Distributed acoustic detection system and associated detection method

Abstract

The invention relates to a method and a DAS system for distributed acoustic detection configured to probe at least a first fiber and a second optical fiber. The system makes it possible to probe each of the fibers by pulses at independently configurable temporal widths and frequencies and then to receive, by special optical receivers, a backscattered signal coming from each scattering point along the entire length of the probed fibers. Figure 1

Description

Title of the invention: Distributed acoustic detection system and associated detection method Technical field

[0001] The present invention relates to a distributed acoustic detection system configured to probe at least a first fiber and a second distinct optical fiber.

[0002] The invention also relates to a distributed acoustic detection method for probing at least a first fiber and a second optical fiber.

[0003] The present invention allows the improvement of a vibration measuring device whose sensor comprises an optical fiber.

[0004] The invention allows in particular the increase in the number of optical channels interrogated by a single vibration measuring device and a single optical interrogator.

[0005] The invention applies to the field of transport, in particular rail transport.

[0006] The technical field concerns the monitoring of railway infrastructure for the detection of events (such as rock falls, landslides, intrusions), preventive maintenance, rail traffic management, etc. With this in mind, the system comprises an interrogator which connects to optical fibres deployed along the tracks and which transforms them into a linear vibration detector. State of the art

[0007] There is no equipment equivalent to the distributed acoustic sensing system (or DAS) for measuring vibrations because the measurement is done continuously, distributed along the optical fiber.

[0008] Indeed, to take vibration measurements and monitor a long distance it is necessary to multiply the number of point sensors along the area to be monitored.

[0009] Thus, to measure vibrations in the railway sector, the techniques used are based on the use of point sensors such as accelerometers, geophones, microphones. These sensors only allow vibrations to be measured at the location where they are positioned.

[0010] Thus, if we want to measure the vibrations along a railway track over a distance of 5 km with a resolution of 5 m, it is then necessary to install a point sensor every 5 m over the entire area, i.e. 1000 sensors.

[0011] The DAS system has been developing for several years and its main advantage is to take advantage of the sensitivity of optical fibers to acoustic vibrations, and particularly of their internal structural defects which constitute diffusing centers of the backscattered light. Since detection is linear, a single fiber can replace thousands of successive point sensors.

[0012] However, the maximum length that a single system can monitor is not infinite; there is a physical limit, due to the technology itself. This limit is commonly around 50 km. For railway applications, it is tempting to increase this maximum length in order to be able to monitor railways longer than 50 km.

[0013] Furthermore, it is useful to be able to monitor two or more railway lines with a single DAS system given the existence of telecom premises positioned between the stations forming a mesh of the railway network in France.

[0014] This is why work has been carried out to improve the system in order to increase this maximum length and / or multiply the sensor fibers.

[0015] The principle used to increase or extend the vibration monitoring / detection capacity is the physical separation / decoupling of the signal transmitted over two or more fibers in order to increase the detection length or to cover separate areas or directions.

[0016] In more detail, the principle used to detect vibrations along the fiber consists of sending from the DAS interrogator, using a laser source, a very brief light pulse within this fiber and recording the backscattered signal which travels along the fiber in the opposite direction, and which therefore returns towards the interrogator.

[0017] Each time a pulse is sent into the fiber, a backscattered signal is recorded. In the absence of vibration, the backscattered signal is invariable. When a vibration reaches the fiber, the backscattered signal is modified and it is the exploitation of this modification which makes it possible to identify the source vibration.

[0018] To be able to record the source vibration finely, it is necessary that it be well sampled and this is possible by sending the pulses at a high frequency. The higher the sending frequency (also called "repetition frequency") of the pulses, the more precise the recorded vibration signal will be and the more the high frequency components of the vibration signal can be recorded.

[0019] Indeed, the repetition frequency of the pulses will limit the maximum frequency of the vibratory signal, namely, it will only be possible to record a signal with a maximum frequency equal to half the repetition frequency of the pulses, in accordance with the sampling theorem (also called the Nyquist-Shannon theorem).

[0020] For example, if the pulses are sent at a frequency lower than twice the frequency of the vibration, the recording obtained will correspond to a signal which will not be faithful to the vibration, which will be distorted.

[0021] We then feel that it is sufficient to arrange for the repetition frequency of the pulses in the fiber to be as high as possible to be certain that we en- registers an undistorted vibration.

[0022] However, we cannot choose the pulse repetition frequency arbitrarily high because it depends on the length of the optical fiber being interrogated.

[0023] Indeed, each time a pulse is sent into the optical fiber to generate the backscattered signal, it is necessary to wait until the pulse has traveled the entire fiber and the entire backscattered signal has returned to the corresponding photodetector before sending another, and this is to prevent the backscattered signals generated by two successive pulses from disturbing each other.

[0024] It can be deduced that the longer the optical fiber being interrogated, the more the rate at which the pulses are sent must decrease, thereby limiting the maximum frequency of the vibration signal that can be measured.

[0025] Other limitations are also imposed by the high length of the fiber such as the emitted optical power which will be reduced in order to avoid non-linearities and the spatial resolution of the DAS system which will be reduced to compensate for the loss in optical power.

