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Claims  |
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I claim:
1. A method for sensing temperature and for generating optical signals
related to said temperature, the method comprising the steps of:
(a) providing a fiber optic through which at least one light beam from at
least one associated light source is transmitted from a first end of the
fiber optic to a second end of the fiber optic;
(b) providing an integral fluorescent solid illuminated by at least one
light beam from the one or more associated light sources, the integral
fluorescent solid further having a shape which is radially symmetric about
a longitudinal axis and having a surface which is tapered along the
longitudinal axis from a proximal end to a distal end, the proximal end
having a diameter substantially the same as the diameter of the core of
the fiber optic, and the proximal end being attached to the second end of
the fiber optic such that the longitudinal axis of the integral
fluorescent solid is substantially coaxial to the longitudinal axis of the
fiber optic;
(c) sensing fluorescence produced by the integral fluorescent solid and
collected and transmitted by the same fiber optic used to transmit light
beams from the one or more associated light sources; and
(d) determining temperature from a predetermined
fluorescence-versus-temperature relationship for the integral fluorescent
solid.
2. The method as recited in claim 1 wherein the fluorescent spectrum of
said integral fluorescent solid includes a plurality of emission lines
whose relative intensities vary with temperature, and wherein said step of
detecting comprises measuring the relative intensities of at least two of
the plurality of emission lines produced by said integral fluorescent
solid, and said step of determining temperature comprises looking up
temperature from a predetermined fluorescent intensity
ratio-versus-temperature relationship for said integral fluorescent solid.
3. The method as recited in claim 1 wherein said at least one associated
light source generates light pulses of predetermined duration, and wherein
said step of sensing comprises measuring fluorescence lifetime, and said
step of determining temperature comprises looking up temperature from a
predetermined fluorescence lifetime-versus-temperature relationship for
said integral fluorescent solid.
4. An apparatus for sensing temperature and for generating optical signals
related to said temperatures, the apparatus comprising:
(a) a fiber optic having a core through which at least one beam of light
from at least one associated light source is transmitted from a first end
of the fiber optic to a second end of the fiber optic;
(b) an integral fluorescent solid illuminated by at least one light beam
from the at least one associated light source, the integral fluorescent
solid further having a shape which is radially symmetric about a
longitudinal axis and having a surface which is tapered along the
longitudinal axis from a proximal end to a distal end, the proximal end
having a diameter substantially the same as a diameter of the core of the
fiber optic and greater than a diameter of the distal end, and the
proximal end being attached to the second end of the fiber optic such that
the longitudinal axis of the integral fluorescent solid is substantially
coaxial to a longitudinal axis of the fiber optic; and
(c) detection means for measuring the fluorescence produced by the integral
fluorescent solid and collected and transmitted by the same fiber optic
used to transmit at least one light beam from the at least one associated
light source, the detection means being operationally associated with the
first end of the fiber optic.
5. The apparatus as recited in claim 4 wherein said integral fluorescent
solid is an integral solid-state laser material.
6. The apparatus as recited in claim 5, wherein said integral solid-state
laser material comprises rare earth-and transition metal-doped glasses.
7. The apparatus as recited in claim 6, wherein said rare earth-and
transition metal-doped glasses include neodymium-doped glass.
8. The apparatus as recited in claim 6, wherein said rare earth-and
transition metal-doped glasses include chromium-doped glass.
9. The apparatus as recited in claim 5, wherein said integral solid-state
laser material comprises rare earth-and transition metal-doped crystals.
10. The apparatus as recited in claim 9, wherein said rare earth-and
transition metal-doped crystals include ruby.
11. The apparatus as recited in claim 9, wherein said rare earth-and
transition metal-doped crystals included ruby countered doped with ferric
ions.
12. The apparatus as recited in claim 9, wherein said rare earth-and
transition metal-doped crystals include neodymium-doped yttrium aluminum
garnet.
