Simple approach to coupling in textured planar waveguides

We develop a self-consistent Green's function technique to calculate the full photonic band structure for guided modes in slab waveguides that have been textured in one dimension with a thin surface grating. The technique is conceptually simple and can easily be reduced to an eigenvalue problem by use of approximations similar to those used in conventional coupled-mode theories. We show that this approach yields the correct TMTM coupling coefficient at oblique incidence and provides a transparent interpretation for its vanishing at a critical angle.     

Three-dimensional step discontinuities in planar waveguides: Angular-spectrum representation of guided wavefields and generalized matrix-operator formalism

For obliquely incident guided modes the light propagation in slab waveguides with step discontinuities can be described by means of an angular-spectrum representation of the electromagnetic fields together with a generalized matrix-operator formalism. By means of a discretization of the radiation and evanescent modes, the computation of the reflection and transmission coefficients can be reduced to the numerical solution of algebraic equations. The TE-TM coupling, which takes place for oblique incidence, and the different evanescent field effects are discussed in detail.     

Factorization method for inverse obstacle scattering problem in three-dimensional planar acoustic waveguides

In this paper, we consider the inverse scattering problem of reconstructing a bounded obstacle in a three-dimensional planar waveguide from the scattered near-field data measured on a finite cylindrical surface containing the obstacle and corresponding to infinitely many incident point sources also placed on the measurement surface. The obstacle is allowed to be an impenetrable scatterer or a penetrable scatterer. We establish the validity of the factorization method with the nearfield data to characterize the obstacle in the planar waveguide by constructing an outgoing-to-incoming operator which is an integral operator defined on the measurement surface with the kernel given in terms of an infinite series.     

Neutron resonances in planar waveguides

We report on the results of the experimental investigation of the spectral width of neutron resonances in planar waveguides using the time-of-flight method and recording the microbeam emerging from the waveguide end. Experimental data are compared with the results of theoretical calculations.     

High-transmission planar x-ray waveguides

We have studied the propagation of hard x rays in a planar x-ray waveguide with a sub-20 nm guiding layer. To optimize the transmission and to minimize absorption losses, a novel waveguide design based on a two-component cladding was implemented. Optimized transmission is achieved by placing an appropriate interlayer between the cladding and the guiding core. The experimental results along with simulations of field propagation show that high transmission values can be obtained in waveguide optics at parameters relevant for x-ray imaging. These are small beam diameters below 20 nm and the relatively long guiding length necessary for efficient blocking of multi-keV photon energy beams.     

Step discontinuities in dielectric waveguides (fibres)

The scattering of surface modes by step discontinuities in dielectric waveguides (lightguides) is investigated by the Wiener-Hopf technique and the variational method. The simple analytical expressions for the scattering characteristics are derived both near and far from cut-off. The results are compared with those obtained by other methods.     

Fundamental losses in planar Bragg waveguides

This paper considers a planar Bragg waveguide. The guided modes and their dissipation due to the fundamental absorption are described. In the interacting-wave approximation, an analytical relation between the characteristics of the modes and parameters of the Bragg-waveguide geometry was established. Absorption losses in hollow Bragg waveguides were shown to be significantly lower than in a regular waveguide fabricated from the same material. Quantitative estimates of the fundamental and radiation losses in Bragg waveguides are given for materials used at wavelengths of 1.55, 10.6, and 0.245 μm.     

Modes of UV-written planar waveguides

The UV-induced refractive-index increase in photosensitive planar waveguides is modeled by an exponential profile. Closed-form field solutions are given and dispersion curves are calculated for TE and TM modes. Modes in air-covered waveguides can be less sensitive to the UV-written profile than modes in buried waveguides.     

Kerr modulation of planar waveguides

We describe two methods for employing materials with giant Kerr nonlinearities for modulating waveguides. A beam, of relatively intense light, is used to vary the propagation constant of a waveguide with the guiding layer, or the substrate, made from a nonlinear material.     

