KR20050022007A - Optical scanning device including a tilt tolerant objective system - Google Patents

Abstract

A radiation source 11 for scanning the information layer 4 of the optical record carrier 2, generating a radiation beam 12, and an objective 18 for focusing the radiation beam 12 on the information layer. In one optical scanning device, the ratio of the root mean square of the optical path differences (OPDs) generated by the objective system 18 when the oblique beam is incident on the objective system is expressed in the field of the objective system (1). Where OPD (A31) is the contribution of the third Zernike Koma's aberration to the root mean square root aberration, and OPD (A51) is the fifth-order Zernike Koma's root mean square root aberration. An optical scanning device 1 is provided, which is a contribution value of aberration.

Description

Optical scanning device with an objective system resistant to tilt {OPTICAL SCANNING DEVICE INCLUDING A TILT TOLERANT OBJECTIVE SYSTEM}

The present invention provides an optical scanning device for scanning an optical record carrier, a lens system suitable for use as (but is not limited to) an objective lens in such a scanning device, methods of manufacturing the device, and such a system. It is about.

In optical recording, the increase in information density on the optical record carrier must involve smaller radiation beam spots that scan the information. Such smaller spots can be realized by increasing the numerical aperture NA of the objective system used to focus the radiation beam on the record carrier in the scanning device. Examples of optical recording media include compact discs (CDs) and digital versatile discs (DVDs).

Due to the diffraction limit, the radiation spot is not perfect. However, in such an optical recording system, it is preferable that the spot is limited not by the influence of aberration but by diffraction. In general, the tolerance of the root mean square of the optical path difference (OPD) of approximately 0.07λ (where λ is the wavelength of the associated radiation beam) is permitted for wavefront aberration, as a whole. This diffraction is limited. It may be convenient to indicate the OPD in mλ (where 0.001λ = 1mλ). From this total tolerance, aberrations of approximately 30-40 mλ are allowed for the total wavefront aberration of the objective system, of which 15 mλ is allowed for the influence of coma aberration. The remaining parts and effects of the recording system (eg, temperature, wavelength error, misalignment of other components) contribute to the total amount of aberration.

Many optical recording (and optical reading) systems have one or more additional beam spots focused on the optical record carrier adjacent to a central beam spot used to write to (or read from) the record carrier. Such additional spots are usually used to provide information about the positioning of the scanning (e. G. Reading or writing) radiation spot on the record carrier. An example of such a system is a "3-spot push-pull" system with two auxiliary spots, one on each side of the main read or write spot. These additional spots are used to ensure that the center spot is in focus and placed in the desired position (eg the desired track) of the record carrier. Since the main spot is usually disposed on the optical axis of the objective system, it can be seen that the radiation beams forming the adjacent spots are obliquely incident on the objective lens system.

When assembling the optical system, it is important not only that the objective lens is correctly positioned, but also precisely oriented (eg tilted) with respect to both the optical axis of the optical system and the recording medium, so as to minimize the effects of aberrations including coma aberration.

The optical axis of the objective lens system preferably coincides with the optical axis of the optical system (i.e., the radiation beam converted into parallel light) and is aligned perpendicular to the surface of the recording medium.

If the optical axis of the radiation beam incident on the recording medium is not perpendicular to the recording medium, coma aberration occurs inside the beam when the beam passes through the transparent cover layer. Most optical record carriers are not completely planar and may bend. Thus, when the radiation beam scans across the surface of the record carrier, the radiation beam encounters different regions with the tilted surface out of the desired direction. In order to compensate for coma aberration caused by such different tilts of the recording medium (for example, various disk tilts), it is known to actively tilt the objective lens using an actuator portion.

Conventional lens structures are limited to a relatively small range of tilt compensation. Although the optical axis of the central scanning beam is disposed on the optical axis of the objective system, the three-spot push-pull system is particularly limited by the coma aberration inherently caused by the auxiliary spots obliquely entering the objective lens system.

After all, it is an object of the present invention to provide an optical system capable of improving the range of disc tilt compensation that can be achieved by tilting the objective lens system.

