensor has been fabricated by splicing a small section of PCF with SMF, followed by collapsing of air holes of PCF at its tip. The interferometer is dip-coated with chitosan-PVA (polyvinyl alcohol) and glutathione functionalized gold nanoparticles. Three probes have been fabricated, and the maximum sensitivity has been found to be 0.031 nm/ppb for lead ions whereas the detection range has been considered from 0 ppb to 50 ppb. The probe has been found to have a faster response time of ~ 10 s. Furthermore, the sensor has been found to be less responsive towards other heavy metal ions, thereby demonstrating its selectivity towards lead ions. Besides, a section of FBG (fiber Bragg grating) has been embedded into the interferometer and the temperature response of FBG peak along with interference spectra has been investigated for better accuracy. Finally, we experimentally demonstrated a biocompatible, reliable and quick responsive fiber optic sensor based on Mach-Zehnder interferometer (MZI) for nitrate analytes tracing. The sensor was constructed by collapsing the air holes of a small PCF with the single-mode fiber on both ways. The proposed sensor has been coated with a graphene-PVA membrane using the thermal coating technique to make the sensor attractive to the nitrate ions in the aqueous solution. The maximum response is found to be 15 pm/ppm on the nitrate measurement scale of 0 ppm to 100 ppm with an average reaction time of ~10 s. The sensor has high selectivity to nitrate compounds than other fertilizers. Besides, a short length of FBG is implanted with SMF to improve the sensing accuracy of the presented sensor.