GB2181231A

GB2181231A - Optical sensing arrangement

Info

Publication number: GB2181231A
Application number: GB08524354A
Authority: GB
Inventors: John Phillip Dakin
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1985-10-02
Filing date: 1985-10-02
Publication date: 1987-04-15
Also published as: GB2181231B; GB8524354D0

Abstract

An optical sensing arrangement for remote measurement of temperature or solution concentrations in which light derived from a source 12 at a measuring and/or detecting location is conveyed by optical fibre means 13 to a remote location where said light is plane polarised, if it is not already so polarised. At the remote location it is transmitted through an optically-active liquid (e.g. bath of sugar solution or liquid-filled optical fibre 15) which produces rotation of plane of polarisation of the polarised light. The polarised light after emerging from the optically-active liquid is applied to orthogonal polarisers 18, 19 which may be situated at the remote or at the measurement location, the polarised light or the light emerging from the polarisers being conveyed by further optical fibre means 16 from the remote location to the measurement location. Photo-detectors 20, 21 receive the light outputs from the orthogonal polarisers to generate electrical outputs which can be compared to give an indication of the degree of rotation of the polarised light produced by its passage through the optically-active liquid,which in turn is an indication of the temperature or concentration of the liquid. <IMAGE>

Description

SPECIFICATION Improvements relating to optical sensing arrangements This invention relates to optical sensing arrangements and more especially to such arrangements for the remote measurement and/or detection of (variations in) such parameters as temperature and solution concentrations.

According to one aspect of the present invention there is provided an optical sensing arrangement in which light derived from a light source art a measuring and/or detecting location is conveyed by optical fibre means to a remote location where said light is plane polarised, if it is not already so polarised, and transmitted through an optically-active liquid which produces rotation of the plane of polarisation of said polarised light, in which the polarised light after emerging from said optically-active liquid is applied to orthogonal polarisers at said remote or measurement location, and in which the polarised light orthe light emerging from the polarisers, as the case may be, is conveyed by further optical fibre means from the remote location to the measurement location whereat photo-detectors are provided for receiving the light outputs from the orthogonal polarisers and to generate electrical outputs which can be compared for affording an indication of the degree of rotation of the polarised light produced by its passage through the optically-active liquid.

In carrying outthe above aspect ofthe invention the optically-active liquid may, for example, comprise sugar solution the concentration of which isto be measured. The concentration will affect the degree of rotation of polarisation ofthe polarised light transmitted through the solution, as will also the temperature of the solution and the length of the path through said solution. Consequently, ifthetem- perature and the length of the light path remain sub stantially constant then the concentration of the solution which will be located remotely can be measured by the arrangement according to the invention.

According to another manner of carrying outthe above aspect ofthe present invention, the opticallyactive liquid maycomprisetheliquid coreof a prefer- ably coiled length of liquid-filled optical fibre. Plane polarised light entering one end of the fibre is rotated as it passes along the fibre but the temperature ofthe liquid core ofthe fibre determines the degree ofsuch rotation. The polarised lightemergingfromthefar end of the liquid core is sp!it into two components which are transmitted through the orthogonal polarisers and on to photo-detectors for detecting rotation ofthe light and thus the temperature ofthe liquid core of the optical fibre.

The coiled liquid-filled optical fibre is located remotely from the light source and the photo-detectors at the measuring point. Ordinary, or at least nonoptically active, birefringement optical fibres may be used to convey the light to and from thetemperature sensing fibre. If the ordinary optical fibres for this use were to have an unacceptable degree of scrambling or rotation of polarisation then the polarisers may be positioned adjacent the sensing coil atthe remote location.

The present invention will now be described by way of example with reference to the accompanying drawings in which Figure lisa diagram of an arrangement for measuring solution concentration; and, Figure 2 shows an arrangementforthe remote measurement of temperature Referring to Figure 1 of the drawings the optical arrangement for measuring the concentration of sugar solution say or of any other optically active liquid (e.g. dextrose solution) comprises a cell 1 of standard or known length which contains sugar solution 2 the concentration of solute in which is to be measured. Monochromatic unpolarised light, conveniently derived from laser source 4, is transmitted down an optical fibre to a polariser 5 which plane polarises the light.The polarised lightthen enters the cell 1 at one end and in passing through the cell the plane of polarisation of the light is rotated by the sugar solution 2, the degree of such rotation being dependent upon the concentration of the solution, as well as upon the temperature of the solution 2 and the length of the cell 1. Thus, if the temperature and cell length are substantially constant the solution concentration can be determined from the degree of rotation of the plane of polarised light. With this end in viewthe light arriving at the far end ofthe cell is split into two components by vertical and horizontal polarisers 6 and 7, respectively, and these components are transmitted through optical fibres 8 and 9to photo-detectors 10 and 11.By comparing the outputs ofthe photo-detectors 10 and lithe degree of rotation of the polarised light and thus the strength or concentration of the sugar solution contained in the cell 1 can be determined.

