- Gopinathan, Unnikrishnan

###### Options

# Gopinathan, Unnikrishnan

Preferred name

Gopinathan, Unnikrishnan

Official Name

Gopinathan, Unnikrishnan

## Research Output

14 results Back to results

### Filters

##### Author

##### Subject

##### Has files

##### Type

### Settings

Sort By

Results per page

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.294 - 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.400Scopus© Citations 27 - 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.1517Scopus© Citations 85 - 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.279 - PublicationA known-plaintext heuristic attack on the Fourier plane encryption algorithm(Optical Society of America, 2006-04-17)
; ; ; The Fourier plane encryption algorithm is subjected to a known-plaintext attack. The simulated annealing heuristic algorithm is used to estimate the key, using a known plaintext-ciphertext pair, which decrypts the ciphertext with arbitrarily low error. The strength of the algorithm is tested by using this estimated key to decrypt a different ciphertext which was also encrypted using the same original key. We assume that the plaintext is amplitude-encoded real-valued image, and analyze only the mathematical algorithm rather than a real optical system that can be more secure. The Fourier plane encryption algorithm is found to be susceptible to a known-plaintext heuristic attack.561Scopus© Citations 204 - PublicationControlling speckle using lenses and free space(Optical Society of America, 2007-12-01)
; ; ; The correlation properties of speckle ﬁelds are studied for general paraxial systems. The previous studies on lateral and longitudinal speckle size for the case of free-space propagation (Fresnel transform) are generalized to the case of the linear canonical transform. These results have implications for the control of speckle size, through appropriate design of optical systems, with particular relevance for speckle interferometry.345Scopus© Citations 17 - 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.1012Scopus© Citations 58 - 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.556Scopus© Citations 57 - PublicationStatistical investigation of the double random phase encoding technique(Optical Society of America, 2009-08-24)
; ; ; ; The amplitude-encoding case of the double random phase encoding technique is examined by deﬁning a cost function as a metric to compare an attempted decryption against the corresponding original input image. For the case when a cipher–text pair has been obtained and the correct decryption key is unknown, an iterative attack technique can be employed to ascertain the key. During such an attack the noise in the output ﬁeld for an attempted decryption can be used as a measure of a possible decryption key’s correctness. For relatively small systems, i.e., systems involving fewer than 5x5 pixels, the output decryption of every possible key can be examined to evaluate the distribution of the keys in key space in relation to their relative performance when carrying out decryption. However, in order to do this for large systems, checking every single key is currently impractical. One metric used to quantify the correctness of a decryption key is the normalized root mean squared (NRMS) error. The NRMS is a measure of the cumulative intensity difference between the input and decrypted images. We identify a core term in the NRMS, which we refer to as the difference parameter, d. Expressions for the expected value (or mean) and variance of d are derived in terms of the mean and variance of the output ﬁeld noise, which is shown to be circular Gaussian. These expressions assume a large sample set (number of pixels and keys). We show that as we increase the number of samples used, the decryption error obeys the statistically predicted characteristic values. Finally, we corroborate previously reported simulations in the literature by using the statistically derived expressions.937Scopus© Citations 18 - 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.399Scopus© Citations 13