An over-under double pass interferometer in which the beamsplitter area and thickness can be reduced to conform only with optical flatness considerations was achieved by offsetting the optical center line of one cat's-eye retroreflector relative to the optical center line of the other in order that one split beam be folded into a plane distinct from the other folded split beam. The beamsplitter is made transparent in one area for a first folded beam to be passed to a mirror for doubling back and is made totally reflective in another area for the second folded beam to be reflected to a mirror for doubling back. The two beams thus doubled back are combined in the central, beamsplitting area of the beamsplitting and passed to a detector. This makes the beamsplitter insensitive to minimum thickness requirements and selection of material

Schindler, R. A.

English

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https://ntrs.nasa.gov/search.jsp?R=19780010452 2020-03-22T05:54:48+00:00Z
NASA CASE NO.#^^-^.Y-j
-1PRINT FIG...... i . . ...................... A ....,,
NOTICE
The invention disclosed in this document resulted from
research in aeronautical and space activities performed under
programs of the National Aeronautics and Space Administration.
The invention is owned by NASA and is, therefore, available
for licensing in accordance with the NASA Patent Licensing
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Address inquiries and all applications for license for
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above address.
d V
n ..1%1 NRLO
( N ASA-C ase-NPO-13999 -1) 	 OVER-UNDER N78-18395
DOUBLE-PASS INTERFEROMETER Patent
Application 
	 (NASA)	 12 p HC A 02/1F A01
CSCL
	 14B tlnclas
G3/35 0849E
Inventor. Rudolf A. Schindler
	
	 NASA Case No. NPO-13999
JPL Case No. _13999
Contractor: Jet Propulsion Laboratory September 10 0 1977
OVER-UNDER DOUBLE-PASS INTERFEROMETER
The invention relates to interferometers utilizing
double-pass retroreflectors, and more particularly to a new
structure of the beamsplitter and an over-under mode of
operation for the retroreflected beam paths.
The beamsplitter 10 is shown in 'FIG. 1 standing on end
in the plan view of an interferometer cmbodying.the invention.
The face of the beamsplitter is shown in FIG. 2 as having a
beamsplitting surface 10b in the middle to split infrared light
from a source 11 into two beams in the usual manner. A mirror
12 directs one beam into a cat's-eye retroreflector 13 disposed
in a position parallel to a second cat's-eye retroreflector 14.
That is for convenience only, although having included the
mirror 12, it may be used for making small, mid-frequency changes
in the path length of one beam. The retroreflector 13 is oriented
to return the beam through it in a path directly above its input
path. Consequently the upper part of-the beamsplitter is pro-
vided as a transparent window 10a as shown in FIG. 2. The
retroreflector 14 is oriented to return the beam through it %n
a path directly below its input path. Consequently, the lower
part of the beamsplitter is provided with a reflective surface
10c as shown in FIG. 2. The beam thus split and retroreflected
is combined through a mirror 15 having a transparent window (or
hole) 15a as shown in FIG. 3. The upper and lower portions 15b
and 15c of the mirror 15 have reflective coatings, the top (15b)
to return the beam to the retroreflector 13, and the bottom (15c)
to return the beam to the retroreflector 14. FIGs. 4 and 5
are cross sections of the respective retroreflectors 13 and 14
showing the beams in dotted lines. The respective input beams
to the retroreflectors 13 and 14 are identified as Al and A2,
and the respective output beams are identified as Bl and B2.
The over-under double-pass beam paths may be better understood
from the isometric diagram of the beam paths shown in FIG. 6.
The novelty of the invention is the new structure of
the beamsplitter and the over-under mode of operation of the
retroreflectors. The advantage is that the beamsplitter area
and thickness can be reduced to conform only with optical flatness
considerations, as compared to the beamsplitter configurations
required in the side-by-side mode of operation of the retrore-
flectors disclosed in U.S. patent 3,809,481 by the same inventor.
Serial No. _	 y - -_
KK
15b
15a ^^ /`
15c
FIG. 3
REFLECTIVE
COATING
TRANSPARENT
WINDOW
NG
13
10
FIG. 2
rA2) 8^ 11 4ii ^_it
TRANSPARENT
WINDOW
BEAMSPLITTING
SURFACE
REFLECTIVE
SURFACE
FIG. 4	 FIG. 5

