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Claims  |
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What is claimed is:
1. A fiber-optic temperature-measuring apparatus comprising:
a body of a temperature-dependent photo-luminescent solid semi-conductive
material wherein said body is subjected to a temperature to be measured;
a source of exciting radiation directed to said body so as to cause said
body to luminesce and produce luminescent radiation;
fiber-optic means for directing said exciting radiation to said body;
measuring means operatively associated with said body;
fiber-optic means for directing said luminescent radiation from said body
to said measuring means; and
two photodetectors having mutually dissimilar spectral response curves
operatively associated with said measuring means and provided for
receiving said luminescent radiation via said fiber optic means for
directing said luminescent radiation from said body to said measuring
means.
2. Measuring apparatus as claimed in claim 1, in which said two
photodetectors monitor the radiation from the body over said dissimilar
spectral response curves, one of which corresponds to said exciting
radiation reflected from the body.
3. Measuring apparatus according to claim 1 or claim 2, characterised in
that the body is of a semiconductor material selected from the group
consisting of AlP, AlAs, GaP, GaAs, InP, InAs, In.sub.1-x Al.sub.x P,
In.sub.1-x Ga.sub.x P, Ga.sub.1-x Al.sub.x P, In.sub.1-x Al.sub.x As,
In.sub.1-x Ga.sub.x As, Ga.sub.1-x Al.sub.x As, InAs.sub.1-y P.sub.y,
GaAs.sub.1-y P.sub.y, and y being between 0 and 1, ZnTe, ZnSe, ZnS, ZnO,
CdTe, CdSe and CdS.
4. Measuring apparatus according to claim 3, characterised in that the
semiconductor material is GaP doped with one of Zn and Cd and with O.
5. Measuring apparatus according to claim 3, characterised in that the
semiconductor material is ZnSe doped with one of Cu and Mn.
6. Apparatus as claimed in claim 1 or claim 2, in which a part of the
fiber-optic means for feeding exciting radiation to the body is the same
as a part of the fiber-optic means used for feeding luminescent radiation
from the body to the measuring means.
7. Measuring apparatus according to claim 6, in which each of said two
photodetectors is provided with an optical filter, one filter transmitting
the radiation emitted by luminescence in the body and the other filter
transmitting exciting radiation reflected by said body.
8. Measuring apparatus according to claim 7, in which a function generator
is arranged to receive the output signals from the two photo-detectors,
the output signal from the function generator providing a measure of the
temperature of the body of luminescent material.
9. Measuring apparatus according to claim 8, in which the function
generator is a quotient-forming circuit.
10. A fiber-optic temperature-measuring apparatus comprising first and
second bodies of temperature-dependent photo-luminescent solid material,
the first of which bodies is subjected to the temperature to be measured,
thermal control means for modifying the temperature of the second body,
means for measuring the temperature of the second body, at least one
source of exciting radiation for the said bodies which will cause each to
luminesce, fiber-optic means for feeding exciting radiation to the two
bodies and for feeding luminescent radiation from the bodies to
electro-optical measuring means, means for determining from the electrical
output of the measuring means when the luminescent radiation from the two
bodies is the same and thus when the temperature of the first body equals
the measured temperature of the second.
11. Measuring apparatus according to claim 1 or claim 2, in which the
exciting radiation is derived from a light-emitting diode.
12. Measuring apparatus according to claim 6, in which a partially
transparent mirror is arranged between the end of the fiber-optic means
closest to the body and the body of luminescent material.
13. Measuring apparatus according to claim 1 or claim 2, in which radiation
within two different wavelength ranges is employed, only one of said
wavelength intervals including the wavelength range within which the
luminescent material may be excited.
14. Measuring apparatus according to claim 13, in which an optical element
is arranged between the end of the fiber-optic means closest to the body
and the body of luminescent material, said optical element transmitting
radiation within the wavelength interval of the exciting radiation and
within the wavelength interval of the luminescent radiation but reflecting
radiation within any other wavelength interval.
15. Measuring apparatus according to claim 14, in which the radiation
emitted by the luminescence and the radiation reflected by said optical
element are adapted to be conducted via fiber-optic means, to said two
photo-detectors.
16. Measuring apparatus according to claim 15, in which the two
photo-detector means each include an optical filter, one filter
transmitting the radiation emitted by the luminescence and the other
filter transmitting the radiation reflected by the said optical element.
17. Measuring apparatus according to claim 16, in which the output signals
from the two photo-detectors are adapted to be supplied to a function
generator, the output signal of which is a measure of the temperature of
the luminescent material.
