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BHARATHIYAR COLLEGE OF ENGINEERING & TECHNOLOGY KARAIKAL – 609609 (U.T OF PUDUCHERRY) DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING A Seminar on Holographic Memory Submitted by: Name : Reg no : Year : 2021 - 2022 Under the Guidance of : Mr.P.SHANKARANARAYANAN ACKNOWLEDGEMENT I express my sense of gratitude to my seminar guide Prof. Shankar Narayanan sir, EC Department, Bharathiyar college of engg & Technology, Pondicherry University, for all of his guidance, encouragement and patience which enable me to carry out my seminar work. He inspired me with his original and innovative ideas and provided his technical guidance in the seminar. Appreciation is expressed for all those who directly or indirectly helped me inthe course of my seminar work. I am also thankful to Bharathiyar college of engg & Technology Management for providing me an opportunity. T.Hariharan (18TC0001) BTech (ECE) ABSTRACT In data storing technologies, high data density per volume and high data transfer speed, both are highly demanded. There are two types of data storing devices that are magnetic and optical. Now a day optical devices are more preferable as they support more data transfer speed. A Holographic Memory is an optical data storing technology which gives these both features that are high data density per volume and high data transfer speed. In this report it is explained that how it is getting these much data density and high data transfer speed by using components like spatial light modulator (SLM), multiplexing agent, lenses and charge coupled device (CCD). Holographic Versatile Disc (HVD) is an example of holographic memory. In this entire volume of disc is used to store data as interference pattern of signal beam and reference beam is to be stored which increases data density per volume. INDEX SI No. 1 2 3 4 5 6 7 8 9 10 Title PageNo. Acknowledgement - Abstract - Index - List of Figures - List of Tables - Introduction 1 1.1 Brief History 2 1.2 What is Holography? 2 1.3 Recording and Reading of Hologram 2 1.4 Holography Vs. Photography 3 1.5 Applications of Holography 4 Holographic Memory 5 2.1 Roadmap of Memory 5 2.2 Recording to HVD 6 2.3 Reading from HVD 7 Holographic Versatile Disc (HVD) Components 8 3.1 Spatial Light Modulator (SLM) 8 3.2 Multiplexing Agent 9 3.3 Storage Medium 10 3.4 Charge Coupled Device (CCD) 10 Advanced HVD 11 4.1 Comparison 11 4.2 Advantages and Drawbacks 11 4.3 Potential Competitors 12 4.4 Companies working on Holographic Memory 12 HOLOGRAPHIC MEMORY vs. EXISTING MEMORY TECHNOLOGY HOLOGRAPHIC MEMORY MOLECULAR MEMORY COLLINEAR SYSTEM COLLINEARHOLOGRAPHIC MEDIA EVALUATION SYSTEM S-VRD CONCLUSION TM 13 14 19 24 25 26 LIST OF FIGURES Fig. No. Fig. Name Page No 1 Recording of Hologram 3 2 Reading of Hologram 3 3 Recording of data to HVD 6 4 Reading data from HVD 7 5 Spatial Light Modulator 9 6 Common CCD Image Sensor Formats (Dimensions in Millimeters) 10 LIST OF TABLES Table No. Table Name Page No. 1 Roadmap of Memory 5 2 Comparison of HVD with other discs 11 1. INTRODUCTION We know that data storing capacity of processors and buses roughly double every three years, data storage has struggled to close the gap. CPU is shaving capacity of executing instructions at every nanosecond. Single magnetic disk is required almost six times more time than this CPU execution speed. There are so many research been done to lose this gap between CPUs and data storage. Pipelining, cache, optimizing compilers and RAM are some advances of these type. For almost two decades for data storing devices are used that use light for storing and reading data. In the early 1980’s revolution happens in data storing field as a Compact Disc (CD) which is having 12 centimetres of diameter and 1.2 millimetres of thickness. These CDs allows multimegabit of data to be stored on disc. Digital Versatile Disc (DVD) is an improved version of CD which was invented in 1997. The full-length movie can be stored a single DVD. At initial level CDs and DVDs are famous to store data like music, personal computing, software and video. A CD is having capacity of data storing up to 783 megabytes, which is almost equal to one hour and 15 minutes of music. DVDs which are double-sided, doublelayer having data storing capacity of 15.9 GB, which is approximately 8 hours of movie. Today’s needs are met by these conventional storage mediums, but consumer demands also increases so companies working on storage technologies have to make changes to keep pace. In these all technologies data is stored bit by bit on a recording medium. However, technology is either optical or magnetic, data is combination of bits and each bit is stored as distinct changes of magnetic or optical recording medium on its surface. This technique of storing data bit by bit is approaching to physical limits because bit is a very small part of whole data. If data can be stored on entire volume of storing material, it will increase high density of data storing. With a purpose of increasing storage capabilities, researchers and scientists are now majorly working on a new optical storage technique, called Holographic Memory. Holographic memory is technology which uses entire volume of storing material to store data instead of only the surface area. So, this three dimensional data storing technique will increase data density per volume and also offers faster data transfer rate. Actually, volumetric approach is used for holographic memory which was introduced almost a decade ago. Holographic memory is having capacity storing data up to 1 Terabyte in a single sugar cube sized crystal. The main advantage of holographic memory is that it is storing data page by page not bit by bit and also same for retrieving. Holographic memory system’s capacity is almost equal to 1,000 CDs. 1|Pa ge 1.1 Brief History In 1971, the noble prize in physics was given to Dennis Gabor for his invention and development of the holographic method. The optical holography highly depending on laser, so more research and invention in this was started after development of laser in 1960. At initial stage in 1962 after invention of laser the first optical hologram was recorded at the University of Michigan, USA. Before that for recording holograms silver halide photographic emulsions was used as a recording medium. 