KR20040071703A - Optical scanning device - Google Patents

Abstract

An optical scanning device for scanning a double layer optical record carrier is provided. The apparatus has a spherical aberration compensation optical subsystem comprising a switchable liquid crystal cell 10 that changes the optical path lengths generated in the aperiodic phase structure (NPS) member 14 made of birefringent material.

Description

Optical scanning device {OPTICAL SCANNING DEVICE}

The present invention provides an optical scanning device that scans an optical recording medium such as an optical disk, and includes an radiation source for generating a radiation beam, and an objective lens disposed in an optical path between the radiation source and the information layer to focus the radiation beam into a spot on the information layer. A scanning device and an optical member used in the optical scanning device. In particular, the present invention compensates for spherical aberrations caused by different optical path lengths (herein referred to as information layer depths) passing as the beam travels inside the optical disc to reach the information layer on the disc. The present invention relates to an optical scanning device having an optical member, but is not limited thereto.

There is an increasing need for the manufacture of optical record carriers with high capacity. Thus, for example, a relatively short wavelength radiation beam, such as a 400 nm radiation beam, a high aperture (NA) objective lens system of at least 0.7, for example NA = 0.85, and a thin protective cover layer, for example 80 μm thick, are used. An optical scanning device is preferable. Moreover, capacity can be increased by providing a double layer disk. At the aforementioned wavelengths and NA, layer separation of at least 20-30 μm is desirable to reduce coherent crosstalk to an acceptable level. Without compensatory measures, refocusing from one layer to another causes spherical aberration, resulting in a wavefront error of 200-300 mλ (rms), which degrades the resolution of the formed light spot.

In order to provide spherical aberration compensation, it is known to mechanically adjust the spacing of two or more lens members of a composite objective lens. Another compensation method is to mechanically adjust the position of the collimating lens relative to the radiation source such that the radiation beam strikes the objective lens as a converging or diverging beam instead of the beam converted into parallel light. Each of these methods compensates for the spherical aberration generated in the optical system of the scanning apparatus, thereby at least approximately canceling the spherical aberration generated in the scanning optical disk.

However, it is relatively complicated to use mechanical actuators to provide spherical aberration compensation, especially when separate mechanical actuators are used to provide focus control, thus increasing the manufacturing cost of the injection device.

Another conventional optical scanning device is described in WO-A-124174, where a radiation beam passes through a twisted nematic (TN) liquid crystal cell that selectively rotates the polarization of incident light by 90 °. The beam then passes through the birefringent plate in the converged state, producing spherical aberration in the birefringent plate. The birefringent plate generates different amounts of spherical aberration according to the state of the TN cell, thereby compensating for different information layer thicknesses.

EP-A-08605037 A1 and in R. Katayama, Applied Optics volume 38 (1999) pp 3778-3786, are also suitable for scanning CD recording media and are used to produce objective lenses designed to scan DVD recording media. The structure is described. In general, DVD record carriers are designed to be scanned using radiation beams having wavelengths and aperture ratios different from those used to scan older generations of record carriers such as CDs. This phase structure is composed of stepped acyclic annular zones, so that each zone produces a phase step that is a multiple of 2π with respect to the DVD wavelength (660 nm), so that the phase structure has no effect at this wavelength. However, for CD readout, a different wavelength is used (785 nm). As a result, the stepped phase profile in this case produces phase steps that are no longer multiples of 2π. By properly designing the step heights and zone widths, the phase introduced by the phase structure in the case of CD reduces the wave front aberration caused by the thickness difference of the disc to below the diffraction limit. This structure can reduce wavefront aberration for two separate wavelengths.

JP-A-2001-174614 discloses a diffractive element for an optical head for an optical scanning device that can operate at two different wavelengths. This diffractive element includes a grating made of a birefringent material inserted into a material having a uniform refractive index. The birefringent material is arranged in a periodic structure, ie in a structure that is regularly repeated across the device. For one wavelength, the device transmits light without diffraction at one polarization and diffracts light at vertical polarizations. For the second wavelength, light is transmitted without diffraction at both polarizations. One problem is that due to the large number of elements inside the structure, it is relatively complicated to manufacture a periodic phase structure, ie a grating. Moreover, due to diffraction, a certain amount of incident light is wasted, which is undesirable.

