WO2007129238A1

WO2007129238A1 - Improved reading/writing of information from multiple layer optical discs

Info

Publication number: WO2007129238A1
Application number: PCT/IB2007/051480
Authority: WO
Inventors: Sjoerd Stallinga
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2006-05-05
Filing date: 2007-04-23
Publication date: 2007-11-15
Also published as: RU2008147881A; TW200818162A; CN101438347A; JP2009536421A

Abstract

A three-spots grating is proposed with a pitch resulting in large field use of the objective lens, typically about 2.3 deg. The field curvature related defocus and other aberrations makes the radial modulation zero, which solves the y-error problem. The resulting large spot distance on the disc, typically about 40 µm, results in a spatial separation at the detector of the satellites and the spot reflected of the out-of-focus layer (for dual-layer disc readout), which solves the satellite coherent cross-talk problem.

Description

Improved reading/writing of information from multiple layer optical discs

FIELD OF THE INVENTION

The present invention relates in general to a method for writing/reading information into/from an optical storage disc and to a disc drive apparatus which performs such a method; hereinafter, such disc drive apparatus will also be indicated as "disc drive". The invention relates more particularly to a method for reading/writing information from an optical multiple data layer disc, such as a dual layer disc, and to a disc drive which performs such a method. A dual layer disc drive comprises a lens system comprising at least one lens for focusing light onto at least one data layer, respectively, of the disc, and an actuator for at least moving the lens system along a radial direction of the disc.

BACKGROUND OF THE INVENTION

As is commonly known, an optical storage disc comprises at least one track, either in the form of a continuous spiral or in the form of multiple concentric circles, of storage space where information may be stored in the form of a data pattern. Optical discs may be read-only type, where information is recorded during manufacturing, which information can be read by a user. The optical storage disc may also be a writable type, where information may also be stored by a user. Examples of optical discs are, for instance, CD-ROM, CD-R, CD-RW, DVD, DVD+RW Blu-ray Disc (BD), HD-DVD, etcetera.

For writing information in the storage space of the optical storage disc, or for reading information from the disc, an optical disc drive comprises rotating means for receiving and rotating an optical disc, and optical means for generating an optical beam, typically a laser beam, and for scanning the storage track with said laser beam. The disc drive device comprises an optical head or optical pickup unit (OPU) for directing the optical laser beam towards the surface of the (rotating) disc, receiving the reflected beam reflected by the disc, and directing the received reflected beam onto a light detector, e.g. a photo-detector, which produces an electrical signal. Thus, an optical pickup unit comprises a light beam generator, an optical system for directing the light beam towards the optical disc, a light detector for converting light into an electrical signal, and an optical system for receiving reflected light and for directing this reflected light towards the light detector. The light detector is required to be positioned very accurately with respect to the returning light beam and, in conventional light path geometry, the returning spot is very carefully aligned on the light detector by adjusting the position of the light detector in the z- direction (i.e. along the optical axis) relative to the rest of the optics in the optical system and in the x-y direction perpendicular to the optical axis.

For rotating the optical disc, an optical disc drive typically comprises a motor, which drives a hub engaging a central portion of the optical disc. Usually, the motor is implemented as a spindle motor, and the motor-driven hub may be arranged directly on the spindle axle of the motor. During operation, the light beam should remain focused on the disc.

To this end, the objective lens is arranged axially displaceable, and the optical disc drive comprises focal actuator means for controlling the axial position of the objective lens. Further, the focal spot should remain aligned with a track or should be capable of being positioned with respect to a new track. To this end, at least the objective lens is mounted radially displaceable, and the optical disc drive comprises radial actuator means for controlling the radial position of the objective lens.

