Development of fiber bragg grating-based ethanol sensors using nanostructured sensitive layers

Optical fiber sensors are widely gaining popularity over the past two decades as the call for highly efficient, miniaturised and versatile sensing devices are rapidly becoming a necessity. This research focuses on designing and analysing a standard single mode Fiber Bragg Grating (FBG) to be used as a sensor for the detection of aqueous ethanol. FBG sensors are conventionally used in the telecommunication wavelength for the measurement of physical parameters such as temperature, strain, pressure and so on. In this research, the FBG sensor is exploited in the ultraviolet and visible (UV VIS) region. Most optical fiber sensors require some modification to their structure such as removal of the cladding or tapering, in order to enhance the evanescent wave at the interface between the fiber and the surrounding medium. This makes the fiber fragile and difficult to handle. In the UV VIS region, the gratings in the FBGs function as refractors that push the light out to the cladding enabling the interaction of the light signal with the medium surrounding the fiber. This behaviour of the FBG makes it possible to be used without the removal of the cladding making it a very robust and reliable sensing device. The enhancement of the sensing capability is explored using gold layer to boost the evanescent field surrounding the optical fiber Bragg grating. The effect of nanostructured layers of graphene oxide (GO), carbon nanotubes (CNTs) and zinc oxide (ZnO) are investigated towards the performance improvement of the optical sensor. GO, CNTs and ZnO have emerged as leading materials in wide variety of applications including chemical sensors due to its exceptional thermal, optical and mechanical properties. Their nanostructures have huge surface area that enhances the sensor-analyte interaction and thus, improves the sensing performance. These nanostructured materials also react chemically with ethanol molecules, resulting in an increase in the sensitivity of the sensor.For the first time, a fully cladded FBG optical fiber chemical sensor with a thin gold film and nanostructured sensing layer is reported. The sensor was interrogated in the UV VIS region and both the absorbance response as well as wavelength shift due to Surface Plasmon Resonance (SPR) were investigated. The CNT coated FBG sensor demonstrated high sensitivity in absorbance levels up to 0.25/vol% ethanol and with a response and recovery time of approximately 50 and 70 seconds respectively. The GO coated FBG sensor demonstrated high SPR wavelength shift at an average of 1000nm/RIU. As a result of this PhD research project, several novel FBG sensors with nanostructured thin films were developed and investigated towards ethanol sensing.

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