System and method for rayleigh-based distributed acoustic sensing

Abstract

Systems and methods of Rayleigh-based distributed acoustic sensing are disclosed. The laser source injects a probe light signal periodically modulated at a modulation frequency fm into the optical fiber; coherent detection is performed on the detection optical signal and a response optical signal emitted by an optical fiber in response to the detection optical signal subjected to Rayleigh backscattering, so that a corresponding detection electric signal and a response electric signal are provided; these signals are sampled at a sampling frequency fs = M fm (M is a non-zero positive integer), providing a probe sampled signal and a response sampled signal; identifying, in these sampled signals, a respective array of N consecutive probe samples and N consecutive response samples, where N = G * M, G is a positive integer; providing a Rayleigh response of the optical fiber based on the array of detection samples and the array of response samples; and obtaining information indicative of the acoustic event by processing the Rayleigh response of the optical fiber.

Description

Technical Field

[0001] The present disclosure relates to distributed sensing in optical fiber. In particular, the present disclosure relates to systems and methods for Rayleigh-based distributed acoustic sensing. Background Art

[0002] It is known to use optical fibers as sensors for various physical parameters, such as pressure or temperature. Furthermore, the use of optical fibers enables so-called "distributed fiber optical sensing" (DFOS), where the physical parameter to be sensed can be sensed continuously along the length of the fiber.

[0003] A known solution commonly used for DFOS is the so-called "single-ended" solution, schematically depicted in FIG1 . According to the known single-ended solution, one end of an optical fiber 1, having transmission properties susceptible to the physical parameter to be sensed, is connected to an interrogator 2. The interrogator 2 transmits a probe optical signal into the optical fiber 1 and detects a corresponding response optical signal transmitted back by the optical fiber 1 in response thereto. The interrogator 2 then processes the response optical signal to extract information indicative of the physical parameter to be sensed.

[0004] One specific application of DFOS is distributed acoustic sensing (DAS). DAS enables the detection and localization of "acoustic events" occurring at specific locations and times along a structure to be monitored. Throughout this specification and the claims, the term "acoustic event" will refer to a pressure change that results in local strain (particularly axial strain) along a sensing fiber deployed along the structure to be monitored. An acoustic event can be a vibration, i.e., a periodic pressure change that results in local strain in the sensing fiber, which varies periodically over time.

[0005] Typically, DAS is based on the analysis of a response optical signal, which is generated by an optical fiber when a probe optical signal propagating through the fiber undergoes linear elastic backscattering, in particular Rayleigh backscattering.

[0006] Rayleigh-based DAS (RDAS) can be implemented based on several known techniques, generally referred to as “reflectometric techniques.” Known reflectometric techniques can be divided into two main categories, depending on the nature of the probe light signal utilized.

[0007] The first type of reflectometry technique uses a pulsed probe optical signal. More specifically, in pulse-based reflectometry techniques (such as OTDR, or optical time domain reflectometry), a sequence of optical pulses is injected into the optical fiber, and a measurement of the response optical signal is taken for each injected optical pulse.

[0008] The second category of reflectometry techniques instead utilizes a non-pulsed probe light signal. In non-pulsed reflectometry techniques, such as OFDR (Optical Frequency Domain Reflectometry), the probe light signal is typically a continuous wave whose instantaneous frequency is periodically linearly swept in time. This linearity is a requirement for the technique to be effective. For each cycle of the linear frequency sweep, a single measurement of the response light signal is typically taken. Summary of the Invention

[0009] The Applicant has noted that the above-mentioned known reflectometry techniques for RDAS exhibit some drawbacks.

[0010] In both of the above-mentioned known reflection measurement techniques, in fact, the response light signal of the optical fiber is measured at a period δt r According to the Nyquist–Shannon sampling theorem, the measurement period δt between consecutive measurements is r Set the upper limit of the maximum detectable acoustic frequency to f a ≤f r / 2=1 / (2δt r ), where f r =1 / δt r is the measurement rate in Hz. In the following description, the maximum detectable acoustic frequency will also be referred to as the "acoustic bandwidth".

[0011] For pulse-based reflectometry to be effective, successive light pulses must be spaced in time to pass through the target sensing range L. max At least one round trip propagation time is separated by the target sensing range L max Defined as the maximum length of fiber that can be interrogated. This sets the measurement period δt r The lower limit of δt r ≥ 2 L max / c, where c is the speed of light in the optical fiber. Therefore, the maximum detectable acoustic frequency is related to the target sensing range L max Proportional according to the following equation: f a ≤c / (4L max ). Target sensing range L max The longer the L, the lower the maximum detectable acoustic frequency. max =10km, the maximum detectable acoustic frequency is f a ≤5kHz, and for L max =100km, maximum detectable acoustic frequency f a ≤500Hz(assuming Another way to think about this limitation is to consider that when a probe light pulse propagates through an optical fiber, it illuminates only a specific span of the fiber at a time. Therefore, if an acoustic event occurs at a location z in the fiber that is not traversed by the probe light pulse during the timeframe of the acoustic event, then this acoustic event cannot be detected, resulting in an inherent dead time in which the fiber is not monitored.

[0012] Regarding non-pulse based reflection measurement techniques, in this case the maximum detectable acoustic frequency f a The limitation of is determined by two factors. The first factor is the maximum time rate at which a linear sweep of the instantaneous frequency of the detection light signal can be performed. The second factor is the ratio between the period of the vibration to be sensed and the period of the frequency linear sweep. More precisely, the period of the frequency linear sweep should be shorter than the period of the vibration. On the other hand, for a given target sensing range L max , reducing the period of the frequency linear sweep requires increasing the electrical bandwidth during detection. Due to these requirements, it is quite challenging to realize a long-range RDAS with a large acoustic bandwidth.

[0013] Furthermore, the manner in which the linear sweep of the instantaneous frequency of the probe light signal is achieved means that dead time is inherent in non-pulse-based reflectometry techniques. For example, if a tunable laser source is used to generate the probe light signal, the dead time corresponds to the "kickback" period during which the tunable laser is brought back to its original frequency after the linear sweep is completed.

[0014] The Applicant then faced the problem of providing a system and method for Rayleigh-based distributed acoustic sensing that overcomes the above-mentioned disadvantages.

[0015] In particular, the Applicant has addressed the problem of providing a system and method for Rayleigh-based distributed acoustic sensing that exhibits substantially no dead time and a lower limit that is substantially independent of the target sensing range L max measurement cycle.

