An optical system is described for rotating the plane defined by a pair of parallel light beams. In one embodiment a single pair of rhomboid prisms have their respective input faces disposed to receive the respective input beams. Each prism is rotated about an axis of revolution coaxial with each of the respective input beams by means of a suitable motor and gear arrangement to cause the plane of the parallel output beams to be rotated relative to the plane of the input beams. In a second embodiment, two pairs of rhomboid prisms are provided. In a first angular orientation of the output beams, the prisms merely decrease the lateral displacement of the output beams in order to keep in the same plane as the input beams. In a second angular orientation of the prisms, the input faces of the second pair of prisms are brought into coincidence with the input beams for rotating the plane of the output beams by a substantial angle such as 90 deg

Orloff, K. L.

Yanagita, H.

English

NASA Technical Reports Server

United States Patent [191 
Orloff et al. 
[111 4,355,870 
[45] Oct. 26, 1982 
[54] RHOMBOID PRISM PAIR FOR ROTATING 
THE PLANE OF PARALLEL LIGHT BEAMS 
[76] Inventors: 
[21] Appl. No.: 
[22] Filed: 
Alan M. Lovelace, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of Kenneth L. Orloff; 
Haruo Yanagita, both of Cupertino, 
Calif. 
219,640 
Dec. 24,1980 
[5 11 Int. Cl.3 ................................................ C02B 7/18 
[52] U.S. C1. ..................................... 350/486; 350/287 
[58] Field of Search ................... 350/286, 287, 31, 33, 
350/35, 36,75, 171, 320,486 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,909,106 9/1975 Buhler ................................... 350/51 
Primaly Examiner-Jon W. Henry 
Attorney, Agent, or Firm-Darrell G. Brekke; John R. 
Manning 
[571 ABSTRACT 
The object of the invention is to provide an optical 
system for rotating the plane defined by a pair of paral- 
lel light beams. 
In one embodiment (see FIG. 4) a single pair of rhom- 
boid prisms 36 and 37 have their respective input faces 
15 disposed to receive the respective input beams 21 and 
22. Each prism is rotated about an axis of revolution 
coaxial with each of the respective input beams 21 and 
22 by means of a suitable motor 29 and gear arrange- 
ment 41 to cause the plane of the parallel output beams 
21' and 22' to be rotated relative to the plane of the input 
beams. 
In a second embodiment, two pairs (FIG. 2) of rhom- 
boid prisms 23, 24 and 31, 32 are provided. In a first 
angular orientation of the output beams, prisms 23 and 
24 serve to merely decrease the lateral displacement of 
the output beams for keeping the beams in the same 
plane as the input beams. In a second angular orienta- 
tion of the prisms (FIG. 3) the input faces of the second 
pair of prisms 31 and 32 are brought into coincidence 
with the input beams for rotating the plane of the output 
beams 21' and 2 2  by a substantial angle such as 90". 
6 Claims, 6 Drawing Figures 
https://ntrs.nasa.gov/search.jsp?R=19830005707 2020-03-21T23:37:20+00:00Z
U.S. Patent Oct. 26, 1982 4,355,870 
4.3 55,870 
1 
RHOMBOID PRISM PAIR FOR ROTATING THE 
PLANE OF PARALLEL LIGHT BEAMS 
ORIGIN O F  THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provision of such Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 U.S.C 2457). 
TECHNICAL FIELD 
The present invention relates in general to optical 
systems for rotating the plane defined by a pair of paral- 
lel light beams and, more particularly, to an improved 
optical system utilizing pairs of rhomboid prisms. 
BACKGROUNDART 
Heretofore, both the dove and the pechan prism have 
been proposed for use as optical image rotators. In each 
case, as the prism is rotated, the image passing there- 
through will rotate at twice the angular rate of the 
prism. These prisms, therefore, have the advantage of 
selecting any desired rotation angle. However, they 
have disadvantages with respect to rotating the plane of 
a pair of parallel light beams as follows: 
With regard to the dove prism, this is typically a very 
long prism, the length is usually 4 to 5 times the input 
aperture, i.e., the lateral beam separation in the case of 
rotating the plane of a pair of parallel beams. Where 
compactness is a requirement, this prism is not usually 
acceptable. In addition, to function correctly, the rota- 
tion axis of the dove prism must be precisely aligned 
with the optical axis of the optical system. Furthermore, 
the dove prism is not achromatic and will not perform 
equally for light beams of different wavelengths. 
With regard to the pechan prism, this prism is typi- 
cally very large, i.e., the length and lateral dimensions 
are both nominally twice the input aperture. This is not 
compatible with compactness. This type of prism is 
formed by a pair of air spaced prism elements. These 