[0026] Regarding existing DAS devices today, when they are developed to monitor two or more fibers, one of which is very long range up to 50km, the system is forced to impose the constraints due to the longest fiber on the other channel fibers in the system, which directly impacts the overall performance.

[0027] However, in the railway field in particular, it is very useful to increase the detection range as well as to be able to make vibration recordings in two or multiple directions from the telecom premises of the railway network.

[0028] To achieve this, it is possible to decouple the pulse train between two separate fibers.

[0029] Examples exist in the scientific literature, only this involves limiting the performance and quality of the backscattered signal from certain fibers, the repetition frequency of the light pulses would be divided by two or three or more compared to the maximum possible frequency limiting the maximum vibrational frequency that can be detected.

[0030] The gain obtained by increasing the detection capacity is therefore to the detriment of the quality of the vibration signal that is recorded: a parameter is imposed limiting the performance to the best of one of the directions to be monitored.

[0031] Applying a classic decoupling separation to the emitted pulse simply allows for a multiplication of the number of fibers probed but inevitably leads to deterioration and deformation of the vibration signal to be detected.

[0032] The invention proposes precisely to allow the operation of the DAS system to be extended over two or more detection fibers while preserving maximum performance on each of these fibers without deteriorating the vibration signal.

[0033] One of the objectives of the invention is to extend the range and use of the DAS system without altering the performance and quality of the recordings of the source vibration signals.

[0034] In other words, this objective is to design a system capable of monitoring two separate railway lines with independent and adaptive parameterization such as can be achieved with two separate DAS systems, with a financial advantage, i.e. approximately 30% less expensive, a smaller footprint, i.e. approximately half the space, and better performance since the data from two optical channels arrive on the same acquisition card, so algorithmic manipulation is greatly facilitated. Statement of the invention

[0035] To this end, the invention proposes a distributed acoustic detection system configured to probe at least a first optical fiber and a second optical fiber distinct from the first, the detection system comprising: • a laser source configured to emit a coherent continuous optical signal; • a coupler connected to the laser source and configured to separate the signal following optics: • a first branch intended to supply each of the first and second optical fibers to be probed; and • a second branch forming a local oscillator; characterized in that: • the first branch comprises a modulation stage configured to divide said first branch into two distinct divisions, each division being intended to be respectively connected to one of the optical fibers to be probed by means of a respective circulator, and configured to convert the continuous optical signal into pulses, the modulation stage being, in addition, configured to modify independently, for each division, at least one of the parameters of the respective pulses among: a frequency offset relative to the local oscillator, a pulse width and a repetition frequency; • the acoustic detection system further comprises means for controlling said modulation stage, configured to deliver, to the modulation stage, a control signal indicative, for each division, of the at least one parameter to be modified; and • the detection system further comprises heterodyne coherent detection means comprising: • a first detection module connected to the coupler to receive the local oscillator, and to the circulator associated with the first optical fiber to receive a first signal backscattered by the first optical fiber, the first detection module also being configured to deliver a first detection signal representative of an optical beat signal between the local oscillator and the first backscattered signal; and • a second detection module connected to the coupler to receive the local oscillator, and to the circulator associated with the second optical fiber to receive a second signal backscattered by the second optical fiber, the second detection module also being configured to deliver a second detection signal representative of an optical beat signal between the local oscillator and the second backscattered signal.

[0036] Various embodiments of the invention are provided, integrating according to all of their possible combinations the different optional characteristics set out below.

[0037] According to a particular embodiment, the modulation stage comprises a first modulator configured to shift the frequency of the coherent continuous optical signal by a first shift, followed by a second coupler configured to divide said first branch into said two divisions, each division comprising a second modulator controlled by the control means and configured to: • shifting the frequency of the coherent continuous optical signal from the first modulator by a corresponding second shift; and • convert said continuous signal into pulses.

[0038] Advantageously, the distributed acoustic detection system further comprises a frequency multiplexing module, connected to the output of at least one of the circulators, and configured to divide again one of the divisions of the first branch into two, by introducing a frequency shift in the pulses injected into at least one of the new divisions.

[0039] Also advantageously, the distributed acoustic detection system further comprises a time-division multiplexing module, connected to the output of at least one of the circulators, and configured to divide again one of the divisions of the first branch into two, by introducing a time shift between the pulses injected into the new divisions.

[0040] According to a certain particular aspect, the heterodyne coherent detection means to analyze the backscattered signals comprise a first balanced photodiode and a second balanced photodiode, respectively connected to the local oscillator.

[0041] Preferably, the distributed acoustic detection system comprises one or more amplifiers downstream of the modulators, configured to independently adjust the amplification levels of the signal(s).

[0042] According to another particular aspect, the distributed acoustic detection system further comprises a data acquisition card connected to the heterodyne coherent detection means, configured to acquire the detection signal generated by the heterodyne coherent detection means.

