GB2200204A

GB2200204A - Temperature sensing apparatus

Info

Publication number: GB2200204A
Application number: GB08700980A
Authority: GB
Inventors: Mark Cunningham Farries
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1987-01-17
Filing date: 1987-01-17
Publication date: 1988-07-27
Anticipated expiration: 2007-01-17
Also published as: GB2200204B; GB8700980D0

Abstract

Light is passed into an optical fibre 3 which is doped with holmium to make its light absorbancy highly dependent on temperature. As an alternative to holmium, other materials having a doublet ground state can be used. Another possibility is to use any material having energy states separated by 2.00 cm<-1> or less and having an absorption line in the range 700 nm to 1500 nm. Instead of being doped into the optical fibre, the holmium may be located between the abutting ends of two or more optical fibres. Temperature at two or more locations (e.g. 5, 6) may be sensed using detection of backscatterred light from a pulsed light source 1. <IMAGE>

Description

An Apparatus for Sensing Temperature This invention relates to an apparatus for sensing temperature. Experiments have previously been made in which light from a laser is directed along an optical fibre made of silica (i.e. SiO2) doped with germanium and neodymium. This material has been found to absorb light of a particular wavelength by an amount which depends on its temperature. Thus the amount of light transmitted along the fibre is also dependent on temperature and by detecting this transmitted light a measurement of the temperature of the fibre can be derived. If a pulse of light is transmitted into the fibre and the amount of back-scattered light is detected at a given time after transmission of the pulse: then the detection will give an indication of the temperature at a position along the fibre which is related to the time delay between transmission and reception.A number of measurements can be made after different time delays representing temperatures at different positions along the fibre. Such a system is generally referred to as a "distributed temperature sensor".

Silica doped with neodynium is commonly available and is used for example in lasers; and this is probably why its temperature dependent absorbancy was originally noted.

However, to our knowledge, the reasons for this property have not been thoroughly investigated nor has consideration previously been given to the possibility of using other materials. The present invention arose from experiments with such other materials.

The experiments involved manufacturing SiO2 optical fibres doped with erbium (Er3+), europium (emu3+), holmium Er europium ( praseodymium (Pr3+) and terbium (Tb3+). These materials were selected for exploratory research because there was no published information on them. Having manufactured the fibres, each was tested to measure the percentage absorbancy change per OC at the wavelength which gave the greatest such change. The results of these experiments showed that: erbium was about 1.5 times better than neodymium; europium was worse than neodymium; holmium was about 48 times better than neodymium; praseodynium was worse than neodymium; and terbium was also worse than neodymium.

From these results it was apparent that holmium was dramatically superior than any of the other materials.

Furthermore, for holmium, very good results (over 50% of the best) were obtained at a wavelength of around 900nm.

This wavelength is significant because suitable semiconductor lasers, detectors and fibres are readily available for operation at that wavelength.

Thus, according to a first aspect of this invention there is provided an apparatus for sensing temperature comprising an optical fibre path containing holmium, a light source arranged to direct light along the path, and a detector for detecting light emerging from the path, the intensity of which depends on the absorbancy of the said path and thus on its temperature.

In a preferred form of the invention the light source is pulsed and the detector is arranged to receive light which is back-scattered from the fibre. In this arrangement the detector also includes a timing device for measuring the time delay between the transmission and reception of a pulse. This allows different temperature measurements to be made for different specified positions along the fibre path.

The fibre path is preferably a glass or glass-like material such as silica doped with holmium but it would be possible, in alternative forms of the invention, to have small quantities of holmium located between abutting ends of two or more fibres. Where a doped material is used the whole of the said path need not be so doped. It would be possible just to have sections of the path made of doped material, the remaining portions being of conventional glass-fibre construction.

The inventor has a theory that the good results obtained with holmium are due to its "doublet" ground state i.e. a splitting of the ground state into primarily just two levels (a doublet) . This is described by N Bontemps and J C Rivoal in the Journal of Physics part C 1982 Volume 15 pages 1301 to 1318. The inventor believes that the relative occupancy or population of the two levels must be dependent on temperature and that this gives rise to an increased temperature dependency of the broadening of the main absorption lines. It is speculated that other materials having these doublet ground levels will give similar good results.Thus, according to another aspect of this invention there is provided an apparatus for sensing temperature comprising an optical fibre path containing a material having a doublet ground state, a light source arranged to direct light along the path, and a detector for detecting light emerging from the path the intensity of which depends on the absorbancy of the said path and thus on its temperature.