18. Measuring apparatus according to claim 1 or claim 2, in which a second
body of temperature-dependent photo-luminescent solid material which is
identical with the material used in the first-mentioned body as regards
photo-luminescence, is provided, together with means whereby the
temperature of said second body may be regulated.
19. Measuring apparatus as claimed in claim 10, in which the first and
second bodies are made to luminesce at two different repetition rates, the
two luminescent radiations being fed to a single photo-detector whose
electrical output is processed to divide it into a component corresponding
to the luminescence of the first body and a component corresponding to the
luminescence of the second body.
20. Measuring apparatus according to claim 19, in which a partially
reflecting member is disposed between the first body and the fiber-optic
means to reflect the luminescent radiation from the second body.
21. Measuring apparatus according to claim 19 or 20, in which the divided
output signals of the photo-detector are supplied to a control circuit,
the output signal of which feeds a power amplifier for controlling the
temperature of the second body.
22. Measuring apparatus according to claim 10, in which exciting radiation
from a common source is fed to the first and second bodies, and the
luminescent radiations from the two bodies are fed to different
photo-detectors.
23. Measuring apparatus according to claim 22, in which exciting radiation
reflected from the first body is supplied to one photo-detector of a
detector system and exciting radiation reflected from the second body is
supplied to a second photo-detector of the same detector system.
24. Measuring apparatus according to claim 23, in which the output of each
photo-detector is supplied to a function generator.
25. Measuring apparatus according to claim 24, in which the output signal
from the function generator is adapted to be supplied to a second function
generator together with the output signal from a third detector positioned
to receive luminescent radiation emitted from the first body.
26. Measuring apparatus according to claim 25, in which the output signal
from the second function generator is adapted to be supplied to a control
circuit with an output signal from a fourth detector positioned to receive
luminescent radiation emitted from the second body.
27. Measuring apparatus according to claim 26, in which the output signal
from the control circuit is supplied to a power amplifier for temperature
control of the second body.
28. Measuring apparatus as claimed in claim 24 or 25, in which the function
generator is a quotient-forming circuit. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a fiber-optic temperature-measuring apparatus
which is based on the photo-luminescence of a solid material, which is
subjected to the temperature to be measured.
For many applications of temperature measurement, it is of great interest
to be able to use a non-electrical method, which utilizes light
transmission in optical fibers. Such applications are especially
advantageous in explosive environments or in areas with strong electric or
magnetic fields.
2. Description of the Prior Art
A disadvantage associated with certain known measuring apparatus of this
kind is that they cannot be positioned at any place without special
measures. A further drawback is that it is not possible to distinguish
changes in transmitted radiation which are due to the temperature, from
other radiation changes that may occur for other reasons. In practice,
known fiber-optic temperature-measuring apparatus has to be calibrated
with a known temperature after the apparatus has been positioned at the
place of measurement, and this involves a complication.
One solution to the above-mentioned problems, and other associated
problems, is disclosed in published European patent application No.
0006530 in which the temperature-dependent, spectral absorption capacity
is measured in a material which is exposed to the temperature to be
measured, optical fibers then being used for passing light to said
material and for passing out part of this light after absorption in the
material. The measuring apparatus is provided with means for determining
the absorption capacity at at least two wavelength ranges of the light
falling into the material.
Another measuring apparatus, which may be used for fiber-optic temperature
measurement, is the object of Brogaardh et al's U.S. patent application
Ser. No. 152,746 (related to the instant application by a common
assignee). In this application, an optical fiber is arranged in optical
contact with a thyristor and a light signal emitted from the thyristor is
used, among other things, as a measure of the temperature of the
thyristor.
It is also known to use in temperature measurements, a material which emits
radiation of wavelength that is dependent on the temperature of the
material.
The present invention is an improvement of this known technique which is a
solution to the above-mentioned problems. Furthermore, there is no
requirement for an electric current to pass through the solid material. It
is a parallel solution to the above-mentioned measuring apparatus, which
used the spectral absorption capacity of a material.
SUMMARY OF THE INVENTION
According to one aspect of the invention, a fiber-optic
temperature-measuring apparatus comprises a body of a
temperature-dependent photo-luminescent solid material which body is
subjected to the temperature to be measured, a source of exciting
radiation for the body which will cause the latter to luminesce, a
fiber-optic mechanism for feeding the exciting radiation to the body, an
electro-optical measuring mechanism for generating an electrical output
related to an optical input, and a fiber-optic mechanism for feeding
luminescent radiation from the body to the mechanism measuring means.
