REAL-TIME OPTICAL DISC AND METHOD FOR RECORD AND READOUT

Abstract: Real-time optical disc having a heat-deformable recording layer (33) which, by virtue of its thickness and optical properties, produces a readout signal of high signal-to-noise ratio. The optical disc of the invention comprises a recording layer (33) which is highly transmissive to light of a reading wavelength and which exhibits, with respect to light of the reading wavelength, a relative phase shift (//c) to thickness (t) curve characterized by a region of relatively steep slope followed by a region of relatively gradual slope. The recording layer thickness is selected to be within or proximate a range corresponding to a zone on such curve where there is a transition between steep and gradual slopes; thus, decreases in recording layer thickness (as produced by the recording of pits in the recording layer) produce relatively large changes in phase of a reading light incident thereon, and increases in recording layer thickness (as produced, for example, by optical debris and crater rims) produce relatively small changes in phase of such reading light. Preferably, the thickness and optical properties of the recording layer are also optimized to be highly absorbant to light of a different wavelength (i. e. the wavelength of light used to record information on the recording layer).

Description: REAL-TIME OPTICAL DISC AND METHOD FOR RECORD AND READOUT This invention relates to improvements in "real-time" optical discs of the heat deformable variety, and to methods for recording and reading-out information on such discs. In general, an optical disc is a high den¬ sity optical storage medium which stores information in the form of spiral or circular tracks of small (e.g. about one micron) optically detectable marks (e.g. pits) formed in the surface of a thin recording layer. The size and/or spatial frequency of these marks along the track are representative of the stored information. A "real-time" optical disc refers to that kind of optical disc which is adapted to have infor¬ mation read out therefrom immediately after such information is written or recorded thereon. That is, in contrast with so-called "processed" discs, no chemicals and/or physical processing of the disc (after recording) is required to render it readable. Real-time optical discs of the prior art typically comprise a very thin (e.g. 50 nm.) heat- deformable recording layer (e.g. a vapor deposited metallic film which overlies a reflective surface. To write (i.e. record) information on such a disc, a focused laser beam is scanned across the disc surface to produce, in the exposed areas, a spiral or circular track. The optical properties of the -disc's recording layer are chosen so as to a,bsprb the laser energy. As the laser beam scans the disc surface, the beam intensity is modulated in accordance with information to be recorded. When the intensity of the beam is high, the recording layer absorbs suffi¬ cient thermal energy to cause a "pit" to be formed (by a melting or ablation process) in the recording layer. Ideally, the depth of each pit extends down ■to the disc's reflective surface. As mentioned above, it is the spatial frequency of these pits along the spiral or circular track that is rep¬ resentative of the recorded information. In order to recover (i.e. read-out) the recorded information, a continuous-wave (cw) laser beam (typically emanating the same laser used to record information) is scanned along a track, and the energy reflected by the disc is monitored by a far- field photodetector. Where no pit is formed, the layer absorbs the laser beam energy and the output of the photodetector is low. Where a pit is formed, the laser beam is highly reflected by the underlying reflective surface, and the output of the photo¬ detector is high. Thus, the photodetector's output will be indicative of the size and/or the spatial frequency of the pits. This technique of reading out the recorded information is known as the "amplitude- variation" approach since the photodetector monitors variations in the amplitude of the reflected reading laser beam. In using the amplitude-variation approach for reading-out information stored in real-time opti¬ cal discs of the type described, it is difficult to achieve a high signal-to-noise ratio in the signal provided by the photodetector. In reading-out infor¬ mation stored in real-time optical discs of the type described above, a number of technical problems can be encountered. For example, it is difficult to achieve deeply modulated (i.e. high contrast) readout signals using the above-mentioned amplitude variation approach because of the surface reflectivity of the unrecorded (i.e. non-pitted) areas of the disc. Since these unrecorded areas unavoidably reflect a portion of the reading light to the photodetector, the reference signal level they provide (i.e. the level against which the pit-producing signal is compared) is increased, thereby decreasing the con- trast between these signals. Further, since the readout light beam is of the same wavelength as the recording light beam, the intensity of the readout beam must be substantially lower than that of the recording light beam to avoid erasure of the recorded information. The necessity of using a relatively low power readout beam, of course, has a limiting effect on the signal-to-noise ratio of the photodetector's output signal. An object of the invention, therefore, is to provide a real time optical disc which, by virtue of its structure and optical properties, provides a -read-out signal of high signal-to-noise ratio. According to one aspect of the present invention, there is provided an improved real time optical disc structure which, with respect to a chosen reading light wavelength, has a recording layer that is optimized in thickness and in optical properties (e.g., refractive index and absorption parameters) for high-contrast, phase-shift reading. This recording layer is disposed on a reflective sur¬ face and comprises a heat-deformable material (pre¬ ferably an organic, dye-binder material) that is highly transmissive to light of the reading wave¬ length and relatively absorptive to light of a dif- ferent wavelength used to record information on the layer. It has been found that information (i.e. pits) recorded on such a disc structure will impart substantial shifts in the relative phase of reading light reflected therefrom, which phase shifts, in turn, give rise to readout signals of improved signal-to-noise ratio. Further, it has been observed that the relative phase shift φ does not vary linearly with recording layer thickness t; rather the > vs. t curve exhibits distinct zones of rapid and gradual (near zero) slope. According to the inven- tion, the thickness of the recording layer is chosen to correspond to a shoulder portion on the φ vs. t curve; i.e. a portion at which there is a transition from a relatively steep positive slope to a rela¬ tively gradual slope. At such a thickness, decreases in recording layer thickness, as produced by the recorded pits, produced relatively large changes in phase of the reading light reflected to the photo¬ detector, and increases in recording layer-thickness , as produced by optical debris and crater rims (pro- duced during the recording process), produce rela¬ tively small phase changes in the reading light reflected to the photodetector. Thus, the optical disc of the invention is optimized to maximize the phase shifts produced by the recorded pits and to minimize the phase