KR20040068176A - Optical scanning device - Google Patents

Abstract

The optical scanning device is used to scan the first and second information layers with the first and second radiation beams. The apparatus has a radiation source and an objective lens assembly 31. The lens assembly includes a first objective lens 61 having a first diameter d ₁ and a second objective lens having a second diameter d ₂ having a second cross section smaller than the first diameter. And dual lens systems 61 and 62 having 62). The first and second lenses are configured to convert the first radiation beam into a first focused radiation beam having a first numerical aperture. The objective lens assembly further includes a third objective lens 63 for converting the second radiation beam into a second focused radiation beam having a second numerical aperture smaller than the first numerical aperture. The second and third objective lenses are integrally formed in one body 64.

Description

Optical scanning device {OPTICAL SCANNING DEVICE}

The present invention relates to an optical scanning device for scanning a first information layer of a first optical recording medium with a first radiation beam and a second information layer of a second optical recording medium with a second radiation beam. A radiation source for supplying the first and second radiation beams, (1) a first objective lens having a first diameter in cross section, and a second objective lens having a second diameter in cross section less than the first diameter; A bilens system configured to convert the first radiation beam into a first focused radiation beam having a first numerical aperture to form a first scanning spot at a position of the first information layer; (2) a third for converting the second radiation beam into a second focused radiation beam having a second numerical aperture smaller than the first numerical aperture to form a second scanning spot at the location of the second information layer; It includes an objective lens assembly having an objective lens.

In addition, the present invention converts a first radiation beam into a first focused radiation beam having a first numerical aperture and converts the second radiation beam into a second focused radiation beam having a second numerical aperture smaller than the first numerical aperture. An objective lens assembly for converting a light source, the assembly comprising: (1) a first objective lens having a first diameter in cross section and a second objective lens having a second diameter in cross section smaller than the first diameter, A bilens system configured to convert the first and second objective lenses into a first focused radiation beam having a first numerical aperture, and (2) converting the second radiation beam to the first numerical aperture And a third objective lens for converting into a second focused radiation beam having a small second numerical aperture.

"Information layer scanning" means information reading from the information layer ("reading mode"), information recording in the information layer ("recording mode"), and / or erasing information from the information layer ("erasing mode"). It means scanning with a radiation beam. "Information density" refers to the amount of information stored per unit area of the information layer. In particular, it is determined by the size of the scanning spot formed by the scanning device in the information layer to be scanned. This information density may be increased by decreasing the size of the scanning spot. Since the size of the scanning spot depends in particular on the wavelength λ and the numerical aperture NA of the radiation beam forming the spot, the size of the scanning spot can be reduced by increasing the NA and / or decreasing the λ.

A problem that conventional optical scanning apparatuses generally have is compatibility with optical recording media having different formats, for example, so-called DVD-format and so-called DVR-format, due to the difference in thickness of the transparent layer. Hereinafter, the "first mode" refers to an operation mode of the optical scanning device for scanning the first information layer with the first focused radiation beam having the first numerical aperture NA ₁ . This numerical aperture NA ₁ is suitable for scanning an optical recording medium of a second type (e.g., a so-called DVR-format) having a first information density. "Second mode" refers to an operating mode of an optical scanning device for scanning a second information layer with a second focused radiation beam having a second numerical aperture NA ₂ less than the first numerical aperture NA ₁ . This second numerical aperture NA ₂ is suitable for scanning an optical recording medium of another second type (e.g., a so-called DVD-format) having a second information density. In other words, the first mode is an operation mode of an optical scanning device for scanning a recording medium having a high information density, and the second mode is an operation mode of an optical scanning device for scanning a recording medium having a low information density. The "free operating distance", i.e. the distance between the objective lens and the position of the information layer to be scanned, is reduced as the free working distance decreases as the transparent layer thickness of the recording medium increases, so that the same objective lens is scanned in both the first and second modes. When used to operate at the same airspace distance, that is, the distance between the subject and the lens, it is a threshold parameter when scanning a recording medium having a different format.

