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Gopinathan, Unnikrishnan
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Gopinathan, Unnikrishnan
Official Name
Gopinathan, Unnikrishnan
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Now showing 1 - 10 of 14
- PublicationSpeckle interferometric system for measuring ocular microtremor(Optical Society of America, 2008-10-19)
; ; ; ; ; ; 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.369 - PublicationAn optical encryption scheme that uses polarization of coherent light(Optical Society of America, 2005-10-16)
; ; ; We demonstrate an optical system that encodes two dimensional data as different polarization states. The encrypted image is recorded using a digital holographic setup and the decryption is done numerically.335 - PublicationGeneralized Yamaguchi correlation factor for coherent quadratic phase speckle metrology systems with an aperture(Optical Society of America, 2006-12-01)
; ; ; ; ; In speckle-based metrology systems, a finite range of possible motion or deformation can be measured. When coherent imaging systems with a single limiting aperture are used in speckle metrology, the observed decorrelation effects that ultimately define this range are described by the well-known Yamaguchi correlation factor. We extend this result to all coherent quadratic phase paraxial optical systems with a single aperture and provide experimental results to support our theoretical conclusions.489Scopus© Citations 15 - PublicationKey-space analysis of double random phase encryption technique(Optical Society of America, 2007-09-10)
; ; ; We perform a numerical analysis on the double random phase encryption/decryption technique. The key-space of an encryption technique is the set of possible keys that can be used to encode data using that technique. In the case of a strong encryption scheme, many keys must be tried in any brute-force attack on that technique. Traditionally, designers of optical image encryption systems demonstrate only how a small number of arbitrary keys cannot decrypt a chosen encrypted image in their system. However, this type of demonstration does not discuss the properties of the key-space nor refute the feasibility of an efficient brute-force attack. To clarify these issues we present a key-space analysis of the technique. For a range of problem instances we plot the distribution of decryption errors in the key-space indicating the lack of feasibility of a simple brute-force attack.1609Scopus© Citations 85 - PublicationPolarization encoding and multiplexing of two-dimensional signals : application to image encryption(Optical Society of America, 2006-08-01)
; ; We discuss an optical system that encodes an input signal to a polarization state, using a spatial light modulator (SLM). Using two SLMs the optical system multiplexes two 2D signals in the polarization domain, and we demonstrate the multiplexing of two binary images. The encryption and decryption of two binary images using an XOR operation is also presented.636Scopus© Citations 61 - PublicationNoninterferometric phase retrieval using a fractional Fourier system(Optical Society of America, 2008-01-01)
; ; ; The signal extraction method based on intensity measurements in two close fractional Fourier domains is examined by using the phase space formalism. The fractional order separation has a lower bound and an upper bound that depend on the signal at hand and the noise in the optical system used for measurement. On the basis of a theoretical analysis, it is shown that for a given optical system a judicious choice of fractional order separation requires some a priori knowledge of the signal bandwidth. We also present some experimental results in support of the analysis.474Scopus© Citations 27 - PublicationRole of phase key in the double random phase encoding technique : an error analysis(Optical Society of America, 2008-07-20)
; ; ; ; We perform a numerical analysis of the double random phase encryption–decryption technique to determine how, in the case of both amplitude and phase encoding, the two decryption keys (the image- and Fourier-plane keys) affect the output gray-scale image when they are in error. We perform perfect encryption and imperfect decryption. We introduce errors into the decrypting keys that correspond to the use of random distributions of incorrect pixel values. We quantify the effects that increasing amounts of error in the image-plane key, the Fourier-plane key, and both keys simultaneously have on the decrypted image. Quantization effects are also examined1266Scopus© Citations 39 - PublicationSpread-space spread-spectrum technique for secure multiplexing(Optical Society of America, 2007-05-01)
; ; ; ; ; ; A novel technique for multiplexing complex images is proposed in which each image may be demultiplexed only if a set of random encryption keys is known. The technique utilizes the ability of the double random phase encoding method to spread a signals’ energy in both the space and the spatial frequency domains in a controlled manner. To multiplex, images are independently encrypted with different phase keys and then superimposed by recording sequentially on the same material. Each image is extracted by using the particular key associated with it. During decryption the energy from the other images is further spread, making it possible to minimize its effects by using suitable filters. Wigner analysis is applied to the technique, and numerical results are presented.480Scopus© Citations 51 - PublicationCryptanalysis of optical security systems with significant output images(Optical Society of America, 2007-08-01)
; ; ; The security of the encryption and verification techniques with significant output images is examined by a known-plaintext attack. We introduce an iterative phase-retrieval algorithm based on multiple intensity measurements to heuristically estimate the phase key in the Fourier domain by several plaintext-cyphertext pairs. We obtain correlation output images with very low error by correlating the estimated key with corresponding random phase masks. Our studies show that the convergence behavior of this algorithm sensitively depends on the starting point. We also demonstrate that this algorithm can be used to attack the double random phase encoding technique.543Scopus© Citations 87 - PublicationGeneralized in-line digital holographic technique based on intensity measurements at two different planes(Optical Society of America, 2008-02-10)
; ; ; 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.1106Scopus© Citations 60