GB2202046A

GB2202046A - Optical fibre sensor arrangement

Info

Publication number: GB2202046A
Application number: GB08705770A
Authority: GB
Inventors: Christopher Anthony Wade
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1987-03-11
Filing date: 1987-03-11
Publication date: 1988-09-14
Also published as: GB8705770D0

Abstract

An optical fibre sensor arrangement allows time division multiplexing of frequency modulated continuous wave (FMCW) type sensors. The sensors 17 are arranged in series with reflective spices 19 therebetween. A source 15 provides pulsed sawtooth signals which are incident upon the assembly of the sensors 17 and reflective splices 19, the reflected signals being detected by a photodetector 21. The combined reflected signals constitute at the photodetector 21 a heterodyne frequency signal. Variations in the sensors are detectable by changes in this heterodyne frequency signal. The source 15 may be chirped or swept through a predetermined frequency band. <IMAGE>

Description

~OPTICAL HBRE SENSOR ARRANGEMENT The present invention relates to optical fibre sensor arrangements and more particularly but not exclusively to such arrangements in a multiplexer configuration.

Variations in the length of optical fibres alter certain physical characteristics and responses of the fibres. These alterations may be used to provide a sensor arrangement which may be suitably arranged in a multiplexer configuration. A known type of such a sensor arrangement is that using a frequency modulated continuous wave (FMCW).

In existing methods for passively multiplexing FMCW type sensors, each intefferometric sensor is given a different path imbalance and a frequency division-multiplexed (FDM) output is produced. The output however also contains unwanted "cross term" frequency components due to signals travelling along paths through two or more interferometers and attention must be given to ensure that frequencies associated with sensor signals are not infringed.

These types of interferometers can be arranged in series or parallel when fabricating a multiplexer. The series type configuration uses fewer couplers (or beam splitters) but can only be used for a small number of sensors if interference from "cross term" signals is to be avoided. The parallel type configuration uses more couplers but "cross term" signals can be avoided if the fibre leads connecting the sensors are sufficiently long that light from different sensors is incoherent.These frequency division multiplexed types of FMCW sensors thus have the following disadvantages: (i) "cross term" signals limit the number of sensors that can be multiplexed together; (ii) A large number of couplers are required; (iii) Each of the multiplexed interferometers must have a different path imbalance It is an object of the present invention to provide a multiplexer arrangement that substantially relieves the above disadvantages.

According to the present invention there is provided an optical fibre sensor arrangement comprising a plurality of optical fibre sensor elements with partially reflective splices therebetween being serially connected to a source of pulsed signals at a specific frequency and a detector whereby in operation a pulsed signal from the source is partially reflected to the detector by the partially reflective splices, the partially reflective splices being arranged to ensure incidence of the reflected signals upon the detector such that the combination of the reflected signals at the detector produces a resultant product signal indicative of sensor element displacements.

In one preferred embodiment the coherence length between adjacent pulses transmitted by the source, as for example the period between the leading edges of the two pulses, is greater than the time taken for each pulse to travel the effective length of a sensor element i.e twice the sensor element length or in an alternative embodiment a differential delay loop is incorporated within the arrangement between the source and the plurality of sensor elements.

The sensor elements may be either of equal or varying length.

The pulsed source may be a laser and the detector a photodiode.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a graphical representation of a known sawtooth modulated signal source with a superimposed time delay Figure 2 is a schematic diagram of a known frequency modulated continuous wave sensor; Figure 3 illustrates schematically sensors as shown in figure 2 arranged in serial multiplex; Figure 4 illustrates schematically sensors as shown in figure 2 arranged in parallel multiplex; Figure 5 is a graphical representation of a pulsed sawtooth modulated signal source used with an embodiment of the present invention; Figure 6 is a schematic representation of a multiplexed sensor according to an embodiment of the present invention including reflective splices; Figure 7 illustrates frequency responses from the sensor as shown in figure 6;; Figure 8 is a representation of heterodyne corresponding to each sensing element sequentially; Figure 9 is a schematic representation of a sensor as shown in figure 6 including additionally a differential delay; Figure 10 is a graphical representation of signals received at point B in figure 9; and, Figure 11 is a graphical representation of reflected signals received at the detector of figure 9.

