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When building spot array binary Fourier diffractive optical elements (DOEs) having feature sizes on the order of the wavelength, we noticed remarkable variations in the experimental diffraction efficiency compared to the simulation results. Even with the use of high-cost electron beam lithography and rigorous Fourier modal method simulations, there appear to be no publications, to the best of our knowledge, showing close agreement in diffraction efficiency between the simulation and experimental results. In this Letter, we show that the diffraction symmetry of binary Fourier DOEs can be an efficient and consistent metric for evaluating the limit of the thin-element approximation and the effects of fabrication errors.

We propose an iterative algorithm based on our scalar nonparaxial propagator for the design of Fourier diffractive optical elements (DOEs) having features on the order of the illumination wavelength. The simulation results show that our algorithm, using iterative Fourier transform and iterative projection, obtains higher-performance DOEs than a purely scalar paraxial design with the same order of calculation time. Upon verification with the experimental results, we find that our scalar-based design method is valid for DOEs with surprisingly small feature sizes (about half the wavelength) and diffraction angles up to about 37°.

We present a scalar model to overcome the computation time and sampling interval limitations of the traditional Rayleigh–Sommerfeld (RS) formula and angular spectrum method in computing wide-angle diffraction in the far-field. Numerical and experimental results show that our proposed method based on an accurate nonparaxial diffraction step onto a hemisphere and a projection onto a plane accurately predicts the observed nonparaxial far-field diffraction pattern, while its calculation time is much lower than the more rigorous RS integral. The results enable a fast and efficient way to compute far-field nonparaxial diffraction when the conventional Fraunhofer pattern fails to predict correctly.

The use of bio-fuels and fuel blends, specially in automotive industry, has been increasing substantially in recent years due to market prices and trends on sustainable development policies. Different spectral analysis techniques for quality control, production, purity, and counterfeit detection have been reported as non-invasive, fast, and price accessible. Raman spectra from three different commercial binary E10 fuel-ethanol blends has been obtained by using a low-cost Fourier-Transform Raman spectrometer (FT-Raman). Qualitative comparison between the commercial fuel blends and a laboratory-prepared fuel blend have been performed. The characteristic Raman lines from some additives contained in the commercial gasoline have been also observed. The spectral information is presented in the range of 0 cm-1to 3500 cm-1with a resolution of 1.66 cm-1. These Raman spectra shows reduced frequency deviation (less than 0.4 cm-1when compared to standard Raman spectra from cyclohexane and toluene without compensation for instrumental response). Higher resolution values are possible, since the greater optical path lengths of the FT-Raman are achievable before the instrumental physical effects appear. The robust and highly flexible FT-Raman prototype proposed for the spectral analysis, consisting mainly of a Michelson interferometer and a self-designed photon counter, is able to deliver high resolution and precise Raman spectra with no additional complex hardware or software control. The mechanical and thermal disturbances affecting the FT-Raman system are mathematically compensated by extracting the optical path information from the generated interference pattern of a =632.8 nm Helium-Neon laser (HeNe laser), which is used at the spectrum evaluation.

We introduce a review concerning hydrogen sensors already validated based on palladium, and we discuss the best ways to proceed to achieve an ideal hydrogen sensor. We discuss the performances regarding the configuration of an optical fiber hydrogen sensor as well as the used materials properties. We conclude that hydrogen sensors using plasmonic effects are a seductive way to follow.

Interferometry associated with an external cavity laser of long coherence length and broad wavelength tuning range shows promising features for use in measurements of absolute distance. As far as we know, the processing of the interferometric signals has until now been performed by Fourier analysis or fringe counting. Here we report on the use of an autoregressive model to determine fringe pattern frequencies. This concept was applied to an interferometric device fed by a continuously tunable external-cavity laser diode operating at a central wavelength near 1.5 μm. A standard uncertainty of 4 × 10^-5 without averaging at a distance of 4.7 m was obtained.

