GB2618376A - Fibre optic sensing - Google Patents

Abstract

Detecting stress/strain in optical fibres using coherent optical time domain reflectometry (COTDR) where the apparatus operates in two modes and determines changes in optical path length. A first mode interrogates the sensing fibre using coherent optical radiation having the same frequency. Backscattered Rayleigh signals are processed and analysed to determine a phase value indicative of changes in path length. A first mode signal is output. A second periodically operated mode launches coherent radiation of different frequencies within a defined frequency range into the sensing fibre. Backscatter is processed to obtain a frequency spectral profile of the backscattered signal. Frequency spectra of one operation period are compared to a previous spectrum to determine optical path changes between operation periods. Preferably, any change in optical path length determined from the second mode of operation is used to identify and/or correct demodulation errors in the first mode output signal.

Description

FIBRE OPTIC SENSING

This application relates to methods and apparatus for fibre optic sensing.

Various types of fibre optic sensing are known, where an optical fibre is deployed in an area of interest as a sensing fibre and interrogated using optic radiation to determine information about the environment around the sensing fibre and/or stimuli acting on the sensing fibre.

One type of fibre optic sensing involves interrogating the sensing optical fibre with coherent optical radiation and detecting and analysing optical radiation which is Rayleigh backscattered from within the sensing fibre. The interrogating radiation may be Rayleigh backscattered from intrinsic scattering sites within the optical fibre that are inherently present in an optical fibre. A disturbance acting on the sensing fibre can result in a change in optical path length for the sensing portion, e.g. a physical change in length of that part of the sensing fibre and/or a modulation of the refractive index, which can alter the distribution of the scattering sites and lead to variation in the properties of the backscatter, which can be detected. The backscatter may be processed in time bins corresponding to the return trip time to different sections or sensing portions of the sensing optical fibre according to the principles of optical time domain reflectometry (OTDR), so as to provide independent sensing of a plurality of sensing portions. Such distributed fibre optic sensing may thus be termed COTDR sensing (coherent OTDR).

Such COTDR sensors effectively act as a linear sensing array of sensing portions of optical fibre which are responsive to dynamic disturbances such as strains due to acoustic stimuli, and thus such sensing is sometimes referred to as distributed acoustic sensing (DAS), although the same principles can be applied to detect any stimulus that results in a variation in effective optical path length of the sensing fibre, such as dynamic temperature variations. As the sensing technique makes use of backscatter from the intrinsic scattering sites that inherently occur within an optical fibre, a standard optical fibre, of the type that may be used for communications, can be used as the sensing fibre without requiring any modification. Sensing can be performed over long ranges with a suitably long sensing fibre, for instance sensing can reliably be performed for a range that may be up to several tens of kilometres within the sensing fibre, or even up to 100km or more when the backscatter is mixed with a local oscillator. COTDR sensing has thus been advantageously deployed in a number of different applications.

In some COTDR systems, the backscatter may be processed to determine an indication of phase for the backscatter. Conveniently, to measure any variation in phase, backscattered optical radiation at one optical frequency may be interfered with optical radiation at a different optical frequency to generate a carrier signal component corresponding to the frequency difference. In some implementations, the carrier signal may be generated by interfering backscatter received from the sensing optical fibre with a local oscillator at a different optical frequency. The relevant carrier signal component of the backscatter may be demodulated at the carrier frequency to provide an indication of phase for the carrier. The phase of the carrier signal for a given sensing portion varies with the optical path length and the phase value can be used to provide quantitative data about the variation in path length from interrogation to interrogation and hence the stimulus acting on the sensing fibre.

Such a phase determining COTDR system is an interferometric sensor which can be sensitive to small changes in the strain of the sensing fibre; and some implementations can measure strains that cause a phase change down to the milliradian level and can accurately measure strains over long periods. However, in some cases, some relatively high frequency, large amplitude strains can lead to the phase of the interferometer changing so quickly that the COTDR system is unable to follow it. This can result in demodulation errors, leading to steps in the output signal that are a multiple of 2 rc radians. Such large amplitude strains may often be impulsive, for example due to the structure in which sensing the fibre is mounted being struck by an object. Similar demodulation errors can also occur if the light level from any one section of the sensing fibre drops below a certain level due to coherent fading.

Embodiments of the present disclosure relate to improved methods and apparatus for fibre optic sensing.

Thus, according to an aspect of the disclosure, there is provided a fibre optic sensing apparatus, comprising an optical output path configured to repeatedly interrogate a sensing optical fibre by launching coherent optical radiation into the sensing optical fibre; a detector configured to receive optical radiation that is Rayleigh backscattered from the sensing optical fibre and output a detected backscatter signal in response to each interrogation; and a processor for processing the detected backscatter signal. The fibre optic sensing apparatus is operable in a first mode, in which the sensing optical fibre is interrogated with a first set of interrogations of coherent optical radiation having the same frequency characteristics as one another and the detected backscatter signal from the first set of interrogations is processed to determine, for at least one sensing portion of the sensing optical fibre, a phase value indicative of any changes in optical path length and output a corresponding first mode output signal. The fibre optic sensing apparatus is also operable in a second mode, in which the sensing optical fibre is interrogated with a second set of interrogations of coherent optical radiation having different optical frequencies from one another, within a defined frequency range, and the detected backscatter signal from the second set of interrogations is processed to acquire for said at least one sensing portion a backscatter spectral profile of a measurement value of the detected backscatter signal with frequency. The fibre optic sensing apparatus is apparatus is configured to periodically operate in the second mode and, for said at least one sensing portion, to compare a backscatter spectral profile for that sensing portion determined from one period of operation in the second mode with a backscatter spectral profile determined from a previous period of operation in the second mode to determine an indication of any change in optical path length between said periods.

In at least some examples, the processor may be configured to use the indication of any change in optical path length determined from periods of operation in the second mode of operation to identify and/or correct any demodulation errors in the first mode output signal. The processor may be configured to use the indication of any change in optical path length determined from periods of operation in the second mode of operation to identify any demodulation errors in the first mode output signal by determining whether a difference value between a change in phase value for a sensing portion determined from operating in the first mode and an expected change in phase value based on the indication of any change in optical path length determined from periods of operation in the second mode of operation has a magnitude greater than 7 radians. If so, any such demodulation error may be corrected by applying correction to the relevant phase value determined from operating in the first mode which is an integer multiple of 27 radians and which reduces the difference value between the changes to a magnitude of p radians or less.

The processor may be configured to determine the indication of any change in optical path length between said periods of operation in the second mode by comparing the backscatter spectral profile for that sensing portion determined in a first period with a backscatter spectral profile for that sensing portion determined in a previous period to identify any shift in frequency between the compared backscatter spectral profiles. The processor may be configured to determine the amount of any shift in frequency between the compared backscatter spectral profiles by cross-correlating the backscatter spectral profiles.

In some implementations, the apparatus may be configured such that in the second mode, the second set of interrogations comprise a plurality of interrogations with different frequencies, where the frequency difference between adjacent frequencies of the set corresponds to a variation of 2 radians or less over a sensing portion. In some examples, at least some frequency steps between adjacent frequencies in the second set of interrogations differ from one another.