In other words, the compatibility issue is that the objective lens assembly is designed to be suitable for scanning in the first mode (eg DVR-format disk) and the second mode (eg DVD-format disk). A relatively small free running distance of is available in the first mode. One solution to solving the compatibility problem is to provide two separate sets of objectives in the objective assembly, ie one used for scanning in the first mode and the other used for scanning in the second mode. The design of the objective lens assembly is of great importance and is suitable for such matters as space, cost, ease of manufacture and handling.

Japanese Patent Application No. 2001-067700 discloses an optical scanning device as described in the introduction above with an objective lens assembly having two separate objective lens sets. 1 according to the present description is a conventional objective of scanning a first information layer IL1 with a first radiation beam RB1 (first mode) and a second information layer IL2 with a second radiation beam RB2 (second mode). Represent the lens assembly. The conventional objective lens assembly 1 includes a first objective lens 2 having a large diameter, a second objective lens 3 having a diameter smaller than the diameter of the first objective lens, and a third objective lens 4 having a large diameter. It is provided. These lenses 2, 3, 4 are mounted on the support member 5 to position the lenses 2, 3 along the optical axis AA 'and to position the lens 4 along the other optical axis BB'. The lenses 2 and 3 convert the radiation beam RB1 into a first scan spot SS1 of the information layer IL1, and the lens 4 converts the radiation beam RB2 into a second scan spot SS2 of the information layer IL2.

One disadvantage of the device described in JP2001-067700 is that the objective lens assembly has at least four parts (three lenses 2, 3, 4 and a supporting member 5), in particular with respect to the position of the assembly of the optical scanning device. The assembly is relatively large, where minimization of space occupancy by fraudulent components is a constant concern for the manufacture of optical scanning devices.

Another disadvantage of the device described in JP2001-067700 is that the objective lens assembly is assembled into four parts, making it relatively difficult to manufacture the assembly, especially in terms of the arrangement of the parts for their respective optical axes. In particular, the mounting is important because of the small diameter of the small lens 4 (typically a few millimeters) since there is an inclination between the objective lens and the information layer and a coma is generated at the scanning spot. Such coma aberration is not preferable because it generally adversely affects scanning of the information layer.

In addition, the apparatus described in JP2001-067700 is generally characterized in that the objective lens assembly is configured to (1) each position (2) and / or scan the positions of the first and second scanning spots. It is necessary to be mounted in an actuator that controls the track position of the first and second information layers. The lenses 2, 3 and 4 should be adjusted along their respective optical axes when assembled. There are two ways to assemble these two separate sets of objectives, the first method taking place outside the actuator and the second method taking place inside the actuator. However, the first method requires the additional use of the member to support the first, second and third objective lenses, thereby making the objective lens assembly relatively more expensive and with an inefficient volume in space. Increase In the second method, the objective lens is assembled in the actuator and fixed therein (e.g., glued) therein, and the exclusion of the objective lens assembly for the test purpose also excludes an ineffective actuator.

One object of the present invention is to provide an optical scanning device as described at the outset, which is suitable for operating in the first mode and the second mode while reducing the manufacturing cost.

The above object is achieved with an optical scanning device as described at the beginning when the second and third objective lenses according to the present invention are integrally formed in one body.

The advantage of such a device is that it scans the first information layer of a disc such as a second optical record carrier, so-called DVR-format, and the second information layer of a disc such as a second optical record carrier, eg a so-called DVD format. It is compatible, and only two parts (first objective lens and body) are assembled to form an objective lens assembly, which is cost effective and easy to assemble.

Another advantage of such a device is that once the first objective lens is assembled in the body, the optical axis of the lens can be aligned almost along the optical axis of the second objective lens before the objective lens assembly is mounted in the actuator. will be. Thus, the manufacturer can check that the objective lens assembly complies with the required specifications and, if necessary, exclude the assembly before mounting the assembly in the actuator. Thus, the objective lens assembly can be tested outside the actuator. After all, eliminating the objective lens assembly does not require the exclusion of the actuator, thereby making it more cost effective than the conventional objective lens assembly.

