WO2004090598A1

WO2004090598A1 - Imaging optical element and method of designing it

Info

Publication number: WO2004090598A1
Application number: PCT/JP2004/004762
Authority: WO
Inventors: Kouei Hatade; Norihisa Sakagami; Masato Okano
Original assignee: Nalux Co., Ltd.
Priority date: 2003-04-03
Filing date: 2004-04-01
Publication date: 2004-10-21
Also published as: JP4649572B2; JPWO2004090598A1

Abstract

An imaging optical element capable of improving a diffraction efficiency using two different wavelengths as zero-degree and α-degree diffraction lights. The imaging optical element is provided at least on one plane thereof with a first area including the optical axis and a second area having a diffraction part surrounding the first area, and handles a first light beam having a first wavelength and a second light beam having a second wavelength different from the first wavelength. The diffraction part in the second area has a stairway form with a substrate as a plane, and the step difference of the stairway form is determined based on one of the wavelengths of the first and second light beams so that the diffraction efficiency of the zero-degree diffraction light approaches the peak of a diffraction efficiency at that one wavelength. The number of steps N is determined, when the number of steps is N, the wavelength of one light beam λ0, a diffraction degree other than zero degree α, and m and p integers, so that a ratio between the difference, between a wavelength λi = [N/(N·m + α)] · λ0 · p and the other wavelength of those of the first and second light beams, and the other wavelength is up to a specified value defined from a degree of lowering from the peak value of a diffraction efficiency.

Description

Description Imaging optical element and its design method

The present invention relates to an imaging optical element used for an optical pickup system and an imaging system for optical recording and reproduction of an optical recording medium such as a compact disk (CD) and a digital versatal disk (DVD), and a design method thereof. About .

Further, the present invention relates to an objective lens for condensing light beams having different wavelengths on different surfaces. In particular, it relates to a single objective lens that focuses different wavelengths for each disc on the surface of an optical disc such as a Blu-ray disc (BD), a digital versatile disc (DVD) and a compact disc (CD). . Further, the present invention relates to an optical pickup optical system using the above objective lens. Background art

Fig. 13 shows the configuration of an optical pickup system used for reproducing and recording on CD and DVD optical recording media. Light beams emitted from semiconductor laser elements 7 and 8 of different wavelengths, in which photodiodes are unitized, are split by passing through diffraction gratings 9 and 10 used for tracking and focusing detection. Next, after passing through a collimating lens 12 for converting a light beam from the semiconductor laser into parallel light, the direction of the optical axis is changed by a rising mirror 13. Further, the light flux is converged on the optical recording medium 15 by passing through the objective lens 14. The light beam reflected by the optical recording medium 15 returns along the same optical path, reaches the split photodiode, and controls the tracking and focusing by the split photodiode signal. The mirror 11 is a mirror having wavelength selectivity.

FIG. 14 shows an optical pickup system using one semiconductor laser element array 16 having light emitting portions of different wavelengths. Light beams of different wavelengths emitted from a semiconductor device laser element array 16 in which photodiodes are unitized pass through a diffraction grating 17 and then pass through a collimator lens 18. For collimator lens The light flux becomes parallel light, the direction of the optical axis is changed by the rising mirror 19, and after passing through the objective lens 20, the light is focused on the optical recording medium 21.

The optical systems for DVD and CD optical recording media are common, and various technologies have been developed to make them common.

For example, the light beam passing through the first area at the center of the objective lens is focused on the first optical recording medium and the second optical recording medium to record or reproduce on the optical recording medium. A method has been proposed in which the second area is mainly used for recording or reproduction on the second optical recording medium, and the third area extending outside the second area is mainly used for recording or reproduction on the first optical recording medium. (Japanese Unexamined Patent Publication No. Hei 11-96585). However, in the prior art method described above, the light beam passing through the objective lens

Energy from a semiconductor laser cannot be efficiently guided to an optical recording medium because it corresponds to an optical recording medium with a different thickness depending on the area through which it passes, and recording or reproduction is performed on an optical recording medium such as a DVD or CD. In this case, the optical recording medium must have a light-condensing ability that is almost diffraction-limited. If an objective lens common to DVDs and CDs is condensed only with a normal aspherical shape and refractive power, spherical aberration will occur. Is difficult to capture. In particular, when different wavelengths are emitted from one laser unit due to the semiconductor laser element array, the distance from the laser light source to the surface of the optical recording medium is the same. Therefore, it was not possible to satisfy the permissible value of the amount of wavefront aberration necessary for recording or reproducing on an optical recording medium.

The two wavelengths of a semiconductor laser used for recording or reproducing on optical recording media of different thicknesses such as DVD and CD are relatively close. Therefore, by forming a lens surface using a common diffraction order in a common area necessary for reproducing or recording a DVD or a CD in an area having a diffractive portion, it is possible to record or reproduce on an optical recording medium. The required allowable value of the wavefront aberration cannot be satisfied. For this reason, there has been proposed a method of performing optimization using different diffraction orders in a region having a diffraction portion (for example, supervised by the Japan Society of Applied Physics, Introduction to Diffractive Optical Elements, Optronitus, 1997). The first edition of the first edition was issued on May 20, p.102-105). Regarding the diffraction efficiency in the region having the diffraction portion, the optimum depth of the diffraction grating is generally expressed by the following equation. Here, N is the number of steps, or the wavelength, and n is the refractive index.

The depth obtained by the above equation is the depth at which the first-order diffracted light is maximized for the defined wavelength λ. Therefore, if the optimal depth is the diffraction grating depth, the primary diffracted light occupies most of the light having a wavelength relatively close to the wavelength; L, which passes through the region having the diffraction portion. The diffraction efficiency of the first-order diffracted light is improved for each of two wavelengths from a semiconductor laser for recording or reproduction of optical recording media having different thicknesses and from a semiconductor laser. That is, the diffraction efficiency of the zero-order diffracted light decreases.

Figure 15 is a graph that calculates the diffraction efficiency in a blazed shape by taking the diffraction grating depth on the X-axis and the diffraction efficiency on the Υ-axis. If the visible red wavelength 660 nm is defined as the 0th-order diffracted light and the near-infrared wavelength 780ηηι is defined as the 1st-order diffracted light, the depth at which the diffraction efficiency can be obtained for both the 0th-order diffracted light and the 1st-order diffracted light is about 0.74 zm. . In this case, the diffraction efficiency at different wavelengths is about 40%.

As described above, a Relief-type diffraction grating that gives the above-described diffraction efficiency value in the diffraction section region can also be used. However, the market demands for DVDs that use visible red semiconductor lasers in particular are increasing their demands for higher speeds, while satisfying the optical characteristics required for optical pickup and the energy efficiency of semiconductor lasers. Need to be higher. In other words, there is a great need for an objective lens and a method for designing the objective lens, which efficiently guide light beams having different wavelengths emitted from a semiconductor laser as 0th-order and 1st-order diffracted light to an optical recording medium.

As described above, in an objective lens used for recording or reproducing on an optical recording medium having a different thickness such as a DVD or a CD, the 0th-order and the 1st-order diffracted light are reduced in order to reduce aberration at the time of focusing on the optical recording medium. It is necessary to use it. However, improving both the diffraction efficiency of the 0th order and the 1st order diffracted light is not It has been difficult with conventional technology. For this reason, there is a great need to improve the diffraction efficiency of the 0th order and 1st order diffracted light.

Optical disks are used as recording media for storing large amounts of information. Among the optical discs, compact discs (CD) and digital versatile discs (DVD) are widely used.

It is desirable that the optical pickup system for CDs and DVDs be shared for the purpose of saving space and cost. However, the required spot diameter for reading and writing is about 1.4 to 1.5 m for CDs, but about 0.8 to 0.9 μm for DVDs. The light collection spot diameter is proportional to the wavelength used and inversely proportional to the image-side numerical aperture of the optical system. For this reason, DVDs use smaller wavelengths and larger numerical apertures than CDs to reduce the converging spot diameter. Furthermore, in DVDs, the substrate thickness, which is 1.2 mm in CDs, is set to 0.6 mm in order to suppress coma caused by disc tilt. Therefore, in order to share the optical pickup system between CD and DVD, it is necessary to change the focal position and the numerical aperture by changing the substrate thickness.

For this reason, in the prior art, an optical pickup using a bifocal lens for changing the focal position (for example, Japanese Patent Application Laid-Open No. 2000-81666), an aperture switching type light for changing the numerical aperture by using a liquid crystal. Pickups (for example, Japanese Patent Application Laid-Open No. H10-120620) have been proposed. However, even if the focal position is changed by changing the thickness of the substrate using a bifocal lens, it is necessary to separately change the numerical aperture. Also, if a liquid crystal panel for switching the numerical aperture is separately provided, the number of parts increases and the parts cost increases. Furthermore, it leads to a decrease in yield and an increase in man-hours in the production process.

Thus, no imaging optical element having a numerical aperture selection function has been proposed in the prior art.

Optical disks are used as recording media for storing large amounts of information. Optical disks that are widely used include compact disks (CDs) and digital versatile disks (DVDs). In addition, higher density pull-ray discs (BDs) are being developed. The focused spot diameter that determines the storage density of an optical disk is proportional to the wavelength used and inversely proportional to the image-side numerical aperture of the optical system. For this reason, the focused spot diameter is reduced by reducing the wavelength and increasing the number of apertures. The wavelengths of CD, DVD and BD are about 785 nm, 655 nm and 405 ηm, respectively, and the numerical apertures are about 0.45, 0.65 and 0.85, respectively. is there

The optical pickup system for CD, DVD and BD is desirably shared for the purpose of saving space and cost.

In the prior art, a two-element high NA objective lens capable of coping with information recording media having different thicknesses without changing the lens interval (for example, Japanese Patent Laid-Open No. 2001-174746). No. 97) and an objective lens for an optical head with high light use efficiency that enables recording and reproduction of a plurality of types of optical information recording media with one objective lens (for example, Japanese Patent Application Laid-Open No. 2000-815). No. 6) has been proposed. However, the former has two lenses, so the number of parts is large and the cost is high. Further, the weight of the objective part is increased, so that the load for driving the lens diameter by an actuator is impeded, which hinders speeding up. In addition, the latter cannot be applied to the optical pickup system for BD.

As described above, in the prior art, no single objective lens for collecting different wavelengths for a disc including a BD has been proposed. Disclosure of the invention

The present invention has been made under the above circumstances. That is, the present invention relates to an optical pickup system for an optical pickup system for recording and reproducing optical data having different substrate thicknesses, such as a compact disk (CD) and a digital versatile disk (DVD). It is an object of the present invention to provide an objective lens that improves both the diffraction efficiency of second- and first-order diffracted light and reduces energy loss, and a design method thereof.

It is another object of the present invention to provide an imaging optical element having a numerical aperture switching function that does not require a separate shield such as a liquid crystal panel in an optical pickup system or the like. According to the present invention, the number of parts and the cost of parts increase. In addition, it does not reduce the yield and increase the man-hours in the production process. In addition, on the surface of optical discs such as Blu-ray 'disc (BD), digital' versatile disc '(DVD) and compact' disc (CD), There is a great need for single objectives that focus different wavelengths for discs.

An imaging optical element according to the present invention includes, on at least one surface, a first region including an optical axis and a second region having a diffractive portion around the first region, and a first light beam having a first wavelength. And a second light beam having a second wavelength different from the first wavelength. The shape of the diffractive portion in the second region is a staircase shape when the substrate is flat, and the step amount of the staircase shape, the diffraction efficiency of the 0th-order diffracted light is one of the wavelengths of the first and second light beams. Is determined based on the one wavelength so as to approach the peak of the diffraction efficiency. The number of steps is N, and the wavelength of the one light beam is. , If diffraction orders other than the 0th order are << and m and p are integers, the wavelength

λ i = [N / (N · m + α)] · λ 0 · p

The ratio of the difference between the first and second light beams and the other wavelength to the other wavelength is equal to or less than a predetermined value determined from the degree of reduction from the peak value of the diffraction efficiency. In addition, the number of steps N is specified.

Therefore, in an imaging optical element having a step-like diffraction grating shape, diffraction efficiency can be improved by using two different wavelengths as 0-order and α-order diffracted light.