For portable applications both the disc and the disc drive need to have small dimensions. In order to achieve sufficient data capacity on a small disc the use of a dual layer disc is quite favorable for this application. The left part of Figure 1 shows, in part, a dual layer optical disc system. A lens system comprises an objective lenses OL for focusing light on to a second data layer Ll. The lens system is attached to an actuator AC for radial and/or vertical movement for the purpose of tracking and focusing, respectively. In a dual layer disc DSC a first data layer LO, is located at a depth d below an entrance surface S of the disc DSC. The second data layer Ll is located at a depth d+s. A top layer of thickness d is called the cover layer CL. The intermediate layer of thickness s is called the spacer layer SL. When in optimal focus objective lens OL has focus point FPi on the second data layer Ll. Ideally the light from the objective lens OL will only reach the second data layer Ll. In practice however also part of the light reaches unintentionally the first data layer LO which reflects light back into the objective lens OL as is schematically indicated with dashed arrows. This reflected light seems to be focused in a virtual focus point VFPl somewhere above the first data layer LO. As a consequence this reflected light produces a relatively large (because out of focus) light spot on the light detector which deteriorates proper focusing and/or radial tracking caused by disturbance of the focus spot and/or any satellite spots imaged on the light detector. The right part of Figure 1 shows a similar situation when the light is focused on to the first data layer LO. See focus point FPo. In this situation there seems to be a virtual focus point VFPO somewhere below the second data layer Ll. Of course also in this situation the reflected light produces a relatively large out-of- focus light spot on the light detector. The electrical signal of the light detector contains information on the tracking error, i.e. the radial distance from the centre of the focal spot to the centre of the track being followed. This electrical signal is fed to a control circuit, which processes the electrical signal in order to generate a control signal for the radial actuator means.

One well-known method to process the electrical signal is to generate a so- called push-pull signal. The push-pull method has some disadvantages. One disadvantage of the push-pull method is the sensitivity to (variable) beam landing errors, i.e. a displacement of the light spot with respect to the light detector which are caused by movement of the actuator AC in the radial direction. A well-known solution to this problem is the three-spot push-pull method. In the three-spot push-pull method three spots are imaged on the disc: a main spot which should be focused on a track, a first satellite spot which should be focused in between said track and a first neighbor track of said track, and a second satellite spot which should be focused in between said track and a second ("the other") neighbor track of said track. (See Figure 2) Figure 2 shows a schematic view of the tracks on the disc and the scanning spots. The information layer consists of a main track 1, a first adjacent track 2 and a second adjacent track 3, the tracks are spaced by a distance p, the track pitch. The information is read/written with the main scanning spot 4, for tracking purposes there are also a first satellite spot 5 and a second satellite spot 6, which are halfway between the main track and the adjacent tracks. The push-pull signal is found by taking the difference signal of various detector segments of the light detector as is indicated in the scheme of Figure 3 for the three-spots push-pull tracking error signal. In the nominal case it varies periodically with the radial position of the scanning spot with respect to the tracks:

PP _a = ^sinφ

where PP _a is the push-pull signal corresponding to the main spot, A is the amplitude, and φ = 2πx/p with x the radial position and p the track pitch. Clearly, the push-pull signal is zero when the scanning spot is on the track. Due to a displacement of the spot with respect to the light detector an offset can occur, which is called beam landing. The push-pull signal is then: PP_a = Asin$ + B

where B is the beam landing contribution due to radial stroke of the actuator AC. In order to overcome the beam landing induced tracking offsets, the so-called three-spots push-pull (3 SPP) error signal is used in practice. This signal is a weighted sum of three push-pull signals, the three signals originating from the main scanning spot and from the two (first and second) satellite spots. (See Figures 2 and 3.) The satellite spots contain a power that is a factor ζ ~ 15 smaller than the power of the main spot, and while the main spot is on track, the two satellite spots are halfway this central track and the adjacent tracks. The first (PPb) and second (PP₀) satellite push-pull signals are:

PP_b = -—Asm§ + — B

PP = -— 1 Λ ,si •nφ , + — I B_n

The Tracking Error Signal (TES) is defined as:

TES = PP_a -ζ (PP_b + PP_c)/2

= 2 A sin φ

Clearly, all effects of beam landing are cancelled.

