Joby Joseph

Optical encryption by double-random phase encoding in the fractional Fourier domain

We propose an optical architecture that encodes a primary image to stationary white noise by using two statistically independent random phase codes. The encoding is done in the fractional Fourier domain. The optical distribution in any two planes of a quadratic phase system (QPS) are related by fractional Fourier transform of the appropriately scaled distribution in the two input planes. Thus a QPS offers a continuum of planes in which encoding can be done. The six parameters that characterize the QPS in addition to the random phase codes form the key to the encrypted image. The proposed method has an enhanced security value compared with earlier methods. Experimental results in support of the proposed idea are presented.

Optical encryption system that uses phase conjugation in a photorefractive crystal

We implement an optical encryption system based on double-random phase encoding of the data at the input and the Fourier planes. In our method we decrypt the image by generating a conjugate of the encrypted image through phase conjugation in a photorefractive crystal. The use of phase conjugation results in near-diffraction-limited imaging. Also, the key that is used during encryption can also be used for decrypting the data, thereby alleviating the need for using a conjugate of the key. The effect of a finite space-bandwidth product of the random phase mask on the encryption systems performance is discussed. A theoretical analysis is given of the sensitivity of the system to misalignment errors of a Fourier plane random phase mask.

Fully phase encryption using fractional Fourier transform

We implement a fully phase encryption system, using fractional Fourier transform to encrypt and decrypt a 2-D phase image obtained from an amplitude image. The encrypted image is holographically recorded in a barium titanate crystal and is then decrypted by generating through phase conjugation, a conjugate of the encrypted image. The decrypted phase image is converted into an amplitude image by the phase contrast technique using an electrically addressed spatial light modulator. Experimental results in support of the proposed idea are presented.

Optical image encryption using a jigsaw transform for silhouette removal in interference-based methods and decryption with a single spatial light modulator

Interference-based optical encryption schemes have an inherent silhouette problem due to the equipollent nature of the phase-only masks (POMs) generated using an analytical method. One of the earlier methods suggested that removing the problem by use of exchanging process between two masks increases the computational load. This shortcoming is overcome with a noniterative method using the jigsaw transformation (JT) in a single step, with improved security because the inverse JT of these masks, along with correct permutation keys that are necessary to decrypt the original image. The stringent alignment requirement of the POMs in two different arms during the experiment is removed with an alternative method using a single spatial light modulator. Experimental results are provided to demonstrate the decryption process with the proposed method.

Impulse attack free double-random-phase encryption scheme with randomized lens-phase functions

Security of the conventional Fourier-based double-random-phase encryption (DRPE) technique is prone to impulse attacks, as the Fourier transform (FT) of a delta function results in a unity function. To negate such an attack, the phase factors of the lenses are modified by multiplying these with random-phase functions. Owing to this modification of the FT as a result of the randomized lens phase function, a modified FT (MFTLR) gives the random output for a delta function input. Employing MFTLR in the DRPE technique enhances the security features and makes the encryption system safer from the impulse attack. Numerical and experimental results are given for the validation of the proposed technique.

Reconfigurable Optically Induced Quasicrystallographic Three‐Dimensional Complex Nonlinear Photonic Lattice Structures

