Optical scanning device

An optical scanning device is used for scanning first and second information layers with first and second radiation beams. The device includes a radiation source and an objective lens assembly (31). The lens assembly includes a doublet-lens system (61, 62) including a first objective lens (61) with a first cross-section having a first diameter (d1) and a second objective lens (62) with a second cross-section having a second diameter (d2). The first and second lenses are arranged for transforming the first radiation beam to a first focused radiation beam having a first numerical aperture. The objective lens assembly further includes a third objective lens (63) for transforming the second radiation beam to a second focused radiation beam having a second, smaller numerical aperture. The second and third objective lenses (62, 63) are integrally formed in one body (64). In a preferred embodiment of the objective lens assembly, the second diameter is smaller than the first diameter.

The invention relates to an optical scanning device for scanning a first information layer of a first optical record carrier with a first radiation beam and a second information layer of a second optical record carrier with a second radiation beam, the device including a radiation source for supplying said first and second radiation beams, and an objective lens assembly including: (1) a doublet-lens system including a first objective lens with a cross-section having a first diameter and a second objective lens with a cross-section having a second diameter, said first and second objective lenses being arranged for transforming said first radiation beam to a first focused radiation beam having a first numerical aperture to form a first scanning spot in the position of said first information layer, and (2) a third objective lens for transforming said second radiation beam to a second focused radiation beam having a second, smaller numerical aperture to form a second scanning spot in the position of said second information layer.

The invention also relates to an objective lens assembly for transforming a first radiation beam to a first focused radiation beam having a first numerical aperture and a second radiation beam to a second focused radiation beam having a second, smaller numerical aperture, the assembly including: (1) a doublet-lens system including a first objective lens with a cross-section having a first diameter and a second objective lens with a cross-section having a second diameter, said first and second objective lenses being arranged for transforming said first radiation beam to a first focused radiation beam having a first numerical aperture, and (2) a third objective lens for transforming said second radiation beam to a second focused radiation beam having a second, smaller numerical aperture.

“Scanning an information layer” refers to scanning by a radiation beam for: reading information from the information layer (“reading mode”), writing information in the information layer (“writing mode”), and/or erasing information from the information layer (“erase mode”). “Information density” refers to the amount of stored information per unit area of the information layer. It is determined by, inter alia, the size of the scanning spot formed by the scanning device on the information layer to be scanned. The information density may be increased by decreasing the size of the scanning spot. Since the size of the spot depends, inter alia, on the wavelength λ and the numerical aperture NA of the radiation beam forming the spot, the size of the scanning spot can be decreased by increasing NA and/or by decreasing λ.

A problem commonly encountered with conventional optical scanning devices is the compatibility with optical record carriers having different formats, e.g. the so-called DVD-format and the so-called DVR-format, due to the difference in thickness of the transparent layers. In the following, “first mode” refers to an operating mode of the optical scanning device for scanning a first information layer with a first focused radiation beam having a first numerical aperture NA1. The numerical aperture NA1is suitable for scanning an optical record carrier of a first type (e.g. the so-called DVR-format) having a first information density. “Second mode” refers to an operating mode of the optical scanning device for scanning a second information layer with a second focused radiation beam having a second numerical aperture NA2that is smaller than the first numerical aperture NA1. The numerical aperture NA2is suitable for scanning an optical record carrier of a second, different type (e.g. the so-called DVD-format) having a second information density. In other words, the first mode is a mode of operation of the optical scanning device for scanning record carriers having a high information density and the second mode is a mode of operation of the optical scanning device for scanning record carriers having a low information density. The “free working distance,” i.e. the distance between the objective lens and the position of the information layer to be scanned, is a critical parameter when scanning record carriers having different formats, since the free working distance decreases when the thickness of the transparent layer of the record carrier increases, when the same objective lens is used for scanning in both first and second modes and operates at the same conjugate distance, i.e. the distance between the object and the lens.

