GB2230086A

GB2230086A - Optical fibre sensing systems

Info

Publication number: GB2230086A
Application number: GB8829110A
Authority: GB
Inventors: Charles Robert Batchellor
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1988-12-14
Filing date: 1988-12-14
Publication date: 1990-10-10
Anticipated expiration: 2008-12-14
Also published as: GB8829110D0; GB2230086B

Abstract

A distributed optical sensing system e.g. for temperature comprises a single laser light source 12 for producing light pulses which are launched into one end of an optical fibre sensor 11. Back-scattered light components received back at the launch end of the sensor are fed into a single receiver 14, one of the Stokes and anti-Stokes back-scattered wavelength components being delayed 16 relative to the other component in order to facilitate separate detection of the components in the single receiver. <IMAGE>

Description

IMPROVEMENTS RELATING TO OPTICAL SENSING SYSTEMS This invention relates to optical sensing systems and relates more specifically to reflective optical sensing systems of the kind in which light pulses derived from laser source means are launched into one end of an optical fibre sensor and in which the level of backscattered light arriving back at the launch end of the fibre with respect to time is detected by receiver means which provides an indication of the parameter (e.g. temperature) being measured.

In one known optical sensing system of the above kind a single laser is used to generate pulses of a single frequency which are launched into one end of an optical fibre sensor for sensing the distribution of temperature along the fibre sensor. The backscattered light received back at the launch end of the optical fibre sensor includes light at the Raman wavelengths (i.e. Stokes and anti Stokes wavelengths) and the variations in the intensity levels of this back-scattered light at Raman wavelengths with respect of time is indicative of the distributed temperature along the fibre sensor.

Back-scattered light at the respective Stokes and anti-Stokes wavelengths is filtered out from the total back-scattered light and applied to separate receivers for detection and distributed temperature measurement purposes.

In another known optical sensing system of the abovenumbered kind two lasers are provided for producing light pulses at different wavelengths which are launched alternately into one end of an optical fibre sensor for sensing the distribution of temperature along its length. The Raman wavelengths of the back-scattered light resulting from alternate light pulses launched into the optical fibre sensor, after filtering of the total back-scattered light, are fed to a single receiver which accordingly detects the shifted frequency (i.e. fo + fr) of one laser and the down-shifted frequency (i.e. fo - fr) of the other laser where fr is the Raman shift frequency from a frequency fo corresponding to the wavelength of the receiver.

In both of these known sensing systems wavelength division multiplexers or couplers are required between the laser meansloptical fibre sensor/receiver means and the known systems either require two lasers or two receivers. Such duplication not only increases the cost and complexity of these known systems but problems can be introduced by differential changes in efficiency of the lasers due to variations in the relative output levels or intensities of the light pulses at different wavelengths produced by the respective lasers or differential changes in the efficiency of respective receivers due to different responses of the receivers to signals of the same intensities due to calibration differences.Still further, the single laser/two receiver system requires either the duplication of digital averaging hardware or the separate detection of the Stokes and anti-Stokes back-scattered light signals in alternate laser pulses.

According to the present invention there is provided an optical sensing system of the kind hereinbefore defined, in which a single laser is provided for producing light pulses which are launched into one end of an optical fibre sensor and in which back-scattered light components are fed to a single receiver so that one of the Stokes and anti-Stokes back-scattered wavelength components is delayed relative to the other component in order to facilitate separate detection of the components in the single receiver.

In carrying out the present invention the Stokes wavelength component may be delayed relative to the anti-Stokes wavelength component by feeding it through delay line means preferably comprising a length of optical fibre before being fed to the receiver through time division multiplexer means for both the Stokes and anti-Stokes wavelength components of the back-scattered light.

The time delay provided by the delay line means may be chosen so that it equals the propagation time of a light pulse travelling along the optical fibre sensor and back to the launch end.

When an optical fibre similar to that used as the sensor is used as the delay line it would be twice the length of the optical fibre sensor.

By way of example the present invention will now be described with reference to the accompanying drawings in which: Figures 1 and 2 show block schematic diagrams of known optical sensing systems for measuring distributed temperature; and, Figure 3 is a block schematic diagram of an optical sensing system according to the present invention.

