Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2000-06-14
2004-04-27
Chang, Audrey (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S569000, C359S571000, C359S572000, C359S576000, C385S037000
Reexamination Certificate
active
06728034
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a diffractive optical element that can be used in a variety of applications, such as in optical information processing equipment and optical communication equipment. In particular, the invention relates to a diffractive optical element that polarizes light.
(2) Related Art
FIG. 1
is a cross-sectional drawing showing the construction of a conventional diffractive optical element
200
.
The arrows in
FIG. 1
show the courses taken by light rays. This is also the case in the following drawings.
As shown in
FIG. 1
, the diffractive optical element
200
is composed of a substrate
201
that is made of transparent plate glass and has a diffractive optical element pattern
203
formed on a main surface
202
.
The incident light L
0
is monochromatic and has the wavelength &lgr;. When this light strikes the diffractive optical element pattern
203
on the main surface
202
at an angle of 90°, the light will be diffracted as it passes the diffractive optical element pattern
203
, producing zero-order diffracted light L
1
, and positive first-order diffracted light L
2
and a negative first-order diffracted light L
3
that each form the diffraction angle &thgr;
1
with the light L
1
.
When it is assumed that the refractive index of the substrate
201
is n (where n>1) and the pattern pitch of the diffractive optical element pattern
203
is &Lgr;, the value of the diffraction angle &thgr;
1
can be found easily according to Equation 1 below.
&thgr;
1
=Sin
−1
{(&lgr;/
n
)/&Lgr;} Equation 1
The diffraction angle &thgr;
1
found in this way will usually be below the critical angle for total internal reflection by a surface (to be precise, a boundary face of a main surface) of the substrate
201
. As a result, the zero-order light L
1
, the positive first-order light L
2
, and negative first-order light L
3
all pass through the substrate
201
and exit a main surface
204
on an opposite side to the main surface
202
.
Polarization is known to occur when the pattern pitch &Lgr; is reduced for the diffractive optical element pattern
203
of this kind of diffractive optical element
200
. There are high hopes that this property will enable new kinds of optical elements to be realized.
As Equation 1 clearly shows, reducing the pattern pitch &Lgr; of the diffractive optical element pattern
203
to a value that is equal to or smaller than the wavelength &lgr; of the incident light will result in an increase in the diffraction angle. This gives rise to the problem of the diffracted light being trapped within the substrate
201
.
FIG. 2
shows what happens in this case. A diffractive optical element pattern
303
having a pitch that is no greater than the wavelength of the incident light L
0
is formed on a main surface
302
of a diffractive optical element
300
. Diffraction caused by the diffractive optical element pattern
303
produces positive first-order diffracted light L
2
and negative first-order diffracted light L
3
that each form a larger diffraction angle &thgr;
2
than the diffraction angle &thgr;
1
in the case shown in FIG.
1
. This diffraction angle &thgr;
2
satisfies the condition for total internal reflection by main surface
304
, so that the diffracted beams are completely reflected back into substrate
301
.
Total internal reflection occurs whenever these reflected beams reach a main surface of the substrate
301
, so that the diffracted light ends up being trapped within the substrate
301
.
The following describes the case shown in
FIG. 3
where a diffractive optical element pattern
213
is formed on a main surface
214
on an opposite side of a substrate
210
to a main surface
212
that is incident to the incident light L
0
. In
FIG. 3
, the pattern pitch &Lgr; of the diffractive optical element pattern
213
is greater than the wavelength &lgr; of the light L
0
. In this case, diffraction by the diffractive optical element pattern
213
produces zero-order diffracted light L
1
, as well positive first-order diffracted light L
2
and negative first-order diffracted light L
3
that each form a diffraction angle &thgr;3 with the light L
1
. This diffraction angle &thgr;
3
can be found using Equation 2 below.
&thgr;
3
=Sin
−1
{(&lgr;/&Lgr;)} Equation 2
As shown in
FIG. 3
, the zero-order diffracted light L
1
, the positive first-order diffracted light L
2
and the negative first-order diffracted light L
3
each pass through the main surface
214
and out of the substrate
211
.
However, when a diffractive optical element pattern is formed in this way, there is still the problem of the diffracted light being trapped in the substrate when the pattern pitch &Lgr; of the diffractive optical element pattern is smaller than the wavelength &lgr; of the incident light.
One example of this case is a diffractive optical element
310
shown in FIG.
4
. Diffraction occurs for the incident light L
0
that strikes diffractive optical element pattern
313
formed on a main surface
314
of a substrate
311
to produce the zero-order diffracted light L
1
, the positive first-order diffracted light L
2
and the negative first-order diffracted light L
3
. While the zero-order diffracted light L
1
exits the main surface
314
, the positive first-order diffracted light L
2
and the negative first-order diffracted light L
3
do not satisfy the condition for transmittive diffraction, and so are reflected back at a diffraction angle &thgr;
4
that is found by Equation 3 below.
&thgr;
4
=Sin
−1
{(&lgr;/
n
)/&Lgr;} Equation 3
These diffracted beams are hereafter subjected to total internal reflection by the main surfaces
312
and
314
and so end up being trapped within the substrate
310
.
The above problem means that even if a diffractive optical element pattern is capable of polarizing light, the diffracted beams produced by the diffractive optical element pattern will not exit the substrate. This greatly limits the potential of such substrates as optical elements.
An optical pickup provided in a magneto-optical (MO) disk device reads the information stored on an MO disk by shining a laser beam at an information recording surface of the disk and splitting the light reflected off this surface using a polarizing beam splitter (hereinafter, “PBS”) in the form of a prism. While doing so, the pickup also obtains servo signals, such as the focus error signal and tracking error signal. A prism-shaped PBS is a relatively large component, and so makes miniaturization of the optical pickup difficult.
SUMMARY OF THE INVENTION
The present invention was conceived in view of the stated problems and has a first object of providing a diffractive optical element where light that has been diffracted by a diffractive optical element pattern exits the diffractive optical element even when a pattern pitch of the diffractive optical element pattern is equal to or smaller than the wavelength of the incident light.
The second object of the present invention is to provide a miniaturized optical pickup that uses a diffractive optical element as a polarizing beam splitter.
The first object of the present invention can be achieved by a diffractive optical element that diffracts incident light, including: a substrate with a first main surface and a second main surface, a refractive index of the substrate being equal to n where n is a value greater than one; a first diffractive optical element pattern that is formed on part of the first main surface with a pattern pitch &Lgr; such that &lgr;
<&Lgr;≦&lgr;, where &lgr; is a wavelength of the incident light; and a second diffractive optical element pattern that is formed on one of the first main surface and the second main surface at a predetermined position on an optical path that diffracted light produced by the first diffractive optical element pattern takes within the substrate.
With the above construction, light is incident on the first diffractive optica
Ijima Shin'ichi
Nakanishi Hideyuki
Shiono Teruhiro
Takasuka Shoichi
Yoshikawa Akio
Chang Audrey
Curtis Craig
Matsushita Electric - Industrial Co., Ltd.
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