Dielectric Optical Waveguides Research Articles

Mode-dependent radiation loss in Y junctions and directional couplers

Calculation of radiation loss in symmetric dielectric optical waveguide Y-junction power dividers and evanescent-field directional couplers is presented. The loss is calculated as a function of the separating angle for the odd and even exciting normal modes in several test structures. It is shown that for most practical cases, the even normal mode is more lossy than the odd normal mode. The difference increases with the separating angle and with the guidance strength. This radiation-loss difference is, in general, smaller for directional couplers than for Y-junction power dividers. Introducing a proper curvature to the output waveguides of a structure can serve to minimize this difference.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Very low-loss Y-junction power divider

The design of an efficient wide-angle dielectric optical waveguide Y-junction power divider is outlined. The method has several degrees of freedom that can be used to optimize the device's performance to achieve minimization of the radiation loss for a specified mode at the input and equalization of the modal losses in a dual-mode device.

Coupled-mode theory approach to depolarization associated with propagation in turbulent media

The problem of light depolarization in a turbulent atmosphere is revisited by means of coupled-mode theory [D. Marcuse, Theory of Dielectric Optical Waveguides (Academic, New York, 1974)]. This allows, in particular, evaluation of the depolarization ratio for a plane wave and comparison of its expression with the one obtained in the frame of two distinct approaches predicting different behaviors [A. A. M. Saleh, IEEE J. Quantum Electron. QE-3, 540 (1967); J. W. Strohbehn and S. F. Clifford. IEEE Trans Antennas Propag. AP-15,416 (1967)]. It is shown that both approaches give the same result when calculated to the same order in both smallness parameters relevant to the problem, thus resolving a controversy of more than 20 years.

Three-dimensional analysis of bending losses in dielectric optical waveguides with arbitrary refractive-index profile

A three-dimensional analysis of bending losses in dielectric optical waveguides is presented. It constitutes a nontrivial generalization of previous two- and three-dimensional studies by other authors. Our analysis is based on homogeneous integral equations for the total radiation field and suitable asymptotic approximations for Green’s functions. A key role is played by a new three-dimensional approximation for a relevant Bessel function with large order and argument (the former being larger than the latter). A nontrivial check of the consistency of all those approximations is given. General formulas are presented for the radiated field and the energy flow and for a bending-loss coefficient in three dimensions. Numerical results are also given, in order to assess the difference between the results of other authors and ours. Such a difference is rather small for monomode behavior near cutoff, increases as the behavior of the waveguide changes from monomode to multimode, and decreases as the parameter V increases for a given core radius and propagation mode.

Waveguide-bend configuration with low-loss characteristics

A structure that can reduce abrupt-bend losses in dielectric optical waveguides is proposed. The structure has a relatively low refractive-index region at the outside of the corner; the region operates as a phase-front accelerator of the propagating mode. It has been ascertained by basic experiments that the bending loss of this structure can be significantly reduced from that of the conventional one.

Exact numerical technique for the analysis of step discontinuities and tapers in optical dielectric waveguides

In this paper, we first review the existing analytical methods used to analyze step discontinuities and tapers in optical dielectric waveguides. We then develop an exact numerical technique for determining the transmitted, reflected, and radiated power that is due to a step discontinuity or a taper in an optical dielectric waveguide. We demonstrate the method by evaluating step discontinuities and tapers in various symmetric and asymmetric slab waveguides. Unlike with previous analytical methods, no physical approximations are made throughout the entire analysis. For this reason, our method can be used to evaluate a step discontinuity or a taper in a dielectric waveguide of arbitrary cross section, regardless of the magnitude of the geometrical mismatch between the input and the output waveguide sections.