[0043] The invention also relates to a distributed acoustic detection method for probing at least a first fiber and a second optical fiber, in which: • a laser source emits a coherent continuous optical signal in a first branch intended to probe the first and second optical fibers, as well as in a second branch forming a local oscillator; • a modulation stage: • divides said first branch into two, each division being respectively connected to one of the optical fibers to be probed by circulators; • shifts in each division the frequency of the coherent continuous optical signal; and • converts said continuous signal into pulses; • control means independently adjust at least one of the parameters of the signals propagating in each division, said parameters being defined by the offset of the optical intermediate frequency, the pulse width, the repetition frequency; • heterodyne coherent detection means generate a detection signal representative of signals backscattered by the first and second optical fibers from an optical beat signal between the local oscillator and each of the signals backscattered by the first and second optical fibers.

[0044] Various embodiments of the invention are provided, integrating according to all of their possible combinations the different optional characteristics set out below.

[0045] According to a particular embodiment, the steps carried out in the modulation stage comprise: • the shift by a first modulator of the frequency of the coherent continuous optical signal propagating in the first branch, • division by a coupler downstream of said first modulator of said first branch in two, • in each division of the first branch, the shift by a second modulator of the frequency of the coherent continuous optical signal, in the opposite direction to the direction of the shift carried out by the first modulator, and • the conversion in each division of the continuous signal into pulses by the second modulator.

[0046] Advantageously, a frequency multiplexing module, at the output of at least one of the circulators, divides again one of the divisions of the first branch into two, and introduces a frequency shift into the signals of the new divisions.

[0047] Also advantageously, a time division multiplexing module, at the output of at least one of the circulators, divides again one of the divisions of the first branch into two, and introduces a time shift at the level of the pulses in the signals of the new divisions.

[0048] According to a certain particular aspect, the heterodyne coherent detection means for analyzing the backscattered signals comprise a first balanced photodiode and a second balanced photodiode, respectively connected to the local oscillator.

[0049] Preferably, one or more amplifiers are arranged downstream of the modulators, so as to independently adjust the amplification levels of the signal(s).

[0050] According to another particular aspect, a data acquisition card connected to the heterodyne coherent detection means acquires the detection signal generated by the heterodyne coherent detection means.

[0051] Preferably, the first optical fiber and the second optical fiber are deployed respectively along a first and a second railway lines. Brief description of the figures

[0052] The invention will be better understood on reading the description which follows, given solely by way of non-limiting example and made with reference to the appended drawings in which:

[0053] [Fig-1] [Fig.l] is an illustrative diagram of an embodiment of a system of distributed acoustic detection according to the invention;

[0054] [Fig.2] [Fig.2] is an illustrative diagram of TTL output control signals of the ADC data acquisition card as well as the backscattered signals for the two fibers operated with optimal parametric settings;

[0055] [Fig.3] [Fig.3] is an illustrative diagram of an embodiment of a distributed acoustic detection system according to an improvement comprising a module of frequency division multiplexing (or FDM);

[0056] [Fig.4] [Fig.4] is an illustrative diagram of an embodiment of a system of distributed acoustic detection according to an improvement comprising a time division multiplexing module (or TDM);

[0057] [Fig.5] [Fig.5] is an illustrative diagram of an embodiment of a system of distributed acoustic detection according to an improvement comprising a TDM multiplexing module equipped with amplifiers so as to compensate for insertion losses;

[0058] [Fig.6] [Fig.6] is an illustrative diagram of an embodiment of a system of distributed acoustic detection according to an improvement comprising an FDM and TDM multiplexing module; and

[0059] [Fig.7] [Fig.7] is an illustrative diagram of an architecture featuring rami arrangements allowing the multiplication of fibers to be probed.

[0060] It is understood that the embodiments which will be described below are in no way limiting. In particular, it will be possible to imagine variants of the invention comprising only a selection of characteristics described below isolated from the other characteristics described, if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art. This selection comprises at least one preferably functional characteristic without structural details, or with only a part of the structural details if it is this part which is only sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art.

[0061] In particular, all the variants and all the embodiments described can be combined with each other if nothing prevents this combination from a technical point of view.

[0062] In the figures and in the remainder of the description, the elements common to several figures retain the same reference. Detailed description

[0063] With a conventional DAS system with two separate fibers, physically at the output of the system, it is possible to probe two fibers by receiving the backscattered signal from each fiber on a different channel of the data acquisition card. However, these conventional systems impose the same parameterization on the two fibers probed, i.e. the same repetition frequencies Frep, spatial resolution RES, optical intermediate frequency IF and output amplification level. This restricts the parameterization of the system to the limits determined by the longest fiber. The shorter fiber is then not used optimally.

[0064] On the contrary, the DAS system and method according to the invention are particularly suitable for railway applications which require monitoring two lines or two directions of the same line whose ends are located in one of the multiple telecom rooms which connect the railway stations. Instead of installing two DAS systems, it becomes possible to use only one to instantly monitor two railway lines, with a parameter setting adapted for each fibre probed.

[0065] The DAS system for Distributed Acoustic Sensing according to the invention makes it possible to probe each of the fibers by pulses at independently configurable temporal widths and frequencies and then receive, by special optical receivers, a backscattered signal coming from each diffusing point along the entire length of the fibers probed.

[0066] This backscattered signal is sensitive to vibrations that the optical fiber may experience.