In the second mode, the processor may be configured to determine the measurement value as the signal level of the detected backscatter signal.

The apparatus may, in some cases, further comprise a mixer for mixing the optical radiation that is Rayleigh backscattered from the sensing fibre with a local oscillator derived from the optical output path prior to detection by the detector. The optical output path may be configured such that there is an optical frequency difference between the local oscillator and the optical radiation that is Rayleigh backscattered from the sensing fibre, and the processor may be configured, in the second mode, to determine the measurement value as a carrier level of a carrier component in the detected backscatter signal at a carrier frequency equal to said optical frequency difference.

The optical output path may comprise a coherent optical source and a first optical modulator for applying a controlled frequency modulation to optical radiation from the coherent optical source.

The apparatus may comprise a controller for selectively controlling the mode of operation in the first mode or in the second mode. The controller may be configured to alternate operation in the first and second modes such that the apparatus operates in the first mode between said periods of second mode operation. Alternatively, the controller may be configured to operate continually in the first mode and the said periods of second mode operation may occur simultaneously with operation in the first mode. In some cases, the controller may be configured to operate in the second mode at pre-defined intervals. In some cases, the controller may be configured to operate in the second mode in response to a defined event occurring and/or if an anomaly in the first mode output signal is detected. In some examples, the controller may be further configured to be operable to operate the apparatus continually in the first mode with no operation in the second mode and/or to periodically operate in the second mode without no operation in the first mode in the intervals between second mode operation.

In another aspect there is provided a method of fibre optic sensing comprising repeatedly interrogating a sensing optical fibre by launching coherent optical radiation into the sensing optical fibre, a detecting optical radiation that is backscattered from the sensing optical fibre and output a detected backscatter signal in response to each interrogation; and processing the detected backscatter signal. The method comprises operating in a first mode, in which the sensing optical fibre is interrogated with a first set of interrogations of coherent optical radiation having the same frequency characteristics as one another and the detected backscatter signal from the first set of interrogations is processed to determine, for at least one sensing portion of the sensing optical fibre, a phase value indicative of any changes in optical path length and output a corresponding first mode output signal. The method further comprises periodically operating in a second mode, in which the sensing optical fibre is interrogated with a second set of interrogations of coherent optical radiation having different optical frequencies from one another within a defined frequency range and the detected backscatter signal from the second set of interrogations is processed to acquire for said at least one sensing portion a backscatter spectral profile of a measurement value of the detected backscatter signal with frequency. The method further comprises comparing a backscatter spectral profile for a sensing portion determined from one period of operation in the second mode with a backscatter spectral profile determined from a previous period of operation in the second mode to determine an indication of any change in optical path length between said periods.

The method may be implemented in any of the ways as discussed above with reference to the apparatus of the first aspect. In particular, the method may comprise using the indication of any change in optical path length determined from periods of operation in the second mode of operation to identify and/or correct any demodulation errors in the first mode output signal. The indication of any change in optical path length determined from periods of operation in the second mode of operation may be used to identify any demodulation errors in the first mode output signal by determining whether a difference value between a change in phase value for a sensing portion determined from operating in the first mode and an expected phase value based on the indication of any change in optical path length determined from periods of operation in the second mode of operation has a magnitude greater than ii radians. The method may comprise correcting any such demodulation error by applying a correction to the relevant difference value determined from operating in the first mode which is an integer multiple of 2ft radians and which reduces the difference value to a magnitude of 7 radians or less. Determining the indication of any change in optical path length between said periods of operation in the second mode may comprise comparing the backscatter spectral profile for that sensing portion determined in a first period with a backscatter spectral profile for that sensing portion determined in a previous period to identify any shift in frequency between the compared backscatter spectral profiles.

Note that unless expressly indicated to the contrary or clearly incompatible, any feature of any of the embodiments described herein may be used in combination with any one or more features of any of the other described embodiments.

Embodiments, and feature of embodiments of the present disclosure, will now be described by way of example only with respect to the accompanying drawings, of which: Figure 1 illustrates one example of a coherent Rayleigh backscatter distributed fibre optic sensor; Figures 2a and 2b illustrate backscatter spectral profiles and Figure 3 illustrates a coherent Rayleigh backscatter distributed fibre optic sensor according to an embodiment.

Embodiments of the present disclosure relate to methods and apparatus for fibre optic sensing, and in particular to Rayleigh backscatter COTDR sensing. Embodiments relate to methods and apparatus that can, separately to any ongoing COTDR sensing, determine a characteristic of a sensing optic fibre in a way that allows for quantification of changes, e.g. by comparison with another such characteristic acquired at a different time. Periodic determination of such a characteristic can be used for calibration of the Rayleigh backscatter COTDR sensing.

As noted above, Rayleigh backscatter COTDR is a known technique for sensing for dynamic disturbances acting on a sensing fibre.

Figure 1 illustrates the principles of Rayleigh backscatter COTDR fibre optic sensing and illustrates generally a sensing arrangement 100. An interrogator 101 is, in use, optically coupled to an optical fibre 102 which is to be used for sensing. The optical fibre 102, may be referred to herein as the sensing optical fibre or just sensing fibre (or sometimes as the fibre under test).

The sensing fibre 102 can be many kilometres in length and can be tens of kilometres in length, say up to 40 km or more or up to 100km or more in some implementations. For coherent Rayleigh distributed fibre optic sensing, the sensing fibre 102 may be a standard, unmodified single mode optic fibre such as is routinely used in telecommunications applications, although multimode fibre may be used in some applications (typically with reduced performance). The sensing fibre need not include any deliberately introduced reflection sites such a fibre Bragg grating or the like, but in some implementations some such reflection sites could be present, or the fibre may be one which has been fabricated or processed to provide greater scattering than a conventional telecommunications optical fibre.

The sensing fibre 102 may be deployed in an area of interest to be monitored and, in some cases, may be specifically deployed to allow for sensing. Depending on the particular use case, the sensing fibre may be deployed in a relatively permanent manner, e.g. being buried or otherwise secured in place. The interrogator 101 may be removably coupled to the sensing optical fibre 102 (possibly via some optical connection which may include a connecting optical fibre), and thus in some instances, if continuous monitoring is not required, the interrogator 101 may be removed from the sensing fibre 102 when sensing is not required, possibly leaving the sensing fibre in situ. In some instances, use may be made of an existing optical fibre which is already deployed in the region of interest, and which may have been originally deployed for some other performance, e.g. for communications. Note whilst the sensing fibre may be one continuous optical fibre, the sensing fibre could, in some applications, be formed from various optical fibre sections that have been spliced together or otherwise optically connected.

In use, the interrogator 101 repeatedly interrogates the sensing optical fibre 102 with coherent optical radiation and analyses the backscatter therefrom. The interrogator 101 thus comprises an optical source, in this example a laser 103, for generating coherent optical radiation and a modulator 104 for modulating the output of the laser. The modulator 104 modulates the output of the laser 103 so as to repeatedly interrogate the sensing fibre with optical radiation, which will be referred to herein as interrogating radiation, in a series of interrogations.

