GB2333357A

GB2333357A - Fibre optic temperature sensor

Info

Publication number: GB2333357A
Application number: GB9801168A
Authority: GB
Inventors: Gary Rumbles
Original assignee: Imperial College of Science Technology and Medicine
Current assignee: Imperial College of Science Technology and Medicine
Priority date: 1998-01-20
Filing date: 1998-01-20
Publication date: 1999-07-21
Also published as: JPH11211581A; WO1999036756A1; GB9801168D0

Abstract

A fibre-optic temperature sensor, comprising a laser (1), an optic fibre (3) for transmitting the laser light and at least one dye-doped section (5) provided in optical communication with the fibre, wherein the dye-doped section is doped with at least two different dyes that luminesce upon excitation by the laser. The peak luminescent wavelengths of the dyes are different, and the intensity of the luminescence of the two dyes vary differently with temperature. The temperature is calculated from a ratio of the intensities of the luminescence of the two dyes.

Description

FIBRE-OPTIC TEM'ERATURE SENSOR This invention relates to fibre-optic temperature sensors.

It is known to use an optic fibre to sense temperature. A number of techniques are currently in use to achieve suitable temperature sensors. Particular techniques include Rayleigh scattering in liquid-filled fibres, Raman scattering (anti-stokes\stokes ratio), temperature-dependent absorption and temperature-dependent luminescence.

At least some of these techniques may be used in conjunction with optical time domain reflectometry (OTDR) to obtain distance information. Some of these temperature sensors may be operated in an intrinsic mode whereby the fibre itself is used as the sensing element. Alternatively, the sensors may be operated in an extrinsic mode whereby the fibre is used only as a light guiding medium which couples with the temperature detector.

United States Patent US-A-5,090,818 discloses a fibre optic system for sensing temperature using a luminescent material which has two emissive levels with temperature dependent relative populations. This luminescent material emits luminescence light from two excited energy levels the energies of which differ by a magnitude E, the relative occupancy numbers (populations) of the two levels being a function of temperature. When excited with pulsed, oscillatory or other transient interrogating light, the intensity of the total luminescence emitted by said two levels has a decay time which varies as a sensitive function of temperature over a wide temperature range. US-A-5,090,8 18 suggests that it is possible to measure temperature by comparing the spectrally resolved luminescence intensities from each of the two emissive levels. However, it considers this method to be inconvenient and teaches instead a way of measuring temperature by measuring the decay time of the total luminescence of the material when excited with pulse or AC-modulated light having a decay time much shorter than the luminescence decay time of the probe material. The above mentioned device has the disadvantage that the decay time is difficult to measure. The suggested, less convenient, intensity comparison method of temperature measurement has the disadvantage of two emissive levels that are close together (the energy level E being in the thermal range) and are thus not easy to separate efficiently. Furthermore, at temperatures away from the centre of the operating range one of the signals will be much stronger than the other making accurate measurement of the weaker signal difficult.

In accordance with the present invention, there is provided a fibre optic temperature sensor, comprising: a laser for generating laser light; an optic fibre for transmitting said laser light, and at least one dye-doped section provided in optical communication with said fibre; said at least one section being doped with at least two different dyes that luminesce upon excitation by said laser light, a first of said dyes having a peak luminescent wavelength hl, a second of said dyes having a peak luminescent wavelength X2, wherein the dyes are such that Bl and X2 are not equal and the intensity of the luminescence of one dye varies differently with temperature to that of the other dye.

Thus the present invention overcomes the disadvantages of the prior art by the use of dyes with different peak luminescent wavelengths and different luminescent temperature dependencies. The use of more than one dye means that dyes can be chosen with wavelengths that are easy to separate and measure, the ratio of the two emission intensities providing an indication of temperature that is independent of excitation light intensities. Furthermore, the use of more than one dye means that the system is very flexible, the choice of the different dyes being made in accordance with the desired operating temperature range, the concentration of the dyes being chosen so that their luminescent intensities are similar to each other over that temperature range In preferred embodiments of the present invention, the peak luminescent wavelength he k1 of the first dye is less than the excitation wavelength of the laser light and the peak luminescent wavelength B2 of the second dye is greater than the excitation wavelength of said laser light. Preferably, the peak luminescent wavelength X1 of the first dye is less than the excitation wavelength of said laser light by an amount such that the laser causes luminescence of the first dye in the anti Stokes region. This means that the luminescence of the first dye is temperature dependent.