The objective lens according to the present invention is an objective lens for converging a first light beam having a first wavelength and a second light beam having a second wavelength different from the first wavelength on the first and second surfaces, respectively. Lens. At least one lens surface includes a first region including an optical axis, a second region having a diffractive portion around the first region, and a third region around the second region. The first ray is focused on the first surface after passing through the first and second regions, and the second ray is focused on the second surface after passing through the first, second and third regions. Thus, the lens surface shape in the second region is designed based on one of the optical paths of the first and second rays, and the phase function that defines the diffracting portion in the second region is the first and second rays. Is designed based on the other path of the light beam.

The method for designing an objective lens according to the present invention comprises the steps of: The objective lens is for focusing a second light beam having a second wavelength different from the first wavelength on the first and second surfaces, respectively. A design method comprising: defining at least one lens surface with a first region including an optical axis, a second region around the first region, and a third region around the second region; and a lens surface in the first region. Designing the shape. The design method further includes a step of designing the lens surface shape in the second region based on one of the optical paths of the first and second rays, and a phase defining the shape of the diffraction portion in the second region. Determining a function based on the other optical path of the first and second light rays; and designing a lens surface shape in the third region.

As described above, since the first region has no diffraction portion, there is no energy loss due to the diffraction portion. In the second region, one of the first and second light beams has an optical path defined by refraction according to the lens surface shape, and the other of the first and second light beams has a lens surface shape and a diffraction portion. Since the optical path can be determined according to the phase function of (1), aberration when converging on the first and second surfaces can be reduced. Therefore, energy loss and aberration can be reduced for light beams having two wavelengths condensed on the first and second surfaces, respectively. According to one embodiment of the present invention, the shape of the diffractive portion in the second region is a staircase shape when the substrate is a flat surface, and the step amount of the staircase shape is based on the wavelengths of the first and second light beams. Is determined.

Further, according to one embodiment of the present invention, the wavelength of the light beam defining the lens surface shape in the second region is obtained. , Where n is the refractive index of the lens, n _{0 is} the refractive index around the lens, and 0 is the angle of incidence on the diffractive part, and the step is determined based on an integer multiple of o 'cos e / (n—n ₀ ). .

Therefore, the influence of the diffracting portion on the light beam that determines the lens surface shape in the second region as the zero-order diffracted light is reduced, and the diffraction efficiency is improved.

According to one embodiment of the present invention, the width of the steps in the diffraction section is determined based on the phase function, the amount of steps, and the number of steps. Therefore, the influence of the diffracting portion on the other of the first and second light beams as the first-order diffracted light increases, and the diffraction efficiency is improved.

The objective lens according to one embodiment of the present invention includes a diffractive portion in the second region. Thus, the light rays defining the shape of the lens surface in the second area pass mainly as the 0th-order diffracted light, and the light rays defining the shape of the diffractive portion in the second area mainly pass as the first-order or -1st-order folded light. Therefore, it is possible to reduce aberrations when converging light on the first and second surfaces, respectively.

According to one embodiment of the present invention, the lens surface shapes of the first, second and third regions are aspherical. Therefore, a design with a high degree of freedom can be performed to reduce aberrations.

According to one embodiment of the present invention, there is provided an optical pickup device for recording or reproducing information on first and second optical recording media having different thicknesses, comprising: a first light beam having a first wavelength; A second light beam having a second wavelength different from the first wavelength is used in an optical pickup device used for the first and second optical recording media, respectively. The first surface is a surface of the first optical recording medium, and the second surface is a surface of the second optical recording medium. Therefore, it is possible to reduce energy loss and aberration with respect to light beams of two wavelengths that are respectively focused on the surfaces of the first and second optical recording media of the optical pickup device.

According to one embodiment of the present invention, the first optical recording medium is a CD, and the second optical recording medium is a DVD. Therefore, energy loss and aberration can be reduced for light beams of two wavelengths focused on the CD and DVD surfaces of the optical pickup device.

According to one embodiment of the present invention, assuming that the numerical aperture at which the light beam passing through the first area converges on the optical recording medium is NA1, NA1 satisfies NA1 0.37, passes through the second area, and passes on the optical recording medium. Assuming that the numerical aperture for condensing light at NA2 is NA2, NA2 satisfies 0.3 NA2 0.51. If the numerical aperture at which light passes through the third area and converges on the optical recording medium is NA3, NA3 satisfies 0.4≤NA3≤0.67. By properly determining the numerical aperture in this manner, energy loss and aberration are reduced for light beams of two wavelengths that are respectively focused on the surfaces of the first and second optical recording media of the optical pickup device. Can be reduced.

In the method for determining a diffraction grating shape according to the present invention, a staircase-shaped diffraction grating shape is determined based on a phase function while considering diffraction efficiency using two different wavelengths as zero-order and first-order diffracted light. The method of determining the diffraction grating shape is one of the wavelengths; Wavelength light as zero-order diffracted light; a step of determining the step amount based on the number of steps N, as a positive integer m, 1 any peak wavelength几₂ in order diffracted light, the diffraction grating pattern is upward sloping shape And the step of obtaining as ₂ = N / (Nm-l) XeQ if the diffraction grating shape has a shape that rises to the left. Method of determining the diffraction grating pattern is further while operating the m as example ₂ approaches the wavelength of the other light is first order diffracted light, the steps of determining the number of steps, based on the step amount and the phase function staircase Determining the width of A computer program for determining a diffraction grating shape according to the present invention determines a staircase-shaped diffraction grating shape based on a phase function while considering diffraction efficiency using two different wavelengths as 0th-order and 1st-order diffracted light. A computer program that determines the shape of the diffraction grating sends a light with a wavelength of λ to the computer as the 0th-order diffracted light. A step of determining a step amount based on the number of steps New, positive and integer as m, 1 any peak wavelength lambda 2 in order diffracted light, ₂ = N e When the diffraction grating pattern has a right upward shape / (Nm + l) X λ ο and the diffraction grating shape goes to the upper left

And obtaining as X ₀ Ru is executed. The computer program for determining the shape of the diffraction grating further includes a step of determining the number of steps while manipulating m so that the beam ₂ approaches the wavelength of the other light that is the first-order diffracted light. Determining the width of the stairs based on the phase function.

Therefore, when determining the step-like diffraction grating shape based on the phase function, it is possible to improve the diffraction efficiency by using two different wavelengths as 0th-order and 1st-order diffracted light.

An imaging optical element according to the present invention includes, on at least one surface, a first region including an optical axis and a second region having a diffractive portion around the first region, and has a first wavelength. The first light ray passes through the first area and converges on the image plane, but does not converge on the image plane when passing through the second area, and has a second wavelength different from the first wavelength. The light beam 2 passes through the first area or the second area and is focused on the image plane. Further, in the imaging optical element according to the present invention, the surface shape in the second region is designed based on one of the optical paths of the first and second light beams, and the shape of the diffractive portion in the second region is first and second. It is designed based on the other path of the second ray. A method of designing an imaging optical element according to the present invention includes, on at least one surface, a first region including an optical axis and a second region having a diffractive portion surrounding the first region, and having a first wavelength. The first light ray passes through the first area and converges on the image plane, but does not converge on the image plane when passing through the second area, and has a second wavelength different from the first wavelength. An imaging optical element is designed so that the two rays pass through the first area and the second area and converge on the image plane. Further, the method of designing an imaging optical element according to the present invention includes the steps of designing a surface shape in the second area based on one of the optical paths of the first and second light rays, and a step of designing the diffractive portion in the second area. Designing the shape based on the other optical path of the first and second light beams.

Therefore, the optical paths of the first and second light beams are separated according to the surface shape in the second area and the shape of the diffractive portion, and the second light beam is imaged without focusing the first light beam on the image plane. Light can be collected on a surface.

According to one embodiment of the present invention, the shape of the diffractive portion in the second region is a stepped shape when the substrate is a plane, and the step amount of the stepped shape is such that the diffraction efficiency of the 0th-order diffracted light is the first and the second. The second light beam is determined based on the one wavelength so as to approach a peak at the one wavelength.

According to one embodiment of the present invention, 1 is an integral multiple of the one wavelength. The refractive index of the imaging optical element is n, the refractive index around the imaging optical element is n _Q , and the incident angle with respect to the diffraction section is Θ. 'cos 0 It is determined based on the value of Z (n-n.). ■

Therefore, the majority of one of the first and second rays passes through the diffraction section without being affected by the diffraction section as the 0th-order diffracted light, and the optical path is determined by the surface shape of the second region . Also, very few of the light rays pass through the diffraction portion as diffraction light of an order other than the 0th order and are affected by the diffraction portion.

According to one embodiment of the present invention, the number of steps of the staircase shape is such that the diffraction efficiency of the 0th-order diffracted light at the other wavelength approaches 0%, and the diffraction efficiency of the diffracted lights of orders other than the 0th order is: It is determined based on the first and second wavelengths so as to be as large as possible.

According to one embodiment of the present invention, when the number of steps is N, the wavelength of the one light ray is; L ₀ , the diffraction orders other than the 0th order are a; λ [N / (N-m + a)] · λ ₀ · p

The number of steps N is determined so that the ratio of the difference between the first and second light beams and the other wavelength to the other wavelength is equal to or less than a predetermined value.

Therefore, of the first and second light beams, most of the other light beams pass through the diffraction portion as diffracted light of orders other than the 0th order, and the optical path is determined by the shape of the diffraction portion. Also, very few of the other light beams pass through the diffraction portion as the 0th-order diffracted light and are not affected by the diffraction portion.

The objective lens according to the present invention is provided with a diffraction grating on at least one surface, and focuses light beams of different wavelengths on different surfaces. When the first wavelength and the second wavelength satisfy the relationship, in a region where both the first wavelength and the second wavelength pass, the first wavelength light flux is converted into the first wavelength as the second-order diffracted light. focused on the face, 1 as the second light flux of wavelength lambda ₂ and the diffracted light is focused on the second surface defines a phase function and lenses surface shape of the diffraction grating, the grating depth of the diffraction grating of the diffraction efficiency of first-order diffracted light 2 in the diffraction efficiency and the second wavelength lambda ₂ of the diffracted light at the first wavelength is specified in earthenware pots by greater than a predetermined value.

Therefore, the luminous fluxes of the first and second wavelengths are condensed on the first and second surfaces as second-order and first-order diffracted light, respectively, and the diffraction efficiency becomes larger than a predetermined value.

In the objective lens according to an embodiment of the present invention, if satisfying the relation of the second wavelength lambda _3, the region where the first wavelength, the light flux of the second wavelength lambda ₂ and third wavelength lambda ₃ to pass through the co in, addition, as 1 third light flux with wavelength lambda ₃ order diffracted light is focused on the third surface, defining a phase function and a lens surface shape of the diffraction grating, the grating depth of the diffraction grating, the third the diffraction efficiency of the wavelength lambda ₃ in 1-order diffracted light is set to be larger than a predetermined value.

Therefore, the luminous flux of the third wavelength is converged on the third surface as primary light, and the diffraction efficiency becomes larger than a predetermined value.

In the objective lens according to the embodiment of the present invention, the diffraction grating has a blaze-ridden shape. Therefore, the processing is relatively simple. In the objective lens according to the embodiment of the present invention, n is the refractive index of the lens, and the depth 1 of the diffraction grating is expressed by the following equation.

Is determined by In this case, the diffraction efficiency of the second-order diffracted light at the first wavelength and the diffraction efficiency of the first-order diffracted light at the second and third wavelengths are each 70% or more.

In the objective lens according to the embodiment of the present invention, at least one surface is divided into at least one band-shaped region surrounding the optical axis and a central region including the optical axis, and each region is defined by a separate surface. .

In the objective lens according to the embodiment of the present invention, a step in the optical axis direction is provided between the separate surfaces.

In the objective lens according to the embodiment of the present invention, the separate surfaces are: ζ axis coincides with the optical axis, i is the number of the surface counted from the center, Ri is the radius of curvature, Ki is the eccentricity, Ai4, Ai6, Ai8, AilO is the aspherical coefficient, and di is the step on the optical axis of the other surface with respect to the first surface.