Lee et al's paper on Technical Digest of the International Symposium on Optical Memory 2001, Taiwan, pp 308-309 describes an optical disk system with a so-called polarization phase compensator (PPC) for HD / DVD compatibility. It is. This PPC includes a birefringent material sandwiched between glass substrates and does not affect the first beam with a wavelength of 405 nm and the first polarization, but with a second beam with a wavelength of 650 nm and a second vertical polarization. Annular phase structures that achieve spherical aberration compensation. However, no method of integrating the PPC inside the optical scanning device is described, and probably two different radiation sources are used to provide beams of different wavelengths.

After all, it is an object of the present invention to provide a non-mechanical wavefront compensation system for an optical scanning device.

According to an aspect of the present invention, an object for scanning an optical recording medium including an information layer and disposed in a light source generating a radiation beam and a light path between the radiation source and the information layer to focus the radiation beam on a spot on the information layer. A first state in which the lens and the polarization of light out of the electro-optical device have a first direction with respect to the predetermined polarization of light incident on the electro-optical device, and the polarization of light out of the electro-optical device An electrical switchable between a second state having a second and a different state, thereby providing a first radiation beam having primarily a first polarization and a second radiation beam having primarily a second vertical polarization, depending on the state of the electro-optical device An optical element, the portion providing wavefront aberration compensation in relation to the first radiation beam and the second radiation beam, the portion having a surface formed of a polarization sensing material and disposed in the path of the first and second radiation beams; And a phase structure in which the surface has a shape of stepped annular zones, the zones forming an aperiodic pattern of light paths having different lengths, and the optical path to the first radiation beam. And optical paths for the second and second radiation beams are provided so that the difference in the wavefront aberration compensation of the first and second beams is respectively provided.

Wavefront aberration compensation may be provided for information layers located at various depths within or between optical discs, even with relatively high aperture ratio devices, without the need for mechanical systems to provide such wavefront aberration compensation. Also, no more than one radiation source is needed.

Although a similar effect may be achieved by using a birefringent lens instead of a stepped structure, it is to be noted that the structure of the present invention has the advantage that the astigmatism can be reduced, which means that the birefringent lens is characterized in that This is because astigmatism wavefront aberration generally occurs because it does not move parallel to the optical axis.

In addition, the phase structure according to the present invention has an aperiodic pattern, that is, a pattern that does not repeat regularly in the radial direction, and thus does not constitute a diffraction order. As a result, the phase structure does not have the intrinsic loss of the grating. The optical member introduces the necessary wavefront change without significant loss of radiant energy.

According to yet another aspect of the present invention, an optical recording medium including an information layer is scanned, the radiation source generating a radiation beam, and disposed in an optical path between the radiation source and the information layer, condensing the radiation beam to a spot on the information layer. An objective lens mounted inside the mechanical drive means for performing servo-based calibration of the position of the objective lens during scanning of the optical record carrier, and a first radiation beam and a second vertical beam comprising a first polarization radiation beam. Providing optical wavefront aberration compensation in relation to a second radiation beam comprising a radiation beam of polarization, the optical member including a portion formed of a polarization sensing material and having a surface disposed in the path of the first and second radiation beams Wherein the surface comprises a phase structure in the form of stepped annular zones, the zones forming an aperiodic pattern of light paths having different lengths, for the first radiation beam The difference between the light paths and the light paths for the second radiation beam provides a difference in the wave front aberration compensation of the first and second beams, respectively, wherein the optical member is fixed to the objective lens inside the mechanical drive means. There is provided an optical scanning device mounted with an objective lens.

This integrated mounting provides centering of the optical member associated with the objective lens during scanning without depending on the driving state of the objective lens.

Preferably, an optical member is provided in which the surface comprising the phase structure is in contact with air. This garret configuration allows the weight of the optical member to be reduced, helping the operation of mechanical drive means generally operating at high frequencies.

According to another aspect of the invention, a wavefront aberration compensation is provided in relation to a first radiation beam comprising a radiation beam of a first polarization and a second radiation beam comprising a radiation beam of a second vertical polarization, and detecting polarization A portion having a surface formed of a material and having a surface disposed in the path of the first and second radiation beams, the surface comprising a phase structure in the form of stepped annular zones, the zones having different lengths Forming a non-periodic pattern of light paths, the difference between the light paths for the first radiation beam and the light paths for the second radiation beam provides a difference in wavefront aberration compensation of the first and second beams, respectively, An optical member in contact with air is provided.

As described above, by providing the air interface, the weight of the optical member can be reduced. Moreover, the step heights of the annular zones can generally be reduced compared to the step height required when the surface is in contact with the coating material.

