U.S. patent application number 09/841772 was filed with the patent office on 2001-11-08 for infrared chopper using binary diffractive optics.
This patent application is currently assigned to Raytheon Company. Invention is credited to Baber, S. Charles, Bell, Michael C., Chang, Richard R., Gibbons, Robert C., McKenney, Samuel R..
Application Number | 20010038974 09/841772 |
Document ID | / |
Family ID | 22574484 |
Filed Date | 2001-11-08 |
United States Patent
Application |
20010038974 |
Kind Code |
A1 |
Gibbons, Robert C. ; et
al. |
November 8, 2001 |
Infrared chopper using binary diffractive optics
Abstract
A chopper and method of making same, the chopper being
fabricated by initially generating a photomask in conjunction with
software. The software provides the lens design to be finally
stamped onto the chopper element. A silicon wafer is then etched by
reactive ion etching using the photomask to provide the pattern and
resulting in a silicon wafer master of the chopper pattern with
regions in the shape of lenslets to be formed of desired dimension.
The chopper pattern on the silicon wafer is then replicated with a
hard material which can be easily stripped from the silicon wafer
without damaging either the wafer or the hard material, preferably
deposited nickel. The separated nickel replication is then used in
conjunction with a heavy press to stamp out sheets of an infrared
transmissive flexible film, preferably polyethylene, with the lens
pattern in the replication. The film with the lens pattern thereon
is the chopper element. The system is designed to operate in the 8
to 13.5 micron range. While the software is designed for an
individual lens, each lens is preferably in the shape of a hexagon
with a plurality of such hexagons positioned on the film in a
predetermined pattern, preferably that of an involute or spiral.
The chopper is designed for rotation about its central axis.
Inventors: |
Gibbons, Robert C.;
(Richardson, TX) ; McKenney, Samuel R.; (Dallas,
TX) ; Baber, S. Charles; (Richardson, TX) ;
Chang, Richard R.; (McKinney, TX) ; Bell, Michael
C.; (Garland, TX) |
Correspondence
Address: |
Jerry W. Mills, Esq.
Baker Botts L.L.P.
Suite 600
2001 Ross Avenue
Dallas
TX
75201-2980
US
|
Assignee: |
Raytheon Company
|
Family ID: |
22574484 |
Appl. No.: |
09/841772 |
Filed: |
April 24, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09841772 |
Apr 24, 2001 |
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08159879 |
Nov 30, 1993 |
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6232044 |
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Current U.S.
Class: |
430/321 ;
250/347; 264/1.32; 264/2.5; 359/210.1; 359/569; 359/599 |
Current CPC
Class: |
G02B 26/04 20130101 |
Class at
Publication: |
430/321 ;
264/2.5; 264/1.32; 250/347; 359/210; 359/569; 359/599 |
International
Class: |
G02B 003/00; G02B
005/18; G02B 026/08; G03F 007/26; B29D 011/00; G03C 005/00 |
Claims
1. A method of making a chopper comprising the steps of: (a)
providing an etchable base member; (b) forming a mask on said base
member for providing a predetermined design on said base member;
(c) etching said base member through said mask to form a lens
pattern on said base member; (d) replicating said lens pattern onto
a rigid material depositable on said base member and removable from
said base member with substantially no damage to said pattern
replicated on said rigid material or to said base member; and (e)
stamping the pattern replicated on said rigid material onto an
infrared transmissive deformable sheet capable of retaining the
pattern pressed thereinto.
2. The method of claim 1 wherein said infrared transmissive sheet
is polyethylene.
3. The method of claim 1 wherein said base member is silicon.
4. The method of claim 2 wherein said base member is silicon.
5. The method of claim 1 wherein said rigid material is nickel.
6. The method of claim 2 wherein said rigid material is nickel.
7. The method of claim 3 wherein said rigid material is nickel.
8. The method of claim 4 wherein said rigid material is nickel.
9. A chopper which comprises: (a) an infrared transmissive
deformable film capable of retaining a pattern pressed thereinto;
and (b) a plurality of lenses disposed in said film in a
predetermined pattern, all of said lenses disposed within a
predetermined geometrical shape.