Another advantage of such a device is that it can in particular increase the manufacturing tolerances for coma aberration. The optical axis of the bilens system can be adjusted by positioning the first objective lens in the body when assembling with respect to the third objective lens. If the third objective lens is oriented with respect to the normal direction of the recording medium to generate coma, the optical axis of the bilens system can be positioned at the time of assembly and inclined with respect to the normal direction. Thus, the bilens system also generates coma. Thereafter, the amount of coma generated in the double lens system can be equal to the amount of coma generated in the third lens. Then, the objective lens assembly is mounted in the actuator and the actuator is oriented with respect to the normal direction so that the amount of coma generated in the dual lens system and the third lens can compensate for the amount of coma generated by the tilt of the optical record carrier. If the bilens system and the third lens produce the same coma amount, the disc tilt required to compensate this coma is the same for both systems. Thus, the preferred amount of coma (preferably minimum) is generated by the combination of the objective lens assembly and the optical record carrier.

Another advantage of such a device is that the second small objective lens (which has a typical size of several millimeters) is integrated with the third large objective lens and therefore does not require operation with a small lens.

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

At this time, Japanese Patent Application No. 09115170 discloses an optical scanning device having an objective lens assembly formed of one integrally molded member. 2 of the present description shows a conventional objective assembly. In FIG. 2, the objective lens assembly 10 includes a first objective lens 11 suitable for scanning the information layer 12 of the DVD-format disk 13 and the information layer 15 of the CD-format disk 16. Is provided with a second objective lens 14 suitable for scanning. These lenses 11 and 14 are integrally molded in one single member. However, the lens assembly known from JP 09115170 is incompatible with optical record carriers of higher information density, such as DVR-format disks.

Another object of the present invention is to provide an objective lens assembly as described in the introduction which is relatively small in size and easy to handle.

The object is to convert the first radiation beam into a first convergent radiation beam having a first numerical aperture and to convert the second radiation beam into a second convergent radiation beam having a second numerical aperture smaller than the first numerical aperture, (1) a first objective lens having a first diameter in cross section and a second objective lens having a second diameter in cross section smaller than the first diameter, wherein the first and second objective lenses comprise the first radiation beam; And (2) the second radiation beam as the second convergent radiation beam when the second and third objective lenses are integrally formed in one body in accordance with the present invention. Achieved by an objective lens assembly with a third objective lens for converting.

Hereinafter, the objects, advantages and features of the present invention will become apparent from the more detailed description of the present invention, as shown in the accompanying drawings;

1 shows a conventional objective lens assembly,

2 shows a conventional objective lens assembly,

3a and 3b show optical scanning values according to the invention in two different modes of operation;

4 shows an embodiment of the objective lens assembly of FIG. 2 when assembled,

5 shows the objective lens assembly of FIG. 4 when assembled,

6 shows another embodiment of the objective lens assembly of FIG. 4 when assembled.

3A shows an optical scanning device 20 according to the invention suitable for scanning the first information layer 21 of the first optical record carrier 22 with the first radiation beam 23. 3B shows the same optical scanning device 20 suitable for scanning the second information layer 24 of the second optical record carrier 25 with the second radiation beam 26. 3A and 3B correspond to each of the first mode and the second mode, as will be described later.

Hereinafter, the first mode refers to an operation mode of the optical scanning device 20 that scans the information layer 21 with a radiation beam having the first numerical aperture NA ₁ . This numerical aperture NA ₁ is suitable for scanning an optical recording medium of a first type, for example, a so-called DVR-format, having a first information density. The second mode refers to an operation mode of the optical scanning device 20 that scans the information layer 24 with the radiation beam 24 having the second numerical aperture NA ₂ smaller than the numerical aperture NA ₁ . This numerical aperture NA ₂ is suitable for scanning an optical recording medium of a second form, for example, a 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 for scanning a recording medium having a high information density, and the second mode corresponds to a mode for scanning a recording medium having a low information density.

