The present invention relates to an optical pick-up unit for scanning a first type of record carriers and/or at least a second type of record carriers according to the preamble of claim 1.

The invention further relates to an optical player having such an optical pick- up unit.

An optical pick-up unit for scanning a first type of record carriers and at least a second type of record carriers is known from the US 2002/0101802 Al. An optical pick-up unit (OPU) as a key component of an optical storage system, called optical player, comprising a drive unit, and an optical pick-up unit for scanning an optical record carrier, is generally known. Information-recording media with high-recording density and large capacity are known as CDs (compact discs) and as DVDs (digital versatile discs). DVDs are available as DVD-ROMs, DVD-RAMs, DVD±R(W)s, as well as HD-DVDs and blue-ray discs.

In the context of the present invention, scanning an optical record carrier means reading from and/or writing on an information layer in or on an optical record carrier, called record carrier in the following.

Although the mentioned DVDs belong to the same family of high recording density and large capacity information recording record carriers, the structure of the record carriers, featured in lands and grooves, is different. The information is scanned on tracks situated on lands and/or grooves. Different types of DVDs can be distinguished by a specific distance between adjacent grooves, called a groove pitch. For the DVD±R(W), which is a groove recording type of optical record carrier, the information is recorded in the grooves, wherein each groove comprises a dye or a phase-change material. During the recording process in the dye or phase-change material of the grooves, a train of pits having different reflection is formed on top of the respective groove as recorded information. For the DVD- RAM being a land-groove type of record carrier, the information is recorded on the lands and grooves, wherein both are also made from a dye of phase-change material, and trains of pits

called tracks are formed thereon. Accordingly, the scanning system - the optical pick-up unit - of the optical player for the complete DVD-family is required to have compatibility with the different types of these described optical structures. For the DVD-RAM the groove pitch from groove to groove is qRAM = 123 μm in the area to be scanned and the groove pitch of the DVD±R(W) is q±Rw = 0.74 μm.

During scanning of the information on the record carrier, additionally to a main scanning radiation beam, in general two radiation beams are used in the optical pick-up unit for tracking the radiation beam. As tracking a measurement and adjustment of the position of a radiation beam spot on the land and/or groove is understood. Three radiation beams are used in a three-radiation beam method to perform the tracking. In particular, the information recording radiation beam, called main beam radiation beam, being in general a zero order diffracted beam, is directed as spot of the main radiation beam on the recording tracks, including both lands and grooves in order to scan information pits on the recording track, and spots of the auxiliary radiation beams, which are in general ± first order radiation beams, are positioned on both sides of the spots of the main radiation beam.

The detection of the three spots of the radiation beams, reflected from the record carrier, is performed by a detection element, comprising three separate detection areas to detect the three spots respectively. By this an error tracking signal, called push-pull signal, is detected to obtain a differential push pull signal. The method is called differential push- pull method, abbreviated DPP-method. With this, a three-beam optical pick-up can realize the tracking and the tracking error detection substantially independent o the offset.

Three spots push pull or DPP is applied to compensate the offset due to movement of the spot on the detector generated by an actuator radial stroke and due to misalignment of the center of the spots with respect to the center of the detectors. The three-beam optical pick-up can also be used to suppress cross-talk between adjacent tracks. Hereby the main radiation beam is focused on a track of the information layer of the record carrier and the auxiliary beams are focused on the adjacent tracks. Signals are simultaneously scanned from the adjacent tracks, and these signals are subtracted from a signal originating from the main radiation beam to cancel the cross-talk. This is a cross-talk canceling (CTC) method accomplished with a three-beam optical pick-up.

When a groove recording type record carrier is scanned with a three-beam optical pick-up, it is necessary to focus the auxiliary beams between grooves in order to detect the tracking error signal using the DPP-method. Because the auxiliary radiation beam spots are adjusted between the pre-embossed grooves and the main spot is on the pre-

embossed groove, the modulation of the grooves of the main radiation beam spots and the auxiliary radiation beam spots have a phase difference of 180° of the main spot, while the offset due to the misalignment of radial stroke has the same sign. Consequently, in the DPP radial signal the offset cancel, while the grooves signal amplitudes add. For a land groove recording type of record carrier it is necessary to focus the radiation beam spots on lands and adjacent grooves. Additionally, the groove pitches are different for both types of record carriers. In order to ensure compatibility between the operations of the DPP- and CTC- method, it is necessary to adapt the distances of the spots of the main radiation beam and the auxiliary radiation beams in the radial direction to the different track pitches of the different types of record carriers.

In US 2002/0101802 Al an optical pick-up unit is disclosed that can obtain a tracking error signal by the DPP-method using various types of optical record carriers with different optical structures using five diffracted radiation beams. Additionally, the CTC- method can be applied, whatever record carrier is scanned. An optical pick-up unit is disclosed, which is driven by an error signal and comprises a grating element for receiving a radiation beam to create a zero-order diffracted radiation beam, ± first order diffracted radiation beams, and ± second order diffracted radiation beams when the radiation beam passes through the grating element.

The ± first order diffracted radiation beams and the ± second order diffracted radiation beams are detected by a detection element comprising five different detection element components. By using in summary five radiation beams (the main beam and two auxiliary beam radiation beams), the different groove pitches of the different types of record carriers can be taken into account, while scanning the different types of record carriers by calculating the tracking error signal out of four auxiliary radiation beams. Herein one radiation beam is either focused on a groove or on a land, both being adjacent to the main groove.

