WO2007091184A1

WO2007091184A1 - Optical pick-up unit for use in a multi-disc optical player

Info

Publication number: WO2007091184A1
Application number: PCT/IB2007/050253
Authority: WO
Inventors: Sjoerd Stallinga
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2006-02-06
Filing date: 2007-01-25
Publication date: 2007-08-16
Anticipated expiration: 2008-08-06
Also published as: TW200809825A

Abstract

An optical pick-up unit for scanning a record carrier having at least one information layer, wherein the record carrier is a first type of record carrier and/or at least a second type of record carrier, the optical pick-up unit comprising: at least one radiation source; at least one objective lens; at least one first optical element changeable between a first state and at least a second state; at least one second optical element with at least one structured surface for providing a wavefront aberration compensation comprising a polarization sensitive material, the structured surface having annular zones, each annular zones having a width, the annular zones forming a first stepped profile having a pattern of steps, wherein each step comprises additionally to the width of the annular zone forming the step, a height. The structured surface comprises at least a second stepped profile with a number of steps forming the pattern of steps, wherein the first stepped profile and the at least second stepped profile are separated by an annular zone and the number of steps forming the pattern of steps of the first stepped profile and the number of steps forming the pattern of steps of the second stepped profile are equal in order to form at least one repetitive pattern of steps.

Description

Optical pick-up unit for use in a multi-disc optical player

The present invention relates to an optical pick-up unit for scanning a record carrier having at least one information layer, wherein the record carrier is a first type of record carrier having a first format and/or at least a second type of record carrier having a second format, according to the preamble of claim 1. The invention further relates an optical player having such an optical pick-up unit.

The invention further relates to an optical element having a structured surface.

The invention is further related to a method for compensating wavefront aberrations while scanning a first type of record carriers and a second type of record carrier.

Such an optical pick-up unit is known from the WO 03/049095 A2.

There is currently a need in the field of optical storage for providing optical scanning devices, so-called optical pick-up units (OPU), for scanning a variety of different types of optical record carriers, in particular record carriers with high information density, such as blu-ray disc (BD-format), digital versatile discs (DVD-format), high density digital versatile disc (HD-D VD-format) and compact disc (CD-format). Each of the above- mentioned record carriers have a different physical set-up of the record carrier in order to achieve a different information storage density, in particular a different information layer depth. Scanning an information layer refers to reading information from the information layer and/or writing information onto the information layer and/or erasing information from the information layer of the record carrier.

Information density refers to the amount of stored information per unit area of the information layer. A record carrier comprises a substrate with a reflection layer, at least one information layer and a so-called cover layer which is in general a transparent layer made of a polymer, in particular polycarbonate (PC).

Information layer depth refers to the distance of the information layer with respect to the surface of the record carrier a radiation beam is incident on. Different types of record carriers have different thicknesses of the cover layer. For instance HD-DVD have a cover layer thickness of d = 0.6 mm, wherein BD record carriers have a cover layer of thickness of d = 0.1 mm. Therefore the information layer depth is different for record carriers having a different cover thicknesses, in particular for HD- DVDs and BDs. The record carrier having more than one information layer, like dual-layer BD, comprises information layers at different information layer depths according to a spacing between the information layers.

The information density may be increased firstly by using dual- or triple- layered record carriers, for example the dual-layer BD. The information density may secondly be increased by decreasing the size of the radiation beam spot, called scanning spot, scanning the information onto or from the information layer. The size of the scanning spot depends on the wavelength λ and the numerical aperture NA of the radiation beam forming the scanning spot. The size of the scanning spot can be decreased by increasing the numerical aperture NA and/or by decreasing the wavelength λ. In an optical scanning device for scanning the above-mentioned types of record carriers, mainly three wavelengths, λi = 780 nm, λ₂ = 650 nm and λ₃ = 405 nm (blue laser) are in use. The smallest wavelength λ₃ = 405 nm is used for scanning the high density information storage record carriers, like HD-DVD, dual-layer BD and BD record carriers. The scanning device for scanning the HD-DVD operates at a numerical aperture of NA = 0.65 and the record carrier has a cover layer thickness of d = 0.6 mm.

Whereas the scanning device for scanning the BD record carriers operates at a numerical aperture NA = 0.85 and a record carrier with a cover layer thickness of d = 0.1 mm.

The above-mentioned optical pick-up unit comprises in general at least one radiation source emitting at least one radiation beam, at least one beamsplitter and at least one objective lens forming a scanning spot from the at least one radiation beam and directing the scanning spot onto the information layer. The radiation beam propagates along an optical axis, passes the above-mentioned optical components arranged along the optical axis of the optical pick-up unit, and the scanning spot scans the information in or from the information layer. The scanning spot is reflected by the reflection layer, propagates along the optical axis, and is directed by the beamsplitter towards a detection element, detecting the reflected scanning spot.

In general a grating element, arranged after the radiation source forms out of the emitted radiation beam three radiation beams in order to perform a focus error correction and a tracking error correction. It is to be understood that the three radiation beams propagate accordingly through the optical pick-up unit and are as well detected by the detection element. Using the detected signals, which are transformed into electric signals, the focus and tracking error correction can be performed. According to the differences in the numerical aperture NA of the objective lens and the cover layer thickness d of the different types of record carriers, a different amount ofwavefront aberrations, especially the spherical aberrations, occurs at the scanning spot.

In order to perform a compensation of the wavefront aberrations, different solutions have been proposed. In the WO 03/060892 A2, the US 2005/0063282 Al, the US 2005/0163015 Al and the US 2005/0122883 Al, an optical scanning device for scanning record carriers with different information densities, in particular with different information layer depths are proposed.

The disclosed optical scanning devices comprise a radiation source for emitting three radiation beams, at least one objective lens system for directing the scanning spot onto the respective information layer. The record carriers to be scanned have different information layer depths. An optical element is included having a diffractive structure, called phase structure, with a non-zoneic stepped profile. The optical element includes a birefringent material, sensitive to different polarizations of the radiation beam. The stepped profile of the surface of the optical element is designed to introduce wavefront modifications to the respective radiation beam, wherein the amount of the wavefront modifications is different for the radiation beams with different wavelengths, respectively.

The WO 03/049095 A2 discloses an optical scanning device for scanning two different types of record carriers, in particular a dual-layer BD and a BD record carrier using a radiation beam with a wavelength of λ = 405 nm. The optical scanning device disclosed, comprises a single radiation source, emitting a radiation beam with a wavelength of λ = 405 nm and an optical element including a birefringent material having a surface with a diffractive structure in order to compensate the wavefront aberrations that occurs at the scanning spot in the case of scanning the dual-layer BD record carrier due to the fact that the objective lens is optimized to the information layer depth of the single-layer BD.

This solution is not suitable for the compensation of the wavefront aberration difference occurring during scanning of a BD and a HD-DVD, due to the fact that the information layer depth of the BD and HD-DVD record carriers are quite different compared to that of the dual- layer BD and the BD record carriers. Accordingly, the wavefront aberrations to be compensated are quite high in the case of the BD and the HD-DVD record carriers.

Therefore, it is an object of the present invention to provide an optical pick-up unit of the type mentioned at the outset being able to compensate wavefront aberrations occurring while scanning record carriers of the different types of record carriers, like BDs, HD-DVDs, CDs and DVDs and so on.

It is further an object of the present invention to provide an optical player which is able to scan different types of record carriers, like BD and HD-DVD, CDs, DVDs and so on.

It is further an object of the present invention to provide a method for compensation of wavefront aberrations in an optical pick-up unit scanning different types of record carriers. 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 the structured surface comprises at least a second stepped profile with a second number of steps forming a pattern of steps, wherein the first stepped profile and the at least second stepped profile are separated by an annular zone and the number of steps of the first stepped profile and the number of steps of the second stepped profile are equal in order to form at least one repetitive pattern of steps.

