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BACKGROUND
1. Field of the Invention
This invention relates to temperature sensors and, more particularly, to a
temperature sensor including a semiconductor as the sensing element, the
semiconductor utilizing the band edge absorption principle of the
semiconductor for sensing temperature by absorbing monochromatic radiant
energy at a function of the temperature.
2. The Prior Art
Conventional temperature probes utilizing metallic wires and sensors are
unusable in the presence of electromagnetic fields because of the inherent
electrical interference problems and field perturbation effects.
Accordingly, several non-perturbing temperature probes have been
fabricated and are either commercially available or described in the
literature. For example, fiberoptic temperature probes have been
fabricated and have been reported in the literature. Among these are (1)
the liquid crystal temperature probe (see "A Prototype Liquid Crystal
Fiberoptic Probe for Temperature and Power Measurements in R. F. Fields",
Johnson, C. C., et al, Microwave Journal, volume 18, number 8, pages
55-59, August 1975); (2) the birefringent crystal optical thermometer (see
"A Birefringent Crystal Optical Thermometer for Measurements of
Electromagnetically Induced Heating", Cetas, T. C., U.S.N.C/U.R.S.I" 1975
annual meeting, Boulder, Colorado, Oct. 20-23, 1975); and (3) the Etalon
Fiberoptic Probe (see "Temperature Measurement Using Optical Etalons",
Christensen, D. A. 1975 Annual Meeting of the Optical Society of America,
Houston, Texas, Oct. 15-18, 1974).
The various non-perturbing temperature probes are useful particularly in
the measurement of tissue temperature in the presence of an
electromagnetic field. This is particularly important where temperature
measurements of tissue are performed simultaneously with irradiation by
electromagnetic energy. Such tissue treatments include hyperthermia
treatment of cancer, microwave biohazards studies and microwave heating,
drying, and cooking. To be useful and widely accepted in the trade, it is
desirable that the non-perturbing temperature probe be small in diameter
at the tip region (less than 0.5 mm), have a stable calibration for at
least several weeks, an accuracy of at least 0.2.degree. C. or better,
reasonably simple and inexpensive to use and possess a wide temperature
range suitable for various tissue treatment procedures (33.degree. C. to
about 47.degree. C.), and, additionally, possess a broader temperature
range for microwave heating, drying, and cooking.
Each of the foregoing prior art devices are useful. However, it would be an
advancement in the art to provide a non-perturbing temperature probe which
meets at least some of the previously listed requirements. Such a
temperature probe is disclosed and claimed herein.
BRIEF SUMMARY AND OBJECTS OF THE INVENTION
This invention relates to a fiberoptic temperature probe which relies upon
the wavelength shift of optical absorption in a semiconductor at its band
edge as a function of temperature. Operating at the band edge of the
semiconductor, radiant energy of a specified wavelength is propagated
through the semiconductor and is absorbed as it excites valence band
electrons into the conduction band. Accordingly, the temperature probe
technique utilizing a semiconductor sensor is based upon the temperature
coefficient which the gap energy displays in the semiconductor sensor
material. The semiconductor sensor is obtained from a suitable
semiconductor material such as galium arsenide. Advantageously, the
semiconductor material is, selectively, fabricated as a
reflecting/refracting device as part of the optical components of the
system so as to reflect/refract radiant energy through the semiconductor
from a monochromatic transmitter to an intensity detector. The intensity
of the reflected/refracted radiant energy is measured to provide an
indication of the temperature of the semiconductor sensor. As defined
herein, monochromatic, quasi-monochromatic, and narrow band radiant energy
are terms recognized in the art as representing radiant energy of a
relatively narrow and nearly single frequency or wavelength.
It is, therefore, a primary object of this invention to provide
improvements in non-perturbing temperature probes.
Another object of this invention is to provide an improved method for
sensing temperature particularly in the presence of an electromagnetic
field.
Another object of this invention is to provide a semiconductor as the
sensor for a temperature probe, the semiconductor being fabricated to form
part of the optical system for the temperature probe.
These and other objects and features of the present invention will become
more fully apparent from the following description and appended claims
taken in conjunction with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic plan view of one presently preferred embodiment of
the temperature sensor of this invention;
FIG. 2 is a schematic enlargement of the sensor tip region of the
temperature probe of FIG. 1;
FIG. 3 is an enlarged perspective view of one preferred semiconductor
sensor during manufacture;
FIG. 4 is a schematic plan view of a second preferred embodiment of the
temperature sensor of this invention; and
FIG. 5 is graphical comparison between radiant energy transmission and
temperature for a galium arsenide semiconductor material.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention is best understood by reference to the drawing wherein like
parts are designated with like numerals throughout.