For example, when the optical record carrier 22 is a so-called DVR-format, the numerical aperture NA ₁ is about 0.85 for the read mode and the record mode. For example, when the optical record carrier 25 is in DVD-format, the numerical aperture NA ₂ is about 0.6 for the read mode and about 0.65 for the record mode.

Referring to FIG. 3A, the recording medium 22 includes a transparent layer 27, and an information layer 21 is formed on one side of the transparent layer. The side surface of the information layer 21 facing away from the transparent layer 27 may be protected from environmental pollution by a protective layer. This transparent layer 27 acts as a substrate of the recording medium 22 by providing a mechanical support for the information layer 21. Alternatively, the transparent layer 27 has a single function of protecting the information layer 21, while the mechanical support is a layer on the other side of the information layer 21, such as a protective or additional information layer and the information layer. It is provided by the transparent layer connected to (21). The information layer 21 is a surface of a recording medium including a track. A track is a path followed by a focused beam of radiation, on which are arranged optically readable marks representing information. This mark may be, for example, in the form of a pit or region having a magnetization direction different from the reflection coefficient or the surroundings. For illustrative purposes only, when the optical record carrier 22 is a DVR-format disk, the thickness of the transparent layer 27 is about 0.1 mm.

Similarly, referring to FIG. 3B, the recording medium 25 of the second form has a transparent layer 28, and an information layer 24 is formed on one side of the layer. The thickness of the transparent layer 28 is thicker than the thickness of the transparent layer 27 of the optical recording medium 22 of the first aspect. For illustrative purposes only, when the recording medium 45 is a DVD-format disk, the thickness of the transparent layer 46 is about 0.6 mm.

As shown in FIGS. 3A and 3B, the optical scanning device 20 includes a radiation source 30 and an objective lens assembly 31 having an optical axis 32. The apparatus 20 includes a beam splitter 33, a collimating lens 34, a detection system 35, a servo system 36, a focus actuator 37, a radial actuator 38, and an error correction information processor 39. ) Is further provided.

The radiation source 30 is preferably configured to supply a radiation beam 23 for scanning the information layer 21 of the first medium 22 and a radiation beam 26 for scanning the information layer 24 of the second medium 25. . The radiation source 30 is provided with at least the 1st semiconductor laser which emits the radiation beam 23 of 1st selective wavelength (lambda) 1, and the 2nd semiconductor laser which emits the radiation beam 26 of 2nd selective wavelength (lambda) 2. For illustrative purposes only, when the first medium 22 is a DVR-format disk, the wavelength λ1 is preferably 405 nm, and when the second medium 25 is a DVD-format disk, the wavelength λ2 is 660 nm. It is preferable.

The beam splitter 33 is configured to reflect the radiation beams 23 and 26 toward the collimating lens 34. The beam splitter 28 is preferably formed of a planar parallel plate inclined with respect to the optical axis 32.

The collimating lens 34 is configured to convert the radiation beams 23 and 26 into the first collimation radiation beam 40 and the second collimation radiation beam 55, respectively.

The objective lens assembly 31 converts the radiation beam 40 into a first focused radiation beam 41 having a first numerical aperture NA ₁ , so that the first scanning spot 42 is positioned at the position of the first information layer 21. And convert the radiation beam 55 into a second focused radiation beam 43 having a second numerical aperture NA ₂ to form a second scanning spot 44 at the position of the second information layer 24. Hereinafter, the objective lens assembly 32 will be described in more detail.

When the optical scanning device 20 operates in the first mode, the omnidirectional focused radiation beam 41 is reflected from the information layer 21 and returns on the optical path of the omnidirectional focused radiation beam 41. Forming a diverging radiation beam 46 in the rearward direction. The objective lens assembly 31 converts the rearward radiation beam 46 into a first collimated rearward radiation beam 47 passing through the collimation lens 34.

