This PDF file contains the editorial “Editorial: The Kingslake Medal and Prize” for OE Vol. 34 Issue 08

This PDF file contains the editorial “Guest Editorial: Photorefractive Nonlinear Optics” for OE Vol. 34 Issue 08

Although the concept of using multiplexed holography for data storage has been considered for some time, recent advances in several critical device technologies along with developments in storage materials have greatly enhanced the likelihood of successful implementations. We review several basic architectural concepts along with various multiplexing options and associated techniques for holographic data storage.

We consider the possibility of restoration and/or enhancement of decaying holograms in photorefractive media by using a simple optical readout in conjunction with a phase conjugator. The results indicate that extremely weak holograms can be enhanced provided that the two-beam coupling is sufficiently strong. Steady-state photorefractive holograms can be maintained continuously without decay by using a properly designed readout scheme. The result also provides an explanation for the formation of mutually pumped phase conjugation in terms of the amplification of initial noise gratings.

The performance of recently developed highly efficient photoref_ ract1ve polyme_rs .base~ on the photoconducting polymer host poly( N-vinylcarbazole) 1s 1nvest1gated, and the use of these materials as recording media in dynamic holography and other applications is evaluated.- A diffraction efficiency 'rJ = 86% (limited only by absorption and reflection losses), a two-beam coupling net gain coefficient r = 200 cm - 1 and light-induced refractive index modulations as high as lin=7X 10-3 are demonstrated. Hologram growth rates of the order of 500 ms are observed with. recording light intensities <10 mW /cm2 , using either lowpower laser diodes (675 nm) or a HeNe laser (633 nm). The materials show good sensitivity in this part of the spectrum. Applications such as dynamic time-average interferometry are demonstrated.

Although thick volume photorefractive (PR) crystal offers promise in application to large-storage-capacity holographic memories, the thickness of the material also impairs the shift-invariant properties of the correlator. We discuss the influence of the angular and the wavelength selectivity in a thick PR crystal filter. The shift tolerances of Bragg diffraction-limited PR-based correlators are analyzed, which include the Vander Lugt, joint transform, and reflection-type (RC) correlators. We have shown that the RC correlator performs the best. A simple experiment to Illustrate the shift-invariant capability of PR-based reflection-type correlators is also described.

Optical storage in photorefractive crystals such as UNbO is subject to some of the same problems associated with photogra~hic em~ls1 ons. ~ynamic range limitations are common to both. In a typical o~t1cal Fo~ ner tr~nsfor!': arrangement, the lower spatial frequencies contain the highest intensities and the higher spatial frequencies are much weaker a~d. are ~ubj~ct to being poorly recorded in multiplexed expos~ re~. This invest1gat1on explores and models a simple method for distributing the spectra more evenly throughout the storage media by utilizing a phase diffusing screen.

The anomalies of photovoltaic current in Ce:Fe doped UNb03 crystal at 55 and 75°C temperatures have been investigated. Experiments show that there is about a twofold increase in the photovoltaic current at these temperatures, which is consistent with the anomalies observed in a two-wave mixing experiment reported previously.

Steady-state Bragg diffraction (nondegenerate four-wave mixing) in a Ce:Fe:LiNbO₃ photorefractive crystal as a function of the crystal temperature is described. We have shown that an efficient steady-state frequency-varied conjugate wave (FVCW) can be generated at an elevated crystal temperature of about 120°C, at which a tenfold increase in the steady-state FVCW intensity is observed compared with that at room temperature (20°C). We have found that the temperature dependency is primarily due to the reduction of light-induced scattering by thermally activated ions that neutralize the noise grating in the crystal. We have also observed that, at higher temperatures, steady-state phase-conjugate waves at both pump wavelengths can be obtained.

The effective optical nonlinearity n₂ of a Ce-doped KNSBN crystal is measured by using the Z-scan method. The dependence of n₂ on the illumination wavelength and polarization is characterized. An optical limiter for strong cw illumination is demonstrated, based on self-focusing and spatial self-phase-modulation.

