Optical waveguide vibration sensor and method

An optical sensor that is particularly useful for measuring ground borne vibrations employs a planar optical waveguide structure to form a sensor for detecting ground born velocities and accelerations. An optical interferometer is used to detect phase shifts in optical signals guided by the waveguide caused by external vibrations. A pair of phase modulator electrodes is placed adjacent one of the legs of the interferometer and a coil of wire is connected between the pair of electrodes. A sensing element that produces a voltage in response to vibrations is connected across the electrodes. The voltage produced by the sensing element is applied to the pair of electrodes to cause phase modulation of optical signals guided by the leg of the interferometer adjacent the electrodes. The optical signals output from the interferometer are then processed to determine the velocity and acceleration associated with the vibration.

BACKGROUND OF THE INVENTION 
This invention relates generally to sensors for detecting mechanical 
vibrations such as sonic waves that propagate through the earth or through 
structures formed of solid materials. In particular, this invention 
relates to sensors that include interferometers to detect mechanical 
vibrations. 
There are numerous applications of mechanical vibration sensors. One 
application of interest is the geophone, which may be used to detect 
vibrations resulting from vehicles traveling across the ground or on a 
highway. It is possible to determine the types of vehicle and the number 
of each type of vehicle in a convoy or the like by using a geophone to 
detect vibrations that propagate through the earth and then analyzing the 
resulting electrical signals. 
Commercial applications of geophones include exploration for oil and 
minerals and testing rotating machinery for wear. In exploring for oil, 
for example, a plurality of geophones are placed in an array at various 
locations on the surface of the earth and in bores drilled into the earth. 
An explosive charge is used to create a shock wave, which is detected by 
the geophones. Analysis of the data from the geophone array then is used 
to determine the structure of the earth in the vicinity of the explosion 
and the geophones. 
Mechanical vibrations in rotating machinery such as water or gas driven 
turbines used in electric power generation may be used to measure the 
amount of wear in bearings, gears and the like. 
Prior art sensors for these applications typically use either piezoelectric 
ceramics or other mechanical means to sense the applied field. Prior art 
devices are electrically active in that electric power must be applied to 
produce an output signal. This electrical downlink can prove to be 
undesirable to due electromagnetic interference and undesired detection. 
In addition, the data that is collected and sent back to the remote 
location is also subject to the bandwidth limitations of twisted pair or 
coaxial cables. The output from these devices is electrical. Therefore, to 
transmit the sensed information via an optical telemetry system, a 
converter must be used to change the information from electrical to an 
optical signal, which adds expense and extra components. 
SUMMARY OF THE INVENTION 
The present invention is directed to a unique optical sensor that is 
particularly useful for measuring ground borne vibrations. The present 
invention employs a planar optical waveguide structure (typically lithium 
niobate) to form a sensor for detecting ground born velocities and 
accelerations. The sensor uses an optical interferometer to causing an 
optical phase shift to occur in optical signals guided by the waveguide in 
response to external vibrations. The sensor element as described herein 
yields the magnitude and frequency content of the disturbance and its 
direction. The optical sensing approaches of the present invention offers 
many cost and technical advantages to conventional detection and sensing 
applications. 
The sensor as described is electrically passive in that it does not require 
a power source downlink. Therefore, there is a high immunity to EMI. The 
sensed information is converted to an optical phase shift; and therefore, 
no signal conversion is required and optical transmission (telemetry) can 
be directly used, which allows for higher transmission bandwidths which 
translates to more sensors per return line. An added advantage is that 
crosstalk between sensors is minimized or eliminated. 
A sensor according to the present invention for measuring parameters of 
ground-borne vibrations comprises an interferometer including a first leg 
having a first optical path length and a second leg having a second 
optical path length formed so that the first optical path length is longer 
than the second optical path length. An optical signal generator is 
connected to the interferometer to apply a frequency modulated optical 
signal input thereto. A pair of electrodes is placed adjacent one of the 
legs of the interferometer and a coil of wire is connected between the 
pair of electrodes. A magnet is arranged to have freedom of motion 
longitudinally inside the coil in response to ground motion, thereby 
inducing a voltage in the coil, the voltage being applied to the pair of 
electrodes, which causes phase modulation of optical signals guided by the 
leg of the interferometer adjacent the electrodes. The optical signals 
output from the interferometer are then processed to determine the 
velocity and acceleration associated with the ground motion. 