Note that as used herein the term "optical" is not restricted to the visible spectrum and, as used herein, the term optical refers to any electromagnetic radiation which may be guided by, and scattered from within, an optical fibre. For the avoidance of doubt, optical radiation as used herein includes infrared radiation and ultraviolet radiation. Any reference to "light" should also be construed accordingly.

The interrogating radiation may take different forms. In some examples, a single pulse of optical radiation at a given launch frequency may be used for each interrogation, although in some embodiments each interrogation may comprise two (or more) pulses, in which case the optical pulses may have different launch frequencies from one another, e.g. a frequency pattern as described in GB2,442,745 or as described in W02020/016556, or optical characteristics such as described in W02012/137022, the contents of each of which are hereby incorporated by reference thereto.

The modulator 104 thus modulates the optical radiation generated by laser 103 to provide suitable interrogating radiation. It will be understood that a single modulator 104 is illustrated in figure 1 for clarity, but in practice the modulator functionality could be implemented by multiple modulator components, e.g. there may be one or more modulator components to provide a desired frequency modulation and/or one or more modulator components to provide some amplitude modulation, such as to form distinct optical pulses. It will also be understood that there may be additional components such as optical amplifiers or the like in the optical output path.

The phenomenon of Rayleigh backscattering results in some fraction of the interrogating radiation input into the sensing fibre being reflected back to the interrogator, where it is detected to provide an output signal which can be representative of environmental disturbances acting on the fibre. The interrogator 101 therefore comprises at least one photodetector 105 arranged to detect radiation which is Rayleigh backscattered from within the sensing fibre 102. In some embodiments the backscatter may be mixed with a local oscillator signal prior to detection, e.g. by mixer 106. The local oscillator may be derived from the laser 103 and may be arranged have an optical frequency difference to the interrogating radiation launched into the sensing fibre, and hence the optical radiation which is Rayleigh backscattered.

The signal from the photodetector may be processed by processor 107 of the interrogator 101 to provide a measurement signal which is representative of disturbances acting on the sensing portions or channels of the fibre.

For a coherent Rayleigh distributed fibre optic sensor, the backscatter from the sensing optical fibre 102 will depend, at least partly, on the distribution of inherent scattering sites within the optical fibre, which will vary effectively randomly along the length of the fibre. Thus the backscatter characteristics, e.g. intensity, from any given interrogation will exhibit a random variation from one sensing portion to the next but, in the absence of any environmental stimulus, the backscatter characteristics from any given sensing portion should remain the same for each repeated interrogation (provided the characteristics of the interrogating radiation, such as the optical frequency, amplitude and duration of the pulse or pulses, remains the same for each interrogation). However, an environmental stimulus acting on the relevant sensing portion of the fibre can result in an optical path length change for that section of fibre, e.g. through stretching/compression of the relevant section of fibre and/or a refractive index modulation. As the backscatter from the various scattering sites within the sensing portion of fibre will interfere to produce the resulting intensity, a change in optical path length will vary the degree of interference.

For a phase based coherent Rayleigh distributed fibre optic sensor the processing by processor will generally determine a phase value from the backscattered light, e.g. the phase of a signal component at a defined carrier frequency. A signal component at a defined carrier frequency can be generated in various ways. For instance, as discussed above, each interrogation may comprise two optical pulses at different optical frequencies to one another, where the frequency difference between the pulses defines the carrier frequency. In such a case, backscatter from both pulses may interfere to provide a signal component at the carrier frequency. Alternatively, backscatter from a pulse at a given optical frequency may be mixed with a local oscillator signal LO, where the optical frequency of the local oscillator differs from that of the backscatter by the defined carrier frequency.

The processor 107 may thus demodulate the signal from the photodetector 105 at the relevant carrier frequency to provide a value of phase for the carrier signal, as would be understood by one skilled in the art, for instance as described in any of GB2,442,745, W02012/137021, W02012/137022 or W02020/016556, depending on the form of the interrogating radiation.

Any disturbance acting on the sensing fibre that results in a change in the optical path length of that part of the sensing fibre will result in a modulation to the phase of the carrier signal over that sensing portion, where the extent of the change in phase is proportional to the change in path length. Thus, a stimulus acting on the sensing fibre can be detected by determining the extent of any phase modulation between repeated interrogations of the sensing fibre, and the magnitude of the phase modulation provides an indication of magnitude of the stimulus acting on the sensing fibre.

The form of the optical input and the method of detection and processing allows the sensing fibre, which may be a single continuous optical fibre, to be spatially resolved into discrete longitudinal channel or sensing portions with a desired gauge length. That is, a measurement signal indicative of disturbance at one sensing portion, e.g. indicative of an incident acoustic wave, can be provided substantially independently of a measurement signal for another sensing portion. Note that the term acoustic, as used herein, shall be taken to mean any type of pressure wave or mechanical disturbance or varying strain generated on the optical fibre and will, for instance, include seismic waves or the like. The term acoustic is intended to refer to the type of stimulus acting on the sensing fibre but is not used to imply any particular frequency limitation.

Such a sensor may be seen as a fully distributed or intrinsic sensor, as it uses the intrinsic scattering process inherent in an optical fibre and thus distributes the sensing function throughout the whole of the optical fibre The measurement signals from interrogator 101 may, in some implementations, be passed to a signal processor 108 which may be co-located with the interrogator 101 or may be remote therefrom. Optionally there may also be a user interface/graphical display 109, which may be co-located with the signal processor or may be remote therefrom. The signal processor 108 and user interface/graphical display 109 may be realised by an appropriately specified computing device such as a PC. The signal processor 108 may be configured to process the measurement signals to provide some sensing or monitoring functionality depending on the application that the sensor is being used for.

Whilst, in general, the processor 107 of the interrogator unit may be operable to generate measurement signals from the sensing portions of the sensing fibre and the signal processor 108 may be operable to apply any application specific processing to the measurement signals, in at least some implementations at least some of the processing to generate the measurement signals could be performed by signal processor 107 or at least some processing of the measurement signals could be performed by processor 108.

Such coherent Rayleigh distributed fibre optic sensors, e.g. DAS sensors, can be usefully employed in a range of applications to provide information about environmental disturbances acting on the sensing fibre for each of a plurality sensing portions and, by determining an indication of phase, can provide quantitative data about the magnitude of the stimulus. In some cases, the Rayleigh backscatter COTDR sensor may be deployed to provide relatively long-term monitoring of strains, for instance relatively low frequency strains.

As noted above, however, demodulation errors may occur, especially over the course of relatively long-term monitoring where coherent fading may result in the relevant backscatter signal falling below a noise threshold and/or where instances of a high rate of change of phase may lead to errors in phase unwrapping. This may particularly be the case where the sensing fibre is relatively long, say of the order of several tens of kilometres in length, and the interrogation or ping rate is constrained by the length of the fibre. Whilst the stimuli of interest may be relatively low frequency and may, themselves, result in a rate of change of phase that could be tracked without error, other stimuli, e.g. acoustic noise or impulsive shocks, could lead to relatively high frequency/high amplitude strains that can lead to rapid phase changes and introduce demodulation errors. Such demodulation errors can thus reduce the accuracy of the monitoring.