13. The apparatus as recited in claim 4, wherein the fluorescent spectrum
of said integral fluorescent solid includes a plurality of emission lines
whose relative intensities vary with temperature, said detection means
includes means for measuring the relative intensities of at least two of
the plurality of emission lines produced by said integral fluorescent
solid, and said surface of said integral fluorescent solid is
substantially parabolically tapered along said longitudinal axis of said
integral fluorescent solid.
14. The apparatus as recited in claim 13, wherein said integral fluorescent
solid is ruby.
15. The apparatus as recited in claim 13, wherein said integral fluorescent
solid is ruby counter-doped with ferric ions.
16. The apparatus as recited in claim 13, wherein said at least one
associated light source is a laser.
17. The apparatus as recited in claim 4, wherein said at least one
associated light source generates light pulses of predetermined duration,
and wherein said detection means includes means for measuring fluorescence
lifetime.
18. The apparatus as recited in claim 17, wherein said integral fluorescent
solid is ruby counter-doped with ferric ions.
19. A method for sensing temperature and for generating optical signals
related to said temperature, the method comprising the steps of:
(a) providing a fiber optic through which at least one light beam from at
least one associated light source is transmitted from a first end of the
fiber optic to a second end of the fiber optic;
(b) providing an integral fluorescent solid illuminated by at least one
light beam from the one or more associated light sources, the integral
fluorescent solid further having a shape which is radially symmetric about
a longitudinal axis and having a surface which is tapered along the
longitudinal axis from a proximal end to a distal end, the proximal end
having a diameter substantially tne same as the diameter of the core of
the fiber optic, and the proximal end being attached to the second end of
the fiber optic such that the longitudinal axis of the integral
fluorescent solid is substantially coaxial to the longitudinal axis of the
fiber optic;
(c) measuring the intensity of the fluorescence produced by the integral
fluorescent solid and collected and transmitted by the same fiber optic
used to transmit light beams from the one or more associated light
sources;
(d) detecting the intensity of Raman scattered light emanating from said
fiber optic due to said at least one light beam from said at least one
associated light source;
(e) monitoring the ratio of the intensity of the Raman scattered light and
the ratio of intensity of said fluorescence, the two intensities
substantially corresponding to the common illumination beam frequency
generated by said at least one associated light source; and
(f) determining temperature from a predetermined ratio-versus-temperature
relationship for said integral fluorescent solid and for said illumination
beam frequency.
20. An apparatus for sensing temperature and for generating optical signals
related to said temperature, the apparatus comprising:
(a) a fiber optic having a core through which at least one beam of laser
light from at least one associated light source is transmitted from a
first end of the fiber optic to a second end of the fiber optic;
(b) an integral fluorescent solid illuminated by at least one light beam
from the at least one associated light source, the integral fluorescent
solid further having a shape which is radially symmetric about a
longitudinal axis and having a surface which is substantially
parabolically tapered along the longitudinal axis from a proximal end to a
distal end, the proximal end having a diameter substantially the same as
the diameter of the core of the fiber optic, and the proximal end being
attached to the second end of the fiber optic such that the longitudinal
axis of the integral fluorescent solid is substantially coaxial to the
longitudinal axis of the fiber optic;
(c) Raman reference means for measuring the intensity of Raman scattered
light emanating from said fiber optic due to light transmitted from said
at least one associated light source, said Raman reference means being
operationally coupled to said first end of said fiber optic; and
(d) detection means for measuring the fluorescence produced by the integral
fluorescent solid and collected and transmitted by the same fiber optic
used to transmit at least one light beam from the at least one associated
light sources, said detection means being operationally coupled to the
first end of said fiber optic.