The advantage of an angle difference instead of a pitch difference is that the same detector array can be used in case of the same wavelength because a distance s between the main spot and one of the auxiliary spot remains the same. This requires a complex detection element and high signal processing to obtain the tracking error signal. Further, the detection element has a complicated structure and therefore is expensive to process. Additionally the guidance of five radiation beams on the recording information layer of the respective record carrier requires a more precise alignment

of all the optical components in the optical pick-up unit, resulting in a higher complexity and a higher susceptibility to malfunctioning of the optical player.

It is an object of the present invention to provide an optical pick-up unit of the type mentioned at the outset, which avoids the above drawback. Therefore, the object of the present invention is aimed to provide an optical pick-up unit, an optical record player, operating with a three radiation beam tracking error signal and having improved performance in scanning different types of record carriers, wherein the different types of record carriers are high information density and high capacity media and have different groove pitches between adjacent grooves and/or adjacent lands and grooves.

Furthermore, the system should be easy to handle, not susceptible to malfunctioning and cheap in production.

According to one aspect of the present invention, the object is achieved with respect to the optical pick-up unit as mentioned at the outset, in that both transparent electrodes comprise aligned electrode-stripes, wherein the electrode-stripes of the first electrode and of the second electrode have different orientations with respect to one another.

The working principle of the optical pick-up unit is described shortly in the following. The optical pick-up unit is suitable for scanning a first type of record carriers and/or at least a second type of record carriers and uses an error tracking signal, wherein the first type of record carriers has a first groove pitch and the second type of record carriers has a second groove pitch. The first and second groove pitches are different from each other.

The optical pick-up unit comprises at least one radiation source, preferably a semiconductor laser, emitting at least one radiation beam having a wavelength and a radiation beam propagating along an optical path. The optical pick-up unit further comprises a grating element for receiving the radiation beam to create an n ^th order diffracted radiation beam and at least a (In) ⁴¹ V(I) ⁴¹¹ order diffracted radiation beam when the radiation beam passes through the grating element, wherein m, 1 are not equal n. The radiation beam and the n* ¹¹ order diffracted radiation beam as well as the order diffracted radiation beam are focused on a groove of an information layer of the first or the second type of record carrier so as to form on the groove a spot of the n ⁴¹¹ order diffracted radiation beam and on the land and/or adjacent grooves and/or in between adjacent grooves spots of the at least (m) ^th /(l) ^th order diffracted radiation beams.

The m ^th /l ^th order diffracted radiation beam spots have lands a distance to the groove, the r^ order diffracted radiation beam is focused on.

The information is scanned in or on the information layer of the first and/or at least second record carrier. Due to the pits, the reflected radiation beam from the record carrier is modulated according to the information on the record carrier. The reflected radiation beam and the reflected auxiliary beams then pass through another optical element, preferably a quarter- wave-plate and the radiation beam is imaged onto a detection element. The detection element component, detecting the main radiation beam, is in general a quadrant photo detection element component. The quadrant photo detection element component has advantageously four light receiving surface areas and transduces opto-electrically the radiation beam spot, irradiated hereon into an electric signal. The electric signal is supplied to a focus error detecting circuit. The focus error detecting circuit generates a focus error signal and a tracking error signal. The error signals are used to adjust the optical components in the optical pick-up unit if the radiation beam spot is not in a desired position. To detect the auxiliary radiation beams, aside of the main beam, the detection element comprises additionally on the left and on the right side of the photo-quadrant detection element component two detection element components, which generally have a setup having each two radiation receiving surface areas. The electric signals obtained from these additional photo detection element components are subtracted from the signals originating from the central detection element component, creating by this a so-called three-spot push- pull radial error signal or differential push-pull (DPP), which is defined as: RE = PP _c -γ/2-(PP _a +PP _b ) = γm _Pp -sin(2πx/q) (l-cos(2 πxo/q)), with

PP _C = (C1+C4)-(C3+C2), PP _a = A1-A2, PP _b = B1-B2, with Al, A2, Bl, B2, Cl, C2, C3 and C4 being the photo-sensitive surface areas of the detection element components, γ is the grating ratio, which is the power ratio of the main spot with respect to the auxiliary radiation beam spots, q is the track pitch and m _pp is the push-pull modulation, x the distance of the main spot to the groove (or land), and X ₀ the difference between the between the distance of the main spot to the groove (or land) and the distance of the auxiliary spot to the groove (or land).

The grating element used to create the diffracted radiation beams comprises at least two transparent electrodes having electrode stripes, and a layer of material having a first and a second optical state, wherein the second optical state is achievable from the first optical state by applying an external voltage to the first and/or the second transparent electrode.

The material can be a liquid crystal or any other polymeric or crystalline material, which has the properties of changeable refractive index in an electric field, generated between the at least two transparent electrodes.

It is also possible to use an index grating, wherein two immiscible liquids have different indices of refraction. Also an absorption grating can be used, wherein one of the two immiscible liquids absorbs, the other does not.

The use of such a grating element, which generates the three spots on the first and/or the second record carrier, allows to change the grating structure of the grating element. With this, the positions of the auxiliary radiation beam spots with respect to the track, on which the main radiation beam spot is positioned, are varied and adjusted to the groove pitch of the respective scanned record carrier.

