WO2011067554A1

WO2011067554A1 - Phase based sensing

Info

Publication number: WO2011067554A1
Application number: PCT/GB2010/002192
Authority: WO
Inventors: Roger Ian Crickmore; David John Hill; Magnus Mcewen-King
Original assignee: Qinetiq Limited
Priority date: 2009-12-02
Filing date: 2010-11-29
Publication date: 2011-06-09
Also published as: GB0921062D0

Abstract

A method of interrogating a test fibre to provide distributed acoustic sensing along an optic fibre, wherein pairs of temporally overlapped pulses are introduced into the fibre. Return signals are detected and demodulated to provide phase information representative of disturbances along the fibre. The demodulation effectively separates out the overlapped portion and may comprise separating a constant phase component from a modulated phase component. Alternatively demodulating may comprise differentiating said output and integrating said differentiated output to produce a reconstructed signal.

Description

PHASE BASED SENSING

The present invention relates to fibre optic sensing, and in particular to distributed acoustic sensing (DAS).

Distributed acoustic sensing (DAS) offers an alternative form of fibre optic sensing to point sensors, whereby a single length of longitudinal fibre is optically interrogated, usually by one or more input pulses, to provide substantially continuous sensing of acoustic/vibrational activity along its length. The single length of fibre is typically single mode fibre, and is preferably free of any mirrors, reflectors, gratings, or change of optical properties along its length.

In distributed acoustic sensing, Rayleigh backscattering is normally used. Due to random inhomogeneities in standard optic fibres, a small amount of light from a pulse injected into a fibre is reflected back from every location along the length of the fibre, resulting in a continuous, return signal in response to a single input pulse. If a disturbance occurs along the fibre it changes the backscattered light at that point. This change can be detected at a receiver and from it the source disturbance signal can be estimated. Low noise levels and high discrimination can be obtained using a coherent optical time domain reflectometer (C-OTDR) approach as described above.

DAS systems have also been proposed based on heterodyne interferometry. In this approach light which has passed through a given section of fibre is interfered with light that has not. Any disturbance to this section of fibre causes a phase change between the two portions of light that interfere and this phase change can be measured to give a more accurate estimate of the disturbing signal than is possible with C-OTDR. It can be desirable to reduce the length of the given section of fibre, but difficulties such as folding of the signal typically occur at a certain limit.

It is an object of the present invention to provide improved systems and methods of phase based sensing. According to a first aspect of the invention there is provided a method of interrogating a test fibre comprising introducing first and second pulses into said fibre, said first and second pulses having a predetermined frequency difference, and said second pulse being temporally delayed with respect to said first pulse, wherein said delay is less than the width of the pulses; and detecting light backscattered from the fibre to provide an output signal and demodulating said output signal to produce a phase value representative of disturbances along the fibre.

In this way, the fibre is interrogated to provide distributed acoustic sensing. Pairs of input pulses are introduced into the test fibre, which pairs of pulses are temporally overlapped. In certain embodiments the region of overlap is greater than or equal to a quarter of the pulse width, more preferably greater than or equal to half of the pulse width. The two pulses are preferably of equal width, or duration, but differing pulse widths can be accommodated.

Desirably the step of demodulating comprises separating a constant phase component from a modulated phase component in certain embodiments. As will be explained in greater detail below, this is believed to compensate for the effect of the overlapped portion of the pulse pair, and to isolate the contribution to the output of the non-overlapped portions.

In preferred embodiments, demodulating comprises differentiating said output and integrating said differentiated output to produce a reconstructed signal.

The invention extends to methods, apparatus and/or use substantially as herein described with reference to the accompanying drawings.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa.

Furthermore, features implemented in hardware may generally be implemented in software, and vice versa. Any reference to software and hardware features herein should be construed accordingly. Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 a is a schematic diagram of DAS system

Figures 1 b and 1 c illustrate relative pulse timings

Figure 2 illustrates output data for input signals with pulse delay of 125 ns

Figure 3 illustrates output data for input signals with pulse delay of 25ns

Figure 4 is an Argand diagram of overlapped pulses

Figure 5 shows modelled data for overloaded pulses.

