ERASABLE OPTICAL DATA STORAGE MEDIUM AND METHOD FOR RECORDING DATA ON THE MEDIUM

Abstract: An erasable optical data storage medium including a sub­ strate and a dual layer of a first material and a second material on the substrate. The dual layer is susceptible to expansion and contraction, to writing data thermally, to erasing data thermal­ ly and mechanically, and to reading data optically, the first material and the second material, respectively, being bonded together and remaining in the same physical state upon the ex­ pansion and relaxation of the dual layer. Methods and appara­ tus are also disclosed for recording data bits on the medium.

Description: FRASABLF OPTICAL DATA STORAGE MEDIUM AND METHOD AND APPARATUS FOR RECORDING DATA ON THE MEDIUM Background of the Invention Field of The Invention The present invention relates generally to a data storage medium and methods and apparatus for recording data on the medium, and, more particularly, to an erasable optical data storage medium and methods and apparatus for writing, erasing, and reading data on the data storage medium. Discussion of Background and Prior Art Currently, the practical or commercial techniques for recording data are based substantially on magnetic storage technology. In general, the data are stored on magnetic media, such as discs and tapes, on which logic 1 data bits and. logic 0 data bits are represented by the magnetization of a medium. For example, one direction of magnetization of a given location or bit storage area of the data storage medium can represent a logic 1, while another direction of magnetization of that bit storage area can represent a logic 0. Each data bit is written on the medium by using a recording head to magnetize the given bit storage area, and each data bit can be erased by writing another bit over the given bit storage area using the magnetic recording head. Each data bit is read by using the recording head to sense the magnetization of the given bit area. While the magnetic storage technology is commercially successful and advantageous, since about early in the decade of the l960s, a recording technique known generically as optical recording has been and continues to he considered a very promising alternative for storing data. Optical recording potentially has siqnificant advantages over magnetic recording, including higher data storage density, higher data rates, and longer data archival capabilities. One type of optical recording that has the highest potential is an optical recording apparatus or system which uses, in lieu of the magnetic recording head, a highly focused laser beam as an ultra-fine recording stylus to write and read data at a very high data rate and recording density, and to erase the data. The system includes an erasable optical data storage medium that responds to the laser beam to store the data. For example, the data storage medium responds to the heat generated by the laser heam to erase and write the data, and responds to the light of the laser beam to read the data. In one optical recording system, to write a data bit, a laser beam is focused on the erasable data storage medium to heat the medium and, thereby, induce a stable transition from one morphological or physical state, e.g., an amorphous state, to another morphological or physical state, e.g., a crystalline state. The two physical' states have different optical properties which are the optical transmittance and optical reflectance properties of the respective states. Therefore, to read the data bit, light from the laser beam, which is at a lower power level than is used for writing, is focused on the data storage medium and will be transmitted or reflected by the medium depending on the physical state of the medium, thereby representing a logic 1 or logic 0. The data bit can be erased by again heating the material with the laser beam at a higher power level to return the medium to its original physical state. The above-mentioned erasable optical data storage medium is made of semiconductor or chalcogenide materials that, as already stated, change from one state to another when heated. One problem with this data storage medium is that these changes in state are very small or slight, i.e., the amorphous and crystalline states are not substantially distinguishable optically. Therefore, a high signal-to-noise ratio of a reflected laser beam is not obtainable to distinguish between a logic 1 and a logic 0 upon reading the data bit. Another problem with this data storage medium is that the data rate, in particular the writing and erasing speeds, is undesirably very low, e.g., one microsecond. This results from the relatively long time that is required for a given material to undergo the change from one physical state to the other. Another problem is that a relatively hig-h amount of laser energy or power is required to heat the material so that it can transform from one physical state to the other Yet another problem is that due to this physical state transformation, the data storage medium will fatigue after a relatively few number of erase/write cycles. This fatigue factor will not be competitive with magnetic storage eechnology, which can achieve a number of erase/write cycles on the order of one million. U.S. Patent No. 4,278,734 to Ohta et al, discloses an optical medium in which the material of the medium changes physical state, leading to an increase or decrease in optical density. Ohta et al appears to have solved the data rate problem in that data may be written or erased quickly, e.g. in 50 nsec. Ohta et al also appears to have solved the contrast problem in that the physical states are distinguishable optically. However, one disadvantage of Ohta et al is that the data cannot be erased, bit-by-bit, since a localized bit area location cannot be achieved. Also, the physical change of state or transformation cannot occur on a surface which has anomalies or irregularities in the surface. Furthermore, the materials used for the optical medium are expensive. U.S. Patent No. 4,264,986 to Willis, issued April 28, 1981, discloses another type of erasable optical data storage medium. To write a data bit, a laser beam is focused on the medium to induce, by heating, a volumetric expansion of the bit area being heated, thereby creating a small bump or deformation. The presence of the bump represents one logic state, while the absence of the bump represents the other logic state. Upon this heating, the bit area of the data storage medium changes from one physical state, i.e., crystalline, to another physical state, i.e., amorphous, which is the phenomenon that causes the volumetric expansion, thereby creating the small bump which becomes reversibly fixed. A bit is read by focusing on the bit area a laser beam of lower power than is used for writing, and then detecting the amount or scattering of reflected light. If the bump is present, the reflected light will be substantially scattered, so that the intensity of the detected light will be low. If no bump is present, the reflected light will not be substantially scattered, whereby the intensity of the detected light will be higher. This read/write recording method is attractive, since it provides a good signal-to-noise ratio to distinguish a logic 1 from a logic 0. One problem with the erasable optical data storage medium of U.S. Patent No. 4,264,986 is that the laser beam must be raised to a very high power level above the melting point of the material of the medium to erase or remove the bump. This has the disadvantage of requiring expensive, high-powered lasers. Moreover, this melting of the bump does not leave the surface of the medium smooth, i.e., ripples can form on the surface upon the cooling of the material. Such a smooth surface is needed to be able to continually reproduce a satisfactory bump for properly reading the data bit. Yet another problem is that the data rate is relatively slow due to the need for the change in physical state of the material of the medium, as previously mentioned. Furthermore, fatigue, resulting from the physical change of state, is a problem with this data storage medium in that the number of erase/write cycles which can be achieved is only about one thousand. U.S. Patent No. 4,371,954 to Cornet, issued February 1, 1983, discloses an erasable optical data storage medium including a substrate, having