KR100788699B1 - Compatible objective lens unit, the method of designing the same and compatible optical pick-up - Google Patents

Abstract

A compatible objective lens unit, a design method thereof, and a compatible optical pickup are disclosed. The disclosed compatible objective lens unit comprises a compatible objective lens unit for condensing laser light onto different types of optical discs to record / reproduce information, comprising: a hologram element; An objective lens for focusing the first laser light; And a hollow case accommodating a hologram element and an objective lens, wherein the hologram element includes a hologram element with respect to a second optical disc on which a second laser light having a center wavelength equal to or greater than the center wavelength of the first laser light is collected. And the objective lens has a pattern of a phase transfer function of zero third cubic aberration of the second laser light at an eccentric position due to a tolerance generated when the objective lens is accommodated in the hollow case, and the objective lens has a third coma aberration of the second laser light. It is characterized by having a working distance becomes zero.

According to the structure as described above, even when the objective lens unit having an NA of about 0.85 is used, optical axis misalignment is unlikely to occur and thus no optical axis adjustment at the time of assembly is required.

Description

Compatible objective lens unit, the method of designing the same and compatible optical pick-up}

1A is a schematic cross-sectional view illustrating a compatible objective lens unit according to an exemplary embodiment of the present invention.

FIG. 1B is a schematic diagram illustrating a pattern of the hologram element of FIG. 1A.

FIG. 2A is a schematic diagram illustrating an optical path of a BD optical system when light is transmitted through the compatible objective lens unit of FIG. 1A.

FIG. 2B is a schematic diagram showing an optical path of an HD DVD or a DVD or CD optical system when light is transmitted through the compatible objective lens unit of FIG. 1A.

3 is a graph showing the operating distance dependence between the hologram eccentricity of the compatible objective lens unit of FIG. 1 and the wavefront aberration generated at that time.

4 is a graph showing the aberration behavior of each aberration component with respect to the working distance when the hologram eccentricity is 50 micrometers.

FIG. 5 is a flowchart illustrating a design method of the compatible objective lens unit of FIG. 1A.

FIG. 6 is a schematic diagram illustrating a design method of the compatible objective lens unit of FIG. 1A.

7 is a graph showing the relationship between the inclination angle and the wavefront aberration of the hologram surface of the compatible objective lens unit of FIG. 1A.

FIG. 8 is a graph showing the relationship between wavefront aberration and operating distance for eccentricity from the center distance between the objective lens and the hologram element of the compatible objective lens unit of FIG. 1A.

FIG. 9 is a schematic sectional view illustrating the basic structure of a BD / DVD / CD compatible optical pickup using the compatible objective lens unit of FIG. 1A.

<Explanation of symbols for the main parts of the drawings>

5.Compatible objective lens unit 10 ... Optical axis

25 Hologram element 27 Pattern area

30.Objective lens 33.Opening limiting element

35 Case 35 Optical pickup

40 ... hologram unit for DVD 42 ... dichroic prism

44 ... Dichroic Mirror 46 ... 1/4 Wavelength Plate

50 ... hologram unit for CD 52a, 52b ... collimating lens

60 ... laser element for BD 61 ... secondary lens

62 ... front monitor 63 ... polarization prism

64 ... starting mirror 66 ... detection lens

68.Photodetector 70 Optical disc

The present invention relates to a compatible objective lens unit, a design method thereof, and a compatible optical pickup, and more particularly, to a compatible objective lens unit having a compatibility with next-generation DVDs, DVDs, and CDs, a design method thereof, and compatible optical lenses. It's about pickup.

An optical disc system using a blue-violet laser diode for recording large data of 15 GB or more on a small and portable recording medium. A Blu-ray Disc (hereinafter referred to as BD) or High Definition Digital Versatile Disc ( Trademark, hereinafter referred to as HD DVD). This system is compatible with conventional CDs (Compact Discs) or DVDs (Digital Versatile Discs), and furthermore, is compatible with BDs and HD DVDs.

