OPTICAL DISC WRITE/READ EMPLOYING PASSIVE FOCUS MAINTENANCE

Abstract: Method for maintaining a write/read light beam (B) in sharp focus on the information surface (41) of a rotating flexible optical disc (13) notwithstanding variations in the thickness of the disc's support (43) which alter the distance between such inforamtion surface and a focusing lens (9) for the light beam. Such method comprises the steps of (a) directing the write/read light beam through the disc's support, and (b) positioning the disc relative to the focusing lens so that the disc's neutral plane (NP) is spaced from the lens by a distance equal to FL-(t/n - t/2), where FL is the focal length of the lens, and t and n are the nominal thickness and refractive index of the disc support (43), respectively. By such a method, variations in the support thickness and spherical aberrations induced thereby cause the spot focus to remain in focus on the disc's information surface without any need for refocusing the lens or repositioning the lens relative to the disc.

Description: OPTICAL DISC WRITE/READ EMPLOYING PASSIVE FOCUS MAINTENANCE The present invention relates to methods and apparatus for optically writing and/or reading 5 information on optical discs and more specifically to a technique for maintaining focus of write/read light on the information surface of the optical disc. The currently preferred optical disc technology employs disc elements with spiral or 10 concentric tracks of minute (usually on the order of a micron or less in size), optically-detectable marks. One real-time mode of recording (writing) such marks is by scanning the disc with a small focused spot of radiation (e.g., from a laser) which is modulated "off • ] _5 or on" according to an electrical signal representative of the information to be written. Information is recovered (read) by scanning the tracks with an unmodulated, equivalently small, tightly focused light spot. The recovered information is in 20 the form of a fluctuating electrical signal obtained from a photodetector that senses the modulated read-out light reflected from the recorded disc. In order to write and read information in the form of such minute markings, optical systems of 5 relatively high numerical aperture are used to focus light to equivalently minute spots on the disc's recording surface. Such optical systems have extremely small depths of focus and, unless a proper positional relation between the writing or reading 0 optical system and the optical disc surface is stringently maintained, unacceptable defocusing will occur. To prevent such defocusing, it is desirable to rotate the disc so that its recording surface is precisely maintained in a predetermined plane. 5 International Patent Publication No. W081/03712, published 24 December 1981, addresses this problem and "BUREAU OMPI disclosed a technique for rotating a flexible optical disc so that its neutral plane (i.e. the plane within, a disc along which material is neither compressed nor extended during bending) is substantially stabilized in space. While this technique virtually eliminates defocusing as a result of variations in spacing between a rotating disc's neutral plane and the focusing lens, it does not assist in eliminating the "second-order" defocusing effects which results from thickness variations of the substrate which supports the disc's recording layer. Such thickness variations, which are unavoidably produced during the manufacturing process and are usually symmetrical with respect to he disc's neutral plane, cause different portions of the disc's recording surface to be spaced at different distances from the disc's neutral plane as the disc rotates. Although the disc's neutral plane is stable relative to the focal point of the focusing lens, the disc's recording surface moves relative to that focal point by one-half the thickness variation Δt from a nominal disc thickness t. Applying prior art solutions to the defocusing problem described above would result in the inclusion of some sort of electrooptical focus-servo device for producing focus error signals and for using such signals to continuously adjust the position of the focus lens relative to the disc's neutral plane. Such solutions are, of course, disadvantageous from the standpoint of expense and reliability. The object of this invention, therefore, is to maintain focus of a write/read light beam on an optical disc without resorting to the focus-servo devices of the prior art. According to the invention, it has been found that the above-mentioned second order defocusing effects can be reduced to an acceptable level by exposing the disc's information layer through the disc's support (which is transparent to write/read light) and by spacing the focusing lens (of the write/read light beam) from