A similar effect could well be obtained with materials which do not have the aforementioned doublet ground state but which have energy levels sufficiently close to the ground level to allow thermal excitation to transfer electrons between them. In practice, temperature sensors are commonly required to operate at temperatures of around 1000C. At this temperature one would expect transfer of electrons between energy states having a -1 separation of 2.00cm or less. However not all materials having these characteristics are suitable since the absorption spectrum must also have an absorption line in the transmission window(s) of the fibre; which in the case of silica is between 700 nm and 1500 nm.

Thus, according to another aspect of this invention there is provided an apparatus for sensing temperature comprising an optical fibre path containing a material whose atomic structure has energy states separated by 2.00cm 1 or less and which has an absorption line between 700 nm and 1500 nm, a light source arranged to direct light along the path, and a detector for detecting light emerging from the path the intensity of which depends on the absorbancy of the said path and thus on its temperature.

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

Referring to the drawing, a pulse laser source 1, operating at a wavelength of around 900 nm, i.e. within the transmission window of Sino2 (silica) directs its ou-tput, via a beam splitter 2, into a silica fibre via a lens 4. The fibre 3 extends through a number of locations 5 and 6 whose temperatures are of interest and terminates at an end 7 where some facility is provided to prevent back reflections. In this embodiment of the invention the whole of the fibre 3 is doped with holmium using a technique described by Poole et al in a paper entitled "Fabrication of low-loss optical fibres containing rare earth ions" published in electronic letters 1985, volume 21, pages 737 to 738. In an alternative arrangement just the paths at locations 5 and 6 could be so doped.

Radiation scattered at the different positions 5 and 6 along the fibre is returned to a photo-detector 8 via the beam splitter2. The amplitude of the resulting electrical output is digitised at 9 and the resulting digital outputs at different times, corresponding to the different ranges of locations 5 and 6, are processed in a computer 10 to give measurements of absorption and thence temperature. The resulting temperature measurements are presented as outputs illustrated schematically by lines 10A and 10B. In a modification of the illustrated system a tuneable laser, equivalent to laser 1, is used. This normally operates, like the laser 1, at 900 nm but can be switched to operate at say 950 nm to effect test measurements. These test measurements give a reference against which the actual measurement can be normalised.

This technique eliminates possible errors e.g. due to bending or ageing of the fibre or due to mechanical damage to the fibre.

Claims

1. An apparatus for sensing temperature comprising an optical fibre path containing holmium, a light source arranged to direct light along the path, and a detector for detecting light emerging from the path the intensity of which depends on the absorbancy of the said path and thus on its temperature.

2. Apparatus according to claim 1 in which the light source is a pulsed light source and in which the detector is arranged to receive light back-scattered from the path and includes aiming device for detecting the time delay between transmission and reception of a light pulse.

3. Apparatus according to claim 1 or 2 in which the light path extends through a transparent material which is doped with the holmium.

4. Apparatus for sensing temperature comprising an optical fibre path containing a material having a doublet ground state, a light source arranged to direct light along the path, and a detector for detecting light emerging from the path the intensity of which depends on the absorbancy of the said path and thus on its temperature.

5. Apparatus substantially as described with reference to the accompanying drawing and substantially as illustrated therein.

5. An apparatus for sensing temperature comprising an optical fibre path containing a material having energy

2.60cm -l states separated by 2.60cm 1 or less and having an absorption line in the range of 700 nm to 1500 nm inclusive.

6. Apparatus substantially as described with reference to the accompanying drawing and substantially as illustrated therein.

Amendments to the claims have been filed as follows CLAIMS 1. An apparatus for sensing temperature comprising an optical fibre path containing holmium, a light source arranged to direct light along the path, and a detector for detecting light emerging from the path the intensity of which depends on the absorbancy of the said path and thus on its temperature.

2. Apparatus according to claim 1 in which the light source is a pulsed light source and in which the detector is arranged to receive light back-scattered from the path and includes a timing device for detecting the time delay between transmission and reception of a light pulse.

4. Apparatus for sensing temperature comprising an optical fibre path containing a material having a doublet ground state, a light source arranged to direct light along the path, and a detector Ifor detecting light emerging from the path the intensity; of which depends on the absorbancy of the said path and thus on its temperature.