For example, a laser light of about 780 nm wavelength is condensed using an objective lens having a numerical aperture of 0.45 to reproduce CD, and a wavelength of about 650 nm is used to reproduce DVD. The laser beam is focused using an objective lens having an NA of 0.6. On the other hand, in BD, laser beams having a center wavelength of about 405 nm are collected by using an objective lens having a NA of 0.85 to reduce the spot diameter formed on the optical disc, thereby reproducing an optical disc having a recording capacity of about 5.3 times that of a DVD. .

The cover layer thicknesses of the optical disc differ from each other by 1.2 mm for CD, 0.6 mm for DVD, and 0.1 mm for BD. Therefore, in order to provide compatibility with CDs and DVDs in next-generation DVD systems such as BDs, differences in the NA of the objective lens used for optical pickup, the cover layer thickness, and the wavelength of the laser diode used are obstacles. . For example, when a BD objective lens is used for recording and reproducing a CD, it is difficult to focus because of a large wave front aberration.

In contrast, Japanese Patent Application Laid-Open No. 2005-203004 discloses a compatible optical pickup that corrects wavefront aberration generated during CD or DVD playback by combining an objective lens, a plurality of hologram elements, and a quarter wave plate. . However, in the case of the disclosed compatible optical pickup, the tolerance of the mechanical tolerance of the optical system is 5 to 10 micrometers, whereas the optical axis shift occurs when the optical system is assembled unregulated, about 40 to 50 micrometers. Therefore, the adjustment process becomes important. In addition, correcting such optical axis misalignment requires a complicated adjustment technique, which causes a problem that yield is also lowered.

The present invention is to improve the problems of the conventional compatible optical pickup described above, an object of the present invention is that even if the optical axis staggered 40-50 micrometers, the deterioration of the wavefront aberration accordingly is very small, the optical axis adjustment during assembly To provide a compatible objective lens unit, a design method and a compatible optical pickup that do not require.

In order to achieve the above object, a compatible objective lens unit according to an embodiment of the present invention is a compatible objective lens unit for collecting and reproducing information by condensing laser light on different types of optical discs, ; An objective lens for focusing the first laser light on the first optical disk; And a hollow case accommodating the hologram element and the objective lens, wherein the hologram element includes a second optical disc on which a second laser light having a center wavelength equal to or greater than a center wavelength of the first laser light is collected. In contrast, the objective lens has a pattern of a phase transfer function of zero third order coma aberration of the second laser light at an eccentric position due to a tolerance generated when the hologram element and the objective lens are accommodated in the hollow case. The third laser coma aberration of the second laser light has a working distance that is zero.

According to an aspect of the present invention, there is provided a method of designing a compatible objective lens unit, comprising: an objective lens for focusing a first laser light on a first optical disk, and a center wavelength equal to or greater than the center wavelength of the first laser light; A design method of a compatible objective lens unit having a hologram element for correcting aberration generated when condensing a second laser light on a second optical disk, wherein the second laser light is used to between the objective lens and the second optical disk. Determining a first working distance of; Determining a phase transfer function of the hologram element at the first working distance; Calculating a first third coma aberration at the time of eccentricity of the hologram element at the first working distance; Determining a phase transfer function of the hologram element at a second working distance different from the first working distance; Calculating a second third coma aberration at the time of eccentricity of the hologram element at the second working distance; Determining a third working distance at which the third coma aberration becomes zero by extrapolating the first third coma aberration and the second third coma aberration; Calculating a phase transfer function of the hologram element by using the third working distance, and determining a hologram pattern.

An optical pickup according to an embodiment of the present invention is an optical pickup including a light source of a first laser light, a light source of a second laser light, and a compatible objective lens unit, wherein the compatible objective lens unit includes a hologram element; ; An objective lens for focusing the first laser light on the first optical disk; And a hollow case accommodating the hologram element and the objective lens, wherein the hologram element includes a second optical disc on which a second laser light having a center wavelength equal to or greater than a center wavelength of the first laser light is collected. In contrast, the objective lens has a pattern of a phase transfer function of zero third order coma aberration of the second laser light at an eccentric position due to a tolerance generated when the hologram element and the objective lens are accommodated in the hollow case. The third laser coma aberration of the second laser light has a working distance that is zero.