By using rigorous coupled-wave diffraction theory along with a time-dependent nonlinear formulation, we analyze two- and multiple-wave coupling and the grating kinetics in BaTiO₃ with different boundary interfaces. Efffects of electrostatic and optical anisotropy have been included in the analysis. Significant mode conversion to higher orders is observed only when the boundary interfaces are highly mismatched.

We have observed and explained three types of hexagon pattern formation in photorefractive crystal KNbO₃:Fe. These are: (1) dynamic (laser induced), (2) semipermanent (holographically stored), (3) permanent (induced by a static domain grid) over a wide wavelength range.

We have theoretically investigated the dynamic range compression in a 45 deg-cut BaTiO₃ crystal. Modified coupled wave equations, which take into account different directions of scattered beams and the angle dependence of the coupling coefficient, are used. The same equations are employed to study the behavior of beam fanning with and without biasing illumination. Logarithmic operation is demonstrated using biasing illumination in the beam fanning geometery. Results indicate that there is a choice concerning the dynamic range of biasing illumination and the input range over which log operation is achieved. An all-optical configuration shows a very small dynamic range over which log operation can be obtained. A hybrid optoelectronic module of such a log processor is shown to have large operating range. Experimental results are presented to demonstrate the feasibility of this concept. Once the given crystal is characterized to give a standard lookup table for logarithmic operation, the results can be applied to 2-D (image) information. A configuration is suggested for such a 2-D processor, which is expected to find applications in joint-transform correlation, analog computation, and logarithmic filtering.

This PDF file contains the editorial “Guest Editorial: Optics in Switzerland, Part 2: Universities and Research Institutes” for OE Vol. 34 Issue 08

Although giro forms are used by many people in daily life for money remittance in Switzerland, the processing of these forms at banks and post offices is only partly automated. We describe an ongoing project for building an automatic system that is able to recognize various items printed or written on a giro form. The system comprises three main components, namely, an automatic form feeder, a camera system, and a computer. These components are connected in such a way that the system is able to process a bunch of forms without any human interactions. We present two real applications of our system in the field of payment services, which require the reading of both machine printed and handwritten information that may appear on a giro form. One particular feature of giro forms is their flexible layout, i.e., information items are located differently from one form to another, thus requiring an additional analysis step to localize them before recognition. A commercial optical character recognition (OCR) software package is used for recognition of machine-printed information, whereas handwritten information is read by our own algorithms, the details of which are presented. The system is implemented by using a client/server architecture providing a high degree of flexibility to change. Preliminary results are reported supporting our claim that the system is usable in practice.

Internal reflections occurring in the waveguides of incoherently emitting photonic devices are mapped using a novel optical coherence domain reflectometry (OCDR) technique, where the light source is the device itself. Such reflections may be caused by cleaved facets, inadvertent waveguide defects, or mntentmonal submicrometer structures. How this OCDR technique is applied to the characterization of a 1.3-μm superluminescent diode with unintentional defects and with isolated and periodic structures produced by focused-ion beam milling is described. A two-pole network theory is used to model the observed effects.

Modern semiconductor technologies make it possible to fabricate photosensors whose geometry and functionality are adapted to specific sensing tasks of various optical measurement techniques. Several such smart sensors, realized with commercially available CMOS/CCD processes, have been demonstrated successfully in different optical sensing applications: (1) a novel absolute position encoder with 10-nm resolution and 50-nm differential accuracy, (2) a dynamic CCD image sensor whose pixel size and shape can be varied in real time, (3) an image sensor capable of carrying out convolutions with arbitrary kernel sizes on-chip and during the exposure, and (4) a 2-D, synchronous detector/demodulator ("lock-in CCD") for applications in heterodyne interferometry and time-of-flight (laser-radar) depth imaging. Several of these devices and techniques rely on dynamic, real-time clocking schemes, for which a universal smart sensor controller was developed, capable of controlling and driving almost any existing CCD/CMOS image sensor at clock rates of up to 25 MHz. The availability of design and fabrication facilities for custom photosensors and imagers, produced quickly and quite inexpensively even in prototype quantities, opens up new possibilities for a wide range of traditional and novel optical measurement techniques.

