Download Physical Properties of Lead Molybdate Relevant to Acousto

JOURNAL OF APPLIED PHYSICS
VOLUME 42, NUMBER 6
MAY 1971
Physical Properties of Lead Molybdate Relevant to Acousto-Optic Device Applications
G. A. COQUIN, D. A. PINNOW, AND A. W. WARNER
Bell Telephone Laboratories, Incorporated, Murray Hill, Ne:w Jersey 07974
(Received 5 October 1970)
All of the elastic and photoelastic constants of crystalline lead molybdate (PbMo04) and various optical
and thermal properties of the material have been measured. This information has been used to evaluate
the material for a number of practical acousto-optic device applications. The high figure of merit found
in earlier preliminary studies is only 10% smaller than the maximum figure of merit of the material. Thus
the material is well suited for acousto-optic modulator and deflector applications. However, the material is
not particularly useful for tunable acousto-optic filters because the relevant e1asto-optic coefficient is small.
Acoustic and optical losses, and changes in sound velocity and index of refraction with temperature can
cause operating limitations in practical devices, and these are discussed.
'
I. INTRODUCTION
A recent paperl reported that lead molybdate
(PbMo04 ) has a combination of physical properties
that make this material exceptionally well suited for
acousto-optic device applications. Interest in lead
molybdate for this application has continued at a high
level at this laboratory and elsewhere. Accordingly we
felt it worthwhile to make a more complete determination of its physical properties that are most relevant for
its use in acousto-optic devices, and the results are
presented in this paper. Some data that were given in
Ref. 1 are included here also for completeness.
In an acousto-optic device, i.e., light deflector2 or
modulator,3 there is an interaction between a sound
wave and a light wave. Hence one would like to know
the elastic constants and acoustic loss to characterize
the acoustic properties of the deflector material, the
indices of refraction and optical loss to characterize its
optical properties, and the photoelastic constants to
characterize the strength of the interaction. When all
of these constants are known, the magnitude of the
acousto-optic interaction (figure of merit) can easily
be optimized with respect to crystal orientation. In addition, since conversion of electric and acoustic energy
to heat can cause problems in many device structures,
various thermal properties such as thermal conductivity, thermal expansions, and temperature coefficients
of sound velocity and index of refraction, are of interest.
Measurements of all these quantities have been made,
and the significance of the results for use of lead molybdate for acousto-optic light deflection is discussed at the
end of the paper.
II. CONVENTION FOR CRYSTAL AXES
P16, P6I, and P45, and all constants related to these by
symmetry. In crystallographic terms this problem is
expressed by the fact that the c axis is a fourfold screw
axis whose pitch changes handedness if the crystal is
turned upside down.
Since there is no present convention for selecting
the positive sense of axes for nonpiezoelectric crystals,
we have somewhat arbitrarily chosen +Z such that the
elastic constant CI6 is positive. Our crystals were oriented
with respect to this "convention" by acoustic velocity
measurements. X-ray techniques could be used also to
find the +Z axis, but this would require the measurement of intensities as well as Bragg angles.
III. EXPERIMENTAL MEASUREMENTS
AND RESULTS
A. Elastic Constants and Acoustic Loss
The elastic constants of lead molybdate were calculated from acoustic plane wave velocities measured by
the pulse-echo technique. 5 Table I gives the measured
velocities for a number of propagation directions. The
first two entries in Table I allow the elastic constants
C33 and C44 to be calculated directly. From the other
measurements, which involve quasilongitudinal and
quasishear waves, the remaining elastic constants must
be found by an iterative calculation. The elastic constants and density of lead molybdate are given in
Table II. Since we estimate the accuracy of our velocity measurements to be about 0.1%, the elastic
TABLE I. Measured acoustic velocities in lead molybdate.
