KR20030019962A - Optical lens and optical recording and reproducing system using it - Google Patents

Abstract

PURPOSE: An optical lens, and an optical recording and reproducing system using the same are provided to reduce aberration of a focus lens and reduce spot of focused light, thereby improving focusing property and recording density in the optical recording and reproducing system. CONSTITUTION: A lens has a flat upper surface and a paraboloid lower surface(31). A quarter-wave plate(32) is attached to the flat upper surface. A polarized beam splitter(33) is formed on the quarter-wave plate. A reflective material is coated on the lower surface except a vertex(35) of a center part. The vertex is symmetrical with a focus of the paraboloid lower surface to the top of the polarized beam splitter. Light projected from a light source is converged to the vertex.

Description

Optical lens and optical recording and reproducing system using same {OPTICAL LENS AND OPTICAL RECORDING AND REPRODUCING SYSTEM USING IT}

The present invention relates to an optical lens and an optical lens for an optical recording and reproducing system using the same, and more particularly, to an optical lens for a near field recording system with low light loss and no spherical aberration.

An optical recording medium or a magneto-optical recording medium must have a small bit (or recording mark) size and a narrow track width to have a high density recording capacity. However, since the spot size of the light condensed on the recording medium to form a bit in the recording film of the recording medium is limited by the diffraction limit, there is a limit in improving the recording density.

In light of the trend of large-capacity information, new optical recording / reproducing methods are required to overcome the limitations of existing optical recording / reproducing methods. In recent years, research on near field recording / reproduction using near field, which is expected to significantly improve recording capacity, has been increasing.

The principle of near field optical recording and reproduction is as follows. Light entering the lens at an angle greater than or equal to the critical angle is totally reflected when traveling from a dense refractive index to a small one. At this time, due to total reflection of light, light of very small intensity exists on the surface of the lens, which is called an evanescent wave or dissipation wave. Using this evernet wave, it is possible to achieve high resolution, which is impossible because of the diffraction limit, which is the absolute limit of the resolution due to the diffraction phenomenon of light in the far-field. The near field optical recording and reproducing optical system totally reflects the light in the lens to generate an Evernet wave on the surface of the lens, and records and plays back by coupling the Evernet wave and the recording medium.

1 is an enlarged cross-sectional view of an optical system mounted on a head slider in an optical recording system, which is composed of a hemispherical solid immersion lens (SIL) 12 and a primary condensing lens 11. The SIL has a spherical shape with a top surface of a sphere and a bottom surface of a hemispherical shape, and is installed so that the center of the flat portion of the SIL coincides with the focus of the primary condenser lens. Thus, the light 14 incident on the primary condensing lens is refracted and collected at the center of the lower planar portion of the SIL. In order to record data (bits) on the disc using the SIL, as shown in FIG. 1, the SIL is brought close to the surface of the recording medium 13 at very small intervals, for example, about 10 to 70 nm. In this proximity, an optical near field phenomenon occurs in which a part of the primary energy focused on the lower surface of the SIL is transferred to the recording medium. This near field phenomenon makes it possible to record or reproduce data on the recording medium surface. For example, the energy delivered from the SIL heats part of the surface of the record carrier, causing local phase changes. This phase change forms bits on the surface of the recording medium. That is, to record information. When reading the recorded information, the property of reflectance is changed at the locally changed phase. When light of low intensity is incident through the SIL, and the intensity of the light reflected from the surface of the recording medium and output through the SIL is measured by the optical sensor, the reflectance varies depending on the presence or absence of the bit, so that information can be read.

As described above, in the optical system using the SIL, it is possible to overcome the diffraction limit of the light and reduce the light spot, but there are the following problems.

In general, an optical lens generates an aberration in which light does not collect at a point, and this aberration has a characteristic that the higher the magnification of the lens is, the larger it becomes. Since an optical system using SIL requires a large-scale primary condenser lens, the aberration of the primary condenser lens greatly degrades the primary condensing performance of the optical system.

In addition, the data recording / reproducing apparatus using SIL requires a primary condenser lens, which makes the device bulky and complicated, and makes it difficult to assemble the entire data storage device and the primary condenser lens. In particular, there is a limit in reducing the height of the head slider on which the lens is mounted, and it is difficult to manufacture an ultra-thin optical recording and reproducing system that can be mounted on a portable device or the like.