Vector variational analysis of the guided modes in the three‐dimensional anisotropic optical waveguides

AbstractThe guided wave characteristics in a three‐dimensional dielectric optical waveguide containing anisotropic materials must be investigated by vectorial analysis. Such analyses are extremely complicated if non‐diagonal terms are contained in the dielectric tensor or more than two anisotropic materials are involved. In this paper the guided modes are analyzed by a variational method in the case where the dielectric tensor contains nondiagonal elements of the transverse components. First, a new variational expression and trial functions suitable for analysis of the guided mode in these waveguides are derived. Next, numerical calculations are performed for a rectangular dielectric waveguide in which both the core and the substrate are made of anisotropic materials. With these calculations, we investigate the effects of the directions of the multiple principal axes on accuracy of the propagation constant near the lower limit, coupling of the guided modes and directions of the electromagnetic vectors. It is found that there are tight hybrid modes and weak hybrid modes. In the case of weak hybrid modes the electromagnetic vectors are straight and almost parallel to the principal axes. On the other hand, in tight hybrid modes the electromagnetic vectors bend significantly and the vector direction is rotated as the normalized frequency is varied.

Electromagnetic propagation at interfaces and in waveguides in uniaxial crystals

The use of surface-impedance and surface-admittance concepts for analyzing reflection and refraction at an isotropic dielectric interface (first developed about 1938) is extended to include an interface between uniaxial birefringent dielectrics. Total internal reflection and the polarizing (Brewster) angle at an anisotropic interface are shown to be naturally explainable in terms of surface impedance (for TM polarization) and surface admittance (for TE polarization). The allowable modes in an integrated optical uniaxial asymmetric dielectric slab waveguide are also shown to be directly obtainable using the surface impedance/admittance approach. Numerical examples are presented.

Review of Numerical Methods for the Analysis of Arbitrarily-Shaped Microwave and Optical Dielectric Waveguides

This paper presents are view of the numerical methods for the analysis of the homogeneous and inhomogeneons,isotropic and anisotropic, microwave and optical dielectric waveguides with arbitrarily-shaped cross sections.The characteristics of various methods are compared,and a set of qualittative criteria to guide the selection of an appropriate method for a given problem is proposed. The main approaches discussed are those of point matching, integral equations, finite difference, and finite element.

Vector Analyses of Propagation Constants in Dielectric Optical Waveguides with Perturbed Refractive-Index Profile

A new method is developed to evaluate the propagation constants of guided or leaky modes in cylindrical dielectric optical waveguides with arbitrary cross sections. It is assumed that the refractive-index profiles of the fiber deviate from the step-index ones. Format expressions of the propagation constants are given for waveguides with anisotropically perturbed refractive-index profiles. The method is applied to a circular step-index fiber with an azimuthally perturbed refractive-index profile, and it is shown that scalar analyses cannot properly predict the propagation constants of those modes whose angular order satisfies certain conditions with respect to the Fourier spectrum of the perturbation of the refractive-index profile.

Wide Y-junctions with low losses in three-dimensional dielectric optical waveguides

This paper proposes a simple structure (antenna coupled Y - junction) which reduces radiation losses of Y -junctions in 3-D optical waveguides by decreasing refractive indexes near the branching regions. A principle to reduce radiation losses and a design theory are presented for the new Y -junction by using geometrical optics. It is theoretically shown that losses of the properly designed 3-D Y -junctions can be reduced by around an order of magnitude compared to those of conventional ones when the branching angles are large.

Low radiation loss Y-junctions in planar dielectric optical waveguides

A simple structure is proposed to reduce radiation losses of Y-junctions in planar optical waveguides by decreasing refractive indices near the branching regions. The losses of the properly designed Y-junctions can be reduced by around an order of magnitude compared with those of conventional ones when the branching angles are large.

Failure of scalar analyses in dielectric optical waveguides with perturbed refractive-index profile

It is theoretically shown that scalar analyses cannot properly predict the propagation constant of a particular mode in circular dielectric optical waveguides when the angular order of the mode satisfies some condition with respect to the order of the Fourier spectrum of the perturbed refractive-index profile.

Sharp bends with low losses in dielectric optical waveguides

A new method is proposed for making sharp bends with low radiation losses in dielectric optical waveguides. By modifying the transverse refractive-index profile at curved sections both the pure bend and the transition losses can be minimized. The optimum gradient-index profile requires an inhomogeneous medium. But in practice this can be replaced by a layered medium. By using four homogeneous layers the permitted radius of curvature of a slab waveguide can be reduced, e.g., from 6400 to only 100 wavelengths.