a low coefficient of thermal expansion, which supports a dual layer having a bottom layer of material and a top layer of material. The bottom layer of material is a relatively inextensible metal or polymer having a high coefficient of thermal expansion, and the top layer is a metal alloy which is in a martensitic phase at ambient temperature and has a low coefficient of thermal expansion. Also, the bottom layer and top layer have a low adhesion to one another, i.e., they are not bonded together, and the latter has a transformation temperature Tt above ambient and below the melting point of the former layer. Above the transformation temperature Tt, the top layer is in its "parent" phase. To write a data bit, as described in U.S. Patent No. 4,371,954, a light pulse from a laser beam is absorbed by the dual layer, resulting in the heating of the dual layer at a temperature below the transformation temperature Tut, as well as a differential expansion between the two laynrs. The bottom layer delaminates or disengages from the substrate and volumetrically expands onto the top layer which forms a bump. Upon cooling, the top layer forms a reversibly fixed bump and the bottom layer contracts back onto the substrate. To erase the data bit, as described in rl.S. Patent No. 4,371,954, the martensitic top layer is raised to a temperature exceeding the transformation temperature Tt, either by, for example, a higher power laser pulse or a slower displacement or movement of the data storage medium across the laser beam, thereby transforming the top layer to its parent phase. The top layer then contracts onto the bottom layer and, upon cooling, returns to its martensitic phase. One problem with the erasable optical data storage medium of t7.S. Patent No. 4,371,974 is that the metallic dual layer, and in particular the bottom layer, is relatively inextensible. Consequently, the bump that can be produced is not as high as is desirable for accurately reading the data bit. Another problem is that the top layer must change between the martensitic phase and the parent phase for erasing the data hut not for writing the data. One disadvantage of this change of phase is that the erase mode is slow and, concomitantly, cannot occur as quickly as the write mode, thereby requiring significantly different data rates for the respective modes. Another disadvantage is that different laser power pulses are required for writing and erasing the data bit, with the latter being significantly higher, thereby requiring the use of expensive high-powered lasers. Yet another problem with U.S. Patent No. 4,371,954 is that the erasable optical data storage medium is highly susceptible to hard bit errors which are errors resulting from imperfections in the medium. More particularly, any anomalies or irregularities in the surface of the medium will affect the ability of the top layer to change between the martensitic and parent phases, resulting in bit errors. Still another problem results from the top layer being metallic or a metal alloy having a low thermal coefficient of expansion. This means that higher power light pulses are needed to expand this type of material, thereby again requiring expensive, high-powered lasers. Another problem is that the bottom layer disengages from the substrate upon writing a data bit. This has the disadvantage of enabling the bottom layer to "creep" about the substrate, thereby creating imperfections during use of the medium and preventing the medium from remaining smooth. Summary of the Invention It is an object of the present invention to provide a novel data storage medium. It is another object of the present invention to provide novel methods and apparatus for optically recording data. Yet another object of the present invention is to provide an erasable optical. data storage medium for storing data at a high data rate. Another object of the present invention is to provide an erasable optical data storage medium for storing data using inexpensive low-power light sources. Still another object of the present invention is to provide an erasable optical data storage medium having a fatigue factor competitive with magnetic storage technology. It is another object of the present invention to use the erasable optical data storage medium in conjunction with any of a plurality of desired wavelengths. Yet another object of the present is to be able to record data on a medium having irregularities or anomalies. Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of instrumentalities and combinations particularly pointed out in the appended claims. To achieve the foregoing and other objects in accordance with the purposes of the present invention, as embodied and broadly described herein, an erasable optical data storage medium of the present invention comprises a substrate, and a dual layer of a first material and a second material on the substrate, the dual layer being susceptible to expansion and relaxation, to writing data- thermally, to erasing data thermally and mechanically, and toreading data optically, the first material and the second material being honded together, and the first material and the second material, respectively, remaining in the same physical state upon the expansion and relaxation of the dual layer. In another aspect, the present invention is a method for writing data on an erasable optical data storage medium having a substrate and a dual layer on the substrate of a first material and a second material honded together, the first material being elastic and the second material having a glass transition temperature, comprising heating the second material above the glass transition temperature to make the second material rubbery, heating the first material to elastically expand the first material within the elastic limit of the first material, allowing the heated first material to expand and push up the heated second material while the heated first material expands elastically, and cooling the expanded second material below the glass transition temperature while the first material is in a stretched, expanded condition, the cooled second material then forming a reversibly fixed deformation and holding the first material in the stretched, expanded condition. In still another aspect, the present invention is a method of recording data on an erasable optical data storage medium having a substrate and a dual layer of a first material and a second material on the substrate, the dual layer corresponding to one data bit stored at one bit area on the erasable optical data storage medium, comprising the steps of erasing the one data bit stored at the one bit area, including heating the second material, and writing another data bit at the one bit area while the second material has not yet cooled. Yet another aspect of the present invention is an apparatus for recording a data bit, comprising an erasable optical data storage medium including a substrate and a dual layer of a first material and a second material on the substrate, the dual layer being susceptible to expansion and relaxation, to writing data thermally, to erasing data thermally and mechanically, and to reading data optically, the first material and the second material being bonded together, and the first material and the second material, respectively, remaining in the same physical state upon the expansion and relaxation of the dual layer, means for generating a first laser beam and a second laser beam, the first material being absorptive of, and the second material being substantially transparent to, the first laser beam, and the second material being absorptive of the second laser beam, means for focusing the first laser beam on the first material to heat and elastically expand the first material onto the second material to write the data bit, means for focusing the second laser beam on the second material to heat the second material and to erase the data bit, and means for moving the erasable optical data storage medium relative to the first laser beam and the second laser beam Among many advantages to be described fully below, the present invention will write, erase and read data at high data rates, using low-power light sources such as lasers, and have a number of erase/write cycles that is competitive with magnetic storage technology. Brief Description of the Drawings The accompanying drawings, which