DEVICE FOR OPTICALLY SCANNING A RECORD CARRIER

Abstract: A device is described for optically scanning two types of record carrier (1; 25) with a radiation beam (15) having a high numerical aperture. The radiation beam is focused through a transparent layer (2; 26) of the record carrier on an information layer (3; 27) by means of an objective lens (10) and a plano-convex lens (11). The magnifying power of the plano-convex lens is equal to 1/n1, i.e. the inverse of the refractive index of the transparent layer (2). The gap between the plano-convex lens and the record carrier can be maintained at a fixed value when the scanning is changed between the first and the second record carrier.

Description: Device for optical]) scanning a record earner The invention relates to an optical scanning device for optical]} scanning a first and second type of record earner, the first type of record earner compnsing a first information layer and a first transparent layer having a first thickness, the second type of record earner compnsing a second information layer and a second transparent layer having a refractive index ni and a second thickness larger than the first thickness, the device compnsing an objective lens and a convex lens for converging a radiation beam through the first or second transparent layer to a focus on the information layer, and means for axially positioning the convex lens, the convex lens having a convex surface facing the objective lens, a substantially planar surface facing the transparent layer, a magnifying power and a refractive index n 2 . The amount of information that can be stored on an optical record earner depends inter alia on the size of the radiation spot formed by the scanning device on the information layer of the record earner. The information density and hence the amount of stored information can be increased by decreasing the size of the spot. The spot size can be reduced by increasing the numencal aperture of the radiation beam forming the spot. When using a single objective lens, such an increase of the numencal aperture is in general accompanied by a decrease of the free working distance of the lens forming the radiation beam, i.e the smallest distance between the record earner and the lens. At higher numencal apertures, the manufactunng costs of such objective lenses become high, the field of the lens reduces and the dispersion of the matenal the lens is made of gives increasing problems. The problems may be mitigated by inserting a convex lens between the objective lens and the record earner. The convex lens may be plano-convex. The convex lens may be a slider lens or a solid immersion lens, and may be arranged at a very small distance above the record earner by means determining its axial position, i.e. its position along the optical axis of the lens Larger distances are also possible The total convergence of the radiation beam is distnbuted over the objective lens and the plano-convex lens. An advantage of the use of the planoconvex lens is that it hardly adds aberrations to the radiation beam. A scanning device having such a plano-convex lens is known from the European patent application no 0 727 777. The device compnses an optical head in which an objective lens and a plano-convex lens converge a radiation beam to a numencal aperture (NA) of 0.84 for scanning the record carrier through a transparent layer. The plano-convex lens is arranged at a small height above the record carrier. The lens may be mounted on a slider in sliding contact with the record carrier or floating on a thin air layer. The lens has a free working distance of several micrometers. It is a disadvantage of the known device, that, when the scanning is changed to a record carrier having a transparent layer with a greater thickness, the free working distance will reduce considerably. For some embodiment it appears to change from 300 to 125 μm when changing the thickness of the transparent layer from 100 to 600 μm. Such a reduction requires a height of the plano-convex lens that can be adjusted in dependence on the type of record carrier to be scanned. It also affects the dynamic behaviour of the plano-convex lens, in particular the behaviour of the servo controlling the axial position and/or the tilt of the lens and increases the risk of collisions between the optical head and the record carrier. It is an object of the invention to provide a scanning device for scanning two types of record carrier with a high numerical aperture radiation beam that does not have the above disadvantages. The object is achieved in accordance with the invention by a scanning device as described in the opening paragraph, which is characterized in that the magnifying power of the convex lens is substantially equal to 1/nι and the means maintain the distance between the convex lens and the record carrier at a predetermined value. The scanning device has the advantage that the distance between the convex lens and the record carrier has substantially the same value for the first and second type of record carrier. Hence, the same suspension of the plano-convex lens can be used for both types of record carrier, thereby simplifying