shifts produced by noise-producing debris and crater rims. Further, since the recording layer is substantially transparent to light of the reading wavelength, a light beam of relatively high power can be used for readout. in addition to optimizing the recording layer thickness and optical properties for light of one wavelength (i.e. the wavelength of the light used for readout) it is preferred that the spectral ab¬ sorption and thickness of the recording layer are also chosen so as to enhance the coupling of light of another wavelength (i.e., the wavelength of the record light) into the recording layer. According to another aspect of the inven¬ tion, there is provided a method for real-time writing and reading of information on an optical disc of the type provided by the invention. This method comprises the steps of scanning the recording layer surface with a first beam of radiation having a wave¬ length at which the recording layer is highly absorp¬ tive, such beam having an intensity which is odu- lated in accordance with information to be recorded, whereby the thickness of the recording layer is selectively decreased at distinct zones; scanning the scanned portions of the recording layer with a second beam of radiation having a wavelength at which the recording layer is highly transmissive; and sensing the light of the second wavelength reflected from the recording element surface with a far-field detector adapted to detect, and produce an output signal indicative of phase changes in the light incident thereon. The following description of preferred embodiments of the present invention is set forth with reference to the attached drawings which form a part hereof and in which: Fig. 1-A is a cross-sectional view of a prior art optical disc structure and Fig. 1-C is a similar view of an optical disc according to one embodiment of the present invention; Fig. 2 is a graph illustrating the variation with respect to recording layer thickness of the reflec¬ tance of writing light for an element such as shown in Fig. 1-A; Fig. 3 is a graph illustrating the variation with respect to recording layer thickness of the reflectance and recording layer absorptance of writing light for an element such as in Fig. 1-B; Fig. is a graph illustrating the variation with respect to recording layer thickness of the phase and amplitude of reflected reading light for an element such as shown in Fig. 1-B; Fig. 5 is a graph superimposing the writing light absorption curve of Fig. 3 and the reading light phase shift curve of Fig. 4; Fig. 6 is a graph illustrating the variation with respect to recording layer thickness of the phase of reflected reading light (like Fig. 4) for different refractive index recording materials for an element such as shown in Fig. 1-B; Fig. 7 is a schematic diagram showing an apparatus embodiment for practice of the present invention; Fig. 8 is a graph indicating pit depth per constant phase shift Dπ/2 and layer absorptance within that pit depth Aπ/2. Superimposed on this graph are dotted line representations of a relative phase shift curve and of a quotient curve Dπ/2÷Aπ/2; Fig. 9 is a graph showing curves like Fig. 5 but of Example I and II recording layer materials; and. Fig. 10 is a graph showing the read-out signal-to noise-ratio of the Example I and II elements when recorded at varying writing power levels. In Fig. 1-A an example of a prior art real- time record element adapted for amplitude-variation read out is illustrated. In Fig. 1-A, the record element typically has a thin light absorptive layer 2 overlying a reflective layer 3 on a support 4. The layer 2 can be formed by vapor depositing a dye that is absorptive to the writing wavelength, e.g., to a thickness in the order of 200 to 1000^Angstroms (A). Focused writing light having high irradiance is used to ablate the layer to form the pit 5, thereby reducing the thickness t of layer 2 to a depth d and increasing the reflectance of the recorded pit area. Upon read-out, with light of the εame wavelength but at reduced intensity, the reading light spot is highly reflected from the pit areas but -absorbed in non-pit areas. This variation in the intensity of the light reflected from the recorded disc is sensed by a far-field photodetector. Amplitude-variation read-out can also be obtained with monolayer discs (constructed with a thin, e.g., 200 to 400 A, metal coating on a substrate) and with trilayer discs (constructed like the bilayer example shown in Fig. 1-A, but with a transparent dielectric layer between the absorbing layer and the reflective layer). In such prior art amplitude- variation systems, any phase differences in the reading light that returns to the detector from pit- ted and non-pitted (i.e., recorded and non-recorded) portions is neglected. There are numerous variations of the real¬ time reflective discs exemplified by the Fig. 1-A elements. Most implementations of the Fig. 1-A approach share the characteristic of being recordable in real-time with high power light of an absorbed wavelength and then being readable with light of the same wavelength at a reduced power level. The recording layers of these discs are very thin, the thickness of the recording layer being substantially equal to the depth of the recorded pits. This assures that the pit areas will have a substantially different (i.e. much higher) reflectance than non- recorded areas. In order to provide high depth of modulation in reading light passing to the detector, and thus in the read-out signals, the^seselements require a read-out spot that is both tightly focused to a size proximate that of the pit and accurately controlled to pass directly over the pit centers. As mentioned, these systems require that the read-out light power not be so high' as to melt, ablate or "BUREA * _OMPI bleach the non-recorded portions of the recording layer. Referring now to Fig. 1-B, one embodiment of the recording element of the present invention will 5. be described. It will be noted that the record element 30, like that in Fig. 1-A, has its reflective layer 31 on its support 32, beneath its recording layer 33. However, the thickness "t" of the recording layer 33 is greater than that utilized in 10 the Fig. 1-A disc types. A pit 34 of depth "d" is shown recorded in the layer 33. In general, the preferred approach of the present invention is to effect real-time writing of pits 34 with a beam of a first, writing wavelength of 5 light to which the recording layer is absorptive and to effect read-out of the record element thus recorded with a beam of a second, read wavelength of light to which the recording layer is substantially transparent, i.e., highly non-absorptive. The read- 0 out light beam responds to the variations in reflec¬ tion phase caused by the different thickness of the recording layer in pitted and non-pitted areas. The amplitude of the reflected read-out light also is affected to a small extent by the recording layer 5 thickness variations, but this effect on read-out signal is small in relation to the effect associated with reflected light phase changes. It would appear that this outlined approach would require relatively precise formation (as to 0 relative thickness) of the pitted and non-pitted regions, in order to obtain substantially constant phase shift differences between the recorded and non- -recorded zones. However, we have found there is a substantial pit depth tolerance that will yield good 5 signal contrast. As will be described in more detail below, we have found that the proper design of the