The rapid growth of information materials is a major characteristic of today’s society. Some people statistics, the number of scientific and technical documents about every 7 years increased by 1 times, while the general intelligence information to every 2 years ~ 3 years doubled the rate of increase. The storage, analysis, retrieval and dissemination of large amounts of information urgently require high-density, high-capacity storage media and management systems.
In 1898, Valdemar Poulsen of the Netherlands invented the world’s first magnetic recording device: the magnetic wire recorder, and from then on, the traditional practice of magnetic recording applications began. Over the next century or so, many different types of magnetic recording devices have emerged: magnetic tape drives, magnetic core memory, disks, and so on. Although a large number of different magnetic storage devices have appeared, the underlying principle of magnetic recording is still the aforementioned property of ferromagnetic materials to maintain the direction of magnetization in an external magnetic field. The writing principle of conventional magnetic recording is to convert the time-varying electrical signal into a spatial variation of magnetization intensity and direction in a linear or rotating ferromagnetic material, and the reading principle of conventional magnetic recording is to convert the spatial variation of magnetization direction and intensity distributed in a magnetic material into a time-varying electrical signal through a linear or rotating motion using magnetoelectric conversion elements.
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However, with the increase in recording density (the current hard disk recording density has been able to reach 30Gb/cm2), the strength of the inductive current and signal-to-noise ratio can be obtained is already too small, resulting in the read-in device BER can no longer meet the requirements. The development of the computer and information industry has enabled more and more information content to be recorded, transmitted and stored in digital form, and the research on large-capacity information storage technology has been heating up. The continuous maturation of laser technology, especially the mature application of semiconductor lasers, has made optical storage develop from the initial microphotography into a fast, convenient and large-capacity storage technology, and various optical ROMs have been produced. Compared with magnetic media storage technology, optical storage has the advantages of long life, non-contact read/write, and low price of information bits.
The basic principle of optical storage
Optical storage technology is the technology of storing information by irradiating the medium with laser and causing physical and chemical changes to the medium through the interaction of laser and medium. The basic physical principle is: the storage medium by laser irradiation, the medium of a certain nature (such as reflectivity, reflected light polarization direction, etc.) change, the different state of the nature of the medium mapped to different storage data, storage data readout is achieved by identifying the change in the nature of the storage unit.
As a light storage method, there has been nearly a century of development history. The common photographic art is the earliest light storage technology. Whether it is film sensitivity, resolution, color, or photographic instruments, great progress has been made, not only to capture still scenes, but also to record and reproduce moving images through film and television. However, including holographic photography, including the photographic art, belong to the category of analog light storage, which is limited in storage capacity, storage density and transmission rate. With the development of information society, especially the emergence of laser and the increasing popularity of computers, digital optical storage technology began to emerge, and the birth of digital optical disk became a major breakthrough in storage technology.
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The recording mechanism of the recording media used in the vast majority of commercial optical disc storage systems to date has been a thermogenic effect. The energy absorbed from the laser beam is used as a highly concentrated and powerful heat source to induce local melting or evaporation of the medium, often referred to as ablative recording. In practice, computers are generally used to process information because they can only recognize binary data, so to store information such as data, audio and video on top of the storage media, the information must first be converted into binary data. Nowadays, common optical storage media such as CD-ROMs and DVD-ROMs are the same as floppy disks and hard disks, which are used to store information in the form of binary data. When writing information, the data sent by the host is encoded and sent to the optical modulator, so that the laser source outputs a beam of different intensities, and the modulated laser beam passes through the optical path system, is focused by the objective lens and then irradiated to the media, and the storage media is ablated with small pits after laser irradiation, so there are two different states of ablated and unablated on the storage media, which correspond to two different kinds of binary data These two states correspond to two different types of binary data.
If the focused beam is projected onto the disc, if the recorded information already exists on the disc, the characteristics of the reflected light, for example, the light intensity, the phase of the light or the polarization state of the light, will change in some way, and the original recorded data information can be reproduced through the electronic system processing, which is the basic read-out process of the disc. Specifically, when reading the information, the laser scans the medium, and at the crater the incident light does not return because the reflected light and the incident light cancel each other, while at the unablative non-cratered place, the incident light mostly returns. In this way, the binary information on the storage media can be read out depending on the reflectivity of the light beam, and then these binary codes can be converted into the original information.
In addition, the storage medium of erasable optical discs is a phase change type medium that causes a change in the crystalline state of the light illumination point. In contrast, the storage medium of optical discs with magneto-optical storage materials produces a change in the direction of magnetization, thus recording or deleting information.
