GB2210451A

GB2210451A - Optical temperature measurement

Info

Publication number: GB2210451A
Application number: GB8722771A
Authority: GB
Inventors: John Philip Dakin
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1987-09-28
Filing date: 1987-09-28
Publication date: 1989-06-07
Anticipated expiration: 2007-09-28
Also published as: GB8722771D0; GB2210451B

Abstract

A distributed temperature measuring arrangement comprises laser means (1, 2) for producing light signals for transmission over optical fibre means (6) comprising an optical fibre temperature sensor and light detector means for determining time-displaced back-scattered or returned light received back from the optical fibre means. A plurality of spaced regions (7, 8) are provided along the optical fibre means and are maintained at different known temperatures to define temperature calibration regions along the fibre means thereby enabling various relative differences (e.g. power differences between lasers, differences in optical fibre composition etc.) to be taken into account in making temperature profile calculations from the time-displaced back-scattered or returned light. <IMAGE>

Description

IMPROVEMENTS RELATING TO TEMPERATURE MEASURING ARRANGEMENTS This invention relates to temperature measuring arrangements and relates more specifically to such arrangements incorporating optical fibres for sensing distributed temperature.

In our co-pending Patent Application No. 2140554A a distributed temperature sensing arrangement of the above kind is described in which the distributed temperature profile along the optical fibre sensor is measured by monitoring the back-scattered light in the Raman anti-Stokes and Stokes wavelengths.

Additionally, in our Patent Applications Nos. 2156513A and 8717155 further distributed temperature sensing arrangements utilising optical-time-domain reflectometry or similar techniques are described in which the sensor fibre is doped along its length with material the degree of light absorbtion of which varies according to temperature or the sensor fibre is doped along its length with material which fluoresces in dependence upon temperature. In these arrangements the time displaced light reflected back along the sensor fibre or the time-displaced fluorescent light transmitted back along the fibre are monitored in order to determine the temperature profile along the path of the sensor fibre.

In temperature measuring arrangements of the above kind detection of the back-scattered or returned (e.g. fluorescent) light may be achieved by the use of avalanche photo-diodes and it has been proposed to control or set the responsivity of these photodiodes in order to take into account relative changes in the power of the laser sources when a plurality of lasers and a single detector are used or to control the relative gains of different avalanche detectors when a single laser and a plurality of photo-diode detectors are used.

The present invention is directed to a distributed temperature measuring arrangement of the kind set forth above in which the need for controlling or setting the gain of the detector or detectors, as the case may be, which can be somewhat complex and costly, is avoided.

According to the present invention there is provided a distributed temperature measuring arrangement comprising laser means for producing light signals for transmission over optical fibre means comprising an optical fibre temperature sensor and light detector means for determining time-displaced back-scattered or returned light received back from the optical fibre means, in which a plurality of spaced regions along the optical fibre means are maintained at different known temperatures to define temperature calibration regions along the fibre means thereby enabling various relative differences (e.g. power differences between lasers, differences in optical fibre composition etc.) to be taken into account in making temperature profile calculations from the time-displaced back-scattered or returned light.

The plurality of regions of the optical fibre means maintained at different known temperatures may be located within temperature controlled enclosures or the regions of the optical fibre may be bonded or otherwise held in intimate thermal contact with respective temperature controlled structures.

The plurality of fibre regions may form parts of the actual distributed temperature sensor fibre or they may define parts of a separate optical fibre cable.

It may be mentioned that if the particular temperature sensing arrangement employed serves to detect the ratio of Stokes and anti Stokes back-scattered light then although this ratio will be independent of optical fibre composition it will in general be a function of the precise wavelength difference from the laser light source and, if two lasers are used, any power difference between the laser outputs. By maintaining regions of the optical fibre means at different precisely controlled temperatures and calibrating the arrangement, then other regions of the fibre at the same temperatures as the control temperatures will exhibit a similar ratio of Raman scattered light even if these regions of the fibre have different compositions.Thus, if the fibre attenuation suffered by the back-scattered light along the optical fibre is similar (or if a small correction is made for any difference) the temperature of an unknown point may be determined more accurately as a result of the calibration regions by extrapolation of the measured ratios at the calibration points using the expected Raman relationship for the ratio of Stokes to anti-Stokes Raman scattered points:

where Ws and Wa are the wavelengths of Stokes and anti-Stokes scattered light; h is Planck's constant; c is the velocity of light; v is the optical frequency; K is the gas constant; and, Tis the absolute temperature.

