【作者】 朱涛；

【导师】 饶云江；

【作者基本信息】 重庆大学 ， 光学工程， 2008， 博士

【摘要】 2000~2003年,笔者所在实验室针对高频CO2激光单侧写入长周期光纤光栅(LPFG)的光学特性和相应器件进行了初步研究,取得的成果得到了国内外专家的初步认可。本文在此基础上进行更深一步的拓展研究,其基本思想是在初步完善非对称折变型LPFG模式耦合理论分析方法和高频CO2激光写入LPFG的成栅机理分析和测量基础上,按照提高环境响应灵敏度、克服传感参量交叉敏感和实现多参数测量的总体思想,比较系统的研究特种LPFG的光栅结构、折变模型、写入方法、传输特性、环境响应特性,以及潜在应用等。论文主要内容如下:①论文按照横截面折变非对称分布的相对大小将光栅横截面分割成多层圆波导,并将各圆环层按周向折射率大小不同进行离散分块,再基于多层圆波导理论和数值计算方法来近似计算光栅各个模式的模场,最后利用模式耦合方程和修正的布拉格条件来讨论这种非对称折变型LPFG的模式耦合特征。非对称LPFG模式耦合数值分析方法的完善为特种LPFG的探索提供了理论基础。②论文研究认为残余应力释放、快速固化、光纤致密化和熔融变形是高频CO2激光写入法能成栅的主要原因,但采用不同写入方法和不同的激光写入能量在不同的光纤类型上制作光栅时其重要性不同。论文测试了激光在光纤上不同作用位置和不同辐射能量所引起的光纤纤芯或包层的折射率平均变化量。这些研究为实现特种LPFG结构的写入提供了实验制作的基础。③在国际上首次提出并利用高频CO2激光写出了边缘折变型LPFG,并比较系统的研究了这类光栅的模式耦合特征和传输特性,最后对这类光栅的部分环境响应特性进行了实验研究,结果表明:(1).由于边缘折变型LPFG折变主要发生在包层,因此环境参量的变化很容易改变包层模场的分布,从而提高了谐振峰对环境参量的响应灵敏度。比如环境折射率响应灵敏度相比普通LPFG提高了约4倍、通过形成周期缺槽型LPFG可将应变灵敏度提高到? 100 pm /με(在0 ~100με的应变范围内); (2).边缘折变型光纤光栅具有非常强的方向耦合性,可基于此设计旋转型光开关等光通信器件; (3).边缘折变型LPFG具有较高的偏振相关损耗(PDL),可通过多边写入的方法降低PDL,最小可达0.22dB。④在国际上首次提出并利用高频CO2激光制作出了旋转非对称折变型LPFG (R-LPFG),并比较系统的研究了这类光栅的模式耦合特征和传输特性,最后对这类光栅的部分环境响应特性进行了实验研究,结果表明:(1).当光栅旋转度逐渐变大时,R-LPFG单一的谐振峰会逐渐分裂成两个,这是与以往所有光栅类型所不同的透射谱特征; (2).从R-LPFG单个谐振峰分裂出来的两个分裂峰的温度灵敏度几乎相同; (3). R-LPFG两分裂峰具有极性相反的应变灵敏度,特别的,缺槽型R-LPFG两分裂峰的应变灵敏度绝对值可达～50 pm /με(在0 ~ 600με的范围内);(4).出现分裂峰的R-LPFG,同向扭曲时,两分裂峰逐渐靠拢,最后合成一个峰;反向扭曲时,两分裂峰逐渐分离。(5).可利用R-LPFG两分裂峰的间距实现无温度补偿的应变、扭曲等物理量的测量。⑤在国际上首次提出非对称折变型超长周期光纤光栅(ULPFG)的结构,并提出利用空间频率的思想来讨论这类光栅的模式耦合特征和传输特性。文中还将边缘折变和旋转折变思想应用于ULPFG中从而形成了边缘折变非对称型ULPFG (E-ULPFG)和旋转折变非对称型ULPFG(R-ULPFG)。在利用高频CO2激光写出这些光栅结构后实验研究了它们的部分环境响应特性,结果表明:(1). ULPFG具有比LPFG更多的谐振峰。(2).由于ULPFG各谐振峰的环境响应特性与实际发生耦合的子光栅级次和包层模阶次密切相关,因此ULPFG每个谐振峰对环境参量如折射率、扭曲等的灵敏度都不一样。(3). E-ULPFG可以进一步加强ULPFG的光学特性,比如边缘缺槽型ULPFG可以实现应变和折射率的高灵敏度测量等;(4).可以利用R-ULPFG各组分裂峰的间距实现无温度补偿的应变、扭曲等物理量的多参数测量等。⑥论文基于特种LPFG实验研究了部分适用于光纤通信或传感领域的新器件:(1).具有方向相关性的光栅型耦合器; (2).利用R-LPFG实现了EDFA的动态增益均衡器;(3).利用ULPFG实现了温度和折射率的同时测量;(4).利用R-LPFG实现了无温度补偿的应变、扭曲等物理量的测量;(5).利用ULPFG等特种光纤光栅实现了温度、应变、扭曲三参数的同时测量; (6).论文提出利用缺槽型E-LPFG和R-LPFG设计可调谐带阻滤波器;(7).利用具有特殊透射谱的LPFG和微加工制作高性能带通滤波器等。论文的主要创新点为:①.独立探索了三大类特种LPFG结构,从而丰富了LPFG的结构体系;②.比较系统的研究了三类特种LPFG的耦合特征、传输特性、环境响应特性等,为光纤传感和通信领域提供了若干可供选择的全光纤型器件;③.基于特种LPFG设计了几种光纤传感和通信用基础器件。通过本论文的研究,笔者已将部分科研成果发表在《Optics Letters》、《IEEE Photonics Technology Letters》、《Applied Optics》、《Optics Communications》、《Electronics Letters》、《Chinese Physics Letters》、《物理学报》等国际、国内著名期刊上,到目前为止以第一作者(或导师第一,学生第二)发表SCI论文13篇,申请发明专利5项,授权1项。更多还原

