WO2004093063A2

WO2004093063A2 - Optical system

Info

Publication number: WO2004093063A2
Application number: PCT/IB2004/050453
Authority: WO
Inventors: Jorrit E. De Vries; Bernardus H. W. Hendriks
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-04-17
Filing date: 2004-04-15
Publication date: 2004-10-28
Also published as: WO2004093063A3

Abstract

An optical system having a first mode for scanning a first optical record carrier of a first type and having an information layer (2) at a first information layer depth (4), and a second mode for scanning a second optical record carrier of a second, different, type having an information layer (3) at a second, different, information layer depth (5), the optical system having a parameter which is variable. The optical system comprises a radiation source system (6 & 18) for generating a first radiation beam (7) in the first mode and a second, different, radiation beam (8) in the second mode; one or more optical elements for focusing either the first radiation beam (7) to a spot on the information layer (2) of the first record carrier in the first mode, or the second radiation beam (8) to a spot (26) on the information layer of the second record carrier in the second mode; and a compensator (16) having a phase structure comprising annular areas forming a non-periodic pattern of optical paths of different lengths. The compensator (16) is arranged to compensate a first wavefront deviation (WI) introduced during scanning of the information layer (2) of the first optical record carrier with said first radiation beam (7) in the first mode and is additionally arranged to compensate a second wavefront deviation (W2) introduced to said second radiation beam (8) upon variation of said parameter of the optical system in the second mode.

Description

Optical system

Field of the Invention

The present invention relates to an optical system, particularly to an optical system for scanning optical record carriers of different types, and to a compensator for use within such an optical system.

Background

In the field of optical recording, information is stored on an information layer of an optical record carrier such as a compact disc (CD) or a digital versatile disc (DVD). An increase in the density of information which can be stored on such an optical disc can be achieved by decreasing the size of a focal spot of a radiation beam which is used to scan the information layer of the optical disc. Such a decrease in spot size can be achieved by using a shorter wavelength and a higher numerical aperture (NA) of the radiation beam. Increased resolution however tends to reduce tolerances which apply to optical elements within the optical system. These reduced tolerances cause the focal spot of the radiation beam for scanning the optical disc to be more susceptible to degradation in quality.

Optical systems are commonly designed to be compatible with different types of optical disc, for example both a CD and a DVD. In such a system a separate radiation beam with an appropriate and different wavelength for scanning each type of disc is used. Each radiation beam is generally directed along a common portion of an optical path within the system along which lies optical elements for focusing the radiation beam to a focal spot on the optical disc. A problem arises during the design of these optical elements, for example an objective lens, as it is needed to ensure that the radiation beam being used to scan the type of optical disc is focused to a spot of sufficient quality on the optical disc.

This problem is caused in part by the different wavelength and numerical aperture of the radiation beam for scanning each type of disc, but also to a difference between an information layer depth of a transparent cover layer of a first and a second type of optical disc through which the radiation beam passes. This cover layer modifies the radiation beam passing through the cover layer. This modification is considered when designing the precise specifications of the objective lens so that a wavefront deviation is introduced into the radiation beam which compensates the modification by the cover layer and ensures that the focal spot achieved is of the highest quality. In the case of a CD and a DVD, the thickness of this cover layer is approximately 1.2mm and 0.6mm respectively. As a result, when an objective lens designed to focus a radiation beam for scanning a DVD to a focal spot is used to scan a CD, a wavefront deviation comprising a spherical aberration is introduced into the radiation beam for compensating the wavefront deviation introduced by the cover layer of the DVD. As the cover layer of the CD is of a different depth than that of the DVD the focal spot is of a reduced quality.

Parameters of the optical system affecting the quality of the focal spot include environmental influences, for example a change in temperature, on the optical system. An optical system of this type generally comprises a collimator lens for modifying the vergence of the radiation beam scanning the optical disc and an objective lens for focusing the radiation beam to the focal spot on the optical disc. The optical system is designed for use at a standard operating temperature and the precise specifications of the optical elements including the collimator and the objective lens are determined based upon this standard temperature.

With a variation from this standard temperature the properties of the optical elements are affected, leading to a decrease in the quality of the focal spot of the radiation beam. In the case of the objective lens a change in temperature causes a refractive index of a material from which the lens is formed, a shape of the lens and a dimension of the lens to vary. Additionally this variation in temperature causes a slight change in a wavelength of the radiation beam being used to scan the disc to occur. The consequent decrease in the quality of the focal spot is typically in the form of a wavefront deviation comprising a spherical aberration of the radiation beam. A non-periodic phase structure (NPS) has a phase structure comprising annular areas forming a non-periodic pattern of optical paths of different lengths. An NPS commonly introduces a wavefront deviation to a radiation beam passing through the NPS and may be used to modify or correct a wavefront deviation of a radiation beam by introducing a further wavefront deviation.