In other words, said compatibility problem is to design an objective lens assembly suitable for scanning both in the first mode (e.g. a DVR-format disc) and in the second mode (e.g. a DVD-format disc), where a relatively small free working distance is available in the first mode. One solution to the compatibility problem is to provide the objective lens assembly with two separate sets of objective lens, one used for scanning in the first mode and the other for the second mode. The design of the objective lens assembly becomes of paramount importance, as well as pertaining concerns like space, cost, ease to manufacture and to handle.

Japanese patent application no. 2001-067700 describes an optical scanning device as described in the opening paragraph, including an objective lens assembly having two separate sets of objective lenses.FIG. 1of the present description shows the known objective lens assembly1for scanning a first information layer IL1with a first radiation beam RB1(first mode) and a second information layer IL2with a second radiation beam RB2(second mode). The known objective lens assembly1includes a first objective lens2with a large diameter, a second objective lens3with a smaller diameter and a third objective lens4with a large diameter. The lenses2,3and4are mounted on a support element5so that the lenses2and3are aligned along an optical axis AA′ and that the lens4is aligned along a different optical axis BB′. The lenses2and3transform the radiation beam RB1to a first scanning spot SS1on the information layer IL1and the lens4transforms the radiation beam RB2to a second scanning spot SS2on the information layer IL2.

A disadvantage of the device described in JP 2001-067700 is that the objective lens assembly includes at least four components (the three lenses2,3and4and the support element5), thereby making the assembly relatively large which is detrimental, in particular with respect to the location of the assembly in the optical scanning device, where the minimization of the occupation of space by the components is a constant concern for the manufacturers of optical scanning devices.

Another disadvantage of the device described in JP 2001-067700 is that the objective lens assembly includes the four components to be assembled, thereby making the assembly relatively difficult to manufacture, especially in terms of alignment of the components with respect to their respective optical axes. The mounting is particularly critical because of the small diameter of the small lens4(typically a few millimeters) since the presence of a tilt between the objective lens and the information layer results in the generation of coma in the scanning spot. Such coma aberration is generally not desired since it negatively affects the scanning of the information layer.

Furthermore, the device described in JP 2001-067700 typically requires the objective lens assembly to be mounted in an actuator for controlling the positions of the first and second scanning spots with respect to: (1) the respective positions of the first and second information layers (2) and/or the position of a track of said first and second information layers which is to be scanned. The lenses2,3and4must be adjusted along the respective optical axes during assembling. There are two manners for assembling the two separate objective lens sets: the first manner takes place outside of the actuator and the second manner inside the actuator. However, the first manner requires the use of an additional element for supporting the first, second and third objective lenses, thereby making the objective lens assembly relatively more expensive and voluminous which is not space-effective. The second manner has the disadvantage that, once the objective lenses are assembled in the actuator and fixed therein (e.g. glued), the rejection of the objective lens assembly for testing reasons results in the rejection of the actuator as well which is not cost-effective.

An object of the invention is to provide an optical scanning device as described in the opening paragraph, which is suitable for operating in both the first mode and the second mode while decreasing the cost of manufacturing.

This object is achieved with the optical scanning device as described in the opening paragraph wherein, according to the invention, the second and third objective lenses are integrally formed in one body.

An advantage of such a device is that it is compatible for scanning a first information layer of a first optical record carrier, e.g. a disc of the so-called DVR-format, and a second information layer of a second optical record carrier, e.g. a disc of the so-called DVD-format, while being cost-effective and easy to assemble since only two pieces (the first objective lens and the body) need be assembled in order to form the objective lens assembly.

Another advantage of such a device is that, once the first objective lens is assembled in the body, the optical axis of that lens can be substantially aligned with the optical axis of the second objective lens, before mounting the objective lens assembly in the actuator. Thus, the manufacturer can check whether the objective lens assembly complies with required specifications and, where necessary, reject the assembly before the mounting of the assembly in the actuator. Thus, the objective lens assembly can be tested outside of the actuator. Consequently, the rejection of the objective lens assembly does not require the rejection of the actuator, which is more cost-effective than the known objective lens assemblies.

