WO2007000681A2

WO2007000681A2 - Astigmatic multi-spot systems

Info

Publication number: WO2007000681A2
Application number: PCT/IB2006/051961
Authority: WO
Inventors: Dominique M. Bruls; Alexander M. Van Der Lee; Albert H. J. Immink
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-06-28
Filing date: 2006-06-19
Publication date: 2007-01-04
Also published as: TW200709201A; WO2007000681A3

Abstract

The invention relates to photo detection, and in particular to an apparatus and a method for focus detection in a multi-spot astigmatic focus system for optical storages in order to improve the focus detection. According to an aspect of the invention there is provided a photo detector system (312) comprising at least one photo detector comprising at least eight segments (402, 404, 406, 408, 410, 412, 414, 416), each segment providing a segment signal in response to an amount of radiation received by the segment so that at least eight signals are provided. The photo detector system (312) is adapted to determine a focus condition of the system (301) from at least one of the signals and to generate a focus error signal based on the determined focus condition in order to achieve an improved S-curve generation and operating displacement of an objective lens accordingly.

Description

Astigmatic multi-spot systems

The invention relates to photo detection, and in particular to an apparatus and a method for focus detection in a multi-spot astigmatic focus system for optical storages in order to improve the focus detection.

In a two-dimensional optical storage system such as TwoDOS or a multi-track DVD, an array of laser spots is used in parallel to readout multiple bit-rows. In order to focus an array of laser spots on the disc, the Foucault-knife focusing method may be used, in which only the central spot of the array is used for focusing. This method however suffers from cross talk from the neighboring spots, which causes the focus S-curve to be distorted or even unusable.

US 6,229, 11 \ describes a focus error detection system for use in a multi-beam optical pickup. A separate set of focus beams is generated for use in determining the focus error. The focus error is detected by introducing astigmatism into the focus beams, and by one or two photo detectors each comprising up to four photo detector segments.

The focus error detection system described in US 6,211, 11 \ makes the use of a separate set of spots, or an extra slit, necessary. In the view of the present inventors a separate set of spots can only prevent cross talk partly. Furthermore, alignment of the focus system may become a cumbersome task. Even further a system using separate focus beams does not seem to be useable in a commercial and/or a compact sized system.

The inventors of the present invention have appreciated that improved focus detection in a multi-spot astigmatic focus system is of benefit, and have in consequence devised the present invention.

The present invention seeks to provide an improved apparatus and method for improved focus detection in a multi-spot astigmatic focus system in connection with reading and/or writing in an optical storage. Preferably, the invention alleviates, mitigates or eliminates one or more of the above or other disadvantages singly or in any combination. Accordingly there is provided, in a first aspect, a photo detector system adapted for focus detection in a multi-spot astigmatic focus system, the photo detector system comprising at least one photo detector comprising at least eight segments, each segment providing a segment signal in response to an amount of radiation received by the segment so that at least eight signals are provided and wherein the photo detector system is adapted to determine a focus condition of the system from at least one of the signals and wherein the system is further adapted to generate a focus error signal based on the determined focus condition.

A photo detector system is provided which is suitable for use in reading and/or writing information in a multiple beam optical system. The detector system may comprise further detectors with e.g. four segments. Such detectors may e.g. be used for detection of a tracking error.

In astigmatic focusing a focus error signal is used to make up a focus S-curve. The focus S-curve is used in a servo-loop in order to catch focus and maintain focus. Because an astigmatic lens is placed in front of this detector, the spot will be round if the spot is properly focused onto the disc, but the spot on the focus detector will be elliptic or distorted if the spot is defocused onto the disc. This makes it possible to detect whether the objective lens is too close or too far away from the disc.

