JP2007122828A - Optical element having diffraction surface and optical pickup apparatus using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element capable of sufficiently improving a diffraction efficiency in a plurality of monochromatic rays when being used for an optical pickup apparatus or the like using the plurality of monochromatic rays. <P>SOLUTION: The optical element has a plurality of diffraction surfaces for diffracting the plurality of monochromatic rays and a plurality of laminated diffraction gratings 24, 25, 27 and 28 comprising at least two kinds of materials having dispersion characteristics different from each other. The plurality of diffraction gratings have the same pitch distribution and are formed so that corresponding parts thereof are opposed to each other. The diffraction efficiency of diffraction rays of a usage diffraction order in the plurality of monochromatic rays is heightened to a prescribed value or more without considering diffraction efficiency in the wavelength region other than that of the plurality of monochromatic rays. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to an optical element such as an objective lens having a high NA (numerical aperture) used for an optical head capable of recording / reproducing with respect to a plurality of types of optical disks having different protective layer thicknesses. In particular, the present invention relates to an optical element having a diffractive surface such as an objective lens having a diffractive lens structure formed on the lens surface of a refracting lens and a collimator lens that refracts laser light to produce a collimated light beam.

Conventionally, as optical pickup devices, CD-ROMs and CD-RWs for recording / reproducing by focusing infrared laser light on the disc have been commercialized corresponding to CD discs. Recently, an optical pickup device using red laser light for DVD and infrared laser light for CD has been commercialized, corresponding to both DVD discs and CD discs. More recently, optical pickup devices that are compatible with the Blu-ray standard and the HD-DVD standard, in which the recording density is increased by using blue laser light, have been put into practical use.

However, at present, an optical pickup device corresponding to three types of media with one objective lens is not yet commercially available. The reason for this is as follows. One reason is that the three types of media are made according to different standards, and the protective layer thickness of the optical disc is 1.2mm for CD, 0.6mm for DVD, 0.1mm for Blu-ray, and HD- DVD may be 0.6mm. Another reason is that it is necessary to deal with laser beams of different wavelengths with different NAs, and there are many development problems.

In order to cope with the different thicknesses of the protective layers, the paraxial condensing position can be moved by moving the objective lens in the optical axis direction. However, since the spherical aberration changes as the thickness of the protective layer changes, simply moving the objective lens disturbs the wavefront of the laser beam, making it impossible to converge the spot to the required diameter, and recording / reproducing information. Is impossible. For example, if an objective lens designed to correct spherical aberration when using a DVD is used for CD playback, the paraxial focusing position is made to coincide with the recording surface by moving the objective lens in the optical axis direction. Even so, the spherical aberration becomes over, and information cannot be reproduced.

Accordingly, an optical system that makes a laser beam suitable for each optical disc incident on an objective lens in accordance with the thickness of the protective layer is conventionally known. For example, in one proposed example, a hologram lens is provided in front of the objective lens to separate laser light emitted from a single semiconductor laser into 0th order light and 1st order light. Further, a technique for forming a spot for an optical disk having a thin protective layer with zero-order light that is parallel light and forming a spot for an optical disk with a thick protective layer by using primary light that is divergent light has been proposed (Patent Document). 1). According to this optical system, a hologram lens is designed so as to obtain an optimum laser beam according to the thickness of the protective layer, thereby suppressing the occurrence of spherical aberration and obtaining a spot having diffraction limited performance for each optical disc. I can do it.

In another proposed example, an optical pickup compatible with two media is provided by providing a surface on which a diffraction grating is formed focusing on the difference in wavelength from a laser beam with a wavelength of 780 nm for a CD and a laser beam with a wavelength of 650 nm for a DVD. Objective lenses are disclosed (see Patent Document 2). In this proposed example, a plurality of media can be dealt with by using the first-order diffracted light at each wavelength, but the deterioration of the diffraction efficiency due to the wavelength is not considered.

The deterioration of the diffraction efficiency depending on the wavelength will be described below. FIG. 15 shows the characteristics of diffraction efficiency for a specific diffraction order when a diffractive optical element provided with a diffraction grating composed of one layer is formed on a certain surface in the optical system. In FIG. 15, the horizontal axis represents the wavelength, and the vertical axis represents the diffraction efficiency. This diffractive optical element is designed to have the highest diffraction efficiency in the used wavelength region in the first-order diffraction order (the diffraction efficiency is indicated by reference numeral 211). That is, in FIG. 15, the design order is primary. Further, FIG. 15 also shows the diffraction efficiencies of diffraction orders in the vicinity of the design order (first-order ± first-order zero-order light, second-order light, and third-order light). The diffraction efficiencies of the zero-order light, the second-order light, and the third-order light are indicated by reference numerals 212, 213, and 214, respectively.

