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 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, beam-splitting area of the beamsplitter and passed to a detector. This makes the beamsplitter insensitive to minimum-thickness requirements and selection of material.

BACKGROUND OF THE INVENTION 
This invention relates to interferometers, and more particularly to 
interferometers utilizing double-pass 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 ray to cause them to double back through separate 
retroreflectors, as shown in U.S. Pat. No. 3,809,481 by the same inventor. 
Changes in optical path length are achieved by linear displacement of both 
retroreflectors using a motor-driven lead screw on one for large, 
low-frequency changes, a moving-coil actuator on the other for smaller, 
mid-frequency 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 the optical axis of the first retroreflector by 
using a mirror at a 45.degree. angle of incidence. This mirror may then be 
displaced in a direction normal to its reflecting surface for smaller 
mid-frequency changes. A piezoelectric actuator on one of the 
retroreflectors is used for small, high-frequency changes as before. In 
either case, a "cubic" beamsplitter must be fabricated with precision to 
form two plane mirrors on the outside normal to the incident 
retroreflected beams, or a beamsplitter according 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 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 INVENTION 
In accordance with this invention, the spatial reflection orientation of 
retroreflectors in a double-pass interferometer is so arranged that one 
retroreflector is in a plane offset from the other retroreflector in order 
that the double-pass retroreflected beams combine. To accomplish that, the 
axes of the two double-pass retroreflected output beams coincide in a 
plane between the single-pass reflected beams of the two retroreflectors. 
Thus, the input beam, A.sub.1, of one retroreflector emerges as a 
single-pass output beam, B.sub.1, offset in one direction. Upon being 
reflected for the second pass, the beam emerges from the one 
retroreflector on the axis of the input beam, A.sub.1. The input beam 
A.sub.2 of the other retroreflector emerges as a single pass output beam 
B.sub.2 offset in a direction opposite the one direction because the other 
retroreflector is oppositely offset. Upon being reflected for the second 
pass, the beam emerges from the other retroreflector on the axis of the 
input beam A.sub.2. The beamsplitter need only split the original beam 
from the source into the beams A.sub.1 and A.sub.2 with virtually the same 
axis, so that the emerging double-pass beams can recombine. Consequently, 
the beamsplitter thickness can be reduced to conform only with optical 
flatness. 
The novel features that are considered characteristic of this invention are 
set forth with particularly in the appended claims. The invention will 
best be understood from the following description when read in connection 
with the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to the drawings, a plan view is shown schematically in FIG. 1 
of a high-speed double-pass interferometer very similar to that shown in 
the aforesaid Pat. No. 3,809,481, but with the improvement of a 
beamsplitter 10 having an area and thickness reduced to conform only with 
optical flatness considerations. That patent discloses a Fourier 
interference spectrometer of the double-pass retroreflector type in which 
a single mirror is employed in the path of both split beams of an incoming 
beam to cause them to double back through separate retroreflectors. 
Changes in optical path length are achieved by linear displacement of both 
retroreflectors using a motor-driven lead screw on one for large, 
low-frequency changes, a moving-coil actuator on the other for smaller, 
mid-frequency changes and a piezoelectric actuator on one of these two for 
small, high-frequency changes. Different arrangements are disclosed for 
the beamsplitter to function as a splitter for the incoming beam, a 
"window" for one split beam and a mirror at 45.degree. 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.degree. 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 flatness. The present invention offsets the axis of 
one retroreflector from the other so that while both receive split beams 
in a common plane, one retroreflects in a plane offset in one direction 
while the other retroreflects 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, 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.sub.1 and 
A.sub.2 illustrated schematically in FIG. 6. The first beam A.sub.1 is 
directed to a mirror 12 which reflects the beam into a cat's-eye 
retroreflector 13. This "corner" mirror is useful not only to so fold the 
path of the beam A.sub.1 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.sub.1. However, 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.sub.1 enters the retroreflector at one (lower) level, and exits 
at another (upper) level as a beam B.sub.1 in the manner shown in FIG. 6. 
The beam B.sub.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 mirror 15 which has a reflective coating on an upper area 15b to 
reflect the beam B.sub.1, thereby to cause it to double back through the 
central area 10b of the beamsplitter and through the transparent central 
area 15a of the mirror 15. The beam A.sub.1 which has thus doubled back is 
passed to a photodetector 16. That beam is to be combined with the beam 
A.sub.2 similarly doubled back, but instead of doubling back through an 
optical path over the path for the beam A.sub.2, as in the case of the 
beam A.sub.1, that beam doubles back through a path under the path for the 
beam A.sub.2, as shown in FIG. 6. 
The beamsplitter 10 shown at a 45.degree. angle for convenience may be 
placed at some other angle for optical considerations. It transmits 50% of 
the input beam from the source 11 to the cat's-eye retroreflector 14. That 
is accomplished by the central area 10b of the beamsplitter as shown in 
FIG. 2. The retroreflector 14 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 B.sub.2 to be 
reflected from the lower area 10c of the beamsplitter 10 shown in FIG. 2, 
and onto the lower area 15c of the mirror 15. From there it doubles back, 
emerging from the retroreflector 14 over the path of the beam A.sub.2 to 
impinge the beamsplitter 10 in the central area 10b. There the back side 
of the beamsplitting surface reflects the beam A.sub.2, thereby to combine 
the beams A.sub.1 and A.sub.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 beamsplitter is better understood from 
FIGS. 4 and 5, which are sectional views of the retroreflectors 13 and 14 
taken along respective lines 4--4 and 5--5. FIG. 4 shows the split beam 
A.sub.1 at the bottom and the retroreflected beam B.sub.1 emerging at 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 looking to the left. Similarly, FIG. 
5 shows the split beam A.sub.2 at the top and the retroreflected beam 
B.sub.2 emerging at the bottom. This entire arrangement is then fully 
clarified by the schematic diagram of FIG. 6 referred to hereinbefore 
wherein all of the elements referred to in 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 retroreflectors, such 
that one retroreflector has its optical center line offset vertically with 
respect to the optical center line of the other, it is recognized that 
modifications and equivalents may readily occur to those skilled 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 area in the mirror 15, there may be a smaller mirror in front 
at an angle to reflect the combined beam off to one side for detection, or 
a hole, preferably a circular hole in the mirror 15. In any case the 
beamsplitter area and thickness can be reduced to conform only with 
optical flatness considerations which require a ratio of thickness to 
length of about 1 to 7, or 8. For example, a beamsplitter 4" long would 
have to be about 1/4" thick to assure optical flatness. The beamsplitting 
surface and reflective surface are on the same face of the plate. Typical 
materials for the beamsplitter are borium fluoride for wavelengths from 
visible light to about 11.5 microns and potassium bromide for wavelengths 
from visible light to about 16 microns.