Another advantage of such a device is to allow larger manufacturing tolerances, especially in respect of coma correction. The optical axis of the doublet-lens system can be adjusted with respect to the third objective lens by positioning the first objective lens in the body during assembling. If the third objective lens is oriented with respect to the normal direction of the record carrier so as to generate coma, the optical axis of the doublet-lens system can be positioned during assembling so as to be tilted with respect to that normal direction. Thus, also the doublet-lens system generates coma. The amount of coma generated by the doublet-lens-system can then be made to equal the amount of coma generated by the third lens. Subsequently, the objective lens assembly is mounted in the actuator and the actuator can be oriented with respect to the normal direction such that the amount of coma generated by the doublet-lens system and the third lens can compensate the amount of coma generated by a tilt of the optical record carrier. When the doublet-lens system and the third lens generate the same amount of coma the required disc tilt to compensate this coma is for both systems the same. Hence, a desired amount (preferably, a minimum) of coma is generated by the combination of the objective lens assembly and the optical record carrier.

Another advantage of such a device is that the objective lens assembly can be made of a smaller size than the lens assembly known from JP 2001-067700.

It has been noted in the Japanese patent application no. 09115170 discloses an optical scanning device including an objective lens assembly formed by one integrally molded element.FIG. 2of the present description shows the known objective assembly. InFIG. 2, an objective lens assembly10comprises a first objective lens11suitable for scanning an information layer12of a DVD-format disc13and a second objective lens14suitable for scanning an information layer15of a CD-format disc16. The lenses11and14are integrally molded in one single element. However, the lens assembly known from JP 09115170 is not compatible with optical record carriers having higher information density, like a DVR-format disc.

In a preferred embodiment of the objective lens assembly, the first diameter in respect of the first objective lens is larger than the second diameter in respect of the second objective lens. An advantage of such assembly is that it does not require the handling of small lenses, since the second, small objective lens (with a typical size of a few millimeters) is integrated with the third objective lens.

Another object of the invention is to provide an objective lens assembly as described in the opening paragraph, which is relatively small in size.

This object is achieved with the objective lens assembly for transforming a first radiation beam to a first converging radiation beam having a first numerical aperture and a second radiation beam to a second converging radiation beam having a second, smaller numerical aperture, the objective lens assembly including: (1) a doublet-lens system including a first objective lens with a cross-section having a first diameter and a second objective lens with a cross-section having a second diameter, the first and second objective lenses being arranged for transforming the first radiation beam to the first converging radiation beam, and (2) a third objective lens for transforming the second radiation beam to the second converging radiation beam, wherein, according to the invention, the second and third objective lenses are integrally formed in one body.

FIG. 3Ashows an optical scanning device20according to the invention, which is suitable for scanning a first information layer21of a first optical record carrier22with a first radiation beam23.FIG. 3Bshows the same optical scanning device20which is suitable for scanning a second information layer24of a second optical record carrier25with a second radiation beam26.FIGS. 3A and 3Bcorrespond to the first mode and the second mode, respectively, as described below.

In the following, the first mode refers to an operating mode of the optical scanning device20for scanning the information layer21with the radiation beam having a first numerical aperture NA1. The numerical aperture NA1is suitable for scanning an optical record carrier of a first type, e.g. the so-called DVR-format, having a first information density. The second mode refers to an operating mode of the optical scanning device20for scanning the information layer24with the radiation beam24having a second numerical aperture NA2that is smaller than the numerical aperture NA1. The numerical aperture NA2is suitable for scanning an optical record carrier of a second type, e.g. the so-called DVD-format, having a second information density smaller than the first information density. In other words, the first mode corresponds to a mode of scanning a record carrier having a high information density and the second mode corresponds to a mode of scanning a record carrier having a low information density.

