Now showing 1 - 10 of 96
  • Publication
    Comparison of Laplacian differential reconstruction of inline holograms recorded at two different wavelengths and distances
    (Optical Society of America, 2010-04-11) ; ;
    We record two holograms using two different illuminating wavelengths. Subtracting these holograms, the resulting reconstruction is an approximation to the second order Laplacian differentiation of the object wave.
  • Publication
    Speckle interferometric system for measuring ocular microtremor
    A compact optical system has been developed capable of measuring minute movements of the eye. Eye movement is simulated through the application an electrical signal to a piezoelectric material which acts as the eye’s surface. Using this device, both the amplitude and frequency components of this movement can be accurately measured.
  • Publication
    The approximate model for holographic grating formation in photopolymers
    (Optical Society of America, 2006-10-10) ; ; ;
    Nonlocal Polymerisation Driven Diffusion model describes grating formation in photopolymer materials and gives valuable insight into the processes taking place during formation. For weak exposures, NPDD reduces to a simple approximate model describing polymer concentration
  • Publication
    Constraints to solve parallelogram grid problems in 2D non separable linear canonical transform
    The 2D non-separable linear canonical transform (2D-NS-LCT) can model a range of various paraxial optical systems. Digital algorithms to evaluate the 2D-NS-LCTs are important in modeling the light field propagations and also of interest in many digital signal processing applications. In [Zhao 14] we have reported that a given 2D input image with rectangular shape/boundary, in general, results in a parallelogram output sampling grid (generally in an affine coordinates rather than in a Cartesian coordinates) thus limiting the further calculations, e.g. inverse transform. One possible solution is to use the interpolation techniques; however, it reduces the speed and accuracy of the numerical approximations. To alleviate this problem, in this paper, some constraints are derived under which the output samples are located in the Cartesian coordinates. Therefore, no interpolation operation is required and thus the calculation error can be significantly eliminated.
  • Publication
    Wigner cross-terms in sampled and other periodic signals
    (Optical Society of America, 2009-10-11) ; ;
    If we sample a scalar wave field, it becomes periodic in frequency. We examine the cross-terms which occur between these periodic replicas in the Wigner-Ville distribution function of such a signal. We present analytic results for Gaussian signals. The results also have implications for physical systems which contain periodic gratings.
  • Publication
    Dual wavelength digital holographic Laplacian reconstruction
    (Optical Society of America, 2010-09-15) ; ;
    Access to the spatial derivatives of an optical wave field can be used to enhance edge detection, focusing, and holographic imaging. It was recently shown that, by using digital holographic techniques, the Laplacian of an object field can be extracted. Here it is demonstrated that equivalent results can be found using two holograms captured at either two distances or with two appropriately related wavelengths. Experimental and numerical results confirming the theoretical analyses are presented. The proposed two-wavelength-based system requires no mechanical repositioning of the object and is shown to provide superior performance.
      565Scopus© Citations 18
  • Publication
    Optical image encryption by random shifting in fractional Fourier domains
    (Optical Society of America, 2003-02-15) ;
    A number of methods have recently been proposed in the literature for the encryption of two-dimensional information by use of optical systems based on the fractional Fourier transform. Typically, these methods require random phase screen keys for decrypting the data, which must be stored at the receiver and must be carefully aligned with the received encrypted data. A new technique based on a random shifting, or jigsaw, algorithm is proposed. This method does not require the use of phase keys. The image is encrypted by juxtaposition of sections of the image in fractional Fourier domains. The new method has been compared with existing methods and shows comparable or superior robustness to blind decryption. Optical implementation is discussed, and the sensitivity of the various encryption keys to blind decryption is examined.
      1242Scopus© Citations 427
  • Publication
    Holographic data storage : optimized scheduling using the nonlocal polymerization-driven diffusion model
    (Optical Society of America, 2004-08-01) ; ;
    The choice of an exposure schedule that maximizes the uniformity and capacity of a holographic recording medium is of critical importance in ensuring the optimum performance of any potential holographic data storage scheme. We propose a methodology to identify an optimum exposure schedule for photopolymer materials governed by the nonlocal polymerization-driven diffusion model. Using this model, the relationship between the material properties (nonlocality and nonlinearity), the recording conditions and the schedule are clarified. In this way, we provide a first-order comparison of the behavior of particular classes of photopolymer materials for use as holographic storage media. We demonstrate, using the nonlocal polymerization-driven diffusion model, that the exposure schedule is independent of the number of gratings to be recorded and that the optimum schedule may necessitate leaving unpolymerized monomer at the end of the recording process.
      341Scopus© Citations 47
  • Publication
    Nonlocal photopolymerization kinetics including multiple termination mechanisms and dark reactions. Part II. Experimental validation
    (Optical Society of America, 2009-08-19) ; ; ;
    In the first of this series of papers [J. Opt. Soc. Am. B 26, 1736 (2009)], a new kinetic model, which includes most of the major photochemical and nonlocal photopolymerization driven diffusion effects, was proposed. Predictions made using the model were presented, and the numerical convergence of these simulations were examined when retaining higher-concentration harmonics. The validity and generality of the model is examined by applying it to fit experimental data for two different types of photopolymer material appearing in the literature. The first of these photopolymer materials involves an acrylamide monomer in a polyvinylalcohol matrix. The second is a more complex photopolymer in an epoxy resin matrix. Using the new model, key material parameters are extracted by numerically fitting experimentally obtained diffraction efficiency growth curves. The growth curves used include data captured both during exposure and post-exposure, allowing examination and analysis of "dark reactions".
      560Scopus© Citations 77
  • Publication
    Non-local polymerization driven diffusion based model : general dependence of the polymerization rate to the exposure intensity
    The nonlocal diffusion model proposed by Sheridan and coworkers has provided a useful interpretation of the nature of grating formation inside photopolymer materials. This model accounts for some important experimental facts, such as the cut-off of diffraction efficiency for high spatial frequencies. In this article we examine the predictions of the model in the case of a general dependence of the polymerisation rate with respect to the intensity pattern. The effects of this dependence on the different harmonic components of the polymerisation concentration will be investigated. The influence of the visibility on the different harmonic components will also be studied. These effects are compared to the effects of varying RD and σD.
      355Scopus© Citations 20