In a further preferred embodiment, the peak luminescent wavelength B2 of the second dye is greater than the excitation wavelength of the laser light such that the laser causes luminescence of the second dye in the Stokes region. Thus the luminescence of the second dye does not increase with temperature, being substantially temperature independent or decreasing with temperature.

In preferred embodiments, the intensity of luminescence of said first dye increases with temperature and the intensity of luminescence of said second dye decreases with temperature. This results in a very sensitive system, the ratio between the two signals varying greatly with temperature, the variations therefore being easy to detect.

Advantageously, the first dye may be rhodamine 101, and the second dye may be phthalocyanine dye. Rhodarnine 101 has a luminescent intensity that increases strongly with temperature whereas phthalocyanine dye has a luminescent intensity that decreases with temperature (see Figures 4 and 5). Furthermore, these two dyes can be used with a relatively inexpensive helium-neon laser having a wavelength of approximately 633nm. In other embodiments a laser diode may be preferred, these being compact and robust.

The use of two different dyes means that the concentration of the two dyes can be selected such that within a preferred operating temperature range the two signals are of similar magnitude such that neither one is too small to detect easily.

This is an advantage over the prior art disclosed above, wherein the use of one dye with two emissive levels means that the level of the two signals is set by the properties of the dye and cannot be optimised by varying relative concentrations as is the case in the present invention. In a preferred embodiment of the present invention, the concentrations of rhodamine 101 and phthalocyanine dye are chosen so that within an operating range of -100 C to 80"C of the sensor, the ratio of the intensities of luminescence of the two dyes is less than 100.

In preferred embodiments, the sensor further comprises an optical separator for separating the at least two luminescent wavelengths, a processing unit for calculating the ratio of the intensities of the two wavelengths and for calculating the temperature of the substance from said ratio.

Preferably, it also comprises a calibration unit for generating a calibration curve from the ratio of the intensities of the luminescence of the two dyes over a range of different temperatures and a memory for storing said curve for use by said processing unit. The unit is thus simple to calibrate and can be used to calculate the temperature of a substance accurately and without being affected by any lack of uniformity in the optical fibre or sensor or by variations in the intensity of the excitation light.

In one embodiment, one dye-doped section is provided in abutment and optical communication with one end of the fibre. This embodiment provides an inexpensive, reliable and simple temperature probe.

In another embodiment, a plurality of dye-doped sections are provided at intervals along the fibres length. This embodiment is particularly well suited for use as distributive optical temperature sensor.

In yet another embodiment, the fibre has a plastic cladding, said cladding being doped along substantially all of its length with dye. Alternatively, the cladding may be doped at intervals along its length with dye. Both these embodiments are suitable for use as distributive temperature sensors.

Alternatively, the fibre's core may be doped with dye.

In any of the above embodiments, the fibre may be a plastic fibre.

Alternatively, for some embodiments, the fibre may be a glass fibre.

In another embodiment of the present invention, there is provided a temperature sensing system which includes a fibre-optic temperature sensor according to any of the above mentioned embodiments, the system comprising a controller for operating said temperature sensor in an optical time domain reflectometry mode.

In this way, the present invention provides a fibre-optic temperature sensor that alleviates the problems associated with the prior art by providing a sensor that may be manufactured from inexpensive materials and which may be used over large distances. In addition the sensor provides two signals that are easily separated and detected, and whose ratio provides an indication of temperature that is independent of excitation light intensity.

Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 illustrates a first embodiment of the present invention; Figures 2a to 2c illustrate further embodiments of the present invention; Figure 3 illustrates the luminescence spectra of a material containing rhodamine 101 and phthalocyanine dye; Figure 4 illustrates the variation in the intensity of luminescence of rhodamine 101 with temperature; and Figure 5 illustrates the variation in the intensity of luminescence of phthalocyanine with temperature.