(Summary of invention)

According to the first aspect, the present invention relates to a radiation source 11 which scans an information layer 4 of an optical record carrier 2 and generates radiation beams 12, 15, 17, 20, and an information layer. In an optical scanning device (1) having an objective (18) for focusing a radiation beam, the square of optical path differences (OPDs) generated by the objective (18) upon incidence of the oblique beam into the objective. The ratio of the root mean square satisfies the following conditions in the field of objective 18:

In this case, OPD (A31) is a contribution value of the third Zernike coma aberration to the root mean square root aberration, and OPD (A51) is a contribution of the fifth order Zernike coma aberration to the root mean square root aberration. It provides an optical scanning value characterized in that the value.

According to another aspect, the present invention provides a lens system having at least one lens for focusing a radiation beam, the square mean of optical path differences (OPDs) generated by the lens system upon incidence of the oblique beam into the lens system. The ratio of the square root meets the following conditions within the field of the lens system:

In this case, OPD (A31) is the contribution value of the third-order Zernike coma aberration to the root mean square wavefront aberration, and OPD (A51) is the contribution value of the fifth-order Zernike coma aberration to the root mean square wavefront aberration. It provides a lens system characterized in that.

According to yet another aspect, the present invention provides a method for manufacturing a lens system having at least one lens for focusing a radiation beam 17, wherein the optical path differences generated by the lens system at the incidence of the oblique beam into the lens system ( Forming said lens system such that the ratio of the root mean squared of OPDs) satisfies the following conditions within the field of the lens system:

In this case, OPD (A31) is the contribution value of the third-order Zernike coma aberration to the root mean square wavefront aberration, and OPD (A51) is the contribution value of the fifth-order Zernike coma aberration to the root mean square wavefront aberration. It provides a method of manufacturing a lens system 18, characterized in that.

According to another aspect, the present invention provides an optical scanning device 1 for scanning an information layer 4 of an optical record carrier 1,

Installing a radiation source 11 for generating a radiation beam;

In the manufacturing method of an optical scanning device comprising the step of providing a lens system 18 for condensing a radiation beam on the information layer (4),

The ratio of the root mean square of the optical path differences (OPDs) generated by the lens system upon incidence of the oblique beam into the lens system satisfies the following conditions within the field of the lens system:

In this case, OPD (A31) is the contribution value of the third-order Zernike coma aberration to the root mean square wavefront aberration, and OPD (A51) is the contribution value of the fifth-order Zernike coma aberration to the root mean square wavefront aberration. It provides a method for manufacturing an optical scanning device, characterized in that.

Another aspect of the invention is described in the dependent claims.

For the understanding of the present invention and to provide a way for the embodiments of the present invention to be realized, reference is made to the following accompanying drawings:

1 shows an apparatus for scanning an optical record carrier including an objective system,

2A shows an objective system comprising two plano-aspherical members,

Figure 2b shows an objective system including one spherical-aspherical member,

Figure 2c shows an objective system including a double aspherical member,

3 illustrates how the lens system and / or medium can be tilted,

FIG. 4 is a graph showing representative OPDs due to different Zernike terms due to the coma aberration of the beam respectively generated by the tilt of the disc and the tilt of the conventional lens structure.

1 shows an apparatus 1 for scanning an optical record carrier 2. The recording medium includes a transparent layer 3, and an information layer is disposed on one surface of the transparent layer. The surface of the information layer on the opposite side of the transparent layer is protected from external influences by the protective layer 5. The face of the transparent layer facing the device is called the incident face 6. The transparent layer 3 serves as a substrate for the recording medium by providing mechanical support for the information layer.

In contrast, the transparent layer has a unique function of protecting the information layer, while the mechanical support is on a layer on the other side of the information layer, for example another layer connected to the protective layer 5 or the information layer 4. And a transparent layer. The information may be stored in the information layer 4 of the record carrier in the form of optically detectable marks arranged in nearly parallel, concentric or helical tracks, not shown. These marks may have an optically readable form, for example in the form of pits, or in the form of regions having a reflection coefficient or magnetization direction different from its periphery, or a combination of these forms.