1.2 What is Holography? The word “Holography” comes from Greek words which are having meaning of “whole” and “Drawing”. In normal photography we store only intensity level of image but in holography with intensity we also store phase for image while construction an image. So while reconstructing of an image we can get 3D view of image. But here one thing is to be noticed that we can get feel of 3D image only in horizontal direction that means if we either move our eyes or an image horizontally then only we can feel 3D image. If we move our head in vertical direction then it will image will be looked like rainbow i.e. all seven colors. 1.3 Recording and Reading of Hologram As we discussed earlier that in Holography, information is stored as page by page. At one instance entire page is to be stored by an optical interference pattern within a thick, photosensitive optical material as shown in fig.1. This pattern will be generated by intersection of two laser beams which are coherent which are object beam and reference beam within the storage material. Here object beam is having data what to be stored and reference beam is for perfect generation of hologram at reading. The photosensitive storing medium’s chemical and physical properties will be changed according to the interference pattern. According to the interference pattern, the changes occur in properties of the photosensitive medium like the refractive index, absorption or thickness. Either object beam or reference beam is illuminated on the interference pattern and from that we can get either reconstructed reference beam or reconstructed object beam which is shown in fig. 2. Here one thing is to be noticed that at time of reconstruction position, angle and wavelength of laser beam should be same as time of construction, otherwise there may be possibility that we will get a different 3D pattern or say some portion of pattern which is unexpected. So, these requirements make holography very complex and costly because proper arrangements of all components are very time worthy. 2|Pa ge Fig. 2 Reading of Hologram[1] Fig. 1 Recording of Hologram[1] 1.4 Holography Vs. Photography In a normal photography information is only from one direction whereas in holography information is from different direction and due to that only it generates a 3D view of image so from different direction observer can observe it. A photograph may be recorded using normal light sources like sunlight or electric lighting whereas a hologram is recorded using laser lights. To store a normal photograph a lens is required whereas to store a hologram storing medium required on which interference pattern will be stored by scattering a light from object. In normal photography only light from object is required whereas in holography object bema as well as reference beam is also required. 3|Pa ge An observer or viewer can observe photograph in any light condition whereas hologram can be observed only in very specific illumination’s form. If a photograph is cut from half, each piece will show half image whereas if hologram is cut from half, each piece will show full hologram. This happens because in normal photography each point related with specific point of scene or image only whereas in holography each point is having information of all points of scene or image because in this laser is scattered. So it is like window either you see from 4 ft * 4 ft window or 8 ft * 8 ft window, you can see the same scene but range of horizontal and vertical visibility changes that you have move your head more to see same scene from 4 ft window compare to 8 ft window. 1.5 Applications of Holography Holographic Interferometry It is a one of the technique to identify static and dynamic change of object’s position. In which two interference patterns of object are again interfaced and gives another interference pattern which indicates the displacement of object. Security Holograms are very useful in security and it is already used by many countries with a purpose of security on their currency notes. It is also used by banks for security of their credit cards. Now a days passports are also having holograms on them and with that sports equipment, books, ID cards are also having holograms on them. Dynamic Holography Holography is of two types that are static and dynamic. Till now we have seen only static holography where hologram is constructed, stored and reconstructed. But in dynamic holography some type of materials are used that is having capacity of recording hologram very quickly and instantly we can reconstruct it using proper arrangements. These can be used in image processing to recognize pattern images which are time varying. Data Storage Holography is having capacity of storing different data using entire volume of storing material like crystals or photopolymers. Now a day researchers are majorly working in the field of minimizing size of data storage devices which means they want to increase data density within specific volume of material. So, now we will focus more on holography’s application of data storing which is having special features of high volume density and high data transfer speed. 