According to another aspect of the invention, a wavefront aberration compensation is provided in relation to a first radiation beam comprising a radiation beam of a first polarization and a second radiation beam comprising a radiation beam of a second vertical polarization, and detecting polarization A surface structure formed of a material and disposed in the paths of the first and second radiation beams, the surface comprising a phase structure in the form of stepped annular zones, the zones having a different length of light paths Forming a periodic pattern, wherein the difference between the light paths for the first radiation beam and the light paths for the second radiation beam provides a difference in wavefront aberration compensation of the first and second beams, respectively, and a non-polarization sensing material. Wherein the non-polarization sensing portion is in contact with the polarization sensing portion along the surface, and the polarization sensing material exhibits a first index of refraction for the radiation beam of the first polarization and of the second polarization An optical member is provided that exhibits a second index of refraction for the radiation beam and is selected such that the index of refraction of the non-polarization sensing material matches the second index of refraction.

This corresponds to the special case of the embodiment where the surface comprising the phase structure is in contact with the coating material. By matching the refractive indices, the required step heights can be significantly reduced.

Features and advantages of various embodiments of the present invention will become apparent from the following detailed description of the preferred embodiments of the present invention, with reference to the accompanying drawings in which:

1 is a schematic illustration of an optical scanning device disposed in accordance with embodiments of the present invention,

FIG. 2 is a schematic illustration of optical components used in the arrangement of FIG. 1,

3 is a cross-sectional view of the phase adjusting member according to the first embodiment of the present invention;

Figure 4 shows the wave front aberration before and after compensation,

5 is a cross-sectional view of a phase adjusting member according to a second embodiment of the present invention;

6 is a cross-sectional view of the phase adjusting member according to the third embodiment of the present invention;

7 is a cross-sectional view of the phase adjusting member according to the fourth embodiment of the present invention.

FIG. 1 schematically shows components common to an apparatus for scanning an optical record carrier according to respective embodiments of the present invention described below for example. The recording medium is, for example, an optical disc as described below for example.

The optical disc OD includes a substrate 1 and a transparent layer 2, at least one information layer 3 being disposed therebetween. In the case of a dual-layer optical disc, as shown, two information layers 3 and 4 are spaced apart by the transparent layer 2 at different depths inside the disc at a distance of 20 μm ± 10 μm. Another transparent layer 5 separates the two information layers. The transparent layer 2 having a thickness of about 80 μm (± 30 μm) has a function of protecting the uppermost information layer 3, while mechanical support is provided by the substrate 1.

The information may be stored in the information layers 3, 4 of the optical disc in the form of optically detectable marks arranged in nearly parallel, concentric or helical tracks not shown in FIG. 1. The marks may have any optically readable form, such as, for example, in the form of pits, regions having a reflection coefficient or magnetization direction different from the periphery, or a combination of these forms.

The scanning device includes an optical pickup device (OPU) mounted on an arm that can move in a radial direction. The OPU 1 includes all the components shown in FIG. 1 other than the disk OD. Radiation source 6, for example one semiconductor laser, emits a diverging radiation beam 7 having a wavelength of 400 nm (± 10 nm). The beam splitter 8, in the present embodiment, reflects the radiation beam inside the lens system. The lens system includes a collimating lens 9, an objective lens 12, and a condenser lens 11. The objective lens 12 is firmly mounted to a movable mounting 13 supported inside a mechanical actuator (not shown) to perform radial tracking servo and focus servo adjustment of the position of the objective lens 12. In addition, the apparatus further comprises a twisted nematic (TN) liquid crystal cell 10 and a phase adjuster 14 as described in detail later. The regulator 14 is firmly mounted to the mounting 13.

The collimating lens 9 refracts the radiating radiation beam 7 to form a beam 15 converted to parallel light. Converting into parallel light means a beam that is almost parallel, and for this beam, the composite objective lens has a lateral magnification of almost zero value. If the birefringent phase adjuster 14 and other optical elements in the beam path converted to parallel light are designed to be used for a beam that is ideally converted (parallel) to parallel light as in this embodiment, There is a need for a beam. However, if the components in the beam path are designed to be used for diverging or converging beams, no beam converted to parallel light is needed. Even using components designed for use in beams ideally converted to parallel light, depending on the required efficiency of the optics, certain tolerances associated with the beam's verbosity may be tolerated. In order to obtain the efficiency required for the present optical system, it is preferable that the beam converted into parallel light has a congestion causing an absolute magnification of the objective lens smaller than 0.02.

