KR20080026016A - Optical pickup, recording/playback apparatus and method of manufacturing lens unit - Google Patents

Abstract

An optical pickup, a recording/playback device and a method for manufacturing a lens are provided to align lenses easily and to improve error tolerance limits by forming one side of an objective lens into a planar surface. An optical pickup has a lens unit(40) having at least two lenses including a near field forming lens(42) and an objective lens(41). The near field forming lens has optical aberration. The objective lens is aligned on the same axis as the near field forming lens and corrects the optical aberration of the near field forming lens. One side of the objective lens is planar.

Description

Optical pickup, recording / playback apparatus and method of manufacturing lens unit

1 is a block diagram showing the configuration of a recording / reproducing apparatus that constitutes one embodiment of the present invention.

2 is a block diagram showing an embodiment of the optical pickup provided in the recording and reproducing apparatus of the present invention.

3 is a schematic cross-sectional view showing the lens portion of the optical pickup constituting an embodiment of the present invention together with the recording medium.

4 is a correlation diagram showing a change in the gap error signal GE according to the distance between the lens portion and the recording medium.

5 is a flowchart illustrating a procedure of a gap control method according to an embodiment of the present invention.

6 is a schematic cross-sectional view showing specifically an objective lens of the present invention.

7 is a side view showing a spherical lens for manufacturing the near-field forming lens of the present invention.

8 is a correlation diagram showing the change of spherical aberration according to the change in the thickness d of the near field forming lens.

9A to 9C are cross-sectional views showing specific embodiments of the near field forming lens constituting the recording and reproducing apparatus of the present invention.

10A and 10B are perspective and side views illustrating another specific embodiment of the near field forming lens of the present invention.

11 is a schematic view showing both the lens unit and the recording medium according to the embodiment of the present invention, and an enlarged partial view of the dotted line portion in an enlarged manner.

12 is a schematic cross-sectional view showing, for example, a lens unit including a near field forming lens and an objective lens that compensates for spherical aberration of the near field forming lens.

FIG. 13A is a schematic diagram showing a case where the central axis of the objective lens and the near field forming lens deviate, and FIG. 13B is a correlation diagram illustrating a comparison of changes in optical aberration according to the objective lens.

FIG. 14A is a schematic diagram illustrating a case where the inclination of the center axis of the objective lens is different from the inclination of the center axis of the near field forming lens, and FIG. 14B is a correlation diagram illustrating a comparison of changes in optical aberration according to the objective lens.

* Description of the symbols for the main parts of the drawings *

1: optical pickup 2: signal generator

3: control unit 4: gap servo driving unit

5: Tracking Servo Drive 6: Sled Servo Drive

7: Decoder 8: Encoder

10: light source 20, 30: separation and synthesis

40: lens unit 41: objective lens

42: near-field shaping lens 50: recording medium

100: micom 111: interface

The present invention relates to an optical pickup and recording and reproducing apparatus and a lens manufacturing method, and more particularly to an optical pickup and a recording and reproducing apparatus having the same, and a method for manufacturing a lens that can be used for the optical pickup or recording and reproducing apparatus. It is about.

In general, an optical recording / reproducing apparatus is a device for reproducing data recorded on the recording medium or recording data on the recording medium using a disc such as a compact disc (CD) or a digital versatile disc (DVD) as a recording medium. As consumer preferences require high quality video processing and video compression technology develops, high density recording media are required. One of the key technologies necessary for developing a high density recording medium is an optical head, that is, a technology related to an optical pickup.

In the above recording medium, the recording density may depend on the diameter of light irradiated onto the recording layer of the recording medium. In other words, the smaller the diameter of the focused light irradiated onto the recording medium, the higher the recording density. At this time, the diameter of the focused light is largely determined by two factors. One is the number of effective apertures (Numeric Aperture, NA), which is the performance of the lens used for focusing, and the other is the wavelength of light focused to the lens.

Since the focused light increases the recording density as the wavelength is shorter, light having a shorter wavelength is used as a method for increasing the recording density. In other words, when blue light is used as compared with red light, the recording density can be further increased. However, in the case of a Far Field recorder head using a general lens, there is a limit in reducing the diameter of the light because of the diffraction limit of the light. Accordingly, a near field recording (NFR) device using near field optics capable of storing or reading information in units smaller than the wavelength of light has been developed. The near field recording and reproducing apparatus using the near field lens obtains light below the diffraction limit by using the near field lens having a higher refractive index than the objective lens, and the light is propagated to the recording medium near the interface in the form of an evanescent wave. Stores high density bit information. Here, the region forming the dissipation wave as described above is called a near-field for convenience of explanation.

However, the prior art as described above has the following problems.

That is, the recording and reproducing method using the near field has a problem in that it is difficult to manufacture a near field lens that does not deviate from the allowable range of error because the tolerance of manufacturing decreases as the recording density increases.

In addition, when using a high refractive index near field lens that forms a dissipation wave in addition to the objective lens, it is difficult to arrange the objective lens and the near field lens so as not to deviate from the tolerance range.

Accordingly, the present invention has been made to solve the above-mentioned conventional problems, an object of the present invention is to provide a lens and a method of manufacturing such a lens that is easy to manufacture.

Another object of the present invention is to provide an optical pickup using such a lens and a recording and reproducing apparatus including the same.

