WO2006048818A1

WO2006048818A1 - Wavefront aberration compensating objective lens

Info

Publication number: WO2006048818A1
Application number: PCT/IB2005/053555
Authority: WO
Inventors: Teunis W. Tukker; Joris J. Vrehen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-11-08
Filing date: 2005-11-01
Publication date: 2006-05-11
Also published as: TW200632899A

Abstract

The present invention relates to an objective (12) lens comprising a focusing lens (52) and an optical element (54) for providing wavefront aberration compensation, the optical element having a surface with a structure in the form of stepped annular zones, the zones forming a non-periodic phase structure (NPS) as a non-periodic pattern of optical paths of different lengths providing different wavefront aberration compensation for different wavelengths, wherein the objective lens (12) design is optimized so as to generate a minimum wavefront aberration for a first wavelength (λ1) , the non-periodic phase structure provides substantially no wavefront aberration compensation for the first wavelength (λ1) and the non-periodic phase structure provides a wavefront aberration compensation for a second wavelength (λ2) and a third wavelength (λ3), wherein λ2 > λ1 > λ3.

Description

Wavefront aberration compensating objective lens

FIELD OF THE INVENTION

The present invention relates to an objective lens comprising an optical element for providing wavefront aberration compensation.

BACKGROUND OF THE INVENTION

Due to the general need for optical record carriers of high data capacity, there is a tendency to use optical scanning devices that work on the basis of shorter wavelengths. The Blu-ray disc standard (BD) works with a wavelength of about 405 nm. In order to read out DVD (dual layer) a 650 nm laser is required, and for a CD a 785 nm laser is needed. Therefore, optical scanning devices that are able to read out discs of various standards have to be able to work on the basis of laser beams with different wavelengths. Furthermore, the various discs have different cover layers, i.e. 1.2 mm for CD, 0.6 mm for DVD and 0.1 mm for BD. If a single objective lens is to be used to read-out all these discs, a different amount of spherical aberration for each disc type must be generated by the objective in order to cope with a difference in cover layer thickness and wavelength. The different systems also require a different numerical aperture.

It is known to mechanically adjust the spacing of lens elements of a compound objective lens in order to provide spherical aberration compensation. Another method of compensation is by mechanically adjusting the position of a collimator lens with respect to the radiation source, so that the radiation beam impinges on the objective lens as a convergent or divergent, instead of collimated, beam. Each of these methods compensate spherical aberrations generated in the optical system of the scanning device, to at least approximately cancel out the spherical aberration generated in the optical disc being scanned. Besides further known methods of compensating an optical wavefront aberration, non-periodic phase structures (NPS structures) have been proposed (cf. B.H.W. Hendriks, J.E. de Vries, and H. P. Urbach; Application of nonperiodic phase structures in optical systems; Applied Optics 40, 6548-6560, 2001). A NPS structure is able to introduce different amounts of spherical aberration for different wavelengths. Thus, in order to correct for the spherical aberration of two wavelengths, the objective lens can be designed so that the NPS structure is invisible for one wavelength while correcting the spherical aberration for the other wavelength.

It is an object of the invention to provide an objective lens that is designed to correct for the spherical aberration for three wavelengths and discs, particularly for the CD, DVD, and BD standards.

SUMMARY OF THE INVENTION

The above objects are solved by the features of the independent claims. Further developments and preferred embodiments of the invention are outlined in the dependent claims.

In accordance with the invention, there is provided an objective lens comprising a focusing lens and an optical element for providing wavefront aberration compensation, the optical element having a surface with a structure in the form of stepped annular zones, the zones forming a non-periodic phase structure (NPS) as a non-periodic pattern of optical paths of different lengths providing different wavefront aberration compensation for different wavelengths, wherein the objective lens design is optimized so as to generate a minimum wavefront aberration for a first wavelength (λi), the non-periodic phase structure provides substantially no wavefront aberration compensation for the first wavelength (X₁) and the non-periodic phase structure provides a wavefront aberration compensation for a second wavelength (λ₂) and a third wavelength (λ₃), wherein λ₂ > λi > λ₃. Thus, the body design of the lens is optimized for the wavelength lying between two other wavelengths. It has been found that by a non-periodic phase structure, the wavefront aberration can be compensated for the two other wavelengths. According to a preferred embodiment: - the focusing lens (52) leads to an optical path difference OPD₂(r) for the second wavelength and an optical path difference OPD₃(r) for the third wavelength, wherein I OPD₃(r) |>| OPD₂(r) | for all values of the radial coordinate r over the pupil of the objective lens, the optical element (54) has an index of refraction of n and the medium from which the light enters the optical element (54) has an index of refraction of n',

a minimal i (i e N) is existing for which the relations (1)

| is valid, where

Φf = fractionalpart ^{l V v 2 J κ}-^ΛlL

Φf =

OPDf* is the peak amplitude in the pupil of OPD₃, j, k are integers,

h. = ^! and nCλJ-n'Cλ.)