However, in a multi-spot system where e.g. an array of laser spots is used in the read and/or write process problems may arise. Even if the laser spot separation is rather large, some portion of an adjacent defocused spot can also fall onto the detector. It this situation the S-curve becomes shorter and possibly asymmetric. This deteriorates the stability of the system and may cause difficulties during the catching of focus (i.e. closing the focus servo loop). It is particularly but not exclusively advantageous to use a photo detector system comprising at least eight segments, and where the photo detector system is adapted to determine a focus condition of the system from at least one of the signals and wherein the system is further adapted to generate a focus error signal based on the determined focus condition. By using eight segments, the focus error signal may be based on a subset of the segments so that segments with overlapping spots may be ignored or taken into account in the generation of the focus error signal in response to the focus condition. In this way an S- curve may be provided so that a stable focus loop may be obtained. The optional feature as defined in claim 2 may be advantageous since by splitting each segment of a four segments photo detector diagonally, segments having a triangular form are provided. This may be advantageous, since the functionality of a standard segment photo detector may be obtained in conjunction with the functionality of the present invention. Furthermore, a photo detector of the present invention may be fitted into standard detector systems with minimum changes to the standard equipment.

The optional features as defined in claims 3 and 4 may be advantageous since means are provided for setting up a focus error signal in response to a determined focus condition. This is advantageous since different focus error signals may be generated and used in different focus condition situations, thereby ensuring optimum S-curve generation.

The optional features as defined in claims 5, 6 and 7 may be advantageous since generating the focus error signal in response to a mathematical function means are provided for easy implementation and updating of the system in response to various focus conditions. The optional features as defined in claims 8 and 9 may be advantageous since a system may be provided which may not need the geometrical details of the system in which it operates in advance.

According to a second aspect of the invention there is provided an optical system for focussing multiple radiation beams on an associated record carrier the system comprising a radiation-emitting device capable of emitting radiation for generating multiple beams and for providing corresponding spots on the associated carrier a focussing unit for focussing the spots on the carrier an astigmatic refraction unit - at least one photo detector for detecting the multiple beams reflected from the associated carrier, the photo detector comprising at least eight segments each segment providing a segment signal in response to an amount of radiation received by the segment wherein the photo detector is adapted to determine a focus condition based on at least one of the signals and wherein the system is further adapted to provide a focus error signal based on the determined focus condition.

The photo detector of the first aspect may be used in an optical system according to the second aspect.

According to a third aspect of the invention there is provided a method of focus error detection in a photo detector system, the photo detector system comprising a radiation-emitting device capable of emitting radiation for generating multiple beams and for providing corresponding spots on the associated carrier a focussing unit for focussing the spots on the carrier an astigmatic refraction unit - at least one photo detector for detecting the multiple beams reflected from the associated carrier for providing multiple reflected spots, the photo detector comprising at least eight segments each segment providing a segment signal in response to an amount of radiation received by the segment wherein the method comprises the steps of determining a focus condition of the radiation spots from at least one of the signals, generating a focus error signal based on the focus condition and at least one of the signals, adjusting the focusing unit in accordance with the focus error signal.

Displacing the position of the focussing unit may provide adjusting the focusing unit. The method of the third aspect may be used to operate a detector according to the first aspect or the optical system according to the second aspect of the present invention.

According to a fourth aspect of the invention there is provided a computer readable code adapted to perform the steps of the third aspect.

In general the various aspects of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

Fig. 1 shows a focus detector for a single- spot system according to prior art, two multi-spot systems with a photo detector perpendicular and not perpendicular to radiation beams, and the respective focus error curves.

Fig. 2 shows a focus detector according to prior art for a multi-spot system where adjacent spots overlap on photo detector segments at the focus point and the corresponding focus error curve with a focus offset.

Fig. 3 is a schematic drawing of an optical system comprising a detector system according to the present invention.

Fig. 4 is a detector layout with eight diagonally split photo detector segments. Fig. 5 is the detector layout with eight segments for a multi-spot system when the photo detector is perpendicular to radiation beams and where adjacent spots overlap on photo detector segments.

Fig. 6 is the detector layout with eight segments for a multi-spot system when the photo detector is not perpendicular to radiation beams and where adjacent spots overlap on photo detector segments.

Fig. 7 is the detector layout with eight segments for a multi-spot system where adjacent spots overlap on photo detector segments at the focus point and the focus error curve with a focus offset that is prevented.