the design of the scanning device and improving the tolerance to crashes of the optical head of the scanning device into the record carrier. The servo system of the scanning device for axially positioning the objective lens and/or the convex lens can be relatively simple. The servo system must adjust the position of the objective lens such that the radiation beam is focused on an information layer and it must keep the distance between the convex lens and the record carrier at a constant value. The servo system does not required different settings for different record carriers. It is an advantage of the scanning device according to the invention, that spherical aberration compensation is automatically adjusted to the record carrier being scanned. The magnifying power is preferably 1/nj when scanning the type of record carrier requiring the largest numerial aperture (NA). This is, in general, the type of record carrier having the thinnest transparent layer. When the refractive index of the transparent layer is in a range between 1.5 and 1.6, then the magnification is in a range between 0.625 and 0.667, i.e. 0.646 ± 0.021. The invention also relates to an optical scanning device for optically scanning a record carrier comprising at least two superjacent information layers and a transparent spacer layer having a refractive index ni arranged between the information layers, the device comprising an objective lens and a convex lens for converging a radiation beam to a focus on one of the information layers, and means for axially positioning the convex lens, the convex lens having a convex surface facing the objective lens, a substantially planar surface facing the transparent layer, a magnifying power and a refractive index n 2 , characterized in that the magnifying power of the convex lens is substantially equal to 1/nι, and the means maintain the distance between the convex lens and the record carrier at a predetermined value. When changing the scanning from one information layer to another one, the axial position of the objective lens must be changed to move the focus of the radiation beam to the desired information layer, whereas the distance between the convex lens and the entrance face of the record carrier, also called 'gap', need not be changed. Since the distance between the convex lens and the entrance surface of the record carrier can be kept at a fixed value, the construction of the optical head of the scanning device is simplified. The refractive index ni of the transparent layer is preferably larger than the refractive index n 2 of the convex lens. The spherical aberration caused by the gap between the convex lens, having a refractive index lower than n 2 , and the transparent layer, having a refractive index larger than n 2 , at least partly compensate one another. The amount of spherical aberration to be introduced by the objective lens is then reduced, which in turn further increases the mentioned positional tolerance. The refractive indices are preferably related through (m-1) > 1.03 (n 2 -l). To obtain a minimum spherical aberration at the location of the focus, the size of the gap, d gap , depends preferably on the refractive index n of the convex lens and ni of the transparent substrate according to the following relation: dg a p/d s = (n 2 /nι 3 )*(nι 2 -n 2 )/(n 2 2 -l). The value of d gap preferably complies with the relation within 40% for values of d s larger than d gap , where d s is the thickness of the transparent layer. The value of d s is preferably larger than the value of d gap in order to obtain refractive indices of available materials. The product of the magnifying power and the refractive index nj lies preferably within a range from 0.95 to 1.05. The objects, advantages and features of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. Figure 1 shows a scanning device; Figure 2 shows an objective lens and a convex lens; Figure 3 shows three different types of record carrier; and Figure 4 shows an embodiment of an objective lens and a plano-convex lens. Figure 1 shows a device for scanning an optical record carrier 1. The record carrier comprises a transparent layer 2, on one side of which an information layer 3 is arranged. The side of the information layer facing away from the transparent layer is protected from environmental influences by a protection layer 4. The side of the transparent layer facing the device is called the entrance face 5. The transparent layer 2 acts as a substrate for the record carrier by providing mechanical support for the information layer. Alternatively, the transparent layer may have the sole function of protecting the information layer, while the mechanical support is provided by a layer on the other side of the information layer, for instance by the protection layer 4 or by a further information layer and transparent layer connected to the information layer 3. Information may be stored in the information layer 3 of the record carrier in the form of optically detectable marks arranged in substantially parallel, concentric or spiral tracks, not indicated in the Figure. The marks may be in any optically readable form, e.g. in the form of pits, or