Patent application EP-A-0865037 describes an objective lens of an optical head for scanning optical discs with different thicknesses of cover layer. A radiation beam having a different wavelength is used to scan each optical disc with a different cover layer thickness. A NPS is attached to a surface of the objective lens and is arranged to reduce a spherical aberration of a first radiation beam when scanning a disc of a first type, for example a CD. When a second type of optical disc is being scanned, for example a DVD, a second radiation beam does not have a spherical aberration and the NPS is arranged to have no effect on the radiation beam. International patent application WO 01/48745 describes an optical head for scanning one type of optical record carrier. At a design temperature an objective lens is arranged to focus a radiation beam to a spot on the optical record carrier. At a temperature other than the design temperature the objective lens introduces a wavefront deviation into the radiation beam. A NPS is arranged to introduce a further wavefront deviation into the radiation beam such that the wavefront deviation introduced by the objective lens is reduced. Patent application EP-A-1143429 describes an objective lens of an optical system for scanning two different types of optical disc, for example a CD and a DVD, using radiation beams of different wavelengths. A first surface of the lens has a first NPS. This first NPS produces a phase difference in the radiation beam used when scanning the first type of optical disc and improves an aberration of the radiation beam. A second side of the objective lens has a second NPS. This produces a phase difference in the different radiation beam used when scanning the second type of optical disc and improves an aberration of this radiation beam.

International patent application WO 02/082437 describes an optical scanning device for scanning optical record carriers of a first, second and a third different type with a radiation beam of a first, second or third different wavelength respectively. An objective system is provided for focusing the radiation beam upon the type of optical record carrier being scanned. Additionally a NPS is provided in a path of the radiation beam. The NPS approximates a flat wavefront for the first radiation beam, a spherical aberration wavefront at the second radiation beam and a flat or spherical aberration wavefront at the third radiation beam.

Both International patent application WO 02/29798 and International patent application WO 02/39440 describe an optical device for scanning optical record carriers of a first and a second type with a first and a second radiation beam respectively. Each radiation beam has a different numerical aperture. Both devices include a NPS which does not affect the first radiation beam but introduces a spherical aberration into the second radiation beam. This introduced spherical aberration is for compensating a spherical aberration resulting when scanning through a difference in a cover layer thickness of the first and second optical record carriers. Summary of the Invention.

It is an object of the present invention to provide improvements in performance of a first mode and a second mode of an optical system for scanning optical record carriers of a first and a second type respectively.

In accordance with a first aspect of the present invention there is provided an optical system having a first mode for scanning a first optical record carrier of a first type and having an information layer at a first information layer depth, and a second mode for scanning a second optical record carrier of a second, different, type having an information layer at a second, different, information layer depth, said optical system having a parameter which is variable, the optical system comprising: a) a radiation source system for generating a first radiation beam in the first mode and a second, different, radiation beam in the second mode; b) one or more optical elements for focusing either the first radiation beam to a spot on the information layer of the first record carrier in the first mode, or the second radiation beam to a spot on the information layer of the second record carrier in the second mode; and c) a compensator having a phase structure comprising annular areas forming a non-periodic pattern of optical paths of different lengths, wherein said compensator is arranged to compensate a first wavefront deviation introduced during scanning of the information layer of the first optical record carrier with said first radiation beam in the first mode, characterised in that said compensator is additionally arranged to compensate a second wavefront deviation introduced to said second radiation beam upon variation of said parameter of the optical system in the second mode.

In accordance with a further aspect of the present invention, there is provided a compensator, for use in an optical system having a first mode for scanning an information layer of a first optical record carrier of a first type, and a second mode for scanning a second, different, optical record carrier of a second, different, type, said optical system having a parameter which is variable, the optical system comprising: a) a radiation source system for generating a first radiation beam in the first mode and a second, different, radiation beam in the second mode; and b) one or more optical elements for focusing either the first radiation beam to a spot on the information layer of the first record carrier in the first mode, or the second radiation beam to a spot on the information layer of the second record carrier in the second mode, the compensator having a phase structure comprising annular areas forming a non-periodic pattern of optical paths of different lengths, wherein said compensator is arranged to compensate a first wavefront deviation introduced during scanning of the information layer of the first optical record carrier with said first radiation beam in the first mode, characterised in that said compensator is additionally arranged to compensate a second wavefront deviation introduced to said second radiation beam upon variation of said parameter of the optical system in the second mode.