For instance, in the case where the optical record carrier22is of the so-called DVR-format, the numerical aperture NA1approximately equals 0.85 for both the reading mode and the writing mode. For instance, in the case where the optical record carrier25is of the DVD-format, the numerical aperture NA2approximately equals 0.60 for the reading mode and 0.65 for the writing mode.

With reference toFIG. 3A, the record carrier22comprises a transparent layer27, one side of which is provided with the information layer21. The side of the information layer21facing away from the transparent layer27may be protected from environmental influences by a protective layer. The transparent layer27acts as a substrate for the record carrier22by providing mechanical support for the information layer21. Alternatively, the transparent layer27may have the sole function of protecting the information layer21, while the mechanical support is provided by a layer on the other side of the information layer21, for instance by the protective layer or by an additional information layer and transparent layer connected to the information layer21. The information layer21is a surface of the record carrier22that contains tracks. A track is a path to be followed by a focused radiation beam on which path optically-readable marks that represent information are arranged. The marks may be, e.g., in the form of pits or areas having a reflection coefficient or a direction of magnetization different from the surroundings. By way of illustration only, in the case where the optical record carrier22is a DVR-format disc, the thickness of the transparent layer27approximately equals 0.1 mm.

Likewise, with reference toFIG. 3B, the record carrier25of the second type comprises a transparent layer28, one side of which is provided with the information layer24. The transparent layer28has a larger thickness than the transparent layer27of the optical record carrier22of the first type. By way of illustration only, in the case where the record carrier45is a DVD-format disc, the thickness of the transparent layer46approximately equals 0.6 mm.

As shown inFIGS. 3A and 3B, the optical scanning device20includes a radiation source30and an objective lens assembly31having an optical axis32. The device further includes a beam splitter33, a collimator lens34, a detection system35, a servosystem36, a focus actuator37, a radial actuator38, and an information processing unit39for error correction.

The radiation source30is arranged for supplying the radiation beam23for scanning the information layer21of the first carrier22and the radiation beam26for scanning the information layer24of the second carrier25. Preferably, the radiation source30includes at least a first semiconductor laser that emits the radiation beam23at a first selected wavelength λ1and a second semiconductor laser that emits the radiation beam26at a second selected wavelength λ2. By way of illustration only, in the case where the first carrier22is a DVR-format disc, the wavelength λ1preferably equals 405 nm and, in the case where the second carrier25is a DVD-format disc, the wavelength λ2preferably equals 660 nm.

The beam splitter33is arranged for reflecting the radiation beams23and26toward the collimator lens34. Preferably, the beam splitter28is formed by a plane parallel plate that is tilted with respect to the optical axis32.

The collimator lens34is arranged for transforming the radiation beams23and26to a first collimated radiation beam40and a second collimated radiation beam55, respectively.

The objective lens assembly31transforms the radiation beam40to a first focused radiation beam41having a first numerical aperture NA1, so as to form a first scanning spot42in the position of the first information layer21, and the radiation beam55to a second focused radiation beam43having a second numerical aperture NA2, so as to form a second scanning spot44in the position of the second information layer24. The objective lens assembly32is described in further detail below.

When the optical scanning device20operates in the first mode, the forward focused radiation beam41reflects on the information layer21, thereby forming a backward diverging radiation beam46which returns on the optical path of the forward focused radiation beam41. The objective lens assembly31transforms the backward radiation beam46to a first collimated backward radiation beam47which traverses the collimator lens34.

The beam splitter33separates the forward radiation beam23from the backward radiation beam47by transmitting at least part of the backward radiation beam47towards the detection system35.

Likewise, when the optical scanning device20operates in the second mode, the forward focused radiation beam43reflects on the information layer24, thereby forming a backward diverging reflected beam50which returns on the optical path of the forward focused radiation beam43. The objective lens assembly31transforms the backward radiation beam50to a backward collimated radiation beam51. Finally, the beam splitter33separates the forward radiation beam26from the backward radiation beam51by transmitting at least part of the backward radiation beam51towards the detection system35.

