• 专利附录
  • 专利说明
  • 权利要求
  • 类似技术
  • 同族专利
  • 引用文献
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    • 专利摘要:
    distributed acoustic sensing method and system a distributed acoustic sensing (das) method by which the derivative or rate of change of a fiber signal backscatter is measured. the phase shift or derivative measured in this way has a much smaller amplitude than the signal itself, if the difference between the two times the signal is measured is much smaller than the period of the signal being measured, resulting in lower sensitivity . frequency shifts can be applied to temporarily shifted feedback signals to compare the rate of change, for example by employing an output interferometer arranged to modulate the signal from each arm by a different frequency shift.
    公开号:BR112012007192B1
    申请号:R112012007192-0
    申请日:2010-09-24
    公开日:2021-06-22
    发明作者:David John Hill；Roger Ian Crickmore
    申请人:Optasense Holdings Limited；
    IPC主号:
    专利说明:

    [001] The present invention relates to optical fiber detection and, in particular, distributed acoustic detection (DAS).
    [002] Distributed acoustic detection (DAS) offers an alternative form of optical fiber detection in point sensors, whereby a single length of longitudinal fiber is optically interrogated, usually by one or more input pulses, to provide substantially continuous acoustic/vibrational activity along its length. The only length of fiber is typically single-mode fiber and is preferably free of any mirrors, reflectors, gratings or changing optical properties along its length.
    [003] In distributed acoustic detection, Rayleigh backscatter is commonly used. Due to the inhomogeneities of standard optical fibers, a small amount of light from a pulse injected into a fiber is reflected back from each location along the length of the fiber, resulting in a continuous return signal in response to a single pulse of Prohibited. By analyzing the radiation backscatter within the fiber, the fiber can effectively be divided into a plurality of distinct sensing parts arranged longitudinally along the fiber that may be (but does not have to be) contiguous.
    [004] If a disturbance occurs along the fiber, 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 achieved using a coherent optical time domain (COTDR) reflectometer approach, as described above.
    [005] An alternative approach to DAS is based on heterodyne interferometry. In this approach, light that has passed through a given section of fiber is interfered with by light that has not passed. Any disturbance in this section of fiber causes a phase shift between the two interfering light parts and this phase shift can be measured to provide a more accurate estimate of the disturbance signal than is possible with COTDR. The dynamic range for such a system is limited especially when detecting very long fibers and it is often desirable to use some method to increase the dynamic range.
    [006] A variety of different techniques have been proposed to achieve this goal. A particularly suitable example is the derivative detection technique (DST) as set out in Applicant's copending WO2008/110780, to which reference is directed. This document describes a known sensor package of the type having four fiber optic sensing spirals disposed between five fiber-coupled mirrors. Interrogation of the sensor package is by introducing a pair of optical pulses and the coils and pulses are arranged so that a series of pulses is returned, the information from each sensor coil being derivable from the phase imposed on the respective pulses. WO2008/110780 notes that if the change, or phase derivative, is measured instead of, then it has a much smaller amplitude than the signal itself, if the difference between the two times the signal is measured is much smaller. than the period of the signal being measured. A system and method are then proposed, which manipulate the regulation of the pulses returned from the packet, so that they alternatively contain direct or “normal” phase and derived phase information. Figure 6 of WO2008/110780 is reproduced in accompanying Figure 4 and shows the combination of returned pulse trains 604 and 606, containing derived information (at time 614, for example), temporarily interleaved with the combination of returned pulse trains 602 and 608, which contain direct phase information (at time 612, for example).
    [007] It is an object of the present invention to provide improved methods and apparatus to provide distributed acoustic detection.
