【作者】 陈超；

【导师】 孙洪波； 于永森；

【作者基本信息】 吉林大学 ， 物理电子学， 2014， 博士

【摘要】 苛刻环境广泛存在于工业、国防、航空航天等诸多领域，这包括高温、高压、强辐射、强电磁干扰等，如何在该极端条件下实现精确、瞬时的物理量感知和测量是横亘在科学界和工业界上的难题。光纤光栅作为光纤传感领域的一支生力军，因其具有无源、抗电磁干扰、插入损耗低、波长选择性好以及可实现远程遥感和分布式传感等优点，为该问题的解决提供了契机。光纤Bragg光栅（FBG）的传统制备采用UV激光刻写技术，其诱导的折射率变化（仅为10-4~10-3量级）强烈依赖于光纤光敏性，并在温度高于400℃时极易被擦除/漂白，这限制其在冶金、石油开采、火力发电、核能等苛刻环境中的传感应用。因此，迫切需要一种耐高温的“极端”FBG，已有的成栅方法有高温再生技术和超快激光加工技术，前者形成的再生光栅存在反射率低、光谱特性差、损耗大等缺点，这会增加信号探测难度并影响分辨率。超快激光微加工技术已被证明可以在任何透明电介质（石英、硅酸盐玻璃、非线性晶体等）中诱导永久性折射率变化（10-3~10-2量级），与材料的光敏性和化学构成无关，这归因于高能超快脉冲的高非线性效应，该技术为耐高温FBG的制备带来了崭新的生机。本文既是以飞秒（fs）激光微加工技术为手段，以制备耐高温FBG为目标，旨在解决苛刻环境传感问题。本文主要研究内容和成果如下：（1）探讨了光纤光栅的耦合模理论，分析了均匀FBG和TFBG中模式之间的耦合特性。分析了影响光栅反射率、带宽的因素，包括光栅长度、直流和交流折射率变化、交流耦合系数等，并对FBG的反射谱进行理论仿真。阐述了飞秒激光诱导透明电介质折射率变化的主要机制——高非线性效应。阐述了相位掩模板刻写技术和逐点刻写技术，探讨了光栅刻写涉及的关键问题：如何通过优化的掩模板获得纯正双光束干涉、如何精确地控制聚焦位置等。（2）利用飞秒激光的高非线性效应和高阶相位掩模板（pm=3.33m）制备了均匀高阶FBG和非均匀高阶TFBG，其中高阶TFBG属首次报道，它们均具有高达1000℃的高温稳定性。提出利用单一高阶FBG实现苛刻环境中的轴向应变和温度的辨别。对于高阶TFBG，研究了同级Bragg衍射对应同阶包层模（m-LPvu）谐振的传感特性；通过溅射金反射镜制作了反射型TFBG传感器，并提出基于相对峰值功率-波长解调技术实现应变和高温（800℃）辨别。（3）基于飞秒激光逐点刻写技术的灵活性制备了高局域化FBG，包括低阶和高阶Bragg光栅结构，由于高局域化折射率调制引起的高方位角光纤模式的耦合，其特征光谱呈现了宽带高阶包层模式谐振特性。证实了一种具有多反射峰特性的新颖的类啁啾高阶FBG。多反射峰以准周期或组（系列）的形式分布于1200~1700nm的超宽频范围，其适宜于苛刻环境的多参数传感。并利用9阶类啁啾FBG实现了多波长光纤激光器。（4）利用飞秒激光结合相位掩模扫描技术在单晶蓝宝石光纤（直径250m）中刻写了高阶SFBG。提出石英光纤与蓝宝石光纤直接熔接耦合技术，理论模拟了蓝宝石光纤中多导模激励特性和多模式谐振光谱。证实了SFBG至少可用于高达1600℃苛刻环境中传感应用。本文利用飞秒激光制作传统均匀和非均匀FBG的同时，利用逐点刻写技术的灵活性设计和制作了具有特殊结构、新颖光谱特性的高局域化FBG，这拓展了光纤光栅的功能性。Fs-FBG不仅能够用于苛刻环境传感，亦在高功率和多波长光纤激光器领域展现了广阔的应用前景和商业价值。更多还原