In general, the diffraction efficiency becomes maximum when the difference between the optical path lengths of the crest and trough of the grating becomes an integral multiple of the wavelength. The maximum diffraction efficiency can be expressed by the following equation: The diffraction efficiency η (λ) of a diffraction grating made of a material having a refractive index n (λ) at a wavelength λ in air is d is the grating thickness and m is the diffraction efficiency. When the order is given, it is expressed by the following formula (1).

As can be seen from the graph of FIG. 15, the diffraction efficiency 211 is highest at a certain wavelength (hereinafter referred to as “design wavelength”) at the design order (first order), and gradually decreases at other wavelengths. The decrease in diffraction efficiency at this design order becomes diffracted light of other orders. In addition, when a plurality of diffractive optical elements are used, particularly, a decrease in diffraction efficiency at a wavelength other than the design wavelength leads to a decrease in light utilization efficiency.

In particular, in the pickup corresponding to two wavelengths of infrared and red laser light, the deterioration of efficiency was relatively small. However, when it is assumed that even blue laser light is used, the problem is the deterioration of the light amount utilization efficiency due to the deterioration of the diffraction efficiency. In FIG. 15, the deterioration of the diffraction efficiency in the use wavelength region is suppressed to a small extent by balancing so that the deterioration of the diffraction efficiency becomes the same value at the wavelengths 405 nm and 780 nm at both ends of the use wavelength region. Nevertheless, the diffraction efficiency deteriorates by 32% per surface.

A configuration that can suppress the decrease in diffraction efficiency is further disclosed in another proposal (see Patent Documents 3 and 4). The configuration presented in Patent Document 3 has a cross-sectional shape in which two layers 311 and 312 are overlapped and a diffraction grating surface 321 is provided at the boundary as shown in FIG.

On the other hand, the configuration presented in Patent Document 4 has a grating structure in which three layers 411, 412 and 413 are superimposed on each other as shown in FIG. 17, and a diffraction grating surface 421 provided at the boundary between the two layers. , 422, and a layer 412 is formed. This diffractive optical element is formed by forming diffraction grating surfaces 421 and 422 on the boundary surfaces of the respective materials, and by optimizing the refractive index difference between the materials of the layers before and after the boundary and the depths d1 and d2 of the grating grooves, etc. High diffraction efficiency is achieved.
JP 7-98431 A JP 2000-081566 A JP-A-9127321 Japanese Unexamined Patent Publication No. 9-127322

However, in the conventional example of FIG. 16 and FIG. 17, if the diffraction efficiency in the entire visible range is not improved and unnecessary diffracted light deteriorates the captured image, etc., it is necessary to improve the diffraction efficiency in the entire visible range. It was designed like that. However, it is difficult to sufficiently improve the diffraction efficiency over a wide range of wavelengths 405 nm, 650 nm, and 780 nm, and deterioration of the diffraction efficiency occurs particularly at both ends of the wavelength region.

In view of the above problems, the optical element of the present invention has a plurality of diffraction gratings having a plurality of diffraction surfaces for refracting a plurality of monochromatic lights and made of at least two types of materials having different dispersions. The plurality of diffraction gratings have the same pitch distribution, corresponding portions are formed to face each other, and the diffraction efficiency of the diffracted light of the used diffraction order for each of the plurality of monochromatic lights is set to a plurality of single colors. The diffraction efficiency in a wavelength region other than light is not considered, and is increased to a predetermined value (for example, 70 percent (this is a better value than the degree of deterioration in FIG. 15)) or more. The idea of the present invention is to abandon the idea of improving the diffraction efficiency over the entire wavelength range including a plurality of monochromatic lights, and to ensure sufficient improvement of the diffraction efficiency at least for a plurality of discrete use wavelengths. The guideline is for designing an optical element having the basic configuration as described above.

In view of the above problems, the optical element of the present invention is an optical element having one diffractive surface for refracting a plurality of monochromatic lights. Then, the used diffraction order at the time of monochromatic light imaging at the first wavelength λ1 is m1, the used diffraction order at the time of monochromatic light imaging at the second wavelength λ2 is m2, and at the time of monochromatic light imaging at the third wavelength λ3. When the diffraction order used is m3, the following expressions (2-1) and (2-2) are satisfied.