4|Pa ge 2. HOLOGRAPHIC MEMORY There are many ways of storing data like optical memory, flash memory, magnetic memory etc. But next will be Holographic Memory. Holographic memory is a technology that allows holograms containing millions of bits of data to be written or read in a single flash of light. Thousands of overlapping holograms can be stored in a common volume of recording medium which increases storage capacity per volume. Holography breaks through the density limits of conventional 2D storage by recording throughout the full depth of a 3D medium. Holography can write and read millions of bits of data in parallel, enabling significantly higher data transfer rates than current optical storage devices which gives high data transfer rate. 2.1 Roadmap of Memory Initially in 1970s to store data LASER discs were used which is having capacity of storing 30 minutes or 60 minutes per side in size if 30 cm. After that they were replaced by Compact Discs and Magneto Optical Discs which are having different storage capacity. The 12 cm sized CD can store 700 MB data while 8 cm CD can store 200 MB of data. The 13 cm sized Magneto optical disc can store from 650 MB to 9.2 GB. In 1990s Mini discs were invented which is having storing capacity of 650 MB at initial stage. At present it is having capacity of 1 GB data storing. These all were then replaced by Digital Versatile Discs (DVDs) which is having storage capacity up to 8.5 GB. In these CDs data were written only on single layer which restrict storing capacity so BLURAY discs were invented in 2000s. BLURAY discs are having layers. The single layer disc can store data of 25 GB while dual layer can store data of 50 GB. This also restricts data storing capacity because still data is to be stored on surface of disc. Whole volume of disc is not utilized to store data. So, solution for that is Holographic Versatile Disc (HVD) in which to store data entire volume is to be utilized which increases data storing capacity in small volume. Year 1978 1982 1985 1992 1995 2006 The Future TABLE 1 Roadmap of Memory Memory Type LASER Disc Compact Disc Magneto Optical Disc Mini Disc Digital Versatile Disc (DVD) Blu-Ray Disc Holographic Versatile Disc (HVD) 5|Pa ge 2.2 Recording to HVD Till now we have seen how to store hologram of an image. But, now we are talking about holographic memory which mean there are much more then only an image. Holographic memory is optical type of memory which means we are using lasers in a procedure of storing data. So we are having information in a form a LASER which will be split in to two laser beams by lens. These two beams are signal beam and reference beam. Signal beam is having information to be stored. Now this signal beam pas through the spatial light modulator (SLM) which converts this beam into pixels of black and white as SLM is a 1024*1024 pixels square where each pixel will either allow or stop the laser. On basis of this pixel charge coupled device (CCD) generates a string of 1’s and 0’s and that entire page is stored on hologram with a help of reference beam. On basis of data physical and/or chemical characteristics of storing material will change. This all procedure of data recording is shown in fig. 3. Fig. 3 Recording of data to HVD[3] 6|Pa ge 2.3 Reading from HVD Now data reading is also following the same method used for recording. As shown in fig 4 reference beam is used for reading data from storing material. Once again these data passes through CCD for conversion and Detector will detect that data. Fig. 4 Reading data from HVD[3] Till now we have seen all basic concepts of Holography, how hologram is constructed and reconstructed. With help of this how data can be stored to HVD and can be read from HVD. Now, In next chapter we will see each component of HVD in detail and structure of HVD.. 7|Pa ge 3. HOLOGRAPHIC VERSATILE DISC (HVD) COMPONENTS An HVD is hot topic for researchers at present as it can support more data density per volume and with that high data transfer speed which are highly required for any storage device. So there is possibility that in a few years all CDs and DVDs will be replaced by HVDs. Now a day two sided blu-ray discs are becoming famous because it can store data on more than on surface i.e. it is having multi-layer structure so data storing capacity increases with same area. HVD is having capacity of transferring data at speed of 1 Gigabit per second. Till now researchers have reached up to level that DVD’s same sized HVD can store data up to 1 terabyte of data and they are claiming that they will able to store 1 terabyte of data in just one cube sugar size. We can say that HVD is having 60 times more capacity of storing data than DVD and can support 10 times more data transfer speed than DVD. So, we can say that 1 HVD is equivalent to 830 DVDs and 160 Bluray discs in comparison of data storing capacity. With a purpose of increasing capacity of data storing, holographic memory uses laser beams for storing data in whole volume of HVD. The HVD process uses a blue green laser beam, used for reading and writing data, collimated (made parallel) with a red laser beam, which is used for servo and tracking. HVD uses the concept of holographic memory. For writing to and reading data from HVD, proper setup should be made. For that main components are required that are Spatial Light Modulator (SLM) Multiplexing Agent Storage Medium Charge Coupled Device (CCD) 3.1 Spatial Light Modulator (SLM) An SLM - Spatial Light Modulator is used to convert electrical or optical signal into image. It can create 1000 of images in a second which gives an entire system high speed. It is used in many various applications like optical devices like televisions, projectors. Normally, the medium used in SLM is liquid crystal. It is also used as variable electro-optic mirror which is used in optical systems for beam splitting, shutters and mirrors. SLM is having feature of working at different frequencies. To analyse the image, SLM can be used as it gives output as pixel by pixel of an image. SLM is having capacity of precisely controlling light which is very much required for holographic memory and optical computing as light is just modified in pixels. Basically, SLM is having two types of addressing modes. Addressing mode is depended on the type of signal. Optically-addressed From name it is indicating that when SLM is having optical input so it is optically8|Pa ge addressed mode. Electrically-addressed From name it is indicating that when SLM is having electrical input so it is electrically-addressed mode. As we have discussed earlier to use holography as a storage technology, digital data must be in a form of a laser as an object beam while recording and reading. A device, having this type of feature is SLM. SLM is nothing but made up of pixels and each pixel is an independent microscopic shutter which is having capacity of either pass or block light. Fig. 5 Spatial Light Modulator (SLM)[2] As we know that holographic memory is having data transfer speed of Gbit per second. So, if we want to use SLM as one of the component of HVD, it should also have ability t transfer data at that rate and yes it is can refresh itself 1000 times per second. 