 |
Claims  |
|
|
I claim:
1. Angle-sensitive interferometer apparatus for creating an interference
fringe pattern in relation to the angle of incidence of a beam of
radiation on said interferometer, comprising:
a first beam splitter which reflects a first portion of the beam and passes
therethrough a second portion of the beam, said first beam splitter having
a normal axis oriented at an angle .theta. from the incident beam thereon;
a retroreflector capable of reflecting the outgoing part of a beam parallel
to and offset from the incoming part of a beam positioned in the path of
said first beam portion;
a specularly reflecting mirror positioned in the path of said second beam
portion and having a normal axis oriented at an angle .phi. greater than
zero degrees from the incident second beam portion sufficient to prevent
feedback and oriented to reflect said second beam portion to intersect
said first beam portion outgoing from said retroreflector; and
a second beam splitter positioned at the intersection of said first and
second beam portions and having a normal axis oriented at an angle
.theta.-.phi. from said first beam portion that is incident thereon.
2. The angle-sensitive interferometer of claim 1, wherein said
retroreflector is comprised of a corner cube having three mirror surfaces
all oriented mutually perpendicular to each of the others.
3. The angle-sensitive interferometer of claim 1, wherein said
retroreflector is comprised of a convex cat's eye lens with a flat mirror
positioned on its focal point and normal to the axis of said lens.
4. Angle-sensitive interferometer apparatus for creating an interference
fringe pattern in relation to the angle of incidence of a beam of
radiation in said interferometer, comprising:
a first beam splitter which reflects a first portion of the beam and passes
therethrough a second portion of the beam, said first beam splitter having
a normal axis oriented at an angle .theta..sub.1 from the incident beam;
a second beam splitter positioned a spaced-apart distance from said first
beam splitter and not in the path of either beam portion split out by said
first beam splitter;
a first mirror in the path of said first beam portion for reflecting said
first beam portion toward said second beam splitter, said first mirror
having a normal axis oriented at an angle .theta..sub.2 from said first
beam portion;
a second mirror in the path of said second beam portion for reflecting said
second beam portion toward said second beam splitter, said second mirror
having a normal axis oriented at an angle .theta..sub.3 from said second
beam portion, the second beam splitter having a normal axis oriented at an
angle .theta..sub.4 from said second beam portion in such a manner that
.theta..sub.2 -.theta..sub.1 =.theta..sub.4 -.theta..sub.3 ; and
an angle reversing beam transmission device positioned in the path of said
first beam portion between said first mirror and said second beam
splitter.
5. The angle-sensitive interferometer of claim 4, wherein said angle
reversing transmission device includes two convex lenses separated axially
a distance equal to the sum of their respective focal lengths.
6. The angle-sensitive interferometer of claim 4, wherein said angle
reversing transmission device includes two concave mirrors separated by a
distance equal to the sum of their respective focal lengths and positioned
with their respective axes offset from each other.
7. Apparatus for detecting changes of incident angle of a primary beam of
coherent radiation, comprising:
interferometer means for splitting said primary beam into two secondary
beam portions, reversing the incident angle change of one beam portion
with respect to the other, and then recombining said secondary beam
portions to produce a fringe pattern in such a manner that no part of the
secondary coherent beam portions is directed back parallel to said primary
beam, and
radiation sensitive detector means positioned in the path of said
recombined beam for detecting asymmetric changes in intensity in the
fringe pattern.
8. The apparatus of claim 7, wherein said radiation sensitive detector
means includes a plurality of individual radiation sensitive diodes
positioned adjacent each other in the path of said recombined secondary
beam portions.
9. The apparatus of claim 7, wherein said interferometer means includes
means for creating a wedge angle interference fringe pattern upon
deviation of said primary beam from the axis of said interferometer.
10. The apparatus of claim 9, wherein said interferometer means includes a
first beam splitter in the path of said primary beam of coherent radiation
for splitting said primary beam into first and second secondary beam
portions, retroreflector means in the path of said first secondary portion
of said beam for reversing the direction of said first secondary portion
in an offset parallel relation to itself, specularly reflecting mirror
means in the path of said second secondary portion of said beam for
reflecting said second secondary portion in a direction away from the
incident second secondary beam portion to a point of intersection with
said retroreflected first secondary portion at a point removed from said
first beam splitter, and a second beam splitter positioned at said point
of intersection.
11. The apparatus of claim 10, wherein said retroreflector means includes a
corner cube reflector having three planar mirror surfaces oriented
mutually perpendicular from each other.
12. The apparatus of claim 10, wherein said retroreflector means includes a
convex lens positioned in the path of said first secondary portion with
this first secondary portion offset from the lens axis a distance greater
than the width of the beam and a mirror positioned at the focal point of
the lens to reflect the first beam portion back to the lens.