Thereby, in the plurality of monochromatic lights, the diffraction efficiency of the diffracted light of the used diffraction order takes a peak value. For each of the plurality of monochromatic lights, the diffraction efficiency of the diffracted light of the used diffraction order is set to a predetermined value (for example, 70 percent) or more without considering diffraction efficiencies in other wavelength regions other than the plurality of monochromatic lights. It is increasing. Also here, based on the idea of the present invention, an optical element having a basic configuration including the single-layer diffraction grating as described above is designed.

In view of the above problems, the optical pickup device of the present invention is characterized in that the optical element described above is used as an imaging optical element for optical pickup or a collimator optical element that uses a light beam as a collimated light beam.

According to the present invention, if the diffraction efficiency is sufficiently improved only at the wavelengths of a plurality of monochromatic lights to be used, the deterioration of the diffraction efficiency in other wavelength regions does not need to be particularly problematic. The optical element is designed to have the optical performance. Therefore, when the present optical element is used in an optical pickup device that uses a plurality of colors of monochromatic light, the plurality of colors of monochromatic light are optical elements with sufficiently good diffraction efficiency.

Specific examples will be described below with reference to the drawings to clarify the embodiments of the present invention.

(First Example)
First, a specific configuration of the optical pickup device according to the first embodiment of the present invention will be described with reference to the drawings. The optical system for the optical pickup device of the present embodiment may be configured by any of the following. You may comprise with the collimator for converting the divergent light beam from a light source into substantially parallel light, and the objective lens for condensing this parallel light on an optical information recording surface. In addition, it comprises a conversion lens (coupling lens) for converting the angle of the divergent light beam from the light source into a divergent light beam or a convergent light beam and an objective lens for condensing the light beam from the conversion lens on the optical information recording surface. Also good. Furthermore, the objective lens (finite conjugate objective lens) for condensing the divergent light beam from the light source on the optical information recording surface may be used.

The optical pickup device shown in FIG. 1 includes a semiconductor laser 11 that is a first light source for reproducing a first optical disk, and a hybrid semiconductor laser 12 that has second and third light sources for reproducing second and third optical disks. have. One light source 11 emits laser light having a wavelength of 400 nm. When reproducing the first optical disk, a beam is emitted from the first semiconductor laser 11. The emitted light beam passes through a beam splitter 7 which is a means for combining light emitted from both semiconductor lasers 11 and 12, passes through a polarization beam splitter 6, a collimator 2, and a quarter wave plate (not shown) to be circularly polarized. Becomes a parallel light flux. This light beam is focused by the diaphragm 3, and is focused on the information recording surface 22 by the objective lens 1 through the transparent substrate 21 of the first optical disc 20 (this light beam is indicated by a solid line in FIG. 1). Then, the light beam modulated and reflected by the information pits on the information recording surface 22 is again transmitted through the objective lens 1, the diaphragm 3, the quarter-wave plate (not shown), and the collimator 2 and enters the polarization beam splitter 6. . Here, the light beam is reflected, given astigmatism by a cylindrical lens (not shown), and is incident on the photodetector 13. In this way, a read signal of information recorded on the first optical disc 20 is obtained using the output signal of the photodetector 13.

In addition, focus detection and track detection are performed by detecting a change in the amount of light due to a change in the shape and position of the spot on the photodetector 13. Based on this detection, a two-dimensional actuator (not shown) moves the objective lens 1 so that the light beam from the first semiconductor laser 11 is imaged on the recording surface 22 of the first optical disc 20. At the same time, the two-dimensional actuator moves the objective lens 1 so that the light beam from the semiconductor laser 11 forms an image on a predetermined track.

When reproducing the second optical disk, a beam having a wavelength of 650 nm is emitted from the second light source of the hybrid semiconductor laser 12. The emitted light beam is reflected by a beam splitter 7 which is a light combining unit. Then, like the light beam from the first semiconductor 11, the transparent substrate 21 of the second optical disk 20 is passed through the polarizing beam splitter 6, the collimator 2, the quarter wavelength plate (not shown), the stop 3, and the objective lens 1. The light is condensed on the information recording surface 22 (in FIG. 1, this light beam is indicated by a one-dot chain line). Here, the light beam modulated and reflected by the information pits on the information recording surface 22 is again passed through the objective lens 1, the diaphragm 3, the quarter wave plate (not shown), the collimator 2, the polarization beam splitter 6, and the cylindrical lens (not shown). Then, the light is incident on the photodetector 13. In this way, a read signal for information recorded on the second optical disc 20 is obtained using the output signal of the photodetector 13.

