微结构光纤光栅的理论、实验与应用研究

【作者】 金龙；

【导师】 董孝义；

【作者基本信息】 南开大学 ， 光学， 2008， 博士

【摘要】 光纤光栅作为最重要的光纤器件之一,在光通信和光传感领域发挥着重要作用。近年来,微结构光纤的出现和迅猛发展,为光纤光栅技术的进一步发展提供了新的平台。通过对光纤微结构的合理设计和光纤光栅实现方式的创新,在微结构光纤上制作的光纤光栅将可能具有新颖的光谱特性和应用价值。另一方面,在微结构光纤中进行光纤光栅的写制是分析微结构光纤模式特性和色散特性的重要实验手段。本论文阐述了微结构光纤光栅的模式耦合理论,探索出一整套在多种微结构光纤中制作光纤光栅的工艺,在折射率引导型光子晶体光纤、全固态光子带隙型光纤、Bragg光纤等多种微结构光纤中成功写制了光栅,并对它们的光谱特性、耦合方式和应用前景作了详细分析,主要内容包括:1.基于光纤光栅的耦合模理论和微结构光纤的模式特性,建立了微结构光纤光栅的理论分析方法,这为我们在理论上解释各种微结构光纤光栅的光谱特性和模式耦合建立了基础。2.探索出一套在多种微结构光纤中写制光栅的工艺,包括:微结构光纤与单模光纤的低损耗熔接技术、微结构光纤的载氢处理、紫外写制工艺的改进、超连续光源的搭建等。3.在国内首次报道了在国产柚子型光纤中写制的光纤Bragg光栅(FBG)。在实现了该光纤与单模光纤低损耗熔接的基础上,利用相位模板法在这光纤上成功写制了Bragg光栅,光栅深度达30dB以上;在这种光纤上写制了倾斜光栅,观察到多个包层模谐振峰;在倾斜光栅中填充高折射率匹配液,观察到了包层模谐振的减弱。4.在国内首次报道了在多层空气孔的光子晶体光纤中写制的Bragg光栅。通过载氢处理提高了光子晶体光纤的光敏性,以抵消空气孔散射对光栅写制效率的影响。在纤芯高掺锗、10层空气孔的光子晶体光纤中制作的光栅具有两套高阶模谐振峰,且观察到了深度达10dB的强包层模谐振峰。为了提高写制效率,利用双光子吸收法在这种光纤中写制光栅,在未载氢的情况下,180秒内即可完成光栅的写制。基于两种不同的光子晶体光纤中的Bragg光栅,分析了微结构光纤各结构参数与掺杂纤芯与纯石英的折射率差对光栅光谱特性、光栅写制效率和折射率调制可见度等方面的影响。5.利用193nm波长的ArF准分子激光器进行了在纯石英折射率引导型光子晶体光纤中写制Bragg光栅的尝试,并观察到一个较弱的反射峰。6.在国际上首次报道了在全固态光子带隙光纤中写制的Bragg光栅。这种光栅的折射率调制区域分布在光纤包层的高折射率柱阵列内,我们不仅观察到光纤基模向反向基模的耦合,还观察到基模向传导超模的耦合。基于各超模在高折射率柱阵列中相位关系和模式的对称性,揭示了各谐振峰产生的原因,指出只有某些满足特定相位关系的超模对基模-超模谐振具有贡献。在理论和实验上分析了光栅写制过程中的动态过程,包括带隙的漂移和各谐振波长的变化。分析了光栅的弯曲特性,发现了由于光栅折射率调制的不均匀引起其弯曲响应与弯曲方向有关的特性。此外,还在全固光子带隙光纤中写制长周期光栅,给出了基模-传导超模耦合的相位匹配曲线,并对其温度特性进行了分析。7.在全固态Bragg光纤中写制的Bragg光栅,观察到光纤正反向基模之间的耦合。这一实验结果为下一步设计基于Bragg光纤FBG的折射率传感器奠定了基础。在纤芯小于1μm的Bragg光纤中写入光栅,根据对应的透射谱发现,由于纤芯太小,光纤中模场以无法被光子带隙效应限制,而是转化为折射率引导。8.基于空芯Bragg光纤和弱折射率引导型微结构光纤中的FBG,分别提出折射率传感器结构,该传感器在保证原有光纤对填充样品的探测能力的基础上,实现了对样品折射率的测量。定量分析了传感器灵敏度、模场能量与样品交叠等主要参数随填充样品折射率的变化关系。更多还原