The scanning device 1 has a radiation source 11 capable of emitting a radiation beam 12. The radiation source may be a semiconductor laser. The beam splitter 13 reflects the diverging radiation beam 12 toward the collimating lens 14, which converts the diverging beam 12 into a converted beam 15. The beam 15 converted into parallel light is incident on the optical system 18.

The object system may include one or more lenses and / or gratings. The objective system 18 has an optical axis 19. The objective system 18 converts the beam 17 into a converging beam 20 which is incident on the incident surface 6 of the recording medium 2. The objective system has a spherical aberration correction configured to pass the radiation beam through the thickness of the transparent layer 3. The converging beam 20 forms a spot 21 in the information layer 4. The radiation beam reflected by the information layer 4 forms a diverging beam 22, which is converted into a beam 23 which is converted into nearly parallel light by the objective system 18, by the collimating lens 14. Is converted to a converging beam 24. The beam splitter 13 separates the front beam and the reflected beam by passing at least a portion of the converging beam 24 to the detection system 25. The detector captures the radiation beam and converts it into electrical output signals. The signal processor 27 converts these output signals into various other signals.

One of these signals is an information signal 28, the value of which indicates information read from the information layer 4. The information signal is processed by the information processor for error correction 29. The remaining signals of the signal processor 27 are the focus error signal and the radial error signal 30. The focus error signal represents the axial height difference between the spot 21 and the information layer 4. The radial error signal represents the distance in the plane of the information layer 4 between the spot 21 and the center of the track within the information layer to which the spot follows.

The focus error signal and the radial error signal are supplied to the servo circuit 31, which converts these signals into servo control signals 32 for controlling the focus actuator and the radial actuator, respectively. Actuators are not shown in the figures. The focus actuator controls the position of the object system 18 in the focal direction 330 so as to control this actual position so that the actual position of the spot 21 substantially coincides with the plane of the information layer 4. The radial actuator is By controlling the position of the objective system 18 in the radial direction 34, the radial position of the spot 21 is adjusted so that the radial position of the spot 21 substantially coincides with the centerline of the track to be followed within the information layer 4. In the figure, the tracks run in a direction perpendicular to the plane of the figure.

The apparatus of FIG. 1 can also be configured to scan a second type of record carrier with a transparent layer thicker than the record carrier 2. The apparatus may use a radiation beam having a different wavelength to scan the radiation beam 12 or the recording medium of the second type. The NA of this radiation beam may be modified to match the shape of the recording medium. The spherical aberration compensation of the objective system must be modified accordingly.

It is apparent that the present invention can be implemented for a lens system including a single lens or a plurality of lenses.

For example, FIG. 2A shows the objective lens system 18 including two separate lens members 18A and 18B. Each member 18A, 18B in this embodiment is a flat-aspherical surface. These two members are held fixed to each other using the lens holder 18C.

Likewise, the present invention may be implemented by other lens arrangements or other lens arrangements, such as the single plain-aspherical lens 18 shown in FIG. 2B or the double-aspherical lens 18 shown in FIG. 2C, or the like. . For ease of reference, one record carrier 2 is shown for all the figures in FIGS. 2A-2C. In each of these figures, the Y and Z axes are assumed to exist in the plane of the paper, and the X axis is assumed to be a direction projecting from the paper. Normally, the optical axis of the lens is perpendicular to the surface of the recording medium 2. However, as shown in FIG. 3, the objective lens 18 'or the recording medium 2' may be tilted from their normal ideal positions 18, 2 by rotating about the X and / or Y axis. .