areas with a reflection coefficient or a direction of magnetization different from their surroundings, or a combination of these forms. The scanning device comprises a radiation source 6, for example a semi- conductor laser, emitting a diverging radiation beam 7. A beam splitter 8, for example a semi- transparent plate, reflects the radiation towards a lens system. The lens system comprises a collimator lens 9, an objective lens 10 and a convex lens 11. The collimator lens 9 changes the diverging radiation beam 7 to a collimated beam 12. The objective lens 10, having an optical axis 13, transforms the collimated radiation beam 12 into a converging beam 14 incident on the lens 1 1. The collimator lens 9 and the objective lens 10 may be combined into a single lens. The convex lens 11 changes the incident beam 14 into a converging beam 15, which comes to a focus 16 on the information layer 3. The convex lens 11 has a convex surface and a substantially flat surface, i.e. the lens 11 is a plano-convex lens. The flat surface may have an aspheric profile without significant optical strength. The flat surface may have a curvature, e.g. for aero-dynamic purposes, the curvature being sufficiently small not to affect significantly the optical performance of the convex lens. The flat surface faces the transparent layer 2 and forms a gap between the lens and the layer. Although the objective lens 10 is indicated in the Figure as a single lens element, it may comprise more elements, and may also comprise a hologram operating in transmission or reflection, or a grating for coupling radiation out of a waveguide carrying the radiation beam. Radiation of the converging beam 15 reflected by the information layer 3 forms a reflected beam 17, which returns on the optical path of the forward converging beam 14. The objective lens 10 and the collimator lens 9 transform the reflected beam 17 to a converging reflected beam 18, and the beam splitter 8 separates the forward and reflected beams by transmitting at least part of the reflected beam 18 towards a detection system 19. The detection system captures the radiation and converts it into one or more electrical signals. One of these signals is an information signal 20, the value of which represents the information read from the information layer 3. Another signal is a focus error signal 21, the value of which represents the axial difference in height between the focus 16 and the information layer 3. The focus error signal is used as input for a focus servo controller 22, which controls the axial position of the objective lens 10 and/or the convex lens 11, thereby controlling the axial position of the focus 16 such that it coincides substantially with the plane of the information layer 3. The part of the detection system, including one or more radiation- sensitive detection elements and an electronic circuit processing the output signal of the detection elements, used for generating the focus error is called the focus error detection system. The focus servo system for positioning the lens system comprises the focus error detection system, the focus servo controller and an actuator for moving the lens system. The gap, i.e. the distance between the planar surface of lens 11 and the entrance surface 5 of the record carrier 1, should be maintained substantially at a nominal value, independent of the type of record carrier and of the position of the information layer in the record carrier. This can be attained by using a passive air bearing construction carrying lens 11 and designed to maintain the gap at its nominal value. It is also possible to use an optically derived error signal which represents the deviation of the actual gap from its nominal value; a special actuator then keeps the plano-convex lens at its prescribed distance from the transparent layer by using the error signal as an input signal for the actuator servo loop. The actuator of the lens 10 is controlled by the focus error signal 21 to keep the focus 16 on the information layer 3. The spherical aberration which arises when the radiation beam has to be focused through a transparent layer which is thicker than the design thickness of the layer, is compensated for by a focusing action of the objective lens. The focusing action causes the plano-convex lens to produce, due to the changing magnification, an amount of spherical aberration which cancels the aberration produced by the thicker transparent layer. During the focusing action of the objective lens, the gap should be maintained substantially at its nominal value in the above-mentioned way. The variations in the thickness of the transparent layer of a record carrier are in general of the order of a few percent of the nominal thickness, e.g. less than 10 μm for a nominal layer thickness of 100 μm. The variations for particular applications are within 3 μm p-p. Figure 2 shows an enlargement of the objective lens 10 and the plano-convex lens 11. The objective lens 10 may be a mono-aspherical plano-convex lens or a bi-aspherical lens . The objective lens 10 is designed in a known way to compensate for the spherical aberration introduced by the convex lens 11 and the