Distributed acoustic detection system and associated detection method

Abstract

The invention relates to a method and a distributed acoustic detection (DAS) system configured to probe at least one first and one second optical fiber. The system probes each fiber with pulses at independently configurable time widths and frequencies and then receives, via special optical receivers, a backscattered signal from each scattering point along 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 one first and a second distinct optical fiber.

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

[0003] The present invention enables the improvement of a vibration measurement device whose sensor includes an optical fiber.

[0004] The invention allows in particular the increase in the number of optical channels interrogated by a single vibration measurement 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 rockfalls, landslides, intrusions), preventive maintenance, railway traffic management, etc. In this context, the system includes an interrogator that connects to optical fibers deployed along the tracks and transforms them into a linear vibration detector. Prior art

[0007] There is no equivalent equipment to the distributed acoustic sensing (DAS) system 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 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, and microphones. These sensors only allow vibration measurement at the location where they are positioned.

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

[0011] The DAS system has been developing for some 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 centers diffusers of 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 railway lines longer than 50 km.

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

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

[0015] The principle used to increase or extend the vibration monitoring / detection capacity is the physical separation / decoupling of the signal emitted on two or multiple 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, by means of 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 to the interrogator.

[0017] Each time a pulse is sent through the fiber, a backscattered signal is recorded. In the absence of vibration, the backscattered signal remains constant. When a vibration reaches the fiber, the backscattered signal is modified, and it is the analysis of this modification that allows the source vibration to be identified.

[0018] In order to accurately record the source vibration, 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 the "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 less than twice the frequency of the vibration, the resulting recording will correspond to a signal that will not be faithful to the vibration, which will be distorted.

[0021] It is then suspected 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 an undistorted vibration is recorded.

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

[0023] Indeed, each time a pulse is sent in the optical fiber to generate the backscattered signal, it is necessary to wait until the pulse has traveled the entire length of the fiber and until all the backscattered signal has returned to the corresponding photodetector before sending another one, in order to prevent the backscattered signals generated by two successive pulses from interfering with each other.

[0024] It follows that the longer the interrogated optical fiber, the more the rate at which the pulses are sent must decrease, thus 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 optical power emitted 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 a very long-range optical fiber up to 50km, the system is forced to impose the constraints due to the longest fiber on the other channel fibers of 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 and 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, however this implies 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 at the expense of the quality of the vibration signal which is recorded: a parameter setting is imposed which limits the performance at best on one of the directions to be monitored.

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

[0032] The invention aims 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 scope 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. about 30% cheaper, a smaller footprint, i.e. about half the space, and better performance since the data from two optical channels arrive on the same acquisition card, so the algorithmic manipulation is greatly facilitated. Description of the invention

[0035] To this end, the invention proposes a distributed acoustic detection system configured to probe at least one first optical fiber and a second optical fiber distinct from the first, the detection system comprising: • a laser source configured to emit a continuous coherent optical signal; • a coupler connected to the laser source and configured to separate the following optical signals: • a first branch intended to feed 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 includes a modulation stage configured to divide said first branch into two separate 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, furthermore, configured to independently modify, for each division, at least one of the parameters of the respective pulses from among: a frequency offset from the local oscillator, a pulse width and a repetition frequency; • The acoustic detection system further includes means for controlling said modulation stage, configured to deliver, to the modulation stage, an indicative control signal, for each division, of at least one parameter to be modified; and • The detection system also includes 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 backscattered signal from 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 backscattered signal 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, incorporating, according to all their possible combinations, the different optional features 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 driven by the control means and configured to: • shift the frequency of the coherent continuous optical signal from the first modulator by a second corresponding shift; and • convert said continuous signal into pulses.

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

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

[0041] Preferably, the distributed acoustic detection system includes 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 and a second optical fiber, in which: • a laser source emits a continuous coherent 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 the frequency of the coherent continuous optical signal in each division; 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 backscattered signals from the first and second optical fibers from an optical beat signal between the local oscillator and each of the backscattered signals from the first and second optical fibers.

[0044] Various embodiments of the invention are provided, incorporating, according to all their possible combinations, the different optional features 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, • the division of said first branch into two by a coupler downstream of said first modulator, • in each division of the first branch, the frequency shift of the coherent continuous optical signal by a second modulator, in the opposite direction to the shift performed 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, again divides one of the divisions of the first branch into two, and introduces a frequency shift in the signals of the new divisions.

