Microfiber Bragg Grating Hydrogen Sensors

The monitoring of hydrogen concentration is very important for security in the clean energy and aerospace, and chemical industries. The traditional electronic sensors can cause heat, charge accumulation, electromagnetic effect, and even sparking, and brings extra unwanted danger. In contrast, the fiber optic hydrogen sensors are made of silica glass and are intrinsically safe. The sensors have been fabricated based on Palladium(Pd) coatings. The metal coating absorbs hydrogen and expands in column, which induces changes in light intensity, phase, and/or polarization state. However, these sensors present relatively low sensitivity and the disadvantage of temperature cross sensitivity. In order to solve these problems, we propose and demonstrate Pd-based microfiber Bragg grating hydrogen sensors in this thesis. The refractive index of the Pd coating changes as a result of hydrogen absorption and changes the Bragg wavelength, as a result of the evanescent field interactions. The detection of hydrogen molecules can be performed by monitoring the Bragg wavelength shift. Compared with the existing methods, the present sensor is highly sensitive and the temperature cross response is weakened.This main contents of the thesis is as as follows:(1) Analysis on the optical properties of the microscaled optical fibers and their sensors based the evanescent field interaction. Preparation of optical microfiber with different dimensions by thermal tapering.(2) Analysis on the hydrogen absorption process of Palladium(Pd) and the corresponding change in its volume and the complex refractive index changes. Research and optimization of the uniform deposition of Palladium on the surface of the microfibers.(3) The fabrication of hydrogen sensor based on highly efficient photoinscription of Bragg gratings in microfibers in combination with the deposition of Palladium film. We have measured the response of sensors with different transverse diameters. The measured results suggests that the reflection peak blue shifts with the increment of hydrogen concentration. The wavelength shift reaches-1.08 nm for a sensor with a diameter of 3.3 microns, which is 26 times higher than the grating in an ordinary singlemode optical fiber. Higher sensitivity can be obtained with thinner gratings because of the strengthened evanescent field interaction. In contrast, the temperature sensitivity is only enhanced by 27% as a result of Pd coating, which means that the temperature cross sensitivity is weakened.