An optical scanning device utilizing a source of optical energy such as laser light backscattered from the earth's atmosphere or transmitted outward as in a lidar, a rotating holographic optical element having an axis of rotation perpendicular to the plane of its substrate, and having a stationary focus which may or may not be located on its axis of rotation, with the holographic optical element diffracting the source of optical energy at an angle to its rotation axis enabling a conical scanning area and a motor for supporting and rotating the rotating holographic optical element, is described

Schwemmer, Geary

English

NASA Technical Reports Server

United States Patent 1191 
I11111 111ll Il11 Il11 Il11 11111 11111 Il1 111ll I1 ll1111111 
US005255065A 
[113 Patent Number: 5,255,065 
Schwemmer 1451 Date of Patent: Oct. 19, 1993 
[54] CONICALLY SCANNED HOLOGRAPHIC 4,925,262 5/1990 Yamagishi et al. ................ 350/3.71 
4,957,336 9/1990 Hasegawa et al. ................. 350/3.71 
5,009,502 4/1991 Shih et al. 356/152 
LIDAR TELESCOPE ........................... 
[75] Inventor: Gary Schwemmer, Hampstead, Md. 5,064,258 11/1991 Inokuchi et al. . 
[73] Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 848,885 
[22] Filed: Mar. 10, 1992 
[51] Int. C l . 5  ........................... GOlC 3/08; G02B 5/32 
[52] U.S. Cl. .......................................... 356/5; 359/17; 
359/18 
[58]  Field of Search ........................ 359/17, 18; 356/5 
1561 References Cited 
U.S. PATENT DOCUMENTS 
4,639,070 VI987 Ikeda et al. . 
4,768,847 9/1988 Iwaoka et al. ..................... 350/3.71 
4,832,426 5/1989 Kaser ................................. 350/3.72 
4,923,262 5/1990 Clay ................................... 350/3.71 
Primary Examiner-Stephen C .  Buczinski 
Attorney, Agent, or Firm-Paul S .  Clohan, Jr.; R. Dennis 
Marchant; Guy M. Miller 
[571 ABSTRACT 
An optical scanning device utilizing a source of optical 
energy such as laser light backscattered from the earth’s 
atmosphere or transmitted outward as in a lidar, a rotat- 
ing holographic optical element having an axis of rota- 
tion perpendicular to the plane of its substrate, and 
having a stationary focus which may or may not be 
located on its axis of rotation, with the holographic 
optical element diffracting the source of optical energy 
at an angle to its rotation axis enabling a conical scan- 
ning area and a motor for supporting and rotating the 
rotating holographic .optical element. 
15 Claims, 5 Drawing Sheets 
8 
9 
9 
6 7 
https://ntrs.nasa.gov/search.jsp?R=19940016118 2020-06-17T23:37:13+00:00Z
U.S. Patent Oct. 19, 1993 Sheet 1 of 5 5,255,065 
FIG. 1 
I 
31 
22 
FIG. 2 
US. Patent Oct. 19, 1993 Sheet 2 of 5 5,255,065 
U.S. Patent 
WAVELENGTH 
WAVELENGTH 
WAVELENGTH 
Oct. 19, 1993 Sheet 3 of 5 5,255,065 
32 
3 x 
WAVELENGTHS 
2 x 
I 
x 
FIG. 4 OPTICAL AXIS AND 
R O T A T I O N  AXIS 
r-- FOCUS FOR 
/ \  TRANSMISSION HOE 
I REFLECTION AXIS  ROTATION A X I S  
U.S. Patent Oct. 19, 1993 Sheet 4 of 5 5,255,065 
FIG.6 
U.S. Patent Oct. 19, 1993 Sheet 5 of 5 5,255,065 
FIG.8 P R O T A T I O N  AND OPTICAL AXIS 
54 
56 
OPlC AXIS (3) 
f1g.9 
5,255,065 
1 2 
which is the optical antenna or collector. Prior art in 
CONICALLY SCANNED HOLOGRAPHIC LIDAR this area has not previously used HOES, but rather 
TELESCOPE conventional reflective or refractive optics, with scan- 
ners that rotate out of the plane of the scanning optic. 