The change in the grating structure is realized by arranging two transparent electrodes on a substrate, preferably a transparent plate - made of glass or polymeric material - opposite to each other with a layer of material with changeable optical properties in between them. The electrodes are made of a material, which is transparent for the radiation beam passing the grating element.

The grating structure is realized by arranging on at least one transparent electrode aligned electrode-stripes made of a transparent material in the liquid crystal device. By applying an electric field via the electrode-stripes to the material, the optical properties will be changed in the area of electrode-stripes. With this, the grating structure is generated in the liquid crystal device. Therefore the geometry of the electrode-stripes can influence the pattern of the grating. If the electrode-stripes are for instance straight stripes which are aligned to each other, a pattern of lines with different optical states is realized and with that a grating structure. The grating element has a specific grating structure, only if an external electric field is applied.

Applying different voltages on different electrode-stripes, different grating elements with different grating structures can be produced. In addition, in a flexible way, grating elements with different grating structures can be realized suitable to scan both DVD±R(W) and/or DVD-RAM and/or HD-DVD and/or CDs. The advantage of this optical pick-up unit according to the present invention is that the grating element comprises transparent electrodes having electrode-stripes, wherein the first and the at least second transparent electrodes each has aligned electrical conducting electrode-stripes. Using two transparent electrodes both comprising electrode-stripes allows a flexibility in the realized grating characteristics and performance in the grating element, for

instance a liquid crystal grating element. It allows to create different auxiliary beams, which have different distances to the grooves, which is scanned by the main radiation beam depending on grating structure.

According to a preferred embodiment of the invention, the optical state of the material is the refractive index of the material.

The refractive index can be changed easily. With this, a pattern of lines with refractive index 1 and refractive index 2 can be created. The arrangement of the lines results in irregularities, known as grating structure and performs the diffraction of the radiation beam into one main radiation beam and two auxiliary radiation beams. According to a preferred embodiment of the present invention, the first transparent electrode has a first pattern of the electrode-stripes and the second transparent electrode has a second pattern of the electrode-stripes, wherein the first pattern of electrode- stripes is different from the pattern of the electrode-stripes of the second transparent electrode. Advantageously, the pattern of the first electrode and the pattern of the second electrode are different and allow the generation of different grating structures which are adjusted to the different groove pitches of the first record carrier and the second record carrier.

According to another preferred embodiment of the present invention, the pattern of the electrode stripes of the first transparent electrode and the pattern of the electrode stripes of the second transparent electrodes are equal.

Using transparent electrodes with equal pattern of electrode stripes allows an easy and cheap production of the transparent electrodes.

In another preferred embodiment of the present invention, the direction of alignment of the electrode-stripes of the first transparent electrode and the direction of alignment of the electrode-stripes of the second transparent electrode are different from each other.

The direction of alignment of the electrode-stripes of the transparent electrodes influences the position of the spot on the disc. Therefore record carriers with different groove pitches can be scanned in the optical pick-up unit comprising transparent electrodes with different direction of alignments of the electrode-stripes on the first and the second transparent electrode. Each record carrier with a different groove-pitch requires a specific grating angle θ.

In another preferred embodiment of the present invention, the angle α, wherein α is the difference between the grating angle of the electrode-stripes of the first transparent electrode and the second transparent electrode, follows the relation: α =Θ _RAM - Θ±R( _W ), wherein ΘRAM is the required grating angle for scanning a DVD-RAM, and Θ±R( _W ) is the required grating angle for a DVD-±R(W).

The advantage of a grating element having transparent electrodes comprising electrode-stripes with different grating angles θ, resulting in an angle α, is that it is only necessary to rotate the transparent electrodes with respect to each other to obtain an additional distance between the spots of the main beam and the auxiliary beams. Transparent electrodes with the same pattern of the electrode-stripes can be used in the grating element.

In a preferred embodiment of the present invention, the first and every m ^th , wherein m is an integer number greater/equal to electrode-stripe of the respective transparent electrode, are connectable with each other to form a first set of electrode-stripes.

If m equals 2, this means that every other electrode-stripe of the respective t UraOnU ₀ s ₁ pIOarie-nIIti ' electrode is connectable to each other to form a first set of electrode-stripes. Applying a voltage to these connected electrode-stripes of the transparent electrode, an electric field is applied to the molecules of the material and the material will change the refractive index in this area. The material with changed refractive index and the material with unchanged refractive index will therefore form a pattern of material with refractive index ni and n ₂ will be created.

It is also possible, if m is an integer number greater than 2, for instance 3 to connect every third electrode-stripe, for the first transparent electrode, to connect the second and the fourth electrode-stripe to form a second set of electrode-stripes and to connect the third and the fifth and so on electrode-stripe to form a third set of electrode-stripes. According to a further preferred embodiment of the present invention, the second and every (m+l) ^th -electrode-stripes are connected to each other to form another set of electrode-stripes. With that, at least two sets of electrode-stripes are formed. The electrode stripes are characteristic of the respective transparent electrode and with that for the characteristic of the grating element. The remaining electrode-stripes, which are not connected to one of the sets of the electrode-stripes, do not disturb the radiation beam passing the liquid crystal grating, because they are made of transparent material. This allows a flexible connection of the electrode stripes and a change in the grating structure can easily be applied.