[0067] The main parameters determining the performance of this system are set out below.

[0068] The level of amplification at the input of the fiber under test makes it possible to compensate for the linear attenuation losses of the optical fiber. The level of amplification is limited by the appearance of cumulative non-linear phenomena with distance. These phenomena, which must be avoided, are due to thermal agitation caused by high incident optical powers. The greater the range, the more the amplification level of the emitted pulse must be reduced.

[0069] The spatial resolution RES of vibration detection is determined by the width of the emitted pulse. For large ranges, increasing the pulse width makes it possible to increase the amount of emitted energy and compensate for the decrease in the amplification level. This is done at the expense of the effective spatial resolution.

[0070] The optical intermediate frequency IF carries the backscattered signal photo-detected at the interrogator receiver. Its value affects the choice of the sampling frequency of the ADC data acquisition card, which is an analog-to-digital converter that determines the digital resolution and the size of the recorded data. The spectral width in the frequency domain of the backscattered Rayleigh signal depends inversely on the width of the emitted pulse, in other words, it depends directly on the spatial resolution. A higher optical intermediate frequency IF would be required for the finest spatial resolutions RES in order to cover the entire spectrum of the backscattered signal.

[0071] The repetition frequency Frep, or pulse sending frequency, makes it possible to sample the vibration source to be detected. The higher this frequency, the more finely the vibration will be detected and the more precise the data analysis will be. However, the maximum possible repetition frequency is limited by the length of the fiber, given that it is necessary to wait for the return of the backscattered signal from the end of the fiber under test before transmitting a "probe" pulse again; this is to avoid interference between backscattered optical signals of the same frequency. A second factor limiting the detection range is the loss due to the linear attenuation of the optical fiber.

[0072] The distributed acoustic detection system DAS according to the invention and as represented in [Fig.l] comprises a laser source LAS, a coupler, a modulation stage ETMOD, circulators BS1, BS2, means for controlling the modulation stage and heterodyne coherent detection means.

[0073] As shown in the figure, the coupler is connected to the laser source (and more precisely, at the output of the laser source), and comprises two outputs, respectively connected to a first branch (comprising the modulation stage) and to a second branch, connected to the heterodyne coherent detection means.

[0074] Furthermore, the modulation stage comprises two outputs, each connected to a first port of a respective circulator BS1, BS2.

[0075] Furthermore, each circulator comprises two additional ports, a second port being intended to be connected to a respective optical fiber to be probed, and a third port being connected to the heterodyne coherent detection means.

[0076] In a known manner, each circulator is configured so that the light injected into the first port exits through the second port, and so that the light injected into the second port exits through the third port.

[0077] The LAS laser source is configured to emit a coherent continuous optical signal.

[0078] As indicated previously, the coupler is connected to the output of the laser source. Additionally, the coupler is configured to separate the optical signal: • following a first branch intended to supply each of the first and second optical fibers to be probed; and • following a second branch forming a local oscillator LO.

[0079] For example, and as illustrated by the figure, the coupler is a 25 / 75 coupler. In this case, 25% of the optical signal generated by the laser source is routed to the modulation stage, while the remaining 75% of the optical signal from the laser source forms the local oscillator LO.

[0080] As indicated previously, the first branch comprises an ETMOD modulation stage.

[0081] The modulation stage ETMOD is configured to divide said first branch into two distinct divisions (each corresponding to a respective output of the modulation stage). In particular, each division being intended to be respectively connected to one of the optical fibers to be probed by means of the respective circulator BS1, BS2.

[0082] The modulation stage is also configured to convert the optical signal continuous output from the laser source in pulses.

[0083] Furthermore, the modulation stage is configured to independently modify, for each division, at least one of the parameters of the respective pulses among: a frequency offset relative to the local oscillator, a pulse width and a repetition frequency.

[0084] Advantageously, the ETMOD modulation stage is configured to apply an optical frequency shift belonging to a range between 0 Hz (hertz) and 60 MHz (megahertz).

[0085] Such a range is advantageous, insofar as it avoids imposing an acquisition frequency that is too high on an ADC acquisition card (described later) connected to the output of the heterodyne coherent detection means.

[0086] The control means (RFI, RF21, RF22) of the modulation stage are configured to deliver, to the modulation stage, a control signal indicative, for each division, of the at least one parameter to be modified.

[0087] Furthermore, the heterodyne coherent detection means comprise a first detection module PHD1 and a second detection module PHD2.

[0088] The first detection module PHD1 is connected, on the one hand, to the coupler to receive the local oscillator, and, on the other hand, to the circulator associated with the first optical fiber Fl to receive a first signal backscattered by the first optical fiber.

[0089] The first detection module is also configured to output a first detection signal representative of an optical beat signal between the local oscillator and the first backscattered signal.

[0090] The second detection module PHD2 is connected, on the one hand, to the coupler to receive the local oscillator, and, on the other hand, to the circulator associated with the second optical fiber F2 to receive a second signal backscattered by the second optical fiber.

[0091] Similarly, the second detection module is also configured to output a second detection signal representative of an optical beat signal between the local oscillator and the second backscattered signal.