In order to understand the function of a structured surface according to the present invention, the working principle of the optical pick-up unit (OPU) is shortly described 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, wherein the first type of record carriers, in particular a BD, has a first format and the second type of record carriers, in particular a HD-DVD has a second format. The first type of record carriers and the second type of record carriers have different thicknesses of the cover layer, resulting in a different information layer depth. The numerical aperture NA of the OPU is also different: NA_BD = 0.85 for the BD and NA_HD-DVD = 0.65 for the HD-DVDs.

The information layer of each record carrier may comprise grooves and land between adjacent grooves, wherein the distance between adjacent grooves is different for different types of record carriers. The optical pick-up unit comprises at least one radiation source, preferably a semiconductor laser, emitting a radiation beam having a wavelength of preferably λ = 405 nm, wherein the radiation beam propagates 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 m^th and 1^th order diffracted radiation beams, called auxiliary radiation beams, when the radiation beam pass through the grating element, wherein m, 1 are not equal n. The radiation beam in the n^th order diffracted radiation beam is as well as the m^th and 1^th order diffracted radiation beams are focused onto an information layer of the first or the second type of record carriers so as to form a scanning spot of the n^th order diffracted radiation beam. The spot of the n^th order diffracted radiation beam is focused on the land or a groove on which the information is to be scanned. The m^th and 1^th order diffracted radiation beams may be focused on the adjacent grooves or lands or partly on land- groove transitions depending on the type of record carrier to be scanned.

The scanning spot scans the information onto or from the information layer of the record carrier, wherein the m^th and 1^th order diffracted radiation beam spots may be used to perform the focus error tracking and/or radial error tracking.

Due to marks or pits in the lands or grooves of the information layer, the reflected radiation beam is modulated during reading according to the information on the record carrier. The reflected radiation beam and the reflected auxiliary beams pass through a further optical element, for example a quarter-wave plate, and a collimator lens before being directed onto a detection element. The detection element, comprises detection element components, detecting the main radiation beam and the auxiliary radiation beams. Herein the detection element components are in general photo-quadrant detection element components transforming the light of the detected radiation beam into electric signals. The electric signals are supplied to a focus error detection circuit generating a focus error signal and tracking error signal. The error signals are used to adjust the optical components in the optical pick-up unit if the scanning spot is not in a desired position.

Preferably, the detection element comprises at least three photo-quadrant detection element components, each photo-quadrant detection element component having a photo-sensitive surface for detecting the incident radiation beam.

The radiation beam is focused onto the information layer of the record carrier through the cover layer of thickness d. The objective lens forming the radiation beam spot, called scanning spot, from the radiation beam is designed in such a way that a spherical aberration resulting from focusing through the cover layer is compensated for, in that the scanning spot at the information layer is nominally free from aberrations.

The optical pick-up unit comprises a first optical element changeable between a first state and at least a second state. In order to perform compensation of wavefront aberrations occurring at the scanning spot due to the differences in the information layer depths of the first type of record carrier and the second type of record carrier, a second optical element comprising a polarization sensitive material and having a structured surface is included into the optical path of the radiation beam. Preferably, the optical element with the structured surface is included between the beam splitting component, in particular a polarization beamsplitter, and the at least one objective lens. The structured surface comprises angular zones having a width, wherein a stepped profile having a pattern of steps is formed, wherein the width of each step is formed by the corresponding angular zone. Each adjacent step has a height; steps are combined to a stepped profile, in forming the height of the stepped profile. According to the invention the structured surface comprises at least two stepped profiles with a number of steps forming a pattern of steps, wherein the first stepped profile and the at least second stepped profiles are separated by an annular zone and the number of steps forming the pattern of steps of the first stepped profile and the number of steps forming the pattern of steps of the second stepped profile are equal. With that, advantageously, a repetitive pattern of steps is formed by the two stepped profiles. Hence, the repetitive pattern of steps is forming a periodic diffractive structure.

Herein, the boundaries of the stepped profiles are arranged at the radii where a to be corrected aberration function is equal to an integer times λ.

Depending on the polarization of the radiation beam, the diffractive structure of the second optical element influences the radiation beam or not, because of the polarization sensitive material of the second optical element. Each optical component in the optical path of the radiation beam adds a different amount of wavefront aberrations to the radiation spot.

The pattern of the steps, also called phase steps, can be made effective for one polarization direction and substantially ineffective for another, orthogonal, polarization direction, since the refractive index observed for the one polarization is different than for the orthogonal polarization.

According to a preferred embodiment of the invention, at least one further stepped profile is provided, wherein the number of steps forming the pattern of steps of the at least one further stepped profile is equal to the number of steps forming the pattern of steps, of the first and the second stepped profiles, the stepped profiles forming the at least one structured surface having at least one pattern of steps with equal numbers of steps, which is repeated. The at least one structured surface of the optical element comprises three stepped profiles with an equal number of steps of the pattern of steps. The cross section of the at least one structured surface of the optical element related to the pupil of the objective lens can thus be divided in zones, each zone consists of a stepped profile, that means the stepped profile comprises the same number of steps, wherein each step has a height. The amount of wave front aberrations introduced in the radiation beam with such an optical element can be influenced by the number of steps as well as by the respective chosen height and width of the steps.

The wavefront aberrations introduced in the scanning spot due to the different information layer depths will be briefly explained in the following: A radiation beam propagating along an optical path has a wavefront W with a predetermined shape, given by the following equation:

W _ φ λ ^~ 2π

Therein λ is the wavelength of the radiation beam and Φ the phase of the radiation beam. In each optical component the radiation beam passes, wavefront aberrations

W_abb are introduced, because in practice the optical components are not perfect. Wavefront aberrations for a circular aperture can be described by so-called Zernike polynomials representing a complete set of surface deformation by which an arbitrary wave aberration can be expanded into discrete shapes of definite size. This makes it possible to classify the wavefront aberrations and to quantitatively describe the surface deformations. The Zernike polynomials are defined in the exit pupil of the optical element and in polar coordinates, consisting a radial term R(r) shown as R™ and a term dependent on the azimuthal angle φ by: sin(mφ) for m > 0

Z;(r,φ) = C cos(mφ) for m < 0

1 for m = 0, where the aperture is normalized so that R = O .... 1 and the azimuthal angle φ is measured in the anti-clockwise direction from the x-axis. x = r • cos φ, y = r • sin φ. The indices n and m stand for the radial azimuthal orders, respectively. These indices are due to used convention and are not to be confused with the n and m of the n^th and m^th order diffracted radiation beams.

First order wavefront aberrations are wavefront tilt or distortion, second order wavefront aberrations are astigmatism and curvature of field and defocus, third order wavefront aberrations is coma, for example. Spherical aberrations are wavefront aberrations of the fourth order. Defocus and spherical aberrations are rotationally symmetric and occur for symmetric object points on the optical axis, which means they are independent in any direction in the plane perpendicular to the optical axis. Introduction of an optical element with an optical axis in the optical path of the radiation beam introduces additional wavefront aberrations, and thereby a wavefront modification ΔW in the radiation beam. The scanning spot on the information layer of the record carrier at a certain information layer depth deviating from a designed value of the information layer depth will suffer from spherical aberration. This can be compensated by introducing wavefront aberrations for example by adding spherical aberrations with the opposite sign in the radiation beam towards the information layer depth. The wavefront modification ΔW is a modification of the shape of the wavefront W and maybe of a first, a second, etc. order or may introduce a constant phase change in the radiation beam. A constant phase change means that after taking modulus 2π of the wavefront modification ΔW, the resulting wavefront is constant. The phase change ΔΦ of the radiation beam is given by the following equation:

ΔΦ = — ■ ΔW . λ

The optical element, compensating the amount of wavefront aberrations, in particular spherical aberrations, introduced by the objective lens due to the different information layer depth for the different types of record carriers on the scanning spot is different for a BD record carrier and a HD-DVD record carrier. The difference in the spherical aberration is given by 2.31 mλ/μm-500 μm = 1.16-λ rms. The corresponding peak- peak value of the aberration function is estimated to be 3.9 λ.

