Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
2000-06-16
2004-05-04
Robinson, Mark A. (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S015000
Reexamination Certificate
active
06731432
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to diffractive optical structures and, more particularly, to diffractive optical structures which shape or split a beam of light and in which a uniform output is required. Diffractive optical structures producing off-axis beams according to the invention may be advantageously used for beam shaping and splitting.
DESCRIPTION OF THE RELATED ART
As used herein, a “beam shaper” is an optical element used to alter the shape or energy distribution within a beam of light. Thus, a beam shaper may alter magnification of a light beam, the footprint of the beam when projected on a surface, the energy distribution within a beam, or some combination thereof. An example of altering the energy distribution of a beam is transforming a Gaussian light distribution to a uniform light distribution. Beam shapers may be alternately and interchangeably referred to as “beam transformers.” Also as used herein, “beam splitter” refers to an optical element which divides a beam of light into two or more separate beams having similar characteristics.
FIG. 1
shows a conventional on-axis beam shaping assembly. An input beam
10
, which has a Gaussian energy distribution, is transmitted by the diffractive beam shaper
11
. The resultant shaped beam
12
, which has a uniform energy distribution, strikes beam corrector
13
located a distance d from the beam shaper
11
along the optical axis of the input beam
10
. The diffractive beam corrector
13
corrects a phase shift in the shaped beam
12
caused by the beam shaper
11
. The beam shaping assembly shown is termed “on-axis,” because the output beam
14
is located on the axis of the input beam
10
. If the phase correction function was not desired, the assembly shown could consist of the beam shaper
11
alone.
Manufacturing tolerances can affect the output quality of beam shapers, such as that shown in
FIG. 1
, to a great degree. For example, for diffractive optics which are formed by dry etching, the etching processes are not exact, and the final optical shape may deviate slightly from the “desired” or “perfect” shape designed by an optical designer and sought to be etched. Such manufacturing errors or tolerances also occur with other methods of forming diffractive optics.
FIG. 2
shows the simulated output of an on-axis beam shaper, which is designed to produce a uniform beam, with various amounts of etch depth error. For the ‘perfect’ optic case
20
(i.e., where the designed shape is simulated with no fabrication or etch error), the peak to valley non-uniformity in the intensity of output beam is 2%. For the 0.5% etch depth error case
21
, the peak to valley non-uniformity in the intensity of output beam increases to 10%. For the 3.0% etch depth error case
22
, the peak to valley non-uniformity in the intensity of output beam increases to 46%. A typical etch depth tolerance to achieve a high yield in a conventional dry etching process is ±3.0%, which produces the 46% non-uniformity shown in plot
22
. For many applications of beam transformers, such as lithography or holography, the desired uniformity of the beam is ±3.0%, which corresponds to a lower etch error than 3%, and hence cannot be attained with such a conventional high yield process.
It should be noted that the magnitude of the beam non-uniformity is a function of the magnification of the beam shaper. If the beam shaper produces a uniform beam that is much smaller than the input beam, for example one eighth, the additional non-uniformity caused by a 3.0% etch error can be as small as 2.0%. However, the effects of the 3% etching error quickly increase to 19% for a beam reduced to only one fourth size.
The non-uniformity observed in the output beams
21
and
22
in
FIG. 2
is the result of the undesired orders produced by the diffractive interfering with the desired order of the output beam. Even though the energy in these orders may only be a few percent of the total input energy, they can have a profound affect on the uniformity of the beam, as illustrated in FIG.
2
. The underlying problem is that all of the orders of an on-axis diffractive system are co-located symmetrically about the optical axis. Since a beam which is transformed in this manner is coherent, these co-located multiple order beams interfere and cause the non- uniformity shown in FIG.
2
.
On-axis diffractive beam splitters, such as that shown in
FIGS. 3
a
and
3
b
, suffer from similar problems of interference by undesired diffractive orders. Such an on-axis diffractive beam splitter may have an extremely tight tolerance for the etch depth of the diffractive, hence reducing the yield and making the cost of such a device impractical.
FIGS. 3
a
and
3
b
show perspective and side views, respectively, of an on-axis diffractive beam splitter that creates five beams. An input light beam
30
strikes a diffractive beam splitter
31
, which is designed to split the input beam
30
into a 0th order beam
32
and four diffracted-order beams
33
. The diffractive beam splitter shown is termed “on-axis,” because the output beams
32
and
33
are located along a line which intersects the axis of the input beam
30
.
FIG. 3
c
shows the five beams in their one-dimensional, on-axis arrangement.
For the “perfect” optic case (not plotted), the peak to valley non-uniformity in the intensity of output beams
32
and
33
is 6% and the efficiency of the beam splitter is 92%. For the 3.0% etch depth error case, the peak-to-valley non-uniformity in the intensity of output beams is 26% and the efficiency is 91%. A typical etch depth tolerance to achieve a high yield in a conventional dry etching process is ±3.0%, which produces the 26% non-uniformity. This non-uniformity among split beams is caused by the co-location of the diffracted beams and the 0th order beam along a line. For many applications of beam splitters, such as communications and hole drilling or marking, the desired non-uniformity among the beams is less than ±5.0%, which corresponds to a lower etch error than 3%, and hence cannot be attained with such a conventional high yield process.
It is accordingly apparent that conventional on-axis diffractive beam shapers and splitters have extremely tight tolerances for the etch depth of the diffractive. Such tolerances lower the manufacturing yield, and thus make the cost of such devices impractical. Further, diffractive optics are wavelength sensitive, and the conventional on-axis configurations can only be used at the wavelength for which they are designed .
SUMMARY OF THE INVENTION
An object of the invention is to provide a diffractive optical element which substantially obviates one or more problems or limitations of conventional on-axis diffractive optical elements.
Another object of the invention is to design a diffractive beam splitter and/or diffractive beam shaper which is less sensitive to manufacturing errors and wavelength than conventional elements.
By designing a beam shaper or beam splitter that is off-axis by a defined minimum amount to separate the desired order(s) of the diffractive from the order(s) sensitive to manufacturing tolerances, the manufacturing difficulty of achieving the otherwise necessary tight tolerance in the etch depth needed for a very uniform beam may be eliminated. This off-axis configuration also allows a diffractive beam shaper or beam splitter to work over a large wave band.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided an off-axis beam shaper for producing an output beam of a desired order with a desired energy distribution, including an opti
Amari Alessandro
Keady, Ciccozzi & Olds, PLLC
Mem Optical, Inc.
Robinson Mark A.
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