are incorporated in and form a part of the specification, illustrate embodiments of the present invention, and together with the description, serve to explain the principles of the invention. In the drawings: : Fig. 1 shows legends used to explain features of the present invention; Fig. 2 is a view in perspective of a data storage medium of the present invention, Fig. 3 is a section taken along lines 3-3 of Fig. 2 showing a bit area of the data storage medium in a relaxed condition; Fig. 4 is a view similar to Fig. 3 showing an expanded condition of the data storage medium; Fig. 5A - Fig. 5C are views, similar to Fig. 3 and Fig. 4, used to explain a write mode of the present invention; Fig. 6A - Fig. 6C are views, similar to Fig. 3 and Fig. 4, used to explain an erase mode of the present invention; Fig. 7A - Fig. 7B are views, similar to Fig. 3 and Fig. 4, used to explain a read mode of the present invention; Fig. 8A - Fig. 8B are views, similar to Fig. 4, used to explain an erase/write cycle of the present invention; ; Fig. 9 is a block diagram of one embodiment of an optical recording apparatus of the present inatention; and Fig. 10 is a block diagram of another embodiment of an optical recording apparatus of the present invention. Detailed Description of the Invention Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Fig. 1 shows a plurality of legends L1 - L4 used to explain a number of mechanical forces F1-F4 that are involved with materials of the present invention to be described more fully below. Legend L1 shows a force F1 which is a spring-like force produced in response to the stretching or extending of molecular chains M of a given material. Legend L2 shows a a force F2 which is produced in response to the stretching of chains between atoms or molecules of a given material. Legend L3 indicates a force F3 that is produced in response to the stretching or orientation of a material which has viscoelastic properties. Legend L4 shows a force F4 which is produced in response to a material having a variable modulus of elasticity. Fig. 1 also shows a legend r,5 having a representation of chains of molecules M of the materials of the present invention that are lightly cross-linked via a few bonds B. A legend L6 shows chains of molecules M of materials of the present invention that are highly cross-linked via many bonds B. As will be further described, such materials not only have the above-mentioned characteristics and properties, but a number of other characteristics and properties to carry out the objects of the present invention. Fig. 2 illustrates generally an erasable data storage medium 10 for storing data words in which each of the data words has a plurality of bits that are a logic 1 or a logic 0. The erasable data storage medium 10 can be an erasable optical data storage disc 12 which stores the data words over a plurality of concentric tracks T. The optical disc 12 also is rotatable in the direction shown by the arrow for data recording purposes to be described below. Fig. 3 shows a section of a small bit area BA of the optical disc 12 for storing one data bit of logic 1 or logic 0. In particular, each bit area RA of the optical disc 12 includes a substrate 14 and a dual layer 16 deposited on the substrate 14. As will be further described, the dual layer 16 is susceptible to expansion and relaxation, to writing data thermally (expansion), to erasing data thermally and mechanically (relaxation), and to reading data optically. Fig. 3 shows the dual layer 16 in a condition of relaxation corresponding to one logic state, e.g., logic 0, whereas Fig. 4 shows the dual layer 16 in the condition of expansion representing the other logic state, e.g., logic 1. The dual layer 16 has a bottom layer 18 of one material 18A and a top layer 20 of another material 20A. Neither the material 18A nor material 20A changes physical state upon expansion or relaxation of the dual layer 16, thereby, for example, increasing the speed or data rate and reducing the power requirements at which data bits are written and erased. The material 18A and material 20A are bonded together at their interface 22, so as to, for example, erase a data bit at a high data rate, as will be further described. In addition, the material 18A is deposited on the substrate 14 such that at their interface 24, material 18A does not delaminate from substrate 14 upon expansion of the dual layer 16, as shown in Fig. 4. This substantially eliminates any "creep" of layer 18 about substrate 14, whereby the bit area BA will remain optically smooth over repeated erase/write cycles, as will be further described. Thus, as illustrated in Fig. 4, upon expansion of the dual layer 16, and as a result of the bonding at interface 22 and at interface 24, material 18A and material 20A will continue to he bonded together, while material 18A will still be in contact with, or not have delaminated from, substrate 14. Fig. 4 shows that the material 18A and the material 20A have the mechanical forces F1 - F4 which are produced in response to the expansion of the dual layer 16. As will be further described, these forces F1 - F4 function, in part, to relax the dual layer 16 4 to the condition shown in Fig. 3 for purposes of quickly erasing a data bit. Material 18A has (1) low thermal conductivity, (2) a high coefficient of thermal expansion, 83) a glass transition temperature, Tg, considerably below ambient temperature, below which material 18A is glassy. That is, in connection with property (3), the material 18A is rubbery at ambient temperature as opposed, for example, to being in a glassy or brittle condition. This means that only relatively low-power heating is required to heat the rubbery material, resulting in localized, extensive or large, and rapid expansion of the material 18A due to these three properties, respectively. Also as a result of this localized expansion, high data storage density can be achieved. Material 18A also is relatively highly cross-linked, so that upon expansion there is substantially no viscous flow of this material. In addition, material 18A is elastic, having a high coefficient of elasticity, so that upon expansion it will stretch without exceeding its elastic limit, resulting in the production of spring- like force F1, and tending to return material 18A to its relaxed condition upon cooling or removal of stress, as will be further described. Other properties and characteristics of material 18A will be described below. Material 20A has (1) a glass transition temperature, Tg, for example 1000C, which is higher than ambient temperature. Thus, at ambient or normal temperatures material 20A is glassy or brittle, but when heated above the glass transition temperature, Tg, immediately transforms through a leathery or pliable condition into a rubbery condition. Material 20A also has (2) a relatively low thermal conductivity and (3) relatively low coefficient of thermal expansion. Therefore, low-power heat can be used to quickly and locally, but only slightly, expand material 20A due to these three properties, respectively. Also, material 20A will cool relatively quickly below its glass transition temperature, Tg, due to its top surface being exposed on the outside, thereby "freezing in" the expanded condition of dual layer 16, as shown in Fig. 4. Material 20A may be either lightly cross-linked, as indicated in legend Ls, or be dependent upon chain entanglements, and may be thermoplastic, viscoelastic, and have a relatively low coefficient of elasticity. Therefore, upon expansion, the cross-linking limits the viscous flow and a stretching of material 20A occurs, producing the force 3. Also, the force F2 is produced in material 20A upon expansion, as previously indicated. Furthermore, material 20A has a variable modulus of elasticity that varies with temperature and results in force F4 being produced upon expansion. This modulus decreases with increasing temperature, which means that the material 20A needs less of a stress to induce a strain in the material 20A. This results in a strain being induced very quickly at low light power, resulting in a bump B being formed, as shown in Fig. 4. Substrate 14 is a material preferably having a low coefficient of thermal expansion. Layer 18 and layer 20 are optically coupled, so that substantially all the light that is propagated through layer 20 is absorbed