The amount of wavefront aberration present in the radiation beam is represented by the aberration function Φ (ρ, θ), where ρ and θ are spherical coordinates, ρ is a normalized pupil radius, and θ is an angular coordinate. This aberration function can be represented by Zernike polynomials. Zernike functions are described, for example, in the famous reference book "Principals of Optics" by Max Born & Emil Wolf. The wavefront aberration function Φ (ρ, θ) can be expressed as:

Where A _nm are Zernike coefficients and R _n ^m (ρ) are radial polynomials (see section 9.2 of the book "Principals of Optics", by University Press, Cambridge, Revision, 2002). The development term corresponding to the third Zernicke Coma aberration is given by:

It is given by the following equation corresponding to the fifth Zernike Koma aberration:

The root mean square OPD of wavefront aberration is defined as:

Thus, the contribution OPD (A31) of the third-order Zernike Koma's aberration to the root mean square wavefront aberration is given by:

The contribution OPD (A51) of the fifth Zernike coma aberration is given by:

Conventional objectives are designed to have adequate field of view performance. This means that the generation of the lowest order coma aberration is suppressed as large as possible for all inclined beam incidences.

The inventors have found that, as a result of the lowest order coma aberration being suppressed, the maximum allowed viewing angle of the lens system is generally limited by higher order coma aberration terms.

Coma aberration occurs due to the asymmetrical passage of the beam through the cover layer in the beam incident on the tilted disc. A relatively large amount of lowest order coma and a very small amount of higher order coma occur. As a result, when the disc is tilted to compensate for the tilt misalignment of the objective lens, only a moderate degree of tilt misalignment of the objective lens can be tolerated since higher order coma aberrations do not cancel each other out significantly.

4 is a graph showing the magnitudes of the coma aberrations generated for the different tilt amounts of each of the disks (ie, the recording medium) and for the different A31 and A51 Zernike terms for the conventional objective system. The tilt angles of the objective lens of the disc up to 1 ° were taken into account. In this tilt range, the root mean square optical path difference that occurs for each of the A31 and A51 Zernike terms is nearly proportional to the amount of tilt (ie, linear).

As can be seen in the graph, for a conventional disc at a tilt of 1 °, term A31 gives an OPD of approximately 135mλ and term A51 gives an OPD of approximately 7.5mλ. In contrast, because the lowest order coma aberration is suppressed, the objective lens system has OPD = 12.6 mλ from the A31 term and OPD = 44.2 mλ from the A51 term, that is, the A31 contribution value is smaller than the A51 contribution value at 1 ° tilt.

Assuming that the disk is tilted at about 0.1 °, OPD = 13.5mλ from A31 and OPD = 0.75mλ from A51. It can be seen that in order for the objective lens system to be tilted to compensate for the OPD generated from the term A31, that is, to provide an OPD of 13.5 mλ from the tilt of the lens system, the objective lens system must be tilted by more than 1 °. This tilt will result in a relatively large amount of OPD occurring in the quadrature A51 term (which is 44.2mλ for the objective lens system at 1 °), which greatly exceeds the OPD from the disc A51 term. Therefore, it can be seen that the conventional objective lens system cannot be tilted to simultaneously compensate for both the coma aberration occurring in the A31 anti-disk tilt and the coma aberration occurring in the A51 anti-disk tilt.

In order to provide an objective lens system with even greater tolerance, the inventors have found that the ratio of higher order comma aberration and third order coma aberration as a function of oblique beam incidence is about the same as that occurring when tilting the disc. We propose to design an objective lens to

As a result, when the beam is obliquely incident on the objective lens, a significant amount of lowest order coma occurs for higher order coma. In particular, in order to be resistant to the tilt misalignment of the objective system, the OPDrms (square root mean square OPD) third coma aberration OPD 31 and the fifth coma aberration OPD 51 generated by the objective lens at an oblique incidence The size of must satisfy the following relationship within the field of the lens system:

At this time, the signs of the Zernike coefficients A31 and A51 are the same. The field of the lens system is an area where the oblique beams generate an OPD smaller than 15 mλ. The field of view of the lens system is twice the field.

Better compensation occurs when the following equation is met:

 At this time, the signs of the Zernike coefficients A31 and A51 are the same.

Hereinafter, various embodiments of the present invention will be described with reference to each conventional objective lens design.

Example 1

Objectives in accordance with embodiments of the present invention can also be used to compensate for disc tilt (eg, in a rewritable DVD drive) using the tilt actuator concept. In order to compensate for the disc's deflection, the lower order coma aberration OPD (A31) and the higher order coma aberration OPD (A51) generated by the disc tilt are caused by the tilt of the objective lens. The coma and higher order coma aberrations (which occur as a result of oblique incidence into the objective lens) must therefore be noticeably compensated respectively. In order to obtain an effective compensation, relation (1), or more preferably relation (2) must be satisfied.

