The diffraction of a plane electromagnetic wave by a perfectly-conducting finite sinusoidal grating is analyzed using the Wiener-Hopf technique combined with the perturbation method. Assuming the depth of the grating to be small compared with the wavelength and approximating the boundary condition on the grating surface, the problem is reduced to that of the diffraction by a flat strip with a certain mixed boundary condition. Introducing a perturbation expansion for the unknown scattered field under the small-depth approximation, the problem is formulated in terms of the zero order and the first order Wiener-Hopf equations, which are solved exactly in a formal sense by a factorization and decomposition procedure. Applying a rigorous asymptotics, explicit high-frequency asymptotic solutions are further obtained for the width of the grating large compared with the wavelength. Scattered far field expressions are derived by taking the inverse Fourier transform are applying the saddle point method. Representative numerical examples of the far field patterns are given for various physical parameters, and the scattering characteristics of the grating are discussed.

A mode-matching method in the sense of least squares is applied for analyzing grating couplers having various groove shapes. These couplers are formed on surfaces of core regions of the thin-film waveguides and their periodic parts extend finitely. The grating couplers are analyzed for the plane wave incidence when the Bragg condition is satisfied. The discussion is developed about the grating coupler which is formed on the asymmetric waveguide and has arbitrary triangular grooves. However various couplers, which have triangular, rectangular and sinusoidal grooves and are formed on asymmetric and symmetric waveguides, are analyzed and results for them are presented in this paper. The approximate scattered fields of each region of the coupler are described by the superpositions of the plane waves with band-limited spectra, respectively. These approximate wave functions are determined by the minimization of the mean-square boundary residual. This method results in the simultaneous Fredholm type integral equations of the second kind for these spectra. Results of analyses based on the first order approximate solutions are presented. The comparison between each coupling efficiency derived by the present method for couplers having sinusoidal, triangular and rectangular grooves is presented in this paper.

The Fourier series expansion method is applied to the problem of EM waves guided by a dielectric grating with periodic surface-relief. The periodic structures of dielectric gratings are divided into two types: (1) surface-relief type and (2) modulated refractive index type. For the modulated dielectric grating, the rigorous solution is obtained using the space-harmonics expansion method, but the usual method cannot be applied to the grating whose dielectric constant distribution has discontinuities. Previously, we pointed out this issue and gave an improved method which can be applied to those problems. This method can also be applied to the surface-relief grating by approximating it as an assembly of stratified thin modulated dielectric gratings. In this paper, numerical analysis is given for gratings with sinusoidal surface and triangular surface for both perpendicular polarization (TE waves) and parallel polarization (TM waves). The validity of our method has been checked by increasing the number of modal expansion terms and the number of multilayers. The results are compared with those obtained by other numerical methods. The differences in the propagation characteristics between the surface-relief grating and the modulated dielectric grating in Bragg region are also discussed.

This paper discusses an efficient numerical calculation technique for scattering from three dimensional bodies of perfect conductors and dielectric materials. The key of this method is to express the scattering fields by the field generated by the distributed hypothetical dipoles (or the equivalent sources). Since the scattered field should be analytic in each homogeneous sub-domain of general scattering bodies, the equivalent sources to be used for expressing the scattered field in one of those sub-domains should be distributed outside of this space domain. Unknown vector currents of equivalent dipoles are solved by the generalized mode-matching method. Example scatterers discussed here are finite conducting and dielectric cylinders with rotationary symmetry. The results are discussed with comparative data obtained by other numerical method and also with experiments that we took.

Electromagnetic wave scattering from a perfectly conducting indented body of revolution is analyzed both in the frequency and time domains. The three-dimensional (3-D) scattering process and the effect of polarization on scattering characteristics are revealed. A well-defined scattering matrix representation is adopted for investigating the polarimetric property of the 3-D scattering. First, the scattering cross sections are calculated in the frequency domain by using the Yasuura method (Mode-Matching Method) which is a powerful and reliable numerical method for solving electromagnetic boundary value problems. Next, co-polarization and cross-polarization components of the pulse response waveforms are calculated from the scattering matrix by using the Fourier synthesis technique. It is found out from the numerical results that the pulse responses from the indented objects can be interpreted as three distinctive waves: conventional specularly-reflected wave, creeping wave and complex specularly-reflected wave which is reflected from a complex specular reflection point. These scattering processes are consistently explained by employing the extended ray theory. And the radar polarimetries of these three distinctive waves are clearly observed both in co-polarization and cross-polarization components of scattering cross sections and pulse responses.

