Data storage optical

Rotating optical disk systems were first demonstrated in the 1960s and paved the way for the Philips Videodisc, introduced in 1973. Less than ten years later, following the successful development of the reliable, low-cost gallium-arsenide laser, the compact disc was launched, leading to the first penetration of laser technology into the consumer market. 

The mechanism of WORM recording and indeed of all currently available optical disc media, whether erasible or write-once, depends upon the heat generated by a focused laser beam impinging on an absorbing medium. The energy, normally near-IR radiation, is transmitted to the absorbing material in a time much shorter than the time it takes for the heat generated to dissipate by thermal conduction. 

Most of the pioneering work on IR-ab sorbing dyes for optical recording media was carried out in the 1970s and 1980s, at which time the emphasis was on WORM media [26], Many classes of dyes, based on patent applications filed by the late 1980s, are potentially suitable for WORM media [3,25-30] (Table 6.1). 

Fujitsu, Pioneer, Ricoh, TDK, Canon, Fuji Photo Film 

ICI (now Avecia), TDK, Mitsubishi, Xerox, Hitachi, BASF, Mitsui Toatsu 

when the film of such materials is irradiated with linearly polarized or unpolarized light, optical information can be written, erased, or rewritten on tbe pol5uner film. Namely, the irradiation causes an optically induced birefringence. Tbe information written in this way can be probed by measuring the optical properties of the material. In this way, the information is retrieved. 

On a molecular base, tbe azobenzene converts from tbe tram state into tbe cis state. By this isomerization reaction, the previously unoriented azobenzene groups are aligned perpendicular to the plane of polarization of the incident light. The alignment results in a high birefringence of the irradiated areas. 

Azo groups can be introduced into polymers by monomers based on acrylate or methacrylate units to which side chains that contain azo groups are attached. The synthesis of such a monomer is shown in Figvne 16.7. 

In addition, mesogenic units are introduced in the polymer. The mesogenes are bonded in tbe same way as the azo dyes. They need not necessarily absorb the actinic light since they act as a passive molecular group. Their task is to intensify the light-inducible double refraction and stabilize it after the action of the light.  

In order to improve the solubility of the polymer, other moieties may be incorporated  

The main interest in this book is the use of laser addressable dyes in optical data recording, specifically WORM (write once read many times) used in the industrial and institutional arena for the mass storage of data, and CD-R used in smaller scale computing, educational and entertainment outlets. 

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Optical counters Optical crystals Optical data storage Optical device Optical devices... 

Photopolymers and photothermoplasts are mentioned only in connection with holographic data storage (see Holography). The classical method of optical data storage in silver haUde films (photographic film, microfiche technique) is not discussed (see Photography). 

In general, the commercially used optical data storage media deposit the information on disks or cards (two-dimensional data deposition. Table 1). Data storage systems, which store data in three and more dimensions are being developed. 

Fig. 16. Maximum achievable signal-to-noise ratio (SNR) on read-out of different writable optical data storage systems as a function of the writing energy (laser power) (121). SQS = Organic dye system (WORM) PC = phase change system (TeSeSb) MO = magnetooptical system (GbTbFe). See text.

Photochromic Organic Dyes. Intensive investigations into this category of substances have led to numerous patent appHcations. Copper—phthalocyanine pigments, organic dyes based on cyanine (Ricoh, Pioneer), naphthochinone (Nippon Denki), and ben2othiopyrane (Sony) (123) have been described. They did not lead, however, to any commercial use. Surveys on the possibiUties of optical data storage with photochromic dyes can be found (124,125). 

The application of nonlinear optical recording techniques for reversible optical data storage based on the excitation of photochromic molecules by two-photon processes also has been described (154). 

FiaaHy, the use of photoreversible change of the circular dichroism for optical data storage is of iaterest. This technique offers an advantage over photochromic materials ia that the data can be read ia a way that does not damage the stored information. These chirooptic data storage devices have been demonstrated with the example of chiral peptides with azobenzene side groups (155). 

High demands are placed on the substrate material of disk-shaped optical data storage devices regarding the optical, physical, chemical, mechanical, and thermal properties. In addition to these physical parameters, they have to meet special requirements regarding optical purity of the material, processing characteristics, and especially in mass production, economic characteristics (costs, processing). The question of recyclabiUty must also be tackled. 

The birefringence of substrate materials for optical data storage devices requires special attention, especially in the case of EOD(MOR) disks. Birefringence has no importance for glass substrates (glass does not exhibit any significant birefringence) and is only a subordinate factor for polymeric protective layers of aluminum substrates because of their reflective read/write technique. 

An advantage of aluminum is the high level of knowledge and the automated production plants stemming from the mass production of A1 substrates for magnetic hard disks these can be widely used for the production of substrate disks for optical data storage. 