Embodiments of the present disclosure relate to methods and apparatus of coherent Rayleigh distributed fibre optic sensing that can mitigate for demodulation errors. In particular, embodiments of the present disclosure determine a characteristic of a sensing portion that allows for any changes in the state of that sensing portion to be determined separately from any ongoing Rayleigh COTDR sensing.

In particular, the characteristic may be a profile of the Rayleigh backscatter from a given sensing portion at different interrogating frequencies, i.e. a backscatter spectral profile. Such a characteristic profile has a dependence on optical path length, and a variation in optical path length can lead to a detectable change in the profile that can be used to quantify the change in path length without the aliasing problems of phase determining COTDRs. Thus, the characteristic of a given sensing portion can be determined at first and second different times, which could be separated by any desired interval, and by comparing the characteristics acquired at the different times, and changes in optical path length between the first and second times may be determined just from the determined characteristics. This technique is less sensitive to strain than determining phase changes using Rayleigh backscatter COTDR as discussed above, but the change in optical path length can be measured by comparing the profiles taken during two short but separated periods and so would not be affected by any short duration, high amplitude signals that occur between the two measurement periods. The change in path length determined from the backscatter spectral profiles can therefore be used to calibrate or correct for any errors in the Rayleigh backscatter COTDR.

In embodiments of the present disclosure, a distributed fibre optic sensing apparatus can thus operate in first mode of operation, which may be a standard Rayleigh COTDR sensing mode, and also in a second mode of operation, which may be a frequency swept mode of operation in which a backscatter spectral profile characteristic is determined for at least one sensing portion. The distributed fibre optic sensing apparatus may, in use, generally operate in the first mode to provide generally continuous monitoring of the sensing fibre, but at intervals may operate in the second mode for a period to acquire a backscatter spectral profile characteristic. Subsequent backscatter spectral profile characteristics acquired in periods of operation in the second mode can be analysed to determine the extent of any changes in optical path length, which can then be used to identify and/or compensate for any demodulation errors in the results generated in the first mode of operation.

During the first mode of operation, the apparatus may repeatedly interrogate the sensing optical fibre with a first set of interrogations of coherent interrogating optical radiation, where the frequency characteristics of the interrogating radiation, e.g. the frequency of the interrogating radiation (or frequencies if each interrogation comprises two or more pulses at different frequencies), are substantially the same for each interrogation. The Rayleigh backscatter from the repeated interrogations in the first mode of operation may be detected and processed to determine a change in phase value for one or more sensing portions so as to provide a first mode output signal for each such sensing portion indicative of changes in optical path length of the relevant sensing portion.

During the second mode of operation, the sensing fibre is repeatedly interrogated with a second set of interrogations of coherent optical radiation, and the optical frequency of the interrogating optical radiation is controllably varied between interrogations of the second set. In the second mode of operation, the Rayleigh backscatter for at least one sensing portion is detected and analysed from the second set of different interrogations at different frequencies to determine a backscatter spectral profile, which is a profile of how a measurement value for the sensing portion varies with frequency. In some implementations the measurement value may be the signal level, i.e. backscatter intensity, of the backscatter for that sensing portion for the relevant frequency, although in some embodiments a carrier signal could be generated, as discussed above, and the measurement value may be determined as a signal level of the carrier signal.

As the interrogating radiation is coherent, varying the optical frequency (and hence the wavelength) of the interrogating radiation changes the resulting interference from the relevant sensing portion, which will result in a change in relevant measurement value, e.g. the backscatter intensity or carrier level. Figure 2a illustrates a hypothetical example of how the measurement value may vary with frequency of the interrogating radiation for a sensing portion, in this case in a range from f1 to f2. This variation in measurement value with frequency provides a backscatter spectral profile which can be used as a characteristic for the relevant sensing portion which can be used to subsequently determine changes in state of the sensing portion, i.e. changes in optical path length affecting the sensing portion such as due to strain and/or temperature. Note that figure 2a illustrates the profile of variation in measurement value against frequency, but it will be understood that the profile could instead be represented as a profile of variation of measurement signal with wavelength.

Changes in optical path length of a given sensing portion, that affect substantially the whole of that sensing portion equally, can result in a change in backscatter at a given frequency that is similar to the variation that would result from a variation in frequency (with no change in optical path length). Consider just two reflectors within a sensing portion that are separated by an optical path length L. Changing the frequency, and hence wavelength, of the interrogating wavelength, with no change in optical path length, will vary the ratio of wavelength to optical path length L, in a similar way to changing the optical path length [whilst maintaining the wavelength.

Thus, the backscatter spectral profile of how measurement value varies with frequency of interrogating radiation would be expected to be substantially the same as a profile of how the measurement value (at a fixed frequency of interrogating radiation) would vary with changes in optical path length.

Thus, when operating in the second mode, by interrogating the sensing optical fibre with interrogating radiation at different frequencies and generating a measurement value for each of the different frequencies, a backscatter spectral profile can be generated which provides an indication of how the measurement signal value, at any frequency, may be expected to change with changes in optical path length.

Subsequently, at some later time, the sensing fibre may again be operated in the second mode and interrogated with coherent optical radiation, where the frequency of the interrogating optical radiation varies, over different interrogations, over at least part of the same frequency range, to determine a new backscatter spectral profile.

If there had been no change to the state of the relevant sensing portion, i.e. no change to the optical path length of that portion, the relevant backscatter spectral profile of measurement value with frequency would be expected to be substantially the same as recorded previously. If, however, there has been a change in optical path for the relevant sensing portion, the operating point for the sensing portion will have changed and thus the measurement value for a given frequency of interrogating radiation would be expected to be different to that measured previously. As discussed above, the change in optical path length will provide a similar affect to a change in frequency (and hence wavelength) and thus the effect of the change in optical path length can be seen as having a similar effect as a shift in frequency of the interrogating radiation. However, taking such an effective shift in frequency into account, the variation of measurement value with frequency of interrogating radiation would be expected to exhibit the same general pattern as the previously determined profile.

Figure 2b illustrates this principle. Figure 2b illustrates an example of a backscatter spectral profile 202 of measurement value with frequency of interrogating radiation for the same sensing portion as that illustrated in figure 2a, but where there has been a change in optical path length. The profile 202 has the same general pattern of variation as the previously acquired profile 201, but the change in optical path length results in an apparent shift in frequency af of the newly acquired profile 202 compared to the original profile. The apparent shift in frequency Af is related to the change in optical path length.

Detecting any shift in the backscatter spectral profile for a sensing portion can thus be used as an indication of a change in optical path length and determining the amount of the frequency shift can quantify the amount of change. The amount of any frequency shift may be determined in various different ways, but in general a newly acquired backscatter spectral profile may be compared with a previously acquired profile to determine the amount of any frequency shift. Conveniently the newly acquired profile may be cross-correlated with the previous profile to determine the amount of any frequency shift, but it will be understood that other techniques may be used, for instance feature analysis may identify one or more features which correspond between the relevant profiles, e.g. one or more peaks.