3.2 Multiplexing Agent As we have discussed earlier that on basis of interference pattern storing material properties will be changed and here by changing angle, position or wavelength we can create different holograms so for that multiplexing agents are used. Angular Multiplexing In this type of multiplexing, angle of reference beam is changed to generate different interference pattern and same angle is used at time of reading data. Here it should be noticed that a very small change in angle will generate different data or say wrong data. So, on basis of sensitivity of storing material angle is changed while recording of data for multiplexing. 9|Pa ge Wavelength Multiplexing This type of multiplexing is used majorly when conjunction is there with other multiplexing methods. In this different wavelength’s lasers are used while recording the data so that on same medium more than one pattern can be stored. Spatial Multiplexing In this type of multiplexing, position of point source is changed so that from same two sources different interference pattern will be generated. But in this more mechanical work is required in comparison of other two multiplexing. 3.3 Storage Medium As we have discussed earlier that on basis of interference pattern physical and/or chemical properties of storing medium is changed. There are mainly two storage mediums are used that are 1. Lithium-niobate crystals 2. Photopolymers 3.4 Charge Coupled Device (CCD) CCD is nothing but an array of sensors which gives responses on basis of pixels of SLM. It is mainly used to read interference pattern at time of recording and reading of data. It is able to read 1 Mb of data at one instance. In general CCDs are of one square centimeter and having typical access rate of 1000 frames per seconds or say 1 Gigabit per second. Fig. 6 Common CCD Image Sensor Formats (Dimensions in Millimeters) [7] For CCD there are two types of resolution which are Temporal Resolution CCD is having specific resolution but it is with specific speed. If image rate per second higher than specific rate then resolution of CCD will decrease. As we all know that in our mobile phones or cameras images are having more resolution than videos. It is because of this problem only that is for video image rate per second will increases which will affect the resolution. Spatial Resolution For same resolution at high speed scan and slow speed scan, there should be spatial resolution for 10 | P a g e CCD so that we can get same resolution image and video. 4. ADVANCED HVD 4.1 Comparison TABLE 2 Comparison of HVD with other Discs DVD BLU-RAY HVD Capacity 9.4 GB 50GB Up to 3.9 TB Laser wave length 650 nm 405 nm 407 nm Disc diameter 120 mm 120 mm 120 mm Read/write speed 11.08 Mbps 36.5Mbps 1 Gbps 4.2 Advantages and Drawbacks Advantages High Data Transfer Speed As we have discussed that holographic memory is giving data transfer speed up o 1 Gbps which is highest till date for any storage device. High Data Density Holographic memory can store data up to 3.9 TB in in 120 mm diameter sized disc and researchers are claiming that they will reach up to 1 TB data in just 1 inch sized sugar cube. Data Stability and Reliability One survey says that within a three year of period, 26.5% of seagate’s drive failed, 5.2% of WD’s (Western Digital) drives failed and 3.1% of Hitachi’s drives failed whereas scientists of holographic memory are claiming that Holographic memory is reliable till 50 years which is very higher than these hard drives. Data Security Holographic memory is of WORM (Write Once Read Many) type of memory so once data is written then nobody can edit it which gives high data security. Drawbacks Not guaranteed market leader Holographic memory is offering all required features but still it is not that much popular because recording and reading of data is very hard. If any other storing technique will come in market then it is very hard to find components to record and 11 | P a g e read data form HVD. Expensive Development As we have discussed for recording and reading data, whole setup should be very much perfect. If there is any small displacement in any component, it will cause major errors in data. 4.3 Potential Competitors Blu-ray Discs UV or X-rays 4.4 Companies working on Holographic Memory Akonia Holographics IBM Intel Musion-The world leaders in holographic technology Zebra Imaging Realview medical holography 12 | P a g e 5 HOLOGRAPHIC MEMORY vs. EXISTING MEMORY TECHNOLOGY In the memory hierarchy, holographic memory lies somewhere between RAM and magnetic storage in terms of data transfer rates, storage capacity, and data access times. The theoretical limit of the number of pixels that can be stored using volume holography is V2/3/ medium and 2 where V is the volume of the recording is the wavelength of the reference beam. For green light, the maximum theoretical storage capacity is 0.4 Gbits/cm2 for a page size of 1 cm x 1 cm. Also, holographic memory has an access time near 2.4 s, a recording rate of 31 kB/s, and a readout rate of 10 GB/s. Modern magnetic disks have data transfer rates in the neighborhood of 5 to 20 MB/s [8]. Typical DRAM today has an access time close to 10 – 40 ns, and a recording rate of 10 GB/s. Storage Medium Access Time Holographic Memory 2.4 s Main Memory (RAM) 10 – 40 ns Magnetic Disk 8.3 ms Data Transfer Rate Storage Capacity 10 GB/s 400 Mbits/cm2 5 MB/s 5 – 20 MB/s 4.0 Mbits/cm2 100 Mbits/cm2 Holographic memory has an access time somewhere between main memory and magnetic disk, a data transfer rate that is an order of magnitude better than both main memory and magnetic disk, and a storage capacity that is higher than both main memory and magnetic disk. Certainly if the issues of hologram decay and interference are resolved, then holographic memory could become a part of the memory hierarchy, or take the place of magnetic disk much as magnetic disk has displaced magnetic tape for most applications. 