The main features of optical storage
1、High recording density and large storage capacity. Optical disk storage system uses laser as light source. Due to the good coherence of laser, it can be focused into a small spot with diameter less than 0.001mm. With such a small spot to read and write, the optical disc surface density can be as high as 107bit/cm2~108bit/cm2. a CD-ROM disc can store 300 million Chinese characters. It took 14 years for China to publish the Chinese encyclopedia with 1.2×108 million words, that is to say, the whole encyclopedia is not enough for one CD-ROM.
- CD-ROM uses non-contact reading and writing, and there is usually a distance of about 2mm between the optical read/write head and the recording disc. This structure brings a series of advantages: first, because there is no contact, no wear, so high reliability, long life, recorded information will not be repeatedly read because of information mourning; second, the recording media with a transparent protective layer, so the disc surface dust and scratches, are very little impact on the recorded information, which not only improves the reliability of the disc, but also makes the disc preservation conditions requirements greatly reduced; Secondly, the change of focal length can change the relative position of recording layers, which makes it possible for optical storage to achieve multi-layer recording; fourthly, optical discs can be replaced easily and freely, and still maintain a very high storage density. This not only brings convenience to users, but also equals to the unlimited expansion of the storage capacity of the system.
3, laser is a high-intensity light source, focused laser spot has a high power, so optical recording can reach a fairly high speed;
4, easy to use online with computers, which significantly expands the application areas of optical storage devices;
- Optical disc information can be easily copied, and this feature makes the lifetime of recorded information on optical disc practically infinite. At the same time, the simple pressing process makes optical storage bits of information inexpensive, which creates the necessary conditions for the mass application of optical disc products.
Of course, optical storage technology has its drawbacks and shortcomings. The optical head, no matter its size or quality, is not yet comparable to the magnetic head, which affects the addressing speed of the optical disc and thus its recording speed. Generally speaking, the read/write speed of optical discs is also lower than that of disks. And since the recording density of optical discs is so high, very small defects on the discs can cause errors. The relatively high native bit error rate of optical discs makes it necessary for optical disc systems to employ strong error correction measures, thus increasing equipment costs.
Optical discs and storage types
The types of optical discs are usually: Read only memory (ROM, Read only memory), Write once read memory (WORM, Write once read memory), Rewriteable, and Direct rewrite (Overwrite).
Read only memory discs
The laser beam is focused into ~1um spot, and the general width of the dimple of the disc is 0.4um, the depth is l/4 of the readout light wavelength, which is about 0.11um, and the spiral linear pattern of the stripe pitch is 1.67um.
The modulated laser beam is focused on the glass liner disc with photoresist at different power densities to expose the photoresist, followed by developing, etching, and making the master disc (also called master), and then making the secondary disc (also called printed film, stamper) by spraying and plating, and then “2P” injection molding. ROM disc is formed.
Disc dumping: The glass and other discs are precision ground and polished, then ultrasonically cleaned to obtain a uniform and clean surface disc; the discs are dripped with photoresist and dumped in a high-speed centrifuge to form a uniform photoresist film on the surface of the disc; the discs are removed and placed in an oven for pre-baking to obtain a dense photoresist film that adheres well to the substrate.
Modulation exposure: The film is placed in a high-precision laser recorder and the information is written according to the predetermined modulation signal.
Development etching: If the photoresist is negative, the non-exposed part comes off, so the information pits appear in the information channel in accordance with the modulation signal, and the shape, depth, and spacing of the pits are related to the information carried. This kind of disc with a concave and convex information structure carrying modulated information is the master disc. Since the photoresist used in this process is generally positive, the resulting master disk is a positive image master disk.
Silver coating: A silver film is sprayed on the surface of the master disk. This silver film is used to improve the reflectivity of the information structure in order to check the quality of the master disk, on the other hand, it is also used as one of the electrodes for the next step of nickel plating.
Electroplated nickel layer: The surface of the silver-plated disk is electroplated with nickel by electrolytic method, which makes the main disk grow a metallic nickel film with the required thickness.
The above discs are chemically treated so that the nickel film is peeled off from the primary disc to form a secondary disc. Each of the above-mentioned main discs can be repeated by (5), (6) steps to make a number of secondary image sub-discs – sub-discs; and each sub-disc can be repeated by (5), (6) steps to make a number of positive image sub-discs.
The resulting positive or secondary image sub-disc is used as a “stamper” to process the center hole and outer circle, and is then loaded into a “2P” sprayer for further “2P” duplication. The process of further “2P” duplication is used to produce batch ROM discs.