Scalar theory is a simple and rapid method to analyse and conceive diffractive optical element and gratings, in some limited cases. When we want to deflect wave fronts with large angles (for telecommunication devices, for instance) or when the features of the grating are around the order of the wavelength, scalar optics fail to be accurate, then adapted rigorous electromagnetic tools must be used. To correct the increasing cost in memory saving and time consuming with the size of the grating, we present here an original `field stitching' method adapted for aperiodic gratings (FSA). This method, initially developed for periodic gratings is extended here to analyse locally and rigorously aperiodic gratings with large sizes and small features without having some of the above-mentioned inconvenients.

We were mainly concerned here with multiple exposure gratings that allow interactions and coupling between gratings. We started the study of interactions and coupling with five gratings recorded in dichromated gelatin (DCG). Having determined experimentally the characteristic parameters of these five gratings, we used a method developed by Oxford University to synthesize the modulation profile. Entering these parameters of each grating into our computing programs (based on the alternative theory of diffraction in modulated media) and by varying the amplitude modulation, the latter was deduced by adjusting the numerical curve to the experimental one. We then extended the study to nine multiplexed gratings. This study allowed us to know the range of the amplitude modulation such that a certain number of angular multiplexing can be done in DCG. Several undesirable effects which we had experimentally observed appeared then during the reconstruction of any one of the multiplexed images, such as cross-talk effect. We noted the good concordance between experimental and theoretical results. We finally illustrated cross-talk effect in a diffractive memory where, if the amplitude modulation was first calculated, the number of multiplexing for an angular range could then be determined and the angular separation achieved to avoid cross-talk.

Two thousand images of resolution 512×512 pixels as a regular matrix pattern of 10×10 elements are stored, where each element is angularly multiplexed 20 times in a 25 μm thickness of dichromated gelatin emulsion without cross-talk effect. The surface area of the matrix is 1 cm2. We show good concordance of the angular selectivity between the experimental result and theory. The diffraction efficiency of each 20 multiplexed images is measured and has nearly the same value. Examples of reconstructed images for multiple applications are given, for example, storage of 160,000 images on a 3′1/2 floppy disc format, which is about 100 min of black and white film. Application can be made to automobile cartography and storage of X-ray images as well as weather forecast images. Colored diffractive images are also possible and are illustrated.

An optical-fiber sensor based on twist-induced optical activity has been developed for measuring angular displacements at low temperature. The sensing part is composed of a fiber coil rotated between two points, which induces a twist of two sections of fiber. A theoretical study of the evolution of a general input state of polarization in the sensor gave us its main characteristics. Then experimental investigations permitted the construction of a sensor to take angular measurements over a 100-deg range with an accuracy of 0.2 deg. The thermal sensitivity of this kind of sensor is also briefly reported.

This Letter introduces and discusses a difference in the behavior of a cylindrical diffractive lens encoded with subwavelength structures illuminated with monochromatic coherent light in the cases of TE and TM polarization. The effective medium theory is used to model with new binary phase function the diffractive lens. A new algorithm combines the finite-difference time domain for the propagation in the near field and the radiation spectrum method for the propagation in the far field. We observe the existence in the TM polarization of a second spot at half the distance of the focal length not predictable by scalar theory.

By focusing light with a sphere several wavelengths in diameter, we can obtain a photonic nanojet [Opt. Express13, 526 (2005) ]: if light is focused on the surface of the sphere, the width of the beam stays smaller than the wavelength along a distance of propagation of approximately two wavelengths and reaches a high intensity. We use the rigorous Mie theory to analyze the basic properties of the photonic jet in the general three-dimensional polarized case. This fast algorithm allows us to determine the influence of the radius and the refractive index of the sphere on the photonic jet. The polarization response is also studied. We observe that high-intensity concentrations and subwavelength focusing are two different effects. Their basic properties are analyzed, and explanations are proposed.