The interferometer may be configured as either a Mach-Zehnder or Michelson 
interferometer. A piezoelectric crystal may be used instead of the coil 
and magnet combination to detect the vibrations. 
Apparatus for sensing parameters of vibrations according to the present 
invention may also be formed to comprise an accelerometer sensor, an 
acoustic sensor; and an optical signal source arranged to provide input 
optical signals to the accelerometer sensor and the acoustic sensor. 
Telemetry apparatus connected to the accelerometer sensor and the acoustic 
sensor produces and transmits optical signals indicative of acceleration 
and acoustic pressure produced by the accelerometer sensor and the 
acoustic sensor. An optical receiver is arranged to receive optical signal 
produced by the telemetry apparatus. A frequency synthesizer is connected 
to the optical signal source to modulate the optical signals provided to 
the accelerometer sensor and the acoustic sensor; and a 
demultiplexer/demodulator connected between the optical receiver and the 
frequency synthesizer to demultiplex and demodulate the signals received 
from the optical receiver. 
An appreciation of the objectives of the present invention and a complete 
understanding of its structure and method of operation may be had by 
studying the following description of the preferred embodiment and by 
referring to the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Mach-Zehnder Interferometer and Moving Magnet 
Referring to FIG. 1, an optical velocity sensor 10 that is particularly 
suitable for measuring ground motion includes a plurality of waveguides 
12-14 formed on an integrated optics substrate 16. The substrate 16 may be 
formed to comprise lithium niobate, Which is an electrooptically active 
material. The optical waveguides 12-14 intersect to form a Y-coupler 18. 
An evanescent field coupler 20 is formed between the optical waveguides 13 
and 14 at a location on the substrate 16 at a location spaced apart from 
the Y-coupler 18. The optical waveguides 13 and 14 have different optical 
path lengths between the Y-coupler 18 and the evanescent field coupler 20. 
An optical signal source 22, such as a laser diode, provides an optical 
signal to an optical fiber 23. The optical fiber 23 guides the optical 
signal to an end of the optical waveguide 12 at an edge of the substrate 
16. The optical waveguide 12 then receives the input optical signal and 
guides it to the Y-coupler 18. The optical signal divides at the Y-coupler 
18 between the optical waveguide 13 and the optical waveguide 14, which 
then guide their respective signals to the evanescent field coupler 20. 
The evanescent field coupler 20 combines the waves from the optical 
waveguides 13 and 14 such that the waves interfere and produce an optical 
signal that is indicative of the phase difference of the waves that have 
propagated through the two mismatched optical paths between the Y-coupler 
18 and the evanescent field coupler 20. The combination of the two optical 
waveguides 13 and 14, the Y-coupler 18 and the evanescent field coupler 20 
forms a Mach-Zehnder interferometer 25. The Mach-Zehnder interferometer 25 
produces an interference pattern due to interference between the waves 
that follow the two different optical paths. The interference pattern is 
output from both the waveguide sections 24 and 26. 
The interference pattern between the waves appears in the portions 24 and 
26 of the optical waveguides 13 and 14 that are to the right of the 
evanescent field coupler 20 as shown in FIG. 1. The output of the 
Mach-Zehnder interferometer 25 may be taken at either the waveguide 24 or 
the waveguide 26. FIG. 1 shows the output being taken at the end of the 
waveguide 26. An optical fiber 28 may be butt-coupled to the portion 26 of 
the optical waveguide 14. The optical fiber 28 guides the optical output 
of the Mach-Zehnder interferometer to a photodetector 30, which produces 
an electrical signal indicative of the interference pattern between the 
optical signals guided by the two optical waveguides 13 and 14. Signal 
processing circuitry 32 analyzes the electrical signal output by the 
photodetector 30 to look for phase changes between the interfering waves. 
Suitable signal processing circuitry is well-known in the art of 
processing signals output from optical interferometers. 
A pair of electrodes 40 and 42 are formed on the substrate 16. The 
electrodes 40 and 42 are parallel to a portion 44 of the optical waveguide 
14 and on opposite sides thereof so that a voltage applied to the 
electrodes 40 and 42 produces an electric field that crosses through the 
portion 44 of the optical waveguide 14. The electrodes 40 and 42 form a 
phase modulator that modulates the phase of optical signals propagating 
through the optical waveguide 14 through the well-known electrooptic 
effect. 