According to a feature of the present invention for achieving the above object, the optical pickup of the present invention includes a lens unit having at least two lenses, the lens unit and the first lens having optical aberration and the first lens and And a second lens provided on the same axis to compensate for optical aberration of the first lens, and one side of the second lens is planar. Here, the second lens includes at least one aspherical surface having optical aberration in a direction compensating for the optical aberration of the first lens. Aspheric coefficient is determined corresponding to the diameter of the light incident on the second lens and the radius of the second lens, when the diameter of the light is 2.5 mm and the radius of the second lens is 1.5 mm to 2.0 mm, -0.8 to It is characterized by -0.6.

The recording and reproducing apparatus of the present invention comprises: a lens portion having at least two lenses;

And a control unit configured to generate a control signal by using a signal generated by the light detector and a light detector for receiving light reflected from the recording medium, wherein the lens unit includes a first lens having optical aberration, Compensating for the optical aberration and having a second lens having a flat one side of the lens surface. Here, the second lens includes at least one aspherical surface having optical aberration in a direction compensating for the optical aberration of the first lens. The aspherical surface is determined according to the aspherical surface coefficient, and the aspherical surface coefficient is determined according to the diameter of the light incident on the second lens and the radius of the second lens. The aspherical surface coefficient may be -0.8 to -0.6 when the diameter of the light is 2.5 mm and the radius of the second lens is 1.5 mm to 2.0 mm.

The lens manufacturing method of the present invention (a) generates a first lens having optical aberration, (b) generates a second lens to compensate for the optical aberration of the first lens, (c) the first lens and the Compensating optical aberration by providing a second lens on the same central axis, wherein one surface of the second lens is manufactured in a plane. The first lens may be generated by cutting a portion of the spherical lens. The second lens may include at least one aspherical surface. The aspherical surface may be determined in correspondence with a diameter of light incident on the second lens and a radius of the second lens.

Hereinafter, with reference to the accompanying drawings, a specific embodiment of the optical pickup, recording and reproducing apparatus and the lens manufacturing method according to the present invention as described above will be described in detail. As used herein, the term "recording medium" means any medium on which data is recorded or capable of recording, and specifically, an optical disk can be given. In addition, the term " recording / reproducing device " in the present specification means any device capable of recording data or reproducing the recorded data using the recording medium. In the present specification, a recording and reproducing apparatus using a near field is described as an example for convenience of explanation, but the present invention is not limited to this embodiment.

In addition, the term used in the present invention was selected as a general term that is widely used as possible at present, but in certain cases there is a term arbitrarily selected by the applicant, in which case the meaning is described in detail in the description of the invention, It is to be understood that the present invention should be understood as the meaning of terms rather than names.

Hereinafter, an optical pickup constituting a specific embodiment of the present invention, a recording / reproducing apparatus having the same, and a lens manufacturing method will be described in detail. In adding reference numerals to the components of the following drawings, the same components are used the same reference numerals as much as possible even if they are displayed on different drawings.

Fig. 1 schematically shows the configuration of a recording / reproducing apparatus constituting an embodiment of the present invention. The configuration of the recording and reproducing apparatus will be described in detail with reference to FIGS. 2 and 3.

The optical pickup (P / U) 1 is a portion for irradiating light to a recording medium and condensing the light reflected on the recording medium to generate a signal. An optical system (not shown) constituting the optical pickup 1 may be configured as shown in FIG. 2. That is, the optical system provided in the optical pickup 1 may include a light source 10, separation and synthesis units 20 and 30, a lens unit 40, and a light detector 60 and 70.

As the light source 10, a laser or the like having good directivity may be used. Therefore, the light source 10 is specifically a laser diode. The light emitted from the light source 10 and to be irradiated onto the recording medium may be composed of parallel light. Therefore, it can be configured to include a lens, such as a collimate, to parallel the path of light on the path of light emitted from the light source.

The separation synthesis units 20 and 30 separate portions of light incident in the same direction or synthesize paths of light incident in different directions. In the present embodiment, since the first separation synthesis unit 20 and the second separation synthesis unit 30 are provided, each of them will be described. The first separation synthesis unit 20 is a part that passes a part of the incident light and reflects some of the incident light (in the present embodiment, the first separation synthesis unit 20 is a non-polarized beam splitter (NBS)). In addition, the second separation synthesis unit 30 is a portion that passes only polarization in a specific direction according to the polarization direction (in this embodiment, the second separation synthesis unit 30 is a polarized beam splitter (PBS)). For example, when using linear polarized light, the second separation composition part 30 may be configured to pass only the polarization component in the vertical direction and reflect the polarization component in the horizontal direction. Alternatively, it may be configured to pass only the polarization component in the horizontal direction and reflect the polarization component in the vertical direction.

The lens unit 40 is a portion which irradiates the light emitted from the light source 10 to the recording medium 50 and condenses the light reflected on the recording medium 50 again. Specifically, as shown in FIG. 3, the lens unit 40 constituting the embodiment of the present invention allows light passing through the objective lens 41 and the objective lens 41 to enter the recording medium 50. It may include a high refractive index lens 42 provided on the path. That is, by further comprising a lens having a high refractive index in addition to the objective lens 41, the numerical aperture of the lens unit 40 is increased to thereby form an evanescent wave. In the present invention, the objective lens 41 and the high refractive index lens 42 provided in the lens unit 40 can be variously modified, which will be described later with reference to the accompanying drawings. Here, the high refractive index lens 42 is referred to as a "near lens forming lens" for convenience of description.