[0016] Without being bound by any theory, the applicant has noted that since Rayleigh backscattering is a linear process, its input / output relationship can be fully described in terms of a transfer function in the spectral domain or a response function in the time domain, according to the theory of linear systems. The time domain response function of an optical fiber will also be referred to as the "Rayleigh response" hereinafter. Rayleigh-based distributed acoustic sensing can then rely on an analysis of the Rayleigh response of an optical fiber. Since the value of the Rayleigh response is a complex number, the analysis should consider both the amplitude and phase of its value. Therefore, if a(t) is the probe optical signal and b(t) is the response optical signal, the relationship between the signals a(t) and b(t) in the spectral domain can be written as:

[0017] b(ω)=H(ω)a(ω)[1]

[0018] in is the spectrum of the detection light signal, is the spectrum of the response optical signal, and H(ω) is the frequency domain Rayleigh response of the fiber, where Indicates the Fourier transform. According to H(ω), the Rayleigh response of the optical fiber can be obtained by the inverse Fourier transform:

[0019]

[0020] in Indicates the inverse Fourier transform. The Rayleigh response can then be converted into a spatial-domain Rayleigh response using the speed of light propagation in the fiber. By appropriately analyzing the temporal variations of the spatial-domain Rayleigh response, information indicative of the acoustic event being sensed can be obtained.

[0021] From the above, the applicant has realized that if H(ω) is replaced by its frequency sampled version H(ω k ),ω k =ω0+2πkf m , And if the inverse Fourier transform is replaced by the inverse discrete Fourier transform, equation [2] expressed in the continuous time domain still holds. In this case, the discrete-time signal h[n·dt] is obtained, which corresponds to the periodic repetition of the samples of the continuous-time function h(t), where n is the sample index.

[0022] According to the theory of discrete Fourier transform, the repetition period T will be equal to H(ω k ) The interval between frequency domain samples is f m The reciprocal of , and the time domain sampling interval dt will be equal to T / N, N corresponds to the composition H(ω k ) is the number of samples.

[0023] In view of the above, the applicant has realized that by providing a probe light signal with a carrier frequency ω0 and a modulation period T m =1 / f m The detection light signal a(t) is periodically modulated (modulation can be applied to one or more of the quantities (i.e., amplitude, frequency, and phase) of the detection light signal), and the spectrum a(ω) of the detection light signal has a discrete support Among the points with discrete support A, the assumed value k∈K.

[0024] According to the theory of linear systems, when such a probe light signal is injected into the optical fiber, the spectrum b(ω) of the corresponding response light signal also has a discrete support A. At the point of the discrete support A, the value is assumed to be k∈K.

[0025] Therefore, by performing optical coherent detection on both optical signals a(t) and b(t), a detection electrical signal A(t) and a response electrical signal B(t) are obtained, the spectra of which again have a discrete support A', which is substantially strictly shifted toward the baseband relative to A, and whose corresponding value is still a k and b k (lacking a fixed scaling factor). Then we can make f s =M f m The sampling frequency f s The electrical signals A(t) and B(t) are sampled, where M is a non-zero positive integer such that f s ≥2maxA′, maxA′ is the maximum value in A′, thereby obtaining the detection sampling signal a[n·dt] and the response sampling signal b[n·dt], where dt=1 / f s And n is the sampling index.

[0026] The point-by-point ratio between the discrete Fourier transforms of the sampled signals a[n·dt] and b[n·dt] according to the right side of equation [2] will provide the frequency response coefficient H(ω k ), as long as the points of A' are contained in the set of discrete frequencies on which the spectra of a[n·dt] and b[n·dt] are sampled by discrete Fourier transform.

[0027] In this regard, the applicant has noticed that when T = Q·1 / f m This condition is satisfied when Q is a non-zero positive integer. In other words, this requires that N = G f s / f m =G·M samples, where G is a non-zero positive integer. The arrays identified in the detection sampling signal a[n·dt] and the response sampling signal b[n·dt] are a=[a u ,…,a u+N-1 ] and b=[b v ,…,b v+N-1 ], where u corresponds to the starting index from which samples of a[n·dt] are taken, and v is the starting index from which samples of b[n·dt] are taken. The two arrays can be time-misaligned, which means that the starting indices u and v can have different values. Their discrete Fourier transforms provide two arrays and Then, the array and can be restricted to those samples corresponding to the values ​​contained in A' and the point-wise ratio is performed according to the right side of equation [2] above.

[0028] After performing the point-by-point ratio, the inverse Fourier transform is applied to the result to obtain the discrete Rayleigh response of the fiber h[m·dt]. The discrete Rayleigh response h[m·dt] can then be converted into a discrete spatial domain Rayleigh response h[m·dz], whose spatial sampling step dz is given by the following equation:

[0029]

[0030] Where c0 is the speed of light in vacuum, n g is the group refractive index in the fiber, |A ′ | is the cardinality of the discrete support A' of the spectra of the electrical signals A(t) and B(t), and the factor 2 accounts for the round-trip propagation of the probe light signal through the optical fiber.

[0031] If the array and Restricted to the subset A belonging to the discrete support A' ″ The above discussion is still valid if the sample is .

[0032] The measurement of the Rayleigh response can be iterated at discrete time instants, thereby providing a sequence of measurements of the spatial domain Rayleigh response h[m·dz], the sequence of measurements being equally spaced in time by an iteration period, which essentially represents the measurement period δt r Specifically, as long as samples of the electrical signals A(t) and B(t) are available, a new iteration can be performed by incrementing the values ​​of the starting indices u and v by the same positive integer number q≥1. Then, according to the following equation δt r =δt·q, measurement period δt r Related to the sampling period dt. Then, the measurement period δt r The lower limit is δt r = δt, which is obtained when q = 1, ie when the measurement of the Rayleigh response is repeated each time a new sample of both the detection electrical signal A(t) and the response electrical signal B(t) is available.

[0033] Therefore, advantageously, the measurement period δt r The lower limit is basically independent of the target sensing range L max .

[0034] Target sensing range L max In fact, it affects the modulation frequency f of the detection light signal m , f m should be set so that L max =c / 2f mHowever, as mentioned above, the minimum measurement period δt of the Rayleigh response r Equal to the sampling period dt, which is not only the modulation frequency f m (and therefore L max ) and is inversely proportional to the integer M. Therefore, in principle, the measurement period δt can be arbitrarily reduced by setting an appropriately high value of M. r From a practical point of view, this means that the measurement period δt r The lower limit of is essentially derived from the processing capability of the data processing unit that implements the sampling and processing of the electrical signals provided by the optical coherent detection of the detection light signal and the response light signal.

[0035] The results of the above process (i.e., a sequence of measurements of the spatial domain Rayleigh response h[m·dz] spaced equally in time by the iteration period) are then processed to obtain information indicative of the acoustic event. To this end, known algorithms can be applied, such as spectral correlation analysis or direct phase demodulation as explained in Z. He and Q. Liu, "Optical Fiber Distributed Acoustic Sensors: A Review," (Journal of Lightwave Technology, Vol. 39, No. 12, pp. 3671-3686, June 15, 2021, doi:10.1109 / JLT.2021.3059771).