The objective lens 18 shown in FIG. 1 according to the first conventional design is a flat-aspherical member. The objective lens 18 has a numerical aperture of 0.65 and an incident pupil diameter of 3.58 mm, and operates at a wavelength λ = 660 nm. The objective lens 18 has a thickness of 2.150 mm along the optical axis. The lens body of the objective lens is made of S-LAH66 OHARA glass with refractive index n = 1.768 (manufactured by OHARA Corporation, a company in California). In use the convex surface of the lens body facing the collimating lens has a radius of 2.32 mm. In use, the surface of the objective lens facing the recording medium is flat. The aspherical form embodies a thin layer of UV cured acrylic lacquer (eg, as described in US Pat. No. 4,623,496) on the glass body. This lacquer has a refractive index of n = 1.564. The thickness of this layer along the optical axis is 30 microns. The rotationally symmetric aspherical shape is given by

Where z is the position of the surface in the optical axis direction in millimeters, r is the distance to the optical axis in millimeters, and B _k is the coefficient of the k power of r. The values of the coefficients B ₂ through B ₁₀ are 0.24451798, 0.0053871439, -8.0244002 × 10 ^-5 , -2.4646422 × 10 ^-5 , -1.1188561 × 10 ^-5, respectively. The free working distance (distance between the objective lens 18 and the disk) is 1.160 mm. The cover layer of the disk has a thickness of 0.60 mm and a refractive index of n = 1.580.

When tilting only the disk in such an objective system, the OPDrms of the fifth and third coma aberrations were found to be as follows:

At this time, for the tilt of 1 degree, the amount of the third coma aberration was found to be 133 mλ.

Following the conventional design approach described above, the ratio of OPDrms of the fifth and third coma aberrations was found to be as follows for the oblique beam incidence:

At this time, for the field of 1 degree, the amount of the third coma aberration was found to be 12.6 mλ. Due to the significant difference between (3) and (4), when the disc has tilt misalignment, it is impossible to tilt the objective lens so that higher order coma aberrations can be canceled out noticeably. Taking 15 mλ as the limit, the nominal tolerance (double the field) in a nominal arrangement is given as 0.64 degrees. That is, when the disc is not tilted, the maximum viewing angle at which the oblique beam passes through the lens to obtain OPD <15mλ is 0.64 degrees.

In such a case, the maximum disc tilt that can be compensated for by tilting the objective lens is a disc tilt of 0.39 degrees (ie, the disc is tilted at an angle of 0.39 ° with respect to the optical axis of the radiation beam converted into parallel light).

In such a case, due to the high value of (4), the allowed disc tilt is close to the allowed field of the objective lens (= 0.5 times the field of view).

Thus, here, the lens surface remains almost parallel to the disc to achieve compensation of the disc tilt, while when compensating for the disc tilt, the lens surface is usually at a constant angle with respect to the disc.

The objective lens 18 shown in FIG. 1 in this first embodiment of the present invention is a spherical-aspherical lens. The objective lens 18 has a numerical aperture of 0.65 and an incident pupil diameter of 3.58 mm, and operates at λ = 660 nm. This objective lens 18 has a thickness of 1.843 mm along the optical axis. The lens body of the objective lens is made of S-LAH66 OHARA glass with refractive index n = 1.768. In use, the convex surface of the lens body facing the collimating lens has a radius of 2.32 mm. In use, the surface of the objective lens facing the recording medium is flat. The aspherical shape is implemented with a thin layer of UV cured acrylic lacquer on the glass body. The lacquer has a refractive index n = 1.564. The thickness of this layer on the optical axis is 33 microns. The rotationally symmetric aspherical shape is given by

Where z is the position of the surface in the optical axis direction in millimeters, r is the distance to the optical axis in millimeters, and B _k is the coefficient of the k power of r. The values of the coefficients B ₂ through B ₁₀ are 0.24672704, 0.0051164884, -1.2805998 × 10 ^-4 , -4.1024615 × 10 ^-5 , -9.0856931 × 10 ^-6, respectively. The free working distance (distance between the objective lens 18 and the disk) is 1.316 mm. The cover layer of the disk has a thickness of 0.6 mm and a refractive index of n = 1.580.