A nondestructive technique that provides near-field measurements as well as refractive index profiles and geometry of waveguides in LiNbO₃ within a few minutes is demonstrated. It is a modification of the refracted-near-field (RNF) method well known for optical fiber characterization. Spatial resolution below 1 μm is achieved. The index resolution is about 2 x 10^-4, the reproducibility better than 7%. A new calibration method for RNF measurement is introduced.

Within the wide field of nonlinear dynamics we investigate temporal and spatial behavior of electromagnetic systems. A strange type of laser, the nuclear magnetic resonance (NMR) laser, shows truly chaotic behavior and is therefore ideally suited to analyze experimentally and theoretically a variety of temporal nonlinear effects. Of particular interest is the analysis of its strange attractors in terms of unstable periodic orbits. For the extension of our research from the NMR laser (representing a purely temporal system described by the Bloch-Kirchhoff ordinary differential equations) to spatial and spatiotemporal systems, we chose, as a three-dimensional dynamic system, polarized laser beams interacting nonlinearly with sodium atoms. Among other effects, we have observed beam bouncing, beam splitting, and beam switching. This can be well described by partial differential equations for beam propagation derived from the Schrödinger equation and the Maxwell equations. An intuitive explanation is given in terms of intensity and polarization patterns formed by optical-pumping-induced mutual refractive index modifications between polarized resonant laser beams.

Scanning near-field microscopy (SNOM or NSOM) is a versatile and attractive scanning probe technique for imaging with subdiffraction-limited spatial resolution using visible light. At least three different types of images can be recorded simultaneously of the selected sample area, such as the topography, the near-field optical transmission, and the fluorescence from excited chromophores. We have built such a microscope, especially designed for achieving the high resolution and the sensitivity needed for single molecule detection. We report on optical near-field investigations of surface structures and thin polymer films that are doped with fluorescent dye molecules. The effective aperture diameters of the fiber tips used in the SNOM experiments were determined by a photon-scanning tunneling microscope (PSTM) giving values between 70 and 160 nm. The transmission imaging of transparent polymer phase gratings reveals the existence of different contrast mechanisms, which are either based on the inherent distance dependence of the optical near field or on the periodic change of boundary conditions for the electric field component of the light between the aperture and the sample. Furthermore, we demonstrate selective irreversible photobleaching of dye molecules at moderate concentration (10^-5 M) induced locally by the subwavelength-sized probe tip. Finally, we present fluorescence images showing single molecule detection in a thin solid film. The chromophores (rhodamine 6G) were embedded at low concentration (10^-7 M) in a 25-nm thin polyvinylbutyral film. A lateral resolution of 160 nm was achieved. We find that the signal strengths of the brightest fluorescent features vary considerably in a sequence of images (a typical single-molecule behavior), whereas the fluorescence background exhibits the usual photobleaching behavior of a large ensemble.

When modern spectral hole burning applications for high-density information storage under noncryogenic temperatures are envisioned, it is necessary to develop new frequency-selective photoactive materials for this purpose. Mixed compounds of the PbFCI family, doped with samarium (II) ions, exhibit promising and true room-temperature hole burning capabilities. We investigate this class of systems (and related ones) by combining material synthesis and high-resolution spectroscopy. Whole groups of isomorphous crystals were synthesized with varying degrees of halide anion and/or cation substitutions. Thin films of fluoride-based materials were made in a laboratory-built molecular beam epitaxy system. An extended x-ray study, differential thermal analysis, luminescence, and Raman measurements allowed the characterization of the materials. Formal models were developed for both the inhomogeneous zero-phonon optical line shapes of the Samarium (II) and the time evolution of hole burning.

Biomolecules performing specific biological functions on material surfaces are progressively employed in the development of miniaturized bioassays, biosensors, bioelectronic devices, and medical equipment. Device performance is improved with covalently immobilized bioconstituents. The unique advantages of using light-controlled reactions to achieve biomolecule immobilization on surfaces are addressed. On activation of introduced light-sensitive reagents, biomolecules are covalently linked to material surfaces. Procedures leading to light-dependent engineering of surfaces are exceptionally facile. Immobilization by light is compatible with biological functions, enabling surface patterning and molecular coating of materials. Current strategies and protocols are illustrated with selected examples of biomolecule photoimmobilization.