【Abstract】 Fiber grating technologies in step-index fibers have developed dramatically to perform an important role in fiber communication and optical fiber sensing in more than 15 years. Recently, the introduction of microstructured optical fibers (MOFs) with a wide range of novel properties results in unique spectral characteristics of fiber Bragg gratings in these fibers. Fiber gratings over MOFs can probably present promising potential applications by designing the fiber microstructure and developing new methods to produce periodic index or permanent structural modulation along the fiber length. On the other hand, fiber grating fabrication in the MOFs has been considered as an effective approach to probe the modal properties and dispersion slope of an MOF.This dissertation describes the theory of mode couplings of the fiber gratings in MOFs and the processes of the fabrication of fiber gratings over various MOFs, based on which fiber gratings over grapefruit fibers, index-guided photonic crystal fibers (PCFs), photonic bandgap fibers (PBGFs) and Bragg fibers are fabricated successfully. The spectral characteristics, mode couplings and potential applications are illustrated as well.The contents of the dissertation include the followings in detail:1. The theory of fiber gratings over MOFs are described based on coupled mode theory (CMT) and the modal properties of the MOFs, based on which the spectral characteristics and mode couplings of fiber gratings over various MOFs can be explained.2. The fabrication processes of fiber gratings over MOFs are developed, including: fusion splice the MOFs with single mode fibers, hydrogen loading of the MOFs, improvement of UV exposure, the implementation of a supercontinuum light source which covers over 1000nm spectrally, etc.3. Fabrication of fiber Bragg gratings (FBGs) in home-made grapefruit fibers is first reported domestically. Low splice loss between this MOF with singlemode fibers is realized. FBGs with depths over 30dB are achieved by standard phase mask method. Tilted FBGs are inscribed in the MOF, which presents multiple cladding mode resonance peaks. High-index gel is infused into the holes of the MOF, which reduces the strengths of the cladding mode resonances by an extent.4. Fabrication of FBGs in a germanosilicate PCF with 10 layers of air holes is first reported domestically. Since the air hole lattice decreases the FBG fabrication efficiency, the PCF is loaded in high-pressure hydrogen in advance to improve the photosensitivity of the PCF. Two series of cladding mode resonance peaks are obtained, which correspond to cladding modes which distribute in pure silica region and air/silica region, respectively. An enhanced cladding mode resonance peak as deep as 10dB is observed. Moreover, FBGs can be inscribed in this PCF within 180 seconds with a 193nm ArF excimer laser, without hydrogen loading. We analyze the influences of structural parameters and index distribution of fiber microstructure on the spectral characteristics of FBGs in index-guided PCFs, grating inscription efficiency and the visibility of the index modulation, based on the fabrication of FBGs into two different PCFs.5. FBG is inscribed into a Ge-free air/cladding PCF, by use of the 193nm ArF excimer laser, by observing a weak reflection peak.6. FBGs inscribed in the cladding high-index rods of all-solid photonic bandgap fibers are first reported. Not only the guided mode resonance, but also supermode resonances are observed. We demonstrate that only those supermodes with certain phase relationships and symmetric mode field profiles are responsible for the supermode resonances. The dynamics of the FBG growth in this kind of fiber, including bandgap shift and the variations of the resonance peaks are described theoretically and experimentally. The bend response of the FBG is investigated, which is direction-dependant because of the asymmetric index modulation over the cladding rods. Long period gratings (LPGs) are fabricated in all-solid PBGFs as well. Phase-matching curves are demonstrated and its temperature response is investigated.7. FBGs are inscribed into solid-core Bragg fibers and the fundamental mode coupling is obtained. We fabricated FBGs in such a fiber with a core diameter less than 1μm as well. According to its transmission and reflection spectra, we found that the way of guidance of the fiber is index-guiding but photonic bandgap.8. Two refractive-index sensors based on fiber Bragg gratings inscribed in hollow-core Bragg fibers and weak index-guided microstructured optical fibers are proposed, respectively. The sensors can measure the loss spectrum and the refractive index of the filled material simultaneously. The relations between the measurement resolution, air-filling factor and the refractive index of the material are calculated.更多还原