13 | P a g e 6 HOLOGRAPHIC MEMORY Wide possibilities in this case are provided by technology of optical recording, it's known as holography: it allows high record density together with maximum data access speed. It's achieved due to the fact that the holographic image (hologram) is coded in one big data block, which is recorded at one access. And while reading this block is entirely extracted out of the memory. For reading and recording of the blocks kept holographically on the light-sensitive material (LiNbO3 is taken as the basic material) they use lasers. Theoretically, thousands of such digital pages, which contain up to a million bits each, can be put into a device measuring a bit of sugar. And theoretically they expect the data density to be 1 TBytes per cubic cm (TBytes/cm3). In practice, the developers expect around 10 GBytes/cm3, what is rather impressive when comparing with the current magnetic method that allows around several MBytes/cm2 and this without the mechanism itself. With such recording density an optical layer which is approx 1 cm in width will keep around 1TBytes of data. And considering the fact that such system doesn't have any moving parts, and pages are accessed parallel, you can expect the device to be characterized with 1 GBytes/cm3 density and higher. FIGURE-1 SYSTEM OF HOLOGRAPHIC MEMORY 14 | P a g e Exceptional possibilities of the topographic memory have interested many scientists of universities and industrial research laboratories. This interest long time ago poured into two research programs. The first of them is PRISM ( Photorefractive Information Storage Material), which is targeted at searching of appropriate light-sensitive materials for storing holograms and investigation of their memorizing properties. The second program is HDSS (Holographic Data Storage System). Like PRISM, it includes fundamental investigations, and the same companies participate there. While PRISM is aimed at searching the appropriate media for storing holograms, HDSS is targeted at hardware development necessary for practical realization of holographic storage systems. How does a system of holographic memory operate? For this purpose we will consider a device assembled by a task group from the Almaden Research Center. At the first stage in this device a beam of cyan argon laser is divided into two components - a reference and an object beam (the latter is a carrier of data). The object beam undergoes defocusing in order it could entirely illumine the SLM (Spatial Light Modulator) which is an LCD panel where a data page is displayed in the form of a matrix consisting of light and dark pixels (binary data). The both beams go into the light-sensitive crystal where they interact. So we get an interference pattern which serve a base for a hologram and is recorded as a set of variations ofthe refractive exponent and the reflection factor inside this crystal. When reading data the crystal is illuminated with a reference beam, which interacts with the interference factor and reproduces the recorded page in the image of "chessboard" of light and dark pixels (the holograms converts the reference wave into the copy of the object one). After that, this image is transferred into the matrix detector where the CCD (Charge-Coupled Device) serves a base. While reading the data the reference beam must fall at the same angle at which the recording was made; alteration of this angle mustn't exceed 1 degree. It allows obtaining high data density: measuring the angle of the reference beam or its frequency you can record additional pages of data in the same crystal. However, additional holograms change properties of the material, and such changes mustn't exceed the definite number. As a result, the images of holograms become dim, what can lead 15 | P a g e to data corruption when reading? This explains the limitation of the volume of the real memory that belongs to this material. The dynamic area of the medium is defined by the number of pages which can be virtually housed, that's why PRISM participants are investigating limitations to the light sensitivity of substances. The procedure in 3-dimensional holography which concludes in enclosure of several pageswith data into the same volume is called multiplexing. Traditionally the following multiplexing methods are used: of angle of dip of the reference beam, of wavelength and phase; but unfortunately they require complicated optical systems and thick (several mm) carriers what makes them unfit for commercial use, at least in the sphere of data processing. But lately Bell Labs have invented three new multiplexing methods: shift, aperture and correlative, which are based on the usage of change in position of the carrier relative to the beams of light. The shift and aperture multiplexing use a spherical reference beam, and the correlative uses a beam of more complicated form. Besides, considering the fact that the correlative and shift multiplexing enable mechanical moving elements, the access time will be the same as that of the usual optical discs. Bell Labs managed to build an experimental carrier on the same LiNBO3 using the technology of correlative multiplexing but this time with 226 GBytes per square inch. Another problem standing on the way of development of holographic memory devices is a search of