recording layer thickness and its optical constants, in conjunction with the write and read approach of .our invention, (1) allows increased writing sensiti¬ vity, (2) provides a degree of immunity to potential "frozen-in" noise sources such as pit rims or trans¬ parent debris on the layer surface and (3) provides a high depth of modulation read-out signal for a far- field photodetector. Thus, a highly desirable, real¬ time, optical disc system is provided. As mentioned, one preferred approach involves predeterminedly designing the record element 30 for a high sensitivity writing with a first light wavelength λ and for high contrast reading with a different light wavelength λ . Such design involves the proper selection of recording stratum thickness, refractive index and absorption in connec¬ tion with the chosen reading and in certain preferred embodiments, additionally the writing wavelength. The principles underlying such selections can be explained more simply by first considering the write and read mechanisms separately, in more detail, and then explaining how a single element can be designed to achieve excellent results for both the writing and reading operations. However, some explanation of terminology is appropriate first. The term "optical constants" is used herein with reference to the recording stratum to collectively denote both the real component (i.e., the real refractive index) and the imaginary component (i.e., the extinction coef- ficient or absorption parameter) of the complex refractive index. For convenience thξ>se two compo¬ nents will in some instances hereinafter be referred to respectively as the refractive index (which affects surface reflectance of a material and the speed of light therein) and the absorption parameter (which characterizes the extent to which light of a particular wavelength is coupled into a material). The "absorption factor" of a recording layer as used herein is defined as the product of the weight per¬ cent of dye in the layer and the extinction coef- : ficient of the dye at the writing light wavelength divided by the molecular weight of the dye. The word "light," as used herein, is used in its broad sense to mean both visible (i.e. 4000-7600 A) and invisi¬ ble (e.g. ultraviolet and infrared) electromagnetic radiant energy. Considering the writing operation in more detail, in certain materials good writing sensitivity requires that writing energy be effectively coupled into a recording layer (from the viewpoint of power efficiency in forming pits). An excellent discussion of how the coupling of writing energy into the recording layer can be enhanced by proper selection of the thickness of the recording layer is presented by A. E. Bell and F. W. Spong in the July, 1978 issue of IEEE Journal of Quantum Electronics at page 487. In general this article explains that optimal write situations (i.e., high absorption of writing light within a recording layer such as layer 2 in Fig. 1-A) exist when strong destructive interference, i.e., a substantial antireflection condition, exists between writing light reflected back through layer 2 from the surface 3 and light initially incident on, and reflected from, the surface of layer 2. See Fig. 1-A. This condition can be obtained by designing the record layer with its optical thickness about one- fourth the wavelength of the recordiηg ,ligbt in that layer, and by selecting the absorption parameter of the layer 2 so that the amplitude of the internally reflected light that traverses layer 2 approximately equals that of the light reflected from the surface of layer 2. The graph of Fig. 2 illustrates how the net reflectance R of record light from an element such as shown in Fig. 1-A varies as a function of recording layer thickness t. The above article by Bell and Spong further expains that this anti- reflection thickness also provides a good signal- to-noise ratio read-out for such a bilayer system, if light of the same wavelength as the writing beam is used for read-out and if the pit depth formed in recording is such as to disrupt the antireflection condition. (As indicated above, the preferred pit depth condition removes all light absorbing material from above the reflective layer). Note, however, that read-out light intensity must be lowered to avoid affecting the non-recorded areas during read- out. Also, the read spot's tracking and size must be carefully controlled. Thus, the element described in this article is an optimized Fig. 1-A type. Certain elements such as shown in Fig. 1-B, having a thicker recording layer, will exhibit a similar variation of net reflectance with respect to recording layer thickness. Fig. 3 provides an exem¬ plary plot of such reflectance variation (curve R) , for a Fig. 1-B type element having an absorption parameter selected to yield relatively strong reflec- tion minima. There is superimposed, using the same thickness abscissa, a plot of the variation of the recording layer's writing-light-absorption with respect to thickness (curve A). It will be noted that besides the first of the reflection minima (which occurs at a recording layer thickness of λ /4n ) , the curve R has a second reflection w w ^ , minima (which occurs at a recording layer thickness of about 3λ /4n ; λ being the write light wavelength and n being the real refractive index of the recording layer with respect to the writing light wavelength) . As would be expected the absorp- tion to thickness maxima, i.e., the maxima of the absorption curve A occur at thicknesses which yield minima of the reflectance curve R. However, one aspect of the absorption curve is not so expected. ■ That is, even though the first reflectance minimum is deeper than the second, the second absorption maximum is stronger than the first absorption maximum. This indicates that, although less light escapes the thinner element (having a thickness producing the first net reflectance minimum), more light is coupled into the thicker element (having a thickness produ¬ cing the second net reflectance minimum). Viewed in retrospect this apparent anomaly can be attributed to the fact that with a thinner element more light interacts with (and is absorbed by) the reflective layer, such as layer 31, than with a thicker element. In this regard it should be noted that curve A represents absorption of only the recording layer, not of the entire recording element-. Based on the foregoing, it can be seen that for an element such as shown in Fig. 1-B, operation at or near the second absorption maximum (second reflection minimum) thickness can be at least equally advantageous from the viewpoint of recording efficiency as operation at or near the first absorption maximum. It .will be understood that such a point of operation is substan¬ tially aligned in thickness with the second reflec¬ tion minimum of a recording stratum such as 33 in Fig. 1-B, i.e., at a thickness yielding antireflec- tion from the record element of Fig. 1-B. v. Now let us consider in more "detail the read¬ out mechanism of an element such as shown in Fig. 1-B, utilizing a light wavelength to which the recording layer is substantially transparent. We have found that, for certain recording element struc¬ tures of the type shown in Fig. 1-B, the relative phase shift φ imparted to light reflected by the recording element does not vary linearly with respect .to variations in recording layer thickness, but instead exhibits distinct zones characterized respectively by more rapid and more gradual slope, i.e., wherein a given thickness variation