In figure 1 prior art photo detectors for a single-spot system 102, a multi-spot system 106 with a photo detector perpendicular to radiation beams, also a multi-spot system 108 with a photo detector not perpendicular to radiation beams and the respective focus error curves or focus S-curves 104, 109 and 110 are shown.

For the single-spot system 102 a radiation beam that rise spot 120 is reflected from an associated optical storage carrier (not shown, see figure 3) onto a photo detector 118 through an astigmatic lens (not shown, see figure 3). For the multi-spot systems 106 and 108, three reflected spots are shown in three different focus situations. A photo detector 118 with four quadratic segments A, B, C and D is shown in the three different focus situations 112, 114 and 116 for all three systems.

In the focus situations 112 to the left in the drawing an objective lens (not shown, see figure 3) is focussing the radiation beam too far, i.e. the focal point of the radiation beam is behind the surface of the optical carrier storage, yielding a negative focus error. In the focus situations 116 to the right in the drawing an objective lens (not shown, see figure 3) is focussing the radiation beam too close, i.e. the focal point of the radiation beam is in front of the surface of the optical carrier storage, yielding a positive focus error. In the focus situations 114 the radiation is in focus.

In astigmatic focusing, the focus error signal, which makes up the focus S- curve, is obtained by focusing the radiation beam, which is reflected form the optical storage carrier, onto a split focus detector. Because an astigmatic lens is placed in front of this detector, the spot 120 will be round if the spot is properly focused onto the carrier, but the spot on the focus detector will be elliptic if the spot 120 is defocused onto the carrier. If the objective lens is too close to or too far from the disc the orientation of the elliptic defocused spots on the focus detector will be 90 degrees different, as shown in the focus situations 112 and 116 in figure 1. This makes it possible to detect whether the objective lens is too close or too far away from the carrier. Depending on the orientation of the astigmatic lens, 116 may represent the too far condition as explained but may in another example denote the to close condition and similarly for 112.

As the photo detector 118 is split into 4 segments ABCD, it is possible to detect if the objective lens is too close, too far or at the right position. The used Focus Error (FE) signal in this case is:

FocusError = (A+D)-(B+C)/(A+B+C+D) [1]

In this way, an focus error S-curve 104, 109 and 110 is obtained for each of the systems. The focus error can be used in a servo-loop in order to catch focus and maintain focus by displacing the objective lens. Also, a method using double normalization of the error signals can be used, in this case the focus error signal is ({[A-B]/[A+B]} + {[D-C]/[D+C]}). Note the FocusError equals zero in case of focus.

If an array of laser spots is used such as for the multi-spot systems 106 and 108, problems may arise. Even if the laser spot separation is rather large, some portion of a defocused side spot 122 can fall onto the detector 118. From formula [1], it can be seen that in that case, the S-curve 109 becomes shorter, as the measured difference between (A+D) and (B+C) becomes smaller and the sum (A+B+C+D) becomes larger. This makes catching focus more difficult, and the system becomes less stable.

If a laser array generating radiation beams is set to an angle e.g. not perpendicular to the detector 118 as shown in the system 108, which is normally the case in multi-track/parallel systems, the situation may become worse. Due to the position of defocused side spots 124 the S-curve 110 becomes both shorter and also a-symmetric, which deteriorates the stability of the system and again may cause difficulties during the catching of focus i.e. closing the focus servo loop. Furthermore, the angle of the laser array may change while reading/writing the disc at different radii. In figure 2 the side spot 210 overlap the B segment 204 along with the middle spot 208 and the side spot 212 overlaps the C segment 206 along with the middle spot 208. 214 is the focus error S-curve, the arrow 218 indicates the direction of the offset and 216 indicates the offset S-curve. Due to the limited length of an optical path (not shown) and an limited field of view of an objective lens, it will be difficult to obtain a large spot separation in a commercial system, therefore spot overlap of adjacent spots on the detector 202 may occur, even at the focus point. As the (C+B) signal becomes too large at the focal point, the S-curve will shift causing a focus offset. As normally the laser array is not set perpendicular to the focus detector, this problem will normally occur. Figure 3 schematically shows an embodiment of an optical system 301 and a detection system 312 according to the invention. The associated optical carrier 302 is fixed and rotated by holding means.