the appropriate material. The most of the investigations in the sphere of holography were carried out with usage of photoreactive materials (mainly the mentioned LiNBO3), but they are not suitable for data recording especially for commercial use: they are expensive, weak sensitive and have a limited dynamic range (frequency bandwidth). That's why they developed a new class of photopolymer materials facing a good perspective in terms of commercial use. Photopolymers are the substances where the light cause irreversible changes expressed through fluctuation of the composition and density. The created material have a longer life circle (in terms of storing data) and are resistant to temperature change, besides, they have improved optical characteristics and are suitable for WORM (write-once/read many). One more problem concludes in the complexity of the used optical system. For holographic memory the LEDs based on semiconductor lasers used in traditional optical devices are not suitable, since they have insufficient power, give out a wide beam angle, and at last it's too difficult to get a semiconductor laser generating radiation in the middle range of the visible 16 | P a g e spectrum. There you need as powerful laser as possible which gives the most exact parallel beam. The same we can say about the SLM: yet some time ago there were no any such devices which could be used in the holographic memory systems. But time flies and today you can get inexpensive solid-state lasers; besides, there appeared the MEM technology (Micro- Electrical Mechanical). The devices on its base consist of the arrays of micromirrors around 17 micron in size, they suit very much for the role of SLM. Since the interference patterns fill up the whole substance uniformly, it gives another useful property to the holographic memory - high reliability of the recorded information. While a defect on the surface of the magnetic disc or tape destroy important data, a defect in holographic medium doesn't cause a loss of information, it leads only to tarnish of the hologram. The small desktop HDSS-devices are to appear by 2003. Since the optical discs. Bell Labs managed to build an experimental carrier on the same LiNBO3 using the technology of correlative multiplexing but this time with 226 GBytes per square inch. Another problem standing on the way of development of holographic memory devices is a search of the appropriate material. The most of the investigations in the sphere of holography were carried out with usage of photoreactive materials (mainly the mentioned LiNBO3), but they are not suitable for data recording especially for commercial use: they are expensive, weak sensitive and have a limited dynamic range (frequency bandwidth). That's why they developed a new class of photopolymer materials facing a good perspective in terms of commercial use. Photopolymers are the substances where the light cause irreversible changes expressed through fluctuation of the composition and density. The created material have a longer life circle (in terms of storing data) and are resistant to temperature change, besides, they have improved optical characteristics and are suitable for WORM (write-once/read many). One more problem concludes in the complexity of the used optical system. For holographic memory the LEDs based on semiconductor lasers used in traditional optical devices are not suitable, since they have insufficient power, give out a wide beam angle, and at last it's too difficult to get a semiconductor laser generating radiation in the middle range of the visible spectrum. There you need as powerful laser as possible which gives the most exact parallel beam. The same we can say about the SLM: yet some time ago there were no any such devices which could be used in the holographic memory systems. But time flies and today you 17 | P a g e can get inexpensive solid-state lasers; besides, there appeared the MEM technology (Micro- Electrical Mechanical). The devices on its base consist of the arrays of micromirrors around 17 micron in size, they suit very much for the role of SLM. Since the interference patterns fill up the whole substance uniformly, it gives another useful property to the holographic memory - high reliability of the recorded information. While a defect on the surface of the magnetic disc or tape destroy important data, a defect in holographic medium doesn't cause a loss of information, it leads only to tarnish of the hologram. The small desktop HDSS-devices are to appear by 2003. Since theHDSS equipment use an acoustooptical deflector for measuring angle of dip (a crystal which properties change with a sound-wave passing through it), the extraction time for adjacent data pages will constitute 10ms. Any traditional optical or magnetic memory device needs special means for data access of different tracks, and this access time constitutes several milliseconds. The holographic memory is not a completely new technology since its basic conceptions were developed about 30 years ago. The only that has changed is availability of the key components - the prices considerably fell down. The semiconductor laser, for example, is not unusual. On the other hand, SLM is a result of the same technology which is used in production of LCD-screens for notebooks and calculators, and the CCd detector array is taken right from a digital video camera. Well, the new technology has more than enough highlights: apart from the fact that information is stored and recorded parallel, you can reach very high data rate, and in some cases high speed of random access. And the main advantage is that mechanical components are practically absent (those that typical for current storage devices). It ensures not only a fast data access, less probability of failures, but also lower power consumption, since today a hard disc is one of the greatest power-consuming elements of a computer. However, there are problems with adjustment of optical devices, that's why at the beginning the data of the device will probably "fear" exterior mechanical effects. 