causes relatively high magnitude and relatively low magni¬ tude phase shift changes to reading light reflected from the element. Fig. 4, which is a plot of the variation of the relative phase shift φ with respect to recording layer thickness t, illustrates such a characteristic. This non-linear dependance of the phase-shift on thickness (i.e., the elements' non-linear phase shift to thickness relation) occurs because light reflected from the element is a combi¬ nation of at least two significant constituents, which can differ in phase, namely: (1) the light that is reflected directly from the surface of recording layer 33 and (2) the light reflected from layer 31 that traverses through layer 33. Referring to Figs. 1-B and 4 and considering the passage of a recorded track of the Fig. 1-B recording layer beneath a read spot, it will be appreciated that the combined or net phase of light reflected to a far-field photodetector from a pitted (recorded) area such as 34 will be different than the net phase of light reflected to such photodetector from a non-pitted (non-recorded) area, because the recording layer thickness is different for recorded and non-recorded areas. The extent of difference in net phase between the light from thes,e fwo areas depends upon the original layer thickness and the depth of the pit. Thus, referring to the Fig. 4 graph it can be seen that if the nominal recording layer thickness were selected at a point proximate the shoulder portion of the phase shift to thickness curve (the curve portion wherein there is a transi¬ tion from a zone of steep slope to a zone of gradual, or near zero slope) a phase shift imparted by the recording element would increase relatively rapidly in magnitude, with small decreases in layer thickness (e.g., 30 nm) compared to the phase shift imparted by similar thickness changes commencing from a nominal thickness further to the right on the curve. For example, if the point of operation (nominal recording layer thickness) were selected at 125 nm, forming a pit having a depth of about 40 nm would cause a phase shift of about π/2 or about 90°. Such a point of operation (i.e., a nominal recording stratum thick¬ ness cooresponding to a point proximate such a shoulder portion of the relative phase shift to thickness variation curve) is thus desirable from the viewpoint of providing decreases such as caused by pit formation. An additional attribute of operating at a thickness proximate such a point is that beyond this point (i.e., to the right, toward greater thick¬ ness) , a relatively flat portion follows for a sub¬ stantial amount increased thickness. This character¬ istic of the recording layer minimizes the subscepti- bility of the read out detection system to phase shift caused by pit rims or transparent debris on the recording layer surface, i.e., renders the recording element relatively insensitive to increases in thick¬ ness of the type mentioned. Thus, in one aspect, it is a significant feature of the present invention that the recording element be designed with nominal X * ". thickness and predetermined optical constants rela¬ tive to the reading light so that such advantages are provided. Now let us refer to Fig. 5 and note the synergistic results for writing and reading which can be obtained when recording layer thickness and opti- cal constants are appropriately selected. In Fig. 5 the writing light absorption to thickness curve A of -Fig. 3 and reading light relative phase shift to thickness curve φ of Fig. 4 have been superimposed with the common abscissa "layer thickness" t. It is another significant aspect of the present invention to select a recording layer composition and configu¬ ration, in regard to predetermined write and read light wavelengths, so that an operative thickness point for writing light, proximate or near an absorp¬ tion maximum (e.g. M~) on its absorption versus thickness curve (such as Fig. 3), will be generally aligned as to its thickness locus with an operative thickness point P for reading light, proximate or near the shoulder portion of its phase shift versus layer thickness curve (such as Fig. 4). When the recording layer material is designed with such opera¬ ting points having generally corresponding thickness (and the recording element is formed at that nominal thickness) , the following complimenting advantages evolve. First, the recording element has an excep¬ tional sensitivity. This occurs to a significant extent because the pit depth variation required to produce a substantial change in phase at the photo¬ detector is relatively small. Thus, the intensity of the record beam need not be as great as in those prior art systems which exhibit a linear relative phase shift to thickness characteristic. Note in Fig. 5 that within the operative thickness variation range (which includes the highly slo n^ curve por¬ tion to the left of such operating point P and the flat or gradually sloping curve portion to the right of that point) pit depth increases that decrease the layer thickness from the nominal thickness (i.e. from point P) will effect rapid 1 change in phase that will be imparted to the read-out light. In addition, however, sensitivity is enhanced because the record¬ ing energy is efficiently coupled in the recording layer (since the recording layer absorption at such a • nominal thickness is near the second absorption maxi¬ Second, because the predetermined nominal thickness is near the shoulder portion, (i.e., at the left side of the flat zone portion) of the phase shift to thickness variation curve, any transparent debris on the recording layer and any rim formation around recorded pits (which add overall layer thick¬ ness within the operative thickness variation range) will have minimal effect upon the phase of light from the unrecorded portions of the recording layer. Further, since pit rims are virtually invisible, the range of spatial frequencies that can be recorded is increased (because recorded pits are " optically defined by their inside diameter only and not by sur- rounding crater rims). Third, because the recording layer is sub¬ stantially transparent to the read light there is no danger of adversely affecting the layer during read out. Thus the read light power can be of a magnitude that is greater than in other real time systems, e.g. , of a power level similar to the recording power level. This moves the obtainable signal-to-noise ratio (S/N) for the system closer to a photon shot noise limit (i.e., amplifier and similar noise sources can be more readily overcome). Fourth, because the recordi g layer facili¬ tates phase shift, as opposed to amplitude variation read out, it enables the use of larger read out scan¬ ning spot sizes and less critical tracking and focusing control during playback. There is a cost incident to some of the advantages outlined above, viz., the use of two dif¬ ferent wavelength lasers. However, for many applica¬ tions the advantages of this real time writing, phase - shift reading system will more than warrent such ad¬ ditional cost. Further, in certain embodiments certain advantageous aspects of the present invention can be retained in a system using the same wavelength light for writing and reading. From the foregoing explanations, it will be understood that the present invention contemplates record elements having predetermined nominal thick¬ ness and optical constants that are optimized with respect to read light and to both read and write light respectively. Specifically, the elements' optical constants are