[0036] Thus, according to a first aspect, the present disclosure provides a system for Rayleigh-based distributed acoustic sensing of acoustic events, the system comprising:

[0037] -optical fiber;

[0038] - a laser source configured to inject a probe light signal a(t) into the optical fiber, the probe light signal a(t) being modulated at a frequency f m a periodically modulated optical signal;

[0039] a first optical detector and a second optical detector, the first optical detector being configured to perform coherent detection of the probe light signal a(t) injected into the optical fiber, thereby providing a probe electrical signal A(t), and the second optical detector being configured to perform coherent detection of a response light signal b(t) emitted by the optical fiber in response to the probe light signal a(t) undergoing Rayleigh backscattering through the optical fiber, thereby providing a response electrical signal B(t); and

[0040] - a data processing unit configured to: s =M f m The sampling frequency f sThe detection electrical signal A(t) and the response electrical signal B(t) are sampled to provide a detection sampling signal a[n·dt] and a response sampling signal b[n·dt], wherein M is a non-zero positive integer; a first array of N consecutive detection samples and a second array of N consecutive response samples are identified in the detection sampling signal a[n·dt] and the response sampling signal b[n·dt], respectively, wherein N=G·M, G is a non-zero positive integer; a Rayleigh response of the optical fiber is provided based on the first array of N consecutive detection samples and the second array of N consecutive response samples; and information indicative of the acoustic event is obtained by processing the Rayleigh response of the optical fiber.

[0041] According to some embodiments, the periodically modulated probe light signal a(t) is a non-pulsed light signal, wherein “non-pulsed light signal” means that the amplitude of the light signal may vanish only in a discrete set of moments.

[0042] According to the embodiment:

[0043] - the laser source is further configured to provide an unmodulated optical signal c(t) to the first optical detector and the second optical detector, the unmodulated optical signal c(t) having a carrier frequency shifted relative to a carrier frequency of the detection optical signal a(t); and

[0044] The first optical detector and the second optical detector are configured to perform heterodyne coherent detection of the detection optical signal a(t) and the response optical signal b(t) using the unmodulated optical signal c(t).

[0045] According to an embodiment, the system further comprises two polarization controllers, which are configured to place the unmodulated optical signal c(t) in corresponding mutually orthogonal polarization states, and wherein the second optical detector comprises two optical detectors, each operating in a corresponding polarization state.

[0046] According to an embodiment, the system further comprises an optical amplifier configured to increase the optical power of the probe light signal a(t) before the probe light signal a(t) is injected into the optical fiber.

[0047] Additionally or alternatively, the system further comprises a further optical amplifier configured to increase the optical power of the response optical signal b(t) before the response optical signal b(t) is received by the second optical detector.

[0048] According to a second aspect, the present disclosure provides a method for Rayleigh-based distributed acoustic sensing of acoustic events, the method comprising the following steps:

[0049] (a) Injecting a detection light signal a(t) into the optical fiber, wherein the detection light signal a(t) is modulated at a frequency f m a periodically modulated optical signal;

[0050] (b) performing coherent detection on the detection light signal a(t) injected into the optical fiber, thereby providing a detection electrical signal A(t);

[0051] (c) performing coherent detection of a response optical signal b(t) emitted by the optical fiber in response to the detection optical signal a(t) undergoing Rayleigh backscattering through the optical fiber, thereby providing a response electrical signal B(t);

[0052] (d) so that f s =M f m The sampling frequency f s Sampling the detection electrical signal A(t) and the response electrical signal B(t) to provide a detection sampling signal a[n·dt] and a response sampling signal b[n·dt], where M is a non-zero positive integer;

[0053] (e) identifying a first array of N consecutive detection samples and a second array of N consecutive response samples in the detection sampling signal a[n·dt] and the response sampling signal b[n·dt], respectively, where N=G·M and G is a non-zero positive integer;

[0054] (f) providing a Rayleigh response of the optical fiber based on the first array of N consecutive probe samples and the second array of N consecutive response samples; and

[0055] (g) obtaining information indicative of the acoustic event by processing the Rayleigh response of the optical fiber.

[0056] According to an embodiment, step (d) comprises synchronously sampling the detection electrical signal A(t) and the response electrical signal B(t).

[0057] According to an embodiment, step (e) comprises identifying the first array of N consecutive detection samples and the second array of N consecutive response samples in a time-unaligned manner.

[0058] According to an embodiment, step (f) comprises:

[0059] - applying a discrete-time Fourier transform to the first array of N consecutive detection samples and the second array of N consecutive response samples, thereby obtaining the arrays and

[0060] -Pair Array and Perform point-by-point ratios to obtain another array as well as

[0061] - applying the inverse discrete-time Fourier transform to the further array

[0062] According to an embodiment, the array and The point-by-point ratios are:

[0063] - By passing the array and Construct two restricted arrays, each restricted to the frequency values ​​contained in its discrete frequency domain support and as well as

[0064] -by Get another array in, Stands for Hadamard division.

[0065] According to an embodiment, step (g) comprises determining a spatial domain Rayleigh response h[m·dz] of the optical fiber, and obtaining information indicative of the acoustic event based on the spatial domain Rayleigh response h[m·dz].

[0066] According to an embodiment, steps (d) to (g) are periodically iterated at discrete time instants, thereby providing a sequence of spatial domain Rayleigh responses h[m·dz] that are equally spaced in time.

[0067] According to an embodiment, the method further comprises applying a spectral correlation analysis on at least two spatial domain Rayleigh responses h[m·dz] measured at different times t1 and t2.

[0068] For the purposes of this specification and the appended claims, unless otherwise indicated, all numbers expressing amounts, quantities, percentages, and the like are to be understood as being modified in all instances by the term "about." Furthermore, all ranges include any combination of the maximum and minimum points disclosed, and include any intermediate ranges therein, which may or may not be specifically enumerated herein.

[0069] For the purpose of this specification and the appended claims, the word "a" or "an" should be understood to include one or at least one, and the singular also includes the plural, unless it is obvious that it is otherwise indicated. This is done merely for convenience and to give a general sense of the disclosure.

