Patent Grant
6927915
1. Field of the Invention
The present invention relates to diffractive optical elements, and more particularly to diffractive optical elements having a blazed-binary grating, as well as optical systems and optical apparatuses provided with the same.
2. Description of the Related Art
Conventional diffractive optical elements serving as diffraction lenses with the purpose of reducing chromatic aberration are disclosed in SPIE Vol. 1354 International Lens Design Conference (1990), for example.
Color separation gratings utilizing the fact that the diffraction angle differs depending on the wavelength of the incident light in order to perform color separation are disclosed for example in Japanese Patent Publication No. 1993-46139 (corresponding to U.S. Pat. No. 5,113,067). Furthermore, recently, a diffractive optical element known as SWS (sub-wavelength structure) grating, whose grating period has a microscopic periodic structure that is smaller than the used wavelength, has been disclosed in “Kougaku” [Optics], vol. 1 of 27 in series (1998), pp. 12 to 17.
Due to its grating structure, this SWS grating is used for elements with various functions, such as birefringent wavelength plates, anti-reflection structures and polarization beam splitters. Moreover, it has been reported that for these functions, there are only small performance variations due to changes of the angle of incidence.
Among SWS gratings, Applied Optics, Vol. 31, No. 22, p. 4453 (1992) discloses the structure shown in
In the diffractive optical element of
With this structure, the effective refractive index changes gradually even though the grating height (depth of the grating grooves) is constant, and as a result, it is possible to attain a performance that is substantially the same as that of a blazed diffraction grating with a constant refractive index in which the height of the grating portions 10 changes gradually, as shown in FIG. 14.
Moreover, recently, the assignee of this application has proposed, as the structure of a diffractive optical element, the diffractive optical element shown in FIG. 12. In this diffractive optical element, the grating thickness (grating height) of two grating portions 10′ and 11′ made of different materials is changed gradually (monotonously), and by stacking the two gratings 10′ and 11′ on top of one another in the thickness (height) direction, a high diffraction efficiency in the entire visible wavelength region can be achieved, as shown in FIG. 13.
In these conventional examples, the structure of blazed-binary gratings has come to a stand-still at obtaining the same performance as the blazed-binary grating shown in
It is an object of the present invention to present a diffractive optical element having a blazed-binary grating with which a high diffraction efficiency can be achieved over substantially the entire used wavelength region.
A diffractive optical element according to the present invention has a periodic structure taking a grating unit including a plurality of grating portions with different grating width and grating interval as one period. The diffraction element further includes a plurality of first grating portions made of a first material and a plurality of second grating portions made of a second material having a different refractive index than the first material that are arranged between the first grating portions.
These and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.
FIG. 1(A) is a front view of a diffractive optical element according to Embodiment 1 of the present invention, and FIG. 1(B) is a lateral view of that diffractive optical element.
FIG. 5(A) is a cross-sectional view of a diffractive optical element according to Embodiment 3 of the present invention, and FIG. 5(B) is a plan view of that diffractive optical element.
FIG. 8(A) is a front view of a camera according to Embodiment 5 of the present invention, and FIG. 8(B) is a lateral sectional view of that camera.
The following is a description of embodiments of the present invention, with reference to the accompanying drawings.
Embodiment 1
FIGS. 1(A), 1(B) and 2 show the structure of a diffractive optical element according to Embodiment 1 of the present invention. The diffractive optical element 1 is configured by providing a transparent blazed-binary diffraction grating 3 on a transparent substrate 2.
The blazed-binary diffraction grating 3 has a periodic structure in which one-dimensional comb-tooth-shaped grating units are formed in repetition, and with respect to the direction A-A′ in FIG. 1(A), it has a grating unit period Pt, which is larger than the wavelength of the incident (used) light (used wavelength). Moreover, light that is incident on the diffractive optical element is diffracted only in a specific direction, which depends on the grating unit period Pt and the design order m.