Referring to Figure 1 of the drawings the optical sensing system shown comprises a laser 1 providing light pulses having a frequency fo The light pulses are launched into an optical fibre sensor 2 through an optical wavelength division multiplexer or optical coupler 3. A proportion of each light pulse launched into tbe optical fibre sensor 2 will be reflected back to the launch end of the optical fibre sensor 2 due to back-scattering and the intensity of the reflected signal relative to time will be indicative of the distributed temperature along the optical fibre sensor. It may here be mentioned that the wavelength components of such back-scattered light that vary most consistently with temperature changes along the sensor fibre are the Raman wavelengths (i.e.Stokes and anti-Stokes wavelengths) and, consequently, it is light at these Raman wavelengths which is utilised to measure temperature measurements along the sensor 2.

In the present case where the light from the laser 1 has a frequency fg the Raman wavelength components which have frequencies of fo + fr and fo - fr and back-scattered components of these frequencies will be derived from the multiplexer 3 and fed to respective receivers 4 and 5. The intensities of these detected components with respect of time will provide an indication of the temperature distribution along the optical fibre sensor 2. However, with this arrangement differences in the calibrationlefficiency of the receivers 4 and 5 may give rise to inaccuracies in the temperature measurements obtained.

Referring to Figure 2 of the drawings this shows another known optical sensing system for measuring the distribution of temperature along an optical fibre sensor 6 in which two lasers 7 and 8 are provided for producing light pulses at respective frequencies fro + fur and fo - fi. These light pulses are launched alternately into the optical fibre sensor through a wavelength division multiplexer 9 and Raman frequency components of the back-scattered light signals are fed back through the multiplexer 9 to a receiver 10. The receiver 10 is arranged to detect back-scattered signals at frequency fg which comprise the up-shifted back-scattered Raman component of the light signal of frequency fo - fr and the down-shifted back-scattered Raman component of the light signal of frequency fo + fr.

The measured intensities of these Raman components of backscattered light appertaining to the alternate pulses launched into the fibre sensor provide an indication of the temperature distribution along the fibre sensor 6. However, relative changes in the intensities of the light pulses from the lasers can give rise to inaccuracies in the temperature measurement obtained.

Referring now to Figure 3 of the drawings, this shows an optical sensing system according to the present invention for measuring the distribution of temperature along an optical fibre sensor 11. The system comprises a single laser 12 for producing pulses of light at frequency fo. These light pulses are launched through a multiplexer 13 into one end of the sensor 11. The intensity of back-scattered light arriving at the launch end of the sensor with respect of time will be representative of the temperature distribution along the sensor 11.

The Raman wavelength components of these back-scattered signals having frequencies fo + fr and fo - fr are fed to a receiver 14 through a multiplexer 15 but the Raman component of frequency fo - fr is delayed with respect to the other component fo + fr by an optical fibre delay line 16 connected between the two multiplexers 13 and 15. The time delay provided by the optical fibre 16 corresponds to the propagation time of a pulse travelling from the launch end of the optical fibre sensor 11 to the far end and back again. Consequently, using like optical fibres, the length of the delay line fibre 16 may be twice the length of the sensor fibre 11. The delayed Raman components of the back-scattered signal are thereafter detected and measured separately by the receiver 14. The received timeseparated outputs 01 and 02 are indicative of the distribution of temperature along the optical fibre sensor 11.

Claims

CLAIMS:

1. An optical sensing system of the kind hereinbefore defined, in which a single laser is provided for producing light pulses which are launched into one end of an optical fibre sensor and in which backscattered light components are fed to a single receiver so that one of the Stokes and anti-Stokes back-scattered wavelength components is delayed relative to the other component in order to facilitate separate detection of the components in the single receiver.

2. An optical sensing system as claimed in Claim 1, in which the Stokes wavelength component is delayed relative to the anti-Stokes component by feeding it through a delay line.

3. An optical sensing system as claimed in Claim 2, in which the delay line comprises a length of optical fibre of twice the length of the optical fibre sensor.

4. An optical sensing system as claimed in any preceding claim, in which multiplexers are provided between the laser and the optical fibre sensor and between the delay line and the receiver.

5. An optical sensing system for measuring the distribution of temperature along an optical fibre sensor substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.