GB2138936A

GB2138936A - Optical sensor systems

Info

Publication number: GB2138936A
Application number: GB08410094A
Authority: GB
Inventors: Edmund Sergio Robert Sikora; Anthony John Walkden; Peter Richard Wallace; Roger Michael Wood
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1983-04-26
Filing date: 1984-04-18
Publication date: 1984-10-31
Also published as: GB2138936B; GB8410094D0

Abstract

An optical sensor system including a light source (1,3) arranged to produce light within two different wavelength bands ( lambda 1, lambda 2), and an optical sensor unit (9). The source (1,3) and unit (9) are linked by an optical fibre (5) an element (7) which is effective to reflect light of wavelength lambda 1, and transmit light of wavelength lambda 2 being interposed between the fibre (5) and unit (9). A detector (15, 17) provides a first measurement of the intensity of light of wavelength lambda 1, after it has passed through the fibre (5) and been reflected at the element (7), and a second measurement of the intensity of light of wavelength lambda 2 which has passed through the fibre (5) and passed through the unit (9). The first measurement is then used to compensate the transmission efficiency of light of wavelength lambda 2 through the fibre (5) and unit (9) for any changes in the optical transmission characteristic of the fibre (5). <IMAGE>

Description

SPECIFICATION Optical sensor systems This invention relates to optical sensor systems of the kind which employ a light source, an optical sensor unit whose optical transmission characteristic changes in response to a parameter being monitored bythe system, and a detectorfor measuring the amount of light passing from the light source through the sensor unitto the detector, the sensor unit being remote from the light source and detector, and linked thereto by an optical fibre.

Whilst multimode optical fibres are insensitive to, for example, stray electromagnetic fields which can affect an electrically linked system, variations in the optical transmission characteristic of an optical fibre can occur, due, for example, to distortion ofthe fibre.

This will then affectthe amount of light received bythe detector unit via the sensor unit.

It is an object ofthe present invention to provide an optical sensor system of the kind specified in which changes in the optical transmission characteristic of the optical fibre may be compensated for.

According to the present invention there is provided an optical sensor system including: a light source arranged to produce light within a first wavelength band, and light within a second different wavelength band; an optical sensor unit whose optical transmission characteristic changes in response to a parameter being monitored bythe system; an optical fibre linking said source and said sensor unit; an element effective to reflect light within said first wavelength band, andtotransmit light within said second wavelength band said element being interposed between said source and said sensor unit; and a detector means effective to provide a first signal representative of the intensity of light within said first wavelength band produced by said source which has been reflected at said element, and to provide a second signal representative ofthe intensity of light within said second wavelength band produced by said source which has passedthroughthesensorunit, such that said first measurement may be used to compensate the transmission efficiency of said light within said second wavelength band through said fibre and said sensor unitforany changes in the optical transmission characteristic of said fibre.

Two optical sensorsystems in accordance with the invention will now be described, byway of example only, with reference to the accompanying drawings in which: Figure lisa schematic diagram ofthe firstsystem; Figure 2 shows schematicallyto a largerscalethe part ofthe system shown in dotted lines in Figures 1 and 3; and Figure 3 is a schematic diagram ofthe second system Referring firstlyto Figure 1 ,the first system comprisestwo light sources 1,3 each source being capable of emitting light within a different wavelength band A1, A2 respectively. The output from each ofthe sources, 1, 3 is coupled into a single optical fibre 5.Referring now 'also to Figure 2, atthe end ofthefibre 5 remote from the sources 1, 3there is a reflective surface7 designed to reflect light of wavelength A1 andtotransmit light of wavelength A2. Connectedtothesurface7 is a sensor unit in the form of a Fabry-Perotinterferometer 9, having the same diameterasthefibre Sand a length L. Reflective surfaces 11, 13at either end ofthe interferometer 9 are designed respectively to partially reflect, and to totally reflect light of wavelength A2.

Connected to the fibre 5, adjacent to the light sources, 1,3 are two photodiode detectors 15,17 each responsive to lightofone of thetwo wavelengthsA1, A2 respectively, as indicated in the diagram. An electrical outputfrom the detector 15 isconnectedto a comparator 19. The output ofthe comparator is connected to a controller 21, which is, in turn, connected via a multiplier device 23, to the light source 3. An output from the controller 21 is also connectedtothe lightsource 1.

In use ofthe apparatusthe interferometer9 is located at a point where it is intended to monitor, for example, temperature, an optical link being main "rained with the rest ofthe system via the optical fibre 5.

