Realization of true-time-delay using cascaded long-period fiber gratings: theory and applications to the optical pulse multiplication and temporal encoder/decoder

This paper proposes and demonstrates a technique for repetition-rate multiplication of an optical pulse train having an arbitrary period or pattern and the optical temporal encoding/decoding for the optical-code-division multiple-access (O-CDMA) system. The technique exploits the difference in the propagation speeds between the core and copropagating cladding modes of a fiber to obtain true-time-delay between the modes traveling in the core mode and the cladding mode, which can be used to achieve pulse multiplication. For the coupling to the cladding mode, long-period fiber gratings (LPGs) were used. A series of cascaded LPGs imprinted in a fiber with a specific separation has been employed to obtain a specific rate of pulse multiplication with a single input pulse. Second, by controlling the separations among the gratings, the temporal encoder/decoder for O-CDMA could be implemented. The principle and the applications of the proposed device are investigated in detail. The effect of the birefringence of fiber and fiber gratings on the system performance in the time and spectral domains is presented. The sensitivity of the cladding modes in a conventional fiber to the perturbations at the cladding has been overcome by replacing the conventional fiber with inner-cladding fiber. The properties and the benefits of using the inner-cladding mode are investigated.     

双包层手征光纤中的半漏模

研究了纤芯和内外外层都有手征的双包层手征光纤中的半漏模，给出了单包层和双包层手征光纤中几个低阶模的有效折射率和功率损耗系数随手征参数的变化关系曲线。结果表明，双包层手征光纤中的半漏模特性与半单包层手征光纤中的相比有很大的不同；当只有包层中有手征时，单包层手征光纤中也存在半漏模。     

The Relations Between Scalar Modes in a Lenslike Medium and Vector Modes in a Self-Focusing Optical Fiber (Short Papers)

The relations are established between the scalar modes in an infinite lenslike medium and the vector modes in a self-focusing optical fiber with a finite homogeneous cladding. It is shown that both the transverse fields and the longitudinal fields of the vector modes can be expressed in terms of the scalar modes, provided the fiber is operated in the core mode region. Otherwise, significant discrepancies could arise. The scalar modes, however, cannot describe the cladding modes which are caused by the index discontintuity at the outer surface of the cladding.     

Bending-induced birefringence of optical fiber cladding modes

The bend-induced lateral stresses that generate birefringence in an optical fiber are analytically determined throughout the fiber cross section. This stress profile is used to determine the birefringence of a cladding mode in a single-mode fiber. The result demonstrates that the birefringence of cladding modes may be much smaller than that of the core mode and knowledge of the mode profile and stress distribution is necessary for an accurate prediction.     

Chasi-Guided Modes and Related Radiation Losses in Optical Dielectric Waveguides with External Higher Index Surroundings

Mode filter actions are found theoretically in an optical dielectric waveguide consisting of a core and a thin cladding layer which is further surrounded by an external higher index region. The propagating waves, which are usually considered to be cutoff modes, can be guided with a small amount of loss under a certain condition. These waves are defined here as quasi-guided modes. These modes tend to the guided modes of the guide when the cladding thickness increases infinitely. A method is given to estimate the losses. As an example, the radiation losses are formulated for a symmetric slab waveguide, and are found to be approximately proportional to the cube of the mode number of the quasi-guided mode. Therefore, losses of the quasi-guided modes depend strongly on the mode number. It is suggested that fibers with large core diameters can be used as quasi-single mode fibers by covering the clad-type multimode fibers with external higher index surroundings and choosing the parameters properly.     

Analysis of Guided Waves Along the Cladded Optical Fiber: Parabolic-Index Core and Homogeneous Cladding

This paper is concerned with the determination of the propagation characteristics of modes in the cladded optical fiber which consists of a parabolic-index core and a uniform cladding. The propagation constants of propagating modes are obtained analytically. The results are given in simple form. The asymptotic forms of the propagation constants are also given, which are valid for lower order mode propagation.     

负折射率及零折射率光纤的光学特性

研究了芯层折射率为正,包层折射率为负的光纤导模光学特性,通过色散曲线分析比较了与常规光纤的不同,表现为低阶的简并的模式更少,单模存在的区域更多。进而分析了当包层折射率趋于零时的光学特性,发现与前两种光纤的光学特性都不同。分析发现当包层的介电常数和磁导率有一个为零时,在较小的区域各自出现了一条单模,与常规光纤的HE11模特性有很大不同,其色散曲线偏离方式与常规光纤相反,且出现了无模式存在的区域。     

基于细芯光纤内嵌马赫曾德尔干涉仪的应变传感器

研究了基于细芯光纤内嵌马赫曾德尔干涉仪的光纤应变传感器,通过将一根细芯光纤熔接在两根单模光纤(SMF)之间,构成了一种光纤内干涉的马赫曾德尔干涉仪。当单模光纤中的光耦合进细芯光纤时,一部分光耦合进细芯光纤纤芯作为芯模传输,另一部分光耦合进细芯光纤包层中激发出包层模沿包层传输,当芯模和包层模再耦合进单模光纤时发生干涉。当应变作用在细芯光纤上时,干涉条纹发生漂移。通过解调干涉条纹对应变的漂移量实现应变测量,在0~2000 με的测量范围下,测得的应变灵敏度为-1.83 pm/με,并且实验结果与理论分析有很好的一致性。该传感器具有体积小、制作简单、灵敏度高等优点。     

