Half a century of optics in computing--a personal perspective [Invited].

Optical signal processing and computing was triggered by the invention of the laser. Starting practically in 1960, it really took off with the introduction of the spatial-matched filter in 1964. Almost half a century later, research and engineering activity in the field continues unabated but in directions that could not have been anticipated in those early days. This paper presents an overview of the developments in the field, discussing the advantages, disadvantages, and limitations of optics in computing paradigms to indicate where and how optics can be exploited in this area. Initially, optical methods were introduced for processing analog signals. Early attempts to extend optical methods toward digital processing failed because the differences between photons and electrons were not properly appreciated. In the last part of the paper we show that some novel concepts and advanced technology may revitalize also optical processes within the digital computing world. This latter development is demonstrated by digital logic functions implemented on simple electro-optic networks. (My personal perspective on the role of optics in computing is deeply rooted in many years of collaboration with my late friend, H. John Caulfield, and I dedicate this paper to his memory.).

Detecting motion through dynamic refraction.

Refraction causes random dynamic distortions in atmospheric turbulence and in views across a water interface. The latter scenario is experienced by submerged animals seeking to detect prey or avoid predators, which may be airborne or on land. Man encounters this when surveying a scene by a submarine or divers while wishing to avoid the use of an attention-drawing periscope. The problem of inverting random refracted dynamic distortions is difficult, particularly when some of the objects in the field of view (FOV) are moving. On the other hand, in many cases, just those moving objects are of interest, as they reveal animal, human, or machine activity. Furthermore, detecting and tracking these objects does not necessitate handling the difficult task of complete recovery of the scene. We show that moving objects can be detected very simply, with low false-positive rates, even when the distortions are very strong and dominate the object motion. Moreover, the moving object can be detected even if it has zero mean motion. While the object and distortion motions are random and unknown, they are mutually independent. This is expressed by a simple motion feature which enables discrimination of moving object points versus the background.

Selection of dark modes in resonators with conical reflectors.

Selection of modes containing different dark regions was studied in resonators with conical reflectors. The possibility of selecting whole subgroups of such modes was shown in circularly symmetric resonators. To handle single-mode selection employing extra intracavity spatial filters, modified integral equations and a numerical method of their analysis are proposed. Usage of the filter symmetry reduces the size of the four-dimensional matrices corresponding to the equation kernels, and they are analyzed by algorithms for two-dimensional matrices with the best convergence. The optimum resonator parameters for effective selection of different dark modes are found.

Absorption enhancement by matching the cross-section of plasmonic nanowires to the field structure of tightly focused beams.

Nanostructured materials, designed for enhanced light absorption, are receiving increased scientific and technological interest. In this paper we propose a physical criterion for designing the cross-sectional shape of plasmonic nanowires for improved absorption of a given tightly focused illumination. The idea is to design a shape which increases the matching between the nanowire plasmon resonance field and the incident field. As examples, we design nanowire shapes for two illumination cases: a tightly focused plane wave and a tightly focused beam containing a line singularity. We show that properly shaped and positioned silver nanowires that occupy a relatively small portion of the beam-waist area can absorb up to 65% of the total power of the incident beam.

Material classification of nanoparticles by focused beam scattering.

Advanced science and technology frequently encounters the need to detect particles in the micrometer and nanometer range of a given composition. While the scattering process of light by small particles is well documented, most conventional analytic methods employ wide illumination of large ensembles of particles. With such an approach, no information can be obtained about single particles due to their weak interaction. In this paper, we show that single particles can be classified with respect to their material composition by analyzing the scattering pattern of a focused Gaussian beam.

Scattering of singular beams by subwavelength objects.

In recent years, there has been a mounting interest in better methods of measuring nanoscale objects, especially in fields such as nanotechnology, biomedicine, cleantech, and microelectronics. Conventional methods have proved insufficient, due to the classical diffraction limit or slow and complicated measuring procedures. The purpose of this paper is to explore the special characteristics of singular beams with respect to the investigation of subwavelength objects. Singular beams are light beams that contain one or more singularities in their physical parameters, such as phase or polarization. We focus on the three-dimensional interaction between electromagnetic waves and subwavelength objects to extract information about the object from the scattered light patterns.