In addition, the optical system of the optical pickup 1 including the lens portion 40 is located very close to the recording medium 50. The concrete example will be described as follows. When the lens portion 40 and the recording medium 50 are brought close to about 1/4 or less of the optical wavelength (that is, λ / 4) or less, a part of the light incident on the lens portion 40 at a critical angle or more is transferred to the recording medium. Dissipation waves are formed on the surface of 50 without penetrating through the recording medium 50 to reach the recording layer. The dissipated wave that has reached the recording layer can be used for recording and reproducing. However, when the distance between the lens portion 40 and the recording medium 50 is greater than λ / 4 or more, the wavelength of the light loses the property of the dissipation wave and returns to the original wavelength, and total reflection on the surface of the recording medium 50 occurs. do. In this case, high-density recording and reproduction cannot be made. Therefore, in the recording and reproducing apparatus using the near field in general, the lens portion 40 is required to be controlled so that the distance from the recording medium 50 does not exceed approximately [lambda] / 4. Is the limit of the near field.

The light detectors 60 and 70 may receive and photoelectrically convert the reflected light to generate an electrical signal corresponding to the light amount of the reflected light. In this embodiment, the first light detector 60 and the second light detector 70 are provided. The first light detector 60 and the second light detector 70 are two optical detection elements (PDA, PDB) that are specifically divided, for example, divided in the signal track direction or the radial direction of the recording medium 50. It can be configured as. The photodetectors PDA and PDB generate electrical signals a and b proportional to the amount of light received. Alternatively, the photo detectors 60 and 70 may be configured by four photo detectors PDA, PDB, PDC, and PDD divided into two in the signal track direction and the radial direction of the recording medium 50, respectively. Here, the configuration of the photodetecting elements constituting the photodetectors 60 and 70 is not limited to this embodiment, and various modifications may be made as necessary.

The signal generator 2 of FIG. 1 uses the signal generated by the optical pickup 1 to produce an RF signal RF for data reproduction, a gap error signal GE for servo control, and a tracking error signal TE. ), And so on.

The controller 3 receives a signal generated by the light detector or the signal generator 2 and generates a control signal or a drive signal. For example, the control unit 3 processes the GE and outputs a drive signal for controlling the gap between the lens unit 40 and the recording medium 50 to the gap servo driver 4. Alternatively, the signal is processed by TE to output a driving signal for tracking control to the tracking servo driver 5. In addition, the controller 3 may limit the dynamic range of the lens unit 40 or the recording medium 50 according to the tilt limit angle α to be described later.

The gap servo driver 4 moves the optical pickup 1 or the lens portion 40 of the optical pickup up and down by driving an actuator (not shown) in the optical pickup 1. As a result, the distance between the lens unit 40 and the recording medium 50 can be kept constant.

The gap servo driver 4 may also serve as a focus servo. For example, the optical pickup 1 or the lens portion 40 of the optical pickup may follow the up and down movement along with the rotation of the recording medium 50 in accordance with a signal for focus control of the controller 3.

The tracking servo driver 5 drives the tracking actuator (not shown) in the optical pickup 1 to correct the position of the light by moving the optical pickup 1 or the lens portion 40 of the optical pickup in the radial direction. do. As a result, the optical pickup 1 or the lens unit 40 of the optical pickup can follow a predetermined track provided on the recording medium 50. In addition, the tracking servo driver 5 may move the optical pickup 1 or the lens unit 40 of the optical pickup in the radial direction in response to the movement command of the track.

The slad servo driver 6 may move the optical pickup 1 in the radial direction in response to a movement command of the track by driving a slat motor (not shown) provided to move the optical pickup 1. .

A host such as a PC may be connected to the recording and reproducing apparatus as described above. The host transmits a recording / reproducing command to the microcomputer 100 through the interface, receives the reproduced data from the decoder 7, and transmits data to be recorded to the encoder 8. The microcomputer 100 controls the decoder 7, the encoder 8, and the controller 3 according to a recording / reproducing command of the host.

In this case, the interface may typically use ATAPI (Advanced Technology Attached Packet Interface, 110). The ATAPI 110 is an interface standard between an optical recording / reproducing apparatus such as a CD or DVD drive and a host, and is proposed to transmit data decoded by the optical recording / reproducing apparatus to a host, and the decoded data is processed by the host. It converts and transmits the packet data which is possible data.

Hereinafter, in the optical pickup 1 constituting an embodiment of the recording and reproducing apparatus, the operation sequence is specifically described based on the traveling direction of the light emitted from the light source 10 within the optical system and on the basis of the signal flow other than that. Explain.

The light emitted from the light source 10 of the pickup 1 is incident on the first separation composition part 20, a part of which is reflected, and a part of the light is incident on the second separation composition part 30. The second separating synthesis unit 30 passes the vertically polarized light component and reflects the horizontally polarized light component from the linearly polarized light (or vice versa). A polarization conversion surface (not shown) may be further included on the path of the light passing through the second separation synthesis unit 30, which will be described later in detail.

The light passing through the first separation and synthesis unit 30 is incident on the lens unit 40. The light incident on the objective lens 41 of the lens unit 40 passes through the near field forming lens 42 to form a dissipated wave. Specifically, light incident on the near field forming lens 42 at an angle greater than or equal to the critical angle is totally reflected on the surface of the recording medium 50 or the near field forming lens 42. However, when the near field forming lens 42 and the recording medium 50 form a near field at an interval close to nanometer level, for example, 100 nm or less, they are not all reflected. In such a case, some of the light is transmitted through the recording medium without being reflected by the Evernetn coupling effect. Therefore, the dissipation wave generated in this process reaches the recording layer of the recording medium 50 to perform recording / reproducing.

