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Description  |
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FIELD OF THE INVENTION
This invention relates to systems for the measurement of ultrasonic waves,
and more particularly to interferometers of the type employing two
phase-quadrature reference beams of coherent light which interfere with a
signal beam of coherent light reflected from a spot of a displaceable
signal mirror insonified by an ultrasonic wave.
BACKGROUND OF THE INVENTION
U.S. Pat. No. 4,019,818 to D. H. R. Vilkomerson discloses a
quadrature-dual-interferometer which simultaneously employs two
phase-quadrature beams of coherent light which interfere with a signal
beam of coherent light from a spot of a displaceable signal mirror
insonified by an ultrasonic wave and which permits an output signal to be
derived which is proportional to the sum of the squares of the ultrasonic
frequency component of the interference between the signal beam and each
respective one of the reference beams. This output signal is inherently
substantially proportional to the intensity of the ultrasonic wave then
insonifying the spot of the signal mirror and independent of random phase
drift and environmental vibrations in the interferometer. Thus, in this
quadrature-dual-interferometer, by squaring and summing the signals from
the two channels, a stable signal, proportional to the square of the
displacement of the measuring mirror, is obtained.
The squaring operation destroys the phase information about the signal and
imposes circuit difficulties in that the squaring operation doubles the
frequency and cuts the dynamic range attainable. If the signal is of a
bandwidth greater than an octave, the squaring procedure introduces
distortion in the form of new components of the signal appearing due to
cross-products between spectral components of the signal. Therefore there
is a need for an improved arrangement which avoids these problems.
A preliminary search of the prior art relating to the subject matter of the
present invention reveals the following U.S. patents of interest:
Rambauske, et al., U.S. Pat. No. 3,202,052
Mezrich, et al., U.S. Pat. No. 3,969,528
Wang, U.S. Pat. No. 4,005,935
Redman, et al., U.S. Pat. No. 4,005,936
Vilkomerson, U.S. Pat. No. 4,019,818
SUMMARY OF THE INVENTION
A main object of the invention is to provide a novel and improved
quadrature-dual-interferometer which overcomes the above-mentioned
defeciencies and problems of the prior art interferometer systems.
A further object of the invention is to provide an improved
quadrature-dual-interferometer wherein the displacement amplitude of the
measuring mirror can be obtained directly and wherein compensation is made
for variations in the interferometer state, such as ambient vibrations,
drifts, and the like.
A still further object of the invention is to provide an improved
quadrature-dual-interferometer system for measuring ultrasonic waves which
employs a vibratory reference mirror producing a pilot signal of known
amplitude and phase from which correction terms can be obtained, needed to
compensate for variations in the interferometer state, such as ambient
vibrations, drifts, laser variations, variations in beam overlap,
variations in coherence, and the like.
The system of the present invention avoids the above-mentioned deficiencies
of the system disclosed in U.S. Pat. No. 4,019,818 by the addition of one
optical component, and by employing improved electronic signal-processing
circuits for directly obtaining the displacement amplitude.
The additional optical component is a "mini-wiggler", which introduces a
phase shift into the reference beam, as by moving the reference mirror
back and forth by means of a piezo-electric crystal. Such a "wiggler" is
described in U.S. Pat. No. 3,969,578, to Mezrich, et al, but in said
patent the wiggler is required to produce phase excursions corresponding
at least to more than one-half the wavelength of the laser light being
employed. In the system of the present invention the mini-wiggler is
operated to produce phase perturbations of just a few nanometers (namely,
of the order of one-thousandth of the wavelength) at a frequency at either
edge or even in the midst of the signal frequency band. The purpose of the
mini-wiggler is to produce a "pilot" signal of known amplitude and phase,
from which can be obtained the correction terms needed to compensate for
the variations in the interferometer state, i.e., the vibrations, drifts,
etc. As the signals obtained from each channel of the
quadrature-dual-interferometer are generated by changes in path-length
difference, whether these signals arise from motion of the reference
mirror or measuring mirror (pellicle) are indistinguishable.
BRIEF DESCRIPTION OF THE DRAWINGS
Further objects and advantages of the invention will become apparent from
the following description and claims, and from the accompanying drawings,
wherein:
FIG. 1 is a block diagram of an ultrasonic wave measuring system
constructed in accordance with the present invention.
FIG. 2 is a schematic block diagram showing an embodiment of a signal
processing circuit employed in an ultrasonic wave measuring system
according to the present invention.