    [008] According to a first aspect of the invention, a distributed acoustic detection (DAS) method is provided by interrogating a length of the optical fiber, said optical fiber providing a phase shift of signal propagation, in response to a parameter detected, said method comprising: introducing an input signal over a length of optical fiber; receiving a back-scattered feedback signal from said optical fiber in response to an input signal; comparing a first backscattered feedback signal from a part of said fiber at a first time, and a second backscattered feedback signal from the same part of said fiber at a second different time; and deriving from said comparison a measure of the rate of phase change with time of said backscattered signal. In a particularly preferred embodiment, the first feedback signal is changed in frequency with respect to said second feedback signal.
    [009] In this way, the derivative detection technique (DST) can be applied to distributed acoustic detection (DAS), despite the fact that DAS provides a substantially continuous return signal. This contrasts with the pulsed returns provided by point sensor formations, which naturally allow normal and derivative outputs to be interleaved.
    [0010] DAS provides the advantage that a substantially continuous, unmodified standard fiber length (eg, SMF28) can be used, requiring little or no modification or preparation for use. Preferably, Rayleigh backscattered signals are detected and analyzed. An example of a distributed acoustic detection arrangement operates with a longitudinal fiber up to 40 km in length and is capable of solving data read in multiple channels corresponding to lengths of 10 m. A suitable DAS system is described in GB 2442745, for example.
    [0011] Since the fiber has no discontinuities, the length and arrangement of the fiber sections, corresponding to each channel, are determined by interrogating the fiber. These can be selected according to the physical arrangement of the fiber and the structure or space being monitored, and also according to the type of monitoring required. In this way, the distance along the fiber and the length of each fiber section, or channel resolution, can easily be varied with interrogator adjustments by changing the input pulse width, pulse separation and input pulse duty cycle. , without any changes in fiber.
    [0012] The frequency change imposed between the first and second feedback signals can be performed by a shift applied to one or the other of the feedback signals, e.g. eg, using an AOM. In such a case, a minimum frequency shift of approximately 40 MHz would be expected. In a preferred embodiment, the first feedback signal is modulated by a first frequency change and the second feedback signal is modulated by a second frequency change. This allows smaller differences to be realized and more flexibility in choosing the frequencies used. A convenient way to accomplish this is to pass the received return signals through an output interferometer, called an output interferometer arranged to modulate the signal of each arm by a different frequency change. Michelson or Mach-Zehnder interferometers could be used.
    [0013] The signal input to the fiber under test comprises a pair of temporarily spaced pulses in certain embodiments. These pulses will typically have different frequency changes and heterodyne interferometry can be used in the analysis of backscattered signals. The frequency change(s) applied to the first and second feedback signals and the frequency changes in the input pulses are desirably selected to allow simple isolation of the desired output components, i.e. the various carrier frequencies , as explained below. Alternatively or additionally, the wavelength of the input pulses can be manipulated to allow more control over the output components, as described in the examples below.
    [0014] The first and second feedback signals are separated by approximately 125ns in one embodiment, however this can be varied to suit the application, separations of less than 500ns or less than 250ns or 100ns may be desirable in different embodiments.
    [0015] In embodiments, the method further includes comparing a first backscattered feedback signal from a first part of said fiber at a first time, and a second backscattered feedback signal from a second different part of said fiber at substantially the same time ; and deriving from said comparison a phase measurement of said backscattered signal. This provides a measure of 'normal' phase and hence the 'direct' acoustic signal. It is advantageous that the phase measurement and the phase change rate measurement are determined substantially simultaneously in response to a common input signal. Embodiments demonstrating this aspect can be exploited in methods of providing multiple sensitivity outputs, as described in PCT Application Number GB2009/001480, published as WO2010/004242.
    [0016] Another aspect of the invention provides a distributed acoustic detection system (DAS) for interrogating a length of optical fiber, said optical fiber providing a change in signal phase propagation, in response to a second parameter, said system comprising: a receiver for receiving a backscattered signal from said optical fiber in response to an input signal; an output output interferometer adapted to combine a first signal received backscattered from a part of said fiber at a first time, and a second signal backscattered from the same part of said fiber at a second different time, wherein said output interferometer includes a frequency modulator in at least one arm for imposing a frequency difference between said first and second feedback signals; and a phase detector for receiving said combined signals and determining the rate of phase change with time of said backscattered signal.