0 < A < 0.3 and the NPS at the interface between the optical element and the medium having step heights that are multiples of Ih₁.

The parameters j and k have to be chosen such that, when added to the mentioned fractional parts, the relations (1) can be fulfilled. For example, for the DVD (index 1), CD (index 2), BD (index 3) scenario, OPD₂(r)/OPD₃(r) is approximately equal to 1/2 over the pupil. Thus, the parameters j and k have to be selected such that (Φf + j) / (Φf + k) is approximately equal to 2. If the ratio of the optical path differences to be corrected has a, preferably constant, value K (K = OPD₂(r)/OPD₃(r)) different from 1/2, the parameters j and k have to be selected such that this value K is compensated in the above relations (1). Fractional part means the part of a real number that is obtained when the integer part is subtracted. For example, fractionalpart(2.379) = 0.379. "A" serves as a parameter that reflects the quality of the correction. The smaller A, the better is the correction.

Preferably, the relations (1) are still fulfilled with A<0.2, more preferably with AO.1.

According to a particular embodiment, the first wavelength (X₁) ranges between 650 and 670 nm, the a second wavelength (λ₂) ranges between 780 and 800 nm, and the third wavelength (λ₃) ranges between 398 and 412 nm. Thus, the present invention is applicable for a DVD, CD, BD system, wherein the cover layer thickness t of a DVD lies more or less in-between the CD and the BD cover layer thickness. f

Thus the optical path difference in units of mm for a lens substantially optimal for DVD can be balanced between BD and CD in such a way that it is possible to introduce phase steps that compensate the optical path differences (OPD) for CD and BD at the same time.

According to a preferred embodiment of the invention the wavefront aberration compensation for the second wavelength (λ₂) and the third wavelength (λ₃) is performed in an inner region of the objective lens for a numerical aperture of NA = 0.5 for the second wavelength (λ₂). Thus, the "3λ problem" can be limited to an inner area of the lens, namely for NA = 0.5 for CD. At larger radii of the objective lens the 3λ problem reduces to a two or one wavelength problem.

According to a particularly preferred embodiment, the optical element has refractive indices for the three wavelengths of n(λ]) between 1.558 and 1.560, n(λ₂) between 1.563 and 1.565, n(λ₃) between 1.598 and 1.600 and the step height h that provides substantially no wavefront aberration compensation for the first wavelength (λi) is calculated as h = λi/(n(λ_!)-l). Thus, for a material having the mentioned refractive indices and air with n'=l as the entrance medium, step heights of 1.170 μm can be used. This creates an OPD of one wavelength for DVD. In the case of CD and BD, these step heights lead to phase differences of which the fractional parts Φ² and Φ³ for the CD (index 2) and the BD (index 3) cases as mentioned in the table below:

Preferably, nine annular zones are provided, the relative height h of which correspondir*.g to the radial coordinate range Δr as follows

The number of annular zones has to be a compromise between the qualit^y of spherical aberration correction and the complexness of the manufacturing process. An element with nine annular zones leads to satisfying results as to the aberration correction and it is easy to manufacture.

This holds particularly in the case in which the optical element is manufactured from diacryl material, in a so-called 2P replica process. The diacryl material in a liquid phase is put between a mould and a substrate. The substrate is often glass, but c an be any suitable transparent material. Then UV light is applied which causes the diacryl to cure, i.e. cross links are formed, and the diacryl becomes a solid that retains the shape of the mould. The NPS can also be integrated on or in the lens body, it can be completely macLe from plastics, e.g. diacryl, and it can be provided as an interface layer between two surfaces.