(11 Ri) χ ί _{r + Ai4 χ] ι + Ai6 xh} 6 _{+ AfS χ h} % _{+ Ail0x} o + _di l + ^ jl- (l + Ki) x (l / Ri) ² xh ―

In the objective lens according to the embodiment of the present invention, the light beam of the first wavelength is collected by the image-side numerical aperture NA1, the light beam of the second wavelength is collected by the image-side numerical aperture NA2,

NA1> NA2

In this case, the outermost part of the light beam of the second wavelength divides at least one surface into at least one band-shaped region surrounding the optical axis and a central region including the optical axis, and separates each region separately. Is defined by

NA1> NA2

In this case, a diffraction grating is provided in a region away from the optical axis through which only the light beam of the first wavelength passes, and the phase of the diffraction grating is adjusted so that the light beam of the first wavelength is focused on the first surface. The function defines the lens surface shape. 4762

13 Therefore, the degree of freedom of the lens surface shape is increased, and by adjusting various coefficients, it is possible to further reduce aberration when light beams having different wavelengths are condensed on different surfaces. When condensing light on each surface, the wavefront aberration should be set to RMS0.07 or less in wavelength units.

In the objective lens according to the embodiment of the present invention, the lattice slope has a shape in which at least a portion is steeper than the blazed slope. ·

Therefore, when the light is incident on a portion where the inclination of the slope of the lattice shape according to the present invention is steep, the incident angle becomes larger as compared with the prior art (blazed shape), and the transmitted light is totally reflected without being generated. The totally reflected light reenters the adjacent grating shape, where it is combined with another incident light by phase superposition, repeatedly reflected, and finally transmitted through a very small angle with respect to the slope (diffraction). Light) is emitted. As a result, the diffraction efficiency is improved. As described above, the diffraction efficiency is improved in a portion where the grating period is short.

In the case where the objective lens according to the embodiment of the present invention focuses light on each surface, the wavefront aberration is designed to be equal to or less than RMS 0.07 in terms of wavelength.

Therefore, with respect to each wavelength, it is possible to collect light with high precision on individual surfaces. In the optical pickup optical system according to the embodiment of the present invention, the first wavelength is a wavelength for a blue-ray disc, and the second wavelength is The wavelength is the wavelength for Digital Versatile's disc.

Therefore, a single lens can support Blu-ray discs and digital versatile discs, providing a compact optical pickup optical system. Therefore, high speed and low price are realized.

In the case of handling the third wavelength in the optical pickup optical system according to the embodiment of the present invention, the third wavelength is a wavelength for a compact disk.

Therefore, since a single lens can be used for Blu-ray 'disks, digital discs, and compact discs, a compact optical pickup optical system is provided. Therefore, high speed and low price are realized. In the optical pickup optical system according to the embodiment of the present invention, the luminous flux having the wavelength for the Blu-ray disc and the wavelength for the digital versatile disk is incident as parallel light on the objective lens, and the light having the wavelength for the compact disk. The source and the elephant have a finite conjugate relationship.

Therefore, an image-side numerical aperture of a wavelength for a compact disk can be realized. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an optical pickup system using an objective lens according to an embodiment of the present invention.

FIG. 2 shows an optical pickup system using a conjugate lens without using a collimator lens according to an embodiment of the present invention.

FIG. 3 shows a lens shape of the objective lens according to the embodiment of the present invention.

FIG. 4 shows the shape of the diffraction portion of the objective lens according to the embodiment of the present invention.

FIG. 5 shows a method for designing an objective lens according to an embodiment of the present invention.

FIG. 6 shows a method for designing a diffraction portion of an objective lens according to an embodiment of the present invention. FIG. 7 shows a design result by the design method of the objective lens according to the embodiment of the present invention.

FIG. 8 shows the result of calculating the diffraction efficiency with respect to the design result by the design method of the objective lens according to the embodiment of the present invention.

FIG. 9 is a numerical example of an optical system including an objective lens according to an embodiment of the present invention. '

FIG. 10 is a numerical example of the objective lens according to the embodiment of the present invention.

FIG. 11 is a diagram comparing the amount of spherical aberration in the objective lens according to the embodiment of the present invention with the spherical aberration in the objective lens constituted only by the aspherical shape having no diffraction area.

FIG. 12 shows spherical aberration in the objective lens according to the embodiment of the present invention. Fig. 13 shows a conventional optical pickup system.

Fig. 14 shows a conventional optical pickup system using a semiconductor laser element array. FIG. 15 shows the calculation results of the diffraction efficiency in the placed shape.

FIG. 16 is a diagram for explaining the optimum tilt amount.

FIG. 17 is a flowchart showing a method of designing an objective lens according to an embodiment of the present invention.

FIG. 18 is a flowchart showing a method for designing the shape of the grating portion in the objective lens according to the embodiment of the present invention.

FIG. 19 shows the shape (the number of steps N = 2) of the diffraction portion according to the embodiment of the present invention. FIG. 20 shows the shape (the number of steps N == 3) of the diffraction section according to the embodiment of the present invention. FIG. 21 shows an optical path of the objective lens according to the embodiment of the present invention.

FIG. 22 shows the spherical aberration of the objective lens according to the embodiment of the present invention.

FIG. 23 shows a point image intensity distribution (P S F) of the objective lens according to the embodiment of the present invention.

FIG. 24 shows the shape (the number of steps N = 2) of the diffraction section according to another embodiment of the present invention.

FIG. 25 is a diagram showing an optical path of an objective lens according to another embodiment of the present invention.

FIG. 26 shows the diffraction efficiency of the first-order diffracted light and the second-order diffracted light with respect to the wavelength. FIG. 27 shows a flow chart of the objective lens designing method of the present invention.

FIG. 28 shows an optical path diagram of a BD light beam by the objective lens according to one embodiment of the present invention.

FIG. 29 shows an optical path diagram of a DVD light beam by the objective lens of one embodiment of the present invention.

FIG. 30 shows an optical path diagram of a CD light beam by the objective lens according to one embodiment of the present invention.

FIG. 31 shows an intensity distribution diagram of a BD light beam by the objective lens according to one embodiment of the present invention.

FIG. 32 shows an intensity distribution diagram of a DVD light beam by the objective lens according to one embodiment of the present invention.

FIG. 33 shows an intensity distribution diagram of a CD light beam by the objective lens according to one embodiment of the present invention. FIG. 34 shows an optical path diagram of a light beam for BD by an objective lens according to another embodiment of the present invention.

FIG. 35 shows an optical path diagram of a DVD light beam by an objective lens according to another embodiment of the present invention.

FIG. 36 shows an intensity distribution diagram of a BD light beam by an objective lens according to another embodiment of the present invention.

FIG. 37 shows an intensity distribution diagram of a DVD light beam by the objective lens according to another embodiment of the present invention.

FIG. 38 is a flowchart showing the procedure for designing the diffractive optical element of the present invention.

FIG. 39 is a flowchart showing the procedure for designing the diffractive optical element of the present invention.

FIG. 40 is a conceptual diagram showing the behavior of the diffracted light.

FIG. 41 shows the relationship between the grating period and the diffraction efficiency in the present invention and the conventional diffractive optical element.

FIG. 42 shows the cross-sectional shape and the first-order diffraction efficiency of the diffractive optical element of the present invention.

FIG. 43 shows the relationship between the numerical aperture and the diffraction efficiency in the diffractive optical element of the present invention.

FIG. 44 is a diagram showing a diffraction grating having a special shape according to the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

One embodiment of an objective lens according to the present invention used in an optical pickup system will be described with reference to FIG.

The light beam 6a emitted from the semiconductor laser 1 has a visible red wavelength, and this light beam can be changed its optical axis direction by the rising mirror 2. Next, the light beam 6a emitted from the semiconductor laser 1 passes through an element 3 (for example, a collimator lens) that converts the light beam 6a into a parallel light, so that the light becomes a parallel light. Further, after passing through the objective lens 4a, the light is focused on a thin optical recording medium 5a (DVD).

The light beam 6b emitted from the semiconductor laser 1 has a near-infrared wavelength, and this light beam can be changed its optical axis direction by the rising mirror 2. Next, the light beam 6b emitted from the semiconductor laser 1 passes through the element 3 for converting the light beam into substantially parallel light, so that the light beam becomes substantially parallel light. Further, after passing through the objective lens 4b, the thick optical recording medium 5b ( Focus on CD).

The laser beam condensed on the optical recording medium must have a light-condensing ability that is almost diffraction-limited, and the wavefront aberration amount must be 0.07: RMS or less. Further, for DVD, 0.035 L RMS or less is preferable.

The semiconductor laser 1 may be an array of semiconductor laser elements emitting two different wavelengths. Semiconductor lasers commonly used in DVDs have visible red wavelengths, and semiconductor lasers used in CDs have near infrared wavelengths. Here, the light beam 6a has a visible red wavelength (for example, 660 nm), and the light beam 6b has a near-infrared wavelength (for example, 780 nm).

Although the objective lenses 4a and 4b according to the present invention are common objective lenses, the objective lenses 4a and 4b are shown moving in the optical axis direction due to the difference in wavelength and the difference in thickness of the optical recording medium.

The numerical aperture for focusing on a thin optical recording medium (eg, a DVD with a thickness of 0.6 mm) is 0.6, and the numerical aperture for focusing on a thick optical recording medium (eg, a CD with a thickness of 1.2 mm) Is approximately 0.45. The diffraction grating section is arranged on the optical recording medium side of the objective lens.

In the above embodiment, a collimator lens for making the light of the semiconductor laser parallel between the semiconductor laser and the objective lens for condensing the light of the semiconductor laser on the optical recording medium is used. Also, the present invention can be applied to a conjugate lens excluding the collimator lens as shown in FIG.

FIG. 3 shows a lens surface shape on the optical recording medium side of an objective lens including a diffraction grating portion according to one embodiment of the present invention. The shape of the objective lens optical recording medium side is a central region having no diffraction portion, that is, a first region, a diffraction region surrounding the central region having no diffraction portion, that is, a second region, and diffraction surrounding the diffraction region. It is divided into a region having no part, that is, a third region. Each region has an aspheric shape, and the aspheric shape of each region is different.

The aspherical shape in each area is represented by the following formula, _ + Υ

04762

18 Figure 4 shows only the shape of the diffraction grating when the substrate in the diffraction area is flat. The width of the staircase is variable according to the phase function of the staircase-shaped diffraction grating within one period.

The phase function that determines the shape of the diffraction grating is represented by the following equation. ≠ f) = C ² + C ₂ r ⁴ + C ₃ r ⁶ + C ₄ r ⁸ + When the phase function is set according to the above equation, the ray tracing equation is 1 in the cosine of the ray in the X direction and 1 in the y direction. Is m,

, λ δφ

/ ―

2π ox λ φ

m =-

2π oy

It becomes. The phase function is determined by determining the coefficients of the equation by ray tracing so as to minimize the aberration of the light beam for CD.

As described above, in the objective lens according to the present invention that records or reproduces data on optical recording media having different thicknesses such as DVDs and CDs, the aspherical shape is changed for the region where the spherical aberration increases, and the diffraction by the diffraction grating is performed. Aberration correction is performed using the effect described above.

The design procedure of the objective lens according to the present invention will be described based on the flowchart of FIG. In step S5010 of FIG. 5, first, second and third regions on the surface of the objective lens are determined. The first area is a DVD and CD shared area. Since the first region does not have a diffraction grating structure, no energy loss occurs. Therefore, it is desirable to make the first area as large as possible. For this reason, the first area is set as large as possible so that the amount of aberration is allowed to an allowable value while checking the amount of aberration of DVD and CD. Assuming that the numerical aperture at which the light beam passing through the first area is focused on the optical recording medium is NA1, NA1 is preferably NA1 0.37. Since the light beam for the CD passes only through the first and second areas, the outer diameter of the second area is determined by the numerical aperture (NA) required for recording and reproducing the CD. Depending on the specification of the CD, NA is in the range of 0.3-0.5. The outer diameter of the third area is determined by the numerical aperture (NA) required for recording and reproducing to and from DVD. NA is 0, depending on DVD specifications The range is 4-0.

In step S5020, the lens surface shape in the first area is designed. Since the first area is a common area for DVD and CD, the lens surface shape is determined in consideration of the optical path and aberration of both DVD light and CD light.