    [0017] This system optionally includes a light source to provide an input signal to a fiber under test.
    [0018] The invention extends to methods, apparatus and/or use substantially as described herein with reference to the accompanying drawings.
    [0019] Any detail of one aspect of the invention may be applied to other aspects of the invention, in any suitable combination. In particular, method aspects can be applied to apparatus aspects and vice versa.
    [0020] Furthermore, details implemented in hardware can generally be implemented in software and vice versa. Any references to software and hardware details here should be interpreted as such.
    [0021] Preferred details of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: - Figure 1 shows a first embodiment of the present invention; - Figure 2 illustrates an alternative pulse arrangement; - Figure 3 shows an alternative embodiment of the invention; - Figure 4 illustrates the pulsed output of a prior art technique.
    [0022] Referring to Figure 1, two optical pulses 102 and 104 are generated with the frequency changes of f1 and f2 and a separation between their starts of x meters. These pulses comprise an input signal, which is propagated through a circulator 106 into the fiber under test (FUT) 108 which, as explained, may be an unmodified single-mode fiber length. Light that is backscattered, in response to the input pulses, passes back through the circulator and then the output interferometer 110 before reaching the photodetector 112. In preferred embodiments, the interferometer and photodetector are adapted to operate on signals Rayleigh backscatters. The output interferometer has 116 and 118 acusto-optic modulators (AOMs) in each arm that continuously operate by applying frequency changes of f3 and f4 Hz, respectively. An arm also has a delay coil 120 to impose a delay equal to the pulse separation, ie of length x meters.
    [0023] Starting from the positions shown in figure 1, it can be seen that, moving to and from the circulator, the light from pulse f2 104, which passes through the delay arm of the output interferometer, travels the same distance as the pulse light f1 102, which passes through the shorter arm of the interferometer. Thus, if light from these pulses arrives at the photodetector at the same time, they must have been reflected from the same fiber section, but at different times, and thus generate a derivative signal. That is, when they have followed the same optical path, the phase difference between them is exactly the change in optical path length during the time separation between the two pulses. The frequency changes of these two pulses are f2 + f4 and f1 + f3 and so they mix together to form a frequency-carrying signal:

    [0024] The f2 pulse light passing through the shorter arm of the interferometer must travel an extra 2x m (ie double xm pass) on the fiber under test in order to reach the photodetector at the same time as the f1 pulse light that passed through the retard arm. This will produce a normal signal (ie not a derivative) that corresponds to x meters of the FUT. The pulses that generate this normal signal have frequencies of f2+f3 and f1+f4 that are mixed together to provide a frequency-carrying signal:

    [0025] The other carrier frequencies that are generated result from both pulses going through the same interferometer arm:
    , from the same pulse going through both arms of the interferometer, giving

    [0026] Through proper selection of f1-4 we can ensure that C1-C4 are all different and it is possible to separate each carrier signal. For example, if
    note that the frequencies shown for pulses f1 - f4 are all relative to an arbitrary reference and thus can have a zero or negative value as well as a positive one.
    [0027] The carrier that has the normal signal with the highest spatial resolution is C3 and results from the pulses reflecting from the fiber sections separated by x/2 m. However, this interferometer output consists of two versions of this signal (corresponding to the delayed and non-delayed interferometer arms) of separate x/2 fiber sections, superimposed on top of each other. This problem could be avoided if, as shown in Figure 1, the output fiber were split and a branch went to photodetectometer 114, which only sees a single carrier (C3) with this high normal signal of high spatial resolution.
    [0028] GB 2442745 describes how a number of pulse pairs, each generating a different carrier frequency, can be simultaneously used to interrogate a DAS system. In this document, the purpose of multiple carriers is to provide redundancy to mitigate the problem of coherence fading caused by a carrier's amplitude becoming too low to demodulate.
