Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2001-07-05
2003-09-23
Juba, John (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S566000, C359S570000, C359S571000, C359S573000, C359S574000, C359S575000, C359S576000
Reexamination Certificate
active
06624943
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 light of a plurality of wavelengths or a specific wavelength concentrate onto diffraction light of a specific order (design order) of diffraction, and to an optical system having the diffractive optical element.
2. Description of Related Art
Heretofore, there have been known various methods of correcting chromatic aberration in an optical system. According to one of the known methods, two glass (lens) materials which differ in dispersion are combined with each other to be used for abating chromatic aberration.
According to another known method, chromatic aberration is abated by using a diffractive optical element having a diffracting function for an optical system which includes a refracting lens, as disclosed, for example, in the optical literature such as SPIE Vol. 1354 “International Lens Design Conference (1990)” and also in 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.
This method has been developed by utilizing a physical phenomenon that the direction in which chromatic aberration arises in a ray of light of a certain wavelength with respect to a ray of light of a reference wavelength on a refractive surface becomes reverse to that on a diffractive surface.
Further, such a diffractive optical element can be provided with an effect of serving as an aspheric lens by varying the period of a periodic structure of its diffraction grating, so that aberrations can be abated advantageously.
Comparing refractive and diffractive surfaces in respect of a refracting action of rays of light, one ray of light remains one ray after refraction on a lens surface, whereas one ray of light is splint into rays of different orders when it is diffracted by a diffraction grating.
Therefore, in using a diffractive optical element for a lens system, it is necessary to design the grating structure in such a manner that light fluxes of a useful wavelength region concentrate onto diffraction light of a specific order (design order) of diffraction. With light fluxes concentrating onto diffraction light of the design order, in order to lower the luminous intensity of diffraction light of orders other than the design order, it becomes necessary that the diffraction efficiency of diffraction light of the design order is sufficiently high. Further, if there are some rays of light of diffraction orders other than the design order, these rays become flare light, because they are imaged in a place different from the imaging place of the rays of the design order.
For an optical system having a diffractive optical element, therefore, it is important to pay sufficient heed to the spectral distribution of the diffraction efficiency of diffraction light of the design order and also to the behavior of diffraction light of orders other than the design order.
FIG. 11
shows a case where a diffractive optical element
1
having one diffraction grating
4
formed on a base plate
2
is formed on a certain surface in an optical system. In this case, the diffraction efficiency for diffraction light of a specific order of diffraction is obtained as shown in
FIG. 12
, which shows in a graph the characteristic of the diffraction efficiency. In
FIG. 12
, the abscissa axis of the graph indicates wavelength and the ordinate axis indicates diffraction efficiency. The diffractive optical element
1
is designed to have the highest diffraction efficiency at the first order of diffraction (shown in a full line curve in
FIG. 12
) in the useful wavelength region.
In other words, the design diffraction order of this diffractive optical element is the first order. The graph of
FIG. 12
also shows the diffraction efficiency of diffraction light obtained at diffraction orders near the design order, i.e., a zero order and a second order (1±1).
As shown in
FIG. 12
, the diffraction efficiency at the design order becomes highest at a certain wavelength (540 nm) (hereinafter referred to as the design wavelength) and gradually lowers at other wavelengths. Such a lowering portion of the diffraction efficiency obtained at the design order becomes diffraction light of other orders, thereby appearing as flare light. Further, in a case where a plurality of diffractive optical elements are used, a drop in diffraction efficiency at wavelengths other than the design wavelength eventually causes a decrease in transmission factor.
The arrangement of lessening such a drop in diffraction efficiency is disclosed in Japanese Laid-Open Patent Applications No. HEI 9-127321, No. HEI 9-127322, etc. The diffractive optical element disclosed in Japanese Laid-Open Patent Application No. HEI 9-127321 is in a sectional shape formed by laminating two layers
4
and
5
, as shown in FIG.
13
.
The diffractive optical element disclosed in Japanese Laid-Open Patent Application No. HEI 9-127322 is of such a grating structure that three layers
4
,
5
and
6
are laminated as shown in FIG.
14
. As shown in
FIG. 14
, the layer
5
, which is sandwiched between two diffraction grating surfaces
8
and
9
provided at the boundaries of the layers
4
,
5
and
6
, has a thickness which varies with portions thereof. In this diffractive optical element, each of the diffraction grating surfaces
8
and
9
is formed between two different materials. A high diffraction efficiency is attained by optimizing a difference in refractive power between the layer materials located in front and in rear of the boundary and the depth of the grating grooves.
Since the arrangements of the above-mentioned diffractive optical elements necessitate a wavelength characteristic of the difference in refractive power between the materials in front and in rear of each of grating areas to have desired values, it is impossible to have a larger difference in refractive power than in a case where a grating area has air on one side thereof instead of a layer material. As a result, their gratings must be arranged to have a relatively large grating thickness. In the case of the diffractive optical element disclosed in Japanese Laid-Open Patent Application No. HEI 9-127321, for example, the grating thickness is 10 &mgr;m or thereabout.
In the case of the diffractive optical element disclosed in Japanese Laid-Open Patent Application No. HEI 9-127322, the number of layers of three different materials is increased to three and the number of gratings is increased to two. One of the two gratings measures at least 7 &mgr;m in thickness, so that a considerably deep grating shape would be recognized.
In manufacturing diffractive optical elements, the above-stated grating shapes can be formed by cutting. A product thus obtained by cutting either may be used directly as a diffractive optical element or may be used as a mold for duplicating diffractive optical elements.
It is conceivable as a simplified manufacturing method to form an arcuate diffraction grating surface with a cutting tool edge
17
by rotating a base plate
2
as shown in FIG.
15
. In this method, while the cutting tool edge
17
is moved in the direction of a grating pitch, the cutting process is carried out by varying a distance between the base plate
2
and the cutting tool edge
17
to obtain a desired shape of grating.
According to this manufacturing method, if the grating thickness is large as mentioned above, the amount of cutting by the cutting process increases to cause the cutting tool edge to be greatly abraded. As a result, the shape of the tool edge obtained at the commencement of cutting differs from its shape obtained at the end of cutting. Such abrasion causes the grating thickness at the point where the cutting comes to an end to become thinner than a desired value. In addition to this problem, since the cutting tool edge is rounded by the abrasion, the grating shape comes to deviate from a desired saw-tooth like shape.
Nakai Takehiko
Setani Michitaka
Boutsikaris Leo
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Juba John
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