The objective lens 12 converts the radiation beam 15 converted into parallel light into a converging beam 16 having a high aperture ratio NA of 0.85 in this embodiment, which is being scanned information layer 3 or 4. It becomes the spot 18 of an image.

The radiation beam of the converging beam 16 reflected by the information layer 3 or 4 forms a diverging reflection beam 20, which is returned along the optical path of the forward converging beam. The objective lens 12 converts the reflected beam 20 into a reflected beam 21 converted into nearly parallel light, and the beam splitter 8 transmits at least a portion of the reflected beam 21 toward the condenser lens 11. This separates the front beam and the reflected beam.

The condenser lens 11 converts the incident beam into a converging reflected beam 22 which is focused on a detection system roughly represented by one element 23, although a plurality of detector elements are used. The detector captures the radiation beam and converts it into an electrical signal. One of these signals is an information signal 24, whose value indicates information read from the information layer being scanned. Another signal is a focus error signal, which represents the axial height difference between the spot 18 and each of the information layers 3 and 4 being scanned. Another signal is a tracking error signal, which indicates the radial deviation of the spot from the track being scanned. Respective signals 25 and 26 are input to the focus servo and tracking servo mechanical actuators to control the position of the mounting 13 during scanning.

Another signal input to the TN cell 10 is a spherical aberration control signal 30. The spherical aberration control signal 30 represents the selected information layer 3 or 4 inside the optical disc currently being scanned.

2A and 2B schematically illustrate one embodiment of the present invention that includes components that form part of a spherical aberration compensation optical subsystem. The TN cell 10 is a flat cell composed of a liquid crystal layer interposed between two transparent flat plates constituting the electrodes of the TN liquid crystal cell 10 and having a conductive transparent layer formed on the inner surface of the flat plate. As is known in the TN liquid crystal cell art, in addition to the electrode layers, the surfaces of the electrodes adjacent to the liquid crystal layer are covered with an alignment material. The material on one side of the liquid crystal cell 10 aligns the liquid crystal molecules in an orientation perpendicular to the alignment direction when the material on the other side of the liquid crystal cell aligns the liquid crystal molecules. Therefore, when the cell 10 is in the off state, a 90 degree twist is formed in the bulk of the liquid crystal layer between the two surfaces of the liquid crystal cell. The liquid crystal cell 10 is connected to a voltage generation source controlled by the spherical aberration control signal 30. When the power is turned on, the voltage generator turns the liquid crystal cell 10 on, so that the liquid crystal molecules are aligned almost parallel to the optical axis of the objective lens 12. In the off state of the liquid crystal cell 10, the polarization of the incident radiation beam rotates by 90 ° as it passes through the liquid crystal cell 10. In contrast, in the on state, the liquid crystal cell 10 has no influence on the polarization of the radiation beam passing through the cell 10.

The liquid crystal layer in the TN liquid crystal cell 10 is relatively thin, usually 4-6 탆. Thus, the response speed of the spherical aberration compensation optical subsystem is fast, allowing the cell to switch between on and off states within 10-50 ms.

Another component of the spherical aberration compensation optical subsystem is the passive phase adjuster 14A. Phase adjuster 14A consists of a linear birefringent material such as a cured liquid crystal compound in which its molecules are aligned along the optical axis. The spherical aberration compensation wavefront aberration generated in the phase adjuster 14A depending on the optical path length of the birefringent material can be changed between two distinct states by switching the TN liquid crystal cell 10. The refractive index of the ideal modulator 14A changes by Δn = n ₀ -n _e in accordance with the polarization of the incident radiation beam. The refractive index of the birefringent phase adjuster is n ₀ when the polarization of the incident radiation beam is perpendicular to its optical axis, while the refractive index is n _e when the polarization of the incident radiation beam is parallel to its optical axis.

By making a 90 [deg.] Rotation in the polarization between the reflected beam and the incident beam in the polarizing beam splitter 8, in combination with the polarizing beam splitter 8 to improve the light efficiency of the device, a quarter wavelength retarder plate A polarization rotating member 14B, such as), is placed between the birefringent phase adjuster 14A and the optical disk OD.