The optical system comprises an optical head 308, sometimes called an optical pick-up unit (OPU), the optical head 308 being displaceable by actuation means 310, e.g. an electric stepping motor. The optical head 308 comprises a photo detection system 312, a radiation source 314 such as a semi-conductor laser, a polarizing beam splitter 316, an objective lens 318, and objective lens displacement means 319. The length of the optical path may be the length from the radiation means 314 to the surface of the carrier 302 and/or the distance to the surface of the detection system from a given point of the optical system. The optical head 308 also comprises radiation dividing means 320, such as a diffraction grating or a phase grating that is capable of dividing the radiation beam 306 into multiple components. In this example five beams are shown and three beams are referenced to, the beam 322 and two beams 324 and 326. The beams 324 and 326 may denote side beams and may denote the first order diffraction on each side of the beam 322. The beam 322 may denote the main beam. In this example a collimator lens 328 for obtaining a parallel beam and a beam-shaper 346 is provided before the radiation dividing means 320. Normally, a semi-conductor laser does not generate a circular spot profile. To correct for this, the beam- shaper 346 is used, which transforms the elliptical beam profile into one or more circular spots with a symmetric intensity profile. The shown optical system is for multi-spot reading from the carrier 302. The radiation source 314 for emitting a radiation beam 306 can e.g. be a semi-conductor laser with a variable power, possibly also with variable wavelength of radiation. Relevant wavelengths of the radiation source 314 may comprise IR, visible light, UV, and soft X-rays. In this embodiment, the radiation source in combination with the beam dividing means (or grating) constitute the radiation-emitting device. This is a cost effective way of designing the radiation-emitting device. Equivalent means may however be envisioned, such as an array of aligned laser diodes or a multi-spot laser diode capable of emitting radiation with different intensity. The detection system 312 can also be used for a combined reading and writing system or a writing system capable of recording information on the carrier 302. For writing purposes the radiation source 314, which comprises a single semi-conductor laser for a reading system, and the radiation dividing means 320 are replaced by a multi-beam laser, from which all individual laser beams can be modulated individually.

The optical head 308 is optically arranged so that the radiation beam 306 is directed to the optical carrier 302 via the radiation dividing means 320, and an objective lens 318.

Radiation beams reflected from the carrier 302 falls on the photo detection system 312 after passing through the quarter wave plate 350, the polarized beam splitter 316 and the astigmatic lens 344.

The function of the photo detection system 312 is to convert radiation beams 322, 324 and 326 reflected from the carrier 302 into electrical signals.

The photo detection system 312 in this example comprises five photo detectors. The photo detector 352 in the middle of the five photo detectors comprises eight photo detector segments (also referred to as segments), e.g. photodiodes, charged-coupled devices (CCD), etc., each segment capable of generating one or more electric output signal signals. The photo detection system may also comprise a control- logic 354 and/or a processor. These electric signals may comprise signal representing e.g. information being read from the carrier 302, but for the present invention it will mainly be explained how the detection system detects a focus error (FE) of the optical system 301. The middle photo detector in this example used for detecting a focus error and here the other photo detectors are used for HF detection and possibly tracking. When further ones of the photo detectors are multiple segments photo detectors and/or eight segment detectors additional information of e.g. the alignment and/or orientation of the laser array with relation to the detectors can be obtained. Furthermore, it may not be the middle detector that is used for focus detection but one of the other detectors.

The photo detection system 312 may be placed perpendicular to the radiation beam 306 as shown but may also be placed at any other angle to the radiation beam. The photo detector system is shown as a single unit that comprises photo detectors and a control logic 354, but the photo detector system 312 may e.g. also be provided as photo detectors only and not comprising a control- logic but being in electrical connection with e.g. a control logic and/or a processor. The one or more signal(s) from the detection system is used to generate the S- curve and operate the objective lens displacement means accordingly in order to obtain focus. The objective lens displacement means may be an actuator.

