| In recent years,harsh environmental sensors have gradually become the research hotspots in the scientific and industrial area.Harsh environments such as high temperature,high voltage,strong radiation,strong electromagnetic interference,etc.are widely found in industries,country defense,aerospace and other fields.However,how to detect real-time environmental changes effectively and accurately is always a problem.Optical fiber sensors have many advantages such as anti-electromagnetic interference,their small size and their light weight that makes them become a hot research topic in the sensing area.Their good physical and chemical properties play significant roles in many harsh environments.Fiber Bragg Grating(FBG)which has high sensitivity and high stability is one of the most common fiber optic sensors that it is favored in temperature sensing and strain sensing applications.The type I FBG written by the traditional ultraviolet laser has a limitation that the grating structure is easily erased at a high temperature.The emerging ultra-fast laser nanotechnology,by changing the molecular structure,makes the grating structure hard to be erased at high temperature that effectively solved the shortcomings of type I FBG.At present,FBGs for high-temperature applications are mostly fabricated by femtosecond lasers,and the fiber materials of FBGs are gradually changing from traditional quartz fibers to emerging sapphire crystal fibers.In recent years,sapphire fiber has become the focus of research for sensing applications in ultra-high temperature and harsh environments.The sapphire crystal fiber has stable physical and chemical properties and a high melting point(over 2000 ?).Compared with ordinary quartz fiber,it has a better application prospect in the field of high temperature sensing.However,the preparation of sapphire crystal fiber is not easy.High price,no single-mode transmission and no cladding to create total reflection limits the accuracy of high-temperature sensing.Some researchers have studied and made improvements for the shortcomings of sapphire crystal fiber.They improve application performance of sapphire crystal fiber by growing cladding and reducing patterns.There are people who proposed to wrap a quartz tube on a crystal fiber to obtain an active fiber with crystal core-quartz cladding structure.They successfully fabricated YAG crystal core fiber and sapphire crystal core fiber by this method.The advantage of this technology is that the good optical properties of the original crystal fiber are maintained and easily fused with the standard quartz fiber for having the structure of quartz cladding.This paper aims at solving the strain sensing at high temperature with the method of fiber Bragg grating fabricated by femtosecond laser technology and we tested the high temperature and strain sensing performance of the fiber Bragg gratings fabricated by different kinds of optical fibers.The research contents and results of this paper are as follows:(1)Using the femtosecond laser direct writing technology,the Bragg grating is written in the quartz fiber,the pure quartz core fiber,the sapphire crystal derivative fiber,and the YAG crystal derivative fiber.Quartz fibers and pure quartz core fibers are commercial fibers,which strictly conform to the standard of single mode fibers.The sapphire crystal derivative fiber and YAG crystal derivative fiber are fabricated by rod-in-tube technique.The quartz sleeve is made by a quartz tube as cladding and a crystal fiber as core and then the fiber is fabricated by a drawing tower.The diameter of the fiber is about 10-12 ?m.However this fiber is not a standard single mode fiber,but a few-mode fiber.(2)For all kinds of fiber Bragg gratings,the performance of high temperature and strain sensing is tested.The testing method is combined with a testing machine and a high temperature furnace.First,the performance of temperature is tested,and then the performance of high temperature strain is tested.The test results are characterized in this paper. |
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