With reference to Figure 1, interrogating light from a helium-neon laser 1 is launched into a plastic fibre-optic cable 3. A length of dye-doped poly(methyl methacrylate) (PMMA or Perspex) 5 is attached to one end of the fibre. The interrogating light from the laser 1 excites the two dyes in the PMMA 5 which luminesce. Some of that luminescence becomes trapped inside the fibre 3 by total internal reflection and is guided back along the fibre. The luminescent light travelling back towards the fibre's launch end emerges and is collimated by a launch lens 7. The luminescent light is subsequently split into two beams and passed through two different bandpass filters 9 which each transmits in the region of respective peak luminescent wavelengths X1 and B2 and rejects scattered laser light and light in the region of the other peak luminescent wavelength. The two transmitted beams are subsequently detected by a photomultiplier tube 11. A processing unit determines the ratio of the intensities of the two beams, compares it with a calibration curve and thereby calculates the temperature.

It will be apparent that the above mentioned embodiment has been described with regard to a particular wavelength of interrogating light which has been chosen for use with particular dyes. Other interrogating light wavelengths and other suitable dyes will be apparent to the man skilled in the art. Similarly, it will be apparent to the man skilled in the art that the optic fibre 3 need not be a plastic fibre. Indeed, a conventional glass fibre could be used with the PMMA slab 5 fixed to an end thereof, as described above. Similarly, whilst a PMMA or Perspex material is preferred for use with the dye, it will be apparent that other materials, such as sol-gel glass, may be alternatively utilised.

In this embodiment, the fibre is being utilised in an extrinsic mode and thus only serves to direct the interrogatory pulse to the doped material.

This embodiment of the present invention is particularly well suited for use as a temperature probe in hostile environments or other environments of reduced accessibility. In this embodiment, the laser is preferably operated in continuous mode operation. However, it will be apparent that a pulsed mode laser may equally well be utilised.

Figures 2a to 2c illustrate further embodiments of the present invention whereby the principles described above have been utilised to provide a distributive optic fibre temperature sensor. These embodiments are particularly well suited for long range temperature sensing of, for example, undersea cables and the like.

With reference to Figure 2a, the optic fibre 3 includes a plurality of dye doped PMMA sections 5 at spaced intervals along its length. The luminescence lifetime of the particular dyes in use with the optic fibre temperature sensor according to the present invention dictate the required spacing for the sections in order to obtain adequate resolution of the returned signal. For rhodamine 101 and phthalocyanine dyes it is proposed that the dye-doped section 5 are spaced approximately eight metres apart as these dyes have a luminescence lifetime of approximately five ns.

By utilising the above mentioned fibre with an OTDR process and/or pulsed mode laser operation, an apparatus for the sensing of temperature fluctuations along the entire length of an optic fibre may be created.

Figure 2b illustrates an alternative arrangement for a distributive optic fibre temperature sensor. In this arrangement, the fibre 3 comprises a fibre core 13 and cladding 15. The fibre core 13 is doped at intervals 17 along its entire length. As above, the spacing of these intervals is governed by the luminescence lifetime of the dyes in use. This arrangement is particularly useful with plastic optic fibres.

However, it is conceivable that a glass fibre could be spliced, at intervals along its length, onto a series of doped PMMA sections.

Alternatively, for shorter lengths of plastic optic fibre, the entire core of the fibre could be doped with dyes. However, in such a case, to reduce attenuation, interrogating light of a longer wavelength is preferably used to measure temperature increases above a threshold.

Figure 2c illustrates an alternative arrangement of the present invention. In this arrangement, the cladding 15 of the fibre 3 is doped with dye along its entire length. However, in order to reduce attenuation of the interrogatory signal, that signal would have to be provided with a longer wavelength, in a manner similar to that discussed above. In this arrangement, the temperature could not then be measured unless it exceeded a certain threshold. However, this arrangement would still be useful for the sensing and detection of "hot spots" above a temperature threshold.

Figure 3 illustrates the luminescence spectra of a material containing rhodamine 101 and sulphonated aluminium phthalocyanine when excited by a heliumneon laser, as a function of temperature. The plot shows a peak at 607nm due to anti Stokes luminescence from rhodamine 101, a sharp peak at 633nm which is the excitation wavelength and a peak at 679 nm due to luminescence from sulphonated aluminium phthalocyanine. As can be seen from Figure 3 (and more clearly from Figure 4) the luminescence intensity of rhodamine 101 increases with temperature.