A coil 50 of wire is formed so that a pair of electrical leads 52 and 54 
extend from the ends of the coil 50 and the pair of electrodes 40 and 42, 
respectively. A magnet 60 is allowed to move freely along the central axis 
of the coil 50 due to vibrations from a source such as ground motion. 
Motion of the magnet 60 along the axis of the coil 50 creates a voltage as 
a result of Faraday's law of electromagnetic induction. The induced 
voltage is given by 
##EQU1## 
where N.PHI..sub.B is the flux linkage and N is the number of turns in the 
coil. The combination of the magnet 60 and coil 50 may be chosen to 
produce peak voltages on the order of a few hundred millivolts in a 
compact size. These voltages will produce phase shifts on the order of a 
few tens of milliradians in the optical signal guided by the optical 
waveguide 14. 
It should be noted that the motion of the magnet 50 is driven by both the 
frequency and magnitude of the external disturbance. The time varying 
voltage that is generated is applied across the electrodes 40 and 44, 
causing phase modulation of the light passing through the waveguide 14. 
The amount of phase shift that is obtained is related to the optical 
wavelength, size (length and gap), the applied voltage and the 
electrooptic (Pockels) coefficients of the waveguide 14. The phase shift 
may be written as 
##EQU2## 
where .eta. is the refractive index, V is the applied voltage across the 
electrodes 40 and 44, L is the length of the electrodes 40 and 44, t.sub.g 
is the gap between the electrodes 40 and 44, .lambda..sub.o is the free 
space wavelength of the optical signal and r.sub.41 is the electro-optic 
coefficient of the optical waveguides. A Typical conversion efficiency is 
approximately 1 rad/1 V. A low power CMOS amplifier 62 may be included in 
the circuit between the coil 50 and the electrodes 40 and 42 to boost the 
voltage if it is necessary to generate larger phase shifts. The amplifier 
62 preferably is powered by a lithium battery (not shown) which has a life 
time of several years. 
The phase modulated signal appears at the output of the interferometer and 
is proportional to the input disturbance. In this scheme the applied 
mechanical disturbance that causes movement of the magnetic 60 along the 
longitudinal axis of the coil 50 is transformed to an optical phase 
difference in the waves guided by the optical waveguides 13 and 14 via the 
electro-optic properties of the waveguide material. 
Michelson Interferometer and Moving Magnet 
FIG. 2 illustrates an embodiment of the invention that includes a Michelson 
interferometer 95 instead of the Mach-Zehnder configuration of FIG. 1. The 
optical signal source 22 provides optical signals to an optical fiber 72. 
The optical fiber 72 guides the optical signals to optical waveguide 82 
formed in a substrate 83. 
Optical signals then propagate in the optical waveguide 82 to an evanescent 
field coupler 90 formed between the optical waveguide 80 and a second 
optical waveguide 82 that is also formed in the substrate 83. The 
evanescent field coupler 90 couples optical signals between the optical 
waveguides 80 and 82. After passing through the evanescent field coupler 
90, optical signals in the optical waveguides 80 and 82 propagate to a 
mirror 92 formed on an edge 94 of the substrate 83. The combination of the 
optical waveguides 80 and 82, the evanescent field coupler 90 and the 
mirror 92 cooperate to form a Michelson interferometer 95. The optical 
waveguides 80 and 82 preferably have different optical paths between the 
evanescent field coupler 90 and the mirror 92. 
A portion 100 of the optical waveguide 82 passes between the electrodes 40 
and 42. The leads 52 and 54 are connected between the electrodes 40 and 42 
and the coil 50 as described above with reference to FIG. 1 to form the 
phase modulator 46. The magnet 60 is arranged as described above with 
reference to FIG. 1 so that ground motion that causes the magnet 60 to 
move along the axis of the coil 50 induces a voltage across the coil 50. 
The phase modulated light propagates in the optical waveguide 82 to the 
mirror 92, which reflects the light back through the phase modulator 46. 
The light then receives an additional phase shift as it propagates between 
the electrodes 40 and 42 of the phase modulator 46. Because the reflected 
light travels the opposite direction in the waveguide portion 100, the 
phase shifts from the two passes of the waves through the phase modulator 
46 are cumulative. The reflected light in the waveguides 80 and 82 then 
impinges upon the evanescent field coupler 90, which combines the 
reflected waves and forms an interference pattern. The interference 
pattern then propagates in the waveguide 80 to the optical fiber 78, which 
directs the light to a photodetector 102. The photodetector 102 produces 
electrical signals that are indicative of the light intensity in the 
interference pattern. These electrical signals are then directed to signal 
processing circuitry 104 that may be substantially identical to the signal 
processing circuitry 32. 