transparent layer 2, thereby making the radiation beam near the focus 16 nominally substantially free from spherical aberration. The tolerance of the optical head for thickness variations of the transparent layer 2 can be relatively large if the magnifying power B of the plano-convex lens 11 is made substantially equal to 1/nj, i.,e. the inverse of the refractive index of the transparent layer. This can be understood as follows. When the distance between lens 10 and lens 11 changes, the conjugate distances of lens 11 change. If the image conjugate of lens 11 changes by an amount of Δl, the spherical aberration W in the radiation beam 15 changes by - AW. =-(l - B 2 ) NA Λ Al 8 The spherical aberration introduced by a change Δdj in the thickness of the transparent layer 2 into the radiation beam 15 is equal to ΔW, = 1 1 -~( " \-n ,ϊ2), A 4 d Jx N Λ „A4* 8 «, If the image conjugate of lens 11 changes by Δl and the thickness of the transparent layer by Δdi, then the distance d 2 between the lens 11 and the entrance face of the record carrier must change by Δd 2 to keep the focus of the radiation beam on the information layer, where n, The spherical aberration introduced by the thickness variation may be compensated by choosing ΔWi = -ΔW 2 , resulting in a value of Δl equal to The resulting change Δd gap in the distance d gap between the flat surface of the plano-convex lens 11 and the entrance face 5 of the transparent layer 2 is The term between square brackets can be made zero by taking the value of B equal to l/nj. In that case the change Δd gap in the distance d gap is in this approximation independent of the variations Δdi in the thickness of the transparent layer 2. In other words, the free working distance of lens 11 is independent of the thickness of the transparent layer. The scanning device is designed for scanning two different types of record carrier. Figure 3 shows the first type of record carrier 25, comprising a transparent layer 26 having an entrance face 29 on which the radiation of the scanning device enters the record carrier. The record carrier 25 also comprises an information layer 27 and a protective layer 28. The second type of record carrier is the record carrier 1 shown in Figure 1, having the transparent layer 2, information layer 3 and protective layer 4. The information layers 3 and 27 may have different or similar information densities and the information may be stored in different or similar types of marks in different or similar formats. The thickness of transparent layer 2 is larger than that of transparent layer 26. The thickness of the transparent layer 26 may be zero, making the record carrier a so-called air-incident record carrier 30, as shown in Figure 3, and comprising an information layer 31 and a protective layer or substrate 32. In a particular example, the transparent layer 26 is a 100 μm thickness poly-carbonate foil and the transparent layer 2 is a 600 μm thickness polycarbonate layer, whereas the information density of the information layer 27 is higher than that of information layer 3. The gap between the convex lens 11 and the entrance surface 5 is chosen sufficiently small to realize the required high numerical aperture of the radiation beam 15 and sufficiently large to avoid crashes of the plano-convex lens 11 or its holder against the record carrier. When the scanning is changed from a record carrier 25 of the first type to a record carrier 1 of the second type, the convex lens 11 is positioned at the same height above the entrance face 5 as it had above the entrance face 29. The axial position of the objective lens 10 is controlled by the focus servo system such that the focus 16 coincides with the information layer 27. The positioning of both lenses 10 and 11 will have changed the distance between them in such a way that the lenses compensate the change in spherical aberration caused by the change in thickness of the transparent layer. Since the gap between the planoconvex lens 11 and the entrance face of both types of record carrier is substantially the same, the dynamic behaviour of the optical head will also be very similar for both types of record carrier. Figure 4 shows an objective lens 45 and a convex lens 46 for the above scanning device. The wavelength of the radiation is 650 nm. The numerical aperture for scanning the record carrier 25 having a 100 μm polycarbonate transparent layer is equal to 0.85. The numerical aperture for scanning the record carrier 1 having a 600 μm polycarbonate transparent layer is equal to 0.60. The refractive index of polycarbonate at 650 nm wavelength is equal to 1.5806. The plano-convex lens 46 is made of the Schott glass FK3 with a refractive index of 1.46 and one flat surface. The radius of curvature of the convex surface of the lens 46 is 1.25 mm, and the thickness of the lens on the optical axis is equal to 1.25 mm. The value of niB for the lens 11 is 0.985. The distance between the piano surface 47 and the entrance face 29 of the record carrier 25 is designed to be 100 μm. When the scanning is changed to record carrier 1, the distance between the piano surface and the entrance face 5 changes to 93 μm to obtain optimum