FIG. 3 is a schematic block diagram showing another embodiment of a signal
processing circuit employed in an ultrasonic wave measuring system
according to the present invention, wherein the mini-wiggler signal is
employed as a standard to correct for laser variations and variations in
beam overlap, coherence, etc.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to the drawings, FIG. 1 schematically illustrates an ultrasonic
wave imaging system generally similar to that disclosed in U.S. Pat. No.
4,019,818 but containing improvements in accordance with the present
invention. In this system, as described in said prior U.S. patent, laser
100 generates a beam of monochromatic coherent light 102, which
illuminates beam splitter and phase quadrature reference beam derivation
means 104, which serves to split the light wave energy beam 102 into two
effectively coincident, or side-by-side, reference beams 106 and a signal
beam 108. The two reference beams 106, after reflection from a rigid
reference mirror 110 and a second passage through beam-splitter and phase
quadrature reference beam derivation means 104, form two effectively
coincident, or at least side-by-side, phase quadrature reference
components of output beam 112. Signal beam 108, under the control of
deflection optics 113, raster-scans the surface of signal mirror 114 in
both the horizontal and vertical directions. Acoustic wave derivation
propagation means 116, which includes a liquid acoustic wave propagation
medium in which signal mirror 114 is immersed, insonifies signal mirror
114 with an ultrasonic wave field. Each point of the surface of signal
mirror 114 vibrates with a displacement amplitude determined by the
intensity of the insonifying ultrasonic wave field at that point.
After reflection from signal mirror 114 and a reverse passage through
deflection optics 113 and beam splitter and phase quadrature reference
beam derivation means 104, signal beam 108 forms a signal component of
output beam 112. Output beam 112 illuminates photodetector and signal
translating means 118, which serves to derive a video signal output 120
which has a magnitude at any instant that is substantially proportional to
the intensity of ultrasonic radiation insonifying the spot on the surface
of the signal mirror being illuminated by signal beam 108 at that instant.
Video signal output 120 along with the same horizontal and vertical
deflection control signals 122 which control deflection optics 113 are
applied to a CRT display to produce a display of the ultrasonic wave field
insonifying signal mirror 114.
The detailed structure of deflection optics 113 may be substantially
identical with that shown in the above-mentioned U.S. Pat. No. 3,969,578
to Mezrich, et al, FIG. 2, and the detailed structure of acoustic wave
derivation propagation means 116 may be substantially identical with that
shown in FIGS. 3a, 3b or 3c of said U.S. Pat. No. 3,969,578. The
beam-splitter and phase quadrature reference beam derivation means may be
similar to either of the embodiments thereof described in U.S. Pat. No.
4,019,818.
As described in U.S. Pat. No. 4,019,818, a portion of reference beam 106
(identified as 106a in said patent) together with a portion of reflected
signal beam 108 form interfering signal components 112a (see FIG. 2) of
the output beam 112. Another portion (identified as 106b in said patent)
together with another portion of reflected signal beam 108 form
interfering components 112b (see FIG. 2) of the output beam 112. Output
beams 112a and 112b comprise side-by-side portions of the resultant output
beam 112 shown in FIG. 1.
U.S. Pat. No. 4,019,818 employs a stationary rigid reference mirror 110 and
a photodetector and signal translating means for providing a signal
proportional to the sum of the squares of the respective intensities of
the therein-derived beams 112a and 112b (i.e., proportional to the squares
of the displacements of the measuring mirror 114). As above-mentioned, the
squaring operation destroys the phase information about the signal and
imposes circuit difficulties in that the squaring operation doubles the
frequency and cuts the dynamic range attainable. Also, if the signal is of
bandwidth greater than an octave, the squaring procedure introduces
distortion in the form of new components of the signal appearing due to
cross-products between spectral components of the signal.
To avoid the above problems, in the present invention the rigid reference
mirror 110 is driven back and forth by a "mini-wiggler" 216, as by means
of a piezo-electric crystal, similar to that described in U.S. Pat. No.
3,969,578 to Mezrich, et al., which introduces a phase shift into the
reference beam. Whereas in U.S. Pat. No. 3,969,578 the wiggler is driven
at a relatively large amplitude, to produce phase excursions of equal to
or greater than one-half wavelength and operates at a frequency much below
the signal frequency, the mini-wiggler 216 of the present invention is
driven at a relatively small amplitude, to produce phase perturbations of
just a few manometers (i.e., about 0.001.lambda.) at a frequency at either
edge or even in the middle of the signal frequency band. Also, as will
presently be described, the mini-wiggler 216 acts to produce a "pilot"
signal of known amplitude and phase from which can be obtained the
correction terms needed to compensate for the operating variations in the
interferometer state, such as ambient vibrations, drifts, and the like.
