JP2004077722A - Diffraction optical element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diffraction optical element being easy to manufacture and having a high diffraction efficiency, which has a diffraction grating which doesn't diffract one of two kinds of light having different wavelengths but diffracts the other. <P>SOLUTION: In a diffraction optical element 1 which makes two lights having different wavelengths incident thereto, a diffraction grating 2 has many echelon gratings 4 provided with the prescribed number of stages of stepped parts 7 comprising rise surface parts 5 being along the incidence direction and diffraction surface parts 6 which are bulged from the rise surface parts 5 and are formed with narrow width, and such a height is given to the stepped parts 7 that an optical path difference being integer times as long as the wavelength of one light may be given to one of two lights passing adjacent diffraction surface parts 6. The diffraction grating 2 comprises a narrow part 8 where diffraction surface parts 6 are formed with narrower width and a wide part 9 where diffraction surface parts 6 are formed with wider width, and the number of stages of echelon gratings 4 in the narrow part 8 is made smaller than that in the wide part 9. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
[Industrial applications]
The present invention relates to a diffractive optical element provided with a diffraction grating composed of a step-like grating in which two lights having different wavelengths are made incident and one light is diffracted and the other light is not diffracted, and is particularly easy to manufacture and has high diffraction. The present invention relates to a diffractive optical element provided with an efficient diffraction grating.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in an optical pickup for reading signals from two types of media, CD and DVD, a laser beam having a wavelength of 785 nm used for reading a CD and a laser beam having a wavelength of 655 nm used for reading a DVD are read. . In such an optical pickup, light from a laser diode that oscillates laser light of two wavelengths (655 nm and 785 nm) is converted into parallel light by a collimator lens, and a signal of a CD medium and a DVD medium is collected by a condenser lens. A light is collected and irradiated on the surface, the reflected light modulated on the medium signal surface is collected on a photodiode, and the light energy incident on the photodiode is converted into electric energy to obtain an electric signal. I have to.
[0003]
However, since the thickness of the protective layer is different between the CD and the DVD, if the optical system is configured so that the laser is focused according to the thickness of the protective layer of the DVD, a large spherical aberration occurs when reading the CD, The signal cannot be read. For this reason, in the conventional optical pickup, a diffraction grating is formed on the incident surface of the collimator lens.
[0004]
The diffraction grating has a large number of stair-step gratings provided on the incident surface of the collimator lens. The height of the step is such that the light incident on the adjacent step has an optical path that is an integral multiple of the wavelength of the laser light used for reading the DVD. It is formed to give a difference. As a result, the optical path difference of the laser beam used for reading the DVD becomes an integral multiple of the wavelength. Therefore, this diffraction grating allows the laser beam used for reading the DVD to go straight, and the laser beam used for reading the CD having a longer wavelength. Can be diffracted.
[0005]
In this case, the diffraction grating has a function as a concave lens with respect to the laser light used for reading the CD, and the laser light used for reading the CD is emitted from the collimator lens as divergent light. As described above, by using the diffraction grating, of the two laser beams having different wavelengths, only the laser beam used for reading the CD is made divergent light, so that the laser beam can be focused on the signal surface of the CD medium and irradiated. it can.
[0006]
An optical element having such a diffraction grating is made of a resin material, and is mass-produced by injection molding using a mold. As described above, when an optical element is manufactured by injection molding using a die, it is necessary to precisely perform fine processing on the die, and the die is manufactured by cutting using a cutting tool.
[0007]
[Problems to be solved by the invention]
However, an optical element having a diffraction grating used in a conventional optical pickup has the following problems.
In order to make laser light divergent light, the grating width must be reduced toward the outer periphery of the optical element. Further, in order to increase the diffraction efficiency, the number of steps of the staircase grating having the staircase shape of the diffraction grating must be increased. However, when the number of steps is increased, the processing of the die for forming the uppermost step of the staircase lattice at the outer peripheral part of the optical element becomes smaller than the limit that can be processed by the cutting tool. On the other hand, if the number of steps is reduced so that the die can be machined with a cutting tool, the diffraction efficiency decreases.