The light reflected by the recording medium 50 is incident again to the second separation composition part 30 through the lens part 40. In this case, as described above, a polarization converting surface (not shown) may be provided on a path incident to the second separation synthesis unit 30. The polarization converting surface converts the polarization directions of the light incident on the recording medium 50 and the reflected light. For example, if a quarter wave plate (QWP) is used as the polarization converting surface, the quarter wave plate polarizes the light incident on the recording medium 50 to the left circle, and the reflected light proceeds in the reverse direction. Polarize. As a result, the reflected light passing through the quarter wave plate is converted in the polarization direction in a direction different from the incident light, and has a difference of 90 degrees from each other. Therefore, when only the horizontal polarization component is passed by the second separation synthesis unit 30, the incident light is reflected by the recording medium 50 to have a vertical polarization component when it is incident on the second separation synthesis unit 30 again. . Therefore, the reflected light of the vertical polarization component is reflected by the second separation synthesis unit 30, and the reflected light is incident on the second light detection unit 70.

On the other hand, since the numerical aperture (Numeric Aperture, NA) of the lens unit 40 in the near field recording and reproduction apparatus of the present invention is larger than 1, distortion occurs in the polarization direction of light in the process of being irradiated and reflected through the lens unit 40. . That is, a part of the reflected light incident on the second separation compound part 30 has a horizontal polarization component due to the distortion in the polarization direction, and passes through the second separation compound part 30. The passed reflected light is incident to the first separation synthesis unit 20. The first separation synthesis unit 20 passes a part of the incident light and reflects the part. The light reflected by the first separation synthesis unit 20 is incident to the first light detection unit 60.

The first light detector 60 and the second light detector 70 output an electrical signal corresponding to the amount of reflected light received. The signal generator 2 generates a gap error signal GE, a tracking error signal TE, or an RF signal using the electrical signals output from the light detectors 60 and 70.

The signal generated by the signal generator 2 will be described in detail with reference to FIG. 4 as follows. Here, the case where the first light detector 60 and the second light detector 70 are each composed of two light detectors will be described as an example.

Two photodetectors constituting the first photodetector 60 output electrical signals a and b corresponding to the amount of light received. The two photodetectors constituting the second photodetector 70 output electrical signals c and d corresponding to the amount of light received.

The signal generator 2 generates a gap error signal GE for controlling the distance between the lens and the recording medium 50 by using the a and b signals output from the first light detector 60. can do. The gap error signal GE may be generated by adding all of the signals output from the photodetector constituting the first photodetector 60. The gap error signal GE generated as described above is expressed as follows.

Since the gap error signal GE corresponds to the sum of electrical signals corresponding to the amount of light, the gap error signal GE is proportional to the amount of reflected light received by the first light detector 60.

In addition, the signal generator 2 may use an RF signal RF for recording and reproducing by using the c and d signals output from the second optical detector 70 or a tracking error signal TE for tracking control. Can be generated. The RF signal RF may be generated as RF = c + d by adding all signals output from the photodetecting device constituting the second photodetector 70. In addition, the tracking error signal TE may be generated as TE = c-d as a difference signal of a signal output from the photodetecting device.

As shown in FIG. 4, the gap error signal GE increases exponentially as the distance g between the lens unit 40 and the recording medium 50 increases in the near field, and moves away from the near field. Chapters have a constant size. This will be described in detail as follows. Light incident at a critical angle or more is larger than or equal to λ / 4 when the distance g between the lens unit 40 and the recording medium 50 is out of the near field, that is, the above-mentioned limit of the near field (that is, the boundary between the near field and the remote field). In this case, total reflection is performed on the surface of the recording medium 50 or the near field forming lens 42. On the other hand, if the distance g between the lens unit 40 and the recording medium 50 is smaller than λ / 4 to form a near field, the lens unit 40 and the recording medium 50 may be at or above the critical angle even if they do not contact each other. Part of the light incident on the light passes through the recording medium 50 and reaches the recording layer. Therefore, as the distance g between the lens portion 40 and the recording medium 50 gets closer, the amount of light passing through the recording medium 50 increases, and the amount of light totally reflected on the surface of the recording medium 50 decreases. As the distance g increases, the amount of light passing through the recording medium 50 decreases, and the amount of light totally reflected on the surface of the recording medium 50 increases. Accordingly, the intensity of the gap error signal GE, which is proportional to the intensity of the reflected light, also increases exponentially as the interval g increases in the near field as shown in FIG. (Maximum value). Based on this principle, the gap error signal GE has a constant value when the distance g between the lens unit 40 and the recording medium 50 is kept constant in the near field. That is, by controlling the feedback so that the gap error signal GE has a constant value, the gap g between the lens unit 40 and the recording medium 50 can be controlled to be kept constant.

As described above, a method of controlling the gap between the lens unit 40 and the recording medium 50 to be kept constant by using the gap error signal GE will be described in detail with reference to FIG. 5.

A distance x between the lens unit 40 and the recording medium 50 suitable for detecting the signal of the reflected light is set (S10). The gap error signal GE y detected at the set interval x is detected (S11). The detected gap error signal GE is stored y (12). Here, y may be set to a value larger than 10-20% of the near field limit lambda / 4 so that the lens unit 40 and the recording medium 50 do not have a high risk of collision. In addition, y may be set to a value smaller than 80 to 90% of the near field limit lambda / 4 so that the lens unit 40 and the recording medium 50 are far from each other so that there is a high possibility of escaping the near field. The above process may be performed before the process of recording / reproducing data on the recording medium 50.