elements must be accurately positioned relative to one 
another and the dimensions and uniformity of the air 
space must be exact if the rotation is to function cor- 
rectly. Moreover, two of the prism surfaces must be 
reflectively coated. If aluminized coatings are used, 
measureable light losses will occur. If dielectric coat- 
ings are used, the prism becomes non-achromatic. 
[STATEMENT OF INVENTION] 
Disclosure of Invention 
In the present invention, the plane defined by first 
and second parallel light beams, such as laser beams, is 
rotated by directing such beams onto the respective 
input faces of a pair of rhomboid prisms disposed in a 
Dlane aenerallv Demendicular to the outical axis of the 
tated about the optical axis of the input beams to rotate 
the plane of the output beams about the optical axis. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a perspective view of a prior art rhomboid 
prism of the type employed in the present invention. 
FIG. 2 is a perspective view of an optical system of 
the present invention for rotating the plane defined by 
parallel light beams, 
FIG. 3 is a view similar to that of FIG. 2 depicting a 
position of the optical system for rotating the plane of 
the output beam through 90", 
FIG. 4 is a view similar to that of FIG. 2 depicting an 
alternative embodiment of the present invention, 
FIG. 5 is a view similar to that of FIG. 4 depicting an 
alternative embodiment of the system of FIG. 4, and 
FIG. 6 is a schematic perspective view depicting a 
series arrangement of optical rotating systems of the 
present invention. 
DETAILED DESCRIPTION O F  THE 
INVENTION 
Referring now to FIG. 1 there is shown the prior art 
rhomboid prism 11, as previously employed for produc- 
25 ing a lateral displacement between an input beam 12 and 
the resultant output beam 13. Briefly, the rhomboid 
prism 11 comprises a generally rhomboid shaped prism 
element 14 having an input face 15 disposed normal to 
3o the incident input beam 12. An internally reflecting end 
face 16 receives the input beam as transmitted through 
the input face 15 and internally reflects the beam longi- 
tudinally through the prism element 14 to a second 
internally reflecting face 17 which internally reflects 
35 the beam in a direction parallel to and laterally spaced 
from the input beam 12. The laterally displaced inter- 
nally reflected beam exits through an output face 18 
normal to the output beam 13. The internally reflecting 
end faces 16 and 17 are generally inclined at an angle of 
Referring now to FIGS. 2 and 3, there is shown an 
optical system 19 of the present invention for rotating 
the plane defined by a pair of parallel input beams 21 
and 22 from a first output plane which is coplanar with 
45 the input beams to a second output beam plane which is 
orthogonal to the plane of the input beams. More partic- 
ularly, a first pair of rhomboid prisms 23 and 24, respec- 
tively, are disposed on a ring 25 having an axis of revo- 
lution coaxial with an optical axis 26 of the system 19 
50 defined by the optical axis 26 of the pair of input beams 
21 and 22. The input face portions 15 of the respective 
rhomboid prisms 23 and 24 are positioned to receive the 
respective rhomboid prisms 21 and 22. The prisms 23 
and 24 are oriented such that their longitudinal axes 
55 extend laterally in opposite directions from the optical 
axis 26. The resultant output beams 21' 22' remain in the 
same plane as the input beams 21 and 22, Le., are copla- 
5 
10 
l5 
20 
40 45" to the plane of the input face 15. 
- .  . 
parallg input beams. The output beams from the pair of 
rhomboid prisms are parallel. Means are provided for 
rotating the rhomboid prisms about an axis or axes par- 60 
allel to the optical axis of the input beams so as to rotate 
the plane defined by the pair of parallel output beams. 
In one embodiment, a single pair of rhomboid prisms is 
employed. Each prism is rotated about an axis of revo- 
lution coaxial with the respective input beam. In a sec- 65 
ond embodiment two pairs of rhomboid prisms are 
employed, one pair for each angular orientation of the 
plane defined by the output beams. The prisms are ro- 
nar and are increased to a lateral spacing bl, of gener- 
ally twice the length 1 of the respective rhomboid 
prisms 23 and 24 plus the input beam spacing bo. 
The ring 25 is supported upon a pair of rollers 29 and 
28, roller 28 being driven by a motor 29 for rotation of 
the ring 25 about the optical axis 26. 
A second pair of rhomboid prisms 31 and 32 are dis- 
posed on the ring 25 with their respective input faces 15 
laterally spaced apart by the lateral spacing bo of the 
pair of input beams 21 and 22 and with their longitudi- 
nal axes of the first pair of rhomboid prisms 23 and 24. 
4,355,870 
3 4 
Referring now to FIG. 3, when the ring 25 is rotated ments relative to the optical axis 26, employ only total 