The detection system35is arranged for capturing the backward radiation beam47,51and converting it into one or more electric signals. One of the signals is an information signal Idata, the value of which represents the information scanned from the information layer21,24. The information signal Idatamay be processed by the information processing unit39for error correction of the information extracted from the information layers21,24. Other signals from the detection system35are a focus error signal Ifocusand a radial tracking error signal Iradial. The signal Ifocusrepresents the axial difference in height along the optical axis33between the scanning spot42,44and the information layer21,24; it is used for maintaining the scanning spot in focus in the information layer (as described below). The signal Ifocusis formed by the commonly used “astigmatic method” which is known from, inter alia, the book by G. Bouwhuis, J. Braat, A. Huijser et al, “Principles of Optical Disc Systems,” pp. 75–80 (Adam Hilger 1985) (ISBN 0-85274-785-3). The signal Iradialrepresents the distance in the plane of the information layer21,24between the scanning spot42,44and the center of a track in this information layer to be followed by this scanning spot; it is used for maintaining the scanning spot42,44on track in the information layer21,24as described below. The signal Iradialis formed by the commonly used “radial push-pull method” which is known from, inter alia, said book by G. Bouwhuis et al., pp. 70–73.

The servosystem36is arranged for, in response to the signals Ifocusand Iradial, providing actuator control signals Icontrolfor controlling the focus actuator37and the radial actuator38, respectively. The focus actuator37controls the positions of the objective lens assembly31along the optical axis32, thereby controlling the actual positions of the scanning spots42and44such that they coincide substantially with the planes of the information layers21and24, respectively. The radial actuator38controls the position of the objective lens assembly31in a direction perpendicular to the optical axis32, thereby controlling the radial positions of the scanning spots42and44such that they coincide substantially with the center lines of the tracks to be followed in the information layers21and24, respectively.

The objective lens assembly31is mounted in the actuator in a manner known in the art, for instance, by using a rotating actuator as described in JP 2001067700 or by using two prisms or dichroic mirrors as described in JP 09115170.

The objective lens assembly31is now described in further detail.FIG. 4shows one embodiment of the objective lens assembly31ofFIG. 3. The objective lens assembly31A includes: (1) a doublet-lens system including a first objective lens61and a second objective lens62, and (2) a third objective lens63.

The structure of the doublet-lens system is known from WO 00/38182 for scanning a DVR-format disc and a DVD-format disc. The known structure has an optical axis and includes two objective lenses having each an optical axis aligned with the optical axis of the doublet-lens system. The two lenses are separated, along the optical axis of the doublet-lens system, by a distance which can be adjusted so that the spherical aberration arising when switching from a DVR-format disc to a DVD format disc, having a difference in thickness of the transparent layer, is compensated. It is noted that the doublet-lens system in WO 00/38182 has the disadvantage that, when the scanning spot is changed from the information layer of the DVR-format disc to the information layer of the DVD-format disc, the distance between the lenses in the doublet-lens system needs to be adjusted requiring an additional actuator. This additional actuator makes the system rather complicated and, therefore, difficult to manufacture.

Each of the lenses61and62has an optical axis aligned with the reference axis of the doublet-lens system, a reference axis AA′.

The lens61has an input surface61A and an exit surface61B; it further has a circular cross-section S1having a first diameter d1. The lens62has an input surface62A and an exit surface62B; it further has a circular cross-section S2having a second diameter d2that is smaller than the diameter d1. Likewise, the lens63has a circular cross-section S3having a third diameter d3. In the present description, the “diameter” of a lens corresponds to the optically effective diameter of the lens, that is, the diameter within which an incident beam is transformed by the lens according to the specified properties of the lens. By way of illustration only, if the record carrier22is a DVR-format disc, the diameter d1is of the order of 3 mm and the diameter d2is of the order of 1.4 mm. If the record carrier25is a DVD-format disc, the diameter d3is of the order of 3.6 mm.

