GB2183331A

GB2183331A - Optical sensing arrangements

Info

Publication number: GB2183331A
Application number: GB08528663A
Authority: GB
Inventors: John Philip Dakin
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1985-11-21
Filing date: 1985-11-21
Publication date: 1987-06-03
Also published as: GB2183331B; GB8528663D0

Abstract

A fibre optic loop oscillator sensor which comprises an optical transmitter 1 and receiver 8 interconnected by means of an optical fibre loop connection 2, 7 including a transducer sensor 3 for sensing parameters and for introducing a time delay into an optical signal being transmitted through the transducer sensor. A feedback control arrangement 9-12 is provided between the optical receiver and its input to maintain the amplitude or intensity of the optical signal input to the receiver at a substantially constant level notwithstanding variations in the attenuation of the loop connection. As shown, the feedback control arrangement includes a twisted nematic liquid crystal device 9 having infrared polarisers 10, 11 connected in the input to the receiver and a control circuit 12 which receives an electrical signal dependent on the light level received by the receiver. The control circuit controls the bias voltage to the liquid crystal device in dependence upon the electrical signal input thereto. <IMAGE>

Description

SPECIFICATION Improvements relating to optical sensing arrangements This invention relates to optical sensing arrangements and relates more specifically to such sensing arrangements embodying so-called fibre optic loop oscillator sensors.

Afibre optic loop oscillator sensor comprises an optical transmitter (e.g. light-emitting diode) connected to an optical receiver (e.g. photo-diode) by means of an optical fibre loop with the interposition in the loop connection between the transmitter and receiver of a transducer/sensor adaptedtoconvertthe measurand orparameter(e.g.

displacement or pressure) being sensed into a time delay which the tra nsducer/sensor introduces into the optical signal being transmitted therethrough.

Oscillation ofthefibre optic loop oscillator sensor takes place provided there is sufficient gain in the loop connection between the transducer and the receiver and the frequency of oscillation will be dependent upon the delay ofthe optical signal around the loop. Thus the loop oscillation frequency can provide an indication of the value of the transducer measurand.

However, it has been found that changes in the amplitude or level of intensity ofthe optical signal input to the receiver due, for example, to changes in the optical attenuation around the loop resulting from bending ofthe optical fibre, for instance, have produced significant changes in the measured oscillation frequency even though the time delay of the optical signal around the loop has remained constant This very undesirable feature of existing loop oscillator sensors, which give rise to serious inaccuracies in the measured oscillatorfrequency, and thus the measured parameter, may be due to variations in parasitic capacitances in the electronics ofthe oscillatorasthe optical signal level changes.

According to the present invention, therefore, there is provided a fibre optic loop oscillator sensor comprising an optical transmitter and receiver interconnected by means of an optical fibre loop connection having interposed therein a transducer/sensorfor sensing parameters and for introducing a time delay into the optical signal being transmitted through the transducer/sensor, in which a feedback control arrangement is provided between the optical receiver and its input to maintain the amplitude or intensity ofthe optical signal inputto the receiver at a substantially constant level notwithstanding variations in the attenuation of the loop connection.

By maintaining the amplitude ofthe optical signal input to the receiver at a substantially constant level the previously referred to undesirable consequential changes in oscillatorfrequencywith changes in receiver input signal level are avoided.

The feedback control arrangement may include a voltage controlled optical attenuator preferably consisting of a transmissive, twisted nematic liquid crystal device which has been modified for use in the infra-red partofthe light spectrum having suitable integral polarisers for operation at infra-red frequencies and which is controlled by the output from a crystal control circuit.

By way of example an embodiment of the present invention will now be described with reference to the accompanying drawing in which: Figure 1 shows a block schematic diagram of a known fibre optic loop oscillatorsensor; and, Figure2shows a blockschematicdiagram ofa fibre optic loop oscillator sensor having feedback control according to the present invention.

Referring to Figure 1 ofthedrawings,thefibre optic loop oscillatorsensorshown comprises an optical transmitter 1 (e.g. light-emitting diode having an associated driver circuit), the modulated IR light output from which is transmitted through an optical fibre 2to the input side of a sensing transducer3 which, in the present example, is adapted to vary the gap represented by the gap 4 between two sphere lens expanded beam connectors 5 and 6 in response to changes in the parameters (e.g. displacement, pressure) to be measured by the oscillator sensor.

The output side of the sensing transducer 3 is connected by means of an optical fibre 7 to the input of an optical receiver 8 (e.g. photodiode with amplifier means and automatic gain control), the a.c.

electrical output from which is then transmitted back to the transmitter 1 to complete the connection between the transmitter via the sensing transducer 3. Provided the gain of the loop connection is sufficiently high,thesensorwill oscillate and the frequency of oscillation, which may be measured by suitable means (e.g. counterortimercircuit) in the receiver, provides an indication of the value ofthe time delay introduced into the loop signal by the transducer gap 4. As the transducer gap 4 is modified, sothefrequency of oscillation will vary.

As previously mentioned, variation in the attenuation ofthe optical fibre connections 2 and 7 and the transducer 3 produces changes in the amplitude or intensity levels ofthe optical signals at the receiver input and such variations give rise to changes in the frequency of oscillation even though thetimedelayortransducergap remain constant.

This in turn leadstoseriouserrorsinthe measurement of various parameters. To overcome this problem the present invention provides a feedback control arrangement as shown in the exemplary embodiment depicted in Figure 2.

As can be seen from Figure 2, the amplitude or intensity level ofthe optical signal received by the receiver 8 is maintained substantially constant by means of a feedback co ntrol arrangement which comprises a twisted nematic liquid crystal device 9 having infra-red polarisers 10 and 11 connected in the input to the receiver 8. The device 9 is controlled by means of an electrical bias (ac) voltage input derived from a control circuit 12 which receives an electrical signal from the receiver 8 dependent upon the light level received by the latter. Thus, if the light input to the receiver 8 tends to increase, the control biasvoltageoutputfromthecircuit12willtendto reduce the light transmitted through the liquid crystal device to the receiver whereas, if the light input to the receiver tends to fall, the control bias voltageincreasesto increasetheoutputfromthe liquid crystal device 9. In this way the level of light input to the receiver 8 is maintained at a substantially constant level thereby avoiding the aforesaid problems experienced with existing fibre optic loop oscillator sensors due to changes in intensity levels of optical signals.

Claims

1. Afibre optic loop oscillator sensor comprisng an optical transmitter and receiver interconnected by means of an optical fibre loop connection including a transducersensorforsensing parameters and for introducing atime delay into an optical signal being transmitted through the transducer sensor, in which a feedback control arrangement is provided between the optical receiver and its input to maintain the amplitude or intensity ofthe optical signal input to the receiver at a su bstantial Iy constant level notwithstanding variations in the attenuation of the loop connection.

2. Afibre optic loop oscillator sensor as claimed in claim 1, in which the feedback control arrangement includes a voltage controlled optical attenuator.

3. Afibre optic loop oscillatorsensoras claimed in claim 2, in which the voltage controlled optical attenuator consists of a transmissive twisted nematic liquid crystal device suitable for use with infra-red light and having suitable integral polarisers for operation at infra-red frequencies and which is controlled bytheoutputfrom a crystal control circuit.

4. Afibre optic IQOP oscillator sensoras claimed in any preceding claim, in which the transducer sensor comprises means responsive to physical parametersto producevariations in the gap between two expanded beam connectors.

5. Afibreopticlooposcillatorsensor substantially as hereinbefore described with reference to Figure 2 ofthe accompanying drawing.