WO2004012187A1

WO2004012187A1 - Optical scanning device

Info

Publication number: WO2004012187A1
Application number: PCT/IB2003/003253
Authority: WO
Inventors: Sjoerd Stallinga; Bernardus H. W. Hendriks
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2002-07-25
Filing date: 2003-07-16
Publication date: 2004-02-05
Also published as: AU2003247091A1; JP2005534133A

Abstract

An optical scanning device for scanning an optical data storage medium. For the purpose of generating a focus error signal for maintaining a focus state of the scanning spot on the storage medium and that of generating a radial error signal for maintaining a tracking state of the scanning spot in a radial direction on the storage medium, an optical arrangement (OA) is arranged in the optical path of the radiation beam between the position of the storage medium and the detection system (25, 27, 29). The optical arrangement splits the radiation beam into at least a first sub-beam (A) and a second sub-beam (B) and generates a first astigmatic aberration having an appertaining focal line in the first sub-beam and a second, different astigmatic aberration having an appertaining focal line in the second sub-beam. The direction of the focal line of the first sub-beam is inclined with respect to a track-equivalent direction at a positive angle and the direction of the focal line of the second sub-beam is inclined with respect to the track-equivalent direction at a negative angle. The detection system (25, 27, 29) is arranged to generate the radial error signal using both the first sub-beam and the second sub-beam.

Description

Optical scanning device

The present invention relates to an optical scanning device for scanning an optical data storage medium, such as an optical disk.

To read data out from an optical disk, the objective lens must be located at the correct height above the disk and directly above the track being read out. This is achieved by the use of two servo loops, which operate in reliance on the generation of focus and radial error signals. One known focusing method is astigmatic focus error detection. One problem with astigmatic focusing is its sensitivity to astigmatism at 45° (A₂₂), which causes crosstalk between the error signals, referred to as radial-vertical crosstalk (RVC). The radial error signal may be generated using either a differential phase detection (DPD) method (for ROM- disks) or a push-pull (PP) method. The PP method is used in particular for (re)writable disks, since DPD does not generate a sufficient signal for unwritten disks. A problem with the push- pull method is that it is sensitive to decentring of the spot on the detector. This can be solved by using two satellite spots, which are generated using an extra diffraction grating.

United States patent 5,233,444 describes a focus error detecting apparatus in which a hologram diffracts a light beam to generate plus and minus first-order diffraction beams with different types of astigmatism, in order to simplify adjustment of the radiation detectors during manufacture. However, the manner in which radial tracking control is carried out is not described.

In accordance with the present invention there is provided an optical scanning device for scanning an optical data storage medium having data storage regions arranged in substantially parallel track sections, the device comprising: a radiation source arranged to produce a radiation beam for scanning the storage medium to produce a scanning spot; a detection system having detector elements arranged to detect radiation after interaction of the radiation beam with the storage medium, the detection system being arranged to generate a focus error signal for maintaining a focus state of the scanning spot on the storage medium and to generate a transverse error signal for maintaining a tracking state of the scanning spot in a transverse direction with respect to the track sections on the storage medium; and an optical arrangement, arranged in the optical path of the radiation beam between the position of the storage medium and the detection system, for splitting the radiation beam into at least a first sub-beam and a second sub-beam and for generating a first astigmatic aberration having an appertaining focal line in the first sub-beam and a second astigmatic aberration having an appertaining focal line in the second sub-beam, wherein the detection system is arranged to detect radiation from each of said first sub-beam and said second sub-beam to generate first sub-beam detection signals and second sub-beam detection signals respectively, characterised in that the optical arrangement is arranged such that the direction of the focal line of the first sub-beam is inclined with respect to a track-equivalent direction at a positive angle, and the direction of the focal line of the second sub-beam is inclined with respect to the track-equivalent direction at a negative angle, and in that the detection system is arranged to generate said transverse error signal using both said first sub-beam detection signals and said second sub-beam detection signals.

Preferably, when the spot is in the well-focused state on the storage medium, the spots on the detection system have a round cross section. However, when the spot on the storage medium is out-of focus the generation of the aberrations as described will produce differently aligned elliptical spots on the detection system. By the given arrangement of the optical arrangement with respect to the track equivalent direction and by generating the transverse error signal using both said first sub- beam detection signals and said second sub-beam detection signals, transverse tracking control may be achieved with reduced sensitivity to decentring of the spot on the detection system, whilst obviating the need for a three-spot diffraction grating, thereby to simplify the construction of the optical lightpath components in an optical scanning device.

Further advantages of the invention include providing focus control whilst reducing sensitivity to A₂₂ astigmatism and to reducing radial-vertical crosstalk (RVC).

It should be noted that the astigmatic aberration in each of the two sub-beams has two appertaining focal lines, spaced along the axis of the beam and mutually oriented at a right angle. The position of the focal line in each sub-beam which is furthest away from the optical arrangement is in general behind the detection system. Since the detection system interrupts the sub-beam, the furthest focal line is not_ractually formed. Each of the focal lines referred to in the above paragraph is the focal line of a sub-beam closest to the optical arrangement and in general having a position between the optical arrangement and the detection system.