21. A method for sensing temperature and for generating optical signals
related to said temperature, the method comprising the steps of:
(a) providing a fiber optic through which at least one light beam from at
least one associated light source is transmitted from a first end of the
fiber optic to a second end of the fiber optic;
(b) providing an integral fluorescent solid illuminated by at least one
light beam from the one or more associated light sources, the integral
fluorescent solid further having a shape which is radially symmetric about
a longitudinal axis from a proximal end to a distal end, the proximal end
having a diameter substantially the same as the diameter of the core of
the fiber optic, and the proximal end being attached to the second end of
the fiber optic such that the longitudinal axis of the integral
fluorescent solid is substantially coaxial to the longitudinal axis of the
fiber optic;
(c) measuring the intensity of the fluorescence produced by the integral
fluorescent solid and collected and transmitted by the same fiber optic
used to transmit light beams from the one or more associated light
sources;
(d) determining temperature from a predetermined ratio-versus-temperature
relationship for said integral fluorescent solid and for said illumination
beam frequency.
22. An apparatus for sensing temperature and for generating optical signals
related to said temperature, the apparatus comprising:
(a) a fiber optic having a core through which at least one beam of laser
light from at least one associated light source is transmitted from a
first end of the fiber optic to a second end of the fiber optic;
(b) an integral fluorescent solid illuminated by at least one light beam
from the at least one associated light source, the integral fluorescent
solid further having a shape which is radially symmetric about a
longitudinal axis from a proximal end to a distal end, the proximal end
having a diameter substantially the same as the diameter of the core of
the fiber optic, and the proximal end being attached to the second end of
the fiber optic such that the longitudinal axis of the integral
fluorescent solid is substantially coaxial to the longitudinal axis of the
fiber optic;
(c) detection means for measuring the fluorescence produced by the integral
fluorescent solid and collected and transmitted by the same fiber optic
used to transmit at least one light beam from the at least one associated
light sources, said detection means being operationally coupled to the
first end of said fiber optic. |
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Claims |
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Description |
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This invention relates generally to temperature sensing, and more
particularly, to optical means for remote temperature sensing, especially
in harsh or inaccessible environments.
Temperature sensing and control are crucial in a variety of situations
arising in medicine, industrial operations, and scientific research.
However, in many cases, temperature measurements must be conducted
remotely because the processes or machinery to be monitored is
inaccessible or involves hazardous components, such as, high pressures,
high radiation levels, high intensity magnetic or electrical fields,
corrosive materials, or the like.
In medicine the need for temperature monitoring arises in several contexts.
Cardiac output and blood flow rates are measured by the thermal dilution
method, wherein a bolus of room temperature solution is injected into a
vein and blood temperature changes are monitored by one or more
temperature sensing devices located down stream. For accurate
measurements, it is desirable to use sensors with frequency responses
greater than the major frequency components of the arterial pressure
pulse, i.e., faster than about 20-30 Hz. In operations involving
artificial hypothermia, such as heart and neurological cases, temperature
monitoring is critical. Accurate temperature monitoring is also critical
where local hyperthermia is induced as a means of cancer therapy. The
differences between normal cells and malignant cells in their
sensitivities to thermal killing is often no more than a few tenths of a
degree. The presence of metallic sensors greatly complicates temperature
control in this mode of therapy as the presence of such sensors can alter
the thermal characteristics of the tissue in which they are imbedded.
Furthermore, electromagnetic heating of tissues causes special
difficulties: electromagnetic interference is induced in the thermometry
electronics, excessive and artifactual heating occurs in sensors
constructed of resistive material (both thermistors and themocouples), and
sensors, especially those contained in highly conductive (and hence
reflective) shields, perturb the electromagnetic fields used for heating.
Thus, application of metallic, electrically-based temperature sensors
presents significant problems: reduced accuracy due to noise and
inadequate frequency response, and direct electrical hazard to the
patient, especially when more than one sensor is employed.
In industrial process control the most common techniques for temperature
measurement utilize thermocouples, thermistors, and resistance
thermometers. These devices generate electrical signals which are
amplified and then converted into temperature readings or employed in
control functions. Frequently, these devices are impractical because the
process is inaccessible, too hazardous, or too corrosive for in situ
placement of sensors. For example, temperature monitoring of nuclear
reactor vessels and coolant systems, underground nuclear waste-disposal
sites, chemical dumping sites, working zones of coal-liquefaction
reactors, oil refinery processes, and like processes, all involve
conditions which make the use of standard electrically-based sensors
difficult or impractical.