A problem arises when the satellites are not properly oriented with respect to the tracks. Figure 4 schematically shows the so-called y-error: nominal case (left) in which the main spot is on a line through the center of the spiral tracks (no y-error), and case with y- error (right) in which the main spot is a distance y from the original line through the center of the spiral tracks. In the nominal, well-aligned case the angle between the tracks and the line through the main and satellite spots is:

β = arcsin — 2* J (with p the track pitch and t the spot distance). Because of disc eccentricity or misalignment the disc may be shifted over a distance y. The tracks are then rotated with respect to the grating over an angle:

γ = arctan —

with R the radius of the spiral track at the spot location. The satellites are then no longer halfway between the track and the adjacent tracks. The phase offset between the main and satellite signals is thus no longer π but π+χ, where:

. t . 2πty χ = 2π — sinγ ~ — — p pR

The resulting error signal turns out to be:

TES = 2(l - sin ² (χ I 2))A sin φ

i.e. the amplitude depends on the y-error through χ. This variation of the error signal amplitude as a function of the y-error must be sufficiently small. For that reason the spot distance for BD is limited to about t = 10 μm. As already shortly indicated with reference to Figure 1 another problem arises with the readout of dual-layer discs. Signals emanating from the out-of- focus information layer can end up at the detector parts a, b, and c (see Figure 3), and can give rise to interference there. Figure 5 shows a picture of the overlapping spots. As an example the spots are at the detector plane for a satellite spot distance on the disc of about 10 μm. The top picture corresponds to focusing on the first layer LO, the bottom picture to focusing on the second layer Ll. The main spot and two satellite spots are drawn, and the spot resulting from the reflection of the main spot on the other, out-of- focus layer. Clearly, the spot distance on the disc is too small to avoid overlap between the satellite spots and this out-of- focus spot. Figure 6 shows the interference pattern of a satellite spot of a 3 -spots astigmatic focusing system due to interference between the out-of- focus light and the astigmatic satellite spot. The interference pattern consists of alternating bright and dark regions, depending on this pattern the net effect on the push-pull signal is an offset. The offset can be varying due to spacer thickness variations. Typical interference pattern of a satellite spot and a main out-of- focus spot on a satellite part of the light detector are shown. The x and y-coordinates are in μm, so the detector area measures by way of example 100 μm by 100 μm, and the center is a distance 150 μm from the optical axis, in this example. The sign and magnitude depends on the overall phase difference between the two interfering beams. This results in noisy fluctuations on the push-pull signal, which are called Coherent Cross-Talk (CCT). This is of particular importance for the two satellite spots, where the total power of the satellite beams is a factor ζ smaller than the total power of the main spot reflected by the out-of- focus layer. Therefore it is an object of the invention to provide a method for solving the y- error problem and the satellite CCT-problem which does not have one or both of the above mentioned problems or at least in a less degree.

To achieve the object of the invention a method for reading information from an optical storage disc is provided comprising the steps of: - generating a main light spot on a target track of an optical storage disc, generating a pair of satellite spots aligned in a manner that an imaginary straight line approximately intersects the centre of the main light spot and the centers of both satellite spots, the spot distance of both satellite spots to the main light spot approximately being equal and defined as a satellite spot distance, - detecting light spots reflected back from the optical storage disc, which are modulated by information from the optical storage disc, on a light detector having segments aligned in order to separately detecting the reflected light spots, and generating the pair of satellite spots in a manner that the amplitude of a normalized push-pull signal, provided by a segment of the light detector which correspond to the pair of satellite spots, is at least five times smaller than the amplitude of a normalized push-pull signal provided by a segment of the light detector which corresponds to the main light spot.

Preferably the generating of the pair of satellite spots is in a manner that the amplitude of the normalized push-pull signal corresponding to the pair of satellite spots is at least twenty times smaller than the amplitude of the normalized push-pull signal corresponding to the main light spot.