In
Within one grating unit period Pt, the grating width (the width of a grating portion) and the grating interval (the interval between the grating portions adjacent to each other) of the first grating portions 4 increases or decreases gradually (monotonously). That is to say, the width w that is occupied by the first material within the grating pitch p1 changes gradually from w1 to ws. A SWS diffraction grating is configured through a periodic structure taking such a grating unit as one period (grating unit period Pt, which is larger than the used wavelength).
The first grating portions 4 are formed integrally with a base portion that constitutes a first region R1 made of the same first material as the first gratings 4 and in contact with the blazed-binary diffraction grating 3 on the side on which light is incident.
The second grating portions 6 are formed in portions inside the grooves between the first grating portions 4 that contact the base portion, and have a grating height d2 that is lower than the height of the first grating portions 4.
The side on which the light emerges from the blazed-binary diffraction grating 3 is in contact with a second region R2 made of air. That is to say, the blazed-binary diffraction grating 3 is formed on the border between the first region R1 and the second region R2.
The portions of the grooves between the first grating portions 4, where the second grating portions 6 are not provided in the grating height direction are also filled with air (third material), like the second region R2.
When light is incident from the side of the substrate 2 into the diffractive optical element with this structure, then the light that has passed via the first region R1 through the blazed-binary diffraction grating 3 is diffracted in a diffraction direction that depends on the grating unit period Pt, in the direction of only a specific diffraction order (first order in FIG. 2), and is propagated through the second region R2.
The following is an explanation of the structure and the optical effect of the diffractive optical element 1, using concrete numerical values. In order to provide a grating structure having a microscopic grating pitch with the properties of a SWS, diffraction grating, the grating pitch needs to be sufficiently small compared to the used wavelength. More specifically, of the grating pitches p1 that satisfy the grating equation (1), a grating pitch p1 should be selected at which the equation becomes true only for zero-th order diffracted light as propagated light.
n3(λ)sin θ3−n1(λ)sin θ1=mλ/p1 (1)
In Equation (1):
A simple numerical example shall supplement these explanations. The used wavelength λ is taken to be 400 nm, the refractive index of the first material on the incident side is
n1(λ)=1.5,
and the refractive index of the third material on the emergent side is
n3(λ)=1.0.
Now, when light from a direction perpendicular to the grating period direction is incident at an incident angle θ1=5°, the diffraction order m is set to 1 and the emergent angle θ3 is set to −90°, the grating pitch p1 according to Equation (1) becomes 0.35 μm.
Consequently, when the grating pitch p1 is 0.35 μm or smaller, there is no solution to Equation (1), so that there is no light that is diffracted by the microscopic periodic structure. Therefore, the only light that is propagated will be the zero-th order diffracted light with respect to the microscopic periodic structure. The equation for the diffraction of zero-th order diffracted light corresponds to the case that m=0 in Equation (1), and is equivalent to Snell's law of refraction.
Adopting these conditions, the microscopic periodic structure has the structural birefringence that is the unique nature of SWS diffraction gratings. Furthermore, with these conditions, there is no diffraction at the SWS diffraction grating, so that it becomes unnecessary to consider diffraction at the microscopic periodic structure when contemplating diffraction at the blazed-binary diffraction grating.
As explained above, in the blazed-binary diffraction optical grating 1 having this microscopic periodic structure, the grating pitch needs to be decided such that the period (grating pitch) of the microscopic periodic structure satisfies the above-described conditions over the entire region of used wavelengths and over the range of used incident angles.
The height d1 of the first grating portions 4 is set to 10.71 μm, and the height d2 of the second grating portions 6 is set to 7.88 μm. Moreover, the grating pitch p1 of the first grating portions 4 is set to 0.2 μm, and the grating width w of the first grating portions 4 is changed gradually from 0.2 μm to 0 μm.
As can be seen in
The following is an explanation of the grating width. In the diffractive optical element (SWS diffractive optical element) 1 of this embodiment as shown in
The following-example is for the left-most step of a nine-step grating unit in FIG. 9. In the diffractive optical element, a first grating portion having nine steps in one period and a second grating portion 11 having nine steps in one period are layered with each other in the grating height direction. The height d1i (i=1˜9) of the first grating portion 10 gradually decreases from the left side to the right side in the figure, and the height d2i (i=1˜9) of the second grating portion 11 gradually increases in the same direction.
