Patent Application
20050232121
The present application claims priority to Japanese Patent Application No. 2004-241630 filed Aug. 20, 2004 and No. 2004-107953 filed Mar. 31, 2004, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to refracting objective optical systems, more specifically to a refracting objective optical system used in condensing incident light at a microscopic spot, more specifically used in recording and/or reproducing information with the near field light, and to an optical recording/reproducing device using the same.
2. Description of the Related Art
Recently, in the optical recording/reproducing technique, various proposals in using the near field light generating structure to obtain a microscopic light spot have been made. The proposal is such to have the evanescent wave and the recording medium interact by bringing the light condensed by the optical system and the recording medium close.
To achieve recordation/reproduction with the near field light, a solid immersion lens (SIL) or a solid immersion mirror (SIM) is conventionally used to bring the microscopic spot and the recording medium close. High-density in recordation or reproduction of information is thereby achieved.
However, in condensing light only with SIL or SIM, the size of the light condensing spot has a limitation of about the wavelength or about half the wavelength at minimum, and is difficult to be made smaller. In order to go beyond such limit, a method of arranging a near field light generating structure near the light condensing spot is proposed and various types of near field light generating structures are disclosed in the Research Journal of Micro-optics “Micro-Optics”.
Further, in Japanese Laid-Open Patent Publication No. 2000-163794, the microscopic spot is realized by combining the SIL or SIM with an elongate slit serving as a near field light generating structure. Further, in Japanese Laid-Open Patent Publication No. 11-203712, a specific example of design of an element for condensing a parallel light at one lens is disclosed. For similar purpose, in Japanese Laid-Open Patent Publication No. 11-339310 and Japanese Laid-Open Patent Publication No. 2000-19315, a design arranged with Fresnel lens or diffraction lens is proposed, and in Japanese Laid-Open Patent Publication No. 2000-82231, a design formed into a gradient index lens is proposed. Further, in Japanese Laid-Open Patent Publication No. 2000-207764, a design in which light is condensed with a hologram lens and the like is proposed.
However, as disclosed in Japanese Laid-Open Patent Publication No. 2000-163794, a problem arises that an objective lens becomes necessary when condensing light using the SIL, which objective lens must synchronously operate with SIL during recordation/reproduction, and thus the control system of operations becomes complicating. Further, as the distance between the objective lens and the SIL requires adjustment at high precision of about the wavelength, the difficulty of manufacturing becomes high.
Further, a configuration including the SIM using one part of a paraboloid of revolution and the near field light generating structure of slit shape, or the catadioptric SIM is disclosed in Japanese Laid-Open Patent Publication No. 2000-163794. The SIM does not require an objective lens as with the SIL, but the lens shape becomes complicating and besides having a problem of the manufacturing becoming difficult, the precision required for the reflecting surface becomes harder than a transmission optical system, and thus the manufacturing cost increases.
As disclosed in Japanese Laid-Open Patent Publication No. 11-203712, in the near field light recording method of condensing the light at only one lens, the difficulty of manufacturing is low and the configuration is simple, but has limitations in making the diameter of the light condensing spot small.
Further, since there is only one optical surface, the aberration of the axial ray may be suppressed, but problems in that there is no degree of freedom for performing off-axis aberration correction (when the incident light is slightly tilted) thus lowering the off-axis performance and in that the light condensing spot becomes large arise.
In the Fresnel lens or diffraction lens disclosed in Japanese Laid-Open Patent Publication No. 11-339310 and Japanese Laid-Open Patent Publication No. 2000-19315, a problem arises that discontinuous points are formed at the lens, and thus the efficiency is lowered from scattering at the discontinuous point and high NA cannot be achieved due to manufacturing limitations of a pitch at the peripheral part of the lens.
In the gradient index lens disclosed in Japanese Laid-Open Patent Publication No. 2000-82231, a problem arises in light condensing performance since the control of the refractive index profile for completely correcting the wave aberration is difficult. Further, in the hologram lens disclosed in Japanese Laid-Open Patent Publication No. 2000-207764, a problem in that diffraction efficiency and chromatic aberration are low arises.
The present invention mainly aims to provide a refracting objective optical system that has a simple configuration and thus is easy to manufacture and that forms a microscopic near field light at high light-use efficiency.
The present invention also aims to provide a refracting objective optical system that satisfactorily corrects off-axis aberration in addition to axial aberration.
The present invention further aims to provide an optical recording/reproducing device using the near field light generated at the refracting objective optical system.