The difference in the amount of spherical aberration that can be compensated for between both layers of a dual-layer BD is about 10 mλ/μm-25 μm = 0.25 λ rms, resulting in a corresponding peak-peak value of the aberration function of about 0.84 λ.

An appropriate diffractive structure to generate an aberration with a peak-peak value smaller than λ is a non-periodic structure, for example the sequence of step heights of the annular zones is not a repetitive sequence. Such non-periodic diffractive structures are described in the WO 03/049095 A2. For correcting peak-peak values larger than λ of difference in two spherical aberration between two types of record carriers a non-periodic phase structure is not preferable, especially when the scanning wavelength of the first and the second type of record carrier is about the same, for example as for scanning BDs and HD- DVDs.

Hence, the second optical element performing the compensation of the wavefront aberrations needs to comprise a different structure, in particular a periodic structured surface. For the scanning BDs as well as HD-DVDs the above-described periodic structured surface with at least two stepped profiles, which are repeated, is advantageous.

Polarization sensitive materials, such as birefringent materials, are materials influencing the radiation beam while passing. An optical element made of a polarization sensitive material acts differently on a radiation beam depending on its polarization. Such polarization sensitive materials for such optical elements can be, for example polymeric and/or crystalline materials or liquid crystal polymers.

— > Polarization describes the plane of polarization for the electric field vector E relative to a direction of propagation of the radiation beam, which is the optical axis.

Polarization describes that the radiation beam has one direction of the electric field vector relative to the direction of the propagation of the radiation beam, which is in general the optical axis. First polarization relates to a radiation beam having first orientation of the electric field vector and second polarization relates to a radiation beam having a second orientation, wherein the first orientation is in general orthogonal to the second orientation.

The polarization of a radiation beam can be modified by an electro-optical element: the polarization is changed or not, depending on the state of the electro -optical element.

The radiation beam exciting the electro-optical element in the first state has a first polarization relative to the radiation beam exciting the electro-optical element in the second state, which has a second polarization different from the first polarization.

According to a further preferred embodiment of the invention the pattern of steps comprises at least three steps, wherein the stepped profile has a first height. The influence of the heights h and the step and the refractive index n of the material on the corresponding phase, is given by the equation:

_^ 2π (n - 1) ^■ h Φ = λ Depending on the polarization of the radiation beam incident in the optical element, n is ri_e (the extraordinary refractive index of the material of the optical element) or n₀ (the ordinary refractive index), wherein n₀ is different from n_e.

This results in two different corresponding phases for the extraordinary and the ordinary mode:

2π (n_e - 1) ^■ h Φ = and

R

_ 2π (n_o - l) h R expressing the different amount of wavefront aberrations introduced to the scanning spot on the respective information layer of the respective record carrier. The ratio of the phases Φo and Φ_e scales with:

which is in practice, depending on the material used, quite closed to 0.75, that means 3 : 4. This suggests to take a number of step N = 3 or N = 4. For N = 3, the ordinary mode may be taken for scanning the BD record carrier and the extraordinary mode for scanning the HD-DVD record carrier. The heights of the profile formed by the sum of the heights of the three single steps with j = 0, 1, 2 is given by:

, • λ n₀ - 1

With that, the phases for the BD record carrier are integer multiples of 2π and the phases for the HD-DVD are multiples of 8-π/3. This implies that the ordinary mode corresponds to the m = 0^th diffraction order, and the extraordinary mode corresponds to the m = -1^th diffraction order of the stepped profiles of the optical element. The diffraction efficiency for HD-DVD is nearly 27/4 ^■ π² = 68 %.

The corresponding width of the smallest angular zone is about 11.4 μm. According to a further preferred embodiment, the pattern of steps comprises at least four steps, resulting in a second height of the stepped profile.

Four steps in the stepped profile corresponds to N = 4 and j = 0, 1, 2, 3 and a heights of: so that the phases for the BD record carriers are integer multiples of 2π, and the phases for the HD-DVD are multiples of 3π/2. This implies that the extraordinary mode corresponds to the m=0 diffraction order and the ordinary mode to the m=l diffraction order. The diffraction efficiently for the HD-DVD is then very close to 8/π² = 81 %. The to be corrected aberration function and the heights profile of the diffractive structure is shown in Fig. 4. The aberration function (measured in radials) is given by:

Φ = 61.224 x r² -32.375 x r⁴ - 9.459 x r⁶ - 1.209 x r⁸, where as the radial pupil coordinate is given in mm. The pupil diameter of the corresponding objective lens was taken to be 3.0 mm for scanning a BD record carrier. The effective pupil diameter for scanning a HD-DVD record carrier is then given by: 0.65 / 0.85 ^■ 3.0 mm = 2.29 mm.

This is because the pupil diameter for the HD-DVD record carrier scales with the numerical aperture NA, being NA = 0.65 for the HD-DVD record carrier and NA = 0.85 for the BD record carrier.

The smallest zone is the outer zone and has a width of 8.5 μm, the preferred step height is 0.5226 μm and the largest step height is 1.57 μm (for a material with no = 1.575 and rie = 1.775). It can be seen that the stepped profile, each stepped profile having four steps, results in a higher efficiency than the stepped profile with three steps. The width to be used is smaller, compared to a stepped profile with three steps.

According to a further preferred embodiment of the invention the pattern of steps comprises steps having different widths.

The width of the steps influences the total width of the stepped profile. Steps with different widths are advantageously used to adapt the width of a stepped profile using a fixed number of steps.

According to a further preferred embodiment of the invention, the wavefront aberration compensation includes spherical aberrations.

Spherical aberration occurs in particular if the objective lens of the optical record player focuses the scanning spot on an information layer having an information layer depth different from the information layer depth for which the objective lens is corrected. This occurs in an optical pick-up unit, which is designed for scanning a first type of record carrier with a first format, wherein the first format includes a first information layer depth and which is used to scan a second type of record carrier having a second format with a second information layer depth. Spherical aberrations can be corrected by adding additional spherical aberrations introduced by the optical element comprising a sensitive-sensitive material and a structured surface having stepped profiles. An amount of spherical aberration suitable to compensate wavefront aberrations introduced to a radiation beam, which has passed the optical element with a structured surface having stepped profiles, wherein the stepped profiles have a repetitive pattern of steps.

According to a further preferred embodiment of the invention, the material of the optical element is a birefringent material having a first refractive index ri_e for the radiation beam having a polarization parallel to the optical axis and at least a second refractive index no for the radiation beam having a second polarization perpendicular to the first polarization, the first refractive index rie and the second refractive index no being different, resulting in an introduction of a different amount of spherical aberrations for the radiation beam with the first polarization than for the radiation beam with the second polarization.

Birefringence results in a division of a ray of a radiation beam into two rays, the ordinary ray and the extraordinary ray when passing an optical component of a birefringent material, depending on the polarization of the radiation beam. Examples for birefringent materials are calcitive crystals, cellophane paper or liquid crystals.