the disc's neutral plane by a distance equal to FL-(t/n - t/2) , where FL is the focal length of the lens, and t and n are the nominal thickness and refractive index of the disc's support. The subsequent description of preferred embodiments of the present invention refers to the attached drawings wherein: Figure 1 is a schematic diagram of an optical write/read system useful in the practice of the present invention; Figure 2 is a perspective view of one configuration useful for rotating an optical disc for practice of the present invention; Figure 3 is a fragmented cross-sectional view of the Figure 2 configuration; Figure -• is an enlarged diagrammatic view illustrating a preferred relationship, in accord with the present invention, between a nominal thickness portion of a rotating optical disc and a focused write/read light beam; Figure 5 is a view similar to Figure 4 but illustrating a disc portion which exceeds nominal thickness; and Figure 6 is a graph which is useful in selecting preferred modes of the present invention and which indicates the variation in radius of a light beam's spherically aberrated blur circle plotted versus lens numerical aperture for various index of refraction of disc support materials. Referring to the drawings, Figure 1 schematically illustrates an optical disc write/read apparatus. The writing means comprises a writing laser 1 which directs light of a first wavelength through a polarization rotator 2 to acoustooptic modulator 3 adapted to receive an electrical information signal and modulate the writing light beam in response thereto. Light exiting modulator 3 passes through beam-expanding optics 4 and is directed via mirrors 5 and 7 and beam-splitter 6 to a negative correction lens 8 and the focusing objective 9. By this optical means, light from the laser 1 can be controlled to form tracks of micron-sized marks on the rotating disc 13 as it moves above the stabilizing reference surface 34. Preferred disc and reference surface configurations are explained in much more detail subsequently. The reading means shown in Figure 1 comprises a reading laser 10 which directs light of a second wavelength sequentially through polarizing rotator 11, polarizer 12, tracking modulator 14, beam-expanding optics 15, polarizing beamsplitter 16, one-quarter wave plate 17, beamsplitter 6 and mirror 7 to elements 8 and 9. Light from the focused read spot is reflected from the disc 13 and returns through elements 9-6 and 17 to polarizing beamsplitter 16. The reflected read light is then directed to beamsplitter 20 where it is divided and directed both to a signal detection system 21 and a focus detection system 23. The focus detection system 23 receives read light via bandpass filter 24 and a field lens 25. Focus system 23 can be one of various kinds known in the art, e.g., including a Foucault knife and a split diode. The read detector system 21 similarly can take various forms known in the art, e.g., such as disclosed in U.S. Patent No. 4,065,786. Referring now to Figure 2, one preferred embodiment for effecting precise stabilization of a flexible optical disc is shown. Within the write/read apparatus shown in Figure 1, a rotatable drive shaft 31 is mounted in housing 30 with its rotational axis normal to the deck 33. The stabilizing reference surface 34 is adjustably supported on the deck 33 in concentric relation around shaft 31. The optical disc 13 is clamped by cap 36 to the top 35 of the drive shaft 31, which is located at a predetermined height above the reference surface 34. More detail of the Figure 3 apparatus is disclosed in the aforementioned International Patent Publication No. W081/03712. In general, the shaft 31 is coupled to a motor (not shown) which is adjustably supported to facilitate adjustment of the shaft axis to a condition generally normal to the deck 33, with its top surface 35 at a predetermined height above stabilizing reference surface 34. The top surface 35 is precisely machined to assure it is normal to the shaft axis of rotation. In accordance with a highly preferred mode, the disc-aligning portion of this surface has a variation of less than +.2 milliradians from the plane normal to the axis of rotation, when measured by rotating the shaft by hand. Adjustable elements (not shown) are provided to assure reference surface 34 is precisely parallel to top surface 35 and precisely normal to the axis of rotation of shaft 31. As shown best in Figure 3, clamp 36 has a protruding annular clamping portion and a centering portion which is adapted to fit snugly in a recess in the top of shaft 31. A precisely centered and sized aperture in disc 13 thus can cooperate with the centering portion of clamp 36 so as to precisely radially align the disc center with the axis of shaft rotation. When clamp 36 is in