The optical arrangement may include a single optical component capable of both splitting the beam and generating the different aberrations, or two or more optical components. For example, one optical component may be used for splitting the beam into two sub-beams and one or more further optical components may be used for generating astigmatic aberration in the two sub-beams.

In a preferred embodiment, the data storage medium is in the form of a disk, in which the substantially parallel track sections are substantially concentric, and in which the transverse direction corresponds to a radial direction on the disk.

The features and advantages of the present invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, made with reference to the accompanying drawings, wherein:

Figure 1 is a schematic illustration of components of an optical scanning device in accordance with an embodiment of the invention;

Figure 2 is a schematic illustration of the operation of an optical arrangement as used in an embodiment of the invention; Figure 3 is a cross-sectional view of two sub-beam spots as produced in an embodiment of the invention;

Figure 4 is a plan view of spots formed on a detector array in accordance with an embodiment of the invention;

Figures 5(A) to 5(C) show spots formed on a detector array in different focus states of the optical scanning device;

Figures 6(A) and 6(B) show a birefringent optical arrangement in cross- section in accordance with an embodiment of the invention;

Figure 7 shows a diffractive optical arrangement in plan view in accordance with an embodiment of the invention; Figure 8 shows a laser detector grating unit (LDGU) in accordance with an embodiment of the invention; and

Figure 9 shows a schematic illustration of a birefringent plate arrangement in accordance with an embodiment of the invention. Figure 1 shows elements of a typical optical scanning device, including an optical head scanning an optical record carrier 2. It is to be noted that the embodiments of the invention, to be described subsequently, include similarly arranged components in each optical beam path. Hence, the description should be taken to apply to the components in each optical beam path of the embodiments of the invention.

The record carrier 2 is in the form of an optical disk comprising a transparent layer 3, on one side of which an information layer is arranged. The side of the information layer 4 facing away from the transparent layer 3 is protected from environmental influences by a protection layer 5. The side of the transparent layer 3 facing the device is called the entrance face 6. The transparent layer 3 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 5 or by a further information layer and a transparent layer connected to the information layer 4. Information may be stored in the information layer 4 of the record carrier in the form of optically detectable marks arranged in substantially concentric, for example concentric or spiral, track sections, not indicated in Figure 1. The marks may be in any optically readable form, e.g. in the form of pits, or areas with a reflection coefficient different from their surroundings.

The scanning device comprises a linearly polarised radiation source in the form of a semiconductor laser 9 emitting a radiation beam 7. The radiation beam is used for scanning the information layer 4 of the optical record carrier 2. A polarising beam splitter 13 reflects the diverging radiation beam 12 on the optical path towards a collimator lens 14, which converts the diverging beam 12 into a coUimated beam 16. The beam 16 is incident on an objective system 18. The objective system may comprise one or more lenses and/or a grating. The objective system 18 in Figure 1 consists in this example of two elements, a first lens 18a and a second lens 18b arranged between the lens 14 and the position of the record carrier 2. The objective system 18 has an optical axis 19. The objective system 18 changes the beam 16 to a converging beam 20 incident on the entrance face 6 of the record carrier 2. The objective system has a spherical aberration correction characteristic adapted for passage of the radiation beam through the thickness of the transparent layer 3. The converging beam 20 forms a spot 21 on the information layer 4. Radiation reflected by the information layer 4 forms a diverging beam 22, transformed into a substantially coUimated beam 23 by the objective system 18 and subsequently into a converging beam 24 by the collimator lens 14. The beam splitter 13 separates the forward and reflected beams by transmitting at least part of the converging beam 24 towards a detector array 25. An optical arrangement OA splits the beam 24 into two sub-beams A and B. The detection system captures the radiation from the two sub-beams and converts it into electrical output signals, carried on wiring flex 26, which are processed by signal processing circuits 27, 29 and 31 which are located in the scanning device separately from the optical head 1. A signal processor 27 converts these output signals to various other signals.

One of the signals is an information signal 28, the value of which represents information read from the information layer 4. The information signal is processed by an information processing unit for error correction 29. Other signals from the signal processor 27 are the focus error signal (not shown) and the radial error signal 30. The focus error signal represents the axial different in height between the spot 21 and the information layer 4. The radial error signal represents the distance in the plane of the information layer 4 between the spot 21 and the centre of a track in the information layer to be followed by the spot.

