A TiO2 Film/K+ Ion-Exchanged Glass Composite Optical Waveguide and Its Application to a Refractive Index Sensor

A composite optical waveguide for biological and chemical sensors was successfully developed by sputtering a thin TiO₂ film onto the surface of a low loss potassium ion-exchanged optical waveguide. The electric field of the evanescent wave at the film surface was made strong through adiabatic transition of the guided light. The attenuation of the guided light was sufficiently small in the air, and the guided light intensity was changed sensitively with the refractive index of the cladding layer (n_c). Thus, n_c can be sensitively monitored with a detecting sensitivity of about 10^-4.     

Cladding index modulated fiber grating

A new type of fiber grating with only cladding index modulation is presented. Characteristics of both cladding index modulated short-period fiber grating (FBG) and long-period fiber grating (LPFG) are analyzed. The calculation of the modes involved in this paper is based on a model of three-layer step-index fiber geometry. Transmission of a mode guided by the core through a cladding index modulated grating when evanescent field coupling occurs is analyzed with couple-mode theory. Evanescent field coupling causes a power flowing from the core to the cladding, so the attenuation of the new grating is analyzed as well. Lower attenuation, flexible spectral characteristics are demonstrated in comparison with traditional fiber core index modulated grating.     

Guided propagation in a step-index, multi-mode fiber: effect of index difference variation on allowable TM propagation constants

The effect of variation of core and cladding index difference, Δn=n_core−n_cladding, on allowable values for the guided mode transverse magnetic propagation constants within a step-index, multi-mode optical fiber is investigated. We use a iterative computational technique to calculate the propagation constants for modes inside and outside the core that satisfy the boundary conditions contained within the characteristic eigen-equation for the TM field components. Evidence of a strong dependence of the allowable propagation constants on changes of Δn is shown.     

非线性包层光纤中TE0模的传播特性分析

本文对具有Kerr非线性包层和线性匀芯的光纤在不同光功率下的传播特性进行了理论分析和数值计算.结果表明,在适当光功率下,当模折射率N等于芯折射率n_f时,芯场变得相当均匀,而包层场迅速衰减.对于自聚焦包层,当从N<n_f逐渐变到N>n_f时,芯场剖面越过N=n_f从凸变凹.在拐点,芯场成为均匀的.论文给出了模场分布和光功率与模折射率的关系.指出纤芯出现匀场预期会在光纤器件、光电子器件、光子器件中得到应用.     

Investigation on characteristics of W-type fiber with an inner cladding made of negative refractive index materials

A doubly clad optical fiber with an inner cladding made of negative refractive index material was proposed and the modes distribution, dispersion and time delay were investigated by fully vectorial numerical method. The results indicate that HE₁₂ mode can be operated as a single mode, the cut-off frequency of modes shifts to low normalized frequency with increasing thickness of inner cladding made of negative index materials, large negative dispersion and time delay can be obtained and tailored by the inner cladding. These results provide theoretical basis for designing novel fibers or optical devices.     

Complete leaky mode coupling in dual-core photonic crystal fibre based on the coupled-mode theory

This paper theoretically investigates the dependence of leaky mode coupling between inner core fundamental mode and outer core defect mode on phase and loss matching in pure silica dual-core photonic crystal fibres with the multi-pole method. The complete mode coupling can take place when both the phase and loss matching conditions are satisfied at the avoided anti-crossing wavelength. It shows the influences of cladding structure parameters including the diameters of cladding air holes d₁, diameters of outer core holes d₂ and hole to hole pitch Λ on the characteristics of leaky modes coupling. The coupled-mode theory is used to analyse the mode transition characteristics and the complete coupling can be clearly indicated by comparing the real and imaginary parts of propagation constant of the leaky modes.     

长周期光纤光栅中的包层模谐振

本文给出了光纤中包层模的电场形式,指出包层膜的电场形式与弱导光纤中导模的电场形式不同,对光纤中基模与包层模电场沿光纤横截面分布的重叠情况的分析表明,长周期光纤光栅除了能将HE₁₁导模耦合到HE_1p包层模外,也可以将其耦合到EH_1p包层模。基于耦合模理论的计算表明,耦合到EH_1p和EH_1,p-1包层模的耦合波长相近,对于典型的单模光纤,当p>4时耦合到这两个模式的耦合系数是可比的。     

双包层手性光纤中导模的光功率特性

在对W型阶跃双包层手性光纤进行解析求解的基础上,对几个低阶导模的功率特性进行了仔细的研究.对典型的内包层厚度,计算了几个低阶模留在纤芯中的光功率随纤芯和内外包层的手性参数的变化关系曲线,讨论了纤芯和内外包层的手性参量以及内包层厚度对不同符号模式的功率特性的影响.内包层厚度越大,功率越被限制在纤芯中.随着纤芯中手性参量的增大, HE－11模和HE－21模（HE01模）限制在纤芯中的光功率缓慢增加,而HE11模和HE21模（HE02模）限制在纤芯中的光功率则逐渐减小.包层手性参量的作用正好与此相反.     

