GB2277147A - Optical fibre distributed sensing - Google Patents

Abstract

In order to provide extended range of distributed sensing of a physical parameter, such a temperature, an optical time domain reflectometry apparatus, comprising a laser source 1, an optical coupler 4A, a sensing fibre 2 and sensing means 5, 7, 8 and 9, operates with the wavelength of the source 1 as near as practicable to 1550 nm in the range from 1510 nm to 1590 nm. At 1550 nm there is an attenuation coefficient minimum for particular single mode fibres, for which the attenuation of light at the source wavelength and inelastically backscattered Raman Stokes and/or anti-Stokes light, produced therefrom from along the length of the fibre 2, is a minimum. Operating the source outside the desired range leads to the source wavelength and one of the backscattered signals being attenuated, at lower source wavelengths, by elastic scattering, or absorption by OH<-> ions, or at higher source wavelengths, by infra-red absorption. <IMAGE>

Description

DISTRIBUTED SENSING The present invention relates to a method of distributed sensing using optical time domain reflectometry (OTDR) and apparatus for carrying out such a method.

Injecting a signal into a waveguide such as an optical fibre results in both propagation and attenuation of the signal. Attenuation may be the result of the signal interacting with certain fibre impurities or with the fibre glass structure. Such interactions with the signal are either elastic or inelastic, depending on the scattering process, and produce forward and backscattered signals. For example, Rayleigh scattering produces elastically scattered signals with a wavelength distribution substantially the same as the injected signal.

Brillouin and Raman scattering on the other hand are inelastic scattering processes which each produce pairs of spectral bands. Each pair of bands comprises one at a higher wavelength (the Stokes line) than that of the injected signal and the other at a lower wavelength (the anti-Stokes line) than that of the injected signal, such that the pair is centred on the injected signal wavelength. The spectrum would normally contain several successive orders resulting from a particular scattering process, the intensity of the bands decreasing as the order increases.

Brillouin anti-Stokes and Brillouin Stokes backscattered signals have respective sidebands immediately above and below that of the injected signal and have intensities dependent on the physical parameters affecting the backscatter in the fibre, such as temperature. Raman Stokes and Raman anti-Stokes backscattered signals are shifted by a constant frequency from the injected signal frequency, giving, for silica, for a 904 nm injected signal a wavelength shift of 34 nm and for a 1.06 pm signal a shift of 50 nm. These backscattered signals also have intensities dependent on physical parameters affecting backscatter in the fibre, such as temperature.

Typically for silica fibres at room temperature, the Raman Stokes and anti-Stokes signals are less intense than those of the Brillouin backscattered signals, the Raman signals having first order intensities which are lower than the injected signal at 1.064 pm by about 18dB and 28dB respectively, compared to the Brillouin signals which are about 13 to 16dB lower than the injected signal. With 7 ns 50W pulses in multimode fibres, the intensity of anti-Stokes wavelength light is typically only about 50 nW at the receiving/injecting end of the fibre.

In a known OTDR method of distributed sensing, such as that described in EP 0 213 872, a 1.5 W modulated optical signal of wavelength 854 nm at 4 kHz and having a pulse width of 40 ns is injected into one end of an optical fibre of more than 1 km in length. A backscattered signal is returned to the first end and comprises the aforementioned elastically and inelastically backscattered signals which are then filtered to remove substantially all but the Raman anti-Stokes signal which passes to detecting means for measurement of its intensity. From the change in intensity with the elapsed time from the injected signal, the distribution of a particular physical parameter such as the temperature along the fibre may be deduced.

In an earlier known OTDR method, such as that described in GB-2140554, pulsed light is launched into one end of an optical fibre and backscattered Raman Stokes and anti-Stokes signals are separated and measured. Ratios of the measurements are then obtained from which a temperature distribution for the fibre is derived.