In one aspect of the present invention, the refracting objective optical system includes an aspherical lens in which an entrance surface is a continuous aspheric surface of convex shape and an exit surface is a flat surface, and a transparent flat plate joined to the flat surface of the aspherical lens; where the ray entering the aspheric surface of the aspherical lens is refracted at the aspheric surface, transmitted through the flat surface or the joint surface, and condensed near the exit surface of the transparent flat plate; and the transparent flat plate includes a microscopic structure for generating a near field light smaller than a light condensing spot near the light condensing point of the exit surface, which when the thickness of the transparent flat plate is tP, satisfies the equation 0.01<tP/tL<1.0.
According to the above refracting objective optical system, it is configured from an aspherical lens and an antireflection coating integrally joined together, the objective lens becomes unnecessary, and the manufacturing is facilitated. Particularly, the aspherical lens is configured from a continuous aspherical lens and a flat surface, and thus is easily produced from the conventional glass molding method or a plastic molding method. Further, since the entrance surface is a continuous aspheric surface, the aberration is satisfactorily corrected and the light is efficiently transmitted. Moreover, since the microscopic structure for generating the near field light smaller than the light condensing spot is arranged near the light condensing point of the transparent flat plate, the microscopic light spot is obtained at high efficiency.
In addition, as the light condensing surface is a flat surface, the microscopic distance between the light condensing point and the recording medium is easily measured/controlled, and the light condensing spot is easily brought close to the recording medium. When the thickness of the aspherical lens is tL and the thickness of the transparent flat plate is tP, the equation 0.01<tP/tL<1.0 is satisfied, and thus a satisfactory optical performance is obtained while reducing the thickness of the entire optical system.
In another aspect of the present invention, the numerical aperture NA of the aspherical lens of the refracting objective optical system is preferably greater than 0.6. This is because the refraction at the continuous aspheric surface excels in light condensability at high NA.
The microscopic structure is preferably formed by a metal that generates plasmon resonance. Through the use of electric field amplification effect by the surface excitation plasmon resonance, a satisfactory light condensing efficiency is obtained. Such microscopic structure preferably has, for example, a configuration in which the distal ends of one pair of projections face each other and are brought close to each other, and a light of a polarizing direction substantially parallel to the direction the pair of distal ends face each other enters therein.
The film thickness of the adhesive interposed between the aspherical lens and the transparent flat plate is preferably less than or equal to 50λ when the wavelength of the incident light is λ. The variation of the light condensing position involved in change of film thickness of the adhesive is suppressed as small as possible.
In the refracting objective optical system according to the present invention, the aspherical lens is configured from a first lens made of a first material having a predetermined index of refraction, and a second lens made of a second material having a predetermined index of refraction, where the opposing surfaces of the first and the second lens are adhered at a substantially same shape, and the incident ray is transmitted in the order of the first lens, the second lens and the transparent flat plate and condensed near the exit surface of the transparent flat plate.
The first lens and the second lens are combined to form a single aspherical lens, and thus the aberration is more freely corrected with the two optical surfaces of the entrance surface of the first lens and the cementing surface of the second lens, and the off-axis aberration is effectively corrected in addition to the axial aberration.
Preferably, the cementing surface of the first lens and the second lens are aspheric surfaces for the purpose of correcting the off-axis aberration.
The first lens and the second lens may each be molded through the glass molding method or the injection molding method and then adhered by way of an adhesive layer, or may be a compound lens in which one of the first lens or the second lens is molded through the glass molding method or the injection molding method and the other lens is integrated thereto by molding the transparent resin on the relevant lens.
The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description taken in conjunction with the accompanying drawings.
FIGS. 5(A)-(H) are graphs showing an aberration property of embodiment 1;
FIGS. 6(A)-(H) are graphs showing an aberration property of embodiment 2;
FIGS. 7(A)-(H) are graphs showing an aberration property of embodiment 3;
FIGS. 8(A)-(H) are graphs showing an aberration property of embodiment 4;
In the following description, like parts are designated by like reference numbers throughout the several drawings.
A refracting objective optical system and an optical recording/reproducing device according to the present invention will now be explained based on the drawings but the present invention is not limited in any way to the following embodiments.
