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Description  |
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BACKGROUND OF THE INVENTION
The field of the invention is in the interferometer art, and more
particularly, that of an interferometer-type device for measuring the
velocity of a moving reflective surface through the Doppler shifts of
reflected laser beams.
Velocity measuring devices utilizing reflected laser beams are known.
Leslie E. Drain in his paper, "Doppler Velocimetry," in Laser Focus for
October 1980, pages 68-73, discusses various types of laser
interferometers including heterodyne, fringe crossing, and differential
Doppler concepts. U.S. Pat. No. 3,432,237 entitled VELOCITY MEASURING
DEVICE, to patentees Flower and Gamertsfelder, discloses a velocity
measuring device utilizing the signal generated by the passing of
successive reflected lobes of a laser beam by an aperture. The paper
"Laser Velocimeter" published in IEEE Conference Record of 1969 Fourth
Annual Meeting of the IEEE Industry and General Applications Group, Oct.
12-16, 1969, at pages 307-314, by R. A. Flower, and the paper "A Laser
Velocimeter" by Gus Stavis, published in TNB Volume 8, No. 4, 1965, by GPL
Division of General Precision Incorporated, further discusses velocimeters
utilizing the passing of side lobes of reflected laser beams by an
aperture. A commercially available laser Doppler velocimeter is described
in the brochure "Laser Doppler Velocimeter" by Cambridge Physical
Sciences, available through U.K. agents: Survey and General Instrument
Company Ltd., Fircroft Way, Edenbridge, Kent, U.K. Further background
information on optical heterodyning may be found in the paper "The Antenna
Properties of Optical Heterodyne Receivers" by A. E. Siegman, published in
Proceedings of the IEEE for October 1966, at pages 1350-1356.
SUMMARY OF THE INVENTION
The invention is for a new combination of elements providing a novel device
for measuring the velocity of a surface without making any contact other
than that of a laser beam directed onto the surface. Some prior art
velocity measuring devices also utilize only a laser beam contacting the
moving surface. Many of these sense light intensity variations as a lobed
reflection pattern passes an aperture or grating. Some compare the
frequency of a reflected signal with the transmitted signal.
The present invention generates a fringe pattern formed by the interference
of a reflected signal having a positive Doppler shift with a reflected
signal having a negative Doppler shift. The frequency of the intensity
variation of the fringe pattern is a function of the velocity of the
surface onto which the laser beam is directed. In prior art devices, the
output indication is also a function of the angle of incidence of the
laser beam onto the surface. This requires very accurate knowledge of the
angle of incidence for accurate velocity measurement. The present
invention does not require a highly accurate determination of the angle of
incidence. Also, external mechanical stability, i.e., stability of the
device relative to the surface being measured, is not critical and no
longer a problem as in the prior art devices. The prior art two beam
devices, frequently termed differential Doppler devices, as discussed in
the previously stated references, require the two laser beams to intersect
at the surface being measured to produce a region of interference at the
surface. What is detected then are reflections from individual scattering
centers traversing this region.
The invention utilizes homodyne operation with single frequency lasers.
Either homodyne or heterodyne operation may be used with multi-mode lasers
wherein two different frequency probe beams are utilized. The spread in
Doppler shift with reflection angle variation is substantially eliminated
by a unique system of apparatus for collapsing the frequency spread by
spatially inverting either the positive or negative Doppler signals.
It is therefore an object of the invention to provide a laser velocimeter
comprising a laser beam directed onto the surface of which the velocity is
desired with reflections from the surface having positive Doppler shift
combined with reflections having a negative Doppler shift to provide a
fringe pattern having an intensity variation frequency responsive to the
velocity of the surface.
It is another object of the invention to provide a system for collapsing a
Doppler shift spread due to an angular spatial spread of Doppler shifted
reflections.
It is another object of the invention to provide a laser Doppler
velocimeter having a probe beam and reflected beams in separate paths in a
common plane.
It is another object of the invention to provide a laser Doppler
velocimeter that determines the sense of direction of a moving surface by
injecting a tilt to single frequency systems and detecting the shift in
the frequency of the reflected signal, and in two-frequency embodiments
detecting the variation in magnitude between the absolute frequency of the
positive Doppler shifted signal and the absolute frequency of the negative
Doppler shifted signal.