in layer 18. This optical coupling is provided by the material 20A having about the same index of refraction as material 18A. Thus, since substantially all this light can be coupled through top layer 20 to bottom layer 18, there is no loss of light, and therefore, low-power light sources can be used. Material 18A and material 20A are also capable of being optically tuned to any of a plurality of desired or given wavelengths of light. To accomplish this tuning, material 18A and material 20A can each constitute a light-transparent material that has added to it a dye or pigment to cause the respective material 18A and material 20A to absorb the given wavelength of light. For reasons to be discussed more fully below, material 18A and material 2QA will be tuned to absorb different wavelengths of light with material 20A being substantially, but not entirely, transparent to the wavelength of light that can be absorbed by the material 18A. This optical tuning has the advantage of making the optical data storage medium 12 highly flexible in that it can be tuned to operate with a variety of different laser sources that are currently available and inexpensive or that might be available in the future. Layer 18 and layer 20 also are thermally coupled together. That is, the material 18A and the material 20A have different heat capacities, and with material 18A being on the inside and material 20A being on the outside of dual layer 16, the latter can cool more quickly, as previously mentioned. write Mode Fig. 5A - Fig. 5C illustrate a method for writing a data bit thermally at the bit area BA. with reference to Fig. 5A, assume that the bit area BA of the erasable optical data storage medium 12 is in the relaxed condition described in connection with Fig. 3. In this relaxed condition, at ambient temperature, as previously mentioned, material 18A is above its glass transition temperature, Tg, and, therefore, rubbery, whereas material 20A is below its glass transition temperature, Tg, and, therefore, glassy. Also, at this time none of the mechanical forces Fl - F4 have been produced. Furthermore, the material 18A and material 20A have the other properties and character#stics previously described, including, for example, material 18A and material 20A being highly elastic and slightly elastic, respectively. Then, to write a data bit thermally, the method includes heating the material 18A to expand the material 18A within its elastic limit. At this time of heating material i8A, the material 20A also is heated above its glass transition temperature, Tg, which makes the material XOA rubbery. Preferably, as shown in Fig. 5A, the heating of material 18A and material 20A is accomplished by generating a laser beam LB that has a wavelength which is substantially 1 transparent to the material ROA and absorptive by the material l8A, and focusing the laser beam LB1 through the material 20A onto the material 18A. The light of the laser beam LB1 is slightly absorbed by the material 20A to produce the heat for making this material 20A rubbery and is substantially absorbed by material 18A for elastically expanding the material 1EA onto the material 20A. Then, as shown in Fig. 5B, the next step is allowing the heated material 18A to expand and push up the heated material 20A creating a deformation or bump, while the heated material 18A stretches elastically. The next step, shown in Fig. 5C, includes cooling the expanded material 20A below its glass transition temperature, Tg, while the material l8tA is in the stretched expanded condition. Upon cooling, the material 20A forms a reversibly fixed deformation or bump B and holds the material 18A in the stretched, expanded condition. Thus, at this time, a data bit of one logic level, e.g., logic 1 has been written with the bit area BA being in the expanded condition described in connection with Fig. 4. if a logic 0 were to be written at the bit area BA, then laser beam LB1 would not be actuated and the bit area BA would remain in the relaxed condition shown in Fig. 3. As an alternative to the above-described heating step, in which material 20A slightly absorbs the light of the laser beam LB1, material 20A may absorb no such light. Rather, a small amount of the heat that is absorbed in material 18A is conducted or transferred to material 20A to heat the latter above its glass transition temperature, Tg. As another alternative to the heating step shown in Fig. 5A, in which only the laser beam LB1 is used, the method can include first heating the material 20A above its glass transition temperature, Tg, by generating a laser beam LBn shown in dotted lines and focusing the laser beam Lib2 on the material 20A. The laser beam LB2 is of a wavelength that is substantially absorbed by material 20A. Immediately thereafter, the laser beam LB1 is actuated and focused on the material 18A as described above, with the method for writing the data bit thermally continuing also as described above. One advantage to using the laser beam LB2 initially is to morguickly bring the material 20A to a rubbery condition and, thereby, be able to write a data bit more quickly. Erase Mode Fig. 6A - Fig. 6C disclose the method for erasing a data bit thermally and mechanically. Assume that the bit area RA has a data bit written as illustrated in Fig. 6A, which shows the same expanded condition for the bit area BA as in Fig. 5C in which the material 20A is holding the material 18A in an expanded, stretched condition at ambient temperature. Then, the method includes heating the material 20A to a temperature above the glass transition temperature, Tg, to make the material XOA rubbery. This heating of the material 20A, as shown in Fig. 6A, can be accomplished by generating and focusing the laser beam LB2 onto material 20A. The heating of material 20A causes a relaxation of the holding by the material 20A of the material 18A, thereby allowing the mechanical forces F1 - F4 to rapidly return the bit area BA to the relaxed condition shown in Fig. 5A. In particular, the elastic force F1 in the material 18A is the dominant force causing a quick return of the dual layer 16 to the relaxed condition. Upon this relaxation, as shown in Fig. 6B, the material 20A is pulled back in by force F1, and force F3 and has an intermediary condition forming a pit P. Thereafter, as shown in Fig. 6C, after being totally cooled to ambient temperature, the material 20A and the overall dual layer 16 returns to the fully relaxed condition, as was described in connection with Fig. 3. The dual layer 16, upon cooling, returns to an optically smooth condition in that any peaks and valleys (not shown) within the material 20A are no higher and lower than · wavelength of light. -Moreover, and as indicated above, the dual layer 16 has a relaxation time which is dependent, in part, on forces produced in response to the viscoelastic properties of the material l8A. This relaxation time accelerates upon increasing temperature of the material 20A. Read Mode Fig. 7A - Fig. 7B illustrate a method of reading a data bit optically. Assume that a data bit, for example, a logic 1, has been written at the bit area PA as shown in Fig. 7A in which the bump B has been formed, as described above. To read this data bit, the laser beam LB2, which is at a lower power level than is used for writing or erasing the data bit, is generated and focused on the bump B of the material 20A. Similarly, assume a data bit of logic 0 has been written in the bit area BA, as shown in Fig. 7B, i.e., no bump B is produced. Again, the laser beam R2 at lower power. level than is used for writing or erasing the data bit is generated and focused on the material 20A. In both read instances, the light of laser beam LB2 will be reflected from the material 20A. Due to the difference in thickness or height H between the expanded condition of dual layer 16 shown in Fig. 7A and the relaxed condition of the dual layer 16 shown in Fig. 7B, there is a phase shift between the light of laser beam LBn that is reflected from the material 20A, respectively. This phase shift or difference can he detected with a high signal-to-noise ratio to distinguish a logic 1 from a logic 0 bit. Alternatively, there will be a difference in the light scattering and hence difference in amplitude of the reflected light between the Fig. 7A and Fig. 7B conditions of bit area BA. This difference in