It is pointed out that the invention is not limited to the optical system of the type illustrated in figure 3, and the figure is provided only as an example.

In figure 4 an example of an embodiment of the layout of the detector 352 shown in figure 3 is shown in a close up view. Instead of the commonly used 4-segment A, B, C and D focus detector every A, B, C and D detector segment is split up diagonally into two separate detector elements Al 402 and A2 404, Bl 406 and B2 408, Cl 410 and C2 412, Dl 414 and D2 416 as shown in figure 4.

The two-segment split may though be provided in any other way with the aim to provide a segment split where focussed or defocused spots do not overlap on one segment. Such an alternative split may be a concave or convex line or by any line or any number of lines connecting two sides of a non-split A, B, C or D quadrant. In this way the electric signals from certain of the segments can be ignored and/or the electrics signals can be multiplied by a constant during the generation of the focus error S-curve. This may be done in a mathematical function comprising a part for each segment. Each part of the mathematical function may be a constant multiplied with the signal. The constant may, e.g. in response to a focus condition of the optical system, for at least one of the parts is zero. Alternatively, the constant may be a finite value in the range between zero and 1.

In figure 5 the three focus situations also described in figure 1 for a multi-spot system with a radiation array perpendicular to the photo detector 352 shown for the photo detector of the present invention. In the situation 502 the elliptical defocused spots 508 and 510 are rotated 90 degrees with respect to each other, i.e. the defocused spots 512 and 510 is rotated 45 degrees in a clockwise and counter-clockwise direction, respectively, from a vertical orientation of the major axis of the ellipse. The focussed spots 514 have a form of a circle.

In can be seen from the figure that adjacent defocused spots overlap the A quadrant but are separated on the two separate split segments Al and A2. The segment is in the embodiments of the invention shown here split diagonally as seen from e.g. figure 3, 4 and 5. In this configuration the S-curve becomes shorter due to the overlap of the elliptical defocused spots on a single quadrant as seen in figure 1. The 8-segment focus detector can solve this problem. Normally the S-curve would be formed using:

FocusError=(Al+A2+Dl+D2)-(Bl+B2+Cl+C2)/(Al+A2+Bl+B2+Cl+C2+Dl+D2) [2]

We can use the configuration of the 8-segment detector to obtain an improved S-curve and e.g. a more stable focus detection, e.g. with the curve 104 as the aim, by determining a focus condition of the optical system. The focus condition may be determined e.g. by the following logical determinations:

if B1+B2+C1+C2 > Thresholdl then Al=O₅Dl=O [3]

and

if A1+A2+D1+D2 > Threshold2 then Bl=O₅Cl=O [4]

Firstly, by comparing one or more of the provided photo detector signals with one or more threshold(s), as shown in [3] and [4], the focus condition of the optical system can be determined. If e.g. B1+B2+C1+C2 is larger than a threshold then a focus condition that is equal or similar to 502 (focal point to close to objective lens) may be present.

Secondly, when multiplying a constant of zero with Al and Dl in the mathematical function [2] will generate a focus error signal where Al and Dl are ignored and an improved S-curve is obtained. Normally, in order to determine a most stable focus error correction, the signal of two of the outer segments Al, Bl, Cl and Dl is set to zero, thus reducing the size of the detector. A less stable correction may be obtained when only one of the signals Al, Bl, Cl and is set to zero.

The thresholds in [3] and [4] depends on one or more of e.g. the spot size, the detector size, the angle of the array with relation to the detector array, the laser power, the spot separation and the strength of the astigmatic lens. For example, thresholdl can be equal to the value of (A1+A2+D1+D2) and threshold2 can be equal to the value of (B1+B2+C1+C2).