10. The chopper of claim 9 wherein said film is a polymeric
material.
11. The chopper of claim 9 wherein said film is polyethylene.
12. The chopper of claim 9 wherein said lenses are polygonal.
13. The chopper of claim 10 wherein said lenses are polygonal.
14. The chopper of claim 11 wherein said lenses are polygonal.
15. The chopper of claim 9 wherein said geometrical shape is an
involute.
16. The chopper of claim 10 wherein said geometrical shape is an
involute.
17. The chopper of claim 11 wherein said geometrical shape is an
involute.
18. The chopper of claim 12 wherein said geometrical shape is an
involute.
19. The chopper of claim 13 wherein said geometrical shape is an
involute.
20. The chopper of claim 14 wherein said geometrical shape is an
involute.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a chopper formed in a plastic
sheet for use primarily in forward looking infrared (FLIR) systems
and principally, but not limited to such systems, of the type
utilizing uncooled ferroelectric infrared pyroelectric
detectors.
[0003] 2. Brief Description of the Prior Art
[0004] Forward looking infrared (FLIR) systems generally utilize a
detector and a chopper system in conjunction with the detector for
calibration of the detector. Such calibration is generally
performed on-line and between detector scanning operations. Prior
art infrared detectors have generally been of the cooled variety,
operating at temperatures in the vicinity of liquid nitrogen, about
77.degree. K. More recently, FLIR systems have been developed which
use uncooled detectors, such systems being preferred when
sufficient sensitivity can be obtained therefrom. An uncooled
detector system utilizing a ferro-electric detector is
intrinsically a differencing detector whose signal is the
difference between that of the viewed scene and that of a reference
source. In order to minimize dynamic range problems in the
detectors, it is desirable to match the reference flux as closely
as possible to the average scene flux. This is typically
accomplished with the chopper which alternately permits the
detector to view the scene and then view a reference source
representing the average scene flux.
[0005] For purposes of minimizing the scene flux/reference flux
delta, some FLIR systems have used as a reference source an image
of the system exit pupil or an approximation of the system exit
pupil. The most simple technique to approximate the exit pupil is
to defocus the optical system. In the present day systems, this is
accomplished in one of two ways, these being either (1) with a
thick flat plate which is cut out in appropriate areas to pass the
scene radiation, whereby, in solid areas, an optical defocus
occurs, resulting in a pupil approximation or (2) with a solid flat
plate which is covered with small ground lenslets in a pattern
matching the solid area of a scanner, these lenslets accomplishing
the defocus.
[0006] A problem with prior art choppers of the second type
described above has been cost. In order to provide a chopper of the
above described second type having a plurality of lenslets, it has
been necessary to grind the lenslets individually, generally in
germanium, to provide a predetermined pattern. Such prior art
choppers have also been fabricated using binary diffractive optic
pattern generated photomasks in conjunction with a high precision
laser writer followed by etching of the desired lens patterns into
the germanium wafer. Such processes have been costly. It is
therefore desired to provide choppers at greatly reduced cost,
preferably at a small fraction of the present cost.
SUMMARY OF THE INVENTION
[0007] In accordance with the present invention, there is provided
a chopper and a method of fabrication thereof which meets the above
noted economic goals. The chopper is designed for rotation about
its central axis.
[0008] Briefly, the chopper in accordance with the present
invention is fabricated by initially generating a photomask. The
photomask is generated in conjunction with software, the preferred
software being set forth herein in the APPENDIX. The software,
which uses the language AutoLISP by AUTOCAD, consists of four macro
routines which generate an exact scale graphical pattern of the
lens array. Several lens design and chopper system variables are
input and the software generates a two level data file of the
graphical pattern.
[0009] Upon initiation of the first macro, the operator is queued
for the design variable and then performs the calculations
generating the spiral shaped boundary and fills the space within
the boundary with an array of Fresnel phase plate lens structures.
Several operator design inputs are required during the construction
of the file. Once the first macro is loaded, all remaining macros
self-load and pass calculated data to the next macro.
