In the present state of the art relative to optical recording, the theoretical limit with respect to optical resolution has been closely approached. The optical resolution is determined by the wave- length of the writing radiation, typically a laser, and the numerical aperture of the objective lens. The numerical aperture of the lenses currently used in optical heads is near to the maximum which still preserves both an adequate working distance and a reasonable lens mass. The semiconductor lasers cur¬ rently used in optical recording have the shortest wavelength which satisfies the requirements of suf¬ ficient power and access time.

Thus, the data capacity has closely approached a maximum for digital recording. With the optical systems so near the theoretical limit, the focus of attention has turned to developing more efficient methods for encoding the data before it is written to the optical recording element. For ex- ample, run—length—limited modulation encoding methods attempt to improve the linear packing density by increasing the fineness to which pit edge locations are determined. (Siegel, "Recording Codes fcrr Digital Magnetic Storage", IEEE Transactions on Magnetics, Vol Mag 21 No. 5, 1344-1349 (Sept. 1985); Heemskerk and Immink, "Compact Disc: System Aspects and Modulation", Philips Tech Rev., Vol 40 No. 6, 157-164 (1982)) This method makes very high demands on the tolerances of all system components resulting in high equipment costs.

One method for increasing the packing dens¬ ity in an optical recording element is to exploit an additional degree of freedom. With binary recording, information is recorded as either the presence or the absence of a deformation on the recording layer. If the information could be recorded as different in¬ tensity deformations as well as the presence or absence of a deformation, more information could be stored in the same amount of space. The relative amount of information that can be stored by ^' a ternary system, for example, could be as much as 1.6 times that which can be stored by a binary system.

In U. S. Patent 4,097,895 issued June 27, 1978 there is disclosed an optimized recording element. The element is characterized in that the recording layer is coated at a thickness such that an anti—reflecting condition is achieved. Using this condition as a starting point, Japanese KOKAI 61—94249 discloses a ulti—level recording system. Rather than using a single power level writing beam, this disclosure teaches the use of lower power level writing beams to remove only a portion of the re¬ cording layer. By starting with a layer having the anti—reflecting condition, several levels of signal can be attained. ■ This additional degree of freedom provides an opportunity for increased packing density. In the system- of KOKAI 61-94249, in addition to the minimum reflectance of the quarter-wave condi¬ tion of the original layer, two higher reflectance conditions correspond to the two power levels which are used. One condition corresponds to the removal of substantially all of the recording layer thereby exposing the highly reflective substrate. The re¬ maining condition is intermediate between the two extremes and corresponds to the removal of a portion of the layer.

However, in the method of this KOKAI, only very small power differences are needed to write the different types of marks on the layer. The method is therefore difficult to control. In accordance with the present invention there is provided an optical recording element and method for multi-level, at least ternary, recording. Accordingly, there is provided an optical recording element comprising a reflective support having there— on a deformable optical recording layer having a thickness corresponding to about the half—wave thick¬ ness with respect to the reading wavelength, said layer having information recorded therein in the form of: a) deformations which are of a depth so as to leave remaining about a quarter-wave thickness of the layer with respect to the reading wavelength and, interspersed with said deformations, b) deformations which are at least substantially through to the reflective support. In accordance with another aspect of the invention, there is provided a method for recording using the described element. A method for recording informa¬ tion on an optical recording element using a con- centrated beam of radiation, said element comprising a reflective support having thereon a deformable optical recording layer having a thickness corres¬ ponding to about tfte half-wave thickness with respect to the wavelength of the radiation used to read said element comprises the steps of: a) causing relative motion between said layer and said beam-and b) modulating the power of said beam in accord¬ ance with the information to be recorded between two power levels, one level corresponding to the

power that is required to produce deformations which are of a depth so as to leave remaining about a quarter—wave thickness of the layer with respect to said reading radiation and the second level corresponding to the power that is required to produce deformations which are at least sub¬ stantially through to the reflective support. The present invention is to be contrasted with the invention described in the KOKAI mentioned above. In that element and process, the unrecorded layer has a very low reflectance. The information is recorded in the form of deformations of at least two types , each having a reflectance above the unrecorded layer. In the present invention, the unrecorded layer has a reflectance that is intermediate between one type of deformation and the other type. Thus, the information appears in the layer as interspersed "positive" deformations, i.e. those having a reflectance lower than the unrecorded material and "negative" deformations, i.e. those having a reflec¬ tance that is higher than the unrecorded layer.

With the method of the invention, there is a larger difference in the power required to write the different types of marks than in the above mentioned KOKAI. Therefore, the method is easier to control. Further, the interspersed negative and positive marks according to the invention are more reliably recovered in the presence of focusing and tracking errors . Figure 1 is a plot of the reflectance vs. thickness for a typical material that is used in deformation recording.

Figure 2 is a plot of the signal vs. time for an element according to the prior art and accord- ing to the invention.