The birefringence is quantified by: Δn = n_e - no, where no is the refractive index for the ordinary ray and n_e if the refractive index for the extraordinary ray. Both the ordinary and extraordinary rays are refracted. Only in the specific case that is considered here, namely that of a stepped profile such that each step represents a phase that is a multiple of 2π for a first polarization state, the optical element has no effect on the beam when it is in this first polarization state but a definite effect when it is in the second orthogonal polarization state. The polarization itself is relatively influenced by the first optical element, mentioned above, arranged after the polarization beamsplitter. Therefore choosing the right polarization, additional spherical aberration is introduced to the radiation beam having the second polarization state by the stepped profile of the optical element. The radiation beam having the first polarization state is not influenced, which means that no additional spherical aberration is introduced to the radiation beam.

With that an optical element performing the compensation of wavefront aberrations, in particular the spherical aberration is realized. According to a further preferred embodiment, the first refractive index n_e and the second refractive index n₀ are related to each other according to an equation, wherein the equation follows the relation: O.65-(n_e-l)<(no-l)<O.85-(ne-l).

The difference between the extraordinary refractive rie and the ordinary refractive index n_o is specific for each material. Examples of a material having birefringent properties for use in an optical pick-up are: calcite, quartz, mechanically stretched-polymer films.

According to a further preferred embodiment, the birefringent material is a liquid crystal polymer. Liquid crystal polymer phase structures can be manufactured by replicating in the monomer phase onto a substrate with a mould that presses the stepped structure into the film, after which the structure is fixed by polymerization as can be achieved by e.g. UV- illumination. The difference of the refractive index Δn = ne - no is in the range of about between 0.1 and 0.3. According to a further preferred embodiment, the amount of spherical aberration is substantially zero for the radiation beam of the second polarization.

Advantageously, the second optical element is designed such that an amount of spherical aberration valued to zero is introduced to the radiation beam passing the optical element. According to a further preferred embodiment, the amount of spherical aberration is substantially zero for the radiation beam of the first polarization.

With that a spherical aberration is introduced using a radiation beam with the reverse polarization. Advantageously, the second optical element is designed such that an amount of spherical aberration valued to zero is introduced to the radiation beam passing the optical element.

According to a further preferred embodiment of the invention, the optical element is arranged in the optical pick-up unit in order to receive the first radiation beam in a collimated state.

The reason for placing the element in a collimated beam is that its position is preferably fixed with respect to the objective lens in order to avoid comatic aberrations when the (lateral) position of the element with respect to the objective lens changes due to e.g. misalignment. The beam incident on the objective lens is preferably collimated in order to avoid changes in the magnification for disc or record carrier to detector which can occur when the axial position of the objective lens changes in order to keep the scanning spot in focus.

According to a further preferred embodiment of the invention, the optical pick-unit comprises a collimator lens, wherein the second optical element is arranged between the collimator lens and the at least one objective lens.

Using a collimator lens in front of the optical element is a simple way to collimate the radiation beam.

According to a further preferred embodiment of the invention, the optical pick-up unit comprises a second objective lens, being arranged in order to form a scanning spot onto the information layer of a third type of record carrier in order to obtain an optical pick-up unit for scanning of at least three different types of record carriers having different formats.

While scanning of BDs and HD-DVDs is performed with a radiation beam having a first wavelength of λi = 405 nm, scanning of DVDs and/or CDs is performed using a second wavelength, in particular λ₂ = 780 nm for CDs and/or λ₃ = 650 nm for DVDs. Therefore, in order to scan the different types of record carriers, an optical pick-up unit comprises for example three radiation sources emitting the above-mentioned wavelength λ_{l s} λ₂ and λ₃. In order to optimize the forming of the scanning spot on the information layer of a CD and/or DVD a second objective lens is advantageously arranged in the optical pick-up unit. This is due to the fact that the wavelength, the numerical aperture and the information layer depth for the CD and the DVD are different from that of the BD and the HD-DVD and thus require an objective lens designed to correct different amount of spherical aberration.

According to a further preferred embodiment of the invention, the at least one objective lens is mountable in an actuator for mechanically changing the position of the objective lens relative to the depth of the information layer of the record carrier.

While scanning the record carrier, the focus error correction is performed by detecting the focus of the scanning spot with the detection element and adjusting the position of the objective lens. The adjusting of the position of the objective lens relative to the depth of the information layer of the record carrier is in general performed mechanically by an actuator.

Such an actuator is comprised as a standard component in the optical pick-up unit. According to a further preferred embodiment of the invention, the optical pick-up unit comprises further optical elements for introducing a defocus of the radiation beam directed onto the information layer of the record carrier.

During operation this additional defocus is compensated by the focus action of the actuator such that the scanning spot is focused at the depth of the information layer in the record carrier.

By adding a defocus to the aberration function, an increase of the minimum size of the scanning spot, called minimum feature size, is achievable. With that a maximum value of the slope of the aberration function can be decreased and hence a minimum annular zone width can be increased.

The added defocus increases the free working distance for HD-DVD, and simultaneously the compensation of the spherical aberrations is not affected. For example, additional free working distance of about 14 μm gives a to be corrected aberration function of: Φ = 102.552 ^■ r² - 28.118 ^■ r⁴-9.368 ^■ /-1.633 ^■ r⁸, wherein the aberration function differs from the previously mentioned one primarily in the first term, which is the defocus term. The value of the optimal defocus-spherical aberration ratio A20/A40 is given by the following aberration function: W = A₄o(6p⁴-6p²+l)+A_2O(2p²-l), wherein the amount of Zernike-type spherical aberration A40 is fixed and where the amount of Zernike-type defocus A20 is variable. W is measured in unity of λ. The radial variable p is defined at the ratio between the radial variable r and the pupil radius, such that it takes the value: 0<p<l.

This is aimed to minimize the largest value that the absolute value of the slope — takes in the range 0<p<l . Depending on the ratio A20/A40 this largest value is found at dp

the pupil rim p=l or at the local extreme defined by = 0. With that, the following dp² relation is given: = 24A₄₀p³ + (4A₂₀ - 12A₄₀)p and dp

^- = 72A₄₀p² + 4A₂₀ - 12A₄₀. dp² With that the local extreme is at: 1 / 2

A.

1 - given to: ^{p =} 7f 3A 40

3 / 2 dW A

1 - 2, 0 dp V^ 3A₄ The slope value at p=l is:

The absolute value of the two slopes are equal if:

which gives the solution if:

A, 3_

A 40 2

With this the aberration function is given by:

W = A₄₀Up⁴ - 9p² + I

and the maximum absolute value of the slope is given by:

The maximum width of the smallest diffractive ring of a stepped profile having N-steps and a pupil radius a is then approximately given by: a w min <

6A₄₀N

A preferred design of the stepped profile considers a=1.15 mm, N=4 and A4o=V5xl.l6=2.59 and w_min=18.4 μm. This can advantageously performed by adding a de focus as discussed above.

In a further preferred embodiment, the optical pick-up unit comprises at least a second radiation source emitting a radiation beam with a second wavelength λ₂ for scanning at least a third type of record carriers having a third format.

The optical pick-up unit is suitable for scanning CD, DVD, BD and HD-DVD record carriers. The comprised optical element including a birefringent material is designed to push the cycle operation gap from BD to HD-DVD. In order to have a feasible quality of the scanning spot while scanning a record carrier of the third type of record carriers, in particular a CD, the stepped profile of the structured surface of the optical element is influencing the quality of the scanning spot negatively, when the same objective lens is used for both wavelengths. Therefore, an optical pick-up unit being able to scan the above- mentioned four types of record carriers includes a radiation source having a wavelength being suitable for scanning a CD and a second objective lens being adapted to the information layer depths of the third type of record carriers, namely the CDs.