13. The apparatus of claim 7, wherein said interferometer means includes a
first beam splitter in the path of said primary beam of coherent
radiation, a first mirror positioned in said first secondary portion of
the beam to reflect this first secondary portion out of its initial path
and away from said first beam splitter, a second mirror in the path of the
second secondary portion of the beam to reflect this second secondary
portion to a point of intersection with the reflected first secondary
portion of the beam, and angle reversing optical transmitter means
positioned in one of said secondary portions for reversing any angle of
deviation of a said primary beam of coherent radiation from the
interferometer axis.
14. The apparatus of claim 13, wherein said reversing optical transmitter
means is comprised of a telescope having two convex lenses separated
axially from each other a distance equal to the sum of their respective
focal lengths.
15. The apparatus of claim 13, wherein said reversing optical transmitter
means is comprised of a telescope having two concave mirrors facing each
other in offset relation to each other in their respective axes and
separated a distance equal to the sum of their respective focal lengths.
16. Angle measuring and control apparatus, comprising:
electromagnetic radiation producing means for producing a coherent beam of
radiation;
first beam splitter means for splitting said beam into first and second
beam portions;
reversing means for reversing the incident angle change of the first beam
portion with respect to the incident angle change of the second beam
portion;
specular reflector means positioned for directing the second beam portion
incident thereon to a point of intersection with said angle-reversed first
beam portion in a direction away from the incident beam portion to prevent
feedback and at a position removed from said first beam splitter; and
second beam splitter means positioned at said point of intersection of said
reflected first and second beam portions for recombining at least parts of
said first and second beam portions.
17. The angle measuring and control apparatus of claim 16, wherein said
second beam splitter means and said specular reflector means are oriented
in such a manner that an incident angle change .delta..theta. in said beam
results in a wedge angle of 2.delta..theta. between said recombined parts
of said first and second beam portions.
18. The angle measuring and control apparatus of claim 17, wherein said
reversing means is an optical retroreflector.
19. The angle measuring and control apparatus of claim 18, wherein said
optical retroreflector is a corner cube having three planar mirror
surfaces, each of which is oriented mutually perpendicular to the other
two mirror surfaces.
20. The angle measuring and control apparatus of claim 18, wherein said
optical retroreflector is a cat's eye, a lens with a mirror positioned at
its focal point.
21. The angle measuring and control apparatus of claim 18, including
detector means in the path of said recombined parts of said first beam and
second beam portions for detecting asymmetry in the intensity of the
interference fringe pattern and outputting signal data corresponding to
the magnitude of such asymmetry.
22. The angle measuring and control apparatus of claim 18, wherein said
first beam splitter means is comprised of a partially reflective mirror
having a normal axis at an angle .theta. from the beam, said first
reflector means is for retroreflecting said first beam portion in an
outgoing manner away therefrom in parallel relation to said incoming first
beam portion and in spaced apart relation thereto, said second reflector
means is a specularly reflecting mirror in the path of said second beam
portion oriented with its normal axis at an angle .phi. not equal to zero
from said second beam portion, and said second beam splitter means is a
second partially reflective mirror positioned in the paths of both said
reflected first beam portion and said reflected second beam portion, with
the normal axis of said second mirror at an angle of .theta.-.phi. from
said reflected first beam portion incident thereon.
23. The angle measuring and control apparatus of claim 22, including
detector means positioned in the path of said recombined beam parts, said
detector means being comprised of four radiation sensitive diodes
positioned adjacent each other in the form of quadrants of a circle.
24. The angle measuring and control apparatus of claim 23, including angle
measurement instrumentation means connected to said diodes for converting
output signals from said diodes to units of angle measurement.
25. The angle measuring and control apparatus of claim 23, including
servo-mirror means positioned between said radiation producing means and
said first beam splitter for reflecting and directing the beam produced by
said radiation producing means, third beam splitting means positioned
between said servo-mirror means and said first beam splitter means for
splitting said beam; and servo-electronic means connected to said diodes
and to said servo-mirror means for activating said servo-mirror means to
rotate in response to an asymmetric fringe pattern intensity change on
said diodes.
26. The angle measuring and control apparatus of claim 23, including
servo-mirror means positioned between said radiation producing means and
said first beam splitter for reflecting and directing the beam produced by
said radiation producing means toward said first beam splitter, and
servo-electronics means connected to said diodes and to said servo-mirror
for activating said servo-mirror to rotate in response to an asymmetric
fringe pattern intensity change on said diodes.
