Optical data

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. 

However, optical fiber communications are not useful only for long-distance communication links. Fiber-optic data links are also used in a variety of short-distance systems, for example in computer—computer links and for internal communications on ships and aircraft. Figure 16 shows some possible appHcations for fiber-optic communications, with respect to length and bit rate. The common carrier appHcations, like telephone links. He to the upper right of the dashed line labeled 100 MHzkm. However, a wide variety of other lower performance appHcations, illustrated to the lower left of the dashed line, are in use or under development. 

Pigure 10 shows the typical commercial performance of LEDs used for optical data communication. Both free-space emission and fiber-coupled devices are shown, the latter exhibiting speeds of <10 ns. Typically there exists a tradeoff between speed and power in these devices, however performance has been plotted as a function of wavelength for purposes of clarity. In communication systems, photodetectors (qv) are employed as receivers rather than the human eye, making radiometric power emitted by the devices, or coupled into an optical fiber, an important figure of merit. 

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. 

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