Single-polarization single-mode photonic crystal fibers

A new structure of single-polarization single-mode (SPSM) photonic crystal fiber (PCF) is proposed and analyzed by using a full-vector finite element method with anisotropic perfectly matched layers. From the numerical results it is confirmed that the proposed fiber is low-loss SPSM-PCF within the wavelengths ranging from 1.48 to 1.6 /spl mu/m, where only the slow-axis mode exists and the confinement loss is less than 0.1 dB/km.     

Finite element beam propagation method for three-dimensionaloptical waveguide structures

A beam propagation method (BPM) based on the finite element method (FEM) is described for longitudinally varying three-dimensional (3-D) optical waveguides. In order to avoid nonphysical reflections from the computational window edges, the transparent boundary condition is introduced. The present algorithm using the Pade approximation is, to our knowledge, the first wide-angle finite element beam propagation method for 3-D waveguide structures. To show the validity and usefulness of this approach, numerical results are shown for Gaussian-beam excitation of a straight rib waveguide and guided-mode propagation in a Y-branching rib waveguide     

Modeling of two-dimensional photonic crystal resonant cavities incorporating elliptically shaped dielectric cylinders

Modeling of photonic crystal structures is an ongoing challenge. This letter is devoted to the theoretical modeling of photonic crystal resonant cavities, composed of elliptically shaped dielectric pillars. Through this rigorous study, we can examine the impact of the elliptical-shape deformations to important quantities related to the photonic crystal cavity performance, such as the quality factor and the resonance frequency.     

Formation of uniform GaAs multi-atomic steps with 20–30 nm periodicity and related structures on vicinal (111)B planes by MBE

We studied morphology of GaAs surfaces and the transport properties of two-dimensional electron gas (2DEG) on vicinal (111)B planes. Multi-atomic steps (MASs) are found on the vicinal (111)B facet grown by molecular beam epitaxy, which will affect electron transport on the facet. We also studied how the morphology of GaAs epilayers on vicinal (111)B substrates depends on growth conditions, especially on the As₄ flux. The uniformity of MASs on the substrates have been improved and smooth surfaces were obtained when the GaAs was grown with high As₄ flux, providing step periodicity of 20 nm. The channel resistance of the 2DEG perpendicular to the MASs is reduced drastically with this smooth morphology. These findings are valuable not only for fabricating quantum devices on the (111)B facets but also those on the vicinal (111)B substrates.     

Photonic bandgap fibers with high birefringence

A highly birefringent air-guiding photonic crystal fiber which provides guidance by a full two-dimensional photonic bandgap (PBG) effect is proposed. The fiber core is formed by introducing a large air hole that has an area of four unit cells of the cladding materials. It is shown from computed results by a full-vector finite element method that the proposed PBG fiber has birefringence of the order of 10/sup -3/.     

More on Security of Public-Key Cryptosystems Based on Chebyshev Polynomials

Recently, a public-key cryptosystem based on Chebyshev polynomials has been proposed, but it has been later analyzed and shown insecure. This paper addresses some unanswered questions about the cryptosystem. We deal with the issue of computational precision. This is important for two reasons. Firstly, the cryptosystem is defined on real numbers, but any practical data communication channel can only transmit a limited number of digits. Any real number can only be specified to some precision level, and we study the effect of that. Secondly, we show that the precision issue is related to its security. In particular, the algorithm previously proposed to break the cryptosystem may not work in some situations. Moreover, we introduce another method to break the cryptosystem with general precision settings. We extend the method to show that a certain class of cryptosystems is insecure. Our method is based on the known techniques on the shortest vector problem in lattice and linear congruences.     

Dynamic mechanical properties of photopolymerizable poly(vinyl alcohol)–acrylate monomer blends

Dynamic mechanical properties of photopolymerizable poly(vinyl alcohol) (PVA)–monoacrylate blends were investigated by measuring dynamic shear modulus G′ and loss tangent, tan δ. The dynamic mechanical properties of the blends before being exposed to UV irradiation were governed by the weight percent of the monomers which act as plasticizers. On the other hand, the UV-irradiated blends seemed to be typical two-phase materials since they revealed two tan δ maxima whose positions were independent of the monomer content. Those two maxima were assigned to PVA and photopolymerized acrylates with reference to the dynamic mechanical data of PVA and a PVA-polyacrylamide polyblend. Those dynamic mechanical data suggested that insolubilization of the blend type photopolymers should be caused by a decrease in solubility due to graft polymerization of acrylate monomers onto PVA.     

Approximate scalar finite-element analysis of anisotropic optical waveguides

An approximate scalar finite-element program for the analysis of anisotropic optical waveguides with a diagonal permittivity tensor is described. The accuracy of the method has been checked by calculating the eigenmodes of two-dimensional, anisotropic asymmetric slab waveguides. The results obtained for a channel waveguide embedded in LiNbO3 agree well with the results of the earlier vectorial finite-element method.     

Noniterative two-dimensional phase-retrieval method from two Fourier intensities by use of an exponential filter

A noniterative method of retrieving the two dimensional phase of a wave field from two intensity measurements is proposed. In the measurements, one records two far-field intensities of the wave field modulated and unmodulated with an exponential filter. The phase retrieval method is based on the solution of the simultaneous equations with unknown coefficients of the two-dimensional discrete Fourier transform for the phase. Then there is no need for the information about the wave field, which is used in iterative phase-retrieval methods. The usefulness of this method is shown in computer-simulated examples of the reconstruction of two-dimensional complex amplitude objects.