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This invention relates to the use of a birefringent crystal as the sensing
element in a probe thermometer constructed of certain basic optical
components utilized in three separate modes. Thus, all of the modes of the
invention are directed to a probe thermometer which uses the temperature
dependence of the birefringence of certain single crystals as the
temperature sensitive parameter. One such crystal is a Y-cut single
crystal of LiTaO.sub.3. Alternative crystals having adequate sensitivity
in the desired temperature range may be constructed from LiNbO.sub.3 or
BaTiO.sub.3. Polarized light propagates through the crystal in two modes,
the ordinary ray and the extraordinary ray, which have indices of
refraction n.sup.o and n.sup.e. For LiTaO.sub.3 at room temperature, n =
2.2, B = n.sup.e - n.sup.o = 0.004, and dB/dT = 4.4 .times. 10.sup.-5
/.degree. C. The intensity of light passed through a sandwich of aligned
sheet polarizer, crystal, and optical analyzer is a function of B and
hence also is temperature dependent. A thermometer probe is constructed by
bonding this sandwich to a bundle of optical fibers along with a
dielectric mirror so that the sensor will be at the probe tip. Half of the
fibers conduct light from a light source which is a monochromatic light
source, such as a light emitting diode (LED) or white light with a
suitable filter, to the sensor tip, while the remainder conduct light from
the sensor to a light detector which is a photodetector, such as a
photodiode, photomultiplier tube, or phototransistor. One example uses a
crystal 0.1 mm thick of LiTaO.sub.3, with which a temperature range of
33.degree. C. or from 15.degree.-48.degree. C. with a 0.1.degree. C.
resolution is attained and another example uses a separate analyzer
oriented with its axis perpendicular to that of the polarizer where the
analyzer is interposed between the crystal and the dielectric mirror. In
this case a satisfactory temperature range of 12.5.degree.-65.degree. C.
is achieved.
An alternative configuration utilizes only a single optic bundle, instead
of two, leading from a beam splitter to the sensor. The operation of the
second configuration is as follows:
Light is generated by a light source, such as a light emitting diode,
(LED). The beam splitter generates two light beams, one entering the
optical fiber which leads to the sensor and one to be used to generate a
reference signal. Light returning from the sensor is reflected by the beam
splitter into an optical fiber which leads to a photodetector such as a
photodiode. Light from the reference beam can be used to produce a signal
which is proportional to the LED output and through known circuitry
eliminates the effects of drifting in the LED output.
The reference signal is used to send signals into an optical fiber past a
chopper wheel to a mirror and back through the beam splitter to the
photodetector. The photodetector signal consists of two time-modulated
parts. When the chopper is closed, only the thermometer signal is
received. When the chopper is open, the signal is the sum of the
thermometer plus reference signals.
A third configuration and an alternate way to use the reference is to send
the reference signal via fiber optics to a second photodetector circuit.
This method is more facile but requires matched photodetectors and
amplifier channels and does not permit the detection or elimination of
relative drifting between the two channels.
PRIOR ART
The closest literature prior art are devices manufactured from liquid
crystal devices based upon the selective reflection of red light such as
disclosed in the following:
C. C. Johnson et al, "Discussion Paper: Fiberoptic Liquid Crystal Probe for
Absorbed Radio-Frequency Power and Temperature Measurement in Tissue
During Irradiation," (In Biologic Effects of Non-Ionizing Radiation,
Annals of the New York Academy of Sciences, ed. Paul E. Tyler,
247:527-531, Feb. 1975. This paper discusses a probe that uses the
reflectance of red light from a mixture of liquid crystals as the
sensitive parameter.
T. C. Rozzell, et al, "A Nonperturbing Temperature Sensor for Measurements
in Electromagnetic Fields," Journal of Microwave Power, 9:241-249,
September 1974.
Illustrative of the use of a solid crystal as in the present device is the
following published art:
T. C. Cetas, "A Birefringent Crystal Optical Thermometer for Measurements
of Electromagnetically Induced Heating," International Union of Radio
Science (URSI) Annual Meeting, October 20-23, 1975, Reports, B11-3, pages
274-276 (Abstract).
In the patent art the following U.S. Pat. is noted: No. 3,453,434 Takami et
al. This patent relates to measurement of birefringence (difference in
refractory indices in two crystalline directions) at column 4, lines
55-63, but the patentees critically do not measure the temperature of the
crystal. Thus, the Takami et al patent is directed to an infrared ray
detector rather than to a thermometer probe.