place in the shaft recess and secured, e.g. by a bolt 40, the annular clamping portions around its periphery secure the underlying portions of the disc in precise parallel relation to the shaft top 35 and thus the top of "BU EAU OMPI A reference surface 34. Additionally, this peripheral clamping Isolates the exterior peripheral portions of the disc 13 from any stresses created by the formation of its central aperture. It is also highly preferred that peripheries of clamp 36 and shaft 31 have a variation not exceeding +.0025 mm. with respect to their radial distance from this axis of rotation. This is important so that the engagement zone with the disc is axially symmetrical and will initiate uniform stresses within the disc. It also is highly preferred that the shaft have at least a predetermined minimum diameter, in relation to a given diameter disc, so that the disc is not excessively stressed, either in the radial or tangential directions during rotation. For a 30 cm. disc rotating at 1800 RPM, a shaft diameter of 29 mm. is highly useful. It will be noted that the central aperture in reference surface 34 exceeds the diameter of shaft 31 so as to leave an annular air passage from the atmosphere to the radially central region of the zone area between disc 13 and reference surface 34. In accord with the illustrated configuration, an annular passage between a 29 mm. outer diameter shaft 31 and an inner diameter opening of 32 mm. in reference surface 34 is highly useful. However, the size of this opening can vary so long as air is not so severely throttled as to cause contact between the disc and reference surface, during rotation, and the air flow into the gap is quiet (i.e. , not turbulent) . it is preferred that the annular passage communicate freely with ambient atmosphere via the space beneath the reference surface and the interior of housing 30. Before proceeding to a description of how the apparatus shown in Figures 2 and 3 rotates disc 13 for writing and reading by the optical system shown in Figure 1, it is appropriate to describe features of the disc 13 itself in more detail, for certain disc features are important. Figure 4 shows, in cross- section, a portion of a disc 13 which typically comprises a record layer 41 and a plastic support 43. To carry out the method of the invention (as discussed in detail below) , write/read light is directed to the record layer 41 through the support 43 while the disc record surface 41 opposes the apparatus reference surface 34. For detecting read light with this arrangement, it is preferred that read light be reflected back through the support to the detector. The support 43, therefore, is constructed to be substantially transparent to the write/read wavelengths utilized and also preferably is substantially non-birefringent and substantially homogeneous and free from striae and significant occlusions. The support should be of thickness which accommodates well stabilized "flight", i.e. rotation of the disc above reference surfacd 34. A highly preferred range is .05 to .15 mm.; however, other thicknesses are useful. As noted, one preferred detection mode is for light to be reflected back through support 43 to the detector and therefore it is preferred that record layer 41 be reflective to read light (and absorptive to write light) . Alternatively the record layer can be absorptive to both write and read light and an additional reflective layer (not shown) provided over the record layer. One preferred construction for the record layer 41 is of sputtered or vapor-deposited tellurium. However, the record layer can be formed of many other reflective materials, e.g. titanium or bismuth. The particular mechanisms of recording mark formation and light readout are not critical as to practice of the invention except insofar as they should not adversely affect the disc flatness and thickness characteristics outlined below. For proper rotational stabilization of the disc's neutral plane, it is important that optical discs have good flatness and thickness. It is also highly preferred that the disc have a smooth, substantially cylindrical periphery which is radially symmetric to its centering aperture. With respect to flatness, it is highly desirable that the support, and disc itself have extremely little curl and core set. As a test for a preferred support material, a thin, e.g. 0.1 mm., 30 cm. diameter disc element formed of the proposed material should be placed on a planar surface in an unconstrained condition. It is preferred that no peripheral or central portion of the disc element raise more than about 0.8 mm. from the planar surface. It is highly preferred that such element have no portion that raises more than about 0.4 mm. in such test. It is to be understood, however, that useful results can be obtained