Propagation
direction&
Displacement
direction
Longitudinal
velocity
Shear
velocity
Lead molybdate is a tetragonal crystal having point
group symmetry 4/m. The crystallographic a and c
3632 m/sec
(0,0, 1)
(0, 0, 1)
axes are chosen by well-known conventions, and lattice
1961 m/sec
(0,
0,
1)
(1,0,0)
spacings are a= 5.435 1, c= 12.110 1.4 Tensorial quan4003 m/sec
x, y plane
(1,0,0)
tities are referred by convention to a right-handed
1312 m/sec
4339 m/sec
x, y plane
(,,3/2, !, 0)
Cartesian coordinate system with the Z axis along c,
2198 m/sec
3970 m/sec
x, y plane
(!, VJ/2, 0)
and the X and Yaxes along equivalent a axes. However,
3860 m/sec
y, z plane
(0, 1/v'1, 1/v'1)
there is a problem in chom:ing the positive sense of the
Z axis; a reversal of sign of the Z axis causes a change
a Given as components of a unit vector n = (nt. n2. 1t3) pointing in direcin sign of the elastic constant C16, photoelastic constants lion of [lro[lagation.
2162
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2163
PHYSICAL PROPERTIES OF LEAD MOLYBDATE
constants as shown should be correct to a few tenths
of a percent.
Figure 1 shows a plot of the variation of acoustic
velocity with propagation direction when both the
propagation and displacement directions are in the X, Y
plane. For propagation directions 29° and 74° from the
X axis in the positive X, Y quadrant (as defined by
the convention of Sec. II), both longitudinal and shear
velocities have extrema, and the wave motions are
purely longitudinal and shear. These pure mode directions are significant because they can be used to
facilitate the measurement of photoelastic constants
as will be described in Sec. III.C.
Measurements of acoustic loss as a function of frequency were made for two cases: (1) a longitudinal
wave propagating along the c axis, and (2) a shear
wave propagating in the 29° pure mode direction in
the X, Yplane. The longitudinal wave (V = 3632 m/sec)
X-AXIS
/
PURE MODE
DIRECTIONS
I
5000....-----11------l----,
c:;
,!
!
3000
>-
~
o
o
oJ
'"
>
30
60
90
TABLE II. Elastic constants and density of lead molybdate.
CI1 =
1.092 X 1011 N/m2
caa=0.917
C44=0.267
c6ft=0.337
cI2=0.683
=0.528
c16=O.l36
Cia
p
= 6950 kg/rna
were measured with a precision spectrometer at various
helium spectral lines which range throughout the visible
spectrum. The index data is shown in Table III, and
for wavelengths greater than "'-'0.5 }J. can be fitted to a
single oscillator Sellmier equation6
(1)
as shown in Fig. 3. For shorter wavelengths approaching the energy gap there is, as expected, an increasingly
greater departure from the single oscillator equation.
The optical transmission of an ordinary wave through
a to-mm thick sample of lead molybdate is shown in
Fig. 4. This particular sample was of exceptionally good
optical quality and looked almost clear under white
light illumination. In the region from 0.42 to 3.9 }J.
the transmission loss is due almost entirely to surface
reflections. The transmission of an extraordinary wave
is similar although the short wavelength cutoff extends
approximately 90 A further into the ultraviolet.
Impurities in much of the lead molybdate grown
to date cause some additional absorption in the bluegreen portion of the spectrum as shown by the dotted
curve in Fig. 4, which represents more typical material.
This mateni:t1 appears slightly yellowish rather than
clear. The impurities have not been identified at present,
6.r-----.--r--r--r-r-r-'T""T'"1
8 (DEGREES)
FIG. 1. Acoustic plane wave velocities in lead molybdate for
waves propagating in the X-V plane, as a function of the angle 0
between the acoustic wave vector and the X axis.
3.
has been utilized in a number of experimental acoustooptic light deflectors, and the shear wave (V = 1311
m/sec) is the lowest velocity wave in a lead molybdate
crystal. The measurements were made by probing
optically along the propagation direction to determine
attenuation with distance. The results are plotted in
Fig. 2 as loss in dB/}J.sec vs frequency. One can see that
in both cases the loss increases as the square of the frequency to within the accuracy of the measurements.
