Optical: systems and elements – Diffraction – From zone plate
Reexamination Certificate
2000-01-27
2002-10-15
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Diffraction
From zone plate
C359S569000, C359S575000, C359S900000
Reexamination Certificate
active
06466371
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to lenses. In particular, this invention relates to diffractive lenses. This invention also relates to lenses used in conjunction with magnetic, magneto-optic or phase change optical recording media. This invention also relates to methods for making lenses.
FIG. 1
illustrates a portion of a magneto-optic disk drive
2
comprising a magneto-optic recording disk
4
and a laser source
6
(typically a diode laser). During use, laser source
6
provides a laser beam
8
that pass through a beam splitter
9
. Laser beam
8
then passes through and is focused by a diffractive lens
10
into a spot
11
on disk
4
. Beam
8
is then reflected back through lens
10
to beam splitter
9
. Beam
8
is then reflected by beam splitter
9
through a lens
12
which focuses beam
8
onto a sensor
14
.
Unfortunately, during reading operations, a portion of laser beam
8
can reflect off of lens
10
. This portion of laser beam
8
is then reflected by beam splitter
9
through lens
12
to sensor
14
, where it introduces noise into the signal detected by sensor
14
. We have experimented with tilting lens
10
as shown in
FIG. 2
, so that light reflecting off lens
10
does not interfere with the operation of sensor
12
. However, we have found that if diffractive lens
10
is tilted, it is highly desirable to modify lens
10
so that it continues to form an aberration-free or substantially aberration-free focused spot on disk
4
.
FIG. 3
schematically illustrates lens
10
in plan view. Lens
10
is arbitrarily drawn as a circle. Also shown in
FIG. 3
are X and Y axes to facilitate a discussion of lens
10
. Lenses such as lens
10
are commonly described using an equation as set forth below:
P
(
x
,
y
)
=
∑
i
=
1
n
α
i
(
x
2
+
y
2
)
i
(
1
)
where P is the phase profile (in radians) of the lens at a particular point x, y of the lens surface. In other words, light striking a point x, y of lens
10
has its phase modified by a number of radians equal to mod[2&pgr;]P(x,y) where mod[2&pgr;] is the modulus operator. In equation 1, the variable &agr;
i
is called the aspheric coefficient.
If lens
10
were a refractive lens, function P(x, y) would be proportional to the height profile of the lens. In other words,
H(x,y)=P(x,y)/&lgr; (2)
where H(x,y) is the height of the lens (in the z direction) at point x,y and &lgr; is the wavelength of light within the lens material. (See
FIG. 3A
, which illustrates a plano-convex refractive lens in cross section along lines A—A.)
As mentioned above, lens
10
is a diffractive lens. One type of diffractive lens is a blazed zone plate lens.
FIG. 4
illustrates in cross section a blazed zone plate lens
10
′. Blazed zone plate lenses can be used to focus light at a focal point, but as can be seen, they have a cross section that is somewhat different from a piano-convex refractive lens. In particular, lens
10
′ comprises a set of concentric ridges and valleys. Blazed zone plate lenses are discussed in “Microoptics”, by Sinzinger et al., published by Wiley-VCH Verlag GmbH in 1999, incorporated herein by reference.
Referring back to
FIG. 1
, for the case in which lens
10
is not tilted, lens
10
is designed such that &agr;
i
is not a function of x or y alone. Rather, (X is a function only x
2
+y
2
(i.e. a function of the radial distance to the center of the lens). However, as mentioned above, if lens
10
is tilted about the x axis to prevent the above-mentioned reflection problem, lens
10
must be modified or it will no longer be able to narrowly focus laser light at spot
11
. It would be desirable to modify lens
10
so that it can be tilted and still be able to narrowly focus light on spot
11
within the data recording disk.
SUMMARY
A diffractive lens constructed in accordance with the invention comprises diffraction gratings. The spacing of the gratings varies over the lens surface so that the lens can focus light onto a small spot on a data recording medium even though the lens is tilted with respect to the laser beam. In one embodiments the lens is a blazed zone plate lens. In another embodiment, the lens is a phase zone plate lens. In another embodiment, the lens comprises alternating opaque and transparent regions. Because the spacing of the gratings is varied, any aberration in the focused spot of light provided by the lens reduced compared to what would be produced without varying the grating spacing. In one embodiment, the amount of aberration is substantially eliminated or completely eliminated.
In one embodiment, rather than having circular diffraction gratings, the gratings are somewhat oval shaped in order to reduce aberrations. For example, the gratings can have the shape of the intersection of a hollow cone and a slanted plane passing through the cone. However, as explained below, the gratings can have other shapes as well.
REFERENCES:
patent: 3586412 (1971-06-01), Leith
patent: 4218111 (1980-08-01), Withrington et al.
patent: 5122903 (1992-06-01), Aoyama et al.
patent: 5138495 (1992-08-01), Shiono et al.
patent: 5257132 (1993-10-01), Ceglio et al.
patent: 5315095 (1994-05-01), Marom et al.
patent: 5446565 (1995-08-01), Komma et al.
patent: 5561558 (1996-10-01), Shiono et al.
patent: 5966244 (1999-10-01), Mukai et al.
patent: 5986779 (1999-11-01), Tanaka et al.
patent: 59-105605 (1984-06-01), None
Sinzinger et al., “Microoptics”, published by Wiley-VCH, 1999, pp. 167-173.
Daschner Walter
Kress Bernard
Aerial Imaging Corporation
Jr. John Juba
Spyrou Cassandra
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