General Discussion
Recent work with semiconductor materials has shown that the energy bands
for electrons in an intrinsic direct-gap semiconductor are separated by
what is referred to in the art as a forbidden zone. The spacing of the
forbidden zone for a typical semiconductor material corresponds to a
certain infrared or visible radiant energy wavelength. At the temperature
ranges of interest herein, KT (Boltzmann's Constant times temperature in
degrees Kelvin) is very small compared to the forbidden gap energy
(.DELTA.E) so tha the lower band (valence band) electron states are almost
completely occupied while the upper band (conduction band) electron states
are nearly empty. If radiant energy of a specified wavelength is
propagated through the semiconductor, it will be absorbed as it excites
valence band electrons into the conduction band. The amount of radiant
energy absorbed will be dependent upon the extent that its photon energy
(hf) (Plank's Constant times frequency) exceeds the minimum required
excitation energy (.DELTA.E). Since the density of electrons states is
relatively large, the wavelength range between very little absorption and
heavy absorption (the band edge) is extremely narrow.
Accordingly, the present temperature probe technique is based upon the
negative temperature coefficient which the gap energy (.DELTA.E) displays
in most semiconductor materials. This temperature dependence, coupled with
the steep change in absorption as a function of wavelength, leads to a
significant temperature-dependent variation in the absorption of any
wavelength which lies within the region of rapidly changing radiant energy
transmission by the semiconductor material.
The measurement of transient surface temperatures using a semiconductor
sensing material and a monochromatic radiation is described in "Novel
Method for Measuring Transient Surface Temperatures with High Spatial and
Temporal Resolution", Journal of Applied Physics, Vol. 43, No. 7, July
1972, page 3213.
Utilizing the foregoing principles, a temperature probe system has been
developed in which narrow band radiant energy is obtained from a suitable
monochromatic radiant energy source such as a suitable, commercially
available monochromator, a light emitting diode (LED) source, or a laser
diode. This monochromatic light is transmitted to a small semiconductor
sensor located at the tip of the temperature probe. The semiconductor
material is suitably shaped and polished so as to provide the necessary
optical coupling for the system. The monochromatic light transmitted to
the sensor and through the same is reflected to a detector such as a
commercially available silicon photodiode. Changes in the sensor
temperature are observed as changes in received light intensity by the
detector. Accordingly, the light intensity may be related to the
temperature of the sensor by a standard calibration curve. Importantly,
the sensor material itself is inherently stable. Additionally, the
elimination of monochromatic radiant energy source fluctuations involving
a conventional reference channel and electronic ratioing before display
are believed useful in improving the accuracy of the temperature probe of
this invention.
Referring now more particularly to FIG. 1, a schematic illustration of a
first preferred embodiment of the semiconductor sensor of this invention
is shown generally at 10 and includes a fiberoptic probe 12. Fiberoptic
probe 12 includes at least two optical fibers 14 and 16 optically coupled
by a semiconductor sensor 20. Semiconductor sensor 20 is fabricated as a
prism as will be set forth hereinafter with respect to the description of
the invention set forth in FIGS. 2 and 3.
Optical fiber 14 is optically coupled to a monochromatic radiant energy
light source 22 and serves as a conducting means for conducting the
monochromatic radiant energy from source 22 to the semiconductor sensor
20. Optical fiber 16 is optically coupled between semiconductor 20 and a
receiver/display 24 and serves as a conducting means for conducting the
temperature-variable radiant energy transmitted by semiconductor sensor
20.
The wavelength of the radiant energy source 22 is specifically chosen so as
to be propagated through the semiconductor material and absorbed by
semiconductor sensor 20 as a function of temperature. For example, a
semiconductor sensor 20 fabricated from a galium arsenide sample 0.25
millimeters thick exhibits a 20% optical transmission at 40.degree. C. and
52.5% optical transmission at 25.degree. C. at a wavelength of 0.899
micrometers. An increase in the wavelength to 0.903 micrometers of the
radiant energy directed to the galium arsenide semiconductor sensor
resulted in optical transmission values of 45% at 40.degree. C. and 80% at
25.degree. C. Since the temperature range of interest in most tissue
experiments is approximately 33.degree. C.-47.degree. C., a wavelength for
radiant energy source 22 was chosen as 0.906 micrometers.
In one presently preferred embodiment of this invention, the monochromatic
radiant energy was obtained at a wavelength of 0.906 micrometers from a
commercially available monochromator. The monochromator was optically
coupled to the transmitting optical fiber, optical fiber 14. The intensity
of the monochromatic radiant energy transmitted by semiconductor 20 was
received in receiver-display 24 where the intensity of the same was
detected with a conventional silicon photodiode and digitally displayed.
The changes in sensor temperature were observed as changes in received
radiant energy intensity which were readily correlated to the temperature
by a previously determined calibration curve, for example, as set forth
herein at FIG. 5 for galium arsenide.