Also, as in the case of the first optical disk, the shape change of the spot on the photodetector 13, the light amount change due to the position change is detected, focus detection and track detection are performed, and a two-dimensional actuator (not shown) The objective lens 1 is moved for focusing and tracking. When reproducing the third optical disk, laser light having a wavelength of 780 nm is emitted from the third light source of the hybrid semiconductor laser 12 as in the case of reproducing the second optical disk. Then, the light detector 13 performs beam detection, focus detection, and track detection using the same optical path, and the objective lens 1 is moved for focusing and tracking by a two-dimensional actuator (not shown).

As described above in the background art, the thickness of the transparent substrate 21 is standardized by the first, second, and third optical discs. That is, the HD-DVD standard is 0.6 mm (blue laser), 0.6 mm (red laser), and 1.2 mm (infrared laser), respectively, and the Blu-ray standard is 0.1 mm (blue laser). ), 0.6 mm (red laser) and 1.2 mm (infrared laser).

FIG. 2 is an explanatory diagram for explaining the shape of the diffraction grating 22 which at least one surface of the objective lens 1 has. FIG. 2 (a) shows the grating ring zone 23 when the diffraction grating 22 is viewed from the direction of the optical axis (indicated by reference numeral 4 in FIG. 1). FIGS. 2 (b) and 2 (c) FIG. 4 shows cross-sectional shapes of diffraction gratings having different laminated forms.

It is particularly noted that the diffraction grating has a characteristic that the dispersion characteristic is significantly different from that of a general optical material such as glass. Specifically, the Abbe number νd and the partial dispersion ratio θgF can take negative values as shown in the following equations (3-1) and (3-2).

Therefore, it is very effective for controlling chromatic aberration. In this embodiment, a diffraction grating is also used to control chromatic aberration in a wide wavelength range exceeding the visible range of 405 nm, 650 nm, and 780 nm.

The diffraction grating 22 shown in FIG. 2 (a) can change the power of refraction due to diffraction by changing the pitch of the grating in the radial direction with respect to the optical axis. Therefore, it is possible to give an aspherical effect by changing the pitch of the grating according to the radial position. FIG. 2 (b) shows a laminated diffraction grating of the type in which a diffraction grating 24 is provided in the base diffraction grating 25 with an air gap 26 therebetween. The diffraction grating 25 and the diffraction grating 24 are made of different materials, and usually one or both of the gratings are made of an ultraviolet curable resin or the like.

In addition, in the laminated form shown in FIG. 2 (c), the diffraction grating 28 is made of a glass mold or a plastic mold, and the diffraction grating 27 is made of ultraviolet curable resin or glass or plastic whose curing temperature is lower than that of the material of the diffraction grating 28. Created by molding. This type does not need to be driven as shown in FIG. 2 (b) and can be produced at a relatively low cost. That is, in any of the stacked forms of FIG. 2 (b) and FIG. 2 (c), the two diffraction gratings have the same pitch distribution and the corresponding parts are formed to face each other. The laminated form shown in (c) is relatively easy. This corresponding form is basically the same as the conventional example shown in FIG.

Since it is created in such a form, assuming that the structure consisting of two diffraction gratings is integrated, the light incident on the diffractive optical element is modulated substantially simultaneously, and the phase amplitude that characterizes the phase shift action is both diffraction It can be thought of as the sum of the phase shift effects at the grating. Therefore, the diffraction efficiency can be obtained by applying the above formula (1) for two diffraction gratings that are considered to be integrated. In this example, for each of the above three wavelengths, the diffraction order, the grating thickness of each diffraction grating, and the material of each layer are created for the three wavelengths to determine the peak value of the diffraction efficiency. It is obtained by using the plurality of formulas (1). The diffraction orders may be the same or different at the three wavelengths. Of course, the grating thickness of each diffraction grating, the material of each layer, etc. are common to the three wavelengths.

Based on the idea of the present invention that if the diffraction efficiency is improved only with a plurality of wavelengths, the deterioration of the diffraction efficiency in other wavelength regions does not have to be a problem. The result obtained by obtaining the necessary conditions by the above derivation method to satisfy the above is the following conditional expression.

Here, the wavelength of the blue laser is λ1, the wavelength of the red laser is λ2, the wavelength of the infrared laser is λ3, and the refractive index of the first material (the material of the diffraction grating 24 or 27) is n ₁ (λ). The refractive index of the second material (the material of the diffraction grating 25 or 28) at the wavelength λ is n ₂ (λ). M1 is the diffraction order when using the blue laser, m2 is the diffraction order when using the red laser, m3 is the diffraction order when using the infrared laser, and the first material to be laminated (material of the diffraction grating 24 or 27) The converted Abbe number ν is expressed by the following equation (4-1).