18 | P a g e 7 MOLECULAR MEMORY Another approach in creation of storage devices is a molecular method. A group of researchers of the "W.M. Keck Center for Molecular Electronic" with Professor Robert R. Birge as a head quite a long time ago received a prototype of memory subsystem which uses digital bits of a molecule. These are protein molecules which is called bacteriorhodopsin. It's purple, absorbs the light and presents in a membrane of a microorganism called halobacterium halobium. This bacterium lives in salt bogs where the temperature can reach +150 °C. When a level of oxygen contents is so low in the ambient that to obtain power breathing (oxidation) is not enough, it uses protein for photosynthesis. Bacteriorhodopsin was chosen because a photocycle (a sequence of structural changes undergone by a molecule when reacting with light) makes this molecule an ideal logically storing element of "&" type or a type of a switch from one condition into another (trigger). According to Birge's investigation, bR-state (logical value of the bit "0") and Q-state (logical value of the bit "1") are intermediate states of the molecule and can remain stable during many years. This property (which in particular provides a wonderful stability of protein) was obtained by an evolutional way in the struggle for survival under severe conditions of salt bogs. Birge estimated that the data recorded on the bacteriorhodopsin storage device must "live" around 5 years. Another important feature of the bacteriorhodopsin is that these both states have different absorption spectra. It allows easily defining the current state of the molecule with the help of a laser set for the definite frequency. They built a prototype of memory system where the absorption spectrum stores data in 3- dimensional matrix. Such matrix represents a cuvette (a transparent vessel) filled up with polyacryde gel, where protein is put. The cuvette has an oblong form 1x1x2 inch in size. The protein which is in the bR-state is fixed in the space with gel polymerization. The cuvette is surrounded with a battery of lasers and a detector array based on the device using a principle of CID (Charge Injection Device), they serve for data recording and reading. 19 | P a g e When recording data first you need switch on a yellow-wave "page" laser - for converting the molecules into Q-state. The SLM which represents an LCD-matrix creating a mask on the beam way stimulates appearing of an active (excited) plane in the material inside the cuvette. This poweractive plane is a page of data which can house 4096x4096 bit array. Beforereturning of protein into the quiescent state (in such state it can remain quite a long time keeping the information) a red-wave recording laser lights on; it's positioned at the right angleto the yellow one. The other SLM displays binary data, and this way creates the corresponding mask on the way of the beam, that's why only definite spots (pixels) of the page will be irradiated. The molecules in these spots will convert into Q-state and will represent a binary one. The remaining part of the page will come into the initial bR-state and will represent binary zeros. In order to read the data you will need again the "page" laser which converts the read page into Q-state. It's implemented so that in the future one can identify binary one and zero with the help of difference in absorption spectra. 2ms later the page is plunged into low- intensive light flux of the red-wave laser. Low intensity is necessary to prevent jumping into Q- 20 | P a g e state. The molecules that represent a binary zero absorb red light, and those that represent a binary one let the beam pass by. It creates a "chess" picture of light and dark spots on the LCD-matrix which takes a page of digital information. For erasing information a short impulse of a cyan laser is enough in order to convert themolecules from Q-state back into bR-state. This beam can be obtained not necessary with thelaser: you can erase the whole cuvette with a usual ultraviolet lamp. In order to ensure theentirety of data when erasing only the definite pages there used caching of several adjacentpages. For read/write operations two additional parity bits are also used to prevent errors. Thedata page can be read without corruption up to 5000 times. Each page is traced by a meterand after 1024 reading the page gets refreshed (regenerated) with a new recording operation. Considering that a molecule changes its states within 1 ms, the total time taken for read orwrite operations constitutes around 10 ms. But similar to the system of a holographic memorythis device makes a parallel access in the read-write cycle, what allows counting on the speedup to 10 Mbps. It is assumed that combining 8 storing bit cells into a byte with a parallel access, you can reach, then, 80 Mbps, but such method requires a corresponding circuitrealization of the memory subsystem. Some versions of the SLM devices implement a pageaddressing, which in cheap constructions is used when sending the beam to the required pagewith the help of rotary system of galvanic mirrors. Such SLM provides 1ms access but costsfour times more. Birge states that the system suggested by him is close to the semiconductor memory in operating speed until a page defect is come across. On detecting of a defect it's necessary to resend the beam for accessing these pages from the other side. Theoretically, the cuvette can accommodate 1TBytes of data. Limitation of capacity is mainly connected with problems oflens system and quality of protein. 