with more generally flat support materials, not meeting the above, preferred criterion. For example, good stabilization of discs over useful annular portions thereof have been attained with disc supports having peripheral curl of 6 mm. or even 12 mm. With respect to uniformity of thickness, it is preferred that the disc support, and the disc itself, have a thickness variation of about +2.5 microns or less. Again, it is to be understood that - support materials of more generally uniform thickness may be useful in certain applications in accordance with the present invention. For example, where depth of focus is less critical such as in some read-only applications, useful materials could have thickness variations in excess of the defined criterion. Having described the useful and preferred disc characteristics, the preferred structure and mode for rotating the disc with its neutral plane in a highly stabilized condition will be explained. In general, it is extremely important, for optimal stabilization of the plane of rotation, that the stabilizing reference surface 34 be properly configured and that the height of the top surface 35 of shaft 31 (and thus the disc's orientation plane) be properly positioned relative to reference surface 34. More specifically, the preferred reference surface configuration has three important characteristics. First, it should be aerodynamically smooth and featureless. That characteristic of "featureless" denotes a continuous surface in distinction to prior art techniques in which grooves, apertures or sensors were incorporated in the surface. Turned and polished metal surfaces or chrome plated surfaces have been found to be preferred from the smoothness viewpoint. Second, it is preferred that the reference surface be radially symmetric with respect to the axis of rotation of shaft 31 and non-convex with respect to the plane of the shaft top 35 (i.e., the plane of disc orientation). The planar surface provided by the annular reference member 34 in the illustrated embodiment of course meets this requirement. However, it may be preferred to modify the surface configuration of reference member 34 to make it symmetrically concave, e.g., as a surface of revolution defined by a curve sloping upwardly from the central axis to the periphery. Such modifications can be useful to cause the stabilized condition of the rotating disc to be more planar in nature. In regard to symmetry, we have found it to be highly preferred that, at given radii, the reference surface have less than .025 mm. (most preferably +.012 mm.) variation in -BU EA OMPI 4& iPo _ height from a plane normal to the axis of rotation of shaft 31. Finally, the periphery of the reference surface should extend at least to the periphery of the disc. That is, it should not terminate substantially radially inwardly from the disc periphery or air turbulence will occur, disrupting any effective stabilization of the rotating disc. As mentioned above, the height of the reference surface 34 in relation to the top surface 35 of shaft 31 is also important. The preferred limits for the variation of this distance ("h" in Figure 3) can best be described functionally in regard to the certain peripheral conditions created between the disc and reference surface during disc rotation. That is, the maximum preferred distance for height h is one which, when the disc is rotating at its nominal speed, just avoids the condition of excessive turbulence at the disc periphery. For a disc having a .1 mm. hickness and a 30 cm. diameter, rotating at 1800 RPM over a planar reference surface, such as shown in the drawings, this maximum height "h" occurs at about .375 τnιn , The minimum for the height h is a function of particles which are present in air and that it should in no event be less than as to maintain a peripheral air gap of about .025 mm. between the disc and reference surface. Smaller peripheral gaps have two compounding adverse effects which prevent any effective stabilization. First, dust particles of .025 mm. will become caught In the passage and disturb the laminar nature of the air flow. Second, even, if particles are not "caught" to an extent blocking air passage, they forcefully impact the too closely opposing surfaces causing a build up of electrostatic charges. Such charges have been recognized in the art as a significant deterrent to stabilized disc flight. Another problem creating electrostatic disturbing forces can occur during the start of disc rotation, i.e. , its take-off. At this stage there is inherently a frictional rubbing between the disc and the reference surface. The extent of this effect can be lessened to some extent by reference surface configuration (e.g., making it concave) and by choice of material (i.e., a chrome-coated reference surface exhibits