According to the embodiments, when recording and reproducing the second optical disc using an objective lens having an NA of about 0.85, the third coma aberration and the second optical disc generated by passing the second laser light through the hologram element The third-order coma aberrations generated by C are canceled with each other to obtain a compatible objective lens unit capable of suppressing wavefront aberration generated in the compatible objective lens unit, a design method thereof, and an optical pickup.

In the present invention, the first laser light is focused onto a first optical disk having a cover layer thickness of approximately 0.1 mm using an objective lens having an NA of about 0.85. Also, in order to maintain compatibility with the second optical disk including CD or DVD, the hologram element for correcting spherical aberration or chromatic aberration even when the cover layer thickness is 0.6 mm or more with respect to the second laser light is opposed to the objective lens. Secure it to the case. At this time, the optical axis shift caused by the hologram element or the case due to the general machining tolerance or the general assembly tolerance, that is, the eccentricity of the hologram element is almost equal to or slightly larger than the tolerance. Even in this eccentric position, there is a working distance that minimizes wavefront aberration. Since the working distance is near the position where the third order coma is zero, the phase transfer function of the hologram element can be designed such that the third order coma is zero.

In the design method of the present invention, after designing the phase transfer function of the hologram element at two working distances, the third order coma aberration at the time of eccentricity is calculated as each. When the relationship between these working distances and the third-order coma aberration is shown on the graph, extrapolation of the point where the third-order coma aberration becomes zero is made by connecting these two points in a straight line. This point is the working distance determined, and the pattern of the hologram element is determined by the phase transfer function calculated from this point.

As a result, the third coma aberration caused by the optical axis stagger between the objective lens and the hologram element can be made very small, thereby making it possible to form a compatible objective lens unit, and compatible optical pickup including next-generation DVD, DVD, CD, etc. And an optical drive device.

Hereinafter, a compatible objective lens unit, a design method thereof, and a compatible optical pickup according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1A is a schematic cross-sectional view illustrating a compatible objective lens unit according to an exemplary embodiment of the present invention, and FIG. 1B is a schematic diagram illustrating a pattern of the hologram element of FIG. 1A.

Referring to FIG. 1A, the compatible objective lens unit 5 of the present exemplary embodiment has a hollow case in which a hologram element 25, an objective lens 30, and the hologram element 25 and the objective lens 30 are accommodated. 35 is provided.

The hologram element 25 is formed of a glass substrate having a diffraction grating pattern region 27 toward an optical disk (not shown), that is, toward the objective lens 30. The hologram element 25 may be made of anisotropic crystals such as quartz and liquid crystal, and in this case, it becomes a polarization hologram.

The objective lens 30, for example, is designed according to the BD standard, has an NA of approximately 0.85, and condenses the first laser light for BD.

The hologram element 25 and the objective lens 30 are accommodated in the hollow case 35 so that the pattern region 27 surface of the hologram element 25 and the convex surface of the objective lens 30 face each other. As the hologram element 25 and the objective lens 30 are disposed in this manner, it is possible to prevent dust from adhering to the pattern region 27 surface or the convex surface of the objective lens 30.

The hologram element 25 serves to correct spherical aberration and chromatic aberration generated when, for example, the objective lens 30 designed in the BD standard corresponds to the HD DVD standard, the DVD standard or the CD standard. The pattern of the hologram element 25 depends on the distance between the hologram element 25 and the objective lens 30 or the working distance between the optical disc and the objective lens 30. Therefore, if the pattern is not suitable, the optical characteristics, assembly tolerances, etc., may also be different, and the optical axis may be weakened, so the optical axis 10 may need to be adjusted.

As shown in Fig. 1B, an unequal pitch curve group of concentric arcs is formed in the pattern region 27 of the hologram element 25. Figs. The hologram element 25 has a function of generating diffracted light such as primary light when light is incident on the pattern region 27. Such a pattern may be formed by, for example, making a pattern mask on the remaining portion and using dry etching by wet etching, chemical vapor deposition (CVD), or the like, or by cutting. As for the space | interval between the hologram element 25 and the objective lens 30, 0.5-1.0 mm is suitable here, for example. As the objective lens 30, for example, a single mold lens made of glass having a focal length of 1.98 mm and an aperture diameter of 3.37 mm can be used.