[0092] Advantageously, each detection module comprises a balanced photodiode PHD1, PHD2. In this case, each balanced photodiode is connected, via a 50 / 50 coupler, on the one hand to the local oscillator LO, and on the other hand to the respective circulator.

[0093] Such an arrangement is advantageous, in that it leads to a reduction in shot noise, which improves the sensitivity of the system.

[0094] Advantageously, an ADC data acquisition card is connected to the heterodyne coherent detection means, to acquire each detection signal generated by the heterodyne coherent detection means.

[0095] Preferably, and as illustrated by [Fig.l], the modulation stage comprises a first modulator AOM1, a coupler and two second modulators AOM21, AOM22.

[0096] More precisely, the first modulator AOM1 is configured to apply a first frequency shift to the optical signal coming from the laser source LAS.

[0097] Furthermore, the coupler, connected to the output of the first modulator A0M1, is configured to divide the first branch into two divisions.

[0098] Finally, each division comprises a respective second modulator A0M21, AOM22. Each second modulator is configured to apply a second frequency shift to the continuous optical signal from the first modulator and received via the coupler. Furthermore, each second modulator is configured to convert said continuous signal into pulses.

[0099] Such an architecture is advantageous, insofar as it allows independent adjustments, for each of the first and second divisions, of the repetition frequency and the duration of the pulses which propagate there.

[0100] In such an architecture, the first modulator A0M1 and the second modulators A0M21, AOM22 are driven to apply, to the optical signal passing through them, opposite frequency shifts. In other words, if the first modulator is driven to operate in upshift (frequency shift upwards), then the second modulators are driven to operate in downshift (frequency shift downwards), and vice versa.

[0101] Such control is advantageous, insofar as it makes it possible to take advantage of the short response times of modulators operating at high frequency (for example, around 200 MHz), while limiting the frequency of the optical beats at the detection module level, which avoids imposing an acquisition frequency that is too high on an acquisition card.

[0102] The control means RF21, RF22 of said second modulators are configured to control independently of each other at least one of the parameters of the following signals: the frequency shifts, the width of the emitted pulses, the optical intermediate frequency.

[0103] As shown in the exemplary embodiment in [Fig.l], the control means comprise tunable drivers, for controlling a frequency shift of the first and second modulators in a frequency range of ±30 MHz around the central frequency 200 MHz of said modulators, which allows independent control of the optical intermediate frequency IF in a range varying from 0 to 60 MHz. This value is chosen according to the spatial resolution RES limit or chosen for a given use.

[0104] Note that it is possible to apply the same optical intermediate frequency IF for both channels since these are physically separated on two independent channels of the data acquisition card.

[0105] Advantageously, TTL1 and TTL2 outputs of the ADC data acquisition card are connected to the drivers of the two second modulators. This makes it possible to separately adjust the pulse width at the input of these drivers for each of the two fibers to be probed, which corresponds to independent control of the RES spatial resolutions.

[0106] Advantageously, the system represented in [Fig.l] comprises one or more amplifiers EDFA1, EDFA2 downstream of the modulators, so as to independently adjust the level of optical amplification associated with the fiber, and thus prevent the appearance of non-linear phenomena.

[0107] In order to increase the number of fibers that can be probed by a single DAS system, the invention proposes improvements with the addition of modules at the output of the system leading to the fibers to be probed, i.e. downstream of the circulators BS1, BS2.

[0108] The structure of these modules is based on principles of FDM (Frequency Division Multiplexing) and TDM (Time Division Multiplexing).

[0109] [Fig.3] illustrates the structure of an example of an FDM module, the addition of which to the DAS system constitutes a first improvement.

[0110] As illustrated by the figure, the FDM module comprises a main circulator connected, downstream, by its second port, to a coupler (preferably a 50 / 50 coupler) to divide the optical signal from the circulator into two.

[0111] In one of the two divisions, the signal is intended to maintain the same frequency as the pulsed optical signal injected at the first port of the circulator, namely Acoll.

[0112] In the other division, the emitted signal is likely to be modified in optical frequency according to a new frequency shift Acol2, by means of a modulator (such as an acousto-optic modulator). More precisely, a circulator loop, associating two secondary circulators, is used.

[0113] More precisely, a first secondary circulator is connected by its second terminal to the output of the coupler. The first secondary circulator is also connected, by its third port, to the input of the modulator, and by its first port to the third port of a second secondary circulator. Furthermore, the second secondary circulator is connected, by its first port, to the output of the modulator, and by its third port to the first port of the first secondary circulator.

[0114] In this way, the pulse signal applied to the second port of the first cir- The secondary circulator then passes through the modulator to be emitted through the second port of the second secondary circulator, and the backscattered signal, recovered at the second port of the second secondary circulator, is routed to the second port of the first secondary circulator, then to the coupler, until it reaches the main circulator.

[0115] The backscattered signals of both divisions 1.1 and 1.2 are mixed and received at the same time at the main circulator, but they are separable in the frequency domain. The data around each optical frequency corresponds to one of the two divisions.

[0116] Note that in an FDM module, the repetition frequency Frep, and the spatial resolution RES of the signals of the two divisions are identical.

