| With the continuous development of technology and industry,people are also facing a harsher application environment while pursuing the goal of faster,higher and stronger.In the fields of metal smelting,energy exploration,aerospace and other fields,there are many types of harsh environments such as high temperature and high pressure,strong radiation,strong chemical corrosion,and strong electromagnetic interference.How to realize the accurate monitoring of parameters and the health assessment of equipment in this extreme environment is an important problem to be solved urgently in various related fields,and it also promotes the continuous development of sensing technology.In the sensing and monitoring of the above-mentioned harsh environment,optical fiber sensors have incomparable advantages over electronic sensor due to its advantages of high temperature resistance,electromagnetic interference resistance and easy multiplexing and networking.Optical fiber sensors represented by silica fibers have played an important role in the past few decades and ultrafast laser processing technology provides a prerequisite for the preparation and application of high temperature fiber gratings.However,due to the element diffusion and low softening point of doped quartz fiber at high temperature,it is difficult to apply it to high temperature environment above 1000 ℃ for a long time.Single crystal sapphire fiber has been widely used in ultra-high temperature fiber sensing due to its high melting point(2053 °C),excellent optical properties,and stable physical and chemical properties.However,because sapphire fiber has no cladding,high refractive index and large mode field diameter,it is highly multimodal and the surface is easily contaminated,which puts forward higher requirements for the preparation and further applications of gratings based on sapphire fiber.Based on the above-mentioned problems and challenges in the development of quartz fiber and sapphire fiber,this thesis takes femtosecond laser processing as the main technical method to improve the stability and tolerance of optical fiber sensor in extremely harsh environment.The main research contents and results are as follows:1.The theoretical model of uniform Bragg grating is deduced by the coupled mode theory of fiber grating,and the characteristic parameters affecting the spectral quality are determined.The spectrum of uniform Bragg grating is simulated and analyzed by MATLAB software,and the effects of different parameters on spectral reflectivity and wavelength are explored.The nonlinear interaction mechanism between femtosecond laser and transparent materials is analyzed.The generation and relaxation mechanism of plasma are discussed in terms of physical process and time scale.The relationship between the bandgap energy of germanium-doped quartz fiber and the laser photon energy is analyzed,and the influencing factors and optimization methods of Gaussian beam focusing characteristics and machining accuracy are analyzed.2.The Bragg grating with a reflectivity of 15%,a full width at half maximum of5.72 nm and a signal-to-noise ratio of 10 d B is fabricated in a 60 μm diameter sapphire fiber by femtosecond laser line by line scanning method.In addition,the helical Bragg grating with high structural symmetry is prepared in sapphire fiber by femtosecond laser helical scanning method.It is a true three-dimensional volume grating,and three gratings with different wavelengths cascaded together are further fabricated.When the helical diameter is 30 μm,the spectral reflectance is 40%,the full width at half maximum is 1.56 nm and the signal-to-noise ratio is 16 d B.The bending test shows that the helical Bragg grating has good bending resistance,and can still maintain good spectral quality when the minimum bending radius is 5 mm.The high temperature and strain response characteristics of the sapphire fiber Bragg grating prepared by the line by line scanning method and the helical scanning method are tested and analyzed respectively.The temperature test is up to 1600 ℃ and the stress test is up to 1.5 N.The results show that the prepared grating has good high temperature stability and sensitivity.The temperature sensitivity in the high temperature region is 35 pm/℃ and the strain sensitivity is 1.45 pm/με(@1600 ℃).The preparation method of crystal fiber,the physical and chemical properties of sapphire fiber and the cause of transmission loss are summarized and analyzed.By optimizing the fusion splicing parameters,the stable coupling between sapphire fiber and quartz fiber is realized and the different treatment methods of sapphire fiber end face are compared.The sapphire fiber is cut at different angles of 0°,8° and 45° by using the femtosecond laser galvanometer scanning system.3.The Bragg grating with a reflectivity of 15% are fabricated in parallel in the sapphire fiber with a diameter of 60 μm and a length of 8.5 mm by a femtosecond laser point by point method.Combining the advantages of good compatibility and low loss of quartz fiber and high melting point and large thermal optical coefficient of sapphire fiber,the sapphire fiber probe sensor is prepared by integrating them together.The temperature sensitivity in high temperature region can reach 30.19 pm/℃,which is more than twice higher than that of quartz fiber gratings.The end-face mode fields of sapphire fiber with different lengths are tested and analyzed by a beam quality analyzer,and the results show that the high order mode is easier to be excited with the increase of sapphire fiber length.At the same time,the antireflection spherical lens structure is prepared at the end of sapphire fiber to effectively improve the signal-to-noise ratio to 22 d B.The stability of the sensor at 1200 ℃ and 1300 ℃ is tested and analyzed.The results show that the sensor can work stably at 1200 ℃ for a long time(more than 24 hours)and at 1300 ℃ for a short time.4.A method of energy apodization is proposed to prepare apodized grating efficiently.The method adopts the combination of half wave plate and polarization splitting prism to realize the real-time regulation of laser energy through stepping motor and host computer software.The gratings with reflectivity of 75% and side-mode suppression ratios of 25 d B and 32 d B are fabricated by this method in standard single-mode fiber and thin-core fiber respectively.The high temperature(800 ℃)test shows that the energy apodized gratings have good high temperature stability and is Type II gratings.5.The sapphire crystal-derived fiber and YAG crystal-derived fiber are prepared by the fusing core method respectively,and their end face element composition and refractive index distribution are characterized.The results show that these two crystal derived fibers are few mode fibers with gradual distribution of core refractive index.The uniform FBG and apodized FBG are prepared in these two crystal derived fibers by femtosecond laser point by point method,and their high temperature and strain response characteristics are tested and analyzed.The test temperature is up to 1000 ℃,the temperature sensitivity is 15.64 pm/℃ and 13.74 pm/℃ respectively,and the strain sensitivity is 1.33 pm/με and 1.25 pm/με respectively.This thesis aims to prepare high temperature resistant fiber gratings for high temperature and harsh environment.The research is carried out from the aspects of fiber grating structure preparation,technology optimization,sensor packaging,special fiber drawing and sensing characteristic analysis.Combined with femtosecond laser processing technology,the preparation of high-temperature resistant fiber gratings is realized.The prepared fiber devices and the adopted technology can not only be used in high-temperature and harsh environment sensing and detection,but also in high-power fiber laser,which further expands its application value and scope. |
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