填充多层介质的共焦椭圆同轴线传播特性

采用在椭圆柱坐标系中分离变量的方法,得到了填充多层介质共焦椭圆同轴线模式特征方程.研究表明:当椭圆退化为圆时,利用角向和径向马修函数的渐进关系,可得到填充多层介质的圆形同轴线的模式特征方程,由此可见,圆形同轴线可看作椭圆形同轴线的特例;当椭圆同轴线内导体半长轴大小为零时,则椭圆同轴线就变成椭圆波导,同样的方法,可得到填充多层介质的椭圆波导模式特征方程.当填充一层介质时,得到了椭圆同轴线和椭圆波导的模式特征方程,结果和相关文献所得结果相同.作为示例,对填充一层介质和两层介质的椭圆同轴线中一些模式色散特性进行了数值计算,分析了椭圆同轴线中介质参数的变化以及椭圆同轴结构的变化等对其传播特性的影响.     

Role of controlled mode conversion in leaky feeder mine communication systems

Using an idealized model, we consider VHF wave transmission in a straight mine tunnel that contains a braided coaxial cable. The controlled interchange of energy between the propagation modes can be used to optimize the communication range and allow for coupling to and from portable hand-held antennas. In particular, we show that the optimum lengths for maximum transmission through the leaky cable sections or mode converters predicted here are compatible to those predicted by approximate coupled mode theory.     

Designing air-core photonic-bandgap fibers free of surface modes

It is known that the coupling of core modes to surface modes in air-core photonic-bandgap fiber (PBF) can give rise to large propagation losses. Using computer simulations, we analyze the relationship between the air-core geometry and the presence or absence of the surface modes in air-core PBFs with a triangular hole pattern. We identify ranges of core radii for which the fiber supports no surface modes over the entire wavelength range of the bandgap, i.e., only core modes are present. In particular, for a hole radius /spl rho/=0.47/spl Lambda/, where /spl Lambda/ is the hole spacing, the core supports a single mode and no surface modes for core radii between 0.8/spl Lambda/ and 1.1/spl Lambda/. The absence of surface modes suggests that fibers within this range of configurations should exhibit a very low propagation loss. We also show that the existence of surface modes can be predicted quite simply from a study of the bulk modes alone, which is much simpler and faster than carrying out a full analysis of the defect modes.     

Bending effects in optical fibers

Mode coupling at bends in optical fibers supporting one or only a few guided modes is analyzed by considering the local normal modes for the corresponding straight waveguide. Matrix elements giving the strength of coupling between guided modes at a corner bend, and for coupling between guided modes and radiation modes, are calculated as a function of guiding strength for this "geometrical" effect. The correction to these matrix elements due to the longitudinal strain in a bent fiber is also determined. The increase in propagation constant for the fundamental mode of a fiber wrapped in a coil of constant radius is calculated from information on the coupling strengths and mode propagation constants. The phase shift and attenuation of the fundamental mode caused by a spatially periodic microbending of the fiber axis are also considered. Finally, potential applications of these effects in fiber-optic devices such as mode converters, phase shifters, switches, and sensors are discussed.     

Analysis of dielectric loaded hybrid mode coaxial horns

This article presents a numerical solution for radiation from a novel coaxial horn with a partial dielectric loading. The horn is represented as a set of three-layer dielectric loaded coaxial waveguide sections. Characteristic equation has been obtained to calculate phase coefficients for hybrid modes in each section. Applying mode matching technique generalised scattering matrices for each junction between the sections have been obtained, besides that power coupling integrals for each of three different type of junctions have been found in closed-form solutions. Cascading these scattering matrices with transmission matrices of each section yields a generalised scattering matrix of the horn. For the given excitation at the throat of the horn, amplitude and phase of eigenmodes at the aperture of the horn have been determined. Finally, Fourier transform integral of the aperture electric field has been applied to solve radiation problem of the horn. Numerical results on propagation and radiation characteristics of the first four hybrid modes of the horn are presented and it is validated that this solution can be successfully used for full-wave electromagnetic analysis and optimisation of multiband dielectric loaded coaxial feeds.     

Influence of the fiber index profile on vectorial fiber modes and application to tapered fiber devices

The correspondence between the scalar modes LP and the nearly degenerate true vectorial modes in circular two-layer fibers is well established. For example, LP/sub 21/ corresponds to a combination of EH/sub 11/ and HE/sub 31/, while LP/sub 02/ is composed only of HE/sub 12/. For the multilayer fibers, inversion of the correspondences for certain modes can occur. We illustrate this inversion for a three-layer fiber. We propose a method permitting to distinguish the hybrid modes HE/sub /spl nu//spl mu// and EH/sub /spl nu//spl mu// in such fibers and we show the possibility of major taper-induced power coupling between these modes in SMF-28 fiber.     

Inherent gain flattening due to two mode interference in erbium doped coaxial fibers

In this paper we propose the use of an Erbium doped fiber (EDF) in coaxial configuration for gain flattening. The coaxial fiber supports two symmetric modes and can be designed to exhibit exchange of power between the amplifying doped inner core and the undoped outer core around the gain peak (1530 nm) due to interference among the two modes as it propagates along the coaxial fiber. This results in inherent gain flattening across the C-band as well as an increase in the average gain in the 1540–1560 nm band.     