In step S5030, the lens surface shape in the second area is designed. In this embodiment, the lens surface shape in the second area is the optical path of only the DVD light beam.

Determined in consideration of aberration.

In step S5040, the shape of the diffraction section in the second area is designed. The shape design of the diffraction section will be described later in detail.

In step S5050, the lens surface shape in the third area is designed. Since the third area is a DVD-only area, the lens surface shape in the third area is D

Determined in consideration of the optical path and aberration of only the VD light beam.

The design of the shape of the diffraction portion in the second region of the objective lens according to the present invention will be described with reference to the flowchart of FIG. Here, the wavelength of the light beam for DVD is, for example, 655 nm, and the wavelength of the light beam for CD is, for example, 790 nm. Steps

In S6010, an optical path difference function or a phase correlation number is determined so as to correct the optical path of the CD light beam. In the present embodiment, the lens surface shape in the second area is D

Since the optical path and aberration of only the VD light beam are taken into consideration, the optical path of the CD light beam is corrected using a phase function.

In step S6020, the step amount (per step) of the staircase-shaped grating portion is determined from the wavelength of the DVD light beam (for example, 655 nm), using the DVD light beam as the 0th-order diffracted light. Since the light beam for DVD is the 0th-order diffracted light, the step amount is the wavelength.

The refractive index of the lens is n and the refractive index around the lens is n. As λ ₀ / (η—η.

). In the present embodiment, η = 1.5407, η. = 1. In step S6030, assuming that the number of steps is Ν and a positive integer is m, an arbitrary peak wavelength λ ₂ in the first-order diffracted light is obtained.If the diffraction grating shape has an upward-sloping shape, l _z = W (Nra + 1) X λ ₀ , and if the diffraction grating shape has a shape that rises to the left, Two

20

X ₂ = N / (Nm-l) XX ₀ The above equation was empirically obtained by data analysis and data analysis. ·

In the present embodiment, the following equation is used because the shape of the diffraction grating has an upward-sloping shape as shown in FIG. Obtain the 0th order diffracted light wavelength. The refractive index of the lens is n and the refractive index around the lens is n. As a step amount. (n−n ₀ ), the positive integer is m = 1, and the results when N = 5, 6, 7, 8 are shown in the first to fourth rows of the table shown in FIG. The peak wavelengths (unit: nm) of the first-order diffracted light are 818.75, 786, 764.17, and 748.57, respectively. Also, the 0th-order diffracted light wavelength is L ₀ , the refractive index of the lens is η, and the refractive index around the lens is η. As a step amount. The result when (= 3, 4, 5, and 6 is set to 倍 = 3, 4, 5, and 6 when the positive integer is πι = 1 and the result is twice as large as / (η−η ₀ ). The peak wavelengths (unit: nm) of the first-order diffracted light are 786, 748.57, 727.78, and 714.55, respectively.

In step S6040, it is determined whether the peak wavelength λ c obtained as described above is sufficiently close to the other CD light wavelength; I k (for example, 790 nm). Specifically, the following equation is calculated as the first-order diffracted light peak error.

I (λ k-n c) / 'λ k I Of the first to eighth rows of the table shown in FIG. 7, the first-order diffracted light peak errors in the second and fifth rows are small. , (For example, 1% or less), so that the number of steps is six (the zero-order diffracted light wavelength is obtained. The refractive index of the lens is n, the refractive index around the lens is n, and the step amount is ぇ ./ ( n-n.), or three steps (obtain the 0th-order diffracted light wavelength, the refractive index of the lens is n, and the refractive index around the lens is n.) and the amount of step is λ ₀ / (η — Twice of η ₀ )). As described above, the step amount and the number of steps of the staircase shape are determined based on the wavelength of the light beam for CD and the light beam for DVD.

The allowable range of the above peak error is determined from the relationship between the wavelength near the diffraction efficiency peak and the diffraction efficiency. In general, when the diffraction efficiency is allowed to fall from the peak value by about 20%, the allowable range of the above peak error is 10 to 15% for a two-stage diffraction grating, and for a three-stage diffraction grating. 4 to 8%, 2 for 4-stage grating ~ 7%, 2 ~ 5 ° / for a 5-stage grating. Range.

In FIG. 7, the optimum tilt amount (t l) indicates the tilt amount with respect to the diffraction grating bus as shown in FIG. Tilt by 2 X t 1 with respect to the grating bus. Thereby, the efficiency balance between the first-order diffracted light and the -1st-order diffracted light is adjusted. The value of the optimum tilt amount (t 1) is, for example, 1 of the peak wavelength of the zero-order diffracted light.

In step S6050, as shown in FIG. 4, the width of the stair is determined from the amount of the step based on the shape of the phase function. That is, the width of the staircase is made variable by matching the staircase-shaped diffraction grating with the phase function within one period.

In step S6060, the diffraction grating depth is recalculated based on the diffraction grating shape obtained by the above procedure, taking into account the angle of the light beam incident on the diffraction section. Specifically, assuming that the incident angle is e, a new step amount corresponding to the incident angle is obtained based on a value obtained by multiplying the step amount obtained above by cosΘ. The depth of the diffraction grating is obtained by multiplying the amount of step by (the number of steps is 1).

As described above, in the present embodiment, the lens surface shape is determined based on the optical path and aberration of the DVD light beam in the second region, and the optical path and aberration of the CD light beam are corrected by the phase correlation number. As another embodiment, the lens surface shape may be determined based on the optical path and aberration of the CD light beam in the second area, and the optical path and aberration of the DV light ray may be corrected by the position correlation number.

FIG. 8 is a graph showing the result of calculating the diffraction efficiency of the diffraction grating shape obtained by the design method of the present embodiment by vector calculation. This graph plots wavelength on the X-axis and diffraction efficiency on the Y-axis. The diffraction efficiency of the 0th-order diffracted light at the wavelength of 655 nm for DVD light is approximately 83% and slightly more than 6◦%, respectively, for the CD light at 790 nm. It can be seen that high diffraction efficiency is obtained. In the present embodiment, the angle of the light beam incident on the diffraction section is 18 degrees, the number of steps of the diffraction grating is 5, and the depth of the diffraction grating is 4. As described above, the optimal number of steps is 6 from the point of the peak wavelength, but the number is set to 5 considering the overall diffraction efficiency and workability including other orders.

(Numerical example 1)

9 and 10 show numerical examples of the objective lens according to the present embodiment. One embodiment shown here is a laser diode and a pick-up lens. A collimator lens that converts the light from the semiconductor laser into parallel light during scanning is arranged. In FIG. 9, APL indicates a cyclic olefin copolymer, and PC indicates polycarbonate. The paraxial R of the aspherical coefficient in FIG. 10 indicates the paraxial radius l Zc in the above equation representing the aspherical shape, and 'conic' indicates a constant k.

FIG. 11 shows a comparison between the spherical aberration of one embodiment of the present invention and the spherical aberration of an objective lens composed of only an aspherical surface having no diffraction region under the same conditions. According to the comparison result of FIG. 11, the spherical aberration of the embodiment of the present invention is very small, and shows sufficient optical characteristics for recording or reproducing on an optical recording medium. On the other hand, it can be seen that the use of an objective lens composed of only an aspherical surface exceeds the standard value for recording or reproduction on an optical recording medium.

FIG. 12 shows a graph of spherical aberration in one embodiment of the present invention. The two figures above show the spherical aberration for a thin optical recording medium (DVD) and the spherical aberration for a thick optical recording medium (CD). The vertical axis indicates the lens height (pupil radius), and the horizontal axis indicates the aberration at the corresponding lens height. The figure below shows the spherical aberration of a thick optical recording medium (CD) given the numerical aperture necessary for recording and reproducing on a thin optical recording medium. The vertical axis shows the numerical aperture, and the horizontal axis shows the aberration at the corresponding numerical aperture. The spherical aberration of light passing outside the lens area corresponding to the numerical aperture required for a thick optical recording medium (CD) increases sharply, and becomes flare light on a thick optical recording medium. , Indicating that no light was collected.

The objective lens in one embodiment of the present invention can also be manufactured by force using a cyclic olefin copolymer as a material, or by other plastic materials. Another embodiment of the present invention will be described below for an objective lens in an optical pickup system for CD and DVD. The first light beam is a laser beam having a wavelength of 785 nm for CD, and the second light beam is a laser beam having a wavelength of 660 nm for DVD.

A method for designing an objective lens as an imaging optical element according to the present invention will be described with reference to the flowcharts of FIGS.

In step S1010 in Fig. 17, the optical paths of the first and second rays are considered. The lens surface shape in the first area and the second area is designed with due consideration. Note that the lens surface shape of the second region is designed so as to converge on the image forming surface in consideration of only the optical path of the second light beam. The surface shape of the surface other than the surface having the first and second regions is also designed at the same time. Here, the first region includes the optical axis, and is, for example, within a certain distance from the optical axis, and is a region where the first and second light beams are focused on the imaging surface. The size of the first area is determined by the numerical aperture on the image side of the objective lens for the CD. The numerical aperture on the objective lens image side for CD is, for example, 0.457. Further, the second region exists around the first region, and is, for example, within a certain distance from the optical axis, and does not focus the first light beam on the imaging surface, but transmits the second light beam on the imaging surface. This is the area where light is collected. The size of the second area is determined from the numerical aperture on the image side of the objective lens for the DVD. The numerical aperture on the objective lens image side for DVD is, for example, 0.652.

In step S1020 in FIG. 17, the shape of the diffraction portion in the second region is designed so that the first light beam, that is, one laser beam for CD, is not converged on the image plane. Specifically, when passing through the second area, most of the second light beam, that is, the laser beam for DVD, passes through the diffracting portion as the 0th-order diffracted light, and most of the first light beam has the first or higher order. It is designed to pass through the diffraction section as diffracted light of 1st order or less. Most of the second light beam that has passed through the diffraction portion as the 0th-order diffracted light is condensed on the image plane because the optical path is determined by the lens surface shape. Most of the first light rays that have passed through the diffraction portion as first-order or first-order or less primary light do not converge on the image plane. The details of the shape design of the diffraction section will be described later in detail based on the flowchart of FIG.

In the above description, the lens surface shape in the second region is designed so that the second light beam, that is, the laser beam for DVD, is focused on the imaging surface, and the first light beam, that is, the laser beam for CD, is formed into an image. The shape of the diffractive portion in the second area was designed so as not to focus on the surface, but the lens in the second area was designed so that the first light beam, that is, one laser beam for CD, was not focused on the imaging surface. The surface shape may be designed, and the shape of the diffractive portion may be designed so that the second light beam, that is, the DVD laser beam, is focused on the imaging surface.

The details of the shape design of the diffraction section are described below based on the flowchart in Fig. 18. I do. The shape of the diffraction portion in the second region is a stepped shape when the substrate is a flat surface.

In step S210 of FIG. 18, a phase function that determines the shape of the diffraction section is designed so that the first light beam, that is, one laser light beam for CD is not converged on the image plane. In the XYZ rectangular coordinate system, when the optical axis is set to the Z axis, the phase function is expressed by the following equation, for example.

^ = C2xA ² + C4xh ⁴ + C6xh ⁶

Here, h is a distance from the Z axis in a plane perpendicular to the Z axis, and h = ^ jx ² + y ² . The ray tracing equation when the phase function is expressed by the above equation is expressed by the following equation, where the cosine in the X direction of the ray is 1 and the cosine in the y direction is in.

I λ δφ

2π ox λ όφ

m =

2π dy

The coefficients in the above equation of the position correlation coefficient are determined by ray tracing so as not to focus the first ray on the image plane.

It should be noted that the form of the phase function is not limited to the above equation as will be described later as a numerical example.

Next, in step S220, the step amount of the staircase shape is determined so that the diffraction efficiency of the 0th-order diffracted light approaches the peak in the wavelength of the second light beam, that is, the wavelength of the DVD laser light beam. Specifically, 1 is determined based on the following formula, assuming that the level difference (per step) is 1.

1 = λ 0 · cos θ / \ η—— n ₀ j .1 j

Here, since the second light beam, ie, the laser beam for DVD, is the 0th-order diffracted light, p is an integer.

e. = p66 nm (2)

It is. Also, 0 is the angle of incidence with respect to the diffraction part, n is the refractive and refractive index of the objective lens, n. Is the refractive index around the objective lens. Next, in step S 2030, the number of steps N is determined so that the difference between the wavelength obtained from the following equation; L i and the first wavelength, that is, the wavelength of the laser beam for CD, is equal to or less than a predetermined value. .