In addition, the convergence angle θ of the focused light is determined by the numerical aperture of the focusing lens, which is a factor that determines the spot size of the incident light. The existing SIL has a limit to increase θ, so the spot of the focused light is limited. It is difficult to reduce the size anymore.

In addition, as the numerical aperture becomes larger, it becomes difficult to control the thickness tolerance and alignment between the SIL and the objective lens. If the thickness and alignment of the focusing lens and the objective lens are not accurately controlled, the operation of the optical recording apparatus will be adversely affected.

It is an object of the present invention to provide an optical system with a low aberration of a focusing lens to improve light condensing performance and recording density in an optical recording and reproducing system.

In addition, an object of the present invention is to reduce the volume and thickness occupied by the primary condenser lens and the focusing lens to facilitate the assembly of the optical system and the entire system, and to enable an ultra-thin optical recording and reproducing system.

Other objects and features of the present invention will appear in detail from the following specific examples and claims.

1 is a cross-sectional view showing a SIL of a conventional near field optical recording system.

2A is a cross-sectional view showing an example of a parabolic mirror.

2B is a cross-sectional view showing an example of a parabolic lens.

3 is a cross-sectional view showing an embodiment of the present invention.

4 is a schematic diagram showing an example in which an incident beam is separated by a polarizing beam splitter.

5 is a schematic diagram showing that the polarization characteristics of the incident beam are changed by the quarter wave plate.

6 is a schematic diagram showing a process of convergence of the incident beam to the optical lens of the present invention.

7 is a cross-sectional view showing another embodiment of the present invention.

8 is a cross-sectional view showing an embodiment of an optical recording system using the optical lens of the present invention.

*** Explanation of symbols for main parts of drawing ***

31: Parabolic surface 32: 1/4 wave plate

33: polarizing beam splitter 35: vertex

In order to achieve the above object, the present invention provides a lens having an upper surface and a parabolic surface, a quarter wave plate attached to the upper surface of the lens, and a polarized beam splitter formed on the quarter wave plate. The parabolic surface of the lower surface of the lens provides an optical lens for an optical recording and reproducing system coated with a reflective material except for a central portion.

At the center of the parabolic surface of the lower surface of the lens, a point where the focal point of the parabolic surface is symmetrical with respect to the upper surface of the lens is located. Light incident on the lens is first reflected at the parabolic surface, secondly at the polarization beam splitter on the upper surface of the lens, and then converges to all of the symmetry points.

In the optical recording and reproducing system using the optical lens of the present invention, the light incident on the upper lens finally converges to the center of the lower surface of the lower lens and magneto-optically interacts with the recording medium located near the lower surface of the lens.

Since the lower surface of the lower lens is formed as a parabolic surface, it is possible to eliminate the spherical aberration of the light that finally converges, and to minimize the spot size of the light used for optical recording and reproduction.

In the optical lens of the present invention, all the light incident on the lens converges at the center portion of the lower surface of the lens, and the incident beam is not used for optical recording and reproduction, thereby acting as noise, thereby preventing the loss of light. In addition, the optical system required for the near field optical recording system may be implemented using only a focusing lens without an objective lens, thereby significantly reducing the size and thickness of the system. Therefore, the present invention not only enables the implementation of an optical memory device requiring high integration, but also can be applied to various applications by significantly reducing its size and thickness.

The optical system of the present invention is applicable not only to the optical recording and reproducing system using the near field, but also to the system using the existing far-field.

In general, in spherical lenses, spherical aberrations in which light does not converge to one point occur. Due to such spherical aberration, the light passing through the lens increases in spot size converged at the focus. The larger the spot size of the converging light, the smaller the recording density in the optical recording system.

In the present invention, to eliminate spherical aberration by using a parabolic lens instead of a spherical lens. 2A is a schematic diagram showing a cross section of a parabolic mirror. The parallel beams 22 incident on the parabolic mirror 20a all converge at the focal point 24a of the parabolic surface due to the characteristics of the parabolic mirror. In this case, the converged light has no spherical aberration, and therefore the spot of light becomes very small.