The beam splitter 33 separates the backward radiation beam 47 and the forward radiation beam 23 by transmitting at least a portion of the backward radiation beam 47 directed to the detection system 35.

Similarly, when the optical scanning device 20 operates in the second mode, the omnidirectional focused radiation beam 43 is reflected by the information layer 24, so that the omnidirectional focused radiation beam 43 is employed. A back diverging reflective beam 50 is formed to return on the optical path of. The objective lens assembly 31 converts the rearward radiation beam 50 into a rearwardly collimated radiation beam 51. Finally, the beam splitter 33 separates the backward radiation beam 51 and the forward radiation beam 26 by transmitting at least a portion of the backward radiation beam 51 toward the detection system 35.

The detection system 35 is configured to capture the rearward radiation beams 47 and 51 and convert them into one or more electrical signals. One of the signals is the information signal I _data , and the value of the signal represents the information scanned from the information layers 21 and 24. This information signal I _data may be processed by the information processing unit 39 which corrects an error of the information extracted from the information layers 21 and 24. The other signals from the detection system 35 are the focus error signal I _focus and the radial tracking error signal I _radial . The focus error signal I _focus represents the axial height difference along the optical axis 33 between the scanning spots 42 and 44 and the information layers 21 and 24, which is in focus on the information layer (as described below). It is used to maintain the injection spot. The focus error signal I _focus can be found in G. Bouwhuis, J. Braat, A. Huisjser et al, "Principles of Optical Disc Systems," pp. 75-80 (Adam Hilger 1985) (ISBN) 0-85274-785-3), and is formed by the "astigmatism method" known in the book. The radial tracking error signal I _radial represents the distance in the plane of the information layers 21, 24 between the scanning spots 42, 44 and the center of the track of this information layer following the scanning spot, which is below It is used to hold the scanning spots 42 and 44 on the tracks of the information layers 21 and 24 as described in. The radial tracking error signal I _radial is formed by the " radial push-pull method " which is generally used, in particular the book known by G. Bouwhuis et al., Pp. 70-73.

The servo system 36 is configured to provide an actuator control signal I _control for controlling the focus actuator 37 and the radial actuator 38, respectively, in accordance with the signals I _focus and I _radial . The focus actuator 37 controls the position of the actual scanning spots 42 and 44 by controlling the position of the objective lens assembly 31 along the optical axis 32 so that they can be controlled by the information layers 21 and 24. The planes almost coincide with each other. Since the radial actuator 38 controls the position of the objective lens assembly 31 in the direction perpendicular to the optical axis 32, they control the radial position of the scan spots 42 and 44, so that they are the information layer 21. In Fig. 24, they almost coincide with the centerlines of the following tracks.

The objective lens assembly 31 is mounted in the actuator using a method of the prior art, for example using a rotary actuator such as described in JP 2001067700 or two prisms or dichroic mirrors as described in JP 09115170.

Hereinafter, the objective lens assembly 31 will be described in more detail. 4 illustrates an embodiment of the objective lens assembly 31 of FIG. 3. The objective lens assembly 31A includes (1) a dual lens system composed of the first objective lens 61 and the second objective lens 62, and (2) the third objective lens 63.

In WO 00/38182, the structure of a bilens system for scanning DVR-formatted discs and DVD-formatted discs is known. This conventional structure includes two objective lenses each having an optical axis and each having an optical axis aligned along the optical axis of the bilens system. These two lenses are spaced apart along the optical axis of the bilens system by an interval that can be adjusted to compensate for spherical aberration that occurs when converting a DVR-formatted disc to a DVD format disc having a different thickness of the transparent layer. In this case, the bi-lens system of WO 00/38182 needs to adjust the distance between the lenses of the bi-lens system to be required by an additional actuator when the scanning spot is changed from the information layer of the DVR-format disc to the information layer of the DVD-format disc. There is a drawback to this. These additional actuators make the system somewhat complicated, making it difficult to manufacture.