TO ALL WHOM IT MAY CONCERN:
BE IT KNOWN THAT RUDOLF A. SCHINDLER, a citizen of the
United States, residing at 452 E. Sierra Madre Boulevard,
	
5	 Sierra Madre, in the County of Los Angeles, State of
California, has invented a new and useful
OVER-UNDER DOUBLE-PASS INTERFEROMETER
of which the following is a specification:
ABSTRACT OF THE DISCLOSURE
	
10	 An over-under double-pass interferometer in which
the beamsplitter area and thickness can be reduced to conform
only with optical flatness considerations is achieved by
offsetting the optical center line of one cat's-eye retro-
reflector relative to the optical center-line of the other
15 in order that one split beam be folded into a plane distinct
from the other folded split beam. The beamsplitter is made
transparent in one area for a first folded beam to be passed
to a mirror for doubling back and is made totally reflQctive
in another area for the second folded beam to be reflected
	
20	 to a mirror for doubling back. The two beams thus doubled
back are combined in the central, beam -splitting area of
the beamsplitter and passed to a detector. This makes the
beamsplitter insensitive to minimum-thickness requirements
and selection of material..
Serial No.	 858,596
Filing Date December 8, 1977
C,) n4 r -- - - .	 CaltechlJPL
	
^v
Contra :_- t No. H107-100
PasaderL ___ .__. CA
(City)	 (State)	 (Zip)
	
1
The invention described herein was made in the
performance of work unuer a ;NASA contract and is subject
'to the provisions of Section 305 of the National Aeronautics
	
5	 and Space Act of 1958, Public Law 85-568 (72 Stat. 435;
42 USC 2457).
BACKGROUND OF THE INVENTION
This invention relates to interferometers, and
more particularly to interferometers utilizing double-pass
	
10	 retroreflectors.
In a Fourier interference spectrometer of the double-
pass "cat's-eye" retroreflector type, a single mirror is
employed in the path of both split beams of an incoming
rah • to cause them to double back through separate retro-
	
15	 reflectors, as shown in U.S. patent 3,805,481 by the same
inventor. Changes in optical path length are achieved by
linear displacement of both retroreflectors using a motor-
driven lead screw an one for large, low-frequency changes,
a moving;-coil actuator on the other for smaller, mid-frequency
	
20	 changes and a piezoelectric actuator on one of these two
for small, high-frequency changes. Alternatively, one of
the retroreflectors may be fixed in space while the other
is displaced for large, low-frequency changes. The optical
axis of the movable retroreflector is then made parallel to
	
25	 the optical axis of the first retroreflector by using a
mirror at a 145° angle of incidence. This mirror may then
i
be displaced in a directiun normal to its reflecting; surface
for :smaller mid-frequency changes. A piezoelectric actuator
2
.. L	 _ _ 
changes as before. In either case, a "cubic" beansplitter
must be fabricated with precision to form two plane mirrors
on the outside normal to the incident retroreflected bears,
	
5	 or a beamsplitter accordin to the aforesaid U.S. patent
must be employed. However, even a "thick" beam splitter
according to that invention involves a cost significantly
greater than if a "thin" beamsplitter were used. Therefore,
an object of this invention is to provide an arrangement for
	
10	 an interferometer utilizing double-pass retroreflectors
that will permit using a beamsplitter reduced in thickness
to conform only with considerations of optical flatness.
SUMMARY OF THE INVI NTIOV
In accordance %. ith this invention, the sp tial
	
15	 reflection orientation of cat's-eye retroreflectors in a
double-pass interferoneter is so arranged that one cat's-eye
retroreflector is in a plane offset from the other cat's-eye
retroreflector in order that the double-pass retroreflected
beams combine. To accomplish that, the axes of the two
	
20	 douLle-pass retroreflected output beams coincide in a p1,ane
between the single-pass reflected beams of the tc•:o retro-
E
	 reflectors. Thus, the input beam, A 1 , of one cat's-eye
I	 retroreflector erierges as a single-pass output beam, H13,
offset ;n one direction. Upon being reflected for the second
	