The focus error signal and the radial error signal are fed into a servo circuit (not shown) which converts these signals to a focus error signal and a radial error signal for controlling mechanical focus actuators (not shown) in the optical head. The mechanical focus actuators control the position of the objective system 18 in the focus direction 33, thereby controlling the axial position of the spot 21 such that it coincides substantially with the plane of the information layer 4, and in the radial direction 34, thereby controlling the radial position of the spot 21 such that it coincides substantially with the track currently being scanned. A further mechanical actuator, such as a radially movable arm, alters the position of the optical head 1 in the radial direction 34 of the disk 2, thereby coarsely controlling the radial position of the spot 21 to lie above a track to be followed in the information layer 4. The tracks in the record carrier 2 run in a direction perpendicular to the plane of Figure 1. The optical arrangement OA is placed in the reflected beam which splits the beam into two sub-beams, such that the two sub-beams have a different wavefront tilt, and have astigmatisms of a different sign. The difference in wavefront tilt ensures that the sub- beams are laterally separated whereby two spots are placed onto the detector array. The sub- beams A and B have different astigmatisms, preferably at +45° with respect to the track- equivalent direction. Sub-beams A and B thus do not have a unique focus point, but each has two mutually orthogonal focus lines, as illustrated in Figure 2.

Figure 2 shows the operation of the optical arrangement OA in greater detail than in Figure 1 , with the illustration being rotated through 45° about the optical axis 19 with respect to the illustration shown in Figure 1. Referring to Figure 2, the coUimated beam 24, on passing through the optical arrangement OA, is split into two sub-beams A and B, each having an added astigmatism of a different sign. The first sub-beam A has a first focus line FL1(A) which is oriented perpendicular to the plane of the drawing in the illustration in Figure 2. The first focus line FL1(A) is located above the detector plane DP. For the purposes of illustration in Figure 2, the detector is not present, such that the radiation passes through the detector plane DP. Below the detector plane, the first sub-sub-beam A has a second focus line FL2(A), oriented parallel to the plane of the drawing as illustrated in Figure 2. The second sub-beam has focus lines FL1(B) and FL2(B) which are oriented perpendicular to the focus lines FL1(A) and FL2(A), respectively. Between the focus lines, the shapes of the sub- beams vary between generally elliptical above the detector plane DP, round and once again generally elliptical below the detector plane. The shape of the sub-beams immediately above the detector plane, taken across the plane X parallel to the detector plane and between the detector plane and the first focus lines FL1(A) and FL1(B) is illustrated in Figure 3.

Referring to Figure 3, in the plane X, the first sub-beam A has an elliptical cross-section X(A), and the second sub-beam B has an elliptical cross-section X(B). The two cross-sections have long axes LA(A) and LA(B) which are perpendicular, and which are each arranged at an angle to a track equivalent direction (TED). The track equivalent direction is the optical equivalent direction of the tracks on the optical disk 2. Namely, a line located at the pupil of the scanning objective lens parallel to the direction of a track on the optical disk is imaged to a line parallel to the track equivalent direction TED at the detector. The first sub- beam cross-section X(A) has a long axis LA(A), parallel to the first focus line FL1(A) of the first sub-beam A, which is arranged at a positive angle α with respect to the track equivalent direction TED. The second sub-beam cross-section X(B) has a long axis LA(B), parallel to the first focus line FL1(B) of the second sub-beam B, which is arranged at a negative angle β with respect to the track equivalent direction. Preferably, α is in the region of +45° and β is in the region of -45°.

Halfway between the two focus lines, the cross-section of each sub-beam is round (referred to herein as the "circle of least confusion"), and to each side thereof elliptical. By placing a detector array between these two focus lines, two spots are formed on the detector array, as illustrated in Figure 4. The element OA is arranged such that the "circle of least confusion" for both sub-beams is located the same distance from the optical arrangement when the system is in focus. When the system is out of focus, the focus line at 45° of sub-beam A will be closer to the detector array than the focus line at -45°, or vice versa. This results in the spot S(A) on the detector array becoming elliptical, with the long axis of the ellipse lying at 45°. With sub-beam B being astigmatic with the opposite sign, spot S(B) will also be elliptical with its long axis perpendicular to that of spot S(A).

Referring to Figure 4, which illustrates the position and shape of the spots S(A) and S(B) on each of the quadrant detectors 25(A) and 25(B) in accordance with this embodiment of the invention, each of the spots is shown in a well-focused state. Thus, each spot has a round outline. The first quadrant detector 25(A) has four quadrants A_l5 A₂, A₃ and A₄, the detector having a centre on which the spot S(A) is centred. The second detector 25(B) includes four quadrant detector elements _\, B₂, B₃ and B₄, the detector having a centre on which the spot S(B) is centred. Due to the diffraction of the spot by the data tracks on the optical disk, the coUimated beam 24 has first order (±1) sidebeams. The first spot S(A) thus has a first order sidebeam spot a(+l) and a first order sidebeam spot a(-l), the two sidebeam spots overlapping the main spot S(A) and being separated therefrom in a direction perpendicular to the track equivalent direction TED. The second spot S(B) has similarly arranged side spots b(-l), b(+l). The track equivalent direction TED makes an angle of 45° with the line connecting the centres of the two quadrant detectors. Note, however, that this is not necessarily the case; an angle of, for example, 0° is possible as well.