amplitude can be detected with a high signal-to-noise ratio as logic 1 and logic 0 bits, respectively. More light scattering and, hence, reduced amplitude will be detected in the Fig. 7A condition than the Fig. 7B condition. Erase/Write Cycle Fig. 8A - Fig. 8B are used to explain an erase/write cycle for erasing one data bit and writing another data bit at one bit area BA. As will be further described, the erase/write cycle occurs rapidly within the same view of an objective lens used to focus laser beam LB1 and LB2 on the material 18A and the material 20A, respectively, as the bit area BA moves across the lens. Assume, as shown in Fig. 8A, that a bit has been written with bit area BA in the expanded condition. The erase/write cycle then includes first heating the material 20A at the bit area BA to erase the bit. The heating can be accomplished by generating and focusing the laser beam R on the material 20A to heat the material above its glass transition temperature, Tg, to a rubbery condition. Then, while the material 20A is still so heated, and before the dual layer 16 has relaxed, another data bit, e.g., a logic 1, is written at the bit area BA in the manner previously described for the Write Mode. This step of writing includes generating and focusing the laser beam LP1 on the material 18A, as shown in Fig. 8R, whereby the elastic expansion previously described occurs, followed by the cooling of the material 20A. However, if a logic 0 were to be written during an erase/write cycle, then after heating the material 20A, as shown in Fig. 8A, the dual layer 16 is allowed to relax and cool as previously described for the Erase Mode. The material 18A and the material 20A preferably are polymers, and particularly, amorphous polymers. Specifically, the material 18A can be, for'example, elastomers having the abovementioned thermoelastic properties and characteristics. The elastomers can include butyl rubbers, silicone rubbers, natural rubbers, ethylene-copolymers, styrene-butadiene rubbers, and a number of other synthetic rubbers. The amorphous polymers of the material 20A are those having the thermosetting or thermoplastic properties with characteristics mentioned above. These may include, for example, cellulose acetate, cellulose acetate-butyrate, polystyrene, polysulfonamide, polycarbonate, cellulose nitrate, hydroabietyl alcohol (sold by Hercules Chemical Company as Abitol AUK 257), poly (ethyl-methacrylate), poly (vinyl butyral), aromatic polyester, polyamides and a wide variety of combinations thereof. Other amorphous polymers that can also be used include, for example, acrylic polymers, polyvinyl acetate, silicone resins, alkyd resins, styrene-butadiene copolymers, vinyl chloride-vinyl acetate copolymers, n-itrocellulose, ethylcellulose, polyvinyl alcohol, gelatin glue, casein, egg albumin, and various other polymers. Examples of dyes or pigments that can be used to tune the material 18A and the material 20A are nigrosine dye, aniline blue, Calco Oil Blue, chrome yellow, ultramine blue, Quinoline Yellow, methylene blue chloride, Monastral Blue, Malachite Green Ozalate, Rose Bengal, Monastral Red, Sudan Black BM, and mixtures thereof. Other examples of dyes include those disclosed in U.S. Patent No. 3,689,768 at column 3, lines 1-22, and U.S. Patent No. 4,336,545 at column 8, lines 53-68, which patents are incorporated herein by reference for these dyes or pigments. Preliminary laboratory fabrication and testing procedures have been performed based on the abovementioned principles of the present invention. For fabrication, an epoxy resin dyed with a red-orange dye manufactured by the Sandoz Corporation, New Jersey, was used for the material 20A. This material 20A was placed on a mandrel, coated with a release agent, which was spun at 3,000 RPM, resulting in a 3 micron thick layer 20A which was then cured. For the material 18A, Dow Corning 734, a clear silicone rubber, having a coefficient of thermal expansion of 920 x 10 6 cm/cm/0 C, was mixed with a blue-green dye manufactured by the Sandoz Corporation. The material 18A was then applied to the material 20A by using a "knife spread" technique. Then, the dual layer 16 was released or taken off the mandrel and mounted on a substrate 14 of aluminum. The fabricated substrate 14 and the dual layer 16 were then mounted in a "single-shot" laser system having a Krypton laser at a wavelength of 647 nanometers. Then, to write a data bit,-the laser was actuated for 50 nanoseconds using an electro-optical crystal, with the laser light then propagated through a shutter, a 25 micron pinhole and a .45 N.A. objective lens which was anti-reflective coated. The laser power was at 820-880 milliwatts, with about 20-30% of this power being provided at the dual layer 16. Bumps B were created in the range of 2.5-5 microns in diameter. It was found that even though the fabricated substrate 14 and the dual layer 16 were not optically smooth and had relatively large pits or valleys resulting from a non-uniform mandrel surface, that the bumps B were also formed in these pits or valleys. This has the advantage of minimizing hard bit errors, since satisfactory bumps B can be formed at bit areas BA that heretofore appeared unusable. Furthermore, it was found that upon erasing the data bits that were so written, as will be described below, the pits or valleys were annealed or smoothed to provide an optically smooth surface. A similar fabrication and testing procedure for writing data bits, as described above, was performed in which the mandrel was spun at 7,500 RPM. This resulted in a thickness of 0.5 microns for the material 20A. Data bits were written with the Krypton laser being at about 150 milliwatts of power and about 4Q-45 milliwatts of this power being at the dual layer 16. Thus, it was found that less power was required for writing data bits at the dual layers 16 of reduced thickness. To erase the data bits written on the dual layer 16, a "single-shot" erase system having an Argon laser at a wavelength of 514 nanometers-w.as used. The laser beam was modulated using an acousto-optical modulator and was focused on bumps B through a microscope system using a 20X objective lens. The laser was operated at 1 microsecond pulse duration times and at 55 milliwatts of power, with about 11-12% of this power being applied at the dual layer 16. The bumps B were erased, as previously described. The data bits that were written on the fabricated substrate 14 and the dual layer 16 were read using white light and a microscope to project images of the bumps B on a glass plate. Then, using a fiber optic bundle hooked to a photomultiplier, the intensity of the light at the glass plate was detected. It was found that a satisfactory contrast ratio of about 2:1 to 3:1 existed between areas of the glass plate having the bumps B and areas of the glass plate having no bumps B. Fig. 9 illustrates one embodiment of a practical electro-optical system 26 for carrying out the optical recording of data on the erasable optical data storage medium 12. In particular, Fig. 9 shows one bit area BA, similar to the bit area BA shown in Fig. 3, for writing, reading and erasing a bit as described above. Electro-optical system 26 includes a digital data processing circuit 28 whose outputs on respective lines 30 and 32 control respective pulsed variable-intensity lasers 34 and 36. Laser 34 outputs the laser beam LB2 and laser 36 outputs the laser beam LB1. A pair of lenses 38 and 40 collimate the laser beams LB2 and LB1, respectively, which are then reflected by a mirror 42 and propagated through a beam splitter 44. The laser beam LB2 and laser beam LB1 are then passed through a filter 46 (e.g. h waveplate) and then propagated through an objective lens 48 which focuses the laser beams on the moving bit area BA. Light reflected by the material 20A of bit area BA is collected by the lens 48 and propagated through filter 46 to the beam splitter 44, where the reflected light is then propagated to a light sensor 50. As illustrated, laser beam LB2 and laser beam LB1 are spatially separated from one another. Also, the laser beam LR is focused onto the bit area BA ahead of the laser beam LB1 relative to the direction of movement of the bit area BA for erase/write purposes to be described below. In the operation of the electro-optical system 26, assume that a bit has been written on