In a further known OTDR method such as that described in EP 0 502 283, optical signals of wavelength 1.32 pm from a source comprising a diode-pumped solid state laser are sent through a length of optical fibre with enhanced Raman scattering properties, an attenuator and an optical filter to emit therefrom a test signal of wavelength 1.40 pm for injection into a sensing optical fibre for measuring temperature therealong. The conversion of the wavelength between 1.32 pm and 1.40 pm is achieved by Stimulated Raman Scattering (SRS) of the first wavelength to produce the second, longer wavelength.

Raman anti-Stokes and Raman Stokes signals of respective wavelengths 1.32 pm and 1.50 pm which are subsequently backscattered from positions along the optical fibre are then detected and processed in the same way as the above first mentioned OTDR method.

The range of test signals in optical fibres is known to be limited by dispersion and attenuation. For a given fibre, therefore, the test signal is desirably selected to be at a wavelength corresponding to a minimum in the attenuation/dispersion characteristics of the fibre. For a fibre material such as GeO2 doped silica, a dispersion minimum for the material itself occurs at a wavelength of 1.3 pm. The waveguide dispersion factor of a particular waveguide, such as a fibre, can vary the material dispersion minimum wavelength by an appropriate choice of the refractive index profile and the core diameter. For such a doped material, an attenuation coefficient minimum of about 0.2 dB/km exists at a wavelength k = 1.55 pm.However, in the above known OTDR methods, the implementation of long-range measurement systems based on low propagation loss fibres using measurement of Raman Stokes and/or anti-Stokes light has not been possible owing either to the specific methods used or to other limiting factors, such as the decrease in scattered light intensity with high injected signal wavelengths, which have made long range sensing an unreasonable possibility.

Accordingly, it is desirable to provide a method of, and apparatus for, distributed sensing using optical time domain reflectometry which can enable measurements to be taken over a comparatively long range.

According to one aspect of the present invention there is provided an optical time domain reflectometry method of sensing respective values of a physical parameter at different locations along an optical fibre in which a test signal operating at a predetermined wavelength is injected into one end of the fibre and selected inelastically backscattered radiation is used to produce output signals dependent upon the values being sensed, wherein the said predetermined wavelength is in the range from 1510 nm to 1590 nm and lies at or near an attenuation coefficient minimum of the attenuation characteristic of the said fibre.

The injected test signal is desirably amplitude modulated.

The selected radiation may be Raman Stokes and/or anti-Stokes backscattered signals.

According to a further aspect of the present invention, there is provided optical time domain reflectometry apparatus for sensing respective values of a physical parameter at different locations along an optical fibre, comprising a source of optical radiation, arranged for launching said radiation into one end of an optical fibre deployed in a region of interest, and sensing means operatively arranged for receiving a selected wavelength of radiation inelastically backscattered along the fibre to produce therefrom output signals dependent upon said values, wherein the source is operable to produce pulses of radiation having a wavelength in the range from 1510 nm to 1590 nm, and the optical fibre has an attenuation coefficient minimum within the said range.

In a method or apparatus embodying the present invention the said predetermined wavelength is desirably within the range from 1520 nm to 1580 nm, and is preferably 1550 nm.

In one arrangement, the fibre is looped such that its two ends are disposed adjacent to one another and the source is arranged so as to be selectively operable to launch radiation into either end of the fibre.

The selected wavelength preferably corresponds to the Raman anti-Stokes and/or Stokes wavelengths of the optical radiation.

The physical parameter is desirably, but not essentially, temperature.

The injected test signal is desirably produced by an Er (Erbium) doped fibre amplifier and an amplitude-modulated semiconductor laser.

Alternatively, the injected test signal may be produced by a 9-switched Er doped fibre laser, which may be pumped using a semiconductor laser at any one of 980, 1480 or 810 nm, or may be pumped using other lasers, for example, at 514 nm.

The fibre is desirably, but not essentially, a single mode fibre.