Yet another object of the invention is to provide a laser Doppler
velocimeter that produces a fringe pattern by heterodyning the separate
reflections of two different frequency probe signals from a common
multi-mode laser, one reflection having a positive Doppler shift, and the
other reflection having a negative Doppler shift, providing a fringe
pattern having an intensity variation responsive to the velocity of the
surface being measured substantially independent of any overall shifting
of the laser frequency.
Other objects and advantages of the invention will be apparent from the
following description, the accompanying drawings, and the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block, pictorial, schematic drawing of a generally preferred
embodiment of the invention;
FIG. 2 is a block schematic end view, looking in the direction of movement
of the moving surface of a typical embodiment;
FIG. 3 is a block, pictorial, schematic drawing of another embodiment of
the invention;
FIG. 4 is a pictorial schematic end view of the embodiment illustrated in
FIG. 3;
FIG. 5 is a block, pictorial, schematic drawing of another embodiment of
the invention;
FIG. 6 is a schematic drawing illustrating a typical spatial filter;
FIG. 7 is an illustrative frequency spectrum plot of a typical frequency
distribution;
FIG. 8 is a schematic illustration showing the resolving of orthogonally
polarized beams by a typical polarizer;
FIG. 9 is a partial schematic drawing illustrating an embodiment having
probe beams and receive beams in the same plane;
FIG. 10 is a pictorial schematic drawing of an embodiment having a single
laser probe beam;
FIG. 11 is an exaggerated schematic drawing illustrating Doppler frequency
spread with beam angle;
FIG. 12 is a schematic representation of collapsing a frequency spread by
placing a pentaprism in one beam;
FIG. 13 is a schematic representation of collapsing a frequency spread with
a positive lens in one leg and a negative lens in another leg of the
reflected beams;
FIG. 14 is a schematic representation illustrating the obtaining of the
direction of the moving surface by altering the angles of impingement and
reflection of the laser beam on a moving surface; and
FIG. 15 illustrates the frequency spectrum change by the tilt illustrated
in FIG. 14.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, it is desired to measure the velocity of surface 21.
The surface characteristics are not critical to the operation of the
invention. The surface may be a flat sheet, or a curved wire, it may
provide a specular or diffused reflection, it may be highly reflective or
of relatively low reflectivity, the velocity may be relatively constant or
rapidly varying, and the invention will provide an indication of the
instantaneous velocity 22 and the direction of motion. Of course, the
greatest usable reflections requiring the least amount of filtering and
amplification are from surfaces providing an intense diffuse reflection.
Multi-mode laser 23 provides a laser beam 24 comprising a first frequency
25 and a second frequency 26 orthogonally polarized to the first. (A
typical, and suitable, laser is a Helium-Neon laser having a cavity
approximately 33.3 centimeters in length providing a primary mode of
illumination of approximately 6328 angstroms [i.e., a wavelength of 632.8
Nanometers and a frequency of 4.74.times.10.sup.14 Hz] and an orthogonally
polarized beam having a stable intermode frequency difference of
approximately 450 MHz.) Beam 24 enters a conventional polarizing beam
splitter 27, such as a Spectra-Physics type 515, which effectively splits
the orthogonal components of beam 24 into beam 28 having frequency and
polarization 25 and beam 29 having frequency and polarization 26.
These beams 28 and 29 are reflected, respectively, by conventional, flat,
adjustable mirrors 30 and 31 angled so as to cause the beams to impinge on
the moving surface 21 at spot locations 32 and 33, respectively. The spot
separation is not critical and separation is not a requirement, but it is
generally desirable to reduce forward scatter from the probe beams 34 and
35 from entering the reflection beams. Spot separations of approximately
one millimeter or slightly less are generally suitable. The direction of
spot separation is not critical. In the embodiments illustrated, the
separation is shown to be in a generally transverse direction to the
direction of motion. The separation may just as well be longitudinal with
the direction of motion, or at any other angle. The spot separation is
typically made quite small relative to the other parameters involved so
that direction of separation is thus inconsequential.
