Zhijuan Hu

Optical image encryption using optical scanning and fingerprint keys

Abstract Fingerprint authentication is a preferable biometric technique for securing images. Optical scanning cryptography (OSC) is a hybrid optical/digital method to encrypt and decrypt an image using optical scanning technique. In this paper, we propose a new strategy to combine OSC with fingerprints to enhance security. We have verified the effectiveness of the proposed idea in simulations. Subsequently practical implementation of the idea has also been performed with optical heterodyning to confirm the results from simulations. We have also discussed the added security and flexibility of implementation when optical heterodyning is employed in cryptography.

Novel optical scanning cryptography using Fresnel telescope imaging.

We propose a new method called modified optical scanning cryptography using Fresnel telescope imaging technique for encryption and decryption of remote objects. An image or object can be optically encrypted on the fly by Fresnel telescope scanning system together with an encryption key. For image decryption, the encrypted signals are received and processed with an optical coherent heterodyne detection system. The proposed method has strong performance through use of secure Fresnel telescope scanning with orthogonal polarized beams and efficient all-optical information processing. The validity of the proposed method is demonstrated by numerical simulations and experimental results.

Optical cryptography with biometrics for multi-depth objects

We propose an optical cryptosystem for encrypting images of multi-depth objects based on the combination of optical heterodyne technique and fingerprint keys. Optical heterodyning requires two optical beams to be mixed. For encryption, each optical beam is modulated by an optical mask containing either the fingerprint of the person who is sending, or receiving the image. The pair of optical masks are taken as the encryption keys. Subsequently, the two beams are used to scan over a multi-depth 3-D object to obtain an encrypted hologram. During the decryption process, each sectional image of the 3-D object is recovered by convolving its encrypted hologram (through numerical computation) with the encrypted hologram of a pinhole image that is positioned at the same depth as the sectional image. Our proposed method has three major advantages. First, the lost-key situation can be avoided with the use of fingerprints as the encryption keys. Second, the method can be applied to encrypt 3-D images for subsequent decrypted sectional images. Third, since optical heterodyning scanning is employed to encrypt a 3-D object, the optical system is incoherent, resulting in negligible amount of speckle noise upon decryption. To the best of our knowledge, this is the first time optical cryptography of 3-D object images has been demonstrated in an incoherent optical system with biometric keys.

Dynamic characteristics of two-dimensional finite boundary photorefractive gratings recording and fixing in LiNbO3:Fe crystals

The formation dynamics of crossed-beam photorefractive gratings formed by the in Fe doped LiNbO3 crystal is studied based on jointly solving the material equations and the two-dimensional coupled-wave theory. The numerical calculations of the temporal and the spatial evolutions of the space-charge field and the light modulation depth are given, and the influence of the recording conditions, including the recording wavelength, the two-dimensional sizes of the holographic grating are presented, and the time evolution of diffraction efficiency during holographic recording is also presented.

High speed and low side lobe optical phased array steering by phase correction technique

Laser beam scanners are important optical elements with a large variety of applications in the measurement techniques, optical communications, laser imaging ladar, etc. Agile beam steering of optical radiation using phased arrays offers significant advantages, such as weight, stability and power requirements over conventional beam steering systems. There are several kinds of optical phased array technology to be developed, such as Lithium tantalite phase shifters, lithium niobate electro-optic prism deflectors, and liquid crystal and ferroelectric liquid crystal phase modulations. However, one of the major drawbacks of these approaches is low light efficiencies because of the side lobes in far-field pattern of the steering beam. In this paper, a new low side lobe optical beam steering technique using phase correction technique is proposed. A stable and fast single-lobed far-field pattern in steering beam can be obtained from the optical phased array with a phase plate by optima design. It can result in a substantially increased light efficiency and beam quality. The quantitative calculation results of typical optical phased array are demonstrated. The low side lobe optical beam steering technology of optical phased array will benefit many practical applications such as laser ladar, laser communications and high resolution displays.