Modification of BPA-PC for adaptation to the conditions of production of CD and CD-ROM disks, and of substrate disks for WORM and EOD was necessary. BPA-PC standard quaHties for extmsion and injection mol ding have, depending on molecular weight, melt flow indexes (MEI), (according to ISO 1130/ASTM 1238 in g/10 min at 300°C/1.2 kg, between less than 3 g/10 min (viscous types) up to 17 g/10 min. For CDs and optical data storage disks, however, MEI values exceeding 30 g/10 min, and for exceptionally short cycle times (5—7 s) even >60 g/lOmin are demanded at an injection mass temperature of 300°C (see Table 5). 

Copolymers nd Blends of PC. Numerous co- and terpolymers as well as polymer blends of BPA-PC have been developed and their suitabihty as substrate materials for optical data storage media has been tested (Table 8) (195). From these products, three lines of development have been chosen for closer examination. 

Table 8. Substrate Materials for Optical Data Storage...

Cyclic Polyolefins (GPO) and Gycloolefin Copolymers (GOG). Japanese and European companies are developing amorphous cycHc polyolefins as substrate materials for optical data storage (213—217). The materials are based on dicyclopentadiene and/or tetracyclododecene (10), where R = H, alkyl, or COOCH. Products are formed by Ziegler-Natta polymerization with addition of ethylene or propylene (11) or so-called metathesis polymerization and hydrogenation (12), (101,216). These products may stiU contain about 10% of the dicycHc stmcture (216). 

Table 9 compares the most important properties of substrate materials based on BPA-PC, PMMA, and CPO (three different products) (216,217). The future will prove if the current disadvantages of CPO against BPA-PC regarding warp, processibiUty (melt viscosity), and especially cost can be alleviated. CycHc polyolefins (CPO) and, especially cycloolefin copolymers (COC) (218) and blends of cycloolefin copolymers with suitable engineering plastics have the potential to be interesting materials for substrate disks for optical data storage. 

Special, uv-curable epoxy resins (qv) for substrate disks for optical data storage (Sumitomo BakeHte, Toshiba) excel by means of their very low birefringence (<5 nm/mm) and high Young s modulus. Resistance to heat softening and water absorption are similar to BPA-PC, but impact resistance is as low as that of PMMA. 

Table 10 compares the values of different experimental uv-curable cross-linked polymers with those of BPA-PC for the most important properties of substrate materials (220). In spite of this remarkable progress in the development of fast curing cross-linked polymers, BPA-PC and, to a small extent, glass are still the materials of choice for substrates for optical data storage. 

Other Polymers. Besides polycarbonates, poly(methyl methacrylate)s, cycfic polyolefins, and uv-curable cross-linked polymers, a host of other polymers have been examined for their suitabiUty as substrate materials for optical data storage, preferably compact disks, in the last years. These polymers have not gained commercial importance polystyrene (PS), poly(vinyl chloride) (PVC), cellulose acetobutyrate (CAB), bis(diallylpolycarbonate) (BDPC), poly(ethylene terephthalate) (PET), styrene—acrylonitrile copolymers (SAN), poly(vinyl acetate) (PVAC), and for substrates with high resistance to heat softening, polysulfones (PSU) and polyimides (PI). 

It has been reported that block copolymers with appropriately chosen partners and mixing ratios yield materials suitable for use in substrate disks for optical data storage. An example is polyarjiate—polystyrene block copolymer with a PS content between 40 and 60% (225). 

The acceptance of optical data storage iato the mass storage market, which is as yet exclusively dominated by magnetic systems, will be fundamentally boosted if optical drives and media are subject to uniform standards and become fully compatible, and multiuser drives are offered which enable the user to employ alternatively CD-ROM and EOD disks, and maybe WORM disks as well (and CD-R disks, respectively). A prerequisite, however, will be whether rewritable optical memories will use the MOR or the PCR process. This accord especially will be hard to reach. 

D. Mergel, P. Hansen, and D. Raasch, Proc. SPIE 1663, Optical Data Storage 1992, 240 (1992). 

C. J. Robiuson, T. Suzuki, and C. M. Ealco, eds., Materia/s for Magnet-Optic Data Storage, Materials Research Society Proceedings 150, Pittsburgh, Pa., 1989. 

This new optical data storage device is reported to be robust and nonvolatile. The response time for the write—read beam is in the subnanosecond range, and no refreshing is requked for long-term retention of trapped charges (95). The basic principle may be appHed to other, similar photoconductive materials. 

PMMA has not been able to compete in the field of compact discs, the market having gone to the polycarbonates (see Chapter 20). It is, however, suitable for optical data storage using large video discs. Large-scale acceptance in the field of optical fibres has been held back by problems of obtaining material of an acceptable level of purity. 

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