The phase structure of the compensator of the present invention compensates the first wavefront deviation, generally comprising a spherical aberration, of the first radiation beam. The first wavefront deviation is introduced to the first radiation beam during scanning of an optical disc of the first type having a first information layer depth, for example a CD, when an optical element of the system, for example an objective lens, with specifications for scanning an optical record carrier of the second type, for example a DVD, with the second radiation beam, is used.

Additionally the phase structure of the compensator is optimised to compensate the second wavefront deviation, comprising a spherical aberration, of the second radiation beam at a given value of a parameter of the system. The second wavefront deviation is introduced to the second radiation beam by a deviation in the parameter of the optical system, for example a deviation in temperature.

By compensation of both the first and the second wavefront deviations, improvements in performance of the first mode and the second mode of the system for scanning optical record carriers of a first and a second type respectively are provided.

Preferably, the phase structure of the compensator comprises an inner structure and an outer structure, wherein the inner structure is arranged to compensate said first wavefront deviation and the outer structure is arranged to compensate said second wavefront deviation. By arranging the inner structure of the phase structure to compensate the first wavefront deviation and arranging the outer structure of the phase structure to compensate the second wavefront deviation the compensator of the optical system for compensating both the first wavefront deviation and the second wavefront deviation is relatively compact and inexpensive to manufacture. Preferably, said inner and outer structures are both arranged on a single surface of one of said optical elements.

By arranging the inner and the outer structures on a single surface of the compensator a manufacture process, for example an injection moulding process, of the compensator is relatively simple, efficient and inexpensive.

Further features and advantages of the present invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, made with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 shows schematically an optical system according to an embodiment of the present invention for scanning an optical record carrier of a first and a second type.

Figure 2 shows graphically a side profile of a phase structure of a compensator of the optical system in accordance with an embodiment of the present invention. Figure 3 shows graphically a wavefront deviation introduced to a radiation beam by an optical element of the optical system in accordance with an embodiment of the present invention.

Figure 4 shows graphically a wavefront deviation having been at least partly compensated by controlling a focal position of an optical element of the optical system in accordance with an embodiment of the present invention.

Figure 5 shows graphically the at least partly compensated wavefront deviation of Figure 4 now with a compensation of a temperature of the optical system in accordance with an embodiment of the present invention.

Detailed Description of Preferred Embodiments of the Present Invention

Figure 1 shows schematically an optical system having a first mode and a second, different, mode for scanning an optical record carrier of a first and a second, different type, respectively. In an embodiment of the present invention the first optical record carrier is a compact disc (CD) and the second optical record carrier is a digital versatile disc (DVD). The position of the DVD, although not shown directly, is indicated in outline in Figure 1. Both the CD and the DVD optical discs to be scanned by the optical system include at least one information layer 2, 3. Information may be stored in the information layer or layers of the optical disc in the form of optically detectable marks arranged in substantially parallel, concentric or spiral tracks. The marks may be in any optically readable form, for example in the form of pits or areas with a reflection coefficient different from their surroundings.

Each optical disc has a transparent cover layer arranged parallel the information layer 2, 3 such that an upper surface of the cover layer lies in contact with the information layer 2, 3 and a lower surface of the cover layer, substantially parallel the upper surface, is an outer surface of the optical disc. A thickness of the transparent layer between said upper and lower surface is an information layer depth. The CD has a first information layer depth 4 of approximately 1.2mm which is greater than a second information layer depth 5 of the DVD of approximately 0.6mm. Optical elements of the optical system are held in a rigid housing which is formed of moulded aluminium or suchlike. The first or second optical record carrier to be scanned is located in a planar scanning area adjacent to the optical system, mounted on a motorised rotating bearing in an optical scanning device including the optical system, whereby the disc is moved relative to the optical system during scanning. The optical system includes two optical branches for scanning optical discs with radiation beams of two different wavelengths. In this embodiment, with the system in the first mode, a first radiation beam 7 for scanning a first optical record carrier is of a first wavelength λi within the range of approximately 775-795nm and preferably approximately 785nm with the system in the second mode. A second radiation beam 8 for scanning a second optical record carrier is of a second wavelength λ₂ within the range of approximately 645nm- 665nm and preferably approximately 650nm. The second wavelength λ₂ is substantially different to the first wavelength λi by at least lOnm and in this embodiment by approximately 135nm.