The two sources 1,3 are alternatelyswitched on,the detector 15 measuring the intensity of light of wavelength A1 afterit has passed downthefibre 5 and been reflected by the su rface 7 back up the fibre. An electrical signal indicative ofthe intensity of light of wavelength afterthe light has passed through the fibre is produced by the detector 15, and is compared with a reference signal, indicated as 25, in the comparator 19.Thecomparatorthen gives an output signal which is indicative of any changeswhich have occurred inthetransmissionefficiencyoflightof wavelength A1 through the fibre.This signal is then used bythe controller 21 to produce an electrical output signal, which is multiplied by afactor Kin the multiplier device 23. The output signal ofthe multi plier device 23 is then effective to control the intensity ofthe light source 3 such that the intensity of light of wavelength A2 incident on the surface 7 remains constant, it being assumed thatfor any attenuation Aoc, of light of wavelength A1 due to, for example, damage to the fibre 5, a corresponding attenuation K toys, of light of wavelength A2will be produced, K being a constant or a known single valued function. The outputfrom the controller 21 to the light source 1 is used to maintain the intensity ofthe light source 1 a constant. Light of wavelength A2 shone down the fibre 5 istransmittedthrough the surface 7, and partially reflected by surface 11 ofthe interferometer 9, and partiallytransmitted to be reflected at the surface 13.

Any change in the length ofthe material between the two surfaces 11,13 dueto, for example, changes in temperature will cause constructive and destructive interference of light of wavelength A2, changes in light intensity duetothis interference being detected bythe detector 17 from the light of wavelength returning back up the fibre 5. Thus the detector 17 is able to give an output which is independent of variations in the optical transmission coefficient of the fibre 5 linking the sensor unit9 and the rest of the apparatus.

Referring nowto Figure 3, the second system to be described is similar in form to the first system and consequently the same items are correspondingly labelled. In the second system, however, the output from the detector 15 is connected, via the multiplier device 23, to a comparator 27, a second inputto the comparator 27 being taken from the output ofthe detector 17.

in use, as in the first system, the two sources 1,3 are alternately switched on. In the second system, howev er,the intensities of each of the sources 1,3 are both maintained at a constant level. The electrical signal produced bythe detector 15, indicative ofthe intensity of light of wavelength A1, after it has passed through the fibre 5 is multiplied by a factor K in the in the multiplier device 23, the output ofthe multiplier device 23 being subtracted in the comparator 27 from the output ofthe detector 17. Thus, assuming as in the first methodthatforanyattenuation Aoc, of light of wavelength A1,through the fibre 5 there will be a corresponding attenuation ncx, of light of wavelength A2,the output ofthe comparator 27 will be a signal which is independent of attenuation, or variations in attenuation, ofthe fibre 5.

Itwill be appreciated thatwhilst in both systems described herebefore by way of example, a Fabry Perot interferometer is used as the sensor unit, the methods are equally applicable foruse with other sensor units connected to the rest ofthe system via an optical fibre.

Claims

1. An optical sensor system including: a light source arranged to produce light within a first wavelength band, and light within a second different wavelength band; an optical sensorunitwhose optical transmission characteristic changes in re sponsetoa parameter being monitored bythe system; an opticalfibre linking said source and said sensor unit; an element effective to reflect light within said first wavelength band, and to transmit light within said second wavelength band said element being interposed between said source and said sensor unit; and a detector means effective to provide a first signal representative ofthe intensity of light within said first wavelength band produced by said source which has been reflected at said element, and to provide a second signal representative of the intensity of light within said second wavelength band produced by said source which has passed through the sensor unit, such that said firstmeasurement may be used to compensate the transmission efficiency of said light within said second wavelength band through said fibre and said sensor unitfor any changes in the optical transmission characteristic of said fibre.

2. A system according to Claim 1 including a control means responsive to said first signal to control the intensity of light within said second wavelength band produced by said source so as to effect said compensation.

3. Asystem according to Claim 1 including means responsive to said first signal to estimate any changes in the transmission efficiency of light within said second wavelength through said fibre, and a means which subtracts said estimate from said second measurement.

4. A system according to any one ofthe preceding claims in which said sensor unit is a Fabry-Perot interferometer.

5. An optical sensor system substantially as hereinbefore described with reference to Figures 1 and 2, or Figures 2 and 3 ofthe accompanying drawings.