Following the design of the first embodiment described above, the ratio of OPDrms of the fifth and third coma aberrations was found to be as follows for the oblique beam incidence:

At this time, for the tilt of 1 degree, the amount of the third coma aberration was found to be 75.8 mλ. Note that at this time (5) is much closer to (3), which indicates that higher order contributions almost compensate each other. Taking 15 mλ as the limit, the permissible disc tilt that can be compensated by tilting the objective lens is 0.55 degrees, while the field of view tolerance is given as 0.34 degrees. Thus, the allowed disk tilt is improved by 40%.

Example 2

The objective lens 18 shown in FIG. 1 in the second embodiment of the present invention is a double-aspherical member. The objective lens 18 has a numerical aperture of 0.65 and an incident pupil diameter of 3.58 mm, and operates at a wavelength λ = 660 nm. The objective lens 18 has a thickness of 2.614 mm along the optical axis. The lens body of the objective lens is made of COC plastic with refractive index n = 1.530. The front and back surfaces each have a rotationally symmetric aspherical form given by:

Where z is the position of the surface in the optical axis direction in millimeters, r is the distance to the optical axis in millimeters, and B _k is the coefficient of the k power of r. The values of the coefficients for the surface (ie the face facing the disk in use) are 0.27449755, 0.010768742, -0.0014658405, 0.0026677659, -0.0021173668, 0.00094500757, -0.00021921749, 2.0032412e-005 respectively for B ₂ to B ₁₆ .

The values of the coefficients for the back side (i.e., the face facing the collimator lens in use) are -0.13747401, 0.067465541, -0.0550585, 0.04365398, -0.021116527, 0.00040009845, 0.0038992694, -0.0010731055 for B ₂ to B ₁₆ , respectively.

The free working distance (distance between the objective lens 18 and the disk) is 1.000 mm. The cover layer of the disk has a thickness of 0.6 mm and a refractive index of n = 1.580.

When tilting the disc only, the OPDrms of the 5th and 3rd coma were found to be:

At this time, for the tilt of 1 degree, the amount of the third coma aberration was found to be 133 mλ. The ratio of OPDrms of the fifth and third coma aberrations to the oblique beam incidence in the objective lens was found to be:

At this time, for the field of 1 degree, the amount of the third coma aberration was found to be 29.53 mλ. Taking 15 mλ as the limit, the disc tilt allowed is 1.0 degrees while the field of view tolerance in a nominal arrangement is given as 0.9 degrees. According to this, the allowable disk tilt is improved by 256% compared with the conventional first designed lens described in the first embodiment.

Example 3

Increasingly, the overall dimensions of the optical system are decreasing. This requires precise assembly constraints with increasing recording density, especially with respect to the orientation of the objective lens. It is very difficult to orient the small objective lens correctly during assembly.

One way to avoid the problem that the objective lens is incorrectly oriented (ie tilted) during assembly is to allow for misalignment of the objective lens and to compensate for this misalignment by tilting the disc. This compensation for tilting the disk can be done during assembly of the optical pickup (ie, tilting the disk motor during assembly so that the orientation of the motor determines the orientation of the disk).

The objective lens 18 shown in FIG. 1 according to the third embodiment design consists of two members. Each member is flat-aspherical. The objective lens 18 has a numerical aperture of 0.85 and an incident pupil diameter of 1.0 mm, and operates at a wavelength λ = 405 nm. The two members are made of a cyclic olefin copolymer (COC) with a refractive index n = 1.550. The first member of the objective lens 18 facing the laser has a thickness of 0.40 mm along the optical axis, while the second member facing the disk has a thickness of 0.527 mm. The distance on these two members on the optical axis is 0.075 mm. The rotationally symmetric aspheric shape of the first surface of the first member is given by the following equation:

Where z is the position of the surface in the optical axis direction in millimeters, r is the distance to the optical axis in millimeters, and B _k is the coefficient of the k power of r. The values of the coefficients B ₂ through B ₁₆ are 0.52915187, -2.6064464, 59.745801, -728.17842, 5097.5432, -20502.8, 44013.121 and -39077.168, respectively.