25	 pass, the Leam emerges from the one cat's-eve retroreflector
on the axis of the input bear:i, A l - The input beam A 2 of the
other cat':-eye retroreflector emerges as a .in,^le pass
output beam 3 2 oft^rt in a direction opposite the one direction
because the other cat's -eye retroreflector is oppositely offset.
3
Upon bcirnf; reflected for the second pass, the beam emerges
from the other cat's-eve retroref lector,
 on t,ie axis of the
input beam A 2 . The beamsplitter need only split the original
beam from the source into the beams A, and k with virtually
5	 the same axis, so that the emerging cioublc!-pass beams can
recombine. Consequently, the beamsplitter thickness can
be reduced to cunforn only with optical flatness.
The novel features -that are considered characteristic
of this invention are set forth with particularity in the
10	 appended claims. The invention will best be understood from
the following description when read in connection with the
accompanying drawings.
D'.:SCRIFTT0'1 OF T'r._ DRA:41NGS
FIG. 1 is a plan view of the optical configuration
15	 of an over-under double-pass interferometer embodying thy,
present invention.
FIG. 2 is a front elevation view of a beamsplitter in
the optical configuration of FIG. 1.
FIG. 3 is a front elevation view of a fixed reflecting
20 mirror used to double back split beams in the optical configu-
ration of FiG. 1, and to pass the combined beams at the output
of the interferometer to a photodet`ctor.
FIG. it is a sectional view taken along a line Li -4 it:
FIG. 1 to show the under and over paths, of the retroreflected
5	 beam on one side of the buamsplit ter.
FIG. 5 i:; a sectional view taken along, a line 5-5 in
FIG. 1 to show the over and under path;.: of the retrorefl.ected
beaus on the other sida of the beamsplitter.
ij
FIG. 6 is an isometric diagram of the over-under
double-pass beam paths in the optical confiuuration of FIG. 1.
DESCRIPTION 01' PREr' I:R'),f:D M'MOD7Mc:i1 S'
Referring; now to the drawings, a plan view is
	
5	 shown schematically in FIG. 1 of a high-speed double-pass
interferometer very similar to that shown in the aforesaid
patent 3,809,481, but with the improvement of a beamsplitter
10 }laving an area and thickness reduced to conform only with
optical flatness considerations. That patent discloses a
	
10	 Fourier interference spectrometer of the dout '.e-pass retro-
reflector type in which a single mirror is e;-.-ployed in
the path of both split beams of an incoming b-am to cauue them
-to double back through separate retroreflec tors . Changes in
optical path length are achieved by linear displacement of
	15	 both retroreflectors using a rotor-driven lead screw on one
for large, low-frequency changes, a moving-coil actuator can
the other for smaller, mid-frequency changes and a piezoelectric
actuator on one of these two for small, high -fregtency changes.
Different arrangements are disclosed for the beanspl.ittcr to
	
20	 function as a splitter for the incoming; beam, a "window"
for one split beam and a mirror at 45° with the beamsplitter
for the other reflecting beam. A problem with that arrangement
was that both retroreflected beams were reflected by the same
mirror at 45° with the beam splitter so that while one split
beam is reflected by a reflective surface on the beamsplitter,
the other split beam needed to be displaced so as to pass
through the window onto the mirror. That required special
design considerations for the beamsplitter beyond just optical
5
T
I5
10
15
20
25
F.1 otnv q!;. The r)rrescnt invention offset, the axis of one
retroreflector from the other so that while both receive
split reams in a common plane, one retroreflects in a
plane offset in one direction while the other rctroreflects
in a plane in the opposite direction. An advantage of this
is that it reduces cost and improves the latitude of selection
of materials for the beamsplitter.
A beamsplitter 10 shown in a plan view (i. e. ,
standing on end) is divided into three horizontal areas
as shown in an elevation view in FIG. 2 9 a top area 10a
that is clear to provide a transparent window, a central
area 10b that is 50% coated to provide an IR beamsplitting
surface, and a bottom area to provide a reflective surface.
The beam from the source 11 is directed at the beamsplitting
surface to provide two beams A l and A2 illustrated scherr.atically
in FIG. 6. The first beam A l is directed to a mirror 12
which reflects the beam into a cat's-e ye retroreflector 13.
This "corner" mirror is useful not only to so fold the path
of the beam A l that the retroreflector 13 is disposed
adjacent a cat's-eye retroreflector 14, for convenient
packaging, but also to provide a way of making; small, mid-
frequency changes in the path length of the beam A 1 . f!o-, cvPr,
that is not essential, since the present invention can be
practiced in the interferometer arrangement of the
aforesaid patent, or with the mirror 12 at some other angle.
The beam A l
 enters the retroreflector at one (lower)
level, and exits at another (upper) level as a beam B I
 in
the manner shown in FIG. 6. The beam B 1
 is reflected by
the mirror 12 to the beamsplitter 10 where it passes through
the transparent upper area 10a of the beamsplitter onto a
6
mirror 15 which lia5 a reflective coating; nn an upper area VA)
to reflect the beam H 1 , thereby to c.iu:,-e it to double back
through the central area 101) of the beamsplitter and through .
the transparent central area 15a of the mirror 15. The beam Al
	