The present invention will be described in detail with reference to ternary recording. It will be understood however, that deformations with reflec¬ tivities other than just the reflectivities corres- ponding to the optical quarter-wave thickness can also be recorded, as is illustrated in the KOKAI mentioned above. Thus, the invention is adaptable to any form of multi-level information storage and retrieval. The principle of the present invention is best understood by referring to Figure 1. There is illustrated a typical reflectance vs. thickness curve for a material that is used in deformation record¬ ings . The wavelength that is used to make the curve is the wavelength that is used to read the informa¬ tion on the element. Point A corresponds to the uncoated substrate having a high reflectance. As the thickness of the recording layer increases, the reflectance decreases up to thickness 31 corresponding to the quarter-wave thickness of the material with respect to the reading wavelength. This is also referred to in the art as the "anti-reflection con¬ dition". As the thickness of the layer increases, the reflectance increases achieving a maxima at point £ where the layer approximates the half-wave thick¬ ness .

In accordance with the present invention, the optical recording layer is coated at a thickness that corresponds to about the half— ave thickness with respect to the reading radiation. During recording, power levels are chosen so that some of the deformations leave about a quarter-wave thickness corresponding to decreased reflectance points £, and some of the deformations are at least substantially through to the support corresponding to point A.

The variation of reflectance with thickness is very well known in the art and has been applied to advantage in binary recording on optical elements . Reference is made to U. S. Patent 4,097,895 issued June 27, 1978 to Spong particularly Fig. 4 and the description relating thereto. However, the present ternary recording is not disclosed.

As is well known, the exact reflectance vs. thickness curve is determined by the material that is used for the recording layer and the wavelength that is used to measure the reflectance. It will there¬ fore be understood, that when selecting a thickness either for the unrecorded layer or the thickness that is about the quarter—wave thickness, the wavelength of the recording and reading wavelength must be con¬ sidered.

In the description, the thickness of the unrecorded layer is "about" the half—wave thickness and some of the deformations are such so as to leave "about" the quarter-wave thickness. It is contem¬ plated that the layer may be coated at a thickness that is different from the exact peak corresponding to the half—wave thickness and that the remaining thickness for the quarter— ave deformations may not be exactly the quarter—wave thickness. These parameters can be adjusted so as to optimize the element as necessary.

For example, it may be that for a particular thickness, the difference between the reflectance of the unrecorded layer and the negative deformations is such that the signal to noise ratio (SNR) for that portion of the recording is more than is needed while the difference between the unrecorded layer and the positive deformations might be less than is needed for optimum SNR. Also, it may be desirable with some materials to vary the thickness slightly so as to

inimize the power difference needed to make the different deformations. Therefore, by varying the thicknesses slightly, the overall performance of the recording can be optimized. In general, it is pre- ferred to have the thicknesses within about 20% of the quarter- and half-wave thicknesses.

The optical recording elements that can be used in the preparation of the recorded elements of the invention are conventional except for requiring a certain thickness for the optical recording ' layer . Typically, they comprise a reflective support and a radiation sensitive optical recording layer. The preferred optical recording elements comprise, in the following order, an aluminum support, a smoothing layer, a metal reflective layer and an optical recording layer. The element can also include an overcoat layer as is known in the art, for example, U.S. Patent 4,527,173.

The aluminum support is coated with the smoothing layer formulation prior to the coating of the reflective layer and the recording layer. The preferred smoothing layer compositions are described in U. S. Patents 4,446,223 and 4,619,890 cited above. In some embodiments it may be desirable to first coat the aluminum support with an anti- reflecting layer as is described in EPO Application No. 89 101 190.0 filed 24 January 1989, entitled OPTICAL RECORDING ELEMENT HAVING AN ANTIREFLECTIVE LAYER. Following coating, polymerization of the fluid produces a smooth surface on the support. The thus formed smoothing layer is made reflective by vacuum metallization of the smooth surface. Useful metals for this purpose include gold and aluminum. Useful recording layers, preferably compris¬ ing a dye and a binder, are coated by any of a wide

variety of methods. Most conveniently, the dye and binder are coated from a common solvent or, alterna¬ tively, from a mixture of miscible solvents. The dye-binder composition is coated by spray coating, air knife coating, whirl coating or by any other suitable method. Useful dyes and binders are dis¬ closed in U.S. Patent 4,380,769 and U.S. Patent 4,499,165. These patents also describe in detail methods for making the recording layer. Other useful recording layers include metallic or alloy layers. The currently preferred optical recording layer is a dye—binder layer described in U. S. Patent 4,499,165. That patent describes an optical record¬ ing layer comprising a binder mixture and a dye, which layer is characterized in that the binder mix¬ ture (a) is amorphous at high dye to binder ratios, (b) exhibits a single thermal transition with no phase separation after annealing, (c) is solid at about 20° C, and (d) comprises at least two different compounds each having at least two linking components joining one multivalent organic nucleus with at least two organic nuclei wherein at least one of the multi¬ valent organic nucleus and the organic nuclei is a multicyclic aromatic nucleus . Another preferred dye—binder composition comprises an amorphous layer of a binder and a oxo— indolizine or oxoihdoliziniu dye as described in United States Patent 4,446,223 issued May 1, 1984. - The preparation of these dyes is described in United States Patent 4,577,024 issued March 18, 1986. The dye in the preferred dye—binder optical recording layer is not critical. Innumerable dyes are available and well—known. A preferred group of dyes which are especially compatible with the pre- ferred binder mixtures are metal complexes of bis-[cis-l,2-bis-(alkyl, hydrogen, aryl or hetero- cyclic)ethylene-l,2—dithiene] . They are commonly

referred to as metal dithiene complexes. Mixtures of these dyes can also be used.