Many of the above-mentioned difficulties with current information-gathering
technology can be overcome by using remote, in situ optical probes coupled
to a detector by optical waveguides, or fiber optics. Fiber optics are
durable, corrosion-resistant, heat-resistant, impervious to electrical or
magnetic interference, and are available in very small diameters, which
make them amenable for use with miniature probes.
In particular, optically-based probes generating fluorescent signals
modulated by ambient conditions are well suited for use with optical
fibers. However, a major drawback with such probes is that fluorescent
intensity depends not only on ambient conditions but also an excitation
beam intensity, which even when generated by a stabilized laser source can
have a short term drift as high as 2 percent.
Wickersheim in U.S. Pat. No. 4,075,493, dated Feb. 21, 1978 discloses a
device which utilizes fiber optics and which senses temperature by
monitoring fluorescent emission line intensity ratios of certain
phosphors. Wickersheim teaches the use of a class of oxysulfide-phosphor
materials characterized by the formula (RE).sub.2 O.sub.2 S:X, wherein RE
is lanthanum, godolinium, or yttrium, and X is selected from a specified
group of rare earth elements. Wickersheim further teaches the use of these
phosphors in the form of fine crystalline powders held by a nitrocellulose
or silicate binder. The oxysulfide powders limit the applicability of the
Wickersheim device. Unless sheathed in a protective coating the
oxysulfides are susceptible to oxidizing in a strongly oxydizing
environment and to reduction in a strongly reducing environment. Practical
application of the apparatus for remote sensing requires that the phosphor
emission lines be closely spaced so that frequency dependent fiber optic
transmission losses do not affect the signal.
As recognized in the art the use of powders causes orders of magnitude
reduction in the optical efficiency of the temperature probe. First, the
density of the crystalline powder held by a binder is only about 50
percent that of the pure crystal. Second, the thermal insulation
interposed by a binder material and protective sheath, and the powdered
nature of the sensor prevents the frequency response of the probe from
being any better than about a Hertz. And finally, phosphor excitation and
signal collection efficiencies are greatly reduced because scattering from
grain boundaries in the powder disperses the excitation beam, causes light
loss, and makes the emitted fluorescence hard to collect within the
limited fiber diameter.
Samulski in U.S. Pat. No. 4,245,507, dated Jan. 20, 1981 mentions the use
of three distinct fluorescent phenomena for measuring temperature:
intensity, emission line frequency shifts, and lifetime. Intensity, when
measured alone, leads to the least reproducible temperature
determinations, because it varies with excitation beam strength as well as
temperature. Emission line frequency shifts do not depend on excitation
beam strength, but they are difficult to measure since the shifts are
typically small (on the order of a few tenths of a nanometer wavelength
change per 100.degree. C.), and are often associated with considerable
broadening, even over relatively narrow temperature ranges (e.g.,
50.degree.-100.degree. C.).
The foregoing illustrates the limitations of the current technology. It is
apparent that it would be advantageous to provide an alternative to
available methods, particularly in regard to remote temperature sensing
apparatus.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide an optical method
and apparatus for remote temperature sensing in hostile or inaccessible
environments, wherein information-bearing optical signals are internally
calibrated by Raman scattered light generated by the fiber optic
transmitting the information-bearing optical signals.
Another object of this invention is to provide a fiber optic sensor for
high-speed remote measurement of temperature.
Another object of this invention is to provide a corrosion resistant
temperature probe with high optical efficiency, suitable for use with
communications-type, small diameter fiber optics, hundreds of meters or
more in length.
Still another object of this invention is to provide a fluorescence-based
temperature probe, utilizing fiber optics, which is chemically more
impervious and optically more efficient than those that current technology
provides.