[0070] In at least one of the aforementioned aspects, the present disclosure may be implemented according to one or more than one of the embodiments (optionally in combination). BRIEF DESCRIPTION OF THE DRAWINGS

[0071] The present disclosure will provide further details in the following detailed description, given by way of example and not limitation, with reference to the following drawings, in which:

[0072] - Figure 1 (described above) shows a known single-ended solution commonly used for DFOS;

[0073] - Figure 2 Schematically illustrating a system for Rayleigh-based distributed acoustic sensing according to an embodiment of the present invention;

[0074] - Figure 3 According to an embodiment of the present invention, Figure 2 A flowchart of the operation of the data processing unit in the system;

[0075] - Figures 4A-4D Four possible variations of interrogator devices according to the invention are shown;

[0076] - Figure 5 The structure of the laser source according to an embodiment of the present invention is shown in more detail;

[0077] - Figure 6 is a graph showing experimental results of a single measurement of the Rayleigh response provided by a system according to an embodiment of the present invention;

[0078] - Figure 7 is a more detailed flow chart of the steps for providing information indicative of an acoustic event to be sensed based on a Rayleigh response;

[0079] - Figure 8 is a graph showing experimentally obtained information indicative of an impulsive acoustic event; and

[0080] - Figure 9 is a graph showing experimentally obtained information indicating periodic acoustic events (vibrations). DETAILED DESCRIPTION

[0081] Figure 2 A system 1000 for Rayleigh-based distributed acoustic sensing according to an embodiment of the present invention is shown.

[0082] The system 1000 includes an optical fiber 100 and an interrogator device 200 connected to one end of the optical fiber 100 according to a single-ended scheme.

[0083] The optical fiber 100 is arranged along the structure to be monitored ( Figure 2The optical fiber 100 is deployed in a manner not depicted in the drawings and has transmission properties that are susceptible to variations in strain caused by possible acoustic events (impulse events or vibrations) affecting the structure to be monitored. Applicants have conducted forward tests using single-mode ITU-T G.652 optical fiber. As an example, the length of the optical fiber 100 can be up to 50-100 km.

[0084] The interrogator device 200 comprises a laser source 3 configured to generate a probe light signal a(t) and inject it into the optical fiber 100. The probe light signal a(t) is modulated at a frequency f m The optical signal a(t) is periodically modulated with a carrier frequency ω0. The detection optical signal a(t) thus has a discrete frequency domain support Modulation may be applied to one or more of the quantities (i.e., amplitude, frequency, and phase) of the probe light signal. The modulation may be nonlinear modulation. The periodically modulated probe light signal a(t) may be a non-pulsed light signal, where "non-pulsed light signal" means that the amplitude of the light signal may only vanish at a discrete set of instants. The modulation frequency f m The desired target sensing range L max Specifically, the modulation frequency f m can be set so that L max =c / 2f m The carrier frequency ω0 of the detection optical signal a(t) can be selected within the frequency range in which the optical fiber 100 is single-mode. For example, the laser source 3 has a carrier frequency ω0 equal to at least 2·L max The coherence length of .

[0085] The interrogator device 200 also includes two optical detectors 4a, 4b. Each optical detector 4a, 4b is configured to perform coherent detection of a corresponding optical signal. More specifically, the optical detector 4a is configured to perform coherent detection of a probe optical signal a(t) injected into the optical fiber 100, while the optical detector 4b is configured to perform coherent detection of a response optical signal b(t) emitted by the optical fiber 100 in response to the probe optical signal a(t) undergoing Rayleigh backscattering through the optical fiber 100. The coherent detection of the probe optical signal a(t) and the response optical signal b(t) generates a probe electrical signal A(t) and a response electrical signal B(t), respectively; both electrical signals A(t) and B(t) have discrete frequency domain support. where f int is such that f int ≥|minK|f m IF.

[0086] In order to be able to detect both the probe optical signal a(t) and the response optical signal b(t), according to an embodiment, the interrogator device 200 includes an optical splitter 5 and an optical circulator 6 .

[0087] The optical splitter 5 has an input connected to the output of the laser source 3 and two outputs connected to one of the ports of the optical detector 4a and the optical circulator 6, respectively. The other two ports of the optical circulator 6 are connected to the optical fiber 100 and the optical detector 4b, respectively. In this way, the detection light signal a(t) is split into two parts: one part is sent to the optical detector 4a, and the other part is sent to the optical fiber 100 via the optical circulator 6. In addition, the response light signal b(t) provided by the optical fiber 100 is sent to the optical detector 4b via the circulator 6. The optical splitter 5 can have a splitting ratio between 85:15 and 95:5 (for example, 90:10), and the main part of the detection light signal a(t) is guided toward the optical fiber 100 via the optical circulator 6. The applicant conducted a forward test using a 1×2 optical splitter 90:10TN1550R2A1 from Thorlabs (Newton, New Jersey, USA) and a 3-port single-mode optical circulator 6015-3-APC from Thorlabs (Newton, New Jersey, USA).

[0088] The interrogator device 200 further comprises a data processing unit 7 cooperating with the optical detectors 4a, 4b. The data processing unit 7 is configured to process the detection electrical signal A(t) and the response electrical signal B(t) to provide information indicative of the acoustic event to be sensed.

[0089] Now refer to Figure 3 The flowchart of FIG. 1 describes the operation of the data processing unit 7 in detail.

[0090] According to the embodiment, the data processing unit 7 samples the detection electric signal A(t) and the response electric signal B(t) (step 301). For both the electric signals A(t) and B(t), so that f s =M f m The sampling frequency f s Sampling is performed, where M is such that f s =M f m ≥2maxA′ non-zero positive integer, thereby obtaining the detection sampling signal a[n·dt] and the response sampling signal b[n·dt], where dt=1 / f s is the sampling period and n is the sampling index. The detection electric signal A(t) and the response electric signal B(t) may be sampled synchronously.

[0091] Then, according to an embodiment, the data processing unit 7 identifies a first array a=[a[n·dt]] of N consecutive detection samples in the detection sampling signal a[n·dt] and in the response sampling signal b[n·dt], respectively. u ,…,a u+N-1 ] and a second array b=[b v,…,b v+N-1 ], where u corresponds to the starting index of the sample from which a[n·dt] is taken, and v is the starting index of the sample from which b[n·dt] is taken. The number N is equal to N=G·M, G being a non-zero positive integer. Two arrays of N consecutive samples may be time-unaligned, meaning that the starting indices u and v may have different values.

[0092] Then, the data processing unit 7 is based on the two arrays a=[a u ,…,a u+N-1 ] and b=[b v ,…,b v+N-1 ]Provide a measurement of the Rayleigh response of the optical fiber 100 (step 303).

[0093] According to an embodiment, at step 303, by adding two arrays a=[a u ,…,a u+N-1 ] and b=[b v ,…,b v+N-1 ] Applying a discrete time Fourier transform (e.g., FTT (Fast Fourier Transform)) to obtain a measure of the Rayleigh response, thereby obtaining the array and Then, according to the right side of equation [2], the array and To this end, according to the embodiment, respectively according to and Two more arrays were constructed and First, we can select a continuous subset from Starting direction from Take (maxK ′ +1) samples to build The first part is taken once every G samples (e.g. ).from Starting direction Take |minK′| samples to construct The second part is taken once every G samples. Apply the same process to obtain another array Thus, the array and is essentially restricted to points corresponding to the frequency values ​​contained in its discrete frequency domain support.