According to the invention, the objective lenses62and63are integrally formed in one body64. For instance, the body64may be formed by using an injection molding process which is commonly used in the field of lens manufacturing. By way of illustration, the body64and therefore the lenses62and63are made of the same plastic material and the lens61is made of glass with an aspherical polymer layer on top of it, as shown inFIG. 4. The objective lens61shown inFIG. 4, in this example, is a plano-aspherical element. The objective lens61has thickness on the optical axis of 2.819 mm and entrance pupil diameter of 3.0 mm. The body of the objective is made of FK5 Schott glass with refractive index 1.4989 at wavelength of 405 nm. The convex surface of the lens body which is directed towards the collimator lens has radius 2.07 mm. The surface of the objective lens61facing objective lens62is flat. The aspherical shape of is realized in a thin layer of acryl on top of the glass body. The lacquer has refractive index 1.5987. The thickness of this layer on the optical axis is 0.019 mm. The rotational symmetric shape of the surfaces can be described by the equation
z(r)=B2r2+B4r4+B6r6+ . . .
with z being the position of the surface in the direction of the optical axis in millimeters, r the distance to the optical axis in millimeters, and Bkthe coefficient of the kthpower of r. The value of the coefficients B2until B16are 0.26447094, 0.0088460392, 0.00014902273, 0.0014305415, −0.0015440542, 0.00082680417, −0.00023319199, 0.0000025911741, respectively. The objective lens62is made of COC (Topas) and is plano-aspherical. The refractive index of COC is 1.5499. The objective lens62has thickness on the optical axis of 0.9 mm and the beam entrance diameter of the objective lens62is 1.352 mm. The surface of the objective lens62facing the disc is flat. The rotational symmetric shape of the surface facing objective lens61can be described by the equation
z(r)=B2r2+B4r4+B6r6+ . . .
with z being the position of the surface in the direction of the optical axis in millimeters, r the distance to the optical axis in millimeters, and Bkthe coefficient of the kthpower of r. The value of the coefficients B2until B16are 0.60369741, 0.22447301, 0.029061701, 0.33507029, −1.1373531, 3.5133805, −5.6443868, 3.1481201, respectively. The free working distance, hence the distance between the objective lens62and the disc is 0.15 mm. The disc has cover layer of 0.1 mm thickness made of Polycarbonate having refractive index 1.6223.

Objective lens63is also made of COC and bi-aspherical. The refractive index of COC at wavelength 660 nm is 1.5309. The objective lens63has thickness on the optical axis of 2.194 mm and the beam entrance diameter of the objective lens63is 3.3 mm. The rotational symmetric shape of the surfaces of objective lens63can be described by the equation
z(r)=B2r2+B4r4+B6r6+ . . .
with z being the position of the surface in the direction of the optical axis in millimeters, r the distance to the optical axis in millimeters, and Bkthe coefficient of the kthpower of r. The value of the coefficients B2until B16for the surface facing the collimator lens are 0.30688174, 0.012537039, 7.46112311 10−5, 0.00034483975, 6.5753831 10−5, −0.00010465506, 2.3627344 10−5, −1.2396363 10−6, respectively. For the surface facing the disc these coefficients B2until B16are given by −0.1114228, 0.02852619, −0.0046668186, −0.0036752428, 0.0063619581, −0.007503492, 0.0046641069, −0.0010757204, respectively. The free working distance is 0.990 mm. The disc has a cover layer of 0.6 mm thickness made of Polycarbonate having refractive index 1.5796 at wavelength 660 nm.

Once the lens61is mounted in the body64, the doublet-lens system formed by the lenses61and62transforms the first radiation beam40to the first focused radiation beam41having a first numerical aperture NA1, so as to form the scanning spot42in the position of the information layer21. The lens63transforms the second radiation beam55to the second focused radiation beam43having a second, smaller numerical aperture NA2, so as to form the second scanning spot44in the position of the second information layer24. The lens63has an input surface63A and an exit surface63B; it further has an optical axis BB′.

