The axisymmetric modes in a longitudinal inhomogeneous SELFOC fiber

The authors consider the propagation characteristics and mode conversion of axisymmetric modes in an imperfect SELFOC fiber with longitudinal gradually varying dielectric constant K=ϵ/ϵ₀=K₀-K ₂(z)r². An analytic solution which is expressed in terms of generalized Laguerre polynomials is found     

Minimum pulse broadening in multimode fibers with index imperfections

Minimum pulse broadening of multimode graded-index fibers is investigated theoretically. Exact solutions of the rms pulse-width σ for a fiber with a power-law index profile is obtained by using the integral expression of group delay time based the multilayer approximation. It is shown that the minimum value of σ and the optimum α for the power-law index profile calculated by the multi-layer approximation in the steady-state modal power distribution are nearly equal to the well-known WKB solution. Next,sigma_{min}and the corresponding αoptare evaluated for certain index imperfections such as a central index dip and sinusoidal index ripple along a radial direction. The results offer the possibility to avoid the bandwidth degradation due to an inevitably generated index tipple in an MCVD graded-index fiber.     

Analysis of fields and coupling associated with optical fibers withpower-law profiles

The fundamental mode fields of optical fibers are of importance for various applications. The field and relevant properties are directly related to the index profile of the fiber itself. In practice, index profiles for many fibers can be represented by power-law functions. Within the framework of a field approximation method the authors systematically study fundamental mode fields of optical fibers with clad power-law profiles, as well as coupling occurring between two such fibers     

Measuring the longitudinal distribution of four-wave mixingefficiency in dispersion-shifted fibers

We measured and analysed four-wave mixing (FWM) efficiency and fluctuation along the actual dispersion-shifted fibers (DSF's). In the short, 1.1-km DSF, our results agreed with theory, and we also obtained zero-dispersion wavelength λ₀, zero-dispersion slope dD/dλ, and nonlinear refractive index n₂ simultaneously. In the long, 23-km, DSF, a high FWM efficiency was observed in the wide wavelength region, due to the longitudinal zero-dispersion wavelength distribution. The fluctuation range was about 3.5 nm and the maximum slope about 1.1 km/nm     

General solutions for stress-induced polarization in optical fibers

General solutions of thermal stresses and material birefringence in polarization-maintaining optical fibers are presented. The solutions give the stresses and birefringence at every point in the cross section of various fibers. The fundamental solution of the stress field is constructed by using the complex variable method in elasticity. When the cores, inner claddings, or stress-inducing regions are ellipses, circles, or bow-tie shapes, exact closed-form solutions can be found. The average core birefringence and center core birefringence of elliptical core, elliptical cladding, twin core, Panda, and bow-tie fibers are calculated and compared. Among these fibers, the bow-tie fiber shows the highest birefringence     

Deformation of single-mode optical fibers under static longitudinal stress

The deformation of single-mode fibers resulting from a longitudinally applied static force has been measured experimentally by means of high resolution heterodyne interferometry and analyzed theoretically using the second-order theory of elasticity and the photoelastic effect. Both the elongation of the fiber and the phase change of light propagating through the fiber have been measured as a function of tensile force. The values of the elastic constants measured for fibers with pure silica core and B2O3doped cladding areE = 6.41 times 10^{10}N/m²for the Young's modulus,delta = -4.0for the nonlinearity constant of the longitudinal strain, andbeta = -2.3for the nonlinearity constant of the transverse strain. For unit elongations up to 0.3 percent, no creep, hysterisis, or relaxation effects have been observed within a resolution of one part in 10⁴.     

BER performance of multiple-antenna OFCDM with imperfections

This letter investigates the performance degradation due to the imperfect parameters in orthogonal frequency- and code-division multiplexing (OFCDM) systems with multiple antennas. For the performance evaluation, the average bit error rate (BER) expression is generalized to the multiple-antenna case in the presence of the channel estimation error and frequency offset. Derived results show that the multiple-antenna OFCDM systems experience a severe BER degradation regardless of the number of users.     

Axial cross polarization in reflector antennas with surface imperfections

Expressions for the probability density function and mean of boresight cross polarization field intensity in terms of the rms surface errorepsilonare derived. It is shown that for small errors the mean on-axis cross polarization field intensity is directly proportional to the rms surface error. For relatively large errors (epsilon > 0.04 lambda) forward cross polarization is proportional to the square root of (1 - exp (-2sigma^{2})) wheresigmais the rms phase error. It is also shown that forward cross polarization is proportional to the correlation diameter and is a function of the polarization efficiency. Experimental measurements are revealed and results are compared with theoretical findings.     