λ i = [N / (Nm + α)]-λ ₀ (3)

Here, m is an arbitrary integer, and is a diffraction order other than the 0th order. Equation (3) above is an equation based on an empirical rule for finding peak wavelengths of diffraction orders other than the 0th order. When the wavelength approaches the first wavelength, the peak wavelength of the diffraction order approaches the first wavelength. Therefore, the energy of the first light beam increases in the diffracted light of the diffraction order α, and approaches 0 in the 0th-order diffracted light.

Next, in step S2040, the width of the steps is determined based on the phase function, the amount of steps, and the number of steps.

Next, in step S205, the diffraction efficiency of the diffraction section is obtained by vector calculation.

In step S2060, it is determined whether or not the diffraction efficiency is within a desired range. If the diffraction efficiency is within the desired range, the process ends. If not, repeat steps S2020 to S2050 while adjusting various parameters.

Here, steps S2020 and S203 will be described based on the following numerical examples.

η = 1.54

n _Q = 1.0

0 = 25 degrees

Here, if p = 3, from the above equation (2)

. = 3660 = 1980

It becomes.

Substituting this value into equation (1) gives

1 = λ a · cos θ / κ η-- η 0

= 1 980-cos (π25 / 1 80) / (1.54—1.0)

= 3323

In this way, the step difference per step 3.323 μηι is obtained.

e. Into the equation (3), and m = 3

a =-l

Then, when N = 2, 'λ i = (2/5)-1 980 = 792

It becomes. This value is very close to the first wavelength, ie, the wavelength of the CD laser beam, 785 nm. Since the difference is 7 nm, the ratio (peak error) to the first wavelength is 0.89%. Therefore, the first-order diffracted light approaches a peak near the first wavelength. Since the energy of the first ray increases in the first-order diffracted light, it is expected to approach zero in the zero-order diffracted light. That is, the zero-order diffracted light approaches the peak at the second wavelength, and approaches zero at the first wavelength. In this way, the number of steps N == 2 is obtained. For example, the number of steps may be determined so that the above ratio (peak error) is 5% or less.

The allowable range of the above peak error is determined from the relationship between the wavelength near the diffraction efficiency peak and the diffraction efficiency.

Also, if P = 2, from the above equation (2)

λ. = 2 6 60 = 1 320

It becomes.

Substituting this value into equation (1) gives

1 = L 0cos Θ / \ n one n ₀

= 1 320-cos (π-25/1 80) / (1.54-1.0)

= 22 1 5

In this way, 2.215 / zm, the level difference per step, is obtained.

λ. Into the equation (3), and

m = 2

α = — 1

Then, when Ν = 3,

λ i = (3/5) 1 320 = 792

It becomes. This value is very close to the first wavelength, ie, the wavelength of the CD laser beam, 785 nm. Since the difference is 7 nm, the ratio (peak error) to the first wavelength is 0.89%. Therefore, the first-order diffracted light is near the first wavelength At the peak. Since the energy of the first ray increases in the first-order diffracted light, it is expected to approach zero in the zero-order diffracted light. That is, the zero-order diffracted light approaches the peak at the second wavelength, and approaches zero at the first wavelength. In this way, the number of steps N = 3 is obtained. For example, the number of steps may be determined so that the above ratio (peak error) is 5% or less.

FIGS. 19 and 20 show the case where the grating shape of the diffraction section obtained by the above procedure is combined with the aspherical shape of the lens surface and the case where it is used alone. FIG. 20 shows the case where the number of steps is N = 2, and FIG. 20 shows the case where the number of steps is N = 3.

(Numerical example 2)

A numerical example of the present embodiment will be described based on FIG. The number of steps is N = 2. The optical configuration is shown in Table 1 below. table 1

The laser beam from the semiconductor laser (LD) light source is a collimated second beam when using DVD. When using a CD, use an objective lens (first lens in Table 1) at a distance of 48.6 mm from the stop surface. If necessary, provide a collimator lens before the objective lens.

The center-to-center distance of the objective lens is 2.2 mm. The distance from the image side of the objective lens to the substrate is 0.961 mm for DVD and 1.161 mm for 〇0. Substrate thickness is 0.6 mm for DVD, and for CD To 1.2 mm. The image-side numerical aperture (NA) of the objective lens is 0.652 for DVD. It is 0.45 7 when it is 0. The focal length of the objective lens is 2.74 for DVD and 2.75 for CD.

The surface on the light source side of the objective lens (the stop surface in Table 1) is a compound aspheric surface. The composite aspheric surface is three surfaces j = 1, 2, and 3 separated by two coaxial circles centered on the optical axis. The three faces are defined by the formula below and Table 2 below. Note that h in the following equation is

Defined by

z = A4j Xh ⁴ + A6j xh ⁶ + ASj xh ^s + AlOj xh ¹⁰ + d-

Here, kj is a constant indicating the shape of the curved surface, Rj is the central radius of curvature, and A4j to A10j are correction coefficients. Also, dj is the shift amount of the surface along the Z axis.The innermost radius of the second surface in Table 2 is set so that the aberration is within a predetermined range while checking the aberration state of DVD and CD. Determine as large as possible. The innermost radius of the third surface is determined by the image-side numerical aperture of the CD.

JP2004 / 004762

29 Table 2

The surface on the image side of the objective lens (the two surfaces in Table 1) is a special DOE surface with a diffraction part. The aspheric surface of this surface is defined by the following equation and Table 3. z =

Here, k is a constant indicating the shape of the curved surface, R is the center radius of curvature, and A4 to A10 are correction coefficients.

Table 3

The phase function of the diffraction part is defined by the following equation (Table 4).

Where C2, C4, and C6 are coefficients. Table 4

In Table 4, the inner diameter of the phase function is defined as the first area including the optical axis on the image side surface of the objective lens and the diffraction area provided around the first area, and the shape is determined by the phase function. This is the distance between the optical axis and the boundary with the second region. The outer edge of the second region whose shape is determined by the number of phase correlations is determined by the effective diameter of the image-side surface of the objective lens.

FIGS. 21 to 23 show the application results of the above numerical examples. The upper part of FIG. 21 is an optical path diagram of a DVD light beam (second light beam). The middle part of FIG. 21 is an optical path diagram when the CD light beam (first light beam) is converted to first-order diffracted light. The lower part of FIG. 21 is an optical path diagram when the CD light beam (first light beam) is the first-order diffracted light. According to the optical path diagram, in the CD optical system, light rays outside the required image side numerical aperture flare on the image plane due to the wavelength selection diffraction grating, and in the DVD optical system, light rays are emitted at the specified numerical aperture. It is focused on one point on the image plane. Thus, it can be seen that the image-side numerical aperture is controlled by the wavelength selection diffraction grating.

FIG. 22 shows a spherical aberration diagram. In the spherical aberration diagram, the horizontal axis represents the distance in the optical axis direction, the vertical axis represents the height at which the ray enters the entrance pupil, and the position where the ray intersects the optical axis is plotted. The upper part of Fig. 22 is for DVD light (second light), the middle part of Fig. 22 is for CD light (first light) as first-order diffracted light, and the lower part of Fig. 22 is In this case, the light beam for CD (first light beam) is the first-order diffracted light. In the optical system of DVD, the spherical aberration is almost optimized, and the power of the system is increased. Similarly, when the first-order light is used in the wavelength-selective diffraction grating and the first-order light is used in the optical system of the CD, the spherical aberration outside the required image-side numerical aperture is large, and the CD is read and written. It can be seen that only the required number of rays for the image-side numerical aperture are captured. Figure 23 shows the point spread function (PSF). The upper part of Fig. 23 is for DVD light (second light), the middle part of Fig. 23 is CD light (first light) as first-order diffracted light, and the lower part of Fig. 23 is This is the case where the CD light beam (first light beam) is the first-order diffracted light. Since the aberration is suppressed at the required image-side numerical aperture for both DVDs and CDs, a point image intensity distribution (PSF) that satisfies recording and reproduction for DVDs and CDs is formed. Table 5 shows the values of the focused spot diameter and sidelobe. The side rope value (%) is the ratio of the side lobe height to the main beam height in the PSF diagram. In Table 5, the diameter of the converging spot without the diffraction grating is 1.14, which is smaller than the converging spot diameter of the CD. On the other hand, the diameter of the converging spot with the diffraction grating is 1.44, which is the specification of the converging spot diameter of CD. Table 5

Table 6 shows the diffraction efficiencies of the light of the first wavelength 785 (thigh) and the light of the second wavelength 660 (run) in the 0th order and the 1st order of the soil for the number of steps N = 2 and 3. . For example, when the number of steps is N = 2, the first wavelength has a diffraction efficiency of 37% for the first-order and first-order diffracted light, and a diffraction efficiency of 0% for the 0th-order diffracted light. . In the second wavelength, the diffraction efficiencies of the first-order and _first-order diffracted lights are each 0%, and the diffraction efficiency of the zero-order diffracted light is 80%. Thus, the numerical values in Table 6 show that the order is properly switched between the first and second wavelengths. Table 6

(Numerical example 3)

As another numerical example, when the phase function of the diffraction unit is defined by the following equation and Table 7, the grating pitches of the diffraction unit are equally spaced.

Φ (h) = C X h

Where C is a coefficient. Table 7

Fig. 24 shows the case where the grating shape of the diffraction part is combined with the aspherical shape of the lens surface and the case where it is used alone. Fig. 25 shows the application results when the data in Tables 1 to 3 are used in the above case where the grating pitch of the diffraction section is at equal intervals. The upper part of FIG. 25 is an optical path diagram in the case where the light beam for CD (first light beam) is the first-order diffracted light. The lower part of FIG. 25 is an optical path diagram when the CD light beam (first light beam) is the first-order diffracted light. According to the optical path diagram, in the CD optical system, light rays outside the required image-side numerical aperture flare on the image plane due to the wavelength-selective diffraction grating. It can be seen that the numerical aperture is controlled.

The objective lens in one embodiment of the present invention uses a cyclic olefin copolymer as a material, but can also be manufactured from other plastic materials. In yet another embodiment of the present invention, a BD, DVD, and CD A description will be given of a single objective lens used. The first wavelength light is for BD light (wavelength 405 nm), the second wavelength light is for DVD light (wavelength 655 nm), and the third wavelength light is for CD It is a light beam (wavelength 785 nm). The image side numerical apertures are 0.85, 0.65 and 0.47, respectively.

An objective lens according to yet another embodiment of the present invention includes a diffraction grating on at least one surface, wherein the light beam of the first wavelength passes through the diffraction grating as second-order diffracted light, and the second and third wavelengths Are designed to pass through the diffraction grating as first-order diffracted light.

The diffraction grating is a blazed grating. The blazed grating is used for the following reasons. The stepped shape having both spherical and aspherical surfaces is very difficult to process, and it is almost impossible to install dozens of zones.

The depth 1 of the diffraction grating is calculated so that n is the refractive index of the lens and the diffraction efficiency for the first or second order diffracted light of each wavelength is 70% or more.

(7/4) X / (n-l) 1 (9/4) X Z (n-1)

Determined by In this case, the diffraction efficiency of each wavelength is specifically shown in Table 8 below.

JP2004 / 004762

34 Table 8

When 2χλι / (η-1)

7 / 4χλι / (η-1)

9 / 4χλι / (η-1)

Figure 26 shows the first and second order diffracted light when the grating depth is optimized (maximizing the diffraction efficiency) for the second order diffracted light having the first wavelength (405 nm for BD). 6 shows diffraction efficiency with respect to wavelength. In FIG. 26, the solid line indicates the diffraction efficiency of the second-order diffracted light, and the dashed line indicates the diffraction efficiency of the first-order diffracted light. The diffraction efficiency of the first-order diffracted light is nearly optimized at 1 at the second wavelength (655 nm for DVD) and at 3rd wavelength (785 nm for CD). I have. Therefore, the diffraction efficiency is almost optimized for the second and third wavelengths by optimizing the diffraction efficiency for the first wavelength with the grating wavelength being the second-order diffracted light and optimizing the grating depth for the first wavelength. Is done.