[0094] Then, Computational Array in stands for Hadamard division. Then the array Perform a discrete-time inverse Fourier transform (e.g., IFFT (Inverse Fast Fourier Transform)) to obtain |K ′ The time domain Rayleigh response h[m·dt] of | samples corresponds to the spatial domain Rayleigh response h[m·dz] of the optical fiber 100. The spatial sampling step dz is equal to dz=L max / (|K ′ |-1).

[0095] The data processing unit 7 then processes the spatial domain Rayleigh response h[m·dz] to provide information indicative of the acoustic event to be sensed (step 304). Further details on this step will be provided below.

[0096] When the data processing unit 7 continuously receives and samples the electrical signals A(t) and B(t) from the optical detectors 4a and 4b, the measurement of the Rayleigh response according to steps 302-304 can be iterated at discrete moments until the end of the measurement period (step 305). Such iteration results in a sequence of measurements of the spatial domain Rayleigh response h[m·dz] equally spaced in time with an iteration period, which essentially represents the measurement period δt r Specifically, as long as samples of the electrical signals A(t) and B(t) are available, a new iteration can be performed by incrementing the values ​​of the starting indices u and v by the same positive integer number q≥1. Then, according to the following equation δt r =δt·q, measurement period δt r Related to the sampling period dt. Then, the measurement period δt r The lower limit is δt r =δt, which is obtained when q=1, ie, when steps 302-304 are repeated each time a new sample of both the detection electrical signal A(t) and the response electrical signal B(t) is available.

[0097] Therefore, advantageously, the measurement period δt r The lower limit is basically independent of the target sensing range L max .

[0098] Target sensing range L max It does affect the modulation frequency f of the detection light signal m , f m should be set so that L max =c / 2f m However, as mentioned above, the minimum measurement period δt of the Rayleigh response r Equal to the sampling period dt, which is not only the modulation frequency f m (and therefore L max) and is inversely proportional to the integer M. Therefore, in principle, the measurement period δt can be arbitrarily reduced by setting an appropriately high value of M. r From a practical point of view, this means that the measurement period δt r The lower limit of is essentially derived from the processing capability of the data processing unit that implements the sampling and processing of the electrical signals provided by the optical coherent detection of the detection light signal and the response light signal.

[0099] In the following, some variations of the interrogator device 200 will be described.

[0100] Figure 4A An interrogator device 201 according to a first variant is shown.

[0101] According to a first variant, in addition to the detection light signal a(t), the laser source 3 of the interrogator device 201 is also configured to provide an unmodulated light signal c(t). The coherence length of the unmodulated light signal c(t) may be equal to at least 2·L max The carrier frequency of the unmodulated optical signal c(t) can be selected within the frequency range in which the optical fiber 100 is single-mode. The carrier frequency of the unmodulated optical signal c(t) can be shifted by f relative to the carrier frequency of the detection optical signal a(t). int Shift. Frequency shift f int For example, in the RF range and making f int ≥|minK|f m The frequency shift may be in the range of, for example, 100-500 MHz. According to an embodiment, the probe light signal a(t) and the unmodulated light signal c(t) are emitted by separate outputs of the laser source 3 .

[0102] According to a first variant, the interrogator device 201 further comprises an additional optical splitter 8. The additional optical splitter 8 has an input connected to the output of the laser source 2 (through which the unmodulated optical signal c(t) is emitted), and two outputs connected to the optical detectors 4a, 4b. The optical splitter 8 may be a 50:50 optical splitter. The applicant performed forward testing using an optical splitter TN1550R5A1 from Thorlabs (Newton, New Jersey, USA).

[0103] According to a first variant, as will be described in further detail below, the optical detectors 4a, 4b use the unmodulated optical signal c(t) to perform heterodyne coherent detection of the optical signals a(t) and b(t), respectively. More specifically, each optical detector 4a, 4b may include an optical coupler 2×2 and a photodiode. The optical coupler 2×2 may be a 50:50 coupler, whose input receives the unmodulated optical signal c(t) and the optical signal a(t) or b(t), respectively, and whose output provides the received optical signal to the photodiode. The applicant performed a forward test using a C-band 50:50, 2×2 single-mode optical coupler TN1550R5A2 from Thorlabs (Newton, New Jersey, USA) and a balanced photodiode PDB-480C-AC from Thorlabs (Newton, New Jersey, USA).

[0104] In each optical detector 4a, 4b, each optical signal a(t), b(t) can be mixed with the unmodulated optical signal c(t) to generate heterodynes. Therefore, the spectrum of each resulting electrical signal A(t), B(t) has a discrete frequency domain support Such electrical signals are then sampled by the data processing unit 7 as described above to provide a detection sampling signal a[n·dt] and a response sampling signal b[n·dt]. The data processing unit 7 can then perform digital quadrature demodulation on the sampling signals a[n·dt] and b[n·dt] to provide a baseband representation of such signals, i.e., a discrete frequency domain representation. The sampled signals a[n·dt] and b[n·dt] are then processed according to the above steps 302 - 304 .

[0105] Figure 4B An interrogator device 202 according to a second variant is shown.

[0106] In addition to the components of the interrogator device 201 according to the first variant, the interrogator device 202 according to the second variant further comprises an optical amplifier 9 inserted between the optical splitter 5 and the optical circulator 6. The optical amplifier 9 increases the optical power of the detection light signal a(t). The optical amplifier 9 may be an erbium-doped fiber amplifier (EDFA). Optionally, the optical amplifier 9 may be followed by an optical filter 10, in particular a bandpass optical filter. The optical amplifier 9 advantageously increases the optical power of the detection light signal a(t), which then allows increasing the target sensing range L of the optical fiber 100. max ..

[0107] Figure 4C An interrogator device 203 according to a third variant is shown.

[0108] Unlike the interrogator device 202 according to the first variant, in the interrogator device 203 according to the third variant, the optical amplifier 9 and the optional optical filter 10 are inserted between the circulator 6 and the optical detector 4b. In this case, the optical amplifier 9 advantageously increases the optical power of the response optical signal b(t) before it is received by the optical detector 4b, thereby increasing the target sensing range L of the optical fiber 100. max ..

[0109] Figure 4D An interrogator device 204 according to a fourth variation is shown.