In each of
The parallel ray first enters the first surface 12 which is an aspheric surface. Only the first surface 12 has refractive power in the refracting objective optical system 10, and thus the aspheric surface shape for condensing the light without aberration at the fourth surface 17 acting as an exit surface, to be hereinafter described, is uniquely derived. The light refracted at the first surface 12 enters the second surface 13 which is a flat surface. The second surface 13 is joined with the third surface 16 of the flat plate 15, and thus the light transmitting through the second surface 13 is condensed in the vicinity of the fourth surface 17 of the flat plate 15.
Here, the construction data relating to embodiments 1 to 4 are shown in the following tables 1 to 4. The definitional equation of the aspheric surface is expressed with the following equation (1).
X=CY2/{1+(1−(1+K)·C2·Y2)1/2}+πAi·Yi (1)
where
Ai: aspheric surface coefficient of ith power
The aberration property in each of the above embodiments 1 to 4 is shown in
In each of embodiments 1 to 4, as the first surface 12 is formed into an appropriate aspheric surface shape, the light condensing spot has small aberration and exhibits a satisfactory optical performance. Therefore, if the light condensing spot has small aberration with incident light of uniform lighting, light is condensed to a size of about 1.22×wavelength+NA (NA is numerical aperture).
The first surface 12 is a continuous aspheric surface and not a Fresnel surface nor a diffraction surface. In case of the Fresnel surface or the diffraction surface, discontinuous points form in the shape and thus the efficiency is lowered from scattering at the discontinuous point, which is not preferred. In case of a continuous surface as with the first surface 12, on the other hand, the light is efficiently transmitted, which is preferred.
The optical refractive power is also achieved with holography or gradient index lens. However, obtaining a satisfactory imaging performance or diffracting efficiency over a large NA is difficult. Therefore, refraction with continuous aspheric surface excels at condensing light at high NA. Preferably, the following conditional equation (2) is satisfied.
NA>0.6 (2)
The conditional equation (2) defines the numerical aperture (NA) of the optical system 10. NA=nsin θ, where n is the index of refraction of the light condensing point, and θ is the maximum axial angle of incidence. Since NA becomes small when exceeding the lower limit of the conditional equation (2), the light condensing spot becomes large and the light condensing efficiency of energy worsen, which is not preferable. Similarly, in case of when NA is less than or equal to the lower limit, the relatively efficient optical system is manufactured even when using the diffractive optical element or the gradient index lens. It is to be noted that NA of embodiments 1 to 4 is a value each noted in
The second surface 13 is joined with the third surface 16 with an adhesive (suitably a transparent ultraviolet curing adhesive). As warp forms at the flat plate 15 from contraction when the adhesive is thick, the thickness of the adhesive is preferably made thin. Further, the variation of the thickness of the adhesive changes the position of the fourth surface 17 acting as a light condensing position, and thus if made as thin as possible, the positional change of the fourth surface also becomes small, which is preferable.
More specifically, the film thickness of the adhesive is preferably less than or equal to 50λ when the wavelength of the incident light is λ. This guarantees the variation of the fourth surface 17 to be less than or equal to a few wavelength when the change of film thickness is a few % of the total. More preferably, the film thickness of the adhesive is less than or equal to 10λ.
The light condensing spot is formed near the fourth surface 17 of the flat plate 15. A large NA is achieved with higher index of refraction at the vicinity of the light condensing spot, and as a result, a small spot is obtained. Therefore, the index of refraction of the flat plate 15 is preferably larger than the index of refraction of the aspherical lens 11 and the adhesive.
The sum of the thickness of the flat plate 15 and the thickness of the aspherical lens 11 determines the thickness of the entire optical system 10. In order to have the entire optical system 10 compact, the thickness of both components is preferably reduced. However, even if only the aspherical lens 11 or only the flat plate 15 is made thin, the refracting objective optical system with a satisfactory optical performance is difficult to obtain. More specifically, the following conditional equation (3) must be satisfied.
0.01<tP/tL<1.0 (3)
where
The conditional equation (3) shows the ratio of the respective thickness of the aspherical lens 11 and the flat plate 15. When exceeding the upper limit of the conditional equation (3), the flat plate 15 becomes too large with respect to the aspherical lens 11. As a result, the diameter of the aspherical lens 11 cannot be made large, a large NA cannot be obtained, and thus the light condensing spot increases, which is not preferable. Preferably, the diameter of the lens (i.e., NA) is set large to obtain a satisfactory optical performance.
On the other hand, when exceeding the lower limit, the flat plate 15 becomes thin compared to the aspherical lens 11. Due to limitation in manufacturing, the lower limit of the thickness of the flat plate 15 substantially implies increase of aspherical lens 11. In this case, the optical system 10 becomes large as a whole, which is not preferable. If the upper limit of the conditional equation (3) is 0.5, the optical system 10 that is small and has satisfactory light condensing efficiency is obtained.