27. The angle measuring and control apparatus of claim 23, including
servo-beam deflector means positioned between said radiation producing
means and said first beam splitter for deflecting the beam produced by
said radiation producing means, third beam splitter means positioned
between said servo-beam deflector means and said first beam splitter means
for splitting said beam, master beam deflector means positioned in the
beam path between said first and third beam splitter means for deflecting
the portion of said beam between the first and third beam splitter means,
and servo-electronic means connected to said diodes and to said
servo-deflector means for activating said servo-deflector means in reponse
to an asymmetric fringe pattern intensity change on said diodes.
28. The angle measuring and control apparatus of claim 17, wherein said
specular reflector means is a first mirror, and said reversing means
includes a second mirror for reflecting said first beam portion toward
said second beam splitter and angle reversing transmission means for
outputting a beam that enters therein at an angle variation .delta..theta.
from a normal incoming beam in a reverse angle -.delta..theta. from a
normal outgoing beam.
29. The angle measuring and control apparatus of claim 28, wherein said
angle reversing transmission means includes two convex lenses separated
axially by a distance equal to the sum of their respective focal lengths.
30. The angle measuring and control apparatus of claim 28, wherein said
angle reversing transmission means includes two concave mirrors facing
each other with their axes offset from each other and separated by a
distance equal to the sum of their respective focal lengths.
31. The method of producing angle-sensitive interference fringe patterns
from a coherent beam of radiation, comprising the steps of:
dividing said beam into a first beam portion and a second beam portion of
equal intensity;
reversing the direction of the first beam portion in parallel, offset
relation to itself;
specularly reflecting said second beam portion to a point of intersection
with the reversed first beam portion in such a manner that said reflected
second beam portion is not parallel with said incident second beam portion
so there is no feedback of radiation into the coherent beam; and
recombining a part of said first beam portion with a part of said second
beam portion at said point of intersection in such a manner that an
interference fringe pattern is produced around said recombined beam parts
and in such a manner that there is no feedback of any of the beam parts
into said coherent beam.
32. The method of claim 31, including the steps of reversing the direction
of said first beam portion by positioning a corner cube having three
mirror surfaces oriented in separate planes mutually perpendicular to each
other in the path of said first beam portion in such a manner that said
first beam portion initially strikes only one of said mirror surfaces from
where it is reflected to the other mirror surfaces and ultimately reversed
in direction.
33. The method of claim 31, including the steps of reversing the direction
of said first beam portion by positioning a cat's eye lens in the path of
said first beam portion with a mirror positioned axially behind the lens
at its focal point and normal to the axis of the lens, said lens being
positioned in the path of said first beam portion with the beam offset
from said lens axis a distance greater than one-half the width of the
beam.
34. The method of producing angle-sensitive interference fringe patterns
from a coherent beam of radiation, comprising the steps of:
dividing said beam into a first beam portion and a second beam portion;
reflecting both said first and second beam portions to a point of
intersection a spaced distance from the point of dividing the beam and not
in alignment with either of the initially divided portions;
positioning an angle reversing beam transmitter in the path of said first
beam portion in front of the point of intersection with said second beam
portion; and
recombining parts of both said first and second beam portions with each
other at the point of intersection.
35. The method of claim 34, including the step of reversing the angle of
the first beam portion by passing it through two convex lenses separated
axially by a distance equal to the sum of their respective focal lengths.
36. The method of claim 34, including the step of reversing the angle of
the first beam portion by reflecting it between two concave mirrors facing
eachother in axially offset relation and separated by a distance equal to
the sum of their respective focal lengths.
37. The method of detecting an angle change in a coherent beam of
radiation, comprising the steps of:
passing the beam through the full aperture of an interferometer capable of
separating and recombining the beam in a manner that produces an
asymmetric fringe pattern intensity change in response to an angle change
in the beam with no part of the beam being reflected back parallel to the
incoming beam so that there is no feedback into the coherent beam of
radiation anywhere in the full aperture of the interferometer;
positioning a plurality of individual radiation sensitive diodes adjacent
each other in the recombined beam path for detecting asymmetric changes in
the interference fringe pattern.
38. The method of claim 37, including the steps of splitting the beam in
the interferometer and reflecting the split beam portions in such a manner
as to recombine them with the angle of change in the original incident
beam diverging apart in the recombined parts of the beam to produce a
wedge interference fringe pattern.
39. The method of claim 38, including the step of reversing the angle of
deviation in one split beam portion by passing it through a retroreflector
which reverses direction of the beam portion in parallel, offset relation
to its unreflected path.