In the diffractive optical element shown in
The grating heights of each right-most step of the first and second grating portion 10 and 11, respectively, are:
d1a=1/9D1
d2a=9/9D2
Furthermore, the refractive index of the material constituting the grating portions with respect to the wavelength λ is set to n1(λ) for the first grating portion (first material) and is set to n2(λ) for the second grating portion (second material). One border is a border between the second grating portion 11 and air (third material). The optical path length Lbi(λ) of the i-th step in this case is
Lbi(λ)={n1(λ)−1}·d1i+{n2(λ)−1}·d2i (2)
where, i=1˜s, and s is 9 in FIG. 10.
On the other hand, the optical path length Li(λ) of the i-th grating portion in one of the grating units of the diffractive optical element 1 of the present embodiment can be determined to be
Lai(λ)={n1ieff(λ)−1}·(d1−d2)+{n2ieff(λ)−1}·d2 (3)
where, i=1˜s, and s is 9 in FIG. 10.
Herein, the effective refractive index n1ieff(λ) of a first grating unit constituted of the first grating portions (the first material) is
n1ieff(λ)=[fi·{n1(λ)}2+(1−fi)·{n3(λ)}2]1/2 (4)
(i=1˜s), and (5)
the effective refractive index n2ieff(λ) a second grating unit constituted of the second grating portions (the second material) is
n2ieff(λ)=[fi·{n1(λ)}2+(1−fi)·{n2(λ)}2]1/2 (6)
(i=1˜s), and (7)
since the third material in this embodiment is air,
n3(λ)=1
Furthermore,
fi=wi/p1
If the value of fi is determined from the above-noted relations such that
Lai(λ)=Lbi(λ) (6)
(i=1˜s)
is satisfied, then the i-th step of the 9-step units shown in
The grating widths should be determined for each step such that Expression (6) is satisfied.
In this embodiment, the heights d1 and d2 of the gratings 4 and 6 may be
d1=D1
d2=D2,
but there is no limitation to this, and d1 and d2 may also be determined giving consideration to the grating widths that can be manufactured and the like.
In this embodiment, it is preferable that the number of first grating portions 4 within the grating unit period Pt is determined such that the optical path length difference
The diffractive optical element 1 of the present embodiment is configured such that a performance that is equivalent to that of the diffractive optical element shown in
To address this, d1 and d2 may be selected such that the desired properties are met for TM-polarized light while meeting the afore-mentioned performance for TE-polarized light.
The above-described diffractive optical element 1 of this embodiment is formed on the substrate 2, but it is also possible to use a quartz substrate, and to form the shape of the diffractive optical element directly on the substrate by etching that substrate.
Embodiment 2
With this structure, when the first grating portions 4 are fabricated for example by etching with a material other than a resin, the substrate 2 can serve as an etching stopper layer, and the grating height can be controlled with high precision.
Furthermore, it becomes possible to select the refractive index of the material of the substrate 2 so as to minimize the reflection loss occurring at the border 9 between the grating bottom surface and the substrate surface.
Embodiment 3
In addition to the structure explained in Embodiment 1, the diffractive optical element 1″ of this embodiment has a periodic structure, in which the second and the third material are arranged alternately with regard to the direction perpendicular to the period of the blazed-binary diffraction grating 3. Then, by making the repetition period smaller than the used wavelength it is possible to provide the properties of a SWS diffractive optical element.
Moreover, with this structure, setting the height of the first and second gratings 4 and 6 uniformly to d1, the same effect as in Embodiment 1 can be attained. By matching the heights of the two grating portions 4 and 6, the overall diffractive optical element can be made flat, making the fabrication of the grating shape by etching as described above as well as the handling of the element easier.