Figure 6 illustrates unfolded data with 25 ns pulse delay.

A schematic diagram of a Distributed Acoustic Sensing (DAS) system based on heterodyne interferometry is shown in figure 1 . Two optical pulses 102 and 104 are generated with frequencies of f1 and f2 and a separation between their starts of x metres (and equivalent^ a temporal delay according to the speed of light in the fibre). These propagate through a circulator 106 into the fibre under test (FUT) 108 from where some light is backscattered and passes back through the circulator and then an Erbium Doped Fibre Amplifier (EDFA) 1 10 before reaching the photodetector 1 12.

Light backscattered from the fibre mixes at the photodetector to form a carrier signal with a frequency†2-f1. As the f2 pulse enters the fibre x metres before the f1 pulse it follows that in order for light backscattered from them to reach the photodetector at the same time, the light from the f2 pulse must be backscattered from xl2 m further along the fibre. Any disturbance acting on this section of the fibre will cause the carrier signal to be phase modulated and it can then be demodulated to provide an output signal caused by and representing the disturbance at that section of the fibre.

A first example was carried out in which a pair of 100 ns pulses were generated with a 25m (125ns) separation between them. This pulse pair arrangement is illustrated in Figure 1 b. Their frequency difference was 40kHz and pulse pairs were transmitted at a rate of 160kHz which defines the sample rate of the system. An 80 m section of the FUT was wrapped around a piezo-electric cylinder to which a 40 Hz sine wave was applied. Signals were recorded at two positions 10m apart in the section of fibre being modulated. A sample of the data is displayed in figure 2, which shows a distorted sine wave as expected. The distortion occurs because the stretching of the fibre not only modulates the separation between where the two pulses are reflected but also the effective reflection point of each pulse. This also accounts for the signal amplitudes from the two channels being slightly different, namely 34 radians in channel 1 and 29 radians in channel 2.

If the input signal becomes too high it is not possible to demodulate it and the system is said to be overloaded which leads to a very distorted output signal. This occurs if the change in phase between successive samples is greater than π/2. As the length of the fibre being modulated is longer than the separation between the pulses one method to reduce the phase modulation is to decrease the pulse separation. This means that a greater input signal could be measured without the system overloading.

It is found that if the pulse separation is reduced to less than the pulse width, i.e. the pulses partially overlap, then folding of the output signal occurs. To demonstrate this effect the pulse separation in the set up of the earlier example was reduced to 25 ns as illustrated in Figure 1c, resulting in the pulse pairs overlapping. Here the right hand pulse has been represented in dashed line for ease of viewing. The signals shown in figure 3 were collected. This data still repeats after 200 samples (= 25 ms or 40Hz) but clearly the signal is folded a number of times within each period.

Although not limited by such a hypothesis, it is believed that a signal of this form is created because the overlapped and non overlapped regions of the two pulses have to be considered separately. Light reflected from the overlapped region has followed the same optical path for both pulses and so is not affected by any disturbance of the fibre. Thus an output signal is only produced from interference between the two sections that do not overlap. In the demodulation of the carrier it is first multiplied by either a cosine or sine wave at the carrier frequency of 40 kHz and then low pass filtered to produce the I and Q terms which represent the real and imaginary parts of a complex number. The output signal is the phase of this complex number and so is obtained by taking the arctan of the I and Q values.

For the case of the overlapped pulses the I and Q terms can be considered to represent the sum of two complex numbers: one with a constant phase resulting from the overlapped region of the pulse and one with modulated phase from the non overlapped regions. The situation is illustrated on the Argand diagram shown in figure 4. Vector 402 represents the signal from the overlapped region and so remains constant. Vector 404 is the signal from the non overlapped region which is modulated and so (ideally) moves around the circle 406. The sum of the two vectors is shown as 408 and it is the phase of this that is calculated from the arctan of the I and Q components.