[0117] [Fig.4] illustrates the structure of an example of a TDM module, the addition of which in the The DAS system is a second improvement.

[0118] As illustrated in the figure, the TDM module comprises a main circulator connected, downstream, by its second port, to a coupler (preferably a 50 / 50 coupler) to divide the optical signal from the circulator into two. Each division comprises a respective circulator loop, as described previously, except that the modulator is replaced by an optical switch.

[0119] At the output of the main circulator, the pulse signal is divided into two. This module makes it possible to separate the two signals in the time domain on the same acquisition channel, thus simplifying signal processing. The idea is to divide the repetition frequency Frep by two in the case of two fibers (particularly of the same length) and to send a pulse alternately in each of the two fibers 1.1 and 1.2.

[0120] The pulse signal transmitted on channel 1 of the DAS is therefore separated into two divisions. The paths of the pulse and backscattered signals are separated by the circulator loops. The transmitted pulse signal passes instantly to the input of the two modulators two circulator loops.

[0121] Advantageously, the switches of the two circulator loops are controlled by a pulse generator synchronized with the internal clock of the ADC acquisition card, thus allowing precise and rapid optical switching between the two divisions. The two backscattered signals are received alternately on the same channel of the ADC acquisition card. The data of the odd pulses correspond to channel 1.1 and those of the even pulses correspond to channel 1.2. Note that between these two channels, the repetition frequency Frep, the spatial resolution RES, the optical intermediate frequency IF and the amplification level of the transmitted signal are the same.

[0122] Advantageously, the optical switches are implemented by means of mo modulators, in particular electro-optic modulators (or OEMs) as optical switches. This is advantageous, as such modulators have a shorter response time, greater speed and, above all, a longer lifetime than conventionally known optical switches.

[0123] Advantageously, and as shown in [Fig.5], in FDM modules as in TDM modules, optical amplifiers (for example erbium-doped fiber amplifiers, known as “EDFAs”, for “Erbium Doped Fiber Amplifiers”) can be added downstream of the modulators and / or optical switches. Such a characteristic is advantageous, insofar as it allows insertion losses to be compensated.

[0124] [Fig.6] illustrates the structure of an example of FTDM frequency and time multiplexing modules, the addition of which in the DAS system constitutes a third improvement. In this case, the modulators are controlled both to achieve a frequency shift (for frequency multiplexing) and to selectively allow certain pulses to pass and not others (time multiplexing).

[0125] [Fig.7] shows the logic of multiplication of the number of optical fibers at the output of the DAS system according to the invention with 8 fibers for this example. The abbreviations on this diagram are as follows: • Ml: Physical separation method (DAS system with two independently configured channels); • M2: Frequency division multiplexing separation method FDM; and • M3: TDM time division multiplexing separation method.

[0126] As is apparent from this figure, in the case of FTDM frequency and time multiplexing, it is possible to simultaneously interrogate 2M optical fibers, each with a given repetition frequency Frep, by injecting, at the input of the stage comprising a succession of FTDM modules, an optical signal having a repetition frequency equal to 2M x Frep.

[0127] Operation

[0128] Now concerning the distributed acoustic detection method, which implements the DAS system according to the invention, this method comprises the following steps.

[0129] The LAS laser source emits a coherent continuous optical signal which is separated by a PM 25 / 75 coupler, following a first branch to probe the two optical fibers, and a second branch forming a local oscillator LO.

[0130] The signal arriving in the second branch forming a local oscillator LO is then divided into two by a 50 / 50 PM coupler in order to ensure optical beating with the backscattered signal from each of the two fibers.

[0131] Advantageously, the heterodyne coherent detection means comprise two balanced photodiodes PHD1, PHD2 connected respectively to one of the divisions of the local oscillator LO.

[0132] The continuous optical signal of the first 25% branch is modulated by the modulation stage to form optical pulses and, preferably, to apply an offset of the optical intermediate frequency IF.

[0133] More precisely, in the example of [Fig.l], the first modulator A0M1 shifts the frequency of the coherent continuous optical signal, then the signal is divided into two, each division being respectively connected to one of the optical fibers to be probed by circulators BS1, BS2. Second modulators A0M21, AOM22 again shift the frequency of the signals of the divisions and convert them into pulses.

[0134] Thus, the first modulator A0M1 is used in continuous mode in order to create a frequency shift of the optical signal to be transmitted. The optical coupler SM 50 / 50 divides the signal at the output of this modulator in order to be able to transmit it through two fibers, i.e. two channels.

[0135] In each of the divisions of the first branch, the second modulator AOM2 makes it possible to modulate the continuous light into pulses and to create a frequency shift in a direction opposite to that introduced by the first modulator A0M1.

[0136] Control means RF21, RF22 of said second modulators control the frequency shifts of the modulators independently of each other.

[0137] To do this, tunable drivers are used, with a frequency range that varies around the central frequency of the modulators 200 MHz ±30 MHz, which allows independent control of the optical intermediate frequency IF in a range varying from 0 to 60 MHz. This value is chosen according to the spatial resolution RES limit or chosen for a given use.