Additional objects, advantages and novel features of the invention will be
set forth in part in the description which follows, and in part will
become apparent to those skilled in the art upon examination of the
following or may be learned by practice of the invention. The objects and
advantages of the invention may be realized and attained by means of the
instrumentalities and combinations particularly pointed out in the
appended claims.
These and other objects are attained in accordance with the present
invention wherein, generally, there is provided a fiber optic and at least
one associated light source for illuminating a suitable fluorescent solid
attached to the end of the fiber optic. The associated light source
delivers light to a first end of the fiber optic which, in turn, transmits
the light to its other, or second, end where the attached fluorescent
solid is illuminated and caused to fluoresce. The same fiber optic
delivering light from the associated light source also collects and
transmits fluorescent light from the fluorescent solid back to a detection
means for analysis. For accurate temperature measurements in remote and
chemically hostile environments it is critical that the temperature probe
be optically efficient, in that a maximal amount of light energy delivered
to the probe is recollected as fluorescent signal; that the optical signal
be independent of the intensity of the illuminating beam from the
associated light source; and that the probe be constructed of a
corrosion-resistant fluorescent solid with high thermal conductivity.
In accordance with the present invention, the fluorescent solid used in the
temperature probe must be used in the form of a single piece of material,
e.g., a single crystal or a single piece of suitably doped glass. The
material must be suitable for the transmission of the illumination beam
and fluorescent signal with minimal loss or diffusion, especially by
scatter that would occur, for example, in material that was in granular or
powdered form.
Fluorescent signal collection efficiency is further enhanced, in accordance
with the present invention, by providing the fluorescent solid in a shape
which is radially symmetric about a longitudinal axis and whose surface is
tapered along the longitudinal axis. Preferentially, the face of the wide
end of the tapered solid is optically smooth and is perpendicular to the
longitudinal axis. This end, hereinafter referred to as the proximal end,
has substantially the same diameter as the fiber optic core and is
attached to the end face of the fiber optic core. This shape allows a
greater fraction of fluorescence being emitted from any point within the
solid to be collected and transmitted by the fiber optic than would be the
case with a cylindrical-shaped solid.
In accordance with the invention, means are provided for precisely
monitoring the intensity of the illumination beam impinging on the
fluorescent solid and for measuring fluorescent signals generated by the
fluorescent solid which are independent of variations in illumination beam
strength. Unless such means are employed accuracy is severely limited by
short term photometric drift in the output of the light source and
possible misalignments in the optic system. Measuring the intensity of
Raman scattered light produced by the fiber optic core is a means employed
by the subject invention for monitoring illumination beam intensity.
Another means involves monitoring the ratio of intensities of multiple
fluorescent emission lines and correlating the ratio, instead of the
intensities themselves with temperature. The use of fluorescence lifetime
instead of intensity also avoids errors due to power fluctuations and
optical system misalignments.
In accordance with the invention, the class of suitable fluoresent solids
are selected with particular regard to their durability, their ability to
resist corrosion, and their ability to conduct heat, which is critical for
high-response time applications. Several solid-state laser materials
fulfill these criteria far better than fluorescent materials currently
being employed in remote temperature sensing technology. Ruby probes
fabricated from sapphire fibers, in accordance with the present invention,
are particularly well suited for high-speed temperature measurements in
harsh environments.
A sapphire fiber whose tip has been doped by exposure to Cr.sub.2 O.sub.3
and Fe.sub.2 O.sub.3 vapors in accordance with the invention is
particularly well suited for high speed temperature sensing. Here
fluorescence lifetime, and therefore response delay, is reduced by the
quenching effect of the iron.
Since the invention employs optical signals in a manner analogous to the
use of electrical signals in electrodes, the sensor is referred to as an
optrode, the descriptor "optrode" being derived from its electrical
analogue.
