Advanced fibre gratings in near- and mid-infrared region and their applications for structure monitoring and biosensing

Abstract

This thesis emphasises an elaborate research finding on the fabrication, analysis of resonance spectral response and sensing applications in various fields using different type of optical fibre grating devices over an entire wavelength range from near- to mid-infrared (IR). Firstly, the major contribution described in the thesis is a thorough investigation of sensor structures, detailed in respect to writing techniques for optical fibre gratings irradiated by a frequency-doubled Argon ion laser. Thereafter spectral modulation for these UV-inscribed fibre Bragg gratings (FBGs), long period gratings (LPGs) and tilted fibre gratings (TFGs) of small and large angled structures are analysed. Special LPG devices with both dual peaks and 1st&2nd orders, and excessively tilted TFGs (Ex-TFGs) are also achieved with the resonances in the mid-IR range for enhanced sensitivity. Investigations of different sensing measurements, such as temperature, strain, bending and surrounding refractive index (SRI) for these fibre grating devices are performed. Another important contribution is the study on experimental investigation for the fabrication of FBGs into multicore fibre such as four core fibre (4CF) with two different core spacings and seven core fibre (7CF) are explained. A selective inscription method is utilised for inscribing FBGs into different cores of multicore fibre (MCF). The measurement performance with vector bend/twist sensing results in an enhanced sensitivity for FBGs in the distributed cores around the circumference of 4CF and 7CF is analysed in detail, showing the effective detection of both amplitude and direction. Whereas, in 7CF the central core FBG acts as the temperature reference having low bending sensitivity of -8.83 pm/m-1, presenting extra function for eliminating temperature cross-talk effect. The application for these grating devices is largely associated to structural monitoring in astronomy, biomedical sciences, and robotics. Finally, I have investigated different enzyme functionalised and nano-deposited LPG devices for bio and environmental sensing applications. The experimental findings for these sensors are discussed in glucose sensing measurements by observing resonance wavelength shift. Whereas LPGs fabricated with mono or multi-layered deposition of 2D nanomaterials, such as graphene oxide (GO) solution and single walled carbon nanotubes (SWCNT) are demonstrated with SRI measurement. Here, the fabricated devices show a significant intensity change into the transmission spectrum. The resonance response is observed in the near- to mid-IR ranges. This enhanced sensitivity is utilised for haemoglobin sensing in the detection of anaemia in human body and relative humidity sensing for monitoring environmental condition respectively. All demonstrated optical fibre grating based sensors have potential for a wide range of future applications in industry, medical and environmental sectors.