Splice loss in dispersion-modified single-mode silica fibers:effect of non-Gaussian fields

The nature of non-Gaussian fields in dispersion-modified single-mode silica fibers is examined to show the limitations of using two well-known Gaussian approximations for the theoretical prediction of splice loss caused by lateral and angular misalignments. To quantitatively evaluate this limitation, four dispersion-modified silica fiber designs were taken from the literature (a single-, a double-, a triple-, and a quadrupole-clad design) and splice losses calculated with Gaussian approximations were compared to that obtained by numerically integrating the overlap integral of the exact field distribution of the fundamental mode. The accuracy of each is discussed     

The effect of wiggler imperfections on nonlinear harmonicgeneration in free-electron lasers

The generation of harmonics through a nonlinear mechanism driven by bunching at the fundamental has sparked interest in using this process as a path toward an X-ray free-electron laser (FEL). An important issue in this regard is the sensitivity of the nonlinear harmonic generation to wiggler imperfections. Typically, linear instabilities in FELs are characterized by increasing sensitivity to both electron beam and wiggler quality with increasing harmonic number. However, since the nonlinear harmonic generation mechanism is driven by the growth of the fundamental, the sensitivity of the nonlinear harmonic mechanism is not severely greater than that of the fundamental. In this paper, we study the effects of wiggler imperfections on the nonlinear harmonics in a 1.5-Å FEL, and show that the decline in the third harmonic emission with increasing levels of wiggler imperfections roughly tracks that of the fundamental     

Location of imperfections in optical glass-fibre waveguides

An echo-pulse technique for the location of fibre imperfections is described. The technique has been successfully applied to a plastics-coated Selfoc fibre and a plastics-coated silica-based fibre. Both showed a break. In the Selfoc fibre, an additional fibre defect was located. The same method may be used to measure rapidly the fibre length.     

AC losses in HTS tapes in applied longitudinal magnetic fields at variable temperatures

In most power devices, the conductor is carrying an ac transport current while it is exposed to an ac magnetic field transverse to the current path. In certain applications, such as power cables or a control winding in a controllable reactor, the conductors are exposed to a magnetic field component longitudinal to the tape axis that is parallel to the current path. To create an improved base for the design of such power devices it is of interest to study the losses in high-temperature superconductor tapes due to longitudinal field in detail. We have investigated the losses at several temperatures of a nontwisted multifilamentary Bi-2223 tape when it was exposed to a longitudinal magnetic field. The losses were measured with a calorimetric method and the results were compared with the critical state hysteresis loss model. The hysteresis losses are dominating at power frequencies (50, 60 Hz) in the investigated field range 2-200 mT and are accurately described by the critical state hysteresis model.     

Digital precompensation of imperfections in quadrature modulators

Residual carrier, balance, and quadrature error imperfections that are normally present in phase/quadrature modulators limit the use of direct modulation techniques in some applications. The classical precompensation techniques are revised and an adaptive solution proposed that improves performance by some order of magnitude     

Analysis of optical fibers with graded-index profile by acombination of modified Airy functions and WKB solutions

An almost exact eigenvalue equation for optical fibers with graded-index profile is derived mathematically based on a combination of the modified Airy functions and the Wentzel, Kramers, Brillouin (WKB) trial solution. By applying proper boundary conditions, a phase shift correction term δ is found out which improves the inherent error problems of the conventional WKB method. It is shown through computer simulations that results of the derived eigenvalue equation are in excellent agreement with those of the finite-element method     

The capacity of a spread spectrum CDMA system for cellular mobileradio with consideration of system imperfections

There has been much interest in the use of spread spectrum code division multiple access (CDMA) techniques for cellular mobile radio. To date, spread spectrum has been used mainly for military applications, in which the inherent transmission security and immunity to deliberate jamming are important. Spread spectrum systems, however, possess various other features such as multiple access and multipath rejection capability, which make their use attractive within the mobile radio environment. However, the current interest has been principally motivated by the work of Gilhousen et al. (see IEEE Trans. Vehic. Technol., vol.VT-40, no.2, p.303, 1991) in which it is claimed that the CDMA option may offer capacity improvement over more conventional frequency and time division multiple access, FDMA and TDMA, techniques. Within this paper, the relative capacities of a basic FDMA and CDMA system are examined. It is shown that, in the absence of capacity-enhancing features such as voice activity detection and cell sectorization, the capacity of each system is comparable. The paper then assesses the sensitivity of the CDMA system to typical propagation conditions, power control errors, and realistic antenna patterns and shows that the capacity of a CDMA system may be significantly reduced under nonideal conditions     

Nonlinear longitudinal waves of interacting fields of deformation and defect concentration in germanium and silicon

A system of equations is formulated to describe the self-consistent behavior of elastic displacement fields and the concentration field of point defects in irradiated crystals with a symmetry center (germanium, silicon). In accordance with the values of the defect relaxation times, the model evolution equations are derived, describing the steady-state nonlinear longitudinal waves, with regard to the flexoelectric effect. The effect is due to the dielectric polarization induced by nonuniform elastic deformations of the lattice. For a particular relationship between the coefficients of these equations, i.e., between the parameters of the subsystem of defects and those of the nonlinear elastic medium, the exact solutions are obtained, which describe the generation of solitons and low-intensity shock waves. The contributions of the strain-defect interaction and the flexoelectric effect to the linear velocity of sound and the dispersion properties of the medium are estimated.     