Reference will now be made, by way of example, to the accompanying drawings, in which: FIGURES 1 and 2 are schematic diagrams illustrating the layout of distributed temperature sensing apparatus embodying the present invention, FIGURES 3 and 4 are graphs showing the variation of the transmission coefficient with wavelength of the coupling devices of Figure 1, FIGURE 5 is a graph showing the variation of the attenuation coefficient of a typical fibre with wavelength, and FIGURE 6 is a graph showing a typical spectral distribution of backscattered radiation obtained using an OTDR method.

As illustrated in Figures 1 and 2, in a method embodying the present invention, pulses of optical radiation are generated by an optical source 1 and launched by appropriate launching optics 3, via a directional coupler 4, into one end A of any one of a number of selected sensing fibres 2 towards, in one arrangement, a far end B or, in another arrangement, towards an alternative launching end B using known fibre components. The source produces pulses of optical radiation having a typical half power duration of 40 to 80 ns at a selected wavelength in the range 1510 to 1590 nm, and preferably, between 1520 and 1580 nm, the Raman shift in this wavelength range being about 100 nm and has a peak power which can be varied between around 1W to many tens of Watts.The source is conveniently but not necessarily an amplitude modulated semiconductor laser, the output of which is amplified by an Er (Erbium)-doped fibre amplifier before being launched into the sensing fibre. This source has the benefit of being available from many suppliers of telecommunications equipment and, moreover, it allows the mean power launched into the fibre to be increased by employing pulse compression coding schemes without degrading the spatial resolution. Such schemes are relatively simple to implement owing to the fact that the semiconductor laser can be easily.modulated by varying its bias current.

Alternatively, the source 1 may be a diode-pumped solid state laser, in particular a 9-switched Er-doped fibre laser, the output spectral width of which is preferably below 20 nm with a pulse half power width which can be controlled by varying the length of the fibre in its cavity. Such a Q-switched laser, which is cheaper than the source mentioned above, may be pumped at 980, 1480 or 810 nm using a semiconductor laser or at 514 nm using other lasers such as Tl:sapphire lasers, certain dye lasers and the like. Semiconductor lasers are likely to be the most convenient pumping source, being potentially cheap, energy efficient and compact.

Control of the pulse width of the source used is required to maximise, for particular fibre types, the injected energy by increasing its peak and/or duration since this determines the temperature resolution of the device whilst maintaining a minimum spatial resolution which decreases with increased signal duration.

In Figure 2, the directional coupler 4 is preferably an optical fibre dichroic beam splitter (also known as a wavelength division multiplexer) which separates the forward and backward travelling waves and the wavelengths of predetermined values.

The fibre 2 may be any one of the multi-mode, single mode or single polarisation types, depending upon its particular application, but for long range distributed sensing using a method embodying the present invention must be such that it has a linear attenuation coefficient minimum at or near 1.55 pm.

The attenuation characteristic of such a fibre type is shown in Figure 5.

Whilst not always giving the best possible performance, measurements on single mode fibres also have the advantage of using comparatively cheap fibre and, more importantly, often fibre which is already installed alongside the structure to be monitored, for example a power cable.

In Figure 1, there is shown an arrangement particularly suitable for single mode fibres for which there is a convenient way of splitting the three backscattered wavelengths. The arrangement includes two fused-taper fibre-couplers, 4A and 4B, which are designed to have wavelength-selective properties such that the coupling coefficient of these devices is an approximately sinusoidal function of wavelength, the period depending on the processing of the device. Such couplers 4A, 4B are commercially available from a number of suppliers.

The first coupler 4A, having a transmission characteristic as shown in Figure 3, is designed to transmit a high fraction of the light entering Port 1 to Port 2, provided that this light is at the operating wavelength hg, which may be, for example, 1537 rim. The period of the coupling coefficient as a function of wavelength is such that the coupling at the Stokes and anti-Stokes wavelengths X -1 (1650 and 1440 nm if is 1537 nm) does not couple between Ports 1 and 2 but between Ports 2 and 4. In the return direction, this allows the desired wavelengths to be separated, whilst rejecting a good fraction of the Rayleigh scattered light.

