Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
1998-07-24
2002-11-12
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S576000, C359S569000
Reexamination Certificate
active
06480332
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to diffractive optical elements, and more particularly to a diffractive optical element having such a grating structure that rays of a plurality of wavelengths or rays of a specific wavelength band concentrate at a specific order (a design order) of diffraction, and to an optical system having the diffractive optical element.
2. Description of Related Art
Heretofore, as one of the methods of correcting chromatic aberration of an optical system, there is known a method of combining two glass (lens) materials which differ in dispersion.
In contrast to the method of reducing chromatic aberration by combining glass materials, there is known another method, which is disclosed in the optical literature, such as “International Lens Design Conference (1990)”, SPIE Vol. 1354, etc., and the specifications of Japanese Laid-Open Patent Applications No. HEI 4-213421 and No. HEI 6-324262 and U.S. Pat. No. 5,044,706. In the case of that method, chromatic aberration is corrected by means of a diffractive optical element which is provided with a diffraction grating for a diffracting action and is disposed on a lens surface or a part of an optical system. That method is based on a physical phenomenon that the direction in which chromatic aberration arises for a ray of light of a reference wavelength becomes opposite between a refractive surface and a diffractive surface in an optical system.
Further, the diffractive optical element of such a type can be arranged to produce an advantageous correcting effect, like an aspheric lens, on the aberration by varying the period of a periodic structure of its diffraction grating.
Here, compared with a refracting action of rays of light, while one ray of light remains one even after refraction at a lens surface, one ray of light is split into rays of a plurality of orders after diffraction at a diffractive surface.
Therefore, in using a diffractive optical element for a lens system, it is necessary to decide the grating structure in such a way as to cause a light flux of a useful wavelength region to concentrate at a specific order of diffraction (hereinafter referred to as a design order). With the light flux concentrating at the specific order, rays of diffraction light other than the light flux of the specific order have a low degree of intensity. When the intensity becomes zero, the rays of diffraction light would not exist.
In order to attain the above-stated feature, the diffraction efficiency of a ray of light of the design order must be sufficiently high. Further, in a case where there are some rays of light having diffraction orders other than the design order, these rays are imaged in a place different from the imaging place of the ray of light of the design order, and thus appear as flare light.
For an optical system using a diffractive optical element, therefore, it is important to pay sufficient heed to the spectral distribution of diffraction efficiency at the design order and the behavior of rays of diffraction light of orders other than the design order.
FIG. 12
shows a case where a diffractive optical element
1
, which has a diffraction grating
3
and is composed of one layer on a base plate
2
, is formed on a surface of an optical system. In this case, diffraction efficiency for a specific order of diffraction is obtained as shown in
FIG. 13
, which shows the characteristic of the diffraction efficiency. This diffractive optical element is made of plastic material which is PMMA (nd=1.4917 and &ngr;d=57.4). The grating thickness d is set at 1.07 &mgr;m. In
FIG. 13
, the abscissa axis of a graph indicates wavelength (nm) and the ordinate axis indicates the diffraction efficiency (%).
The diffractive optical element
1
is designed to have the diffraction efficiency become highest at the first order of diffraction (shown in a full line curve in
FIG. 13
) in the useful wavelength region (a wavelength of 530 nm and thereabout). In other words, the design order of the diffractive optical element
1
is the first order.
Further,
FIG. 13
shows also the diffraction efficiency of a diffraction order near the design order, i.e., zero-order light and second-order light (first order +first order). As shown in
FIG. 13
, at the design order, the diffraction efficiency becomes highest at a certain wavelength (hereinafter referred to as a “design wavelength”) and gradually decreases at other wavelengths. In this case, the design wavelength &lgr; is set at 530 nm. The lower portion of the diffraction efficiency obtained at the design order becomes diffraction light of other orders and comes to appear as flare light. Further, in a case where the optical system is provided with a plurality of diffractive optical elements, a drop in diffraction efficiency at wavelengths other than the design wavelength eventually causes a decrease in transmission factor.
A diffractive optical element having the structure capable of lessening the drop in diffraction efficiency is disclosed in Japanese Laid-Open Patent Application No. HEI 9-127321. More specifically, the diffractive optical element disclosed is constructed by laminating on a base plate a plurality of layers of different materials and forming a relief pattern of diffraction grating on an interface between the layers of different materials.
The diffractive optical element of the kind having a grating structure formed by laminating a plurality of layers on a base plate can be formed to have a high degree of diffraction efficiency by combining component layers in various manners. Some of such combinations, however, make the thickness of grating of the diffractive optical element thicker than in the case of an ordinary element composed of a single layer, because materials forming diffraction gratings at their boundary faces cannot be otherwise arranged to have a sufficiently large difference in refractive index. Such a combination that causes an increase in thickness of grating presents a problem in a case where the refractive index is caused to vary by temperature variations to lower the diffraction efficiency.
In the event of temperature variations, some of such combinations thus cause the diffraction efficiency of the diffractive optical element of this kind to become lower than that of a diffractive optical element composed of a single layer in a conventional manner.
BRIEF SUMMARY OF THE INVENTION
It is an object of the invention to provide a diffractive optical element, or an optical system having the diffractive optical element, which is arranged to be capable of keeping its diffraction efficiency not much degraded by changes of refractive index resulting from temperature variations.
To attain the above object, in accordance with a first aspect of the invention, there is provided a diffractive optical element having a diffraction grating formed at an interface between different materials, in which a total sum of values obtained by multiplying rates of change in refractive index due to temperature variations of the materials by a grating thickness of the diffraction grating is smaller than a useful wavelength.
In accordance with a second aspect of the invention, there is provided a diffractive optical element having diffraction gratings formed by a plurality of layers made of at least two kinds of materials of different dispersions to enhance diffraction efficiency of a specific order (design order) over an entire useful wavelength region, in which a total sum of values obtained by multiplying rates of change in refractive index due to temperature variations of the materials forming the respective layers by grating thicknesses of the respective diffraction gratings is smaller than a useful wavelength.
In accordance with a third aspect of the invention, there is provided a diffractive optical element having a first diffraction grating surface formed at a boundary between first and second layers made of materials of different dispersions and a second diffraction grating surface formed at a boundary b
Assaf Fayez
Spyrou Cassandra
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