A simulation was run to demonstrate the general form of this type of signal and its output is shown in figure 5. As can be seen it seen it has a number of similarities with the observed output shown in 3.

The signal which is desired is the phase of the vector 404 shown in figure 4. The end of the resultant signal 408 should move in a circular path around the end of vector 402 which is expected to be constant. This form of vector would enable us to estimate the amplitude and phase of the fixed vector 402 which could then be separated and subtracted from the resultant vector 408 to yield vector 404 which represents the modulation. In practice the resultant vector is unlikely to move exactly in a circle as shown which would make this approach more complicated. To assist in this calculation an estimate of the ratio between the magnitude of the constant and modulated vectors could be made knowing the degree to which the pulses are overlapped.

In certain arrangements, only the phase of the output is provided, and not the I and Q components from which this phase is derived, and without knowledge of the I and Q components, unfolding of the data shown in figure 3 may not possible using the technique described above. 0

An alternative approach is to unfold the output is by differentiating and then to reconstruct by integration. This is explained by way of the example below:

The differentials of the first 100 samples are calculated and the absolute value of these is taken. The second 100 output samples are similarly differentiated, and then the negative of absolute value of these is taken. In the example used here, the effect of rising and falling quadrants is taken into account, by taking negative values. The differentials of the samples are then integrated to reconstruct the signal.

The result of this procedure is shown in the figure 6. Note that only the first 200 samples of the data in figure 3 are reconstructed, but these are plotted twice to give the two cycles. The signals can clearly be seen to be in the form of sine waves containing a degree of noise. The amplitude of the sine waves are of the order 5 times lower than those shown in figure 2; five being the ratio of the two pulse separations that were used.

In order to observe signals over a larger dynamic range it would be possible to transmit two sets of pulse pairs into the fibre at the same time. One set would have the pulses separated and would be used to measure smaller input signals while the other set would have overlapping pulses that could be used to measure larger signals that would overload the signal from the separated pulse pairs. The signals from the two different sets of pulse pairs could be measured simultaneously if they were imposed on different carrier frequencies or different wavelengths. This method provides outputs of the same sensor or length of test fibre but having different sensitivities, and can be used in conjunction with techniques taught in Applicant's publication WO2010/004249, to which reference is directed.

It will be understood that the present invention has been described above purely by way of example, and modification of detail can be made within the scope of the invention.

Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

Claims

1. A method of interrogating a test fibre comprising:

introducing first and second pulses into said fibre, said first and second pulses having a predetermined frequency difference, and said second pulse being temporally delayed with respect to said first pulse, wherein said delay is less than the width of the pulses; and

detecting light backscattered from the fibre to provide an output signal and demodulating said output signal to produce a phase value representative of disturbances along the fibre.

2. A method according to Claim 1 , wherein the step of demodulating comprises separating a constant phase component from a modulated phase component.

3. A method according to Claim 2, wherein demodulating comprises differentiating said output and integrating said differentiated output to produce a reconstructed signal.

4. A method according to Claim 3, wherein the absolute value of differentials of output samples are taken.

5. A method according to Claim 3 or Claim 4, wherein the negative of absolute value of differentials are taken of falling quadrant output samples.

6. A method according to any preceding claim, wherein said delay is less than or equal to half the pulse width.

7. A method according to any preceding claim, further comprising introducing third and fourth pulses into said fibre, said third and fourth pulses being temporally delayed with respect to one another by an amount greater than the pulse width.

8. A method according to Claim 7, wherein said third and fourth pulses have a different frequency difference or wavelength to said first and second pulses.

9. Fibre sensing apparatus comprising:

an interrogator adapted to provide distributed acoustic sensing along a section of optic fibre, wherein said interrogator is adapted to introduce into said optic fibre pairs of pulses having a defined frequency difference and a temporal delay therebetween, said delay being less than the pulse width; a detector adapted to detect backscattered radiation from the fibre and produce an output signal therefrom; and

a processor, adapted to demodulate said output signal to produce a phase value representative of disturbances along the fibre.