The second coupler 4B, having a transmission characteristic as shown in Figure 4, is chosen to have a coupling period of twice this wavelength separation, so that, of the two wavelengths entering its Port 1, the anti-Stokes wavelength exits through Port 2 and the Stokes wavelength exits through Port 3. This provides a very efficient optical circuit because the light never leaves the fibre (no reflection or re-launching losses) and the intrinsic losses of the devices can be very low, for example 0.1 dB. Furthermore, the losses in the splices interconnecting the devices marked as x's along the fibres, can also be very low.

In the apparatus of Figures 1 and 2, a proportion of the injected radiation is backscattered along the selected fibre and is guided back towards the launching end A. Such backscattered light has a spectrum as shown diagrammatically in Figure 6. Typically, for a 1550 nm injected signal with 80 ns, 1W pulses and a numerical aperture 0.11, the backscattered anti-Stokes wavelength signal has a power of about 600 pW.

On reaching the directional couplers 4 or 4A and 4B this backscattered light is directed to low, high or band-pass optical filters 5 which pass selected Raman anti-Stokes and/or Stokes wavelengths of the injected radiation but which attenuate the unwanted parts of the spectrum. The filtering element may however, take the form of any device which selectively reflects, absorbs, scatters, deflects, polarises or otherwise separates the different spectral components of the selected radiation or, as described above, may be combined with the coupler 4 of Figure 2, which may be an optical fibre dichroic beam splitter or other such directional separating devices as above.

For measurement of the returned signal, either both Raman wavelengths or a single wavelength, preferably the anti-Stokes wavelength, may be used in accordance with the methods described in GB-2 140 554A or EP 0 213 872B. In the case of a signal processing arrangement where the unfiltered light is used as a reference, then a slightly different coupler arrangement would be required than that shown in Figure 1. In such an arrangement, a first coupler, partially transmissive at the injected signal wavelength from Port 1 to Port 2 and highly transmissive at the wavelength used for temperature sensing from Port 2 to Port 4 would be required. The second coupler in this case would be identical to the first coupler where the anti-Stokes/Stokes ratio is used with characteristics similar to those shown in Figure 4.

As in the above-mentioned prior art, the filtered and returned signals are then passed to an optical receiver or receivers 6 which may each conveniently comprise a detector 7 followed by a low noise preamplifier 8 and possibly further stages of amplification and electrical filtering. The electrical signal thus produced is converted into a digital signal and processed by a processor 9 which produces therefrom a set of readings representative of a temperature distribution along the fibre 2 from one end A to its other end B. This process is preferably repeated and averaged over many returned pulses to calculate to a sufficient accuracy the temperature distribution along the fibre. The processor 9 may also control, for example, the source 1, the selected fibre 2 or the filters 5.Ideally, owing to the high and possibly variable output power of the laser, all measured signals should be taken from successive laser pulses in order to enhance the speed and accuracy of the results.

The results may be further improved by performing the measurement from each end of the fibre to separate variations in the signal caused by temperature fluctuations from those from fibre loss. By calculating the geometric means of the backscattered signals measured from both ends of the fibre returning from a particular location, the effects of any propagation losses can be eliminated, leaving only the effects of changes in the back-scatter of the injected signal, i.e. changes of numerical aperture of the scattering coefficient of the spectral line of interest.

The fibre may be calibrated for temperature independent variations either of the scattering coefficient or of the capture fraction of the fibre prior to installation by testing the fibre with a known temperature distribution.

The spatial resolution along the fibre may be improved by winding coils as shown diagrammatically in Figures 1 and 2 of the fibre to monitor, by way of an extended length of fibre in one position, the physical properties at that position. The overall resolution of the equipment is, however, determined by the width of the pulse launched into the fibre and by the bandwidth of the processing circuitry and optical filters employed to separate the signal to be measured from other backscattered signals.