Each of the lenses 61 and 62 has an optical axis aligned with the reference axis of the bilens system, that is, the reference axis AA '.

The lens 61 has an incident surface 61A and an exit surface 61B, and has a circular cross section S ₁ having a first diameter d ₁ . The lens 62 has an incident surface 62A and an exit surface 62B, and has a circular cross-section S ₂ having a second diameter d ₂ smaller than the first diameter d ₁ . Once again, the lens 63 has a third diameter d ₃ of circular cross-section S _3. In the present description, the "diameter" of a lens corresponds to the optically effective diameter of the lens, ie the diameter of converting the incident beam by the lens in accordance with the specific characteristics of the lens. For illustrative purposes only, if the recording medium 22 is a DVR-format disk, its diameter d ₁ is about 3 mm and its diameter d ₂ is about 1.4 mm. If the recording medium 25 is a DVD-format disk, its diameter d ₃ is about 3.6 mm.

According to the present invention, the objective lenses 62 and 63 are integrally formed in one body 64. For example, the body 64 may be formed using an injection molding process generally used in lens manufacturing. For the sake of explanation, the body 64 and thus the lenses 62, 63 are made of the same plastic material, and the lens 61 is made of glass having an aspheric polymer layer thereon, as shown in FIG. In this example, the objective lens 61 shown in Fig. 4 is a flat aspherical member. The objective lens 61 has a thickness of 2.819 mm and an incident pupil diameter of 3.0 mm on the optical axis. The body of the objective lens is made of FK5 Schott glass having a refractive index of 1.4989 at a wavelength of 405 nm. The radius of the convex surface of the lens body disposed toward the collimating lens is 2.07 mm. The surface of the objective lens 61 opposite to the objective lens 62 is flat. Its aspherical shape is realized by a thin layer of acrylic on its glass body. The refractive index of the lacquer is 1.5987. The thickness of this layer on the optical axis is 0.019 mm. The rotationally symmetrical shape of the surface can be described by the following equation:

z (r) = B ₂ r ² + B ₄ r ⁴ + B ₆ r ⁶ + ....

At this time, z is the surface position in the optical axis direction expressed in millimeters, r is the distance to the optical axis expressed in millimeters, and B _k is a coefficient of k squares of r. The values of the coefficients B ₂ to B ₁₆ are 0.26447094, 0.0088460392, 0.00014902273, 0.0014305415, -0.0015440542, 0.00082680417, -0.00023319199, and 0.0000025911741, respectively. The objective lens 62 is made of COC (Topas) and is a flat aspherical surface. The refractive index of the COC is 1.5499. The thickness on the optical axis of the objective lens 62 is 0.9 mm and the beam incident diameter of the objective lens 62 is 1.352 mm. The surface of the objective lens 62 facing the disk is flat. The rotational symmetry of the surface with respect to the objective lens 61 can be described by the following equation:

z (r) = B ₂ r ² + B ₄ r ⁴ + B ₆ r ⁶ + ....

At this time, z is the surface position in the optical axis direction expressed in millimeters, r is the distance to the optical axis expressed in millimeters, and B _k is a coefficient of k squares of r. The values of the coefficients B ₂ to B ₁₆ are 0.60369741, 0.22447301, 0.029061701, 0.33507029, -1.1373531, 3.5133805, -5.6443868, and 3.1481201, respectively. Therefore, the free working distance, that is, the distance between the objective lens 62 and the disk, is 0.15 mm. The cover layer of the disk is made of polycarbonate having a thickness of 0.1 mm and a refractive index of 1.6223.

The objective lens 63 is composed of COC and both aspherical surfaces. The refractive index of COC with a wavelength of 660 nm is 1.5309. The thickness on the optical axis of the objective lens 63 is 2.194 mm and the beam incident diameter of the objective lens 63 is 3.3 mm. The rotationally symmetrical shape of the surface of the objective lens 63 can be described by the following equation:

z (r) = B ₂ r ² + B ₄ r ⁴ + B ₆ r ⁶ + ....