According to this embodiment, when the DVD or CD is recorded and reproduced using the objective lens 30 having an NA of about 0.85, the third coma aberration and the disc generated by the laser beam for the DVD or CD pass through the hologram element 25. Since the third coma aberrations generated by C are canceled with each other, the wave front aberration generated by the compatible objective lens unit can be suppressed to be small, thereby obtaining a compatible objective lens unit 5 that does not require adjustment of the optical axis 10. have.

2A and 2B are schematic diagrams showing the optical path when light is transmitted through the compatible objective lens unit 5 of the present embodiment. FIG. 2A is an optical path of the BD optical system, and FIG. 2B is an HD DVD or DVD or CD. It is the optical path of the optical system. Here, the objective lens 30 has an NA of 0.85, a center wavelength of BD and HD DVD is, for example, about 405 nm, a DVD of 650 nm, and a CD of 780 nm.

Referring to the drawings, the pattern region 27 of the hologram element 25 has an outer peripheral region 27b having a pattern different from the central region 27a and the central region 27a provided around the hologram element 25. The planar dimension of the central region 27a is designed to be smaller than the opening of the opening limiting element 33.

As shown in Fig. 2A, in the BD, a parallel light beam having a center wavelength of about 405 nm is incident on the hologram element 25 through the aperture limiting element 33, and is not diffracted as zero-order light in the hologram element 25. Without being transmitted as a parallel light beam and converged on the optical disk having a cover layer thickness of about 0.1 mm through the objective lens 30. Here, the hologram pattern formed in the hologram element 25 is designed so that the light of the wavelength of BD is transmitted as it is without diffraction in the pattern area 27 by making the depth into integer multiple of the wavelength used by BD.

On the other hand, as shown in FIG. 2B, when the parallel light flux of the wavelength used in the DVD or CD is incident on the hologram element 25 through the opening limiting element 33, the center portion of the pattern region formed in the hologram element 25 is formed. It is diffracted in the area 27a to be divergent light, and is condensed on the optical disk having a cover layer of about 0.6 mm thickness or about 1.2 mm thickness through the objective lens 30. Here, light near the optical axis is diffracted at the central region 27a of the hologram pattern 27 to become divergent light and enter the objective lens. On the other hand, the light incident on the outer circumferential region 25b is scattered in a direction different from that of the objective lens 30 and is not incident on the objective lens 30.

Furthermore, the compatible objective lens unit 5 employs a polarization hologram as the hologram element 25, and uses P-polarized light in BD and S-polarized light in HD DVD, so that the BD and the HD DVD are shown in FIGS. 2A and 2B. It is possible to have an effect as shown in the above, that is, compatibility.

Thus, according to this embodiment, it is possible to cope with one objective lens 30 with respect to various types of optical discs using laser light of different wavelengths, and is generated by eccentricity caused by general mechanical tolerance or assembly tolerance. A compatible objective lens unit 5 can be obtained which minimizes the deterioration of wavefront aberration.

By the way, the inventors have found that the tolerance of the eccentricity of the hologram element (hereinafter, referred to as hologram eccentricity) can be increased by optimizing the working distance of the objective lens. The compatible objective lens unit 5 developed on the basis of this viewpoint will be described below.

3 is a graph showing the operating distance dependence between the hologram eccentricity and the wave front aberration of the compatible objective lens unit 5 according to the present embodiment.

Here, the horizontal axis is hologram eccentricity (micrometer), and the vertical axis is wavefront aberration (λrms) when the wavelength is about 650 micrometers. The working distance WD evaluated the dependence every 0.1 mm from 0.65 to 0.71 mm. The wavelength band used is about 650 nm.

In Fig. 3, it can be seen that when the working distance is 0.68 mm, the allowable value of the hologram eccentricity is maximized and the hologram eccentricity characteristic is improved. For example, when the upper limit of the wave front aberration is 0.03 lambda rms, the allowable value of the hologram eccentricity is 50 micrometers. In addition, if the operating distance is not correctly selected, the wave front aberration is expected to be larger than 0.03 lambda rms, so that a focusing error or a tracking error cannot be detected correctly.

