GB2096784A

GB2096784A - Optical fibre temperature sensors

Info

Publication number: GB2096784A
Application number: GB8111095A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1981-04-09
Filing date: 1981-04-09
Publication date: 1982-10-20
Also published as: GB2096784B

Abstract

An optical temperature sensor arrangement includes a sensor element (2) providing two mutually interfering different length light paths (A, B). A change in temperature of the element causes corresponding expansion or contraction and this alters the phase relationship of the two paths. The temperature change can be determined by measurement of the changes produced in the interference pattern. The element has a focussing system to produce a pencil beam 5, a half-silvered mirror to and a plane mirror 7. in another arrangement the combined interfering beams out along another fibre optic. <IMAGE>

Description

SPECIFICATION Optical temperature sensors This invention relates to temperature sensor devices, and in particular to such devices in which the temperature sensitive element is constructed from an optical fibre.

According to the invention there is provided a temperature sensing arrangement wherein the temperature sensitive element comprises an optically transparent device providing first and second mutually interfering temperature dependent light paths.

According to another aspect of the invention there is provided a fibre optic temperature sensor arrangement, including a fibre optic element providing first and second light paths of different path lengths, wherein light beams traversing said paths are combined to generate an interference pattern, and wherein thermal expansion or contraction of the element results in a change in the relative lengths of the light paths thus causing a corresponding change in the interference pattern.

As the sensor arrangement employs no moving parts nor does it involve electrical currents it may be employed in hazardous environments where the use of conventional devices would be difficult or even dangerous.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 shows an optical fibre temperature sensor element; Fig. 2 shows a temperature sensor arrangement employing the sensor element of Fig. 1; Fig. 3 shows a dual fibre temperature sensing element for use in the arrangement of Fig. 2.

Referring to Fig. 1, monochromatic light is launched from a source S, which may comprise a solid state laser down a conventional optical fibre 1 and is guided along the optical fibre to a sensor element 2. The sensor element includes a focussing element 4 adjacent the end 3 of the optical fibre, the focussing element consisting preferably of a small portion of graded index fibre.

Light travelling down the fibre is focussed by the element 4 into a thin parallel light beam 5 which beam is incident on a half-silvered mirror 6.

Preferably the mirror 6 is disposed at the focus of the element 4. A proportion of the light is reflected off the mirror along path B, the rest of the light being transmitted through the mirror along path A. The light travelling along path A is reflected back on itself by a plane mirror 7 positioned at the end of the portion of graded index fibre. The light travelling along path B is reflected back on itself by a mirror 8 which lies on the side surface of the graded index fibre. This mirror 8 has the same radius of curvature as the sides of the graded index fibre and thus acts as a refocussing device to reconcentrate the light to a pencil thin beam back to the mirror.The two reflected light beams interfere at the mirror, the interference fringes formed being due to the difference in phase of the two beams of light caused by the difference between the path length A and the path length B. On recombination of the two reflected beams the resultant beam is propagated back down the fibre to a detector arrangement D. The optical fibre is split into two branches, one branch leading to the source and the other branch to the detector arrangement.

It will be clear that if the sensor element is heated or cooled the absolute difference between the optical path lengths A and B will alter thus producing a changing of relative phase between the two light paths. This produces an output which is alternately light and dark, the number of transitions from light to dark corresponding to the temperature change that has taken place. Thus, where monochromatic light feeding to a single detector is employed an indication of the absolute magnitude of a temperature change can be determined by feeding the detector output into a counter. The output of the counter can then be translated into a temperature change. It will be apparent that this technique does not give an immediate indication of the direction of the temperature change.However, in many applications, for example in a chemical process where hazardous materials are to be heated from ambient temperature to a predetermined temperature, the direction of the temperature change is already known and only the absolute magnitude of the change is required.

In applications where it is necessary to determine the direction of a temperature change the sensor arrangement shown schematically in Fig. 2 may be employed. In this arrangement light signals of two different wavelengths are employed to give an unambiguous indication of a temperature change. In this arrangement the light source S produces light signals at two wavelengths, each signal producing its own interference pattern in the sensor element 2. The light signal returned from the fibre 1 is fed via e.g.

a prism or diffraction grating 21 such that light signals of the one wavelength are directed on to a first detector and light signals of the other wavelength are directed on to a second detector, said first and second detectors being incorporated in the detector assembly D. The outputs of the detectors are fed to respective counters, CA and CB, which in turn may be coupled to a microprocessor MPU programmed to determine not only temperature changes from the number of light and dark signals detected via the detectors but also the direction of the changes from the phase relationship between the detector outputs.

Fig. 3 shows an alternative arrangement which enables the sensing device to be used at a greater distance from the source and the detector. In this arrangement the detector arrangement D is placed at the end of a second optical fibre 10 which is coupled to the side of the first optical fibre 1 at the position of the sensing device 2.

The sensing device 2 has the same construction as previously described but, on recombination of the two beams of light the resultant beam is directed along path C, out of the sensing device, into the second optical fibre 10.

The resultant beam is reflected off a piane mirror 11 positioned at the end of the fibre 10 and propagates down the second fibre to an externally calibrated detector placed at the fibre end.

In order to measure the temperature of an object the sensing device is placed in contact with the object. As before, changes in the relative optical lengths of the two light paths cause corresponding changes in the interference pattern observed via the fibre 1 0.

Claims

1. A temperature sensing arrangement wherein the temperature sensitive element comprises an optically transparent device providing first and second mutually interfering temperature dependent light paths.

2. A fibre optic temperature sensor arrangement, including a fibre optic sensor element providing mutually interfering first and second temperature dependent light paths, means for directing monochromatic light signals along said paths, means for responding to a composite light signal produced by the combination of the signals traversing said paths, and means for determining from said composite signal a temperature change in said sensor element.

3. A temperature sensor arrangement as claimed in claim 2, wherein said temperature change determining means includes a photodetector coupled to a counter.

4. A temperature sensor as claimed in claim 2, wherein said light directing means provides light signals of two different wavelengths and said temperature change determining means includes first and second detectors each fed with a respective said wavelength signal.

5. A temperature sensor arrangement substantially as described herein with reference to Fig. 2 together with Fig. 1 or Fig. 3 of the accompanying drawings.

6. A fibre optic temperature sensor element including a fibre optic device providing first and second light paths of different path lengths, wherein light beams traversing said paths are combined to generate an interference pattern, and wherein thermal expansion or contraction of the element results in a change in the relative lengths of the light paths thus causing a corresponding change in the interference pattern.

7. A sensor element as claimed in claim 6, wherein the fibre optic device includes a guided index self focussing portion.

8. A sensor element as claimed in claim 6 or 7, wherein said first and second light paths are provided by interaction of incident light with a partially silvered mirror.

9. A sensor element as claimed in claim 8, wherein said mirror is disposed at the focus of said self focussing portion.

1 0. A fibre optic temperature sensor element substantially as described herein with reference to Fig. 1 or Fig. 3 of the accompanying drawings.

11. A fibre optic temperature sensing arrangement, including a fibre optic sensor element as claimed in any one of claims 6 to 10.

12. A method of temperature sensing substantiaily as described herein with reference to the accompanying drawings.