In a further preferred embodiment of the present invention, the aligned electrical connectable electrode-stripes are parallel to each other in a plane of transparent electrodes facing the radiation source.

To form a grating, it is advantageous to arrange the electrode-stripes aligned parallel to each other in the plane of the transparent electrode.

In a further preferred embodiment, the electrode-stripes have a distinct distance, called a pitch, to each other, wherein the pitch between the electrode-stripes of one set of electrode-stripes is equal. With that, a regular set of electrode-stripes can be realized.

According to a further preferred embodiment of the present invention, the pitch between the electrode-stripes of two sets of electrode-stripes can be different.

This is advantageous, because with a different pitch between the electrode- stripes, the realized grating element is different and can by this be adapted to the requirements of a record carrier to be scanned.

According to a further preferred embodiment of the present invention, the pitch of the electrode-stripes of the first transparent electrode is equal to the pitch of the electrode-stripes of the at least second transparent electrode.

Using the same pitch between the electrode-stripes of the first transparent electrode and the second transparent electrode allows a cheap production of the transparent electrodes. This is especially important, if the transparent electrodes are produced by evaporation of the material of electrode-stripes via a masking process. Accordingly, the same mask can be used to produce both transparent electrodes.

According to a further preferred embodiment of the present invention, the respective electrode-stripes are positioned in the plane of the respective transparent electrodes including an angle θ with the main axis of the plane. The transparent electrode is in general a glass plate of a size with an x-axis and a y-axis according to the Cartesian coordinate system. The main axis is generally meant to be the y axis. This is the direction of alignment of the electrode-stripes. In general, in an optical pick-up unit the direction of alignment of the electrode-stripes, resulting in the alignment of the grating, is performed in the y-axis of the Cartesian coordinate system. The angle θ, called grating angle θ, is then defined between the y-axis and the direction of alignment of the electrode-stripes.

In a further preferred embodiment of the present invention, the parallel electrode-stripes of the first transparent electrode include a first angle, grating angle G ₁ , and the parallel electrode-stripes on the second transparent electrode include a second grating angle θ ₂ to the main axis of the respective transparent electrode.

The first grating angle θi is referred to the necessary grating angle for scanning a first record carrier and the second grating angle θ ₂ is referred to scan the at least second record carrier. Frthermore, the first transparent electrode is used to realize a liquid crystal grating according to the first type of record carriers and the second electrode is used to realize a liquid crystal grating according to the at least second type of record carriers.

According to a further preferred embodiment of the present invention, the difference between the first angle θi and the second angle Θ2 equals zero.

According to a further preferred embodiment of the present invention, the difference between the first angle θi and the second angle Θ2 is equal to the difference between the required grating angle Θ _RAM and the required grating angle Θ± _RW , wherein the grating angle Θ _RAM is defined by the first groove pitch of the first record carrier, preferably the DVD-RAM and Θ± _R(W) is defined by the second groove pitch of the at least second record carrier, preferably DVD±R(W).

If the angles θi and Θ2 of the electrode-stripes are different, the direction of alignment of the electrode-stripes is slightly rotated with respect to each other, wherein the difference is the angle α, which is accordingly the difference between the angles θi and Θ2 and is referred to the required grating angle necessary for the first record carrier, preferably a DVD-RAM, and at least a second record carrier, preferably a DVD±R(W). The realization of the liquid crystal grating according to each of the mentioned record carriers is realized by selecting the applied voltage to the set of electrode-stripes.

The first set of electrode-stripes is set to a defined voltage and the other sets of electrode-stripes are set to 0 V. A grating according to the scanning of the DVD±R(W) is realized as a result. A grating for scanning a second type of record carrier, being preferably a DVD-RAM is selected by applying a voltage with a value greater than 0 to another transparent electrode with a set of electrode-stripes having the angle Θ2, according to the angle ΘRAM.

The other set of electrode-stripes is set to 0 V. With this, the grating in the liquid crystal device is slightly rotated by an angle α = ΘRAM - Θ±R _W and with that the distance between the main spot and the auxiliary spots is adapted to the track pitch of the record carrier to be scanned. With this, it can be switched between at least two types of record carriers, by choosing the required set of electrode-stripes of the first and the at least second transparent electrode respectively. This is easily achieved by applying different voltages to different sets of electrode-stripes.

According to a further preferred embodiment of the present invention, the first transparent electrode comprises a first and at least a second set of electrode-stripes and the second transparent electrode comprises a third and a fourth set of electrode-stripes.

Choosing a first and at least a second set of electrode-stripes on both transparent electrodes, four sets of electrode-stripes can be utilized for realizing a specific liquid crystal grating.

According to a further preferred embodiment of the present invention, the grating element suitable for scanning the first record carrier, preferably the DVD±RW, is created by applying an external voltage to the first set of electrode-stripes and interconnecting the second set of electrode-stripes and the third set of electrode-stripes and the fourth set of electrode-stripes.

According to a further preferred embodiment of the present invention, the grating element suitable for scanning the at least second record carrier, preferably the DVD- RAM, is created by applying an external voltage to the third set of electrode-stripes and interconnecting the first set of electrode-stripes and the second set of electrode-stripes and the fourth set of electrode-stripes.