Here, the outer diameter tolerance of the objective lens 30 is, for example, the upper limit 0 / lower limit minus 20 micrometers, and the hologram outer diameter tolerance is, for example, both the upper limit and the lower limit are 25 micrometers, and the hollow case 35 The tolerance of the fitting portion of the objective lens 30 is, for example, an upper limit of 20 micrometers / lower limit, and the tolerance of the fitting portion of the hologram element 25 of the hollow case 35 is, for example, an upper limit. If the lower limit is 20 micrometers, then the sum of the upper and lower tolerances of each part is 65 micrometers, and in terms of rms (standard deviation), it is 35 micrometers. Therefore, if the working distance is 0.68 mm, the compatible objective lens unit 5 can have a margin (margin) of approximately 15 micrometers, so that it can be assembled with mechanical precision without adjusting the optical axis.

On the other hand, there is also a method of adjusting the optical axis 10 while measuring aberrations and spot shapes, but since the general assembly tolerance of the optical system is only about 5 to 10 micrometers, complex adjustment techniques are required to correct the optical axis 10 staggering. The problem arises that the assembly time takes time and the yield decreases. On the other hand, according to the present embodiment, even when mounted with mechanical precision, the stagger of the optical axis 10 can be suppressed within the allowable range, so that the compatible objective lens unit 5 can be assembled without adjusting the optical axis 10. And the yield can be improved.

Next, the optical behavior of each aberration component when the hologram eccentricity is set to 50 micrometers in order to investigate the relationship between the working distance and the eccentricity characteristic will be described.

4 is a graph showing the aberration behavior of each aberration component with respect to the working distance when the hologram eccentricity is generated at 50 micrometers.

Here, the horizontal axis is the working distance (mm) and the vertical axis is the aberration (λrms) when the wavelength is about 650 micrometers. Each aberration component is astigmatism AS, 3rd order coma (COMA3), 5th order coma (COMA5), and these total components (Total). There are spherical aberrations, etc. in addition to each of these aberration components, but since they are very small, they are included in the overall components without being marked as individual aberration components.

It can be seen from FIG. 4 that even if the hologram eccentricity is larger than the general mechanical tolerance, there exists an operating distance that shows a small value of the total component of the wavefront aberration. This is because the optical axis 10 caused by the optical axis stagger between the hologram element 25 and the objective lens 30 is reversed, thereby causing the third order coma aberration generated in the objective lens 30 and the disk 70 and the hologram element 25. ) And the third coma aberration caused by the optical axis stagger of the objective lens 30 is canceled out.

Next, a design method of the hologram pattern of the compatible objective lens unit 5 will be described.

5 is a flowchart showing a design method of the compatible objective lens unit 5 according to the present embodiment. 6 is a graph illustrating a relationship between the working distance and the third coma aberration.

First, the distance between the objective lens 30 and the hologram element 25 is determined in advance using the objective lens 30 having NA of 0.85. As for this distance, about 0.5-1 mm is preferable here, for example.

Then, the first working distance is determined using the laser light of the DVD or CD (step S100). Here, this distance can be 0.6 mm, for example. Then, the phase transfer function of the hologram element 25 is determined using the first working distance (step S110). For the design of this phase function, "Binary Optics2" of ZEMAX (trademark) can be used, for example. Subsequently, the first third-order coma aberration is calculated with the hologram eccentricity set to 50 micrometers, and the point A is determined as shown in FIG. 6 (S120).

Next, the phase transfer function is determined using an arbitrary second working distance different from the first working distance (step S130). The second third order coma aberration is calculated with the hologram eccentricity set to 50 micrometers, and the point B is determined as shown in Fig. 6 (step S140).

Next, by extrapolating a straight line connecting the points A and B, the third working distance at which the third coma aberration becomes zero, as shown in FIG. 6, for example, determines the point C (step S150). The hologram pattern is determined by calculating a phase transfer function at the third working distance (step S160). This makes it possible to obtain the compatible objective lens unit 5.

In the above, the design method of the compatible objective lens unit 5 according to the present embodiment has been described.

However, since the objective lens 30 side of the hologram element 25 is planar, when parallel light is incident, there is a possibility that the reflected light returns to the laser element and causes a mode hop. In order to prevent this, wavefront aberration when the hologram element 25 is tilted will be described below.

7 is a graph showing the relationship between the inclination angle and the wave front aberration of the surface of the hologram element 25 of the compatible objective lens unit 5 according to the present embodiment.