Somewhat surprisingly the slow shear wave has less
loss in these units ("-'3.0 dB/}J.sec GHZ2) than the
longitudinal wave (,,-,5.3 dB/}J.sec GHZ2) , but the
measurements were not made on the same crystal.
We would expect that the acoustic losses for other
directions of propagation in the crystal will not differ
by significant amounts from the values shown in Fig. 2.
B. Index of Refraction and Optical Transmission
The indices of refraction for both the ordinary and
extraordinary polarization states of lead molybdate
u
:
~
~
.,.,
o
-'
o
;::
CI)
5
o
'"
0.3
1-)
FIG. 2. Acoustic loss in lead molybdate as a function of
frequency. Open circles are data for longitudinal mode propagating
parallel to the c axis. Squares are for slow shear mode propagating
29° from the X axis in the X-V plane.
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2164
COQUIN, PINNOW, AND WARNER
TABLE III. The index of refraction of lead molybdate.
Index of refraction
Wavelength (II-)
Ordinary
Extraordinary
0.66782
0.6328"
0.58756
0.5145"
0.50157
0.49219
0.48800.47131
0.44715
2.371
2.386
2.409
2.469
2.483
2.496
2.502
2.528
2.576
2.252
2.262
2.277
2.315
2.324
2.331
2.335
2.349
2.375
a Interpolated values.
however, there is some evidence to suggest that they
may be lead in an improper oxidation state. 7 The bulk
loss of even the impure material is quite low beyond
the short wavelength absorption region. For example,
the loss of an impure sample was determined to be
0.5%/cm at A= 1.06 p. by observing the change in pumping threshold when a sample was inserted into a
Nd:YAG laser cavity.s The loss of a pure sample at
A= 1.06 p. was too small to be measured by this technique, i.e., less than O.OS%/cm.
namic and static techniques, using 6328-A light. Dynamic measurements consisted of applying the technique of Dixon and Cohen9 to determine the acoustooptic figure of merit relative to fused silica for various
acoustical and optical polarization states. Static
measurements consisted of placing samples in an interferometer to observe changes in the indices of refraction
with an applied uniaxial stress. The dynamic measurements, which are inherently more accurate than the
static measurements, yield a numerical value for some
linear combination of photoelastic constants, but the
sign is indeterminate. Thus the static measurements,
which yield both magnitude and sign of a linear combination of piezo-optic constants, were used only to aid
in sign determinations., although no large inconsistencies
between the two techniques were observed.
To simplify the extraction of photo elastic constants
from the data, the photoelastic constants were first de100
REFLECTION LOSS
80
z
Q
'"'"i
'"z
...'""
I
,
I
60
I
I
I
I
40
I
I
#.
C. Photoelastic Constants
The 10 independent photoelastic constants of lead
molybdate were measured by a combination of dy-
20
~~.3-L-~05~-L~~I~---~-~3~.0~~75.70~
WAVELENGTH
X(MICRONS)
FIG. 4. Transmission of ordinary polarization state through a
I-em-thick sample of lead molybdate. Transmission of extraordinary state is similar although the short wavelength cutoff
extends approximately 90 A further into the ultraviolet. Dotted
curve is for a sample containing impurities.
.270.--_--'0".B'-0,.7'--0,S::.,-=.;0.5 5o....::,;0.5j=--..:.;0·45o.......-,
r
r
2.2
.260
.250
*'
(1')
. 240
2.3
.230
.220
,,
.210
2.4
.200
2.5
.190
.IBO
2.6
xt (MICRONS-
2
)
FIG. 3. Index of refraction data from Table III fit to a single
oscillator Sellmier equation [see Eq. (1) J.
termined relative to a coordinate system rotated 29°
about the c axis, so that the rotated X axis is coincident
with the +29° pure mode direction mentioned in Sec .