[0110] The fourth variant implements a coherent polarization diversity scheme. Therefore, in addition to the components of the interrogator device 201 according to the first variant, the interrogator device 204 according to the fourth variant includes two polarization controllers 11 and 12 connected to the corresponding outputs of the optical splitter 8. Such polarization controllers 11 and 12 are configured to put the unmodulated optical signal c(t) in corresponding mutually orthogonal polarization states. Instead of the optical detector 4b, two optical detectors 4b1 and 4b2 are provided, each operating in a corresponding polarization state. Between the circulator 6 and the optical detectors 4b1 and 4b2, a polarization beam splitter 13 is provided, which separates the two polarization states of the response optical signal b(t) and provides each of them to the corresponding optical detectors 4b1 and 4b2 for separate heterodyne detection, which results in two separate response electrical signals B1(t) and B2(t). Then, the respective response electrical signals B1(t) and B2(t) are subjected to the heterodyne detection according to Figure 3 The processing of steps 301-304 of the flowchart of FIG. 4 results in a pair of Rayleigh responses, one for each polarization state. This may be beneficial in providing additional information about the acoustic event to be sensed.

[0111] The above-mentioned variations can be combined with each other.

[0112] Figure 5 The laser source 3 comprised in the interrogator devices 201, 202, 203, 204 is shown in more detail.

[0113] The laser source 3 includes, for example, a CW laser 30 , a beam splitter 31 , a frequency shifter 32 , a light wave modulator 33 , and an RF signal generator 34 .

[0114] The CW laser 30 is configured to emit an unmodulated optical signal c(t), for example. The optical frequency of the CW laser 30 may be included in the optical C-band ranging from 1530 nm to 1565 nm. The applicant conducted a forward test using a CW laser source Koheras Adjustik X15HP from NKT Photonics (Birkeroed, Denmark).

[0115] The optical splitter 31 has an input connected to the output of the CW laser 30 to split the unmodulated optical signal c(t) into two parts. The splitting ratio of the optical splitter 31 can be included between 85:15 and 95:5, for example 90:10. The optical splitter 31 has two outputs: one output (the output of the main part of the unmodulated optical signal c(t)) is connected to the frequency shifter 32, and the other output (the output of the secondary part of the unmodulated optical signal c(t)) is connected to the input of the optical splitter 8, which further splits the secondary part of the unmodulated optical signal c(t) and provides each part to the corresponding optical detector 4a, 4b, which uses the part to perform heterodyne demodulation as described above. The applicant performed a forward test using a 1×2 optical splitter 90:10TN1550R2A1 from Thorlabs (Newton, New Jersey, USA).

[0116] The frequency shifter 32 is configured to shift the optical frequency of the main part of the unmodulated optical signal c(t) by a frequency f int The applicant conducted a forward test using an acousto-optic modulator FIBER-Q T-M110-0.2C2J-3-F2S(FS).

[0117] The light wave modulator 33 is configured to modulate the m The output of the frequency shifter 32 is modulated to provide a probe optical signal a(t). As described above, modulation can be applied to one or more of the quantities (i.e., amplitude, frequency, and phase) of the optical signal. Applicants conducted forward testing using an optical phase modulator LN81S-FC(MOD) from Thorlabs (Newton, New Jersey, USA).

[0118] The applicant uses a length of Some tests were conducted on an optical fiber link as optical fiber 100, which consists of two connected spools of single-mode ITU-T G.652 optical fiber with lengths equal to 4.8 km and 1.3 km respectively.

[0119] Figure 6 The graph in shows the result of a single measurement of the spatial domain Rayleigh response h[m·dz] obtained by a single iteration of the above steps 302 - 304. The following parameters are applied:

[0120] -f m =10kHz, which corresponds to Where the group index n g =1.466;

[0121] -f s=500MHz, which corresponds to M=50000;

[0122] -G=2;

[0123] -X1=X2=3M=100000;

[0124] -u=v=M; and

[0125] -|K ′ |=5393, which corresponds to

[0126] exist Figure 6 In the graph of FIG, the horizontal axis represents the position z along the optical fiber 100, while the vertical axis represents the normalized amplitude in dB of the spatial domain Rayleigh response h[m·dz]. To reduce the detrimental visual effects of coherent Rayleigh fading, a 20-meter moving average filter (11 samples) has been applied to the amplitude.

[0127] As mentioned above, the measurement of the spatial domain Rayleigh response h[m·dz] can be repeated at discrete time instants, thereby providing a measurement period δt r A sequence of measurements of the spatial domain Rayleigh response h[m·dz] that are equally spaced in time.

[0128] As described above, the data processing unit 7 processes the spatial domain Rayleigh response h[m·dz] to provide information indicative of the acoustic event to be sensed ( Figure 3 304 in the flowchart of FIG. 1 ).

[0129] According to an embodiment, the data processing unit 7 processes the spatial domain Rayleigh response h[m·dz] by applying spectral correlation analysis (SCA), which allows the identification of variations in the strain applied to the segment Z of the optical fiber 100 between two different times t1 and t2. More specifically, the spatial domain Rayleigh response h[m·dz] is measured at times t1 and t2. The two measurements of the spatial domain Rayleigh response h[m·dz] are then windowed to select only the spatial points belonging to the segment Z, thereby obtaining two arrays h1 and h2. The spectra of the arrays h1 and h2 are then calculated and cross-correlated, as described in ST Kreger et al., "High Resolution Distributed Strain or Temperature Measurements in Single- and Multi-mode Fiber Using Swept-Wavelength Interferometry" (Optical Fiber Sensors, p. ThE42, Cancun, Mexico).

[0130] Due to the spectral shift property of the Rayleigh response, whenever a change in applied strain Δ∈≠0 occurs between times t1 and t2, the spectral cross-correlation exhibits a peak at a frequency lag Δf∝Δ∈≠0. On the other hand, if no change in strain occurs between times t1 and t2, the spectral cross-correlation exhibits a peak at Δf=0. Thus, performing this analysis on, for example, spatial partitions of the entire optical fiber 100 allows for monitoring strain changes in a distributed manner along the entire length of the optical fiber 100. Furthermore, by continuously extracting the Rayleigh response of the optical fiber 100, this analysis can be iterated in time, for example, keeping time t1 fixed as the reference time and moving forward to time t2.

[0131] More specifically, refer to Figure 7 According to the flowchart of FIG. 1 , the data processing unit 7 performs the following processing according to the embodiment of the present invention.

[0132] First, the data processing unit 7 performs R measurements of the spatial domain Rayleigh response by repeating steps 302-304 at R times (step 801). The data set obtained by the data processing unit 7 at step 801 can be represented as an R×|K′| matrix H=[h r,m ], where the first index identifies the measurement time as t r = rq dt, and the second index identifies a spatial sample within the spatial domain Rayleigh response such that m∈[0,…,|K ′ |-1].

[0133] Then, the data processing unit 7 selects a sample with a length of L along the second index of H. w N w spatial windows, thus generating a three-dimensional data matrix HH = [h r,p,w ], where p∈[0,…,N w -1] and w∈[0,…,L w –1]. In this case, the second index indicates a specific spatial window and is denoted by p. Therefore, the new third index w identifies the spatial sample within the p-th window.