An advantage of the objective lens assembly31, in addition to those mentioned above, is that the spacing between the optical axes AA′ and BB′ is decreased with respect to the spacing in the known objective lens assemblies, since (1) integration of the body and one objective lens set makes the objective lens assembly smaller and (2) the integrated body does not require additional element for supporting the first, second and third objective lenses. This is advantageous in comparison with the known objective lens assemblies where two separate objective lens sets need be mounted in an additional body.

The assembling of the objective lens assembly ofFIG. 4is now described.

FIG. 5shows the disassembled objective lens assembly31ofFIG. 4where the first lens61has an optical axis CC′ and the second lens62has an optical axis which is the reference axis of the doublet-lens system, the reference axis AA′.

Firstly, the coma aberration generated by the lens63of the body64is measured by known techniques.

Secondly, the lens61is mounted in the body64so that the optical axes CC′, AA′ and BB′ are aligned so as to meet predetermined specifications. Meanwhile, the amount of coma generated by the doublet-lens system formed by the lenses61and62is measured. Thus, the lens61is positioned so that the doublet-lens system generates the same amount of coma than the amount generated by the lens63.

Thirdly, the distance between the lenses61and62is adjusted so that the radiation beam emerging from the doublet-lens system has a fixed value of spherical aberration. For instance, that fixed value may compensate the amount of spherical aberration generated by the optical record carrier. It is noted that the optical properties of the objective lenses are improved when making more surfaces aspherical, e.g., the exit surface of the first objective lens or of the second objective lens aspherically curved as shown, e.g., in the article by B. H. W. Hendriks and P. G. J. M. Nuyens, “Designs and manufacturing of far-field high NA objective lenses for optical recording,” 413–414, SPIE 3749 (1999).

It is noted that the measurements are done before the mounting of the objective lens assembly31in the actuator, which has the advantage that the objective lens assembly can be checked beforehand and, where necessary, can be rejected before assembly. This is cost-effective since the rejection of the objective lens assembly already assembled (glued) but not mounted in the actuator does not require the rejection of the actuator.

It is to be appreciated that numerous variations and modifications may be employed in relation to the embodiments described above, without departing from the scope of the invention which is defined in the appended claims.

FIG. 6shows an alternative embodiment31′ of the objective lens assembly31shown inFIG. 4. As shown inFIG. 6, the objective lens assembly31′ is arranged so that the exit surface62B′ of the lens62′ and the exit surface63B′ of the lens63′ are in two different planes. An advantage of the second embodiment is that the position of the exit surface62B′ can be chosen so that the position of the objective lens assembly31′ does not need to be changed along the optical axis, in order to keep both scanning spots42and44in focus with the information layers21and24, respectively.

A second alternative embodiment31″ (not shown) of the objective lens assembly31ofFIG. 4will be described. Similarly to the objective lens assembly31shown inFIG. 4, the objective lens assembly31″ includes a first objective lens62″, a second objective lens61″ and a third objective lens63″, wherein the second and third lenses are integrally formed in one body64″.

In an alternative embodiment of the optical scanning device, radiation beams having different wavelengths and numerical apertures than those described above may be used. For instance, the optical scanning device may be formed to be suitable for scanning, e.g., both a DVD-format disc and a CD-format disc, or both a DVR-format disc and a CD-format disc.

In an alternative embodiment of the optical scanning device, the third objective lens may be further arranged for compatibility with both a CD-format disc and a DVD-format disc. For instance, such compatibility can be achieved by adding a non-periodic phase structure as described in the European patent application no. 0.865.037 or a grating structure as described in the proceedings of the 2000 International Conference on Optical Design and Fabrication (see K. Maruyama and R. Ogawa,Background of Conception of DVD/CD compatible diffractive lens, pp. 93–96, Optical Society of Japan (2000)).

In another alternative embodiment of the optical scanning device, the optical scanning device may be of the type capable of performing simultaneous multi-track scanning. This results in improving the data rate in the reading mode as described, for example, in U.S. Pat. No. 4,449,212.