[0008]
For example, FIG. 5 shows a cross-sectional view of a diffractive optical element when all the step gratings are formed in six steps. The dashed line at the left end of the figure indicates the center line of the diffractive optical element 1. The diffractive optical element 1 has a diffraction grating 2, and the diffraction grating 2 has a large number of step gratings 4 formed in six steps. Then, the width of the staircase lattice 4 becomes smaller toward the outer periphery, so that the outer periphery becomes a narrow portion 8 where the width of the staircase lattice 4 is small, and the other region becomes a wide portion 9 where the width of the staircase lattice 4 is large. In this case, a mold for manufacturing an optical element needs to be very finely processed in a portion corresponding to the narrow portion 8, and it is difficult to manufacture a cutting tool corresponding to the processing. Even if the cutting tool can be manufactured, the rigidity of the cutting tool is insufficient, and it is difficult to machine the mold with a predetermined accuracy.
[0009]
FIG. 6 is a cross-sectional view of a diffractive optical element when all the step gratings are formed in five steps. Similarly to FIG. 5, the dashed line on the left end of the figure indicates the center line of the diffractive optical element 1. In this case, since the staircase grating 4 does not have a portion corresponding to the uppermost stage when formed in six steps, the mold for manufacturing the optical element is processed also for the portion corresponding to the narrow portion 8. can do. However, by reducing the number of stages, the diffraction efficiency is lower than in the case of six stages.
[0010]
The present invention has been made in order to solve the above-described problems, and in a diffractive optical element having a diffraction grating that diffracts one of two lights having different wavelengths without diffracting the other, it is easy to manufacture and diffractive. It is an object to provide a highly efficient diffractive optical element.
[0011]
[Means for Solving the Problems]
In order to solve the above-mentioned problem, a diffractive optical element according to the present invention is a diffraction optical element including an incident surface having a diffraction grating that allows two lights having different wavelengths to enter and diffracts one light without diffracting the other light. In the optical element, the diffraction grating has a large number of stair gratings each having a predetermined number of steps each including a rising surface portion extending along the incident direction and a diffraction surface portion formed to have a narrow width and protrude from the rising surface portion. The diffraction grating is formed at a height that gives an optical path difference that is an integral multiple of the wavelength of one of the lights passing through the adjacent diffraction surface portion, and the diffraction grating has a narrow portion and a width of the diffraction surface portion that are formed with a small width of the diffraction surface portion. The stepped lattice of the narrow portion has a smaller number of steps than the stepped lattice of the wide portion.
[0012]
According to the present invention, since the number of steps of the step grating in the portion where the grating width is small in the diffraction grating is small, it is not necessary to form a difficult-to-machine groove in the processing of the mold for manufacturing the diffractive optical element.
[0013]
Further, in the diffractive optical element according to the present invention, the staircase grating of the narrow portion has one less step than the staircase grating of the wide portion, and the rising surface portion at the top corresponds to the staircase grating of the wide portion. It is configured to be lower than the rising surface portion.
[0014]
ADVANTAGE OF THE INVENTION According to this invention, the staircase grating of the narrow part in a diffraction grating can suppress the fall of diffraction efficiency despite having reduced the number of steps.
[0015]
Further, the diffractive optical element according to the present invention is characterized in that the step grating of the diffraction grating is formed concentrically on the incident surface.
[0016]
According to the present invention, the diffraction grating functions as a Fresnel lens, and one of two incident lights having different wavelengths can be divergent light.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a front view schematically showing the diffractive optical element according to the present embodiment. FIG. 2 is a cross-sectional view schematically showing a cross section of the diffractive optical element according to the present embodiment. FIG. 3 is an enlarged cross-sectional view schematically showing a cross section around a portion where a step of the staircase grating is changed in the diffractive optical element according to the present embodiment.