Therefore, it is seen that the invention may be practiced using either a 
Mach-Zehnder or a Michelson interferometer. In either case, one leg of the 
interferometer has an optical pathlength that is slightly longer the 
other. This slight optical path mismatch causes a phase generated carrier 
when the device is exposed to frequency modulated light. Therefore, the 
optical signal source 22 preferably produces a frequency modulated light 
output. The Michelson configuration has the added advantage of yielding 
twice the optical phase shift as the Mach-Zehnder for a given physical 
length due to the double pass the light makes in the interferometer. 
Triaxial Sensor 
Referring to FIG. 4, a triaxial vibration sensor 120 may be formed using a 
single substrate 124. Three interferometers 126-128 can be made on the 
substrate 124 with corresponding magnet/coil assemblies 130-132 being 
arranged mutually perpendicular to each other to detect vibrations along 
the x, y and z axes. The interferometers may conveniently be formed in the 
manner described with reference to FIG. 1. The magnet/coil assembly 132 is 
oriented to sense vertical displacements. The other two magnet/coil 
assemblies 130 and 131 are arranged to be perpendicular to each other in a 
horizontal plane to sense only horizontal displacements. 
The magnitude and frequency content of the vertical channel gives the 
"signature" of the disturbance while the bearing (direction) is found by 
the superposition of the output from the two horizontal channels 130 and 
131. It should be noted that using only one sensor would give a 
180.degree. bearing ambiguity. Therefore, one sensor is not capable of 
yielding absolute directional information. The sensor 120 as described 
above should be capable of sensing velocity-induced displacements out to a 
range of 1.0 to 2.0 kHz. 
Piezoelectric Crystal and Mach-Zehnder Interferometer 
Referring to FIG. 3, an alternate embodiment of the present invention 
replaces the 50 and magnet 60 with a piezoelectric crystal 150. The 
crystal 150, when stressed, produces a voltage. The voltage output from 
the crystal 150 is coupled to a CMOS amplifier 152 that amplifies the 
crystal output voltage by an amount sufficient to produce phase modulation 
in the waveguide 14. By selecting the piezoelectric crystal 150 for 
specific hoop, beam or longitudinal resonance, the frequency range of the 
sensor element can be extended to nearly 100 kHz. 
Modular Seismic Sensor 
The present invention also includes a novel optical seismic sensor and a 
modular distributed system that has the advantage of proving a low power 
deployable sensor system. The system includes the following elements: 
optical transmitter/receiver module 200, telemetry harness 202, and sensor 
elements 220 and 222. Other sensors (not shown) may be included in 
addition to the sensor elements 220 and 222. 
The function of this unit is to transmit and receive optical signals from 
the sensor elements. Contained within this unit are the optical source 
(laser) and laser drive electronics, optical receiver, demultiplexing and 
demodulation (demux/demod) electronics. A block diagram of the optical 
transmitter/receiver module 200 and interchangeable sensor element is 
shown in FIG. 5. 
A frequency synthesizer 210 provides a signal having frequency .omega. to a 
laser 212 and to a demultiplexer/demodulator 214. The laser 212 provides 
frequency modulated optical signals to a plurality of sensors 220 and 222. 
The sensors 220 and 222 may include both accelerometer sensor elements and 
acoustic sensor elements. 
Signals from the sensors 220 and 222 are input to an optical receiver 224. 
The optical receiver 224 produces signals that are input to the 
demultiplexer/demodulator 214, which then produces signals that can be 
further processed to measure changes in the parameters being sensed. 
The laser optical source 212 and the laser drive electronics are 
interrelated and must be considered together. The laser is frequency 
modulated either directly, by mixing a small amplitude sine wave with the 
DC bias voltage, or by external phase modulation. The frequency modulation 
is done to eliminate signal fading of the interferometric signal at the 
optical receiver 224 and to allow the use of remote interrogation 
techniques such as FM or synthetic homodyne. 
The sensors 220 and 222 are preferably interferometric in nature The 
external disturbance, such as a vibration propagating through the earth, 
sensed by the sensors 220 and 222 is converted to an optical phase shift. 