The present invention applies the characteristics of such a parabolic mirror to form a parabolic lens as one component. 2B is a cross-sectional view showing an example of a parabolic lens. The bottom surface 20b of the parabolic lens is coated with a reflective material so that all light incident to the bottom surface is reflected. If, as shown in the figure, the light reflected from the lower surface of the lens is reflected back from the upper surface of the lens 21, a point inside the lens symmetrical with the focal point of the parabolic surface with respect to the upper surface of the lens (hereinafter referred to as a convergence point) 24b. Will gather all together. At the convergence point 24b, the light of very small spot without spherical aberration is concentrated like the light converged at the focus of the parabolic mirror in FIG. 2A. However, the parabolic lens shown in the drawing needs a means for the incident light reflected from the lower surface of the lens to be reflected again and concentrated at the convergence point inside the lens.

In the present invention, the 1/4 wavelength plate and the beam splitter are provided on the upper surface of the lens so that the light incident on the lens is concentrated to the convergence point of the lower surface of the lens through several reflection processes. 3 shows an embodiment of the optical lens of the present invention. Referring to the drawings, the lower surface 31 of the lens is a parabolic surface and the upper surface is a flat surface, and a first wavelength plate 32 having a predetermined thickness is formed on the upper surface of the lens, and a thin beam splitter 33 film is formed thereon. It is. The lower surface of the lens is coated with a reflective material except for the vertex 35 in the center portion.

The position of the upper surface of the lens is preferably made to coincide with the vertical line passing through the point on the parabolic surface (that is, the convergence point on the drawing) perpendicular to the focal point of the parabolic plane and the center point of the virtual straight line formed by the parabolic focal point. That is, the structure of the lens is designed such that the vertex of the center of the lower surface of the lens is symmetrical with the focal point of the parabolic surface with respect to the upper surface of the polarizing beam splitter.

In this way, all the light incident on the lens concentrates on the vertex 35 on the lower surface of the lens, so that no light is lost.

The quarter-wave plate and the beam splitter reflect light from the light source to the upper surface of the lens at the lower surface of the lens and then again at the upper surface of the lens to finally reach the peak of the lower surface of the lens. The details of this are described as follows.

Light is a kind of electromagnetic wave, which can be seen as a phenomenon transmitted by vibration of an electric field and a magnetic field. In general, natural light is composed of a plurality of waves in which the vibration direction of the electric field is irregularly distributed in an arbitrary direction. When the light is polarized through the polarization means, the direction of the electric field is changed regularly or regularly.

Passing the light past the polarizer perpendicular to the optical axis of the birefringent material divides it into linear polarization with polarization planes perpendicular to each other, all passing through the crystal plate without refracting. However, there is an optical path difference that is determined by the difference between the thickness of the material and the refractive index of each light, so the material is synthesized with some phase difference. At this time, if the phase difference between two lights is π, the synthesized light becomes linearly polarized light. If the phase difference is other than that, the ellipse (circle when the phase difference is π / 4) is ellipsoidal in the plane perpendicular to the direction of light travel. It becomes a special polarized light that rotates. Such light is called rotating polarization (elliptical polarization or circular polarization).

The linearly polarized light ray can be expressed as the sum of two perpendicular components. One component is p-polarized light in which the electric field vibrates parallel to the plane of incidence and the other is s-polarized light in which the electric field vibrates perpendicular to the plane of incidence. . The p-polarized wave and the s-polarized wave have different reflectances, and when the polarized light is incident on a specimen having a thin surface, the polarized state of the reflected light is different from the polarized state when it is incident.

A polarized beam splitter separates linearly polarized light into two components again. The P-polarized wave passes through the polarization beam splitter as it is, but all of the S-polarized wave is reflected. That is, the transmittance of the P-polarized wave passing through the polarization beam splitter is 1 and the reflectance is 0. In the case of the S-polarized wave, the transmittance is 0 and the reflectance is 1, on the contrary.

4 shows an example in which an incident beam is separated by a polarization beam splitter. Two prisms 41 and 42 abut diagonally, and a polarizing beam splitter 43 is located in the middle thereof. The electric field Ei of the incident light 44 may be divided into two waves 45a and 45b perpendicular to each other. P-polarized wave in which the electric field vibrates parallel to the incident surface passes through the polarizing beam splitter, and S-polarized wave in which the electric field vibrates perpendicular to the incident surface is reflected by the polarizing beam splitter. Er and Et represent electric field vectors of a reflected beam (ie, S-polarized wave) and electric field vectors of a transmitted beam (ie, P-polarized wave), respectively.

In FIG. 4, the polarizing beam splitter is inclined at an angle of 45 °, but the P-polarized wave is transmitted and the S-polarized wave is reflected similarly even when positioned vertically at 90 °. In the present invention, the polarizing beam splitter is positioned perpendicular to the incident direction of light.