Hereinafter, referring to FIG. 2A showing the TN liquid crystal cell 10 in the off state, the incident beam indicated by reference numeral 104 generated by the radiation source 6 first passes through a polarizing beam splitter 8 having P-type polarization. do. When passing through the TN liquid crystal cell 10, the polarization of the incident beam rotates to the S-type polarization. The beam passes through the birefringent phase adjuster 14A. In this case, since the birefringent phase adjuster 14A is disposed in the P direction, the birefringent phase adjuster 14A exhibits a refractive index of n ₀ . Then, upon passing through the quarter wave plate 14B, the polarization of the incident beam is transformed into preferential circularly polarized light, and the incident beam is reflected in the scanning information layer 3 or 4 inside the optical disk 1. Thereby, the polarization of the reflected beam is transformed into left linear circular polarization, which is transformed into P-type polarization upon passing through the quarter wave plate 14B.

When passing through the birefringent phase adjuster 14A, the reflected beam undergoes a refractive index of n _e , and when passing through the TN liquid crystal cell 10 in the off state, the P-type polarization effect rotates 90 degrees of the TN liquid crystal cell 10. Is transformed into S-type polarized light. The polarization beam splitter 8 reflects most of the reflected beams in the S-type polarization state toward the detector 23 with the beam indicated by reference numeral 106.

Hereinafter, referring to FIG. 2B, the contents described with reference to FIG. 2A are applied, but in this case, the TN liquid crystal cell 10 is turned on by the spherical aberration control signal 30. Therefore, the polarization of the radiation beam incident on the TN liquid crystal cell 10 is not affected by the passage of the cell 10. Accordingly, when passing through the birefringent phase adjuster 14A, the beam remains in the P-type polarization state and undergoes a refractive index of n _e , which is different from the wavefront aberration pattern generated in the off state of the TN liquid crystal cell described with reference to FIG. 2A. Generate a wavefront aberration pattern. Thus, when the radiation beam passes through the birefringent phase adjuster 14A, the beam is in an S-type polarization state and undergoes a refractive index of n ₀ , thus similarly the wavefront aberration pattern generated when the TN liquid crystal cell 10 is in the off state. And different wavefront aberration patterns. When the reflected beam encounters the polarizing beam splitter 8, the reflected beam is in S-type polarized light, and the polarizing beam splitter 8 reflects most of the beam toward the detector 23.

Thus, the switching of the TN liquid crystal cell 10 creates a difference in the shape of the wavefront of the beam incident on the optical disk. The other wavefront aberration generated in the birefringent phase adjuster 14A is used to compensate for the spherical aberration generated by the passage of the beam through the transparent layers 2 and 5. Accordingly, in this embodiment, although two different depths of the information layer in the dual-layer optical disk need to be read by the scanning device, improved resolution can be obtained in each of the information layers 3 and 4 of the disk. .

FIG. 3 shows a non-periodic phase structure consisting of stepped annular zones of the phase adjuster 114A used in the apparatus of FIG. 1 scanning each of the two information layers 3, 4 of a double layer disc at approximately 400 nm. Periodic phase structure (NPS) shows an example of the surface. The phase adjuster is formed of a physically uniform birefringent material in which at least one surface of the NPS is in contact with air. The opposite side may be in contact with air or may be formed on a flat substrate such as a glass plate. The disk has a transparent cover layer 2 of 80 mu m and a layer separation of 20 mu m. The incident pupil diameter of the lens 12 is 3 mm (that is, the radial distance from the optical axis OA varies from 0 to 1.5 mm), and NA = 0.85.

Consider a case where the spot is focused on an information layer 3 having a thickness of 80 mu m, while the lens is optimized for an information layer of 100 mu m. The smaller information layer depth gives the optical path difference W (ρ) shown in FIG. 4A given by

W (ρ) = λf (ρ) (1)

At this time,

And ρ is relative pupil coordinates. Note that, in the lens design, the defocus constant (0.17138) is selected so that F (1) = 0.2πf (ρ) is the wavefront phase introduced by the variation of the information layer depth. The root mean square of the optical path difference of the wave front aberration is 194 mλ.