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

[0048] According to a certain particular aspect, the heterodyne coherent detection means for analyzing backscattered signals include a first balanced photodiode and a second balanced photodiode, connected respectively 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 and second optical fibers are deployed respectively along a first and a second railway line. Brief description of the figures

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

[0053] [Fig-1] [Fig.1] is an illustrative diagram of one 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 from the ADC data acquisition card as well as the backscattered signals for the two fibers used with optimal parametric settings;

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

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

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

[0058] [Fig.6] [Fig.6] is an illustrative diagram of one 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 ramifications allowing the number of fibers to be probed to be multiplied.

[0060] It is understood that the embodiments described below are in no way limiting. In particular, variants of the invention may be conceived comprising only a selection of the features described below, isolated from the other features described, if this selection of features is sufficient to confer a technical advantage or to differentiate the invention from the prior art. This selection includes at least one preferably functional feature without structural details, or with only a portion of the structural details if this portion alone is sufficient to confer a technical advantage or to differentiate the invention from the prior art.

[0061] In particular, all the variants and embodiments described are combinable with each other if there is no technical obstacle to this combination.

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

[0063] With a conventional DAS system with two separate fibers, physically at the system output, 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 parameters on both probed fibers, i.e., the same repetition frequencies (Frep), spatial resolution (RES), optical intermediate frequency (IF), and output amplification level. This restricts the system parameters to the limits determined by the longer fiber. The shorter fiber is therefore not used optimally.

[0064] Conversely, the DAS system and method according to the invention are particularly well-suited to railway applications requiring the monitoring of two lines or two directions of the same line, the ends of which are located in one of the multiple telecom rooms connecting railway stations. Instead of installing two DAS systems, it becomes possible to operate only one to instantly monitor two railway lines, with parameters adapted for each fiber monitored.

[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 parameterizable time widths and frequencies and then receive, by special optical receivers, a backscattered signal from each scattering point along the probed fibers.

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

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

[0068] The amplification level at the input of the fiber under test compensates for the linear attenuation losses of the optical fiber. The amplification level is limited by the appearance of cumulative nonlinear phenomena with distance. These phenomena, which must be avoided, are due to thermal agitation caused by high incident optical powers. The greater the distance, the lower the amplification level of the emitted pulse must be.

[0069] The spatial resolution (RES) of vibration detection is determined by the width of the emitted pulse. For long ranges, increasing the pulse width allows for an increase in the amount of emitted energy and compensates 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 detected at the interrogator receiver. Its value affects the choice of sampling frequency for 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 transmission frequency, allows sampling of 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 fiber length, since it is necessary to wait for the backscattered signal to return from the end of the fiber under test before transmitting another "probe" pulse; 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.1] comprises a laser source LAS, a coupler, an ETMOD modulation stage, circulators BS1, BS2, means for driving the modulation stage and means for heterodyne coherent detection.

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

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

[0075] In addition, each circulator has 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 sensing 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 continuous, coherent optical signal.

[0078] As previously stated, the coupler is connected to the output of the laser source. Furthermore, the coupler is configured to separate the optical signal: • following a first branch intended to feed 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 previously stated, the first branch includes an ETMOD modulation stage.

[0081] The ETMOD modulation stage 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 is intended to be connected respectively 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 continuous optical signal from the laser source into pulses.

[0083] In addition, 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 offset belonging to a range between 0 Hz (hertz) and 60 MHz (megahertz).

[0085] Such a range is advantageous, insofar as it avoids imposing too high an acquisition frequency on an ADC acquisition card (described later) connected at 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, an indicative control signal, for each division, of at least one parameter to be modified.

[0087] In addition, 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 deliver 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 deliver 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 of a 50 / 50 coupler, on one side to the local LO oscillator, and on the other side to the respective circulator.

[0093] Such an arrangement is advantageous, insofar as 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.1], the modulation stage comprises a first modulator A0M1, a coupler and two second modulators A0M21, AOM22.

[0096] More specifically, the first modulator A0M1 is configured to apply a first frequency shift to the optical signal from the laser source LAS.

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

[0098] Finally, each division includes a second modulator AOM21, AOM22 respectively. 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. In addition, 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 that propagate there.

[0100] In such an architecture, the first modulator A0M1 and the second modulators A0M21, A0M22 are driven to apply opposite frequency shifts to the optical signal passing through them. 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 allows us to take advantage of the short response times of modulators operating at high frequency (for example, around 200 MHz), while limiting the frequency of optical beats at the detection module level, which avoids imposing too high an acquisition frequency on an acquisition card.

[0102] The RF21, RF22 control means of said second modulators are configured to independently control at least one of the following signal parameters: frequency offsets, pulse width, optical intermediate frequency.