in the counterclockwise direction through an angle 4, internal reflections, thereby avoiding losses associated 
which is somewhat less than 90", where tuR@-(b2/b0), with mirrored surfaces and are achromatic in that they 
the input beams 21 and 22 are incident on the respective provide constant optical properties for different operat- 
input faces 15 of the second pair of rhomboid prisms 31 5 ing wavelengths. In the case of an application of the 
and 32 and the respective output beams 21' and 22' present invention to a laser velocimeter, the output 
define a plane orthogonal to the plane of the input beams would preferably have different lateral separa- 
beams 21 and 22. The individual rhomboid prisms 31 tions for the orthogonal positions in order to optimize 
and 32 are chosen of the proper length 1 which is sensitivity to the velocities being measured. 
slightly longer than the length of the first pair of rhom- 10 We claim: 
boid prisms 22 and 23 such that, in a preferred embodi- 1. In an optical system for rotating the plane defined 
ment, the lateral spacing of the output beams 21' and 22' by the first and second parallel light beams that are 
does not change with rotation of the plane of the output parallel to and equally spaced from a reference axis; first 
beams 21' and 2 2  from the zero degree rotation position and second rhomboid prism means each having an input 
to the 90" rotation position. 15 face disposed generally normal to a respective one of 
The optical beam rotation system 19 of FIGS. 2 and said first and second light beams for receiving thereon 
3 is particularly useful for rotating the output beams in and transmitting the beam therethrough into each of the 
a laser doppler velocimeter as disclosed and claimed in respective prisms, each of said prisms means having a 
US. Pat. No. 3,915,572 issued Oct. 28, 1975. first internal face inclined to the respective inputted 
Referring now to FIG. 4 there is shown an alternative 20 light beam for internally reflecting the respective light 
embodiment of the present invention. This optical rota- beam within the respective prism, each of said prism 
tion system 35 is similar to that of FIGS. 2 and 3 with means having an internal face generally parallel to said 
the exception that only a single pair of rhomboid prisms first internal face for internally reflecting the respective 
36 and 37 is employed. Each prism 36 and 37 is affixed beam generally parallel to said respective inputted light 
to an individual disc 38 and 39 having an axis of revolu- 25 beam, each of said prism means having an output face 
tion coaxial with the respective input beam 21 or 22, generally perpendicular to the light beam incident 
such discs 38 and 39 being driven from a drive wheel 41 thereon for directing the light beam out of said prism 
which is driven from a motor 29 for turning each of the generally parallel to said respective input light beam; 
respective prisms in the same sense, i.e., either clock- and said first and second prism means being of substan- 
wise or counterclockwise at the same angular rate. 30 tially identical dimensions; rotating means for rotating 
When the prisms 36 and 37 are oriented in the posi- each of said first and second prism means about an axis 
tion as shown in FIG. 4, the plane defined by the pair of of revolution coincident with the first and second input 
parallel output beams 21' and 22' is orthogonal to the light beams, respectively, for rotating the plane defined 
plane of the input beams 21 and 22. The output beams by said parallel first and second output light beams 
have a lateral spacing b2. However, when the motor 29 35 relative to the plane defined by said first and second 
is energized and the respective prisms 36 and 37 are input light beams; and said prism rotating means being 
rotated in the counterclockwise direction so that each formed and arranged for rotating each prism means 
of the prisms has its longitudinal axis generally coplanar through an angle of the same sense and magnitude so 
with the input beams 21 and 22, the output beams 21' that the parallel output beams are parallel and equidis- 
and 2 2  are coplanar with the input beams 21 and 22 and 40 tant to said reference axis. 
have a substantially greater lateral spacing than in the 2. In an optical system for rotating the plane defined 
orthogonal output position. by first and second parallel light beams; first and second 
A second feature of the beam rotation system 35 is pairs of rhomboid prism means, each prism means hav- 
that the plane of the output beams 21' and 22' is continu- ing an input face disposed generally normally to a re- 
ously variable relative to the plane of the input beams. 45 spective one of the first and second light beams for 
However, as aforementioned, the lateral spacing of the receiving thereon and transmitting the beam there- 
