GB2156513A

GB2156513A - Temperature measuring arrangements

Info

Publication number: GB2156513A
Application number: GB08407945A
Authority: GB
Inventors: John Philip Dakin
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1984-03-28
Filing date: 1984-03-28
Publication date: 1985-10-09
Also published as: GB2156513B; DE3511185C2; US4673299A; DE3511185A1; GB8407945D0

Description

1

SPECIFICATION

Improvements relating to temperature measuring arrangements This invention relates to temperature measur ing arrangements and relates more specifically to temperature measuring arrangements using optical fibre temperature sensing means for sensing temperature distribution over a rela tively long path (e.g. 50 metres to 10 kilo metres).

According to one aspect of the present invention a temperature measuring arrange ment comprises a relatively long optical fibre temperature sensor doped along its length with material that absorbs light in dependence upon temperature, means for launching into one end of the temperature sensor fibre con temporaneous light pulses of two different wave lengths and mcans for comparing varia tions with time in the back-scattered light of the two different wavelengths returned along the sensor fibre in order to provide an indica tion of the temperature distribution along the fibre.

The two wavelengths of the light pulses launched into the fibre sensor are chosen so that they correspond with different absorption bands of the doped material (i.e. bands pro ducing different variations of absorption of light with temperature). Thus by comparing absorptions at different wavelengths any varia tions in the dopant concentration along the sensing fibre will be compensated for. 100 According to a preferred aspect of the pre sent invention a temperature measuring ar rangement comprises a relatively long optical fibre temperature sensor doped along its length with material that fluoresces in re sponse to the absorption of light in depen dence upon temperature, means for launching light pulses into one end of the sensor fibre and means for detecting the variations with time in the levels of fluorescent light emitted by the doped material in response to the light pulses which returns to the launch end of the sensor fibre.

In this latter embodiment the dopant ma terial (e.g. rare earth material) of the sensor fibre effectively produces radiant re-emission of the absorbed light energy (i.e. ffluores cence) at a longer wavelength than that of the light pulses launched into the sensor fibre.

The wavelength of these light pulses and the dopant material will be chosen so that the light content of the light pulses as the latter reach the end of the sensor fibre remote from the launch end will still be at a reasonable level, even after absorption of the light by the dopant material of the sensor fibre has taken place. Temperature-dependent variations in the levels of fluorescent emission along the sensor fibre can readily be detected at the launch end of the sensor fibre using techGB2156513A 1 niques similar to those normally employed for optical time domain reflectometry.

The fluorescent light returned to the launch end of the sensor fibre may be measured in a single narrow wavelength region but it is preferred to measure the fluorescent light in two wavelengths or specific wavelength regions simultaneously in order to compensate for any variations in clopant material concen- tration along the sensing fibre. With single wavelength measurement, the variations in the levels of returned fluorescent light are used to deduce the temperature variations along the sensor fibre, whereas with dual wavelength measurement the variations in the ratio of levels of returned fluorescent light are used to deduce temperature distribution. It will be apparent therefore that in the case of dual wavelength measurement the two wavelengths concerned will need to be selected so that there is a relative difference in levels between the fluorescent emission with temperature at these wavelengths. Notwithstanding whether the single wavelength measurement or the dual wavelength measurement technique is used, the specific wavelength or wavelengths chosen will be subject to low absorption during return of the light to the detector means at the launch end of the sensor fibre.

By way of example two embodiments of the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of one arrangement for measuring the temperature distribution along an optical fibre sensor; Figure 2 is a schematic diagram of an alternative temperature distribution measuring arrangement to that shown in Figure 1; and, Figures 3 and 4 show graphs of logarithmic values of back-scattered light against time in respect of the arrangements depicted in Figures 1 and 2 respectively.

Referring to Figure 1 of the drawings the temperature distribution measuring arrangement illustrated comprises two lasers 1 and 2 arranged to be operated by electrical pulses received from pulse generating means 3 through electrical current splitter means 4. The lasers 1 and 2 accordingly produce light pulses having different wavelengths W1 and W2. These light pulses are fed into power combining means 5 (e.g. ffibre tee or discrete cube beam-combiner) for launching the pulses simultaneously into a sensing optical fibre 6 (monomode or multimode) after being transmitted through conventional connecting optical fibres 7 and 8 and optical connectors 9 and 10. The length of the sensing fibre 6 which has a similar geometry and numerical aperture to the conventional connecting fibre 8 will be predetermined in accordance with the path length or extent of a region to be monitored in the specific application of the 2 temperature measuring arrangement (e.g. process control, fire and overheating detection, thermal sensing etc.). However, the monitoring of fibre path lengths of 50 metres to 10 kilometres is envisaged.