[0138] Note that it is possible to apply the same optical intermediate frequency IF for the two fibers since these are physically separated on two independent channels of the data acquisition card.

[0139] Advantageously, TTL1 and TTL2 outputs of the ADC data acquisition card are connected to the drivers of the two second modulators. This makes it possible to separately adjust the width of the pulse at the input of these drivers for each of the two fibers to be probed, which corresponds to independent control of the RES spatial resolutions.

[0140] Each amplifier EDFA1, EDFA2 makes it possible to independently adjust the level of amplification associated with the channel of the fiber Fl and with the channel of the fiber F2.

[0141] The ADC data acquisition card only accepts a single synchronization signal and imposes the same number of samples to be acquired on its two acquisition channels. To remedy this, the TTL1 and TTL2 pulse outputs are synchronized with the internal clock of the acquisition card. A single frequency sending, and therefore repetition, frequency can be set for both pulse outputs. This frequency is limited and determined by the longest fiber.

[0142] Advantageously and in order to be able to apply a higher repetition frequency on the shorter fiber, a burst transmission mode is chosen, making it possible to send a given number of pulses for a single synchronization signal, and consequently to acquire several backscattered signals at the same time on this channel. Consequently, a maximum repetition frequency is chosen independently for each of the two channels.

[0143] Preferably, the repetition frequency is adjusted as a function of the total number of fibers obtained by multiplexing.

[0144] As for the processing of backscattered signals received by the DAS coming from the two fibers.

[0145] Each optical backscattered signal is converted into an electrical signal, after beating with the signal from the local oscillator LO, by the balanced AC-coupled photodiode making it possible to retain only the signal around the intermediate frequency IF.

[0146] These analog electrical signals are received on the two channels of the ADC data acquisition card and are then recorded in binary format on two separate files in a dedicated server.

[0147] The extraction of the amplitude and the phase is carried out digitally for each fiber by a digital IQ modulation, that is to say with a signal brought back to baseband and transformed into a complex number I+jQ, and an adequate filtering.

[0148] Each synchronization signal corresponds to a known number of backscattered signals for the fiber whose pulse is sent in burst mode.

[0149] In summary, in the case of an application of a conventional system to two railway lines of a length of 5km and 20km respectively, this system would be limited by the longest fiber, i.e. the one with a length of 20km, which gives a repetition frequency Frep of 5 kHz max for the two fibers with a lower amplification level to avoid non-linear effects and a reduced resolution to increase the signal to noise ratio.

[0150] As a result, the 5 km length fiber suffered the limitations of the 20 km fiber. In addition, the optical intermediate frequency IF is constant and fixed in the system design phase giving less flexibility to these systems.

[0151] In the case of an application of a system according to the present invention, to these two railway lines, it is possible to optimize the performance of the 5 km fiber as well.

[0152] As shown in [Fig.2], a repetition frequency Frep of 5 kHz is imposed by the longest fiber, an internal synchronization of the acquisition card of the ADC data with its internal clock is then set to 5 kHz, a burst mode is applied on the shortest fiber of 5 km allowing to transmit 4 pulses separated by 50 us (period of the trigger divided by 4) for each trigg which multiplies the repetition frequency Frep of this fiber by 4 and allows to reach the maximum precision with a repetition frequency Frep of 20 kHz. A much finer resolution can be applied over the distance of 5 km (narrower pulses) with a higher possible amplification level and an optical intermediate frequency IF adapted to the high resolution.

[0153] The DAS system according to the invention as well as the distributed acoustic detection method make it possible in particular to instantly and independently monitor two fibers with identical or different ranges while preserving an optimal balance between the main performance characteristics of the system for each of these two fibers.

[0154] Furthermore, this system allows significant adaptability with the different resolutions, in particular with the intermediate frequency IF, allowing appropriate control of the sampling and the size of the recorded data, as well as an optimized application for the different railway configurations.

[0155] Note that the synchronization of the pulses with the internal clock of the data acquisition card allows very high precision for applications requiring real-time processing.

[0156] Of course, the invention is not limited to the examples which have just been described.

Claims

Claims

1. A distributed acoustic sensing (DAS) system configured to probe at least a first optical fiber (F1) and a second optical fiber (F2) distinct from each other, the sensing system comprising: • a laser source (LAS) configured to emit a coherent continuous optical signal; • a coupler connected to the laser source and configured to separate the following optical signal: • a first branch intended to supply each of the first and second optical fibers to be probed; And • a second branch forming a local oscillator (LO); characterized in that: • the first branch comprises a modulation stage (ETMOD) configured to divide said first branch into two distinct divisions, each division being intended to be respectively connected to one of the optical fibers to be probed by means of a respective circulator (BS1, BS2), and configured to convert the continuous optical signal into pulses, the modulation stage being, in addition, configured to modify independently, for each division, at least one of the parameters of the respective pulses among: a frequency offset relative to the local oscillator, a pulse width and a repetition frequency; • the acoustic detection system further comprises means for controlling said modulation stage, configured to deliver, to the modulation stage, a control signal indicative, for each division, of the at least one parameter to be modified; and • the detection system further comprises heterodyne coherent detection means comprising: • a first detection module (PHD1) connected to the coupler to receive the local oscillator, and to the circulator associated with the first optical fiber (Fl) for receiving a first signal backscattered by the first optical fiber, the first detection module also being configured to deliver a first detection signal representative of an optical beat signal between the local oscillator and the first backscattered signal; and • a second detection module (PHD2) connected to the coupler to receive the local oscillator, and to the circulator associated with the second optical fiber (F2) to receive a second signal backscattered by the second optical fiber, the second detection module also being configured to deliver a second detection signal representative of an optical beat signal between the local oscillator and the second backscattered signal.