Note that the discussion above has focussed on determining an amount of frequency shift. This could be determined by comparison of profiles of measurement value with frequency, as discussed with reference to figures 2a and 2b. It will be understood, however, that the amount of any frequency shift is related to a corresponding wavelength shift, and it would be possible to generate a profile of measurement value with wavelength of interrogating radiation and determine the amount of any wavelength shift to determine the extent of any change in optical path length. It will be understood that a profile of measurement value with wavelength is related to a profile of measurement value with frequency and thus either can be seen as a backscatter spectral profile and, as used herein, a determination of a frequency shift shall be taken as including a determination of a wavelength shift.

Thus, by periodically operating in the second mode to acquire a backscatter profile, the extent of any change in optical path length of a sensing portion can be determined. The indication of any change in optical path length determined from periods of operation in the second mode of operation to identify and/or correct any demodulation errors in the first mode output signal. For instance, the indication of any change in optical path length determined from periods of operation in the second mode of operation, can be used to calculate the phase change that should be measured by the COTDR operating in the first phase for the same sensing portion, and if they differ by more than ±7C radians (i.e. there is a difference value with a magnitude greater than TC radians) it may be assumed that this is due to demodulation error, e.g. steps, being induced in the COTDR signal generated in the first mode for the sensing portion. A correction may then be applied to the COTDR signal of 2/7it radians, where n is an integer and is chosen so that the phase change in the COTDR signal now matches, to within ±12 radians, that which is calculated from repeated operation in the second mode, i.e. the frequency swept mode of operation.

In general, therefore, the interrogator may periodically operate in the second mode to determine an indication of any change in optical path length for one or more sensing portions. Such an indication of optical path length change may be less precise, but less susceptible to demodulation errors, than a corresponding measurement for the same sensing portion(s) when operating in the first mode of operation. The measurements acquired in the second mode of operation may thus be used to calibrate the measurements acquired in the first node of operation, e.g. to identify and correct for any phase unwrapping demodulation errors.

Figure 3 illustrates one example of a distributed fibre optic sensing apparatus 300 according to an embodiment, in which similar components as discussed with reference to figure 1 are identified by the same reference numerals.

The sensing apparatus 300 comprises an interrogator 301 which, in use, is coupled to a sensing fibre 102, as discussed with reference to figure 1.

The interrogator 301 comprises an optical source, e.g. laser 103, and modulator 104 for repeatedly interrogating the sensing optical fibre 102 with coherent optical radiation.

However in the interrogator 301 of figure 3, the frequency of the interrogating radiation may be controllably varied. Figure 3 thus illustrates a frequency modulator 302 which is operable to apply a controlled frequency modulation. The frequency modulator 302 may be controlled by a controller 303. The controller 303 may thus control the frequency modulator 302 so that the interrogator 301 selectively operates in the first mode or in the second mode of operation.

In the first mode of operation, the controller 303 may control the frequency modulator 302 such that the frequency of interrogating radiation launched into the sensing fibre 102 is the same for each interrogation. Thus, the controller 303 may control the frequency modulator 302 in the first mode, so that there is either no frequency modulation applied or that a fixed frequency modulation is applied which is the same for each interrogation.

In the second mode of operation, the controller 303 may control the frequency modulator 302 such that the frequency of interrogating radiation launched into the sensing fibre 102 varies between interrogations of the second mode.

Note that figure 3 illustrates that the frequency modulator 302 is separate to the modulator 104 for clarity, but in practice the functionality of these modulators could be at least partly combined and/or in some embodiments the laser 103 could be a frequency tuneable laser and at least some of the frequency modulation may be applied by tuning the output frequency/wavelength of the laser. In general therefore the interrogator comprises an output optical path, which in this example includes the source 103, first optical modulator 301 and second optical modulator 104, which is configured to repeatedly interrogate a sensing optical fibre by launching coherent optical radiation into the sensing optical fibre, and where, in one mode of operation, the frequency of the interrogating radiation can be controllably varied.

Interrogating optical radiation which is Rayleigh backscattered from the sensing optical fibre is detected by a photodetector 105. In some implementations the backscatter may be mixed, by mixer 106, with a local oscillator signal derived from the laser 103. In some embodiments the local oscillator signal LO may be tapped from upstream of the modulator 104, and the modulator 104 may be configured to apply a fixed modulation of optical frequency to the interrogating radiation, as to introduce a frequency difference between the Rayleigh backscatter and the local oscillator, thus defining a carrier frequency. The modulator 104 may therefore apply a modulation of optical frequency, and hence wavelength, but the amount of this modulation is fixed. The frequency modulator 302 is configured to apply a controllably variable amount of frequency modulation. The local oscillator may be tapped from downstream of the frequency modulator 302, so that the local oscillator signal includes the effect of the controlled frequency modulation. The modulator 104 may then apply the fixed modulation to the interrogating radiation launched into the sensing fibre 102 so as to provide the frequency difference between the Rayleigh backscatter and the local oscillator. In this way the frequency of the interrogating radiation varies, but the carrier frequency remains the same, which may ease the detection and processing of the carrier signal.

The backscatter (possibly mixed with the local oscillator LO) is detected by a photodetector 105 and the detected backscatter signal is processed by processor 202.

The processor 202 may process the detected backscatter signal in different ways depending on the mode of operation, as may be controlled by the controller 303.

In the first mode of operation, the backscatter signal may be down converted to a baseband signal and then demodulated to provide a phase value for different sensing portion of the optical fibre as will be understood by one skilled in the art. The difference in phase value for the different sensing portions from the repeated interrogations in the first mode of operation can be used to provide a measurement signal for each of the relevant sensing portions of the sensing optical fibre (also referred to as channels of the distributed fibre optic sensor).

In the second mode of operation, the detected backscatter signal can be processed in suitable time bins to determine a measurement value for each of one or more sensing portions of the sensing fibre for each of a plurality of different frequencies of interrogating radiation. In some implementations, the measurement value could be the signal level, e.g. intensity, for the relevant sensing portion. In implementations where the backscatter is interfered to generate a carrier signal component at a defined carrier frequency, the measurement value could be the carrier level.

The interrogator 301 is configured to generate a measurement value for each sensing portion for each of the plurality of different frequencies of interrogating radiation, so as to provide a backscatter spectral profile for each sensing portion.

In the second mode, the sensing optical fibre is repeatedly interrogated, and the frequency of the interrogating radiation is controllably varied between interrogations, such that frequencies of interrogating radiation varies for different interrogations. For each individual interrogation in the second mode of operation, the frequency of the interrogating radiation may not vary significantly and, in some cases, the frequency of the interrogating radiation for each individual interrogation of the second mode may be substantially constant. However the frequency can vary between different interrogations in the second mode. The frequency of the interrogating radiation may be varied over a desired frequency range over a set of a plurality of interrogations to allow generation of a profile for each sensing portion. The set of interrogations may thus be performed over a profile acquisition period to allow generation of the backscatter spectral profile.

