Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
1999-09-14
2003-05-06
Chang, Audrey (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S576000, C359S566000
Reexamination Certificate
active
06560019
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a diffractive optical element and an optical system having the same and, more particularly, to a diffractive optical element of such a grating structure that diffracts light (energy) of a plurality of wavelengths or a certain band so that the diffracted light concentrates on a particular order (design order), and an optical system having the same.
2. Description of Related Art
One of the conventional methods of correcting the chromatic aberrations of the optical system is to combine a plurality of glasses (lenses) of different dispersions (Abbe numbers) from one another.
In addition to the above method of lessening the chromatic aberrations by using the combination of glass materials, there is another method of using a diffractive optical element having the diffracting function in the lens surface or the surface of other parts of the optical system, as disclosed in SPIE Vol. 1354 International Lens Design Conference (1990), Japanese Laid-Open Patent Applications No. Hei 4-213421 and No. Hei 6-324262, and U.S. Pat. No. 5,044,706, etc.
This method is based on the physical phenomenon that, for the rays of light in the wavelengths other than a reference wavelength, the refractive surface and the diffractive surface in the optical system produce chromatic aberrations in opposite directions to each other.
Further, in such a diffractive optical element, when the period of its diffraction grating is made to vary depending on the place, the diffractive optical element can take an effect similar to an aspherical lens, giving a great advantage of reducing the aberrations of the optical system.
Here, on comparison of the refracting action of light, for the lens surface, one ray of light, even after being refracted, remains the one. For the diffraction grating, on the other hand, it is typical that one ray of light, when diffracted, is divided into a plurality of rays of light of different diffraction orders.
To employ the diffractive optical element in the lens system, therefore, determination of the grating structure must be made such that, for a useful wavelength region, the light ray diffracts in concentration on a particular one order (design order). In a case where the energy of incident light concentrates on the diffracted light of the particular order, the intensities of the diffracted light rays of the other orders become low. If the sum of the intensities of the diffracted light rays of the other orders is zero, the diffracted light rays of the other orders are considered to be not present.
To this purpose, it becomes necessary that, for the design order, the light ray diffracts with a high enough efficiency (ideally, 100%). It should be also noted that, if the diffracted light of any of other orders than the design order is present, it forms an image at a different place from that of the design order, becoming flare.
In the optical system that utilizes the diffractive optical element, therefore, it is important to fully consider not only the spectral distribution of the diffraction efficiency for the design order, but also the behavior of the diffracted light of the other orders.
Suppose, as shown in
FIG. 1
, when a diffractive optical element
1
is formed with a diffraction grating
3
in one layer on a substrate
2
or a surface in the optical system, then the diffraction efficiencies for particular orders are obtained as shown in FIG.
2
. In the graph of
FIG. 2
, the abscissa represents the wavelength, and the ordinate represents the diffraction efficiency. This diffractive optical element is so designed that, for the diffracted light of the first order (shown by a solid line curve), the diffraction efficiency becomes highest in the useful wavelength region.
That is, the design order is the first order. In addition, there are also shown the diffraction efficiencies for diffraction orders near the design order, i.e., or zero order and second order ((1±1)st orders).
As shown in
FIG. 2
, in the design order, the diffraction efficiency has a highest value at a certain wavelength (540 nm) (hereinafter, referred to as the “design wavelength”), and gradually lowers as the wavelength goes away from the design wavelength. This lowering of the diffraction efficiency in the design order is reflected to the diffracted light of the other orders, thereby producing flare. Also, in a case where a plurality of diffractive optical elements are in use, it particularly results that the diffraction efficiency lowers in the wavelengths other than the design wavelength. This leads to a decreases in the transmittance of the entire optical system.
An arrangement for reducing this lowering of the diffraction efficiency is proposed in U.S. patent application Ser. No. 09/121,685 (Japanese Patent Application No. Hei 9-217103).
FIG. 3
is a sectional view of the main parts of the diffractive optical element
1
proposed in U.S. patent application Ser. No. 09/121,685. The diffractive optical element
1
shown in
FIG. 3
has a laminated cross-section form with two layers
4
and
5
of diffraction gratings on a substrate
2
in superimposed relation to each other. Then, the refractive indices and dispersion characteristics of the materials of the two layers
4
and
5
and their grating thicknesses are optimized to obtain higher diffraction efficiencies throughout the entire range of useful wavelengths.
In the type of diffractive optical element shown in
FIG. 3
, as the material of the diffraction grating for each layer, use may be made of easy-to-cut optical glasses, plastics, or optically transparent, ultraviolet curable polymer. In this case, however, it becomes difficult to take as large a difference in the refractive index as in the mono-layer type. Therefore, the large difference in the optical path length becomes harder to take. For this reason, the diffraction grating becomes considerably thick. For example, in the diffractive optical element
1
of the two-layer structure, the material used for the first layer
4
is assumed to be an ultraviolet curable polymer of refractive index nd=1.525 and Abbe number &ngr;d=47.8, and the material used for the second layer
5
is assumed to be another ultraviolet curable polymer of refractive index nd=1.635 and Abbe number &ngr;d=23.0. In this combination, the grating thicknesses are optimized. Then, the resultant diffraction efficiency is shown in FIG.
4
. It is understandable that the diffraction efficiency of the first order is kept high over the entire visible spectrum. In this case, however, the first diffraction grating
4
has a thickness d1 of 12.70 &mgr;m, and the second diffraction grating
5
has a thickness d2 of 9.55 &mgr;m. On consideration of the usual one-layer diffraction grating whose thickness is about 1 &mgr;m, the two-layer diffraction grating has so much a large thickness. Also, in actual practice of manufacturing, because the second layer
5
shown in
FIG. 3
is sectioned by every grating pitch, the use of the production technique by molding or the like results in a difficulty of transferring the form and detaching from the die.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide a diffractive optical element which is actually more practical to utilize than was heretofore possible. This utilizable diffractive optical element has such a fundamental structure that, as shown in
FIG. 5
, diffraction gratings
4
and
5
which differ in dispersion from each other are first formed in separation, then, both the diffraction gratings
4
and
5
, while keeping their corresponding pitches to each other in alignment, are brought into a near juxtaposition, and through a certain space whose refractive index is “1” (for example, air), the diffraction gratings
4
and
5
are superimposed on each other.
Such diffraction gratings are manufactured by the ruling machine. So, the product can be used directly as the actual optical element. It may otherwise be used as a master grating, from which to
Canon Kabushiki Kaisha
Chang Audrey
Fitzpatrick ,Cella, Harper & Scinto
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