长周期光纤光栅中基模到辐射模耦合的数值分析

我们根据耦合模理论和辐射模式理论针对长周期光纤光栅环境折射率高于包层折射率的情况建立了完整模型,并提出了一种新的数值方法求解辐射模耦合方程组,定量的分析了长周期光纤光栅中基模到辐射模的耦合特性,并总结了长周期光纤光栅透射谱特性随环境折射率的变化规律,与实验结果符合的很好.这种方法可以很容易推广到短周期布喇格光纤光栅的情况.     

腐蚀长周期光纤光栅温度稳定性研究

长周期光纤光栅在光纤通信和传感等方面应用广泛,温度稳定性是其在实际应用中一个十分关键的问题。本文分析了腐蚀长周期光纤光栅(LPFG)温度特性,通过测试包层半径分别为62.5μm5、0μm、40.25μm和34.5μm的长周期光纤光栅的温度特性,得到了光栅温度灵敏度随着包层模半径的减小及耦合包层模阶次的升高而灵敏度减小,提出了提高长周期光纤光栅温度稳定性的新方法。     

环境折射率对长周期光纤光栅谐振波长的影响

长周期光纤光栅是近几年出现的新型光纤器件,表现为前向传播的纤芯导模与同向传播的各阶次包层模在特定波长时的模式耦合,因此它在温度、应变、折射率传感方面比普通的光纤光栅有更大的优越性.运用耦合模理论,首先讨论了长周期光纤光栅的谐振波长,并模拟了其透射谱.其次在理论上详细分析了长周期光纤光栅对环境折射率的敏感特性,讨论了环境折射率敏感度因子与包层模次数的关系,并比较了不同环境折射率下的灵敏度.最后模拟了在环境折射率小于光栅包层折射率的情况下,谐振波长随环境折射率变化的规律.     

长周期光纤光栅耦合特性及模拟分析

本文采用简化的三层光纤模型,分析长周期光纤光栅的耦合特性,着重对包层模场分布及其与纤芯基模的耦合系数进行研究,运用计算机软件包Matlab进行建模计算,得出纤芯基模和一阶低次包层模的场、能量分布及其相互耦合规律.分析中应用了图解法和简化计算的方法,数值计算结果表明纤芯基模与一阶低次奇模的耦合要比其与一阶低次偶模的耦合强得多.该特征(耦合特性和图表)和国外报道相符.     

LPFG折射率模式响应及其在传感建模中的应用

基于光纤光栅的模式耦合理论,采用严格的数学模型,给出了三层介质长周期光纤光栅(LPFG)的纤芯模和包层模的本征方程,分析了长周期光纤光栅中的纤芯模式和包层模式的有效折射率随外界环境折射率变化的情形,以及外界环境折射率变化对长周期光纤光栅谐振波长的影响.通过在长周期光纤光栅外涂覆折射率对外界环境变化敏感响应的特定薄膜可以显著提高其传感测量的灵敏度.研究结果可为长周期光纤光栅传感器结构参数的优化设计和实际应用提供理论支持.     

长周期光纤光栅透射谱与结构参数关系的研究

采用光纤的3层模型,从模式理论和耦合理论出发,分析了长周期光纤光栅(LPG)一阶各次包层模和基模的耦合特性.通过分析指出,以往用来区分HF和EH模的2种方法是不等价的.利用传输矩阵法模拟均匀和非均匀情况下LPG的功率透射谱,分析光栅各结构参数对一阶低次包层模与基模耦合的影响.     

长周期光纤光栅透射谱仿真的新方法

通过对基于三层简化模型的长周期光纤光栅模式耦合机理的研究,提出了一种仿真长周期光纤光栅透射谱的新方法.该方法并不需要象传统的仿真那样需--求出各损耗峰的位置和带宽,而是直接将某波长下纤芯基模与各奇次包层模式耦合的透射率作为该波长的总的透射率并据此求出整个光栅的透射谱.模拟结果与已有的实验结果进行了对比,结果吻合得相当好.与其它方法相比,该方法直观、简单,编程、调试容易,具有强的可操作性.     