Conversely the luminescence intensity of the phthalocyanine dye decreases with temperature, as is shown most clearly in Figure 5.

It will be apparent, of course, that the present invention has been described above by way of example only and that modifications may be made within the scope of the appended claims.

Similarly, it will be apparent that an OTDR system will need other components for operation. However, these components are standard and will not be described herein.

It will also be apparent to the skilled man that the teachings given above may be applied to any of the above embodiments and that those combinations are also to be included within the scope of the appended claims.

Claims

CLAIMS 1. A fibre optic temperature sensor, comprising: a laser for generating laser light; an optic fibre for transmitting said laser light, and at least one dye-doped section provided in optical communication with said fibre; said at least one section being doped with at least two different dyes that luminesce upon excitation by said laser light, a first of said dyes having a peak luminescent wavelength hl, a second of said dyes having a peak luminescent wavelength B2 wherein the dyes are such that hl and B2 are not equal and the intensity of the luminescence of one dye varies differently with temperature to that of the other dye.
2. A fibre optic temperature sensor according to claim 1, wherein the peak luminescent wavelength X1 of said first dye is less than the excitation wavelength of said laser light and the peak luminescent wavelength B2 of said second dye is greater than the excitation wavelength of said laser light.
3. A fibre optic temperature sensor according to claims 1 or 2, wherein the peak luminescent wavelength X1 of said first dye is less than the excitation wavelength of said laser light by an amount such that the laser causes luminescence of said first dye in the anti-Stokes region.
4. A fibre optic temperature sensor according to claim 3, wherein the peak luminescent wavelength X2 of said second dye is greater than the excitation wavelength of said laser light such that the laser causes luminescence of said second dye in the Stokes region.
5. A fibre optic temperature sensor according to any of the preceding claims, wherein the intensity of luminescence of said first dye increases with temperature and the intensity of luminescence of said second dye decreases with temperature.
6. A fibre optic temperature sensor according to any of the preceding claims wherein said first dye is rhodamine 101.
7. A fibre optic temperature sensor according to any of the preceding claims wherein said second dye is phthalocyanine dye.
8. A fibre optic temperature sensor according to claims 6 and 7, wherein the concentration of the two dyes is such that within an operating temperature range of 100"C to 80"C of the sensor, the ratio of the intensities of luminescence of the two dyes is less than 100.
9. A fibre optic temperature sensor according to any of claims 6 and 7, wherein the excitation wavelength of said laser light is approximately 633nm.
10. A fibre optic temperature sensor according to any preceding claim wherein said laser is a helium-neon laser.
11. A fibre optic temperature sensor according to any of claims 1 to 9 wherein said laser is a laser diode.
12. A fibre optic temperature sensor according to any of the preceding claims comprising: an optical separator for separating the at least two luminescent wavelengths; a processing unit for calculating the ratio of the intensities of the two wavelengths and calculating the temperature of the substance from said ratio.
13. A fibre optic temperature sensor according to claim 12 comprising: a calibration unit for generating a calibration curve from the ratio of the intensities of the luminescence of the two dyes over a range of different temperatures; and a memory for storing said curve for use by said processing unit.
14. A fibre optic temperature sensor according to any of the preceding claims, wherein one dye-doped section is provided in abutment and optical communication with one end of the fibre.
15. A fibre optic temperature sensor according to any of claims 1 to 13, wherein a plurality of dye-doped sections are provided at intervals along the fibre's length.
16. A fibre optic temperature sensor according to any of claims 1 to 13, wherein said fibre has a plastic cladding, said cladding being doped along substantially all of its length with dye.
17. A fibre optic temperature sensor according to any of claims 1 to 13, wherein said fibre has a fibre core and a fibre cladding, said fibre core being doped with dye.
18. A fibre optic temperature sensor according to any of the preceding claims, wherein said fibre is a plastic fibre.
19. A fibre optic temperature sensor according to any of claims I to 17, wherein said fibre is a glass fibre.
20. A temperature sensing system including a fibre optic temperature sensor according to any of the preceding claims, comprising a controller for operating said temperature sensor in an optical time domain reflectometry mode.
21. A fibre-optic temperature sensor substantially as hereinbefore described with reference to the accompanying drawings.
22. A temperature sensing system substantially as hereinbefore described with reference to the accompanying drawings.