At this time, in the diffractive optical element of the present embodiment, the combination of the grating thicknesses d and ν of the diffraction grating 25 or 28 of the second material represented by the formula (4-2) is diffractive efficiency at λ1, λ2, and λ3. Is a necessary condition for taking a peak value. Further, the peak value is set to be a predetermined value (for example, 70%) or more.

FIG. 3 shows a graph obtained by calculating ν in the above equation (4-1) from an OHARA glass table. As is apparent from FIG. 3, the range of ν is about 7 to 30, and the lower the refractive index or the lower the dispersion, the fewer the selectable materials.

Actually, the first material often uses a special material such as an ultraviolet curable resin as described above, and in this case, the converted Abbe number ν of the first material takes a limited value. Become. Further, the value of d can be finely tuned to slightly change the peak position of the diffraction efficiency by giving an optical path change within 0.28λ as the optical path length.

In this example, an ultraviolet curable resin material of ν = 8.57 and n (650 nm) = 1.62863 is used as the first material, and an ultraviolet curable resin C001 (trade name, manufactured by Dainippon Ink, Inc.) is used as the second material. ) Is used as follows. That is, n ₂ (405 nm) = 1.541685543, n ₂ (650 nm) = 1.52139598, and n ₂ (780 nm) = 1.517681808. Also, when m1 = 7, m2 = 8, and m3 = 9, when the thickness d of the diffraction grating 25 or 28 of the second material is 110.23 μm, and the thickness of the diffraction grating 24 or 27 of the first material is 82.12 μm There is a configuration in which the diffraction efficiency has a peak value at each wavelength used. Actually, m1, m2, and m3 are integers. Therefore, the diffraction orders are determined by searching for a configuration in which the diffraction efficiency at the three wavelengths to be used becomes a peak value while changing these values.

FIG. 4 is a graph showing diffraction efficiency when the above configuration is adopted. The horizontal axis represents wavelength, the vertical axis represents diffraction efficiency, and a line is written for each diffraction order. As can be seen from the graph, a diffraction efficiency of 96.7% is obtained at the wavelength 405 nm by the seventh-order diffracted light of the line 41 (m1), and a wavelength of 650 nm is obtained by the ninth-order diffracted light of the line 42 (m3).
100% diffraction efficiency can be obtained with the 8th order diffracted light of line 43 (m2), and 99.9% diffraction efficiency can be obtained with a wavelength of 780 nm.

Further, as shown in FIG. 4, it can be seen that there is diffracted light having a peak in a portion other than the above-described wavelength. For example, the line 44 is 10th-order diffracted light, and has a peak in the vicinity of a wavelength of 520 nm. However, the laser emits light of only three wavelengths of 405 nm, 650 nm, and 780 nm, and there is no problem even if it has a peak at another wavelength. Although diffraction orders of other orders are also shown, these are not a problem.

In the actual objective lens design, the 7th-order diffracted light is optimal for laser light with a wavelength of 405 nm, the 9th-order diffracted light for laser light with a wavelength of 650 nm, and the 8th-order diffracted light for laser light with a wavelength of 780 nm. It is necessary to design according to the purpose so as to obtain optical performance.

FIG. 4 shows one configuration in which the diffraction efficiency is improved. As another example, only the first-order diffracted light may be used. In this case, when the grating thickness of the first material is 12.2 μm and the grating thickness of the second material is 9.1 μm, the diffraction efficiency is 83.3% at 405 nm, 100% at 650 nm, and 95.5% at 780 nm. . At this time, the orders of the diffraction efficiency peaks are all first-order diffracted light. If the 83.3% degradation in diffraction efficiency at 405 nm is acceptable, this is also a solution.

(Second embodiment)
FIG. 5 is an explanatory view showing the shape of the diffraction grating according to the second embodiment of the present invention. FIG. 5 (a) shows the annular zone 53 when the diffraction grating 52 is viewed from the optical axis direction. FIG. 5 (b) shows a cross-sectional shape. As shown in FIG. 5, the diffractive optical element of this embodiment has a diffraction grating 52 of a single layer 54.

In a single-layer diffraction grating, when diffracted light of the same order is used, it has the maximum diffraction efficiency at the design wavelength, but the diffraction efficiency deteriorates as the wavelength deviates from the design wavelength.

