Ra'ed Malallah

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.

Self-bending of optical waveguides in a dry photosensitive medium

Optical waveguide trajectories formed in an AA/PVA a photopolymer material photosensitive at 532 nm are examined. The transmission of light by this materials is discussed. The bending and arching of the waveguides which occur are investigated. The prediction of our model are shown to agree with the observed of trajectories. The largest index changes taking place at any time during the exposure, i.e. during SWW formation are found at the positions where the largest light intensity is present. Typically, such as maxima exist close to the input face at the location of the Primary Eye or at the location of the Secondary Eyes deeper with in the material. All photosensitive materials have a maximum saturation value of refractive index change that it is possible to induce, which is also discussed.

Optical characterization of photopolymer material using λ=532nm and investigation into future use at λ=850nm and λ=1300nm

The propagation of a light beam through a photo-sensitive photopolymer Polyvinyl Alcohol/Acrylamide (PVA/AA), and the creation of self-written waveguides (SWWs), has received much attention. Here we explore the manufacture and characterization of SWWs in PVA/AA for applications at near infrared communication wavelengths 850nm and 1300nm. The SWWs are fabricated using visible light at wavelength 532nm. The insertion and optical loss of the SWWs at different wavelengths will be interrogated. An optical loss and attenuation profile is to be built up for each of the three wavelengths as they propagate down the resulting SWWs.

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.

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 μm 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.

Investigating nonlinear distortion in the photopolymer materials

Propagation and diffraction of a light beam through nonlinear materials are effectively compensated by the effect of selftrapping. The laser beam propagating through photo-sensitive polymer PVA/AA can generate a waveguide of higher refractive index in direction of the light propagation. In order to investigate this phenomenon occurring in light-sensitive photopolymer media, the behaviour of a single light beam focused on the front surface of photopolymer bulk is investigated. As part of this work the self-bending of parallel beams separated in spaces during self-writing waveguides are studied. It is shown that there is strong correlation between the intensity of the input beams and their separation distance and the resulting deformation of waveguide trajectory during channels formation. This self-channeling can be modelled numerically using a three-dimension model to describe what takes place inside the volume of a photopolymer media. Corresponding numerical simulations show good agreement with experimental observations, which confirm the validity of the numerical model that was used to simulate these experiments.

Security analysis of quadratic phase based cryptography

The linear canonical transform (LCT) is essential in modeling a coherent light field propagation through first-order optical systems. Recently, a generic optical system, known as a Quadratic Phase Encoding System (QPES), for encrypting a two-dimensional (2D) image has been reported. It has been reported together with two phase keys the individual LCT parameters serve as keys of the cryptosystem. However, it is important that such the encryption systems also satisfies some dynamic security properties. Therefore, in this work, we examine some cryptographic evaluation methods, such as Avalanche Criterion and Bit Independence, which indicates the degree of security of the cryptographic algorithms on QPES. We compare 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 than that of using the conventional Fourier and Fresnel based encryption systems.

Reconstruction of Digital Holograms by Iterative Phase Retrieval Algorithm:Improvements and Analysis

Iterative phase retrieval usually requires a good initial guess for the phase. In this paper, we consider two improvements to a recently proposed scheme (Opt. Eng. 55 (3), 033106 (2016)) to acquire the initial guess from a digital holographic system. This set-up is simpler than phase-shifting interferometry, and suppresses the twin image and dc terms without the resolution trade-off involved in off-axis holography. In this paper, we show the benefits of ensuring the propagation algorithm is unitary. We also use a recently proposed iterative phase retrieval algorithm which uses a combination of the Gerschberg-Saxton and hybrid input-output algorithms, and demonstrate the benefit of this algorithm for avoiding stagnation of the algorithm.

A fast converging and error reducing phase retrieval algorithm for Fourier Ptychographic microscopy

Fourier ptychography is a recently developed computational framework for wide-field, high-resolution imaging. Here, we propose to use a fast converging and error reducing phase retrieval algorithm to retrieve the complex sample information.

Self-Written Waveguide Formation in the Dry Photopolymer Material, Using a Single Mode Fiber Optics

Self-Writing of optical waveguides has been demonstrated in the photopolymerizable materials. The unique self-writing phenomenon is due to the balance of light wave diffraction and confinement through the increase in index of refraction during polymerization.