【Abstract】 Harsh surroundings are widely existed in industry, national defense, aviation area, this includes high temperature, pressure, strong radiation and electromagnetic interference, et al. How to realize precise, transient physical quantity sensing and measurement in harsh condition is a problem that blocks the way of science and industry.As a fresh troop in fiber sensing area, fiber gratings have the merits of passivity, anti‐electromagnetic interference, low insert loss, good wavelength selectivity and can realize long haul and distributed sensing that provide an opportunity for solving the above problems. The traditional fabrication of fiber Bragg gratings (FBG) is based on UV laser writing technology, the induced refractive index (10‐4‐10‐3) strongly depends on the photosensitivity of the fiber, which is readily erased when the temperature is more than400℃, limiting its sensing applications in hash environment, such as metallurgy, oil exploration, thermal power generation and nuclear energy. Therefore, the need for a high‐temperature resisting “extreme” FBG is urgent. The existed fabrication method includes high temperature re‐generated method and ultrafast laser machining method, the formed re‐generated gratings of the former have the defects of low reflectivity, bad spectrum characteristics and big loss that will increase the signal detecting difficulty and affects the resolution. The ultrafast laser micro‐machining technology have been proven to induce perpetual index changes (10‐3‐10‐2) in any transparent dielectrics (quartz, silicate glass, non‐linear crystal) that has nothing to do with the photosensitivity and chemical composition, which is ascribed to the high non‐linear effects of the high‐powered ultrafast pulse. This technology brings new vitality for the fabrication of high temperature resisting FBG.This paper is based on the method of femtosecond laser (fs) micro‐machining and aimed at fabricating high temperature resisting FBG to solve the harsh environment sensing problems. The study contents and achievements of the paper is listed as bellows:(1) Discussed the coupled mode theory of fiber gratings, analyzed the coupling characteristics among modes of uniform FBG and TFBG. Analyzed the factors that influence the grating reflectivity and bandwidth, which contain grating length, DC and AC index changes and AC coupling coefficients, then conducted the simulation to the reflection spectrum of FBG. Described the main mechanism of femtosecond laser induced index modulation to transparent dielectric medium, which is the high nonlinear effects. Described the phase mask and point‐by‐point writing technology and discussed the main problems of gratings writing: how to get pure double beam interference with the optimized phase mask and how to control the focal position precisely.(2) Fabricated the uniform high order FBG and non‐uniform high order TFBG by means of the high nonlinear effects of fs laser and high order phase mask(pm=3.33m), among which the high order TFBG was reported for the first time, they all have the high temperature stability up to1000℃. Realized the discerning of axial strain and temperature in harsh environment with single high order FBG. For high order TFBG, studied the sensing characteristics of the same order cladding mode (m‐LPvu) resonances corresponding to the peer Bragg diffraction; fabricated the reflective TFBG sensor by sputtering the gold reflective mirror, proposed the discerning of strain and high temperature (800℃) based on relative peak power‐wavelength demodulation technology.(3) Fabricated the high localized FBG based on the flexibility of fs laser PbP technology, which contains the low order and high order Bragg gratings structure, because the coupling of the high azimuth angle fiber mode caused by high localized index modulation, its characteristic spectrum showed wideband high order cladding mode resonance. Confirmed a kind of novel quasi‐chirped high order FBG that has multi‐reflective peaks. Multi‐reflective peaks distributed in the ultra‐wide frequency range of1200~1700nm in the form of quasi‐period or groups (series), which is suitable for the multi‐parameter sensing in harsh environment. Realized the multi‐wavelength fiber laser by using the9order quasi‐chirped FBG.(4) Inscribed high order SFBG in Single‐crystal sapphire fiber (250m) by means of fs laser combined to phase mask scanning technology. Proposed the direct fusing coupling method between the silica fiber and sapphire fiber, simulated the multi guiding mode stimulating characteristics and multi‐modes resonance spectra. Proved SFBG can be used in the sensing application in harsh environment at least as high as1600℃.This paper described the fabrication of the traditional uniform and non‐uniform FBG, at the same time, we devised and fabricated highly localized FBG with special structure and novel spectral characteristics taking advantage of the flexibility of PbP technology, which expanded the functionality of fiber gratings. Fs‐FBG can not only be used to sense in harsh environment, but also shows wide application prospect and commercial value in high power and multi‐wavelength fiber laser area.更多还原