A first optical branch includes a first radiation source 6, for example a semiconductor laser of a radiation source system, operating at a predetermined wavelength, in this example the first wavelength λi, which produces the first radiation beam 7. The first branch further includes a beam splitter 9 for reflecting a returning first radiation beam 7 towards a detector system 10, and a collimator lens 11 for producing a more collimated beam. A dichroic beam splitter 12 reflects the collimated first radiation beam 7 towards the first optical record carrier along a common optical path portion between the dichroic beam splitter 12 and the first optical record carrier. The common optical path portion is shared by both the first and the second radiation beams 7, 8 of the optical system. In this embodiment the common optical path portion is coincident with an optical axis OA. The first radiation beam 7, having been reflected by the dichroic beam splitter 12 is focused by an objective lens 14, the beam having a numerical aperture (NA) of approximately 0.5, and compensated by a compensator 16 to a first focal spot 17 on the information layer 2 of the first optical record carrier. The first radiation beam 7 is reflected by the information layer 2 of the first optical record carrier and follows a return path which includes travelling along the common optical path portion. The dichroic beam splitter 12 reflects the returning first radiation beam 7 to the beam splitter 9 which reflects it towards a photodiode detector array arranged in the detector system 10, where data, focus error and tracking error signals are detected. The objective lens 14 is driven by servo signals derived from the focus error signal to maintain the focused state of the first focal spot 17 on the first optical record carrier and from the tracking error signal to maintain alignment with a track on the first optical record carrier being scanned.

A second optical branch includes a second radiation source 18, for example a semiconductor laser of the radiation source system, operating at a predetermined wavelength, in this example the second wavelength λ₂, which produces the second radiation beam 8. The second branch further includes a beam splitter 20 for reflecting a returning second radiation beam 8 towards a detector system 18, and a collimator lens 24 for producing a collimated beam. The collimated second radiation beam 8 is transmitted substantially fully by the dichroic mirror 12. Travelling along the common optical path, the second radiation beam 8 is focused by the objective lens 14, the beam having a numerical aperture (NA) of approximately 0.65, and compensated by the compensator 16 to a second focal spot 26 on the information layer 3 of the second optical record carrier. The second radiation beam 8 is reflected by the information layer 3 of the second optical record carrier and follows a return path which includes travelling along the common optical path portion. The beam splitter 20 reflects it towards a photodiode detector array arranged in the detector system 22, where data, focus error and tracking error signals are detected.

The objective lens 14 is driven by servo signals derived from the focus error signal to maintain the focused state of the second focal spot 26 on the second optical record carrier and from the tracking error signal to maintain alignment with a track on the second optical record carrier being scanned.

The objective lens 14 has a numerical aperture (NA) of between approximately 0.55 and 0.75 and preferably approximately 0.65 and a focal length (f) of between approximately 2.65mm and 2.85mm and preferably approximately 2.75mm. The lens is made of COC (Cyclo Olefinic Copolymer). Specifications of the objective lens 14 are determined at a design temperature T₀ of the optical system of between approximately 10°C and 30°C and preferably approximately 20 °C At this design temperature To of approximately 20°C and where the objective lens 14 is focusing the first radiation beam 7 of the first wavelength λi of approximately 785nm the objective lens 14 has a refractive index (n) of approximately 1.537.

At the design temperature To of approximately 20°C the objective lens 14 has a refractive index (n) of approximately 1.5312, 1.5310 and 1.5309 when the second wavelength λ₂ of the second radiation beam 8 being focused is approximately 650nm, 655nm and 660nm respectively.

During scanning of the information layer 2 of the first optical record carrier with the first radiation beam 7 and with the system being in the first mode, a first wavefront deviation Wi comprising a spherical aberration is introduced into the first radiation beam 7. This first wavefront deviation Wi is introduced by a difference between the first information layer depth 4 and the second information layer depth 5.

During scanning of the information layer 3 of the second optical record carrier with the second radiation beam 8 and with the system being in the second mode, the second information layer depth 5 introduces a cover layer wavefront deviation comprising a spherical aberration. Specifications of the objective lens 14 are designed such that a compensatory wavefront deviation comprising a spherical aberration is introduced into the second radiation beam 8 which compensates the cover layer wavefront deviation introduced by the second information layer depth 5. Consequently the second focal spot 26 of the second radiation beam 8 on the information layer 3 has substantially no wavefront deviation and is of a high quality.