The design method of the objective lens of the present invention will be described based on the flowchart of FIG.

In step S1000 in Fig. 27, the entrance pupil diameter is set and the outermost ray angle is determined as a constraint so as to realize the image-side numerical aperture for each wavelength of light. The lens surface shape and the phase function of the diffraction grating are determined so as to satisfy the constraint on the outermost ray angle. In step S1030, the aberrations of the light beams of each wavelength on the respective image planes are calculated. ―

In step S104, it is determined whether the aberration is within an allowable range. For example, if the wavefront aberration is RMS0.07 or less in wavelength units, it is within the allowable range. RMS is the value obtained by calculating the average value of the square of the wavefront aberration over the entire reference wavefront and taking the square root thereof. If it is within the allowable range, the process ends. Step S if not within tolerance

Proceed to 1 0 5 0.

In step S1005, the correction amount of the lens surface shape and the phase difference function of the diffraction grating is determined, and the process returns to step S100.

A numerical example of the present invention will be described below with reference to the drawings and tables.

(Numerical example 4)

28 to 30 show optical path diagrams of BD, DVD, and CD of the objective lens of Numerical Example 4, respectively. Table 9 shows the lens data of Numerical Example 4. Figure

As shown in FIGS. 28 and 29, the luminous flux for BD and DVD is incident on the objective lens as parallel light. The light beam for CD is incident on the objective lens as a light beam with a spread angle from the light source at a distance of 21 mm from the stop surface. That is, the CD light source and the image constitute a finite conjugate system. On the stop surface (incident side surface) of the lens, a diffraction grating in the form of a plate is provided. The exit side surface of the lens is composed of a special surface that is divided into at least one band-shaped region surrounding the optical axis and a central region including the optical axis, and each region is defined by a separate surface.

4004762

36

Table 10 shows the specifications of the incident surface.

Table 10

The lens shape of the entrance side is expressed by the following aspherical formula

Here, the z-axis coincides with the optical axis, R is the radius of curvature, K is the eccentricity, and A4, A6, A8, and A10 are the aspheric coefficients. Here, d is assumed to be zero. H is the distance from the optical axis and is represented by the following equation. h = ^] x ² + y ^{2 The} phase function that determines the grating interval of the diffraction grating on the incident surface is represented by the following equation.

^{O = C2xr 2 + C4xr + C6xr} 6 + C8xr 8 + C10xr 10 + C12xr 12 (2) where, C2, C4, C6, C8 , C10, C12 represents the phase function coefficients. R is represented by the following equation. the specification of r = ^ x ^² + ² exit surface shown in Table 1. The exit side is divided into two strips surrounding the optical axis and a central area containing the optical axis, each area being defined by a separate surface. The surface of the central region including the optical axis is referred to as a first surface, the surface of the region surrounding the first surface is referred to as a second surface, and the surface of the region surrounding the second surface is referred to as a third surface.

Surface 1 Surface 2 Surface 3 Boundary radius 0.3744 0.8722

0.00115 0.00168 Radius of curvature 3.6267 4.7991 4.4313

4th order coefficient0.4560 -0.0285 -0.0532

6th order coefficient 2.8942 0.0141 0.0067 Aspheric coefficient 8th order coefficient 係数 9.4556 -0.0822 0.0016

10th order coefficient 3.1137 0.0921 -0.0081

12th order coefficient 42.1682 -0.0356 0.0041 The shapes of the first to third surfaces are represented by the following aspherical expressions. .- ^{/ Rl xn} + Ai4 xh ⁴ + Ai6 xh ⁶ + Ai8 xh ^s + AilO h '° + di l + ^ l- (l + Ki) x (l / Rf) ² h ² _{(3 )} here, z-axis coincides with the optical axis, i is represents the surface number counted from the center, Ri is the radius of curvature, Ki is eccentricity, Ai4, ai6, Ai8, AilO represent aspherical coefficients. di represents the movement amount (step amount) of the other surface on the optical axis with respect to the first surface. The amount of movement in the traveling direction of light is positive. H is the same as in the case of equation (1).

Here, the boundary radius and the step amount are determined based on the following concept. In the design method of Fig. 27, the boundary radius is set so that the optical path difference (ie, wavefront difference) of the light beam condensed on the image plane is minimized. The step is set so that the variance of the optical path difference of the light beam condensed on the image plane is minimized, that is, the shape is numerically and differentially (slope) continuous in the boundary region.

31 to 33 show point image intensity distributions according to Numerical Example 4.

(Numerical example 5)

34 and 35 show the optical path diagrams of BD and DVD of the objective lens of Numerical Example 5, respectively. Table 12 shows lens data of Numerical Example 5. As shown in FIGS. 34 and 35, the luminous flux for BD and DVD is incident on the objective lens as parallel light. The aperture surface (incident side surface) of the lens is divided into a common area for DVD and BD on the inside and a dedicated area for BD on the outside, with different surface shapes and different diffraction gratings provided. Has been. The exit side of the lens is composed of a special surface that is divided into at least one band-like region surrounding the optical axis and a central region containing the optical axis, each region being defined by a separate surface.

Table 12

Table 13 shows the specifications of the incident side surface. The lens shape on the incident side surface is represented by the above-mentioned aspherical expression (1). However, outside the radius of 1.45 mm around the optical axis, a plane perpendicular to the optical axis is formed. The part represented by the aspherical formula (center area) is a common area for DVD and BD, and the part (band-shaped area) forming a plane perpendicular to the optical axis is a special area for BD. That is, the above boundary radius is determined from the DVD entrance pupil diameter of 2.9 mm.

The phase function that determines the grating interval of the diffraction grating on the incident surface is represented by the above equation (2). Table 13 shows the phase function of the common area (center area) for DVD and BD and the phase function of the dedicated area (band-shaped area) for BD.

Table 13

Table 14 shows the specifications of the emission side surface. The exit side surface is divided into one strip region surrounding the optical axis and a central region containing the optical axis, each region being defined by a separate surface. The surface of the central region including the optical axis is referred to as a first surface, and the surface of a region surrounding the first surface is referred to as a second surface. The shapes of the first and second surfaces are represented by the above equation (3). Table 14

Here, the boundary radius and the step amount are determined based on the following concept.

The radius is determined from the optical path of the outermost ray of DVD. The value of the step amount is determined so that the optical path difference between the light beam passing through the outermost part of the central area and the light ray passing through the most 內 side of the strip-shaped area is as small as possible.

Figures 36 and 37 show point image intensity distributions according to Numerical Example 5.

In Numerical Example 5, a specially shaped diffraction grating (diffractive optical element) described below is used instead of the blazed grating as the diffraction grating in the BD-only area on the incident side surface.

In the diffractive optical element of the present invention, at least a part of the grating period

λ <Α≤4λ

A diffractive optical element having a grating shape in the range of the above on the substrate, and in the periodic portion, a portion where the grating slope is steeper than the saw-shaped slope to increase the diffraction efficiency. It is configured to provide for at least a part of. When light is incident on a steeply inclined portion of the lattice shape according to the present invention, the incident angle becomes larger as compared with the conventional technology (blazed shape or saw-shaped shape), and transmitted light is totally reflected without generation. . The totally reflected light is re-incident on the adjacent grating shape. At this time, it is combined with another incident light by phase superposition, and is repeatedly reflected and finally transmitted light at an extremely small angle with respect to the slope ( Diffracted light) Is done. As a result, the diffraction efficiency is improved.

Generally, the phase function of the diffraction grating is Ψ, the distance from the optical axis is r, and the grating period is

2 π = Λ (d / d r)

Is established. This equation

λ <λ <4 λ

Substituting into

(2 π / 4 λ) <(ά / ά r) <(2 π / λ)

Here, substituting the first wavelength Xi-OSnm,

3879 (rad / mm) <(d / dr) <15514 (rad / mm)

It becomes.

On the other hand, from the phase function of the band-shaped region (peripheral region) in Numerical Example 5, if the range of the band-shaped region is 1.45 mm <r <l.9 mm,

8900 (rad / mm) <(d φ / d r) 110 11070 (rad / mm)

It becomes. Therefore, the condition of λ <Α <41 is satisfied.

According to one embodiment of the present invention, the steep portion is configured so that incident light causes total reflection. Therefore, coupling with another incident light by phase superposition is reliably performed, and the diffraction efficiency is improved.

According to one embodiment of the present invention, a line representing a grid slope in a grid cross section forms at least one inflection point. Therefore, there are parts where the grid slope is steeper than the saw-shaped slope.

According to one embodiment of the present invention, a line representing a lattice slope in a lattice cross section is composed of two or more curves having different curvatures. Therefore, there are parts where the grid slope is steeper than the saw-shaped slope.

According to one embodiment of the present invention, the refractive index of the substrate is the refractive index of the medium on the emission light side, and the height of the grating in the periodic portion is

The range is set to increase the diffraction efficiency. Therefore, the grating height is optimized taking into account the phase change that is proceeding in the grating structure. According to one embodiment of the invention, a central portion having a period greater than 4 λ, and λ <Α≤4λ

And a peripheral edge having a period in the range of. Therefore, by increasing the diffraction efficiency of the portion where the grating period of the diffractive optical lens is short, the light collection intensity is improved, and as a result, the numerical aperture can be increased. Further, by using a diffractive optical element having higher performance than before as a part of an optical device such as a reading optical system, the performance of the device can be improved.

In Numerical Example 5, the above-described peripheral portion is used as a diffraction grating for the BD-only area (the band-shaped area in Table 6) on the incident side surface.

According to one embodiment of the present invention, incident light is transmitted to the substrate. Ie

Thus, a transmission type diffractive optical element is obtained.

According to one embodiment of the present invention, it corresponds to incident light from above the grating portion. That is, a diffractive optical element that responds to incident light from above the grating can be obtained.

In the following, a diffractive optical lens will be described as an example of a diffractive optical element, but the present invention is not limited to a diffractive optical lens.

The design method of the diffractive optical element according to the present invention will be described with reference to the flowcharts of FIGS.

In step S310 in FIG. 38, initialization is performed. Initial settings include wavelength, refractive index, element size, target numerical aperture, and diffraction efficiency. In step S3200, a phase function is calculated. In step S300, the grid height of the element is calculated. In step S340, the lattice shape of the element is determined from the phase function.

In step S350, it is determined to which region each grid belongs. If the period of the grating structure of the diffractive optical element satisfies the following formula, the grating belongs to region 2; otherwise, the grating belongs to region 1.

λ <Α≤4λ

'In Numerical Example 5, the above-mentioned region 2 is used as the diffraction grating of the BD-only region (the band-shaped region in Table 6) on the incident side surface.

For the lattice with the period belonging to region 2, in step S360, the lattice Perform shape optimization. Here, “e” indicates the wavelength used. If the value is below the lower limit of the above equation, the first-order diffracted light does not appear and only the 0th-order light is transmitted, and condensing properties cannot be obtained. When the value exceeds the upper limit, the shape of the diffraction grating that maximizes the first-order diffracted light is a sawtooth shape known in the prior art. That is, since the grating period is sufficiently long with respect to the wavelength, there is no need to optimize the grating shape. Therefore, optimization of the lattice shape is not performed for a lattice with a period belonging to region 1 that does not satisfy the above expression. In step S370, it is determined whether the determination of each grid has been completed. If not, the process returns to step S3050.

In step S3800, the numerical aperture and the diffraction efficiency are calculated, and in step S3900, the calculation result is output.

Next, a method of optimizing the grid shape of the grid in the area 2 (step S3060 in FIG. 38) will be described with reference to FIG.

In step S4010 in FIG. 39, each grid shape is finely divided into M, and each height position coordinate value is set to P (I) (I = 1,..., M). In step S 4020, an arbitrary number R in the range of 0 to 1 is specified by a predetermined optimization algorithm for the area of P (I). In step S430, if R is larger than 0.5, P (I) is increased by a predetermined value, and if R is 0.5 or less, it is decreased by a predetermined value. As the optimization algorithm, for example, the simulated annealing method (Simulated Annealing Method) or the transmission algorithm (Genetic Algorithm) is used. In step S4040, the corrected shape is updated. In step S405, it is determined whether or not I = M. If I = M, the process returns to step S4020. If I = M, proceed to step S4060.

