Tunable ultrashort electro-optical power divider using coupled photonic crystal waveguides

Photonic crystals (PCs) have many potential applications because of their ability to control lightwave propagation. We have investigated a tunable ultrashort electro-optical power divider in two-dimensional PC structures. The power divider, composed of a dielectric cylinder in air, is studied by solving Maxwell's equations using the plane wave expansion method and finite-difference time-domain method. The power-splitting mechanism is analogous to that of conventional directional couplers, utilizing coupling between guided modes supported by line defect waveguides. To increase the coupling coefficient of the PC coupler, the radius of the rods between two waveguides is reduced. The switching mechanism is a change in the conductance in the coupling region between the waveguides and hence modulating the coupling coefficient, and eventually switching is achieved. Such a mechanism of wavelength multiplexing should open up a new application for designing components in photonic integrated circuits.     

Finite element approach to propagation in elliptical and parabolic waveguides

This paper deals with the application of the finite element method to the solution of propagation problems in elliptical and parabolic waveguides. Mode classification in empty waveguides is discussed, followed by extensions of the method to problems where metallic ridges lie along either major or minor axes.     

Decoupling co-existing surface plasmon polariton (SPP) modes in a nanowire plasmonic waveguide for quantitative mode analysis

Knowledge of surface plasmon polariton (SPP) modes in one-dimensional (1D) metallic nanostructures is essential for the development of subwavelength optical devices such as photonic circuits,integrated light sources,and photodetectors.Despite many efforts to characterize the propagation parameters of these subwavelength 1D plasmonic waveguides,such as Ag nanowires,large discrepancies exist among available reports owing to their sensitivity to the relative weights of co-existing SPP modes and the lack of a method of decoupling these modes and analyzing them separately.In this work,we develop an interference method to distinguish different SPP modes that are simultaneously excited in a Ag nanowire waveguide and measure their propagation parameters separately.By extracting information from the propagation-distancedependent intensity oscillations of the scattered light from the nanowire tip,the effective refractive indices,propagation lengths,and relative mode weights of co-existing SPP modes supported by the nanowire are derived from a mode interference model.These parameters depend strongly on the nanowire diameter and excitation wavelength.In particular,we demonstrate the possibility of selective excitation of different SPP modes by varying the nanowire diameter.This new mode analysis technique provides unique insights into the development and optimization of SPP-based applications.     

Lossy multilayer channel optical waveguides analyzed by the transmission line matrix method

We demonstrate that the three-dimensional vectorial transmission line matrix (TLM) method is applicable to the analysis of lossy multilayer optical waveguiding structures. Any lossy multilayer waveguide geometry, including sharp discontinuities in the transverse plane, can be treated taking into account the coupling between all optical field components. The complex propagation constants (propagation constants and the attenuation coefficients) for the fundamental TE-like and TM-like modes can be determined. These parameters of the fundamental TM-like mode of a typical lossy multilayer rib dielectric waveguide are obtained as functions of free-space wavelength. Calculation of the electric-field pattern is also included. Numerical comparisons with the argument principle method (for the case of lossy multilayer slab waveguides) and the spectral-index technique (for the case of lossy multilayer rib waveguides) are also included, and it is shown that the application of the TLM method to lossy multilayer optical waveguides is accurate.     

High-index-contrast waveguides and devices

We present a theoretical and experimental study of high-index-contrast waveguides and basic (passive) devices built from them. Several new results are reported, but to be more comprehensive we also review some of our previous results. We focus on a ridge waveguide, whose strong lateral confinement gives it unique properties fundamentally different from the conventional weakly guiding rib waveguides. The ridge waveguides have distinct characteristics in the single-mode and the multimode regimes. The salient features of the single-mode waveguides are their subwavelength width, strong birefringence, relatively high propagation loss, and high sensitivity to wavelength as well as waveguide width, all of which may limit device performance yet provide new opportunities for novel device applications. On the other hand, wider multimode waveguides are low loss and robust. In addition, they have a critical width where the birefringence is minimal or zero, giving rise to the possibility of realizing intrinsically polarization-independent devices. They can be made effectively single mode by employing differential leakage loss (with an appropriate etch depth) or lateral mode filtering (with a taper waveguide). Together these waveguides provide the photonic wire for interconnections and the backbone to build a broad range of compact devices. We discuss basic single-mode devices (based on directional couplers) and multimode devices (multimode interferometers) and indicate their underlying relationship.     

Characterization of surface-plasmon modes in metal-clad optical waveguides

Finite-element analysis, based on the vector H-field formulation and incorporating the perturbation technique, is used to calculate the complex propagation characteristics of metal-coated dielectric waveguides. The propagation and attenuation characteristics of the surface-plasmon modes at the metal/dielectric interfaces are presented. The effects on the optical properties of metal-clad optical fibers with infinite and finite cladding thickness and the formation of the supermodes due to the coupling between the surface-plasmon modes in the presence of different surrounding materials are also investigated.     