Propagation constants for linearly polarized modes of arbitrarily shaped optical fibers or dielectric waveguides

A simple, approximate formula (valid for modes not too close to cutoff) is given for the propagation constants of linearly polarized modes on a step-index, weakly guiding optical fiber with a core of arbitrary shape. The formula requires only knowledge of the value of a Dirichlet eigenvalue of a similarly shaped region; not only is it significantly more accurate than previously obtained approximate formulas, but it also has a particularly intuitive physical explanation in terms of the depth-of-field penetration into the cladding region.

Loss reduction in curved dielectric optical slab waveguide

Formulas of the pure bending loss and the transition loss valid for TM modes in dielectric optical slab waveguides are presented. The characteristic of the total transmission loss of a step-index slab waveguide containing a circular bend having an asymmetric waveguide structure are analysed numerically by applying derived loss formulas. It is found that the features of the total transmission loss of TM modes are closely analogous to those of TE modes except for small discrepancies in the numerical values.

Analysis of optical waveguide junctions by means of an integral equation method

AbstractUsing a slab waveguide model we analyze the problem of waveguide mode transmission and reflection that occurs at the junctions of dielectric optical waveguide due to lateral offset, angular tilt, and gap between the guides, and deformations of the end faces. the analysis first develops a set of integral equations for the unknown equivalent electric and magnetic surface currents over the end faces. the integral equations are discretized through partitioning of the boundaries of both ends into small elements, and their solutions are expanded in a Neumann series, which makes it possible to obtain the solutions iteratively. Using reciprocity and mode orthogonality properties appropriately, the mode coupling coefficients are obtained. After discussing the relation between partition number of the boundaries and convergence of solutions, and the satisfaction of energy relations, we give numerical examples for the reflection and transmission coefficients of guided modes, the radiation losses and the radiation power patterns, choosing end shapes and their locations as the parameters.

GaAs/GaAlAs Curved Rib Waveguides

Curved dielectric optical waveguides suffer from radiation loss due to bending. To minimize the bending loss and reduce the radius of curvature, it is necessary to fabricate guides which provide strong optical confinement. This paper gives a brief review of curved waveguide analysis and presents some experimentally measured loss values for GaAs/GaAlAs curved rib waveguides. The rib waveguides, fabricated using ion beam milling, have a large rib height and are tightly guided structures. When corrected for reflection losses and input coupling efficiency, a minimum loss of approximately 3 dB has been achieved for a multimode 90° curved guide with a radius of curvature of 300 µm, and 8.5 dB for a single-mode curved guide with a radius of curvature of 400 µm. It is believed that most of this residual loss is not radiation loss due to bending, but rather scattering loss due to rib wall imperfections.

GaAs/GaAlAs curved rib waveguides

Curved dielectric optical waveguides suffer from radiation loss due to bending. To minimize the bending loss and reduce the radius of curvature, it is necessary to fabricate guides which provide strong optical confinement. This paper gives a brief review of curved waveguide analysis and presents some experimentally measured loss values for GaAs/GaAlAs curved rib waveguides. The rib waveguides, fabricated using ion beam milling, have a large rib height and are tightly guided structures. When corrected for reflection losses and input coupling efficiency, a minimum loss of approximately 3 dB has been achieved for a multimode 90° curved guide with a radius of curvature of 300 μm, and 8.5 dB for a single-mode curved guide with a radius of curvature of 400 μm. It is believed that most of this residual loss is not radiation loss due to bending, but rather scattering loss due to rib wall imperfections.

Low loss dielectric optical waveguide bends

Abstract The pure bend loss of a curved dielectric optical waveguide can be reduced effectively with only a slight change in the transverse field distribution by replacing the cladding material outside the caustic by another material with a smaller refractive index. This is demonstrated by microwave model experiments.