光子晶体光纤中，纤芯和包层没有严格的分界面。在采用有效折射率模型计算时，纤芯半径的选取具有一定的任意性。本文以色散的计算为例，通过有效折射率模型和多极化模型计算结果的对比，给出了有效折射率模型中纤芯选取的原则。

There is no clear boundary between the core and the cladding in the photonic crystal fiber. So, there is some random in the determined the radius of the core. In this paper, the rule to select the radius of the core is given according to the comparative calculation between the effective index model and multipole method.

光纤结构设计、模间色散求解、光纤光栅模式耦合等问题的研究，都需要对光纤纤芯及包层模的有效折射率进行精确计算。本文以光纤三层结构模型为基础，结合该模型下的模式本征方程，使用截弦法求解了纤芯模有效折射率，并将计算结果与COMSOL软件模拟的对应纤芯模的传输光场进行对比，验证了计算结果正确。使用区间遍历算法对包层模有效折射率进行了求解，与已有的传统方法相比，该方法可以有效防止求解过程中根的遗漏、避免特征方程产生的奇点，并能保证模式的正规性。本文采用Mathematica软件对求解过程进行仿真，获得了纤芯模和包层模有效折射率与波长关系曲线。

The accurate computation of effective refractive index of optical fiber core and the cladding mode are needed for the fiber structure design, mode dispersion analysis and fiber grating mode coupling research. In this paper, based on three layered optical fiber waveguide model and its accurate eigenvalue equation, by using the cut string algorithm the effective refractive index of core mode is solved. At the same time the computation results are verified by using COMSOL software. By using interval traversal algorithm the effective refractive index of cladding mode is computed. Compared with the existing algorithms, this method can avoid the loss of root during the calculation. It can also avoid the generation of odd point and can keep the normality of each mode. The simulation is based on Mathematica software. The curves relating effective refractive index of core and cladding mode to wavelength are obtained.

高折射率棒光子带隙光纤受高折射率填充材料的影响很大，对温度、外加电场等参数非常敏感。为获得高灵敏度的光子晶体光纤温度传感，提出了在其包层空气孔注入温敏液体的方法，并利用全矢量有限元法对提出的方法进行了仿真。仿真结果表明，随着温度的升高，光纤的有效折射率减小，模场面积增大，温度灵敏度能达到20．2dB／m／℃。

Highly influenced by filler materials with high refractive index,PBG fibers with high-index rods are very sensitive to such parameters as temperature and applied electric field.In order to obtain temperature sensing of photonic crystal fibers with high sensitivity,we propose a method,in which temperature-sensitive liquid is inj ected into the air holes of their claddings,and simulate the method by using the full vector finite element method.The results indicate that with the increase of temperature, the effective refractive index decreases,the mode field area increases,and the temperature sensitivity can be as high as 20.2 dB/m/℃.

增益光纤的折射率和离子掺杂分布是决定光纤激光器输出功率和光束质量的重要因素,针对大模场光纤弯曲效应对模场面积和模场畸变的影响进行了数值分析,采用有限元方法计算了不同折射率和掺杂离子分布光纤的模场面积和增益系数。提出了高斯复合型折射率和掺杂离子分布的大模场增益光纤结构,该结构可有效提高增益光纤的增益系数和高阶模抑制系数,并具有较强的抗弯曲特性,较好的平衡了模场面积与抗弯曲特性的矛盾。根据计算结果设计了直径为65μm的高斯复合型折射率和掺杂分布的增益光纤,在波长为1.064μm的条件下,基模有效模面积达到1.17×103μm2,基模相对增益系数和高阶模相对抑制系数分别达到0.58和0.2088,有效地提高了光纤激光器和放大器的输出光束质量。

The gain fiber is the important indicators to determine the output power and beam quality of fiber laser. The paper focused on the large-mode-area fiber (LMA) bending effect on fundamental mode (FM) area and mode field distortion by numerical analysis, the FM area and gain coefficient of LMA fiber with different refractive index (RI) and doped distributions were solved by finite-element method. The Gaussian hybrid refractive index distribution and doping profile were proposed firstly. It can effectively improve the gain coefficient and suppression coefficient of high-order modes (HOMs). Moreover, the bending against of the fiber can be strengthen, the design can balance the contradiction the mode area and bending resistance. Based on the numerical calculation, the gain fiber with Gaussian hybrid RI distribution and doping profile were designed, the fiber diameter was 65μm. At the wavelength of 1.064μm, the FM area can be 1.17×103μm2, the FM relative gain coefficient and r

基于长周期光纤光栅(LPFG)包层有效折射率与包层半径、折射率和环境折射率的良好相关性,提出一种LPFG的新颖结构。利用传输矩阵法和三包层光纤的色散方程对其建模,mathcad15计算软件进行数值仿真和模拟。得到新型结构LPFG谐振峰发生分裂,即一个透射峰分裂为两个;两个分裂峰谐振波长间距随着腐蚀段包层半径的减小或填充材料厚度的增大而增加,模式越高增加越快;同时分裂峰间距在填充材料折射率小于1.4和大于1.48时基本不变,而在1.4和1.48之间分裂峰间距变化显著,在1.44附近达到极值。此种结构LPFG设计上的特殊性即可弥补半腐蚀LPFG容易断的不足,又可通过填充敏感材料且利用分裂峰间距定标而提高气体或液体浓度传感灵敏度。

A novel LPFG structure was proposed based on the excellent relation between effective refractive index of cladding,radius and refractive index of cladding and environment refractive index. A theoretical model was built using transmission matrix method and dispersion equation of three-layered fiber,and then numerical simulation was carried out by Mathcad15 software. The simulation results show that the resonance peak is divided into two peaks, and the gap between them is increased when decreasing the cladding radius of the etched section or increasing the thickness of the filling materials. The gap increase is more rapid for the higher order mode. Moreover, the gap between the divided peaks remains unchanged when the refractive index of the filling materials n is less than 1 . 4 and more than 1 . 48 . The gap change is remarkable when the refractive index is in the 1 . 4