Achromatic nanostructured gradient index microlenses.

We present a development of microlenses achromatically corrected in near-infrared spectral windows. We show that the standard fiber drawing technology can be successfully applied to the development achromatic gradient index microlenses by means of internal nanostructurization. These gradient index microlenses can achieve similar performance to standard aspheric doublets, while utilizing a simpler, singlet element geometry with flat surfaces. A nanostructured lens with a parabolic profile was designed using a combination of the simulated annealing method and the effective medium approximation theory. Measurements on the fabricated lenses show that the microlenses have a nearly wavelength-independent focal plane at a distance of about 35 µm from the lens facet over the wavelength range of 600-1550 nm. The successful design and fabrication of achromatic flat-parallel rod microlenses opens new perspectives for micro-imaging systems and wavelength-independent coupling into optical fibers.

Nanostructured elliptical gradient-index microlenses.

We show theoretical and experimental characterizations of a nanostructured gradient-index lens. The elliptical lens is a nonguiding element fabricated using the mosaic method, which is widely used for the fabrication of photonic crystal fibers. For the first time we show experimental data in the optics regime that confirm the effective medium approximation for discrete mosaic structures with subwavelength feature size. This opens the door for the development of general asymmetric gradient-index materials.

Design and fabrication of nano-structured gradient index microlenses.

We present a novel fabrication technology for nano-structured graded index micro-optical components, based on the stack-and-draw method used for photonic crystal fibres. These discrete structures can be described with an effective refractive index distribution. Furthermore we present spherical nano-structured microlenses with a flat facet fabricated with this method and designed using an algorithm based on the Maxwell-Garnett mixing formula. Finally we show theoretical verification by using FDTD simulations for a nano-structured lens as well as experimental data obtained in the microwave regime.

Dual-wavelength operation diffractive phase elements for pattern formation.

We report on the design and fabrication of novel diffractive phase elements that reconstruct distinct intensity patterns in the far-field on illumination with two specific wavelengths. The elements contain deep surface-relief structures that represent phase-delays of greater than 2p radians. The design process incorporates a modified version of the iterative Fourier transform algorithm. A 16 phase-level element for dual wavelength (blue and red) operation, with high diffraction efficiency, is demonstrated experimentally.

A neural-network packet switch controller: scalability, performance, and network optimization.

We examine a novel combination of architecture and algorithm for a packet switch controller that incorporates an experimentally implemented optically interconnected neural network. The network performs scheduling decisions based on incoming packet requests and priorities. We show how and why, by means of simulation, the move from a continuous to a discrete algorithm has improved both network performance and scalability. The system's limitations are examined and conclusions drawn as to its maximum scalability and throughput based on today's technologies.

Comparison of simulated quenching algorithms for design of diffractive optical elements.

We compare the performance of very fast simulated quenching; generalized simulated quenching, which unifies classical Boltzmann simulated quenching and Cauchy fast simulated quenching; and variable step size simulated quenching. The comparison is carried out by applying these algorithms to the design of diffractive optical elements for beam shaping of monochromatic, spatially incoherent light to a tightly focused image spot, whose central lobe should be smaller than the geometrical-optics limit. For generalized simulated quenching we choose values of visiting and acceptance shape parameters recommended by other investigators and use both a one-dimensional and a multidimensional Tsallis random number generator. We find that, under our test conditions, variable step size simulated quenching, which generates each parameter's new states based on the acceptance ratio instead of a certain theoretical probability distribution, produces the best results. Finally, we demonstrate experimentally a tightly focused image spot, with a central lobe 0.22-0.68 times the geometrical-optics limit and a relative sidelobe intensity 55%-60% that of the central maximum intensity.

Diffractive optical elements for beam shaping of monochromatic spatially incoherent light.

Fresnel-type diffractive optical elements (DOEs) for general beam shaping of monochromatic, spatially incoherent light are demonstrated. Direct and indirect methods, i.e., adding a lens' phase to the designed Fraunhofer-type DOEs, are used for the design. The indirect method can reduce the calculation time by approximately half without loss of design accuracy. Two different design examples are shown. For one design the direct method gives a maximum sidelobe intensity of 5.0% of the maximum intensity in the signal window. For the second design the indirect method gives 23.0% of this value. The generated patterns can maintain their basic shapes over a long distance. The elements have been fabricated by directly using gray-scale commercial slides as masks. Experimental results are in close agreement with numerical predictions.

Comparison of one- and two-dimensional dielectric reflector geometries for high-energy laser pulse compression.

We present a high-efficiency reflective lamellar grating geometry, based on a two-dimensional photonic bandgap structure, that we predict will provide significantly improved resistance to laser-induced damage. Two independent numerical methods are used to compare the performance of this geometry with that of a conventional multilayer dielectric stack.

Optimized quantization for diffractive phase elements by use of uneven phase levels.

Many applications of diffractive phase elements involve the calculation of a continuous phase profile, which is subsequently quantized for fabrication. The quantization process maps the continuous range of phase values to a limited number of discrete steps. We present a new scheme with unevenly spaced levels for the design of diffractive elements and apply it to the design of intracavity mode-selecting elements. We show that this modified quantization can produce significantly better results than are possible with a regular or even the bias-phase-optimized quantization scheme that we reported here earlier. In principle this process can be employed to a greater or lesser extent in any quantization process, allowing the fabrication of diffractive elements with much improved performance.

Analysis of gratings with large periods and small feature sizes by stitching of the electromagnetic field.