In general, the recording medium for read-only storage discs is photoresist, and the recording method is to burn the information on the medium with an acousto-optical modulated argon ion laser, and then to make a primary disc and a secondary disc, which is then used as the original mold for mass reproduction of video discs or digital audio and video recordings.
Write-On-Off Discs
Write-once disc is a disc that uses laser spot to produce irreversible physicochemical changes in the micro-region of the storage medium to record information in the following ways.、
Ablation type: The storage medium can be metal, semiconductor alloy, metal oxide or organic dye. Using the thermal effect of the medium, it is the micro-region of the medium that melts and evaporates to form information pit holes.
Foaming type: The storage medium consists of a polymer-high melting point metal two-layer film. Laser irradiation causes the polymer to decompose and discharge gas, and the bubble formed between the two layers causes the upper film to bulge, forming a difference in reflectivity with the surrounding area to achieve the recording of information.
Fused velvet type: The storage medium uses ion-etched silicon with a velvet-like structure on the surface, and the laser spot makes the irradiated part of the velvet surface melt into a mirror surface to achieve contrast recording.
Alloying type: Double-layer structure made of Pt-Si, Rh-Si or Au-Si, and the laser-heated micro-region is fused into an alloy to form a contrast record.
Phase change type: The storage medium is mostly made of sulfur compounds or metal alloys into thin films, and the thermal and optical effects of metals are used to cause phase change from amorphous to crystalline phase in the illuminated micro-region.
Erasable rewritable discs
Erasable rewritable optical discs are divided into two main categories in terms of the mechanism of writing, reading and erasing the recording media.
Phase change discs: These discs use structural phase change materials formulated with multi-semiconductor elements as the recording media film, and use the thermal and optical effects of laser light to cause reversible phase change between the crystalline and glass states when the laser light interacts with the media film to achieve repeated writing and erasing requirements, and can be divided into thermal phase change discs and optical phase change discs.
Magnetic discs: These discs use magnetic phase change media made of rare earth-transition metal alloys as recording films, which have an easy magnetization axis perpendicular to the surface of the film and use the photodemagnetization effect and the positive or negative orientation of magnetization intensity under the effect of bias magnetic field to distinguish “0” or “1” in binary. “1” in binary.
Principle of erasable rewritable phase change discs
RW phase change discs use reversible phase structure changes between two stable states of the recording medium to achieve repeated writing and erasing. The common phase structure changes are as follows: 1. reversible phase change between crystalline state I and crystalline state II, which is too small to be of use. 2. reversible phase change between amorphous state I and amorphous state II, which is also too small to be of practical value. 3. reversible phase change between crystalline state of glass, which is of practical value.
Storage principle and process: the near-infrared wavelength of the laser action on the medium, can intensify the vibration of atoms and molecules in the medium structure, thus accelerating the phase change. Therefore, the action of near-infrared laser on the medium is based on the thermal effect.
The recording of information: corresponds to the transition of the medium from the crystalline state C to the glassy state G. The laser pulse with high power density and pulse width of tens to hundreds of sodium seconds is selected to make the spot micro-region enter the liquid phase because the medium temperature momentarily exceeds the melting point Tm, and then the phase transition to the glass state is completed by fast quenching of the liquid phase.
Readout of information: with low power density, short pulse of laser scanning information channel, from the size of the reflectivity to identify the information written. General medium in the glass state (i.e., write state) when the reflectivity is small, in the crystal state (erase state) when the reflectivity is large, in the process of reading out, the phase structure of the medium remains unchanged.
Erasure of information: corresponds to the transition of the medium from the glass state G to the crystalline state C. A laser of medium power density and wider pulse is selected to make the spot micro-region as the medium temperature rises close to Tm, and then the crystallization is completed by nucleation-growth. In this process, the light-induced defect center can become the new nucleation center, therefore, the nucleation rate and growth rate are greatly increased due to the laser action, which leads to a higher rate of laser thermal crystallization than monothermal crystallization.
New technology of optical information storage
With the rapid development of information technology, the demand for massive information storage is growing rapidly. However, the information storage system is still a relatively weak and critical link in the development of information superhighway networks and miniaturization of starting-level computers that are emerging worldwide. The current storage density and data transfer rate of optical storage are still far from meeting the requirements of the rapidly developing information science and technology.
In order to improve the storage density and data transmission rate, optical storage is being developed from long-wave to short-wave, low-dimensional to high-dimensional (i.e., from flat to three-dimensional), far-field to near-field, photothermal effect to photonic effect, point-by-point storage to parallel storage.