At this time, z is the surface position in the optical axis direction expressed in millimeters, r is the distance to the optical axis expressed in millimeters, and B _k is a coefficient of k squares of r. The values of the coefficients B ₂ to B ₁₆ for the surface opposite the collimating 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 surfaces facing the disk, these coefficients B ₂ through B ₁₆ are -0.1114228, 0.02852619, -0.0046668186, -0.0036752428, 0.0063619581, -0.007503492, 0.0046641069, -0.0010757204, respectively. Free working distance is 0.990mm. The cover layer of the disk is made of polycarbonate having a thickness of 0.6 mm and a refractive index of 1.5796 at a wavelength of 660 nm.

Once the lens 61 is mounted in the body 64, the bilens system formed of lenses 61 and 62 converts the first radiation beam 40 into a first focused radiation beam 41 having a first numerical aperture NA ₁ . The scanning spot 42 is formed at the position of the information layer 21. The lens 63 converts the second radiation beam 55 into a second focused radiation beam 43 having a second numerical aperture NA ₂ smaller than the first numerical aperture, so as to be located at the position of the information layer 24. The scanning spot 44 is formed. The lens 63 has an incident surface 63A and an exit surface 63B, and also has an optical axis BB '.

In addition to the above, the objective lens assembly 31 has a space between the optical axes AA 'and BB' to (1) make the objective lens assembly smaller by the integration of the body and one objective lens set (2 Since the integrated body does not require an additional member to support the first, second and third objective lenses, there is an advantage that it is reduced compared to the space of the conventional objective lens assembly. This is desirable in comparison to a conventional objective lens assembly in which two separate sets of objective lenses need to be mounted in an additional body.

Hereinafter, the assembly of the objective lens assembly of FIG.

5 shows the disassembled objective lens assembly 31 of FIG. 4 in the case where the first lens 61 has the optical axis CC 'and the second lens 62 has the reference axis of the dual lens system, that is, the optical axis which is the reference axis AA'. Indicates.

First, the coma aberration generated in the lens 63 of the body 64 is measured by a conventional technique.

Secondly, by mounting the lens 61 in the body 64, the optical axes CC ', AA' and BB 'are aligned to meet certain specifications. Meanwhile, the amount of coma generated in the bilens system formed by the lenses 61 and 62 is measured. Thus, lens 61 is positioned such that the bilens system generates the same amount of coma as compared to the amount generated in lens 63.

Thirdly, the distance between the lenses 61 and 62 is adjusted so that the radiation beam emitted from the bilens system has a fixed spherical aberration value. For example, the fixed value may compensate for the amount of spherical aberration generated by the optical recording medium. In this case, the optical characteristics of the objective lens may be, for example, as shown in BHWHendriks and PGJMNuyens, "Designs and manufacturing of far-field high NA objective lenses for optical recording," 413-414, SPIE 3749 (1999). For example, it is improved when the surface is made more aspherical, such as the exit surface of the first objective lens or the exit surface of the second objective lens curved aspherically.

Fourth, the lens 61 is fixed, for example glued, to the body 64.

At this time, after performing the measurement, the objective lens assembly 31 is mounted in the actuator, and this measurement has the advantage that the objective lens assembly can be inspected in advance and, if necessary, can be excluded before assembly. This is cost effective since the exclusion of the objective lens assembly, which has already been assembled (glued) but not mounted in the actuator, does not require the exclusion of the actuator.

It will be appreciated that various changes and modifications may be made with respect to the above-described embodiments without departing from the scope of the invention as set forth in the appended claims.

FIG. 6 shows another embodiment 31 ′ of the objective lens assembly 31 shown in FIG. 4. As shown in FIG. 6, the objective lens assembly 31 ′ is disposed such that the exit face 62B ′ of the lens 62 ′ and the exit face of the lens 63 ′ are in different planes. An advantage of the second embodiment is that the exit surface 62B does not need to change the position of the objective lens assembly 31 'along the optical axis in order for both scanning spots 42 and 44 to be maintained in the information layers 21 and 24, respectively. Is to select the position of '.

