GB2256481A - Optical fibre sensor array - Google Patents

Abstract

An optical fibre sensor in which the phase of reflected optical pulses is perturbed by the parameter being sensed has the advantage that a number of sensors can be mounted on a single downlead, but this configuration can lead to crosstalk between the sensor signals caused by non simultaneity errors on either the interrogating or return optical pulses, depending on the architecture. This crosstalk between optical fibre sensors is cancelled by measuring the value of the parameter sensed by a plurality of sensor in an array (P1 etc.), operating on each sensed value (at INT) to generate a correction factor which is added to the value sensed by each subsequent sensor in the array. <IMAGE>

Description

Removing Crosstalk in Multiplexed Signals from an Optical Sensor Arrav This invention relates to arrangements and methods for removing cross talk in multiplexed signals from an optical sensor array, particularly time domain multiplexed signals in an optical fibre.

Multiplexed fibre optic sensors arrays are known for example from UK patent GB 2126820B. Such an arrangement is shown in Figure 1 and, as is known, comprises a plurality of sensors S1-S4 disposed in series along a fibre light guide 1.

Each sensor comprising a length of fibre light guide bounded by discontinuities in the light guide. Pairs of pulses are repetitively launched down the light guide, the sensor spacing and inter pulse spacings being such that, for each respective sensor, a portion of the first pulse of each pair reflected from the discontinuity at the far end of the sensor arrives at the optical detector simultaneously with a portion of the second pulse of the pair reflected from the discontinuity at the near end of the sensor. As both pulses are produced from a coherent light source, the two reflected pulses heterodyne together in the optical detector. Only the first reflected pulse has traversed the length of the sensor, and in its passage through the sensor its phase is changed as a function of the physical properties e.g. length, of the sensor.These physical properties are perturbed by the parameter being sensed, thereby perturbing the change of phase (phase shift) by a corresponding amount. When the two reflected pulses, one being shifted in phase by the sensor, the other not, are heterodyned, a signal is produced containing phase modulation corresponding to the change of phase.

Reflection and heterodyning take place at progressively different times as the pulse pairs progress down the light guide and interact with the series of sensors, producing a sequence of phase modulated output pulses at the output of the optical detector. By launching a number of pairs of pulses down the light guide, a serial stream of pulses is received by the photo detector. The photo detector produces at its output a corresponding time-domain multiplexed data stream comprising bursts of phase modulated carrier. For example in an acoustic array the carrier might typically be 1MHz and may be phase modulated at (say) lkHz by a lkHz signal incident on a given fibre sensor.

In an ideal system the output pulses from each of the sensors are entirely separate and are not influenced by the passage of the signals through other sensors in the series.

In practice however the passage through other sensors in the series can alter the output pulse from the sensor, this alteration is known as crosstalk.

One source of crosstalk between sensors in such a reflectometric configuration is caused because the two integrating pulses are separated in time (i.e. not simultaneous). Hence this form of crosstalk is known as non-simultaniety error or NSE for short. As the first pulse passes through an an optical fibre sensor the effective path length will be modified by the acoustic pressure, when the second pulse passes through the same sensor, (2b) seconds later, the pressure field may have changed causing a relative phase change between the first and second pulses as they travel down the array. With this source of crosstalk, for any sensor only earlier sensors in the series will produce crosstalk. The same effect can be generated by any other parameter which alters the effective path length through the sensors.

The length of optical fibre before the first sensor S1, generally known as the downlead of an array of optical sensors, is normally relatively insensitive to acoustic pressure effects, and because it is distributed in space any pressure effects will tend to be uncorrelated over its length, which in most cases will make its contribution to this effect negligible. However if the downlead is sensitive, there may be crosstalk generated in this way in the downlead as well as in the sensors earlier in sequence than a given sensor.

This invention was intended to allow crosstalk produced in this way to be at least partially removed from the output signal of a sensor.

This invention provides a method of removing crosstalk in multiplexed signals from an optical sensor array including the steps of: (i) measuring the values of a parameter sensed by a plurality of parts of the array, (ii) operating on the parameter value sensed by each part of the array to generate a correction factor which is added to the parameter value sensed by each subsequent part of the array to correct the parameter value sensed by each subsequent part of the effects of crosstalk generated in the first part.This allows crosstalk to be eliminated The invention will now be described by way of example only, with reference to the accompanying diagrammatic figures in which: Figure 1 shows a fibre optic sensor array suitable for use with the invention, Figure 2A is an explanatory diagram showing pulse movements through the system, Figure 2B is a graph showing pressure changes over time, Figure 3A shows an algorithm putting the invention into effect, Figure 3B shows a detailed breakdown of an operation used in the algorithm of Figure 3, Figure 4 shows an analogue circuit for carrying out the algorithm of Figure 3, Figure 5 shows a digital circuit for carrying out the algorithm of Figure 3 and Figure 6 shows a simple linear interpolation and extrapolation suitable for use in the circuit of Figure 5.