The height li of the grid in region 2 is in the range of the following expression.

3 λ, .3 λ

4 | η-κ ₀ | 2 | η- « ₀ | where 7 is the refractive index of the substrate, and is the refractive index of the medium on the exit light side. At this time, if the height h is out of the range of the above expression, the first-order diffraction efficiency is reduced, and the condensing intensity of the diffractive optical element is reduced. In region 2, the grating height is constant for the Fresnel lens. λ

h =-n-\

The reason is that in the region 2 where the grating period is close to the wavelength, it is necessary to consider the phase change while traveling in the grating structure. That is, the final phase difference differs from the case where the grating period is sufficiently large with respect to the wavelength. As a result, it is necessary to change the grating height in order to balance the phase difference. In step S460, the diffraction efficiency is calculated using an exact analysis method of electromagnetic waves. In step S 470, an evaluation function is calculated. The evaluation function φ is obtained by calculating the difference between the calculated value of the diffraction efficiency η and the target value for each order i of the diffracted light, and adding the weight W i to obtain the sum. In step S470, it is determined whether the value of the evaluation function φ is less than a predetermined value. If it is not less than the predetermined value, the process returns to step S4202, and if it is less than the predetermined value, the process ends.

Figure 41 shows the results of calculating the dependence of the first-order diffraction efficiency on each grating period. (a) shows the case of 偏光 -polarized light, and (b) shows the case of TM-polarized light. Here, the calculation used Rigorous Coupled Wave Analysis (RCWA) as a method to strictly reproduce the behavior of electromagnetic waves. Figure 41 shows the results for the sawtooth shape of the conventional technology and the results for the shape optimized by the above procedure to maximize the first-order diffraction efficiency. The refractive indices on the optical element substrate and the outgoing diffracted light side are 1.5 and 1.0, respectively. At this time, if the period is extremely long with respect to the wavelength regardless of the polarization direction in the conventional saw-tooth shape, the diffraction efficiency can obtain a value of about 90% or more, but as the period becomes shorter, It can be seen that the diffraction efficiency gradually decreases, and only about 20% can be obtained when the period is around 2λ. On the other hand, in the shape optimized to increase the first-order diffraction efficiency, the obtained diffraction efficiency is almost the same as that of the conventional technology (saw shape) when the period is longer than the wavelength. However, it can be seen that the optimized shape maintains about 80% even if the period is short.

Figure 41 (c) shows the difference between the first-order diffraction efficiencies of the incident light with respect to the polarization directions of the ΤΕ and ΤΜ waves. The prior art (saw shape) is indicated by a dotted line, and the effect of the present invention is indicated by a solid line. The difference in diffraction efficiency due to polarization is up to about 25% as the period becomes shorter in the saw-shaped shape, whereas the difference is It is about 13%, and the dependence of the diffraction efficiency on the polarization direction is smaller than that of the saw-shaped shape. This indicates that the dependency of the diffraction efficiency on the polarization direction of the incident light can be improved by the diffractive optical element of the present invention.

'FIG. 42 shows the cross-sectional shape of the diffractive optical element and the diffraction efficiency at each point when the grating shape of region 2 is optimized. For comparison, the results of the diffraction efficiency according to the prior art are also indicated by dotted lines.

The lattice cross-sectional shape in region 2 is represented by a curve including an inflection point, and has a slope that is steeper than the saw-shaped slope. As shown in Fig. 40 (b), when light is incident on the steep slope of the grid-shaped slope, the angle of incidence is larger than in the conventional technology, and there is no transmitted light and total reflection occurs. . The totally reflected light re-enters the adjacent grating shape, where it is combined with another incident light by phase superposition, repeatedly reflected, and finally transmitted through a very small angle with respect to the slope (diffraction). Light) is emitted. As a result, the diffraction efficiency is improved.

Here, “coupling by phase superposition” means superposition of waves. Since the light intensity difference between the light due to total reflection and the adjacent light due to direct incidence is small, the superposition of waves on the grating slope is effectively performed. As a result, as shown in (b) of FIG. 40, the intensity of the waves is increased by the superposition of the waves, and the wavefront satisfying a certain condition advances as reflected light again. To reinforce each other, it is necessary that the directions of the amplitudes match. As the shape condition of the grating structure, it is considered that a structure in which the traveling distance of the reflected wave to meet the adjacent incident wave is less than the wavelength is the minimum.

It should be noted that even in the prior art, the above-described effect is considered to be confirmed in a region where the grating period is almost equal to the wavelength. However, in the prior art, since the intensity of the adjacent incident light is larger than the intensity of the reflected light, the superposition of the waves predominates on the incident light, and the grid is directly used as shown in (a) of FIG. It passes through as 0th order light and does not result in an increase in 1st order diffracted light.

Thus, the grating shape of the present invention makes it possible to obtain diffracted light having higher diffraction efficiency than the sawtooth shape of the prior art.

Referring back to FIG. 42, in region 1 where the grating structure having a period sufficiently longer than the wavelength is arranged, the values of the diffraction efficiency according to the prior art and the present invention are almost the same. The characteristic differences due to Ming appear little and little. However, in region 2 where the grating structure having a short period is arranged, the diffraction efficiency is significantly reduced in the conventional grating structure as compared with region 1, but in the present invention, the reduction in diffraction efficiency is improved as compared with the conventional technology. ing. This greatly affects the light-collecting intensity. Conventional technology cannot maintain a sufficient light-collecting intensity due to the influence of region 2, but the diffractive optical element according to the present invention can maintain the light-collecting intensity. It becomes. Fig. 43 shows a simple graph of this.

FIG. 43 shows the average value of the diffraction efficiency obtained with respect to the numerical aperture, which is a factor that determines the condensing intensity of the diffractive optical element. It can be seen that the results of the present invention maintain a high diffraction efficiency even when the numerical aperture is large, as compared with the results of the prior art indicated by the dotted line. Therefore, it is possible to realize a diffractive optical element having a large numerical aperture that is higher in performance than the conventional technology, and it is possible to provide an optical device having higher performance by using this.

In the above-described embodiment of the diffractive optical element, it is assumed that the polarization direction of the incident light is the TE polarization, but the invention is applicable to any polarization.

In addition, the substrate material of the diffractive optical element according to the embodiment of the present invention is not limited to glass, plastic, optical crystals, and the like as long as the material has a sufficient transmission band in the wavelength region to be used.

In addition, diffractive optical elements can be manufactured using lithography technology (light source is ultraviolet light, X-ray, electron beam, etc.) by semiconductor manufacturing technology or cutting. Alternatively, since it is a diffractive optical element with a continuous shape, a master is made by lithography technology or cutting, and a mold is manufactured, so that molding for the purpose of mass production with plastic glass etc. it can.

As described above, the diffractive optical element of the present invention reduces the spherical aberration as compared with the lens, and also increases the light collection efficiency by arranging the optimized grating structure at the peripheral edge, thereby reducing the problems of the prior art. It is possible to prevent a reduction in diffraction efficiency, which is a point. The light-collecting efficiency of the entire element increases with the increase of the light-collecting efficiency of the peripheral portion, and an optical element having a high numerical aperture can be realized. As a result, by using the diffractive optical element of the present invention, the performance of the optical system and the optical device can be improved as compared with the conventional diffractive optical element, and the diffractive optical element structure can be used. T / JP2004 / 004762

According to 48, the weight of the apparatus can be reduced and the size of the optical system can be reduced.

FIG. 44 shows the diffraction grating having the special shape described above.

Claims

The scope of the claims

1. At least one surface includes a first region including an optical axis and a second region having a diffractive portion surrounding the first region, a first light beam having a first wavelength, and a first wavelength. An imaging optical element for handling a second light beam having a second wavelength different from the above, wherein the shape of the diffractive portion in the second region is a stepped shape when the substrate is a flat surface, and the step amount of the stepped shape is , The diffraction efficiency of the 0th-order diffracted light is determined based on the one of the first and second rays based on the one wavelength such that the diffraction efficiency approaches the peak of the diffraction efficiency at the negative wavelength.

The number of steps is N, and the wavelength of the one light beam is obtained. , Where the diffraction orders other than the 0th order are a, m and p are integers, the wavelength

λ; = [N / (N · m + a)] · / 1 ₀ · p

The ratio of the difference between the first and second light beams and the other wavelength to the other wavelength is equal to or less than a predetermined value determined from the degree of reduction from the peak value of the diffraction efficiency. An imaging optical element in which the number of steps N is determined.

2. An imaging optical element that focuses a first light beam having a first wavelength and a second light beam having a second wavelength different from the first wavelength on the first and second surfaces, respectively. The objective lens as

At least one lens surface includes a first region including an optical axis, a second region having a diffraction portion around the first region, and a third region around the second region. And after passing through the second region, the second region is focused on the first surface, and the second light beam is focused on the second surface after passing through the first, second and third regions. The lens surface shape at is designed based on one of the optical paths of the first and second rays, and the phase function defining the diffractive portion in the second region is based on the other optical path of the first and second rays. The objective lens as the imaging optical element according to claim 1, wherein the objective lens is designed as follows.

3. The shape of the diffractive portion in the second area is a stepped shape when the substrate is a plane, and the step amount of the stepped shape is determined based on the wavelengths of the first and second light beams. Objective lens.

4. Obtain the wavelength of the light beam that determines the lens surface shape in the second area. N is the refractive index of the lens, and n is the refractive index around the lens. , Let に対する be the angle of incidence with respect to the diffraction part, and obtain the amount of step. 4. The objective lens according to claim 3, wherein the objective lens is determined based on an integral multiple of 'cos ^ Z (n-n.).

5. The objective lens according to claim 3, wherein the width of the step in the diffraction section is determined based on the phase function, the amount of the step, and the number of steps.

6. In the diffractive portion in the second region, the light beam that determines the lens surface shape in the second region passes mainly as the 0th-order diffracted light, and the light beam that determines the shape of the diffractive portion in the second region is mainly the first or first-order light. 6. The objective lens according to claim 2, wherein the objective lens passes as folded light.

7. The objective lens according to any one of claims 2 to 6, wherein the lens surface shapes of the first, second, and third regions are aspherical.

8. An optical pickup device for recording or reproducing information on first and second optical recording media having different thicknesses, wherein a first light beam having a first wavelength and a second light beam different from the first wavelength are provided. An objective lens used in an optical pickup device that uses a second light beam having a wavelength for the first and second optical recording media, respectively, wherein the first surface is a surface of the first optical recording medium. The objective lens according to any one of claims 2 to 6, wherein the second surface is a surface of a second optical recording medium.

9. Let NA1 be the numerical aperture at which the light beam passing through the first area converges on the optical recording medium, and NA1 satisfy NA, and let NA2 be the numerical aperture that passes through the second area and converge on the optical recording medium. NA2 satisfies 0.3 ≤ NA2 ≤ 0.51, and NA3 satisfies 0.4 ≤ NA3 ≤ 0.67, where NA3 is the numerical aperture that passes through the third region and converges on the optical recording medium. Item 9. The objective lens according to Item 8.

10. At least one surface includes a first region including an optical axis and a second region having a diffraction portion around the first region, and a first light beam having a first wavelength has a first region. Light passes through and condenses on the image plane, but does not converge on the image plane when passing through the second area. A second light beam having a second wavelength different from the first wavelength passes through the first area and the second area and is focused on an imaging surface; The surface shape at is designed based on one of the optical paths of the first and second rays, and the shape of the diffractive portion in the second region is designed based on the other optical path of the first and second rays. Item 2. The imaging optical element according to Item 1.

1 1. The shape of the diffractive portion in the second region is a stepped shape when the substrate is a plane, and the stepped amount of the stepped shape, the diffraction efficiency of the 0th-order diffracted light, and the first and second light beams 10. The imaging optical element according to claim 10, wherein the imaging optical element is determined based on the one wavelength so as to approach a peak at the one wavelength.

1 2. An integer multiple of the one wavelength. The refractive index of the imaging optical element is n, and the refractive index around the imaging optical element is n. The incident angle to the diffractive part is Θ, and the amount of step in the diffractive part is: The imaging optical element according to claim 11, wherein the imaging optical element is determined based on a value of cos 0 Z (n-n.).