Here, the horizontal axis is the inclination angle (°) of the surface of the hologram element 25, and the vertical axis is the wavefront aberration (λrms). Moreover, the working distance was 0.68 mm, for example.

7 shows that, for example, when the surface of the hologram element 25 is inclined by 1 °, since the wave front aberration is about 0.007 lambda rms, the wave front aberration is not affected. If a hologram eccentricity of about 50 micrometers occurs with an inclination angle of 1 °, the wave front aberration is 0.031 lambda rms and can converge within the allowable range.

8 shows the wave front aberration RMS and the operating distance WD for the deviation from the center distance between the objective lens 30 and the hologram element 25 of the compatible objective lens unit 5 according to the present embodiment. Graph showing the relationship.

Here, the horizontal axis is the deviation of the distance (micrometer) between the objective lens 30 and the hologram element 25, and the left vertical axis is wavefront aberration (λrms) when the wavelength is about 405 micrometers, and the right vertical axis is the working distance. (Mm). The center distance of the objective lens 30 and the hologram element 25 is 0.78 mm, for example.

Thus, even when the distance between the objective lens 30 and the hologram element 25 (the optical axis direction) is shifted by about minus 0.1 mm from the center value, for example, the wave front aberration converges within the allowable range.

In the above, the compatible objective lens unit 5 which concerns on a present Example was demonstrated.

Next, the optical pickup 37 using the compatible objective lens unit 5 of this embodiment will be described.

9 is a schematic sectional view illustrating the basic structure of a BD / DVD / CD compatible optical pickup 37 using the compatible objective lens unit 5 of the present embodiment.

The optical pickup 37 includes a DVD hologram unit 40, a dichroic prism 42, a second dichroic mirror 44, a quarter wave plate 46, and a hologram element 25. And the objective lens 30, the aperture limiting element 33, the CD hologram unit 50, the collimating lenses 52a and 52b, the BD laser element 60, and the auxiliary lens ( 61, a front monitor 62, a polarizing prism 63, a starting mirror 64, a detection lens 66, and a photodetector 68 are provided.

First, the optical path of the optical system of the DVD will be described.

The divergent light having a wavelength of about 650 nm emitted from the DVD hologram unit 40 is incident on the dichroic prism 42 and reflected through the dichroic prism 42 to the dichroic mirror 44, It enters into the hologram element 25 through the quarter wave plate 46. The dichroic prism 42 has wavelength selectivity, and serves to transmit light having a wavelength of about 650 nm and reflect light having a wavelength of about 780 nm. The dichroic mirror 44 serves to transmit only light having a wavelength of about 405 nm. The divergent light is converted into circularly polarized light by the quarter wave plate 46. The divergent light incident on the hologram element 25 through the aperture limiting element 33 has a cover layer thickness of 0.6 mm through the objective lens 30 as shown in FIG. 2 (b). It converges to the signal recording layer (path).

In the case where optical axis staggers occur in the hologram element 25, third-order coma aberration occurs when a laser beam having a wavelength of about 650 nm passes. According to the present embodiment, an operation distance with very little generation of third-order coma aberration is generated. Deterioration of wavefront aberration due to optical axis staggering of the hologram element 25 can be suppressed by forming a pattern using the phase function calculated by

The reflected light from the optical disc 70 takes an optical path that almost returns to the royal road. That is, the reflected light passes through the objective lens 30, the hologram element 25, and the aperture limiting element 33 in order, is converted into linearly polarized light in the quarter wave plate 46, and the dichroic mirror ( 44 is reflected by the dichroic prism 42 and diverged to a different light path from the path to be received by a photodetector for a DVD, not shown, installed in the DVD hologram unit 40. The received light is converted into an electrical signal by the photodetector, and the signal information of the sub-signal and the optical disc 70 is detected (path).

Next, the optical path of the optical system of the BD will be described.

