Fabrication And Sensing Properties Of Microchannels And Microcavities In Optical Fibers By Femtosecond Laser

In recent years, microchannel and microcavity structures in optical fibers have attracted much attention. The microchannels and microcavities in optical fibers are with small sizes and compact structures. Light in the fiber core reflected by the microchannels and microcavities can form Fabry-Perot (FP) interference, and its phase can be modulated by the change of environmental parameters such as temperature, liquid refractive index and strain. Therefore the microchannel and microcavity structures in optical fibers can have great application in temperature, liquid refractive index and strain sensing due to their higher sensitivities. In this dissertation, we proposed a novel method of fabricating microchannels and microcavities in optical fibers by using femtosecond laser-induced water breakdown. Sensing properties of microchannels and microcavities in optical fibers were investigated theoretically and experimentally. The research work has been summarized in details as follows.Single microchannel in optical fiber was fabricated by using femtosecond laser-induced water breakdown and its temperature sensing properties were investigated. The fabricated microchannel is with uniform diameter, precise location, smooth interface and high fabrication efficiency. The fabrication process takes only1-2minutes. We investigated the relationship between femtosecond laser pulse energy and microchannel diameter. Then we fabricated single microchannel at the end of fiber by using femtosecond laser-induced water breakdown and smoothed its interface by using arc discharge. Two sides of microchannel and the end surface of the fiber act as three reflective mirrors, which constitute three-wave FP interferometer. We measured and simulated its refection spectrum and investigated its temperature sensing properties. The experiment result shows that the temperature sensitivity reaches13pm/°C, which is about ten times higher than that of common two-beam FP interference sensing structures. The fabricated single microchannel structure in optical fiber can be used as high temperature senor in harsh environments with high sensitivity.Three-cascaded microchannels and H-type microchannel in optical fibers were fabricated by using femtosecond laser-induced water breakdown and their liquid refractive index and humidity sensing properties were investigated respectively. Firstly, we fabricated three-cascaded microchannels across the fiber core by using femtosecond laser-induced water breakdown. Part of light transmitted in the fiber core is scattered into the fiber cladding, and the light transmitted in the fiber core and cladding respectively form interference. Its liquid sensing properties were investigated and its sensitivity reaches2406.1nm/RIU and156.8dB/RIU. The fabricated three-cascaded microchannels structure in optical fiber can be used as liquid refractive index sensor with high sensitivity. Then, we fabricated H-type microchannel in optical fiber by using femtosecond laser-induced water breakdown. Air and liquid can be brought into the H-type microchannel and can interact with the light in the fiber core directly. Its humidity sensing properties were investigated and its sensitivity reaches1.0dB/1%RH. The fabricated H-type microchannel structure in optical fiber can be used as humidity sensor with high sensitivity.Rectangle microcavity in optical fiber was fabricated by using femtosecond laser-induced water breakdown and its liquid refractive index sensing properties were investigated. Light in the fiber core reflected by the two interfaces of microcavity can form FP interference. We investigated the liquid refractive index sensing properties of the microcavity in optical fiber theoretically. And, we also analyzed the influence of shape, length and interface smoothness of microcavity on sensitivity and accuracy. Based on the theoretical analysis results, we fabricated rectangle microcavity in optical fiber by using femtosecond laser-induced water breakdown together with arc discharge. Its refractive index sensing properties were investigated experimentally. The sensitivity reaches1147.5nm/RIU and the accuracy reaches1.29×10-4. The fabricated rectangle microcavity structure in optical fiber can be used as liquid refractive index sensor with high sensitivity, high accuracy and low crosstalk.Strip microcavity in optical fiber was fabricated by using femtosecond laser micromachine followed by fiber fusion and its strain sensing properties were investigated. We investigated the strain sensing properties of the microcavity in optical fiber theoretically, and simulated the deformations of microcavities in optical fiber with different shapes. By analyzing the influence of microcavity shape and size on strain sensitivity, it is showed that the strip microcavity with smaller cavity length is expected to be with higher sensitivity. Then we fabricated two strip microcavities in optical fibers with cavity length of18μm and15μm by using femtosecond laser micromachine followed by fiber fusion. The strain sensing experiment results show that, their sensitivities reach22pm/mN and26pm/mN respectively. The fabricated strip microcavity structure in optical fiber can be used as strain force sensor with high sensitivity and low crosstalk.