OPTICAL DATA SECURITY ARUNKUMAR P.S S 7 EC A ROLL NO- 16

1.INTRODUCTION 2.OPTICAL DATA STORAGE PRINCIPLE 3.BASIC COMPONENTS 4.RECORDING AND READING OF DATA 5.ENCRYPTION TECHNIQUES 6.APPLICATIONS 7.CONCLUSIONS 8.REFERENCES

Optical data storage is an alternative to magnetic disk data storage. Currently data access times are extremely slow for magnetic disks when compared to the speed of execution of CPUs so that any improvement in data access speeds will greatly increase the capabilities of computers, especially with large data and multimedia files.

Optical memory uses a three dimensional medium to store data and it can access such data a page at a time instead of sequentially, which leads to increases in storage density and access speed. Optical memory uses the basic principles of holography for the recording purposes.

Holographic method was a two step coherent image forming process in which a record is made of the interference pattern produced by the interaction of the waves diffracted by the object and a coherent background or a reference wave. When this hologram is illuminated, the original wave front is reconstructed.

1.LASER 2.LENS AND MIRRORS 3.SPATIAL LIGHT MODULATORS 4.PHOTOSENSITIVE MATERIALS 5.CHARGE COUPLED DEVICES 6.PHASE MASKS FOR ENCRYPTION

Laser is a device for the generation of coherent, nearly monochromatic and highly directional electromagnetic radiation emitted. Fundamental attributes of a laser are directionality, mono chromaticity, coherence and brightness. To record holograms on the crystals usually argon ion lasers, krypton lasers and diode lasers are used.

Mirrors are used to reflect laser beams to the desired direction. Lenses are usually used to converge the laser to a point. A special type of lens is used in the case of optical recording called the Fourier lens. The lens has the property of obtaining the Fourier transform and the inverse transform system.

SLM is an optical device that is used to convert the real image or data into a single beam of light that will intersect with the reference beam during recording. It basically consists of an array of pixels which are usually microscopic shutters or LCD displays. Each pixel of the SLM corresponds to bit of data. Depending on whether the bit is a 1 or a 0 the pixel will go dark or transparent in the case of a LCD.

1.PHOTOREFRACTIVE CRYSTALS The recording medium usually used is a photo refractive crystal that has certain optical characteristics. One characteristic of the recording medium that limit the usefulness of holographic storage is the property that every time the crystal is read with the reference beam the stored hologram at that location is disturbed by the reference beam and some of the data integrity is lost.

1.PHOTOPOLYMERS Photopolymers have been developed that can also be used as a holographic storage medium. Typically the thickness of the photopolymers is much less than the thickness of photo refractive crystals because the photopolymers are limited by mechanical stability and optical quality.

Polaroid Corporation recently developed a holographic recording system that exhibits significantly less shrinkage than conventional photopolymers. The Polaroid polymers uses monomers that polymerize using a cationic ring-opening (CROP) mechanism to replace more conventional free-radical monomers. Shrinkage during hologram recording for CROP monomers is partially compensated by a volume increase produced by the ring-opening polymerization mechanism.

The charge-coupled device is the most common mechanism for converting optical images to electrical signals. CCD’s are silicon devices. A CCD is a silicon-based semiconductor arranged as an array of photosensitive elements, each one of which generates photoelectrons and stores them as a small bucket of charge or potential wells.

In the case of encryption, a plane polarized monochromatic wave front illuminates the encrypted phase mask, which consists of a random array of phase-shifting pixels. These phase-masks are produced by electronically scrambling the original information, to be encrypted, with a random pattern and using this to generate an encrypted phase mask.

The decrypting key effectively reverses the scrambling operation in the optical domain and results in the production of a wave front in which the information of interest is encoded as a relative phase shift between different sections of the wave front, in this case corresponding to the pixels.

1.OPTICAL RECORDING In holographic data storage, light from a coherent laser source is split into two beams, signal (data- carrying) and reference beams. Digital data to be stored are encoded onto the signal beam via a spatial light modulator. The o’s and 1’s of the data pages are translated into pixels of the spatial light modulator.

The light of signal beam traverses through the modulator and is therefore encoded with the “checkerboard” pattern of the data page. This encoded beam then interferes with the reference beam through the volume of a photosensitive recording medium, storing the digital data pages.

2. OPTICAL READING OF DATA The interference pattern induces modulations in the refractive index of the recording material yielding diffractive volume gratings. The reference beam is used during readout to diffract off of the recorded gratings, reconstructing the stored array of bits. The reconstructed array is projected onto a pixilated detector that reads the data in parallel. This parallel readout of data provides holography with its fast transfer rates.

The readout of data depends sensitively upon the characteristics of the reference beam.By varying the reference beam, Many different data pages can be recorded in the same volume of material and read out by applying a reference beam identical to that used during writing. This process of multiplexing data yields the enormous storage capacity of holography.

There are many different types of encryption techniques available. Here three main techniques used for encryption has been described. They are : 1. Encrypted Memory Using Double Random Phase Encryption. 2. Encrypted Memory Using Three-Dimensional keys in the fresnel Domain 3. Encrypted Memory Using Wavelength-Code and Random Phase Masks

IN ULTRA SHORT COMMUNICATIONS Encrypted Memory used Double Random Phase encryption can be used in secure communication network using ultra short pulses. In this system the original data is stored in an encrypted memory system. The encrypted data read out from the memory is converted into a 1D temporal pulse using the space to time converter and then is transmitted to users via optical fibers.

At the receivers the temporal signal is converted again into a spatial signal by the time to space converter. The authorized users can decrypt the data using the correct key. This system can be expected to communicate at an ultra high speed of more than 1 Tb/s.

 Three encrypted optical memory systems have been discussed here.  These systems are secure because the total number of mathematical possibilities of the multidimensional keys, which consists of two dimensional phase masks, their three dimensional positions, and wavelengths of light, is extremely large.  It is expected that the encrypted memory system is to play an important role in ultra-fast secure communication systems.

 It is believed that the substantial advances in recording media, recording methods and the practical consideration of optical data storage greatly enhance the prospects for Hollography to become a next generation storage technology.

Journals O.Matoba &B.Javidi, “ENCRYPTED OPTICAL STORAGE WITH ANGULAR MULTIPLEXING”, Applications of Optics Vol.38 F.H Mok, “ANGLE MULTIPLEXED STORAGE OF 5000 HOLLOGRAMS IN Lithium Niobate”, Optics letters Vol.11