The present invention is further directed to an optical scanning device for scanning an optical carrier comprising an objective lens according to the present invention.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic set-up of a scanning device comprising an objective lens according to the present invention; Figure 2 shows a diagram illustrating a non-periodic phase structure according to the present invention;

Figure 3 shows optical path differences (OPD) of an objective lens for three different wavelengths; Figure 4 shows optical path differences after correction with a non-periodic phase structure for DVD;

Figure 5 shows optical path differences after correction with a non-periodic phase structure for CD;

Figure 6 shows optical path differences after correction with a non-periodic phase structure for BD;

Figure 7 shows three representations of an objective lens according to the present invention for the BD (a), the DVD (b) and the CD (c) modes.

DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 shows a schematic set-up of a scanning device comprising an objective lens 12 according to the present invention. The optical disc, generally denoted by reference numeral 50, comprises a substrate and a transparent layer, between which at least one information layer is arranged. In the case of a dual- layer optical disc, two information layers are arranged behind the transparent layer, at different depths within the disc. A further transparent layer separates the two information layers. The transparent layer has the function of protecting the uppermost information layer, while the mechanical support is provided by the substrate. Information may be stored in the information layers in the optical disc in form of optically detectable marks arranged in substantially parallel, concentric or spiral tracks. The marks may be in any optically readable form, e.g. in the form of pits, or areas with a reflection coefficient or a direction of magnetization different from their surroundings, or a combination of these forms.

The scanning device includes an optical pickup unit which can be moved in the radial direction. The optical pickup unit includes all components illustrated in Figure 1, other than the disc 50. A radiation source 6, emits a diverging radiation beam 7 with different selectable wavelengths. A beam splitter 8, reflects the radiation within a lens system. The lens system includes a collimator lens 9, an objective lens 12, and a condenser lens 11. The collimator lens 9 refracts the diverging radiation beam 7 to form a collimated beam 15. By collimated it is intended to mean a substantially parallel beam, for which the objective lens has a transverse magnification substantially equal to zero. The objective lens 12 transforms the collimated radiation beam 15 into a converging beam 16.

Radiation of the converging beam 16 reflected by the information layer forms a diverging reflected beam, which returns along the optical path of the forward converging beam 16. The objective lens 12 transforms the reflective beam 20 to a substantially collimated reflected beam 21, and the beam splitter 8 separates the forward and reflected beams by transmitting at least part of the reflected beam 21 towards the condenser lens 11.

The condenser lens 11 transforms the incident beam to a convergent reflected beam 22 focused on detection systems, generally indicated by a single element 23 although a plurality of protector elements are used. The detection systems capture the radiation and convert it into electrical signals. One of these signals is an information signal 24, the value of which represents the information read from the information layer being scanned. Another signal is a focus error signal 25, the value of which represents the axial difference in height between the spot 18 and the respective information layer 3 being scanned. Another signal is a tracking error signal 26, the value of which represent a radial deviation of the spot from the track being scanned. Each of the signals 25, 26 are input to the focus servo and tracking servo mechanical actuators controlling the position of the objective lens during scanning.

In the present drawing, the beam at the region of the objective lens 12 is drawn as a bundle of light rays in order to better illustrate the path of the light rays through the system and in order to be in conformity with the representations in Figure 7 below. In the remaining part of the Figure, for the sake of clarity, only the edge of the beam is shown. The objective lens 12 comprises a convex focusing lens 52 and an optical element 54 providing a non-periodic structure (NPS). According to the present disclosure, the NPS element 54 is described as being a part of the objective lens 12. An equivalent description is to take the NPS structure as an element external from the objective lens. The structure can also be integrated on the lens surface. The NPS structure can also be placed on the joining surface between two surfaces.

Figure 2 shows a diagram illustrating a non-periodic phase structure according to the present invention. In this Figure the NPS height is shown in dependence of the lens radius. The height in the middle of the lens is defined as 0. In the following table, the values of the NPS height in dependence on the lens radius are given.

In the case that a material with refractive indices for the three wavelengths of n(λc_D) = 1.558768, n(λ_Dvϋ) = 1.563999, n(λ_BD) = 1.598689 is used, such a NPS structure is invisible for the wavelength used for DVD read-out, and it corrects for the spherical aberration for the wavelengths for CD and BD read-out. For example, diacryl, so-called "replica" can be used as a basis for the NPS structure.

Figure 3 shows optical path differences (OPD) of an objective lens for three different wavelengths without the correction structure. It can be seen that the basic design of the objective lens shows almost no optical path differences with respect to the DYD' wavelength. However, for CD and BD considerable optical path differences are present that have to be corrected. In the present representation, the pupil coordinate is normalized to the pupil radius for BD which is 1.87 mm.