[0134] Then, the data processing unit 7 follows the matrix HH=[h r,p,w ]'s third axis for F points (F≥L w ) applies a discrete-time Fourier transform, providing the matrix Where f∈[0,…,F-1] (step 803).

[0135] Then, the data processing unit 7 selects the data element with index r * The reference spatial domain Rayleigh response of , and for each value of index r other than r and for each value of p, the cross-correlation is calculated according to the following equation:

[0136]

[0137] in is an array and ★ represents the cross-correlation of two discrete series with lag l. This operation provides a three-dimensional data matrix The superscript r * A reference spatial domain Rayleigh response is identified, and the third subscript 1 indicates a cross-correlation lag (step 804).

[0138] Then, the data processing unit 7 receives the Information indicative of an acoustic event to be sensed is extracted from the sensor, as will be discussed in further detail below (step 805).

[0139] Then, the data processing unit 7 returns to step 801 to obtain a new R′×|K to which the processing according to steps 802 to 805 is applied. ′ |Dataset H = [h r′,m ], r'∈[R, ..., R+R'-1]. Thus, the iteration of steps 801-805 enables continuous monitoring of the optical fiber 100 until the end of the measurement period (step 806).

[0140] The applicant has carried out some tests of the above procedure to process the spatial domain Rayleigh response h[m·dz] to provide information indicative of different types of acoustic events.

[0141] The first type of acoustic event considered was an impulse event. Specifically, the system's ability to sense two 25 μs long pulses separated by 1.25 ms was tested.

[0142] The tested optical fiber 100 consists of two spools of standard ITU-T G.652 optical fiber having lengths equal to 4.8 km and 1.3 km, respectively, connected by a fiber stretcher.

[0143] according to Figure 3 Flowchart for applying the following parameters to each single measurement of the spatial domain Rayleigh response:

[0144] -f m =10kHz, which corresponds to Where the group index n g =1.466;

[0145] -f s =500MHz, which corresponds to M=50000;

[0146] -G=1;

[0147] -X1=X2=35M=1.75·10 6 ;

[0148] -u=v=M;

[0149] -|K ′ |=5393, which corresponds to as well as

[0150] -q=2500, which corresponds to f r =200kHz.

[0151] For the spectral correlation analysis of the measurements of the spatial domain Rayleigh response, the following parameters were applied:

[0152] -L w =128;

[0153] -N W = 746 windows, where each window has a relative shift of 4 samples;

[0154] -F=128; and

[0155] -r * =140.

[0156] Figure 8 is to show that by fixing the correlation lag l to l = 0, The obtained two-dimensional matrix Graph of the amplitude of the spatial window. The starting coordinate of the spatial window (p, i.e. the distance along the fiber 100) is reported on the horizontal axis, time (r) is plotted on the vertical axis, and the normalized correlation value for a lag l = 0 is represented by the grayscale. Only windows completely belonging to the fiber 100 have been selected; normalization has been performed with respect to the maximum amplitude of the plane.

[0157] The temporal localization of two acoustic events can be performed by examining the row-by-row minima of the amplitude of the plane. Such a curve is Figure 8 The two time indices r0 and r1 (corresponding to time t0 = r0 / f r and t1=r1 / f r ) are identified as two local minima, highlighted by two marks and corresponding to t0 = 0.94 ms and t1 = 2.19 ms.

[0158] The correlation lag can be fixed to l = 0 and the time index to r = r0, r1. The two one-dimensional arrays obtained and Instead of performing a spatial identification of two acoustic events, the two curves are Figure 8 The locations of the acoustic events (in both cases corresponding to ).

[0159] It must be noted that two acoustic events occurring at the same spatial position along the optical fiber 100 can be detected even if the temporal separation of the two acoustic events is not an integer multiple of the measurement period of an equivalent pulse-based RDAS system such as an OTDR.

[0160] The second type of acoustic event considered is a periodic acoustic event, namely vibration. p =System capability of vibration at acoustic frequency of 5100 Hz.

[0161] The optical fiber 100 consists of two spools of standard ITU-TG.652 optical fiber having lengths equal to 4.8 km and 1.3 km, respectively, connected by a fiber stretcher.

[0162] according to Figure 3 Flowchart for applying the following parameters to each single measurement of the spatial domain Rayleigh response:

[0163] -f m =10kHz, which corresponds to Where the group index n g =1.466;

[0164] -f s =500MHz, which corresponds to M=50000;

[0165] -G=1;

[0166] -X1=X2=35M=1.75·10 6 ;

[0167] -u=v=M;

[0168] -|K ′ |=5393, which corresponds to as well as

[0169] -q=2500, which corresponds to f r =200kHz.

[0170] For the spectral correlation analysis of the measurements of the spatial domain Rayleigh response, the following parameters were applied:

[0171] -L w =45;

[0172] -N W = 746 windows, where each window has a relative shift of 4 samples;

[0173] - F = 45; and

[0174] -r* =140.

[0175] In this case, the relevant lag l can be fixed to l = 0. Get a two-dimensional matrix

[0176] Figure 9 (a) shows the matrix for P=6 and The phase difference between the two windows is shown on the horizontal axis (p+P, i.e. the distance along the fiber 100), the starting coordinates of the spatial window are reported on the horizontal axis (p+P), the time (r) is reported on the vertical axis, and the phase of the correlation value for the lag l=0 is represented by grayscale. Only the differences between the windows that are completely dependent on the fiber 100 are selected.

[0177] The spatial localization of the vibration can be performed by calculating the power of the phase signal along time for each spatial window (i.e., column by column). The curve obtained is Figure 9 (b), normalized relative to its own maximum value; distance is reported on the x-axis and amplitude is reported on the y-axis. The spatial location of the vibration can be identified as the global maximum of the curve and corresponds to the index p0+P, which in turn represents the position within the optical fiber 100

[0178] To determine the frequency of the vibration, one can extract the one-dimensional array The phase of the spectral analysis is performed, for example, by FFT. The amplitude obtained is Figure 9 (c) is illustrated. In this diagram, the frequency is plotted on the horizontal axis and the normalized amplitude is plotted on the vertical axis; normalization is done relative to the maximum value. This diagram allows the perturbation frequency to be identified as It corresponds to the frequency f closest to the actual disturbance p =5100 Hz sampling frequency. It must be noted that for the equivalent known pulse-based RDAS system, assuming the group index n g =1.466, for the test distance length The upper limit of the maximum detectable acoustic frequency will be 5000Hz.