In this example, m1 is the diffraction order used when imaging the laser beam with the first wavelength λ1, m2 is the diffraction order used when imaging the laser beam with the second wavelength λ2, and m3 is the third wavelength λ3. The following conditions are satisfied when the diffraction order is used at the time of laser beam imaging. That is, by satisfying the above formulas (2-1) and (2-2), the diffraction efficiency of the diffracted light at the used laser wavelength is increased. Also in this example, as in the first example, at the three wavelengths, the diffraction order, the grating thickness of the diffraction grating, and the material of the layer, the diffraction efficiency takes a peak value as described above. Obtained using equation (1). The result is that the equations (2-1) and (2-2) are satisfied.

FIG. 6 is a graph showing the existence ranges of the above equations (2-1) and (2-2) when λ1 = 405 nm, λ2 = 650 nm, and λ3 = 780 nm. In FIG. 6, the horizontal axis represents the used diffraction order (m1) at a wavelength of 405 nm, and the vertical axis represents the used diffraction order of m2 or m3. The line 61 in the graph indicates the median value in the range of m2 that satisfies the above formula (2-1), and the line 62 indicates the median value in the range of m3 that satisfies the above formula (2-2). Since m1, m2, and m3 are integer values, the portion surrounded by a circle on the graph is a range that satisfies the expressions (2-1) and (2-2).

When the combination of m1 = 8, m2 = 5, m3 = 4 is actually selected, it becomes as follows. When the diffraction order used at wavelength 405 nm is 8th, the diffraction order used at wavelength 650 nm is 5th, the diffraction order used at wavelength 780 nm is 4th, and the best value at 780 nm for grating thickness d is calculated, 6.109 μm.

FIG. 7 is a graph showing the diffraction efficiency at this time. In FIG. 7, the horizontal axis represents wavelength, and the vertical axis represents diffraction efficiency. Line 71 shows the diffraction efficiency of the eighth-order (m1) diffracted light, line 72 shows the diffraction efficiency of the fifth-order (m2) diffracted light, and line 73 shows the diffraction efficiency of the fourth-order (m3) diffracted light. From the graph, the diffraction efficiency of the blue laser (λ1 = 405 nm) is 100%, the diffraction efficiency of the red laser (λ2 = 650 nm) is 91%, and the diffraction efficiency of the infrared laser (λ3 = 780 nm) is 100%. All are 70% or more, which is a satisfactory value.

In this example, regarding the refractive index of the material of the single-layer diffraction grating 52, the converted Abbe number ν is expressed by the following equation (5-1) as the refractive index n1 at the wavelength λ1, the refractive index n2 at the wavelength λ2, and the refractive index n3 at the wavelength λ3. ), Use a material that satisfies the following formula (5-2). Usually, the Abbe number is determined by the refractive indices of the F-line and C-line wavelengths with respect to the d-line wavelength (νd = (nd-1) / (nF-nC). In the present invention, a limited number (here, 3 Therefore, in this embodiment, the converted Abbe number is defined as the equation (5-1) according to the refractive index with respect to the used wavelength.

The glass material used in this example is calculated by substituting λ1 = 405, λ2 = 650, and λ3 = 780 nm into the equation (5-2) to calculate the converted Abbe number. From the glass material near ν = 27.1, HOYA NBf1 (trade name). This glass material is a material frequently used for glass molds, and the converted Abbe number ν = 25.2.

The solution of the single-layer diffraction grating in this example is slightly lower in diffraction efficiency than that of the laminated structure, but can be adequately handled depending on the required specifications because the amount of deterioration is relatively small. However, since the combinations of diffraction orders that can be used are limited, the degree of freedom in design is somewhat reduced.

(Third example)
FIG. 8 is an explanatory view of an optical element according to the third embodiment of the present invention. In FIG. 8, FIG. 8 (a) shows areas 81, 82, and 83 having different diffraction gratings when the diffraction gratings are viewed from the optical axis direction. That is, in the diffraction grating of the optical element of the present embodiment, there are three areas 81, 82, and 83 expressed by different phase coefficients (optical path difference function coefficients). FIG. 8B shows the cross-sectional shape of the lens 85, 84 is a stop, and 85 is an optical disk.

In each of the areas 81, 82, and 83, as described above, the area through which the light beam passes is different due to the different NA and wavelength. That is, three wavelengths pass through area 81, and two wavelengths pass through area 82. Area 83 passes one wavelength.

In the configuration of FIG. 8, in the area 81, the diffraction efficiency can be improved by providing the above-mentioned laminated type or single layer type diffraction grating corresponding to three wavelengths. Actually, the intensity profile of the laser beam has a Gaussian distribution with a peak centered on the optical axis, and the amount of light at the center is the highest, so improving the diffraction efficiency of this part as a whole is significant. It will be effective.