optimized with respect to the chosen reading light wavelength so that decreases from said predetermined nominal thickness cause rela¬ tively high magnitude phase shifts of reflected read light and increases from said predetermined nominal thickness cause relatively low magnitude phase shifts of reflected read light. Further, the elements' optical constants can be optimized with respect to the chosen writing light wavelength so that a record- ing layer absorption to thickness maximum exists near the predetermined nominal recording stratum thick¬ ness. Approaches for designing such elements will be explained in more detail subsequently; however, a description of some useful and preferred record ele- ment materials is appropriate first. There are a wide variety of 'materials suitable for the supports and recording stratum of recording elements of the present invention. Impor¬ tant support characteristics are that it have a melt- ing point that avoids deformation and that it have a smooth reflective surface. Materials such as reflec- ' ■ gVjREAtT _ MPL- tive metals or glass, resin films such as poly¬ ethylene terephthalate coated with thin metal layers, .e.g. 500 A of aluminum, are exemplary, but there are many others. Although there are other recording layer materials that can be used in implementing element design according to the present invention, we have found dye-binder material to be particularly suitable. A number of useful binders and dyes are dis- closed in U.S. Patent Application Serial No. 23,434, filed March 23, 1979. That disclosure is incorpor¬ ated herein for its teachings of such binders and dyes as well as for teachings of other examples of other element structures such as supports. In general it is preferred according to the present in¬ vention that the recording layer be an amorphous material. Thus the mixture of binder and absorptive material (e.g., the dye which absorbs at the writing wavelength) desirably are compatible to mix in such a way that no crystalline structure is formed. Useful binders include for example cellulose acetate butyrate, polystyrene, polysulfonamide, Lexas (polycarbonate), cellulose nitrate, hydroabietyl alcohol (sold by Hercules Chemical Company as Abitol AUK 257), poly(ethyl methacrylate), poly(vinyl butyral) , and combinations thereof. In some in¬ stances it may be useful to include a plasticizer in the mixture to achieve desired deformation capabili¬ ties. It is highly desirable that the selected recording layer be soluble in a solvent (e.g., -such as cyclohexanone, acetone, benzene xylene or the like) to facilitate coating. There are a wide variety of useful dyes that can be considered for their absorptivity at the writing wavelength and compatibility with the binder of choice. A number of such dyes that are useful for ' ■ gϋREAt; 0MP _ consideration when writing with an argon-ion laser (wavelength = 488 nm) and reading with a helium-neon laser (wavelength - 633 nm) are described in the above-cited application, such disclosure being incor- porated herein by reference. Additional useful light absorber-binder materials are disclosed in U.S. Patent Application Serial No. 001,519, filed January 8, 1979 which is also incorporated herein by refer¬ ence for that purpose. in designing optimized recording elements according to the present invention, certain system parameters can be determined by general apparatus and materials characteristics. For example, the writing and reading light wavelengths could be selected on the basis of commercially available laser apparatus. The components of the recording layer's dye-binder mixture could be selected for their known optical characteristics relative to the chosen wavelengths (i.e., substantially transparent to the read wave- length and at least moderately absorptive to the write wavelength) and for their known physical characteristics relative to the physicsl dynamics of the desired write-read system (e.g. , the glass tran¬ sition temperature, viscosity, heat capacity, thermal diffisuvity and thermal decomposition temperature of the dye-binder combination relative to the laser power, optics and recording speed desired for the write-read system). Also, it will be helpful to con¬ sider Fig. 6 in regard to selection of a preliminary dye-to-binder mixture. That figure illustrates that the general steepness (indicated by dotted straight lines) of relative phase shift to thickness curves increases with increased refractive index of the recording layer. Thus from the viewpoint of record sensitivity (with respect to read light) , higher refractive index material combinations will be advan- IJUREATT J3MPI tageous, i.e., as the refractive index increases, the pit depth required to achieve a given phase shift decreases. However, it will be noted that the steeper the curve, the shorter is the flat portion following the shoulder. Therefore, it is usually desirable to accept some compromise as to the curve steepness in order to maintain the tolerance to crater rims and transparent debris that is provided by extended flat portions of the curve. After such consideration a dye-binder mixture is prepared in a preliminary concentration, e.g., a 1:1 ratio by weight and coated in a desired manner. As pointed out in the aforementioned Application No. 23,434, it is usually preferred that the recording layer have an absorption factor of 20. or greater with respect to the selected writing light wavelength. The refractive index of this preliminary mixture is measured at the reading light wavelength and based on this measurement, a curve (such as the one shown in Fig. 4) indicating relative phase shift with varying layer thickness is plotted. It will be understood by those skilled in the art that such curves can be calculated without the necessity of measuring a large number of different-thickness coatings. Having plotted the relative phase shift to thickness curve for a preliminary dye-binder mixture, the selection of a preliminary nominal thickness is to be made. As discussed, it is desired that the final nominal thickness be within, or proximate, a thickness zone that corresponds to the ^houlder por¬ tion of the phase shift curve (i.e., its transition zone from a generally flat to much steeper slope) . The preliminary nominal thickness is selected accord- ing to this criterion, for example see point P on the curve φ of Fig. 5 which defines a preliminary -gυ EAt OMPI thickness t' In certain instances selection based on such criteria will be adequate to provide .highly useful elements. However, in certain instances it will be desirable to further optimize design by improving recording efficiency. Thus given such a selected preliminary nominal thickness t'n and based on the understanding that it is desired to have a maximum, e.g., the second absorption maximum 2 , of the recording layer's absorption to thickness curve in general alignment with t' (and thus point P of curve φ) , the following relation can be stated: where λ W is the writing light wavelength and nW is the refractive index of the recording layer (with respect to writing light) that would cause the desired alignment to exist. (Recall the second ab¬ sorption maximum for a recording layer having index n r w τ will occur at a thickness very J near the point 3λ w .) Thus a desired refractive index nw for a preliminary nominal layer thickness t' of the pre¬ liminary dye-binder mixture can be calculated. The actual refractive index (to writing light) of the preliminary dye-binder mixture is next measured and compared to the desired