Plasmonic resonance scattering from silver nanowire illuminated by tightly focused singular beam.

We investigate scattering features of tightly focused singular beams by placing a cylindrical nanowire in the vicinity of a line phase singularity. Applying an illumination wavelength corresponding to silver cylinder plasmonic resonance, we compare the scattering response with that of a perfect conductor. The rigorous modeling employs a 2D version of the Richards-Wolf focusing method and the source model technique. It is found that a cylinder with a plasmonic resonance produces a strong scattering response by deflecting the power flow toward the optical singularity region, where otherwise the power approaches zero.

Singular beam microscopy.

Optical singularities are localized regions in a light field where one or more of the field parameters, such as phase or polarization, become singular with associated zero intensity. Singular beam microscopy exploits the fact that the strong variations of the optical field around the singularities are highly sensitive to changes in their neighborhood. As a consequence, analysis of the light field scattered from the object during a scanning process can yield useful information about the object features. We present a theoretical background, numerical simulations, and experimental results. Preliminary experiments have demonstrated a sensitivity of 20 nm in the position and size of simple objects, with theoretically estimated 1 nm capability under the assumption of a reasonable and conservative 30 dB signal to noise ratio.

Optics inspired logic architecture.

Conventional architectures for the implementation of Boolean logic are based on a network of bistable elements assembled to realize cascades of simple Boolean logic gates. Since each such gate has two input signals and only one output signal, such architectures are fundamentally dissipative in information and energy. Their serial nature also induces a latency in the processing time. In this paper we present a new, principally non-dissipative digital logic architecture which mitigates the above impediments. Unlike traditional computing architectures, the proposed architecture involves a distributed and parallel input scheme where logical functions are evaluated at the speed of light. The system is based on digital logic vectors rather than the Boolean scalars of electronic logic. The architecture employs a novel conception of cascading which utilizes the strengths of both optics and electronics while avoiding their weaknesses. It is inherently non-dissipative, respects the linear nature of interactions in pure optics, and harnesses the control advantages of electrons without reducing the speed advantages of optics. This new logic paradigm was specially developed with optical implementation in mind. However, it is suitable for other implementations as well, including conventional electronic devices.

Paradigms for bit-oriented holographic information storage.

One of the difficulties encountered during the many years of research on holographic information storage was the lack of an easy theoretical way to assess proposed paradigms. I exploit the fact that for bit-oriented holographic storage, Gaussian beams are usually involved. For this case I show that the reconstructed wave can be represented as a superposition of simple Gaussian beams, regardless of the exact recording condition, and a virtual source for this wave can be determined. This theoretical result is used to explore several holographic storage architectures, in particular thick volume holograms and layered volume holograms. Simulation results demonstrate the power of the method, show good correspondence with earlier experimental studies, and provide clues for further developments.

Simple online recognition of optical data strings based on conservative optical logic.

Optical packet switching relies on the ability of a system to recognize header information on an optical signal. Unless the headers are very short with large Hamming distances, optical correlation fails and optical logic becomes attractive because it can handle long headers with Hamming distances as low as 1. Unfortunately, the only optical logic gates fast enough to keep up with current communication speeds involve semiconductor optical amplifiers and do not lend themselves to the incorporation of large numbers of elements for header recognition and would consume a lot of power as well. The ideal system would operate at any bandwidth with no power consumption. We describe how to design and build such a system by using passive optical logic. This too leads to practical problems that we discuss. We show theoretically various ways to use optical interferometric logic for reliable recognition of long data streams such as headers in optical communication. In addition, we demonstrate one particularly simple experimental approach using interferometric coinc gates.