IILA. In this rotated coordinate system the elastic
constant C16' is equal to zero. Thus the simple strain
patterns associated with waves propagating along the
rotated X axis allows a rather straightforward determination of the photoelastic constants Pu', P12', Pat',
P16', P6t', and P66', the primes indicating rotated constants. Measurements with longitudinal and shear
acoustic waves propagating 45° from the Z axis in the
X' -Z plane allow P44', and P4;' to be determined. The
photoelastic constants P13' and Paa' (= P13 and paa) are
given directly by measurements with longitudinal waves
along the c axis. When all the constants had been determined in this way, they were referred back to the
crystal axes by a tensor transformation.
The complete set of photoelastic constants for lead
molybdate is shown in Table IV. Also given in Table IV
are very rough estimates of possible errors in the photo-
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2165
PHYSICAL PROPERTIES OF LEAD MOLYBDATE
elastic constants. These estimates are based on the facts
that an individual measurement of the figure of merit is
repeatable to about ±10%, somewhat worse when the
light propagates near the optic axis; that some of the
smaller constants (e.g., P44 and P45) are measured as
a sum of terms involving the larger constants, resulting
in additional loss of accuracy; and that some inconsistencies were observed in a number of redundant
measurements used as a check. Although our estimated
accuracies may seem unimpressive, they are realistic
and more than adequate for a practical evaluation of
the Bragg scattering efficiency as a function of crystal
orientation. This is done in Sec. IV.
As reported previously,1 for longitudinal acoustic
waves propagating along the c axis, there is an appreciable increase in the figure of merit, greater than 20%
for M 3, in going from 6328 to 514.5 A. Most of this increase is due to dispersion in the indices of refraction,
but there is some increase in the photoelastic constant
P33, as shown in Table IV, over this wavelength range.
The dispersion of the other photoelastic constants was
not measured.
D. Thermal Properties
Measurements made of the thermal conductivity
parallel and perpendicular to the c axis indicated that
lead molybdate is thermally isotropic to within the
accuracy of the measurements, with a conductivity of
15±2 mW /cmoC at room temperature. The value of
conductivity is comparable with those of other crystalline dielectric solids. The specific heat was not measured,
but from the Dulong and Petit value (c.=3R), we estimate c.",0.5 J/cm3 °C near room temperature.
The coefficients of thermal expansion were measured
with the aid of a dilatometerlO and were found to be
o:n = 10, 0:33= 25 ppm;oC at room temperature.
For the longitudinal wave propagating along the
c axis the temperature coefficient of delay was measured
to be 186 ppm;oC. Subtracting this from the thermal
expansion coefficients 0:33, we find the temperature
coefficient of velocity to be -161 ppm;oC.
Temperature coefficients of the ordinary and extraordinary indices of refraction were determined by illuminating a plane parallel plate of lead molybd3;,te
with divergent light from a He-Ne laser ('-=6328 A)
and monitoring changes in Haidinger's fringes ll with
sample temperature. These concentric fringes are due
to interference of the front and rear surface reflections
from the sample and arc thus a sensitive measure of
the optical thickness
LoPt=nL,
(2)
where L is the actual thickness and n is the index of
refraction. Since the change in optical thickness with
temperature is
dLopt/dT=n(dL/dT)+L(dn/dT),
(3)
the temperature coefficient of the index of refraction
TABLE
IV. Photoelastic constants of lead molybdate.
>..=6328 A:
pn=0.24 (±1O%)
PI2=0.24 (±1O%)
PI3=0.255 (±5%)
PI6=0.017 (±10%)
P31=0.175 (±5%)
h3=0.300 (±5%)
P•• =0.067 (±20%)
P46= -0.01 (±50%)
P'I=0.013 (±20%)
P66=0.05 (±20%)
>..=5145 A:
PI3=0.254
P33=0.309
may be determined as follows:
(l/n)(dn/dT) = (I/Lopt) (dLopt/dT)-o:,
(4)
where 0: is the thermal expansion coefficient for the
plate thicknes". Results of measurements on two
samples of different orientation were consistent. The
temperature coefficients of the ordinary and extraordinary indices of refraction were found to be -30±2
and -18±2 ppm;oC, respectively, at room temperature. The negative sign indicates that the indices of
refraction decrease with increasing temperature.