Regarding the arrangement of the diffraction orders in the spots S(A) and S(B), the line parallel to the tracks in the pupil plane is imaged by the optical arrangement OA (containing the splitting functionality and functionality of adding astigmatism at +/-45 degrees to the two sub-beams) into the line TED at the detector plane. Due to the astigmatism of both sub-beams the TED line is perpendicular to the line in the pupil that it is the image of. In addition, the diffraction orders are mirrored, due to the astigmatism, about lines LA(A) and LA(B), shown in Figure 3, respectively. As a consequence of these two effects, the radial disc diffraction orders at the detector plane should be oriented perpendicular to the line TED, as is shown in Figure 4.

Figures 5(A), 5(B) and 5(C) show the spots S(A) and S(B) in various focus states. In Figure 5(A), the spots are shown in a state in which the objective lens 18 is too far away from the entrance face 6 of the disk. The spots in Figure 5(B) are shown in the state when the objective lens 18 is well-focused. The spots shown in Figure 5(C) are shown in the state when the objective lens 18 is too close to the entrance face 6 of the disk.

With a quadrant detector structure as illustrated in Figure 4, the focus error signal FES is in one embodiment generated using an astigmatic method, using the combination of signals:

FES = ASA - AS_B (1)

AS_A = A₁ + A₃ - A₂ - A₄ (2)

AS_B = B₁ + B₃ - B₂ - B₄ (3)

To reduce the effect of difference in intensity between A and B, ASA and ASB are preferably normalised with respect to the total signal on A and B respectively. If there is A₂₂ astigmatism in the main beam, then this will alter the signals from AS_A to ASA+C'A₂₂ and from AS_B to AS_B+c'A ₂, with c' being a constant. The total focus error signal FES therefore is not affected. The sensitivity to A ₂ astigmatism is thus minimised or at least reduced. The radial error signal is in one embodiment generated using a push-pull method. It should be noted that the focus line at 45° for sub-beam A lies in front of the detector and thus locations of the first order satellites on the detector is mirrored with respect to their location in the pupil of the objective. For sub-beam B, the focus line at -45° lies in front of the detector. This results in the areas of the spots S(A) and S(B) being mirrored with respect to each other. The resulting radial error signal RES is then the difference between the following push-pull signals from A and B :

RES = PP_A - PPB (4)

PP_A = A₁ + A₂ - A₃ - A₄ (5)

PP_B = B₁ + B₂ - B₃ - B₄ (6)

Here also, any intensity variations are preferably normalised. In the case of to decentring of the spots with respect to the detector, that is to say in case of an alignment fault or due to disk tilt, the sub-beams do not fall directly in the middle of the detectors but are shifted to one side of the centre. However, the radial error signal does not alter because the radial error signal is generated by means of the difference between the push-pull signals from A and B. A detector decentring error ε causes a variation such that the signal PPA alters to PP_A+C8 and PP_B alters to PPβ+cε, with c being a constant. The total radial error signal is therefore not affected. A three spots diffraction grating for the generation of two extra satellite spots is thus not necessary.

As an alternative to the use of a push-pull radial error detection method, a Differential Phase Detection (DPD) or Differential Time Detection (DTD) radial error detection method may be used for the generation of radial error signals in the case of readout from read-only (ROM) disks. For ROM disks, DPD or DTD radial error detection may be preferable, because of the relatively low push-pull signal produced.

Embodiment with Birefringent Optical Element

Figures 6(A) and 6(B) illustrate an optical arrangement in accordance with one embodiment of the invention in two perpendicular cross sections. Figure 6(A) shows the lens in cross section through plane X-X shown in Figure 6(B), and Figure 6(B) shows the lens in cross section through plane Y-Y shown in Figure 6(A). In this embodiment, the optical arrangement includes a birefringent material, such as a liquid crystal (LC) polymer. Birefringent materials have a refractive index which is dependent on the polarisation of the radiation passing through it. If the polarisation is parallel to the optic axis of the liquid crystal, the refractive index is then n_e (extraordinary mode); if perpendicular to the optic axis then the refractive index is n_o (ordinary mode). For the generation of two sub-beams of different astigmatism in this embodiment, a birefringent astigmatic lens 100 is used. The lens 100 includes an upper glass substrate 102 and a lower glass substrate 106, between which the birefringent material 106 is positioned.

The birefringent material has an optic axis which is arranged perpendicular to a selected one of the planes X-X and Y-Y shown in Figures 6 A and 6B. The polarisation of the radiation beam reflected from the disk 2, before reaching the element 100, is arranged to have either a linear polarisation which is aligned at 45° to the optic axis of the birefringent material or a circular polarisation. One of the sub-beams is the ordinary mode ("o-mode") beam, and the other is the extraordinary mode ("e-mode") sub-beam.

As shown in Figure 6(A) the upper substrate 102 has an inner surface 112 which is planar and tilted with respect to a plane perpendicular to the optical axis by angle γ. the lower surface 104 has an inner surface having a saddle-form lens surface part 108, and an outer part 110, surrounding the lens surface part 108, which is a three-dimensional form which may follow the contours of the lens surface part. The lens surface part 108 has, in the cross section shown in Figure 6(A), a curvature Ri>0. The lens surface part 108 has, in the cross section shown in Figure 6(B), a curvature R₂<0.