The present invention is addressed to problems associated with remote
temperature monitoring in hostile or inaccessible regions. It
advantageously overcomes many of these problems by combining rugged, high
quality fiber optics with simple in situ probes for generating optical
signals related to ambient temperature. The problem of increasing the
response time of probes is overcome by providing small probes made of
materials, e.g., ruby, with extremely high thermal conductivity. The
problem of low signal collection efficiency is overcome by providing a
probe consisting of a single, integral piece of material having a
maximally efficient shape for fluorescent collection. The problem of
reduced reproducibility due to illumination beam fluctuations is overcome
by monitoring beam intensity by Raman scatter generated by the fiber optic
core.
Furthermore, all particular embodiments of the invention are amenable for
use with a multi-position sensing system which comprises many sensors, all
of which feed signals to a single station for analysis. Such a
configuration can reduce costs by obviating the need for separate
analyzers for each sensor, and can increase reproducibility between
sensors by having all signals analyzed by the same instrument.
BRIEF DESCRIPTION OF THE DRAWINGS
Further objects of the invention, together with additional features
contributing thereto and advantages accruing therefrom will be apparent
from the following description of a preferred embodiment of the invention
which is shown in the accompanying drawings, which are incorporated in and
form a part of the Specification. In the drawings:
FIG. 1 diagrammatically illustrates a system for obtaining fluorometric and
Raman scatter information from a single remote location in accordance with
the present invention.
FIG. 2 is an embodiment of a coupler arrangement suitable for use with the
present invention.
FIG. 3 is an embodiment of a double-filter/spacial filter combination
suitable for use with some embodiments of the present invention.
FIG. 4 illustrates a configuration incorporating a mechanical chopper for
measuring fluorescent lifetimes in accordance with some embodiments of the
present invention.
FIG. 5 is a graph which contains signal intensity-versus-temperature
calibration curves for temperature probes made of chromium-doped glass and
neodymium-doped glass.
FIG. 6 graphically illustrates signal intensity-versus-distance
relationships for non-tapered and tapered probes made of neodymium-doped
glass.
FIG. 7 graphically illustrates the spectrum of ruby fluorescence at five
different temperatures when the probe was illuminated by 12 milliwatts of
514.5 nanometer light.
FIG. 8 graphically illustrates the temperature dependence of the intensity
ratio of the R.sub.1 and R.sub.2 emission lines of ruby.
FIG. 9A shows a non-tapered temperature probe and diagrammatically
illustrates the solid angle over which an attached fiber optic collects
fluorescence emitted from a particular point within the probe.
FIG. 9B shows an example of the preferred shape of the temperature probe
and diagrammatically illustrates the solid angle over which an attached
fiber optic collects fluorescence from a particular point within the probe
.
DETAILED DESCRIPTION OF THE INVENTION
Reference will now be made in detail to the present embodiment of the
invention, examples of which are illustrated in the accompanying drawings.
In accordance with the present invention a method and apparatus are
provided for remote temperature sensing via in situ fluorescent probes
optically connected to an excitation source and a detection system by a
single fiber optic. The crux of the present invention is coupling the use
of signals which are independent of excitation beam intensity with the use
of temperature probe materials and shapes that are, respectively,
chemically highly impervious and optically efficient.