With this, every angle θi and θ ₂ is realized by two sets of electrode-stripes arranged on the respective transparent electrodes. This is advantageous, because it allows a redundancy during operation of the diffraction grating. For instance, if there is a failure in the first set of electrode-stripes on the first transparent electrode, the second set of electrode- stripes, which have the same angle θi as the first set of electrode-stripes, can be used for further operation mode of the grating element. Then the fourth set of electrode-stripes have to be chosen on the second transparent electrode.

According to a further preferred embodiment of the present invention, the liquid crystal device comprises four transparent electrodes, wherein at least two have aligned electrode-stripes.

It is advantageous to use two separate liquid crystal devices, wherein the two different liquid crystal devices have transparent electrodes with different grating angles θi and Θ2, because it is only necessary to add the missing liquid crystal device to realize an additional grating angle. It is advantageous, because it is possible to use the prefabricated liquid crystals.

The object of the present invention is achieved by an optical player for a first type of record carrier, preferably a DVD-RAM, and at least a second type of record carrier,

preferably a DVD±R(W), using an optical pick-up unit according to the embodiments described above.

It is to be understood, that the aforementioned features and those still to be explained below are not only applicable in the combinations given, but also in other combinations or in isolation without departing from the scope of the invention.

These and other objects and advantages of the present invention will become more apparent from the following description with reference to the accompanying drawings, wherein:

Fig. Ia is a schematic view of an optical pick-up unit;

Fig. Ib is a schematic view of a detection element;

Fig. 2 is a schematic view of tracks and grooves for different types of record carriers;

Fig. 3 is a schematic view of a set-up of a first embodiment of a grating element;

Fig. 4 is a schematic view of a grating element according to the present invention;

Fig. 5 is a schematic top view of two transparent electrodes with electrode- stripes, which have different direction of alignments of alignment;

Fig. 6 is a schematic view of a grating element according to the present invention.

Now, the present invention will be described below with reference to the accompanying figures of the drawing in accordance with the embodiments. For convenience of the description, information storage media are called record carriers.

Fig. Ia shows a schematic view of an optical pick-up unit (OPU) for use in an optical player, suitable for scanning a record carrier 12 having tracks 14 to store information on the record carrier 12. The optical pick-up unit 10 according to the present invention is suitable for scanning information on record carriers 12 having a high recording density and a large capacity for the recorded information. These are preferably DVDs (digital versatile discs). The DVDs to be scanned in an optical player using an optical pick-up unit 10 according to the present invention are DVD-RAMs, DVD±R(W) and so on. The different

types of record carriers, especially the different types of DVDs, are distinguishable by a different structure of the record carrier 12. In particular the different optical structure is related to a different distance, shown as an arrow 16, between the adjacent tracks 14, called track pitch 18. The track 14 comprises in general a train of pits 20, which are in general realized in a dye or a phase-change material of the track 14. It is also possible that the record carrier comprises pre-content. For this case, the pits are embossed in the groove and/or land. Among the tracks 14 to be scanned in an optical recorder, there is one type of record carrier which has a groove recording type structure, which means that grooves are formed on the information layer, not shown here, of the record carrier 12, which is made from a dye or a phase-change material, and a train of pits 20 processing different reflectance if formed on top of the grooves as recorded information. Still another optical structure is known, which is called a land-groove recording type optical record carrier structure. In the land-groove type structure, lands and grooves are formed from the dye or phase-change material, and trains of pits 20 are formed on the lands and grooves.

Accordingly, the scanning system, the optical pick-up unit 10 for the complete DVD family, is required to be compatible with different types of optical record carrier structure. Additionally the record carriers, particularl the DVD-RAM and DVD±R(W), are distinguished by the groove pitch 18 of adjacent grooves 14. The groove pitch 18 of a DVD±RW has a value of 0.74 μm, while the value of the groove pitch for a DVD-RAM is 1.23 μm.

The scanning procedure and the components of the optical pick-up unit itself will be described shortly in the following. A radiation beam 22 is emitted from a radiation source 24, which is preferably a semiconductor laser. The radiation beam 22 enters a diffraction grating element 26, named grating element 26 in the following, which converts the radiation beam 22 into a main radiation beam and two auxiliary radiation beams each being aside of the main beam. The main radiation beam and the auxiliary radiation beams are not shown here. The main radiation beam and the two auxiliary radiation beams are important for the tracking mechanism of the optical pick-up unit 10 and will be described later.

A diffracted radiation beam is in the following assigned with reference number 28. A diffracted radiation beam 28 comprising at least three radiation beams (main and auxiliary) is generated from a radiation beam 22 after having passed the grating element 26, even if the main radiation beam and the auxiliary radiation beams are not shown separately in

the figure. The radiation beam 28 comprises an n ⁴¹¹ order diffracted radiation beam, which is preferably a zero order diffracted radiation beam and an (mfflilf ¹ order radiation beam, which are preferably a ± first order diffracted radiation beam. It is also possible to choose other diffraction orders of the diffracted radiation beam to realize the diffracted radiation beam 28.

The radiation beam 28 propagates along an optical path 30 of the optical pickup unit 10 and passes a beam splitter 32. The beam splitter 32 is preferably a polarized beam splitter. With a polarized beam splitter 32, only that part of the radiation beam 28 passes through the beam splitter 32 thatis polarized - for instance parallel to the surface of the paper. The radiation beam, which has passed the beam splitter 32, is in general collimated by a collimating element 34 and directed by a reflecting element like a mirror 36 and to an objective lens 38.