ORIGIN OF THE INVENTION 5 What is unique about this invention is that it provides a 
The invention described herein was made by an em- device which is capable Of collecting light over its en- 
ployee of the United States Government, and may be tire aperture and focusing it to a small Spot which will 
manufactured and used by or  for the Government for not move when the device is rotated. This spot may or 
governmental purposes without the payment of any may not lie on the rotation axis of the HOE, but in the 
royalties thereon or therefor. 10 preferred embodiment it does lie on the rotation axis of 
the HOE because this reduces the dynamic torque and 
any requisite balancing on the system. In addition the TECHNICAL FIELD 
This invention relates to Holographic Optical Ele- ability to focus various wavelengths along the optic axis 
ments (HOE) and more particularly to a scanning tele- to separate detectors is unique. None of the spots will 
scope that employs an HOE spinning in a plane which l5 move when the HOE rotates. This creates a compact 
allows conical scanning of a light source in transmission system for wavelength multiplexing and demultiplex- 
and reception. ing. 
BACKGROUNDART BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is an illustration of the production of a con- 
sents a brand new technique in remote sensing instru- 
mentation, and an innovative application of a technol- FIG. 2 is an illustration of the use of an HOE in a lidar 
ogy that has been developing along for the past twenty 
some years. Recent developments in HOE'S Using VOl- FIG. 3 is an illustration of a Conically Scanned Holo- 
class of light-weight telescopes with specific advantages 
for lidar remote sensing instruments. In particular, a 
conical scanning configuration can be constructed with 
a much smaller rotating mass compared to conventional 30 of a unique focal length. 
designs which require scanning either the entire tele- 
scope assembly including the primary mirror, second- 
ary, baffling, and the detector package or rotating a 
separate flat scanning mirror which is larger in size than 
ing systems utilize a spectrally wavelength band 
The Present h o h w v h i c  scanning telescope repre- 2o 
ventional HOE. 
application, 
ume phase holograms enables the development of a new 25 graphic Lidar Telescope according to the teachings of 
the present invention. 
FIG. is an illustration of a configuration in which 
system with each wavelength diffracted with an HOE 
FIG. is an illustration of an alternate embodiment of 
FIG. 6 is also an illustration of an alternate embodi- 
FIG. 7 is also an illustration of an alternate embodi- 
large aperture, narrow Of view in a multiple wavelengths are employed in a lidar receiver 
the HOE arrangement in the present invention. 
the primary collection optic. Because laser remote sen+ 35 ment Of the 
or bands and require optical blocking and interference ment Of the arrangement in the present invention. 
filters to reject unwanted wavelengths, especially scat- * is a Of an embodi- 
tered solar radiation, the holographic telescope can be ment Of the HOE arrangenlent in the present invention. 
arrangement in the present invention' 
made diffract only a 
bands thus reducing or even eliminating the require- 
ments for optical blocking and interference filters. 
An HOE is essentially a hologram of a lens or curved 
mirror. It is a diffractive optic that has optical power, 
that is, the ability to form images. The hologram itself 45 
wavelength band or 40 FIG. 9 is also an illustration of an alternate embodi- 
ment of the HOE arrangement in the present invention. 