the bit area BA, that this bit is to be erased and that another bit is to be written at the bit area BA. This will constitute an erase/write cycle. In describing this cycle, reference should be made to the above disclosure of the erase/write cycle of the present invention. As the bit area BA moves across the view of the objective lens 48, the laser 34 is modulated by the output of the data processing circuit 28 on line 30 to produce a high intensity laser beam LB2. The laser beam LB2 is then focused on the material 20A by lens 48 to heat it to a rubbery condition. Then, while the material 20A is still rubbery, and with the bit area BA continuing to be in the same view of the objective lens 48, the laser 36 is ^immediately modulated by the output of data processing circuit 28 over line 32 to produce the laser beam LB1. Consequently, the laser beam LB1 is focused by the objective lens 48 onto the material 18A to heat it and cause the elastic expansion previously described. Thereafter, the bit area BA will move out of the view of the objective lens 48 and cool, so that either a logic 1 or a logic 0 will be written. In other words, the erase/write cycle can occur within one bit cell time, which means within the same view of the objective lens 48. Assume now that instead of an erase/write cycle only an erase cycle is required, in which the bit written at the bit area BA is to be erased with no other bit written at the bit area BA. As the bit area BA moves within the view of the objective lens 48, the data processing circuit 28, via line 30, modulates the laser 34 to output a high intensity laser beam LB2 which is then focused on the material 20A. Therefore, the material 20A is heated and then cools as the bit area moves outside the view of the objective lens 48. The result is that the bit will be erased in the manner previously described. Assume now that it is desired to read a bit stored at the bit area BA, which bit is either a logic 1 or a logic 0. As the bit area BA moves within the view of the objective lens 48, the data processing circuit 28, via line 30, modulates laser 34 to produce a low intensity laser beam LB2 that is focused on the material 20A. The reflected light is then collected by lens 48 and propagated by filter 48 and beam splitter 44 to sensor 50 which ,then detects the logic state of the bit. As previously mentioned, differences in phase or amplitude of the reflected light can be detected by sensor 50 to distinguish a logic 1 from a logic 0. Fig. 10 is similar to Fig. 9 except that rather than having spatially separated laser beams LB1 and LB2, the two laser beams are concentric or co-linear. The co-linear beams LB1 and LB2 are propagated so that they are focused co-linearly on the material 18A and the material 20A as the bit area BA moves within the view of the objective lens 48. The co-linear laser beams LB1 and LB2 are produced by having laser 34 and laser 36 arranged, as shown, whereby light from each laser is propagated through a pellice beam splitter BS*. The erase/write cycle, the erace cycle and the read cycle are performed in a similar manner in Fig. 10 as described in connection with Fig. 9. The foregoing description of preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modification as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.

Claims: CTjAINS 1. An erasable optical data storage medium, comprising: a) a substrate; and b) a dual layer of a first material and a second material on said substrate, said dual layer being susceptible to expansion and relaxation, to writing data thermally, to erasing data thermally and mechanically, and to reading data optically, said first material and said second material being bonded together, and said first material and said second material, respectively, remaining in the same physical state upon said expansion and said relaxation of said dual layer. 2. An erasable optical data storage medium, according to claim 1, wherein said first material and said second material have mechanical forces, produced in response to expansion of said du#1-Iayer, for relaxing said dual layer. 3. An erasable optical data storage medium, according to claim 2, wherein said first material and said second material, respectively, have cross-linked chains of molecules, and said mechanical forces include forces produced in response to stretching of said cross-linked chains of molecules upon expansion of said dual layer. 4. An erasable optical data storage medium, according to claim 3, wherein said first material is highly cross-linked. 5. An erasable optical data storage medium, according to claim 3, wherein said second material is lightly cross-linked. 6. An erasable optical data storage medium, according to claim 2, wherein said second material has chains of polymer molecules comprising atoms joined by multiple bonds, and said mechanical forces include forces being produced in response to stretching of said chains of polymer molecules upon expansion of said dual layer. 7. An erasable optical data storage medium, according to claim 2, wherein said second material is viscoelastic to allow for a small amount of flow of said second material upon expansion of said second material, and said mechanical forces include forces produced in response to the viscoelastic properties of said second material upon expansion of said second material. 8. An erasable optical data storage medium, according to claim 7, wherein said dual layer has a time for relaxation which is dependent on said forces produced in response to the viscoelastic properties of said second material and wherein said relaxation time accelerates upon increasing temperature of said second material. 9. An erasable optical data storage medium, according to claim 7, wherein said second material is lightly cross-linked to prevent viscous flow of said second material, wherein said dual layer has a time for relaxation which is dependent on said forces produced in resonse to the viscoelastic properties of said second material, and wherein said relaxation time highly accelerates upon increasing temperature of said second material and due to said second material being lightly cross-linked. 10. An erasable optical data storage medium, according to claim 2, wherein said second material has a variable modulus of elasticity produced upon expansion of said second material, said variable modulus of elasticity decreasing with increasing temperature, and wherein said mechanical forces include forces produced in response to said variable modulus of elasticity. 11. An erasable optical data storage medium, according to claim 1, wherein said second material has a glass transition temperature and is in a glassy state at ambient temperature. 12. An erasable optical data storage medium, according to claim 11, wherein said glass transition temperature is higher than said ambient temperature. 13. An erasable optical data storage medium, according to claim 12, wherein said second material is rubbery above said glass transition temperature. 14. An erasable optical data storage medium, according to claim 1, wherein said second material has a low modulus of elasticity. 15. An erasable optical data storage medium, according to claim 14, wherein said low modulus of elasticity is variable and decreases with increasing temperature. 16. An erasable optical data storage medium, according to claim 1, wherein said second material has a low thermal conductivity. 17. An erasable optical data storage medium, according to claim 1, wherein said second material has a low thermal coefficient of expansion. 18. An erasable optical data storage medium, according to claim 1, wherein said first material is rubbery. 19. An erasable optical data storage medium, according to claim 1, wherein said first material has a high thermal coefficient of expansion. 20. An erasable optical data storage medium, according to claim 19, wherein said high thermal coefficient of expansion increases with increasing temperature. 21. An erasable optical data storage medium, according to claim 1, wherein said first material expands and relaxes while being fully in contact with said substrate. 22. An erasable optical data storage medium, according to claim 1, wherein said first material comprises a light-transparent material and a substance to cause said light-transparent material to absorb light at a desired wavelength. 