On the other hand, a quarter wave plate is used as a means for changing linearly polarized light into circularly polarized light or circularly polarized light into linearly polarized light. The quarter wave plate adjusts the birefringent material to have a constant thickness so that the component having the high refractive index among the components of the polarized light has a phase difference of 90 ° (ie, 1/4 wavelength) than the component having the small refractive index. The generation of this phase difference acts to convert linearly polarized light into circularly polarized light and vice versa.

In the case of P-polarized wave, it becomes right circularly polarized light when it passes through 1/4 wave plate, and in the case of S-polarized wave, it is changed to left circle polarized light when it passes through 1/4 wave plate. On the contrary, when the left circularly polarized light passes through the quarter-wave plate, it is converted into the S polarized wave, and in the case of the right circularly polarized light, it becomes P polarized wave.

5 is a schematic diagram showing that the polarization characteristics of the incident beam are changed by the quarter wave plate. When incident light is linearly polarized light, it passes through a quarter wave plate and is converted into circularly polarized light. On the contrary, when incident light is circularly polarized light, it is converted into linearly polarized light. In the drawing, the upper incident beam 52a is a P-polarized wave, and when it passes through the quarter wave plate 51, the transmitted beam 52b is changed into a right circularly polarized light. On the contrary, when the right circularly polarized beam passes through the quarter wave plate, it is converted into P polarized wave. The incident beam 53a at the bottom is an S-polarized wave, and passes through the quarter wave plate 51 to be turned into a left circularly polarized beam 53b. On the contrary, when the left circularly polarized beam passes through the quarter wave plate, it is converted into S polarized wave.

The present invention transmits only the P-polarized light through the polarization beam splitter through the polarization beam splitter, and then changes the circularly polarized light back to linearly polarized light through the quarter-wave plate to convert the linearly polarized light back into linearly polarized light. At the convergence point of the lower surface.

A detailed process of converging the incident beam will be described with reference to FIG. 6. In the drawing, the incident beam passes through two reflection processes and passes through the quarter-wave plate three times. In this process, the P-polarized wave first passed through the polarizing beam splitter is changed from linearly polarized to circularly polarized light, and finally is converted to S-polarized wave and collected at the convergence point.

In the drawing, the polarization beam splitter 63, the quarter-wave plate 62, and the parabolic lens 61 are separated from each other for the sake of understanding, but in reality, the respective components are integrally formed as shown in FIG. Is formed. The light 65a incident in parallel to the plane of the polarizing beam splitter 63 passes through only the P-polarized wave 65b and reflects the S-polarized wave 66. The P-polarized wave transmitted through the polarization beam splitter is changed into the right circularly polarized beam 65c while passing through the quarter-wave plate 62. The right circularly polarized beam is reflected by the reflective material on the parabolic surface 61a at the bottom of the lens, and in this process, the right circularly polarized beam is changed to the left circularly polarized beam 65d. The left circularly polarized beam changes to the S-polarized wave 65e while passing through the quarter-wave plate, and the S-polarized wave does not pass through the polarizing beam splitter and is reflected. The reflected S-polarized wave passes through the quarter-wave plate and changes to the left circularly polarized beam 65f, and finally converges to the center apex 61b of the lower surface of the lens. The size of the vertex may vary depending on the spot of converged light, and the vertex is not coated with a reflective material to allow the converged light to travel outside the lens.

On the other hand, as the light incident on the lens, it is also possible to use light obtained by transforming light generated from the light source into linearly polarized light through a separate polarizer, or use only P-polarized wave.

The parabolic lens of this structure converges all incident light to the vertex located on the lower surface of the lens. In near field optical recording systems, the light passing through the lens has a very small distance to the surface of the recording medium, for example, within several nanometers. Close.

The optical lens of the present invention all converges to the center vertex of the lower surface of the lens without losing light incident on the lens from the light source, thereby improving the light efficiency in the optical recording system.

On the other hand, the light converged at the apex of the lower surface of the lens interacts photoelectrically with the recording medium located near the lower surface of the lens, and the surface of the recording medium and the lower surface of the lens cannot be inclined due to the shaking of the optical recording system. In this case, a physical defect occurs on the surface of the recording medium or the distance between the recording medium and the lens is changed, making it difficult to transmit the optical signal. In the present invention, since the lower surface of the lens is a parabolic surface, the area between the lower surface of the lens and the surface of the recording medium is greatly reduced, thereby solving the above problems.