index n . Based upon the difference between the measured refrapt^ve index and the desired index n , concentration adjustments can then be made to the dye-binder mixture. It will be understood that varying relative concentrations of the dye-binder mixture to adjust its refractive index relative to writing light may have some small effect -glTREATT OMPI on its refractive index relative to reading light, but this latter shift will be much less because the mixture is substantially transparent to read light. The above steps can be reiterated (selecting a new t ' ) if the shift is significant. Elaborating somewhat on the mode of concen¬ tration adjustment mentioned above, one skilled in the art will understand from the foregoing that a shift in the thickness locus of an absorption curve maximum depends on the refractive index of the dye- binder mixture (to writing light), and that the strength of an absorption maximum depends on the absorption factor of the dye-binder mixture. The refractive index of the dye-binder mixture varies with dye and binder concentrations according to the relation: n2 mixture =n2 binder + < n2 due- n2 binder> ~~ d β where x dye is the mole fraction of dye. In the event that adjustment of the mixture concentration (to obtain a useful general alignment of the phase shift curve shoulder portion and an absorption curve maximum) results in an unacceptably low strength absorption maximum another dye can be selected for use in the mixture. One skilled in the art will recognize that comparison of the optical constants a first-tried dye with known optical constants of other similar dyes will greatly facilitate such alternate dye selection. .,, At this stage, however, it έiioqld be re- emphasized that the present invention does not require precise coincidence of the writing light's absorption curve maximum with a selected point on the reading light's phase shift curve. Highly useful results can be obtained when the finally selected nominal thickness differs somewhat from either or .both of the points M 2 and P in Fig. 5. That is, highly useful elements can be designed wherein the predetermined nominal thickness is not precisely aligned with either a shoulder portion of the read light phase shift curve nor an absorption maximum of the write light's absorption curve, but is in general alignment with respect to both. As useful guide-- lines, we have found it desirable that the finally selected nominal thickness be selected with respect to the absorption curve maxima so that at least 50% of the writing light is absorbed by the recording layer. From the viewpoint of final nominal thickness vis-a-vis the phase shift curve, it highly preferable that the nominal thickness be selected sufficiently close to the shoulder portion of the phase shift curve that crater rims formed during pit recording will fall in a thickness zone corresponding to the phase shift curve's flat portion. That is, suffi¬ cient space should be left on the back of the curve's "stair step" to accommodate crater rim height. Simi¬ larly, the nominal thickness should not be so far over the shoulder (i.e., on the relatively rapid slope curve portion) that rims effect significant phase shift changes. One other aspect pertaining to final nominal thickness selection will be of interest to those designing record elements according to the present invention. As will be understood by those skilled in the art, the power required for forming^ a pit of given depth will be related to some extent to the intralayer interference charcteristics of a particu¬ lar recording stratum (thickness and optical con- stants). The selection of a final nominal layer thickness can be further optimized by taking such characteristics into account. Reference to Fig. 8 will clarify how this can be done, bearing in mind that the general desired condition is to have a high energy concentration near the upper portion of the recording layer. In Fig. 8, the phase shift to thickness curve φ for an exemplary element is plotted in superposition with a Dπ/2 curve, an Aπ/2 curve and a Dπ/2÷Aπ/2 curve. The Dττ/2 curve is illustrative of the physical pit depth D required to yield a τr/2 phase shift at varying recording layer thickness, and the A π/2 curve is illustrative of the percentage of light absorbed in the depth Dπ/2 at varying overall layer thicknesses (those curves can be calculated based on the recording stratum's optimal constants). The Dπ/2÷Aπ/2 curve thus is a plot indicating the power requirement for obtaining the desired phase shift (i.e., a τr/2 shift) during recording and will be of assistance in final selection of nominal thickness. It is to be noted that the same analysis can be undertaken regarding other desired phase shifts, e.g., τr/4, that might be used in given detection systems. However, it is to be noted that the minimum on the "power" curve should only influence nominal thickness selection where it does not require a nomi¬ nal thickness on the high slope portion of the phase shift curve φ. (Such a selection of nominal thick¬ ness would render the element highly sensitive to crater rims and debris). It will be noted in Fig. 8 that only the first power curve minimum^ does not align with a high slope portion of the phase shift curve and it will be useful in determination of a nominal recording layer thickness. This analysis also serves as a check regarding the appropriateness, from intralayer energy absorption viewpoint, of the material and thickness, parameters chosen based on general alignment of the phase shift and absorption curves. Briefly summarizing then, the optical con¬ stants of the recording stratum are first adjusted (e.g., by varying dye-binder concentration) to obtain general thickness alignment of the phase shift curve shoulder and absorption curve maximum and then, if desired, the final nominal thickness can be selected within the range of such general alignment based on the foregoing type analysis regarding the intralayer energy profile of that recording layer. It will be appreciated, of course, that for some recording layer materials and/or for some applications, the level of absorption may be adequate in thickness zones not generally aligned with an absorption maximum. In such circumstances the design of the element can be optimized solely based on the reading light phase shift curve aspect of the present invention. How- ever, even in these situations it may be useful to consider the intralayer interference affects in accord with the Fig. 8 approach to minimize power required for the needed phase shift. The following Examples of specific bilayer elements will be useful in further understanding cer¬ tain aspects of the present invention: Example I A recording layer coating solution was formed containing one part cellulose nitrate binder, one part of the dye SK-1 and 60 parts solvenέ * (cyclo- hexanone) . The dye SK-1 has the structural formula: and the mode of preparation and utility of coumarin compounds such as SK-1 are described in Research Disclosure Publication No. 16167 by Industrial Oppor¬ tunities Ltd., U.K. The coating was filtered and 5. whirl coated onto a smoothed circular glass substrate of radius, of about 150 nm and a thickness of about 1/4-inch, having a reflective aluminum layer of about 500A. The coating dried and resulted in a record¬ ing layer thickness of about 1100A. The following 10 parameters characterized the significant optical con¬ stants of the element so formed with respect to writing light (488 nm) and reading light (633 nm) . λ w (488) λ r (633) Recording Layer: n - 2.25 + 10.45 n = 1.80 5 Reflective Layer: 0.70 + 14.8 nc * 1.0 + 16.0 Substrate: n = 1.53 n ~~ 1.53 The absorption curve I A and phase shift curve I shown in Fig. 9 respectively illustrate the variation of the Example I recording layer . 