To compensate for spherical aberration generated in the disk, an NPS is used with zones that produce wavefront deviations that approximate spherical aberration. These zones are annular zones of a plate arranged concentrically with the normal to the optical axis of the member. In the same part of each zone, for example outer periphery, center or inner periphery, the wavefront deviation generated (for one polarization state of the radiation beam) is closest to the compensated phase aberration, while larger (but not Aberrations (which are still reduced) will remain in different parts of each zone. Moreover, for different polarization states, the step height is selected such that a phase having a multiple of 2 [pi] is generated by each of the zones. In the present embodiment, for example, n ₀ = 1.5 and If you take

h _e = λ / (n _e -1) = 0.72 μm (3)

The step height increment of is used. As a result, a step with a height mh _e (m is an integer) introduces the following phases for a typical luminous flux:

The number of zones is selected based on two competing criteria, the first criterion is to increase the number of zones to provide the desired amount of wavefront aberration compensation, and the second criterion is to increase the manufacturing efficiency of the device. Will reduce the number of people. The number of these zones is preferably selected from 5 to 25 zones, more preferably 10 to 20 zones. In this embodiment, the number of zones is chosen to be 3, each zone as shown in Table 1 It has a radial size.

area Relative pupil coordinates (ρ) m Φ (2π) Mod 1 12345678910111213 0.31120.39660.47110.54240.61840.75250.76600.86660.90490.93520.95460.97351.0000 109876545678910 0.00.10.20.30.40.50.60.50.40.30.20.10.0

Table 1 lists the relative pupil coordinates of the outermost points r ₁ to r _{13 of} each of the 13 zones of the NPS, along with the number m of step height increments used in each zone and the corresponding relative phase. . In the present embodiment, the number m of step height increments used for each zone is such that the zone that generates the largest phase deviation has the smallest thickness and the zone that has generated the smallest phase shift has the largest thickness. Is selected. By this choice, the variation in the thickness of the zones of the NPS is then configured to be less than half the value of the number of zones multiplied by the step height increment, in this case only 4.3 μm.

4B shows compensated wavefront aberrations using the structure described above. The wavefront aberration was reduced from 194 mλ (rms) to 37 mλ (rms), which is smaller than the diffraction limit at the operating wavelength, resulting in diffraction limited spot size.

Fig. 5 shows another embodiment of the present invention in which the birefringent phase adjusting member 214A is integrated with the quarter wave retardation plate 214B, and the two members are bonded to each other at a flat interface. This phase adjuster 214A includes an aperiodic phase structure 200 similar to that shown in FIG.

FIG. 6 shows another embodiment of the invention in which wavefront aberration occurs using a phase adjuster integrated with a portion of birefringent material 314A integrated with a non-polarization sensing material, i.e., a portion having only one refractive index. will be. Surface 300 of the aperiodic phase structure forms an interface between portions 314A and 302. In this case, the zones have the radial size given in Table 1, but the step heights increase in that the step height increment is defined as:

h _e = λ / (n _e -n _l ) (5)

In this case, n ₁ is the refractive index of the non-polarization sensing material, and in the present embodiment, h _e = 7.19 μm, n ₁ = 1.45, n _o = 1.5 and n _e = 1.5056 for illustrative purposes.

A phase adjuster in accordance with the present invention is integrated with quarter wave retardation plate 314B, with portions 314A and 314B bonded through a planar interface.

FIG. 7 shows another embodiment of the present invention in which part 414A of the birefringent phase adjuster is integrated with part 402 made of non-polarization sensing material having a refractive index that matches the general refractive index n _o of the birefringent material of this part 414A. An example is shown. Due to the coincident index of refraction at one polarization, the interface 400 of the aperiodic phase structure surface becomes invisible to light at one polarization irrespective of the step height.

When n _l is the refractive index of the non-polarization sensing material, by matching the two refractive indices, i.e., n _l = n _e , a greater degree of freedom is provided in selecting the step height. Another criterion was applied to the structure shown in FIG. 7, that is, a criterion was applied to reduce the amount of birefringent material required in an aperiodic phase structure by keeping the step heights to a minimum. The formula used to select each section height is:

ρ _s is the relative pupil coordinates in the selected portion of the zone (eg outer periphery, center or inner periphery), q is an integer (…, -2, -1, 0, 1, 2,…) In embodiments, this value is zero to reduce the step height.

In this embodiment, the zones are arranged to have a constant step height increment, but this is not necessary. Due to the matching of the refractive indices, irregular fluctuations in the step height increment may be used. In this embodiment, the (constant) incremental value of the step height used is h _e = λ / 10 (n _e -n _o ) = 0.24 μm. Moreover, the number and radial arrangement of these zones is the same as in the previous embodiments. The number n of step height increments selected in each zone is given in Table 2 below.

area Relative pupil coordinates (ρ) n Φ (2π) 12345678910111213 0.31120.39660.47110.54240.61840.75250.76600.86660.90490.93520.95460.97351.0000 0123456543210 0.00.10.20.30.40.50.60.50.40.30.20.10.0

At this time, it should be noted that the step height increment values in this embodiment are considerably smaller than the embodiment shown in FIG. 6, and furthermore, the total step heights used are smaller than the embodiment shown in FIG. 3. The thickest zones produce the largest phase deviation.