Reflections 36 and 37 from the probe beams 34 and 35, respectively, from a
surface providing specular reflection will comprise substantially light of
the same polarization as the respective probe beams. Surfaces that produce
a diffused reflection will substantially be de-polarized to the extent of
containing all polarizations with accordingly less intensity of light
having the desired polarization. The desired paths of the reflected beams
36 and 37 are substantially, but not exactly, along the paths of the
respective probe beams 34 and 35. (Other reflections not along these paths
36 and 37 are not utilized.) Conventional spatial filters 38 and 39 are
adjusted to pass only the reflections from the desired respective
illumination spots 32 and 33. This further aids in eliminating forward
scatter from the opposing probe beam.
The primary function of the spatial filters is to correct for phase
distortion in the reflected wave. It is desirable that the wave front at
the observation plane 40 of the photodetector 41 be smoothly varying and
preferably flat. These are separate and independent parameters from the
Doppler-shifted frequency of the reflected beam which is to be sensed. The
operation of a typical spatial filter is schematically diagrammed in FIG.
6. Typical returning wave front 601 has been greatly distorted in phase.
When this wave 601 is brought to a focus by lens 602, it will consist of a
central maximum plus energy in the side lobes due to diffraction and due
to the phase distortion present, as schematically illustrated 603. When an
aperture stop 604 is placed at the focus of the lens and a second lens 605
is positioned behind the aperture stop a focal distance away, then a plane
wave 606 with greatly reduced phase distortion will emerge. To condition
laser beams in the laboratory is a typical common usage of spatial
filters. A typical, and suitable, commercially available spatial filter is
the Model 900 manufactured by the Newport Research Corporation, Fountain
Valley, Calif.
Spatially filtered reflected beam 42 has a major component of polarization
similar to probe beam 28. This component, not being in the plane of the
polarization element in the polarizing beam splitter 27, passes
substantially straight through. Spatially filtered reflected beam 43 has a
major component of polarization similar to probe beam 29. This component,
being in the plane of the polarizing element of the polarizing beam
splitter 27, is reflected so that a beam 44 emerges the polarizing beam
splitter having orthogonal components of polarization. (Reflections in the
return beams 42 and 43 having polarizations orthogonal to that desired are
passed out of the beam splitter and are not used as indicated by the
dotted arrow.) These polarization components of composite beam 44 are
represented in FIG. 8 by vectors 801 and 802. A linear polarizer 55 is
employed to permit the two orthogonal components, 801 and 802, comprising
beam 44 to mutually interfere. This is done by resolving each component
into two subcomponents 803, 803a and 804, 804a, parallel and perpendicular
to the polarizer axis respectively. The parallel subcomponent 803 and 804
for each component is then passed and permitted to interfere at the
detector.
In the embodiment illustrated in FIG. 1, the beam 45 has intensity
amplitude variations determined by the frequency difference between the
two component beams. Thus, in this particular embodiment being described
in detail in which a multi-mode laser 23 having a intermode difference
frequency of 450 MHz is used, the frequency of the amplitude variations of
the intensity of beam 45 is 450 MHz for zero velocity 22 of the surface
21, i.e., when the surface is stationary. For surfaces moving as indicated
by the velocity vector 22, reflected beam 36 has a positive Doppler
frequency shift, while beam 37 acquires a negative Doppler frequency
shift. For reverse directions of motion of surface 21, beam 37 acquires a
positive Doppler and beam 36 a negative Doppler frequency shift. The
invention combines these two beams interferometrically to provide a beam
46 focussed by lens 47 on photosensitive element 40 of detector 41. (If
photodetector 41 contains a focussing lens, separate lens 47 is not
needed.)
The electrical output of the conventional photodetector 41 is selectively
filtered by conventional band pass filter 48, frequency counted by
conventional counter 49, direction indicated by left-right direction
indicator 50, appropriately scaled by conventional scaling amplifier 51,
and the resultant displayed in units of length per unit of time by display
52. The electrical signal from scaler amplifier 51 may be conventionally
used as a control signal in conventional circuits to actuate drive motors
for regulating the surface velocity.
