Long period fiber gratings (LPFGs) have been used as the optical signal generation and transmission component for the development of optical based high temperature sensors for in-situ monitoring hydrogen and carbon dioxide in the fossil fuel power plants. The sensing principle is based on the change in the refractive index of the sensing materials coated on the cladding layer of the optical fiber, resulting in the shift of the resonance wavelength of the LPFGs. This dissertation is dedicated to understanding the interaction between the target gases and sensing materials, exploring the film coating methods on optical fiber and evaluating the sensing performance of integrated optical sensors.

First, palladium is selected as the hydrogen sensing material. The palladium-coated LPFGs exhibit higher sensitivity compared to other palladium based optical hydrogen sensors reported in literature due to the high sensitivity of LPFGs. Hydrogen in the partial pressure range of 0-0.16 atm can be measured at a temperature up to 200 degree Celsius.

Terbium doped strontium cerate (SCTb) is another sensing material investigated for high temperature hydrogen monitoring. When this material is exposed to hydrogen, a decrease in its refractive index is observed. This is due to the partial reduction of quadrivalent cerium ion on the surface into a trivalent cerium ion. The resonance wavelength of SCTb-coated LPFGs shifts to higher wavelengths in a hydrogen and helium mixture compared to helium. The resonance wavelength increase rate depends on hydrogen partial pressure and can be used as the sensing signal. The sensor exhibits a high sensitivity and fast response.

Barium strontium cobalt iron (BSCF) is selected as the carbon dioxide sensing material. It can react with carbon dioxide at high temperatures to form carbonates. The conversion of BSCF into carbonates depends on the carbon dioxide partial pressure. The BSCF-coated LPFGs has a lower resonance wavelength in a carbon dioxide and air mixture than in air. The detection of carbon dioxide at a partial pressure in the range of 0.01 atm to 0.3 atm can be achieved at 600 and 700 degree Celsius. The response time of the sensor is relatively long.