GB2211605A

GB2211605A - Optical fibre distributor sensor

Info

Publication number: GB2211605A
Application number: GB8829109A
Authority: GB
Inventors: David Andrew James Pearce
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1987-12-23
Filing date: 1988-12-14
Publication date: 1989-07-05
Anticipated expiration: 2008-12-14
Also published as: GB2211605B; GB8730061D0; GB8829109D0

Abstract

An optical fibre distributed sensing system, e.g. for temperature, includes a multi-channel averaging circuit arrangement comprising an analogue-to-digital converter 10 which receives an analogue electrical input corresponding to back-scattered or returned light along an optical fibre sensor 5. The digital output therefrom is fed sequentially to respective digital storage devices 11 of a digital storage arrangement and the data stored for corresponding elements of each back-scattered or returned light signal is then clocked out simultaneously into a digital adder 15. The output from the adder is fed to a processor 24 for providing an indication of the averaged signal waveform in respect of successive waveforms with reduced noise. <IMAGE>

Description

IMPROVEMENTS RELATING TO OPTICAL SENSING SYSTEMS This invention relates to optical sensing systems of the kind in which successive optical pulse signals of a particular wavelength are transmitted along an optical fibre sensor which extends over a path the temperature profile or some other parameter profile of which it is required to monitor. Time-related back-scattered or returned light resulting from the pulses propagating along the fibre sensor may be detected and measured by means of an optical time-domain reflectometer which accordingly provides an indication of the temperature distribution along the fibre sensor.

The utilisation of back-scattered or returned light for determining the profile of an external parameter causing variations in scattering or return of the light along the fibre sensor enables optical fibres to be monitored either to check their basic transmission characteristics or, alternatively, to monitor external physical parameters, such as temperature, along the fibre.

However, the back-scattered or return signals from the fibre sensor are generally weak and give rise to a very noisy electrical signal in the reflectometer. Consequently, a high degree of signal averaging of the return signals in response to successive optical pulses propagating along the fibre sensor is required in order to establish accurate parameter distribution measurement.

For the purpose of signal averaging an optical sensing system of the kind set forth above may be provided with a principally analogue multi-channel signal averaging circuit comprising a plurality of charge-coupled device arrays each consisting of a multiplicity of devices capable of storing all the successive elements of a signal waveform corresponding to the return optical signal of back-scattered light, in which the charge-coupled device arrays are arranged to be rendered operative sequentially for storing incoming signal waveforms under the control of timing means, in which following the storage of a predetermined number of signal waveforms in the respective charge-coupled device arrays the groups of signal waveform elements stored in the charge-coupled device arrays are fed sequentially into an averaging circuit and in which the averaged groups of signal elements may be fed into an analogue-to-digital converter prior to being fed into a processor for providing an indication of the averaged signal waveforms in respect of the successive incoming waveforms to the circuit arrangement.

One disadvantage of such a multi-channel signal averaging circuit is that the charge-coupled devices produce additional noise and temperature offsets.

According to the present invention there is provided an optical fibre sensing system utilising back-scattered or returned light signals along an optical fibre sensor for establishing the scattering or light return profile along the fibre sensor, in which a multi-channel averaging circuit arrangement comprises an analogue-to-digital converter which receives an analogue electrical input corresponding to back-scattered or returned light along said fibre sensor, in which the digital output from the analogue-to-digital converter is fed sequentially to respective digital storage devices of a digital storage arrangement and in which the digital data stored in the respective storage devices corresponding to groups of elements of the back scattered or returned light signal is clocked out simultaneously into a digital adder the output from which may, if an analogue output is required be fed into a digital-to-analogue converter and which may thereafter be fed into a processor for providing an indication of the averaged signal waveform in respect of the successive incoming waveforms from the optical sensing system.

The present invention replaces the analogue charge-coupled device storage components of the previously referred to averaging circuit arrangement with digital components and thereby eliminates from the arrangement the aforesaid additional noise and temperature offsets produced by the charge-coupled device arrays.