Unified Approach for Coupling to Cladding and Radiation Modes in Fiber Bragg and Long-Period Gratings

Couplings of core mode to guided cladding and unguided radiation modes in fiber Bragg and long-period gratings are investigated by a unified approach based on complex mode matching method (CMMM). With the combination of the perfectly matched layer (PML) and the perfectly reflecting boundary (PRB), the continuous radiation modes can be well represented by a set of discrete complex modes so that simulation of coupling to radiation modes is greatly simplified and may be treated in the same fashion as guided modes. Numerical results of fiber Bragg and long-period gratings with refractive index of the outer cladding lower, equal, and higher than that of the inner cladding indicate that the unified approach is highly effective in the simulation of couplings to cladding and radiation modes.      

双包层椭圆光纤的保偏特性

本文对双包层椭圆光纤进行了解析求解，得到了模式特征方程。研究了基模的保偏特性，给出了不同椭圆比下的归一化双折射和模间色散随归一化频率，纤芯内包层相对折射率差，内包层和纤芯半长轴比等参数的变化关系曲线。     

Characteristics of cladding index modulated fiber gratings for ambient refractive index sensing

Characteristics of cladding index modulated fiber gratings for ambient refractive index sensing is studied in this paper. The modes propagation properties and coupling properties in cladding index modulated FBG are studied based on the classical coupling mode theory while considering interactions among multiple modes and developed on a three-layer step-index fiber geometry. Evolution of cladding mode resonance spectra of cladding index modulated FBG with the decreasing of fiber diameter is simulated and compared with the core grating. Sensing characteristics of cladding modes resonance of cladding index modulated FBG to the variation of ambient refractive index is detailed and emphasized for cladding modes resonances has a much narrower bandwidth. Results show that cladding index modulated fiber grating has better spectrum for ambient refractive index sensing applications under the same sensitivity with core index modulated fiber grating and may be applied to many field including biochemical sensing and environmental monitoring.     

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。     

TM-polarized surface-plasma modes on metal-coated dielectriccylinders

TM-polarized surface plasma modes on a thin metallic cylindrical shell embedded in dielectric media are investigated. The dispersion characteristics of both bound modes and leaky waves are obtained for two sets of material parameters, the first corresponding to an optical fiber in which the metallic shell occupies a region between the core and the cladding and the second corresponding to a metal coated cylindrical rod in vacuum     

Guided optical waves in fibers with negative dielectric constant

We classify and discuss all optical guided modes able to propagate in ideal metallic-dielectric structures having the symmetry of a step index fiber. The cases of a metallic core with a dielectric cladding and a dielectric core with a metallic cladding are considered. In each case, the intensity profile of the allowed guided modes is shown. The geometric and frequency dispersion of the effective index of the modes is discussed as a function of the optogeometrical parameters of the fibers     

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.     

基于单侧几何形变结构的全光纤Michelson干涉高温传感器

利用CO2激光脉冲对标准单模光纤(SMF)单向形成较小的几何形变,把部分基模能量耦合到光纤包层,制作了一种新型的在线型Michelson干涉传感器。理论分析了这种几何微扰激发的包层模特性,并利用芯模-包层模较大的热光系数差,把这种传感器应用于高温测量。实验结果表明,这种干涉传感器的温度灵敏度为0.093 7nm/℃,并且在800℃温度范围内具有良好的线性和重复性。这种制作简单、结构稳定、体积小和灵敏度高的全光纤在线型Michelson温度传感器将具有较大的潜在应用价值。     

Compact all-fibre on-line power monitor via core-to-cladding modecoupling

An on-line all-fibre power monitor is demonstrated in which light from the core mode is first coupled to the cladding mode using an acoustooptic tunable filter, and then coupled to an InGaAs pin detector bonded to the fibre. Detection efficiency for different cladding modes and fibre cladding diameters are presented     

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。     

Linear edge and temperature characteristic of tilted fiber Bragg gratings cladding-mode envelope

In this paper, a system of linear edge filter demodulation based on weakly tilted fiber Bragg gratings in fiber sensor was proposed. The linear edge of the cladding-mode envelope and temperature characteristic between cladding modes and core mode of tilted fiber Bragg grating (TFBG) were experimentally investigated. The results show that falling edge (1505–1535 nm) and rising edge (1545–1565 nm) of the cladding-mode envelope is approximately linear and can be used as an edge filter. Moreover, the temperature characteristics of the core mode and cladding modes are similar to FBG’s. It can dynamically compensate the temperature induced wavelength shift of the FBG and the effect of temperature on the demodulated sensing signals is minimized. The advantages of this simple system are an all-fiber design, quasi static and dynamic operation, high stability and lower cost.     

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。