The thermal properties of lead molybdate are summarized in Table V.
IV. DISCUSSION OF RESULTS
A. Light Scattering Efficiencies
The main purpose of measuring the elastic and photoelastic constants of lead molybdate was to investigate
in detail the variation of Bragg scattering efficiency with
crystal orientation. This is easily done analytically once
all the constants are known, whereas it would be practically impossible to do so experimentally. As an aid
in acousto-optic materials evaluation, it has been found
extremely useful to define a figure of merit involving
several material parameters. Three different figures of
merit have in fact been used in the literature. 12- 14 For
light deflectors and modulators, the most relevant of
these is the figure of merit M3=p2n7/pVZ (Dixon's14
notation) were n is the index of refraction, P is the
photoelastic constant, p the density, and V the acoustic
velocity. The distinction between M3 and other figures
of merit Ml and M2 is discussed by Pinnow. 15
For modulators and deflectors, the light and sound
propagate approximately orthogonal to each other,
and the scattered light is nearly collinear with and has
the same polarization as the incident light. In this case
the effective photoelastic constant p appearing in the
definition of the figure of merit is
L
(5)
PijkldidjUknl,
ijkl
where d, u, and n are unit vectors in the directions of
the optical frequency electric displacement, the mechanical displacement, and the acoustic wave vector,
respectively.
p=
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2166
COQUIN, PINNOW, AND WARNER
TABLE V. Thermal properties of lead molybdate.
Specific heat&
Thermal conductivity
Thermal expansion
Temperature coefficient velocityb
Temperature coefficient index
",0.5 J/cm3 °C
Kll~K33= 15±2 mW /cm °C
O'n = 10,
0'33=25 ppm;oC
(l/v) (dv/dT) = -161 ppm;oC
(l/no) (dno/dT) = -30±2,
(l/n.) (dne/dT) = -18±2 ppm;oC
• Estimated from Dulong and Petit value.
The maximum value of M3 in lead molybdate is
36X 10-12 cm sec2/g, and this occurs for a longitudinal
acoustic wave propagating in the direction nsound =
(0.35, 0.67, 0.65), and an extraordinarily polarized
light wave propagating in the direction nlight= (0.43,
0.50, -0.75). This is about 27 times the M3 of fused
silica. For comparison, with a longitudinal acoustic
wave propagating along the c axis, the figures of merit
relative to fused silica are 24 and 23 for 0- and e-polarized light, respectively, propagating in the X -Y plane.
This latter crystal orientation has been the only one
used thus far in a number of experimental light deflectors, and it appears that no great advantage can
be obtained from changing. It is interesting to note
that for all directions of propagation for longitudinal
waves in lead molybdate, there is a range of propagation directions for o-polarized light where the relative
M3 is greater than 19.
In a tunable acousto-optic filter,16 the light and sound
propagate collin early, and light is scattered from one
polarization state to another. The most obvious way
to obtain such an interaction in lead molybdate is to
propagate an acoustic shear wave, polarized along the
c axis, in the X - Y plane. The relevant photoelastic
constant is P45, which is quite small as shown in Table
IV. An investigation of other directions of propagation
shows that the effective photoelastic constant for this
type of interaction reaches a maximum of about 0.035,
but this is still less than what is obtainable with other
available materials such as lithium niobate and calcium
molybdate.
B. Isotropic Scattering
As mentioned in Ref. 1, with a longitudinal acoustic
wave along the c axis, and light propagating in the
X, Yplane, the amount of light scattered is independent
of the polarization of the incident light, a useful and
convenient property for practical light deflection systems. This does not mean that PI3 is equal to Paa, but
rather n o3PI3=n e3p33. This relation is not required by
symmetry and although it may be coincidental, it is
also observed in several other materials, such as lead
tungstate, strontium and calcium molybdate, calcium
tungstate, and lead glass. 17 Furthermore, in lead molybdate, the relation holds throughout the visible spectrum
even though both the indices of refraction and the
photoelastic constants show an appreciable dispersion.