The objective lens 38 focuses the radiation beam 28 onto the information layer of a record carrier. The radiation reflected from the record carrier 12, in the following called the returning radiation beam 40, is introduced into the detection element 42 by passing the optical path up to the reflection transmission element 32, following then the detection optical path 44. Especially the returning radiation beam 40 passes again through the beam splitter 32 via the objective lens 38, a quarter wave length plate 37, which can be introduced into the optical path 30, the mirror 36 and the collimator lens 34. The optical path of the returning radiation beam 40 is varied to a direction different from the direction directed toward the radiation source 24 by the beam splitter 32. The returning radiation beam 40 is guided to the detection element 42 in general through an optical element 46, which is in general a cylindrical lens. The returning radiation beam 40 includes the main radiation beam as well as the auxiliary radiation beams. The detection element 42 comprises radiation receiving detection element components, wherein each one is equipped with radiation sensitive surface areas, which convert the incident radiation beam into an electrical signal. The detection element 42 is shown in detail in Fig. Ib.

The detection element 42 consists in general of three radiation detection element components 48, 50 and 52. The detection element component 48 has two radiation receiving surfaces 54 and 56, which are electrically separated from each other, to allow the distinguishing between radiation incident on the radiation receiving surface 54 and receiving surface 56, resulting in two different electrical signals for light incident on the different surface areas. The radiation receiving element 52 has the same structure comprising radiation

receiving surface 58 and 60. The radiation receiving element 50 is a so-called four-quadrant radiation receiving element having four separated radiation receiving areas 62, 64, 66 and 68, resulting in four separate electrical signals. The radiation receiving surface areas of the radiation receiving elements 48 and 52 are the same, and the areas of the four radiation receiving areas 62, 64, 66 and 68 are equal.

The estimation of the position of the radiation beam 40 on the detection element 42 will be described in the following. The main radiation beam is directed onto the radiation receiving element 50 and the auxiliary radiation beams are directed to incident on the surface of the radiation receiving elements 48 and 52. The position of the main spot with respect to the groove is measured by subtracting the sum of the signals resulting from the surface areas 68 and 64 from the sum of signals resulting from surface areas 62 and 66. The position of the auxiliary beams with respect to the grooves is measured by subtracting 56 from 54 and subtracting 60 from 58. The radial error signal is given by the following equation: Radial error signal = PP _c -γ/2 (PP _a +PP _b )

= γmp _P (sin(2πx/q) (l-cos(2πxo/q)), wherein PP _c =(62+66)-(68+64), PP _a =(54-56) and PP _b =(58-60), γ is the grating ratio, that means the power ratio between the main radiation beam spot with respect to the auxiliary radiation beam spot, q is the track pitch and m _pp is a pull push modulation. The distance of the auxiliary radiation beam spots in relation to the grooves is adjusted to half the groove pitch (xo=q/2), which means that the main beam is for instance centered on the groove and the auxiliary beam spots are centered on the land between the neighboring grooves. This means that the amplitude and the slope are maximal when the radiation error signal fulfils the following equation: Radiation error signal = γm _pp sin(2πx/q) 2.

The grating pitch has to have a specific value to result in a specific separation between the main and auxiliary spots on the disc. With the alignment θ, the position of the auxiliary spots relative to the position of the main spot on the disc, which is the distance X ₀ , can be adjusted. The specific diffraction of the radiation beam 22 results in a relative distance between the main radiation spot and the auxiliary radiation beam spot.

From this it is clear that a record carrier with a specific track pitch requires a specific grating element with a specific grating angle θ of the pattern of the grating structure.

The grating element 26, according to the present invention, comprises a material which has different optical states. In particular, the material is a liquid crystal material, but it can also be an organic polymer or an inorganic polymer. It is only necessary that the material has at least two optical states which are different from each other and allows the formation of a grating structure by changing the optical state. In particular with the optical state, the index of refraction of the material is meant.

The function and the set-up of the grating element is explained by way of example for a liquid crystal grating in accordance with Fig. 3. Here it should only be mentioned that the grating element comprises a grating, wherein the grating can be varied by applying an external voltage to transparent electrodes in order to adapt the grating angle θ to the requirements of the track pitch of the different types of record carriers. Therefore with the grating element, in particular the liquid crystal grating, being adjustable, at least two types of record carriers can be scanned in an optical player comprising the optical pick-up unit according to the present invention. The spots of the auxiliary radiation beams are varied by this by varying the grating angle θ of the diffraction structure in the liquid crystal grating.

Fig. 2 describes the spots of the main radiation beam 70 and the auxiliary radiation beams 72 and 74 positioned on the track 14 on the record carrier. It can be seen that the main radiation beam spot is focused on a central track 76 and the auxiliary beams 72 and 74 are focused in between adjacent track 14. The distance of the tracks marked with arrows 16 can be recognized as being different. For instance, the distance between the grooves of the right record carrier is 1.23 μm and the distance between the grooves of the second record carrier is 0.74 μm, relative to the groove pitch 18 of a DVD-RAM and a DVD±R(W). With arrow 78 half of the track pitch 18 is described. In order to obtain a usable error tracking signal, the main spot is positioned on the track 14 and the auxiliary radiation beam spots 72 and 74 are positioned beside the track 14, wherein the position is centered in the groove 15 adjacent to the track 14.