积分法求解长周期光纤光栅透射谱研究

基于光纤光栅的模式耦合理论,用德拜势能法对纤芯、包层内的电磁场进行矢量分析,得到纤芯及各包层中的各矢量场分量的表达式,求解出纤芯和包层模的耦合常数和有效折射率后,利用数值积分的方法通过求解耦合模方程得到光纤光栅的透射率。积分法是一种较为精确的求解透射谱方法,为长周期光纤光栅的优化设计和其在光纤通信、光纤传感领域的高精度测量提供理论依据。     

积分法求解长周期光纤光栅透射谱研究

基于光纤光栅的模式耦合理论,用德拜势能法对纤芯、包层内的电磁场进行矢量分析,得到纤芯及各包层中的各矢量场分量的表达式,求解出纤芯和包层模的耦合常数和有效折射率后,利用数值积分的方法通过求解耦合模方程得到光纤光栅的透射率。积分法是一种较为精确的求解透射谱方法,为长周期光纤光栅的优化设计和其在光纤通信、光纤传感领域的高精度测量提供理论依据。     

LPFG包层模对环境折射率的灵敏度分析

从理论和实验两个方面分析了长周期光纤光栅（LPFG）前3阶包层模的透射谱特性与光栅外包层厚度间的关系。LPFG的谐振波长漂移范围随着其外包层厚度的减小而扩大,同时,高阶耦合模LP04模的谐振波长漂移强于低阶模。研究表明,通过设计光纤光栅结构能改善其各阶包层模谐振峰对环境折射率的灵敏度。     

Numerical analysis of core-mode to radiation-mode coupling inlong-period fiber gratings

This letter presents, to the best of our knowledge, the first analysis of core-mode to radiation-mode coupling in long-period fiber gratings (LPGs) that are surrounded with dielectric material whose refractive index is higher than that of the cladding. We calculated core-mode transmission spectra through LPGs by integrating numerically the coupled-mode equations between the core mode and a continuum of radiation modes. We show calculated core-mode transmission spectra that exhibit such loss band continuums as have already been observed experimentally but not yet theoretically estimated     

Nonlinear property analysis of long-period fiber gratings usingdiscretized coupled-mode theory

We present generalized mathematical expressions for coupled-mode equations for nonlinear pulse propagation in fiber gratings using discretized coupled-mode theory and quantitatively analyze the nonlinear properties of long-period fiber gratings, considering multimode coupling between the core mode and several cladding modes. The calculations yield nonlinear responses for the case of long-period fiber gratings, including pulse shaping and all-optical switching in the self- and cross-phase modulation regimes. In addition, we briefly discuss the group delay properties of long-period fiber gratings and present several numerical examples of nonlinear pulse compression, which is related to strong dispersion and soliton-like behavior in fiber gratings     

Ultra-broadband optical filter based on chirped long-period fiber grating using leaky mode coupling

An ultra-broadband optical filter was proposed and demonstrated based on leaky mode coupling in a coated chirped long-period fiber grating (CLPFG). The CLPFG was coated with a material whose refractive index (RI) was higher than that of the fiber cladding, enabling the coupling of the core mode to leaky modes, to achieve a desired coupling efficiency. Complex coupled-mode theory was used to investigate the power evolution of the core mode that resulted from the coupling. From this, the conditions in which the core mode power attenuates the most rapidly were identified. In addition, phase matching turning point (PMTP) was used in the design, to overcome the conflict between the range of grating period change and the grating length in the CLPFG. Finally, an optimized CLPFG-based filter with a length of 3.5 cm was obtained, which has a symmetrical attenuation band with an operating bandwidth over 300 nm. Within the operating bandwidth, the flatness is less than 2.5 dB and the transmittance is lower than 0.1%.     