The focus lengths for the ordinary mode sub-beam are as follows: f_0k8 =R_I/(n_β -n_B) (7) f_0,y =R₂/(n₀ -n_g) (8) The focus lengths for the extraordinary mode sub-beam are as follows: f_{β X} =R₁/(n_β -n_B) (9) f_e,y =R₂/(n_e -n_g) (10)

Herein, n_g is the refractive index of the glass. In one embodiment, the optical arrangement is designed such that R = -Ri and n_g= (n_o+n_e)/2, thus f₀,_x = f_e d n_{g y} < 0 and f_0ιy = fe,_x > 0. Thus the astigmatism generated has a different sign for each of the two sub-beams, and the circle of least confusion lies, for each sub-beam, the same distance away from the optical arrangement.

Where the astigmatic focus lines of the sub-beams lie can be calculated as follows. These are the two representative points of the focus point of the original coUimated beam in the X and Y directions. Given that the birefringent astigmatic lens lies a distance v in front of the original focus point, then the lateral magnifications are (given v is much smaller than f_x and f_y):

1 . . r. _(π)

M_x wl- -v/f_x l+v/f_x

1 M M_y^τ^- *11-- -vv//ff,_y (12)

1+v/f y

This applies to the e-mode sub-beam. The analysis for the o-mode sub-beam is found by interchanging x and y. The focus lines thus lie distances M_xv and M_yv behind the optical arrangement. The axial separation between the two focus lines is then (with Δn = n_e - no):

The location of the detector and the diameter of the spots on the detector array are now also calculable. With the main beam in front of the optical arrangement having a numerical aperture NA, then the e-mode sub-beam has a numerical aperture NA/M_X and NA M_y in either direction. If the detector is located a distance z^! behind the first focus line of the e-mode sub-beam, then the elliptical spot has a long axis length d_x and a short axis length d_y as follows: d_x =2(Δz-z')NA/M_x (14) d_y =2z*NA/M_y (15)

From the condition d_x= d_y= d (round spot), it follows that:

, 2ΔzNA . _{T A} .., _. d= «ΔzNA (17)

^• M_x +M_y

The appearing focal lengths f_x and f_y are equal to ±f c-R/Δn.. Using (13) and (17) it follows that:

f_Lc =1 v²^NA (18) d In one example d is in the region of lOOμm, v in the region of 5mm and NA in the region of 0.15. It follows that f_Lc is in the region of 38mm. With a typical value of Δn=0.1 for liquid crystal polymers, a typical curvature R=38cm results. The numerical aperture of the astigmatic birefringent lens is then for both directions |NA_LC| * vNA/2f c = d/2v = 0.01. The amount of astigmatism produced by the astigmatic lens is A₂₂ =f _CNA_LC =3.8μm,that is to say in the region of 6λ for the case that λ=0.65μm (a typical wavelength for DVD-type systems).

The difference in wavefront tilt, necessary for the lateral separation of the two spots, can be generated by using the field of the birefringent lens. By tilting the lens through an angle γ (see Figure 6 - [please describe]), the original focus point is located at a distance h=vγ away from the optical axis of the birefringent lens. The focus points of the two sub- beams therefore lie distances M_xh and M_yh away from the optical axis (the two sub-beams are refracted through different angles, namely h/f_x and h/f_y). The separation on the detector is then:

This separation should be greater than the spot diameter d, thus h≥vNA or γ≥NA.

In an alternative embodiment, the composite of the LC layer in the two glass substrates is formed in a wedge shape, hi this case, field angles in the order of the NA may also be used to achieve the desired lateral separation of the spots.

A result of this method of lateral separation is that both sub-beams receive an extra amount of astigmatism. Firstly, astigmatism is generated by the glass substrate of thickness d_g which the sub-beams pass through with angles +h/2f_Lc and - h/2f_Lc respectively. This provides:

Taking n_Lc * 1 -5, d_Lc « 10μm, γ » NA = 0.15, leads to an A₂₂ astigmatism of approximately 21 mλ, a negligible amount. The second additional contribution is made by the liquid crystal layer. This provides:

Taking nι,c « 1.5, d^ » 10μm, γ « NA = 0.15, leads to an A₂₂ astigmatism of approximately 0.5mλ, also a negligible amount. The third contribution is the consequence of astigmatism being inherent in the birefringent layers. This is only generated in the e-mode sub-beam. This is approximately a fraction dτχ/R of approximately 3xl0^"3 of the original astigmatism, that is to say in the region of 18mλ. This is also a negligible amount. By the appropriate selection of refractive indices n₀, n_e, and n_g it is possible to balance the astigmatisms generated in the e-mode and o-mode sub-beams.

Embodiments using a Diffractive Optical Element (DOE) In alternate embodiments of the invention, a diffractive optical arrangement is used. For this, a diffraction grating is provided which splits the beam into two such that the sub-beams receive different astigmatisms. The diffraction grating is a two dimensional phase object, instead of a one dimensional phase objects as in the case of standard diffraction gratings. It is also possible to use structures which employ combinations of diffraction patterns and birefringence in order to achieve the desired effect.