40. The method of claim 38, including the step of reversing the angle of
deviation in one split beam portion by passing it through an angle
reversing telescope optical transmitter. |
|
 |
|
 |
Claims |
|
|
|
Description |
|
|
BACKGROUND OF THE INVENTION
This invention is related to methods and apparatus for measuring and
controlling angles between objects and, more specifically, to a method and
apparatus, including angle sensitive interferometer apparatus, for
detecting and controlling angle changes in an incident beam of coherent
radiation such as lasers, and utilizing same for measuring and controlling
angles between objects and devices.
There are many circumstances or situations in which it is desireable to
measure very accurately and even control very precisely the angles or
relative positions and orientations between two or more objects or points.
Laser beams, which are very intense, coherent beams of monochromatic
light, have been used quite successfully for accurate angle detection and
alignment application such as surveying devices and guidance systems.
However, even these highly accurate laser instruments which are based on
the nature of a laser beam to not spread out widely in space, are limited.
There is a need for even more accurate angle measuring and control
capability to detect and control more minute angle changes than possible
with present laser and other devices for many applications, including
command guidance systems for space vehicles, materials testing devices and
the like. This invention utilizes radiation interference phenomenon to
achieve such desired extremely accurage angle measuring and controlling
results.
When two or more trains or beams of coherent electromagnetic radiation,
such as laser light, microwaves, radio frequency waves, and the like,
cross each other or are superimposed on each other, the resultant wave
displacement at any point and at any instant is the sum of the
instantaneous displacement that would be produced at the point by the
individual wave trains if each was present alone. Where the respective
waves are in phase with each other, their displacements would add
resulting in increased intensity. However, where the respective waves are
out of phase with each other, they cancel resulting in decreased
intensity. Such increasing or decreasing of intensity by crossing or
superimposing trains or beams of electromagnetic radiation on each other
is referred to as interference. Where the waves are in phase and add
respective displacements to increase intensity, the interference is
constructive. where the waves are out of phase and their respective
displacements cancel to decrease intensity, the interference is
destructive.
It is known that when a beam of laser light or other coherent
electromagnetic radiation is split into two parts to produce two beams and
each of the two parts is made to travel a different path and then joined
together again, they will produce a fringe pattern of intense, bright
bands separated by less intense, dark bands. Such fringe patterns can be
observed, and the distances between the bands in the fringe pattern are
related to the wave lengths of the radiation. Thus, it has long been
recognized that the wave lengths of light can be measured by producing an
interference fringe pattern, measuring the separation of the bands in the
fringe pattern, and, through mathematical equations, calculating the wave
length of the light.
The essential feature of the formation of fringes is the division of a beam
of light by partial reflection at a surface, and the subsequent
recombination of the two disturbances or separated parts of the beam.
Apparatus for producing interference fringes in this manner are referred
to as interferometers.
There are a number of two-beam interferometers that have been developed and
used for measuring the wavelength or frequency of radiation. For example,
in the Michelson interferometer, which is well known to persons skilled in
this art, the beam is divided into two beams of approximately equal
intensity by a beam splitter. The beams are reflected at respective front
silvered mirrors and recombined at the beam splitter. Other standard
interferometer apparatus known to persons skilled in this art include the
Twyman and Green interferometer, the Jamin interferometer, and the
Mach-Zehnder interferometer. These standard known interferometers can be
designed to have very high sensitivity for wavelength or frequency
measurements. Other interferometer apparatus, such as the Koster's
interferometer and Dowell's interferometer have been designed for such
purposes as measuring slip gauges and distances very accurately. However,
all of these prior art interferometer apparatus suffer from feedback and
are severely limited in their angle sensitivity and effectiveness in angle
measuring and controlling applications.
Feedback in prior art interferometers results from a portion of the
radiation emitted from a radiation source, such as a laser, being
reflected from the interferometer directly back into the laser source.
Such beam or radiation feedback into the laser or radiation source
produces interaction between the reflected light and the laser medium
resulting in undesireable changes in frequency, intensity, and even the
angle of the laser beam. Other detrimental effects in prior art devices
can result from wavefront distortion due to physical limitations of size
and structure of reflecting devices that limit the useable portion of beam
diameter.
Another characteristic in the prior art interferometers, such as Michelson,
Mach-Zehnder, and the like, is that the superimposed output beams are
parallel even when the angle of the incoming incident beam is varied.
Therefore, the intensity distribution in the fringe pattern of a normal
Michelson interferometer or the like is described by the equation:
p.lambda.=2t cos .theta., (1)
where p is the order number of the interference fringe, .lambda. is the
wavelength of the radiation (related to the radiation frequency .gamma. by
the definition .lambda.=C/.gamma., where C is the speed of light), t is
the difference in the optical path length for the two separated beams
within the interferometer, and .theta. is the angle of incidence of the
light beam on the interferometer. The change of intensity for a small
angle .theta. is proportional to .theta..sup.2. Such a relationship does
not provide a particularly angle-sensitive interferometer.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a
novel, highly sensitive and accurate method and apparatus for measuring
and controlling angles between two or more objects or points.
