Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2002-02-13
2003-12-30
Chang, Audrey (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S569000, C359S571000, C359S573000, C359S575000
Reexamination Certificate
active
06671096
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a diffractive optical element and an optical system, and more particularly to a diffractive optical element for white light and an optical system including such a diffractive optical element.
2. Description of the Prior Art
A diffractive optical element has useful optical properties that are not found in a well-known refractive optical element. For example, a diffractive optical element having a light-condensing ability offers the following advantages. First, by providing a diffractive optical element on a lens surface of an ordinary refractive optical element, it is possible to give a single optical element both light-diffracting and light-refracting abilities. Second, a diffractive optical element makes effective correction of chromatic aberration possible because, in it, the quantity that corresponds to the dispersive power of a refractive optical element has the opposite sign.
Although a diffractive optical element has useful properties as described above, it also suffers from problems resulting from the fact that diffraction efficiency is wavelength-dependent. For example, except at the design wavelength, the diffracted light of orders other than the intended order is too intense. This causes ghosts, and thereby degrades imaging performance. In particular, an optical system designed for white light, i.e. one that needs to cope with a wide range of wavelengths, suffers greatly from this problem.
To solve this problem, U.S. Pat. No. 5,847,877 and a report written by Steven M. Ebstein (the Sep. 15, 1996 issue of Optical Society of America) each propose a diffractive optical element of the type that has a relief pattern constituting a diffraction grating formed at the cementing interface between two different optical materials. Here, by exploiting the fact that the difference in refractive index between two optical materials depends on the wavelength, the wavelength-dependent variation in phase difference is successfully prevented, and thereby higher diffraction efficiency is achieved over a wide wavelength range.
However, even though this diffractive optical element exhibits satisfactory diffraction efficiency for white light incident thereon, it does not automatically follow that it is fit for use in an optical system designed for white light. For such application, the diffractive optical element needs to exhibit satisfactory diffraction efficiency not only over a wide wavelength range extending from the g line to the C line, but also over its entire diameter, i.e. for axial as well as off-axial rays.
SUMMARY OF THE INVENTION
A first object of the present invention is to provide a diffractive optical element that exhibits satisfactory diffraction efficiency for white light.
A second object of the present invention is to provide an optical system that includes a diffractive optical element and that offers satisfactory diffraction efficiency not only over a wide wavelength range but also for axial as well as off-axial rays.
To achieve the above objects, according to one aspect of the present invention, a diffractive optical element for white light is composed of a plurality of layers of optical materials and has a relief pattern constituting a diffraction grating formed at least at one cementing interface between two different optical materials. The grating height (i.e. the trough-to-ridge height) of the relief pattern constituting the diffraction grating is defined by the following formula:
h=&lgr;/|n−n′|
where
h represents the grating height of the relief pattern constituting a diffraction grating;
&lgr; represents the wavelength (here it is assumed that &lgr;≦450 (nm));
n represents the refractive index of the optical material abutting the interface from the object side for light of the wavelength &lgr;; and
n′ represents the refractive index of the optical material abutting the interface from the image side for light of the wavelength &lgr;.
According to another aspect of the present invention, an optical system is provided with a diffractive optical element composed of glass and resin cemented together and has a relief pattern constituting a diffraction grating formed at the cementing interface therebetween. The diffractive optical element is so designed that the relief pattern is placed on the object side of the pupil or the aperture stop of the optical system. If the diffraction grating has a positive optical power, the diffractive optical element is composed of, from the object side, the resin, the relief pattern, and the glass. If the diffraction grating has a negative optical power, the diffractive optical element is composed of, from the object side, the glass, the relief pattern, and the resin.
According to another aspect of the present invention, an optical system is provided with a diffractive optical element that is composed of glass and resin cemented together and has a relief pattern constituting a diffraction grating formed at the cementing interface therebetween. The diffractive optical element is so designed that the relief pattern is placed on the image side of the pupil or the aperture stop of the optical system. If the diffraction grating has a positive optical power, the diffractive optical element is composed of, from the object side, the glass, the relief pattern, and the resin. If the diffraction grating has a negative optical power, the diffractive optical element is composed of, from the object side, the resin, the relief pattern, and the glass.
According to another aspect of the present invention, a method for manufacturing an optical system comprises: a step of disposing a diffractive optical element that is composed of glass and resin cemented together and has a relief pattern constituting a diffraction grating formed at the cementing interface therebetween with the relief pattern placed on the object side of the pupil or the aperture stop of the optical system; and a step of, if the diffraction grating has a positive optical power, composing the diffractive optical element of, from the object side, the resin, the relief pattern, and the glass, and, if the diffraction grating has a negative optical power, composing the diffractive optical element of, from the object side, the glass, the relief pattern, and the resin.
According to another aspect of the present invention, a method for manufacturing an optical system comprises: a step of disposing a diffractive optical element that is composed of glass and resin cemented together and has a relief pattern constituting a diffraction grating formed at the cementing interface therebetween with the relief pattern placed on the image side of the pupil or the aperture stop of the optical system; and a step of, if the diffraction grating has a positive optical power, composing the diffractive optical element of, from the object side, the glass, the relief pattern, and the resin, and, if the diffraction grating has a negative optical power, composing the diffractive optical element of, from the object side, the resin, the relief pattern, and the glass.
According to another aspect of the present invention, a diffractive optical element is composed of a plurality of layers of optical materials and has a relief pattern constituting a diffraction grating formed at least at one cementing interface between two different optical materials. The optical material abutting the relief pattern from one side is glass that fulfills Conditions (A1), (B1), and (C1) below. The optical material abutting the relief pattern from the other side is resin that fulfills Conditions (A2) and (B2) below:
nd≦1.62 (A1)
nd≦1.6 (A2)
55≦&ngr;d (B1)
30≦&ngr;d≦60 (B2)
Tg<600 (C1)
where
&ngr;d represents the Abbe number (which equals (nd−1)/(nF−nC));
nd represents the refractive index of the optical material, for the d line;
nF represents the refractive index of the optical material, for the F line;
nC represents the refractive in
Chang Audrey
Minolta Co. , Ltd.
Sidley Austin Brown & Wood LLP
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