It is to be noted that tP/tL in embodiments 1 to 4 is a value each noted in
The aspherical lens 11 has a very simple configuration made of a continuous aspheric surface (first surface 12) and a flat surface (second surface 13), and is formed using a conventional glass molding method or an injection molding method and is thus preferable. It is preferred that light weight is achieved if the aspherical lens 11 is manufactured with the injection molding method.
The incident ray is condensed near the fourth surface 17 of the flat plate 15. A light condensing spot of a size appropriate for the NA of the aspherical lens 11 is formed at the light condensing point. More specifically, if the incident ray is uniform, a pattern referred to as a so-called Airy disc is formed, which diameter D of the central spot thereof is generally known to be expressed with the following equation (4).
D=1.22λ/NA (4)
Therefore, if NA is 1.0 and the wavelength is 780 nm, the light condensing spot having a diameter D of 952 nm is obtained. However, the light condensing of wavelength order is the limit with the size of the spot determined by NA.
In order to obtain a smaller spot, a near field light generating structure having a dimension of less than or equal to the wavelength must be arranged in the vicinity of the light condensing spot. Preferably, such microscopic structure is a microscopic structure that generates a surface excitation plasmon, and particularly, that is manufactured from a metal that generates plasmon resonance.
Even with a structure having a dimension of less than or equal to the wavelength, the energy must be efficiently condensed at the microscopic spot, and a satisfactory light condensing efficiency is obtained through the use of a electric field amplification effect by the surface excitation plasmon resonance. It is, for example, known that gold or silver at 780 nm, aluminum or magnesium at 405 nm etc. has large electric field amplification effect.
The plasmon resonance is effectively used by selecting the metal material and microscopic structure as well as thickness according to the wavelength, and the microscopic spot is efficiently obtained. The magnitude and the like of the electric field amplification effect are calculated using Finite Difference Time Domain (FDTD) Method.
The microscopic structure 20 shown in
The microscopic structure 30 shown in
The microscopic structure 40 shown in
The microscopic structures 20, 30, 40 are each applicable to any one of embodiments 1 to 4. With regards to the metal materials, aluminum and magnesium are preferable when using light of short wavelength of about 405 nm, and gold and silver are preferable when using light of long wavelength of about 780 nm for generating plasmon resonance of large electric field amplification.
The plasmon resonance is a phenomenon that depends on the incident polarization. In order to generate a strong resonance near the minimum structure dimension (between projections 23 and between projections 32) in the microscopic structures 20, 30, the polarizing direction of the incident light must be parallel to the direction in which the projections 23 and projections 32 face each other.
The microscopic structures 20, 30, 40 are arranged near the light condensing point of the fourth surface 17 of the flat plate 15 and there may only be one microscopic structure or a plurality of the same may be arranged in a matrix form. When arranging a plurality of microscopic structures as shown in
When light acts on the microscopic structure of less than or equal to the wavelength, the microscopic spot corresponding to the size of the structure is formed in the vicinity thereof. The microscopic light is referred to as the near field light and is a light localizing near the surface of the substance. The near field light is not propagation light and attenuates exponentially. Therefore, in performing recordation/reproduction, the recording/reproducing medium must be brought close to the microscopic structure (within three times the minimum structure dimension of the microscopic structure). More specifically, in microscopic structures 20, 30, the distance from the medium is preferably set to less than or equal to 60 nm when the minimum structure dimension Δ is 20 nm.
In the near field recordation/reproduction, the size of the near field light from the microscopic structure is known to be about that of the microscopic structure and the attenuating distance to be about the same extent. Therefore, to perform a more efficient recordation/reproduction, the medium is preferably brought close to less than or equal to the same extent as the minimum structure dimension.
In each embodiment 1 to 4, the aspherical lens 11 and the flat plate 15 are separately manufactured and then adhered together. Therefore, when forming the microscopic structure with a technique such as EB lithography, it is performed on a flat plate such as a wafer. It is manufactured through a simple step of cutting the wafer, on where the microscopic structure is formed, into a desired size to form the flat plate 15 and then joining the same with the aspherical lens 11.