DETAILED DESCRIPTION OF THE 
INVENTION 
Referring now to FIG. 1, an illustration of the pro- 
usually consists of a film emulsion containing a diffrac- duction of a conventional HOE is shown in its simplist 
tion pattern as a surface relief or as index modulation form. As those skilled in the art will recognize, HOE 1 
throughout the thickness of the film as in the volume iS produced in a film emulsion deposited on an optically 
phase hologram. It may be fixed to a planer substrate or transparent substrate, by the interference of two mutu- 
to one that has curvature contributing to the optical 50 ally coherent laser beams 2 and 3- Beam 2, generally 
power. Being a diffractive optic, an HOE has spectral referred to as the object beam, contains spherical wave- 
dispersion which can be utilized to advantage in a lidar fronts emanating from a point source 20. The second is 
system in which only one or more particular laser wave- a collimated beam 3, generally referred to as the refer- 
lengths are being observed. The device described herein ence beam, containing Plane wavefronts After expo- 
may also be applied to advantage in passive instruments 55 sure to beams 2 and 3, the photo-sensitive material, 
requiring spectral filtering or dispersion. typically ammonium dichromate, is washed from the 
emulsion and the film's interference pattern fixed by 
chemical processing commensurate with the type of 
It is therefore an object of the present invention to film emulsion used. For  example, in dichromated gela- 
provide a light weight large aperture (1 meter diameter 60 tin, the photo-exposed regions produce chemical cross- 
or  greater), narrow field of view scanning telescope, links which increase the gelatin hardness and refractive 
particularly useful for lidar applications. index. This aspect of the instant invention follows well 
Another object of this invention is to provide a de- established procedures for producing HOES. 
vice to focus a large collimated beam of light to a sta- Referring now to FIG. 2, an illustration of the use of 
tionary focal point while rotating an HOE to observe 65 HOE 1 in a lidar application is shown. Laser light back- 
light coming from various directions. scattered by the earth's atmosphere acts as a reconstruc- 
These and other objects are achieved by providing an tion beam 4, which is conjugate to the original reference 
HOE imbedded in the receiver of an optical system beam 3. Properly made, HOE 1 will diffract most of the 
STATEMENT OF THE INVENTION 
5,255,065 
3 J 
desired laser radiation into a converging beam 5, which 
is the conjugate of the original object beam 2, and is 
brought to a focus on a detector 31 as shown. Back- 
ground light at wavelengths outside the diffraction 
bandpass of HOE 1 is transmitted undiffracted through 
HOE 1 to be trapped by a black absorber 22 on the rear 
surface of HOE 1. Thus, scattered light in the receiver 
is minimized by the use of the holographic optic instead 
of a reflective optic. A field stop (not shown), if desired, 
in the focal plane, would further limit the bandpass of 
the light reaching detector 31, as well as light at the 
laser wavelength that falls outside of the field of view. 
The basic design for a Conically Scanned Holo- 
graphic Lidar Telescope according to the present in- 
ventive concepts is shown in FIG. 3. In this design, flat, 
rotating HOE 1 replaces the primary mirror in a con- 
ventional lidar telescope. HOE 1 is designed so that the 
field of view (for the specific narrow band of wave- 
lengths about the laser transmitter wavelength) is at an 
angle (in this example, 45”) with the axis of rotation of 
HOE 1. HOE 1 is made with its optical axis on a perpen- 
dicular line through its center and coincides with the 
rotation axis of HOE 1. Thus, the field of view sweeps 
out in a conical scan. However, instead of a bulky, 
cantilevered large mirror and support structure being 
rotated, spinning HOE 1 will accomplish the scanning, 
focusing, and part of the optical filtering. HOE 1 and its 
associated motor 6 (and angular momentum compensat- 
ing flywheel for spacecraft use) will be the only moving 
components of this telescope, allowing a significantly 
lighter structure 7 for supporting any secondary optic 8, 
(which may also be an HOE) and baffling 9, both of 
which remain static. 