It can be seen that for different groove pitches 18 of different record carriers, the distance between the main spot and the auxiliary spots 80 is a different one. Because the distance between the main spot and the auxiliary spots is adjustable by the grating angle θ of the grating 26 in the optical pick-up unit 10, it is necessary to use the grating element 26, which can realize different grating angles θ, to allow the scanning of record carriers 12 having different groove pitches 18. To be more exact, a first type of record carriers 12 having a first groove pitch 18 requires a first grating angle θi for the grating element 26, and a

second type of record carriers 12 having a different groove pitch 18 requires a different grating angle θ ₂ of the grating element 26, to realize a distance between the track on which the spot of the main radiation beam and spots of the auxiliary radiation beams are positioned. The distance between the track on which the spot of the main radiation beam is positioned and the spots of the auxiliary beams to scan the first type of record carriers are different from the distance between track on which the spot of the main radiation beam is positioned and the spots of the auxiliary beams spots required to scan the second type of record carriers 12. The distance 80 and the grating angle θ as well as the groove pitch 18 follow the following equation: X ₀ = p/2 = s sin θ. wherein p is the groove pitch 18 and s is the distance 80 between the spots and X ₀ the distance to be adjusted. Because the applied grating angles θ are small angles, sin θ can be approximated by θ:

X ₀ = p/2=s ' θ From this it is obvious, that the distance X ₀ can be adjusted by varying the grating angle and that the grating angle θi and the grating angle θ ₂ have to be different in order to scan different types of record carriers, in particular DVD-RAM and DVD±R(W).

In Fig. 3 a schematic view of the grating element 26, realized by a liquid crystal device as generally known, is shown. The liquid crystal device comprises two glass plates 82, a layer 84 of liquid crystal molecules 85 and a first transparent electrode 86 and a second transparent electrode 88. The transparent electrodes 86 and 88 are made o a material thatis transparent on the one hand and electrically conducting on the other hand. The transparent electrodes are connected by connectors 90, 92 and 94, which allows to apply a voltage to the transparent electrodes 86 and 88. The transparent electrodes 88 and 86 are positioned opposite each other with the liquid molecules 85 in between. The surface of the transparent electrodes 86 and 88 are arranged in such a way that they face the transparent plates 82 which are preferably made of glass or a transparent polymer or another transparent and electrically insulation material. In case of the transparent electrode 88, the electrode area is divided into different electrode-stripes, 104, 106, 108, wherein the electrode-stripes are insulated electrically from each other.

Preferably the electrode-stripes are directly positioned on the plate 82, in that the plate acts as a support medium for the electrode-stripes, which allows a very thin set-up of the electrode-stripes. It can be imagined, to produce the transparent electrodes 86 and 88

by evaporating an electrically conducting material onto a glass plate. It is also conceivable to produce very thin and flexible electrode-stripes and glue them onto a plate 82 to form a transparent electrode. It should be kept in mind that the transparent electrodes have to be transparent for the radiation beam used and that the liquid crystal device can be produced in a very small size to fit in an optical pick-up unit of an optical player.

The first electrode-stripe 104 is connected electrically with the (l+m) ^th electrode 108. In the example, shown in Fig. 3, m equals 2, which means that every other electrode is electrically connected. But in general it is also possible to connect every third electrode or every fourth and so on. The electrode 106 next to first electrode is connected to the electrode next to the electrode 108, that is the electrode 109. This means in this example shown, electrode 104 and 108 form the first set of electrode-stripes, and electrode-stripes 106 and 100 form a second set of electrode-stripes.

The working principle of a liquid crystal grating is explained shortly in the following. If a certain voltage V is applied to the first set of electrodes and the second set of electrodes and the electrode 86 are connected to ground potential, an electrical field is built up between the electrodes 88, consisting of electrode-stripes, and the ground electrode 86, here in particular between the electrode-stripe 104 and the ground electrode 86 and electrode 108 and the ground electrode 86. The electrical field is marked by arrows 112 and leads to a changed index of refraction U ₀ end in this area. The area with the changed index of refraction is marked by reference Nos. 114 and 115. The areas of the layer of liquid crystal molecules seeing no electric field will remain with an unchanged refractive index no.

With this, the layers of liquid crystal molecules 84 having a uniform refractive index no without an electric field, now have with the electric field a structure of alternating stripes of different refractive indices, one stripe has n _c and the other has no. The optical path length for rays passing though the liquid crystal is different in areas with different refractive indices. The liquid crystal device has become a grating element with this.

The arrangement of the electrode-stripes as shown in Fig. 3 is only an example and has been used to explain the working principle of a liquid crystal grating.

According to the present invention, both transparent electrodes 86 and 88 comprise electrode-stripes, which are connectable. The electric field is built between the electrode-stripes of the electrode 86 and the electrode 88 as described for the second transparent electrode 88.

Fig. 4 shows a grating element 26, in cross section, realized by a liquid crystal grating according to the present invention, which includes two transparent electrodes,

wherein every electrode comprises at least one set of electrode-stripes, a first type of transparent electrodes and a second type of transparent electrodes 116 and 118, with the layer 84 of liquid crystal molecules 85 in between the sets of electrode-stripes 116 and 118 arranged opposite each other. The glass plates 82 can also be seen. Preferably the transparent electrode-stripes 102, 104 and so on are produced by evaporation of a electrical conducting material on the glass plates 82. This is the easy way of producing electrodes.