2010 WILEY-VCH Verlag Gm Quasicrystals (QCs) are materials that possess a long-range order with defined diffraction patterns, but lack the characteristic translational periodicity of crystals. From the discovery of the non-crystallographic icosahedral quasiperiodic symmetry found in Al6Mn in 1984, [2] the distinct properties of quasicrystallographic structures attracted a great deal of interest in different realms of science in recent years. Another field of technological interest in the recent past is that of photonic crystals (PCs), the structured materials with a translational periodic modulation of the refractive index. Merging these two fields, a new class of material structures called photonic quasicrystals (PQCs) has drawn the attention of researchers stemming from a cumulative effect from both fields. This is mainly due to the fact that the higher rotational symmetry of QCs leads to more isotropic and complete photonic bandgaps (PBGs) even in materials with a low refractive index contrast. However, as in the case of PCs, the fabrication of 3D PQCs is much more involved in comparison to 2D PQCs and remains a real challenge today. Moreover, many of the conventional methods become technically either unsuitable or extremely complicated for the fabrication of 3D PQCs. Therefore, the fabrication and optimization of higher rotational symmetry 3D PQCs demand an approach that is flexible as well as reconfigurable. The purpose of the present Communication is dual fold. On the one hand, we demonstrate for the first time the generation of well-defined reconfigurable 3D quasi-crystallographic photorefractive nonlinear photonic structures with various rotational symmetries, which are experimentally realized in an externally biased cerium doped strontium barium niobate (SBN:Ce) photorefractive material as the nonlinear optical material of choice. These complex structures are envisaged to form a reconfigurable platform to investigate advanced nonlinear light–matter interaction in higher spatial dimensions with various rotational symmetries. On the other hand, we present a generalized versatile experimental approach for the fabrication of complex 3D axial PQCs with higher order rotational symmetry and variants of complex 3D structures similar to those having icosahedral symmetry, using a real-time reconfigurable holographic technique. It involves a programmable spatial light modulator (SLM)-assisted single step optical induction approach based on computer-engineered optical phase patterns. It is also important to note that the versatility of the experimental approach, we present, is not limited to photorefractive materials alone. It can be easily well adapted to various photosensitive materials as per the application requirement in the diverse fields of material science. Among various photosensitive materials, reconfigurable nonlinear photonic lattices can be easily generated by means of a so-called optical induction technique at very low power levels ( micro watts) in a photorefractive material, exploiting the wavelength sensitivity of these materials. The process of refractive index modulation, which leads to photonic lattice formation in such a medium is caused by a two-step process out of the incident light intensity distribution. Under the influence of an externally applied electric field, the incident light intensity distribution causes a charge carrier redistribution that results in a macroscopic space charge field in the photorefractive material. This, in turn, leads to a space-dependent refractive index modulation via the electro-optic effect thereby representing a nonlinear optical effect of third order that creates the refractive index modulation out of the incident intensity distribution. Apart from the possibility of permanent fixing of the generated structures in a photorefractive crystal, the recorded structure is reconfigurable: it can also be erased by the flush of white light so that new patterns could be again recorded in these materials. Therefore, photorefractive materials are ideal materials for reconfigurable PQC generation either to optimize the required photonic structure on the one hand or to be used as a reconfigurable platform to investigate novel nonlinear wave dynamics. From the optical properties point of view, the photonic lattices formed in SBN:Ce show both polarization as well as orientation anisotropy. In order to obtain refractive index modulated structures that mimic the intensity pattern, o-polarized writing beams are used causing a low modulation due to the appropriate electro-optic coefficient addressed. For the case of using e-polarized writing beams, as the relevant electrooptic coefficient is much higher, a strongly nonlinear refractive index modulation can be obtained for the fabricated lattices. Moreover, as maximum refractive index modulation is induced in the direction parallel to the crystal c axis, there exists also orientation anisotropy in SBN:Ce.

Homogenized Fourier transform holographic data storage using phase spatial light modulators and methods for recovery of data from the phase image

We present a method for homogenizing the Fourier spectrum for holographic digital data storage by use of a phase spatial light modulator (SLM), and methods for the recovery of data from a phase image are implemented and discussed. Binary digital data are displayed on a phase SLM operating in 0 and pi phase modes to optimally remove the intense dc peak so as to obtain a homogenized Fourier spectrum. Methods based on holographic interferometry have been developed and employed for recovery of the original amplitude data page from the phase-data page. A new edge-detection-based method also has been demonstrated and analyzed for reconstruction of the original data. Experimental results are presented to confirm the feasibility of these novel techniques.

Single shot high resolution digital holography

We demonstrate a novel computational method for high resolution image recovery from a single digital hologram frame. The complex object field is obtained from the recorded hologram by solving a constrained optimization problem. This approach which is unlike the physical hologram replay process is shown to provide high quality image recovery even when the dc and the cross terms in the hologram overlap in the Fourier domain. Experimental results are shown for a Fresnel zone hologram of a resolution chart, intentionally recorded with a small off-axis reference beam angle. Excellent image recovery is observed without the presence of dc or twin image terms and with minimal speckle noise.

Fully phase encryption using digital holography

We demonstrate a fully phase encryption system that uses digital holography. The input amplitude image to be encrypted is phase encoded, and either its Fourier or Fresnel transform is obtained. Using interference with a wave from a random phase mask, a Fourier or Fresnel hologram (encrypted data) is recorded digitally. The decryption key is also recorded as a digital hologram, called the key hologram. An electronic key is generated and multiplied with the encrypted hologram. A Fourier transform (encrypted image) is then obtained. The decryption key hologram, the electronic key, and the encrypted image can be transmitted through communication channels. The retrieval is carried out by all-digital means.

Three-dimensional optically induced reconfigurable photorefractive nonlinear photonic lattices.

We experimentally investigate the formation of reconfigurable three-dimensional (3D) nonlinear photonic lattices in an externally biased cerium doped strontium barium niobate photorefractive crystal by a spatial light modulator-assisted versatile simplified single step optical induction approach. The analysis of the generated 3D nonlinear photonic lattices by plane wave guiding, momentum space spectroscopy, and far field diffraction pattern imaging is presented, which points to the embedded potential of these 3D structures as reconfigurable platform to investigate advanced nonlinear light-matter interaction in periodic structures.