Finite-element analysis of planar dielectric waveguide discontinuities

The finite element method is combined with the mode-matching method and the multi-mode network method to analyze the scattering and radiation characteristics of a class of planar dielectric waveguide discontinuities. Unlike the conventional method to treat the radiation as a source-field problem, in the present approach, the dispersion characteristics of dielectric guided-wave structures are calculated first, and then the radiation problem is transferred to the propagation problem of a series of surface-waves and space waves from the viewpoint of scattering, thus the analysis is tremendously simplified.     

High-temperature stable gratings in germanosilicate planar waveguides

Negative index gratings in planar germanosilicate waveguides were found to be stable up to 500 degrees C. The annealing properties are similar to those of negative index fiber gratings.     

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.     

Radiation losses due to discontinuities in asymmetric three-layer optical waveguides

In this paper, the radiation losses caused by film thickness and refractive index discontinuities in asymmetric slab waveguides are examined theoretically. A technique developed by Mahmoud and Beal for dealing with the continuous part of the radiation spectrum is used. Several numerical examples which show the radiation loss for various waveguide configurations are presented and discussed and, where appropriate, comparisons with other related work made. In the case of asymmetric thickness steps, the radiation losses are found to be appreciably higher than those computed for symmetric guides. The radiation losses for guides having the same thickness were found to decrease as the guide thickness increased. Once the TE₁ mode become guided, the losses decreased rapidly. However, if the two guides had different thickness then it was found that the losses exhibited a minimum.     

Complex photosensitivity observed in germanosilica planar waveguides

Photosensitive effects distinguished as type I and type IIA photosensitivity within optical fibers were observed in a much more pronounced form within germanosilica waveguides deposited by hollow-cathode plasma-enhanced chemical-vapor deposition. With increasing exposure to 193-nm UV light, positive index changes greater than 2 x 10(-3) were observed, followed by negative index changes greater than -5 x 10(-3) . These behaviors are attributed to an increase in macroscopic polarizability and a reduction in material density, respectively. The negative index change is more temperature resistant and is fully annealed only at 900 degrees C, whereas the positive one is annealed at 500 degrees C.     

Stress-induced birefringence control in optical planar waveguides

For what is to our knowledge the first time, the thermal-stress formula in analytical form is derived for the core layer in an optical planar waveguide by the thin-film approximation, which is based on a closed-form solution in multilayered isotropic structures off the edges. This formula indicates that elastic, thermal, and structural parameters can affect the magnitude of the stress. Using the formula, one can remove thermal stress and hence stress-induced birefringence by proper waveguide design.     

Disappearance of modes in planar Bragg waveguides

We show how the forbidden bands of a Bragg reflector may shrink to points and how some classes of the guided modes of a planar Bragg waveguide may disappear altogether by shrinking with the forbidden bands. We derive the general conditions to determine the missing modes and explain these conditions with examples. It is possible, for example, to design a Bragg waveguide that rejects all antisymmetric modes and supports only symmetric modes for the TE polarization. We also highlight the effect of Brewster incidence on the interpretation of the missing modes for the TM polarization.     

Second harmonic generation in planar optical waveguides

We study optical second harmonic generation (SHG) in planar waveguide structures composed of several layers with different dielectric constants. We develop a general formalism for the calculation of mode generation by a planar antenna embedded in the waveguide. As an application we consider a monolayer of high second-order susceptibility adsorbed at the interface between two layers of the waveguide structure. Periodic modulation of the nonlinear susceptibility allows phase matching leading to dramatically enhanced second harmonic intensities. We investigate the SHG-efficiency of various experimentally realizable geometries.     

Modal coupling in traveling-wave resonators

High-Q traveling-wave-resonators can enter a regime in which even minute scattering amplitudes associated with either bulk or surface imperfections can drive the system into the so-called strong modal coupling regime. Resonators that enter this regime have their coupling properties radically altered and can mimic a narrowband reflector. We experimentally confirm recently predicted deviations from criticality in such strongly coupled systems. Observations of resonators that had Q>10(8) and modal coupling parameters as large as 30 were shown to reflect more than 94% of an incoming optical signal within a narrow bandwidth of 40 MHz.