FILT (Fast Inversion of Laplace Transform) method combined with MPMM (Modified Point Matching Method) is used to solve the problem of electromagnetic wave scattering by a circular cylinder with longitudinal slots. Some numerical results for the far field back scattering transient response are presented and discussed by comparing with the response for the perfect cylinder. The physical meaning of the transient wave form is discussed in detail. Also the precision of the analysis is checked carefully.

The characteristics of transient electromagnetic responses between two dipole antennas are investigated theoretically and experimentally for the case where one dipole antenna is located above an interface of a lossy ground half-space and another is buried underground. First, an asymptotic expression for the refracted electric fields is derived based on the saddle point method when a horizontal dipole is buried in the lossy half-space. A suitable saddle point which differs from the conventional one is selected for evaluation when the observation point is located far enough away from the buried antenna. The resultant formula indicates that the refracted electric fields can be interpreted by the sum of two waves. When the observation point is very close to the interface, one of which is a direct wave which propagates from the buried antenna toward the interface point just beneath the observation point and the other a lateral wave which propagates from the buried antenna to the interface with an angle of total reflection and then propagates along the interface. Next, the transient reception voltages of the antenna over the ground surface are measured. It is shown that the experimental results agree very well with the theoretical results, confirming the validity of the theory.

A simple and practically accurate method for studying rectangular dielectric waveguides is proposed. This method can provide good evaluations of propagation constant and field distribution of the waveguides by analyzing improved equivalent guide models. In this paper, it is applied in studying a novel taper-formed waveguides system. The waveguides system was experimentally demonstrated of being advantageous in its lightwave transmission characteristics. Experimentally obtained results exhibited good agreement with the performances predicted by the system numerical analyses using the proposed waveguide study technique.

Five channel optical waveguides are investigated numerically by using mode-matching method. The precise numerical results of the dispersion relations near the cutoff and the field distributions are presented for the lowest five HE modes. When the geometric parameters of the waveguides can be optimized so as to satisfy the phase-matching condition, it is shown that the five channel optical waveguides operate as power divider.

This paper presents a useful numerical approach based on a self-consistent finite-element method for solving stationary properties of third-order nonlinear guidedwave phenomena in a planar optical waveguide which supports nonlinearly coupled transverse-electric and transverse-magnetic modes. This method can be useful for the stability analysis as well by tracing intermediate solutions generated through iterative processes. Depending on the transitional behavior of the intermediate solutions we can identify the nonlinear excitation under consideration to be absolutely stable, quasi-stable, or unstable.

An analytical theory of the quasi LP-modes in anisotropic optical fibres with a homogeneous circular core and homogeneous cladding is developed. Analytical expressions for the quasi LP-mode fields are obtained by the separated variable method and by the perturbation method under the following conditions. The Cartesian permittivity tensors of the core and cladding are diagonal and have the same orientation in the crosssection of the fibre. The longitudinal components of these permittivity tensors are equal, and the differences between the transversal and longitudinal components of these tensors are sufficiently small. The transversal components of the quasi LP-mode fields are derived from Maxwell's equations in the linear approximation on these differences. Solutions of the mode characteristic equation for the quasi LP-modes are reduced to the known solutions of the characteristic equation for the LP-modes in the isotropic optical fibre.

A design method to realize the large modal birefringence and the low dispersion (within 2 ps/km/nm) over the spectral range 1.30-1.55µm is investigated for the polarization-maintaining optical fibers with the crescent or circular hollow pits. When the low dispersion is satisfied for the HE₁₁^o mode, one of two orthogonally polarized HE₁₁ modes, it is found that (1) the crescent pits type has a larger modal birefringence than the circular pits type for Δ₁1.21% (Δ₁: the relative index difference), and (2) the maximum modal birefringence of the crescent pits type is nearly equal to that of the circular pits type for Δ₁1.21%.