Modes in weakly guiding elliptical optical fibres

Approximate and much simplified dispersion relations are obtained for the problem of optical wave propagation within weakly guiding elliptical fibres. The refractive index difference between the core and its cladding of weakly guiding optical fibres that are contenders for use as practical optical communication lines is very small; i.e., (n ₁/n₀–1) 1 wheren ₁ is the core index andn ₀ is the cladding index. These greatly simplified dispersion relations are then used to calculate the propagation constants for several higher order modes on an elliptical optical fibre.Supported partly by NELC, San Diego; this paper was presented at 1974 URSI Electromagnetic Waves Conference, London.     

Four-layer metal-clad thin film optical waveguides

An investigation of the low order modes supported by an asymmetrical four-layered metal-clad optical waveguide is presented showing the attenuation characteristics and the field profiles. The attenuation and phase constants are examined as functions of both the thickness and refractive index of the buffer layer as well as the mode order. The results for small and large buffer layer thicknesses are discussed in terms of the modes supported by simpler asymmetrical three-layered metal-clad and dielectric-clad waveguides respectively. It is shown that the coupling of the TM polarized modes to the lossy surface plasma wave depends upon the buffer layer thickness, the refractive indices of the buffer layer and dielectric cladding and the mode order. This coupling is very dependent upon the mode order with the TM₀ mode exhibiting far weaker coupling than the higher order TM modes. Methods of controlling the amount of coupling and hence the attenuation of the TM modes are discussed.This work was performed while the author was with the Department of Electrical Engineering, University of Queensland, Brisbane, Australia, 4067.     

All-optical amplification via the interaction among guided and leaky propagation modes in lithium niobate waveguides exploiting the cascaded second-order nonlinearity

The aim of this paper consists of performing a theoretical investigation of an all-optical amplifier and discussing the problems for a realistic implementation. An original periodically poled lithium niobate channel waveguide, having the optical c-axis opportunely oriented on the xz plane was considered. The waveguiding structure, designed to operate in the second communication window of optical fibers, shows versatile properties as was theoretically demonstrated. In fact, it enables both the selection and the coupling strength of four different propagation modes, two guided and two leaky, interacting via the cascaded second-order nonlinearity. The coupling among guided and leaky modes is obtained by means of two different techniques: (i) birefringence phase-matching (BPM) via a suitable orientation of the optical c-axis crystal on the xz plane and (ii) quasi-phase-matching by periodical inversion of the lithium niobate ferroelectric domains. A strong diffusion of magnesium in the region external to the periodically poled waveguiding channel is needed in order to achieve the suitable BPM and to maximize the bounding of the electromagnetic field and, thus, the overlapping integrals. Finally, a powerful refinement of device operation via the use of the seeded second harmonic waves is proposed. Gains of the order of 20 dB are achieved with reduced bias powers. The order of magnitude of the bias power decreases, with respect to the unseeded second harmonic operation, from 145 to 32 W.     

A partially reflecting metal cladding: its effect on the modes of dielectric optical waveguides

If the metal cladding of a dielectric optical waveguide is sufficiently thin to be only partially reflecting, then the waveguide modes differ from those of the conventional metal-clad waveguide. The TE modes are little affected by a variation in the metal thickness but the TM modes change considerably due to a coupling between the waveguide modes and the surface plasma waves supported by both the metal: dielectric interfaces and the metal film. It is the refractive index of the dielectric cladding which is remote from the guiding core that determines whether the lowest order TM mode is the TM₀ or TM₁ mode. This dielectric cladding also strongly influences the attenuation of the TM modes and, if the guide supports a TM₀ mode, then the attenuation of the TM modes far from cut-off are an order of magnitude higher than that of the corresponding modes when the guide cannot support a TM₀ mode. If the guide can support a TM₀ mode then its dimensions can be chosen such that it will support the TE_N and TM_N modes with equal phase velocities. A lossless approximation is used to develop an expression which will specify the required guide dimensions directly and parametric plots of these dimensions are discussed.     