Since the figure of merit for this orientation is only
about 10% less than the maximum, there is no reason
b
Longitudinal wave along c axis.
not to take advantage of the isotropic scattering
practical devices.
10
c. Thermal Effects
\
Although lead molybdate is one of the most efficient
currently available for acousto-optic devices,
apprecIable acoustic power levels, typically in the range
1-2 W, are required to deflect most of the light in the
large aperture devices we have built for X-V-deflection
systems. Power dissipation can cause temperature
changes which might affect device performance. Uniform temperature changes and small linear temperature
gradients will cause a displacement in the position of
the deflected light beam without loss of resolution. Such
displacements are of little consequence for display applications, but they must be carefully controlled for
applications such as optical memories and laser machining where precise beam positioning is essential. Nonlinear temperature gradients cause defocusing which
results in loss of resolution. Generally, temperature
changes and gradients caused by heat sources located
outside or on the surface of the lead molybdate crystal
can be controlled effectively by suitable engineering.
However, this is not the case for internal heat sources
which result from bulk acoustic and optical attenuation.
These mechanisms set fundamental limits on device
performance.
To give some feel for the thermal tolerances, we
will use as a typical example a deflector operating over
the acoustic bandwidth of 100--200 MHz with 100 resolvable beam positions. We further assume that this
deflector utilizes the isotropic scattering orientation
discussed in the last section, that its dimensions are
1 cm on each side, and that the wavelength of the incident light is 6328 A. Acoustic loss is not a problem
in this low frequency range. Optical power densities
of up to 103 W / cm 2 have not caused observable defocusing at the 6328-A wavelength. However, at shorter
optical wavelengths, such as 5145 A, the optical loss in
typical material (see Sec. III.B) results in detectable
defocusing at power dem,ities above approximately
200 W/cm2 •
For applications that require precise beam positioning it is reasonable to require that the beam should
not wander by more than 10% of the spacing between
adjacent resolvable positions. Since the angle of deflection () is given by
materi~ls
()=f..j/V,
(6)
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PHYSICAL PROPERTIES OF LEAD MOLYBDATE
where A is the vacuum optical wavelength, f the acoustic
frequency, and V the acoustic velocity, the worst case
is at the highest frequency in the band, 200 MHz where
a change in () of 1 part in 2000 corresponds to 10% of the
spacing. Considering the variation of acoustic velocity
with temperature, -161 ppm/DC, a 3°C change in
temperature will deflect the beam by 1 part in 2000.
Hence the temperature of the deflector must be held
constant during its operation to within ±3°C.
A constant change in the index of refraction does
not change the angle of deflection, but a gradient in
index of refraction combined with a thermal-expansion
gradient will create a prism effect which does cause
beam displacement. The amount of displacement is
proportional to the optical path length through the
sample, and, fortunately, the thermal expamion and
temperature coefficient of index cancel to some extent.
Nevertheless, for an optical path of one centimeter,
a temperature gradient of 0.4°C/cm along the c axis
will displace an a-polarized light beam by 10% of a
resolution element in our example. The corresponding
figure for e-polarized light is 1.2°C/em. Thus relatively
small temperature gradients can significantly affect device performance.
There are two heat sources located on the surfaces
of the lead molybdate crystals in the devices we have
constructed. The first is due to power dissipation at the
ultrasonic transducer. Approximately 10% of the electrical power delivered to the transducer is dissipated
due to either resistive heating of the thin film electrodes
or acoustic losses in the transducer bond. The other
heat source is a metallic block attached to the crystal
face opposite the transducer which serves as an acoustic
terminator and absorber. Controlling the temperature
of this metallic piece is not difficult if it has a reasonably high thermal conductivity. However, the thermal
conductivity of the lead molybdate, 15 mW/cmoC, is
so small that removal of heat from the transducer by
conduction through the crystal to the metallic terminator results in substantial temperature gradients. For example, if 10% of an assumed one-watt drive power is
dissipated at the transducer, the resulting temperature
gradient necessary for conduction of this power to the
terminator is 6.7°C/cm which is far in excess of the
O.4°C/cm desired. In certain cases where the deflector
is operated continuously, this large gradient may be
tolerable provided that it does not fluctuate in time
by more than ±OAoC/cm. However, if the deflector
is operated intermittently, the transducer must be directly cooled by an adjacent heat sink or by forced convection in order to limit beam wander to 10% of the
spacing between resolvable positions.
