Scattering characteristics of a beam mode in a dielectric-slab optical waveguide. Part II

The radiated power of scattered light caused by refractive-index inhomogeneities and waveguide wall irregularities in an asymmetric slab waveguide is investigated by using the stationary phase method on the basis of a first-order perturbation theory. The correlation between both imperfections of the refractive index and the waveguide wall is taken into consideration. It is found that the correlation between both imperfections hardly affects the radiated power caused by air radiation modes while it modifies the radiated power caused by substrate radiation modes.     

PECVD SiC optical waveguide loss and mode characteristics

We report the fabrication of single mode SiC (silicon carbide) waveguides and the measurement of their propagation loss. By studying the effect of sidewalls scattering loss due to surface roughness and by reducing it, minimal propagation loss of 2.3 dB/cm for the TM polarization is measured in the visible at 0.633 μm. This loss can be used as a benchmark for further development of SiC microphotonic components and circuit for sensor systems in harsh environment.     

Nonlinear optical beam interactions in waveguide arrays

We report our investigation of Kerr nonlinear beam interactions in discrete systems. The influence of power and the relative phase between two Gaussian shaped beams was investigated in detail by performing numerical simulations of the discrete nonlinear Schr?dinger equation and comparing the results with experiments done in AlGaAs waveguide arrays. Good agreement between theory and experiment was obtained.     

Subwavelength-sized beam spot confinement using an optical thin core dielectric ridge-type waveguide in leaky mode

Optical near-field plasmon-enhancement techniques have made marked contributions to the field of subwavelength optical beam spot generation for photonic devices. However, the metal material used is not sufficiently heat resistant for high-temperature operating requirements, such as laser-assisted magnetic recording storage. Therefore, it is preferable to use a dielectric material as a heat source for this application. To realize a subwavelength optical beam spot size using a dielectric material, we extensively analyze a leaky mode dielectric waveguide with a thin core. Finally, we show a new ridge-type dielectric waveguide using a leaky mode operation for a subwavelength beam spot.     

A propagating beam analysis of optical waveguide couplers

We study the mode coupling among the local normal modes of couplers formed from planar parabolic graded-index tapered optical waveguides with the propagating beam method. Our results indicate that couplers whose radiia (z) depend on axial length as \Lambda /2$$ " align="middle" border="0"> , have many desirable properties not possessed by other couplers.     

Transmission characteristics of optical waveguide corners

Two-dimensional dielectric waveguide corners with air reflector or metal reflector have been investigated by means of the method of plane-wave expansion, which has the advantage of being compact for optical integrated circuit applications. The operating principle of these waveguide corners is based on the total internal reflection or large partial reflection. The corner losses and field profiles are presented for various waveguide parameters. It is found that the corner loss can be small if the waveguide corner is properly fabricated.     

Scattering in a Pekeris waveguide from a rough bottom using a two-way coupled mode approach

Scattering from a rough ocean bottom is described numerically with a two-way coupled-mode formalism that contains scattering effects to all orders and provides an exact solution to the wave equation. Both scattered field and direct blast components are computed within the formalism framework. A comparison of the scattered component solution from the coupled mode with the Born approximation (BA) solution for scattering from a rough bottom Pekeris waveguide shows that the BA predicts correctly the scattered field levels but not detailed structure. The transition from direct blast to scattered field dominance is identified in the total field time series.     

Scattering in optical fibres: analysis of mode degeneration

Mode degeneration and mode conversion in multimode optical fibres are the main subjects of this work. In particular it is shown how fluctuations of the core shape can cause a rapid degeneration among the modes with the same azimuthal mode number. This allows a rigorous single variable approach to the second problem obtaining evident simplifications. The formalism proposed is applied to the study of mode distribution in the general case of microbending, of core-radius fluctuations in fibres with power law profiles and for simultaneous presence of microbending and ellipticity in parabolic fibres with differential mode attenuation.     

Two-dimensional coupled mode equation for grating waveguide excitation by a focused beam

The problem of grating coupling of a focused incident beam under non-normal incidence into a slab waveguide is given a complete three-dimensional (3D) solution. The diffracted field is expressed as the Fourier integral of a regular part and of a singular part resulting from the existence of the coupled guided mode. A suitable expression of the field in the neighborhood of the pole and a rigorous definition of the modal field lead to a generalized coupled mode equation relating the incident field and the two-dimensional (2D) modal field propagating in the plane of the slab waveguide. The phenomenological parameters involved in the coupled wave equation: the propagation constant, the radiation coefficient as well as the modal field shape are derived from the exact treatment of plane wave diffraction in the same structure. The solution of the complete coupling problem is given in the particular case of a Gaussian incident beam, and of a high index step-index waveguide.     