According to a further preferred embodiment, the respective objective lens is mountable in an actuator for mechanically changing the position of the objective lens relative to the depth of the information layer of the record carrier. In every standard optical pick-up unit, the objective lens is mounted in an actuator in order to perform the focus error correction and/or the tracking error correction.

According to a further preferred embodiment of the invention, the optical element is connected to the first objective lens.

This is advantageous, because the rather tight tolerance for decent ring the optical element with respect to the objective lens can be achieved. Herein the tolerance can be estimated as follows: a decent ring of the optical element with respect to the objective lens would give rise to an additional wave front aberration, namely coma expressed by the relation:

wherein Δ is the amount of displacement measured in units of the pupil radius. In terms of the RMS aberration values:

For required RMS aberration values of A₃i^RMS < 30 mλ, the decent ring must satisfy the relation: Δ <0.0041, that concerns an accuracy of about 4.7 μm for the pupil diameter of 3 mm for a BD. This is an accuracy requiring high precise manufacturing of the optical components and the relative position between the optical element and the objective lens.

According to a further preferred embodiment of the invention, the first and the second objective lens are mounted in one actuator. With that, the relative position of the first objective lens and the second objective lens is fixed. A further advantage is that both objective lenses are movable with one actuator, reducing the costs for production. The object of the invention is solved by an optical player comprising the optical pick-up unit with the above discussed embodiments.

The optical player is suitable for scanning the different types of record carriers, namely CDs, DVDs, BDs, and HD-DVDs. The use of an optical element performing the compensation of spherical aberrations due to the different information layer depths for BDs and HD-DVDs allows an optimized scanning of the high density record carriers BDs and HD-DVDs. The use of at least a second radiation source and a second objective lens allows additionally the scanning of CDs and DVDs.

The object is further solved by an optical element for providing compensation of wavefront aberrations in an optical pick-up unit like mentioned above, wherein the optical element comprises a sensitive-sensitive material and has a structured surface with at least one repetitive step profile having a pattern of steps, each step having a height and a width.

In general an optical pick-up unit can be endorsed by the above-mentioned optical element. Herein the optical element has all the features of the features of the optical pick-up unit discussed above.

With that the optical element can introduce wavefront aberrations to the scanning spot at the information layer of the record carrier.

The object is also solved by a method to be applied in an optical pick-up unit of an optical player for performing compensation of wavefront aberrations while scanning a record carrier, according to a method mentioned at the outset, in that a further step of modifying the optical characteristics of at least one optical element of the optical pick-up unit in order to compensate the wavefront aberrations generated in a scanning spot due to the fact that the first information layer depth of the first type of record carriers is different from the second information layer depth of the second type of record carriers by applying a wavefront aberration to the scanning spot which is larger than λ.

This is advantageous, because the spherical wavefront aberrations introduced in the scanning spot results in a bad scanning performance. This is corrected by applying the above-mentioned method.

According to a further preferred aspect of the method, the compensation is performed by introducing an amount of spherical aberration to the scanning beam spot depending on the polarization of the radiation beam incident on an optical element according to the one mentioned above in an optical pick-up unit as aforementioned. It is to be understood, that aforementioned features and those shall 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. 1 shows in Fig. Ia a schematic view of an optical pick-up unit and in Fig. Ib a schematic view of a detection element used in such an optical pick-up unit; Fig. 2 shows in Fig. 2a a schematic view of an optical element having a structured surface and in Fig. 2b a schematic view of the optical element of Fig. 2a in the plane AA, wherein only the right part is shown;

Fig. 3 shows in Fig. 3a a schematic view of the optical element, the optical lens and the record carrier to be scanned for a BD record carrier with a numerical aperture NA of 0.85 and in Fig. 3b a schematic view of the optical element, the optical lens and the HD-DVD record carrier with a numerical aperture NA = 0.65;

Fig. 4 is a schematic view of the aberration function (upper curve) and a structured surface of one embodiment of the optical element (first embodiment);

Fig. 5 is a schematic view for a second embodiment of the profiles of the structured surface of the optical element (lower curve) and the corresponding aberration curve (upper curve);

Fig. 6 is a schematic view of the profiles of the structured surface of the optical element for a third embodiment (lower curve) and the corresponding aberration function (upper curve); Fig. 7 is a schematic view of a part of an optical pick-up unit including two optical lenses.

Now, the invention will be described below with reference to the accompanying figures of the drawing in accordance with embodiments.

For convenience in the description, information storage media are called record carriers. Scanning information may include writing onto, reading from and/or erasing information from an information layer of a record carrier. Fig. Ia shows a schematic view of an optical scanning device, in particular an optical pick-up unit 10 (OPU) for use in an optical player, suitable for scanning a record carrier 12. The information is stored on an information layer 14 of 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, preferably HD-DVDs (high density digital versatile discs) and BDs (Blu-ray discs). A preferred embodiment of the present invention is able to scan four types of record carriers: CDs, DVDs, BDs and HD-DVDs.

The record carrier 12 comprises a substrate 24 and a transparent layer, between which at least one information layer 14 is arranged. In the case of a dual-layer record carrier, as for example a dual-layer BD, two information layers are arranged behind the transparent layer, called also cover layer 18 at a different depth within the record carrier 12, separated by about 25 μm. A further transparent layer not shown here separates the two information layers. The transparent layer respective the cover layer has different thicknesses for the different types of record carriers 12. The transparent layer, also called cover layer 18, has the function of protecting the upper most information layer 14, while the mechanical support is provided by the substrate 24.

Information may be stored in the information layer 14 of the record carrier 12 in the form of optically detectable marks arranged in substantially parallel, concentric or spiral tracks, not indicated in Fig. 1. The marks may be in any optically readable form, for example in the form of pits or areas with reflection coefficient or direction of magnetization different from the surroundings, or a combination of these forms.

The different types of record carriers 12, are distinguishable by a different structure of the record carrier 12, namely in the difference of a thickness 16 of the cover layer 18, being arranged at the surface 20 of the record carrier radiation beam 21 is incident on. The record carrier 12 comprises further a reflection layer 22, which is arranged between the information layer 14 and the substrate 24 of the record carrier.

The first type of record carriers, namely the HD-DVDs have typically a thickness d of the cover layer 18 of d = 0.6 mm, whereas the BD record carriers has typically a thickness d of the cover layer of d = 0.1 mm. For a further type of record carriers, the dual- layer BD record carriers, where a second information layer 14, not shown here, is arranged, an additional spacing layer, not shown here, is designed to separate the two information layers. A distance between the surface 20 of the record carrier 12 and the information layer 14 is called information layer depth 25. The first type of record carriers comprises hence a first information layer depth 25' and the second type of record carriers comprises a second information layer depth 25". The radiation beam 21 is emitted by a radiation source 26, which is preferably a semiconductor laser. The emitted radiation beam has typically a wavelength of λ = 405 nm.

Even if in the embodiment shown in Fig. Ia and discussed here, only one radiation source 26 is shown and discussed, according to the invention the OPU may also comprise a second and/or third radiation source emitting a second and/or third radiation beam with a wavelength λ₂ = 780 nm and/or λ₃ = 650 nm in order to scan different types of record carriers in one optical pick-up unit. According to the invention the radiation source 26 emits the radiation beam 21, having a wavelength λi = 405 nm suitable to scan HD-DVD and BD record carriers.

The radiation beam 21 enters a diffraction grating element 28, named grating element 28 in the following, which converts the radiation beam 21 into a main radiation beam and at least two auxiliary radiation beams, each being adjacent to the main radiation beam. The main radiation beam and the auxiliary radiation beams are not shown here separately. The main radiation beam and the two auxiliary radiation beams are used for performing the tracking error correction and/or the focus error correction in the optical pick-up unit 10 as will be described later.