BACKGROUND
The need for new thermometers that can be used in the presence of strong
radio frequency electromagnetic fields has been demonstrated dramatically
in recent years (See, for example, C. C. Johnson and A. W. Guy,
"Non-ionizing Electromagnetic Wave Effects in Biological Materials and
Systems," Proc. of IEEE, 60:692-718, 1972; and T. C. Cetas, "Temperature
Measurement in Microwave Diathermy Fields: Principles and Probes,"
presented at the International Symposium on Cancer Therapy by Hyperthermia
and Radiation, April 28-30, 1975) and developments now are appearing to
meet this need. The two basic approaches which have been followed have
nearly opposite points of view. One method is to start with a well
established thermometric technique, in particular, the use of thermistors.
The objective, then, is to reduce the electromagnetically induced heating
(both electric dipole and magnetic loop currents) through the use of high
resistance leads and by minimizing the area enclosed by the circuitry.
Examples of these are the MIC devices (see L. E. Larsen et al, "Microwave
Decoupled Brain-temperature Transducer," IEEE Trans. on Microwave Theory
Tech. MTT-22, 438-444, 1974) and that described by Bowman (see R. R.
Bowman, "A Probe for Measuring Temperature in Radio Frequency Heated
Material," presented at the International Microwave Power Symposium,
Waterloo, Ontario, May 1975, and IEEE Trans. on Microwave Theory Tech.,
MTT-24, 43-45, 1976). The other approach has been to begin with materials
which do not interact with the electromagnetic radiation and then to make
a good thermometer, that is, a device which has the appropriate range, has
adequate temperature resolution, and can be calibrated. Probe thermometers
following this approach are optical devices which use fiber optics to
communicate with the sensor and relate its temperature to the intensity of
the light reflected from the sensor. Examples of these are a liquid
crystal device which is based on the selective reflection of red light
(see C. C. Johnson, et al, Discussion paper: Fiberoptic Liquid Crystal
Probe for Absorbed Radio-frequency Power Temperature Measurement in Tissue
During Irradiation, (in) Biologic Effects of Non-ionizing Radiation, Ann.
N.Y. Acad. Sci., February 1975, 247:527-531; and T. C. Rozzell et al, "A
Non-perturbing Temperature Sensor for Measurements in Electromagnetic
Fields," J. of Microwave Power, 9:241-249, 1974) and the birefringent
crystal sensor which is the subject of this patent application. This
latter development by T. C. Cetas appears in a paper, "A Birefringent
Crystal Optical Thermometer for Measurements of Electromagnetically
Induced Heating," which was presented at the International Union of Radio
Science (URSI), Boulder, Colorado, October 1975. Two inherent advantages
of the birefringent crystal sensor are the stability of the sensor itself
(a solid single crystal in contrast to a semi-ordered liquid) and the
existence of a simple, physically based, expression to characterize the
thermometer calibration.
The present invention consists of three configurations of a probe-type
thermometer utilizing a birefringent crystal at the tip of the probe and
these variations have been generally described above. As noted above, the
first configuration is an assembly of a probe thermometer made by mounting
a polarizing disc, a birefringent crystal (anisotropic) with temperature
dependent indices of refraction and a mirror at the end of a bundle of
optical fibers. Half of the fibers in the bundle lead to a light source
such as a light emitting diode, the other half lead to a photodetector,
such as a photodiode. Appropriate electronics energize the light emitting
diode and amplify and measure the signal received by the photodetector.
One difficulty with the device is that a slow drift may occur in the
calibration as a result of drifting characteristics of the light source
and light detector. The present application discusses another method of
assembly which eliminates the effects of drifting electro-optic
components.
The second basic configuration is sketched in FIG. 3. Instead of two fiber
optic bundles going to the sensor, only one leads from a beam splitter to
the sensor. This sensor arrangement is simpler and is easier to construct
and to miniaturize.
A third configuration substitutes a second reference photodetector circuit
for a chopper and wheel.