As the specifications of the objective lens 14 are determined in order to introduce the compensatory wavefront deviation for the cover layer wavefront deviation introduced by the second information layer depth 5, rather than the first information layer depth 4, the first wavefront deviation Wi of the first radiation beam 7 results. This first wavefront deviation Wi leads to a decrease in the quality of the first focal spot 17. The spherical aberration introduced into the first radiation beam 7 by the difference in the cover layer thickness has approximately the following spherical Zernike coefficients: ₄₀ = -0.49807 λi, Λ₆₀ = -0.04210 λi, A_s = 0.002593 λi, A_m = -0.07095 λi, A_uo = 0.04685 λi. The compensator 16 comprises a transparent plate, a surface of which is a phase structure which is rotationally symmetrical about the optical axis OA. A second surface of the transparent plate and on an opposite side to the phase structure is attached in this embodiment to a first surface of the objective lens 14 such that the phase structure faces the cover layer of the first or second optical record carrier. The phase structure has a plurality of annular areas arranged about the optical axis OA and is a non-periodic phase structure (NPS). Each annular area is a ring with a height h from the surface of the phase structure of the transparent plate, and a beginning normalised radial coordinate p_b and an ending radial normalised coordinate p_e from the optical axis OA. The beginning normalised radial coordinate pb and the ending normalised radial coordinate p_e describe an annular width and position of each ring. Each individual height h of one of the rings is an optical path with a different length. A side profile 28 of the phase structure of the compensator 16 is shown graphically in figure 2 and has a non-periodic stepped pattern. The side profile 28 of the NPS is a plot of the individual heights h of the rings on a first axis 30 as a function of their beginning normalised radial coordinate pb and ending normalised radial coordinate p_e from the optical axis OA on a second axis 32.

The NPS of the compensator 16 has an inner structure 34 and an outer structure 36. The inner and outer structures 34, 36 include annular zones in the form of a series of the individual rings of different heights. In this embodiment the inner structure 34 preferably has a beginning and an ending normalised radial coordinate p_b, p_e of approximately 0 and above 0.5, in this embodiment 0 and 0.769, respectively. The outer structure 36 preferably has a beginning and an ending normalised radial coordinate pb, p_e of above 0.5 and approximately 1, in this embodiment 0.769 and 1, respectively. A boundary 37 between the inner and the outer structures 34, 36 is marked by a change between a first sequence of decreasing heights of adjacent rings of the inner structure 34 and a second, different, sequence of decreasing heights of adjacent rings of the outer structure 36. The height of the ring of the inner structure 34 adjacent the boundary 37 is less than the height of the ring of the outer structure 36 adjacent the boundary 37. The boundary 37 is at a numerical aperture (NA) of between 0.4 and 0.6, preferably approximately 0.5. With the optical system being in the first mode, the inner structure 34 of the

NPS of the compensator 16 is arranged to approximately compensate the first wavefront deviation Wi introduced into the first radiation beam 7 by the difference between the first and the second information layer depths 4, 5. The approximate compensation in this embodiment is such that a root mean square optical path difference (RMS OPD) of the first radiation beam 7 is reduced to below 70mλ and more preferably below approximately 45mλ, for example 40mλ. Table 1 gives approximate data of the heights h and the beginning and the ending normalised radial coordinates pb, p_e of each of the individual rings of the inner structure 34. A height of a ring of the inner structure 34 having a negative value indicates that the ring is a depression of the surface of the compensator 16 having the NPS.

A unit height ho is determined to introduce a phase shift of approximately 2π to the second wavelength λ₂ during scanning in the second mode and at the design temperature To of approximately 20°C The article "Application of Nonperiodic Phase Structures in Optical Systems" by B.H.W. Hendriks, J.E. de Vries & H.P. Urbach in Applied Optics Vol.40 (2001) p6548- 6560, describes the design of an NPS based upon multiples of a unit height ho to produce a wavefront modification for compensating a wavefront deviation of the radiation beam.

K = — n - _Λ\ ⁽i⁾ Equation 1 is used to calculate the unit height ho, wherein n is the refractive index of the material from which the ring of the NPS is formed. For the material COC the unit height ho has a value approximately between the range 1.18-1.28μm and preferably approximately 1.23μm. Table 1 gives approximate data of the radial coordinates and the heights of the rings of the inner structure 34:

Table 1

During scanning of the first optical record carrier the first radiation beam 7 travelling along the optical axis OA having passed through the objective lens 14 enters the compensator 16 at the surface attached to the objective lens 14. The first radiation beam 7 is modified by the optical paths of different lengths of the inner structure 34. Consequently the first radiation beam 7 emerging from the surface of the compensator 16 having the NPS has a compensatory wavefront deviation comprising a spherical aberration. Each optical path of a different length introduces a phase shift into the first radiation beam 7. The different phase shifts introduced by the different rings of the inner structure 34 together form the compensatory wavefront deviation. The compensatory wavefront deviation comprising a spherical aberration is arranged to compensate the first wavefront deviation Wi, thus reducing the spherical aberration of the first radiation beam 7.