In a lidar operation, an outgoing laser beam 10, is 
transmitted through a hole 11 in lower baffling 9, 
through f-matching optics 12, to a folding mirror 13, 
positioned above HOE 1, which deflects beam 10 to 
HOE 1. Laser beam 10 is collimated by and diffracted 
off the center of HOE 1, to be transmitted coaxially 
along the receiver’s field of view 24. In systems in 
which the transmitted laser energy is too high for nor- 
mal HOE materials, a small high energy mirror 15 may 
be attached to the center of HOE 1 so as to reflect 
transmitted beam 10 into the proper direction to match 
the field of view of HOE 1. To prevent scattered light 
from transmitted laser beam 10 from getting into the 
receiver detector 14, two additional conically shaped 
baMes (not shown) may be added to the system nested 
between the principal baffles 9 so as to act as a conduit 
for the outgoing laser beam. 
Multiple wavelength systems may be produced in a 
variety of ways. Multiple HOE’s may be produced in 
the same film, each responding to a different laser wave- 
length or atmospheric Raman shifted wavelength. Al- 
ternatively, individual HOE’s may be sandwiched one 
on top the other, each transparent to the other wave- 
lengths but diffracting the single wavelength of its de- 
sign. Multiple H O E s  may occupy separate areas of the 
substrate, such as pie shaped or annular ring areas. This 
technique is not as efficient as overlapping HOEs, but 
may be required in systems where the disparity of the 
wavelengths may be such that the HOE material for 
one may not be compatible with the others, for example 
in using 10 micron C02 laser radiation with visible laser 
wavelengths. 
FIG. 4 is an example of a configuration in which 
multiple wavelengths are employed in a receiver sys- 
tem, and each wavelength is diffracted with an HOE 1 
5 
10 
1s 
20 
25 
30 
35 
40 
45 
50 
55 
60 
65 
4 
of a unique focal length. Hence the different wave- 
lengths are separated along a common optical axis 
which is also the axis of rotation of HOE 1. This ar- 
rangement facilitates keeping the foci 29, 30 and 31 
stationary, while HOE 1 rotates and the field of view 
sweeps out a cone in the viewing space. Optical fibers 
26, 27 and 28 can be employed to carry the light to the 
individual detectors 32, 33 and 34 so as to minimize the 
amount of obscuration of the longer focal length images 
by the detectors. Note that the entire area of HOE 1 
may be responsive to each wavelength, or different 
areas of HOE 1 may be responsive to different wave- 
lengths (e.g., segments or annuli). In either case, focal 
positions will not move when HOE 1 is rotated about 
the optical axis which is centered on HOE 1. 
Alternate embodiments of the arrangement of the 
H O E s  is shown in FIGS. 5-9. In FIG. 5, a configura- 
tion is shown of a holographic scanning telescope using 
a reflection HOE 42 (having infinite focal length) for 
the rotating scanner and a reflection HOE 40 (or mir- 
ror) for the collecting optic (solid rays), o r  a transmis- 
sion HOE 40 or lens for the collecting optic (dashed 
rays). The collecting optic may also perform wave- 
length separation either on-axis or off-axis. 
FIGS. 6 and 7 show holographic telescope configura- 
tions combining a rotating transmission HOE scanner 
(which may contain aberration correction) and a reflec- 
tion HOE(s) or conventional mirror@) for focusing. 
FIG. 7 shows a cassegrain configuration having a rotat- 
ing scanner HOE 44 and reflection H O E s  46 (or mir- 
rors) while FIG. 6 shows an off-axis configuration hav- 
ing a rotating scanner HOE 48 and a collector HOE 50 
(or off-axis parabolic mirror). In the off-axis configura- 
tion, the foci of different wavelengths may be posi- 
tioned at various locations around the periphery of the 
system, totally unobscured by each other. 
FIG. 8 shows a transmission HOE 52 combining 
functions of scanner and collecting (focusing) optics 
that has the advantage of large cost and weight savings. 
FIG. 9 shows a wavelength multiplexed holographic 
telescope with one transmission HOE 54 providing the 
scanning function and one transmission HOE 56 provid- 
ing the collecting and wavelength division functions. 
The advantage of this arrangement is that the detectors 
are unobscured and stationary. 