PHYSICAL BASIS
The physical basis of the birefringent crystal sensor can be understood
(see Born and Wolf, Principles of Optics, 4th ed, Oxford, Pergamon, 1970
pages 694-696) by considering the two orthogonal principal optic axes, D'
and D", respectively. If a polarizer is placed in front of the crystal and
is oriented so that its axis, P, is at angle .phi. with respect to the
crystal axis, D', and if an analyzer is placed behind the crystal with its
axis, A, at angle, X, with respect to the polarizer P, then the light
intensity, I, which passes through the three pieces, is related to the
intensity, I.sub.o, which is passed by the polarizer, by
I = I.sub.o (Cos.sup.2 X - sin 2.phi. sin 2 (.phi. - X) sin.sup.2
.delta./2) (1)
The phase shift, .delta., between the rays propagated along the two
principal axes is given by
.delta. = (2.pi./.lambda.) h (n" - n') (2)
where h is the crystal thickness and .lambda. is the wavelength of the
light. When the polarizer and analyzer axes are parallel (X = 0), Equation
1 becomes
I = I.sub.o (1 - sin.sup.2 2.phi. sin.sup.2 .delta./2) (3)
The factor sin.sup.2 2.phi. is a constant, .alpha., for a particular
arrangement of the polarizer, analyzer, and crystal. The ideal case would
be for .phi. = .pi./2 which gives .alpha. = 1. We assume that the
birefringence, n" - n', depends linearly on the temperature, T, and set
.delta./2 = (T - .theta.). Hence,
I = I.sub.o (1 - .alpha. sin.sup.2 .beta. (T - .theta.)) (4)
where .theta., .alpha., .beta., and I.sub.o are calibration constants for
an individual optical thermometer. Once these are determined,
temperatures, T(I) can be calculated from measured values of I by
inverting Equation (4) to yield
T (I) = .theta. + (1/.beta.) sin.sup.-1 ((I-I.sub.o)/I.sub.o
.alpha.).sup.1/2 TM ( 5)
equation (4) is a multiple-valued function of the temperature and so some
provisions must be made to determine unique temperatures from measurements
of I. One approach is to select the parameters h, .lambda., n' and n" (or
.beta. and .theta.) such that only one-half of the sine function is
covered in the temperature region of interest, say, 15 to 50.degree. C.
Alternatively, the parameters could be chosen so that the maxima in the
function repeat frequently with temperature, e.g., every degree. Then an
unknown temperature would be determined by counting the number of maxima
from some reference temperature (as in fringe counting in interferometry)
with interpolation between maxima for finer resolution.
The birefringence of a Y-cut single crystal disc of lithium tantalate
(LiTaO.sub.3) shows a rather large temperature dependence (see Miller and
Savage, "Temperature Dependence of the Optical Properties of Ferroelectric
LiNbO.sub.3 and LiTaO.sub.3," Appl. Phys. Lett., 9:169, 1966). In
particular, for this material, n" .perspectiveto. n' + 2.18, n" - n' = 4.4
.times. 10.sup.-3 near room temperature and d(n" - n')/dT = 4.5 .times.
10.sup.-5 /.degree. C. If red light (.lambda. = 660 nm) is used to sense
the birefringence of a 0.2 mm thick crystal, .delta./2 will range from
.pi. radians near 12.3.degree. C. to 3.pi./2 radians near 49.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 are schematics of two examples of methods of assembly of the
first configuration of this invention showing a split optical bundle.
FIG. 3 depicts a diagramatic illustration of a single optical bundle and
beam splitter embodying a second configuration of this invention and with
reference signal via chopper wheel.
FIG. 4 is a diagram of the beam splitter optical fiber arrangement.
FIG. 5 is a diagramatic sketch of a beam splitter using a lens system.
FIG. 6 is a diagramatic sketch of the third configuration showing a
reference signal utilizing a second photodetector circuit.
FIG. 7 is a block diagram of an electronic tabulating unit.
THERMOMETER CONSTRUCTION
Several thermometers have been constructed according to the ideas suggested
above. FIGS. 1 and 2 are schematic representations of two examples of
methods of assembly. A bundle of optical fibers was bifurcated such that
half of the fibers, 11A, went to a light source, 12, and half, 11B, went
to a photodetector, 14. At the sensor end, 13, where the source and
detector fibers were well randomized, the bundle was potted in clear epoxy
and ground flat. In the first example, FIG. 1, a sandwich composed of a
properly aligned dichroic polarizer, 15, the crystal, 16, and a mirror,
17, was bonded to the tip. In this prototype, the crystal was 0.1 mm thick
(optical thickness was 0.2 mm) and the light source was a red light
emitting diode (.lambda. = 660 nm). The useful temperature range was 15 to
48.degree. C. The sensitivity of this thermometer was better than
0.1.degree. C. The diameter of the discs (polarizer, crystal, and mirror)
were 2 mm, but this is not restrictive; these could be reduced.
In the second example, FIG. 2, a second polarizing film, the analyzer, 18,
is inserted between the crystal and the mirror. The orientation of its
optical axis is perpendicular to that of the polarizer, 15. A mathematical
analysis similar to that above leads to a dependence of the reflected
light on the fourth power of the cosine of the phase shift .delta.