[0018]
The diffractive optical element 1 in the present embodiment is formed in a substantially circular shape as shown in FIG. 1, and a large number of concentric staircase gratings 4 are formed on the incident surface 3 in a concentric manner. 2 are formed. The diffraction grating 2 is divided into concentric regions shown by thick lines in FIG. 1, and the pitch of the staircase grating 4 differs for each region. The pitch of the staircase lattice 4 is smaller in the outer peripheral region, and therefore, in the outer peripheral portion, the staircase lattice 4 is formed with the smallest pitch.
[0019]
The step grating 4 of the diffraction grating 2 provided on the incident surface 3 of the diffractive optical element 1 is formed in a step shape as shown in FIG. FIG. 2 is a schematic diagram showing the diffraction grating 2 in an exaggerated manner. Therefore, the step grating 4 of the diffraction grating 2 is actually provided in a larger number than that shown in FIG. In FIG. 2, light is incident from above. Note that the chain line at the left end of FIG. 2 indicates the center line of the diffractive optical element 1.
[0020]
The staircase grating 4 is provided with a predetermined number of step portions 7 each having a rising surface portion 5 along the light incident direction and a diffraction surface portion 6 formed to have a narrow width extending from the rising surface portion 5 in the vertical direction. . In the present embodiment, the number of diffraction surface portions 6 is six or five. (Hereinafter, the number of steps of the staircase grating 4 refers to the number of the diffractive surface portions 6. That is, in this case, there are six steps or five steps.)
[0021]
Here, assuming that the height of the rising surface portion 5 of the staircase grating 4 is Δ, an optical path difference (n−1) Δ (where n is the refractive index of the diffractive optical element 1 and the refractive index of air is 1) for passing light. And the step of the staircase grating 4 is set so that this optical path difference becomes an integral multiple of 655 nm, which is the wavelength of the laser beam used for reading the DVD. Thus, the diffraction grating 2 allows incident light having a wavelength of 655 nm used for reading a DVD to travel straight. On the other hand, incident light having a wavelength of 785 nm used for reading a CD can be diffracted, that is, the diffraction grating 2 has a concave lens function for laser light used for reading a CD.
[0022]
As described above, since the diffraction grating 2 functions as a so-called Fresnel lens with respect to the laser beam used for reading a CD, the width of the diffraction surface portion 6 of the staircase grating 4 needs to be reduced toward the outer periphery. A region where the width of the diffraction surface portion 6 of the staircase grating 4 is large is called a wide portion 9, and a region where the width of the diffraction surface portion 6 of the staircase grating 4 is small in the outer peripheral portion is called a narrow portion 8. In this case, the number of steps of the staircase lattice 4 of the wide part 9 is formed in six steps. On the other hand, in the narrow portion 8, the number of steps of the staircase lattice 4 is formed in five steps.
[0023]
As shown in FIG. 3, in the six-step stair grating 10 in which the number of steps of the stair grating 4 in the diffraction grating 2 is six, each of the step portions 10a, 10b, 10c, 10d, 10e, and 10f is sequentially arranged from the bottom. The diffraction surface section 6 is provided with an equal width. On the other hand, in the five-step stair grating 11 in which the number of steps of the stair grating 4 in the diffraction grating 2 is five, step portions 11a, 11b, 11c, 11d, and 11e are provided in order from the bottom. Here, as for the steps 11a, 11b, 11c, and 11d, the respective diffraction surfaces 6 are provided with equal widths. On the other hand, since the diffraction surface portion 6 of the uppermost step portion 11e is formed without providing the step portion 7 corresponding to the uppermost step portion 10f in the case of six steps, two diffraction surface portions 6 are provided. It has a width equivalent to minutes.
[0024]
Further, in the five-step staircase lattice 11 in the narrow portion 8, the height of the uppermost step 11e is formed lower than the other steps 11a, 11b, 11c, 11d. For this reason, the height of the step portion 11 e of the narrow portion 8 is slightly lower than that of the step portion 10 e of the wide portion 9. In the present embodiment, the height of the step 11e is set to be 50 nm lower than the height of the other steps 7.