It is well known that positive and negative Doppler frequency shifts are
not reciprocal in magnitude. That is, for example, the shift in frequency
of an approaching train whistle is different in magnitude as well as sign
from the perceived pitch change as the train is departing the observer
(the observer being stationary and the train moving at constant velocity
while emitting a steady frequency whistle). Thus, it can readily be shown
that the change in frequency of the wave in reflected beam 36 is increased
over that of probe beam 34 by the velocity of light plus the effective
velocity component of the surface all divided by the velocity of light
minus the effective velocity component of the surface, when the movement
of the surface is approaching the source as shown. Expressed
mathematically, the frequency of the reflected beam f.sub.r is:
##EQU1##
in the approaching instance, where f.sub.p is the frequency of the probe
beam, V is the velocity of light and v is the velocity of the surface, and
.theta. is the angle between the light beam and the surface in the plane
of the velocity. Likewise, when the surface is receding from the beam, the
reflected beam f.sub.r is:
##EQU2##
It can now be seen that when the two probe beams are of different
frequencies, that is, i.e., f.sub.p.sbsb.1 and f.sub.p.sbsb.2, and the
received return beams are heterodyned, that for zero velocity, 22, of the
surface 21, the beat signal or frequency of the intensity variation on the
photodetector 41 will be the same as the frequency of the mode separation
in the laser 23. In the specific embodiment being discussed in detail, it
is 450 MHz. Considering the fact that the Doppler principle decrees that
the increase in frequency with closing range is greater than the decrease
in frequency with opening range, other conditions being the same, and the
fact that all normal velocities of surface movement that will be measured
are many times removed from the velocity of light, it is apparent that,
when the velocity of surface is in the direction approaching the higher
frequency probe beam, that the output signal, that is, fringe intensity
frequency, will be greater in frequency than the mode separation, and when
the surface is receding from the higher frequency probe beam, the output
signal will be less in frequency than the mode separation frequency. In
the particular embodiment being described, with the beams making
approximately 45.degree. angles with the surface, the shift in frequency
from 450 MHz is a little greater than 4 MHz up (higher) for a surface
moving at approximately one meter per second in a direction toward the
high frequency probe beam and a little less than 4 MHz down lower in
frequency for a surface moving away from the higher frequency probe beam.
These variations are taken into consideration in the conventional
calibration of the instrument.
FIG. 2 is a pictorial schematic end view of an embodiment similar to that
of FIG. 1. The angular departure of the beams from the vertical plane is
enlarged for clarity. Separate polarizing beam splitters 70 and 71 are
shown in the probe and return receiving beams. Generally, a common beam
splitting element 27 as illustrated in FIG. 1 can be used because the
angle of separation of the beams generally is relatively small. When the
angle between the probe beams and return beams is relatively large, it is
desirable to use separate, appropriately angled polarizers as illustrated
in FIG. 2. These variables of polarizer angles of acceptance are well
known in the art, and those practicing the invention will adjust the
configurations of the apparatus accordingly.
A somewhat similar physically, but more complex operationally, embodiment
of the invention is illustrated schematically in front view in FIG. 3 and
in end view in FIG. 4. In this embodiment, laser 80 is a non-polarized,
single mode, i.e., single frequency, conventional laser. The beam 81
contains components in all directions. Conventional non-polarizing beam
splitter 82 conventionally separates out two beams 83 and 84. These beams
are of the same frequency. They are reflected by conventional adjustable
flat mirrors 87 and 88 to impinge at the common point on moving surface
90. While two slightly separated points of impingement, as described in
the previous embodiment, are generally preferred, some surface conditions
may require that a single point be used. This embodiment, schematically
illustrated in FIG. 3, may be operated either with a single point of beam
impingement as illustrated in this FIG. 3 or point separation may be used.
As in the previously described embodiment, point separation will provide
for greatly reduced or eliminated, forward scatter from the opposing probe
beam. Thus, return beam 85 will contain reflections from its respective
probe beam 83. These reflections will contain a Doppler frequency shift.
In addition, returning beam 85 will contain forward scatter from beam 84.
These forward scattered beams from opposing probe beam 84 will not contain
any Doppler shift. These same conditions apply to return beam 86 with
respect to probe beams 84 and 83.
Return beams 85 and 86 are phase corrected by spatial filters 91 and 92,
respectively. The action of these filters has been described. Beam halves
passed out of beam splitter 82 along path 100 are not used. Thus, there
are effectively four beam components in composite beam 93. One component
is the reflection of probe beam 83 having a Doppler frequency shift in
accord with the movement of surface 90, another is the forward scatter
from probe beam 84. This component has no Doppler frequency shift. Another
component is the return reflection from probe beam 84 having a Doppler
shift; and the fourth component is the forward scatter from probe beam 83
and having no Doppler shift. As in the previous embodiment, lens 95
focusses the beam onto observation plane 96 of photodetector 97.
