Long period fibre grating as gas sensor for environmental pollution monitoring.

The principle objective of the work presented in this thesis is to investigate and demonstrate the possibility of utilising the inherent properties of long-period gratings (LPGs) to detect the existence and concentration of a given gas in the region immediately surrounding the fibre cladding. The principle and the viability of using LPGs for sensor applications is supported by the preliminary results reported here. However, the sensitivity of the sensor requires further improvement before it can in any way challenge the existing sensors in this field. The operational characteristics and limitations of existing optical sensors are reviewed and the advantages of a LPG based optical sensor highlighted. Thorough explanations of the theory and principles of light propagation, mode formation, and mode coupling in optical fibres are presented. Computer simulations predicting the optical effects due to changes in ambient indices from theoretical conditions are successfully derived, confirming the results obtained by experimental investigation. Various established coating methods are investigated and utilised in the application of optically sensitive compounds adsorbed onto the cladding with different levels of success, the poly-electrolyte self assembly (PESA) and evaporation methods proving most suitable. A novel method of monitoring the build up of PESA layers in-situ using surface plasmon resonance (SPR) methods is introduced. The coating chemicals used in this investigation showed some optical sensitivity at the optimum wavelengths used in optical fibres to the various gases being monitored, in most cases causing a detectable change in the optical characteristics of the modes in the LPG.This study has shown the possibility of using a suitably prepared LPG as a gas sensor. The LPG is coated with a chemical whose refractive index is changed by absorption of a given gas and thus the change in the coupling wavelengths being caused by the existence of the gas. Maximum shifts in coupling wavelength of +/-1.5 nm for relatively high gas concentrations are observed. The possibility of using a coating material which absorbs water, or surrounding the LPG with a suitable liquid is also demonstrated, the refractive index of the coating or liquid and thus the coupling wavelength shift being affected by the reaction of the gas. The possibility of using a single temperature immune LPG for ambient index sensing by observing different coupling wavelengths in the same grating is also reported.