Radial mode dependence of optical beam shifts

It is known that orbital angular momentum (OAM) couples the Goos-H?nchen and Imbert-Fedorov shifts. Here, we present the first study of these shifts when the OAM-endowed LG(?,p) beams have higher-order radial mode index (p>0). We show theoretically and experimentally that the angular shifts are enhanced by p while the positional shifts are not.     

Coordinate stretching for finite difference optical waveguide mode solvers

Numerical methods are necessary to calculate propagating modes in optical waveguides. The finite difference method is widely used because it is applicable to waveguides with arbitrary refractive index profiles and it is easy to implement. To improve the efficiency and to reduce the size of the resulting large sparse matrix, the finite difference method is often used with a variable grid size strategy. This is related to the technique of coordinate stretching. In this paper, we develop a technique for optimizing the coordinate stretching function based on discrete reflection coefficients. We demonstrate our method using a scalar model which is valid for weakly-guided optical waveguides.     

The effect of an inhomogeneous cladding of a gradient optical waveguide on the mode and envelope characteristics of a soliton pulse

The nonlinear dynamics of a short pulse in a gradient waveguide layer is studied analytically taking into account the occurrence of a cladding layer and the longitudinal inhomogeneity of the two layers. The class of functions is presented in terms of which the transverse profiles of the refractive indices of both the gradient and the cladding layers can be represented, and, at the same time, the mode structure of the pulse can be explicitly described in terms of a hypergeometric equation. Expressions are presented that describe the variations of the propagation constant and transverse distribution of the wave field under the action of the longitudinal inhomogeneities of the gradient and cladding layers. It is shown that the envelope of the pulse satisfies the generalized nonlinear Schrödinger equation the coefficients of which are functions of the longitudinal coordinate and are expressed via the refractive indices of the waveguide layer and cladding.     

Coupling of tilting Gaussian beam with hybrid mode in the corrugated waveguide

The mode-conversion loss in the matching between the gaussian beam emanated from the gyrotron and the hybrid mode in the circular corrugated waveguide with the diameter of 2a is discussed. By numerical calculation, it is found that the loss considerably increases and optimum waist sizew ₀ changes when TEM₀₀ mode with the wavelength λ is injected with offset or tilt. By fitting numerical data to the polynomial function, it becomes evident that the scaling formulas of the losses for the off-axis shiftr _d and for the tilt angle θ are derived to be 2.3(r _d /a)² – 2.2(r _d /a)⁴ and 3.9(aα/λ)² – 5.6(aθ/λ)⁴ for fixedw ₀/a=0.643, respectively. To keep the mode-conversion loss ≤1% for the frequency of 168 GHz and 2a=88.9 mm, tilting angle and offset should be less than 0.1 degrees and 2.9 mm, respectively.     

Polarization mode conversion in an optical microdisk resonator vertically coupled to a waveguide bus

Due to the existence of non-zero, and asymmetrics cross-polarization coupling coefficients between a microdisk and a waveguide bus, high Q (∼10,000) overcoupled microdisks with minimal internal losses and sharp bending curvature (small radii) vertically coupled to a narrow-width ridge waveguide can be engineered as a proper candidate for a narrow-band polarization rotator. A semi-analytical calculation method is presented to realize microdisk-waveguide system as a polarization rotator.     

Slow light propagating characteristics through a two-dimensional silicon-based coupled-resonator optical waveguide

The light propagating characteristics of the coupled-resonator optical waveguides (CROWs) are studied by the finite-difference time-domain method. The circular CROWs are constructed by arranging the micro-cavities at a certain distance, which is constructed by removing the air holes along the edge of a circle from a two-dimensional (2D) triangular-lattice photonic crystal (PC). With the increasing of distance between the adjacent cavities, the group velocities of the guiding modes reduce significantly. The circular CROW studied in the paper have much minibands within the band gap, and their respective group velocities can be quite different from each other. This kind of CROW structure can provide different group velocities for the light signals with different frequencies, and avails to the separating and controlling the light signals in the all optic integrated circuits.     

Vectorial vortex mode transformation for a hollow waveguide using Pancharatnam-Berry phase optical elements

Transformation and inverse transformation between a free-space linearly polarized beam and the vectorial vortex mode of a circular hollow waveguide by use of Pancharatnam-Berry phase optical elements is proposed. Demonstration was achieved by fabricating GaAs subwavelength gratings and utilizing a 300 microm diameter hollow metallic waveguide for 10.6 microm wavelength CO(2) laser radiation. The mode transformations and the excitation of a single vectorial mode inside the hollow waveguide were verified by full polarization measurements. In addition, the inverse mode transformation of the single vectorial mode excitation in the waveguide enabled us to experimentally obtain a linearly polarized bright spot with a high central lobe.     

Effect of noise on characteristics of optical waveguide systems

Transient regimes of an optical waveguide system in the presence of additive and multiplicative noises are studied. The presence of noise is shown to result in a transformation of the spectrum of radiation modes. The effect of noise on the efficiency of input of electromagnetic energy into a waveguide system is studied.     

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.