Roberto Torroba

Three-dimensional joint transform correlator cryptosystem

We introduce for the first time, to the best of our knowledge, a three-dimensional experimental joint transform correlator (JTC) cryptosystem allowing the encryption of information for any 3D object, and as an additional novel feature, a second 3D object plays the role of the encoding key. While the JTC architecture is normally used to process 2D data, in this work, we envisage a technique that allows the use of this architecture to protect 3D data. The encrypted object information is contained in the joint power spectrum. We register the key object as a digital off-axis Fourier hologram. The encryption procedure is done optically, while the decryption is carried out by means of a virtual optical system, allowing for flexible implementation of the proposal. We present experimental results to demonstrate the validity and feasibility of the method.

Optical-data storage-readout technique based on fractal encrypting masks

We propose the use of fractal structured diffractive masks as keys in secure storage-readout systems. A joint transform correlator based on a photorefractive crystal in the Fourier domain is implemented to perform encryption and decryption. We discuss the advantages of encrypting information using this kind of deterministic keys in comparison to conventional random phase masks. Preliminary experimental results are presented to demonstrate the effectiveness of the proposed system.

Moiré beating digital technique to collimation testing

A digital collimation technique based on the self-imaging phenomenon is proposed. Basically, a moire pattern is observed by viewing the Fresnel image of an original self-imaged grating through a second grating. Decollimation of the laser beam causes a variation in the self-image period. In our method, the moire beating is produced through a digital subtraction operation of a replica of the original grating and the self-images provided by a decollimated beam. The variation of the moire period is related to the amount of decollimation of the illuminating beam. Lenses of different focal lengths were used in the collimation procedure, as well as gratings with different periods. The validity of the approach is experimentally demonstrated and setting sensitivities are compared.

Positioning method based on digital moiré

An alternative object positioning technique based on digital moire is presented. The moire pattern is obtained by the digital subtraction of different self-images under predetermined conditions including a grating rotation. The principle of the method relies on the fringe visibility matching in the moire process while moving from one self-image plane to another. The motion produces both loose of fringe contrast and rotation of the moire fringes. The theoretical model of this matching is provided. It applies the concept of measuring a global maximum contrast to each image, which allows to obtain both the best contrast and the adequate rotation. This allows a precise identification of each Talbot plane associated to an object to be positioned.

Dual random phase encoding: a temporal approach for fiber optic applications

We analyze the dual random phase encoding technique in the temporal domain to evaluate its potential application for secure data transmission in fiber optic links. To take into account the optical fiber multiplexing capabilities, the noise content of the signal is restricted when multiple channels are transmitted over a single fiber optic link. We also discuss some mechanisms for producing encoded time-limited as well as bandwidth-limited signals and a comparison with another recently proposed technique is made. Numerical simulations have been carried out to analyze the system performance. The results indicate that this multiplexing encryption method could be a good alternative compared with other well-established methods.

Optimized random phase only holograms

We propose a simple and efficient technique capable of generating Fourier phase only holograms with a reconstruction quality similar to the results obtained with the Gerchberg-Saxton (G-S) algorithm. Our proposal is to use the traditional G-S algorithm to optimize a random phase pattern for the resolution, pixel size, and target size of the general optical system without any specific amplitude data. This produces an optimized random phase (ORAP), which is used for fast generation of phase only holograms of arbitrary amplitude targets. This ORAP needs to be generated only once for a given optical system, avoiding the need for costly iterative algorithms for each new target. We show numerical and experimental results confirming the validity of the proposal.

Focal length assessment by self-imaging

Several classical and modern approaches were developed in the literature for measuring the focal length of a lens. We focus our attention on those that employ self-imaging methods. In our proposal, a collimated laser beam illuminates a test lens. After the lens, the wavefront becomes convergent or divergent according to the lens power. Attached to the lens, we place a Ronchi grating. This procedure gives rise to several classical Talbot images, although magnified due to the non-parallel illumination. Key to our presentation is the use of the property that the pitch of each self-image is directly related to the focal length of the collimating lens. To obtain the pitches, a lensless CCD camera is positioned in any two consecutive self-images planes. The respective images are captured and processed. We designed software that allows first to precisely focus each self-image plane by a best-contrast algorithm, and then takes an intensity histogram along a direction perpendicular to the grating lines. By measuring the distance between consecutive self-image planes and their pitches, the focal length is determined. This method provides a reference testing with higher setting sensitivity and an increase in the accuracy compared to previous methods. Note that the proposed method represents a practical improvement over other existing methods.

Holographic nondestructive testing by an optically generated zone plate.

An improvement for holographic nondestructive testing in the field of information processing is proposed. It makes use of an optically generated zone plate for obtaining an interferometric hologram of an object suffering a deformation. This allows quick data acquisition and can be used outside laboratory conditions. Experimental results are discussed, and a brief mathematical analysis from the point of view of Fourier optics is also given.

Optimized random phase only holograms in the Fresnel domain

We present a fast and simple technique to generate phase only Fresnel holograms of 2D intensity objects. This technique uses a modified Gerchberg-Saxton (G-S) algorithm to optimize a random phase. This optimization takes into account the resolution, pixel size, reconstruction plane, and optical characteristics of the system that will reconstruct the holograms. The resulting Optimized Fresnel RAndom Phase (OFRAP) is then multiplied with the desired intensity target, and after performing a Fresnel transform and a phase extraction we obtain the corresponding phase only hologram. Numerical results show that generation of holograms with OFRAP achieves performance close to the traditional G-S algorithm directly applied to the intensity target. The proposed technique has the additional advantage that a single ORAP can be used to generate any number of holograms, thus eliminating the need for any further iterative algorithms. This proposal is ideal for the generation of holographic videos and other applications where dynamic generation of phase holograms to manipulate the light field are necessary, like neuronal photostimulation, holographic displays and aberration correction.

Optimized random phase encryption

We propose for the first time, to the best of our knowledge, the use of optimized random phases (ORAPs) in a double random phase encryption scheme (DRPE). In DRPE schemes the convolution between two random phase functions encrypts the information to be secured. However, in actual encryption applications, this convolution of random phases also results in unwanted effects like speckle noise. In this Letter we show that under certain conditions this noise can be drastically reduced. These conditions can be easily achieved by using ORAPs. These ORAPs, besides containing information about the parameters of the optical system and maintaining all the security properties of a random phase function, ensure that the encrypted data is a phase-only function. This leads to a great increase in system performance, with decryption quality similar to the reconstruction of a phase-only hologram generated with the Gerchberg-Saxton algorithm. We show both numerical and experimental results confirming the validity of our proposal.