(Equation 2), where, in this case, the optical thickness, h, is the same
as the physical thickness of the crystal. This configuration led to a
useful temperature range of 15 to 65.degree. C. for the thermometer.
In the above, the apparatus is utilized by pulsing the LED, which permits
greater drive currents and results in greater light intensity. In the
present first configuration, the LED and photodiode are potted in the body
of BNC connectors with the optical fibers butted and potted tightly to
them. Thus, the thermometer probes are terminated electrical connections
rather than optical ones which are more difficult to reassemble
reproducibly.
The improved configuration, illustrated in FIG. 3, overcomes the effect of
drifting electro-optic components. As shown in FIG 3, instead of two fiber
optic bundles going to the sensor 13A, only one leads from a beam splitter
to the sensor 13A. The sensor arrangement of FIG. 3 is simpler than that
of FIG. 2 and thus is easier to construct and to miniaturize. The
operation is as follows: Light is generated by a light source 12A, such as
a light emitting diode (LED). The beam splitter 19 generates two light
beams, one entering the optical fiber bundle 11 which leads to the sensor
13A and a second beam used to generate a reference signal. Light returning
from the sensor 13A is reflected by the beam splitter 19 into an optical
fiber 11C which leads to a photodetector 21 such as a photodiode. Light
from the reference beam can be used to produce a signal which is
proportional to the LED output and hence through proper circuitry
eliminates the effects of drifting in the LED output.
One alternative in using the reference signal is to send the reference
signal into an optical fiber bundle 11D past a chopper wheel 23 to a
mirror 24 and back through the beam splitter 19 to the photodetector 21.
This is illustrated in FIG. 3 where the light is guided through optical
fibers to each of the components.
As one form of the device, the photodetector signal consists of two time
modulated parts. When the chopper 23 is closed, only the thermometer
signal is received by photodetector 21. When the chopper is open, the
signal received by photodetector 21 is the sum of the thermometer plus
reference signals. Appropriate electronics 25, described hereinafter,
operate on these signals to yield an output which is proportional to the
ratio of the thermometer signal to reference signal.
An alternative method of using the reference is to send the reference
signal via fiber optics to a second photodetector circuit (see FIG. 6).
This method is simpler but requires matched photodetectors and matched
dual amplified channels. It does not permit the detection or elimination
of relative drifting between the two channels.
FIG. 4 illustrates an alternative method of constructing the beam splitter
optical fiber arrangement. In this arrangement the materials should have
about the same index of refraction so that spurious optical reflections
from interfaces are minimized.
A second alternative form of the beam splitter arrangement is shown in FIG.
5 in which the light is guided through a lens system.
DESCRIPTION OF THE ELECTRONIC BLOCK DIAGRAM FOR THE OPTICAL THERMOMETER
The system 25 can be divided into two sections with the PREAMP and
INTEGRATOR, A-D Converter, Scratchpad Memory, and Control Logic as Section
I. Section II, which is composed of Program Data Storage, Arithmetic
Processor, Digital Display, D-A Converter, and Program Control, is
essentially a Central Processing Unit that treats the other section as a
peripheral device, operates upon the data from the device and presents the
resultant data on a digital display as well as in analog form (see FIG.
7).
Section I
The PREAMP and INTEGRATOR section samples and integrates exactly 10 pulses
from the optical detector. Upon command of the Control Logic, signals from
the temperature modulated beam (sample), from the sample and reference
beam combined (sample plus reference), and from the dark current plus
analog offset voltages (zero) are read. These signals are digitized
sequentially by the A-D converter and are stored in the scratchpad memory.
Section II
Under control of the Program Control Logic which is stored in the PROM
memory, the data are transferred to the Program data storage where they
are available to be manipulated by the Arithmetic Processor. The Zero
reading is subtracted algebraically from sample and sample plus reference.
This negates the effects of drifts in the analog circuitry. Next sample is
subtracted from sample plus reference. The ratio of sample to reference is
computed and the outputs (digital display and analog voltage) are updated.
When the data are transferred to Section II, Section I is triggered
simultaneously to begin accumulating new data for the next reading. The
period required for each data cycle (that is, the time interval between
output dates) is 500 ms.
Alternative Method
Alternative to the circuitry above is replacement of this circuitry with
microprocessor techniques. This will permit a reduction in the complexity
and cost of the electronics package and at the same time increase its
capability to include an output in terms of temperature units (.degree.
C.).
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