Also other schemes are possible, by e.g. comparing the signals of different segments, like e.g. if B2 > A2 and/or C2 > D2 then Al=O, Dl=O [5]

and

if A2 > B2 and/or D2 > C2 then Bl=O, Cl=O [6]

Another possibility would be If A2+D2 > B2+C2 then Bl=O, Cl=O and If B2+C2 > A2+D2 then Al=O, Dl=O. E.g. when one or both of the conditions in [5] is true a focus condition equal to or similar to 502 (focal point to close to objective lens) may be present. Thus by comparing one or more of the provided photo detector signals with one or more of the other signals as shown in [5] and [6], the focus condition may be determined and one or more of the signals can be set to zero or ignored in the generation of the focus error signal S-curve. Normally, in order to determine a most stable focus error correction, the signal of two of the outer segments Al, Bl, Cl and Dl is set to zero. A less stable correction may be obtained by only setting one of the signals Al, Bl, Cl and Dl to zero.

In figure 6 the three focus situations also described in figure 1 for a multi-spot system, with a radiation array not perpendicular to the photo detector 352 as shown in figure 3, 4 and 5, is shown.

In this configuration the S-curve becomes asymmetric. The asymmetry of the focus S-curve is due to the overlap of the elliptical defocused spots on a single quadrant B and even an overlap on a single segment 602 of the photo detector 352 and the non-overlap of spots in the too far focus situation as shown at 604. The 8-segment focus detector can solve or at least partly prevent this problem. The focus error signal, used for the generation of the S-curve focus error signal is given by formula [2] and the asymmetry in the S-curve can be reduced by one or both of the following methods [7] and [8].

By reducing the effective size of the detector i.e. setting Al=Bl=Cl=Dl=O. This completely eliminates the a-symmetry, however, the S-curve may become too short. [7]

By using a normalization iactor n for element Bl and Cl . [8] When using a normalization iactor n for the element Bl and Cl the FocusError is given by:

FocusError=(Al+A2+Dl+D2)-(nBl+B2+nCl+C2)/(Al+A2+Bl+B2+Cl+C2+Dl+D2) [9] For which 0<n<l.

The constant n multiplied to one or more of the signals may be called a normalization factor. E.g. n=0.6 would work fine in the example. A more exact value of n can be determined by evaluating the maximum values of Al (+ A2) and Dl (+ D2). The size of the constant n is dependent on the strength of the astigmatic lens, the spot separation, spot size, and detector size etc., i.e. the orientation of the spot array with relation to the detector array, n may be determined as an example given by the area of a satellite spot which falls onto the detector when the spots have the most elliptical shape, see 124. n is determined by:

n=max(Area of central spot on element C2) / total area of spots on C2

The area of central spot on element C2 is the signal from the segment C2 as well as the total area of the spots on C2. As the area of the central spot on element C2 equals the maximum signal on the segment Al these can be chosen to be equal and n can be determined. Thus n can be used and the asymmetry can be partly suppressed.

In figure 7 the two adjacent spots overlap one quadrant on the B quadrant 710 and on the C quadrant 708. This overlap of adjacent spots on a quadrant will normally introduce a shift and a focus off-set in the S-curve. Using the 8-segment focus detector can solve this problem.

Reducing the size of the detector after focus has been established, although there is a focus off-set until then, can prevent the shift of the S-curve, just by setting Bl=Cl=O. After Bl=Cl=O has been provided, the proper FocusError signal is obtained and the focus offset will not be present. This method can be combined with the rules set by [3], [4], [5] and [6], which facilitates an improved generation of the focus S-curve and thereby the capture of focus.

In general, by evaluating all the values of the segments of the 8-segment focus detector, the geometry of the laser array with respect to the focus detector can be determined. After the geometry of the laser array with respect to the focus detector has been determined and/or after the determination of whether the focal point is too close or to far from the objective lens, the proper strategy of normalizing and/or ignoring certain segments at certain times can be chosen and the focus error signal can be generated in response to the proper strategy. The geometry of the laser array with respect to the focus detector may e.g. be perpendicular or non-perpendicular as described herein. The geometry and/or whether the focal point is too close or too far from the objective lens may be denoted the focus condition of the optical system 301.

Although the present invention has been described in connection with preferred embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims.