23. An erasable optical data storage medium, according to claim 22, wherein said substance is a dye. 24. An erasable optical data storage medium, according to claim 1, wherein said second material comprises a light-transparent material and a substance to cause said light-transparent material to absorb light at a desired wavelength. 25. An erasable optical data storage medium, according to claim 24, wherein said substance is a dye. 26. An erasable optical data storage medium, according to claim 1, wherein said first material and said second material each comprises a light-transparent material and a substance to cause said light-transparent material to absorb light at a desired wavelength, said first material absorbing light at a different wavelength than said second material and said second material being substantially transparent to said wavelength absorbed by said first material. 27. An erasable optical data storage medium, according to claim 26, wherein said substance of said first material and said substance of said second material are different dyes. 28. An erasable optical data storage medium, according to claim 1, wherein said first material and said second material are optically coupled so that substantially all light of a desired wavelength is propagated through said second material to said first material. 29. An erasable optical data storage medium, according to claim 1, wherein said first material and said second material absorb light at different wavelengths and said second material absorbs substantially all light at the wavelength not absorbed by said first material. 30. An erasable optical data storage medium, according to claim 1, wherein said first¯material and said second material each includes a light-transparent material being susceptible to optical tuning to absorb light at any of a plurality of wavelengths. 31. An erasable optical data storage medium, according to claim 30, wherein said first material and said second material are optically tuned to absorb light at different wavelengths. 32. An erasable optical data storage medium, according to claim 1, wherein said first material and said second material are optically coupled so that substantially all light of a desired first wavelength is propagated through said second material to said first material, wherein said second material absorbs substantially all light of a desired second wavelength different than said desired first wavelength and no light of said desired second wavelength is propagated into said first material, and wherein said first material and said second material each includes a light-transparent material being susceptible to optical tuning to absorb light at any of a plurality of different wavelengths. 33. An erasable optical data storage medium, according to claim 1, wherein said first material and said second material are thermally coupled -so that, upon writing, a small amount of heat absorbed in said first material is transferred to said second material to make said material rubbery. 34. An erasable optical data storage medium, according to claim 1, wherein said first material and said second material are thermally coupled so that, upon erasing, substantially no heat absorbed in said second material is transferred to said first material. 35. An erasable optical data storage medium, according to claim 1, wherein said first material and said second material are thermally coupled so that, upon writing, a large amount of heat is absorbed in said first material and a small amount of heat is absorbed by said second material to make said second material rubbery, and upon erasing, substantially no heat absorbed in said second material is transferred to said first material. 36. An erasable optical data storage medium, according to claim 1, wherein, upon an expanded condition of said dual layer, said second material holds said first material in said expanded condition. 37. An erasable optical data storage medium, according to claim 1, wherein said first material and said second material are amorphous. 38. An erasable optical data storage medium, according to claim 1, wherein said first material and said second material are polymers. 39. An erasable optical data storage medium, according to claim 1, wherein said first material and said second material are amorphous polymers. 40. An erasable optical data storage medium, according to claim 1, wherein said first material and said second material cool at different#rates. 41. An erasable optical data storage medium, according to claim 40, wherein said second material cools faster than said first material. 42. An erasable optical data storage medium, according to claim 1, wherein said second material is a thermoplastic material. 43. An erasable optical data storage medium, according to claim 1, wherein said second material is a thermosetting material. 44. An erasable optical data storage medium, according to claim 1, wherein said first material is an elastomer. 45. An erasable optical data storage medium, according to claim 1, wherein said substrate has a low coefficient of thermal expansion. 46. An erasable optical data storage medium, according to claim 1, wherein said second material has a surface irregularity, and wherein said dual layer is susceptible to writing data thereat regardless of said surface irregularity. 47. An erasable optical data storage medium, according to claim 1, wherein said second material has a surface irregularity, and wherein said surface irregularity is susceptible to annealing in response to erasing data. 48. An erasable optical data storage medium, comprising: a) a substrate; and b) a dual layer of a first material and a second material on said substrate, said dual layer being susceptible to expansion and relaxation, to writing data thermally, to erasing data thermally and mechanically, and to reading data optically, said first material and said second material being bonded together, said second layer, upon an expanded condition of said dual layer, holding said first layer in the expanded condition. 49. An erasable optical data storage medium, according to claim 48, wherein said first material is elastic and is stretched in said expanded condition. 50. An erasable optical data storage medium, according to claim 48, wherein, upon heating said second material to erase said data, said first layer assists said dual layer to relax. 51. An erasable optical data storage medium, according to claim 50, wherein said first material is rubbery below ambient temperature. 52. An erasable optical data storage medium, according to claim 51, wherein said second material has a glass transition temperature above ambient temperature and is rubbery above said glass transition temperature. 53. An erasable optical data storage medium, comprising: a) a substrate; and b) a dual layer of a first material and a second material on said substrate, said dual layer being susceptible to expansion and relaxation, to writing data thermally, to erasing data thermally and mechanically, and to reading data optically, said first material and said second material, respectively, remaining in the same physical state upon said expansion and said relaxation of said dual layer. 54. An erasable optical data storage medium, according to claim 53, wherein said first material and said second material are amorphous polymers. 55. A method of writing data on an erasable optical data storage medium having a substrate and a dual layer on the substrate of a first material and a second material bonded together, said first material being elastic and said second material having a glass transition temperature, comprising: a) heating the first material to elastically expand the first material within the elastic limit of the first layer, a small amount of heat also heating the second material above the glass transition temperature to make the second material rubbery; b) allowing the heated fir-st--material to expand and push up the heated second material as the heated first material expands elastically; and c) cooling the expanded second material to below the glass transition temperature while the first material is in a stretched, expanded condition, the cooled second material then having a reversibly fixed deformation and holding the first material in the stretched, expanded condition. 