0 material's relative phase shift and net absorptance to thickness variation in a manner such as described previously with respect to Fig. 5. Tracks were recorded on the record element at varying power levels using an argon-ion laser 5 light beam (488 nm) focussed with a numerical aper¬ ture NAg = 0.525 while the disc was.rotating at 1800 RPM. (The term NAg represents the numerical aperture of the focused gaussian beam of light measured to its e irradiance diameter.) The recorded tracks were 0 then read back with a similarly focused helium-neon laser light beam (633 nm), having a -o^er of about 1 mW on the disc surface, and a plot such as shown by curve I of Fig. 10 was made to illustrate the read¬ out signal-to-noise ratio obtained for differing 5 recording power levels. (The ordinate of Fig. 10 denotes the demodulated TV signal flat field SNR) . Example I I A recording layer coating solution was .formed containing one part cellulose nitrate binder, one part SC-1 dye and 60 parts solvent (cyclo- hexanone). The dye SC-1 has the structural formula: and the mode of preparation of the dye is disclosed in U.S. Application Serial No. 124,382, entitled α, α'-BIS-(DIALKYLAMINOBENZYLIDENE) KETONE DYES AND THEIR USE IN OPTICAL RECORDING ELEMENTS, and filed of even date herewith, in the names of Specht and Thomas. The layer was coated on a reflectively coated circular disc such as described in Example I and the dried coating resulted in a recording layer of about 1000A thickness. The following parameters characterize the significant optical constants of the element so formed with respect to writing light (488 nm) and reading light (633 nm) : λ w ( 488 ) λ r ( 633 ) Recording Layer: n c = 1.5 + 10.60 n « 1.77 Reflective Layer: n c = 0.70 + 14 .8 n c «= 1.0 + 16.0 Substrate: n « 1.53 n -= 1.53 The absorption curve II. and phase shift curve II shown in Fig. 9 respectively illustrate the variation of the Example II recording layer material's relative phase shift and net absorptance to thickness variation in a manner sύch as described previously with respect to Fig. 5. Tracks were recorded and read back from the disc in the same manner described to Example I and a plot such as shown by curve II of Fig. 10 made to illustrate the resultant read-out signal-to-noise ratio obtained for different recording power levels. Considering the results of these examples .several factors are noteworthy. First it can be seen that both the Example I and Example II discs attain a - good signal-to-noise ratio by implementing the aspect of the present invention whereby nominal recording stratum thickness is selected in general alignment with the shoulder portion of the phase shift to thickness curve (curves Iφ, and l l__ )' . On com- parison of curves I and II of Fig. 10, however, it will be noted that the disc of Example II (curve II) required considerably less power throughout the range of comparison, in obtaining an equivalent εignal-to- noise ratio read-out with the Example I disc. This is believed significantly attributable to the fact that the Example II disc implemented the additional aspect of the present invention whereby an absorptiom maximum of the absorption-to-thickness curve (curve II A ) is also generally aligned with the nominal recording layer thickness. Note also, that because the absorption of the Example II element is generally high throughout a wide thickness range, it is one in which analysis of the type described with regard to Fig. 8 will be of significant use. xhe foregiong discussion has specifically illustrated only bi-layer element embodiments of the present invention (i.e., elements comprising only a reflecting layer and a recording layer on the support). However, it is to be understood that the practice of the present invention is not limited to such bi-layer configurations; other nnjlξi-layer con¬ figurations can be optimized according to the present invention. In this regard the term "recording stratum" is used to denote the layer(s) over the reflective layer of both bilayer and other multilayer recording elements. For example, a dielectric spacer layer(s) can be provided between the recording layer and the reflective layer to cooperate in design opti¬ mization according to the present invention. In this regard the term "recording stratum" is used to denote the layer(s) over the reflective layer of both bi¬ layer and other multilayer recording elements. For example, a dielectric spacer layer(s) can be provided between the recording layer and the reflective layer to cooperate in design optimization according to the present invention. Such spacer layers can be sub¬ stantially transparent to both write and read light wavelengths and preferably have a refractive index creating an interface of low dielectric contrast with the recording layer. Beyond these characteristics, however, the spacer layer(s) are designed with thick¬ nesses) and optical constants that cooperate with the thickness and optical constants of the recording layer in providing an element with: (1) the high absorption to writing light and (2) the high sensiti- vity phase shift to thickness variation for reading light, that were described above regarding bi-layer structures. To accomplish these results the following routine is useful. First a desired recording layer material (e.g., amorphous dye-binder mixture) and thickness is selected. This selection is again based upon optical characteristics (substantial trans¬ parency to read light and absorptivity to write light) and upon other physical heat-deformation characteristics. Having selected a recording layer material and thickness, its refractive ^indices to writing and reading light are measured. Next, absorption versus thickness curves and phase shift versus thickness curves are calculated and plotted for the recording layer, cooperating in a stacked relation with various alternative dielectric aterials meeting the general requirements stated above (i.e., transparency and low dielectric contrast relative to the recording layer). A visual com¬ parison of these curves will readily show when a par- ticular dielectric spacer-recording layer composite yields a useful thickness alignment between the writing light's absorption-to-thickness curve maxima and the reading light's phase-shift-to-thickness curve shoulder portion. The recording element is then fabricated with that dielectric spacer element in thickness corresponding to the thickness alignment point of the curves. For example, given a specific recording layer material and thickness, increases in the thick- ness of a given spacer element will progressively shift the shoulder portion of the element's phase shift curve to the left as the absorption curve undu¬ lates as to absorption-to-thickness curve maxima. This is caused by the differing affect of thickness changes to the differing wavelength write and read light. Appropriate general alignments can be achieved in this way as well as various others. One advantage which pertains to the above- described spacer embodiments of the present invention is that light absorption is concentrated in a thinner recording layer thickness and the thermal effective¬ ness in deforming the layer thus is enhanced. Further advantage can be obtained by choosing the dielectric spacer element as