Three-dimensional storage technology
Three-dimensional body storage is an important way to achieve ultra-high-density information storage, and the research area is mainly focused on two aspects of body holographic storage and photonic three-dimensional storage.
Volume holographic storage
Volume holographic storage is a large-capacity, high-density storage method that emerged with the development of optical holography technology in the 1960s. With the rapid development of the computer industry and the breakthroughs in the field of optoelectronic devices and holographic storage materials, great progress has been made in the field of holographic storage, which has made holographic storage a hot spot for research in the field of ultra-high density optical storage.
The general optical body holographic data storage mechanism is as follows: the data to be stored (digital or analog) is modulated to the signal light by the spatial light modulator (SLM) to form a two-dimensional information page, and then interferes with the reference light in the recording medium to form a body hologram so as to complete the recording of information read out using the same reference light addressing as the original, which can read out the corresponding hologram stored in the crystal. By using the Bragg selectivity of the body hologram and changing the incident angle or wavelength of the reference light, multiple images can be multiplexed in a unit volume to realize multiple storage and achieve the purpose of ultra-high density storage.
Holographic storage has the following characteristics.
(1) high storage density and capacity: the storage density in the visible spectrum can reach 1012bits/cm3 [8];
(2) High data redundancy: holographic recording is distributed, and defects and damage to the storage medium will only reduce the intensity of all signals without causing data loss;
(3) high data transfer rate: information is read and written in parallel in pages, thus achieving a very high data transfer rate. The current holographic storage system using multi-channel parallel probe array, the data transfer rate is expected to reach 1Gbyte / s;
(4) fast addressing speed: reference light can be used acousto-optical, electro-optical and other non-mechanical addressing methods, data access time can be reduced to sub-millisecond range or lower;
(5) Long storage life: the information recorded by the storage medium can be maintained for more than 30 years.
The goal of the development of body holographic storage is to achieve terabytes of storage capacity and 1Gbps data transfer rate, Inphase of the United States and Optware of Japan have made impressive achievements and have made great progress in the commercialization process . At the same time, there are many challenges in the development of body holographic storage, mainly to find a storage material with the advantages of performance, capacity and price.
Photonic three-dimensional storage
The activation center in the storage material, under the excitation of light to make electron leap and achieve the purpose of optical storage, called photon storage ( photo induced opTIcal memory). It is a kind of optical storage formed without the stage of thermal effect after the absorption of photons by the material, which is different from the current general application of photothermal storage method. The main research includes spectral burnt-hole storage and two-photon absorption three-dimensional storage.
1、Spectral burnt-hole storage
The doped molecules in the solid mechanism appear non-uniform broadening of energy levels due to the difference of local environment. When irradiated with a narrow-band laser, a reduction in absorption occurs at the laser frequency within the absorption band of the dopant molecule, a phenomenon known as spectral hole-burning. This burnt hole can be read out with a laser of the same frequency. Since multiple holes can be burned within the absorption band by varying the laser frequency, i.e., information can be recorded using the frequency dimensional variable, thus allowing multiple information to be stored in a single spot.
Spectral burn holes include single-photon spectral burn holes and two-photon spectral burn holes. Both types of materials have photon electron-selected burn holes at low temperatures. Since the current materials have shallow electron capture trap depths, resulting in shallow hole depths for burn holes, and the holes burned first are prone to gradual filling during sequential burn holes, it is crucial to find materials that can burn holes at room temperature. At present, the domestic and foreign research mainly two types of material systems: Sm ion doped inorganic material system and organic material system for the electron transfer reaction of the donor and acceptor.
2, two-photon absorption three-dimensional storage
Two-photon absorption three-dimensional record of the basic principle is: two photons at the same time when acting on a medium, can make the medium of the atoms of a particular energy level on the electron excitation to another steady state, and make its optical properties change, if the two beams from two directions focused to the same point in the space of the material, it can achieve three-dimensional space addressing and reading and writing. The use of material refractive index, absorbance, fluorescence or electrical property changes to achieve storage [10], can achieve T bits/cm3 bulk density, can reach 4MB/s transmission rate. The most representative one internationally is the 100-layer recording method of the University of California, San Diego and Call&Recall. Tsinghua University in China has been engaged in research in this area since 1995, and has initially established a physical model of recording for organic media and completed the development of special equipment for testing the characteristics of two-photon recording media.
The two-photon absorption three-dimensional storage principle is based on the energy level jump, the response time of the material can reach the picosecond level, can achieve high-density bulk storage, theoretical resolution can reach the molecular scale. However, the small two-photon absorption cross-section of most materials limits its application, so that the research on storage materials is necessary to make two-photon three-dimensional storage practical.