[0103] As shown in the exemplary embodiment in [Fig. 1], the control means comprise tunable drivers for controlling a frequency shift of the first and second modulators within a frequency range of ±30 MHz around the 200 MHz center frequency of said modulators, thus enabling independent control of the optical intermediate frequency IF within a range from 0 to 60 MHz. This value is chosen according to the limiting spatial resolution RES or selected for a given application.

[0104] Note that it is possible to apply the same optical intermediate frequency IF for both channels since they 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 allows the pulse width at the input of these drivers to be adjusted separately for each of the two fibers to be probed, which corresponds to independent control of the spatial resolutions RES.

[0106] Advantageously, the system represented in [Fig.1] includes 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 occurrence of non-linear phenomena.

[0107] In order to multiply 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 FDM (Frequency Division Multiplexing) and TDM (Time Division Multiplexing) multiplexing and demultiplexing principles.

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

[0110] As illustrated by the figure, the FDM module includes 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 capable of being modified in optical frequency according to a new frequency shift Acol2, by means of a modulator (such as than an acousto-optical modulator). More precisely, a circulator loop, combining two secondary circulators, is used.

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

[0114] In this way, the pulsed signal applied to the second port of the first 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 level of 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 from the two divisions 1.1 and 1.2 are mixed and received simultaneously at the main circulator, but they are separable in the frequency domain. The data around each optical frequency correspond 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] Figure 4 illustrates the structure of an example TDM module, the addition of which in the The DAS system represents a second improvement.

[0118] As illustrated in the figure, the TDM module includes a main circulator connected downstream, via its second port, to a coupler (preferably a 50 / 50 coupler) to split the optical signal from the circulator in two. Each split includes 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 split in two. This module allows the two signals to be separated 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 emitted on channel 1 of the DAS is thus split into two parts. The paths of the pulse and backscattered signals are separated by the circulator loops. The emitted pulse signal passes instantaneously to the input of the two modulators via the 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 enabling 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 odd-numbered pulse data correspond to channel 1.1 and the even-numbered pulse data 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, optical switches are implemented by means of modulators, in particular electro-optical modulators (or OEMs, from the English "Electro-Optic Modulator") as optical switches. This is advantageous because such modulators exhibit 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 both FDM and TDM modules, optical amplifiers (for example, erbium-doped fiber amplifiers, known as "EDFAs," for "Erbium Doped Fiber Amplifier") can be added downstream of the modulators and / or optical switches. This feature is advantageous because it allows for compensation of insertion losses.

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

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

[0126] As can be seen from this figure, in the case of FTDM frequency- and time-division multiplexing, it is possible to simultaneously query 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] Regarding the distributed acoustic detection method, which implements the DAS system according to the invention, this method includes the following steps.

[0129] The LAS laser source emits a continuous coherent 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 LO oscillator.

[0130] The signal arriving in the second branch forming a local LO oscillator is then divided in two by a PM 50 / 50 coupler in order to ensure optical beat 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 from the first branch at 25% is modulated by the modulation stage to form optical pulses and, preferably, apply an optical intermediate frequency shift IF.

[0133] More specifically, in the example of [Fig.1], the first modulator A0M1 shifts the frequency of the coherent continuous optical signal, then the signal is split in two, each division being respectively connected to one of the optical fibers to be probed by circulators BS1, BS2. The second modulators A0M21, A0M22 shift the frequency of the signals from the divisions again and convert them into pulses.

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

[0135] In each of the divisions of the first branch, the second modulator AOM2 allows the continuous light to be modulated into pulses and creates a frequency shift in the opposite direction to that introduced by the first modulator AOM1.

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

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

[0138] Note that it is possible to apply the same optical intermediate frequency IF for both fibers since they 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 allows the pulse width at the input of these drivers to be adjusted separately for each of the two fibers to be probed, which corresponds to independent control of the spatial resolutions RES.

[0140] Each amplifier EDFA1, EDFA2 allows independent adjustment of the amplification level associated with the Fl fiber channel and the F2 fiber channel.

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

[0142] Advantageously, and in order to be able to apply a higher repetition frequency to the shorter fiber, a burst transmission mode is chosen, allowing a given number of pulses to be sent for a single synchronization signal, and consequently acquiring several backscattered signals simultaneously on this channel. Therefore, a maximum repetition frequency is chosen independently for each of the two channels.

[0143] Preferably, the repetition frequency is adjusted according to the total number of fibers obtained by multiplexing.