In another embodiment of the optical scanning device, a radiation beam having a wavelength and a numerical aperture different from those described above may be used. For example, the optical scanning device may be configured to be suitable for scanning, for example, a DVD-format disk and a CD-format disk, or a DVR-format disk and a CD-format disk.

In another embodiment of the optical scanning device, the third objective lens may also be arranged to be compatible with the CD-format disk and the DVD-format disk. For example, such compatibility may include aperiodic phase structures, such as those described in European Patent Application No. 0,865,037, or the 2000 International Optical Design and Manufacturing Conference (K. Maruyama and R.Ogawa, Background of Conception of DVD / CD compatible diffractive lens). , pp. 93-96, Optical Society of Japan (2000)).

In another embodiment of the optical scanning device, the optical scanning device may be of a type capable of performing simultaneous multitrack scanning. This improves the data transfer rate in the read mode as described, for example, in US Pat. No. 4,449,212.

Claims (6)

The first information layer 21 of the first optical recording medium 22 with the first radiation beam 23 and the second information layer 24 of the second optical recording medium 25 with the second radiation beam 26. Inject, A radiation source 30 for supplying the first and second radiation beams, The first end face (S ₁₎ of claim 2 having a first diameter (d _1), the first objective lens 61 and the second section a second diameter (S ₂₎ smaller than the first diameter (d ₂₎ having a And an objective lens 62, wherein the first and second objective lenses convert the first radiation beam into a first focused radiation beam 41 having a first numerical aperture NA ₁ , thereby providing the first information. Second focusing having a bilens system 61, 62 configured to form a first scanning spot 42 at the position of the layer, and a second numerical aperture NA ₂ , wherein the second radiation beam is smaller than the first numerical aperture. Scanning apparatus having an objective lens assembly (31) comprising a third objective lens (63) for converting into a radiation beam (43) to form a second scanning spot (44) at a position of said second information layer ( 20), And the second and third objective lenses are integrally formed in one body (64). The method of claim 1, Each of the second and third objective lenses 62 and 63 has emission surfaces 62B and 63B facing the position of the information layer to be scanned, and the emission surface 62B of the second objective lens 62 is An optical scanning device (20) characterized in that it is flush with the exit surface (63B) of the third objective lens (63). The method of claim 1, Each of the second and third objective lenses 62 and 63 has emission surfaces 62B and 63B facing the position of the information layer to be scanned, and the emission surface 62B of the second objective lens 62 is Optical scanning device (20), characterized in that it is in the same plane as the exit surface (63B) of the third objective lens (63). The method of claim 1, A detection system 35 configured to provide a focus error signal I _focus and / or a radial tracking error signal I _radial , a servo system 36, the focus error signal and / or the radial tracking error signal The first and second information layers scanned and / or the positions of the first and second scan spots 42 and 44 with respect to the respective positions of the first and second information layers 21 and 24 in response thereto. Optical scanning device (20), characterized in that it further comprises an actuator (37, 38) for controlling the position of the track. The method of claim 1, And an information processing unit (39) for error correcting the information extracted from said first or second information layer. The first radiation beam 40 is converted into a first convergent radiation beam 41 having a first numerical aperture NA ₁ and the second radiation beam 55 is a second numerical aperture NA smaller than the first numerical aperture. ₂ ) a second converging radiation beam 43 having The first end face (S ₁₎ of claim 2 having a first diameter (d _1), the first objective lens 61 and the second section a second diameter (S ₂₎ smaller than the first diameter (d ₂₎ having a A dual lens system (61, 62) including an objective lens (62), wherein the first and second objective lenses are configured to convert the first radiation beam into the first convergent radiation beam; In the objective lens assembly (31) having a third objective lens (63) for converting the second radiation beam into the second convergent radiation beam, And the second and third objective lenses are integrally formed in one body (64).