Thus, in one embodiment, excitation light is adapted to be conducted, by
means of at least one optical fiber, towards a semiconductor material with
temperature-dependent luminescence which is in optical contact with the
fibre, whereby luminescence occurs in the irradiated material, and the
light thus emitted is adapted to be conducted, via at least one optical
fiber, possibly wholly or partially the same as the former, to an
electronic measuring means. The light emitted by the luminescence, and
possibly reflected excitation light are adapted to be conducted via
separate fibers or via a common fiber and possibly also via a fiber branch
to two photo-detectors having mutually different spectral response curves,
or that the light emitted by the luminescence is adapted to be supplied
via a fiber, possibly also via a fiber branch, to at least two
photo-detectors having mutually different spectral response curves.
The invention thus concerns a new principle of temperature measurement, in
which fiber optics are utilized. It is a reliable, economical and sturdy
measuring apparatus which may be used to advantage in difficult
environments and/or in environments which are difficult to reach, for
example in plants which are under voltage.
In a preferred embodiment, the body is composed of a semiconductor material
with temperature-dependent luminescence. As advantageous examples of such
semiconducting materials may be mentioned GaP, suitably doped with Zn and
O or Cd and O, or ZnSe, suitably doped with Cu or Mn.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
According to a further aspect of the invention a fiber-optic
temperature-measuring apparatus comprises first and second bodies of
temperature-dependent photo-luminescent solid material the first of which
bodies is subjected to the temperature to be measured, thermal control
means for modifying the temperature of the second body, means for
measuring the temperature of the second body, at least one source of
exciting radiation for the said bodies which will cause each to luminesce,
fiber-optic means for feeding exciting radiation to the two bodies and for
feeding luminescent radiation from the bodies to electro-optical measuring
means, means for determining from the electrical output of the measuring
means when the luminescent radiation from the two bodies is the same and
thus when the temperature of the first body equals the measured
temperature of the second.
Wavelengths in the range 0.1 to 10 microns would normally be employed.
BRIEF DESCRIPTION OF DRAWINGS
The invention will now be exemplified in greater detail with reference to
the accompanying drawings, in which:
FIG. 1a shows a first embodiment of the invention utilizing both the light
emitted through luminescence and the excitation light reflected from a
surface of the temperature-sensing body,
FIG. 2a shows a second embodiment in which the light within two wavelength
intervals, emitted through the luminescence, is detected,
FIGS. 1b and 2b show the corresponding wavelength intensity curves relating
to the embodiments, respectively of FIGS. 1a and 2a,
FIG. 3 shows a third embodiment of apparatus which is based on a
comparative principle, and
FIG. 4 shows a fourth embodiment of the invention.
The invention concerns a fiber-optic temperature-measuring apparatus in
which the photo-luminescence of a semiconductor, or of any other body of
solid material having photo-luminescent properties, is utilized. Two
different effects may then be used alternatively, namely the temperature
dependence of the luminescence effect as well as the temperature
dependence of the spectral distribution of the luminescence. By
simultaneous detection of the reflection of the exciting radiation,
variations in intensity caused by losses in the fiber-optic system may be
compensated for by dividing the luminescence signal by this reflected
signal. The advantage of this method, as compared with previous
fibre-optic methods, is that the temperature dependence of the measuring
apparatus may be varied within wide limits by selection of a suitable
luminescent material and by suitable doping of that material.
A few embodiments of the invention are shown in the accompanying Figures
and are described in the following.
DESCRIPTION OF PREFERRED EMBODIMENTS
In a first embodiment, which is shown in FIG. 1a, a solid material 1, for
example, a semiconductor material, is optically coupled to a fiber 2 which
branches off at 2a into fibers 3 and 4. Fiber 4 is in turn branched off at
4a into fibers 5 and 5a. A light-emitting diode 6 (LED) is optically
coupled to the fiber 3 and is capable of exciting photo-luminescence in
the material 1. A photo-diode 7 is optically coupled to the fiber 5 via an
optical filter 9, selected to transmit only radiation from the LED 6 which
is reflected by the material 1. Radiation detected by the photo-diode 7
has thus passed through fibers 3 and 2 and after reflection from the
material 1 passes through fiber 4 and fiber 5. If the attenuation of the
radiation in its passage through the fiber system in one direction is
equal to B, the signal detected by the photo-diode 7 becomes approximately
equal to the product of B.sup.2 and L.sub.O, where L.sub.O is the
intensity of radiation emitted by the LED 6 which enters the fiber 3.
A second photo-diode 8 is optically coupled to fiber 5a, via a second
filter 10 selected to transmit only the radiation that is emitted by the
luminescence of the material 1.