The laser performance of Nd:KGW (neodymium-doped potassium gadolinium tungstate) end pumped by cw or quasi-cw high-power diode laser bars is studied and compared to Nd:YAG. In cw operation, thermal problems in Nd:KGW are quite important, whereas in quasi-cw operation, the optical and slope efficiencies attained with Nd:KGW are clearly better than with Nd:YAG.

This brief survey of picosecond diagnostics in the accelerator domain examines new electro-optics methods. In the past decade, the main progress in picosecond techniques has been in laser technology and electro-optics. Laser technology cannot be used directly for accelerator diagnostics, but the new, emerging techniques in optoelectronics are useful for single-shot measurement and picosecond diagnostics. Such techniques are needed for single-shot, multidimensional measurements of very short incoherent light pulses, and several devices have been developed at the European Organization for Nuclear Research (CERN) making use of the very wide spectrum of synchrotron light emitted by the particle beam. A very intense x-ray detector has been developed using picosecond photoconductivity. A new method using the photon storage ring and stochastic sampling of single photons enables single-shot measurement of some tens of picoseconds of light pulses, emitted at a wavelength of 1.3 μm. Visible, UV pulse light in a real-time 3-D measurement is analyzed using a new streak-camera method, giving longitudinal resolution of about 0.6 mm (2 ps) and 15 μm in transverse dimensions. All three instruments are certain to find applications in other fields.

The output power of cw monomode fiber lasers, commonly operated far below 1 W, can be enhanced using a double-clad fiber structure. Double-clad fibers can be excited with powerful multimode laserdiode arrays. An output power of several watts has already been demonstrated in a Nd³⁺-doped double-clad fiber. A further enhancement of the laser power can be achieved by side pumping the fiber. An upper limit of the order of 100 W given by the damage threshold of the fiber can be estimated for a multi-side-pumped system. As for the case of crystal lasers, even more output power in the fundamental mode can be obtained by coherent combination of several lasers. This procedure can be performed by active phase adjustment between identical lasers: The adjustment of the phase can be realized rather elegantly in fibers by squeezing or stretching the fibers with a piezoelectric element. Even if we are still far from the practical realization of a 100-W monomode fiber laser, all the different steps leading to high output power have been demonstrated to work.

We describe a concept for an instrument to measure 2-D space plasma distributions by remote sensing of neutral atoms. The instrument measures in one dimension, and from a spinning spacecraft one obtains 2-D (line-of-sight) maps of the neutral flux. Because we want to employ this instrument for measurements in the magnetosphere, the main species of interest are neutral H and O atoms with kinetic energies ranging from about 10 eV to 1 keV. The instrument makes use of a low-work-function surface to convert neutral atoms efficiently to negative ions. The ions are then accelerated away from the surface and brought to an intermediate focus by a large aperture lens. After further acceleration, the ions are deflected by a spherical electrostatic analyzer into a time-of-flight mass spectrometer. Mass resolution of the device is sufficient to resolve H, D, He, and O. Energy and azimuth angle information are obtained by position imaging of the secondary electrons produced at the carbon foil. The large geometric factor combined with simultaneous angle-energy-mass measurement eliminates the need for cycling and provides the necessary high sensitivity for imaging at short time intervals. On a spinning spacecraft this instrument is capable of producing 2-D maps of low-energy neutral atom fluxes.

Total internal reflection (TIR) holographic lithography is applied to the fabrication of binary diffractive optical elements with submicrometer surface relief features. The recording conditions for the intermediate TIR volume hologram, used for high-resolution proximity printing, are discussed. In particular, the fabrication of efficient high-carrier-frequency fan-out gratings is considered and experimental results are presented for an off-axis 9 x 1 fan-out element in photoresist with a carrier frequency of 1000 lines/mm.

