Diffractive optical element

Details Diffractive optical element Diffractive optical element

2001-11-20

2004-11-23

Assaf, Fayez (Department: 2872)

Optical: systems and elements

Diffraction

From grating

C359S571000, C359S576000

Reexamination Certificate

active

06822796

ABSTRACT:

RELATED APPLICATION
This application is based on application No. 2000-356961 filed in Japan, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a diffractive optical element, and more particularly, to a diffractive optical element in which a dielectric multilayer film is provided on a diffraction grating.
2. Description of the Prior Art
Diffractive optical elements that diffract light are used in various types of apparatus that handle light, such as optical pickups, optical communication devices, laser beam printers, copiers and microscopes. Diffractive optical elements include a transmissive type that transmits light and diffracts the transmitted light, and a reflective type that reflects light and diffracts the reflected light. Both types comprise a substrate on a surface of which a diffraction grating, comprising periodically repeated minute depressions and projections, is formed. In reflective type diffractive optical elements, a reflecting film of a metal such as aluminum is provided on the diffraction grating. In transmissive type diffractive optical elements and reflective type optical elements used with light incident from the reverse side of the substrate, a transparent substrate is used.
Diffraction gratings are broadly divided into a blaze type with an inclined surface and a binary type with a horizontal surface. Blaze type diffraction gratings include one with a unidirectional surface inclination and one with a bidirectional surface inclination. Although both are triangular in cross section, the former has a sawtooth cross section and the latter has a V-shaped cross section. Binary type diffraction gratings include a two-level grating with only the highest level and the lowest level, and a multilevel grating with levels intermediate the highest and the lowest levels. The former has a rectangular cross section, and the latter has a step-shaped cross section comprising a set of rectangles. In blaze type diffraction gratings with a unidirectional surface inclination and two-level binary type diffraction gratings, a level difference perpendicular to the entire surface of the substrate is present on the borders between the depressions and projections. In multilevel binary type diffraction gratings, a perpendicular level difference is also present on the borders between the levels.
In recent years, it has been proposed to provide on the diffraction grating a dielectric multilayer film formed by alternately and periodically laminating dielectric layers with a high reflective index and dielectric layers with a low refractive index to thereby improve the reflectance of the diffractive optical element and improve the wavelength selectivity and the polarization selectivity of reflection. Such a diffractive optical element, which can function as the transmissive type or function as the reflective type, is high in utility.
An example of a diffractive optical element in which a dielectric multilayer film is provided on the diffraction grating is shown in FIG.
4
. This diffractive optical element
5
comprises a substrate
51
and a dielectric multilayer film
53
comprising two kinds of dielectric layers
53
a
and
53
b
that are alternately laminated. A diffraction grating
52
is formed on a surface of the substrate
51
. The diffraction grating
52
is a blaze type with a unidirectional surface inclination.
In a case where reflected light is diffracted, the first-order diffraction efficiency is highest when the optical level difference between the depressions and projections of the diffraction grating is ½ the wavelength of the light. Therefore, in a case where light is made incident from the reverse side of the substrate to obtain first-order reflected light, the physical level difference h
0
between the depressions and projections of the diffraction grating is set to h
0
=&lgr;/2n
0
where the wavelength of the light to be reflected is &lgr; and the refractive index of the substrate is n
0
. In a case where light is made directly incident on the dielectric multilayer film from the obverse side of the substrate, that is, from an air interface, to obtain first-order reflected light, since the refractive index of air is 1, the physical level difference h
0
is set to h
0
=&lgr;/2.
The reflectance of the dielectric multilayer film is highest when the optical thicknesses of the layers are ¼ the wavelength of the light. Therefore, the physical thicknesses h
1
and h
2
of the two kinds of layers of the dielectric multilayer film are set to h
1
=&lgr;/4n
1
and h
2
=&lgr;/4n
2
, where the refractive indices of the layers are n
1
and n
2
.
By setting the diffraction grating and the dielectric multilayer film as described above in consideration of the wavelength of the light to be reflected, the wavelength selectivity is improved, and the desired light can be efficiently extracted as the reflected light and can be prevented from being mixed in transmitted light of a different wavelength. In addition, the polarization selectivity of a dielectric multilayer film that transmits p-polarized light and reflects s-polarized light is improved, so that separation between p-polarized light and s-polarized light is ensured.
However, in the conventional diffractive optical element in which the dielectric multilayer film is provided on the diffraction grating, although the physical level difference h
0
between the depressions and projections of the diffraction grating and the physical thicknesses h
1
and h
2
of the two kinds of layers of the dielectric multilayer film are set as mentioned above, these are individually decided and no consideration is given to the relationship among the level difference h
0
between the depressions and projections of the diffraction grating and the thicknesses h
1
and h
2
of the layers of the dielectric multilayer film. For this reason, in a structure in which the diffraction grating has a level difference, layers of the dielectric multilayer film having different refractive indices are in contact with each other at the level difference of the grating, so that the reflectance decreases.
This problem will be described with the diffractive optical element
5
of
FIG. 4
as an example. Assuming now that the refractive index n
0
of the substrate
51
is 1.5 and the refractive indices n
1
and n
2
of the layers
53
a
and
53
b
of the dielectric multilayer film
53
are 2.5 and 1.875, respectively, a relationship &lgr;/2n
0
=&lgr;/4n
1
+&lgr;/4n
2
+&lgr;/4n
1
exists. That is, h
0
=2h
1
+h
2
, and the physical level difference h
0
between the depressions and projections of the diffraction grating
52
is the sum of twice the physical thickness h
1
of the layers
53
a
of the dielectric multilayer film
53
and the physical thickness h
2
of the layers
53
b
. Therefore, odd-numbered layers
53
a
with a high refractive index and even-numbered layers
53
b
with a low refractive index are in contact with each other in the region of the level difference G of the diffraction grating
52
.
When light is obliquely incident on the diffractive optical element
5
from the reverse side of the substrate
51
(as depicted by the arrow) and reaches the area of the level difference G of the diffraction grating
52
, the effective thicknesses h
1
and h
2
of the layers
53
a
and
53
b
for the light are shifted from &lgr;/4n
1
and &lgr;/4n
2
where the reflectance is highest, and become smaller than &lgr;/4n
1
and &lgr;/4n
2
or become twice &lgr;/4n
1
and &lgr;/4n
2
, for example, &lgr;/2n
1
and &lgr;/2n
2
for the case of the light shown by the arrow. Consequently, the reflectance of the dielectric multilayer film
53
decreases, so that the light passes through the diffractive optical element
5
rather than being reflected.
Since light perpendicularly incident from the reverse side of the substrate
51
does not obliquely traverse the film in the area of the level difference G, it appears that the reflectance of the diele

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