It will be understood by one skilled in the art that some distributed fibre optic sensors are known which use an optical source which generates a frequency swept output which is launched into the sensing fibre, for instance fibre optic sensors based on optical frequency domain reflectometry. In such sensors the optical frequency (and hence wavelength) is varied significantly over the course of each interrogation and frequency analysis is applied to the backscatter. This requires the use of relatively complex spectral analysis to determine how the backscatter power varies with frequency and the range of such OFDR sensors is relatively extremely limited.

By contrast, where each interrogation in the second mode comprises interrogating optical radiation which is substantially constant at a given frequency, there may be a relatively large amount of optical radiation launched at that wavelength, which means than a backscatter signal above the noise floor of the sensor apparatus can be received from a much longer range into the sensing fibre. In particular, where the backscatter is mixed with a local oscillator, which, as will be understood by one skilled in the art, provides a degree of amplification for the backscatter, a sensing range of up to 100km or more may be achievable. In addition, as the backscatter from each interrogation is simply processed to provide a single measurement value, complex spectral analysis, e.g. FFTs and the like or multiple filters, are not required.

The difference or separation in frequency between the frequencies for the different measurement values in the second mode is preferably selected so as to provide sufficient detail in the resulting backscatter spectral profile to allow correlation between profiles acquired at different times. Generally the backscatter spectral profile has a characteristic scale of 27 radians, i.e. the characteristic separation between peaks in profile 201 of figure 2a is such that when that frequency change is applied over the length of a sensing portion it leads to a phase change of 27 rad. Thus the separation in frequency, between adjacent frequencies of the set of interrogations that form the backscatter spectral profile in the second mode, may differ from one another by an amount that causes a phase change, say, 2 radians or less over a sensing portion. For instance, in some examples a frequency variation between adjacent frequencies in the set that corresponds to about 1 radian or so for the sensing portion should be sufficient to allow characterisation of the backscatter spectral profile.

The frequency range over which the backscatter spectral profile is determined may be based on an expected maximum level of change in optical path length it is wished to be able to determine between successive profile acquisitions. For instance, if it is wished to periodically operate in the second mode to measure for any change in strain of a sensing fibre and the maximum expected strain is 0.05 millistrain, then the frequency range over which the backscatter spectral profile is determined may correspond to 0.1 millistrain. If, for example, the laser 103 generates optical radiation at a wavelength of 1500nm say (where, as is conventional, the quoted wavelength is the relevant free space wavelength), the relevant wavelength range may thus correspond to a range of about 0.118nm, and the frequency range may thus correspond to a frequency range of about 15.8GHz.

As mentioned above, it may be desirable in order to suitably characterise the backscatter spectral profile, for the frequency difference between adjacent frequencies to correspond to about 1 radian. If the gauge length of the sensor, i.e. the length over which the measurement value is determined or length of the sensing portion, is 10m, then for the example of a variation in wavelength of 0.118nm from a base wavelength of 1500nm may represent a change of about 10,000 radians (e.g. the difference in the number of wavelengths of 1500nm light in 20m (allowing for the double pass of light through the fibre),compared to the number of wavelength of 1500.118 nm light in 20m is about 1570, which corresponds to about 10,000 radians).

Thus, in this example, in the second mode, the sensing fibre may be interrogated about 10,000 times with different frequencies over a frequency range of about 15.8GHz (i.e. a wavelength range of about 0.118nm around the laser wavelength of 1500nm). However, other examples may use a different number of interrogations in the set of interrogations used to form the backscatter spectral profile in the second mode.

It will be understood that in the embodiment of figure 3, the same optical output path is used in the first mode as in the second mode, and thus the interrogator 301 selectively operates in either the first mode or the second mode and during the time that the interrogator is operating in the second mode, operation in the first mode is paused.

In general, therefore it may be desirable for periods of operation in the second mode be relatively short, so as to minimise the amount of time in which the interrogator is not operating in the first mode.

As noted above, the amount of time taken to acquire the relevant backscatter spectral profile in the second mode of operation depends on the frequency range and the number of distinct frequency steps As noted above, the backscatter from a given interrogation in the second mode may be detected and processed together without any significant filtering to separate different wavelengths. Thus, the interrogator 301 may be configured so that backscatter from only one interrogation reaches the detector at any time (at least from the part of the sensing fibre which is of interest for sensing) so that there is no ambiguity in whether the backscatter is from one interrogation from one distance into the fibre or from a previous interrogation from further into the sensing fibre. Thus, the rate of interrogations, referred to as the ping-rate, may be set so as to allow time for the interrogating radiation to travel to the end of the fibre (or distance into the fibre beyond which any backscatter is insignificant) and the backscatter to travel back to start of the sensing fibre before the next interrogation is launched into the fibre. If the sensing fibre has a length of say, 10km, the round-trip travel time to the end of the fibre (assuming, as an example, a refractive index of 1.5) would be 0.1ps and thus the ping rate could be set to 10kHz. Performing, a set of 10000 successive interrogations with different wavelength, such as discussed in the example above, would thus take 1.0s in total, if each interrogation was at a different wavelength. The profile acquisition period would thus be about 1.0s in this example and could be longer for longer lengths of sensing fibre where the ping rate may be slower. For long terms monitoring applications, such a time gap in data from operating in the first mode, i.e. the COTDR data, may be acceptable for the improved reliability in being able to detect and correct for demodulation errors.

The spectral profile acquisition period may be lower in applications where such a wide frequency range is not required, for example if the maximum strain that was expected in the period between successive periods of operation in the second mode were say 0.025 millistrain, instead of the example of 0.05 millistrain given above, the relevant frequency range, and hence the number of interrogations required to acquire the profile, could be halved. Additionally or alternatively, the set of frequency steps, i.e. the different frequencies within the set, may not be evenly spaced, which may allow for a lower number of interrogations to be used to provide the profile and may allow the strain resolution to be maintained whilst reducing the burden of acquisition.

To provide the variation in frequency in the second mode, the frequency modulator may vary the frequency in a stepwise fashion, e.g. from a first frequency fl to a second frequency f2 via a series of steps of wavelength corresponding to a change of about 1 radian, although as mentioned in some cases the size of the frequency steps could vary.

Alternatively, in the second mode, the frequency modulator may slowly ramp the frequency from the first frequency f1 to a second frequency f2 in a relatively continuous fashion over the acquisition period, with the modulator 104 generating the relevant interrogating radiation at the ping-rate. This may mean there is some small ramping of frequency over the course of an individual interrogation, but the interrogating radiation may be a relatively short pulse, say of the order of a few tens or hundreds of nanoseconds, and the frequency ramp occurs over an acquisition period of the order of seconds, the amount of variation during an individual interrogation may be relatively low and the backscatter can be deemed to correspond to a given frequency, e.g. the centre/midpoint frequency that applies for that interrogation.