Consider first in brief the manner in which a DOE operates. The surface of a DOE is divided into zones, labelled by an integral value j, the zone-index. The locations f = (x, y) fall within a zone j in accordance with:

j≤^≤j+l (22)

The function W( r ) is called the zone function. Within zone j the variable s varies in accordance with:

between 0 and 1, of which [x] the largest integral value is smaller than x. The phase which is added to the radiation within each zone is:

The function f(s) is called the structure function. The transmission function of the DOE is then given by:

T(r)=e^iφ(?) =∑C_me^imW(ϊ) (25) m with:

1

C_m = Jdse^if(s)e-²"^im (26)

0

Here, m = ..., -2, -1, 0, 1, 2, ... the index of the diffraction orders which exist. Every diffraction order m therefore has an amplitude C_m and a phase mW( f ). The efficiency of the diffraction order m is then: ϊ/_m H C ² (27)

Overall, the DOE is characterised through two functions, the zone function W(f ) which gives the form of the zones, and the structure function f(s) which gives the phase within each zone.

Embodiment with Double Astigmatic DOE

Figure 7 shown an optical arrangement in the form of a DOE 200 in plan view in accordance with an embodiment of the invention, with the zone structures formed thereon. The element includes a plurality of zones %_\, z₂, etc.

The DOE 200 is provided which diffracts radiation mainly in two diffraction orders and which adds astigmatism and wavefront tilt to these two orders. In this embodiment, the -1^st and +l^st diffraction orders are used, with the zone function:

W(r)=A_π +A₂₂(x² -y²) , (28) where x and y are normalised pupil coordinates. Thus, the astigmatism of the -1^st and +l^st diffraction order sub-beams is equal and opposite. The wavefront tilt Ai ₁ and the astigmatism A₂₂ depend on the numerical aperture (NA) of the collimator lens, the diameter d of the two spots on the detector, the separation b between the centres of the spots, and the wavelength λ as follows:

A„ -- - (29)

2λ

A₂₂ =^ (30)

4Λ In one example λ = 780nm, NA = 0.15, d = lOOμm and b = 150μm. This provides a wavefront tilt of 14.4λ and an astigmatism of 4.8λ. The zone borders satisfy the relation

^χJ -y^{2 +2} d ^x=^ dNA <³¹> The zone borders are therefore hyperbolical. Figure 7 shows an example of an optical arrangement with appropriate zone borders for b/d = 1.5 and dNA/4λ = 3.0. The number of zones required can be estimated by taking the astigmatic term to be negligible. Then it follows that:

Ns,^ (32) λ In this example, the result is N « 29. If the distance between the DOE and the detector v = 5mm, then the beam diameter in the DOE is approximately 2vNA = 1.5mm, and the zone width is approximately 2vNA/N = 52μm.

Regarding the structure function, a function which diffracts a large amount of radiation into the -1^st and the +l^st orders is preferred. Thus the phase functions — W(?) and +W( f ) are used. This can be achieved with a stepped structure: f- 7r/2, for 0≤ s <l/2 f(s)H (33)

[+ τr/2, forl/2 <s <l

The step height h whereby this is achieved is equal to:

Where n is the refractive index of the DOE and p = 0, 1, 2, .... In this example λ = 780nm and n = 1.5. Thus, the smallest step height h = 0.78μm. This DOE only generates uneven orders (m = +1, +3 ...), and these have an efficiency:

τ m This ensures that the -1^st and +l^st orders together receive 8/π², that is approximately 80%, of the radiation; thus 20% of the radiation is lost to unused orders. For a two spot diffraction grating, this is optimum. More importantly, this loss only appears in the part of the optical path from the disk towards the detector, where efficiency is less important. The efficiency of the optical path towards the disk thus is also improved compared to a light path with a three-spots grating. Embodiment with Double Astigmatic Birefringent DOE

Figure 8 shows a schematic cross-sectional diagram of an LDGU (laser diffraction grating unit) containing an optical arrangement 300 according to an embodiment of the invention. In this embodiment, a DOE zone structure is used which is similar to that described above in relation to Figure 7. By combining diffractive optics with birefringent optics, it is possible to provide a component which allows us to make the entire optical path compact. By locating a LC-layer 306 above the DOE 308, the operation of the DOE may be made polarisation-dependent. The ordinary and extraordinary refractive indices of the LC are selected such that:

2π ^{h(lle ~n)} =2p_e^r (36) λ

2*^=^=(2p„ +l)* (37)

Herein, p_e and p₀ are integral values. The extraordinary mode will receive in the region of 100% of the radiation of the zeroth order, and no phase function is applied to the beam. For the ordinary mode, in contrast, the astigmatic DOE as described above in relation to Figure 7 appears. By further adding a neutral transparent substrate 304 and a quarter wavelength plate 302 to the integrated optical arrangement 300, an LDGU is provided for writable systems, with polarising optics. On the path from the laser 310 towards the disk, the polarisation of the laser output beam 316 (shown parallel to the plane of the drawing) is such that the radiation is not effected by the DOE. The operation of the quarter-wavelength plate 302 causes oppositely circularly polarised radiation beams 318 and 320 respectively in the scanning beam before and after reflection from the disk. Thus, in the reflected beam path the linear polarisation is perpendicular to the initial beam path and the DOE 308 operates fully. By placing the detectors 312 and 314 next to the laser diode to receive the first and second sub-beams 320, 322, respectively, a compact optical path can be provided. A beam splitter is not needed in a separate path towards the detector. It should be mentioned that the operation of the ordinary and extraordinary modes may be reversed relative to that described.