This optical phase shift is detected at the optical receiver 224 as an 
intensity variation. The optical receiver 224 converts the optical signal 
into a corresponding time varying voltage given by 
EQU V(t)=A+B cos(.PHI..sub.m cos.omega..sub.m t+.PHI.(t)) 
where, .PHI..sub.m and .omega..sub.m are laser modulation parameters, and 
.PHI.(t) is the detected information. This voltage is then sent to the 
demultiplexer/demodulator 214 where the desired information is extracted. 
Signal extraction preferably is done via the differentiate and cross 
multiply technique as described by Dandridge et al. "Homodyne Demodulation 
Scheme for Fiber Optic Sensors Using Phase Generated Carrier," pp 
1647-1653, IEEE Journ. of Quant. Elec., Vol. QE-18, No. 10, October 1982. 
To complete the discussion of the optical transmitter/receiver module 200, 
various types of multiplexing architectures will be discussed. The scope 
of this discussion is limited to a few telemetry architectures, without 
going into great details of their workings, merits and faults. The two 
most popular forms of multiplexing presently used are time division (TDM) 
and frequency division (FDM) multiplexing as described by Kersey, 
"Multiplexed Interferometric Fiber Sensors," pp 313-319, 7th Optical Fiber 
Sensors Conference Proceedings, December 1990. 
In the TDM technique, the sensors are arranged in such that their return 
signals are separated in time. Typically a pulsed optical source is used 
for this type of multiplexing. Therefore, the timing of the return pulse 
identifies the sensor position. 
In the FDM approach, the array elements are arranged in an N.times.M/2 
matrix, where N is the number of laser modules and M (M=2N) is the number 
of sensing elements. The optical modules are modulated at discrete 
frequencies, .omega..sub.1, .omega..sub.2 . . . .omega..sub.N. In this 
case the return times of the sensors are nominally the same but are 
separated by their specific carrier frequency, .omega..sub.f. 
A third common multiplexing technique is Wavelength Division Multiplexing 
(WDM). In this case, lasers of different wavelength are used, and the 
sensor returns are separated by color (or optical frequency). Hybrid 
multiplexing techniques using the above multiplexing techniques in 
combination have been proposed and have been shown to be theoretically 
feasible. 
The telemetry approach taken in a detection network is governed by several 
factors: the number of sensors, the proximity of the sensors with respect 
to each other, and the minimum detectable signal. These factors are traded 
off to arrive at an architecture that will allow the system to meet its 
performance goals. 
A system using a trip-wire mode will allow longer utilization time before 
re supply is needed. A single sensor in the array is used to monitor the 
environment. Detection of a signal level that exceeds a preset value 
causes the power-up of the entire array, and begins data collection. Array 
level data is then stored in a CMOS memory for retrieval later (through 
remote means). This store and dump technique will not only lengthen the 
field time of the systems but also reduce the amount of data that needs to 
be processed. Alternatively, the system can be configured for continuous 
operation. 
Systems can be composed of a single sensor type or a combination of sensor 
elements using a modular design approach. Choosing combinations for a 
particular mission will maximize array use and data obtained. As an 
example, an array composed of five seismic and five acoustic sensors laid 
out in a grid would yield the bearing location from the seismic elements 
(multiple directional sensors are required to get rid of bearing 
ambiguities) and target ID via the acoustic grid. Improvements in the 
signal to noise ratio are obtained via array gain. In the above example an 
array gain of 7 db, 10 log N where N is the number of sensors in the 
array, would be realized. This, of course, is a simple sum. However, 
weighted sums can be applied to improve directionality of the array. 
Referring to FIG. 6, a sensor module 230 includes a radio transceiver 232 
and a power supply/processor unit 234. The sensor module 230 thus is 
capable of telemetering information back to a central monitoring location. 
Additionally, a GPS unit can be included if absolute position is required. 
Deployment of a grid of these would enable the localization of targets. 
FIG. 7 illustrates a transmit/receive module power supply/processor unit 
236. A plurality of sensors S.sub.1, S.sub.2, . . . S.sub.n are connected 
to a bus 238, which carries signals from the sensors S.sub.1, S.sub.2, . . 
. S.sub.n to the transmit/receive module power supply/processor unit 236. 
Each sensor may be formed according to the invention as described herein 
with reference to FIGS. 1-4 and 8-10. 