Another thermal problem has arisen in device fabrication. In our first devices aluminum terminators were
epoxy bonded to the lead molybdate, but a number of
crystals cracked when cooled from 50°C, the curing
temperature of the epoxy. We subsequently found that
this was due to a mismatch in the thermal-expansion
coefficients of aluminum (24 ppm;oC) and lead molyb-
2167
date (10 ppm/DC in the plane normal to the c axis).
Later devices were constructed with Kovar heat sinks.
With a thermal expansion of 6 ppm;oC, the Kovar is a
better match to lead molybdate and the cracking problem was eliminated, although the thermal conductivity
of Kovar is 20 times smaller than that of aluminum.
D. Material Quality Evaluation and Handling
A few simple observations suffice to determine the
quality of a given source of lead molybdate. If the material is to be used with intense light in the blue-green
portion of the spectrum, it is essential that it appear
clear under white light illumination to avoid the additional optical losses present in the yellowish material.
The optical homogeneity is most easily checked by
observing the interference figure along the optic axis,
after the two surfaces normal to the c axis have been
polished. This is done using a pair of crossed polaroids
or with a polarizing microscope set for conoscopic viewing. The interference figure, in the form of a cross with
concentric rings, should remain stationary as the
crystal is moved under the microscope. In poor crystals
the cross is seen to open in some areas of the crystal.
Lead molybdate has been found to have a cleavage
plane normal to the c axis. Hence it is important that
the material to be processed for devices be completely
free from cracks, and care must be exercised to prevent
the start of a crack either by a surface scratch or thermal
shock. Since the material is relatively soft and easily
scratched, the handling must be more considerate than
procedures employed with, for example, glass or quartz.
V. CONCLUSIONS
From measurements of the elastic and photoelastic
constants of lead molybdate, we have determined that
the high figure of merit reported previously for acoustooptic light deflector and modulator applications is only
10% smaller than the maximum figure of merit of the
material. We have also determined that the Bragg scattering efficiency for tunable acousto-optic filter applications is quite small. Acoustic and optical losses limit
the use of lead molybdate to ultrasonic frequencies less
than about 500 MHz and optical wavelengths from
0.45 to 4 }.L. There are other materials, such as Ge,18
which are more efficient in the longer wavelength
regions, however.
Problems are encountered in practical device structures due to optical beam displacements and distortions caused by thermal effects, but it appears these
can be overcome to a large extent by good engineering
design. In view of its desirable physical properties and
the relative ease of growing crystals of excellent optical
quality, we conclude that lead molybdate presently is
the preferred material choice for many acousto-optic
applications.
ACKNOWLEDGMENTS
The authors thank D. M. Dodd for determining the
optical absorption characteristics, R. C. Beairsto
Downloaded 27 Jul 2010 to 60.191.99.5. Redistribution subject to AIP license or copyright; see http://jap.aip.org/jap/copyright.jsp
2168
COQUIN,
PINNOW, AND
for measuring the thermal conductivity, and H. M.
O'Bryan for measuring the thermal-expansion coefficients of lead molybdate. The assistance of S. R.
Williamson in other experimental measurements was
greatly appreciated.
1 D. A. Pinnow, L. G. Van Uitert, A. W. Warner, and W. A.
Bonner, App!. Phys. Lett. 15,83 (1969).
2 E. 1. Gordon, Proc. IEEE 54, 1391 (1966).
3 D. Maydan, J. App!. Phys. 41, 1552 (1970).
4 X-ray Powder Data File No. 8-475, American Society for
Testing Materials, Philadelphia, Pennsylvania.
6 H. J. McSkimin, J. Acous. Soc. Amer. 33, 12 (1961).
6 J. M. Stone, Radiation of Optics (McGraw-Hill, New York
1963), pp. 378-385.