[0179] Therefore, the system 1000 according to various embodiments of the present invention is advantageous because the system exhibits substantially no dead time and the lower limit is substantially independent of the target sensing range L. max The system can therefore be used in a variety of applications, including for example:

[0180] Detecting and locating acoustic events caused by electrical damage in power cables. In this regard, the increased acoustic bandwidth of system 1000, relative to known interferometry techniques, increases the sensitivity of the system and its effectiveness in aiding in the search for and repair of damage;

[0181] Detecting the time and location of lightning strikes along overhead lines;

[0182] Measuring the spectra of acoustic events due to underwater leaks in pipelines, thereby facilitating the identification and classification of these acoustic events;

[0183] Measuring reflected sound waves in geological monitoring, allowing for higher frequency sensitivity;

[0184] Identify fluid cavitation along pipelines;

[0185] Measuring the vibration modes of the overhead lines under wind disturbances and calculating the mass of the overhead lines (which is a measure of ice accumulation); and

[0186] • Measure the vibration spectrum of remote equipment (e.g. sensitive equipment like pumps in nuclear power plants) and detect mechanical damage early based on spectral changes.

Claims

1. A system (1000) for Rayleigh-based distributed acoustic sensing of acoustic events, the system (1000) comprising: - an optical fiber (100); - a laser source (3) configured to inject a probe light signal (a(t)) into the optical fiber (100), the probe light signal (a(t)) being modulated at a frequency f m a periodically modulated optical signal; - a first optical detector (4a) and a second optical detector (4b), the first optical detector (4a) being configured to perform coherent detection of the detection light signal (a(t)) injected into the optical fiber (100), thereby providing a detection electrical signal (A(t)), and the second optical detector (4b) being configured to perform coherent detection of a response light signal (b(t)) emitted by the optical fiber (100) in response to the detection light signal (a(t)) undergoing Rayleigh backscattering through the optical fiber (100), thereby providing a response electrical signal (B(t)); and - a data processing unit (7), which is configured to: s =Mf m The sampling frequency f s The detection electrical signal (A(t)) and the response electrical signal (B(t)) are sampled to provide a detection sampling signal a[n·dt] and a response sampling signal b[n·dt], wherein M is a non-zero positive integer; a first array of N consecutive detection samples and a second array of N consecutive response samples are respectively identified in the detection sampling signal a[n·dt] and the response sampling signal b[n·dt], wherein N=G·M, G is a non-zero positive integer; a Rayleigh response of the optical fiber (100) is provided based on the first array of N consecutive detection samples and the second array of N consecutive response samples; and information indicating the acoustic event is obtained by processing the Rayleigh response of the optical fiber (100).

2. The system (1000) of claim 1, wherein: - the laser source (3) is further configured to provide an unmodulated optical signal (c(t)) to the first optical detector (4a) and the second optical detector (4b), the unmodulated optical signal (c(t)) having a carrier frequency shifted relative to a carrier frequency of the detection optical signal (a(t)); and - The first optical detector (4a) and the second optical detector (4b) are configured to perform heterodyne coherent detection of the detection optical signal (a(t)) and the response optical signal (b(t)) respectively using the unmodulated optical signal (c(t)).

3. The system (1000) according to claim 2, wherein: The system (1000) further comprises two polarization controllers (11, 12), wherein the two polarization controllers (11, 12) are configured to place the unmodulated optical signal (c(t)) in corresponding mutually orthogonal polarization states, and wherein the second optical detector (4b) comprises two optical detectors (4b1, 4b2), each optical detector operating in a corresponding polarization state.

4. The system (1000) according to any one of the preceding claims, wherein: The system (1000) further comprises an optical amplifier (9) configured to increase the optical power of the detection light signal (a(t)) before the detection light signal (a(t)) is injected into the optical fiber (100).

5. The system (1000) according to any one of the preceding claims, wherein: The system (1000) further comprises a further optical amplifier configured to increase the optical power of the response optical signal (b(t)) before the response optical signal (b(t)) is received by the second optical detector (4b).

6. The system (1000) according to any one of the preceding claims, wherein: The detection optical signal (a(t)) is an optical signal periodically modulated by nonlinear modulation.

7. A method for Rayleigh-based distributed acoustic sensing of acoustic events, the method comprising the following steps: (a) injecting a detection light signal (a(t)) into an optical fiber (100), wherein the detection light signal (a(t)) is modulated at a frequency f m a periodically modulated optical signal; (b) performing coherent detection on the detection light signal (a(t)) injected into the optical fiber (100), thereby providing a detection electrical signal (A(t)); (c) performing coherent detection of a response optical signal (b(t)) emitted by the optical fiber (100) in response to the detection optical signal (a(t)) undergoing Rayleigh backscattering through the optical fiber (100), thereby providing a response electrical signal (B(t)); (d) so that f s =Mf m The sampling frequency f s Sampling the detection electrical signal (A(t)) and the response electrical signal (B(t)) to provide a detection sampling signal a[n·dt] and a response sampling signal b[n·dt], wherein M is a non-zero positive integer; (e) identifying a first array of N consecutive detection samples and a second array of N consecutive response samples in the detection sampling signal a[n·dt] and the response sampling signal b[n·dt], respectively, where N=G·M, G is a non-zero positive integer; (f) providing a Rayleigh response of the optical fiber (100) based on the first array of N consecutive probe samples and the second array of N consecutive response samples; and (g) obtaining information indicative of the acoustic event by processing the Rayleigh response of the optical fiber (100).

8. The method according to claim 7, wherein: Step (d) includes synchronously sampling the detection electrical signal (A(t)) and the response electrical signal (B(t)).

9. The method according to claim 7 or 8, wherein: Step (e) includes identifying the first array of N consecutive detection samples and the second array of N consecutive response samples in a time-unaligned manner.

10. The method according to any one of claims 7 to 9, wherein: Step (f) comprises: - applying a discrete-time Fourier transform to the first array of N consecutive detection samples and the second array of N consecutive response samples, thereby obtaining the arrays and - for the array and Perform point-by-point ratios to obtain another array as well as - applying the inverse discrete-time Fourier transform to the further array 11. The method according to claim 10, wherein: For the array and Performing the point-by-point ratio includes: - By passing the array and The two restricted arrays are constructed by restricting each array in to the frequency values ​​contained in its discrete frequency domain support. and as well as -by Get the additional array in, Stands for Hadamard division.

12. The method according to any one of claims 7 to 11, wherein: Step (g) comprises: determining a spatial domain Rayleigh response h[m·dz] of the optical fiber (100), and obtaining information indicative of the acoustic event based on the spatial domain Rayleigh response h[m·dz].

13. The method according to claim 12, wherein: Steps (d) to (g) are periodically iterated at discrete time instants, thereby providing a sequence of spatial domain Rayleigh responses h[m·dz] that are equally spaced in time.

14. The method according to claim 13, wherein: The method further comprises applying a spectral correlation analysis to at least two spatial domain Rayleigh responses h[m·dz] measured at different times t and t2.

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