By way of example the present invention will now be described with reference to the accompanying drawings in which: Figure 1 shows an optical distributed temperature sensing arrangement according to the invention having two laser light sources and a single detector; Figure 2 shows typical detected signals derived from the Figure 1 sensing arrangement; Figure 3 shows an optical distributed temperature sensing arrangement according to the invention having a single light source and two detectors; and, Figure 4 shows typical detected signals derived from the Figure 3 sensing arrangement.

Referring to Figure 1 of the drawings two laser light sources 1 and 2 are provided for producing light signals of different wavelengths W1 and W2. These light signals are fed through respective optical fibres 3 and 4 into an optical wavelength division multiplexer!demultiplexer 5 the output from which is fed into an optical fibre 6 having two coiled regions 7 and 8 the respective temperatures of which are accurately controlled, as by locating these coiled regions within temperature controlled (e.g. thermostatically controlled) enclosures 9 and 10. The light output from the optical fibre 6 is then launched through an optical connector 11 into a distributed temperature optical fibre sensor 12 which extends through a temperature measurement zone 13.

In operation of the temperature sensing arrangement the two laser light sources 1 and 2 are operated sequentially by electrical pulses applied to the lasers to produce light outputs of the respective wavelengths W1 and W2. These light outputs are fed in turn through the optical fibre 6 including the coiled regions 7 and 8 which are maintained precisely at different known temperatures T1 and T2 and then into the distributed temperature optical fibre sensor 12.

Back-scattered light at the Stokes wavelength derived from the optical fibre arrangement in response to the operation of the laser source 1 is detected by an avalanche photo-diode detector 14 which produces an electrical output typically having the profile shown on the left hand side of Figure 2. However, back-scattered light at the anti-Stokes wavelength derived from the optical fibre arrangement in response to the operation of the laser source 2 will also be detected by the detector 14 to produce an electrical output signal typically of the profile shown on the right hand side of Figure 1.

These electrical signals corresponding to the back-scattered light signals at Stokes and anti-Stokes wavelengths may then be fed into an electronic processor 15 which enables the ratio of Stokes and anti-Stokes back-scattered light (which ratio is independent of the optical fibre composition) to be deduced and which by reference to the ratio of back-scattered signals obtained from each of the calibration regions 7 and 8 of the optical fibre 6 and calculated from the previously mentioned equation for obtaining the ratio R by extrapolation can make corrections for the relative powers of the light signals emitted sequentially by the laser sources 1 and 2.As will be appreciated, the wavelengths W1 and W2 of the lasers 1 and 2 and the pass-bands of the wavelength division multiplexerldemultiplexer 5 will be chosen so that the Stokes scattered light derived from the laser 1 and the anti-Stokes scattered light derived from the laser 2 are both in a narrow detection wavelength region close to a central wavelength WO.

As will also be appreciated, this arrangement uses only one relatively expensive avalanche diode detector 14 and an electronic processor 15 allowing the ratio of the sequential back-scattered signals at Stokes and anti-Stokes wavelengths to be deduced by an electronic computation from the signal waveform or from the average of repetitive cycles of the waveform.

In the alternative distributed temperature sensing arrangement shown in Figure 3, a single laser 16 produces light at a wavelength WO which is fed through an optical fibre 17 and multiplexerldemultiplexer 18 into an optical fibre 19 having two coiled regions 20 and 21 which are maintained at known temperatures T1 and T2 within temperature controlled (e.g.

thermostatically controlled) enclosures 22 and 23. The light signal is then launched via a connector 24 into a distributed temperature optical fibre sensor 25 for sensing the temperature over a temperature measurement zone 26. Back-scattered light from the optical fibre arrangement at the Stokes and anti-Stokes wavelengths W1 and W2 which are close to a central wavelength WO (i.e. the wavelength of the laser light source 16) pass through the multiplexerldemultiplexer 18 into respective avalanche photo-diode detectors 27 and 28.

The simultaneous output signals produced by these detectors 27 and 28 in response to the received back-scattered light at the Stokes and anti-Stokes wavelengths will typically have profiles of the form shown in Figure 4. These signals are fed into respective electronic processors 29 and 30 which by suitable processing can derive not only the ratio between Stokes and anti-Stokes wavelength signals from which temperatures along the sensor 26 can be derived but can also make adjustments to avoid the need for controlling the gains of the detectors 27 and 28 by reference to the ratio of backscattered light at the coiled regions 20 and 21 maintained at known temperatures.

