GB2183821A

GB2183821A - A temperature sensor

Info

Publication number: GB2183821A
Application number: GB08529212A
Authority: GB
Inventors: Mark Cunningham Farries
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1985-11-27
Filing date: 1985-11-27
Publication date: 1987-06-10
Anticipated expiration: 2005-11-27
Also published as: GB8529212D0; GB2183821B

Abstract

Light is directed into one end of an optical fibre (6, 7) and the light scattered back from within the fibre is analysed by two separate sensing devices. One of them (11) is sensitive to light of the same wavelength as enters the fibre (Rayleigh scattered light). The outer (15) is sensitive to light of a shorter wavelength (anti-Stokes scattered light). The intensities of these two types of scattered light are compared in a ratio device (18) to give an indication of the temperature of the fibre. The arrangement is stated to permit a much faster response then the prior art method of comparing anti-Stokes scattered light with Stokes scattered light, since the Rayleigh scattered light is much larger and easier to measure. <IMAGE>

Description

SPECIFICATION A temperature sensor This invention relates to a temperature sensor.

It has previously been proposed by Dakin, Pratt, Bibby and Ross (in a paper entitled "distributed optical fibre Raman temperature sensor using a semiconductor light source and detector" published in Electronics Letters 20th June 1985 Volume 21 No. 13) to pass a light of a given wavelength into an optical fibre and to detect the so called "Stokes" and "anti Stokes" backscattered light. The Stokes light is of a longer wavelength than the light passed into the fibre and is temperature dependant. The anti-Stokes light is of a shorter wavelength and its intensity is also dependant on temperature. Dakin, Pratt, Bibby and Ross made use of the fact that, for a given temperature, the Stokes and anti-Stokes intensities will be very similar and the ratio of their intensities will vary with temperature. Thus, by comparing the two, a measurement of temperature can be obtained.

This invention arose from a realisation that a similar effect to that achieved by Dakin, Pratt, Bibby and Ross could be obtained by comparing the intensity of the anti-Stokes light with that of the Rayleigh scattered light i.e. light scattered without a change in wavelength. Accordingly this invention provides a temperature sensor comprising a source of radiation arranged to direct radiation of a first wavelength to a member exposed to the temperature to be sensed, sensing means for sensing radiation of the first wavelength and of a second shorter wavelength after being scattered by the said member and means for comparing the intensities of components of scattered radiation having the first and second wavelengths respectively to give an indication of the temperature of the fibre.

Because the Rayleigh scattered light is much more intense than the Stokes scattered light used by Dakin, Pratt, Bibby and Ross, it can be sensed using relatively simple and inexpensive equipment. This makes it practicable to use separate sensing devices for the Raman and anti-Stokes measurements which in turn enables the two measurements to be made simultaneously. This abiiity to make the two measurements simultaneously is of considerable significance since the invention thus enables a very fast response to temperature changes to be obtained; in the order of 0. 1 seconds in the case of coated fibre and in the order of 0.001 seconds in the case of an uncoated fibre.

The first and second wavelengths referred to need not of course be single wavelengths, but will normally be bands of wavelengths of finite width.

It would be possible to employ the invention in a system where the light source directs a beam of light through the surrounding medium to the aforementioned "member" where temperature is to be sensed and in which a telescopic focussing device is used to focus light scattered from the aforementioned "member" onto the sensing means. However, in a preferred form of the invention the light or other radiation is passed from the source into and along an optical waveguide, preferably an optical fibre, which itself serves as the aforementioned "member".

One way in which the invention may be performed will now be described by way of example with reference to the accompanying schematic drawing of a temperature sensor constructed in accordance with the invention.

Referring to the drawing, light from a laser 1 is pulsed by a shutter 2 controlled by pulses generated by a signal generator 3. The light passes from the shutter 2 through a beam splitter 4 and is focussed by a lens 5 onto one end of the fibre 6 along which the light travels in a direction generally from left to right as illustrated. The fibre 6 is wound into a coil 7 in an area where temperature is to be sensed and extends to its opposite end which is immersed in a liquid 8 chosen to have a refracted index similar to that of the fibre. The purpose of this is to avoid reflections from the far end of the fibre.