The development of this technology will allow larger 
optical and infrared planetary and earth observing tele- 
scopes to be deployed while at the same time offering 
significant weight savings over existing telescope mir- 
ror technology. It is quite conceivable that this technol- 
ogy will also find use in multichannel passive instru- 
ments. 
Those features of this invention which are new are 
the use of an HOE in a conically scanned telescope, 
having a stationary HOE optic axis, the design of the 
baffling, secondary, and support structure to facilitate 
conical scanning while rotating only the HOE, and the 
arrangement of multiple foci along the optical axis to 
facilitate the use of static focal plane optics and detec- 
tors and multiple wavelengths while performing the 
conical scanning. The use of multiple HOE’s utilizing 
the same substrate and responsive to different wave- 
lengths in an annular area configuration is also a new 
innovation. 
To those skilled in the art, many modifications and 
variations of the present invention are possible in light 
of the above teachings. It is therefore to be understood 
that the present invention can be practiced otherwise 
5,255,065 
5 6 
than as specifically described herein and still will be 
within the spirit and scope of the appended claims. 
8. The device of claim 1 further comprising f-match- 
ing optics located between said laser and said rotating 
9. The device of claim 2 further comprising second- 
5 ary optics located on said axis of rotation of said rotat- 
10. The device of claim 9 further comprising station- 
ary baffling surrounding said rotating holographic opti- 
cal element. 
for 
supporting and rotating said rotating holographic opti- 
12. The device of claim 11 further including an angu- 
13. An optical scanning device comprising: 
a source of optical energy; 
a plurality of rotating holographic optical elements 
each having a stationary focus, said plurality of 
holographic optical elements each diffracting a 
Particular wavelength of said Source of optical 
energy at an angle to said rotation axis so as to 
define a conical scanning area; 
rotating holographic optical element; 
said plurality of rotating holographic optical elements 
comprises individual holographic optical elements 
sandwiched one on top of the other, each transpar- 
ent to the others wavelengths but diffracting the 
single wavelength of its design. 
14. The device of claim 13 wherein said plurality of 
rotating holographic optical element comprises multi- 
holographic optical element produced in the Same 
film. 
15. The device of claim 13 wherein said plurality of 
35 rotating holographic optical elements occupy separate 
areas of a substrate in a pie shaped or annular ring fash- 
ion. 
I claim: holographic optical element. 
1. An optical scanning device comprising: 
a source of optical energy; 
a rotating holographic optical element having a sta- ing Optical 
tionary focus, said holographic optical element 
diffracting said source of optical energy at an angle 
to said rotation axis so as to define a conical scan- ,o 
ning area; 
graphic optical element; 
a laser transmitting a beam of light to said rotating 
holographic optical element, said beam of light 15 
diffracted by said rotating holographic optical ele- 
ment and thereby emanating outward from said 
rotting holographic optical element in a conical 
manner; and 
a small high damage threshold mirror attached to the 20 
center of said rotating holographic optical element. 
2. The device of claim 1 wherein said stationary focus 
ll. The device of claim wherein said 
means for supporting and rotating said rotating holo- element comp~ses a motor means. 
lar momentum compensating flywheel, 
is located on the axis of rotation of said holographic means for supporting said rotating said plurality Of 
optical element. 
means located at said stationary focus of said rotating 
holographic optical element. 
4. The device of claim 3 wherein said source of opti- 
cal energy comprises laser light backscattered from the 3o 
earth's atmosphere. 
5. The device of claim 3 wherein said source of opti- 
cal energy comprises a natural source of light. 
6.  The device Of claim 5 wherein said natural source 
of light comprises backscattered solar light. 
7. The device of claim 1 further comprising a folding 
mirror positioned above said rotating holographic opti- 
cal element. 
3, The device of claim further comprising a detector 25 
* * * * *  
40 
45 
50 
55 
60 
65 


Conically scanned holographic lidar telescope

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