Fig. 5 shows the first transparent electrode 116 and the second transparent electrode 118 in a top view. Schematically shown are the glass plates 82 by the rectangle frame 120 for the transparent electrode 120, and 122 for the transparent electrode 118. A set of electrode-stripes 124 and 126 is shown as well, wherein the sets 124 and 126 include the electrode-stripes 104, 106, 108, 110, which was discussed in the description according to Fig. 4. Additionally, the electrical connection means 92 and 94 are shown for the electrode 116. These electrical connections were discussed in relation with Fig. 3. Additionally, electrical connections 128 and 130 can be seen for electrode 118.

It can be seen that the electrode-stripes 104, 106, 108, 110 in the set of electrode-stripes 124 are aligned parallel to each other. Every other electrode is connected electrically by connection 92 and 94, respectively, to each other, to form two sets of electrode stripes 132 and 134. Arrow 138 indicates the direction of alignment of alignment of the electrode-stripes. In this example, the direction of direction of alignments 8 is equal to the main axis of the electrode indicated by the arrow 138, which is the y-direction, when applying a Cartesian coordinate system to the glass plates 120 and 122.

The second transparent electrode 118 shows a different direction of alignment indicated by arrow 140. It can be seen that between the direction of alignment indicated by arrow 138 and the direction of alignment indicated by arrow 140, an angle α with reference numeral 142 is formed. According to the present invention, this angle α 142, indicating the difference between the direction of alignment of the electrode-stripes of the first transparent electrode 116 and the direction of alignment of the second transparent electrode 118, is equal to the difference of the required grating angle θ for a first record carrier and a second record carrier, expressed in a different way

By applying a voltage with a specific value to the electrical connection 92 and a zero voltage (V=O) to the electrical connection 94, 128 and 130, a grating angle G ₁ is realized, wherein G ₁ is preferably the grating angle for a DVD-RAM G _RAM - Applying a certain

voltage to the electrical connection 128 and applying zero voltage to the electrical connection 192, 94 and 130, a different grating angle θ ₂ is realized.

It should be mentioned that the selection of applying a certain voltage is just an example of a first embodiment and it does not limit the scope of the present invention, if, to realize a first grating angle, a voltage is applied to the electrical connection 94 instead of 92. Although the present invention is not limited to having two sets 132 and 134 of electrode- stripes on one transparent electrode 116, it is also possible to position several sets of electrode-stripes on one transparent electrode. The only requirement according to the present invention is that the direction of alignment 140 between the electrode-stripes of the second transparent electrode 118 is different to the direction of alignment 136 of the first transparent electrode.

It should be mentioned that the pattern of the electrode-stripes is not discussed here and all known patterns of electrode-stripes are included in the present invention. The patterns can be equal or different from each other. Fig. 6 shows a second embodiment of the present invention of a grating element, in particular of a liquid crystal grating. The embodiment comprises three glass plates 82, two transparent electrodes 86, 88, which do not comprise electrode-stripes and two sets of electrode-stripes, for instance 116 and 118. In between the electrodes 86 and 116 and the electrodes 86 and 118, a layer 84 of liquid crystal molecules 85 is arranged, respectively. That means that the grating element 26 shown in Fig. 6 comprises two liquid crystal elements as shown and explained in Fig. 3, wherein the sets of electrode-stripes 116 and 118 have different directions of alignment, especially an angle α which is the difference between the angle G ₁ and G ₂ , wherein G ₁ belongs to the transparent electrode 116 and θ ₂ to the transparent electrode 118. With this, the grating element is realized by combining two separate grating elements, in particular liquid crystal cells, wherein the first liquid crystal device is suitable for scanning a first type of record carrier, in particular a DVD±R(W), and the second type of liquid crystal device is suitable for scanning a second type of record carrier, in particular a DVD-RAM. The two liquid crystal devices 142 and 144 are separated by a glass plate 82. The working principle is as follows: a zero potential (V=O) is applied to the electrodes 86 and a potential not equal to zero (V≠O) is applied to the grating 116 to realize the grating angle G ₁ . The other liquid crystal grating 144 is just a transparent part of the grating element and does not hinder the passing of the radiation beam. In case a grating angle θ ₂ for a second type of record carrier has to be realized, the potential is applied to the set of

transparent electrodes 118 and the liquid crystal grating 142 is just acting as a transparent part of the grating element.

To summarize the embodiments according to the present invention, it is stated that the grating element has at least two transparent electrodes with a material with two different optical states in between. The transparent electrodes comprise aligned electrode- stripes, wherein the electrode-stripes of the first transparent electrode has a first direction of alignment 136 and the electrode-stripes of the second transparent electrode have a second direction of alignment 140. The direction of alignment 136 is different from the direction of alignment 136 resulting in a rotation of the electrode-stripes in relation to each other. A first distance between the spots of the main radiation beam and the auxiliary radiation beam, measured in the direction perpendicular to the grooves on the disc, suitable for a first type of record carrier is created by using a grating element with a first grating angle θi and a second distance suitable for a second type of record carrier is created by the same grating element with a second grating angle θ ₂ . The different grating angles are realized by having different direction of alignments of the at least two aligned electrode-stripes of the at least two transparent electrodes - the electrode-stripes are rotated against each other.