Other physical parameters, not only temperature, may be measured using a method and apparatus embodying the present invention, and one or more spectral bands sensitive to, or having encoded upon it information relating to, changes in the parameter of interest may be selected accordingly for detection.

With increased range, the present invention is particularly suitable for distributed sensing of power lines to monitor disturbances such as lightning strikes and the like.

Claims (16)

Claims:

1. An optical time domain ref lectometry method of sensing respective values of a physical parameter at different locations along an optical fibre, in which a test signal operating at a predetermined wavelength is injected into one end of the fibre and selected inelastically backscattered radiation is used to produce output signals dependent upon the values being sensed, wherein the said predetermined wavelength is in the range from 1510 nm to 1590 nm and lies at or near an attenuation coefficient minimum of the attenuation characteristic of the said fibre.

2. A method according to claim 1, wherein the selected radiation comprises Raman Stokes backscattered signals.

3. A method according to claim 1 or 2, wherein the selected radiation comprises Raman anti-Stokes backscattered signals.

4. A method according to any preceding claim, wherein the said predetermined wavelength is in the range from 1520no to 1580nm.

5. A method according to any preceding claim, wherein the said predetermined wavelength is 155on.

6. Optical time domain reflectometry apparatus for sensing respective values of a physical parameter at different locations along an optical fibre, comprising a source of optical radiation, arranged for launching said radiation into one end of an optical fibre deployed in a region of interest, and sensing means operatively arranged for receiving a selected wavelength of radiation inelastically backscattered along the fibre to produce therefrom output signals dependent upon said values, wherein the source is operable to produce pulses of radiation having a wavelength in the range from 1510 nm to 1590 nm, and the optical fibre has an attenuation coefficient minimum within the said range.

7. Apparatus as claimed in claim 6, wherein the test signal is injected into the said optical fibre, and the backscattered radiation is directed from the said optical fibre into the said sensing means, via an optical coupler having a transmission characteristic such that it is operable to transmit a high fraction of light at said predetermined wavelength from a first port thereof, to which said source is connected, to a second port thereof, which is connected so as to inject light into, and receive light backscattered from, said fibre, and to transmit a high fraction of the backscattered light at the selected wavelength from the said second port to a third port thereof, but not to the said first port, the said third port being connected to direct light into the said sensing means.

8. Apparatus as claimed in claim 7, wherein said optical coupler is also operable to transmit a high fraction of the back scattered light at a further selected wavelength from the said second port to the said third port, and the said third port is connected to a first port of a further optical coupler of the apparatus, which further optical coupler has a transmission characteristic such that it is operable to transmit a high fraction of light at one of the said selected wavelengths from said first port thereof to a second port thereof, and a high fraction of light at the other of said selected wavelengths from said first port to a third port thereof, which second and third ports are connected to the said sensing means.

9. Apparatus as claimed in claim 7 or 8, wherein the or each optical coupler, as the case may be, is a fused-taper fibre-coupler.

10. Apparatus according to any one of claims 6 to 9, wherein the selected radiation comprises Raman Stokes backscattered signals.

11. Apparatus according to any one of claims 6 to 10, wherein the selected radiation comprises Raman anti-Stokes backscattered signals.

12. Apparatus according to any one of claims 6 to 11, wherein the said predetermined wavelength is in the range from 1520nm to 1580nm.

13. Apparatus according to any one of claims 6 to 12, wherein the said predetermined wavelength is 1550nm.

14. Apparatus according to any one of claims 6 to 13, wherein the said fibre is a single mode fibre.

15. Optical time domain reflectometry method of sensing respective values of a physical parameter at different locations along an optical fibre substantially as hereinbefore described with reference to Figures 1 to 6 of the accompanying drawings.

16. Optical time domain reflectometry apparatus for sensing respective values of a physical parameter at different locations along an optical fibre substantially as hereinbefore described with reference to Figures 1 to 6 of the accompanying drawings.