It is also a general object of the present invention to provide a method
and apparatus for detecting minute angle changes in a beam of radiation.
A more specific object of the present invention is to provide a very
angle-sensitive interferometer for angle changes in an incoming or
incident beam of radiation in any plane or direction with respect to the
interferometer axis.
Still another specific object of the present invention is to provide an
interferometer that has no reflected beam feedback into the incoming
radiation beam or source even when the incoming radiation beam fills the
interferometer aperture.
A further object of the present invention is to provide an angle-sensitive
interferometer that minimizes wavefront distortion to maximize the useable
diameter of the laser or other radiation beam.
A still further object of the present invention is to provide a method and
apparatus for accurately transducing minute angle changes in an incoming
beam of radiation in any direction with respect to the axis of an
interferometer into electrical signals that can be processed and utilized
to measure or control such angle changes.
Another object of the present invention is to provide a method and
apparatus for maintaining a laser beam directed at a precise angle or
orientation.
Yet another object of the present invention is to provide a method and
apparatus for maintaining an accurate angular alignment or orientation in
any direction between two objects or points.
Still another object of the present invention is to provide an accurate
method and apparatus for inducing and controlling a deflection or scanning
laser beam through a precisely controlled angle or sector.
A still further object of the present invention is to provide a method and
apparatus in which the orientation between two or more objects or the
direction of a laser beam can be controlled by varying any one or more of
the three parameters of angle of incidence of the laser beam on the
angle-sensitive interferometer, the wavelength of the laser beam, or the
interferometer path difference between the two portions of the split beam.
The apparatus of the present invention includes an interferometer for
producing an interference fringe pattern that varies greatly in intensity
and intensity symmetry with a change in angle of the beam of radiation
incident on the interferometer. This angle sensitivity is accomplished by
splitting a coherent incoming beam into two portions of approximately
equal intensity, reversing the direction of the first beam portion in a
parallel, offset path with a corner cube retroreflector or a cat's eye
lens and mirror retroreflector, specularly reflecting and diverting the
second beam portion out of its path of alignment with the first beam
splitter to a point of intersection with the reversed first beam portion,
and recombining the first and second beam portions at the point of
intersection with another beam splitter in such a manner that
approximately equal parts of the first and second beam portions are
superimposed on each other in a fashion that will create a good
interference fringe pattern without feedback of any part of the beam
portions through the interferometer aperture or into the incoming beam.
An alternate embodiment of the invention obtains the same results by
passing the first beam portion through an angle reversing optical beam
transmitter before superimposing parts of the first and second beam
portions together to create the interference fringe pattern. One such
angle reversing transmitter is comprised of two convex lenses axially
separated by a distance equal to the sum of their respective focal
lengths. Another embodiment of a suitable angle reversing transmitter is a
pair of concave mirrors separated by a distance equal to their respective
focal lengths.
The invention also includes the use of a quadrant diode detector comprised
of four separate radiation sensitive diodes in combination with the
angle-sensitive interferometer for detecting asymmetrical variations in
fringe pattern intensity indicative of an angle change of the incident
radiation beam. This apparatus is used in several different embodiments
for measuring incident beam angle variations, stabilizing radiation beams
on a target, maintaining precise angular alignment of a laser or radiation
beam or orientation between two bodies, and remote or master controls for
a servo-radiation beam deflector to sweep or scan the beam over precise
sectors or areas.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the present invention will become
more apparent as the description proceeds, taken in conjunction with the
following drawings, in which:
FIG. 1 is a diagramatic view of the angle-sensitive interferometer
apparatus of the present invention;
FIG. 2 is a front elevation view of a corner cube retroreflector device
utilized in the angle-sensitive interferometer of the present invention;
FIG. 3 is a side elevation view of a corner cube retroreflector device
utilized in the angle-sensitive interferometer of the present invention;
FIG. 4 is a front elevation view of the quadrant diode utilized with the
angle-sensitive interferometer of the present invention with an
interference fringe pattern thereon produced by an incident beam normal to
the interferometer;
FIG. 5 is a front elevation view of the quadrant diode with an interference
fringe pattern thereon produced by an incident beam at an angle to normal
coming into the interferometer;
FIG. 6 is a side elevation view of the alternate cat's eye retroreflector
utilized in the angle-sensitive interferometer of the present invention;
FIG. 7 is a diagramatic view of an alternate embodiment of the
angle-sensitive interferometer of the present invention utilizing optical
transmission angle reverser apparatus rather than a retroreflector;
FIG. 8 is a side elevation view of one embodiment of the optical
transmission angle reverser apparatus utilized in the alternate embodiment
of the angle-sensitive interferometer shown in FIG. 7;
FIG. 9 is a side elevation view of a second embodiment of the optical
transmission angle reverser apparatus utilized in the alternate embodiment
angle-sensitive interferometer shown in FIG. 7;
FIG. 10 is a schematic diagram of an accurate angle measuring method and
apparatus according to the present invention;
FIG. 11 is a schematic diagram of an accurate servo control method and
apparatus for maintaining a precise orientation of a laser beam according
to the present invention;
FIG. 12 is a schematic diagram of an accurate method and apparatus for
precision control of the angular orientation of two objects in relation to
each other; and
FIG. 13 is a schematic diagram of an accurate method and apparatus for
controlling a scanning laser beam through a precise sector.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The angle-sensitive interferometer 10, which is the essential component of
the method and apparatus of this invention is shown in FIG. 1. It utilizes
a source of coherent plane wave electromagnetic radiation source, such as
a laser 12, although it can also use a microwave or radio frequency
transmitting antenna as well. For convenience, this description will refer
to a laser radiation source, since it is preferred and is very practical
and accurate in most applications.