[0025]
The step grating 4 formed in a step shape is equivalent to a grating having a depth of a difference between an integral multiple of the wavelength of the laser beam used for reading the CD and the optical path difference of the light passing through the adjacent diffraction surface 6. . Here, when the height of the uppermost step portion 11e in the five-step staircase lattice 11 is lower than that of the other step portions 7, the difference becomes larger than that of the other step portions 7. That is, the five-stage staircase lattice 11 was equivalent to a lattice having a shallow depth at the deepest part as compared with the case of the six-stage staircase lattice, whereas the height of the uppermost step portion 11e in the five-stage staircase lattice 11 was high. By lowering the depth, it becomes equivalent to a deeper lattice at the deepest part, and approaches the case of six stages. As a result, the diffraction efficiency of the laser beam used for reading the CD can be increased as compared with the case where the height of the uppermost step portion 11e is the same as the height of the other step portions 7. It is possible to suppress a decrease in the diffraction efficiency caused by changing the number of steps 4 from six to five.
[0026]
As described above, the region where the staircase grating 4 is formed in five steps, that is, the narrow portion 8 is used when forming the staircase grating 4 when processing the mold 20 used for manufacturing the diffractive optical element 1 described later. This is an area that cannot be processed into a step. When the mold 20 is processed, the portion that requires the finest processing is the portion corresponding to the uppermost step in the staircase grating 4 of the diffractive optical element 1, and the width of the staircase grating 4 is smaller toward the outer periphery. Is done. Therefore, the region where the staircase grating 4 is formed in five steps, that is, the narrow portion 8 becomes the outer peripheral portion of the diffractive optical element 1.
[0027]
As shown in FIG. 3, in the staircase lattice 10 of the wide portion 9, the die 20 corresponding to the sixth step 10 f, which is the uppermost step, can be processed, while the narrow portion 8 is processed. In the staircase lattice 11 described above, it is difficult to process the mold 20 corresponding to the sixth step. For this reason, the staircase lattice 11 of the narrow portion 8 has five steps, and the uppermost step is 11e.
[0028]
Next, a method for manufacturing the diffractive optical element according to the present embodiment will be described. The diffractive optical element 1 can be formed of a resin material or the like. In this case, mass production can be performed by injection molding using the mold 20. The mold 20 is made of a super hard material, and has a shape obtained by inverting the diffractive optical element 1 as shown in FIG. 4, and is manufactured by cutting with a cutting tool.
[0029]
When machining the mold 20 as shown in FIG. 4, when cutting the step groove 21 for forming the uppermost step of the step lattice 4, if the step of the step lattice 4 is to be formed into six steps, the cutting tool is used. In a region where a groove smaller than the limit of the cutting process has to be cut, the step groove 21 is formed in five steps. The step groove 21 is formed to have a smaller width toward the outer periphery corresponding to the diffractive optical element 1 to be manufactured, and thus corresponds to a portion where the narrow portion 8 of the diffraction grating 2 is formed in the processing of the mold 20. A region where the step groove 21 is formed in five steps is formed in the outer peripheral portion. In this way, the mold 20 is processed in which the step groove 21a near the center has six steps and the step groove 21b on the outer peripheral part has five steps.
[0030]
In the stepped groove 21b formed in five steps, the deepest groove corresponds to the step in the uppermost step of the stepped grating 4 formed in five steps of the diffractive optical element 1 corresponding to 6 steps. It is processed to be 50 nm shallower than the fifth step groove of the step groove 21 a formed in the step. When such a mold 20 is used, the stair grid 4 of the narrow portion 8 has five steps, the stair grid 4 of the wide portion 9 has six steps, and the stair grid 4 of the narrow portion 8 has It is possible to manufacture the diffractive optical element 1 in which the upper stage is formed 50 nm lower than the corresponding step 7 of the step grating 4 of the wide portion 9.
[0031]
The embodiment has been described above, but the application of the present invention is not limited to this embodiment, and can be variously applied within the scope of the concept of the present invention.