If the efficiency of the luminescence process in the material 1 is .eta.,
the signal detected by the photo-diode 8 becomes approximately equal to
B..eta..BL.sub.O. The signals from the two photo-diodes 7 and 8 are
supplied to a quotient-forming circuit 11, the output signal of which
therefore becomes approximately equal to .eta.. Because the efficiency is
temperature dependent (.eta.(T)), the temperature of the material 1 may be
obtained from the output signal of the circuit 11 and this temperature
measurement is independent of transmission losses in the fiber optics or
alterations of L.sub.O caused by temperature drift and/or ageing of the
LED 6.
FIG. 1b illustrates spectra for a system consisting of a green-radiant GaP
LED 6, a GaP crystal doped with Zn and O for the material 1, and silicon
diodes as the photo-diodes 7 and 8. In FIG. 1b, the unbroken curve 12 is
the luminescence spectrum of the material 1 and the broken curve 13 is the
excitation spectrum for the luminescent material 1. The curve 14 is the
transmission curve for the filter 10 and the curve 15 is the transmission
curve for the filter 9. The curve 16 represents the output spectrum of a
green-radiant LED. In the abscissa, the unit h.nu. is used, where h is
Planck's constant and .nu. is the frequency of the radiation, whereas the
ordinate indicates the transmission for the respective filters 9 and 10,
the intensity of the radiation emitted by luminescence, the intensity of
the radiation emitted by the LED 6 and the spectral sensitivity of the
luminescent material 1. The luminescence spectrum 12 is displaced along
the abscissa and changes its shape in dependence on the temperature of the
material 1, and in this way a measure of the temperature is obtained at
the output side of the quotient-forming circuit.
Further possible choices for the material 1 may be GaP:Cd,O (i.e. a GaP
crystal doped with cadmium and oxygen), or ZnSe:Cu (i.e. ZnSe doped with
copper), or ZnSe:Mn (i.e. ZnSe doped with manganese).
The semiconductor material may consist of AlP, AlAs, GaP, InP, InAs,
In.sub.1-x Al.sub.x P, In.sub.1-x Ga.sub.x P, Ga.sub.1-x Al.sub.x P,
In.sub.1-x Al.sub.x As, In.sub.1-x Ga.sub.x As, Ga.sub.1-x Al.sub.x As,
InAs.sub.1-y P.sub.y, GaAs.sub.1-y P.sub.y, where x and y lie between 0
and 1, or ZnTe, ZnSe, ZnS, ZnO, CdSe, CdTe or CdS.
The system shown in FIG. 1a can be modified. Thus it is possible to send
the excited radiation from the LED 6 to the material 1 in one fiber and
the radiation emitted from the material 1 through luminescence and/or the
reflected radiation in another separate fiber, provided with the necessary
junctions.
FIG. 2a shows another embodiment, in which radiation is emitted from an LED
6' via fibers 3' and 2' to a material 1', such as a body of semiconductor
material, thus obtaining photo-luminescence. The radiation emitted by
photo-luminescence is sent via a fiber 2', a branch 2a', a fiber 4' and a
branch 4a' to two fibers 5' and 5a', respectively. The transmission curve
for a filter 9' is in this case different from that of the filter 9 in
FIG. 1a. In this embodiment, the quotient between the luminescence signal
from two different wavelength intervals is detected with the aid of two
filters 9' and 10'. (See FIG. 2b.) When the temperature of the sensor
material 1' is varied, the shape and position of the luminescence spectrum
are changed, the detected quotient then providing a measure of the
temperature of the sensor material 1'. Also in this case, the output
signals from two photo-diodes 7' and 8' are supplied to a quotient-forming
circuit 11', the output signal of which is thus a measure of the
temperature. The measuring signal is again independent of fiber losses and
variations in the intensity, etc. of the radiation emitted from the LED
6'.
FIG. 2b is a second spectra curve which corresponds to FIG. 1b with the
exception that the transmission curve for the filter 9', indicated at 15a
in FIG. 2b, is different compared with the curve 15 in FIG. 1b. The
transmission curve of the filter 9' is thus displaced to the left into a
different wavelength range, whereas the transmission curve for the filter
10', i.e. curve 14' in FIG. 2b is unchanged in shape or position compared
to curve 14 in FIG. 1b, as are the other three curves shown in FIG. 2b.