【Abstract】 The optical characteristics and corresponding devices based on side-written long period fiber grating (LPFG) induced by high frequency CO2 laser pulses were studied in our research group during 2000 to 2003. This thesis is an extension of the above mentioned basic study. In order to strengthen the environmental response sensitivities, overcome the cross-affections of sensing and realize the multi-parameters measurement, three types of specific structures of LPFG are explored and proposed, including the model of refractive index, the theoretical analysis of transmission spectrum, fabrication methods, environmental response characteristics and potential applications. The main contents are as follows:1) The transverse refractive index distribution of LPFG with asymmetric refractive index modulation in the cross-section is divided into multilayer circular waveguides, which are then discretized according to the magnitude of index. The mode field of LPFG can be approximately calculated by making use of multilayer circular waveguide theory and numerical method. The mode coupling characteristics of this asymmetric LPFG is discussed by adopting coupled-mode equation sets and the revised Bragg condition. The numerical method of mode coupling for asymmetric LPFG provides theoretical foundation for the exploration of special LPFGs.2) The release of residual stress, rapid solidification, fiber densification and fused distortion are discovered to be the main mechanism of forming gratings induced by CO2 laser, which plays different role for different fiber, scanning method and laser power. The core/cladding index change caused by different inducing position and laser power is measured in this thesis. It provides the basis of experimental fabrication for special LPFGs.3) The structure of edge-written LPFG (E-LPFG) is proposed and fabricated by using high frequency CO2 laser pulses, for the first time to our knowledge. The mode coupling features are investigated and environmental response characteristics are studied experimentally. It’s indicated that: (1).The environmental response sensitivities of E-LPFG with refractive index modulation in the fiber cladding were enhanced because the cladding mode fields were easily affected by the change of the environmental parameters. For example, the environmental index response sensitivity is increased by a factor of 4 compared with general LPFG, and the strain sensitivity of LPFG with periodical grooves si up to ? 100p m /μεfor0 ~100με. (2). E-LPFG has strong orientated coupling behavior, based on which optical communication devices, such as rotated optical switch, can be designed. (3).The PDL of E-LPFG is the greatest, and then is the double-edge-written LPFG. The multi-edge-written grating has the least PDL, which can be as less as 0.22dB.4) The structure of asymmetric LPFG with rotary refractive index modulation (R-LPFG) along the fiber axis is proposed and fabricated by using high frequency CO2 laser pulses, for the first time to our knowledge. The mode coupling features are investigated and environmental response characteristics are studied experimentally. It’s observed that: (1) The transmission spectrum has different features according to the rotation angle. For small angle, resonance peak is unique, however, when the angle becomes greater, the resonance peak will be split into two, which is different from the transmission spectrum of previous all LPFGs. (2). For R-LPFG with greater rotation angle, the two split peaks have almost the same temperature sensitivity. (3). For R-LPFG with greater rotation angle, the two split peaks have opposite wavelength shift direction when it was applied strain. Besides, the sensitivity of R-LPFG with rotary grooves along fiber axis can be up to～50 pm /μεfor0 ~ 600με. (4). For R-LPFG with greater rotation angle, When co-twisted, the two peaks keep close and combine into one peak finally; when counter-twisted, the two peaks walk off. (5). The high sensitivity torsion meter and strain meter without temperature compensation can be designed by adopting the wavelength spacing between the two split peaks to measure the environmental variables.5) For the first time, the structure of asymmetric ultra long period fiber grating (ULPFG) is proposed to our knowledge. Also, the method of combining sub-fiber-gratings linearly with different frequency is firstly proposed. Two new grating structures of ULPFG--asymmetric edge-written ULPFGs (E-ULPFGs) and ULPFGs with rotary refractive index change are formed (R-ULPFGs). The above three ULPFGs are induced by high frequency CO2 laser and their environmental response characteristics are studied systematically. It shows that: (1). ULPFG has more resonance peaks than general LPFGs in that core mode can couple with more cladding modes. (2). Because the environmental response characteristics of each resonance peak has close relationship with the coupled level of sub-fiber gratings and the order of cladding modes, the environmental index and torsion sensitivity of ULPFG are different for each resonance peaks. (3). Based on the features of E-ULPFGs, the optical features of ULPFG can be strengthened, for example E-ULPFGs can be used to measure strain and index with high sensitivity. (4) Based on the features of R-ULPFGs, the variables such as strain and torsion can be measured by using the spacing of split peaks.6) Based on the specific LPFGs, new devices partly applicable to optical fiber communciation or sensor area are studied. (1). Grating coupler is produced by using two special LPFGs. (2). The EDFA gain equalizer is produced by using R-LPFG. (3). The temperature and index can be measured simultaneously by using ULPFG. (4). For R-LPFG, the strain and torsion measurement without temperature compensation can be realized by using R-LPFG with greater rotation angle. (5). The temperature, strain and torsion measurement can be realized simultaneously by using ULPFG. (6). In addition, adjustable band rejection filter can be designed by using E-LPFG or R-LPFG with periodical grooves along the fiber axis. (7). The bandpass filter with high performance can be devised by adopting LPFGs with specific transmission spectrum and micro-processing methods.更多还原