In the process of recording / reproducing data on the rotating recording medium 50, the light irradiated onto the track of the recording medium 50 is reflected and received by the first light detection unit 60. The signal generator 80 generates a gap error signal GE using the signal output from the first light detector 60. At this time, it is determined whether the detected gap error signal GE y1 corresponds to the stored gap error signal GE y (S13). If the detected gap error signal GE y1 corresponds to the stored gap error signal GE y, since the set interval is maintained, the recording / reproducing process is continued in the state (S14). On the other hand, if the detected gap error signal GE y1 does not correspond to the stored gap error signal GE y, a change has occurred in the interval, and thus, the lens unit 40 and the recording medium ( 50) can be adjusted. As such, the gap between the lens unit 40 and the recording medium 50 can be kept constant by feedback control of the lens unit 40 using the gap error signal GE detected in the recording / reproducing process.

Hereinafter, specific embodiments of the lens unit 40 will be described in detail with reference to the accompanying drawings. First, a specific embodiment of the objective lens 41 will be described with reference to FIG. 6. Next, a specific embodiment of the high refractive index lens 42 will be described with reference to FIGS. 7 to 11. In addition, since two lenses are used, the relationship between the objective lens 41 and the high refractive index lens 42 will be described with reference to FIGS. 12 to 14.

6 is a view showing a specific embodiment of the objective lens 41 according to the present invention.

In the first embodiment of the objective lens 41 according to the present invention, the objective lens 41 includes at least one aspheric surface. The aspherical surface is referred to as the case where the spherical surface is not a plan view, and a part of the spherical surface can be cut out or added. Here, the aspherical surface may have an optical system configured to have an arbitrary wavefront abberation by using a Zernike polynomial. That is, an optical system for compensating optical aberrations of the near field forming lens 42 to be described later may be formed. Here, the aberration refers to the deviation of the ideal phase caused by the optical system is out of the ideal image. That is, when the light from one point does not collect at one point, the image is blurred, the image is not flat, bent, or the image is distorted. In addition, wavefront refers to a surface in which light simultaneously started from a point light source arrives after a predetermined time. Therefore, wavefront aberration refers to the measurement of the deviation between the spherical surface centering on the ideal store and the wavefront converging toward the actual store along the ray. Therefore, in the present invention, the wavefront aberration corresponds to a wide concept of optical abberation including spherical aberration and coma aberration formed by the lens unit 40.

Theoretical treatment of aberrations involves the development of a polynomial (polynomial) of a suitable variable, with each term corresponding to its own development. For example, as described above, the optical system may be configured to have an arbitrary wavefront abberation using the Zenike polynomial. Herein, a description will be given of the Jennike polynomial as an example.

Where K is the Conic constant and R is the radius of the objective lens 41. The coefficient of each subsequent term is a Zenike coefficient (also called aberration coefficient). The Zenikee coefficient corresponds to a numerical value that can be determined to determine how much aberration represented by the Zenikee polynomial occurs. The mean of the aberration values may be represented by a root mean square (RMS), where the RMS value is an important measure of the degree of aberration.

In the case of using light in a narrow area close to the optical axis, that is, light having a narrow entrance pupil diameter, the above aberration does not matter. However, in the near field recording / reproducing apparatus requiring a high numerical aperture (Numeric Aperture, hereinafter referred to as NA), aberration may be particularly problematic because light of a large area far from the optical axis is used. Here, the numerical aperture is a value representing the resolution of the lens. In this embodiment, the aspherical surface of the objective lens 41 is formed in consideration of optical aberration such as spherical aberration of the near-field forming lens 42 described later. Therefore, first of all, a specific embodiment of the objective lens 41 and the near field forming lens 42 will be described first, and then the compensation relationship between the objective lens 41 and the near field forming lens 42 will be described in detail.

In the second embodiment of the objective lens 41 according to the present invention, one side of the objective lens 41 is configured as a plane. As the objective lens 41, as shown in FIG. 6A, a bi-convex type lens having both aspherical or spherical lenses may be used. Alternatively, as shown in (b) and (c) of FIG. 6, one lens surface may be a plano-convex type lens having a spherical surface, an aspherical surface, or the other lens surface, that is, a lens including a flat surface.

In this case, the alignment between the lenses is a problem in the present invention constituting the lens unit using at least two lenses. That is, the alignment of the objective lens 41 and the near field forming lens 42 should be able to be controlled. At this time, the objective lens 42 as shown in Fig. 6 (a), both sides are composed of aspherical or spherical surface should be controlled so as not to shift the center of each other. That is, the central axis C1 of one lens surface and the central axis C2 of the other lens surface are not shifted, and in addition, there is a difficulty in considering the relationship with other lenses. Therefore, by configuring one side in a plane, the coupling with the near field forming lens 42 can be facilitated, which will be described later in detail in terms of the relationship between the two lenses.

Meanwhile, a specific embodiment of the near field forming lens 42 will be described in detail with reference to FIGS. 7 to 11. According to an embodiment of the near field forming lens, the near field forming lens 42 may use a solid immersion lens (SIL), and as shown in FIG. 7, the spherical lens 47 may be manufactured by cutting the spherical lens 47. Can be. For example, by cutting the part shown by the dotted line of FIG. 7, the near field forming lens 42 of thickness d can be manufactured. Here, the spherical aberration of the near field forming lens 42 is different depending on the position at which the spherical lens 47 is cut. Here, spherical aberration refers to a phenomenon in which light reflected or refracted at one point does not gather again at a point due to curvature. If the spherical aberration is not compensated for, the data cannot be recorded or reproduced at the correct position of the recording medium 50.

