GB1582768A

GB1582768A - Temperature sensitive optical fibre

Info

Publication number: GB1582768A
Application number: GB3606277A
Authority: GB
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1977-08-26
Filing date: 1977-08-26
Publication date: 1981-01-14

Description

(54) TEMPERATURE SENSITIVE OPTICAL FIBRE (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company, of 190 Strand, London, W.C.2, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - This invention relates to temperature sensing and in particular to a method of temperature sensing in which an optical fibre waveguide is employed as the temperature responsive element.

According to one- aspect of the invention there is provided a method of temperature detection by measurement of the optical transmitting properties of a fibre light guide, the method including cladding the guide with a plastics material having a refractive index which is more temperature dependent than that of the material for which the light guide is made, launching light signals along the light guide, and measuring the attenuation of said light signals after their passage through the guide, said attenuation being determined by the difference in refractive indices between the light guide and the cladding at the temperature of the light guide.

An optical fibre waveguide of the step index type transmits light by virtue of the total internal reflection of light striking the interface between the relatively higher refractive index core region and the relatively lower refractive index cladding region.

The light energy guided in such a fibre is proportional to the difference of the squares of the refractive indices of the core and cladding material. A typical fibre waveguide of this type is the plastics clad silica or glass fibre formed by coating a core fibre of fused silica or glass with a cladding layer of lower refractive index polymeric material, The temperature dependence of the refractive index of many polymers is much greater than that of glass or silica.

Thus if a silica or glass fibre is clad with a suitable plastics material there will be a temperature at which the refractive indices of the core and cladding are equal and below that temperature all guidance of light will be lost. The term 'light' as used herein is understood to include the infrared the ultraviolet and the visible region of the electromagnetic spectrum.

An embodiment of the invention. will now be described with reference to the accompanying drawing which shows, in schematic form, an optical fibre waveguide temperature sensor arrangement.

Referring to the drawing, the temperature sensor arrangement includes a plastics clad silica fibre 11 intro which light is launched from a laser or light emitting diode 12 via an optical launching system 13. An optical detector 14, e.g. a photo diode, is used to monitor continuously the signal level transmitted through the fibre.

The refractive index temperature characteristic of the plastics cladding is such that the refractive indices of the core and cladding are identical at a temperature hereinafter referred to as the extinction temperature. Lowering the temperature of any point of the fibre near to or below the extinction temperature leads to a change in the light guiding properties of the fibre causing a reduction or total loss respectively of the signal received by the photodetcetor.

A typical example of a clad figure for this purpose is a pure fused silica fibre coated with a cladding of Shin-Etsu-RTV 103 (registered Trade Mark) silicone rubber. The refractive indices of these materials at 250 are 1.458 and 1.410 respectively for light of a wavelength 589 nm.

The extinction temperature for this fibre at that wavelength is -550C and lowering the temperature of portion of the fibre to --55"C causes a substantially total loss of signal at the photodetector.

The temperature sensor arrangement may be used with a signal level detector circuit 15 coupled to the photodetector to provide an indication of the temperature of the fibre and/or to operate an alarm circuit. By using most of the optical waveguide in its normal mode, temperature measurements may be made at locations several hundreds of metres from both source and detector equipments.

The temperature detection of the arrangement may be made more accurate by increasing the fibre path length in the zone of interest and/or by selection of a particular pair of core and cladding materials.

An example for operation at higher temperatures is a sodium borosilicate glass fibre having a refractive index of 1.50 clad with polymethylmethacrylate with an index of 1.4% at ambient temperature. At ambient temperature this fibre has a high loss thus providing a low output signal.

Raising the temperature of the fibre causes a corresponding increase in the output signal. For remote applications light is preferably fed to and from such a fibre by lengths of conventional fibre waveguide.

Sensitivity of the arrangement may be increased by introducing controlled bends at the measuring point.

WHAT WE CLAIM IS: - 1. A method of temperature detection by measurement of the optical transmitting properties of a fibre light guide, the method including cladding the guide with a plastics material having a refractive index which is more temperature dependent than that of the material for which the light guide is made, launching light signals along the light guide, and measuring the attenuation of said light signals after their passage through the guide, said attenuation being determined by the difference in refractive indices betwen the light guide and the cladding at the temperature of the light guide.

2. A method as claimed in claim 1, in which the core fibre is a pure silica fibre and the cladding is a silicone rubber.

3. A method as claimed in claim 1, in which the core fibre is a sodium borosilicate glass fibre and the cladding is polymethylmethacrylate.

4. A method of temperature measurement substantially as described herein with reference to the accompanying drawing.

5. Apparatus as described herein when used for carrying out the method of any one of claims 1 to 4.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. circuit. By using most of the optical waveguide in its normal mode, temperature measurements may be made at locations several hundreds of metres from both source and detector equipments. The temperature detection of the arrangement may be made more accurate by increasing the fibre path length in the zone of interest and/or by selection of a particular pair of core and cladding materials. An example for operation at higher temperatures is a sodium borosilicate glass fibre having a refractive index of 1.50 clad with polymethylmethacrylate with an index of 1.4% at ambient temperature. At ambient temperature this fibre has a high loss thus providing a low output signal. Raising the temperature of the fibre causes a corresponding increase in the output signal. For remote applications light is preferably fed to and from such a fibre by lengths of conventional fibre waveguide. Sensitivity of the arrangement may be increased by introducing controlled bends at the measuring point. WHAT WE CLAIM IS: -

1. A method of temperature detection by measurement of the optical transmitting properties of a fibre light guide, the method including cladding the guide with a plastics material having a refractive index which is more temperature dependent than that of the material for which the light guide is made, launching light signals along the light guide, and measuring the attenuation of said light signals after their passage through the guide, said attenuation being determined by the difference in refractive indices betwen the light guide and the cladding at the temperature of the light guide.