The sensing fibre 6 which may be of silica is doped along its length with a material (e.g. neodymium or europium) which absorbs a small proportion of the light content of the light pulses as they travel along the fibre in dependence upon the temperature distribution along the fibre. However, the wavelengths W1 and W2 of the simultaneous light pulses.are chosen so that they correspond with different absorption bands in the sensing fibre and thereby produce different variations of absorption with temperature. By this arrangement any variations in the dopant concentration along the sensing fibre 6 will be compensated for.

As the simultaneous light pulses of wavelengths W1 and W2 travel along the conventional fibre 8 and through the connector 10 and then along the sensing fibre 6 the light pulses will be attenuated, and as regards the attenuation of the respective light pulses travelling along the sensing fibre 6, this will be dependent upon the temperature variations along the sensing fibre. Some of the light content of the pulses is reflected back along the sensing fibre 6 as back- scattered light and the wavelengths of the light pulses are so chosen in relation to the dopant material of the sensing fibre and the temperatures to be experienced by the fibre that the level of back- 100 scattered light, even that from the far end of the optical fibre 6, will be at a reasonably detectable level. As will be appreciated, the level of back-scattered light will diminish with the increase in absorption of the light with temperature by the dopant material of the sensing fibre and consequently the variations with respect to time between the back-scat tered light at the two wavelengths concerned and a comparison between such variations will provide an indication of temperature dis tribution along the optical sensing fibre 6.

For this purpose the back-scattered light at the two wavelengths travels back along the sensing fibre 6 and through the connector 10 and conventional fibre 8 and connector 9 to wavelength separation means 11 (e.g. di chroic mirror or diffraction grating dichroma tor) via an optical fibre 12. The optical out puts at wavelengths W1 and W2 from the separation means 12 are applied to detectors 13 and 14 and corresponding electrical out puts from the detectors are then compared by means of a comparator 15 to determine the ratio between such detected values and to provide an indication of temperature distribu tion along the sensing fibre.

Figure 3 shows a graph having the logarith mic values of the back-scattered light plotted against tirne which is proportional to the dis- 130 GB 2 156 513A 2 tance travelled by the light pulses along the sensing fibre 6. For the sake of convenience and case of understanding it is assumed that two sections of the sensing fibre 6 are sub- jected to uniform temperatures Tl and T2 respectively. As can be seen the variations in the ratio of back-scattered light at or about the wavelengths W1 and W2 at the two temperatures T1 and T2 can be utilised to provide an indication of temperature distribution along the sensing fibre. It will also be seen that the light content of the pulses at the far end of the sensing fibre where there are reflections R is still at a sufficiently high level to ensure an adequate level of back-scattered light for detection purposes. As shown further reflections P may occur at the interface of the connector 10 and the sensing fibre 6.

Referring now to Figure 2 of the drawings the temperature distribution sensing arrangement depicted comprises a pulsed laser 16 providing an output light pulse of wavelength W1 which is launched into one end of a suitably doped sensing fibre 17 after travel- ling along an optical fibre 18 and through a first connector 19 and along a convention fibre 20 and through a connector 21. The sensing fibre 17 is doped with partially light absorbing material which produces fluorescent emission covering the wavelength bands centered on wavelengths W2 and W3 after absorption of light at the wavelength W1.

The relative level of fluorescent light at wavelengths W2 and W3 is dependent upon the level of absorption of light at wavelength W1, which in turn depends upon the temperature of the sensing fibre 17. The fluorescent light at wavelengths W2 and W3 returned along the sensing fibre 17 and the conven- tional optical fibre 20 is diverted by the connector 19 to a twin wavelength selection filter 22 over an optical fihre 23. The outputs at wavelengths W2 and W3 from the filter 22 are applied to detectors 24 and 25 and the ratio between these outputs is determined by a ratiometer 26 which accordingly provides an indication of the variations of temperature with time along the sensing fibre 17.

As with the first embodiment of the inven- tion, the detection of returning light at two different wavelengths compensates for any variations in the concentration of dopant along the sensing fibre.

Figure 4 shows a graph corresponding to that shown in Figure 3 but in respect of the fluorescent detection arrangement of Figure 2. As will be seen from the graph of Figure 4 the sensing fibre 17 for the sake of simplicity is assumed to have two sections at the respec- tive temperatures T1 and T2 and the return from these fibre sections of fluorescent light in two wavelength regions centred on the wavelengths W2 and W3 are shown together with the reflections R1 and S1 from the far end of the sensing fibre and the connector-sensing 3 fibre interface, respectively. As will be seen, in this embodiment the level of returned fluo rescent light-from the conventional fibre 20 is much lower than the corresponding level of reflected light shown in Figure 3. This is due 70 to the low fluorescence of the conventional fibre 20 in the present embodiment.