5	 which has thus doubled back is passed to a photo]etryctor, 16.
That bean is to be combined with the beam A l similarly doubled
back, but instead of doubling back through an optional path over
the path for the bears A 2 , as in the case of the beam A1 , that
beam doubles back through a path under the path for the beam
10	 A2, as shown in FIG. 6.
The beamsplitter 10 shown at a 45 0 .angle for corive-
nience may be placed at some other angle for optical considera-
tions. It transmits 50% of the input beam from the source 11
to the cat's-eye retroreflector 14. That is accomplished by
15	 the central area 10b of the beamsplitter as shown in FIG. 2.
The retroreflector 1 1; has its optical axis offset from the
optical. axis of the retroreflector 13 in a vertical direction,
i.e., normal to the plane of the drawing of FIG. 1. This causes
the beam 11 2 to be reflected from the lower area 10c of the beam- 	 #
	
20	 splitter 10 shorn in FIG. 2. and onto the lower area 15c of the
mirror 15. From there it doubles back, emerging from the r^tro-
reflector 14 over the path of the beam A 2 to impinge the
beamsplitter 10 in the central area 10b. There the back
side of the beamsplitting surface reflects the beam A2,
	
25	 thereby to combine the beams A l and A 2 passing through the
transparent window 15a into the photo detector 16.
This novel arrangement for an over-under double-
pass interferometer utilizing a flat thin plate as a beam-
splitter is better understood from FIGS. 4 and 5, which are
	
30	 sectional views of the retroreflectors 13 and 14 taken
7
a	 diUuy iespU(.;Llve lines 4-4 	 5-5. eR,.	 shows the split
beard Al at the bottom and the retroreflected beam B 1 emerging
ac the top. The top is shown on the left in FIG. 4 because
it is a Section taken on a line in the plan view of FIG. 1
	
5	 looking to the left. Similarly, FIG. 5 shows the split
beam A 2
 at the top and the retroreflected bearr, p2 emerrinr
at the bottom. This entire arrangement is then fully
I
clarified by the schematic diagram of FIG. 6 referred to
hereinbefore wherein all of the elements referred to in
	
1u	 FIGs. 1 to 3 are identified by the same reference numerals.
Although a particular embodiment of the invention
has been described using vertical reflection orientation in
the cat's-eye retroref?ectors, s l.ich that one retroreflector
has its optical center line offset vertically with respect
	
15	 to the optical center line of the other.
	 is recognized
that modifications and equivalents may r<<rdily occur 'to
those s),illcd in the art and consequently it is intended
that the claims be interpreted to cover such modifications
and equivalents. For example, in place of a transparent
	
20	 area in the mirror 15, there may be a ' caller mirror in
front at an angle to reflect the coml y ined beam off to one
side for detection, or a hold, preferably a circular hose in
the mirror 15. In any case the beamsplitter area and thicr:ness
can be reduced to conform only with optical flatness con-
	
25	 s	 i.	 .	 r
„lcferation.. which require a ratio of thickness to length
of about 1 to 7, or, 8. Yor example, a beamsplitter 4" long
would have to be ibotit 1/4" thick to a:.sure optical fl.at-
ne S• Th y ! bOaMSp.Lltting -nut-face and reflective surfilce
are on th(! same Lace of the plate. Typical materials for
3b the I) cams 1 Itter at, c! bori.um f luor idt! l Or wave lc!ris , the from
v.i.^.ible li.t;ht to .ibout 11.5 mic:roci : -ind hota:caium br(mlide
for wavelov,", t h s } r •orc vi : i bl- l i;911. Lo aboii t if" microns.
4
1h	 i


Over-under double-pass interferometer

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