Referring to Figure 1, a fibre optic pressure sensor array is shown which operates as described previously.

For the purposes of this example it is assumed that the downlead of the array is not affected by acoustic pressure, while the sensors S1 to S4 are and are subjected to an acoustic pressure varying overtime.

The return from the first sensor S1 is not affected by NSE crosstalk because the pulses have not passed through any previous sensors. Thus the return pulse from the first sensor S1 contains only the required phase modulation generated by the acoustic pressure.

The return from the second sensor S2 will contain the required phase modulation plus a term caused by the time delay between the transmit pulses as they pass through the first sensor; for sensors further down the array their return will have crosstalk contributions from all prior sensors.

Referring to Figure 2A the movements of a pair of pulses through the first sensor S1 to generate a heterodyne return pulse from the second sensor S2 is shown.

The pulses are drawn to a different scale to those shown in Figure 1 for clarity.

At time t = 0 the first forward pulse enters the length of optical fibre forming sensor S1.

At time t =tthe first forward pulse enters the length of optical fibre forming sensor S2 At time t = 2 tthe first forward pulse leaves the sensor S2 and is partially reflected generating a first return pulse. The second forward pulse enters the sensor S1.

At time t = 3 t the second forward pulse enters the sensor S2 and is partially reflected, generating a second return pulse. The first return pulse leaves the sensor S2 and the two return pulses travel together back to the photodetector.

Referring to Figure 2B the graph shows the variation of acoustic pressure over time from t = 0 to t = 3t. As can be seen the pressure acting on sensor S1 between times t = 0 and t = tas the first forward pulse passes through it is different to the pressure acting on it from t = 2to t = 3t as the second forward pulse passes through it.

As an optical pulse passes through a sensor the total phase change will be determined by the average pressure it senses during its travel time in the sensor; this is why in Figure 2 the timing of the first results is shown as seconds from the first pulse incident on the first semi-reflector rather than (25) seconds which is when the two pulses overlap. In the same way the average values of the crosstalk terms are estimated using the value when the pulse is half way through the sensor.

The return signals can be expressed:

or generally:

Where: is is the measured pressure signal from the mth sensor m is the number of the sensor p, is the actual pressure signal on the mth sensor without crosstalk.

The factor of 'li is due to the fact that the crosstalk term is due to a one way transit through a sensor, while the required term has a two way transmission.

The process to remove the crosstalk starts with the evaluation of the pressure signal on the first sensor that has no crosstalk, then by interpolation and/or extrapolation using the previous data the crosstalk terms for the second sensor can-be calculated. This allows the second sensor's actual pressure signal to be calculated. The crosstalk term for the first sensor will be the same for all other sensors, so once it has been calculated the same value can be removed from all sensors. In the same way the crosstalk components for sensors 2,3 etc. can be avalanched together for all successive sensors.

Figure 3A shows a generalised algorithm for this process. The measured pressure value P1 from the first sensor S1 is outputted as the pressure value of P1 out since it is not subject to crosstalk.

The measured pressure value of P1 is operated on by the operator "int" and the resulting correction factor added to the measured pressure value of P2 from the second sensor S2 to give the output pressure value P2 out with crosstalk effects removed.

The measured pressure value P3 then has the sum of the correction factors of the measured values P1 and output value P2 out operated on by operator "int" added to it to give the output pressure value P3 out with crosstalk effects removed.

This procedure is followed for all of the pressure values from all of the sensors.

The operator "int" is shown in detail in Figure 3B. The measured pressure value is supplied on a line 1, then advanced by 3t/2 by an advance system 2. A portion of the signal is then passed to a delay 3 having a delay of 285.

The delayed output signal from the delay 3 is subtracted from the current signal in a subtractor 4 and the resultant multiplied by 0.5 in a multiplier 5. The signal from the multiplier is the correction factor output on a line 6.

It should be noted that the 32 Advance term is a second order correction, and can probably be ignored in most circumstances.

The processing required to achieve this crosstalk cancellation would be implemented after the signal processing required to demultiplex and demodulate the sensor signals.