The diffracted radiation beams are in the following assigned with reference number 30. The diffracted radiation beam 30 comprises a n^th order diffracted radiation beam, which is preferably a zero order diffracted radiation beam and an m^th and 1^th order radiation beam, which are preferably a ± first order diffracted radiation beams. It is also possible to choose other diffraction orders of the auxiliary diffracted radiation beams in order to realize the diffracted radiation beams 30.

The diffracted radiation beams 30 propagate along an optical path 32 of the optical pick-up unit 10 and pass a beamsplitting element 34. The beamsplitter 34 is preferable a cube polarizing beamsplitter 34. The beamsplitter can also be a plate polarizing beamsplitter.

The transmitted beam 36 is mostly polarized parallel to the plane of incidence (P -polarized) and the reflected beam is mostly polarized perpendicular to the plane of incidence (S-polarized). The radiation beam 36, which has passed the beamsplitter 34 is collimated by a collimating element 38, for example a collimator lens, and directed by a reflection element, such as a mirror 40, to an objective lens 42. The OPU comprising two objective lenses is also part of the invention. The at least one objective lens 42 focuses the radiation beam 36 onto the information layer 14 of the record carrier 12. Herein a radiation beam spot, called scanning spot 44 is formed.

The radiation beam spot 44 is reflected from the reflection layer of the record carrier 12 and propagates as returning radiation beam 45 along the optical path, being reflected by the beamsplitter 34 and is impinging on a detection element 46 after having passed a cylindrical lens 48, which focus the returning radiation beam 45 on the detection element 46. The returning radiation beam 45 includes the main radiation beam as well as the auxiliary radiation beams.

The detection element 46 comprises radiation receiving detection element components 50, 52 and 54, wherein each detection element component 50, 52 and 54 is equipped with at least one radiation-sensitive area that converts the incident radiation beam into an electrical signal. A preferred embodiment of the detection element 46 is shown schematically in details in Fig. Ib.

The detection element 46 comprises in general three radiation detection element components 50, 52 and 54, being in general quadrant detection element components having four separated radiation-sensitive surfaces. In Fig. Ib for example the four radiation- sensitive surfaces Al to A4 for detection element component 50, Cl to C4 for a detection element component 52 and Bl to B4 for detection element component 54 are shown. It is also possible to use detection element components having only two radiation-sensitive areas or to use detection element components having more than four radiation-sensitive areas. The electrical signals obtained from the radiation beams incident on each radiation-sensitive area Al to C4 can be used to perform the radial tracking error correction and/or focus error correction.

The optical pick-up unit 10 according to the present invention comprises further a first optical element 56, which is changeable between a first state and at least a second state, wherein the first optical element 56 in the first state influences the polarization of the first radiation beam 36 in a different way than the first optical element 56 in a second state. For example, the first optical element 56 is an electro -optical element comprising liquid crystal molecules interposed between two transparent planar plates having conductive transparent layers formed on the inner surface thereof, which forms the electrodes for the liquid crystal, and with that of the first optical element 56.

The first optical element 56 rotates the polarization of the incident radiation beam by 90° in a first state, and does not affect the polarization of the incident radiation in a second state.

The first optical element 56 is in particular a planar cell, comprising a liquid crystal layer interposed between two transparent planar plates having conductive transparent layers formed on the inner surface thereof, forming the electrodes of the electro-optical element. Applying an electric voltage to the electrodes allows the switching of the electro- optical element from a first state to a second state and vice versa. The application of the voltage results in an alignment of the liquid crystal molecules, in general parallel to the optical axis of the objective lens 42. In the second state, called the off state with no applied voltage, the polarization of the incident radiation beam is rotated by 90° when passing through the liquid crystal cell. In the on state, the liquid crystal cell 56 has no effect on the polarization of the radiation beam passing through the liquid crystal cell. Preferably the liquid crystal layer is relatively thin, typically 4-6 μm.

With that the first optical element 56 is able to switch the polarization of the radiation beam between the first polarization and the second polarization by applying an external voltage to the electrodes of the first optical element 56. Preferably the external voltage is controlled by a device, not shown here, which has an input signal related to an amount of wavefront aberrations in the scanning spot 44.

An alternative for an optical element is a half-wave plate, which may be rotated over an angle of 45 deg around an axis parallel to the optical axis by mechanical means. In one orientation the polarization of the incident radiation beam is unaffected, in the second orientation the polarization of the incident beam is rotated over 90 deg.

A further optical component, a second optical element 60 including a polarization sensitive material is included in front of the objective lens 42 in the optical path. The second optical element 60 comprises in particular a birefringent material such as a liquid crystal polymer component having its molecules aligned along an optic axis of the material of the second optical element.

The material of the second optical element 60 has two refractive indices: the extraordinary refractive n_e and the ordinary refractive index n₀. The refractive index of the birefringent optical element is experienced to be n_o for an incident radiation beam having a polarization perpendicular to the optic axis of the material and n_e for an incident radiation beam having a polarization parallel to its optic axis.

With that dependence on the polarization of the radiation beam, a different optical effect on the radiation beam is achievable. The optical effect is achieved by the second optical element 60 that comprises at least one structured surface having a stepped profile comprising annular zones with a certain width.

The radiation beam 36 with a first polarization incident on the optical element 60 is diffracted with the refraction coefficient n₀, when the first polarization is perpendicular to the optic axis and with the refractive index ne when the first polarization is parallel to the optic axis. With that, wavefront aberrations can be introduced to the radiation beam, such as spherical aberrations to compensate the occurring wavefront aberrations according to the different information layer depth of a second type of record carrier compared to a first type of record carrier.

Herein, the distance of the objective lens 42 has been optimized to the information layer depth 25 of the first type of record carrier requiring a further introduction of spherical aberrations to the radiation beam performing the scanning of the second type of record carriers.

Further, a changing-changing element 62 is preferably introduced, which is in particular a quarter wavelength retarder plate. The changing-changing element 62 is interposed between the optical element 60 and the objective lens 42. Herein a 90° rotation in polarization between the reflected and the incident radiation beam in the polarizing beamsplitter 34 is achieved.

In summary the optical pick-up unit 10 according to the present invention comprises a radiation source 26 emitting a radiation beam in particular of a wavelength of λ = 405 nm, an electro-optical element 56 and an optical element 60, wherein the electro- optical element 56 selectively changes the polarization of the radiation beam and the optical element 60 introduces spherical aberrations to the radiation beam. The objective lens 42 comprised by the optical pick-up unit 10 is either mounted in an actuator performing a mechanical change of the position with respect to the distance to the information layer of the record carrier. Preferably, in an embodiment suitable for scanning four different types of record carriers, a second objective lens, not shown in Fig. Ia, is comprised.

Fig. 2a and Fig. 2b illustrate schematically the situation concerning the layout of the optical pick-up unit 10, wherein only the record carrier, the objective lens 42 and the optical element 60 is depicted. The situation shown in Fig. 2a is related to the BD record carrier having a numerical aperture of NA = 0.85 and a cover layer thickness d = 0.1 mm and the situation in Fig. 2b is related to the situations in scanning the HD-DVD record carrier with a numerical aperture of NA = 0.65 and a cover layer thickness d = 0.6 mm.

It can be clearly seen that forming the scanning spot 44 on the information layer 14 of the record carrier 12 is performed by the objective lens 42 and the cover layer 18 with the thickness 16. This is due to the fact that the material of the cover layer 18 is a transparent material, for example a polycarbonate acting as an optical component.

Therefore, a distance between an exit surface 62 of the objective lens 42 and the information layer 14, which is optimized for a record carrier having an information layer depth 16 is not suitable for a second type of record carrier having a second information layer depth, because the additional focusing effect of the cover layer 18 is different.