Stokes, anti-Stokes and Rayleigh scattered light, scattered in the fibre 6, travels in both directions along the fibre. Such light travelling in the direction generaily from right to left as illustrated returns to the beam splitter 4 and passes through an optical aperture 9 to a filter 10. This is a narrow band interferance filter and is composed in known manner of a number of layers of transparent material. The filter is tuned by setting its angle relative to the direction of propogation of the light thus adjusting the path length through each layer. In the illustrated system the adjustment is made so that only the anti-Stokes wavelengths pass through it. All wavelengths, which in practice are dominated by Rayleigh scattered light, are reflected from the front surface of the filter to a photodetector in the form of a pin diode 11.

The output 12 of the pin diode photodetector 11 represents the intensity of the Rayleigh scattered light and this is amplified at 13 and passed to a sample and hold circuit 14.

The anti-Stokes scattered light is of relatively weak intensity and passes into a photo multiplier 15. The output of the photo multiplier 15, representing the intensity of the anti Stoke scattered light, passes to a phase sensitive detector 16. The phase sensitive detector receives, as a reference signal, the output from the signal generator 3 and serves to provide, at its output, a signal representing the component of the output of 15 which is in phase with the pulses of light fed into the optical fibre 6. The phase sensitive detector thus serves to eliminate the effects of noise arising for example from ambient light entering the fibre 6 and from the photo multiplier 15.

The outputs of the sample and hold circuit 14 and of the phase sensitive detector 15 are converted into digital form at 17 and 17A.

They are then compared in a comparator 18 which takes the form of a micro computer.

The latter first weights the outputs from 14 and 16 so as to make them equal when the fibre 7 is at a particular temperature. It then divides the anti-Stokes intensity value from 16 by the Rayleigh value from 14 to obtain an output which represents the temperature of the fibre 7. This output is passed to a display device 19.

It is believed that a device constructed along the lines as described and illustrated could provide a relatively simple and low cost component useful in many commercial situations. It operates from minus 196"C to 1500C with an accuracy of at least 2"C. The sensor has all the advantages of an optical fibre, being non-metallic and having a high resistence to chemical attack. Furthermore it can cover the temperature range from near absolute zero to 6000C if suitable coatings are applied to fibre. The opticai-fibre can be inserted into sub-millimetre spaces and for this reason can have many useful applications in the medical field. Furthermore the sensor serves to average the temperature along the optical fibre and this property is useful for temperature control for example of large rooms where hot and cold spots upset conventional temperature sensors. A further, and very significant, advantage is that the sensor has a fast response; a requirement which is needed in many process control applications.

Claims

1. A temperature sensor comprising a source of radiation arranged to direct radiation of a first wavelength to a member exposed to the temperature to be sensed, sensing means for sensing radiation of the first wavelength and of a second shorter wavelength after being scattered by the said member and means for comparing the intensities of components of scattered radiation having the first and second wavelengths respectively to give an indication of the temperature of the fibre.

2. A temperature sensor according to claim 1 in which the said member is an optical waveguide and the source is arranged to direct the radiation of the first wavelength into one end of the waveguide.

3. A temperature sensor according to claim 2 in which the optical waveguide is an optical fibre.

4. A temperature sensor according to claim 2 or 3 in which the sensing member is arranged to sense radiation of the first and second wavelengths emerging from the same end of the optical waveguide as that into which the radiation is directed by the source.

5. A temperature sensor according to any preceding claim in which the sensing means comprises a first sensor for sensing radiation of the first wavelength and a separate second sensor for sensing radiation of the second wavelength.

6. A temperature sensor according to claim 5 including a filter arranged to reflect radiation of one of the said wavelengths towards one of the sensing means and to transmit radiation of the other wavelength towards the other sensing means.

7. A temperature sensor substantially as described with reference to the accompanying drawing and substantially as illustrated therein.