Mode classification and loss mechanism in air-core Bragg fibers

With an equivalent mode-solving model, the mode spectra in air-core Bragg fibers are systematically studied by using an improved full-vector finite-difference method. All supported modes are classified into four categories, namely, guided modes, cladding modes, leaky modes and PML modes, among which the leaky modes can be further subdivided into radiation-like and evanescence-like leaky modes. To ensure that the modes are solved accurately and efficiently with this model, the strategy for choosing model parameters is suggested. Benefiting from this convenient mode solver, the characteristics of cladding modes are observed in details, and potential applications are suggested. Moreover, the rapid loss of a hybrid mode at lower frequencies is explained by an evanescence-like leaky mode induced cutoff, which is different from that of TE₀₁ mode due to the failure of band gap confinement.     

MICROWAVE PROPAGATION IN ROUND GUIDING STRUCTURES BASED ON DOUBLE NEGATIVE METAMATERIALS

This paper presents a theoretical study of guided propagation in a waveguide containing ordinary and negative refractive index (NRI) media. The cut-off frequency of positive/negative combined media is examined. It is calculated for each propagating mode as a solution of a characteristic equation that is solved in terms of Bessel functions, taking into account the implications of the negative refraction. In contrast to what happens in ordinary propagation, mode mixing and crossing and no cut-off modes are found for negative values of ε, μ and refractive index n. The investigation considers the range 1-80 GHz where metamaterials have been demonstrated to work with negative parameters. The results are also compared with the negative core/negative cladding case. Some applications are suggested for devices operating in the microwave frequency domain.     

Cladding mode coupling in long-period gratings formed in photonic crystal fibers

We extend the improved effective index method (IEIM) to analyze the cladding mode of the photonic crystal fiber (PCF) and to predict cladding mode coupling in gratings. By introducing a new step-index fiber model for the PCF, the cladding mode coupling between a LP₀₁ core mode and HE₁₁ cladding mode in long-period gratings formed in PCF is accurately predicted by the IEIM. The fiber model works well for the PCF used here not only in predicting resonant mode coupling, but also in analyzing core and low-order cladding modes.     

Characterization of coupling power for single-mode fiber fusion

Fabrication of single-mode fiber coupler by heating fibers on a flame is very common and popular. However, controlling the input and the output power quantities of the fiber coupling process is neither easy nor similar. Power losses occur at the coupling length as the effect of the geometry and structure of fibers during and after fusion. In application, the power losses will affect sensor devices, e.g. optical switch. This paper proposes power splitting into the second fiber junction as a new model by deriving and integrating the coupling power with two conditions of refractive index changes. First is the change in the vertical part and second is the change in the vertical and the horizontal parts. Both conditions are studied by perpendicular and parallel directions of coupling power. The model is examined with a linear change of refractive index where power absorption and reflection are accumulated by power losses. The result shows that power increases as the effect of losses by simulation. These power losses are lost by radiation out of cladding and fiber heating.     

Limitations in the perturbation analysis of bent finite-clad fibres and waveguides

The transverse shift in the field distribution and the correction to the propagation constant of the fundamental and symmetric cladding modes on bent finite-clad single-mode fibres and slab waveguides are evaluated from perturbation theory for effective index values extending below the cladding index. Analytical results are derived in both geometries for the step-profile that are valid within the overall limitations of the theory. However, it is found that, for the fibre geometry only, the method breaks down at certain discrete wavelengths because of degeneracies that occur between the HE₁₂ (LP₀₂) and TE₀₁ (LP₁₁) mode propagation constants.     

TM mode optical characteristics of five-layer MOS optical waveguides

The field distribution and complex eigenvalue equation of the TM mode are solved from the wave equation for a five-layer optical waveguide with finite metal cladding and a dielectric buffer layer. For air–Au–SiO₂–GaAs–AlGaAs MOS waveguides, numerical results for the propagation constant and absorption loss of the TM mode are computed in the complex plane from the eigenvalue equation. The effects of some guided structural parameters on the mode propagation and absorption loss are analysed and discussed.     

Coupling to circularly asymmetric modes via long-period gratings made in a standard straight fiber

We demonstrate long-period fiber gratings made in a straight standard optical fiber via a CO₂ laser side irradiation that couples light between modes of different azimuthal symmetry. We show that such coupling can be even stronger than that to the azimuthally symmetric modes, provided the photoinduced asymmetry in the refractive index change across the fiber cross-section is high enough. This suggests that some previously-observed phenomena with CO₂-written LPGs not observed with other types of LPGs may be attributed to LPG resonances that couple light into the cladding modes with higher azimuthal symmetry. We show it on an example of bend-induced LPG resonance position tuning with the resonance depth maintained constant.