    [0029] It is further possible to transmit sets of pulse pairs with different frequencies to overcome the problem of coherence fading in embodiments of the present invention. For example, using values of f1 = -5 MHz, f2 = 15 MHz and then f1 = -10 MHz, f2 = 20 MHz, would give C1 = 50 and then 60 MHz, respectively, with all other frequencies remaining at 30 MHz or less. At photodetector 114, normal signals with 10, 20, and 30 MHz carriers would be produced for the three sets of input pulses.
    [0030] In a proposed embodiment, the delay between the two pulses will be approximately 215ns. The amplitude of the derived signal is proportional to the separation between the two pulses and, with this relatively small delay, the derived signal in some applications can have a very low SNR when the normal signal is overloaded, especially if the frequency of the disturbance is low. The amplitude of the derived signal could be improved by increasing the pulse separation, however this would have an adverse effect on the spatial resolution of the system.
    [0031] A proposed method of avoiding this problem would be to transmit a series of three pulses, as shown in the embodiment of figure 2.
    [0032] The input pulses f1 and f2 would again have a spacing of x m and would mix in the photodetector 114, to form the normal signal for a section of the fiber of length x/2 m. The f1 input pulses and the newly introduced f5 pulse would have a much greater separation than yme, after passing through the f5 output interferometer pulse (now with a ym delay coil), it would blend with the f1 pulse in the photodetector 112, to provide a derivative signal based on a time separation of yn/c, where n is the refractive index of the fiber and c is the speed of light.
    [0033] As each of these pulses should preferably have a different frequency, it increases the number of carrier frequencies, which are generated making it more difficult to find a set of frequencies that produce the desired signals on the carriers well separated from any others. Alternatively, the system could generate pulse pairs at two different wavelengths, using the arrangement shown in Figure 3. Wavelength 2 (input pulses 302 and 306) would have a large pulse separation to produce the derived signals. , while wavelength 1 (input pulses 302 and 304) would have a shorter one to produce normal signals. On the receive side, the wavelengths would be separated by a wavelength demultiplexer 310 with A2 passing through the output interferometer to photodetector 312 and A1 passing directly to photodetector 314. The same pair of frequency changes (f1 and f2) could be used for both wavelengths.
    [0034] It should be understood that the present invention has been described above purely as an example and modification of detail can be made within the scope of the invention.
    [0035] Every detail described in the description and (where appropriate) in the claims and drawings may be provided independently or in any suitable combination.
    权利要求:
    Claims (15)
    [0001]
    1. A distributed acoustic detection (DAS) method by interrogating a length of optical fiber (108), the optical fiber (108) providing a change in signal propagation phase in response to a detected parameter, the method comprising: inputting a input signal (102, 104, 302, 304, 306) over a length of optical fiber (108); receiving a back-scattered fiber optic feedback signal (108) in response to the input signal (102, 104, 302, 304, 306); characterized by: comparing a first backscattered return signal from a part of the fiber at a first time, and a second backscattered return signal from the same part of the fiber at a second different time; wherein the first feedback signal is modulated by a first change in frequency and the second feedback signal is modulated by a second change in frequency such that the first feedback signal is changed in frequency with respect to the second feedback signal; and deriving from the comparison a measure of the rate of phase change with time of the backscattered signal.
    [0002]
    2. Method according to claim 1, characterized in that the method comprises passing the received return signals through an output interferometer (110), the output interferometer (110) arranged to modulate the signal in each arm by the first and second frequency changes.
    [0003]
    3. Method according to any one of claims 1 or 2, characterized in that the input signal comprises a pair of temporally spaced pulses (102, 104, 302, 304 306).
    [0004]
    4. Method according to claim 3, characterized in that the pulses have different frequency changes.
    [0005]
    5. Method according to any one of claims 1 to 4, characterized in that it further comprises obtaining a phase value based on the measure of the phase change rate.