JOURNAL OF APPLIED PHYSICS
WARNER
W. A. Bonner (unpublished).
R. B. Chesler, D. A. Pinnow, and W. W. Benson (unpublished).
9 R. W. Dixon and M. G. Cohen, App!. Phys. Lett. 8, 205
(1966).
10 H. A. Sauer, Rev. Sci. Instrum. 39, 562 (1968).
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Optics (Freeman, San
Francisco, 1958), p. 231.
12 T. M. Smith and A. Korpel, IEEE J. Quantum Electron. 1,
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17 D. P. Shinke (unpublished).
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(1970) .
7
8
VOLUME 42, NUMBER 6
MAY 1971
Experimental Hydroacoustic Imaging System*
G. C.
KNOLLMAN,
A. E.
BROWN,
J. L.
WEAVER, AND
J. L. S.
BELLIN
Lockheed Research Laboratory, Palo Alto, California 94304
(Received 14 September 1970)
An underwater acoustic imaging system is described which has been developed especially for closerange, high-resolution (on the order of millimeters) viewing in very turbid water. The system operates
at nominal 2.5-MHz frequency and has a range up to 10 m in water of suspended ocean sediment concentration as high as several thousand parts per million. Larger range capability is possible with lower
sediment concentration. Real-time kinescope displays are presented by the system of the acoustic field
of view, which is that insonified by a high-intensity sound transmitter of total beamwidth variable up to
30° in 10° increments. The sensitive (better than 10-10 W fcm') piezoelectric image converter is augmented
by a mechanical scanning system to provide in effect a 10 OOO-element pickup matrix. A Plexiglas lens
moved by remote-control drive is employed for focusing. System parameters and performance are discussed,
and some typical hydroacoustic images are displayed.
I. INTRODUCTION
The ever-increasing variety of activities being
performed in the oceans and especially on or near the
ocean floor has stimulated considerable interest in
underwater observation and recognition of objects.
However, in many oceanic areas, particularly in harbors
and estuaries, the concentration of suspended sediment
is high at all times. Furthermore, because of the fine
grain size of many ocean sediments, slight disturbances
of the ocean bottom can becloud the proximate neighborhood for a lengthy period. This situation often
exists in undersea mining, recovery of submerged
objects, underwater salvage, and deep-submersible
operations.
Underwater turbidity frequently restricts the range
of optical viewing, a limitation that can exclude optical
systems from practical use in many ocean operations.
Thus, one turns to acoustic imaging for viewing in
turbid environments. Acoustic images1- a can be formed
of insonified objects in a manner analogous to that
in optical imaging. These acoustic images (which
consist of spatial amplitude and phase variations of
the ultrasonic field in an image plane) can be detected
with acoustic sensors and displayed as in conventional
television. Underwater sound-wave propagation at
low-megahertz frequencies (the frequency region for
high resolution in water of high turbidity concentration) is not affected appreciably by the presence of
typical ocean sediments. Although highly turbid sea
water can effect considerably greater acoustic attenuation than that arising in clear water, high-resolution
viewing by acoustic means is definitely practical, at
least over short ranges, in turbidity concentrations
as high as several thousand parts per million of suspended ocean sediment; longer ranges are possible
with lower sediment concentrations.
In this paper we describe an experimental underwater acoustic imaging system that was developed
for real-time, high-resolution viewing in highly turbid
water, with the ultimate goal of deep-sea operation on
or near the ocean floor. Recently, hydroacoustic
imaging systems have received attention elsewhere4- 7
as well. However, no other megahertz image-conversion
techniques are known by the authors to provide high
sensitivity and resolution together with the potential
for operation under great hydrostatic pressure and
severe mechanical constraints.
The system presented here is based on solid-state
image conversion technology that we devised earlierll· 9
and now have incorporated, together with a transistorized switching matrix, in a packaged hydroacoustic
image converter. A lOO-element, linear piezoelectric
array plus a mechanical scanning system constitute,
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