Asymptotic Coupling Coefficients of Well-separated Single-mode Optical Waveguides

Abstract

Simple general formulae for the coupling coefficients of the fundamental modes on well-separated parallel optical waveguides are derived. The calculation merely requires accurate knowledge of the propagation constants and core fields of the modes on the individual waveguides, while the far-from-core fields need not be known. Results for coupled circular fibres are identical with the asymptotic expansions of previously derived exact coupling coefficients. Analytical expressions for the coupling coefficients between the fundamental modes on identical weakly guiding channel waveguides are obtained.     

Refractive indices and thicknesses of optical waveguides fabricated by silicon ion implantation into silica glass

Planar optical waveguides formed by Si ion implantation into PECVD SiO₂ have been characterized by the dark mode spectroscopy method at a wavelength of 0.6328 μm. The measured effective index values of the guided modes have been used to investigate the optical properties of the core layers of the waveguides after different pre-implantation treatments. It was found that annealing the specimens before implantation, affected both the refractive index and thickness of the core layers. In the annealed specimens a thicker core layer and a larger relative refractive index difference between the core and the buffer layer resulted.     

Acoustic modes in optical fiberlike waveguides

A perturbation analysis of guided and leaky modes in fiber acoustic waveguides with core and cladding parameters that are slightly different is presented. The perturbing parameter is the shear-velocity difference between the core and cladding material epsilon(s). Acoustic fields and eigenvalues are expanded in power series of epsilon (s)(1/2) for radial and flexural modes and in powers of epsilon(3) for torsional modes. Expansion of leaky longitudinal modes is also in terms of epsilon(s), but the nature of perturbation analysis for these modes is somewhat different from that of guided modes. Zero-order solutions for all types of modes are obtained, and some important higher-order effects are discussed. Common features of optical and acoustic modes in weakly guiding fibers are addressed. It is shown that with respect to zero-order solutions of guided modes, optical and acoustic fibers have identical propagation characteristics. Exact and zero-order propagation characteristics for several lower-order shear-type modes are calculated and compared.     

A New Approach for Optimization of Wavelength Multiplexers with Phased Waveguide Arrays by Use of the Beam Propagation Method

An approach to the optimization of wavelength multiplexers with phased waveguide arrays is presented. It is based on a simulation of the structure by the beam propagation method. The approach takes into account not only the symmetry of the structure but also the perturbations due to both emitted radiation and coupling between the waveguides at the ends of the phased waveguide array. Low-loss cosine-shaped bends are used. Both the design principles and the optimization of real wavelength multiplexers are described. The optimized multiplexers, based on silica waveguides on a silicon substrate, have sizes of about 50 mm 2 30 mm, fibre-to-fibre losses of 1–2 dB and cross-talk values between m 40 and m 29 dB.     

Analysis of the left-handed corrugated circular waveguide

Infinite circular corrugated waveguide is analysed to investigate its ability to support modes with backward wave behaviour. Such waveguides provide an alternative structure, easier to manufacture than those already reported based on rectangular symmetry with corrugated walls or filled with frequency selective surfaces. The corrugations if sufficiently deep provide a guiding structure with the required series capacitance and shunt inductance to allow left-handed propagation within some frequency bands. These backward waves are analysed using the surface impedance model of propagation in corrugated waveguides to predict their properties. Interpreting the physical meaning of the analysis, the authors discuss how backward waves are related to resonances in corrugated structures. The relationship between power flows in the guide and the behaviour of the group velocity for such guides is shown. A full wave simulator is also applied to validate these results and the case of a dielectric filled waveguide is considered showing the improved ability to support left-handed modes. The authors present the results of a parametric study of how left-handed propagation depends on the corrugation depth. Potential applications of backward waves in corrugated circular waveguides are proposed.     

Quantum statistics and dynamics of nonlinear couplers

Abstract

We investigate both codirectional and contradirectional nonlinear couplers composed of two nonlinear waveguides operating by second-harmonic generation. We take into account possible mismatches inside the waveguides and between them. We calculate the photon number distribution, its factorial moments, quadrature and integrated intensity variances and quadrature uncertainty product taking into account the effects of transmission of light between waveguides. Incident beams are assumed to be coherent, squeezed and mixed with external noise. Second-harmonic modes are assumed to be pumped with strong coherent beams. We show that non-classical behaviour of beams generated by the nonlinear waveguides can be transferred between them and controlled by linear and nonlinear mismatches. In the contradirectional regime of propagation, asymptotic non-classical states can be generated.     

Enhanced propagation in a plasmonic chain waveguide with nanoshell structures based on low- and high-order mode coupling

We studied the performance of a plasmonic chain waveguide by employing an array of nanoshell structures. The optical properties of the proposed structures are discussed in detail with respect to the mode coupling for both low-order resonances and high-order multipolar modes. We show (a) that the choice of nanoshell particles allows an easy tuning of the structure's resonances according to given wavelength specifications and (b) that the resonances are insensitive to the chain length when high-order multipolar modes are involved. Moreover, chain waveguides that are operated on resonant multipolar modes provide propagation lengths up to 1.88 microm, which is beyond what is maximally achieved by conventional solid particle chains. This is attributed to the large field enhancement within metallic nanoshell structures, as well as to far-field effects, which play an important role in low-loss light guiding along nanoshell chains.     