The operation of the previous embodiment employing two separate frequencies
beating or combining to produce a difference frequency is termed
heterodyne. This (FIG. 3) embodiment having only one frequency plus the
Doppler shifts, operates on the homodyne principle. Thus, the frequency of
amplitude variations of the fringe pattern formed on the observation plane
96 produced by the beating of Doppler shifted return beams are a direct
measure of the velocity 98 of the moving surface 90. Unlike the previous
embodiment, for relatively low velocities of the moving surfaces, the beat
signal approaches zero. It can thus be seen that for very low frequencies,
homodyning of the Doppler frequency shifted return beams tends to become
unsatisfactory from a detection standpoint.
Other beat signals are present due to the forward scatter beams having no
Doppler shift being present in the beam passed to the detector. Thus,
there are the beat signals formed from the combination of the original
laser frequency (forward scatter signals) with each of the Doppler shifted
signals, i.e., the signal having a positive Doppler and the signal having
a negative Doppler frequency shift. The frequency of the intensity
variations of all of these beat signals approach zero as the velocity of
the surface being measured approaches zero. Relatively narrow pass-band
filter 99 is set to encompass the desired operating frequency range of the
received signals. This is illustrated graphically in FIG. 7 wherein the
filter pass-band has high frequency cut-off 110 and low frequency cut-off
111. Beat signal 112 from the Doppler shifts of two probe beams
representing the surface velocity is maintained within the filter range by
conventional feedback control circuitry. The beat signal 113 from one
Doppler shift and the original probe frequency is outside this pass-band.
Directivity of motion with this embodiment may be obtained as later
described.
The use of beam polarization aids in separation of the desired return beam
as does a slight separation in the impingement spots of the two probe
beams. This separation is desirable in that undesirable signals and noise
are reduced thus making the detection of the desired signal easier. An
embodiment using only a single spot has been described and shown in FIGS.
3 and 4. FIG. 5 illustrates schematically an embodiment utilizing
non-polarizing beams but with slightly separated points of probe beam
impingement. The beam 120 from conventional laser 121 is split into two
beams 122 and 123 by conventional beam splitter 119. As in the embodiment
illustrated in FIG. 1, flat mirrors 124 and 125 are adjusted to direct
these beams to slightly separated spots 126 and 127, respectively. The
reflections of each of these beams are Doppler shifted by the movement 128
of surface 129. In the illustration of FIG. 5, return beam 130 is lower in
frequency in the conventional Doppler manner, and beam 131 is raised in
frequency. These return beams are spatially filtered by filters 132 and
133 and combined by beam splitter 119 into substantially identical beams
134 and 135. Beam 135 is not used in the embodiment illustrated. Lens 136
focusses beam 134 onto the photosensitive element 137 of photodetector
138. The beat signal associated with beam 134 is formed by homodyne action
and with zero velocity will produce a zero beat signal.
An embodiment as otherwise illustrated in FIG. 5 but having a multi-mode
laser 121 will provide both homodyne and heterodyne operation. The
multiplicity of beats produced provides a wide range of control
frequencies.
In the embodiments previously described, the return beam paths have been
angled slightly with respect to the probe beam to prevent back scatter
from the probe beam from masking the reception of the return beams. That
is, the paths of the probe beam and the return beams have not been in the
same plane. This is clearly shown in the views of FIGS. 2 and 4 in
addition to the other illustrations. FIG. 9 schematically illustrates an
embodiment in which the probe beams and the return beams are in the same
plane yet separate beam paths are maintained. In this embodiment, separate
mirrors 150 and 151 are used for the probe beams from those 152 and 153
used for the return receive beams. Beam splitter 154 functions in a
conventional manner as previously described, splitting the laser beam into
two beams and combining two return beams into a single beam. Detector 155
contains a focussing lens, as previously described. Aperture stop 156 is
used to exclude stray light intensities. Such a stop may also be used when
desired with the previously described embodiments. This single plane
configuration may be used with all of the previously described dual probe
beam embodiments.