Figures 2 to 4 describe three basic known frequency modulated continuous wave sensors arrangements, the sensor of figure 2 providing the curves as shown in figure 1 where a solid line represents sent pulses and a broken line reflected pulses. It is readily appreciated that a time delay t and a heterodyne signal of frequency f (figure 1) are introduced into the arrangement shown in figure 2 by the presence of a sensor path 1. The length of the sensor path 1 compared to a reference path 3 determines the degree of shift in both t and f. Consequently, the arrangement of figure 2 may constitute a displacement indicator as variation in the sensor path 1 will alter the heterodyne signal.

The optical frequency of a source 5 is sawtooth modulated as in figure 1, the output beam being split into two beams one of which passes along the sensor path 1 and the other passing along the reference path 3 before recombining and becoming incident upon a photo detector 7. The time delay t between signals passing through the sensor path 1 and reference path 3 resulting in a frequency difference of f=s t where sz i.e.

dt the slope of the sawtooth. The two frequencies mixing on the photodetector 7 to constitute the heterodyne signal of frequency f.

Consequently, as the frequency of the signal at the photodetector 7 is proportional to the path imbalance between the sensor pathl and the reference path 3, a change in the length of the sensor path 1 will cause a consequential change in the phase of the detector signal.

Small changes in the length of the sensor path 1 (lie. in the order of a wavelength of light) will result in quite measurable changes in the phase of the heterodyne signal while larger changes will produce a measurable change in heterodyne frequency.

Existing methods of passively multiplexing frequency modulated continuous wave sensors are illustrated in figures 3 and 4 i.e. serial and parallel respectively. Each element is provided with a different path imbalance; corresponding signals therefore have different frequencies, and a frequency-division-multiplexed (FDM) output is produced.

In an embodiment of the present invention a signal source 15 (figure 6) transmits a sawtooth modulated pulsed signal (figure 5).

There being a period between pulses when there is no signal present i.e. the source 15 is switched off or interrupted. This pulsed saw-tooth modulated beam passes into an array of elements 17 comprising a series of identical sections of optical fibre each of the same nominal length 1 and separated by partially reflecting splices or joints 19. A portion of the beam is reflected from each of the partially reflecting splices or joints 19 back up the series array of sensor elements 17 and become incident sequentially as reflected signals on a photodetector 21. In a preferred embodiment the duration of the ramped-frequency pulses is set to be four times the one-way propagation time of light through a single sensor element 17 of the array 9.Thus, the first half of each pulse reflected from the far-end of a given sensing element overlaps at the photodetector 21 with the second half of each pulse reflected from the near end of the same sensing element 17, and the delay T between these two beams is the time taken for light to travel the length of the sensor elements and back: T=2ln/c (see figure 7), where c is the speed of light in the optical fibres and n is the number of elements. This results in a heterodyne signal of frequency: f=Ts. It can be seen that any change in the length 1 of one sensor of array 9 will cause a change in the frequency and phase of the corresponding heterodyne signal.

Heterodyne signals corresponding to each of the sensor elements 17 occur sequentially (figure 8), and the cycle repeated to form a time-division-multiplexed output. This can be demultiplexed and the frequencies or phases of the signals corresponding to each sensor of array 9 extracted using standard techniques that are well known to those skilled in the art.

The frequency of each of the signals is an unambiguous measure of the length of the corresponding sensor element, while the phase provides a highly sensitive means of detecting movements in the length.

One disadvantage of the time division multiplexed embodiment described previously is that the source 15 requires a coherence length between reflected pulses of greater than 21, the effective optical path difference between the two interfering beams.

