Research On Microfiber And Active Optical Fiber Hydrogen Sensors

In recent years, with the developing of the hydrogen energy industry, it is of great importance to monitor the hydrogen concentration in some key environment with the risk of hydrogen leaking. The traditional electronic sensors can generate heat, charge accumulation, electromagnetic effect, and even sparking, which brings extra unwanted danger. We can use optical fiber hydrogen sensor to detect the hydrogen leaking by sputtering the hydrogen sensitive material-Palladium(Pd) on the surface of the fiber. The metal coating absorbs hydrogen and expands in column inducing changes in light intensity, phase, and polarization state of the fiber. Compared with the traditional electronic sensors, the fiber optic hydrogen sensors are made of silica glass, which is intrinsically safe. However, these sensors present relatively low sensitivity and high temperature cross sensitivity. Moreover, palladium cannot absorb hydrogen below zero degree centigrade, leading to failure of sensors in some key area such as aviation and space.In order to solve these problems, we propose and demonstrate microfiber Bragg grating hydrogen sensors and highly sensitive fiber taper interferometric hydrogen sensors based on evanescent field effect of microfiber. Moreover, we propose and demonstrate active optical fiber Bragg grating hydrogen sensors based on rare earth ion absorbing pump light and generating heat to improve activity of the Pd overlay, to achieve the hydrogen detection in low temperature. The main contents of the thesis are as follows:(1) Microfiber Bragg grating hydrogen sensors. The manufacture processes of the sensors are consist of tapering the multimode fiber to microfiber, using excimer laser to fabricate the Bragg grating into the microfiber and sputtering the Pd overlay on the surface of the microfiber. Dielectric constant of Pd changes after absorbing hydrogen, which will induce Bragg wavelength shift by evanescent field interaction. When the hydrogen concentration is 5%, the wavelength shift reaches 1.08 nm, which is 26 times as much as that of the traditional ones.(2) Highly sensitive fiber taper interferometric hydrogen sensors. We use optical fiber micro-processing workstation to taper the fiber to microfiber, and we can control the microfiber structure flexibly by adjusting the arc charge power and the speeds of the motors. The ready-made microfiber will be sputtered with Pd overlay on the surface to detect the hydrogen. Dielectric constant of Pd changes after absorbing hydrogen, inducing the variation of the phase difference between fundamental mode and high-order mode by evanescent field interaction, which will cause wavelength shift of the sensors. When the hydrogen concentration is 5%, the wavelength shift reaches 1.99 nm, which is 47 times as much as that of the traditional ones.(3) Active optical fiber Bragg grating hydrogen sensors in low temperature. We use excimer laser to fabricate the Bragg grating into the Er3+/Yb3+co-doped fiber, and sputter the Pd overlay on the surface to detect the hydrogen in low temperature. The optical fiber can absorb strongly the pump light and can generate quantity of heat, which can improve the temperature of Pd overlay to make it absorb hydrogen and expand in volume to stretch the optical fiber, inducing the shift of the Bragg wavelength. We achieve the detecting of the hydrogen in-50 degree centigrade successfully, and the response time is only 30 seconds.