[0010] Each macro routine is summarized as follows:
[0011] SPIRAL.LSP generates the spiral shaped boundary which is
sized to modulate the detector for a specific period of the
detector sampling time. User inputs are several detector and
chopper wheel assembly shape parameters. The spiral is generated
using an Archimedes spiral math function.
[0012] BDO.LSP generates a single unit cell lens containing all of
the lens structure. User inputs are the index of refraction of the
substrate, lens diameter, spherical radius and design wavelength.
The unit cell lens structure is generated using equations that
model wavefront diffraction theory.
[0013] PGON.LSP takes the single circular lens and trims it to a
hexagon shaped pattern for perfect nesting of the lenses without
overlap or gaps. There are several operator steps to identify which
segments of the lens to eliminate.
[0014] BDOMATRIX.LSP generates a honeycomb array of hexagons and
blocks the hexagon shaped unit cell into each hexagon of the
array.
[0015] The software accordingly generates a mask having the lens
design to be finally stamped onto the chopper element as explained
herein below.
[0016] A silicon wafer is then etched by reactive ion etching,
using the photomask to provide the pattern, resulting in a silicon
wafer master of the chopper pattern with regions in the shape of
lenslets to be formed of desired dimension. The chopper pattern on
the silicon wafer is then replicated with a hard material which can
be easily stripped from the silicon wafer without damaging either
the wafer or the hard material, such material preferably being film
(index of refraction=1.52) and a wavelength of 10 .mu.meters, the
lens structure depth is approximately 9 .mu.meters (0.00035 inch).
Success has been achieved at 0.003 inch film thickness over the
environmental temperature spectrum of the sensor without distortion
of the small lens structure shapes.
[0017] The system is designed to operate in the 8 to 13.5 micron
range and the APPENDIX is designed for operation in this range.
Accordingly, the individual lenses are fabricated for operation in
this range by the software. The software is designed for an
individual lens, each lens having a perimeter preferably in the
shape of a hexagon. A plurality of equally sized such hexagons are
positioned on the film within an envelope in the shape of a spiral
with the radius increasing proportional to the angle of rotation.
An involute spiral and Archimedes spiral are the preferred envelope
shapes. The hexagonal shape is preferred because hexagons can be
fitted together such that they cover all of the area within the
involute or spiral with no spaces between lenses.
[0018] The software generates two file layers. The first file layer
is the spiral shaped pattern and the second layer is the array of
hexagons. The hexagon file layer contains a hidden layer which
compresses the file to a manageable size. These two files are
downloaded to a database pattern compiler which translates the data
into a format that is readable by the laser patterning system which
then fabricates the photomask by writing the pattern onto the
surface of an emulsion covered glass slide, the emulsion being, for
example, AGFA photomask plates, Part No. PF-HD. There are many
different types of emulsions that can be used and these would be
apparent to one skilled in the art.
[0019] The hexagon array file layer is generated by the software in
a rectangular window shaped pattern which overlies the spiral file
layer. The hexagons that are completely exterior to the boundary of
the spiral are eliminated from the file by the operator to reduce
the file size. Those lens cell structures that stagger the spiral
boundary and all those within the boundary are printed by the
photomask writing equipment. The portion of the lens structure that
falls external to the spiral is hidden by the boundary of the
spiral file when the photomask is fabricated and thus it is not
necessary to trim this lens structure away.
[0020] Each individual lens is a diffractive structure designed to
defocus incident energy by a predetermined amount. The purpose is
to achieve a defocused image of the exterior scene for use as a
reference source for image differencing.
[0021] The exact shape of each lens is determined by the modulo
.pi. behavior of the desired wavefront deformation. FIG. 3 shows
this modulo .pi. behavior wherein the resultant shape for the
individual lens is a concentric grating with grating depth
determined from 1 T ( r ) = T opt [ F ( r ) 2 + 1 ]
[0022] where T.sub.opt is the optimum thickness for a 2.pi. phase
shift, and is given by T.sub.opt =.lambda./.DELTA.n, and
.PSI..sub.F(r) is the desired radial phase shift function.