Multi-order optical storage technology
Multi-order optical storage is currently one of the main focuses of optical storage research at home and abroad, because it can greatly increase the storage capacity and data transfer rate. In the traditional optical storage system, binary data sequences are stored in the recording medium, and the recording symbols have only two different physical states, such as the alternating pit bank shape in a read-only CD. If the data stream is modulated into M binary data (M>2), so that the modulated data corresponds to M different physical states of the recording medium, M-order storage can be realized. As shown in the figure of pit depth modulation multi-order storage, is to achieve multi-value storage by changing the depth of the information symbol, the data stream is converted into a variety of different pit depth changes in the disk base by modulation, you can achieve multi-order pit depth storage.
Multi-order optical storage is divided into signal multi-order optical storage and media multi-order optical storage.
Its early scheme is Pit Depth Modulation (PDM: Pit Depth ModulaTIon). In this multi-order read-only optical disc, the width of the information pit is fixed at t min, and the depth of the information pit has M different possibilities, representing different orders. The information pits with different depths present different light intensities for their readout light, thus realizing multi-order pit depth modulation. sony has developed a multi-order information storage by using the variation of the edge of the information pit relative to a fixed clock, i.e., multi-order optical storage by using the variation of the length of the information pit. Both the starting and ending edges of the information pit can be varied in a certain step with respect to the clock edge. If the number of possible positions of both the starting and ending edges of the pit is 8, then 64 states are possible for the edge change of a pit, and the pit can store 6 bits (byte) of information, thus significantly higher than the recording density of conventional optical discs.
There are various media that can be used to implement multi-order optical storage. When the first dopant ion absorbs a photon from a short-wavelength laser, its electron is excited to a high-energy state, which may be “captured” by the second dopant ion, enabling data to be written. The captured electrons are released to their original lower energy state by another long wavelength laser (e.g., red light), and the stored energy is released as fluorescence, which is a linear response to the write/read process because the intensity of the fluorescence emitted is proportional to the number of captured electrons and also to the intensity of the writing laser, making electron-capture materials suitable for digital optical storage. The fast response time of electron-capture optical storage enables ns-time read/write.
Near-field optical storage technology
The traditional optical drive uses an optical head containing an objective lens for write, read, and erase operations. Since the objective lens is mostly a few millimeters from the disk recording layer, it belongs to the far-field optical storage method, and the light cannot be focused into a point with a diameter of less than half a wavelength, and the storage density is limited. Near-field optical storage uses near-field light, which is an occulted light obtained from the recording medium at a distance of less than half-wavelength magnitude from the light source. The occulted light is non-transmitted and decays rapidly to near zero when the distance exceeds the wavelength magnitude. The basic principle of near-field optical storage is to achieve subwavelength size optical point recording by subwavelength size optical head and subwavelength size distance control. As long as the optical storage medium is placed in the near-field optical microscope and the distance between the optical probe and the storage medium is kept within the near-field range, the size of the recording spot formed in the storage medium may be within the subwavelength magnitude, thus overcoming the diffraction limit and realizing high-density storage.
Compared with other ultra-high-density storage methods, near-field optical storage has the following main advantages.
(1) high density and high capacity: reading and writing small optical spot greatly improves the density of storage, making the storage capacity has been greatly improved. With the further improvement of near-field optical storage technology, you can also obtain a relatively high data transfer rate;
(2) can make full use of existing storage technology: such as hard disk drives in the air levitation head technology and optical disk storage in the optical head flight technology, without having to additionally carry out new system design and development, thus helping to reduce the price of the product, increasing competitive advantage.
Development trend and outlook of optical storage technology
High recording density is the most prominent feature of optical storage technology, and is also the most attractive aspect for use as computer peripherals. However, with the development of science and technology and the improvement of manufacturing process, magnetic recording technology is also making new progress. At present, compared with the disk, the storage capacity of optical disk alone has no absolute advantage, while the access speed gap is not significantly reduced. Therefore, increasing the recording density and thus the capacity of optical storage, as well as increasing the read and write speed are the main directions of research work on optical storage technology.
Ultra-high density optical storage technology represents the development direction of information storage, and the competition at home and abroad is very intense. Compared with the development trend of foreign countries, there is still a certain gap in China. Research in the direction of optical storage is to meet the needs of the growing information technology, so various storage technologies are to improve storage capacity, density, reliability and data transfer rate as the main development goal.
Review Editor: Zixiong Tang