In this embodiment, the optical path length difference corresponding to Expression (3) is as follows:
Lai(λ)={n1ieff(λ)−1}·d1 (7)
wherein, i=1˜s,
n1ieff(λ)=[fi·{n1(λ)}2+(1−fi)·{n2ieff(λ)}2]1/2, and (8)
n2ieff(λ)=[fy·{1/n2(λ)}2+(1−fy)·{1/n3(λ)}2]−1/2. (9)
Herein, fy=width wy of each second grating portion 6/grating pitch py of the second grating portions 6.
Embodiment 4
By gradually changing the grating unit period Pt radially in this element, it is possible to use the blazed-binary diffractive optical element as a lens.
Here, the pitch p1 within one period may be changed in accordance with changing of the grating unit period Pt. Alternatively, in a grating unit with a large grating unit period Pt, the number rn of grating portions within one grating unit period may be made larger than in a grating unit with a small grating unit period Pt. What is important is in either case that the grating pitch p1 is set to a pitch that is smaller than the used wavelength, so that a SWS structure is attained.
Embodiment 5
As explained for Embodiments 1 to 3, the diffractive optical element 100 has a diffraction efficiency of diffracted light of the design order that is improved over the entire region of used wavelengths, so that a favorable optical performance can be achieved in an optical system 110 using white light.
Embodiment 6
FIGS. 8(A) and 8(B) are respectively a front view and a lateral sectional view of a camera using the optical system 110 shown in
In the image-taking optical system 201 and the finder optical system 202, the diffractive optical element 100 may be arranged at any suitable position.
Thus, it is possible to improve the optical performance of the optical system by using the diffractive optical element described in the above embodiments in the optical system of an optical apparatus such as a camera or the like.
As explained above, with the above embodiments, diffractive optical elements can be achieved that combine the features that a high diffraction efficiency can be attained in substantially the entire region of used wavelengths (e.g. visible wavelengths), that the performance changes with regard to changes in the angle of incidence, which are a feature SWS gratings, are small, that the grating height, which is a feature of blazed-binary grating structures, is constant, that is to say, the overall element is substantially flat, and that manufacturing and handling are easy.
Furthermore, if the height of the seconds grating is made lower than the height of the first grating, and the heights of the first and the second gratings are respectively constant, then it is possible to fill the portions between the first grating portions where the second grating is not formed in the grating height direction with a third material (such as air) whose refractive index is different from that of the first and the second material.
Moreover, if the first grating portions are formed on a base portion formed by a first material, and if the refractive index of the second material is closer to the refractive index of the first material than that of the third material, then it is possible to provide the second grating portions in the regions contacting the base portion between the first grating portions and to make the reflection loss at the border portion contacting the base portion small.
By satisfying Expression (6), it is possible to achieve an optical performance that is equivalent to the diffractive optical element with high diffraction efficiency shown in FIG. 12.
Here, if the height of the first grating and the height of the second grating are equal and constant, the overall element structure becomes flat, so that handling and manufacture become easy, and the difference between the optical performance at the periphery and at the center can be made small.
More specifically, a periodic structure in which a second material and a third material (e.g. air) whose refractive index is different from the first and the second material are arranged alternately in a direction perpendicular to the direction in which the first grating portions are arranged, may be adopted. Then, by arranging the second and third materials alternately at a period that is smaller than the used wavelengths, it is possible to achieve the characteristics of a SWS diffractive optical element.
While preferred embodiments have been described, it is to be understood that modification and variation of the present invention may be made without departing from the spirit or scope of the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2002-176143 | Jun 2002 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3484153 | Jackson et al. | Dec 1969 | A |
| 5113067 | Nakai et al. | May 1992 | A |
| 5225930 | Land et al. | Jul 1993 | A |
| 5245471 | Iwatsuka et al. | Sep 1993 | A |
| 5742433 | Shiono et al. | Apr 1998 | A |
| 6191890 | Baets et al. | Feb 2001 | B1 |
| 6490393 | Zhou | Dec 2002 | B1 |
| 20040021945 | Tompkin et al. | Feb 2004 | A1 |
| Number | Date | Country |
|---|---|---|
| 5-46139 | Jul 1993 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20030231395 A1 | Dec 2003 | US |