The heights of the rings of the inner structure 34 of the NPS are determined to introduce the desired phase shifts into the first radiation beam 7. Additionally the heights of the rings of the inner structure 34 are arranged to introduce a phase shift of an integral multiple of 2π into the second radiation beam 8 such that no substantial wavefront deviation is introduced into the second radiation beam 8 when a second optical record carrier is being scanned at least during scanning at the design temperature To. During scanning of the first optical record carrier, because of the smaller NA being used, substantially none of the first radiation beam 7 which is focused to the first 17 focal spot passes from the outer structure 36 of the NPS. Additionally any variation of the first wavelength λi due to a manufacture process of the radiation source system lies substantially within the tolerances of the detector system 10.

As described before, specifications of the objective lens 14 and the compensator 16 are determined at a design temperature To of approximately 20°C. At this design temperature To the objective lens 14 introduces a spherical aberration into the second radiation beam 8 when scanning the second optical record carrier to compensate the cover layer wavefront deviation introduced by the second information layer depth 5. When a temperature of the optical system and therefore of the objective lens 14 deviates from the design temperature To, a change in a refractive index n of the objective lens 14 occurs. Additionally a change in a shape and a dimension of the objective lens 14 occurs.

With this deviation in temperature a variation of the first and second wavelengths λi, λ₂ occurs also. At the design temperature To the second wavelength λ₂ is approximately 650 nm and at a temperature of the optical system of approximately 80°C the second wavelength λ₂ is approximately 660nm. This wavelength variation with temperature is approximately 0.2 nmIC¹.

The change of the objective lens 14 with the deviation in temperature cause an additional wavefront deviation comprising a spherical aberration to be introduced into the first or second radiation beam 7, 8 when scanning a first or second optical record carrier respectively. The amount of spherical aberration introduced is proportional to the deviation in temperature. Since the additional wavefront deviation is not required for the compensation of the first wavefront deviation Wi of the first radiation beam 7 or the cover spherical aberration of the second radiation beam 8, the additional wavefront deviation introduced by the first or second information layer depth 4, 5 reduces the quality of the first or second focal spot 17, 26. During scanning of a first optical record carrier with the first radiation beam 7 at least part of the additional wavefront deviation introduced by a deviation from the design temperature To is compensated by adjusting a focal position of the objective lens 14 along the optical axis OA using the servo signals from the focus error signal.

During scanning of a second optical record carrier with the second radiation beam 8 the introduced additional wavefront deviation comprising a spherical aberration is a second wavefront deviation W₂. At a temperature of the objective lens 14 of approximately 80°C the second wavefront deviation W₂ comprises a spherical aberration with approximately the following spherical Zernike coefficients: A₄0 = -0.189 λ₂, Aβo - -0.04732 λ₂₎ A_z. = 0.00570 λ₂, -4₁₀₀ = -0.1803 λ₂, -4ι₂₀ = 0.03372 λ₂. The RMS OPD of this second wavefront deviation W₂ is approximately 88mλ.

Figure 3 shows graphically a cross section of the second wavefront deviation W₂ plotted on a third axis 38 as a function of the normalised radial coordinate p on a fourth axis 40. The second wavefront deviation W₂ when the temperature of the objective lens 14 is approximately 20°C and the second wavelength λ₂ is approximately 650nm, is shown by a first plot line 42 in Figure 3. A second plot line 44 shows the second wavefront deviation W with the temperature being approximately 50°C and the second wavelength λ₂ being approximately 655nm. A third plot line 46 shows the second wavefront deviation W₂ with the temperature being approximately 80°C and the second wavelength λ₂ being approximately 660nm. The second wavefront deviation W₂ illustrated by the third plot line 46 has a greater spherical aberration than that of the first and the second plot lines 42, 44.

With the optical system being in the second mode, an area of the second wavefront deviation W₂ of the second radiation beam 8 having a numerical aperture (NA) between approximately 0 and 0.5 is at least partly compensated by adjusting a focal position along the optical axis OA of the objective lens 14 using the servo signals from the focus error signal. This focal position is adjusted to minimise an RMS OPD of the second radiation beam 8 and at a temperature of 80°C this focal position is a focus offset of between approximately - 0.5 and -1.5 μm and more preferably approximately -l.Oμm from a focal position of the objective lens 14 when scanning the second optical record carrier at the design temperature To.