The advantage of using a stepped annular periodic phase structure instead of a convex birefringent lens with similarly matched refractive indices (n _e = n _o ) contacted along a curved phase surface is that each zone surface perpendicular to the optical axis Due to the members arranged in the beam converted into the parallel light and the planar arrangement of, astigmatism is not caused by the member according to the invention.

In this embodiment, the quarter-wave retardation plate 414b may be integrated with the phase adjuster 414A, and these two parts are bonded through the planar interface.

6 and 7, the phase adjusters 314A and 414A may be integrated with the TN cell. In this case, the liquid crystal layer of the TN cell may be integrated. Note that it is preferable to be inserted between the planar outer surface of the non-polarization sensing portions 302 and 402 and the transparent flat cover plate.

According to the present invention, even if the depth of the information layer of the optical disk being read is changed, a radiation beam having a relatively low wavelength, for example, a wavelength of about 400 nm, is required without using a mechanical actuator or a birefringent lens to perform spherical aberration compensation. It can be seen that it is possible to read a high-capacity optical disk by using a radiation beam having the same, and using a high aperture ratio beam in the optical disk.

Although reference has been made to spherical aberration correction above, it should be noted that the present invention can be used in connection with other forms of wavefront aberration correction. For example, an NPS pattern that can correct for coma aberration wavefront aberration may be used. In such a case, the switching of the TN cell occurs, for example, in response to the detection of excessive tilt of the disk in one event of the TN cell, resulting in a different wavefront aberration of approximate coma aberration. Similarly, the NPS pattern can also be used to correct astigmatism, where different magnitudes of wavefront deviation may occur to correct astigmatism in two different states of the TN cell.

In the above embodiments, the TN liquid crystal cell was used to selectively rotate the polarization of the incident radiation beam by 90 °, but the polarization rotating member was eliminated, instead the direction of 45 ° relative to the axis of the birefringence and / or beam splitter. Similar functionality (but less efficient in terms of complexity and efficiency of the optics) can be achieved by using one radiation source that emits a radiation beam at or two separate radiation sources that emit a vertically polarized radiation beam at each polarization required. It can also provide The required spherical aberration compensation can then be selected according to the selection control signal, for example by a switchable polarization selection filter located at the detector. In the case of installing two such separate radiation sources, the power may be selectively supplied to the radiation sources according to the selection control signal.

Although an objective lens having one flat convex lens member is shown in FIG. 1, the objective lens may have more members, and other lens shapes such as convex-convex or convex-concave lenses may be employed. In addition, the objective system may include a hologram operating at the time of transmission or reflection, or may include a grating that couples the radiation beam from the waveguide for transmitting the radiation beam.

In this specification, the term “annular” is not intended to be limited to regions exhibiting full circular symmetry, and in one embodiment the annular zones of the NPS exhibit such symmetry, but in other embodiments these zones are fully circular. You can also get out of symmetry.

The above-described embodiments should be construed as illustrating the present invention. Another embodiment of the invention can be envisioned. Obviously, features related to one embodiment may be used for the remainder of these embodiments. Moreover, equivalents and variations not described above may be employed without departing from the scope of the invention as defined in the appended claims.

Claims (18)