It will be understood that any significant change in strain or temperature during the acquisition of the backscatter spectral profile could distort the profile. The set of interrogations involved in acquiring the backscatter spectral profile may thus be performed at a time when it can be assumed that the sensing fibre is relatively stable. In many use cases this assumption is valid and other fibre optic sensing techniques such a Brillouin based sensing for instance also rely on such assumption. In some cases, to detect any distortion in a newly acquired profile, the degree of correlation between the newly acquired profile and a reference profile could be determined. The reference profile could be a historic profile based on one or more previously acquired profiles, which may have been acquired at a time when it was known or likely that there was no significant strain variation on the sensing fibre. If the newly acquired profile does not exhibit sufficient correlation (allowing for any frequency shift) to the reference profile, it may be rejected as distorted, and a new profile acquired.

In use for a particular application, the distributed fibre optic sensing apparatus 300 may thus be initially operated in the second mode to acquire an initial profile of the initial state of the sensing fibre for one or more sensing portions or channels of the sensing fibre.

The distributed fibre optic sensing apparatus 300 could be operated in the second mode a number of times to acquire a number of initial profiles for each sensing portion which may be compared to make sure they match sufficiently, so as to avoid a risk of initial profile for a sensing portion being a distorted profile. Subsequently the distributed fibre optic sensing apparatus 300 may switch to the first mode of operation and may generally operate in the first mode of operation to generate COTDR sensing data for the one or more sensing portions or channels of the sensing optical fibre. At various intervals, however, the distributed fibre optic sensing apparatus 300 may switch to the second mode of operation to acquire a new backscatter sensing profile for the one or more sensing portions, before returning to the first mode of operation. For each sensing portion, the newly acquired backscatter spectral profile can be compared to the previous acquired profile to determine an expected change in optical path length or expected phase change in the COTDR sensing data between profile acquisitions and any correction applied to the COTDR data as necessary.

In some instance the intervals between operation in the second mode may be pre-defined and may occur at any desired interval which could be regular or irregular. In some cases, the spacing of the intervals of operation in the second mode could vary according to known or expected noise conditions, e.g. there may be more frequent instance of operation in the second mode when more background noise is expected. In some cases, operation in the second mode could additionally or alternatively be triggered by one or more events. For instance, if a sensing fibre is deployed for long term monitoring near a rail tack, passage of a train on the track may cause high amplitude strain that could lead to demodulation errors, and thus an instance of operation in the second mode could be triggered by passage of a train. In some cases, operation in the second mode could be triggered by an analysis of the COTDR data, i.e. if something appears anomalous in the COTDR sensing data, an instance of operation in the second mode could be triggered to provide calibration.

Embodiments of the disclosure thus relate to distributed fibre optic sensing, in which, in one mode of operation a backscatter spectral profile is generated for at least one sensing portion that provides a characterisation of the state of that sensing portion. In effect the backscatter spectral profile provides an indication of the absolute state of the sensing portion, at least compared to an initial profile. Thus, having acquired a first backscatter spectral profile for the or each sensing portion, the sensing fibre can be interrogated at any time later to acquire another backscatter spectral profile for the or each sensing portion and the amount of any change in strain or temperature can be quantified by analysing just those profiles. Any amount of time can occur between acquisition of the two profiles and there is not a need for any continual monitoring to quantify the change in strain or temperature. The distributed fibre optic sensing may also be operable in a COTDR mode of operation to repeatedly interrogate the sensing optical fibre with interrogating radiation to determine, for the or each sensing portion, an indication of a change of phase between interrogations. The periodic acquisition of the backscatter spectral profile can be used to provide a separate indication of the expected change in phase of between acquisitions, which can be used to correct or calibrate the sensing data from the COTDR mode of operation as necessary.

Whilst embodiments of the disclosure relate to distributed fibre optic sensing apparatus that may be operable in first and second modes, e.g. a COTDR mode and a frequency swept mode respectively, it will be understood that in some applications the apparatus could be selectively operated in just one of these modes. For instance, for a monitoring application, which may be a shorter-term monitoring application, where continuous COTDR data is required and/or high amplitude impulsive stimuli are unlikely, it may be preferable to just continually operate in the first COTDR mode to provide continuous COTDR sensing data. Alternatively, if in some applications it is wished to periodically monitor for any significant changes in strain but continually monitoring is not required, it may be sufficient to just periodically operate in the second mode to acquire profiles that can be analysed to determine any significant strain changes, without any monitoring between the acquisition periods.

In the embodiments discussed above, the interrogator 301 is selectively operable in either the first mode or the second mode and thus each interrogation launched by the interrogator 301 can be seen as a first mode interrogation or a second mode interrogation. In some embodiments, it would alternatively be possible to use techniques, such as wavelength division, to allow a first mode interrogation and a second mode interrogation to be propagating in the sensing fibre at the same time as one another. For example, a first set of interrogations at a first fixed wavelength may be generated and launched into the sensing optical fibre to provide first mode operation, whilst a second set of integrations may comprise interrogation that vary within a frequency range centre about a second, different wavelength, where the first and second wavelengths are sufficiently different from one another that backscatter from the first and second sets of interrogations can be separated by wavelength demultiplexing, e.g. by filtering. Other techniques for discrimination between the two sets of interrogations, such as coding, could alternatively be used. This could allow operation in the first and second modes simultaneously, albeit at the expense of requiring the interrogator to be able to have separate optical paths for generating the respective interrogations and requiring filtering or other techniques for separating the backscatter, which can add to the cost, size and complexity of the interrogator. In some embodiments, therefore, the apparatus may thus comprise an optical output path that is capable of outputting suitable interrogating radiation for operating in the first mode and the second mode simultaneously, for instance the interrogator 301 of figure 3 may comprise first and second lasers (not separately illustrated) and the receive path may be configured to separate the backscatter components from the first and second sets of interrogations.

In general therefore the controller 303 may be operable so that the interrogator is capable of operating in the first mode and the second mode simultaneously.

Embodiments which operate can operate in the first and second mode simultaneously can thus avoid any unwanted gaps in the continuity of operation in the first mode.

Embodiments which selectively operate in either the first mode or the second mode may require limited additional components compared to a conventional COTDR sensor and thus can be relatively low cost but can improve the functionality and accuracy of the sensor.

Embodiments of the present disclosure thus relate to coherent Rayleigh backscatter based distributed fibre optic sensing, where the Rayleigh backscatter is processed according to the principles of OTDR to determine a measurement value, but where, in one mode of operation, which may be a sequential or a simultaneous mode of operation, the wavelength of the interrogating radiation is controllably varied so as to provide, for one or more sensing portions, a backscatter spectral profile of measurement value against wavelength which can be used to characterise the state of that sensing portion.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim, "a" or "an" does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.