In an alternative embodiment described below this problem is avoided and therefore comparatively short coherence length or pulse period sources can be used without shortening 1 such that 21 is much less than the coherent length of the source. This alternative embodiment is shown in figure 9. The arrangement of figure 9 is similar to the previous embodiment except that a differential delay 29 is incorporated immediately after the source 31, thus providing two paths between points A and B in figure 9, a first direct and second delayed by T+ T, where T=21n c and T T. The source modulation, is also similar to that described previously except that the duration of each frequency ramp is now T rather than 2T. The signals transmitted from source 31 as seen at point B are shown in figure 10 and the resulting reflected signals in figure 11.

If T is very much less than T, the direct ramp reflected from the far end of a given sensor overlaps considerably with the delayed ramp reflected from the near end of the same sensor and, during the period of overlap, these two signals mix on the photodiode to produce a beterodyne signal of frequency f=s t where t= T+ Jn and the instantaneous length of C the sensing element is 1+ 1. Changes in the length of a particular sensing element thus result in changes in the frequency and phase of the corresponding heterodyne signal.

It trill be appreciated that the same effect as above may be produced by inserting a differential delay at point C in figure 9.

Having the differential delay after the source reduces the imbalance between the two interfering beams, without recourse to shortening the nominal sensor length 1.

An additional feature of this second alternative embodiment is that the partial balancing reduces the nominal heterodyne frequency, but does not reduce the magnitude of changes in frequency caused by changes in sensor length; these are still given by f=2 lns. The net result is that for a C given change in sensor length, the corresponding change in frequency is the same for both configurations, but this corresponds to a larger fractional change in frequency for the partially balanced configuration.

Whereas in the above specific embodiments optical fibres of equal length are used it will be appreciated that an alternative embodiment may be constructed with varying optical fibre lengths and different apparatus components including lasers and photodiode detectors.

Claims

1. An optical fibre sensor arrangement comprising a plurality of optical fibre sensor elements with partially reflective splices therebetween being serially connected to a source of pulsed signals at a specific frequency and a detector whereby in operation a pulsed signal from the source is partially reflected to the detector by the partially reflective splices, the partially reflective splices being arranged to ensure sequential incidence of the reflected signals upon the detector such that the combination of the reflected signals at the detector produces a resultant product signal indicative of sensor element displacements.

2. An optical fibre sensor arrangement as claimed in claim 1 wherein operation the coherence length between adjacent pulse transmitted by the source is greater than the time for each pulse to travel the length of the sensor element

3. An optical fibre sensor arrangement as claimed in claims 1 or 2 wherein a differential delay loop is incorporated within the arrangement between the source and the plurality of sensor elements giving in operation pulsed signals at least two routes to the plurality of sensor elements of varying length.

4. An optical fibre sensor arrangement as claimed in claims 1 or 2 wherein a differential delay loop is incorporated within the arrangement between the plurality of sensor elements and the detector.

5. An optical fibre sensor arrangement as claimed in any preceding claim wherein the sensor elements are of equal length.

6. An optical fibre sensor arrangement as claimed in claims 1, 2 or 3 wherein the sensor elements are of varying length.

7. An optical fibre sensor arrangement as claimed in any preceding claim wherein the pulsed source is a laser device.

8. An optical fibre sensor arrangement as claimed in any preceding claim wherein the detector is a photo-diode device.

9. An optical fibre sensor arrangement as claimed in claims 4 or 5 wherein the sensor elements are formed of optical fibre.

10. An optical fibre sensor arrangement substantially as hereinbefore described with reference to the accompanying drawings.

11. An optical fibre sensor arrangement comprising an optical transmission channel including an optical fibre having defined therein at nominally equispaced locations a plurality of optically reflective splices, the arrangement comprising a pulsed light source which is arranged to produce spaced pulse which are chirped or swept through a predetermined frequency band and which are of a duration which bears a predetermined relationship to ~ the nominal spacing between the splices, and a frequency detector responsive to the frequency of light reflected from the splice in dependence upon which the parameter to be sensed is measured.