In this section, certain specific details of the disclosed embodiment such as the distance between multiple detectors and distance between multiple spots in other figures, etc., are set forth for purposes of explanation rather than limitation, so as to provide a clear and thorough understanding of the present invention. However, it should be understood readily by those skilled in this art, that the present invention may be practized in other embodiments which do not conform exactly to the details set forth herein, without departing significantly from the spirit and scope of this disclosure. Further, in this context, and for the purposes of brevity and clarity, detailed descriptions of well-known apparatus, circuits and methodology have been omitted so as to avoid unnecessary detail and possible confusion.

Reference signs are included in the claims, however the inclusion of the reference signs is only for clarity reasons and should not be construed as limiting the scope of the claims.

Claims

CLAIMS:

1. A photo detector system (312) adapted for focus detection in a multi spot astigmatic focus system (301), the photo detector system (312) comprising at least one photo detector (352) comprising at least eight segments (402, 404, 406, 408, 410, 412, 414, 416), each segment providing a segment signal in response to an amount of radiation received by the segment so that at least eight signals are provided and wherein the photo detector system (312) is adapted to determine a focus condition of the system (301) from at least one of the signals and wherein the system is further adapted to generate a focus error signal based on the determined focus condition.

2. A photo detector system (312) according to claim 1 wherein the segments is having a triangular form and wherein the triangular form is provided by splitting each segment of a four segments photo detector diagonally into two photo detector segments (402, 404) and (406, 408) and (410, 412) and (414, 416).

3. A photo detector system according to claim 1 wherein the focus condition of the system is determined by comparing one or more of the provided photo detector signals with one or more of the provided photo detector signals.

4. A photo detector system according to claim 1 wherein the focus condition of the system is determined by comparing one or more of the provided photo detector signals with one or more predetermined values.

5. A photo detector system according to claim 1 wherein the focus error signal is generated in response to a mathematical function, the mathematical function comprising a part for each segment.

6. A photo detector system according to claim 5 wherein the part comprises a constant multiplied with the signal and where the constant for at least one of the parts is zero.

7. A photo detector system according to claim 5 wherein the part comprises a constant multiplied with the signal and where the constant for at least one of the parts is a finite value in the range between zero and 1.

8. A photo detector system according to claim 1 wherein the focus condition is determined in response to the geometry of the optical system.

9. A photo detector system according to claim 1 wherein the focus condition is determined in response to the whether a focal point is too close or too far from the objective lens.

10. An optical system (301) for focussing multiple radiation beams on an associated record carrier (302) the system comprising a radiation-emitting device (314) capable of emitting radiation for generating multiple beams and for providing corresponding spots on the associated carrier (302) a focussing unit (318) for focussing the spots on the carrier (302) an astigmatic refraction unit (344) at least one photo detector (352) for detecting the multiple beams reflected from the associated carrier (302), the photo detector comprising at least eight segments (402, 404, 406, 408, 410, 412, 414, 416) each segment providing a segment signal in response to an amount of radiation received by the segment (402, 404, 406, 408, 410, 412, 414, 416) wherein the photo detector (352) is adapted to determine a focus condition based on at least one of the signals and wherein the system (301) is further adapted to provide a focus error signal based on the determined focus condition.

11. A method of focus error detection in a photo detector system (312), the photo detector system comprising a radiation-emitting device (314) capable of emitting radiation (306) for generating multiple beams and for providing corresponding spots (322, 324, 326) on the associated carrier (302) a focussing unit (318) for focussing the spots (322, 324, 326) on the carrier (302) an astigmatic refraction unit (344) at least one photo detector (352) for detecting the multiple beams reflected from the associated carrier for providing multiple reflected spots (332, 334, 336), the photo detector (352) comprising at least eight segments (402, 404, 406, 408, 410, 412, 414, 416) each segment providing a segment signal in response to an amount of radiation received by the segment wherein the method comprises the steps of determining a focus condition of the radiation spots (322, 324, 326) from at least one of the signals, generating a focus error signal based on the focus condition and at least one of the signals, adjusting the focusing unit (344) in accordance with the focus error signal.

12. Computer readable code adapted to perform the steps of claim 11.

13. An apparatus for reading and/or writing information from/to a record carrier, comprising a photo detector system as claimed in one of the claims 1 to 9.