Due to the fact that the difference in the cover layer thickness 16 between a BD and a HD-DVD disc is about 500 μm, it will still be possible to obtain a scanning spot 44 on the information layer 14 of the second type of record carrier, but the quality of the scanning spot 44 is accordingly different for the second type of record carriers, and in practice not sufficient for scanning the second type of record carrier.

According to the invention, an additional wavefront aberration, in particular a spherical aberration is introduced by the second optical element 60 arranged in front of the objective lens 42 in such a way that the radiation beam 36 first passes the optical element 60 before being incident on the objective lens 42. By the second optical element 60 a spherical aberration is introduced to the radiation beam 36, if the polarization of the radiation beam 36 is such that the stepped profile, not shown here, but explained in Figs. 3 to 5 leads to a slight diffraction of the radiation beam 36 resulting in the introduced spherical aberration.

According to the invention, the stepped profile comprising annular zones includes at least one repetitive pattern of steps having a certain height 90 and a width related to the width of the annular zones. Not shown here, but possible in principle, is that an optical element 60 designed with two structured surfaces 61 on opposite sides of the second optical element 60 can be used.

Fig. 3a shows the optical element 60 having stepped profiles 64, 66, 68 and 70, wherein a radius 72 of the stepped profile 64 is different from a radius 94 of the stepped profile 66. Wherein the reference number 74 depicts the center of the optical element 60, which is the optical axis.

Fig. 3b, which is a cut of the optical element in the direction AA of Fig. 3a, shows four stepped profiles 64, 66, 68 and 70. It can be seen that each stepped profile comprises steps 76, 78, 80 and 82 in case of the stepped profiles 64 to 68 and three steps 84, 86 and 88 in the case of the stepped profile 70. Each step comprises a height indicated with the reference number 90 and a width indicated with the reference number 92. The width 92 of a step is denoted by the wording annular zone. The annular zone concerns a circular zone of a certain width 92.

The distance 72 describes the radius with respect to the center 74 of the structured profile 64. The radius of the stepped profile 66 is denoted by the distance 94. The mentioned distances 72 and 94 are the boundaries of the stepped profiles 64 and 66 and are arranged such that the boundaries correspond to a radius where the to be corrected aberration function is equal to an integer times wavelength λ. This will be explained with respect to Figures 3, 4 and 5.

It can be seen that the stepped profiles 64, 66 and 68 comprises four steps, wherein three steps have a specific height 90 and the fourth step is the one with a height equal to zero. The stepped profile 70 comprises accordingly three steps: two steps with a height 90 and a third step with a height equal to zero. The summation of the heights 90 of the stepped profile results in a total height 96' of the stepped profiles 64, 66 and 68 and the summation of the heights 90 results in a total height 96 of the stepped profile 70.

Therefore, it can be seen that the height 96 of the stepped profile 70 is different from the respective height of the stepped profiles 64, 66 and 68. The determining value of the corresponding phase for the extraordinary and ordinary mode is the heights 90 of a single step, because the height 90 (h,) determines pattern of the diffracted rays of the radiation beam, diffracted by the steps of the stepped profile.

In Fig. 4 an example for a structured surface 61 of the second optical element 60 having seven stepped profiles each with three steps can be seen together with a calculated aberration function 100. It can be seen that the stepped profiles 98 comprises steps having a height 90 of the respective steps. The same reference number for the stepped profiles is used, because the stepped profiles all have the same number of steps and thereby regarded as repeated stepped profiles. The widths 92 of the respective steps are different in order to realize the required distances 102, 104, 106, 108, 110, 112, 114 between the boundaries of the profiles 98 and the center of the optical element 60.

Fig. 5 shows a further embodiment of the structured surface 61 of the second optical element 60 comprising seven stepped profiles, in total that are six stepped profiles 116 and one stepped profile 118. It can be seen that the stepped profile 116 comprises four steps, wherein the stepped profile 118 comprises three steps. A calculated aberration function 120 is also depicted in Fig. 5, wherein the aberration function 120 is obtained using the second optical element 60 comprising the structured surface 61 with the stepped profiles 116 and the stepped profile 118. The height 96 of the stepped profile 118 is smaller than the height 96' of the stepped profiles 116, wherein the heights (h,) 90 of the steps are equal. Fig. 6 comprises a further embodiment of the structured surface 61 of the second optical element 60 comprising stepped profiles 124 each having four steps. It can be seen that the height 90 of each of the steps of the profiles 124 is equal, resulting in an equal height 96 for the respective stepped profiles 124 and thereby in a repetitive periodic structure of the structured surface. The material of the optical element with the structured surfaces shown in Figs.

4, 5 and 6 comprises a polarization sensitive material, in particular a birefringent material, which is preferably a liquid crystal polymer. An example for a liquid crystal polymer is a liquid crystal polymer having an ordinary refractive index n₀ = 1.575 and an extraordinary refractive index rie = 1.775 resulting in a ratio of 0.75, that is — suggesting to take a number

N of steps, wherein N=4 or N=3.

In general the material of the optical element is chosen in such a way that the refractive indices of the birefringent medium of the optical element 60 follows the following equation: 0.65<(n_o-l)/(n_e-l)<0.85. The corresponding aberration function is also shown in Fig. 6 depicted with the reference number 126. Fig. 7 shows a part of an embodiment of the optical pick-up unit 10 comprising two objective lenses 42 and 128, which is suitable to scan four different types of record carrier. The second optical element 60 is closely mounted to the objective lens 42. A dichroic mirror 40' is included in order to reflect the radiation beam with λ = 405 nm towards the objective lens 42 and transmits the radiation beam with λ = 650 nm and/or λ = 780 nm. A further optical component 40 is also shown, which optical component reflects the radiation beam with λ = 650 nm and/or λ = 780 nm towards the objective lens 128.

The objective lens 42 is designed to form the scanning spot 44, not shown here from a radiation beam 36 having a wavelength of λi = 405 nm and the second objective lens 128 is designed to form the scanning spot 44 of a different wavelength in order to perform scanning of the third type of record carrier, in particular the CD or DVD. With that no additional spherical aberration is added in case of scanning the CD and/or DVD while using the optical path including the objective lens 128. It can be seen that the two objective lenses 42 and 128 are mounted on a common actuating component depicted with the rectangular symbols 130. With that the objective lenses 42 and 128 can be adjusted according to the tracking error correction or the focus error correction.

The optical element 60 comprises a structured surface 132 comprising one of the stepped profiles shown in Figs. 4, 5 or 6 or further stepped profiles. According to the present invention it is important that the stepped profiles are repetitive in the structured surface forming a repetitive pattern of steps.

The optical element 60 can be provided with the structured surface 132 comprising a first repetitive pattern of steps and/or a second repetitive pattern of steps and/or further repetitive pattern of steps. Herein the first repetitive pattern of steps and the second repetitive pattern of steps are different, in particular the heights of the steps of the first repetitive pattern of steps and the second repetitive pattern of steps is different. Each stepped profile can include steps of an equal width and/or steps of different width.

The scope of the invention includes also an optical player being able to scan at least two types of record carriers, wherein the two types of record carriers have different information layer depth and/or different numerical apertures NA including an optical element 60, which performs the compensation of the occurring wavefront aberrations.

The occurring wavefront aberrations are compensated by a method applied in the optical pick-up unit comprising the step of modifying the optical characteristics of at least one optical element of the optical pick-up unit in order to compensate the wavefront aberrations generated in a scanning spot due to the fact that the first information layer depth of the first type of record carrier is different from the second information layer depth of the second type of record carriers by applying a wavefront aberration to the scanning spot which is larger than λ.