Figure 4 shows graphically a cross section of the second wavefront deviation W₂ with the objective lens 14 being at the focal offset of preferably approximately -l.Oμm. The second wavefront deviation W₂ is plotted on the third axis 38 as a function of the normalised radial coordinate p on the fourth axis 40. A fourth plot line 48 shows the second wavefront deviation W₂ with the temperature being approximately 80°C and the second wavelength λ₂ being approximately 660nm.

An area of the second wavefront deviation W₂ of the second radiation beam 8 has a numerical aperture (NA) between approximately 0.5 and 0.65 and is approximately compensated by the outer structure 36 of the NPS of the compensator 16, the optical system being in the second mode. The approximate compensation in this embodiment is such that a RMS OPD of the second radiation beam 8 is reduced to below 70mλ and more preferably below approximately 45mλ, for example 30.6mλ. Table 2 gives approximate data of the heights h and the beginning and the ending normalised radial coordinates pb, p_e of each of the individual rings of the outer structure 36. Each of these rings preferably has a height difference of approximately 3.7μm from an adjacent ring of the outer structure 36.

Table 2

As mentioned earlier, the article "Application of Nonperiodic Phase Structures in Optical Systems" by B.H.W. Hendriks, J.E. de Vries & H.P. Urbach in Applied Optics Vol.40 (2001) p6548-6560 describes the design of an NPS using the unit height h₀.

The unit height ho of the rings of the outer structure 36 is approximately 1.23μm and, in a similar manner to that described earlier, is determined to introduce a phase shift of approximately 2π to the second wavelength λ₂ during scanning in the second mode at the design temperature To.

With a deviation in temperature from the design temperature To the heights of the rings of the outer structure 36 of the NPS are determined to introduce desired phase shifts which are not equal to 2π into the second radiation beam 8. As a result the second wavefront deviation W₂ is compensated by reducing the spherical aberration of the second radiation beam 8.

Figure 5 shows graphically a cross section of the second wavefront deviation W₂ with the objective lens 14 being at the focal offset of preferably approximately -l.Oμm and the area of the second radiation beam 8 having a NA of approximately between 0.5 and 0.65 being compensated by the outer structure 36. The second wavefront deviation W₂ is plotted on the third axis 38 as a function of the normalised radial coordinate p on the fourth axis 40. A fifth plot line 50 shows the second wavefront deviation W₂ for the area compensated by the outer structure 36. The temperature is approximately 80°C and the second wavelength λ₂ is approximately 660nm. For illustrative purposes the inner and the outer structures 34, 36 and the boundary 37 are shown also.

Table 3 shows approximate values, for this embodiment of the present invention, of the RMS OPD of the second radiation beam 8 at different temperatures of the objective lens 14 and different values of the second wavelength λ₂. The variation of the second wavelength λ may be attributed not only to the deviation in temperature but also to a manufacture process of the radiation source system.

Table 3

Temperature (°C)

20 40 60 80

Second 650 13.3 14.0 23.0 34.3

Wavelength 655 14.9 13.2 24.9 36.9 λ₂ (nm) 660 15.0 21.9 31.9 42.8

RMS OPD mλ

The above embodiment is to be understood as an illustrative example of the present invention. Further embodiments are envisaged.

In a further envisaged embodiment of the present invention the optical system is arranged to scan different other types of optical record carrier from a CD or a DVD, for example a Blu-ray™ disc or a small form factor optical (SFFO) disc. To achieve this, different radiation beams with different wavelengths are used which are appropriate for the type of optical record carrier being scanned. Additionally an optical scanning system having greater than two modes is envisaged for scanning more than two types of optical record carrier whilst maintaining an insensitivity to a variation of a parameter of the system, for example a deviation in temperature.

In the described embodiment of the present invention, optical record carriers with one information layer are scanned by the optical system. It is envisaged further that optical record carriers having multiple information layers are scanned. The compensator of the described embodiment of the present invention is arranged to compensate a wavefront deviation of the radiation beam with a deviation in temperature. In further embodiments of the present invention it is envisaged that the compensator is arranged to compensate for a deviation in other variable parameters of the optical system. Examples of such other variable parameters include a variation in the wavelength of the radiation beam occurring during a switching between a reading and writing power of the radiation beam; a tilting of the objective lens leading to a change in a field angle of the radiation beam entering the objective lens; a humidity of the surroundings of elements of the optical system; a polarisation of the radiation beam scanning the optical disc; and a tilt of the optical disc being scanned.