Normally, .DELTA.n is the deviation of the refractive index of the
zone material from that of the surrounding medium (air) and
.lambda. is the design wavelength.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 shows a chopper in accordance with the present
invention;
[0024] FIG. 2 is an enlarged view of one of the hexagonally shaped
lenslets of FIG. 1; and
[0025] FIG. 3 is a graph of the phase shift function for a Fresnel
phase plate.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0026] The present invention is similar to the second technique
discussed above, however the ground lenslets on germanium in the
shape of an involute are replaced with an array of lenslets formed
from binary diffractive elements on a film. This array performs the
same function as the ground lenslets, but is much easier and
economic to fabricate. The chopper uses a lenslet array composed of
binary diffractive elements to perform the uniform defocusing
required for calibration. The purpose of the lenslets is to defocus
the radiation falling on the detector during the scan dead period
of the imaging system. This defocused radiation is an approximation
of the pupil irradiance and is used for differencing.
[0027] The chopper is fabricated by generating a photomask. The
photomask is generated in conjunction with the software set forth
herein as APPENDIX The software controls a laser which etches out
the programmed pattern on a standard mask material, preferably AGFA
Photomask Plates Part No. PF-HD. The mask now contains the mask
pattern and, accordingly, the lens design to be ultimately stamped
onto the chopper element. A silicon wafer is then etched through
the mask by reactive ion etching in standard manner to provide a
master of the chopper lens pattern on the silicon wafer. The
chopper pattern on the silicon wafer is then replicated with a hard
material which can be easily stripped from the silicon wafer
without damaging either the wafer of the hard material, preferably
a thin layer of silver of about 500.ANG. over which is
electro-plated the nickel to a depth of at least 0.10 inch. The
nickel replication of the pattern on the silicon wafer is then
separated from the silicon wafer by placing the silicon pattern
with silver and nickel layers thereon in a bath which removes the
silver selectively to the nickel, preferably photoresist, although
gold film provides a superior mask at added expense, the separated
replication then being used in conjunc-tion with a heavy press as a
stamp to stamp the lens pattern into individual sheets of an
infrared transmissive material, preferably flexible film which is
preferably polyethylene. The infrared transmissive material is
preferably heated prior to stamping so that the pattern on the
stamp is more easily impressed into the film. The film with the
lens pattern thereon is the chopper element. Such chopper elements
can be continually stamped out on individual sheets of the film in
conjunction with the silicon wafer and the heavy press which drives
the silicon wafer into the film, as above described, the silicon
wafer operating as a mold or die.
[0028] The system is designed to operate in the 8 to 13.5 micron
range and the APPENDIX is designed for operation in this range.
Accordingly, the individual lenses are fabricated for operation in
this range by the software. While the software is designed for an
individual lens, a plurality of such lenses are positioned on the
film in a predetermined pattern, preferably within an involute or
spiral.
[0029] FIG. 1 shows a chopper in accordance with the present
invention which has been fabricated in accordance with the present
invention. There is shown a chopper 1 having a plurality of
lenslets 3, each lenslet being in the shape of a hexagon an being
about 0.2 inches across opposing sides. The lenslets 3 are of the
same dimensions and are positioned to be interfitting with no
spaces therebetween. The lenslets 3 are disposed within an involute
5. The lenslets 3 are impressed into a polyethylene film (not
numbered) having a thickness of at least 0.10 inch which surrounds
the involute 5. An aperture 7 is disposed at the center of the
chopper 1 for securing the chopper to a device which will rotate
the chopper in standard manner.
[0030] Referring now to FIG. 2, there is shown an enlarged view on
one of the lenslets 3 of FIG. 1. FIG. 3 shows how a cross section
through the lenslet would appear. The lenslet is essentially a
binary Fresnel zone plate encoding phase information required for
focusing (or defocusing, in this case) the scene flux.
[0031] Though the invention has been described with respect to a
specific preferred embodiment thereof, many variations and
modifications will immediately become apparent to those skilled in
the art. It is therefore the intention that the appended claims be
interpreted as broadly as possible in view of the prior art to
include all such variations and modification.
* * * * *