[0144] As for the processing of backscattered signals received by the DAS 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 allowing only the signal to be retained 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 amplitude and phase extraction is performed digitally for each fiber by a digital modulation IQ, i.e. with a signal brought back to baseband and transformed into a complex number I+jQ, and an appropriate 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 length respectively of 5km and 20km, this system would be limited by the longer fiber, i.e. the one of length of 20 km, 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] Consequently, the 5 km long fiber has experienced the limitations of the 20 km fiber. Furthermore, the optical intermediate frequency IF is constant and fixed during the system design phase, resulting in less flexibility for 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 Frep repetition rate of 5 kHz is imposed by the longer fiber. Internal synchronization of the ADC data acquisition card with its internal clock is then set to 5 kHz. A burst mode is applied to the shorter 5 km fiber, allowing the transmission of four pulses separated by 50 µs (trigger period divided by 4) for each trigger. This multiplies the Frep repetition rate of this fiber by 4 and allows maximum accuracy to be achieved with a Frep repetition rate of 20 kHz. A much finer resolution can be applied over the 5 km distance (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 and the distributed acoustic detection method make it possible in particular to monitor instantly and independently two fibers with identical or different spans while preserving an optimal balance between the main performance characteristics of the system for each of these two fibers.

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

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

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

Claims

Demands

1. A distributed acoustic detection (DAS) system configured to probe at least one distinct first optical fiber (F1) and second optical fiber (F2), the detection system comprising: • a laser source (LAS) configured to emit a continuous coherent optical signal; • a coupler connected to the laser source and configured to separate the following optical signals: • a first branch intended to feed each of the first and second optical fibers to be probed; and • a second branch forming a local oscillator (LO); the first branch comprising a modulation stage (ETMOD) configured to divide said first branch into two separate 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, furthermore, configured to independently modify, for each division, at least one of the parameters of the respective pulses from among: a frequency offset from the local oscillator, a pulse width and a repetition frequency; the acoustic detection system including, furthermore, means for controlling said modulation stage, configured to deliver, to the modulation stage, an indicative control signal, for each division, of at least one parameter to be modified; and the detection system including, furthermore, 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) to receive a first backscattered signal 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 backscattered signal 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.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 shift, followed by a second coupler configured to divide said first branch into said two divisions, - each division comprises a second modulator (A0M21, AOM22) driven by the control means (RF21, RF22) and configured to: - shift the frequency of the coherent continuous optical signal from the first modulator by a corresponding second shift; and - convert said continuous signal into pulses.

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

3. A distributed acoustic detection system according to any one of the preceding claims, characterized in that it comprises, In addition, a time-division multiplexing (TDM) module, connected at the output of at least one of the circulators (BS1, BS2), and configured to split one of the divisions of the first branch in two again, by introducing a time offset between the pulses injected into the new divisions.

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

5. 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).

6. Distributed acoustic detection system according to any one of the preceding claims, 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.

7. A distributed acoustic detection method for probing at least one first optical fiber (F1) and one second optical fiber (F2), wherein: • a laser source (LAS) emits a coherent continuous optical signal in a first branch intended to probe the first and second optical fibers, and in a second branch forming a local oscillator (LO); • a modulation stage: • divides said first branch in 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.And in which the steps carried out in the modulation stage include: • the shift 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 shift by a second modulator (A0M21, AOM22) of the frequency of the coherent continuous optical signal, in the opposite direction to 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).

8. Distributed acoustic detection method according to claim 7, wherein a frequency multiplexing (FDM) module, at the output of at least one of the circulators (BS1, BS2), again divides one of the divisions of the first branch in two, and introduces a frequency shift in the signals of the new divisions.

9. A distributed acoustic detection method according to any one of claims 7 to 8, wherein a time-division multiplexing (TDM) module, at the output of at least one of the circulators (BS1, BS2), again divides one of the divisions of the first branch in two, and introduces a time offset at the pulse level in the signals of the new divisions.

10. A distributed acoustic detection method according to any one of claims 7 to 9, wherein the heterodyne coherent detection means for analyzing backscattered signals comprise a first balanced photodiode (PHD1) and a second balanced photodiode (PHD2), connected respectively to the local oscillator (LO).

11. A distributed acoustic detection method according to any one of claims 7 to 10, wherein one or more amplifiers (EDFA) are arranged downstream of the modulators, so as to independently adjust the amplification levels of the signal(s).

12. A distributed acoustic detection method according to any one of claims 7 to 11, wherein a data acquisition card connected to the heterodyne coherent detection means acquires the detection signal generated by the heterodyne coherent detection means.

13. A distributed acoustic detection method according to any one of claims 7 to 12, wherein the first optical fiber (F1) and the second optical fiber (F2) are deployed respectively along a first and a second railway line.

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