56. A method, according to claim 55, wherein the step of heating comprises: a) generating a laser beam being substantially transparent to the second material and absorptive by the first material; and b) focusing the laser beam through the second material onto the first material. 57. A method, according to claim 55, further comprising erasing the reversibly fixed deformation. 58. A method, according to claim 57, wherein the step of erasing comprises: a) heating the second material above its the glass transition temperature to a rubbery condition; and b) allowing the heated second material to relax from its deformed state; and c) allowing the expanded first material to contract and assist the heated second material to reform its relaxed state. 59. A method, according to claim #e, wherein the second material is viscous and lightly cross-linked to prevent viscous flow, and the step of allowing the heated second material to relax includes allowing elastic forces of the expanded first material and forces due to the stretched cross-linking of the expanded second material to pull down the second material to its relaxed state. 60. A method, according to claim 58, wherein the step of heating the second material comprises: a) generating another laser beam for absorption by the second material; and b) focusing the other laser beam on the second material. 61. A method of writing data on an erasable optical data storage medium having a substrate and a dual layer on the substrate of a first material and a second material bonded together, said first material being elastic and said second material having a glass transition temperature above ambient temperature, comprising: a) heating the second material above the glass transition temperature to cause the second material to become rubbery; b) heating the first material to elastically expand the first material within the elastic limit of the first material; c) allowing the heated first material to expand and push up the heated second material as the heated first material expands elastically; and d) cooling the expanded second material below the glass transition temperature - while the first material is in a stretched, expanded condition, the cooled second material then having a reversibly fixed deformation and holding the first material in the stretched, expanded condition. 62. A method, according to claim 61, wherein the step of heating the second material comprises: a) generating a laser beam for absorption by said second material; and b) focusing the laser beam on the second material. 63. A method, according to claim 62, wherein the step of heating the first material comprises: a) generating another laser beam being substantially transparent to the second material and absorptive by the first material; and b) focusing the other laser beam through the second material and onto the first material. 64. A method, according to claim 61, wherein the steps of heating the first material and the second material include: a) generating a laser beam for a substantial amount of absorption by the first material and for a small amount of absorption by the second material; and b) focusing the laser beam on the first material. 65. A method, according to claim 61, wherein the steps of heating the first material and the second material include coupling a small-amount of heat in the first material into the second material. 66. A method of recording data on an erasable optical data storage medium having a substrate and a dual layer of a first material and a second material on the substrate, said dual layer corresponding to one data bit stored at one location on the erasable optical data storage medium, comprising the steps of: a) erasing the one data bit stored at the one location on the erasable optical data storage medium, including heating the second material; and b) writing another data bit at the one location on the erasable optical data storage medium while the second material has not yet cooled. 67. A method of recording data, according to claim 66, wherein the step of writing comprises heating the first material to expand the first material onto the second material. 68. A method of recording data, according to claim 67, wherein the step of heating the second material comprises: a) generating a first laser beam; and b) focusing the first laser beam on the second material through an objective lens. 69. A method of recording data, according to claim 68, wherein the step of heating the first material comprises: a) generating a second laser beam; and b) focusing the second laser beam on the first material through the objective lens. 70. A method of recording data, according to claim 69, wherein the erasing of the one data bit and the writing of the other data bit occurs within the same view of the objective lens. 71. A method of recording data on an erasable optical data storage medium having a substrate and a dual layer of a first material and a second material on the substrate, the dual layer being susceptible to expansion and relaxation, to writing data thermally, to erasing data thermally and mechanically, and to reading data optically, the first material and the second material being bonded together, the first material and the second material, respectively, remaining in the same physical state upon the expansion and relaxation of the dual layer, and the expanded or relaxed condition of the dual layer representing one data bit stored at one location of the erasable optical data storage medium, comprising the steps of: a) first erasing the one data bit stored at the one location on the erasable optical data storage medium, including: : i) generating a first laser beam; ii) focusing the first laser beam through an objective lens onto the second material; and iii) heating the second material with the focused first laser beam to a rubbery condition; and b) then writing another data bit stored at the one location on the erasable optical data storage medium, including: i) generating a second laser beam, ii) focusing the second laser beam through the objective lens and the second material onto the first material during the same view by the objective lens of the second material, iii) heating the first material to elastically expand the first material on the second material, with the first material remaining in contact with the substrate; and iv) cooling the second material to form a reversibly fixed deformation in which the second material holds the first material in an expanded condition. 72. A system for recording a data bit, comprising: a) an erasable optical data storage medium, including i) a substrate having a low coefficient of thermal expansion; and ii) a dual layer of a first material and a second material on said substrate, said dual layer being susceptible to expansion and relaxation, to writing data thermally, to erasing data thermally and mechanically, and to reading data optically, said first material and said second material being bonded together, and said first material and said second material, respectively, remaining in the same physical state upon said expansion a-nd said relaxation of said dual layer; ; b) means for generating a first laser beam and a second laser beam, said first material being absorptive of, and said second material being substantially transparent to, said first laser beam, and said second material being absorptive of said second laser beam; c) means for focusing said first laser beam on said first material to heat and elastically expand said first material onto said second material to write the data bit; d) means for focusing said second laser beam on said second material to heat said second material and to erase the data bit; and e) means for moving said erasable optical data storage medium relative to said first laser beam and said second laser beam. 73. A system for recording a data bit, according to claim 72, wherein said means for focusing said first laser beam and said means for focusing said second laser beam comprise a common objective lens. 74. A system for recording a data bit, according to claim 72, wherein said first laser beam and said second laser beam are spatially separated from one another. 75. A system for recording data, according to claim 72, wherein said first laser beam and said second laser beam are colinear.

Inventors: FEYRER, CLYDE DEWAIN, GORDON, NORMAN ROSS, SKIENS, W. EUGENE
Applicants: OPTICAL DATA, INC.
Application year: 1984
Main classification: G11B7
IPC codes: G11B7, B41M5