FIG. 8 is a correlation diagram showing a change in spherical aberration according to the thickness d of the near field forming lens 42, and FIG. 9 illustrates an embodiment of the near field forming lens 42 having different thicknesses d. It is a side cross-sectional view shown. When the near-field forming lens 42 having the thickness d of the spherical lens 47 or more is manufactured, it can be seen that the thickness without spherical aberration is d1, d3. Here, it is said that the case where there is no spherical aberration corresponds to the spherical aberration point. On the other hand, the near-field forming lens 42 having the remaining thickness except d1 and d3 has spherical aberration of the corresponding value. In particular, the near field forming lens 42 having a thickness of d2 has a maximum value of spherical aberration, which is called a local maximum.

According to the first embodiment of the near field forming lens 42 according to the present invention, a near field forming lens 42 without spherical aberration may be used. That is, the near field forming lens 42 of d1 or d3 thickness can be used. Referring to FIGS. 8 and 9, d1 represents a case in which the thickness d of the near field forming lens 42 is the same as the radius of the spherical lens 47, that is, a hemisphere. On the other hand, d3 represents the case where the thickness d of the near field forming lens 42 is larger than the radius of the spherical lens 47 and smaller than the diameter. In the case where the d3 thickness is referred to as 'super-hemisphere' below. The near field forming lens 42 provided in the lens unit 40 of the present invention may correspond to the hemispherical or ultra hemispherical lens without spherical aberration.

According to the second embodiment of the near field forming lens 42 according to the present invention, the near field forming lens 42 with spherical aberration can be used. That is, a near field forming lens 42 other than d1 or d3 thickness may be used. In this case, a means for compensating spherical aberration of the near field forming lens 42 is required. In the present invention, the spherical aberration of the near-field forming lens can be compensated for by using the objective lens 41, which will be described later in detail in relation to the objective lens 41 and the near-field forming lens 42.

In particular, the near field forming lens 42 according to the present invention may use a near field forming lens 42 having a spherical aberration maximum point having a thickness of d2.

In the hemispherical lens, the effective numerical aperture is defined as nsin θ. Here, the numerical aperture (Numeric Aperture) represents the resolution of the lens, and the effective numerical aperture refers to the numerical aperture formed by the objective lens 41 and the near field forming lens 42. Therefore, the effective numerical aperture of the hemispherical lens is relatively lower than the effective numerical aperture of the super hemispherical lens defined by n ² sin θ. This is a problem for the lens used for the near field requiring a high effective numerical aperture.

On the other hand, the ultra-spherical lens has a high effective numerical aperture, but has a difficult manufacturing problem. That is, as shown in FIG. 8, the spherical aberration shows a sudden change in the slope of the tangent line at the d3 position representing the hyperspherical lens. In other words, the hyperspherical lens has a large difference in spherical aberration even if there is a slight error in fabrication in thickness. Therefore, the hemispherical lens has to accurately cut the thickness, and thus there is a difficulty in manufacturing.

On the contrary, the spherical aberration forming lens 42 of the maximum spherical aberration maximum point having a thickness of d2 has a slight slope in the tangential line as shown in FIG. Therefore, the spherical aberration forming lens 42 of the maximum spherical aberration can be used, and the spherical aberration provided by the near field forming lens 42 can be compensated for and used.

According to another embodiment of the near field forming lens 42 according to the present invention, as shown in Figs. 10A and 10B, a surface in contact with the recording medium 50 can be formed in a conical shape. 10A is a perspective view of the near field forming lens 42 according to the present embodiment, and FIG. 10B is a side view.

Specifically, since the side 42a facing the objective lens is hemispherical, it is the same as in the above embodiment, but the side 42b facing the recording medium has the shape of a truncated cone. This increases the tilt limit angle α by minimizing the area of the surface in contact with the recording medium 50 and at the same time has a minimum area for irradiating light onto the recording medium. Specifically, it will be described with reference to FIG. 11 as follows.

In the recording and reproducing apparatus of the present invention, as described above with reference to Fig. 3, the interval between the recording medium 50 and the lens portion 40 maintains a close interval of several tens of nanometers so as to form a near field. FIG. 11 schematically shows the near field forming lens 42 and the recording medium 50 according to the present embodiment. The radius of the bottom surface at the end of the near field forming lens 42 is r, and the distance between the near field forming lens 42 and the recording medium 50 is g. If the limit angle of the physical range that can be tilted as far as possible until the near field forming lens 42 contacts the recording medium 50 is called the tilt limit angle α, the tilt limit angle α is as follows. Can be calculated

In the present specification, for convenience of description, specific examples will be described. In the present invention, when the value of r is large, the tilt limit angle α is reduced. For example, in the embodiment using the hemispherical near field forming lens 42 as shown in Fig. 9A, when the radius of the hemisphere is 1 mm, the tilt limit angle α becomes very small. There may be difficulties in controlling the recording and reproducing apparatus. Therefore, the area in contact with the recording medium 50 is minimized as shown in Figs. 10A and 10B. That is, the face of the near field forming lens 42 which maintains a close distance at the nanometer level from the recording medium 50 corresponds to 42b, and the other face is larger than the recording medium as shown in FIG. There will be a gap. Thereby, the said tilt limit angle (alpha) can be enlarged. Here, if the radius of the bottom surface 42b shown in Fig. 11 is too small, it is difficult to irradiate light onto the recording layer of the recording medium 50. Therefore, a truncated cone having a suitable bottom surface is used, and the radius r of the bottom surface of the near field forming lens 42 in the recording and reproducing apparatus of the present invention can be used in the range of 30 mu m to 40 mu m. However, if it has the same effect as the present embodiment, it is noted that it is not limited to this embodiment.

