Now showing 1 - 9 of 9
  • Publication
    Measuring optical phase digitally in coherent metrology systems
    (Society of Photo-optical Instrumentation Engineers (SPIE), 2017-04-09) ; ; ;
    The accurate measurement of optical phase has many applications in metrology. For biological samples, which appear transparent, the phase data provides information about the refractive index of the sample. In speckle metrology, the phase can be used to estimate stress and strains of a rough surface with high sensitivity. In this theoretical manuscript we compare and contrast the properties of two techniques for estimating the phase distribution of a wave field under the paraxial approximation: (I) A digital holographic system, and (II) An idealized phase retrieval system. Both systems use a CCD or CMOS array to measure the intensities of the wave fields that are reflected from or transmitted through the sample of interest. This introduces a numerical aspect to the problem. For the two systems above we examine how numerical calculations can limit the performance of these systems leading to a near-infinite number of possible solutions.
      396
  • Publication
    2D Non-separable Linear Canonical Transform (2D-NS-LCT) based cryptography
    The 2D non-separable linear canonical transform (2D-NS-LCT) can describe a variety of paraxial optical systems. Digital algorithms to numerically evaluate the 2D-NS-LCTs are not only important in modeling the light field propagations but also of interest in various signal processing based applications, for instance optical encryption. Therefore, in this paper, for the first time, a 2D-NS-LCT based optical Double-random-Phase-Encryption (DRPE) system is proposed which offers encrypting information in multiple degrees of freedom. Compared with the traditional systems, i.e. (i) Fourier transform (FT); (ii) Fresnel transform (FST); (iii) Fractional Fourier transform (FRT); and (iv) Linear Canonical transform (LCT), based DRPE systems, the proposed system is more secure and robust as it encrypts the data with more degrees of freedom with an augmented key-space.
      559
  • Publication
    Generalized in-line digital holographic technique based on intensity measurements at two different planes
    In-line digital holography based on two-intensity measurements [Zhang et al. Opt. Lett. 29, 1787 (2004)], is modified by introducing a π shifting in the reference phase. Such an improvement avoids the assumption that the object beam must be much weaker than the reference beam in strength and results in a simplified experimental implementation. Computer simulations and optical experiments are carried out to validate the method, which we refer to as position-phase-shifting digital holography.
      1122Scopus© Citations 60
  • 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.
      177
  • 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.
      408
  • 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.
      374
  • Publication
    Sparsity based Terahertz reflective off-axis digital holography
    Terahertz radiation lies between the microwave and infrared regions in the electromagnetic spectrum. Emitted frequencies range from 0.1 to 10 THz with corresponding wavelengths ranging from 30 m to 3 mm. In this paper, a continuous-wave Terahertz off-axis digital holographic system is described. A Gaussian fitting method and image normalisation techniques were employed on the recorded hologram to improve the image resolution. A synthesised contrast enhanced hologram is then digitally constructed. Numerical reconstruction is achieved using the angular spectrum method of the filtered off-axis hologram. A sparsity based compression technique is introduced before numerical data reconstruction in order to reduce the dataset required for hologram reconstruction. Results prove that a tiny amount of sparse dataset is sufficient in order to reconstruct the hologram with good image quality.
      569Scopus© Citations 5
  • 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.
      578Scopus© Citations 19
  • Publication
    The choice of optical system is critical for the security of double random phase encryption systems
    The linear canonical transform (LCT) is used in modeling a coherent light field propagation through first-order optical systems. Recently, a generic optical system, known as the Quadratic Phase Encoding System (QPES), for encrypting a two-dimensional (2D) image has been reported. In such systems, two random phase keys and the individual LCT parameters (, , ) serve as secret keys of the cryptosystem. It is important that such encryption systems also satisfies some dynamic security properties. In this work, we therefore examine such systems using two cryptographic evaluation methods, the avalanche effect and bit independence criterion, which indicate the degree of security of the cryptographic algorithms using QPES. We compared our simulation results with the conventional Fourier and the Fresnel transform based DRPE systems. The results show that the LCT based DRPE has an excellent avalanche and bit independence characteristics compared to the conventional Fourier and Fresnel based encryption systems.Keywords: Quadratic Phase Encoding system, linear canonical transform, Double Random Phase Encryption, Avalanche effect and bit independence criterion.
      618Scopus© Citations 2