Loss calculations in weakly-guiding optical dielectric waveguides

The application of perturbation theory to a three-layer weakly-guiding slab waveguide composed of lossy dielectric media yields a simple formula for the attenuation coefficient α of a guided mode: α = (Σ³_i=₁a_iP_i)/ (Σ³_i= ₁P_i, where α_i, P_i are respectively the loss coefficient and model power in region i (i = 1, 2, 3). It is shown that this result can also be obtained from arguments based purely on geometric optics. The result is easily extended to apply to circularly-symmetric optical fibres where it yields confirmation of an earlier approximation for the power ratios P_i/(Σ²_i= ₁P_i).     

Analytic approach to dielectric optical bent slab waveguides

A rigorous classical analytic frequency domain model of confined optical wave propagation along 2D bent slab waveguides and curved dielectric interfaces is investigated, based on a piecewise ansatz for bend mode profiles in terms of Bessel and Hankel functions. This approach provides a clear picture of the behaviour of bend modes, concerning their decay for large radial arguments or effects of varying bend radius. Fast and accurate routines are required to evaluate Bessel functions with large complex orders and large arguments. Our implementation enabled detailed studies of bent waveguide properties, including higher order bend modes and whispering gallery modes, their interference patterns, and issues related to bend mode normalization and orthogonality properties.     

Mode coupling between two connected multimode parabolic-index optical waveguides

Both a displacement of axis and a change in the refractive index distribution function cause mode coupling to occur between two multimode parabolic-index optical waveguides. This coupling is investigated by the coherent states method and the method of the dynamical symmetry group. Expressions for the mode coupling coefficients are obtained and their relationship to the Franck-Condon factors is emphasized. Recurrence relations for the mode overlap integrals and sum rules for the mode coupling coefficients are presented. An analogue of the Franck-Condon principle for the mode coupling coefficients is formulated. A simple geometric method for finding the most intensively coupled modes is also proposed.     

Principal modes in multimode waveguides

We generalize the concept of principal states of polarization and prove the existence of principal modes in multimode waveguides. Principal modes do not suffer from modal dispersion to first order of frequency variation and form orthogonal bases at both the input and the output ends of the waveguide. We show that principal modes are generally different from eigenmodes, even in uniform waveguides, unlike the special case of a single-mode fiber with uniform birefringence. The difference is most pronounced when different eigenmodes possess similar group velocities and when their field patterns vary as a function of frequency. This work may provide a new basis for analysis and control of dispersion in multimode fiber systems.     

Mode energy transformation between two connected multimode general square-law-index optical waveguides

A coherent states method is used to investigate the mode coupling coefficients in the most general buttjoint problem for two multimode general square-law-index optical waveguides, when two different waveguides with elliptical cross-sections are to be butt-jointed with axis displacement, tilt and some turning of symmetry axes with respect to each other. The explicit expressions for the mode coupling coefficients and recurrence relations for the mode overlap integrals are obtained; the sum rules for the mode coupling coefficients are formulated and a method of experimental data analysis is proposed for this general problem. The method of calculating mode coupling coefficients is proposed for the case of the butt-joint problem when the waveguide cross-sections are only partially overlapping and then it is used to investigate the influence of misalignment on mode power distribution and pulse dispersion in the second waveguide. The experimental data are compared and it is shown that the coupling coefficients obtained in square-law-index model can be used in dispersion calculations forα-profile fibres in the case when the valueα is not far from the valueα=2.     

Characteristics of vertical bent coupling between straight and curved rectangular optical waveguides

In terms of the coupled mode theory (CMT) and the transfer matrix method, we present a technique and relative formulas including the bent coupling coefficient, amplitude coupling ratio, amplitude transmission ratio and propagation power for analyzing the vertical bent coupling between a curved rectangular waveguide and a straight one. Investigations show that the bent coupling appears in an effective coupling region by both sides of the central coupling point. Beyond this effective coupling region, however, the bent coupling is very weak, and the propagation power in the waveguide is closer to a constant. As the bent radius increases, or the thickness of the coupling layer between these two waveguides decreases, this effective coupling region becomes wider, in which the coupling behavior becomes stronger.     

Index profiles of multimode optical strip waveguides by field-enhanced ion exchange in glasses

Buried optical waveguides are fabricated by field-enhanced ion exchange processes in AgNO₃/NaNO₃ and NaNO₃ melts with the two-dimensional index distributions being accurately determined by reflectivity profiling with the aid of an optical multichannel detector system.     

Transmission characteristics in plasmonic multimode waveguides

In this article, we examine spectral transmission characteristics based on the self-imaging effect in plasmonic multimode waveguides. For the analysis, we calculate the correlation between an input field and the field in the self-imaging plane. We perform full vectorial computations using the Method of Lines as numerical method. The resulting transmission profile is discussed with regards to the attenuation, the even and odd mode sets and for several structural parameters of the plasmonic waveguide. The introduced transmission characteristic may offer the opportunity for the implementation of filtering operations in plasmonic waveguides.     

Two-photon path-entangled states in multimode waveguides

We experimentally show that two-photon path-entangled states can be coherently manipulated by multimode interference in multimode waveguides. By measuring the output two-photon spatial correlation function versus the phase of the input state, we show that multimode waveguides perform as nearly ideal multiport beam splitters at the quantum level, creating a large variety of entangled and separable multipath two-photon states.     

Field focusing control in multimode plane-layered waveguides

The possibility of controlling localized fields in multimode regular waveguides on the basis of the interference invariant principle is studied by numerical simulation. It is demonstrated that, by tuning the radiation frequency of an array without changing the initial field distribution at the aperture, it is possible to perform the scanning by a focal spot in a waveguide. Estimates of the efficiency of this method depending on the size of a vertical linear array are presented.     

Guided-mode resonances in multimode planar periodic waveguides

<正>Using the effective medium theory and the waveguide eigenvalue equation,we design a multimode planar periodic waveguide.When a plane wave illuminates the grating at the designed angle and wavelength, more than one leaky modes are excited coincidentally.Then the reflection efficiency around this designed angle and wavelength is investigated using a rigorous coupled wave analysis formulation and a gap of reflection peaks is found.Electric field distributions reveal that this high reflectivity gap is due to the coupling between these two coincidentally excited leaky modes.     

Optical characteristics of bent dielectric rectangular waveguides

Bent rectangular waveguide characteristics are analysed using conformal transformation and four-layered waveguide approximation. Using this method the physical model is clear, the optical characteristic expressions given are uncomplicated and the computed result is in good agreement with the results of Marcatili and Marcuse.     

On the pulse broadening in dielectric multimode waveguides

This paper presents theoretical results concerning pulse-broadening in multimode fibres. All the calculations are based on the geometric optic hypothesis. The results are given for different kinds of input functions in the case of a loss-free guide and an energy equidistribution. Subsequently, material and interface losses are included. The influence of angle-dependent energy distributions is also investigated. For all these cases analytical expressions are obtained. Experimental results are presented: they are in good agreement with theory.     

Selective modes excitation in multimode photonic crystal waveguides

We investigate modes excitation with the input field of different positions in two-dimensional multimode photonic crystal waveguides. Odd modes can be selectively excited by the input field of odd symmetry. The input field with different positions can excite different modes due to the field intensity distribution of modes. When the input field locates at the position of the zero field, intensity of waveguide modes is zero and the modes are not excited. The finite-difference time-domain method is used to obtain the excited field distributions.