It may here be mentioned that apart from the corrections that can be made for different laser powers in the Figure 1 arrangement and for avalanche photo-diode detector gain in the Figure 3 arrangement another advantage provided by both arrangements is that if the Raman signals are contaminated by a small degree of residual interference from Rayleigh scattered light, due, for example to imperfect filtering in the multiplexer/demultiplexer, or due to scattering of non-lasing luminescent light from the semiconductor laser source(s), or if each laser wavelength is not exactly as required, it will be possible for the processor(s) to apply correction factors to the previously mentioned equation for the ratio R in order to obtain even more accurate values for the fibre temperatures in the measurement regions 13 (Figure 1) or 26 (Figure 3).

Although in the Figure 1 and Figure 3 embodiments described above Stokes and anti-Stokes back-scattered light is monitored it is also contemplated that the optical fibre means of the arrangements could be doped with suitable material to provide temperature dependent fluorescence in response to light signals produced by the laser light source(s).

Rather than using two discrete calibration regions as in the Figure 1 and Figure 3 embodiments described it is also possible to use a larger number of calibration points, a particularly attractive option for obtaining a continuous series of calibration regions being to wind a length of optical fibre between 20 to 300 metres, say, on to a cylindrical former and by producing a known temperature gradient along the length of the fibre by maintaining the temperature at each end of the former at different known temperature values and allowing thermal conduction in the former to establish the gradient according to the thermal conductivity of the material of the former.

In practice, the temperature calibration regions of the optical fibre may be most conveniently provided by having lengths of the fibre contained within an equipment housing containing the laser light source(s), detector(s) and wavelength division multiplexer. The temperature sensing fibre cable may be connected to the housing through a bulkhead connector. However, if a variety of optical fibres of distinctly dissimilar materials are to be used in the temperature sensor fibre and, in particular, if it is desired to reference to light derived from Rayleigh scattering or light-induced fluoresence which are strongly dependent on composition, it may be necessary to provide a facility for location of regions of the temperature sensor fibre itself in temperature controlled enclosures which may, for example, be let into the external walls of the equipment housing with a suitable thermally-insulating cover plate to reduce heat losses.

Claims

1. A distributed temperature measuring arrangement comprising laser means for producing light signals for transmission over optical fibre means comprising an optical fibre temperature sensor and light detector means for determining time-displaced back-scattered or returned light received back from the optical fibre means, in which a plurality of spaced regions along the optical fibre means are maintained at different known temperatures to define temperature calibration regions along the fibre means thereby enabling various relative differences between components or parts thereof to be taken into account in making temperature profile calculations from the time-displaced back-scattered or returned light.

2. A distributed temperature measuring arrangement as claimed in claim 1, in which the plurality of regions of the optical fibre means maintained at different known temperatures are located within respective temperature controlled enclosures.

3. A distributed temperature measuring arrangement as claimed in claim 1, in which the plurality of regions of the optical fibre means maintained at different known temperatures are held in intimate thermal contact with respective temperature controlled structures.

4. A distributed temperature measuring arrangement as claimed in claim 1, in which the plurality of optical fibre regions form parts of the actual distributed temperature sensor fibre.

5. A distributed temperature measuring arrangement as claimed in claim 1, in which the laser means comprises two laser light sources operated sequentially for producing alternate light signals of different wavelengths which are fed through a multiplexer/demultiplexer to the optical fibre means including the optical fibre regions maintained at different known temperatures, in which the light detector means comprises a single avalanche photodiode detector which detects back-scattered light from the controlled temperature fibre regions and the optical fibre temperature sensor at the Stokes wavelength in response to the operation of one of the laser light sources and which detects back-scattered light from the controlled temperature fibre regions and the optical fibre temperature sensor at the anti-Stokes wavelength in response to the operation of the other laser light source, and in which consequential electrical output signals from the light detector are fed to an electronic processor which calculates the ratio of Stokes and anti Stokes back-scattered light and which in accordance with the deduced ratio of back-scattered light signals from the controlled temperature regions of optical fibre makes corrections for the relative powers of two laser light signals.

6. A distributed temperature measuring arrangement as claimed in claim 1, in which the laser means comprises a single laser light source for producing a light signal which is fed through a multiplexer/demultiplexer to the temperature controlled optical fibre regions and the optical fibre temperature sensor, in which backscattered light from the optical fibre means at the Stokes and anti Stokes wavelengths is fed by the multiplexer/demultiplexer to respective avalanche photo-diode detectors the electrical outputs from which are fed into processor means which calculates the ratio between the back-scattered Stokes and anti-Stokes wavelength signals and takes into account the respective gains of the detectors by specific reference to the ratio of back-scattered light from the controlled temperature optical fibre regions.

7. A distributed temperature measuring arrangement substantially as hereinbefore described with reference to Figs.l and 2 of the accompanying drawings.

8. A distributed temperature measuring arrangement substantially as hereinbefore described with reference to Figs. 3 and 4 of the accompanying drawings.