2. Distributed acoustic detection system according to claim 1, characterized in that the modulation stage (ETMOD) comprises a first modulator (A0M1) configured to shift the frequency of the coherent continuous optical signal by a first offset, followed by a second coupler configured to divide said first branch into said two divisions, each division comprising a second modulator (A0M21, AOM22) driven by the driving means (RF21, RF22) and configured to: • shifting the frequency of the coherent continuous optical signal from the first modulator by a corresponding second shift; and • convert said continuous signal into pulses.

3. Distributed acoustic detection system according to any one of the preceding claims, characterized in that it further comprises a frequency multiplexing module (FDM), connected to the output of at least one circulator (BS1, BS2), and configured to divide again one of the divisions of the first branch into two, by introducing a frequency shift in the pulses injected into at least one new divisions.

4. Distributed acoustic detection system according to any one of the preceding claims, characterized in that it further comprises a time division multiplexing module (TDM), connected to the output of at least one of the circulators (BS1, BS2), and configured to divide again one of the divisions of the first branch into two, by introducing a time shift between the pulses injected into the new divisions.

5. Distributed acoustic detection system according to any one of the preceding claims, characterized in that the heterodyne coherent detection means for analyzing the backscattered signals comprise a first balanced photodiode (PHD1) and a second balanced photodiode (PHD2), respectively connected to the local oscillator (LO).

6. Distributed acoustic detection system according to any one of the preceding claims, characterized in that the system comprises one or more amplifiers (EDFA) downstream of the modulators, configured to independently adjust the amplification levels of the signal(s).

7. A distributed acoustic detection system according to any preceding claim, further comprising a data acquisition card (ADC) connected to the heterodyne coherent detection means, configured to acquire the detection signal generated by the heterodyne coherent detection means.

8. Distributed acoustic detection method for probing at least a first optical fiber (F1) and a second optical fiber (F2), in which: • a laser source (LAS) emits a coherent continuous optical signal in a first branch intended to probe the first and second optical fibers, as well as in a second branch forming a local oscillator (LO); • a modulation stage: • divides said first branch into two, each division being respectively connected to one of the optical fibers to be probed by circulators (BS1, BS2); • shifts the frequency of the coherent continuous optical signal in each division; and • converts said continuous signal into pulses; • control means (RF21, RF22) independently adjust at least one of the parameters of the signals propagating in each division, said parameters being defined by the offset of the optical intermediate frequency, the pulse width, the repetition frequency; • heterodyne coherent detection means (PHD1, PHD2) generate a detection signal representative of signals backscattered by the first and second optical fibers from an optical beat signal between the local oscillator and each of the signals backscattered by the first and second optical fibers.

9. Distributed acoustic detection method according to claim 8, wherein the steps carried out in the modulation stage comprise: • the shifting by a first modulator (A0M1) of the frequency of the coherent continuous optical signal propagating in the first branch, • the division by a coupler downstream of said first modulator of said first branch into two, • in each division of the first branch, the shifting by a second modulator (A0M21, AOM22) of the frequency of the coherent continuous optical signal, in the opposite direction to the direction of the shift carried out by the first modulator, and • the conversion in each division of the continuous signal into pulses by the second modulator (A0M21, AOM22).

10. Distributed acoustic detection method according to any one of claims 8 or 9, in which a frequency multiplexing module (FDM), at the output of at least one of the circulators (BS1, BS2), divides again one of the divisions of the first branch into two, and introduces a frequency shift into the signals of the new divisions.

11. A method of distributed acoustic detection according to any one of claims 8 to 10, in which a time division multiplexing module (TDM), at the output of at least one of the circulators (BS1, BS2), divides again one of the divisions of the first branch into two, and introduces a time shift at the pulse level in the signals of the new divisions.

12. A distributed acoustic detection method according to any one of claims 8 to 11, wherein the heterodyne coherent detection means for analyzing the backscattered signals comprise a first balanced photodiode (PHD1) and a second balanced photodiode (PHD2), respectively connected to the local oscillator (LO).

13. A distributed acoustic detection method according to any one of claims 8 to 12, wherein one or more amplifiers (EDFA) are arranged downstream of the modulators, so as to independently adjust the amplification levels of the signal(s).

14. A distributed acoustic detection method according to any one of claims 8 to 13, wherein a data acquisition card connected to the heterodyne coherent detection means acquires the detection signal generated by the heterodyne coherent detection means.

15. A distributed acoustic sensing method according to any one of claims 8 to 14, wherein the first optical fiber (F1) and the second optical fiber (F2) are deployed along a first and a second railway line respectively.

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