Disadvantages of HD DVD: -
If a HD DVD detects the invisible mark, it means the disc is playing back a copy made from a theatrical print (probably from illegal camcording), and will cause the player to refuse to play the disc. The mark is made by varying the waveform of speech and music in a regular pattern to convey a digital code. Sometimes it is not illegal but is just a water spot on the disk which the disk interpretes to be illegal copy and hence doesn’t play it in the player.
Put the disadv in points)
Photograph of HD DVD: -
Holographic memory is a technique that can store information at high density inside crystals or photopolymers known as holograms.Holography (from the , Όλος-holos whole + γραφή-graphe writing) is the science of producing holograms, an advanced form of that allows an to be recorded in three . The technique of holography can also be used to optically store and retrieve information.
Holograms: -
A hologram is a three-dimensional image, created with photographic projection. Unlike 3-D or virtual reality on a two-dimensional computer display, a hologram is a truly three-dimensional and freestanding image that does not simulate spatial depth or requires a special viewing device.
Holographic data storage:-
Holographic data storage is a technique that can store information at high density inside crystals (à la ) or photopolymers. As current storage techniques such as reach the upper limit of possible data density (due to the limited size of the writing beams), holographic storage has the potential to become the next generation of storage media. The advantage of this type of data storage is that the volume of the recording media is used instead of just the surface.
Using currently available can produce about 1000 different images a second at 1024 × 1024 bit resolution. With the right type of media (probably polymers rather than something like ), this would result in about 1 writing speed. Read speeds can surpass this and experts believe 1 readout is possible.
In 2005, the company has produced a 120 mm disc that uses holographic surface to store data to a possible 1 TB (). See .
Other applications of holograms include:
whole working of hologram(emphasis on holograms)
Working of holographic Storage : -
A holographic data storage system consists of a recording medium, an optical recording system, and a photo detector array. A beam of coherent light is split into a reference beam and a signal beam, which are used to record a hologram into the recording medium. The recording medium is usually a photo refractive crystal such as LiNbO3 or BaTiO3 that has certain optical characteristics. These characteristics are high diffraction efficiency, high resolution, and permanent storage until erasure, and fast erasure on the application of external stimulus such as UV light. A ‘hologram’ is simply the three-dimensional interference pattern of the intersection of the reference and signal beams at 90° to each other. This interference pattern is imprinted into the crystal as regions of positive and negative charge. To retrieve the stored hologram, a beam of light that has the same wavelength and angle of incidence as the reference beam is sent into the crystal and the resulting diffraction pattern is used to reconstruct the pattern of the signal beam. Many different holograms may be stored in the same crystal volume by changing the angle of incidence of the reference beam. One characteristic of the recording medium that limits the usefulness of holographic storage is the property that every time the crystal is read with the reference beam the reference beam disturbs the stored hologram at that “location” and some of the data integrity is lost. With current technology, recorded holograms in Fe- and Tb- doped LiNbO3 that use UV light to activate the Tb atoms can be preserved without significant decay for two years.
The most common holographic recording system uses laser light, a beam splitter to divide the laser light into a reference beam and a signal beam, various lenses and mirrors to redirect the light, a photo refractive crystal, and an array of photo detectors around the crystal to receive the holographic data. To record a hologram, a beam of laser light is split into two beams by a mirror. These two beams then become the reference and the signal beams. The signal beam interacts with an object and the light that is reflected by the object intersects the reference beam at right angles. The resulting interference pattern contains all the information necessary to recreate the image of the object after suitable processing. The interference pattern is recorded onto the photo reactive material and may be retrieved at a later time by using a beam that is identical to the reference beam (including the wavelength and the angle of incidence into the photo reactive material). This is possible because the hologram has the property that if it is illuminated by either of the beams used to record it, the hologram causes light to be diffracted in the direction of the second beam that was used to record it, thereby recreating the reflected image of the object if the reference beam was used to illuminate the hologram. So, the reflected image must be transformed into a real image with mirrors and lenses that can be sent to the laser detector array.