a good position in the triboelectric series relative to plastic supports). It is useful to provide a quick disc take-off, e.g. , preferredly effected by use of a high torque motor and relative lightweight construction of the moving parts. Alternatively peripheral lifter elements, e.g., solenoids, might be used; however, these should be precisely constructed and quickly retracted after disc take-off, to a condition leaving the reference surface featureless. In the embodiment illustrated in Figure 2, a protective coating can, if desired, be provided over the disc's recording layer to prevent damage to the recording layer by abrasion on the reference surface. The nominal disc rotational rate can be selected for the signal format but should be sufficiently high so that dynamic forces straighten the disc to the modified planar condition above the reference surface. The air supply to the central region is preferredly at atmospheric pressure, but we have found that good stabilization can be obtained with slightly negative pressure sources. However, the pressure source should not be significantly positive with respect to atmospheric. It is preferable that the air introduction means provide uniform flow along its circumference, thus the annular opening between the shaft and reference surface is preferred. Thus, if a properly configured disc is properly supported for rotation and if this outward movement of air is highly uniform in all radial directions, a highly stabilized equilibrium condition can be attained between the dynamic, elastic, fluid and gravitational forces acting on the disc and a remarkable stabilization in vertical displacement of a large annular portion of the disc is attained. With the flexible optical disc rotating so that its neutral plane is stabilized in a plane of rotational equilibrium, the focus maintenance technique of the present invention is implemented. This technique is described with reference to Figures 4-6. Consider the situation shown in Figure 4 where a beam of write or read light B is focused by lens 9 (Fig. 1) through a portion of optical disc support 43 of nominal thickness "t" and of refractive index "n" (greater than 1) such that focus occurs on the disc's record surface 41. The aerial paraxial focus f, (i.e., the position of paraxial focus if the support material were removed) occurs at distance L-, from the top of the support material, and the real paraxial focus f^ occurs at distance L2 from the top surface of the support material. It will be understood by those skilled in the art that ..* is related to L. by the equation L - n-L^. In accordance with the invention, one positions the focusing lens with respect to the aerodynamically stabilized support 43 such that L 2 = t, where t is the nominal thickness of the material. Under this condition, L, » t/n. Referring to Figure 5, when the support material thickness changes from the nominal thickness t by an amount Δt, L. and L„ change such that τ i. . t Δt L 1 n + (1) and L' n(- + -j) A. n ^ + 2 (2) From equation (2), it is interesting to note that, if c the refractive index of the support 43 is equal to 2.0, the real paraxial focus f 2 will always occur at a distance L„ » t + Δt (i.e., on the recording layer 41). However, for a material having a refractive index of 1.5 (a typical value for a plastic TQ support 43), L„ will equal t + 0.75 Δt, and the real paraxial focus f„ will fall short of the lower surface by 0.25 Δt. Now consider the effects of spherical aberration. The typical focusing lens 9 is corrected ic to work through a nominal thickness t of index n material; i.e. it is designed to produce diffraction limited focus under such conditions. When working through an additional thickness Δt of material, the lens will produce longitudinal spherical aberration (SA) given by 0 where NA is the numerical aperture of the focusing 5 lens. When Δt is greater than 0,. overcorrected spherical aberration results (i.e. the marginal rays focus further away from the lens than do the paraxial rays). The recording layer 41 of the stabilized disc (whose support has an index n and thickness t+Δt) 0 will be situated a distance d -* Δt(l-n ) beyond 2 the real paraxial focus ** (equation 2) and the radius b of the spherically aberrated blur circle of " light striking the record surf ce is 5 b = -NA{fci7ϊfepr) * - l ( ΪT7 Jk r r)/ 2 . (4) OMPI Plots of the absolute value of b/ Δt vs NA for various values of n are given in Figure 6, and from Figure 6 we see that, for an n a 1.5 support material, we can use an objective of NA as high as 0.79 and still have a blur circle of radius less than 0.065Δt occurring at the record surface. The airy disc central core radius at the focus of a diffraction limited lens is given by r s 0-61λ (5) r A NA J where λ is the wavelength of the light