The output beam characteristics of individual emitting structures are investigated for high-power cw diode laser bars of two types, namely a five-zone design and a 48-widestripe design. The far-field intensity distribution, output power, and wavelength were recorded as a function of the location of the emitting structure within the diode bar and correlated to the emission of the entire diode. Significant variation of the output properties over the extent of the diode lasers was found. Several diodes of each type were examined and the results compared.

We constructed an apparatus for dynamic light scattering (DLS) measurements in the posterior chamber for the eye. The setup uses single-mode technology and, with regard to future clinical use, it is combined with a commercial ophthalmologic microscope. The scattering volume is illuminated with a Gaussian beam realized by coupling a HeNe laser into a single-mode fiber. The scattered light is measured using a single-mode fiber receiver. The signal is processed with a digital autocorrelator. Our measurements on the vitreous of post mortem porcine eyes show that DLS measurements in the posterior chamber are feasible and that it is possible to keep the excitation intensity below the damage threshold of the retina. In addition, absolute measurements of light scattering in toluene confirmed the validity of DLS theory for single-mode receivers.

Integrated optical transducer platforms for the measurement of small changes in the refractive index or thickness of a sensing layer are of considerable interest for the fabrication of a wide variety of sensors. Progress toward the development of two types of such platform is described. A replicated polymer platform fabricated by the embossing or molding of a surface relief microstructure followed by the evaporation of a dielectric waveguide layer forms the basis of a low-cost disposable device for applications such as medical diagnostics. A fully integrated, monolithic III-V platform with laser, interferometer, modulator, sensor pad, and detector is also being developed for compact, rugged sensors required in applications such as environmental sensing and process control. Both transducer platforms are suitable for many different types of sensors and can be extended to multichannel sensor structures for the simultaneous measurement of a number of analytes.

Multiple-wavelength interferometry is, like classical interferometry, a coherent method, but it offers great flexibility in sensitivity by an appropriate choice of the different wavelengths and it can be operated on rough surfaces. The accuracy of this method depends essentially on the signal processing and on the properties of the source. Practical considerations for distances in the range of several meters with micrometer resolution are given.

Microwave radiometry is an extremely powerful tool for atmospheric research. It is specially suited for the investigation of the dynamical and chemical processes that are important in the depletion of the ozone layer and related phenomena of the middle atmosphere. The theoretical and technical concepts of microwave remote sensing of the atmosphere are briefly reviewed. A special weight is given to the approaches as used at the Institute of Applied Physics (lAP) of the University of Bern for ground-based instruments and sensors on aircraft and on the space shuttle. Examples of results from the different instruments in the frequency range of 20 to 200 GHz illustrate the huge potential of this technique for atmospheric research.

We investigated an optical method for characterizing submicrometer structures of photolithographic masks, enabling fast and nondestructive testing over large areas. The scanning spot metrology provides accurate information about edge locations of opaque structures on chrome masks. Algorithms for the extraction of edge locations from the detector signal are discussed and applied to the characterization of a modulated grating mask. Local fabrication errors of the order of 10 to 50 nm can be detected.

Attention mechanisms extract regions of interest from image data to reduce the amount of information to be analyzed by time-consuming processes such as image transmission, robot navigation, and object recognition. Two such mechanisms are described. The first one is an alerting system that extracts moving objects in a sequence through the use of multiresolution representations. The second one detects regions in still images that are likely to contain objects of interest. Two types of cues are used and integrated to compute the measure of interest. First, bottom-up cues result from the decomposition of the input image into a number of feature and conspicuity maps. The second type of cues is top-down, and is obtained from a priori knowledge about target objects, represented through invariant models. Results are reported for both the alerting and the attention mechanisms using cluttered and noisy scenes.

An adaptive multilayer optical neural network with all-optical forward propagation including optical thresholding by a liquid crystal light valve (LCLV) is described. It has a large number of modifiable optical interconnections that are implemented by liquid crystal television screens, and it has a modular structure enabling the cascading of layers, each layer with its own light source. Sigmoid fits to response curves of four LCLVs are evaluated and their suitability as optical thresholding functions is examined on the basis of neural network simulations.