By way of illustration, FIG. 1 shows a block diagram of a detection means
configuration, a light source 2, a fiber optic 4, and a temperature probe
6 for an embodiment employing a single optrode. In this embodiment the
detection means comprises coupler 8 for separating the outgoing
illumination (or excitation) beam from the incoming optical signal; beam
splitter 10 which divides the signal so that fluorescence intensity and
Raman backscatter intensity can be determined separately and substantially
simultaneously; double monochromator 12 and photomultiplier tube 14 for
determining Raman backscatter; monochromator 16 and photomultiplier tube
13 for determining optrode fluorescence; and an electronics and display
means 20 for processing and displaying the signals. An optical signal is
produced by optrode 6, passes through fiber optic 4, and is directed to
beam splitter 10 by coupler 8. Part of the optical signal proceeds from
beam splitter 10 to double monochromator 12 where the portion of the
optical signal corresponding to Raman scattered light from fiber optic 4
is directed to photomultiplier tube 14. Photomultiplier tube 14 generates
an electrical signal related to the intensity of the Raman scattered
light. This electrical signal is processed and displayed by electronics
and display means 20. The other part of the optical signal proceeds from
beam splitter 10 to monochromator 16 where the portion of the optical
signal corresponding to the fluorescent intensity of the optrode 6 is
directed to photomultiplier 18. Photomultiplier tube 18 generates an
electrical signal related to the intensity of the fluorescent emissions
generated by optrode 6. The electrical signal is processed and displayed
by electronics and display means 20. The double monochromator 12 and
photomultiplier tube 14 are components used in ordinary Raman spectroscopy
as illustrated in chapter two, "Experimental Methods", from Tobin, Laser
Raman Spectroscopy (Wiley-Interscience, 1971). Accordingly, this chapter
is incorporated by reference. The components for fluorescent intensity
measurements are also well-known in the art; in particular, U.S. patent
application Ser. No. 194,684 filed Oct. 6, 1980 and entitled "Remote
Multi-Position Information System and Method", discloses fluorescent
detection means suitable for use in accordance with the present invention.
Accordingly, U.S. patent application Ser. No. 194,684 is incorporated by
reference.
In accordance with the invention, the ratio of the intensity of Raman
scatter from the fiber optic and the fluorescent intensity of a
fluorescent solid is determined. The two intensities making up the ratio
are those corresponding to a common illumination beam frequency. The value
of the measured ratio is compared to ratios on a predetermined table,
which relates such intensity ratios to temperature for each kind of
fluorescent solid and for the particular illumination beam frequency
employed. Operationally, such a tabular relationship is stored on a data
acquisition computer, such as a DEC LSI-11 (Digital Equipment Corp.
Waltham, Mass.). Temperatures are determined by an interpolation scheme
implemented by the computer. Such utilization of the Raman scatter
intensity will be referred to as the Raman reference means.
An analogous procedure is followed when intensity ratios of separate
fluorescent emission lines are employed. Namely, once an intensity ratio
is measured, temperature is interpolated automatically from predetermined
intensity ratio-versus-temperature table stored on a small data
acquisition computer.
Coupler 8 allows a single fiber to be used both for transmitting the
illumination beam to optrode 6 and for collecting signals from the optrode
and fiber optic. FIG. 2 is an example of one coupler arrangement which is
used in accordance with the present invention. Illumination beam 24 is
directed into a first end of fiber optic 4 by lens 30. Lens 30 focuses
beam 24 through an aperture 22 in mirror 28. Light 26 exiting the fiber
optic is reflected by mirror 28 to collection lens 32.
Simpler detection means configurations are possible, and in some cases
preferred, for example, in embodiments of the invention which do not
utilize Raman backscatter as a reference signal. Where intensities of
multiple emission lines from the fluorescent solid are the only parameters
measured, the double monochromator may be replaced by a single
monochromator, or both monochomators may be replaced by a system of beam
splitters and filters depending on the nature of the fluorescent emissions
and the conditions to which the optrode is subjected.
An example of a double-filter configuration for analyzing intensity ratios
is illustrated in FIG. 3. This example employs the coupler shown in FIG.
2. After beam 26 passes through collection lens 32 it is collimated by
lens 48 and is split by beam splitter 31. One portion of beam 26 passes
through filter 33 and is focused by lens 35 onto photodetector 37. The
other portion of beam 26 passes through filter 39 and is focused by lens
41 onto photodetector 43.