Laser-mode selection by a combination of biprism-like reflectors with narrow amplitude masks.

In recent work the laser mode selectivity induced separately by a biprism-like reflector and by an absorbing strip was investigated by numerical analysis. It was shown that each of these elements in an otherwise conventional resonator was suitable to cause the laser to oscillate preferentially on the first odd mode that contains a line singularity, which is a useful dark beam (i.e., a laser beam with a dark central region) for high-resolution metrological applications. We study the combined effect of these two mode-selecting elements and show that the unified analysis leads to much better performance than could be expected from a simple superposition of the performance with each element alone. The results indicate that the mode selectivity can be enhanced by at least a factor of 3 compared with that of laser resonators with biprism-like reflectors alone. Thus a laser equipped with such a combined element will oscillate on a pure first-order mode with high power efficiency. Moreover, calculations show that the resultant dark beam, focused for metrological applications, has a significantly improved shape compared with the dark beam obtained by external modulation of a fundamental Gaussian beam.

Achieving stabilization in interferometric logic operations.

Interferometric systems with amplitude beam splitters can implement reversible operations that, on detection, become Boolean operators. Being passive, they consume no energy, do not limit the operating bandwidth, and have negligible latency. Unfortunately, conventional interferometric systems are notoriously sensitive to uncontrolled disturbances. Here the use of polarization in a common-path interferometric logic gate with and without polarization beam splitters is explored as an attractive alternative to overcome those difficulties. Two of three device configurations considered offer significant stability and lower drive modulator voltage as advantages over the previous systems. The first experimental tests of such a system are reported. Common-path interferometry lends itself to even more stability and robustness by compatibility with no-air-gap, solid optics.

Two regions of mode selection in resonators with biprismlike elements.

A resonator structure in which one reflector is replaced by a biprismlike reflecting surface is investigated theoretically. It is shown that such a modification leads to two regions of parameters, each with different regimes of mode selection. The first region has an improved laser power output because of the nearly flat-top mode shape. In the second region the biprism is inverted, with the result that the main oscillating mode can be the first odd mode. The line singularity contained in such a mode is one example of singular beams that are employed in various fields, such as micromanipulators and advanced high-resolution metrology.

Tunable, oblique incidence resonant grating filter for telecommunications.

We have designed a tunable, oblique-incidence resonant grating filter that covers the C band as an add-drop device for incident TE-polarized light. We tune the filter by tilting a microelectromechanical systems platform onto which the filter is attached. The fabrication tolerances as well as the role of finite incident-beam size and limited device size were addressed. The maximum achievable efficiency of a finite-area device as well as a scaling law that relates the resonance peak width and the minimum device size is derived. In good agreement with simulations, measurements indicate a negligible change in shape of the resonance peak from 1526 nm at a 45 degrees angle of incidence to 1573 nm at a 53 degrees angle with a full width at half-maximum of 0.4 nm. In this range the shift of the peak wavelength is linear with respect to changes in the angle of incidence.

Generalized Bragg selectivity in volume holography.

The diffraction efficiency of holographically recorded volume gratings was extensively studied, and it can be accurately predicted as long as the recording wave fronts are simple. The derivation of the diffraction efficiency when complicated wavefronts or images are involved is much more tedious and less explored. In this work we derive operator expressions that can be used to analyze these processes regardless of the shape of the wavefront and the nature of the optical systems through which they propagate. The compact expressions derived are directly applicable to the analysis of volume holographic processes, and the deterioration of the holographic reconstruction quality is derived as a function of the deviations from the recording parameters. The generalized results obtained reduce to the conventional Bragg effect for plane wave recording and reconstruction. Previously unexplored phenomena are discussed and demonstrated through some simple, and practically useful paradigms, including hologram recording and reconstruction in the Fresnel, Fourier transform, and image plane regions, as well as recording with plane and spherical waves. Some prior experimental results are also interpreted mathematically. In subsequent publications the analysis will be explored further to facilitate its application to more complicated architectures.