a thermal insulator. However, the provision of additional layers also raises the level of complexity of ele \menVt fabrication. Referring now to Fig. 7, one exemplary embodiment of write-read apparatus 70 for practice of the present invention is illustrated schematically. he apparatus 70 in general comprises a laser writing subsystem LW, a laser reading subsystem LR, and a platen 71 for rotating a disc D fabricated according to the above teachings. The subassemblies LW and LR are shown diametrically opposite only for ease of illustration; it will usually be preferable to provide such subassemblies more nearly side-by-side for common coarse radial indexing relative to tracks of the disc D. Also, each of the subassemblies desirably will have tracking and focus servo-systems of a type known in the art, so that the actual loca- tion of optical elements will usually vary from that shown in the Fig. 7 diagram. Moreover, a single optical assembly can be used for both (simultaneous) writing and reading so long as both the writing and reading light beams are brought into the assembly such that the single focusing lens therein focuses both light beams in the same general vicinity of the record element. The subassembly LW can comprise a writing laser 72 of a first wavelength, a light modulator 73, such as an acoustooptic cell, and a series of light- refractive elements 74 which provide a tightly focused beam at the surface of the recording layer of. disc D. A signal processor 75 addresses the modula¬ tor 73 to turn the beam on and off in accord with the encoded pit generating signal. The reading subassembly LR comprises a read laser 76 of a second wavelength and focusing optics 77 which provide a read spot that extends at least across one track of the disc. As described, the present invention allows less-tightly focused read- out beams which can be on the order Cf twice the diameter of the writing light spot or greater. Also, it has been pointed out that reading light power can be greater than in prior art real-time techniques. Typically it is useful according to the present inventon to provide read-out power of the same approximate magnitude as the writing light power. Desirably a polarizing beam splitter 78 and a quarter wave plate 79 are provided so that read light reflected from the disc surface will be directed back through a portion of the focusing optics 77 to photodetector means 80. Detection means 80 can be of push-pull (or split detector) type such as described in the U.S. Patent 4,065,786 in which case the desired phase shift would be in the order of one-quarter of the read wavelength. Alternatively detection means can be of the central aperture type which is also discussed in U.S. Patent 4,065,786, in which case the desired phase shift would be in the order of one-half wavelength. A discussion of split field and central aperture detection systems is also set forth in an article entitled "Simplified Dif¬ fraction Theory of the Video Disk" by A. Korpel at page 2037 Applied Optics, July 1978 (Vol. 17, No. 13). In either mode, the detected signal is output to signal processing means 81 for decoding, etc. The disc D is constructed in one of the optimized configurations described above, for the particular wavelengths of the write and read lasers 72 and 76 respectively. In operation the disc so constructed is rotated by platen 71 to effect scanning motion rela¬ tive to the modulated beam from writing laser 72 along its tracks, thus forming alternating pitted and non-pitted track portions. Similarly the rotation of *\* * . the disc relative to the modulated beam from writing laser 72 along its tracks, thus forming alternating pitted and non-pitted track portions. Similarly the rotation of the disc relative to the beam from read- ing laser 76 effects scanning motion between the tracks and the beam to produce a modulated light signal to detector 80. Various alternative optimized disc struc¬ tures according to the present invention will occur to those skilled in the art, as will alternative ap¬ paratus and modes for using such structures. For example, instead of a thin metal film reflector on the disc, a dielectric stack reflector could be uti¬ lized if desired. Also, transparent, thermal- mechanical barrier layers can be provided over the recording layer. The invention has been described in detail with particular reference to preferred embodi¬ ments thereof, however, it will be understood that variations and modifications can be effected within the spirit and scope of the invention. \ v

Claims: What is claimed is: 1. A real-time optical disc adapted to record information upon being exposed to light of a first wavelength and to be read-out upon being exposed to light of a second wavelength, said disc comprising a heat-deformable recording layer which overlies a reflective surface, characterized in that: said recording layer is highly transmissive to light of said second wavelength and exhibits, with respect to light of said second wavelength, a non¬ linear relative phase shift to thickness curve having a first zone of relatively steep slope, indicating a relatively large change in relative phase shift per unit change in recording layer thickness, followed by a second zone of relatively gradual slope, indicating a relatively small change in relative phase shift per unit change in recording layer thickness, and in that the recording layer has a nominal thickness proximate a point on said curve where there is a transition between said first and second zones. 2. The optical disc defined by Claim 1 fur¬ ther characterized in that said recording layer has optical properties which render it highly absorbant to light of said first wavelength. 3. The optical disc defined by Claim 1 fur¬ ther characterized in that the recording layer is disposed with respect to said reflective surface that a substantial antireflection condition exists with respect to light of said first wavelength entering said recording layer. 4. The optical disc defined^by claims 1, 2 or 3 further characterized in that said recording layer comprises an amorphous material including a dye and a binder. 5. x e optical disc defined by claims 1, 2 or 3 further characterized in'that the recording layer comprises an amorphous material consisting es¬ sentially of a thermoplastic binder and a dye, and in .that said recording layer has an absorption factor of 20 or greater with respect to said first wavelength. 6. The optical disc defined by claim 1 further characterized in that said recording layer comprises an SC-1 dye and a binder and has a nominal thickness between 100 and 150 nm. 7. A method for real-time deformation writing and reading of information on an optical disc of the type defined by claim 1, said method being characterized by the steps of: (a) scanning said record element with a beam of light of said first wavelength that is intensity- modulated in accordance with information to be recorded, thereby selectively decreasing the thickness of the scanned portions of said recording layer within an operative thickness range; (b) scanning said scanned portions with a beam of light of said second wavelength; and (c) sensing the light of said second wavelength reflected from said record element with a far- field detector adapted to detect, and produce output signals indicative of, phase changes of the light incident thereon. \ - ϊTREA OMPI , ^ WIPO . ^ ATlO^

Inventors: HOWE D, WROBEL J
Applicants: EASTMAN KODAK CO
Application year: 1981
Main classification: G11B7
IPC codes: B41M5, G03C1, G11B7