The laser 12 projects an incoming beam 80 to the interferometer 10. The
incoming beam 80 strikes a beam splitter 14, such as a half-silvered
mirror, which divides the incoming beam 80 into two beams 82, 90. The
incoming beam 80 strikes the beam splitter mirror 14 at an incident angle
.theta. to an axis 16 normal to the mirror 14 so that the reflected beam
90 leaves the mirror 14 at an angle .theta. from the normal axis 16.
The reflected beam 90 then strikes a retroreflector apparatus R which
reflects a beam back in the opposite direction parallel to the reflected
beam 90 a spaced distance laterally from beam 90. For example, if there is
a distance X between beam 90 and a line of symmetry 22 of the
retroreflector device R, the emerging beam 92 is a distance X on the
opposite side of the line of symmetry 22 and directed in the opposite
direction. Regardless of what angle the beam 90 strikes the retroreflector
R, the emerging beam 92 will be parallel to beam 90, and the two beams 90,
92 will be equal distances X on opposite sides of the line of symmetry 22.
The non-reflected portion 82 of beam 80 emerging from the beam splitter 14
continues in a straight line beyond the line of beam 92 to a mirror 40.
This mirror 40 is tilted so that its normal axis 42 is at an angle .phi.
with beam 82. Consequently, beam 82 is reflected in a beam 84 at an angle
2.phi. from beam 82.
A second beam splitter 44 is positioned at the intersection of beam 84 with
beam 92 at an angle such that its normal axis 46 is at an angle
.theta.-.phi. from beam 92. This beam splitter 44 splits beam 92 into two
parts, and it splits beam 84 into two parts. The transmitted part of beam
92 is beam 94, and the reflected part of beam 92 is beam 88. The
transmitted part of beam 84 is beam 86, and the reflected part of beam 84
is beam 96. As can be seen in FIG. 1, the beams 94, 96 are recombined at
beam splitter 44. Consequently, these recombined beams 94, 96 interfere
with each other and create an interference fringe pattern on plane P. The
retroreflector R can be made adjustable in a direction parallel to beams
90, 92 so that the path distance of beams 90, 92 can be adjusted in order
to adjust the path differences of beams 94, 96 to produce the desired
optimum intensity fringe pattern at plane P.
It is significant to note that beams 86, 88, which are also recombined to
form a beam W, continue from beam splitter 44 at the angle 2.phi. from
beams 80, 82. Therefore, they miss and do not feed back into beam 80 or
laser 12. Consequently, the inaccuracies produced from feedback in prior
art interferometers are avoided in this interferometer according to this
invention.
It is also significant that this interferometer 10 of the present invention
produces a wedge angle between the output beams 94, 96 when there is a
change in the incident angle of input beam 80. Such an angle change
.delta..theta. is illustrated in FIG. 1 with the beams at the new angle
shown in broken lines with corresponding beam designation numerals in
prime for clarity. These beams are illustrated in what appears to be
relatively large angles, also for clarity, but it should be understood
that these actual angle changes are so small in use, and this
interferometer is so angle-sensitive, that the actual beam offset is
negligible compared to the actual diameters of the light beams.
As illustrated in FIG. 1, when there is an angle change .delta..theta. of
the input beam 80' from the original input beam 80, this same angle change
.delta..theta. occurs in the beams 90', 92', 82', 84', 86', 88', 94', and
96'. However, it is significant to note that the beams 94', 96' diverge in
opposite directions from the original beam paths 94, 96. Consequently,
there is a wedge angle of 2.delta..theta. produced between the output
beams 94', 96', when the input beam 80' varies an angle of .delta..theta.
from the original input beam 80. This wedge angle 2.delta..theta. produces
the high angle sensitivity of this interferometer.