The invention also provides a disc drive apparatus which makes use of the inventive method and which is defined in the appended claims 3 and 4. To solve the y-error problem as well as the satellite CCT-problem a grating (well known in the optical storage field for generating multiple beams from a single laser beam) can be used with such a fine pitch that the distance t between the main and satellite spots on the disc is sufficiently large. Figure 10 shows spots at the light detector for a satellite spot distance on the disc of about 40 μm. The top picture corresponds to focusing on layer LO, the bottom picture to focusing on layer Ll. The main spot and two satellite spots are drawn, and the spot resulting from the reflection of the main spot on the other, out-of- focus layer. Clearly, the spot distance on the disc is sufficiently large in order to avoid overlap between the satellites and this out-of- focus spot. This has the following consequences. First, the field use of the objective lens is increased. The field angle is defined as the satellite spot distance t divided by the objective focal length. For example, a spot distance oft = 40 μm corresponds to a field angle of 2.3 deg, assuming a focal length of 1.0 mm. The main effect of large field use is so-called field curvature, which is defocus that increases quadratically with the field angle. It turns out that the amplitude of the radial error signal decreases with increasing defocus and for certain defocus values the amplitude is even zero. This zero radial modulation implies that the satellite push-pull signal now only measures beam landing, so that the error signal amplitude does no longer depend on the orientation of the spots with respect to the tracks. This solves the y-error problem. A second consequence is that the distance between the main and satellite spots at the detector plane is also increased. Above a certain value there is no longer overlap between the satellite spots and the spot arising from the reflection of the out-of-focus layer. This eliminates any interference effect between the two spots and thus solves the satellite CCT-problem. It turns out that values for the spot distance on the disc can be found in the regime t>33 μm (in this example), so that both the y-error problem and the satellite CCT- problem are solved.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail with reference to the accompanying drawings, in which: Fig. 1 shows, in part, a dual layer optical disc system;

Fig. 2 shows a schematic view of the tracks on the disc and the scanning spots; Fig. 3 shows a scheme for the three-spots push-pull tracking error signal; Fig. 4 schematically shows the so-called y-error; Fig. 5 shows spots at the light detector for a satellite spot distance on the disc of about 10 μm;

Fig. 6 shows a typical interference pattern of a satellite spot and a main out-of- focus spot on a satellite part of the light detector; Fig. 7 shows aberrations of an objective lens of the satellite spots as a function of satellite spot distance;

Fig. 8 shows Optical Transfer Function (OTF) as a function of defocus for BD-parameters;

Fig. 9 shows simulated normalized push-pull amplitude (ppn) as a function of spot distance;

Fig. 10 shows change in push-pull amplitude of the two satellites for focusing on the first (LO) and second (Ll) layers as a function of tangential tilt for a satellite spot distance of 40 μm; and

Fig. 11 shows spots at the light detector for a satellite spot distance on the disc of about 40 μm according to the inventive method.

In these Figures parts or elements having like functions or purposes bear the same reference symbols.

DESCRIPTION OF THE EMBODIMENT To solve the y-error problem as well as the satellite CCT-problem a grating

(well known in the optical storage field for generating multiple beams from a single laser beam) can be used with such a fine pitch that the distance t between the main and satellite spots on the disc is sufficiently large. Figure 11 shows spots at the light detector for a satellite spot distance on the disc of about 40 μm. The top picture corresponds to focusing on layer LO, the bottom picture to focusing on layer Ll . The main spot and two satellite spots are drawn, and the spot resulting from the reflection of the main spot on the other, out-of- focus layer. Clearly, the spot distance on the disc is sufficiently large in order to avoid overlap between the satellites and this out-of- focus spot.

This has the following consequences. First, the field use of the objective lens is increased. The field angle is defined as the satellite spot distance t divided by the objective focal length. For example, a spot distance oft = 40 μm corresponds to a field angle of 2.3 deg, assuming a focal length of 1.0 mm. The main effect of large field use is so-called field curvature, which is defocus that increases quadratically with the field angle. It turns out that the amplitude of the radial error signal decreases with increasing defocus and for certain de focus values the amplitude is even zero. This zero radial modulation implies that the satellite push-pull signal now only measures beam landing, so that the error signal amplitude does no longer depend on the orientation of the spots with respect to the tracks. This solves the y-error problem. A second consequence is that the distance between the main and satellite spots at the detector plane is also increased. Above a certain value there is no longer overlap between the satellite spots and the spot arising from the reflection of the out-of-focus layer. This eliminates any interference effect between the two spots and thus solves the satellite CCT-problem. The values for the spot distance on the disc can for instance, depending on the design of the optical pick-up unit, be found in the regime t>33 μm, so that both the y-error problem and the satellite CCT-problem are solved.

The invention is supported by numerical calculations. Figure 7 shows the aberrations of the objective lens as a function of the spot distance on the disc (which is proportional to field angle). These aberrations are calculated with the "Zemax raytracing" software. It appears that the largest effect is indeed field curvature (defocus, coefficient A₂o, depending quadratically on t), but that other aberrations are significant as well. These are astigmatism (A₂₂), spherical aberration (A₄₀), higher order astigmatism (A₄₂), and coma (A₃₁). In addition to this, it turns out that the aberrations for focusing on layer LO (depth 100 μm) and on layer Ll (depth 75 μm) differ slightly.