7 is a cross-sectional view showing another embodiment of the present invention, in which a protrusion is formed at an edge of a lens peripheral portion. In an optical recording system, a focusing lens is generally mounted on a slider. If a projection is formed at the periphery of the lens, the focusing lens is easily mounted on the sulfider.

The optical lens of the present invention described above is mounted on an optical recording system and used for optically recording and reproducing information. Referring to Fig. 8, an optical recording system using the optical lens of the present invention will be described. The figure is sectional drawing which shows an example of a near field optical recording system.

In the deck (not shown), a central portion of the disk 89, which is a recording medium, is mounted to the spindle motor 88 so as to be rotatable. The other side of the deck is provided with a recording and reproducing apparatus. The light generated from the light source 81 passes through the collimator lens 82 and is converted into a parallel beam, and then passes through the beam splitter 83 and through the optical path converting means 86 through the optical lens 87 of the present invention. It reaches the surface of the recording medium in the state of convergence with light of diameter. The light reflected from the recording medium passes through the lens 87, and then the optical path is changed by the conversion means. Then, the light splitter proceeds from the beam splitter to the sensor lens 84 and finally reaches the optical sensing means 85.

Although the size of the lens is relatively large in the figure, it is actually mounted on the head (not shown) in a very small size in the system. Since the size and weight of the lens is very small, the servo of the lens is very easy in the present system, and the lens of the present invention can be applied to both the integrated pickup and the separate pickup. The optical lens of the present invention may be variously modified in the above embodiments, and may be applied to various optical recording systems.

According to the present invention, the aberration of the focusing lens can be reduced, and the spot of focused light can be made very small. Therefore, the condensing performance and the recording density are improved in the optical recording and reproducing system. According to the present invention, by providing an optical system having a very small size and weight, it is possible to record and reproduce information using only a focusing lens without an objective lens, and to provide an ultra-thin optical recording and reproducing system which significantly reduces the overall height of the system. . In addition, the weight of the head portion on which the lens is mounted can be reduced, and the incident pupil of the light source can be reduced, thereby greatly reducing the power consumption of the driving means and the system as a whole.

Claims (10)

The upper surface is flat and the lower surface is a parabolic lens, A quarter-wave plate attached to the lens upper surface, Consists of a polarized beam splitter formed on the quarter-wave plate, The parabolic surface of the lower surface of the lens is coated with a reflective material except for the center part. Optical lenses for optical recording and playback systems. The optical lens for an optical recording and reproducing system according to claim 1, wherein light generated from the light source is incident perpendicularly to the polarization beam splitter and the quarter wave plate on the upper surface of the lens. The optical lens of claim 2, wherein the light generated from the light source is a P magnetic polarization wave. 2. An optical lens for an optical recording and reproducing system as set forth in claim 1, wherein the light incident from the light source converges at the apex of the central portion which is not coated with the reflective material on the lower surface of the lens. 5. An optical lens for an optical recording and reproducing system as recited in claim 4, wherein the apex at the center of the lower surface of the lens is symmetrical with the focal point of the parabolic surface with respect to the upper surface of the polarizing beam splitter. 5. An optical lens for an optical recording and reproducing system as set forth in claim 4, wherein the light that converges to the center apex of the lower surface of the lens is a transverse electric wave. The optical lens for an optical recording and reproducing system according to claim 1, wherein a projection is formed at an edge around the lens. A light source, optical path converting means for converting a path of light generated from the light source, a recording medium on which optical information can be recorded, an optical lens for converging light generated from the light source to the recording medium, and transmitted from the recording medium. It comprises a light sensing means for detecting an optical signal, and a drive unit for enabling rotation to each component, The optical lens includes a lens having a flat upper surface and a parabolic lower surface, a quarter wave plate attached to the upper surface of the lens, and a polarized beam splitter formed on the quarter wave plate. The parabolic surface of the optical recording and reproducing system, characterized in that the reflective material is coated except for the central portion. 10. The optical recording and reproducing system according to claim 8, wherein the light converged to the center portion of the lower surface of the lens interacts magnetically with the surface of the recording medium in close proximity to the lens. The optical recording and reproducing system according to claim 8, wherein the light incident from the lower surface of the lens to the recording medium surface is propagated back to the upper surface of the lens through the center of the lower surface of the lens.