13. The number of steps in the staircase shape is set so that the diffraction efficiency of the 0th-order diffracted light at the other wavelength approaches 0% and the diffraction efficiency of the diffracted lights of orders other than the 0th-order becomes as large as possible. 13. The imaging optical element according to claim 11, which is determined based on the second wavelength.

1 4. The number of steps is N, and the wavelength of the one light beam is obtained. , Where diffraction orders other than the 0th order are α, m and ρ are integers, the wavelength

λ j = [N / (N-m + a)] · ₀ · p

14. The imaging optics according to claim 13, wherein the number of steps N is determined such that a ratio of a difference between the other wavelength and the first and second light rays to the other wavelength is equal to or less than a predetermined value. element.

15. The imaging optical element according to any one of claims 11 to 14, wherein the width of the stairs is determined based on the amount of steps, the number of steps, and the other optical path.

16. The imaging optical element according to any one of claims 10 to 15, which is a lens.

17. At least one surface includes a first region including an optical axis and a second region having a diffraction portion around the first region, and a first light beam having a first wavelength; Design imaging optics to handle a second light beam with a second wavelength different from the wavelength Wherein the shape of the diffractive portion in the second region is a staircase shape when the substrate is flat,

Determining a step amount of the staircase shape based on the one wavelength so that the diffraction efficiency of the zero-order diffracted light approaches the peak of the diffraction efficiency at one wavelength of the first and second light beams;

The number of steps is N, the wavelength of the one light beam; , If the diffraction orders other than the 0th order are a and m and p are integers, the wavelength

λ; = [N / (N · m + α)] · λ. · P

The ratio of the difference between the first and second light beams and the other wavelength to the other wavelength is equal to or less than a predetermined value determined from the degree of reduction from the peak value of the diffraction efficiency. Determining the number of steps N.

18. Focusing a first light beam having a first wavelength and a second light beam having a second wavelength different from the first wavelength on the first and second surfaces, respectively. A method of designing an objective lens as an optical element,

Defining at least one lens surface with a first region including the optical axis, a second region around the first region, and a third region around the second region;

Designing a lens surface shape in the first region;

Designing a lens surface shape in the second region based on one of the optical paths of the first and second light rays; and

Determining a phase function defining the shape of the diffractive portion in the second region based on the other optical path of the first and second light beams;

The design method according to claim 17, further comprising: designing a lens surface shape in the third region.

19. The shape of the diffractive portion in the second area is step-like when the substrate is a plane, and the step of determining the step amount of the step-like shape based on the wavelengths of the first and second light beams is performed. 19. The design method according to claim 18, further comprising:

20. The wavelength of the light beam that determines the lens surface shape in the second area; The refractive index of the lens! ! The refractive index around the lens n. The incident angle with respect to the diffractive part is Θ, and the amount of step is; 'Cos 0 / design method of claim 1 9 for determining, based on the integer multiple of the (n- n _0).

21. The design method according to claim 19, further comprising the step of determining the number of steps of the diffractive portion in the second region in consideration of the diffraction efficiency based on the wavelengths of the first and second light beams.

22. The design method according to any one of claims 19 to 21, wherein the width of the step in the diffraction section is determined based on the phase function, the amount of step, and the number of steps.

23. In the diffractive portion in the second region, the light beam that determines the lens surface shape in the second region mainly passes as the 0th-order diffracted light, and the light beam that determines the shape of the diffractive portion in the second region is mainly the primary or negative beam. The design method according to any one of claims 18 to 22, wherein the design method is designed to pass as first-order diffracted light.

24. The design method according to any one of claims 18 to 23, wherein the lens surface shapes of the first, second, and third regions are designed as aspherical surfaces.

25. An optical pickup device for recording or reproducing information on first and second optical recording media having different thicknesses, wherein a first light beam having a first wavelength and a second light beam different from the first wavelength. A method of designing an objective lens used in an optical pickup device for using a second light beam having a wavelength of 2 for first and second optical recording media, respectively, wherein the first surface is the first light beam. 26. The design method according to claim 18, wherein the second surface is a surface of the second optical recording medium.

26. Assuming that the numerical aperture at which light rays passing through the first area converge on the optical recording medium is NA1, A1 satisfies NA1 ≤ 0.37, passes through the second area and converges on the optical recording medium. If the numerical aperture is NA2, NA2 satisfies 0.3 ≦ NA2 ≦ 0.51 and NA3 satisfies 67 if the numerical aperture passing through the third area and condensing on the optical recording medium is NA3. Design method described in.

27. At least one surface includes a first region including an optical axis and a second region having a diffraction portion around the first region, and a first light beam having a first wavelength has a first region. When the light passes through and converges on the imaging surface, but does not converge on the imaging surface when passing through the second region, the second light beam having a second wavelength different from the first wavelength is transmitted to the first region and the second region. (2) A method of designing an imaging optical element that passes through a region and converges on an imaging surface, Designing a surface shape in the second region based on one of the optical paths of the first and second rays;

18. The design method according to claim 17, further comprising the step of designing the shape of the diffraction portion in the second region based on the other optical path of the first and second light beams.

28. The shape of the diffractive portion in the second region is step-like when the substrate is a plane, and the step amount of the step-like shape, the diffraction efficiency of the 0th-order diffracted light, and the first and second rays 28. The design method according to claim 27, further comprising a step of being determined based on said one wavelength so as to approach a peak at one wavelength.

29. Obtain an integer multiple of the one wavelength. The refractive index of the imaging optical element is n, and the refractive index around the imaging optical element is n. The incident angle with respect to the diffraction part is Θ, and the amount of step in the diffraction part is i. 28. The design method according to claim 27, wherein the value is obtained based on an integer multiple of _COS 0 Z (n—n _Q ).

30. The number of steps in the staircase shape is set to be 1st and 2nd so that the diffraction efficiency of the 0th-order diffracted light at the other wavelength approaches 0% and the diffraction efficiency of the diffracted lights of orders other than 0th-order becomes as large as possible. 30. The design method according to claim 28, further comprising the step of determining based on the wavelength of the light.

3 1. The number of steps is N, and the wavelength of the one light beam is λ. , And m and ρ are integers.

λ i = [N / (N · m + α)] · λ 0 · p

30. The design method according to claim 30, wherein the number of steps N is determined such that a ratio of a difference between the other wavelength and the first and second light rays to the other wavelength is equal to or less than a predetermined value. .

32. The design method according to any one of claims 28 to 31, wherein the width of the stairs is determined based on the amount of steps, the number of steps, and the other optical path. ·

3 3. The design method according to any one of claims 27 to 32, wherein the imaging optical element is a lens. '

34. A method of determining a staircase-shaped diffraction grating shape based on a phase function while considering diffraction efficiencies of the 0th-order and 1st-order diffracted light as two different wavelengths as 0th-order and 1st-order diffracted light, One wavelength. Wavelength as the 0th order diffracted light; L. Where N is the number of steps and m is a positive integer, and any peak wavelength or ₂ in the first-order diffracted light is obtained.If the diffraction grating has a right-up shape, λ ₂ = Ν / (Νηι +1) Χ λ. If the diffraction grating shape has a shape that rises to the left; ₂ = N / (Nm-1) X And the steps required

Determining the number of steps while manipulating m so that 2 approaches the wavelength of the other light that is the first order diffracted light;

Determining the width of the step based on the step amount and the phase function.

35. A computer program for determining a step-shaped diffraction grating shape based on a phase function while considering diffraction efficiencies of the 0th-order and 1st-order diffracted light as two different wavelengths as a 0th-order and a 1st-order diffracted light. To

—Wavelength λ. Is the wavelength λ as the 0th-order diffracted light. A step for determining the amount of step based on

If the number of steps is Ν and a positive integer is m, an arbitrary peak wavelength λ ₂ in the first-order diffracted light is obtained. If the shape of the diffraction grating rises to the right, L ₂ = N / (Nm + l) X. If the shape of the diffraction grating has a shape that rises to the left, ₂ = N / (Nm-1) X λ ₀

determining the number of steps while manipulating ra so that λ ₂ approaches the wavelength of the other light that is the first-order diffracted light;

A computer program for executing a step of determining a width of a stair based on a step amount and a phase function;

3 6. The rays of different wavelengths, a objective lens for focusing on different surfaces, provided with a diffraction grating on one surface even without small, the first and second wavelengths lambda ₂ is

λ! <λ ₂

In the region where both the first wavelength and the second wavelength λ ₂ pass through when the relationship of So that the luminous flux of the second wavelength λ ₂ is focused on the second surface. Determine the phase function of the folded grating and the lens surface shape, and set the grating depth of the grating to the diffraction efficiency of the second-order diffracted light at the first wavelength λ and the diffraction efficiency of the first-order folded light at the _second wavelength λ2. Objective lens set to be larger than the value of.

3 7. The second wavelength λ ₃

λ ₂ \ 8 ₃

In the region where the first wavelength, the second wavelength λ _{2 and} the third wavelength λ ₃ pass together, the light beam of the third wavelength λ ₃ as the first-order diffracted light is as condensed to a third aspect, define a phase function and a lens surface shape of a diffraction grating, the grating depth of the diffraction grating, the diffraction efficiency of first-order diffracted light of the third wavelength lambda ₃ is higher than a predetermined value The objective according to claim 36, which is set to be large.

38. The objective lens according to claim 36, wherein the diffraction grating has a blazed shape.

3 9η is the refractive index of the lens.

(7/4) χλno (η-1) ku 1 <(9/4) χλno (η- 1)

The objective lens according to claim 38, wherein the objective lens is determined by:

40. Any of claims 36 to 39, wherein at least one surface is divided into at least one strip region surrounding the optical axis and a central region including the optical axis, and each region is defined by a separate surface. Or the objective lens according to item 1.

41 1. The objective lens according to claim 40, wherein a step in the optical axis direction is provided between the separate surfaces.

4 2. Separate surfaces, ζ axis coincides with optical axis, i is the number of the surface counted from the center, R i is radius of curvature, K i is eccentricity, A i 4, A i 6, A i 8 Where A i 10 is the aspherical surface coefficient and di is the step on the optical axis of the other surface with respect to the first surface, the expression z— ~~. ( ^{L / Rl} ) ^x- + i4 xh ^{^{+ Ai6xh 6 + Ai8 xh s +}} Ail 0xh lo + objective lens according to claim 4 1, represented by di.

4 3. The light beam of the first wavelength is focused by the image-side numerical aperture NA1, the light beam of the second wavelength is focused by the image-side numerical aperture NA2, NA 1> NA 2

In the case of, the outermost part of the light beam of the second wavelength divides at least one surface into at least one band surrounding the optical axis and a central region including the optical axis, each region being a separate surface. The objective lens according to any one of claims 36 to 39, defined by:

4 4. A light beam of the first wavelength is collected by the image-side numerical aperture N A 1, a light beam of the second wavelength is collected by the image-side numerical aperture N A 2,

N A 1> N A 2

In the case of, the diffraction grating is provided in a region distant from the optical axis through which only the light beam of the first wavelength passes, and the phase of the diffraction grating is so set that the light beam of the first wavelength is focused on the first surface. The objective lens according to any one of claims 36 to 39, wherein a function and a lens surface shape are determined.

45. The objective lens according to claim 44, wherein the lattice slope is shaped so that at least a portion thereof is steeper than the blazed slope.

46. The objective lens according to any one of claims 36 to 45, wherein the wavefront aberration is designed to be less than RMS 0.07 in terms of wavelength when condensing on each surface. .

47. An optical pickup optical system using the objective lens according to any one of claims 36 to 46, wherein the first wavelength is a wavelength for a Blu-ray disc, and the second wavelength is a digital versatile. · Optical pick-up optics that is the wavelength for the disc.

48. An optical pickup optical system using the objective lens according to any one of claims 37 to 46, wherein the third wavelength is used for a compact disc when the third wavelength is handled. Optical pick-up optical system that is wavelength.

4 9. The luminous flux of the wavelength for the Blu-ray disc and the wavelength for the digital versatile disc is incident on the objective lens as parallel light, and the light source for the compact disc and the elephant have a finite conjugate relationship. Item 47. An optical pickup optical system according to Item 47 or 48.