The described embodiment of the present invention has the compensator attached to a surface of the objective lens. It is further envisaged that the compensator is separate from the objective lens. Additionally it is envisaged that in an arrangement unlike that of the described embodiment, the NPS of the compensator is arranged on two surfaces of the transparent plate.

Although rings are shown as flat, when added to a curved surface each zone may have curved surface characteristics. It is to be understood that any feature described above in relation to any one embodiment may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the embodiments, or any combination of any other of the embodiments. Furthermore, 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 optical system having a first mode for scanning a first optical record carrier of a first type and having an information layer (2) at a first information layer depth (4), and a second mode for scanning a second optical record carrier of a second, different, type having an information layer (3) at a second, different, information layer depth (5), said optical system having a parameter which is variable, the optical system comprising: a) a radiation source system (6 & 18) for generating a first radiation beam (7) in the first mode and a second, different, radiation beam (8) in the second mode; b) one or more optical elements for focusing either the first radiation beam (7) to a spot on the information layer (2) of the first record carrier in the first mode, or the second radiation beam (8) to a spot (26) on the information layer of the second record carrier in the second mode; and c) a compensator (16) having a phase structure comprising annular areas forming anon-periodic pattern of optical paths of different lengths, wherein said compensator (16) is arranged to compensate a first wavefront deviation (Wi) introduced during scanning of the information layer (2) of the first optical record carrier with said first radiation beam (7) in the first mode, characterised in that said compensator (16) is additionally arranged to compensate a second wavefront deviation (W₂) introduced to said second radiation beam (8) upon variation of said parameter of the optical system in the second mode.

2. An optical system according to claim 1, wherein the first radiation beam (7) is of a first wavelength λi and the second radiation beam (8) is of a second, different, wavelength λ₂.

3. An optical system according to claim 2, wherein said different lengths of the optical paths are integral multiples of the first wavelength and/or the second wavelength (λi_, λ₂).

4. An optical system according to claim 1, 2 or 3, wherein the first wavefront deviation (Wi) is introduced by a difference between the first information layer depth (4) and the second information layer depth (5).

5. An optical system according to any preceding claim, wherein said parameter is a temperature of the optical system.

6. An optical system according to any preceding claim, wherein the phase structure of the compensator (16) comprises an inner structure (34) and an outer structure (36), wherein the inner structure (34) is arranged to compensate said first wavefront deviation (Wi) and the outer structure (36) is arranged to compensate said second wavefront deviation (W₂).

7. An optical system according to claim 6, wherein said inner structure (34) is arranged to have substantially no effect on the second radiation beam (8) at a selected value of said parameter.

8. An optical system according to claim 6 or 7, wherein the optical system is arranged such that substantially none of the first radiation beam (7) which is focused to the spot (17) on the information layer (2) of the first optical record carrier passes from said outer structure (36).

9. An optical system according to any of claims 6, 7 or 8, wherein said inner and outer structures (34, 36) are both arranged on a single surface of one of said optical elements.

10. An optical system according to any preceding claim, the system including focal position control means, said control means being arranged to control a focal position of one of the optical elements of the optical system to at least partly compensate said second wavefront deviation (W ).

11. An optical system according to any preceding claim, wherein said first and second wavefront deviations (Wi, W₂) comprise spherical aberrations.

12. A compensator (16), for use in an optical system having a first mode for scanning an information layer of a first optical record carrier of a first type, and a second mode for scanning a second, different, optical record carrier of a second, different, type, said optical system having a parameter which is variable, the optical system comprising: a) a radiation source system (6 & 18) for generating a first radiation beam (7) in the first mode and a second, different, radiation beam (8) in the second mode; and b) one or more optical elements for focusing either the first radiation beam (7) to a spot on the information layer (2) of the first record carrier in the first mode, or the second radiation beam (8) to a spot (26) on the information layer (3) of the second record carrier in the second mode, the compensator having a phase structure comprising annular areas forming a non-periodic pattern of optical paths of different lengths, wherein said compensator (16) is arranged to compensate a first wavefront deviation (Wi) introduced during scanning of the information layer (2) of the first optical record carrier with said first radiation beam (7) in the first mode, characterised in that said compensator (16) is additionally arranged to compensate a second wavefront deviation W introduced to said second radiation beam (8) upon variation of said parameter of the optical system in the second mode.