A new design of a fiber optic sensor using Palladium as a sensitive layer is presented. In this new configuration, a transducer layer is deposited on a multimode fiber (without the optical cladding). The transducer layer is a multilayer stack based on a silver, a silica and a Pd layer. The spectral modulation of the light transmitted by the fiber allows to detect hydrogen. The sensor is only sensitive to the Transverse Magnetic polarized light and the Transverse Electric polarized light can be used as a reference signal. The multilayer thickness defines the sensor performance. The Silica thickness tunes the resonant wavelength, whereas the silver and Pd thickness determines the sensor sensitivity. We present some results obtained for different multilayer Pd configurations.

When a city or a local authority wishes to emphasize its historical heritage, for the lighting of its streets, setting up lights during the festive season, they call upon the skills of a lighting designer. The lighting designer proposes concepts, ideas, lighting, and to be able to present them, he makes use of simulation. On the other hand lighting technologies are evolving very rapidly and new lighting systems offer features that lighting designers are now integrating their projects. The street lights consume lot of energy; light projects are now taking into account the energy saving aspect. Lighting systems based on LEDs today provide good lighting needs, taking into account sustainable development issues while enabling new creative dimension. The lighting simulation can handle these parameters. Images or video simulation are no longer sufficient: stereoscopy and virtual reality techniques allow better communication and better understanding of projects. Virtual reality offers new possibilities of interaction, the freedom of movement in a scene, the presentation of variants or interactive simulations.

A robust, flexible Fourier transform Raman spectrometer (FT-Raman) based on a Michelson interferometer and a self-made photon counter is presented. The proposed inexpensive setup has no complex hardware or control systems for optical path compensation. The mechanical and thermal induced errors are mathematically compensated by extracting the optical path information from the generated interference pattern of a = 632.8nm Helium-Neon laser (HeNe laser). This information also permits high frequency precision of the calculated Raman spectrum. This system is flexible and allows the user having complete access to hardware and software. It enables a variety of experimental changes in the system, which are difficult to achieve with commercial devices. Precise, high resolution Raman spectra of cyclohexane with a resolution of 1.66 cm-1to 5.0cm-1have been measured with this device. Higher resolution values can be achieved since longer scanning distances at the Michelson interferometer are possible and its calculated ´etendue (throughput) does not substantially corrupt the obtained interferograms. Other chemical compounds have been also monitored. Additionally, a detailed spectral analysis of different precision optical components and light sources has been performed.

Degraded printed character recognition is a hard and ever-present problem in optical character recognition. Previous work has explored serial combination of multilayer perceptron (MLP) and autoassociators networks for printed character recognition. The MLP is used as a first step classifier for its discrimination capability and is not very well suited for rejection. On the other hand, the autoassociator used as a second step classifier is more appropriate when a very small error and rejection are required. Unfortunately, this better behavior with respect to rejection is either paid in terms of rejection error or in terms of computational complexity, particularly when the number of classes is high. In this paper, we propose a serial combination of the Hopfield and MLP networks in order to achieve accurate recognition of degraded printed characters. We introduce a relative distance and use it as a quality measurement of the degraded character, which makes the Hopfield-based classifier very powerful and very well-suited for rejection. This relative distance is compared to a rejection threshold in order to accept or reject the incoming degraded character by the Hopfield model used as a first classifier. Due to its discrimination capability, the MLP network is used as a second classifier to avoid rejection error and to diminish computational complexity. The proposed method is devoted to solving the problem of recognition of single font characters collected from poor quality bank checks. We report experimental results from a comparison of three neural architectures: the Hopfield network, the MLP-based classifier, and the proposed combined architecture. The proposed method is also compared to five other recognition systems. It is shown that the proposed architecture exhibits the best performance, with no significant increase in the computational burden. In this paper, we propose also a bank check processing procedure for account check number (ACN) detection, localization, and character retrieval. The recognition system is applied to ACNs that are doubly printed in two serial numbers on each side of poor quality bank checks and to verify if they are identical, showing an achievement of 98.33&percent; recognition rate. Experimental results related to the proposed method with various added Gaussian noise levels, as well as tests with broken and incomplete characters, are presented. They confirm that the proposed method can be successfully used for effective recognition of even extremely degraded printed characters.