For example, in the present embodiment, the number of steps 7 in the narrow portion 8 is one less than the number of steps 7 in the wide portion 9, but may be two or more. Further, instead of forming the step portion 7 in the narrow portion 8 in a fixed number of steps, the step portion 7 may be divided into regions and have different numbers of steps.
[0032]
【The invention's effect】
As described above, according to the present invention, in a diffractive optical element including an incident surface having a diffraction grating that causes two lights having different wavelengths to enter and diffracts one light without diffracting the other light, Because the number of steps in the narrow portion of the staircase grating is smaller than that in the wide portion, it is difficult to process the mold for manufacturing the diffractive optical element while maintaining high diffraction efficiency for the wide portion. There is no need to form a simple groove, so that a diffractive optical element that is easy to manufacture and has high diffraction efficiency can be obtained.
[0033]
Further, according to the present invention, the stair lattice of the narrow portion has one less step than the stair lattice of the wide portion, and the uppermost rising surface portion is lower than the corresponding rising surface portion in the wide portion of the stair lattice. Accordingly, in the step grating of a portion where the grating width is small in the diffraction grating, a decrease in the diffraction efficiency can be suppressed even though the number of steps is reduced, and a decrease in the diffraction efficiency of the entire diffractive optical element can be suppressed.
[0034]
Furthermore, according to the present invention, since the step grating of the diffraction grating is formed concentrically on the incident surface, the diffraction grating functions as a Fresnel lens, and one of two incident lights having different wavelengths is regarded as a divergent light. Therefore, it can be used as an optical element having high transmission efficiency in an optical pickup that reads signals from two types of media, CD and DVD.
[Brief description of the drawings]
FIG.
FIG. 2 is a front view schematically showing the diffractive optical element according to the embodiment.
FIG. 2
FIG. 2 is a cross-sectional view schematically illustrating a cross section of the diffractive optical element according to the embodiment.
FIG. 3
FIG. 4 is an enlarged cross-sectional view schematically showing a cross section around a portion where a step of a stair grating is changed in the diffractive optical element according to the embodiment.
FIG. 4
FIG. 2 is a cross-sectional view schematically illustrating a cross section of a mold for manufacturing the diffractive optical element according to the embodiment.
FIG. 5
It is sectional drawing which showed typically the cross section of the diffractive optical element when all the staircase gratings are formed in six steps.
FIG. 6
It is sectional drawing which showed typically the cross section of the diffractive optical element when all the staircase gratings are formed in five steps.
[Explanation of symbols]
REFERENCE SIGNS LIST 1 diffractive optical element 2 diffraction grating 3 incident surface 4 step grating 5 rising surface portion 6 diffraction surface portion 7 step portion 8 narrow portion 9 wide portion 10 6 step stair grating 11 5 step stair grating 20 mold 21 step groove

Claims (3)

In a diffractive optical element having an incident surface having a diffraction grating that causes two lights having different wavelengths to enter and diffracts one light without diffracting the other light, the diffraction grating includes a rising surface portion along an incident direction and the rising surface portion. It has a large number of staircase gratings having a predetermined number of steps including a diffractive surface portion formed to have a narrow width and protrude from the rising surface portion, and the step portion is an integer of the wavelength of one light to the light passing through the adjacent diffractive surface portion. It is formed at a height that gives twice the optical path difference,
The diffraction grating is composed of a narrow portion formed to reduce the width of the diffraction surface portion and a wide portion formed to increase the width of the diffraction surface portion, and the step grating of the narrow portion has fewer steps than the step grating of the wide portion. A diffractive optical element characterized by being performed. The staircase lattice of the narrow portion has one less step than the staircase lattice of the wide portion, and the rising surface portion at the top is lower than the corresponding rising surface portion in the wide portion of the staircase lattice. The diffractive optical element according to claim 1, wherein 3. The diffractive optical element according to claim 1, wherein the step grating of the diffraction grating is formed concentrically on the incident surface.

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