Referring now more particularly to FIG. 2, the sensing tip of temperature
probe 12 is shown schematically and greatly enlarged for ease of
illustration. Optical fiber 14 is configurated with a core 30 and a
cladding 32. Correspondingly, optical fiber 16 is configurated with a core
34 and a cladding 36.
Semiconductor sensor 20 is fabricated as a prism having reflective faces 40
and 42 and an incident face 43 (FIG. 3). Semiconductor sensor 20 is
optically coupled to the ends of optical fibers 14 and 16, and, more
particularly, to cores 30 and 34 therein, respectively.
In the embodiment illustrated herein, the monochromatic light emitted by
monochromatic source 22 (FIG. 1) is indicated schematically as ray 44 and
transverse core 30 of optical fiber 14. Ray 44 strikes face 40 of the
prismatic configuration of semiconductor sensor 20 and is reflected as ray
46 to face 42 where it is reflected a second time by face 42 as
transmitted ray 48. In its traversal of semiconductor sensor 20, the
radiant energy is absorbed by semiconductor sensor 20 as a function of the
temperature of semiconductor sensor 20. Accordingly, the intensity of
transmitted ray 48 will be diminished as the temperature of semiconductor
sensor 20 is increased. The intensity of transmitted ray 48 is then
readable as temperature as set forth in the representative calibration
curve illustrated in FIG. 5.
Only two optical fibers, optical fiber 14 and optical fiber 16, are shown
herein. However, it may be found desirable to include additional optical
fibers in probe 12 for the purpose of providing suitable radiant energy
guide means for the monochromatic radiant energy. Furthermore, increasing
the number of optical fibers from two to four would not substantially
alter the effective diameter of probe 12 to an undesirable degree since
there would be only a slight increase in the diameter with a doubling of
the radiant energy-carrying capacity.
Referring now more particularly to FIG. 3, semiconductor sensor 20 is shown
at a stage of fabrication from a larger prism 21 of a suitable
semiconductor material such as galium arsenide. In one presently preferred
technique of fabrication, a block of suitable semiconductor material is
selected and suitably secured in a machine block, for example, by
releasably bonding one edge of the semiconductor block in a 45.degree.
machined groove. Thereafter, the remainder of the block of semiconductor
material is polished away leaving a prism 21. A plurality of semiconductor
sensors 20 may be cut from semiconductor prism 21 by cutting the same
along a line 41 or line 41a so as to provide the appropriate reflecting
surfaces. A cut along line 41 provides a prism suitable for attachment
across the ends of two optical fibers whereas a cut along line 41a
provides a prism suitable for optically coupling at least four optical
fibers in probe 12. In particular, the prism configuration of
semiconductor sensor 20 is chosen such that the reflective faces 40 and 42
form an isosceles triangle having a 90.degree. angle at the apex. In this
manner, the incoming incident ray 44 (FIG. 2) is reflected as ray 46 (FIG.
2) parallel to face 43 until reflected by face 42 as transmitted ray 48
(FIG. 2) into core 34 of optical fiber 16. Advantageously, faces 40 and 42
of semiconductor sensor 20 do not require a reflective coating in view of
the relatively high index of refraction for a material such as galium
arsenide. For example, the index of refraction for galium arsenide is
approximately 3.4, the index of refraction being dependent upon the
wavelength of the incident electromagnetic radiation.
Clearly, semiconductor sensor 20 could be readily configurated as a double
component system involving a section of semiconductor sensor material in
intercepting relationship to incident light 44 (FIG. 2) with a
conventional prismatic or other suitable light coupling means for
directing the transmitted light 48 into core 34 of optical fiber 16. The
relatively high index of refraction of the semiconductor sensor material
set forth hereinbefore renders the optical coating of the reflective faces
of the prismatic configuration of semiconductor sensor 20 unnecessary. It
is, therefore, presently preferred that the semiconductor sensor 20 be
configurated as a prism as shown herein.
Referring now more particularly to FIG. 4, a second preferred temperature
sensor configuration is shown herein generally at 50 and includes a single
transmitting/receiving optical path 52 optically coupled to a
semiconductor sensor 54. Semiconductor sensor 54 is configurated as a flat
element having a flat reflective face 55 formed thereon. As illustrated,
semiconductor sensor 54 is shown having an exaggerated thickness for ease
of presentation. However, it is presently preferred that semiconductor
sensor 54 be fabricated as a flat disc and optically coupled at one face
to the end of optical path 52. In this manner, semiconductor sensor 54
serves as a temperature sensor and also as part of the optical components
for the radiant energy conducting means of this invention. Optical path 52
may be one or a plurality of optical fibers such as optical fibers 14 and
16 (FIGS. 1 and 2). Regardless of the number of optical fibers therein,
optical path 52 is configurated to both transmit and receive radiant
energy for semiconductor sensor 54.