Periodical corrugated structure for forming sampled fiber Bragggrating and long-period fiber grating with tunable couplingstrength

A novel corrugated structure on an optical fiber is proposed and demonstrated to produce tunable long-period index modulation based on photoelastic effect. The corrugated structure is a periodic variation made by chemical etching on the cladding radius of an optical fiber. By imposing this corrugated structure upon a built-in fiber Bragg grating (FBG), a superstructure grating with tunable reflectance is formed. In addition, couplings between the fundamental core mode and cladding modes take place under such a corrugated structure when the phase-matching condition is satisfied. Thus, the device can also act effectively as a long-period fiber grating (LPFG) with tunable coupling strength. We also develop the coupled-mode theory based on Fourier series expansion to describe such a corrugated sampled Bragg grating. The phase-matching condition for various sampling orders can be derived, and the calculated spectra are compared with those based on the fundamental matrix method. Optical measurements demonstrate some unique characteristics of these devices, and good qualitative agreements between simulation and measurement verify the viewpoint that long periodic index modulation is indeed induced by such a corrugated structure through the photoelastic effect when a tensile force is applied     

Leaky cladding mode propagation in long-period fiber gratingdevices

We investigate leaky-mode propagation of the cladding modes of an optical fiber through coupling with a long-period fiber grating. A one-dimensional ray propagation model is applied. Experimental measurements are compared to a transfer-matrix model to estimate the cladding mode losses. Losses are also measured using an all-fiber Mach-Zehnder mode interferometer. The measured losses are somewhat greater than predicted by the simple theory     

Ag-coated Long-period Fiber Grating

Based on coupled-mode theory, the eigenvalue equation of five-layered long-period fiber grating （LPFG） sensor with Ag film and gas-sensitive film overlays are firstly studied. The problem of resolving complex eigenvalue equation on five-layered LPFG is analyzed, and the method of resolution is also given. Then the eigenvalue equation of three-layered metal cladding LPFG is analyzed, and the complex transcendental equation is also discussed. The computing result shows that the coupling between the low order EH modes and the core mode is much stronger than that between the low-order HE modes and the core mode.     

Theoretical analysis of cladding-mode waveguide dispersion and its effects on the spectra of long-period fiber grating

A new method to control the free spectral range (FSR) of a long-period fiber grating (LPFG) is proposed and theoretically analyzed. As the refractive index decreases radially outward in the silica cladding by graded doping of fluorine, waveguide dispersion in the cladding modes was modified to result in the effective indexes change and subsequently the phase-matching conditions for coupling with the core mode in a LPFG. Enlargement of the FSR in a LPFG was theoretically confirmed.     

包层腐蚀机械写制长周期光纤光栅的耦合特性研究

利用周期为600μm的刻槽板,采用单面受压的方式包层对经过HF溶液腐蚀后的单模光纤施加压力,形成机械写制长周期光纤光栅(LPFG).通过测最具有不同包层直径的机械写制LPFG在各种压力下的透射光谱,研究包层直径对纤芯基模与包层模LP12、LP13间耦合特性的影响.实验结果表明,随着包层直径的减小,谐振波长发生红移,耦合...     

倾斜长周期光纤光栅薄膜传感器特性研究

为设计高灵敏光纤光栅薄膜传感器,基于耦合模理论研究了表面镀制高折射率敏感薄膜的倾斜长周期光纤光栅（TLPFG）耦合特性和薄膜折射率传感特性。TLPFG能激发高阶（角向序数l1）包层模耦合,高阶包层模耦合系数随倾角增大而增大,80时,高阶包层模耦合系数远小于1阶包层模耦合系数。研究表明,光栅倾角变化不影响包层模耦合谐振波长,但影响光谱透过率。进一步研究了TLPFG在80时1阶5次包层模耦合的光谱透过率对薄膜折射率变化的响应,分析了光栅结构参数（L、、）和薄膜参数（h3,n3）对薄膜折射率响应灵敏度的影响,结果表明,响应灵敏度相比于非倾斜LPFG可提高一个数量级,对薄膜折射率的分辨率可达10-9。