The concepts of near-field optical microscopy and experimental and theoretical work carried out in Switzerland over the last 10 years are reviewed. After a description of the pioneering experiments of the mid-1980s, we focus on the recent efforts of the three Swiss laboratories currently working in the field in close collaboration. This newly refreshed initiative in near-field optics is supported by the Swiss Priority Program Optique.

A holographic technique is presented that increases the flux-transfer efficiency of laser speckle generation by a factor of 100 over the integrating sphere method. This makes a wider range of low-power lasers usable for the speckle MTF test method, and increases the number of wavelengths at which the test can be applied.

Various interferometric and fringe projection techniques may result in crossed and closed fringe patterns with the information about the phenomena tested. Recent applications require the analysis on the basis of a single fringe pattern. Here, we address two problems: simultaneous analysis of the information about two events, and analysis of fringe pattern with high phase gradients in both x and y directions. The analysis of a fringe pattern is performed by the spatial-carrier phase-shifting method used sequentially for x and y sampling directions. The error analysis of the method in reference to both problems is presented.

This paper proposes an approach to flexible recognition of patterns based on the asymptotic stationary property of Hopfield-Amari autoassociative memory networks (a synchronously updating randomly generated recurrent network). In this neural network, the weights of the connective matrix are constructed in terms of the energy function, and the pattern-matching problem is reduced to the procedure of retrieving a target memory pattern that is stored in the neural network. Two experiments, the recognition of handwritten Chinese characters and the recognition of plane figures, are performed to verify the approach. The experimental results show that the method is robust and reliable.

The transient behavior of a high electron mobility phototransistor has been calculated theoretically, where radiation is allowed to fall within the gaps between source, gate, and drain and the gate behaves as an opaque material. The parameters calculated are the threshold voltage, drain-source current, and transconductance of the device as functions of time. It is observed that when the light is turned on, the parameters reach the steady-state value at around 40 ps, and when the light is turned off, they reach their dark values at around 300 ps. In the turned-on case, it is the optical relaxation time that controls the behavior, and in the turned-off case it is the photovoltage across the heterointerface, which decreases linearly with time.

Light sectioning of an object surface uses the line deformation imaged by a CCD camera to compute the object profile. Because of the low optical magnification between the object and the CCD image plane, the measured CCD line deformation is small and leads to low resolution in profile measurements. To obtain larger CCD line deformation without decreasing the field of view, we propose to use an additional cylindrical lens to magnify only the horizontal deformation (which contains the profile information) without modifying the vertical field of view. The limitations of this anamorphic system, such as maximum object depth and magnification, are theoretically determined. The improvements given by the cylindrical lens are calculated, and experimental profiles (pyramid, cylinder) show a gain in depth resolution of approximately 2 to 3 over an earlier light sectioning system. The depth resolution, which was 0.36 mm with the previous system, is now improved to 0.15 mm for an average object distance of 350 mm.

Recent studies on low-observable target acquisition relate clutter to traditional true- and false-detection performance parameters. This paper presents modified definitions of the target signature and clutter in the image plane and their relation to alternative metrics. Measured detection results for experienced observers in a controlled experiment strongly correlate with target signal-to-clutter ratios calculated for the various images. Empirical false-alarm rates were quite constant as a function of time during the typical observer search process; a sudden decrease to zero occurred at a typical "saturation time" for the standard observer. The rate of false alarms and the saturation time were invariant to the particular signal and clutter signature. The algorithms derived on the basis of the theory of signal detection suggest an effective decrease of the threshold signal with increased observation period and attribute the detection-probability behavior with time to this factor alone. Although the behavior with time differed from that predicted from random-glimpse statistics, an algorithm using that approach is also presented and serves as a numerical approximation. The signal and clutter metrics may be applied as tools to compare and rank target detectability and background complexity in various images.

This PDF file contains the editorial “Book Rvw: Optical Engineering Fundamentals," by Bruce H. Walker for OE Vol. 34 Issue 08

This PDF file contains the editorial “Book Rvw: Electro-Optical Imaging System Performance," by Gerald C. Holst for OE Vol. 34 Issue 08