Figure 4 shows an optical path difference after correction with a non-periodic phase structure for DVD. Obviously, the NPS structure does not add spherical aberration in the DVD situation, since the NPS step heights are chosen as multiples of 2π.

Figure 5 shows an optical path difference after correction with a non-periodic phase structure for CD. The fractional parts of the phase difference obtained by a non- periodic phase structure as shown in Figure 2 are given in the following table for the CD (index 2) mode and also for the BD (index 3) mode (see Figure 6).

In contrast to Figure 4, in the CD situation the optical path differences are considerably changed, namely to smaller values. The final wavefront aberration in the CD mode is about 10 mλ RMS for the low order aberrations. Figure 6 shows an optical path difference after correction with a non-periodic phase structure for BD. Also in this case it is obvious that a wavefront aberration correction was successful for the BD wavelength. Note that the pupil diameter for BD is larger than shown in the plot according to Figure 6, so that in the outer regions of the pupil a lens design can be chosen that is optimized for BD. Hence, also in this case the total aberration is about only 13 mλ RMS for the low order aberrations.

Figure 7 shows three representations of an objective lens according to the present invention for the BD (a), the DVD (b) and the CD (c) modes. The objective lens according to the present invention is shown together with a Blu-ray disc 502 in (a), a DVD 504 in (b), and a CD 506 in (c). The different thicknesses of the discs 502, 504 and 506 are visible, as well as the different pupil radii and numerical apertures of the different modes. Due to the NPS optical elements 54, in all three cases satisfying results as to the wavefront aberration are obtained.

Equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.

Claims

CLAIMS:

1. An objective (12) lens comprising a focusing lens (52) and an optical element (54) for providing wavefront aberration compensation, the optical element having a surface with a structure in the form of stepped annular zones, the zones forming a non-periodic phase structure (NPS) as a non-periodic pattern of optical paths of different lengths providing different wavefront aberration compensation for different wavelengths, wherein the objective lens (12) design is optimized so as to generate a minimum wavefront aberration for a first wavelength (λi), the non-periodic phase structure provides substantially no wavefront aberration compensation for the first wavelength (λ_\) and the non-periodic phase structure provides a wavefront aberration compensation for a second wavelength (λ₂) and a third wavelength (λ₃), wherein λ₂ > λ] > λ₃.

2. An objective lens (12) according to claim 1, wherein the focusing lens (52) leads to an optical path difference OPD₂(r) for the second wavelength and an optical path difference OPD₃ (r) for the third wavelength, wherein I OPD₃ (r) |>| OPD₂ (r) | for all values of the radial coordinate r over the pupil of the objective lens, the optical element (54) has an index of refraction of n and the medium from which the light enters the optical element (54) has an index of refraction of n', a minimal i (i e N) is existing for which the relations (1)

is valid, where

OPDf* is the peak amplitude in the pupil Of OPD₃, j, k are integers, h. = ^l and nCλ^-n'Cλ,)

0 ≤ A < 0.3 and the NPS at the interface between the optical element and the medium having step heights that are multiples of Ih₁.

3. An objective lens (12) according to claim 2, wherein A < 0.2.

4. An objective lens (12) according to claim 2, wherein A < 0.1.

5. An objective lens (12) according to claim 1, wherein the first wavelength (λi) ranges between 650 and 670 nm, the a second wavelength (λ₂) ranges between 780 and 800 ran, and the third wavelength (λ₃) ranges between 398 and 412 nm.

6. An objective lens (12) according to claim 1, wherein the wavefront aberration compensation for the second wavelength (λ₂) and the third wavelength (λ₃) is performed in an inner region of the objective lens (12) for a numerical aperture of NA = 0.5 for the second wavelength (λ₂).

7. An objective lens according to claim 5, wherein the optical element has refractive indices for the three wavelengths of n(λi) is between 1.558 and 1.560, n(λ₂) is between 1.563 and 1.565, n(λ₃) is between 1.598 and 1.600 and the step height h that provides substantially no wavefront aberration compensation for the first wavelength (λ{) is calculated as h =

8. An objective lens according to claim 5, wherein nine annular zones are provided, the relative height h of which corresponding to the radial coordinate range Δr as follows:

9. An objective lens according to claim 1 wherein the optical element (54) is manufactured from diacryl material on a glass plate.

10. An optical scanning device for scanning an optical record carrier, comprising an objective lens (12) according to any of claims 1 to 9.