As illustrated in FIG. 1, in order to obtain this wedge angle
2.delta..theta. between the wave fronts of the two recombined beam
portions 94', 96' according to this invention, the two split portions 82',
90' of the incoming beam 80' have to be recombined after one split portion
90' undergoes an angle .delta..theta. reversal with respect to the other
split portion 82'. In the preferred embodiment of FIG. 1, the split beam
portion 90' is put through such an angle .delta..theta. reversal with
respect to the split beam portion 82' by the retroreflector 30. The
resulting angle reversed beam portions 92' is produced. However, as also
shown in FIG. 1, the split beam portion 82' is not retroreflected, thus it
does not undergo an angle reversal. On the contrary, split beam portion
82' is specularly reflected by plane mirror 40 to produce beam portion 84'
which has no angle .delta..theta. reversal. Therefore, when non-angle
reversed beam portion 84' is recombined with angle reversed beam portion
92' at the second beam splitter 44, the resulting wedge angle osf
2.delta..theta. is produced between the wave fronts of recombined beam
portions 94' and 96'.
The two output beams 94', 96' superimposed at the detector plane P produce
wedge type interference fringes. The intensity distribution I of the
fringes for a small angle change .delta..theta. in the incoming beam 80'
is described in the ratio:
##EQU1##
where t is the difference in optical path length of the two beams within
the interferometer, .lambda. is the wavelength of the radiation, and Z is
the distance from the center of the fringe pattern measured in the
direction of the angle change .delta..theta..
When the angle of the input beam 80 into the interferometer is zero, the
intensity distribution of the fringe pattern is symmetric around the
center of the laser beam. This symmetric distribution of the fringe
pattern on plane P is illustrated in FIG. 4. The plane P for the purposes
of angle detection, measurement, and control can be comprised of a
quadrant diode 50. The quadrant diode 50 is comprised of four separate
light sensitive diode detectors 52, 54, 56, and 58 shaped and set together
as the quadrants of a circle in a frame 51. These diodes produce an
electrical current which is proportional to the intensity of light
impinging on their surfaces. The type of electronic measuring and control
circuits required would, of course, depend on the kind of light detectors
used. However, specific light detectors and electronic circuits, which can
be obtained or made as necessary by persons skilled in that art, are not
considered to be part of this invention.
The interference fringe pattern 60 around the center laser beam 61 on the
quadrant diode 50, as shown in FIG. 4, is symmetric as would be produced
by an input beam 80 incident at a zero angle on the interferometer. The
electrical output of each diode 52, 54, 56, and 58 is approximately equal.
When the angle of the input beam 80' is varied by .delta..theta., the
intensity I of the fringe pattern becomes asymmetrical, as illustrated in
FIG. 5. The asymmetry produced by this angle change .delta..theta. is in
intensity distribution of the fringe pattern rather than its shape.
Therefore, as shown in FIG. 5, an input beam 80' angle change of
.delta..theta. could produce a more intense fringe pattern 64 on diodes
56, 58, with a less intense fringe pattern 62 on diodes 52, 54. Thus, the
electrical outputs or signals from diodes 52, 54, 56, 58 would vary in
proportion to the fringe intensity on each diode. Of course, a change of
angle of the input beam in a different direction would result in a
different asymmetry and different asymmetrical diode output signals
corresponding to the asymmetry of the fringe pattern intensity. The
difference between integrals over the two halves of the quadrant diode 50
having a height 2B gives the detected signal S as a ratio to the maximum
100% signal S max, as follows:
##EQU2##
for small .delta..theta..
As discussed above, the conditions for a normal Michelson interferometer to
go from zero to maximum intensity as .delta..theta. is increased from zero
are given by:
t.delta..theta..sup.2 =.lambda., (5)
as can be seen from equation (1).
Such conditions for the angle-sensitive interferometer according to this
invention are given by:
B.delta..theta.=.lambda./2, (6)
as can be seen from equation (3).
Therefore, it can be appreciated that the dependence on
.delta..theta..sup.2 for the Michelson interferometer and .delta..theta.
for the interferometer of this invention demonstrates the extremely
superior angle sensitivity of the interferometer of this invention. For
comparison of angle sensitivities, note that for the normal Michelson
interferometer, .delta..theta. for maximum signal depends on the path
difference t of the two arms or beams, and the wavelength .lambda..