Dual Phase Modulator Seismic Sensor 
Referring to FIG. 8, there is shown an embodiment of the invention that 
includes a pair of phase modulators 293 and 297. Three optical waveguides 
254-256 formed in a substrate 260 intersect to form a Y-coupler 262. An 
optical signal may be introduced to the optical waveguide 254, which then 
guides the optical signal to the Y-coupler 262. The optical waveguides 255 
and 256 diverge away from the Y-coupler 262 and then have linear parallel 
sections 264 and 266, respectively. Optical signals input via the optical 
waveguide 254 travel through the parallel optical waveguide sections 264 
and 266. The optical waveguides 255 and 256 then converge to 
closely-spaced parallel sections 268 and 270, respectively, which form an 
evanescent field coupler 272. The Y-coupler 262, the evanescent field 
coupler 272 and the portion of the optical waveguides 255 and 256 between 
these two couplers forms a Mach-Zehnder interferometer 280. 
A magnet 282 and a coil 284 are arranged in manner similar to the magnet 60 
and coil 50 as described above with reference to FIG. 1. Electrically 
conductive leads 286 and 288 extend from the ends of the coil. Referring 
to FIGS. 8 and 10, a first pair of electrodes 290 and 292 are mounted on 
the substrate 260 and arranged to be parallel to the section 264 of the 
optical waveguide 255. The electrodes 290 and 292 are on opposite sides of 
the optical waveguide section 264 and form the phase modulator 293. A 
second pair of electrodes 294 and 296 are mounted on the substrate 260 on 
opposite sides of the optical waveguide section 266 and form the phase 
modulator 297. 
The electrically conductive lead 286 is connected to the electrode 290 in 
the first electrode pair and the electrode 296 in the second electrode 
pair. The lead 288 is connected to the electrode 292 in the first 
electrode pair and the electrode 294 in the second electrode pair. For a 
given direction of movement of the magnet 282 and coil 284, one of the 
leads will be at a higher electrical potential than the other. It is 
assumed for the present discussion that the lead 286 is the positive and 
that the lead 288 therefore is the negative. The electrodes 290 and 296 
will therefore be positive with respect to the electrodes 292 and 294. 
Therefore, as shown in FIGS. 8-10, the electric fields across the optical 
waveguide sections 264 and 266 are oppositely directed. 
For a given voltage, the arrangement described above for connecting the 
coil 284 to the phase modulators 293 and 297 produces twice the phase 
shift between optical signals guided by the optical waveguide sections 262 
and 264 than is produced with a single electrode pair. The increased phase 
shift arises because optical signals in one optical waveguide section 
experience a positive phase shift while optical signals in the other 
optical waveguide section experience a negative phase shift. The 
magnitudes of these positive and negative shifts are the same if the 
electrode size and configuration are the same for each of phase modulators 
293 and 297. 
It should be noted that the dual phase modulator arrangement of FIG. 8 
could be incorporated into a Michelson interferometer. The Michelson 
interferometer with two phase modulators would have four times the phase 
shift of the standard Mach-Zehnder interferometer design of FIG. 1. 
Discussed above is a novel approach for the sensing of ground born velocity 
and the relative bearing of the disturbance. Two configurations of the 
device are available, one with an estimated response bandwidth near 2.0 
kHz, the other with a potential bandwidth near 100 kHz. In addition, an 
acoustic configuration of the sensor element is also pointed out. The 
system level approach incorporates a modular design, that provides 
flexibility of the optical transmitter/receiver module design details and 
longer deployments between re supply due to the trip wire detection 
method. 
It should be noted that an acoustic sensor can be made using either the 
coil/magnet embodiments of FIGS. 1 and 2 or the piezoelectric crystal of 
FIG. 3. Slight reconfiguration of the sensors would be required to form an 
acoustic sensor. In the case of the embodiment of FIG. 3, the 
piezoelectric crystal must be exposed to the dynamic acoustic field. For 
the coil/magnet configurations of FIGS. 1 and 2, the magnet may acted on 
by the acoustic field via a diaphragm. 
The structures and methods disclosed herein illustrate the principles of 
the present invention. The invention may be embodied in other specific 
forms without departing from its spirit or essential characteristics. The 
described embodiments are to be considered in all respects as exemplary 
and illustrative rather than restrictive. Therefore, the appended claims 
rather than the foregoing descriptions define the scope of the invention. 
All modifications to the embodiments described herein that come within the 
meaning and range of equivalence of the claims are embraced within the 
scope of the invention.