Generally speaking, a field generated by a current located on any surface depends on the shape of the surface and the current value and its distribution. The main idea discussed below is based on the existence of such surfaces for which a certain property of fields generated by currents distributed on these surfaces does not depend on the currents and is fully and soley determined by the surfaces. This is such a property that any complete system of functions generates an incomplete system of fields. This means that it is impossible not only to realize but even to approximate any given field by the generated field if the given field does not satisfy this condition. In this case, the corresponding inverse problem of the approximate current synthesis is not solvable. Practical results have been received for the horn construction, the determination of the scattering body shape and so on.

This paper discusses on the methods and their numerical simulation results of the one-dimensional profiling of dielectric targets in the direction of electromagnetic wave incidence. Two different inverse profiling algorithms are proposed. First method is based on the Rayleigh approximation. The inverse profiling is carried out by performing the Fourier transformation of the backscattered field in frequency domain. In this method, the dielectric constant of the target is assumed as known beforehand. The second method is based on the extended physical optics (EPO) method. The EPO method was proposed for calculating the backscattering cross section for a conducting scatterer, however, we apply the EPO method to the inverse problem of the dielectric target whose dielectric constant is not known. The validity and limitation of each method are tested numerically for spherical and spheroidal targets.

A radial line slot antenna (RLSA) is a high gain and high efficiency planar antenna for DBS reception and VSAT terminals. To generalize the slot design of RLSA, it is necessary to estimate the radiation, reflection and the transmission characteristics of a slot. This paper presents an equivalent circuit of a slot on radial waveguide. A periodic structure model is used in the moment method analysis, to simulate the slot coupling in rotationally symmetric antenna operation in the oversized radial line. Radiation and reflection coefficients are expressed in the S-matrix form; the slot length dependence of the matrix element is discussed.

Tunable fiber lasers with ring resonator structure were reported in this paper. A tunable fiber directional coupler links the rear-earth-doped single-mode fiber to from a ring resonator. Laser oscillation should occur in the fiber ring cavity when the rear-earth ions in the fiber are excited by a pumping light. The losses of the directional coupler and rear-earth-doped fiber are very small so that the fiber ring laser has low threshold and high slope efficiency. In this fiber ring laser, the directional coupler plays three functions of transmitting pumping light, coupling lasing oscillation and tuning laser wavelength. We have used both Nd³⁺-doped and Er³⁺-doped single-mode fiber to fabricate the tunable fiber ring lasers. An argon ion laser was used as the pump source. For the Nd³⁺-doped fiber ring lasers with cavity lengths of 3 meters and 5 meters, the lasing threshold were 0.8 mW and 4.9 mW respectively at the lasing wavelength of 1.088 nm. The highest slope efficiency was about 30%. The laser wavelength could be tuned continuously in the Nd³⁺ fluorescent spectrum and the tunning range is greater than 60 nm. The Er³⁺-doped fiber ring lasers had the cavity lengths of 1 meter and 8 meters, lasing at 1550 nm with a tuning range of about 70 nm, the thresholds were 1.9 mW and 14.2 mW and the slope efficiencies were 6.1% and 10.2% respectively. A novel fiber ring laser with single-longitudinal-mode output was demonstrated. This fiber laser has a composite resonator which is composed of two subrings. Longitudinal mode is selected in the subrings coupled each other. A laser linewidth less than 40 MHz was got from the Nd³⁺-doped fiber ring laser.

In this letter, the relations between the generalized beams with the different parameters are investigated by using the formula for the Hermite polynomials and that for the generalized Laguerre polynomials. The adjoint functions for the generalized Laguerre-Gaussian beam functions are also derived concretely. It is shown that the generalized beam is represented by a finite number of another generalized beam with the different parameter.

A method of calculating geometrical optics fields in dielectric tapered waveguides excited at the input ends by a monochromatic Gaussian beam light is presented. It is shown that field distributions expected at the output ends of waveguides can be determined without calculating fields at intermediate steps of propagation between the input and output ends. Examples of multimode propagation and monomode propagation in a linearly tapered waveguide are given.