Where fluorescent lifetimes are measured still other detection-means
configurations are necessary. By way of example, FIG. 4 illustrates a
configuration incorporating a mechanical chopper for measuring fluorescent
lifetimes. Illumination beam 24 is focused by lens 34 and deflected by
mirror 36 so that a chopper blade 38 interrupts the beam at its focal
point. The beam is then recollimated by lens 40 and deflected by mirrors
42 and 44 for injection into fiber optic 4. Lens 30 focuses the beam for
injection. Light 26 exiting the fiber optic is separated from the injected
beam by a coupler, in particular, apertured mirror 28. Light 26 exiting
the fiber optic and separated from the illumination beam is collected,
collimated, and focused by lenses 46, 48, and 50, respectively. The focus
of the beam is located in the path of the chopper blade 38. Lens 52 then
focuses the beam to monochrometer 54.
The class of fluorescent solids best suited for use in accordance with the
present invention are those selected from solid-state laser materials.
Other materials may be used, but the ruggedness and intense fluorescence
of the solid-state laser materials makes them a preferred class.
Crystalline laser materials are preferred among the solid-state laser
materials because of their superior thermal conductivity, sharper
fluorescence emission lines, and higher melting points than, for example,
laser glasses. However, laser glasses may be preferred for particular
applications. Generally, solid-state laser materials suitable for use in
accordance with the present invention may be selected from the materials
described by Koechner in Solid State Laser Engineering (Springer-Verlag,
1976). Accordingly, chapter two, entitled "Properties of Solid-State Laser
Materials", is incorporated by reference. Generally, these materials
include rare earth and transition metal-doped glasses and crystals. The
preferred host crystals are sapphire and garnet, the latter being
available in several suitable forms, such as yttrium aluminum (YAG),
yttrium gallium, gadolinium gallium, and gadolinium scandium aluminum.
These materials comprise the temperature probe which is attached to the
second end of the fiber optic, that is, the end placed adjacent to the
region whose temperature is to be determined.
By way of example, FIG. 5 illustrates the signal
intensity-versus-temperature relationships for chromium-doped glass 60 and
neodymium-doped glass 62. Neodymium and chromium glass fibers were worked
into 200-220 .mu.m fibers about 11/2 inches long and installed with
silicone potting compound in an AMP fiber optic terminator (General Fiber
Optics, Calwell, N.J.). The active end of the fiber was then submerged in
the reservoir of an apparatus for capillary melting point determination,
which was filled with glycerol. The temperature of the glycerol was
determined with partial immersion mercury thermometers. Excitation and
signal collection took place over 200 meters of Valtec PC-10 fiber optic
(Valtec Optical Group, Waltham, Mass.). The optrodes were illuminated by
an Argon laser operating at 514 nm and 2 milliwatts. Fluorescence
intensities were measured at 875 nm and 850 nm for the chromium-doped and
neodymium-doped glasses, respectively.
FIG. 6 illustrates signal intensity versus fiber optic (Valtec PC-10)
length for the neodymium-doped glass optrodes. Curve 64 shows results from
the same neodymium-doped fiber as that used to produce the data in FIG. 5,
except that the laser is operated at 100 mw and measurements are at room
temperature.
Ruby is the preferred solid-state laser crystal for use in accordance with
the present invention. FIG. 7 illustrates the fluorescent spectrum of ruby
in the region of the R.sub.1 and R.sub.2 emission lines for five different
temperatures. Here a ruby rod was mounted in a furnace and excited with 12
mw of 514 nm laser light. Signal collection took place over 200 meters of
Valtec PC-10 fiber optic. For long distance operation excitation with a
Helium-Neon laser is preferred because of lower fiber losses at 633 nm.
FIG. 8 shows how the ruby R.sub.2 -R.sub.1 emission line intensity ratio
varies with temperature. The normalized ratio was obtained with a dual
filter arrangement similar to the one illustrated in FIG. 3. That is,
filters were chosen so that R.sub.1 intensity was taken as the total
intensity of light at wavelengths longer than that of the midway point
between the R.sub.1 and R.sub.2 peaks, and R.sub.2 intensity was taken as
the total intensity of light at wavelengths shorter t | | |
