Research On Key Technologies Of Miniature Interferometric Optical Fiber Sensors For Hydrogen Detection

With the development of society and industry, hydrogen has been an important industrial raw material extensively used in many fields such as chemical industry, electronic industry, and metallurgical industry, etc. However the hydrogen is flammable and explosive; thus it is of great importance to precisely detect hydrogen concentration and monitor leakage. Optical fiber hydrogen sensors have attracted considerable research interests in recent years because of their advantages of small size, light weight, compactness, good reliability and electromagnetic immunity. However, the sensor is not suitable for the limited space or high integration. The recent research efforts have focused on miniaturization and high sensitivity. The traditional methods to fabricate the sensors by welding and capillary encapsulation are complicated, and difficult to be applied in mass production. The femtosecond(fs) laser has provided a new plateform and process for sensor fabrication.In this dissertation, fs laser fabricated miniature interferometric optical fiber hydrogen sensors coated with Pd films are proposed and developed. The optical fiber hydrogen sensors based on interferometer, such as Mach-Zehnder interferometer, Fabry-Perot interferometer, and Michelson interferometer, are analyzed and discussed under different conditions, and the novel sensors have high potential in hydrogen sensing. The main contents are listed as follows:(1) The miniature interferometric optical fiber hydrogen sensor fabricated by a new method is proposed. The proposed sensors in the micrometer size are compact, easy in fabrication and have high potentials in hydrogen concentration detection.(2) The materials micromachining methods and procedures with fs laser micromachining have been investigated systematically. Thus, the miniature interferometric optical fiber hydrogen sensors can be fabricated with optimal processing parameters.(3) The working principle of the miniature optical fiber hydrogen sensor based on Mach-Zehnder interferometer has been analyzed and the transmission characteristic of the sensor is discussed with optical model simulation under different conditions. The sensor is measured in the hydrogen volume concentration range of0-16%, and the sensitivity of the sensor (L=40μm, dfilm=110nm) is-0.155nm/%. Experimental results show that the miniature sensor has high sensitivity, and is featured by small size as compared to the existing hydrogen sensor.(4) The fs laser fabricated miniature Fabry-Perot interferometer hydrogen sensor coated with Pd film has been proposed and investigated. The corresponding reflection characteristics to hydrogen concentration have been discussed, and the wavelength shifts of the sensor (L1=20μm, Li=50μm) are10,30,100, and150pm respectively, corresponding to the hydrogen concentration of2%,4%,6%,8%, respectively. The sensitivity of the miniature Fabry-Perot interferometer hydrogen sensor is lower than that of the Mach-Zehnder interferometer, but the detection of reflection spectra leads to a smaller size.(5) The fs laser fabricated miniature Michelson interferometer with Pd film deposited on the fiber end face is proposed and demonstrated for hydrogen detection. The experimental results show that the reflection spectrum experiences a blue-shift, and the sensitivity of the sensor are-0.155nm/%at1280nm and-0.1625nm/%at1330nm. As the reflection spectra are detected, the sensor is almost the same size as the miniature Fabry-Perot interferometer hydrogen sensor. And it has higher sensitivity than the Mach-Zehnder interferometer and Fabry-Perot interferometer.