spherical aberration compensation, i.e. a change of only 7 μm on a distance of 100 μm. Hence, keeping the air gap equal to 100 μm will result in an effective spherical aberration compensation when changing from a record carrier of the first type to one of the second type. The objective lens 45 is made of Schott glass LAKN22, having a refractive index of 1.648. The convex surface 48 of the objective lens has a radius of curvature τ \ equal to 2.429 mm, the concave surface 49 a radius of curvature r 2 equal to 180.368 mm. The thickness of the lens on the optical axis is 1.9 mm. The distance on the optical axis between the lenses 45 and 46 is 0.010 mm when scanning through the 600 μm transparent layer of record carrier 1 and 0.985 mm when scanning through the 100 μm transparent layer of record carrier 25. Both surfaces 48 and 49 have an aspheric profile, described by the following equation: z +a 4 r 4 + a 6 r 6 +... where Ci = l/ for surface 48 and Ci = l/r 2 for surface 49, z is the coordinate along the optical axis and r is the radial coordinate perpendicular to the optical axis, as indicated in the figure. The values of the coefficients a 2 to aι 6 are as follows Surface 48 Surface 49 a = -0.00064031138 3 4 = 0.018413388 aa = -0.002214109 a ό = -0.023037169 a 8 = -0.00013925445 3 8 = 0.0171954 a, 0 = 0.00029925671 3,o = -0.0071408498 a 12 = -0.00010404658 3, 2 = 0.0015024416 a I4 = 1.2246551 e-5 3, 4 = -0.00012458591 aiό = -5.0598072 e-7 3,6 = 0 The sc3nning device is also eminently suitable for scanning a multi-layer record carrier 35 as shown in Figure 3. The record carrier comprises a transpsrent lsyer 36 with sn entrance face 37, two information yers 38 and 39, separated by a transpsrent spacer layer 40, and a protective cover layer 41. The spacer layer may be a foil glued between the information layers or a UV-hardened spin-coated layer having a thickness of 30 μm. The materisl of the spscer lsyer m3y be a polymer such as poly-carbonate having a refractive index 1.58. When the scanning is changed from information layer 38 to information layer 39, the objective lens 10 is moved axially such that the focus 16 is displaced from information layer 38 to information layer 39. The distance between the plano-convex lens 11 and the entrance face 37 of the record carrier does not change, i.e. the free working distsnce stsys spproximstely the same. The change in distance between the objective lens 10 and the plano-convex lens 11 introduces spherical aberration that compensstes the spheric3l sberration due to the spacer layer 40. The combination of the objective lens and the plano-convex lens operates as an adjustsble spherical aberration compensator.

Claims: CLAIMS: 1. An optical scanning device for optically scanning 3 first snd second type of record carrier, the first type of record carrier comprising a first information layer and a first transparent layer hsving a first thickness, the second type of record carrier comprising a second information layer and a second transparent layer having a refractive index n, and a second thickness larger than the first thickness, the device comprising 3n objective lens and a convex lens for converging a radistion beam through the first or second transparent layer to 3 focus on the information layer, and means for axially positioning the convex lens, the convex lens hsving a convex surface facing the objective lens, a substantially planar surface facing the transparent layer, a magnifying power 3nd a refractive index n , characterized in that the magnifying power of the convex lens is substantially equal to l/n,, and the means maintain the distance between the convex lens and the record carrier at a predetermined value. 2. Optical sc3nning device according to Claim 1, wherein the value of n, is larger than the value of n 2 . 3. Optical scanning device according to Claim 2, wherein the refractive indices n, and n 2 sre related through (n,-l) > 1.03 (n 2 -l). 4. An opticsl scsnning device for opticslly scsnning a record csrrier comprising at least two superjacent information layers and a transpsrent spacer layer having a refractive index n, arranged between the informstion layers, the device comprising an objective lens and a convex lens for converging a radiation beam to a focus on one of the information layers, and means for axislly positioning the convex lens, the convex lens hsving a convex surface facing the objective lens, a substantially planar surface facing the transpsrent lsyer, a msgnifying power and a refractive index n 2 , characterized in that the magnifying power of the convex lens is substantially equal to l/n,, and the means maint3in the distance between the convex lens and the record carrier at a predetermined value. 5. Optical scanning device according to Claim 4, wherein the value of n, is larger than the value of n 2 . 6. Optical scanning device according to Claim 5, wherein the refractive indices n, and n 2 are related through (n,-l) > 1.03 (n 2 -l).

Inventors: BRAAT JOSEPHUS J M, HENDRIKS BERNARDUS H W, BRAAT, JOSEPHUS, J., M., HENDRIKS, BERNARDUS, H., W.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 1999
Main classification: G11B7
IPC codes: G11B7