In the method of the present invention, information is encoded in the form of interspersed deformations in the recording layer. There are several methods available to affect the recording process. The preferred method is to modulate the incident power of the recording radiation. This can be accomplished by changing the power level of the incident radiation or by varying the duty cycle of a laser modulated at high frequency. Either of these methods can be used to vary the energy delivered to the recording layer on a deformation by deformation basis . Algorithms for encoding the written infor¬ mation and for reading the encoded information are within the skill of those in the art. Ternary recording has been ^* shown for magnetic materials and analogous methods for writing and reading information are adaptable to the present invention. Reference is made to Dixon, French and Wolf, J. 1987 Intermag. Conf. Proceedings; Jacoby, IEEE Trans. Mag.-17, &, 3326 (1981); Vinding. IEEE Trans. Mag.-18, £., 1256 (1982) and Chi and Frey, IEEE Trans. Mag.-18, £, 1259 (1982). The described ternary recording can be used in conjunction with run-length-limited encoding methods .

According to the method of the present invention, relative motion must be provided between the optical recording element and the source of the recording radiation. In a typical method, the ele¬ ment is moved relative to a fixed source of radiation as in a typical optical disk system. The alterna¬ tive, where the optical recording element is held stationary and the radiation source is moved, is also within the scope of the invention.

The following example is submitted for a further understanding of the invention. Example

An optical recording element was prepared in the form of an optical disk. A diamond turned 5 1/4" diameter aluminum substrate was coated with a smooth¬ ing layer composition similar to that described in example 1 of U.S. Patent 4,619,890 described above. The smoothing layer was the same except that the solvent was butyl propionate and the "monomer 1" of the example was changed to an 80:20 mixture of 1,2,4—benzenetricarboxylic acid, tris 2—((l-oxo-2- propenyl)oxy)ethyl) ester and 2—propenoic acid—2— (benzoyloxy) ethyl ester. This smoothing layer com- position was cured as described in the patent. The smoothing layer was then coated with a reflecting layer of 200 nm of gold.

The disk was then coated with a solution of a dye and a binder in the ratio of 4:6 by weight and 2% solids. The solvent for the coating composition was a mixture consisting of 20% trichloropropane and 80% bromobenzene. The binder was a mixture as described in United States Patent 4,169,890 and was similar to "No. 11" in Col 21 and 22 of that patent. The mixture was a nonpolymeric, amorphous glass mix¬ ture which was the reaction product of 1,1,3—tri— methyl—5—amino-3(p-aminophenyl) indan with 1-naph— thoyl chloride (50.00 mole%) ; p-bromobenzoyl chloride (16.67 mole7 ₀ ); and p-methoxybenzoyl chloride (33.33 mole%) . ?

The dye in the dye-binder mixture was the dye described in United States Patent 4,577,024 (men¬ tioned above) at Col 45, the second structure in that column. The chemical name of the dye is: 2,3-di(2,4,6,-trimethylphenyl)-7-[2-(l,2- dimethyl-1,2,3,4-tetrahydro-6-quinolinyl) —1—ethenyl]-l—oxo-lE—indolizinium trifluoro— methanesulfonate.

The disk was coated such that the thickness of the dye—binder optical recording layer was 213 nm. This corresponds to a thickness which is near the half-wave thickness for this layer at a reading wavelength of 780nm. (Half-wave thickness is about 235nm. )

To evaluate the disks, test tracks were recorded on the ^' optical recording layer using a diode laser operating at a wavelength of 830nm. The recorded tracks were read back using a similar diode laser operating at 780nm. The test tracks were recorded using a variety of laser power levels and the carrier to noise ratio (CNR) was measured for each power level. The velocity of the recording layer at point of recording was 9.4 meters per second.

At high recording powers, e.g. 10 mW, the playback amplitude showed waveforms corresponding to high reflectivity- arks . At lower recording powers, e.g. 5 mW, the playback amplitude showed waveforms corresponding to low reflectivity. Examination of the resulting tracks indicated that the deformations in the recording layer made using the 10 mW recording power were of a higher reflectivity than the unrecorded areas of the layer while the deformations in the recording layer made using the 5 mW recording power were of a lower reflectivity than the unrecorded layer . The CNR at both 5 and 10 mW was at least 55 dB (with a 30 KHz resolution bandwidth) . This CNR performance is sufficient for practical data recording at conventional data rates.

The reflectivity of the high reflectivity deformations approached the reflectivity of the uncoated substrate indicating that the deformations were substantially through to the support. The reflectivity of the low reflectivity deformations corresponded to about the reflectivity of the quarter—wave thickness.

In a second test, the laser recording power was alternated between the high power and the low power. On playback, both the high reflectivity high power deformations and the interspersed low power, low reflectivity deformations could be distinguished even when a high power deformation was adjacent to a low power deformation.