    [0006]
    6. Method according to any one of claims 1 to 5, characterized in that it further comprises comparing a first backscattered return signal of a first fiber part (108) at a first time, and a second backscattered return signal of a second different fiber part (108) at the same time; and deriving from the comparison a measure of the backscattered signal phase.
    [0007]
    7. Method according to claim 6, characterized in that the phase measurement and the phase change rate measurement are determined simultaneously in response to a common input signal (102, 104, 302, 304, 306).
    [0008]
    8. Method according to any one of claims 1 to 7, characterized in that the input signal comprises three pulses temporarily spaced apart (302, 304, 306).
    [0009]
    9. Method according to claim 8, characterized in that the input pulses include at least two different wavelengths.
    [0010]
    10. Method according to any one of claims 8 or 9, characterized in that the temporal spacing between the first (302) and the third (306) pulses is greater than twice the temporal spacing between the first (302) and the second (304) pulses.
    [0011]
    11. Distributed acoustic detection (DAS) system for interrogating a length of optical fiber (108), optical fiber (108) providing a change in signal propagation phase in response to a measured parameter, the system comprising: a receiver ( 112, 114, 312, 314) for receiving a back-scattered signal from the optical fiber (108) in response to an input signal (102, 104, 302, 304, 306), characterized in that the system further comprises: a output interferometer (110) adapted to combine a first signal received backscattered from a part of the fiber (108) at a first time and a second signal received backscattered from the same part of the fiber (108) at a second different time, wherein the interferometer output (110) includes a frequency modulator (116, 118) in at least one arm for imposing a frequency difference between the first and second feedback signals; and a phase detector (112) for receiving the combined signals and determining the rate of phase change with time of the backscattered signal.
    [0012]
    12. System according to claim 11, characterized in that the output interferometer (110) includes a frequency modulator (116, 118) in each arm of the output interferometer, each frequency modulator (116, 118) imposing a different frequency change.
    [0013]
    13. System according to any one of claims 11 or 12, characterized in that the system includes a second phase detector (114) to determine the phase of the backscattered signal.
    [0014]
    14. System according to any one of claims 11 to 13, characterized in that the system includes a demultiplexer to separate the backscattered signals from inputs having different wavelengths.
    [0015]
    15. System according to claim 14, characterized in that backscattered signals at a first wavelength are passed to the output interferometer (110) and signals at a second wavelength are passed directly to the second detector. phase (114).
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    法律状态:
    2016-09-06| B25A| Requested transfer of rights approved|Owner name: OPTASENSE HOLDINGS LIMITED (GB) |
    2019-01-08| B06F| Objections, documents and/or translations needed after an examination request according [chapter 6.6 patent gazette]|
    2019-07-16| B06T| Formal requirements before examination [chapter 6.20 patent gazette]|
    2020-06-09| B06A| Notification to applicant to reply to the report for non-patentability or inadequacy of the application [chapter 6.1 patent gazette]|
    2021-05-04| B09A| Decision: intention to grant [chapter 9.1 patent gazette]|
    2021-06-22| B16A| Patent or certificate of addition of invention granted|Free format text: PRAZO DE VALIDADE: 20 (VINTE) ANOS CONTADOS A PARTIR DE 24/09/2010, OBSERVADAS AS CONDICOES LEGAIS. PATENTE CONCEDIDA CONFORME ADI 5.529/DF, , QUE DETERMINA A ALTERACAO DO PRAZO DE CONCESSAO. |
    优先权:
    申请号 | 申请日 | 专利标题
    GBGB0917150.5A|GB0917150D0|2009-09-30|2009-09-30|Phase based sensing|
    GB0917150.5|2009-09-30|
    PCT/GB2010/001802|WO2011039501A2|2009-09-30|2010-09-24|Phase based sensing|
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