However, for the angle-sensitive interferometer of this invention,
.delta..theta. for maximum signal is dependent only on the detector
dimension B, or the laser beam diameter, whichever is smaller, and
wavelength .lambda.. For example, for a quadrant diode detector 50 having
a dimension B=1 cm and for red laser radiation, 100 percent signal
intensity is obtained for .delta..theta..apprxeq.3.times.10.sup.-5
radians. However, in order to obtain a 100 percent signal intensity for
such a .delta..theta. angle change of 3.times.10.sup.-5 radians with a
normal Michelson interferometer, the Michelson interferometer would have
to have a path difference t nearly a kilometer long.
Again, as discussed above, such theoretical angle sensitivity is virtually
useless for most significant applications if it is destroyed by feedback
into the incoming or incident laser beam 80 or 80'. If any portion of the
split beam was reflected or directed back into the source beam 80 or 80',
interaction would occur between the reflected beam and the laser medium.
Such interaction would cause detrimental changes in frequency, intensity,
and even in the angle of the laser beam, thus destroying the accuracy and
angle sensitivity of this interferometer. It is essential to accomplish
the desired benefits of this invention that such feedback be avoided. Such
feedback cannot be prevented by merely having a beam splitter, such as the
beam splitter 14, in the path of the reflected beam portion of the beam
incident thereon.
As shown in FIG. 1, and as discussed briefly above, feedback is avoided in
this invention by orienting the specular mirror 40 at an angle .phi.
greater than zero to the incident beam portion 82, thereby reflecting beam
portion 84, 84' out of the path of the incident beam portion 82, 82'. In
this manner, there cannot be any feedback into the source beam 80, 80'. On
the contrary, as shown in FIG. 1, the wavefront W of the reflected beams
86, 86', 88, 88' is directed in a different direction to completely avoid
the source laser beam 80, 80'.
In order to produce a fringe pattern, however, the retroreflected beam
portion 92, 92' and the specularly reflected beam portion 84, 84' must be
recombined in parallel. Such parallel recombination according to the
preferred embodiment illustrated in FIG. 1 is accomplished with the second
beam splitter 44 oriented with its normal axis at an angle .theta.-.phi.
from the retroreflected beam portion 92 as described above.
The retroreflector R of the angle-sensitive interferometer shown in FIG. 1
can be a corner cube 30, such as the one shown in FIGS. 2 and 3. The
corner cube 30 is comprised of three mirror surfaces 32, 34, 36, each of
which is mutually perpendicular to the other two mirror surfaces. An
incoming beam 90 that strikes one mirror surface, such as 34, as
illustrated in FIG. 2, will be reflected first to an adjacent mirror
surface 32, as shown at 39, and then to the third mirror surface 36, as
shown at 38. The third mirror surface then reflects the beam back out at
92 parallel to the incoming beam 90 an equal distance X on the opposite
side of the line of symmetry 22. As shown in FIG. 3, the corner cube 30
will always even reflect a beam 90' deviated an angle .delta..theta. from
original beam 90 back outwardly at 92' parallel to incoming beam 90' and
at the same angle deviation .delta..theta. from original reflected beam
92.
The use of a corner cube 30 for the retroreflector R makes the
interferometer of the present invention achromatic, i.e., it is useable
for any wave length without adjustment. It also gives no distortion of the
light beam when it is positioned so that the beam does not intersect the
joints between the mirror surfaces.
An alternate retroreflector R embodiment 70 having certain advantages is
shown in FIG. 6. This embodiment 70, a cat's eye, is comprised of a small
convex lens 72 with a small, light-weight mirror 74 positioned at the
focal point 76 of the lens 72 and normal to the line of symmetry 77 of the
lens 72. An incoming beam 90 is incident on one side 75 of the lens 72
where it is focused at 78 to the focal point 76 on the mirror 74. The
reflected beam 79 is then directed to the opposite side 73 of lens 72,
where it is projected outwardly at 92 parallel to incoming beam 90. As
with the corner cube 30 described above, this cat's eye retroreflector
embodiment 70 also reflects an incoming beam 90' at an angle
.delta..theta. to original beam 90 back outwardly at 92', parallel to
incoming beam 90'.
This cat's eye retroreflector 70 makes it possible to quickly change the
beam path difference in the interferometer by translating the mirror 74
along the axis 77 of the cat's eye lens 72. It should be noted that the
beam path difference can also be varied easily and quickly by using an
electro-optical crystal (not shown) in one of the beam paths.
From the discussion above of the characteristics of the corner cube and
cat's eye retroreflectors illustrated in FIGS. 2, 3, and 6, and as shown
in FIG. 1, the use of a retroreflector 30 for angle .delta..theta.
reversal in combination with a specularly reflecting mirror 40 and a
properly oriented second beam splitter 44, parallel beam portion
recombination w | | |
