Tamás Sarkadi

Hybrid multinary modulation using a phase modulating spatial light modulator and a low-pass spatial filter.

We propose a method for performing binary intensity and continuous phase modulation of beams with a spatial light modulator (SLM) and a low-pass spatial filtering 4-f system. With our method it is possible to avoid the use of phase masks in holographic data storage systems or to enhance the phase encoding of the SLM by making it capable of binary amplitude modulation. The data storage capabilities and the limitations of the method are studied.

Quantitative security evaluation of optical encryption using hybrid phase- and amplitude-modulated keys

In the increasing number of system approaches published in the field of optical encryption, the security level of the system is evaluated by qualitative and empirical methods. To quantify the security of the optical system, we propose to use the equivalent of the key length routinely used in algorithmic encryption. We provide a calculation method of the number of independent keys and deduce the binary key length for optical data encryption. We then investigate and optimize the key length of the combined phase- and amplitude-modulated key encryption in the holographic storage environment, which is one of the promising solutions for the security enhancement of single- and double-random phase-encoding encryption and storage systems. We show that a substantial growth of the key length can be achieved by optimized phase and amplitude modulation compared to phase-only encryption. We also provide experimental confirmation of the model results.

Micropatterned Polyvinyl Butyral Membrane for Acid−Base Diodes

Until now, polyvinyl alcohol (PVA) gel cylinders have been used in electrolyte diodes as a connecting element between the acidic and alkaline reservoirs. In this paper, a new connecting element is reported: a breath figure templated polyvinyl butyral (PVB) membrane prepared with dip-coating from a dichloromethane solution of the polymer in a humid atmosphere. The procedure gives a 1.5-2 μm thick membrane with a hexagonal pattern, the average characteristic length of which is 1 μm. After an acidic etching, it was found to be a good connecting element. The voltage-current characteristics and dynamic properties of PVA and PVB were measured and compared. The PVB membrane has a faster response to voltage changes than the PVA gel, but in both cases, there was a slow drift in the current that prevented it from reaching a steady state. Reproducible characteristics can be obtained, however, after the current reaches a well-defined quasi-steady state.

Investigation of a 3D head-mounted projection display using retro-reflective screen

We propose a compact head-worn 3D display which provides glasses-free full motion parallax. Two picoprojectors placed on the viewers head project images on a retro-reflective screen that reflects left and right images to the appropriate eyes of the viewer. The properties of different retro-reflective screen materials have been investigated, and the key parameters of the projection - brightness and cross-talk - have been calculated. A demonstration system comprising two projectors, a screen tracking system and a commercial retro-reflective screen has been developed to test the visual quality of the proposed approach.

Measurement of the Jones matrix of liquid crystal displays using a common path interferometer

We propose a robust interferometric method to measure the Jones matrix of polarization components, especially liquid crystal displays. Phase values are measured by a simple common path interferometer containing a birefringent crystal as beam splitter and a polarizer as beam combiner. This solution eliminates the sensitivity of traditional interferometric techniques to vibration, temperature variation or wavefront distortion. The proposed phase measurement method is applicable to the measurement of both spatially homogeneous and binary modulated states, thus the modulation transfer function and inter-pixel interference can also be studied. We demonstrate this technique by the measurement of a liquid crystal on silicon display. The resulting Jones matrix, as a function of displayed gray level, can be efficiently embedded in any numeric model of an optical system containing the analyzed spatial light modulator.

Spatial filters for complex wavefront modulation

In this paper we propose a method to generate independent and simultaneous phase and amplitude modulation by a phase-only spatial light modulator and Fourier filtering. The incident light is modulated by a suitable phase pattern containing high spatial frequencies. The modulated light is transmitted through a 4f optical system having an appropriate spatial filter in the Fourier plane in order to synthesize the expected complex modulated wavefront on the output of the system. We propose a simple method to generate spatial filters applicable for the phase-only to complex modulated wavefront conversion. We analyze the quality of the output image related to the ideal wavefront using the proposed filters. We show that more efficient complex modulation can be realized by the proposed method than by the earlier solutions.

Improved data pages for an interference-based cryptosystem

In this paper, we propose a novel input wave front modulation method to enhance the security level of a Mach-Zender interferometer-based Fourier encryption system. The input data is encoded in the two wave fronts propagated in the arms of the interferometer. Both arms contain a 4f setup, and two independent Fourier keys are used to encrypt these wave fronts. During decryption the encrypted wave fronts are propagated through the interferometer. In the case when correct Fourier keys are used for decryption, the reconstructed data page is shown by the interference pattern of the output. We propose a method to synthesize two phase modulated input images for this cryptosystem. The modulation method has a user defined phase parameter. We prove that the security level of the proposed cryptosystem can be significantly improved compared with previous solutions, by using an optimally chosen phase parameter.

Hologram positioning servo for phase-encoded holographic data storage systems

We propose a method using phase encryption and hologram multiplexing to encode positional information into the hologram, which can be used during readout to find the correct position of the reference beam. We also include a method to align the position of the phase code in the reference beam during readout, with which we achieved approximately 1/100 hologram size (4.4 microm) precision electronically, without the need of a precise mechanical hologram positioning device. We prove the feasibility of the method with experiments.

Optical encryption using pseudorandom complex spatial modulation

In this paper we propose a new (to our knowledge) complex spatial modulation method to encode data pages applicable in double random phase encryption (DRPE) to make the system more resistant to brute-force attack. The proposed modulation method uses data page pixels with random phase and amplitude values with the condition that the intensity of the interference of light from two adjacent pixels should correspond to the encoded information. A differential phase contrast technique is applied to recover the data page at the output of the system. We show that the proposed modulation method can enhance the robustness of the DRPE technique using point spread function analysis. Key space expansion is determined by numeric model calculations.

Simulation of small- and wide-angle scattering properties of glass-bead retroreflectors

Retroreflective materials are extensively used as traffic signs and security patterns. These goods are often realized by spherical glass-beads attached to some reflective substrate. New applications, especially 3D projection, require the precise evaluation and design of the characteristics of light backscattered from retroreflective screens. Simulation of such materials is not straightforward due to the different optical processes taking place: direct retroreflection involving small-angle diffraction effects, and multiple scattering resulting in wide-angle diffuse light. We propose a new complex method to describe the backscattering properties of glass-bead retroreflectors that uniquely combines diffraction calculations with ray tracing based on the microscopic properties of the screen. We validated our simulation method by measurements performed on commercial retroreflective samples.