Modal behavior in single-mode slab waveguides consisting of inhomogeneous cores

We present an analytical method for TE and TM modes in weakly guiding inhomogeneous single-mode slab waveguides. Based on our results, the modal behavior or propagation constants depend on index profiles of the waveguides. It is important to know how the modal behavior depends on the index profile in single-mode waveguides, because it determines wave-front characteristics of propagating modes.     

Fabrication and characterization of molecular beam epitaxy grown thin-film GaAs waveguides for mid-infrared evanescent field chemical sensing

Thin-film GaAs waveguides were designed and fabricated by molecular beam epitaxy for use in mid-infrared (MIR) evanescent field liquid sensing. Waveguides were designed to facilitate the propagation of a single mode at a wavelength of 10.3 microm emitted from a distributed feedback quantum cascade laser, which overlaps with molecular selective absorption features of acetic anhydride. The characterization of the waveguides shows transmission across a broad MIR band. Evanescent field absorption measurements indicate a significant sensitivity enhancement in contrast to multimode planar silver halide waveguides.     

Propagation properties of an elliptical anisotropic metamaterial cylinder

The propagation characteristics were investigated of an open elliptical cylinder comprising a uniaxial anisotropic metamaterial. The dispersion relation for hybrid modes in this cylinder was examined theoretically. It is shown that forward and backward waves can be supported by such a metamaterial cylinder. Even and odd modes have different losses in a realistic anisotropic elliptical cylinder. The forward wave can propagate longer distances than the backward wave because of its lower loss. The light can propagate backward on the cylinder without the limit of structure size. The odd modes have higher operating wavelengths than the even modes. The extraordinary properties in this structure are identified as multilayer plasmons. The validity of the effective medium approximation of this structure is examined.     

Propagation of Laguerre-Bessel-Gaussian beams

New exact solutions to the paraxial wave equation are obtained in the form of a product of Laguerre polynomials, Bessel functions, and Gaussian functions. In the limit of large Laguerre-Gaussian beam size, the Bessel factor dominates and the solution sets reduce to the modes of closed resonators, hollow metal waveguides, and dielectric waveguides. In the opposite limit the solutions reduce to Laguerre-Gaussian modes of open resonators and graded-index waveguides. These solutions are valid for electromagnetic waves traveling through free space, and they are valid for propagation through circularly symmetric optical systems representable by ABCD matrices as well. An interesting feature of the new solution set is the existence of three mode indices, where only two are required for an orthogonal expansion. As an example, Laguerre-Gaussian beam propagation through an optical system that contains a Bessel-like amplitude filter is discussed.     

Beam propagation in parabolically tapered graded-index waveguides

To a good approximation, the electromagnetic-propagation characteristics of graded-index waveguides can be written in terms of polynomial-Gaussian modes. For uniform quadratic-index waveguides the behavior of these modes is well known. However, there are sometimes practical reasons for using tapered waveguides, but detailed propagation solutions are known for only a few specific taper functions. The parabolic taper is perhaps the most important special case, and the solution-generating techniques that we generalize are used to obtain analytic solutions for this case.     

Elaboration,structural characterization and optical properties of the yttrium alkoxide derived Y2O3 planar optical waveguides

Y₂O₃ optical planar waveguides are fabricated by the sol–gel method and the dip-coating technique. High purity yttrium alkoxide (yttrium 2-methoxyethoxide) is used as starting material for the Y₂O₃ sol and acetylacetone is added to improve the stability of the sol. Highly concentrated sol up to 0.8 M is prepared and six stacked layers are enough to support four propagation modes at wavelength from 488 to 632.8 nm. The cubic phase of Y₂O₃ is crystallized after an annealing treatment at 700 °C. X-ray diffraction is conducted on the films for their structural analysis. Optical properties of the waveguides are determined by multi-wavelength m-lines spectroscopy and attenuation coefficient measurement. The attenuation coefficient of the prepared Y₂O₃ waveguiding thin films is in the range between 1 and 2 dB/cm at 632.8 nm.     

Full vectorial simulation of multilayer anisotropic waveguides with an accurate and automated finite-element program

An efficient, accurate, and automated vectorial finite-element software package (named WAVEGIDE), which is implemented within a PDE/Protran problem-solving environment, has been extended to general multilayer anisotropic waveguides. With our system, through an interactive question-and-answer session, the problem can be simply defined with high-level PDE/Protran commands. The problem can then be solved easily and quickly by the main processor within this intelligent environment. In particular, in our system the eigenvalue of waveguide problems may be either a propagation constant (β) or an operated light frequency (F). Furthermore, the cutoff frequencies of propagation modes in waveguides can be calculated. As an application of this approach, numerical results for both scalar and hybrid modes in multilayer anisotropic waveguides are presented and are also compared with results obtained with the domain-integral method. These results clearly illustrate the unique flexibility, accuracy, and the ease of use f the WAVEGIDE program.