We present a new method for the analysis of diffractive optical elements, which we refer to as field stitching. It is suitable for use with grating structures of arbitrarily large period, even when the local feature size is of the order of a wavelength. Furthermore, the concept is straightforwardly extendable to aperiodic structures. To assess its applicability, we have calculated the diffracted orders from a 1 x 81 fan-out grating with periods of 100lambda and 10, 000lambda. The field-stitched calculations agree very well with independent rigorous predictions for the small-period element and scalar-regime predictions for the large-period element. We believe that a variety of areas within the diffractive-optics field will benefit from this new analytical tool. It promises accurate analysis and, by facilitating component optimization, high-performance designs.

Iterative algorithm for the design of diffractive phase elements for laser beam shaping.

An improved iterative algorithm for designing diffractive phase elements for laser beam shaping in free space is presented. The algorithm begins with the Gerchberg-Saxton approach to obtain a stable solution. This is followed by several new iterations, in which modified constraining functions are imposed in the Fourier domain while the phase distribution of each iteration remains unchanged. For super-Gaussian beam shaping suitable for inertial confinement fusion applications the mean-square errors of the amplitude and the intensity profile of the entire beam fitted to the corresponding parameters of the 12th-power super-Gaussian beam are approximately 0.035 and 9.75x10(-3), respectively. Approximately 97.4% of the incident energy is converged into the desired region.

Optimized phase quantization for diffractive elements by use of a bias phase.

Many applications of diffractive phase elements involve the calculation of a continuous phase profile that is subsequently quantized for fabrication. The quantization process maps the continuous range of phase values to a limited number of discrete steps. We report our observation of the influence of this quantization process on the performance of mode-selecting diffractive elements and show that the quantization process produces significantly better results by use of an optimized bias phase. In principle this process can be employed to a greater or lesser extent in any quantization process.

Application of optimization algorithms to the design of diffractive optical elements for custom laser resonators.

We report what we believe to be the first applications of numerical optimization algorithms to the design of diffractive elements that customize the fundamental mode profile of a laser system. Standard design techniques treat these elements as specific phase-conjugation devices, which leads to performance loss when they are quantized to permit fabrication. Numerical optimization can account for quantization of the element to increase the effective performance. Also, it is shown that allowing a slight increase in the intrinsic loss of the cavity can substantially increase the fidelity of the fundamental mode of the customized cavity. The good discrimination qualities of the mode-selection elements are shown to be unaffected by this process.

Effects of dead zones in multiple-quantum-well binary-phase modulators on optical interconnections.

We investigate the effects of inactive regions [dead zones (DZ's)] in multiple-quantum-well binary-phase modulators used for free-space dynamic optical interconnection applications. Results, however, have implications for other types of pixelated spatial light modulators (SLM's). To our knowledge, the effects of DZ's in SLM's have not before been thoroughly studied in a context other than optical correlation. We investigate the DZ's (considered to be either opaque or transmissive) as a feature that may be exploited in system design, calculating light efficiency and fidelity as a function of DZ fractional width. It is shown that in particular cases an appropriate choice of DZ width would lead to an optical interconnection with substantially improved cross-talk performance.

Regenerative ring-laser design by use of an intracavity diffractive mode-selecting element.

We report what we believe to be the first application of diffractive phase elements for transverse mode selection in laser ring resonators. We show that this resonator type offers several advantages over Fabry-Pérot resonators with diffractive mirrors. The design for a regenerative ring resonator that produces an eighth-order super-Gaussian intensity profile beam is presented. Numerical simulations, including modeling of the gain saturation and experimental tests, have been carried out to demonstrate the performance of this approach for cw and pulsed operations.

Interference effects in far-field diffractive optical elements.

The effects of interference between closely packed diffraction orders in the far field are studied for a number of different scalar-domain diffractive optical elements (DOE's). We demonstrate that there are specific order separations that minimize the observed degradation in the far-field output uniformity. Finally, a DOE that is designed to ensure that the order separation lies near one of these minima is compared with a more general design that produces an equivalent far-field output.

High-efficiency, wide-bandwidth optical fanout elements in dichromated gelatin.

We have combined the flexibility of computer-generated binary holograms with the high optical quality of dichromated gelatin holography to fabricate a range of efficient, wide-bandwidth, and compact space-invariant fanout elements. Our approach is based on a certain novel coherent spatial filtering technique, termed the inverse central dark ground method, which permits the use of a binary-amplitude hologram corresponding to the desired binaryphase profile in the object arm of the interferometric recording setup.

Fabrication of space invariant fanout components in dichromated gelatin.

Efficient and compact space invariant fanout holographic optical elements have been fabricated by combining the flexibility in design of computer-generated holograms with the versatility of conventional interferometric holography in dichromated gelatin. Effects of K-ratio variation and the phase distortion in the reconstruction fidelity are discussed. In addition, various preprocessing and postprocessing techniques have been employed to increase the optical damage threshold of these components to well over 100 W cm(-2).

Interconnects for a symmetric-self-electro-optic-effect-device cellular-logic image processor.

An optical circuit is successfully operated by interconnecting two arrays of 128 symmetric self-electro-opticeffect devices. The holographic interconnect used in this cellular-logic image processor is described. The design issues (noise orders, efficiency, and ease of alignment) associated with the interconnect and and extensions of it are discussed.

Multilevel phase-only array generators with a trapezoidal phase topology.

The implementation of a two-stage design process for the design of diffractive optical elements for array illumination is described. Results are presented for on-axis two-dimensional array illuminators for which this method is used. The final designs are theoretically within 5% of the calculated diffraction efficiency upper bound, and theoretical signal reconstruction error is below 1%. Experimental verification of the design theory is given, with experimental diffraction efficiencies within 4% of design values and signal reconstruction error below 6%.