This is achieved by applying an optical element in the optical pick-up unit, wherein the optical element comprises a structured surface comprising at least a second stepped profile with a pattern of steps, wherein the first stepped profile and the at least second stepped profiles are separated by an annular zone and the pattern of steps of the first stepped profile and the pattern of steps of the second stepped profile are equal.

Claims

CLAIMS:

1. An optical pick-up unit for scanning a record carrier (12) having at least one information layer (14), wherein the record carrier (12) is a first type of record carriers having a first format and/or at least a second type of record carriers having a second format, the optical pick-up unit (10) comprising: - at least one radiation source (26) emitting a first radiation beam (21) having a first wavelength for scanning the first type of record carriers and/or the at least second type of record carriers; at least one objective lens (42) for directing a first radiation beam spot (44) formed from the first radiation beam (21) onto the information layer (14) of the first type of record carriers and/or the at least second type of record carriers; at least one first optical element (38) changeable between a first state and at least a second state, wherein a polarization of the first radiation beam is influenced by the first optical element (38) in the first state in a different way than by the first optical element (38) in the second state, resulting in a first polarization of the first radiation beam (21) passing the first optical element (38) in the first state and a second polarization of the first radiation beam (21) passing the first optical element (38) in the second state; at least one second optical element (60) with at least one structured surface (61) for providing a wavefront aberration compensation, the at least one second optical element (60) comprising a polarization sensitive material, the at least one structured surface (61) having annular zones, each of the annular zones having a width (92), the annular zones forming a first stepped profile (64, 66, 68) having a first number of steps forming a pattern of steps (78, 80, 82), wherein each step (78, 80, 82) comprises a height (90) additionally to the width (92) of the annular zone forming the step, characterized in that the at least one structured surface (61) comprises at least a second stepped profile (64, 66, 68) with a second number of steps forming the pattern of steps, wherein the first stepped profile (64, 66, 68) and the at least second stepped profile (66, 68) are separated by an annular zone and the number of steps of the first stepped profile (64, 66, 68) and the number of steps of the second stepped profile (64, 66, 68) are equal in order to form at least one repetitive pattern of steps.

2. The optical pick-up unit of claim 1 , characterized in that at least one further stepped profile (64, 66, 68) is provided, wherein the number of steps forming the pattern of steps (78, 80, 82) of the further stepped profile is equal to the number of steps forming the pattern of steps (78, 80, 82) of the first and the second stepped profiles (64, 66, 68), the stepped profiles (64, 66, 68) forming the at least one structured surface (61) having at least one repetitive pattern of steps.

3. The optical pick-up unit of claim 1 or 2, characterized in that the pattern of steps comprises at least three steps (84, 86, 88) resulting in a first height (96) of the stepped profiles (70).

4. The optical pick-up unit of any of the preceding claims 1 to 4, characterized in that the pattern of steps comprises at least four steps (76, 78, 80, 82) resulting in a second height (96') of the stepped profile.

5. The optical pick-up unit of any of claims 1 to 4, characterized in that the pattern of steps comprises steps (76, 78, 80, 82, 84, 86, 88) having different widths (92).

6. The optical pick-up unit of any of claims 1 to 5, characterized in that the wavefront aberration compensation includes a compensation of spherical aberrations.

7. The optical pick-up unit of any of claims 1 to 6, characterized in that the material of the optical element (60) is a birefringent material having a first refractive index n_e for the radiation beam having a polarization parallel to the optical axis and at least a second refractive index n_o for the radiation beam having a second polarization perpendicular to the first polarization, the first refractive index n_e and the second refractive index no being different, resulting in an occurring of a different amount of spherical aberrations for the radiation beam (36) with the first polarization than for the radiation beam (30) with the second polarization.

8. The optical pick-up unit of claim 6, characterized in that the first refractive index rie and the second refractive index n₀ are related to each other according to the following relation: 0.65-(rvl) < (D₀-I) < 0.85 (rvl).

9. The optical pick-up unit of claim 7 or 8, characterized in that the birefringent material is a liquid crystal polymer.

10. The optical pick-up unit of any of claims 6 to 9, characterized in that the amount of the spherical aberration is substantially zero for the radiation beam (36) of the second polarization.

11. The optical pick-up unit of any of claims 6 to 9, characterized in that the amount of the spherical aberration is substantially zero for the radiation beam (36) of the first polarization.

12. The optical pick-up unit of any of claims 6 to 9, characterized in that the amount of the spherical aberration is zero for the radiation beam (36) of the second polarization.

13. The optical pick-up unit of any of claims 6 to 9, characterized in that the amount of spherical aberration is zero for the radiation beam (36) of the first polarization.

14. The optical pick-up unit of any of claims 1 to 13, characterized in that the second optical element (60) is arranged in the optical pick-up unit (10) in order to receive the first radiation beam (36) in a collimated state.

15. The optical pick-up unit of any of claims 1 to 14, characterized in that the optical pick-up unit (10) comprises a collimator lens, wherein the second optical element (60) is arranged between the collimator lens and the at least one objective lens (42).

16. The optical pick-up unit of any of claims 1 to 15, characterized in that the optical pick-up unit (10) further comprises a second objective lens (128), being arranged in order to form a scanning spot (44) onto the information layer (14) of a third type of record carriers (12), in order to obtain an optical pick-up unit for scanning at least three different types of record carriers having different formats.

17. The optical pick-up unit of any of claims 1 to 16, characterized in that the optical pick-up unit (10) comprises a further optical element for performing a defocus of the radiation beam directed onto the information layer (14) of the record carrier.

18. The optical pick-up unit of any of claims 1 to 17, characterized in that the optical pick-up unit (10) comprises at least a second radiation source emitting a radiation beam with a second wavelength λ₂ for scanning at least a third type of record carriers having a third format.

19. The optical pick-up unit of any of claims 1 to 18, characterized in that the respective objective lenses (42, 128) are mounted in an actuator (130) for mechanically changing the position of the first and the second objective lens (42, 128) relative to the depth of the information layer (14) of the record carrier (12).

20. The optical pick-up unit of claim 18 or 19, characterized in that the second optical element (60) is connected to the first objective lens (42).

21. The optical pick-up unit of claim 19 or 20, characterized in that the first (42) and the second objective lens (128) are movable by means of the actuator (130).

22. An optical player comprising the optical pick-up unit (10) of any of claims 1 to 21.

23. An optical element for providing compensation of wavefront aberrations in an optical pick-up unit (10) of any of claims 1 to 19, the optical element comprising a polarization sensitive material and having a structured surface (61) with at least one repetitive stepped profile having a pattern of steps, each step (78, 80, 82, 86, 88) having a height (90) and a width (92).

24. A method to be applied in an optical pick-up unit (10) of an optical player of claim 22 for performing compensation of wavefront aberrations while scanning a record carrier (12), wherein the record carrier (12) to be scanned is a first type of record carrier having a first format, wherein the first format includes a first information layer depth (25') and/or a second type of record carriers having a second format including a second information layer depth (25"), comprising the step of: - scanning the information layer (14) of the first type or the second type record carrier; characterized by the further step of: modifying the optical characteristics of at least one optical element (60) of the optical pick-up unit (10) in order to compensate the wavefront aberrations generated in a scanning spot (44) due to the fact that the first information layer depth (25') of the first type of record carriers is different from the second information layer depth (25") of the second type of record carriers by applying a wavefront aberration to the scanning spot which is larger than λ.

25. The method of claim 24, characterized in that the compensation is performed by introducing an amount of spherical aberration to the scanning beam spot (14) depending on the polarization of the radiation beam (36) incident on an optical element (60) according to claim 21 in an optical pick-unit (10) of any of claims 1 to 19.