The required defocus for achieving zero radial modulation can be estimated analytically by calculating the so-called OTF (Optical Transfer Function) as a function of defocus. In literature many times the so-called MTF (Modulation Transfer Function) is mentioned in stead of OTF. MTF is the modulus of OTF. The OTF/MTF is a measure for the push-pull amplitude, so the zero's of the OTF/MTF correspond to zero radial modulation. Figure 8 shows a plot of the OTF as a function of the defocus A₂o, assuming BD-parameters The zero's are at 0.431 λ and 0.720 λ. (corresponding to 0.249 λ and 0.416 λ rms-values). Using the data of Figure 7 it follows that the first zero corresponds to a spot distance on the disc of about 41 μm, and the second zero to a spot distance on the disc of about 52 μm. The effect of the other aberrations can be investigated numerically by a model calculation of the push-pull amplitude for the actual set of aberrations for a specific value of the spot distance. (ppn means push-pull signal normalized.) Figure 9 shows the result of such a calculation. The two zero's are shifted to smaller values for the spot distance, and do not coincide for the two layers (the aberrations for the LO and Ll cases are not exactly the same). The optimum values for the spot distance are about 31 μm and 40 μm, as opposed to the values 41 μm and 52 μm for the defocus only case. For the spot distance of 40 μm the amplitude is below 2% of the push-pull amplitude of the unaberrated main spot for both cases. This is sufficient to solve the y-error problem.

It is mentioned that the main shortcoming of the numerical calculations is that the effects of the compatibility plate are not taken into account. This is a diffractive structure that is placed in front of the lens that makes the BD objective lens compatible with DVD and CD. For normal incidence it has no effect on the blue light that is used for BD. However, when the field of the lens is used, the incident beam on the plate is not normal, and aberrations (of yet unknown magnitude) are generated. This may result in different optimum values for the spot distance compared to the numerical calculations presented here. However, from practice it is known that tilting this plate over about 1 deg does not result in significant aberrations. This value of 1 deg is in the regime of field angles that the invention is intended for. There are some tolerance issues. Firstly, the increase in the spot distance with a factor of about 4 decreases the margins for the placement of the grating along the optical axis with the same factor of about 4. Secondly, when there is tangential disc tilt with an angle α, the +lst satellite will be a distance tα closer to the lens, whereas the -1st satellite will be a distance tα further away from the lens. This implies that for one of the satellites the defocus will be increased, whereas for the other satellite the defocus will be decreased. This will affect the push-pull amplitude. Figure 10 shows the results of numerical calculations of this effect. The deviations of the +lst and -1st satellites partly cancel, so that the average of the two spots remains within 2% of the main spot amplitude for tilt angles less than 0.4 deg. This average is used as input for the error signal, so it may be concluded that the margins for tangential disc tilt are sufficiently large.

Fortunately, these values for the spot distance are sufficiently large so that the spots on the detector do not overlap. Figure 11 shows a picture of the spots on the disc

(calculated using "Zemax raytracing" software) for a spot distance of 36 μm. Clearly, this is sufficiently large to avoid overlap, and hence interference effects. It turns out that any value t>33 μm is large enough to solve the satellite CCT-problem. This lower limit may be estimated analytically as follows. The radius of the main and satellite spots on the detector is r, the radius of the spot coming from the out-of- focus layer is R, where r and R are given by:

R = Is NAM In, r = lNAM, where s is the spacer thickness, n the spacer refractive index, 1 the focus s-curve length, NA the Numeric Aperture of the objective lens, and M the disc-to-detector magnification. For a spot distance on the disc t, the spot distance on the detector is Mt. It follows that we must have that Mt > R+r, or:

With NA = 0.85, n = 1.6, s = 25 μm, and 1 = 4 μm, this minimum is 30 μm. In practice, the spots on the detector are not circular, and several aberrations need to be taken into account. This results in a slightly different value of about 33 μm.