21 | P a g e Will the molecular memory be able to compete against the traditional semiconductor memory? Its construction has undoubtedly some advantages. First, it's based on protein which is produced in large volumes and at low price. Secondly, the system can operate in the wider range of temperatures than the semiconductor memory. Thirdly, data are stored constantly - even in case of power switching off, it won't cause data loss. And at last, bricks with data whichare rather small in size but contain gigabytes of data can be placed into an archive for storing copies (like magnetic tapes). Since the bricks do not have moving parts, it's more convenient than usage of portable hard discs or cartridges with magnetic tape. Recently, holographic data storage technologies have again been in the limelight as a next- generation storage system that achieves large storage capacities and high transfer rates simultaneously. This shift was mainly shaped by two USA national projects, the PhotoRefractive Information Storage Materials (PRISM) consortium and the Holographic Data Storage System (HDSS) consortium, launched and conducted from 1994 to 1999. In PRISM, it was shown that photopolymer has entered a commercially practicable realm as a holographic recording material other than crystals. In HDSS, rotating a diskshaped storage medium to continuously [2] even record and reconstruct data was demonstrated [1] [3]. However, a large number of subjects still remain to be tackled regarding commercialization after the completion of these large-scale USA national projects. We have been developing a holographic memory that has compatibility with the conventional optical disk technologies. Since the reference beam and information beam are coaxially arranged to perform recording and reconstruction, the holographic memory is called a coaxial read/write holographic memory or collinear systemThere have been a number of proposals for holographic memories in which holographic recording media were shaped into a disk[4]. However, simply shaping a recording medium into a disk does not assure practicability and a number of problems have to be tackled from the perspective of commercialization 22 | P a g e For the two-beam interference method as shown in Fig.1, the reference beam and information beam are configured in two different optical axes. This makes the optical system complex. There are no precise standards or addresses for the holographic recording media, resulting in very poor removability There is no interchangeability between the holographic recording devices and recording media of the same type Insufficient consideration is given to measures to combat plane deflection or eccentricity during rotation of the disk. Holographic recording media require flatness of the order of the optical wavelength, which is unsuitable for volume production No consideration is given to maintaining upward compatibility with existing storage media such as CDs and DVDs 23 | P a g e Utilization of the existing production infrastructure has not been considered. This means that the production of holographic memories requires investments in new equipment 8 COLLINEAR SYSTEM By studying the conventional problems noted above, it becomes especially important to fundamentally reconsider a holographic recording and reconstruction optical system using the two-beam interference method noted in (1). The collinear system is based on the coaxial read/write type in which the reference and information beams are handled as a pencil of coaxial light, rather than the two-beam interference method that was widely used in the past. This enables the comprehensive optical disk technologies to be easily fused to realize large storage capacity, and high transfer rate memories of a new concept while exploiting the advantages of the holographic memory. The following shows the concept of collinear holographic memories Thick volume-recording media are used to increase the recording capacities A batch of two-dimensional page data are recorded and reconstructed as a hologram to improve the transfer rates. The information beam and reference beam are collinearly optically arranged on the same axis without angles to perform holographic recording and reconstruction. Disk construction with a coated reflection film is employed to configure an optical system that completes on a single side of the disk An optical disk is preformatted with addresses and optical servo information The optical servo technique is applied so that interference patterns are recorded even in the presence of disk rotation, eccentricity, or plane deflection A two-wavelength optical system is configured to read out addresses at a wavelength that does not photosensitize a holographic recording material, and perform optical servo operations. 24 | P a g e A beam for the optical servo is utilized to provide upward compatibility with the existing CDs or DVDs 9 COLLINEAR HOLOGRAPHIC MEDIA EVALUATION SYSTEM S-VRD TM Fig.2 Collinear Holographic Media Evaluation System S-VRD To accelerate the commercialization of collinear holographic memories, it is necessary to promote the research and development of holographic recording media. Fig.2 shows the appearance of a recording media evaluation system (abbreviated product name: VRD) employing the collinear S- system. This evaluation system can record and reconstruct two-dimensional digital page data employing the collinear system and evaluate the characteristics of the recording material such as bit error rates and signal-to-noise ratios (SNR) from different angles. Moreover, it can measure small-piece samples to disk samples (option) using a 6-axes control universal sample holder. 25 | P a g e CONCLUSION Holographic memory is using lasers for recording and reading of data in which signal beam and reference beam are used and interfering pattern of these two beams is to be stored in memory. As interfering pattern is stored, by changing angle, position or wavelength of reference beam different patterns can generated which increases data density. Holographic Memory is having all required features of any storing device like high storage densities – 1 TB in just sugar sized cube, fast data transfer rate - 1 Gbps and with that 3D representation of image or video. So these all make Holographic memory a perfect suitable choice for storing data. 26 | P a g e