Adaptive beamforming with the generalized sidelobe canceller in the presence of array imperfections

Antenna designers often employ linearly constrained adaptive beamforming as an antijamming measure. With minimal a priori knowledge of the signal environment, this technique nulls out jammers while simultaneously preserving the quality of the main lobe so that a friendly look-direction signal can be received with unity gain. Unfortunately, in the absence of special strategies, linearly constrained adaptive beamforming is hypersensitive to array imperfections when the input signal-to-noise ratio exceeds a certain threshold. This hypersensitivity manifests itself as a nailing of the friendly signal as if it were a jammer. Luckily, the signal nulling problem can be easily remedied by artificial receiver noise injection. A particularly simple and general structure for linearly constrained adaptive beamforming was proposed during the 1970's, and is known as the generalized sidelobe canceller. A detailed analysis of the generalized sidelobe canceller in the presence of array imperfections is discussed, and two new artificial receiver noise injection algorithms are proposed. Computer simulations are included to demonstrate that use of these new algorithms alleviates the signal nailing problem without seriously compromising jammer nulling. For the special case of the Capon maximum-likelihood beamformer, simple approximations are presented for: 1) the Wiener output signal-to-interference-plus-noise ratio (SINR_{0}astr), 2) the antenna element error variance that causes a 3 dB loss ofSINR_{0}astrfrom its value for an ideal array, and 3) the optimal artificial receiver noise that maximizesSINR_{0}astr.     

Full-wave solutions for the scattered radiation fields from rough surfaces with arbitrary slope and frequency

Full-wave solutions are derived for the scattered radiation fields from rough surfaces with arbitrary slope and electromagnetic parameters. These solutions bridge the wide gap that exists between the perturbational solutions for rough surfaces with small slopes and the quasi-optics solutions. Thus it is shown, for example, that for good conducting boundaries the backscattered fields, which are dependent on the polarization of the incident and scattered fields at low frequencies, become independent of polarization at optical frequencies. These solutions are consistent with reciprocity, energy conservation, and duality relations in electromagnetic theory. Since the full-wave solutions account for upward and downward scattering, shadowing and multiple scatter are considered. Applications to periodic structures and random rough surfaces are also presented.     

Modeling the effects of electric fields on nerve fibers:Determination of excitation thresholds

We have developed a method to predict excitation of axons based on the response of passive models. An expression describing the transmembrane potential induced in passive models to an applied electric field is presented. Two terms were found to drive the polarization of each node. The first was a source term described by the activating function at the node, and the other was an ohmic term resulting from redistribution of current from sources at other nodes. A total equivalent driving function including both terms was then defined. We found that the total equivalent driving function can be used to provide accurate predictions of excitation thresholds for any applied field. The method requires only knowledge of the intracellular strength-duration relationship of the axon, the passive step response of the axon to an intracellular current, and the values of the extracellular potentials. Excitation thresholds for any given applied field can then be calculated using a simple algebraic expression. This method eliminates the errors associated with use of the activating function alone, and greatly reduces the computation required to determine fiber response to applied extracellular fields.     

Modeling the effects of electric fields on nerve fibers: influenceof tissue electrical properties

The effects of anisotropy and inhomogeneity of the electrical conductivity of extracellular tissue on excitation of nerve fibers by an extracellular point source electrode were determined by computer simulation. Analytical solutions to Poison's equation were used to calculate potentials in anisotropic infinite homogeneous media and isotropic semi-infinite inhomogeneous media, and the net driving function was used to calculate excitation thresholds for nerve fibers. The slope and intercept of the current-distance curve in anisotropic media were power functions of the ratio and product of the orthogonal conductivities, respectively. Excitation thresholds in anisotropic media were also dependent on the orientation of the fibers, and in strongly anisotropic media (sigma z/sigma xy > 4) there were reversals in the recruitment order between different diameter fibers and between fibers at different distances from the electrode. In source-free regions of inhomogeneous media (two regions of differing conductivity separated by a plane boundary), the current-distance relationship of fibers parallel to the interface was dependent only on the average conductivity, whereas in regions containing the source the current-distance relationship was dependent on the individual values of conductivity. Reversals in recruitment order between fibers at different distances from the electrode and between fibers of differing diameter were found in inhomogeneous media. The results of this simulation study demonstrate that the electrical properties of the extracellular medium can have a strong influence on the pattern of neuronal excitation generated by extracellular electric fields, and indicate the importance of tissue electrical properties in interpreting results of studies employing electrical stimulation applied in complex biological volume conductors.