FIG. 2 schematically represents one form of signal-processing circuit which
can be employed in accordance with the present invention. Photo diodes 11a
and 11b respectively receive the output beam components 112a and 112b and
provide electrical outputs I'.sub.S1 and I'.sub.S2 comprising the
respective total electrical signals presented to Channels 1 and 2 of the
circuit.
I'.sub.S1 represents the signal mirror 114 displacement signal (.delta.
signal) plus the mini-wiggler signal, multiplied by sin 2kd, where k is a
constant and d is the relative path length difference around which the
signal mirror displacement of interest is taking place. As shown, in
Channel 1 the mini-wiggler signal is extracted from the total signal by
means of a band pass filter 12a and is amplified in an amplifier 13a and
mixed with the mini-wiggler drive signal in a mixer 14a. The relative path
length difference d around which the displacement of interest is taking
place is obtained from mixer 14a via a low pass filter 15a, in the form of
A sin 2kd, where A is a maximum sensitivity term. This same factor, A sin
2kd, is the relative sensitivity of Channel 1; the displacement amplitude
is essentially modulated by this term.
After extraction of the mini-wiggler signal, filtering through a band pass
filter 16a and amplification by an amplifier 17a, the output of amplifier
17a becomes .delta. sin .omega.t.multidot.sin 2kd, where .delta.
represents the peak amplitude displacement of signal mirror 114. By
multiplying this signal by A sin 2kd in a multiplier 18a, the product term
A.delta. sin .omega.t(sin.sup.2 2kd) is obtained.
In the quadrature channel (Channel 2) the same procedure is followed, with
cos 2kd replacing sin 2kd, giving as the output of a multiplier 18b the
term A.delta. sin .omega.t(cos.sup.2 2kd). The respective outputs of
multiplier 18a and multiplier 18b are summed in a summer 19, providing as
a sum the term A.delta. sin .omega.t, which is the direct-amplitude
displacement signal desired.
The mini-wiggler signal is of known amplitude and can be used as a standard
to correct for laser variations and variations in beam overlap, coherence,
etc., by means of a modified circuit arrangement such as that shown in
FIG. 3. The correction coefficient for variations in beam characteristics
and variations in beam overlap, coherence, etc., which equally influence
both channels, is designated by .gamma.. This coefficient .gamma. is
extracted by squaring and summing the terms .gamma. sin 2kd and .gamma.
cos 2kd obtained in the same manner as in the embodiment of FIG. 2. (The
amplitude of the mini-wiggler signal is normalized to be 1). Thus, for
Channel 1 the mini-wiggler signal is extracted from the total signal
I'.sub.S1 by a band pass-amplifier stage 22a and mixed with the
mini-wiggler drive signal in a mixer-low pass filter stage 23a to derive
the .gamma. sin 2kd output, which is squared in a multiplier 24a to
produce .gamma..sup.2 sin.sup.2 2kd. Similarly, Channel 2 has a
corresponding configuration producing .gamma..sup. 2 cos.sup.2 2kd at the
output of a multiplier 24b. The squared outputs are summed in a summer 25,
producing .gamma..sup.2 at its output. The term .gamma..sup.2 is used
respectively as the divisor for .gamma. sin 2kd and .gamma. cos 2kd in
dividers 26a and 26b to yield respective multiplying factors sin
2kd/.gamma. and cos 2kd/.gamma. for Channels 1 and 2 at the outputs of
dividers 26a and 26b. These factors are respectively multiplied with
.gamma.(.delta. sin .omega.t) sin 2kd and .gamma.(.delta. cos .omega.t)
cos 2kd in multipliers 27a and 27b to yield the terms .delta. sin
.omega.t.multidot.sin.sup.2 2kd and .delta. sin
.omega.t.multidot.cos.sup.2 2kd, which are added in a summer 28. Thus, the
.gamma. term in the quadrature signals is cancelled and the output of
summer 28 yields an amplitude signal .delta. sin .omega.t independent
(within the limits of the circuitry of FIG. 3) of the beam characteristics
as well as of the state of the interferometer.
In the above-discussion the .omega. term represents 2.pi.f, where f is the
frequency of the ultrasonic signal causing displacement of the signal
mirror 114. The frequency of vibration of the mini-wiggler 216 may be at
either edge or in the middle of the signal frequency band, namely, near
the signal frequency.
While certain specific embodiments of an improved system for the
measurement of ultrasonic waves have been disclosed in the foregoing
description, it will be understood that various modifications within the
scope of the invention may occur to those skilled in the art. Therefore it
is intended that adaptations and modifications should and are intended to
be comprehended within the meaning and range of equivalents of the
disclosed embodiments.
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