The example used to demonstrate this process uses two transmit pulses that are separated in time and receive signals that are overlayed. It is possible to have an architecture where there is a single transit pulse and a pair of receive pulses separated in time as they pass back through previous sensors. A similar crosstalk cancellation technique could be implemented for this architecture also.

The patents quoted refer to heterodyne systems: in fact the techniques described here apply to any optical pulses time domain multiplexed interferometer sensor array, and its associated electronic techniques to obtain outputs from each sensor.

This crosstalk problem can exist in any time domain multiplexing optical architecture whether reflectometric or transmissive, although in some transmissive examples, the problem can be reduced to only that of unwanted signals on the downlead being transferred onto the sensors. The solution postulated is the same for these architecture as for the reflectometer described here.

This crosstalk correction algorithm can be implemented as an analogue or a digital system depending on the method of operation of the rest of the system.

Referring to Figure 4 a block diagram of a typical analogue demultiplexer and demodulators is shown with separate output (S1 2, S2 S3) for each sensor. After the sensors a cascaded algorithm is realised, to remove sequentially NSE from each sensor in turn, starting at the beginning of the array. The processor to realise this is based very explicitly on the algorithm in Figure 3, although in the interests of economy and simplicity, the 3 < /2 Advance term has been ignored, since it is a small correcting factor, as already mentioned, and introduces major difficulties in analogue implementation. The delays, summers and multiplier functions can all be realised using standard analogue electronic components.In the analogue implementation noise build up is due to the cascading increase of systems noise entering at P1,P2,P3 etc., and also the noise contribution of the analogue arithmetic functions. From this point of view, the analogue system will be inferior in its noise performance to a digital system.

Referring to Figure 5 a digital implementation is shown.

It is possible to realise the demodulation process using digital techniques to produce a parallel data output in which the sensor data is sequentially presented. Digital processing can be used to realise equation (1) using the algorithm shown in Figure 3. A simple block diagram is shown in Figure 5 for the digital system. The buffer store holds data for all the sensors in fixed relative locations, so that the processor can select the required sensor data to carry out the algorithms; Arithmetic is carried out using standard digital processing techniques. The interpolator operator "int" in Figure 3, generates the advanced and delayed values of the sensor outputs by extrapolation and interpolation respectively.

A simple linear scheme for these two operations is shown in Figure 6, although a more elaborate non linear approach could be taken. In this case the interpolation/extrapolation is calculated from the values of successive digital samples.

The value of the pressure value at a time 3f/2 in the future, is calculated as being equal to, Pm + [(3#/2/T) (Pm - Pm-1) ] While the value of the pressure value at a time #/2 in the past is calculated as being equal to, Pm + [(#/2/T) (Pm - Pm-1) ] Where Pm is the measured pressure value from the mth sensor.

Pm1 is the measured pressure value from the m - 1th sensor.

T is the time delay between the pulses passing through the m - 1th and mth sensors.

In a system where the downlead of the array is sensitive to external parameter values, the downlead will replace sensor S1 in the algorithm of Figure 3.

The examples are all pressure sensitive and form a pressure sensor array. The invention could equally well be applied to a sensor array sensitive to any other parameter.

Claims (7)

1. A method of removing crosstalk in multiplexed signals from an optical sensor array including the steps of: (i) measuring the values of a parameter sensed by a plurality of parts of the array, (ii) operating on the parameter value sensed by each part of the array to generate a correction factor which is added to the parameter value sensed by each subsequent part of the array to correct the parameter value sensed by each subsequent part for the effects of crosstalk generated in the first part.

2. A method as claimed in claim 1 in which in step (ii) the parameter value is operated on by an operator which subtracts the parameter value from the parameter value produced by the same part of the array at a time 2 t earlier and multiplies the resultant by a half to produce the correction factor, where t is the time taken for an optical signal to traverse that part of the array.

3. A method as claimed in claim 2 in which before the operation of claim 2 is carried out the parameter value is altered to its predicted value at a time 3 3//2 in the future.

4. A method of removing crosstalk substantially as shown in or as described with reference to Figures 3 or 3A of the accompanying drawings.

5. Apparatus for carrying out the methods of any of claims 1 to 4.

6. Apparatus for removing crosstalk substantially as shown in or as described with reference to Figure 4 of the accompanying drawings.

7. Apparatus for removing crosstalk substantially as shown in or as described with reference to Figures 5 or 6 of the accompanying drawings.