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INTRODUCTION
As computer systems continue to become faster, they will need a way to access larger amounts of data in shorter periods of time.
As our databases are increasing day by day, our system is growing at rate that is leaving behind our traditional modes of storing the data CD, DVD’s are now very small and inefficient as compared to our needs, today even our hard disks are also not that big enough and even if they stores our databases they take too much space.
Also another problem is that even if we have mediums enough to store our database we need its transfer rate to be high otherwise we would be waiting to much on their transfer to complete because all traditional modes don’t have good transfer rate .
So in our rapidly growing databases today we need an efficient way to store large information which also have good transfer rates.
So one solution to our problem is the HVD : Holographic versatile disk which is currently under development but it has a very large storage capacity and even a good transfer rate which use the holographic memory.
Holographic memory is a three-dimensional data storage system that can store information at high density inside the crystal or photopolymer.
What is Holographic Memory?
Holographic memory is a storage device that is being researched and slated as the storage device that will replace hard drives and DVDs in the future. It has the potential of storing up to 1 terabyte or one thousand gigabytes of data in a crystal the size of a sugar cube.
Holographic data storage is a potential replacement technology in the area of high-capacity data storage currently dominated by magnetic and conventional optical data storage. Magnetic and optical data storage devices rely on individual bits being stored as distinct magnetic or optical changes on the surface of the recording medium. Holographic data storage overcomes this limitation by recording information throughout the volume of the medium and is capable of recording multiple images in the same area utilizing light at different angles.
Additionally, whereas magnetic and optical data storage records information a bit at a time in a linear fashion, holographic storage is capable of recording and reading millions of bits in parallel, enabling data transfer rates greater than those attained by optical storage.
It is a memory that can store information in form of holographic image. As current storage techniques such as DVD reach the upper limit of possible data density (due to the diffraction limited size of the writing beams), holographic storage has the potential to become the next generation of storage media. Like other media, holographic media is divided into write once (where the storage medium undergoes some irreversible change), and rewritable media (where the change is reversible). Rewritable holographic storage can be achieved via the photo refractive effect in crystals.
HVD : HOLOGRAPHIC VERSATILE DISK
HVD (Holographic Versatile Disc) is the next generation in optical disk technology. HVD is still in a research phase that would phenomenally increase the disk storage capacities over the currently existing HD DVD and Blu-Ray optical disk systems. According to published statistics, when produced in full scale, HVDs will have a storage capacity of 3.9 terabytes (39,00 GB) and a data transfer rate of 1 GB/s, which is at least six times more than the speed of DVD players . This would, without a doubt, become a giant step in revolutionizing the disk storage industry.
This inference is a direct reference to the enormous storage capacity HVDs offer compared to HD DVD and Blu-Ray optical disk systems, both of which are yet to replace DVDs for mass optical storage. HD DVD and Blu-Ray optical disk systems offer a storage capacity of 75 and 90 GB respectively, but neither comes anywhere near the massive storage capacity of HVD.
HVD Technology
HVD uses a technology called 'collinear holography,' in which two laser rays, one blue-green and one red, are collimated into a single beam. The role of the blue-green laser is to read the data encoded in the form of laser interference fringes from the holographic layer on the top, while the red laser serves the purpose of a reference beam and also to read the servo info from the aluminum layer - like in normal CDs - near the bottom of the disk. The servo info is meant to monitor the coordinates of the read head above the disk (this is similar to the track, head and sector information on a normal hard disk drive ).
How do the laser beams selectively pass through the layers? A layer of dichroic mirrors that exists between the holographic and servo data layer reflects back the blue-green laser beam, letting only the red laser pass through it to reach the servo information. By doing so, it actually eliminates the possible chances of the interference that can happen due to the refraction of blue-green laser off the servo data pits, a problem that had affected the efficiency of many holographic storage media in the past.
HVD STRUCTURE
The simple but revolutionary concept of HVD comes from inventor, Mr. Horimai’s long time experience in optical disk development and his idea to combine Collinear technology with the conventional optical disk technology.
The figure on the right shows the cross section of an HVD disc. As seen in this diagram, holographic recording layer is formed on top of a reflective layer. The simple optical setup of Collinear Technology has allowed the HVD disc to have a reflective layer on the substrate and address pits formed on this layer. This configuration is the key to apply Collinear Technology to commercial HVD products. These address pits and the servo technology to read them guarantee the interchangeability of HVD discs, ruggedness against vibration in the real environment, wide system margin against variety of HVD discs from different manufacturers. Of course the servo information also make random access easy.
Then, does HVD need special, sophisticated servo technology? The answer is "No." The servo technology and the address pits are, in fact, not different from those used in the current CD and DVD players and disks. The laser which is used to read address pits is 650nm red laser, also common with DVD players in the market.
Another layer called “Dichroic Mirror LayerEis placed between the holographic recording layer and the substrate to block the green or blue laser, which are used to read/write holographic information, to reach address pits, thus eliminates noise.
To make a long story short, HVD is a disc the holographic recording layer of which is formed on top of a conventional optical disk.