In addition, the area 82 has a shape such as an optimum grating thickness for two wavelengths, and the area 83 has an optimum shape for one wavelength, so that the overall diffraction efficiency can be improved. The method for obtaining this optimum shape is basically the same as that described in the above embodiment using the formula (1).

According to the diffractive optical element of this embodiment, each color of laser light corresponds to the thickness of the protective layer of one type of optical disk, and at least two colors of laser light correspond to protective layers of different thicknesses. It can be made easy to be focused on a predetermined surface through. In addition, any one of the surfaces can be easily provided with a plurality of diffractive surface areas represented by different aspherical coefficients and different optical path difference function coefficients.

`` Numerical examples ''
Hereinafter, numerical examples of the present invention will be described with reference to the following table. In Table 1, r is the radius of curvature, d is the surface spacing, and n is the refractive index at each wavelength. In addition, the table whose interval varies depending on the wavelength is summarized in a separate table. In the table, the first surface is the light incident side surface of the lens, the second surface is the light output side surface of the lens, the third surface is the light incident side surface of the transparent substrate of the optical disk, and the fourth surface is the transparent substrate of the optical disk. This is the surface on the light exit side.

Here, the distance (sag amount) from the tangential plane on the aspherical optical axis of the coordinate point on the aspherical surface where the height from the optical axis is h is X (h), and on the aspherical optical axis Curvature (1 / r) is Cu, conic coefficient is K, 4th order, 6th order, 8th order, 10th order, 12th order aspherical coefficients are A4 (A), A6 (B), A8 (C), A10 (D) and A12 (E). At this time, the aspherical shape is represented by the following expression.
X (h) = Cuh ² / (1 + √ (1- (1 + K) Cu ² h ² )) + A4h ⁴ + A6h ⁶ + A8h ⁸ + A10h ¹⁰ + A12h ¹²

In Table 1, the radius of curvature of the aspheric surface (first surface) is the radius of curvature on the optical axis. Table 2 shows the conical coefficient and the aspheric coefficient that define the aspheric surface, and the optical path difference function coefficient that defines the diffractive lens structure formed on the first surface of the optical element.

Also, the amount of addition of the optical path length by the diffractive lens structure is the optical path difference function defined by the following equation using the height h from the optical axis, the nth order (even order) optical path difference function coefficient Cn, and the wavelength λ. Represented by φ (h).
φ (h) = (C1h ² + C2h ⁴ + C3h ⁶ +…) × 2π / λ

FIG. 9 shows an optical path diagram through the lens 91 at a wavelength of 780 nm, a thickness of the substrate 92 of 1.2 mm, and an NA of 0.45 in this numerical example. FIG. 10 shows an optical path diagram through the lens 101 at a wavelength of 650 nm, a thickness of the substrate 102 of 0.6 mm, and NA of 0.65. FIG. 11 shows an optical path diagram through the lens 111 at a wavelength of 405 nm, a thickness of the substrate 112 of 0.6 mm, and NA of 0.65.

FIG. 12 shows the spherical aberration in FIG. Here, 121 indicates spherical aberration at a wavelength of 780 nm, 122 indicates 650 nm, and 123 indicates 405 nm. From FIG. 12, it can be seen that 780 nm shows good imaging performance. FIG. 13 shows the spherical aberration in FIG. In FIG. 13, 131 indicates 650 nm, 132 indicates 780 nm, and 133 indicates 405 nm. From FIG. 13, it can be seen that the imaging performance is good at 650 nm. FIG. 14 shows the spherical aberration in FIG. In FIG. 14, 141 indicates spherical aberration of 405 nm, 142 indicates 780 nm, and 143 indicates 650 nm. From FIG. 14, it can be seen that 405 nm shows good imaging performance.

In this numerical example, as is apparent from FIGS. 9 to 11, good image formation is performed for different substrate thicknesses. The distance between the lens and the disk is the narrowest when the substrate thickness is 1.2 mm at 780 nm (Fig. 9), but it is still a sufficient amount because it is 0.54 mm. As for the spherical aberration, as shown in FIGS. 12 to 14, good imaging performance is shown at each wavelength used.

Since this numerical example uses only the first-order diffracted light, it is necessary to take a laminated type configuration. Thereby, when the first and second materials having the configuration of the first embodiment are used, it is possible to achieve a diffraction efficiency of 83.3% at 405 nm, 100% at 650 nm, and 95.5% at 780 nm. This configuration is described in the last part in the description of the first embodiment.

