Research Output
1 - 10 of 15
Publication
Sparsity based Terahertz reflective off-axis digital holography
2017-04-24, Wan, Min, Muniraj, Inbarasan, Malallah, Ra'ed, Zhao, Liang, Ryle, James P., Rong, Lu, Healy, John J., Wang, Dayong, Sheridan, John T.
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.
Publication
The choice of optical system is critical for the security of double random phase encryption systems
2017-06-14, Muniraj, Inbarasan, Guo, Changliang, Malallah, Ra'ed, Cassidy, Derek, Zhao, Liang, Ryle, James P., Healy, John J., Sheridan, John T.
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.
Publication
Cases where the linear canonical transform of a signal has compact support or is band-limited
2008-02-01, Healy, John J., Sheridan, John T.
A signal may have compact support, be band-limited (i.e., its Fourier transform has compact support), or neither (“unbounded”). We determine conditions for the linear canonical transform of a signal having these
properties. We examine the significance of these conditions for special cases of the linear canonical transform and consider the physical significance of our results
Publication
Reevaluation of the direct method of calculating Fresnel and other linear canonical transforms
2010-04-01, Healy, John J., Sheridan, John T.
The linear canonical transform may be used to simulate the effect of paraxial optical systems on wave fields. Using a recent definition of the discrete linear canonical transform, phase space diagram analyses of the sampling requirements of the direct method of calculating the Fresnel and other linear canonical transforms are more favorable than previously thought. Thus the direct method of calculating these Transforms may be used with fewer samples than previously reported simply by making use of an appropriate reconstruction filter on the samples output by the algorithm.
Publication
Unitary Algorithm for Nonseparable Linear Canonical Transforms Applied to Iterative Phase Retrieval
2017-03-20, Zhao, Liang, Sheridan, John T., Healy, John J.
Abstract:Phase retrieval is an important tool with broad applications in optics. The GerchbergSaxton algorithm has been a workhorse in this area for many years. The algorithm extracts phase information from intensities captured in two planes related by a Fourier transform. The ability to capture the two intensities in domains other than the image and Fourier plains adds flexibility; various authors have extended the algorithm to extract phase from intensities captured in two planes related by other optical transforms, e.g., by free space propagation or a fractional Fourier transform. These generalizations are relatively simple once a unitary discrete transform is available to propagate back and forth between the two measurement planes. In the absence of such a unitary transform, errors accumulate quickly as the algorithm propagates back and forth between the two planes. Unitary transforms are available for many separable systems, but there has been limited work reported on nonseparable systems other than the gyrator transform. In this letter, we simulate a nonseparable system in a unitary way by choosing an advantageous sampling rate related to the system parameters. We demonstrate a simulation of phase retrieval from intensities in the image domain and a second domain related to the image domain by a nonseparable linear canonical transform. This work may permit the use of nonseparable systems in many design problems.
Publication
Digital computation of the complex linear canonical transform
2011-07-01, Liu, Changgeng, Wang, Dayong, Healy, John J., Hennelly, Bryan M., Sheridan, John T., Kim, Myung K.
An efficient algorithm for the accurate computation of the linear canonical transform with complex transform parameters and with complex output variable is presented. Sampling issues are discussed and the requirements for different cases given. Simulations are provided to validate the results.
Publication
Space-bandwidth ratio as a means of choosing between Fresnel and other linear canonical transform algorithms
2011-05-01, Healy, John J., Sheridan, John T.
The product of the spatial and spatial frequency extents of a wave field has proven useful in the analysis of the sampling requirements of numerical simulations. We propose that the ratio of these quantities is also illuminating. We have shown that the distance at which the so-called “direct method” becomes more efficient than the so-called “spectral method” for simulations of Fresnel transforms may be written in terms of this space-bandwidth ratio. We have proposed generalizations of these algorithms for numerical simulations of general ABCD systems and derived expressions for the “transition space-bandwidth ratio” above which the generalization of the spectral method is the more efficient algorithm and below which the generalization of the direct method is preferable.
Publication
Inclusive Teaching & Learning Case Studies in Engineering, Architecture & Affiliated Disciplines
2021-10-14, Padden, Lisa, Buggy, Conor J., Ahern, Aoife, Rogers, Mark S., Cotterill, Sarah, Healy, John J., Faria, Tiago, Sudhershan, Daniel, Fitzpatrick, Miriam, McCrum, Daniel, Keenahan, Jennifer, Shotton, Elizabeth
Diversity and inclusion are core to UCD values. We seek to attract students from a wide range of social and economic backgrounds and students who reflect the true diversity of the country. And as a global university, UCD attracts international students from over 100 countries. This diversity enriches our campus, and the experience of our students. The University's strategy 2020-2024 'Rising to the Future' also recognises the importance of inclusion and diversity, in seeking to "provide an inclusive educational experience that defines international best practice and prepares our graduates to thrive in present and future societies." However, an inclusive educational experience will not be achieved by simply creating diversity in the student body. It requires that we adjust our approach in everything we do to support and encourage our students’ success. We have clearly articulated in our strategy, and further emphasised in our Education and Student Success strategy, that our goal is to "equip all our educators with the tools and resources required to embed Universal Design for Learning on an institution-wide basis".
Publication
Fast linear canonical transforms
2010-01-01, Healy, John J., Sheridan, John T.
The linear canonical transform provides a mathematical model of paraxial propagation though quadratic phase systems. We review the literature on numerical approximation of this transform, including discretization,
sampling, and fast algorithms, and identify key results. We then propose a frequency-division fast linear canonical transform algorithm comparable to the Sande–Tukey fast Fourier transform. Results calculated with an implementation of this algorithm are presented and compared with the corresponding analytic functions.
Publication
Additional sampling criterion for the linear canonical transform
2008-11-15, Healy, John J., Hennelly, Bryan M., Sheridan, John T.
The linear canonical transform describes the effect of first-order quadratic phase optical systems on a wave field. Several recent papers have developed sampling rules for the numerical approximation of the transform. However, sampling an analog function according to existing rules will not generally permit the reconstruction of the analog linear canonical transform of that function from its samples. To achieve this, an additional sampling criterion has been developed for sampling both the input and the output wave fields.