The rotationally symmetric aspheric shape of the first surface of the second member is given by the following equation:

Where z is the position of the surface in the optical axis direction in millimeters, r is the distance to the optical axis in millimeters, c is the curvature of the surface, r ₀ is the normalized radius, and D _k is _k in r Coefficient of power. The value of c is 2.506 mm ⁻¹ , while r ₀ is 0.5 mm. The values of the coefficients D ₂ through D ₁₆ are 0.3376384, -2.484319, 8.0824648, -7.2616424, -24.092634, 64.606397 and -46.405224, respectively. The free working distance, that is, the distance between the objective lens 18 and the disk, is 0.075 mm. The cover layer of the disk has a thickness of 0.1 mm and a refractive index of n = 1.622.

When tilting only the disk in such an objective system, the OPDrms of the fifth and third coma aberrations were found to be as follows:

At this time, for the tilt of 1 degree, the amount of the third coma aberration was found to be 108.5 mλ.

Following the third embodiment design approach described above, the ratio of OPDrms of the fifth and third coma aberrations to the oblique beam incidence in the objective lens was found to be:

At this time, for the field of 1 degree, the amount of the third coma aberration was found to be 66.1 mλ. Due to the significant difference between (8) and (9), when the objective lens has a tilt misalignment, it is possible to appropriately cancel higher order coma aberration by tilting the disc (measured by condition (1)). Taking 15mλ as the aberration limit, the permissible disc tilt misalignment of the objective lens is 0.45 degrees while the field of view tolerance is given as 0.4 degrees.

Similarly, the objective lens 18 shown in Fig. 1 of the fourth embodiment of the present invention is composed of two members. Each member is flat-aspherical. The objective lens 18 has a numerical aperture of 0.85 and an incident pupil diameter of 1.0 mm, and operates at a wavelength λ = 405 nm. Two members are made of COC with refractive index n = 1.550. The first member of the objective lens 18 facing the laser has a thickness of 0.40 mm along the optical axis, while the second member facing the disk has a thickness of 0.444 mm. The distance on these two members on the optical axis is 0.075 mm. The rotationally symmetric aspheric shape of the first surface of the first member is given by the following equation:

Where z is the position of the surface in the optical axis direction in millimeters, r is the distance to the optical axis in millimeters, and B _k is the coefficient of the k power of r. The values of the coefficients B ₂ through B ₁₆ are 0.67904917, -0.7382746, 22.022311, -296.67542, 2236.389, -9564.7477, 21643.036 and -20152.291, respectively. The rotationally symmetric aspheric shape of the first surface of the second member is given by the following equation:

Where z is the position of the surface in the optical axis direction in millimeters, r is the distance to the optical axis in millimeters, c is the curvature of the surface, r ₀ is the normalized radius, and D _k is _k in r Coefficient of power. The value of c is 2.480 mm ⁻¹ , while r ₀ is 0.5 mm. The values of the coefficients D ₂ through D ₁₆ are 0.20750918, -2.667102, 16.934987, -61.236377, 124.45225, -133.28039 and 57.151524, respectively. The free working distance, (distance between the objective lens 18 and the disk) is 0.075 mm. The cover layer of the disk has a thickness of 0.1 mm and a refractive index of n = 1.622.

Following the design of the fourth embodiment described above, the ratio of OPDrms of the fifth and third coma aberrations to the oblique beam incidence was found to be:

At this time, for the field of 1 degree, the amount of third coma aberration was found to be 142.7 mλ. Note that (10) is much closer to (8), which indicates that higher order contributions almost compensate each other. Taking 15 mλ as the limit, the permissible tilt misalignment of the objective lens is 0.8 degrees while the field of view tolerance is given as 0.2 degrees. Thus, the allowed tilt misalignment is improved by almost twice.