output beams varies with angular rotation of the plane through into each of the respective prisms, each prism 
of the output beams relative to the plane of the input means having a first internal face inclined to the input- 
beams. ted light beam for internally reflecting the respective 
Referring now to FIG. 5 there is shown an optical 50 beam within the respective prism, each of said prism 
beam rotation system 45 similar to that of FIG. 4 with means having a second internal face generally parallel 
the exception that the individual prisms 36 and 37 are of to said first internal face for internally reflecting the 
shorter length relative to the lateral spacing bo of the respective beam generally parallel to said respective 
input beams and the prisms are operated in the rota- inputted light beam, and each of said prism means hav- 
tional regime wherein the output faces tend to rotate 55 ing an output face generally perpendicular to the light 
toward each other so that the lateral spacing of the beam incident thereon for directing the beam out of said 
output beams b2 is less than the lateral spacing bo of the prism means generally parallel to said respective input 
input beams 21 and 22. beam; said first pair of rhomboid prism means being 
Referring now to FIG. 6 there is shown an alternative disposed in a first position to receive on their respective 
embodiment of the present invention wherein a plural- 60 input faces said first and second light beams, respec- 
ity of the beam optical rotation systems of FIGS. 2 and tively; rotating means for rotating said first and second 
3 are arranged in series for obtaining a series of select- pairs of prism means about an axis of revolution which 
able rotation angles 61, 62, and 63, as desired. is disposed midway between the two parallel beams and 
The advantages of the beam rotation system of the which is parallel thereto; said second pair of prism 
present invention, as contrasted with the prior art, are 65 means being disposed relative to said first pair of prism 
that the rhomboid systems 19, 35 and 49 of the present means so that when said first and second pairs of prism 
invention are more compact than those proposed in the means are rotated to a second position said input faces 
prior art, are less sensitive to small angular misalign- of said second pair of prisms are brought into coinci- 
5 
4,3 55,870 
dence with said first and second light beams, whereby 
the plane defined by said first and second output light 
beams is rotated relative to the plane defined by said 
pair of input light beams. 
3. The apparatus of claim 2 wherein said prism means 5 
of said first and second pair are elongated and wherein 
the length of said second pair of prism means is longer 
than that of said first pair. 
4. The apparatus of claim 3 wherein said prism rotat- 
ing means is formed and arranged for rotating each of 10 
said prism means through an equal angle of the same 
sense so that the output beams are parallel to said axis of 
revolution. 
5. The apparatus of claim 2 wherein each of said 
rhomboid prism means is elongated and said first and 15 
second internal faces are disposed at 45" relative to the 
plane of said input face. 
6. In a method for rotating the plane defined by first 
and second parallel light beams that are parallel to and 
disposing first and second rhomboid prisms so that 
each of said prism means, having an input face, is 
disposed with its input face generally normal to a 
respective one of the first and second light beams 
equidistant from a reference axis, the steps of: 20 
25 
6 
so that the light beams received on each respective 
input face of each prism is transmitted through the 
input face into the respective prism, each of said 
prisms having a first internal face inclined to the 
respective inputted light beam for internally re- 
flecting the respective light beam within the re- 
spective prism, each of said prisms having a second 
internal face generally parallel to the first internal 
face for internally reflecting the respective beam 
generally parallel to the respective inputted light 
beam, each of said prisms having an output face 
generally perpendicular to the light beam incident 
thereon for directing the light beam out of the 
prism generally parallel to the respective input 
light beam, the output light beams being parallel to 
and equidistant from said reference axis; simulta- 
neously rotating said first and second prisms in one 
direction about an axis of revolution coincident 
with said reference axis for rotating the plane de- 
fined by said parallel first and second output light 
beams relative to the plane defined by said first and 
second input light beams. * * * * *  
30 
35 
40 
45 
50 
55 
60 
65 


Rhomboid prism pair for rotating the plane of parallel light beams

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