A monochromatic radiant energy indicated schematically at ray 62 is
obtained from a conventional monochromatic radiant energy source 56. Ray
62 is directed through a beam splitter 60 where it is optically coupled as
incident ray 64 to optical path 52. Incident ray 64 traverses the length
of optical path 52 where it enters semiconductor sensor 54 and is
reflected by reflective face 55 thereof. Semiconductor sensor 54 absorbs
radiant energy of incident ray 64 as a function of temperature as set
forth hereinbefore. Face 55 reflects the reduced intensity radiant energy
from semiconductor sensor 54 as transmitted ray 66. Transmitted ray 66
strikes the beam splitter 60 and is directed as reflected ray 68 into a
conventional receiver/display 58. The intensity of ray 68 is interpreted
by receiver/display 58 as the temperature of semiconductor sensor 54.
Experimental results have shown that extraneous radiation reflected and/or
refracted by beam splitter 60 will enter receiver/display 58 and give a
spurious reading to the signals received therein unless appropriate steps
are taken to assure that it is excluded. Additionally, source fluctuations
in radiant energy source 56 may result in spurious signals received by
receiver/display 58 unless some form of conventional signal filtering or
ratioing is used. For example, a reference channel 70 may be optically
coupled to beam splitter 60 to receive reflected ray 72 and thereby serve
as a means for detecting and cancelling source fluctuations from the
readings developed by receiver/display 58. The various components of this
system including radiant energy source 22 (FIG. 1), receiver/display 24
(FIG. 1); radiant energy source 56, receiver/display 58, and reference
channel 70 are all commercially available equipment that is well known in
the art.
Importantly, the semiconductor sensor 20 (FIGS. 1-3) and semiconductor
sensor 54 (FIG. 4) of this invention is inherently stable and can be made
extremely small to correspond with the diameter of the overall optical
fiber radiant energy transmitting means for each respective configuration.
For example, a total temperature probe diameter of less than 0.250
millimeters was fabricated and achieved a temperature sensing resolution
better than 0.2.degree. C. in a temperature range of 33.degree.
C.-47.degree. C.
The Method
The method of this invention is practiced by fabricating a temperature
sensor from a suitable semiconductor material such as galium arsenide,
indium phosphide, selenium, galium aluminum arsenide, or the like which
exhibits the appropriate change in optical transmission as a function of
temperature for a given wavelength. The temperature sensor can be
fabricated so as to form a part of the optical path or may be merely
interposed in the optical path as set forth hereinbefore. Optical coupling
of the semiconductor material is advantageously achieved by by utilizing
an optical fiber system consisting of either one or a plurality of the
same so as to suitably optically couple the semiconductor material to the
monochromatic light source and the receiver/display.
The monochromatic radiant energy source may be any suitable commercially
available source including, for example, a Light Emitting Diode (LED) as
set forth hereinbefore, a grating monochromator, laser diode, or the like.
Advantageously, the radiant energy source may be coupled with a reference
channel including conventional electronic ratioing so as to cancel any
source fluctuations in the intensity of the source.
The detector may be any suitable intensity detector such as a silicon
photodiode so as to suitably detect the intensity of the monochromatic
radiant energy transmitted by the semiconductor material as a function of
temperature. Calibration of the detector is readily attained by placing
the semiconductor material in a material of known temperature and
preparing a suitable calibration curve, for example, as shown in FIG. 5.
Thereafter, the intensity of the transmitted monochromatic radiant energy
may be received, interpolated, and digitally displayed as the temperature
of the semiconductor material.
In one embodiment of this invention, a temperature probe system was
developed whereby a monochromatic radiant energy source having a
wavelength of approximately 0.906 micrometers are obtained from a
commercially available grating monochromator. This monochromatic radiant
energy was transmitted through two optical fibers of a 4-fiber bundle.
Each optical fiber had a outer cladding diameter of 0.085 millimeters so
that the overall diameter of the bundle was less than 0.250 millimeters.
A semiconductor sensor was fabricated as a prism from galium arsenide
semiconductor material. The galium arsenide was polished to match the size
of the optical fiber bundle and affixed to the tip of the same.
The radiant energy was transmitted through two optical fibers and
transmitted through the semiconductor material and thereafter reflected to
a silicon photodiode optically coupled to the remaining two fibers of the
optical fiber bundle. The intensity variations of the radiant energy
transmitted by the galium arsenide sensor were detected by the silicon
photodiode and digitally displayed as the temperature of the galium
arsenide sensor.
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 only as illustrative and not
restrictive and the scope of the invention is, therefore, indicated by the
appended claims rather than by the foregoing description. All changes
which come within the meaning and range of equivalency of the claims are
to be embraced within their scope.
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