A drawback is that the radiation does not necessarily pass through the birefringent material perpendicularly. This results in that some radiation is lost between the laser and the disk, and also in that between the disk and the detector the efficiency is also lowered. A second point is spot orientation. The far field beam of a laser diode is typically elliptical and its polarisation stands perpendicular to the long axis of the ellipse. Preferably, the laser diode is oriented such that its polarisation is parallel to the birefringent axis of the LC-layer. It is possible to orient the liquid crystal in the direction of the zones, that is to say perpendicular to the direction in which the wavefront tilt is to be provided. Because this is preferred to be in the same direction as the astigmatism, and because this latter stands at 45°, the birefringent axis of the LC also preferably stands at 45°. From this, it follows that the polarisation of the laser also should stand at 45° and thus the elliptical far field also. This results in a DOS (diagonally oval spots) configuration.

Embodiment for Two Different Wavelengths

In an alternate embodiment, an optical arrangement is provided for a scanning device that operates at two different wavelengths, for example a CD/DVD compatible system. The wavelength and the NA (numerical aperture) of the light beam are different for the CD and DVD formats. This problem can be solved by forming a double astigmatic DOE as described above for the CD wavelength (λc_ϋ) on one side of the substrate and by forming a double astigmatic DOE as described above for the DVD wavelength ( _DVD) on the other side. The step heights on the CD-side are selected so that these steps add for the DVD- wavelength a phase which is a multiple of 2π, and for the CD-wavelength, a phase step of π:

_27r ^hc_D(n-l) _{=(2pcD +1)7r (38)}

With pea and p_DVD being integral values (if the dispersion of the material is not negligible, then the refractive index in these formulas may be different). On the DVD- side of the substrate, the step height hovo is selected so that the opposite occurs.

Both DOEs should be aligned above one another on the two different sides, which requirement tends to reduce the ease of manufacture. In the alternative, both structures may be integrated to form a dual- wavelength DOE on one side of the substrate. Such a structure uses four step levels, 0, hovo, h_CD - h_DvD, and hen + h_DvD-

Embodiment with Birefringent Plane-Parallel Plate

In an alternate embodiment an optical arrangement in the form of a tilted birefringent plane-parallel plate 400 used, as illustrated in Figure 9. The plate 400 is placed in a beam 408'that is in a convergent state when travelling towards the detector array, as illustrated in Figure 9. In this embodiment, the birefringent symmetry axis 402 of the plate 400 is arranged in the plane of the plate, and is shown in Figure 9 to be within the plane of the drawing. The convergent beam 408 is split into two sub-beams 410, 412, which fall on separate sub-beam detectors 404, 406. In Figure 9, the track equivalent direction is, in the detector plane 414, at 45° to the plane of the drawing. With the birefringent plate tilted in the converging beam astigmatism is generated in both polarisation modes. Moreover, the plate is arranged such that astigmatism of the two modes has the opposite sign. For a specific tilt angle (given the values for the ordinary and extraordinary refractive indices n₀ and n_e) the astigmatism in both modes can be made opposite in sign and equal in magnitude. This plate tilt angle β follows from: _sin² /3-=-^_ ₍39)

where Δn = n_e - n₀. The distance a between the focal lines (astigmatic distance) is given by:

where d is the plate thickness. The distance between the tangential focal lines of the two modes is :

_Δz ^dΔnfeM⁾ ₍₄₁₎ ⁿ _eKⁿ _e(n_o - )(n₀n_e -l)J and the lateral separation between the two spots on the detector is:

n₀ (n₀n_e -l)J

In one example n_o = 1.5 and Δn = 0.15. Thus, the tilt angle is 23°, the astigmatic distance is 0.063d, the separation of the tangential focal lines is -0.073d, and the lateral separation is 0.025d. hi one example the astigmatic distance is set at 300μm, thus the thickness d = 4.8mm. The lateral separation of the two spots on the detector is 119μm, and the separation of the tangential and sagital foci is 348μm and 252μm, respectively. The required thickness is quite large compared to other components in an optical pick-up unit. Instead of LC or LC- polymers, materials like calcite are preferred, because of excess radiation scattering by LC materials.