An optical scanning device for scanning an optical record carrier comprising an information layer, A radiation source for generating a radiation beam, An objective lens disposed in an optical path between the radiation source and the information layer to focus the radiation beam on a spot on the information layer; A first state in which the polarization of the light out of the electro-optical device has a first direction with respect to the predetermined polarization of light incident on the electro-optical device, and the polarization of the light out of the electro-optical device is different from the predetermined polarization. An electro-optical element switchable between a second state having two states, providing an first radiation beam having primarily a first polarization and a second radiation beam having primarily a second vertical polarization according to the state of the electro-optic element; , An optical member comprising a portion that provides wavefront aberration compensation in relation to the first radiation beam and the second radiation beam and includes a portion formed of a polarization sensing material and having a surface disposed in the path of the first and second radiation beams and, Wherein the surface comprises a phase structure in the form of stepped annular zones, which form an aperiodic pattern of light paths having different lengths, the light paths for the first radiation beam and the second radiation beam And the difference of the optical paths for the optical paths provides a difference in the wave front aberration compensation of the first and second beams, respectively. The method of claim 1, An optical scanning device, characterized in that the difference in wavefront aberration compensation is close to spherical aberration. The method according to claim 1 or 2, And the wavefront aberration compensation is almost zero with respect to the second radiation beam when the second radiation beam is composed of a predetermined wavelength. The method according to any of the preceding claims, And an optical member disposed inside the optical scanning device to receive the first and second beams when in the converted state into parallel light. The method of claim 4, wherein An optical scanning device comprising a collimating lens, wherein an optical member is disposed between the collimating lens and the objective lens. The method according to any of the preceding claims, An optical scanning device further comprising a portion having an effect of a quarter-wave retarder. The method of claim 6, And the retarder portion is attached to the polarization sensing portion along a planar interface. The method according to any of the preceding claims, And a portion of the non-polarization sensing material, wherein the non-polarization sensing portion contacts the polarization sensing portion along the surface. The method of claim 8, The polarization sensing material exhibits a first index of refraction for the radiation beam of the first polarization and a second index of refraction for the radiation beam of the second polarization, wherein the refractive index of the non-polarization sensing material is selected to match the second index of refraction. Optical scanning device, characterized in that. The method according to any of the preceding claims, And the surface comprises 5 to 25 zones. The method according to any of the preceding claims, And a means for selecting between two information layers, said electro-optical element being responsive to said selection means, configured to scan a multilayer optical record carrier comprising two information layers. The method according to any of the preceding claims, And the beam deviating from the objective lens is configured to be incident on the recording medium having an aperture ratio greater than 0.7. In the method of operating the optical scanning device according to any one of the preceding claims, And reading the information layer of the recording medium during the scanning operation, and changing the optical characteristics of the optical scanning device during the scanning operation to compensate for wavefront aberration generated in the recording medium. In the method of operating the optical scanning device according to any one of claims 1 to 12, Recording data in the information layer of the recording medium during the scanning operation, and changing the optical characteristics of the optical scanning device during the scanning operation to compensate for the wavefront aberration generated in the recording medium. . An optical scanning device for scanning an optical record carrier comprising an information layer, A radiation source for generating a radiation beam, An object disposed in the optical path between the radiation source and the information layer, condensing the radiation beam in a spot on the information layer, mounted inside the mechanical drive means to perform servo-based calibration of the position of the objective lens during scanning of the optical record carrier Lens, Provide wavefront aberration compensation with respect to a first radiation beam comprising a radiation beam of a first polarization and a second radiation beam comprising a radiation beam of a second vertical polarization, formed of a polarization sensing material and formed of the first and second An optical member comprising a portion having a surface disposed in the path of the radiation beam, Wherein the surface comprises a phase structure in the form of stepped annular zones, which form an aperiodic pattern of light paths having different lengths, the light paths for the first radiation beam and the second radiation beam The difference in the optical paths for each provides a difference in wavefront aberration compensation of the first and second beams, And the optical member is mounted together with the objective lens to be fixed to the objective lens inside the mechanical driving means. The method of claim 15, And the surface is in contact with air. Provide wavefront aberration compensation with respect to a first radiation beam comprising a radiation beam of a first polarization and a second radiation beam comprising a radiation beam of a second vertical polarization, formed of a polarization sensing material and formed of the first and second A portion having a surface disposed in the path of the radiation beam, the surface comprising a phase structure in the form of stepped annular zones, the zones forming an aperiodic pattern of light paths having different lengths, Wherein the difference between the light paths for the first radiation beam and the light paths for the second radiation beam provides a difference in wavefront aberration compensation of the first and second beams, respectively, wherein the surface is in contact with air. absence. An optical member for providing wavefront aberration compensation with respect to a first radiation beam comprising a radiation beam of a first polarization and a second radiation beam comprising a radiation beam of a second vertical polarization. An optical path having a surface structure formed of a polarization sensing material and disposed in the paths of the first and second radiation beams, the surface having a phase structure in the form of stepped annular zones, the zones having different lengths Forming a non-periodic pattern, wherein the difference between the light paths for the first radiation beam and the light paths for the second radiation beam provides a difference in wavefront aberration compensation of the first and second beams, respectively; Having a portion formed of a non-polarization sensing material, The non-polarization sensing portion is in contact with the polarization sensing portion along the surface, Wherein the polarization sensing material exhibits a first refractive index for the radiation beam of the first polarization and a second refractive index for the radiation beam of the second polarization, wherein the refractive index of the non-polarization sensing material is selected to match the second refractive index Optical member.