John J. Ottusch

Scalar and Tensor Holographic Artificial Impedance Surfaces

We have developed a method for controlling electromagnetic surface wave propagation and radiation from complex metallic shapes. The object is covered with an artificial impedance surface that is implemented as an array of sub-wavelength metallic patches on a grounded dielectric substrate. We pattern the effective impedance over the surface by varying the size of the metallic patches. Using a holographic technique, we design the surface to scatter a known input wave into a desired output wave. Furthermore, by varying the shape of the patches we can create anisotropic surfaces with tensor impedance properties that provide control over polarization. As an example, we demonstrate a tensor impedance surface that produces circularly polarized radiation from a linearly polarized source.

Holographic artificial impedance surfaces for conformal antennas

We have developed a method for generating arbitrary radiation patterns from antennas on complex objects. The object is coated with an artificial impedance surface consisting of a lattice of sub-wavelength metal patches on a grounded dielectric substrate. The effective surface impedance depends on the size of the patches, and can be varied as a function of position. Using holography, the surface impedance is designed to generate any desired radiation pattern from currents in the surface. With this technique we can create antennas with novel properties such as radiation toward angles that would otherwise be shadowed

Stimulated Brillouin scattering phase-conjugation fidelity fluctuations.

We observe shot-to-shot fluctuations in the far-field fidelity of stimulated Brillouin scattering phase-conjugate mirrors in the saturated reflectivity regime under various experimental conditions. The fluctuations, which are not seen in the reflectivity, reflect a partial breakdown in the discrimination mechanism that selectively amplifies the conjugate mode above the stimulated Brillouin scattering noise seed.

Numerical solution of 2-D scattering problems using high-order methods

We demonstrate that a method of moments scattering code employing high-order methods can compute accurate values for the scattering cross section of a smooth body more efficiently than a scattering code employing standard low-order methods. Use of a high-order code also makes it practical to provide meaningful accuracy estimates for computed solutions.

Adaptive artificial impedance surface conformal antennas

Moving platforms, like manned and unmanned aerial vehicles, require adaptable directive antennas for both communication and radar applications. The integration of these antennas into the body of the platform drives significant development time and expense, and in many cases adversely affects the overall vehicle performance by adversely affecting its aerodynamics or weight. Being able to integrate the antenna function directly in the skin of the platform will result in simpler better performing vehicles. We have developed an AIB with which we have used to demonstrate 2D electronic beam steering. These results show how AIB technology can be utilized for the realization of next generation conformal, electrically steerable directive antennas.

Efficient high-order discretization schemes for integral equation methods

A high-order method is a method that provides extra digits of accuracy with only a modest increase in computational cost. A number of method of moment (MoM) techniques based on high-order basis and testing functions have been presented in the literature. Characteristically, these methods result in a substantial increase in precomputational cost principally due to the expensive numerical integration required for near interactions. This can be accelerated through the use of specialized quadrature schemes when available. Unfortunately, performing the double integration numerically over high-order functions can still be quite computationally intensive. A novel high-order technique based on a locally-corrected Nystrom scheme combined with advanced quadrature schemes is presented. It is shown that this method truly demonstrates high-order convergence for the solution of electromagnetic scattering problems with comparable computational cost to low-order schemes. The elegance of this technique is in its simplicity and ease of implementation. However, the power of the method is its ability to inexpensively provide true high-order convergence.

Efficient anti-Stokes Raman conversion by four-wave mixing in gases

We describe a means for efficient, energy-scalable generation of first-order anti-Stokes light by four-wave mixing, using collimated pump and first-order Stokes seed beams. We performed a one-dimensional, plane-wave analysis of the process, assuming steady-state conditions and monochromatic beams. A prescription for designing the optimum anti-Stokes converter with this technique is derived from this analysis, and we show that energy conversion efficiencies from pump light to anti-Stokes light of ~30% might be possible. We also report an unoptimized, proof-of-concept experiment that produced 40 mJ of anti-Stokes-shifted light at 432 nm from 1.2 J of 527-nm pump light in a 60-cm-long hydrogen cell.

Integral equations and discretizations for waveguide apertures

We present integral equations and their discretizations for calculating the fields radiated from arbitrarily shaped antennas fed by cylindrical waveguides of arbitrary cross sections. We give results for scalar fields in two dimensions with Dirichlet and Neumann boundary conditions and for (vector) electric and magnetic fields in three dimensions. The discretized forms of the equations are cast in identical format for all four cases. Feed modes can be TM, TE, or transverse electromagnetic (TEM). A method for numerically computing the modes of an arbitrarily shaped, cylindrical waveguide aperture is also given.

Energy scaling of phase-conjugate solid-state lasers

Phase conjugation offers a practical, realistic approach for scaling solid-state lasers to high energies and high peak powers with a minimum increase in complexity. The present approach involves coherently combining the outputs of multiple parallel amplifiers in a single phase-conjugate oscillator-amplifier configuration. The use of phase conjugation can eliminate phase distortions that would otherwise result from individual amplifiers having optical lengths that differ from one another by many optical wavelengths. The laser output energy can be scaled well beyond the limits imposed by traditional volume constraints of crystalline media. Hence, instead of selecting a laser medium based solely on the available sizes, a laser system designer can base the medium selection on tradeoffs among many other important material parameters. Since an increase in output energy also requires an increase in the energy incident on the PCM, the energy scalability of PCMs based on SBS has been actively investigated over the past decade. These investigations have shown that, while practical single-cell PCMs offer somewhat limited energy scaling potential, a series combination of two Brillouin cells can accommodate energies of several tens of joules. We summarize our effort in developing such a dual-cell PCM that has achieved excellent performance at energies approaching 5 J and is scalable to even higher energies.

Comments on "Numerical solution of 2-D scattering problems using high-order methods" [with reply]

For original article by L.R. Hamilton et al. see ibid., vol.47, no.4, p.683-91 (Apr. 1999).