Claims (21)

1. CLAIMSA fibre optic sensing apparatus, comprising: an optical output path configured to repeatedly interrogate a sensing optical fibre by launching coherent optical radiation into the sensing optical fibre; a detector configured to receive optical radiation that is Rayleigh backscattered from the sensing optical fibre and output a detected backscatter signal in response to each interrogation; and a processor for processing the detected backscatter signal; wherein the fibre optic sensing apparatus is operable in a first mode, in which the sensing optical fibre is interrogated with a first set of interrogations of coherent optical radiation having the same frequency characteristics as one another and the detected backscatter signal from the first set of interrogations is processed to determine, for at least one sensing portion of the sensing optical fibre, a phase value indicative of any changes in optical path length and output a corresponding first mode output signal; and wherein the fibre optic sensing apparatus is also operable in a second mode, in which the sensing optical fibre is interrogated with a second set of interrogations of coherent optical radiation having different optical frequencies from one another within a defined frequency range and the detected backscatter signal from the second set of interrogations is processed to acquire for said at least one sensing portion a backscatter spectral profile of a measurement value of the detected backscatter signal with frequency; wherein the fibre optic sensing apparatus is apparatus is configured to periodically operate in the second mode and, for said at least one sensing portion, to compare a backscatter spectral profile for that sensing portion determined from one period of operation in the second mode with a backscatter spectral profile determined from a previous period of operation in the second mode to determine an indication of any change in optical path length between said periods.
2. 2. The fibre optic sensing apparatus of claim 1 wherein the processor is configured to use the indication of any change in optical path length determined from periods of operation in the second mode of operation to identify and/or correct any demodulation errors in the first mode output signal.
3. 3. The fibre optic sensing apparatus of claim 2 wherein the processor is configured to use the indication of any change in optical path length determined from periods of operation in the second mode of operation to identify any demodulation errors in the first mode output signal by determining whether a difference value between a change in phase value for a sensing portion determined from operating in the first mode and an expected change in phase value based on the indication of any change in optical path length determined from periods of operation in the second mode of operation has a magnitude greater than ft radians, and to correct any such demodulation error by applying a correction to the relevant phase value determined from operating in the first mode which is an integer multiple of arr radians and which reduces the difference value between the changes to a magnitude of 7 radians or less.
4. 4. The fibre optic sensing apparatus of any preceding claim wherein the processor is configured to determine the indication of any change in optical path length between said periods of operation in the second mode by comparing the backscatter spectral profile for that sensing portion determined in a first period with a backscatter spectral profile for that sensing portion determined in a previous period to identify any shift in frequency between the compared backscatter spectral profiles.
5. 5. The fibre optic sensing apparatus of claim 4 where the processor is configured to determine the amount of any shift in frequency between the compared backscatter spectral profiles by cross-correlating the backscatter spectral profiles.
6. 6. The fibre optic sensing apparatus of any preceding claim wherein the apparatus is configured such that in the second mode, the second set of interrogations comprise a plurality of interrogations with different frequencies, where the frequency difference between adjacent frequencies of the set corresponds to a variation of 2 radians or less over a sensing portion. 7. 8. 9. 10. 12. 13.
7. The fibre optic sensing apparatus of any preceding claim wherein at least some frequency steps between adjacent frequencies in the second set of interrogations differ from one another.
8. The fibre optic sensing apparatus of any preceding claim wherein, in the second mode, the processor is configured to determine the measurement value as the signal level of the detected backscatter signal.
9. The fibre optic sensing apparatus of any of claims 1 to 7 further comprising a mixer for mixing the optical radiation that is Rayleigh backscattered from the sensing fibre with a local oscillator derived from the optical output path prior to detection by the detector.
10. The fibre optic sensing apparatus of claim 9, wherein the optical output path is configured such that there is an optical frequency difference between the local oscillator and the optical radiation that is Rayleigh backscattered from the sensing fibre, and the processor is configured, in the second mode, to determine the measurement value as a carrier level of a carrier component in the detected backscatter signal at a carrier frequency equal to said optical frequency difference.
11. The fibre optic sensing apparatus of any preceding claim wherein the optical output path comprises a coherent optical source and a first optical modulator for applying a controlled frequency modulation to optical radiation from the coherent optical source.
12. The fibre optic sensing apparatus of any preceding claim further comprising a controller for selectively controlling the mode of operation in the first mode or in the second mode.
13. The fibre optic sensing apparatus of claim 12 wherein the controller is configured to alternate operation in the first and second modes such that the apparatus operates in the first mode between said periods of second mode operation.
14. 14. The fibre optic sensing apparatus of claim 12 wherein the controller is configured to operate continually in the first mode and wherein said periods of second mode operation occur simultaneously with operation in the first mode.
15. 15. The fibre optic sensing apparatus of any of claims 12 to 14 wherein the controller is configured to operate in the second mode at pre-defined intervals.
16. 16. The fibre optic sensing apparatus of any of claims 12 to 15wherein the controller is configured to operate in the second mode in response to a defined event occurring and/or if an anomaly in the first mode output signal is detected.
17. 17. The fibre optic sensing apparatus of any of claims 12 to 15 wherein the controller is further configured to be operable to operate the apparatus continually in the first mode with no operation in the second mode and/or to periodically operate in the second mode without no operation in the first mode in the intervals between second mode operation.
18. 18. A method of fibre optic sensing comprising: repeatedly interrogating a sensing optical fibre by launching coherent optical radiation into the sensing optical fibre; a detecting optical radiation that is backscattered from the sensing optical fibre and output a detected backscatter signal in response to each interrogation; and processing the detected backscatter signal; wherein the method comprises operating in a first mode, in which the sensing optical fibre is interrogated with a first set of interrogations of coherent optical radiation having the same frequency characteristics as one another and the detected backscatter signal from the first set of interrogations is processed to determine, for at least one sensing portion of the sensing optical fibre, a phase value indicative of any changes in optical path length and output a corresponding first mode output signal; and the method further comprises periodically operating in a second mode, in which the sensing optical fibre is interrogated with a second set of interrogations of coherent optical radiation having different optical frequencies from one another within a defined frequency range and the detected backscatter signal from the second set of interrogations is processed to acquire for said at least one sensing portion a backscatter spectral profile of a measurement value of the detected backscatter signal with frequency; and the method comprises comparing a backscatter spectral profile for a sensing portion determined from one period of operation in the second mode with a backscatter spectral profile determined from a previous period of operation in the second mode to determine an indication of any change in optical path length between said periods.
19. 19. The method of claim 18 comprising using the indication of any change in optical path length determined from periods of operation in the second mode of operation to identify and/or correct any demodulation errors in the first mode output signal.
20. 20. The method of claim 19 wherein the indication of any change in optical path length determined from periods of operation in the second mode of operation is used to identify any demodulation errors in the first mode output signal by determining whether a difference value between a change in phase value for a sensing portion determined from operating in the first mode and an expected phase value based on the indication of any change in optical path length determined from periods of operation in the second mode of operation has a magnitude greater than rr radians, and the method comprises correcting any such demodulation error by applying a correction to the relevant difference value determined from operating in the first mode which is an integer multiple of 2vr radians and which reduces the difference value to a magnitude of 1T radians or less.
21. 21 The method of any of claims 18 to 20 wherein determining the indication of any change in optical path length between said periods of operation in the second mode comprises comparing the backscatter spectral profile for that sensing portion determined in a first period with a backscatter spectral profile for that sensing portion determined in a previous period to identify any shift in frequency between the compared backscatter spectral profiles.