Referring now to Figure 2 the arrangement shown for measuring temperature remotely comprises a light source 12 (e.g. laser) for producing unpolarised light which is applied to one end of an optical fibre 13 extending to a remote location where the optical fibreterminatesata polariserl4forproducing plane polarisation of the light which is then transmitted along a liquid filled fibre 15 comprising an opticallyactive liquid core (e.g. sugar or dextrose solution) which will be located at the location wheretemperature is to be measured. The other end ofthe liquid-filled fibre 15 may be connected back to the temperature measuring location by means of a nonoptically active optical fibre 16which maintainsthe polarisation state ofthe light emerging from the fibre 15.This optical fibre 16 conveys the polarised lightto a beam splitter 17 which splits lights in two components which are then fed via vertical and horizontal polarisers 18 and 19 having photo- detectors 20 and 21 associated therewith. These detectors produce electrical outputs the relative magnitude of which provide an indication ofthe angle ordegree of rotation of the polarised light emerging from the output end of fibre 1 5. When the liquid filled fibre 1 5 is subjected to changes in temperature the rotation of the plane of polarisation varies accordingly. Consequently, by suitable calibration of the sensing fibre, temperature measurement can be achieved.

In a modification to the embodiment just above de scribedthe beam splitter 17 and polarisers 18 and 19 may be located atthe remote location and the outputs from these polarisers conveyed to the photodetectors 20 and 21 at the measuring location by ordinary optical fibres.

In the various optical arrangements hereinbefore described the optical fibre employed are preferably single modefibressincethey preserve plane polarisation better than multi-mode fibres.

Claims

1. An optical sensing arrangement in which light derived from a light source at a measuring and/ordetecting location is conveyed by optical fibre means to a remote location where said light is plane polarised, if it is not already so polarised, and transmitted through an optically-active liquid which produces rotation ofthe plane of polarisation of said polarised light, in which the polarised light after emerging from said optically-active liquid is applied to orthogonal polarisers at said remote or measurement location, and in which the polarised light or light emerging from the polarisers is conveyed byfurtherop tical fibre means from the remote location to the measurement location whereat photo-detectors are provided for receiving the light outputs from the orthogonal polarisers and to generate electrical out putswhichcan becomparedforaffording an indication ofthe degree of rotation ofthe polarised light produced by its passage through the optically-active liquid.

2. An optical sensing arrangement as claimed in claim 1, in which the optically-active liquid comprises sugar solution and in which the temperature of said solution and the length of the light path through said solution are maintained substantially constant.

3. An optical sensing arrangement as claimed in claim 1, in which the optically-active liquid com prisesthe liquid core of a liquid-filled optical fibre whereby the temperature of the liquid core of the fibre determines the degree of the rotation ofthe plane of polarisation of the light passing therethrough and in which the polarised light emerging from the far end ofthe liquid core is split into two components which are transmitted through the orthogonal polarisers and on to photo-detectors for detecting rotation of the light and thus thetemperature ofthe liquid core ofthe optical fibre.

4. An optical sensing arrangement as claimed in claim 3, in which the liquid-filled fibre is of coiled form and located remotely from the light source and the photo-detectors at the measuring point.

5. An optical sensing arrangement as claimed in claim 4, in which non-optically active birefringent op- tical fibres are used to convey the lightto and from the temperature sensing fibre.

6. An optical sensing arrangement as claimed in claim 1, in which thefurtheroptical fibre meanscomprises a single optical fibre which conveys polarised lightto a beam splitter at the measurement location which splits the light into two components which are fed via the orthogonal polarisersto respective photodetectors.

7. An optical sensing arrangement as claimed in claim 1, in which the further optical fibre means comprises two optical fibres extending from the orthogonal polarisers to the respective photo-detectors.

8. An optical sensing arrangement substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

9. An optical sensing arrangement substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.