Assuming the optical system is configured as an integrated object, the microscopic structure must be fabricated at the bottom surface of the material having the lens structure. The EB lithography process and the like includes a spin coating process etc. and thus easy handling of the fabricating sample influences the difficulty of fabrication. The material having the lens structure is difficult to handle compared to the wafer and thus increases fabrication difficulty and is not preferable. Further, in case of the wafer, by manufacturing in large quantity and then cutting into multiple flat plates 15, the productivity in one process is enhanced, but enhancement of productivity cannot be desired when including the lens structure.
In each of
The cementing surfaces 13A, 13B of the first lens 11A and the second lens 11B are joined in the same shape. The plane of incidence 12A of the first lens 11A and the cementing surfaces 13A, 13B are each configured with an aspheric surface. Further, the exit surface 14 of the second lens 11B is a flat surface and is joined with the surface 16 of the flat plate 15 with an adhesive.
The lens 11 of embodiment 5 shown in
The lens 11 of embodiment 6 shown in
The lens 11 of embodiment 8 shown in
The lens 11 of embodiment 9 shown in
The construction data relating to embodiments 5 to 9 are shown in tables 8 to 12. The definitional equation of the aspheric surface is the same as the equation (1) mentioned above.
In embodiments 5 to 9, since each of the refractive objective optical systems is configured with two aspheric surfaces of the plane of incidence 12A of the first lens 11A and the cementing surface 13A, the on-axis aberration and the off-axis aberration are satisfactorily corrected by setting the aspheric surface coefficient thereof to be optimum. For example, in embodiment 5 shown in
In embodiment 5 shown in
In embodiment 7 shown in
In embodiments 5 to 9, assuming the power of the plane of incidence 12A of the first lens 11A is P1 and the power of the cementing surface 13A with the second lens 11B is P2, the following equation (5) is preferably satisfied to secure satisfactory aberration correction.
0.1<P1/P2<5 (5)
where
When P1/P2 exceeds 5, the power distribution at power P1 becomes too large and the advantage of configuring with a cemented lens or a compound lens is not exhibited, that is, correction of off-axis aberration becomes difficult. When P1/P2 is below 0.1, the power of the cementing surface becomes too large. That is, the curvature radius of the cementing surface becomes too small with respect to the plane of incidence of the first lens 11A, and as a result, the lens becomes difficult to manufacture. The range of 0.3<P1/P2<3 is suitable.
The values of P1/P2 in embodiments 5 to 9 are each noted in
The original aim of configuring the aspherical lens 11 as the cemented lens or the compound lens is for off-axis aberration correction, but further facilitates removing the influence by wavelength variation of the light source by setting a suitable Abbe constant. More specifically, achromatic configuration is desired. When the focal length is a positive lens and a negative lens in a single unit of the first lens 11A and the second lens 11B, color correction is performed by having the Abbe constant of the negative lens smaller than the Abbe constant of the positive lens.
The refracting objective optical system 10 of embodiments 1 to 9 is mounted to the air floating slider of an optical recording/reading device. In this case, the ABS (Air Bearing Surface) structure is suitably formed on the surface 17 of the flat plate 15. The ABS structure is generally fabricated by means of the lithography technique. By fabricating the ABS structure at the exit surface 17 of the flat plate 15 of mother board state with lithography and cutting the above into individual flat plates 15 with a dicer and the like and subsequently joining the same with the lens 11, mass production becomes possible. Fabricating the ABS structure on the lens 11 itself is extremely difficult and attaching a suspension for floating is also difficult.
Further, each embodiment 1 to 9 show the construction optimized for the parallel light (infinity). This is to respond, when the optical system 10 is mounted to the floating slider, to change in floating height of the slider (optical system 10) from undulation of the recording medium surface when the recording medium rotates and the slider follows and floats. In this case, if the incident ray is infinite, the position of the light condensing spot is always in the vicinity of the exit surface irrespective of the floating height of the optical system 10. In case of finite conjugation, on the other hand, the follow variation of the slider becomes the variation of the object distance, and thus the position of the light condensing spot varies, which is not preferable. However, the optical system 10 may be used without trouble at a state of finite conjugation as long as it is within a range that barely influences the position of the light condensing spot even if the incident ray is shifted from infinity.
The refracting objective optical system and the optical recording/reproducing device according to the present invention are not limited to the above embodiments, and may of course be modified into various forms within the scope of the invention. Particularly, the details of the configuration of the aspherical lens and the transparent flat plate are optional and the above mentioned construction data thereof are merely an example.
Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-107953 | Mar 2004 | JP | national |
| 2004-241630 | Aug 2004 | JP | national |