Explanatory drawing of a three-wavelength compatible optical pickup. FIG. 3 is an explanatory diagram of the multilayer diffraction grating according to the first embodiment of the present invention. Graph of converted Abbe number. 3 is a graph illustrating the diffraction efficiency of the first example of the present invention. Explanatory drawing of the single layer diffraction grating of the 2nd Example of this invention. 6 is a graph for explaining diffraction orders of the second embodiment of the present invention. 6 is a graph for explaining diffraction efficiency of the second embodiment of the present invention. Explanatory drawing of the diffraction surface of the 3rd Example of this invention. The optical path figure of a numerical example. The optical path figure of a numerical example. The optical path figure of a numerical example. The spherical aberration figure of a numerical example. The spherical aberration figure of a numerical example. The spherical aberration figure of a numerical example. The graph explaining the diffraction efficiency of the conventional single layer type diffraction grating. Explanatory drawing of the conventional single layer type | mold diffraction grating. Explanatory drawing of the conventional laminated type diffraction grating.

Explanation of symbols

1, 85, 91, 101, 111 Diffractive optical element (objective lens)
11, 12 Semiconductor laser
20, 86, 92, 102, 112 Optical disc
22, 24, 25, 27, 28, 52 Diffraction grating

Claims (10)

An optical element having a plurality of diffractive surfaces for refracting a plurality of monochromatic lights, having a plurality of laminated diffraction gratings made of at least two types of materials having different dispersions, and the plurality of diffraction gratings having an equal pitch And corresponding portions are formed to face each other, and for each of the plurality of monochromatic lights, the diffraction efficiency of the diffracted light of the used diffraction order is set to the diffraction efficiency in a wavelength region other than the plurality of monochromatic lights. An optical element characterized by being raised to a predetermined value or higher without considering the above. The plurality of monochromatic lights are blue laser light, red laser light and infrared laser light,
The wavelength of the blue laser beam is λ1, the wavelength of the red laser beam is λ2, the wavelength of the infrared laser beam is λ3, the refractive index of the first material at the wavelength λ is n ₁ (λ), and the second material at the wavelength λ Refractive index is n ₂ (λ), diffraction order when using blue laser light is m1, diffraction order when using red laser light is m2, diffraction order when using infrared laser light is m3, first material to be laminated When the converted Abbe number ν is expressed by the following equation:

The grating thickness d of the second material is represented by the above formula, and in the plurality of monochromatic lights, the diffraction efficiency of the diffracted light of the used diffraction order takes a peak value, respectively. 2. The optical element according to claim 1. An optical element having one diffraction surface for refracting a plurality of monochromatic lights,
The diffraction order used for monochromatic light imaging of the first wavelength is m1, the diffraction order used for monochromatic light imaging of the second wavelength is m2, and the diffraction order used for monochromatic light imaging of the third wavelength is m3. When the first wavelength is λ1, the second wavelength is λ2, and the third wavelength is λ3,

By satisfying both of these equations, the diffraction efficiency of the diffracted light of the used diffraction order has a peak value in each of the plurality of monochromatic lights, and the diffracted light of the used diffraction order of each of the plurality of monochromatic lights is further reduced. An optical element characterized in that diffraction efficiency is increased to a predetermined value or more without considering diffraction efficiency in a wavelength region other than a plurality of monochromatic lights. The monochromatic light of the first wavelength is blue laser light, the monochromatic light of the second wavelength is red laser light, the monochromatic light of the third wavelength is infrared laser light,
The refractive index at the wavelength λ1 of the material of the optical element is n1, the refractive index at the wavelength λ2 is n2, and the refractive index at the wavelength λ3 is n3,
When the converted Abbe number ν is defined by the first equation below

4. The optical element according to claim 3, wherein a material represented by the second equation above is used. 5. The optical element according to claim 1, wherein the predetermined value is 70%. The plurality of monochromatic lights are blue laser light, red laser light and infrared laser light,
Laser light of each color corresponds to the thickness of the protective layer of one type of optical disc, and laser light of at least two colors corresponds to the protective layer of different thickness so that it is condensed on a predetermined surface through the protective layer 6. The optical element according to claim 1, wherein the optical element is formed. 7. The optical element according to claim 1, wherein the used diffraction orders are different orders for a plurality of monochromatic lights, respectively. 8. The optical element according to claim 1, wherein any one of the surfaces has a plurality of diffractive surface areas represented by different aspheric coefficients and different optical path difference function coefficients. 9. The optical element according to claim 1, wherein at least two light beams of different colors are formed so as to form a light spot with different NA (numerical aperture). 10. An optical pickup device, wherein the optical element according to claim 1 is used as an imaging optical element for an optical pickup or a collimator optical element that uses a light beam as a collimated light beam.

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