From the foregoing descriptions, it is apparent that a plurality of dual
probe beam laser velocimeters may be constructed of varying complexity and
operational characteristics. For instance, in the two frequency types
providing heterodyne operation, all combinations of polarized and
non-polarized, one or two spot combinations may be utilized. Likewise, in
the single frequency types, the same combinations are available. Also,
while generally not as convenient in the two plane embodiments, separate
mirrors may be used for a probe beam and for its reflected beam.
The invention is not restricted to dual probe beams. FIG. 10 schematically
illustrates an embodiment wherein a single probe beam 200 from
conventional single frequency laser 201 impinges substantially
perpendicularly upon surface 202 providing relatively intense scattered
radiation 203. Spatial filters 204 and 205 cooperate with flat mirrors 206
and 207 to select and direct reflected beams 208 and 209, respectively,
upon the beam splitting surface element in beam splitter 210, which in
this instance functions to combine beams rather than to divide them. Beam
211, combined from spatially filtered beams 208 and 209, produces a
modulated intensity on the photodetector 212. (Half the combined beam
energy 213 passes out the other surface of the beam splitter unused.) In
this embodiment, it is to be noted that, unlike the foregoing described
embodiments, there is no Doppler effect applied to the probe beam; only
the reflected beams have a Doppler frequency shift. For surface 202 moving
in the direction 214, the frequency of reflected beam 208 is increased in
accordance with the expression
##EQU3##
where f.sub.r is the frequency of the reflected beam 208, f.sub.p is the
frequency of probe beam 200, V is the velocity of light, v is the velocity
214 of surface 202, and .theta. is the angle between the direction of
motion of the surface in the plane of the reflected beam (normally
45.degree.). Likewise, the frequency of reflected beam 209 is lowered as
indicated by the expression
##EQU4##
The reflections from an impinging probe beam, whether the probe beam be
perpendicular as illustrated in FIG. 10, or at an angle with a surface as
in other embodiments, will be scattered as indicated at 203 in FIG. 10.
The degree of Doppler frequency shift possessed by the reflected beams is
a function of their angle with the surface as previously set forth.
Considering only the reflected beam, and greatly overemphasizing the
spread of the beams utilized, the situation is obtained as illustrated in
FIG. 11. FIG. 11 is not only applicable to the embodiment illustrated in
FIG. 10, but also to the reflected beams of all other embodiments.
Previously, in discussing the operation of embodiments of the invention,
return reflections have been spoken of as a single beam while actually the
reflection is a spread of illumination within the acceptance limits of the
elements utilized. This spread is illustrated, overemphasized, in FIG. 11.
Considering 254 as the median beam with median Doppler shift
.omega..sub.o, the beam on which operation of the device is based, it is
readily apparent that the beam set 250 having positive Doppler shift (for
direction of movement 251 of surface 252), that the lower beam 253 will
have a greater positive Doppler shift than the median beam 254, and the
beam 255 will have less positive Doppler shift that the median beam. The
same situation holds for the set of beams 256 having negative Doppler
shift, i.e., beam 257 will be decreased more in frequency than median beam
258. When these two sets of return beams are combined in the beam splitter
259 (acting as a beam combiner), it can be seen that beam 253 having the
greatest positive Doppler shift is combined with beam 257 having the
greatest negative Doppler shift. Likewise, the beam having the least
positive Doppler shift is combined with the beam having the least negative
Doppler shift. Thus, the magnitude of Doppler shift is greater in beam
261, and less in beam 262 than the desired Doppler shift of beam 260. This
may frequently become appreciable and act to greatly degrade the
operational performance of the invention.
A unique way to correct this discrepancy in the reflected beam Doppler
shift magnitudes has been devised. The frequency may be collasped by
optically inverting the ordering of the beams so as to make the resultant
differential Doppler shift from all the combined beams to have the same
value as that of the median beam. This is accomplished by recognizing that
with equal spread to a first approximation .omega..sub.H +.omega..sub.L
=2.omega..sub.o. One embodiment for doing this has been a pentaprism
inserted in one set of reflected beams (either set) so as to effectively
invert the high and low value Doppler shift beams of that set. This is
schematically illustrated in FIG. 12. (For clarity, details of the system
not involved with frequency collasping are omitted in FIG. 12.)
Conventional pentaprism 270 inverts the beam set 271 such that, when the
respective beam elements of this set are combined with beam set 272 by
combining beam splitter 273, the resultant Doppler values of all beams ar | | |