It may here be mentioned that the tempera ture measuring arrangements according to the present invention as hereinbefore described provide significant advantages over known systems for measuring temperature distribu tion along optical fibres. In such known sys tems errors in temperature measurement may arise due to the presence of solid impurity particles (e.g. dust) or air bubbles within liquid-filled sensing fibres and any variations in the loss, diameter or numerical aperture of the liquid-filled sensing fibre will affect the measurement of temperature. In another known arrangement a number of optical fibre/filter junctions are provided and these suffer from relatively high loss and, moreover, the arrangement provides a method of mea suring the temperatures at a series of points along the sensor rather than temperature dis tribution along a sensing fibre. Still further, in other known arrangements dopant concentra tion variations along the sensing fibre may give rise to inaccurate temperature distribution 95 measurements.

Claims

1. A temperature measuring arrangement comprising a relatively long optical fibre tem perature sensor doped along its length with material that absorbs light in dependence upon temperature, means for launching into one end of the temperature sensor fibre con temporaneous light pulses of two different wavelengths and means for comparing varia tions with time in the back-scattered light of the two different wavelengths returned along the sensor fibre in order to provide an indica tion of the temperature distribution along the fibre, the two wavelengths of the light pulses launched into the fibre sensor being chosen so that they correspond with different absorp tion bands of the doped material.

2. A temperature measuring arrangement as claimed in claim 1, in which the light pulses of two different wavelengths are derived from respective lasers which are pulses simultaneously by electric pulses produced by pulse generating means.

3. A temperature measuring arrangement as claimed in claim 2, in which the light pulse outputs from the two lasers are fed into power combining means which launches the pulses simultaneously into the sensor fibre.

4. A temperature measuring arrangement as claimed in any of claims 1 to 3, in which the back-scattered light of the two different wavelengths returned along the sensor fibre is fed into wavelength separation means which pro- GB2156513A 3 duces respective outputs at the two different wavelengths which are then applied to respective opto-electric detectors for the generation of electrical outputs arranged to be fed to comparator means to determine the ratio between the detected outputs and to provide an indication of temperature distribution along the sensor fibre.

5. A temperature measuring arrangement as claimed in any of claims 1 to 4, in which the dopant material is neodymium or europium.

6. A temperature measuring arrangement comprising a relatively long optical fibre temperature sensor doped along its length with material that fluoresces in response to the absorption of light in dependence upon temperature, means for launching light pulses into one end of the sensor fibre and means for detecting that variations with time in the levels of fluorescent light emitted by the doped material in response to the fight pulses which return to the launch end of the sensor fibre.

7. A temperature measuring arrangement as claimed in claim 6, in which the dopant material (e.g. rare earth material) of the sensor fibre effectively produces radiant re-emission of the absorbed light energy at a longer wavelength than that of the light pulses launched into the sensor fibre, the wavelength of these light pulses and the dopant material being chosen so that the light content of the light pulses as the latter reach the end of the sensor fibre remote from the launch end will still be at a reasonable lelvel even after absorption of the light by the dopant material of the sensor fibre.

8. A temperature measuring arrangement as claimed in claim 6 or claim 7, in which the fluorescent light returned to the launch end of the sensor fibre is arranged to be measured in a single narrow wavelength region.

9. A temperature measuring arrangement as claimed in claim 6 or claim 7, in which fluorescent light in two wavelengths or specific wavelength regions is measured simultaneously in order to compensate for any variations in dopant material concentration along the sensor fibre, the variations in the ratio of levels of returned fluorescent light being used to deduce temperature distribution.

10. A temperature measuring arrangement as claimed in any of claims 6 to 9, in which light pulses of one wavelength are produced by a pulsed laser and launched into the sensor fibre.

11. A temperature measuring arrangement as claimed in claim 9, in which the fluorescent light returned to the launch end of the sensor fibre is applied to filter means to provide light outputs at two different wavelengths and in which these outputs are applied to respective opto-electric detectors to produce electrical outputs which are then compared by means of a ratiometer to provide 4 GB 2 156 513A 4 an indication of variations of temperature with time along the sensor fibre.

12. A temperature measuring arrangement substantially as hereinbefore described with reference to Figures 1 and 3 of the accompanying drawings.

13. A temperature measuring arrangement substantially as hereinbefore described with reference to Figures 2 and 4 of the accom10 panying drawings.

Printed in the United Kingdom for Her Majesty's Stationery Office, Dd 8818935, 1985, 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.