Figure 9 shows simulated normalized push-pull amplitude (ppn) as a function of spot distance. The data points are normalized to the zero aberration value for the main spot. A push pull signal is also normalized in the sense that the normalized push-pull signal is divided by its Central Aperture which will be further denoted as CA. (As is commonly known to a person skilled in the art of optical storage devices, the CA of a spot (either main spot or satellite spot) is defined as the sum of the signal produced by all segments of the light detector which correspond to the corresponding (either main or satellite) spot.) The aberrations are calculated using "Zemax raytracing" software for both focusing on layer LO and focusing on layer Ll. The invention works if the normalized push-pull signal of the pair of satellites is at least a factor 5 lower than the normalized push-pull amplitude of the main spot (indicated as 100% in Figure 9). In this example it means that a satellite spot distance of 28 μm would be sufficient. Preferably however the normalized push-pull signal of the pair of satellites is at least a factor 20 or more lower than the normalized push-pull amplitude of the main spot. Or to put it in other words: preferably the normalized push-pull signal of the pair of satellites is as close as possible to zero. In this example it means that the optimum values for the satellite spot distance are about 31 μm and 40 μm. For the satellite spot distance of 40 μm the amplitude is below 2% of the main spot push-pull amplitude for both cases.

It is to be noted that the spot distances given here are just examples and that the distances can be quite different since it depends on the overall design of an optical storage device (in particular the objective lens OL), e.g. it depends on the used track pitch p (see Figure 2).

Briefly summarized: a three-spots grating is proposed with a pitch resulting in large field use of the objective lens, typically about 2.3 deg. The field curvature related defocus and other aberrations makes the radial modulation zero, which solves the y-error problem. The resulting large spot distance on the disc, typically about 40 μm, results in a spatial separation at the detector of the satellites and the spot reflected of the out-of- focus layer (for dual-layer disc readout), which solves the satellite coherent cross-talk problem. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and those skilled in the art will be capable of designing alternative embodiments without departing from the scope of the invention as defined by the appended claims. The words "comprising" and "comprises", and the like, do not exclude the presence of elements other than those listed in any claim or in the application as a whole. The singular reference of an element does not exclude the plural reference of such elements.

Claims

CLAIMS:

1. Method for reading information from an optical storage disc comprising the steps of: generating a main light spot on a target track of an optical storage disc, generating a pair of satellite spots aligned in a manner that an imaginary straight line approximately intersects the centre of the main light spot and the centers of both satellite spots, the spot distance of both satellite spots to the main light spot approximately being equal and defined as a satellite spot distance, detecting light spots reflected back from the optical storage disc, which are modulated by information from the optical storage disc, on a light detector having segments aligned in order to separately detecting the reflected light spots, and generating the pair of satellite spots in a manner that the amplitude of a normalized push-pull signal, provided by a segment of the light detector which correspond to the pair of satellite spots, is at least five times smaller than the amplitude of a normalized push-pull signal provided by a segment of the light detector which corresponds to the main light spot.

2. A method according to claim 1 characterized in that the generating of the pair of satellite spots is in a manner that the amplitude of the normalized push-pull signal corresponding to the pair of satellite spots is at least twenty times smaller than the amplitude of the normalized push-pull signal corresponding to the main light spot.

3. A disc drive apparatus for reading information from an optical storage disc comprising means for generating a main light spot on a target track of an optical storage disc, means for generating a pair of satellite spots aligned in a manner that an imaginary straight line approximately intersects the centre of the main light spot and the centers of both satellite spots of the pair of satellite spots, the spot distance of both satellite spots of the pair of satellite spots to the main light spot approximately being equal and defined as a satellite spot distance, a light detector for detecting light spots reflected back from the optical storage disc which light spots are modulated by information from the optical storage disc, the light detector having segments aligned in order to separately detecting the reflected light spots, and the generating of the pair of satellite spots being carried out in a manner that the satellite spot distance is large enough so that the amplitude of a normalized push-pull signal, provided by a segment of the light detector which correspond to the pair of satellite spots, is at least five times smaller than the amplitude of a normalized push-pull signal provided by a segment of the light detector which corresponds to the main light spot.

4. A disc drive apparatus according to claim 3 characterized in that the generating of the pair of satellite spots is in a manner that the amplitude of the normalized push-pull signal corresponding to the pair of satellite spots is at least twenty times smaller than the amplitude of the normalized push-pull signal corresponding to the main light spot.