being focused. In a typical optical disc read/write system, λ * 488 nm and NA - 0.79; thus, equation (5) yields r A = 0.3768μ Thus, if we select b * 0.3768μ, Figure 6 shows that a properly stabilized disc having, for example, an n = 1.5 support material can have +££ ) .579μ and still have a geometrical blur circle at the back surface that is smaller than the airy disc central core if the lens is properly positioned relative to the stabilized neutral plane of the rotating disc. In other words, if Δt does not exceed 0.579μ, there is no need to re-positIon the focusing lens to maintain sharp focus because the diffraction limited spot (produced by such re-focusing) would be larger than the blur circle produced without re-focusing. Figure 6 shows that for any index n there are NA and Δt values which will produce acceptable blur circles at the back surface, and diffraction-limited focus can be maintained on the recording layer without moving the lens. Another case of interest would have λ - 830 nm, NA * 0.5 and n - 1.5. From Figure 6 we see that for this case the blur circle radius will be on the order of 0.061 Δt. From equation (5), r. » 1.01 μ. Thus we could have Δt <_ + 1.66μ and not exceed the diffraction limited spot size. Considering the foregoing, it can be seen that, in accord with the present invention, substantial focus compensation for thickness variation of a rotating optical disc can be obtained passively if (1) the write/read light beam is directed to the recording layer through the disc's support and (2) the focusing lens and the disc's neutral plane NP are relatively located in a proper manner. Specifically, the lens and stabilized neutral plane of the disc should be located so that there is a distance "x" between the lens means ' second principal point and the disc's neutral plane, where FL is the focal length of the lens, t is the nominal thickness of the disc support and n is the refractive index of the support material. The value of "x" is derived by solving for the second principal point to neutral plane NP distance (in terms of n and t) for the nominal thickness condition. Referring to Fig. 4, it is apparent that: x = FL - (L 1 - §) or By using the method of the invention, support-induced spherical aberration actually assists in refocusing the write/read beam to compensate for thickness variations in the support, and precise spot-focus of write/read light on the information surface of the disc is passively maintained. In another preferred embodiment, not shown, the optical disc can be rotated with its support side facing the stabilizing reference surface of the write/read apparatus. In this embodiment at least a ""BUREA OMPI portion of the reference surface is transparent to write/read light, e.g., formed of polished glass to maintain proper aerodynamic characteristics. The write/read light source(s) are located to direct light through the stabilizing reference surface and the disc support to the information surface. In this mode the focusing lens focal length (FL) is corrected to operate through the transparent reference surface. This embodiment has the advantage of avoiding abrasion of the record layer by the reference surface. Also the read light detector can, if desired be located on the opposite side of the disc from the read light and in such instance the disc information surface need not be reflective. One skilled in the art will appreciate that the present invention also can be used advantageously in cooperation with a low performance (i.e. low bandwidth) focus servo system. In such embodiments the focus servo controls adjustment of the lens means at low frequencies to initially access the discs (and thus accommodate for slight constructional differences between discs) and/or to accommodate for any changes in the vertical position of the central plane of a stabilized disc (which exist in some embodiments as a function of disc radius) . In such embodiments the present invention again functions to reduce the high frequency defocus caused by thickness variation of the disc support.

Claims: CLAIM : 1. A method for maintaining a light beam in focus on the information layer of a rotating optical disc having a substantially stabilized neutral plane, said layer being supported by a transparent support of nominal thickness t and refractive index n, said method being characterized by: a) directing said light beam to said information layer through a lens (9) of focal length FL and through said transparent support, and b) spacing said lens from the neutral plane of said disc by a distance FL-(t/n - t/2).

Inventors: BARNARD JAMES ARTHUR, HOWE DENNIS GEORGE, BARNARD, JAMES, ARTHUR, HOWE, DENNIS, GEORGE
Applicants: EASTMAN KODAK CO, EASTMAN KODAK COMPANY
Application year: 1983
Main classification: G11B7
IPC codes: G02B7, G11B7