In the present invention, since two lenses are used using the near field forming lens 42 in addition to the objective lens 41 to form the near field, the positional relationship between the two lenses is problematic. Here, the relationship between the two lenses will be described in detail with reference to FIGS. 12 to 14.

First, as shown in FIG. 3, the light passing through the objective lens 41 passes through the near field forming lens 42 and is irradiated to the recording medium 50. In consideration of optical aberration here, it is important to align the central axis of the objective lens 41 and the near field forming lens 42 to be located on the same axis. For example, it should be controlled so that a descent in which the central axis of the objective lens 41 and the central axis of the near-field forming lens 42 are shifted or a tilt in which one central axis is inclined does not occur. Optical aberrations here encompass not only spherical aberrations but also coma abberations and other aberrations that can occur in optical pickups.

According to the first embodiment of the combination of the objective lens 41 and the near field forming lens 42 according to the present invention, the optical aberration included in the near field forming lens 42 by using the objective lens 41 having an aspherical surface is provided. You can compensate.

Specifically, it will be described with reference to FIG. 12. As described above in FIG. 9, the near field forming lens 42 in the case of excluding the hemispherical or ultra hemispherical lens has spherical aberration. In addition, the near field forming lens (FIG. 9B) of the maximum spherical aberration point has the largest spherical aberration. As such, the light irradiated to the near field forming lens 42 having spherical aberration has an optical aberration including spherical aberration as shown in FIG. Such optical aberration can be compensated for by using the objective lens 41 having an aspherical surface.

For example, the wavefront error obtained from the interferometer can be used to determine the position of the lens and to obtain accurate and quantitative alignment values. In this case, the most widely used method is as described above using the Zenike polynomial. By using the Zenike polynomial, the optical system can be readjusted to have any wavefront error as described above. Through this, the objective lens 41 has the same size aberration in the opposite direction as the optical aberration of the near field forming lens 42 to compensate for the optical aberration of the near field forming lens 42. In this process, optical aberrations other than spherical aberration can be compensated together.

The optical aberration of the objective lens 41 is determined differently according to the aspherical surface coefficient of the objective lens 41. Specifically, the diameter of the light incident on the objective lens 41 and the radius of the objective lens 41 should be considered. For example, when the diameter of the incident light is 2.5 mm and the radius of the objective lens 41 is 1.5 mm to 2.0 mm, the koenic coefficient of the above-mentioned Zenike polynomial is determined in the range of -0.8 to -0.6. It is assumed here that the radius of the sphere on which the near field forming lens 42 is formed is 1 mm.

According to the second embodiment of the combination of the objective lens 41 and the near field forming lens 42 according to the present invention, one side of the plane can facilitate alignment with the near field forming lens 42 using the objective lens 41. Can be.

In order to use both lenses simultaneously, the positional relationship between the two lenses is important. In particular, in the case of two lenses having an optical compensation relationship as described above, the positional relationship becomes more problematic. For example, alignment should be made so that no descent or tilt occurs.

As described in FIG. 6, when the objective lens 41 that does not include a plane is used, the descent of the objective lens 41 itself is prevented so that the central axis C1 of one lens surface and the central axis C2 of the other lens surface do not deviate. Or there is a need to control the tilt. Therefore, it is difficult to use with other lenses. Accordingly, in the present invention, it is possible to avoid the need to control the descent or tilt of the objective lens 41 itself by using a lens having one plane on one side. Specifically, for example, as shown in FIG. 12B, an objective lens 41 having one aspherical surface to which light enters or exits an aspherical surface and the other lens surface may be flat may be used.

In addition, the near field forming lens 42 having a flat side has less optical error due to descent or tilt between two lenses that are relatively coupled. Experimental results supporting this are shown in FIGS. 13 and 14.

FIG. 13A illustrates a case where the central axis C1 of the objective lens 41 and the central axis C2 of the near-field forming lens 42 are not coupled on the same axis and are shifted by A to cause a descent. In this case, the optical aberration formed by the lens unit 40 according to the size of the decent A is illustrated in FIG. 13B. In FIG. 13B, ■ shows a case in which both lens surfaces are spherical or aspherical as illustrated in FIG. 6 (a), and ▲ shows one lens surface as spherical or aspherical and the other lens as illustrated in FIG. 6 (c). The face shows the case of plane. In the graph, the horizontal axis represents the size of the decenter, that is, the size of A in μm in FIG. 13A. The vertical axis represents optical aberration generated in the lens unit 40 according to the size of the decenter, and specifically, represents a wave front aberration of RMS wavefront error (unit: lambda). This value represents the optical aberration of the lens including the spherical aberration, and is experimentally determined as described above.

When other conditions are the same, optical aberration of 0.05 lambda occurs even with a descent of only 3.5 µm in case of ■. On the other hand, in the case of ▲, the optical aberration is about 0.05λ when the descent of 10 μm occurs. Therefore, in the case of using the objective lens 41 composed of one plane on one side, the optical aberration generated by the descent is reduced. In other words, the range of errors allowed in manufacturing, that is, the tolerance is increased.

FIG. 14A illustrates a case where tilt occurs because the central axis C1 of the objective lens 41 and the central axis C2 of the near field forming lens 42 are inclined. In this case, the optical aberration formed by the lens unit 40 according to the size of the tilted angle B is shown in FIG. 14B. In FIG. 14B, ■ shows a case in which both lens surfaces are spherical or aspheric as illustrated in FIG. 6 (a), and ▲ shows one lens surface as spherical or aspherical and the other lens as illustrated in FIG. 6 (c). The face shows the case of plane. In the graph, the abscissa indicates the magnitude of the tilt, that is, the magnitude of B in FIG. 14A (unit: ° degree). The vertical axis represents optical aberration generated in the lens unit 40 according to the size of the tilt, as described above with reference to FIG. 13B.