Variational coupled-mode theory of optical couplers

A scalar coupled-mode theory is developed from the variational principle. The formulation is variational in the sense that the mode parameters are adjustable and can be optimized subject to the coupling between the guides. The theory is applied to slab and fiber couplers. In comparison with the conventional scalar coupled-mode theory in which only the amplitudes of the modes in the individual guides are adjustable, the variational scalar coupled-mode theory predicts more accurate propagation constants and field patterns of the normal modes of the couplers     

Mode coupling in coaxial waveguides with varying-radius center andouter conductors

The mode conversion process in a coaxial waveguide with varying-radius center and outer conductors is shown to be described by a system of first-order differential equations-the coupled mode equations. The nondiagonal coefficients of this system are called the coupling coefficients. In this paper, we derive the explicit expressions for the coupling coefficients in a varying-radius coaxial waveguide and discuss some of their important features. These coefficients can be used in determining model conversion in a coaxial cavity with slowly varying walls or designing and analyzing coaxial waveguide tapers and mode converters. Some experimental results of the coupling coefficients for the case of azimuthally symmetric modes, TE_0n modes, are also given     

Analysis of the nonlinear coaxial coupler

The authors present an analysis of the nonlinear properties in the coaxial fiber coupler. Two approaches are used, numerical beam propagation calculations in the radially symmetric waveguide and analysis of the coupled equations describing the interaction of the local modes. The two methods are compared and the region of validity of the local node analysis is then determined. They examine in detail the critical power condition for this structure and show how the design considerations to achieve this conditions can be understood     

New generalized computations of quasi-planar waveguides characteristics

An efficient hybrid field solution to the general multilayer substrates transmission line problems with full shielded and half open cross-sectional geometry is presented by using the waveguide modes coupling and decoupling procedure in the spectral domain. Modal transverse transmission lines corresponding to the LSE and LSM modes are taken into account so that the “Green's elements” for the edge conditions can be derived by a simple iterative method. In this paper, we analyze not only the propagation constants and characteristic impedances but also the available transmission power capacity and power distribution of most commonly used structures. Normalized eigenmode power levels characterizing the propagation features are computed.     

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.     

Loss mechanisms in single-mode fiber tapers

The onset of transmission loss in four single-mode tapers is shown to coincide with the taper slope exceeding the critical slope angle at which fundamental mode cutoff occurs. This implies that the primary loss mechanism in single-mode fiber tapers is cutoff of the fundamental mode from the core. A secondary loss mechanism of coupling to the modes of a coaxial outer waveguide is also observed and modeled. Implications for fused fiber couplers are discussed.     

Efficient single-mode fiber to titanium diffused lithium niobate waveguide coupling for λ = 1.32 µm

We report detailed results on the achievement of very high optical throughput for titanium diffused lithium niobate waveguides coupled between input and output single-mode fibers. By determining appropriate diffusion parameters to obtain excellent dimensional match between the fiber and waveguide modes and simultaneously low propagation loss, we have achieved total measured fiber-waveguide-fiber insertion loss as low as 1 dB for a 1 cm long waveguide atlambda = 1.32 mum. The relative contributions of coupling and propagation loss are determined. Very good correlation is found between the coupling loss and the match between the fiber and waveguide mode dimensions. Design data for diffusion parameters to obtain good mode match for arbitrary fiber dimension are presented.     

光致折变Taper光纤耦合器的耦合波分析

研究利用光纤中光致折变效应研制的光纤波导折射率沿轴向渐变的Ｔａｐｅｒ器件的模式耦合特性．在标量场近似下，用本地模表示的方法分析在阶梯折射率分布光纤和平方律折射率光纤的轴向折射率渐变过渡结构中传输的基模向反向散射模和高价模的耦合。对阶梯折射李光纤两种不同折射率Ｔａｒｅｒ结构的数值计算结果表明：在折射率变化相对缓慢情况下，这种折射率Ｔａｐｅｒ耦合器的基模反向散射和向高阶模的耦合都很小；基模能够自适应地调节以适应波导折射率的渐变，具有优良的传输性能．     

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.     

Calculated Channel Characteristics of a Braided Coaxial Cable in a Mine Tunnel

The braided coaxial cable is studied as a communication scheme in a mine tunnel. A simplified rectangular waveguide model is adopted for the tunnel, and the shield of the cable is assumed to behave as a single inductive transfer impedance. Specific results on the attenuation of the monofilar and bifilar (or coaxial) modes of propagation, taking into account the possible existence of a thin lossy film on the cable, are presented. In order to estimate the maximum possible range of communication, we consider the coupling factors of these modes to transmitting and receiving dipoles inside the tunnel, and we present results on these factors for various cable parameters and over a wide range of frequencies.     

Full Strain Tensor Treatment of Fiber Bragg Grating Sensors

Embedded fiber Bragg gratings can be subjected to arbitrary states of strain including shear strain. Such perturbations can cause coupling between polarization modes. Coupled-mode theory in Bragg gratings so far neglected this effect and only considered forward-backward coupling. Polarization mode coupling within a Bragg grating leads to interdependencies between Bragg reflection peaks which have so far been unaddressed. We formulate a full strain tensor treatment of fiber Bragg gratings, considering the coupling of the polarization modes within the grating. We give an approximation for the coupling coefficients affecting the polarization mode coupling and numerically solve the coupled-mode equations for representative states of strain. We show in which way shear strain affects the optical response of a grating and demonstrate how the fiber's beat length influences this characteristic.     

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     

Effect of Ripple Taper on Coupling Modes in a Coaxial Bragg Structure

Based on the mode-coupling method, numerical analysis is presented to demonstrate the influence of ripple taper on coupling modes on the frequency response in a coaxial Bragg structure. Results show that the interval between the band-gaps of the competing mode and the desired working mode is narrowed by use of a positive taper, but is expanded if a negative taper is employed, and the influence of the negative taper is more obviously advantage than the positive taper. The residual side-lobes of the frequency response on coupling modes can be effectively suppressed by employing the windowing-function technique. These characteristics of a tapered coaxial Bragg structure are favorable to improvement of the performance as a reflector or a filter in its application, also favorable to the mode selectivity and further weaken the excitation of unwanted spurious modes.