Carlo G. Someda

Coupling and decoupling of electromagnetic waves in parallel 2D photonic crystal waveguides

The behavior of two nearby straight photonic crystal waveguides is analyzed. It is shown that the two guides, considered as a single system, may realize a very efficient wavelength selective directional coupler, that may be used as a channel interleaver in a WDM communications system. In addition, we also show that, by properly designing the geometry of the dielectric region between the guide cores, waveguide decoupling can be obtained. Necessary conditions for this feature to be obtained are analytically derived, and an electromagnetic explanation of the decoupling process is given.

The space filling mode of holey fibers: an analytical vectorial solution

We tackle holey fibers in full vectorial terms. From Maxwells equations, we derive the dispersion relations of the modes guided by an infinitely self-similar air hole lattice. We focus in particular on the fundamental mode (the so-called space filling mode), and show that previous numerical results based on vector methods are accurate, but scalar ones are not. We also find the field flow lines, intensity distribution in the cross section, and linear polarization ratio vs. wavelength.

Experimental investigation of linear polarization in high-birefringence single-mode fibers.

We report on measurements of linear polarization ratio in high-birefringence fibers as a function of wavelength. Comparison with standard step-index and with dispersion-shifted fibers reconfirms that the modes of polarization-maintaining fibers are not linearly polarized.

Numerical analysis of propagation and impedance matching in 2D photonic crystal waveguides with finite length

A novel way of approaching wave propagation in two-dimensional (2-D) photonic crystal guides with finite length is presented. It is shown that the main propagation features can be captured by borrowing simple concepts of propagation in transmission lines and combining them with other concepts taken from the theory of periodic structures.

Modeling of enhanced field confinement and scattering by optical wire antennas

We describe the application of full-wave and semi-analytical numerical tools for the modeling of optical wire antennas, with the aim of providing novel guidelines for analysis and design. The concept of antenna impedance at optical frequencies is reviewed by means of finite-element simulations, whereas a surface-impedance integral equation is derived in order to perform an accurate and efficient calculation of the current distribution, and thereby to determine the equivalent-circuit parameters. These are introduced into simple circuits models, directly borrowed from radio frequency, which are applied in order to model the phenomena of enhanced field confinement at the feed gap and light scattering by optical antennas illuminated by plane waves.

FAR FIELD OF POLARIZATION GRATINGS

In a recent Letter [Opt. Lett. 24, 584 (1999)], Gori analyzed polarization gratings and proposed using them as a tool to measure Stokes parameters. Most of his analysis dealt with the near field, but practical exploitation of his approach relies on the far field. A discussion focusing on the far field is presented, and recipes are suggested for actual implementations of such devices.

Stress distribution in optical-fiber ribbons

Previous results indicate that fibers in ribbons are sometimes affected by systematic birefringence superimposed on the random one, their relative weights depending on fiber position in the ribbon. We report new theoretical and experimental results on stress distribution in ribbons, which is shown to depend on thermal and mechanical properties of the common coating. Numerical simulations are based on the theory of elasticity and the finite-element method (FEM). Polarization dispersion measurements vs. temperature match very well with numerical results, and indicate that central fibers in the ribbon exhibit significantly larger birefringence than lateral ones.

RANDOM BIREFRINGENCE AND POLARIZATION DISPERSION IN LONG SINGLE – MODE OPTICAL FIBERS

Non-ideal single-mode fibers are affected by birefringence and coupling, which cause polarization dispersion. They are conveniently modeled in terms of the so-called principal states of polarization and of their differential group delay. We review this formalism and its application to the evolution of polarization along a randomly varying fiber, and to concatenation of group delays over long links consisting of N spliced fibers.

Concatenation of polarization dispersion in single‐mode fibers: A monte‐carlo simulation

We present a model for non-ideal long single-mode fiber links affected by polarization mode dispersion. It leads to simple expressions for the overall polarization delay and for the r.m.s. width of the impulse response, which entail systematic simulation on personal computers within reasonable CPU times. Simulations show that the mean differential group delay and the double r.m.s. width of the impulse response coincide in such a way that both quantities can characterize the fiber polarization mode dispersion. In particular, both grow as the square root of the link length when the correlation length of the perturbation is much smaller then the fiber length. This confirms the results obtained by other authors under restrictive approximations.

Circularly birefringent optical fibres: new proposals

A new family of azimuthally inhomogeneous fibres is proposed and analysed theoretically. In contrast with previous similar structures, in these fibres one can find exact analytical expressions for the guided fields. Such modes could also be used as zeroorder approximations in analysisng other waveguides.This paper presents the basic theory and several examples illustrating how the main parameters (radii, number and width of the sectors, and refractive indices) affect the field profiles. We also present introductory, qualitative arguments on how this improved control over the field could be exploited in order to get either high circular birefringence or good matching of the guided modes to TEM00 or LP01 exciting fields.