Pulse shaping in crossed-beam volume gratings

A detailed study of pulse shaping properties in crossed-beam volume gratings is performed. Analytical expressions for the profiles of the transmitted and diffracted beams are obtained based on the modified coupled wave theory. The spatial and spectrum evolutions and the diffraction efficiency of the diffracted pulsed beams are discussed. It is shown that the profiles of diffraction pulses and the total diffraction efficiency are greatly affected by the grating size, recording angle, and refractive-index modulation. The studies indicate that the shaping properties for different geometry parameters of the crossed-beam volume gratings are different. However, this difference can be used for many special pulse shaping applications.

Group delay properties of linear chirped Gaussian pulse diffracted by volume gratings

Volume gratings are of wide interest in many applications because of their properties of high diffraction efficiency, excellent wavelength selectivity and angular selectivity. Ultrashort pulsed laser has many promising applications in the fields such as optical communication, signal and imaging processing. Because of its abundant spectrum and wide bandwidth, the diffraction properties of volume gratings under illumination by ultrashort optical pulse will be different from that of a monochromatic continuous wave. A periodical structure is highly dispersive and such a dispersive property can be used to control the group velocity of light beams. So the spectral properties and dispersive properties of the periodically layered structures such as volume gratings when a linear chirped Gaussian pulse illuminated are very necessary. These properties can be applicable to addressable filter, switching, and controllable optical delay line. In this paper, a detailed study of linear chirped ultrashort pulsed beams diffracted by volume gratings in dispersive media is performed. The group delay characteristics of the diffraction spectrum of the transmitted and reflected volume gratings were studied based on the modified coupled wave theory. The analysis and observation of this paper will be valuable for the designing optical elements based on diffractive structure for the use with linear chirped ultra-short pulsed waves.

Preparation and Properties of Aluminum-doped ZnO–SiO2 Composite Thin Films

Aluminum-doped ZnO (AZO) thin films are usually used as transparent conductors. For practical applications, however, the relatively high refractive index of AZO can lead to high optical reflection. In this work, we modified the optical properties of AZO thin films by adding different amounts of SiO2 into the AZO thin films to prepare the AZO-SiO2 composite thin films. The AZO-SiO2 films were deposited on quartz substrates using a co-sputtering deposition method. It was shown that the refractive index of the AZO-SiO2 thin films consistently decreased with the amount of SiO2 contained in the films. In the meantime, the resistivity of the AZO-SiO2 thin films was found to increase with the SiO2 content in the films. The effects of the deposition conditions on the optical and electrical properties of the AZO-SiO2 thin films were systematically investigated.

Diffraction characteristics of volume holographic gratings with finite size for the ultrashort pulsed beam readout

Using the two-dimensional coupled-wave theory, the diffraction characteristics of volume holographic gratings (VHGs) with finite size planar are studied for the ultrashort pulsed beam (UPB) readout. Numerical simulations are show for the special case of the overlapping VHGs reconstructed by a Gaussian-shaped UPB in temporal domain. The effects of the material dispersion and the finite size of the grating on the intensity distributions of the diffracted and transmitted pulsed beams, and the total diffraction efficiency are given. Our study also shows the differences between the diffraction characteristics of the finite size planar VHG for the UPB readout and those for the CW readout. And, comparison of the diffraction characteristics between the finite size VHGs and the one dimensional VHGs under the UPB readout is given.

Design of photorefractive volume holographic cylindrical lenses with different recording and readout wavelengths

The photorefractive volume holographic cylindrical lens taking on the wavelength conversion and the wave-front conversion synchronously was designed. Based on coupled wave theory, the wave-front conversion by local photorefractive volume holograms between cylindrical and plane waves which recoded at 632.8 nm and reconstructed at 800 nm is studied. The off-Bragg parameter at arbitrary point of the hologram in the reconstruction process is analyzed. The dependences of diffraction efficiency on the focal length of recording cylindrical wave and on the geometric size of the gratings on the hologram are discussed in detail. The amplitude distribution of the diffracted beam at the output boundary is also analyzed. Furthermore, the effect of selectivity of different on-Bragg reference point for recording hologram on the diffraction efficiency of the grating is discussed.