An alternative system for temperature measurement according to the
invention is shown in FIG. 3. An LED 17 emits radiation with an intensity
L.sub.17, amplitude-modulated at a frequency of f.sub.17, into a fiber 18
and thus to a luminescent material A via a fiber 19. Between the end of
the fiber 19 and the luminescent material A there is interposed a
semi-transparent mirror 20. The radiation emitted as a consequence of the
luminescence of the material A is passed to a photo-detector 21 through
the fiber 19 and a fiber 22 and through a filter 22a which does not
transmit radiation of wavelengths corresponding to the excitation
radiation from the LED 17.
The intensity detected by the photo-detector 21 thus becomes
K.sub.1.L.sub.17.B.sup.2..eta.(T.sub.A). This signal also has a frequency
of f.sub.17.
An LED 23, identical with the LED 17, emits radiation with an intensity of
L.sub.23, and which is amplitude-modulated at a frequency of f.sub.23,
towards a luminescent material B, which has properties identical with
those of material A. The radiation emitted as a consequence of
luminescence in the material B is conducted through a filter 24, which has
a transmission curve identical with that of the filter 22a, and then
through a fiber 25 and into the fiber 19, to be reflected partially by the
mirror 20 back to the photo-detector 21 through the fiber 19 and the fiber
22. The intensity thus detected by the photo-detector 21 becomes
K.sub.2.L.sub.23.B.sup.2..eta.(T.sub.B). The intensity L.sub.17 and/or the
intensity L.sub.23 is/are adjusted such that K.sub.1.L.sub.17
=K.sub.2.L.sub.23. The signal from the photo-detector 21 is amplified in
an amplifier 26. Through the influence of electrical filters 27 and 28,
the two components of the output signal from the photo-detector 21 which
have frequencies of f.sub.17 and f.sub.23, respectively, are separated and
separately supplied to different inputs of a control circuit 29, which
controls a power circuit 30 of a heater/cooler 31. The output of the
circuit 30 is controlled to ensure that the two input signals to the
control system 29 are maintained equal. This occurs when
.eta.(T.sub.B)=.eta.(T.sub.A), i.e. when T.sub.B =T.sub.A. By measuring
T.sub.B, for example by using a thermocouple, T.sub.A is thus obtained
directly. The temperature of the luminescent material A, i.e. T.sub.A, can
thus be determined by this method without knowledge of the value of the
factor B, which represents the attenuation in those parts of the system
which are traversed by the two optical signals, which thus means that, for
example, a reduction in the optical signal, caused by losses in the fiber
19, will not influence the accuracy of the temperature measurement.
A further embodiment is shown in FIG. 4. Radiation from a photo-diode 32 is
led through a fiber system to a luminescent material A, which is exposed
to the temperature to be measured, as well as to a material B which is
identical with A, the temperature of which is to be controlled. A
photo-detector 33A, which is provided with an optical filter 34A
transmitting the excitation radiation, provides an output signal
proportional to L.sub.O.B.sup.2, where B represents the attenuation of the
optical signal during its passage in one direction through the fiber
system 35. A system comprising a photo-detector 33B and a filter 34B is
identical with the system comprising the photo-detector 33A and the filter
34A. The system 33B-34B is supplied with the excitation radiation that is
reflected from the surface of the material B and provides an output signal
which is proportional to L.sub.O. The system 33A-34A is supplied with the
excitation radiation that is reflected from the surface of the material A
and provides an output signal proportional to B.sup.2.L.sub.O. After
amplification in amplifiers 36 and 37, the quotient between these two
signals is formed in a quotient-forming circuit 38, a signal proportional
to B.sup.2 thus being obtained. A system comprising a photo-detector 39A
and a filter 40A and a system comprising a photo-detector 39B and a filter
40B have identical properties. Filters 40A, 40B transmit the radiation
emitted due to the luminescence in the materials A and B. The output
signals from amplifiers 41 and 42 are thus proportional to
L.sub.O.B.sup.2..eta.(T.sub.A) and L.sub.O..eta.(T.sub.B), respectively.
The output signal from a quotient-forming circuit 43, which receives
signals from the amplifier 41 and the quotient-forming circuit 38, is thus
proportional to L.sub.O..eta.(T.sub.A). This signal and the output signal
from the amplifier 42 are supplied to separate inputs of a control circuit
44. The operation of this circuit and of a power circuit 45 and a
heater/cooler 46 is analogous to what has been described in connection
with the system according to FIG. 3. In the FIG. 4 embodiment, as in the
FIG. 3 embodiment, temperature of the material B is measured by any
suitable method to give a measure of the temperature of the material A.
The systems described above with reference to the drawings may be varied in
many ways within the scope of the following claims.
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