Divergence light of about 405 nm is emitted from the BD laser element 60, narrows the divergence in the auxiliary lens 61, and enters the polarizing prism 63. The polarizing prism 63 serves to separate the incident light into linearly polarized light having a longitudinal vibration (hereinafter abbreviated as S-polarized light) and linearly polarized light having lateral vibration (hereinafter abbreviated as P-polarized light). The light of the P polarization component transmitted by the polarizing prism 63 is received by the front monitor 62, the information is fed back to the control circuit (not shown) of the BD laser element 60, and the laser light emitted is predetermined. It should have strength of. The reflected light of the S-polarized component reflected by the polarizing prism 63 is converted into a substantially parallel light beam by the collimating lens 52b, and reflected by the starting mirror 64 and ¼ wavelength through the dichroic mirror 44. Incident on plate 46. The parallel light beam is converted into circularly polarized light in the quarter wave plate 46, and then enters the objective lens 30 through the aperture limiting element 33 and the hologram element 25 in order to record the signal of the optical disc 70. Converges into layers. Light incident on the hologram element 25 is incident on the optical disk 70 in the zero order without being affected by the hologram element 25 as shown in FIG. 2A.

The reflected light reflected from the optical disk 70 passes through the objective lens 30, the hologram element 25, and the aperture limiting element 33 in order, and is converted into P-polarized light by the quarter wave plate 46. And is reflected by the starting mirror 64 through the dichroic mirror 44 and is incident on the polarizing prism 63 through the collimating lens 52b. At this time, the light incident on the polarizing prism 63 in the return path is P-polarized light, so that it passes through the polarizing prism 63 and is received by the photodetector 68 through the detection lens 66. The received light is converted into an electric signal by the photodetector 68, and detected as the signal information of the servo signal and the optical disc 70 (return).

Next, the optical system of the CD will be described.

About 780 nm of divergent light is irradiated from the CD hologram unit 50, converted into collimated light beams by the collimating lens 52a, reflected by the dichroic prism 42, and then reflected by the dichroic mirror 44. Incident on the quarter wave plate 46. Light converted into circularly polarized light at the quarter wave plate 46 is incident on the hologram element 25 through the aperture limiting element 33 and is incident on the objective lens 30 using the zero-order light from the hologram element 25. And converge on the optical disc 70 (return).

The reflected light from the optical disc 70 takes a path substantially the same as that of the optical system of the DVD, and the dichroic mirror is passed through the objective lens 30, the hologram element 25, the aperture limiting element 33, and the quarter wave plate 46. After being reflected at 44, it is reflected at the dichroic prism 42 and received by the collimating lens 52a on the light receiving element of the CD hologram unit 50 (not shown).

Here, the compatible objective lens unit 5 of this embodiment is installed in, for example, a moving electromagnetic actuator not shown to move a light spot to position a target track. At this time, for example, an astigmatism method can be used to detect the focusing error, and a servo method such as a heterodyne method or a push-pull method can be used to detect the tracking error. In the present embodiment, the optical pickup of the BD compatible with the CD and the DVD is illustrated, but the optical pickup of the BD having the compatibility with the HD DVD is not limited thereto.

As described above, according to this embodiment, when the DVD or CD is recorded and reproduced using the objective lens 30 having an NA of about 0.85, the third order generated by the laser beam for the DVD or CD passes through the hologram element 25. Since the coma aberration and the third coma aberration produced by the disc are canceled with each other, the wavefront aberration generated by the compatible objective lens unit can be suppressed to a small degree, and a compatible objective lens unit 5 that does not require optical axis adjustment can be obtained. Can be.

In the above, the Example of this invention was described, referring drawings. However, the present invention is not limited to these examples. That is, even if a person skilled in the art has made various design changes regarding the configuration of an optical pickup and an optical disc recording / recording device equipped with a pattern shape, phase transfer function, working distance, material, and compatible objective lens unit of the present embodiment, the present invention It is included in the scope of the present invention as long as it has the characteristics of.

As described above, according to the present invention, even when a laser beam having a wavelength other than about 405 nm is used for a compatible objective lens unit such as BD, deterioration of wavefront aberration due to optical axis stabilization hardly occurs, and thus optical axis adjustment during assembly is performed. A compatible objective lens unit, a design method thereof, and a compatible optical pickup that do not require the present invention are provided.