At this time, it is apparent that various embodiments of the present invention may be used in connection with various lens systems. Preferably, these embodiments are used in connection with lens systems having a numerical aperture greater than 0.7. Preferably, the lens systems according to the embodiments have an incident pupil diameter smaller than 2 mm, more preferably smaller than 1.5 mm. Preferably, these embodiments are used with a radiation beam having a wavelength less than 600 nm, including a beam having a wavelength of approximately 405 nm.

In light of the foregoing embodiments, embodiments of the present invention improve the assembly of optics, such as small optical drives, or disc tilt when using a tilt actuator (eg, as in a rewritable DVD drive). It can be seen that it can be used to allow compensation.

Claims (11)

A radiation source 11 that scans the information layer 4 of the optical record carrier 2 and generates radiation beams 12, 15, 17, 20, and an objective 18 that focuses the radiation beam on the information layer. In the optical scanning device provided with The ratio of the root mean square of the optical path differences (OPDs) generated by the objective 18 upon inclination of the beam into the objective meets the following conditions within the field of the objective 18: At this time, OPD (A31) is the contribution value of the third Zernike Koma aberration to the square root mean square wavefront aberration, OPD (A51) is a contribution value of the fifth order Zernike Koma aberration to the root mean square wavefront aberration. An optical scanning device (1) characterized in that. The method of claim 1, Optical scanning device (1) characterized in that the values of OPD (A31) and OPD (A51) satisfy the following conditions: The method of claim 1, An optical scanning device (1) characterized by further comprising detection systems (25, 27) for converting radiation beams entering the information layer into information signals, and an information processor (29) for error correction of the information signals. The method of claim 1, An optical scanning device (1), further comprising an actuator portion configured to tilt at least one of the objective system (18) and the optical recording medium (2). In a lens system having at least one lens for focusing a radiation beam, The ratio of the root mean square of the optical path differences (OPDs) generated by the lens system upon incidence of the oblique beam into the lens system satisfies the following conditions within the field of the lens system: In this case, OPD (A31) is the contribution value of the third-order Zernike coma aberration to the root mean square wavefront aberration, and OPD (A51) is the contribution value of the fifth-order Zernike coma aberration to the root mean square wavefront aberration. Lens system 18, characterized in that. The method of claim 5, The lens system (18), characterized in that the lens system is an objective system. The method of claim 5, A lens system (18) comprising at least first and second lenses. In the method of manufacturing a lens system having at least one lens for focusing the radiation beam 17, Forming the lens system such that the ratio of the root mean square of the optical path differences (OPDs) generated by the lens system upon incidence of the oblique beam into the lens system satisfies the following conditions within the field of the lens system: In this case, OPD (A31) is the contribution value of the third-order Zernike coma aberration to the root mean square wavefront aberration, and OPD (A51) is the contribution value of the fifth-order Zernike coma aberration to the root mean square wavefront aberration. Method for producing a lens system (18), characterized in that. The method of claim 8, The method of claim 1, further comprising the step of designing the lens system (18) to satisfy the above conditions. An optical scanning device for scanning the information layer 4 of the optical record carrier 1, Installing a radiation source 11 for generating a radiation beam; In the manufacturing method of an optical scanning device comprising the step of providing a lens system 18 for condensing a radiation beam on the information layer (4), The ratio of the root mean square of the optical path differences (OPDs) generated by the lens system upon incidence of the oblique beam into the lens system satisfies the following conditions within the field of the lens system: In this case, OPD (A31) is the contribution value of the third-order Zernike coma aberration to the root mean square wavefront aberration, and OPD (A51) is the contribution value of the fifth-order Zernike coma aberration to the root mean square wavefront aberration. Method for producing an optical scanning device (1) characterized in that. The method of claim 10, And installing a device for holding the recording medium 2 at a predetermined angle relative to the lens system 18, wherein the predetermined angle is such that coma aberration generated inside the beam due to the recording medium passes through the lens system. A method of manufacturing an optical scanning device, characterized in that it is set to at least partially compensate for coma aberration occurring inside the beam.