When other conditions are the same, optical aberration of 0.05λ ° occurs even with a tilt of 0.08 ° in case of ■. On the other hand, in case of a tilt of 0.3 °, optical aberration is 0.05λ °. Therefore, in the case of using the objective lens 41 composed of one plane on one side, the optical aberration generated by the tilt is reduced. In other words, the range of errors allowed in manufacturing, that is, the tolerance is increased.

Therefore, by manufacturing one side of the objective lens 41 in the plane as in the present embodiment, the light is refracted at both sides of the objective lens 41 and only refracted at one side aspherical so that the near field forming lens 42 more stably. Compensate for spherical aberrations. In addition, in this case, the tolerance range is increased when combined with the near field forming lens 42 to facilitate alignment between the two lenses.

The rights of the present invention are not limited to the embodiments described above, but are defined by the claims, and those skilled in the art can make various modifications and adaptations within the scope of the claims. It is self-evident.

The following effects can be expected in the optical pickup, recording and reproducing apparatus and lens manufacturing method according to the present invention as described in detail above.

That is, there is an advantage that can be easily manufactured a lens that can be used in the near field.

In addition, there is an advantage that the near field lens and the optical pickup and recording / reproducing apparatus including the same can be easily manufactured without departing from the tolerance of the error.

In addition, in the optical system using at least two lenses there is an advantage that the alignment of the lens or control of the mutual relationship is easy.

Claims (27)

Including a lens unit having at least two lenses, The lens unit having a first lens having optical aberration; And a second lens provided on the same axis as the first lens to compensate for optical aberration of the first lens. One side of the second lens is an optical pickup, characterized in that the plane. The method of claim 1, wherein the second lens, And at least one aspherical surface having optical aberration in a direction compensating for the optical aberration of the first lens. The method of claim 2, wherein the aspherical surface, Optical pickup, characterized in that determined according to the aspherical coefficient. The method of claim 3, wherein the aspherical surface coefficient, The optical pickup is determined according to the diameter of the light incident on the second lens and the radius of the second lens. The method of claim 4, wherein the aspherical surface coefficient, And -0.8 to -0.6 when the diameter of the light is 2.5 mm and the radius of the second lens is 1.5 mm to 2.0 mm. The method of claim 1, wherein the first lens, And a refractive index higher than that of the second lens. The method of claim 6, wherein the first lens, An optical pickup, which is a part of a spherical lens, the thickness of which is larger than the radius of the spherical lens and smaller than the diameter. The method of claim 7, wherein the thickness of the first lens, And the thickness when the spherical aberration formed by the first lens is a maximum value. The method of claim 6, wherein the first lens, An optical pickup characterized by a conical surface facing the recording medium. The method of claim 1, wherein the first lens and the second lens, An optical pickup, characterized in that it is made in one piece. A lens unit having at least two lenses; A light detector which receives light reflected by the recording medium; And And a controller configured to generate a control signal using the signal generated by the light detector. And the lens unit includes a first lens having optical aberration, and a second lens compensating optical aberration of the first lens and having one lens surface flat. The method of claim 11, wherein the second lens, And at least one aspherical surface having optical aberration in a direction compensating for the optical aberration of the first lens. The method of claim 12, wherein the aspherical surface, And the aspherical surface coefficient is determined according to the diameter of light incident on the second lens and the radius of the second lens. The method of claim 13, wherein the aspherical surface coefficient, And -0.8 to -0.6 when the diameter of the light is 2.5 mm and the radius of the second lens is 1.5 mm to 2.0 mm. The method of claim 11, wherein the thickness of the first lens, And the thickness when the spherical aberration formed by the lens is a maximum value. The method of claim 11, wherein the first lens, A recording and reproducing apparatus characterized by a conical surface facing a recording medium. The method of claim 11, wherein the control unit, And a distance between the first lens and the recording medium. The method of claim 17, wherein the control unit, And a gap error signal generated by the reflected light to control the distance between the first lens and the recording medium within the limit of the near field. (a) generating a first lens having optical aberration, (b) generating a second lens that compensates for optical aberration of the first lens, (c) compensating optical aberration by providing the first lens and the second lens on the same central axis, The method of manufacturing an optical pickup lens, characterized in that one surface of the second lens is manufactured in a flat surface. The method of claim 19, wherein the first lens, A method of manufacturing a lens of an optical pickup, characterized by cutting a portion of a spherical lens. The method of claim 20, wherein the first lens, And a spherical aberration formed by the spherical lens is formed by cutting the spherical lens to a maximum thickness. The method of claim 19, wherein the first lens, A method for producing a lens for an optical pickup, characterized by forming a surface in contact with a recording medium in a conical shape. The method of claim 19, wherein the second lens, And at least one aspherical surface. The method of claim 23, wherein the aspherical surface, It is formed according to the aspherical surface coefficient, the aspherical surface coefficient lens manufacturing method of the optical pickup, characterized in that determined in accordance with the optical aberration of the first lens. The method of claim 24, wherein the second lens, And forming an optical aberration in a direction opposite to the optical aberration of the first lens. The method of claim 24, wherein the aspherical surface coefficient, And a diameter of the light incident on the second lens and a radius of the second lens. The method of claim 26, wherein the aspherical coefficient, When the diameter of the light is 2.5 mm and the radius of the second lens is 1.5 mm to 2.0 mm, the lens manufacturing method of the optical pickup, characterized in that -0.8 to -0.6.

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