The present inventors compatible objective lens unit, its design method and the compatible optical pickup has been described with reference to the embodiment shown in the drawings for the sake of understanding, but this is merely illustrative, and those skilled in the art It will be appreciated that various modifications and other equivalent embodiments are possible from this. Therefore, the true technical protection scope of the present invention will be defined by the appended claims.

Claims (17)

A compatible objective lens unit for recording / reproducing information by condensing laser light on different types of optical discs, A hologram element; An objective lens for focusing the first laser light on the first optical disk; And a hollow case in which the hologram element and the objective lens are accommodated. When the hologram element and the objective lens are accommodated in the hollow case for the second optical disk on which the second laser light of the center wavelength equal to or greater than the center wavelength of the first laser light is collected. Has a pattern of a phase transfer function that makes the third coma aberration of the second laser light zero at an eccentric position due to the generated tolerance, And the objective lens has an operating distance such that a third coma aberration of the second laser light becomes zero. The method of claim 1, wherein the objective lens, And the first laser light is focused on the first optical disc when the cover layer thickness of the first optical disc is 0.1 mm and the center wavelength of the first laser light is 405 nm. The method of claim 2, A compatible objective lens unit, characterized in that the numerical aperture of the objective lens is 0.85. The method of claim 2, wherein the hologram element, And a phase transfer function pattern in which the third coma aberration of the second laser light is zero when the cover layer thickness of the second optical disc is at least 0.6 mm. The method of claim 4, wherein The second laser light has a central wavelength of any one of the laser light used in HD DVD, DVD or CD. The method according to any one of claims 1 to 5, The hologram device is a compatible objective lens unit, characterized in that the hologram is formed on the forged glass substrate. The method according to any one of claims 1 to 5, The hologram device is a compatible objective lens unit, characterized in that the hologram is formed on a flat substrate having an optical anisotropy. The method according to any one of claims 1 to 5, And the hologram element and the objective lens are disposed such that the pattern region surface of the hologram element and the convex surface of the objective lens face each other. Compensating aberration generated when the objective lens for condensing the first laser light on the first optical disk and the second laser light having a central wavelength equal to or greater than the central wavelength of the first laser light on the second optical disk. In the design method of the compatible objective lens unit having a hologram element, Determining a first working distance between the objective lens and the second optical disc using the second laser light; Determining a phase transfer function of the hologram element at the first working distance; Calculating a first third coma aberration at the time of eccentricity of the hologram element at the first working distance; Determining a phase transfer function of the hologram element at a second working distance different from the first working distance; Calculating a second third coma aberration at the time of eccentricity of the hologram element at the second working distance; Determining a third working distance at which the third coma aberration becomes zero by extrapolating the first third coma aberration and the second third coma aberration; Calculating a phase transfer function of the hologram element and determining a hologram pattern using the third working distance; and designing a compatible objective lens unit. The method of claim 9, The cover layer thickness of the first optical disk is 0.1mm, the central wavelength of the first laser light is 405nm, the cover layer thickness of the second optical disk is at least 0.6mm design method of the compatible optical lens unit. The method of claim 10, The second laser light has a central wavelength of any one of the laser light used in the HD DVD, DVD or CD design method of the compatible objective lens unit. The method according to any one of claims 9 to 11, And the numerical aperture of the objective lens is 0.85. In an optical pickup comprising a light source of the first laser light, a light source of the second laser light, and a compatible objective lens unit, The compatible objective lens unit, A hologram element; An objective lens for focusing the first laser light on the first optical disk; And a hollow case in which the hologram element and the objective lens are accommodated. When the hologram element and the objective lens are accommodated in the hollow case for the second optical disk on which the second laser light of the center wavelength equal to or greater than the center wavelength of the first laser light is collected. Has a pattern of a phase transfer function that makes the third coma aberration of the second laser light zero at an eccentric position due to the generated tolerance, And the objective lens has an operating distance at which the third coma aberration of the second laser light becomes zero. The method of claim 13, And the first laser light has a center wavelength of 405 nm. The method of claim 13, The second laser light has a center wavelength of any one of 650nm and 780nm. The method according to any one of claims 13 to 15, The cover layer thickness of the first optical disk is 0.1 mm, and the cover layer thickness of the second optical disk is at least 0.6 mm. The method according to any one of claims 13 to 15, The numerical aperture of the objective lens is 0.85.

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