There are many different volume holographic techniques that are being researched. The most promising techniques are angle-multiplexed, wavelength-multiplexed, spectral, and phase-conjugate holography. Angle- and wavelength- multiplexed holographic methods are very similar, with the only difference being the way data is stored and retrieved, either multiplexed with different angles of incidence of the reference beam, or with different wavelengths of the reference beam. Spectral holography combines the basic principles of volume holography using a photo refractive crystal with a time sequencing scheme to partition holograms into their own sub volume of the crystal using the collision of ultra short laser pulses to differentiate between the image and the time-delayed reference beam. Phase-conjugate holography is a technique to reduce the total volume of the system (the system includes recording devices, storage medium, and detector array) by eliminating the need for the optical parts between the spatial light modulator (SLM) and the detector. The SLM is an optical device that is used to convert the real image into a single beam of light that will intersect with the reference beam during recording. Phase-conjugate holography eliminates these optical parts by replacing the reference beam that is used to read the hologram with a conjugate reference beam that propagates in the opposite direction as the beam used for recording. The signal diffracted by the hologram being accessed is sent back along the path from which it came, and is refocused onto the SLM, which now serves as both the SLM and the detector.
There are two main classes of materials used for the holographic storage medium. These are photo refractive crystals and photopolymers (organic films). The most commonly used photo refractive crystals used are LiNbO3 and BaTiO3. During hologram recording, the refractive index of the crystal is changed by migration of electron charge in response to the imprinted three-dimensional interference pattern of the reference and signal beams. As more and more holograms are superimposed into the crystal, the more decay of the holograms occurs due to interference from the superimposed holograms. Also, holograms are degraded every time they are read out because the reference beam used to read out the hologram alters the refractive nature of the crystal in that region. Photo refractive crystals are suitable for random access memory with periodic refreshing of data, and can be erased and written to many times. Photopolymers have been developed that can also be used as a holographic storage medium. Typically the thickness of photopolymers is much less than the thickness of photo refractive crystals because the photopolymers are limited by mechanical stability and optical quality. An example of a photopolymer is Dupont’s HRF-150. This film can achieve 12-bits/μm2 with a 100 μm thickness, which is greater than DVD-ROM by a factor of two. When a hologram is recorded, the interference pattern is imprinted into the photopolymer by inducing photochemical changes in the film. Changing the density of exposed areas of the film changes the refractive index modulation. Stored holograms are permanent and do not degrade over time or by readout of the hologram, so photopolymers are suited for read-only memory (ROM).
Explanation of working of holographic storage according to above figure: -
The device first splits a blue argon laser beam into separate reference and object beams. The object beam that carries the information gets expanded so that it fully illuminates a spatial light modulator (SLM). An SLM is simply an LCD panel that displays a page of raw binary data as an array of clear or dark pixels.
To read the stored data, the hologram is illuminated with the reference beam. Each page of the hologram is recorded separately. To record on the hologram, a page composer converts the data in the form of electric signal to optical signals. The controller generates the address to access the desired page. This results in the exposure of a small area of the recording medium through an aperture. The optical output signal is directed to the exposed area by the deflector. Using this beam deflecting mechanism, the light (which carries the information) and the reference beam are made to interact. The interference pattern is thus recorded on the hologram. To record a different page, the aperture is moved and the above process is repeated.
For data retrieval, the laser (reference beam) is focused on the appropriate page according to the address generated. A photo detector array on the other side of the hologram records the image of that sub-hologram.
Problems with the Holographic storage system
A difficulty with the holostore technology had been the destructive readout. The re-illuminated reference beam (i.e. the read beam, see How holographic storage system works), used to retrieve the recorded information, also excites the donor electrons and disturbs the equilibrium of the space charge field in a manner that produces a gradual erasure of the recording. In the past, this has limited the number of reads that can be made before the signal-to -noise ratio becomes too low. Moreover, writes in the same fashion can degrade previous writes in the same region of the medium. This restricts the ability to use the three-dimensional capacity of a photo refractive for recording angle-multiplexed holograms.
The other challenge has been the geometry of the crystal medium. It is difficult to grow large crystals of good optical quality and of limited size. As a consequence of this problem, widespread application of the bulk photo refractive technology has not been occurred, despite an initial surge of development in the decade of the 1970s.