Embodiment with Simplified Flex Manufacturing costs may be reduced by reducing wiring on the flex connecting the detector with the signal processing circuitry. In principle, the two four- quadrant detectors give rise to eight wires on the flex. This can be reduced to four by adding the eight quadrants two by two on the detector itself. Figure 2 shows the quadrants Ai to At and Bi to B₄. In this embodiment the signals are combined in the detector circuitry to produce four discrete signals A, B, C, D as follows:

A = A, + B₄ (43)

B = A₂ + B₃ (44)

C = A₃ + B₂ (45) D = A₄ + B_! (46)

The focus error signal (FES) and radial error signal (RES) are then produced by signal processing as follows:

FES = A + C - B - D (47)

RES = A + B - C - D (48) In practice, these signals are preferably normalised with respect to the central aperture signal:

CA = A + B + C + D (49)

General Remarks Herein, the sub-beams are said to have "generally elliptical" cross sections.

This is intended to allow for variations from a perfect elliptical cross section which may be caused by other aberrations in the beam, such as coma, and other factors such as angle of incidence on the detector.

As the astigmatic focus error detection method and the push-pull tracking error detection method are particularly suited to the scanning of writable, and rewritable, optical disks, the invention is of particular application to, inter alia, the following formats, namely the CD-R, CD-RW, DVD+RW, DVD-RW, DVD-RAM, DVD+R, DVD-R, and Blu-Ray disk formats.

The above embodiments are to be understood as illustrative examples of the invention. Further embodiments of the invention are envisaged. It is to be understood that any feature described in relation to one embodiment may also be used in other of the embodiments. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

CLAIMS:

1. An optical scanning device for scanning an optical data storage medium having data storage regions arranged in substantially parallel track sections, the device comprising: a radiation source (9) arranged to produce a radiation beam for scanning the storage medium to produce a scanning spot; a detection system (25, 27, 29) having detector elements arranged to detect radiation after interaction of the radiation beam with the storage medium, the detection system being arranged to generate a focus error signal for maintaining a focus state of the scanning spot on the storage medium and to generate a transverse error signal (30) for maintaining a tracking state of the scanning spot in a transverse direction with respect to the track sections on the storage medium; and an optical arrangement (OA), arranged in the optical path of the radiation beam between the position of the storage medium and the detection system, for splitting the radiation beam into at least a first sub-beam (A) and a second sub-beam (B) and for generating a first astigmatic aberration having an appertaining focal line in the first sub-beam and a second astigmatic aberration having an appertaining focal line in the second sub-beam, wherein the detection system (25, 27, 29) is arranged to detect radiation from each of said first sub-beam and said second sub-beam to generate first sub-beam detection signals and second sub-beam detection signals respectively, characterised in that the optical arrangement (OA) is arranged such that the direction of the focal line of the first sub-beam is inclined with respect to a track-equivalent direction at a positive angle, and the direction of the focal line of the second sub-beam is inclined with respect to the track-equivalent direction at a negative angle, and in that the detection system (25, 27, 29) is arranged to generate said transverse error signal using both said first sub-beam detection signals and said second sub- beam detection signals.

2. An optical scanning device according to claim 1, wherein the positive and negative angles have substantially the same magnitude.

3. An optical scanning device according to claim 2, wherein the positive angle is substantially +45° and the negative angle is approximately -45°.

4. An optical scanning device according to claim 1, 2 or 3, wherein the transverse error signal (30) is a push-pull error signal.

5. An optical scanning device according to any of claims 1 to 4, wherein the focus error signal is an astigmatic focus error signal.

6. An optical scanning device according to any preceding claim, wherein the detection system comprises a first sub-beam detector (25(A)) and a second sub-beam detector (25(B)), each including four quadrant detector elements providing signals Ai, A₂, A₃, t and Bi, B₂, B₃, B₄ respectively.

7. An optical scanning device according to claim 4 and claim 6, wherein a transverse error signal (RES) is generated according to the following scheme:

RES = (Ai + A₂ - A₃ - A₄) - (Br + B₂ - B₃ - B₄)

8. An optical scanning device according to claim 5 and claim 6 and/or claim 7, wherein a focus error signal (FES) is generated according to the following scheme:

FES = (A₁ + A₃ - A₂ - A₄) - (B₁ + B₃ - B₂ - B₄)

9. An optical scanning device according to any of claims 6 to 8, wherein output signals are combined in the detection system to generate four discrete signals A, B, C, D as follows:

B = A₂ + B₃

C = A₃ + B₂

D = A₄ + B₁

10. An optical scanning device according to any preceding claim, wherein the optical arrangement comprises a birefringent optical element.

11. An optical scanning device according to any of claims 1 to 10, wherein the optical arrangement comprises a diffractive optical element.

12. An optical scanning device according to claim 11 , wherein the diffractive optical element has the zone function:

W(f)=A_nx+A₂₂(x² -y²) .

13. An optical scanning device according to any preceding claim, adapted to scan a storage medium of at least one format selected from the group of the CD-R, CD-RW, DVD+RW, DVD-RW, DVD-RAM, DVD+R, DVD-R, and Blu-Ray disk formats.