By way of example the present invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a block schematic diagram of an optical sensing system with a multi-channel averaging circuit arrangement; and, Figure 2 shows typical waveforms of sensed parameters and signals of the system of Figure 1.

Referring to Figure 1 of the drawings the optical sensing system depicted shows a pulsed laser source 1 which produces successive optical pulses of a particular frequency which are transmitted through optical fibres 2 and 3 and an optical coupler 4 to an optical sensor fibre 5. The optical fibre sensor 5 will be located along the path to be monitored by the system in order to determine a particular parameter profile of said path. In respect of each optical pulse propagating along the fibre sensor 5 a very small proportion of the light at each position along the fibre will be reflected back along the fibre sensor 5 due to backscattering which sill be dependent on the particular external parameter (eg. temperature) influencing the fibre sensor. This very weak time-related back-scattered light signal will be returned to the input end of the sensor.

This reflective action of the sensor fibre 5 is illustrated in the waveform diagram of Figure 2 in which one of the successive optical pulses launched into the optical fibre sensors is shown at 6 in section( a) of the figure. An exemplary temperature profile along the fibre sensor 5 is depicted at 7 in section(b). The resultant backscattered light signal returning along the sensor is shown at 8 in section (c). This signal will be a very low level and would give rise to a very noisy electrical signal if it were fed directly into an optoelectrical reflectometer. Accordingly, these back-scattered signals need to be averaged over a large number of cycles in order to achieve an accurate measurement of the back-scattered light levels.

For this purpose a large number of successive light pulses, such as the pulse 6, need to be transmitted down the fibre sensor and the resultant back-scattered signals, such as the signal 8, suitably averaged.

The averaging of these signals is achieved by feeding a succession of back-scattered light signals, such as the signal 8, into an opto-electric device 9 and then into an analogue-to-digital converter 10 which samples the analogue output from the device 9 (see sampling pulses 20 in section (d)) of Figure 2. The digital outputs corresponding to the respective back-scattered light signals (eg.

signal 8) from the converter 10 are applied sequentially to digital shift registers four of which are shown at 11, 12, 13 and 14 under the control of clock counter 25. When the digital elements corresponding to a predetermined number of back-scattered light signals have been stored in the digital shift registers, the stored digital information will be simultaneously clocked out of all the shift registers, one digital signal element at a time, into a digital adder 15 which effectively averages corresponding digital elements of the stored digital signals corresponding to successive back-scattered light signals to provide a digital averaged signal output shown at 21 in section (f) of Figure 2 the sampled output from the converter 10 in respect of one return signal being shown at 22 in section (e).

This output may be converted to an analogue signal by a digital-to-analogue converter 23 and then fed to a processor 24 which gives an indication of the average signal waveform in respect of the successive incoming signal waveforms to the circuit arrangement, or the output from the adder 15 may be fed directly to the processor 24.

Claims

1. An optical fibre sensing system utilising back-scattered or returned light signals along an optical fibre sensor for establishing the scattering or light return profile along the optical fibre sensor, in which a multi-channel averaging circuit arrangement comprises an analogue-to-digital converter which receives an analogue electrical input corresponding to back-scattered or returned light along said optical fibre sensor, in which the digital output from the analogue-todigital converter is fed sequentially to respective digital storage devices of a digital storage arrangement and in which the digital data stored in the respective storage devices corresponding to groups of elements of the back-scattered or returned light signal is clocked out simultaneously into a digital adder the output from which is applied to a processor which provides an indication of the averaged signal waveform in respect of the successive incoming waveforms from the optical sensing system.

2. An optical fibre sensing system as claimed in claim 1, in which the digital storage devices comprise shift registers which are stepped under the control of a clock counter.

3. An optical fibre sensing system as claimed in claim 1 or claim 2, in which the output from the digital adder is applied to a digitalto-analogue converter before being applied to the processor.

4. An optical fibre sensing system substantially as hereinbefore described with reference to the accompanying drawings.