Optical repeater

Abstract

The invention relates to an optical repeater (100) comprising: an upstream circulator (102), provided for coupling said repeater (100) to an upstream optical fiber (104), a downstream circulator (106), provided for coupling said repeater (100) to a downstream optical fiber (108), a first branch (110) connecting said upstream circulator (102) to said downstream circulator (106) and allowing the passage of a measurement wave from said upstream circulator (102) to said downstream circulator (106), a second branch (112) connecting said downstream circulator (106) to said upstream circulator (102) and allowing the passage of a backscattered wave from said downstream circulator (106) to said upstream circulator (102), said first branch (110) comprises a frequency modulator (114) arranged to shift in frequency an upstream measurement wave, coming of said upstream circulator (102) so as to provide said downstream circulator (106) with a downstream measurement wave of different frequency. Figure for the abstract: Fig. 1

Description

Title of the invention: Optical repeater Technical field

[0001] The present invention relates to an optical repeater used in optical measurement systems in infrastructure monitoring, such as railway monitoring.

[0002] The field of the invention is the field of vibration measuring devices using optical repeaters, in particular in the railway field. State of the art

[0003] Devices are known for measuring vibration in areas to be monitored, such as along railway tracks.

[0004] In particular, in the railway field, these systems are known as DAS systems, from the English Distributed Acoustic Sensing (DAS) and consist of using the sensitivity of optical fibers to acoustic vibrations. The principle consists of sending an optical wave into a measuring fiber and recording a backscattered signal of this wave by the internal defects of the measuring fiber. If the measuring fiber does not undergo any vibration, the backscattered signal is always the same. When a vibration reaches the measuring fiber, it varies the backscattered signal. The use of the backscattered signal makes it possible to trace the vibration of a physical phenomenon, for example the passage of a train. In these measuring systems, the detection is linear, and a single measuring fiber can advantageously replace a plurality of point sensors.However, the maximum length that the DAS system can monitor is not infinite and is typically limited to 50 km. To be able to use the DAS system over greater monitoring distances, optical amplifiers, known as optical repeaters, are used to extend the monitoring distance beyond 50 kilometers.

[0005] Current optical repeaters are functional but pose various problems.

[0006] Indeed, to be able to accurately record a phenomenon of interest creating acoustic vibrations, it is necessary to use measurement pulses comprising high sending frequencies (also called repetition frequencies). The higher the sending frequency of the measurement pulses, the more precise the vibration signal recorded, via the backscattered signal, will be because it is more sampled, and the more the high-frequency components of the vibration signal can be recorded. Indeed, the repetition frequency of the measurement pulses will limit the maximum detected frequency of the vibration signal. According to the Nyquist / Shannon law, the maximum frequency of the vibration signal that can be recorded is equal to half the frequency of repetition of the measurement pulses. Thus, if, for example, the phenomenon of interest includes a vibration phenomenon that has a high frequency (i.e. very rapid vibrations over time) and the measurement pulses are sent at a rate less than twice the frequency of the vibration, then the recording obtained will correspond to a signal that will not be faithful to the vibration but will be distorted because the high (maximum) frequencies of the vibration phenomenon cannot be recorded.

[0007] It is not possible to arbitrarily increase the frequency at which the pulses are sent. The sending frequency depends on the length of the fiber. In fact, 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 there is no other backscattered signal in the optical fiber in which the sending frequency is emitted before sending another one, and this is to prevent the backscattered signals generated by two successive pulses from interfering with each other.

[0008] The invention aims to overcome the aforementioned drawbacks.

[0009] In particular, one aim of the invention is to propose an optical repeater capable of working with high measurement pulse repetition frequencies.

[0010] Another aim of the invention is to limit interference between the signals of the vibratory phenomenon resulting from two successive pulses.

[0011] Another object of the invention is to propose an optical repeater making it possible to record vibration signals comprising high maximum frequencies, thus making it possible to improve the accuracy of the DAS system. Statement of the invention

[0012] The invention makes it possible to achieve at least one of the aforementioned aims by an optical repeater comprising: - a circulator, called upstream, designed to couple said repeater to an optical fiber, called upstream, - a circulator, called downstream, designed to couple said repeater to an optical fiber, called downstream, - a first branch connecting said upstream circulator to said downstream circulator and allowing the passage of an optical wave, called a measuring wave, from said upstream circulator to said downstream circulator, - a second branch connecting said downstream circulator to said upstream circulator and allowing the passage of an optical wave, called a backscattered wave, from said downstream circulator to said upstream circulator. The first branch of the optical repeater according to the invention comprises at least one frequency modulator arranged to shift the frequency of an optical wave, called a wave upstream measurement, coming from said upstream circulator so as to provide said downstream circulator with an optical wave, called downstream measurement wave, of different frequency.

[0013] The repeater according to the invention performs a frequency shift of the optical measurement wave, when it passes through it to pass from the upstream optical fiber to the downstream optical fiber. Thus, the downstream measurement wave and the upstream measurement wave are of different frequencies. Consequently, any backscattered wave coming from the downstream measurement wave will have a different frequency from the upstream measurement wave, so that the backscattered wave coming from a downstream optical fiber does not interfere with the backscattered wave coming from the upstream measurement wave circulating in the upstream optical fiber. The frequency shift of the optical repeater according to the invention therefore allows a downstream backscattered optical wave to be able to circulate at the same time as the upstream measurement backscattered wave, without interfering with it.The frequency shift of the optical repeater according to the invention also allows a downstream backscattered optical wave to be able to circulate at the same time as the upstream measurement wave, without interfering with it, in the upstream optical fiber.

[0014] The repeater according to the invention makes it possible to use high measurement wave pulse repetition frequencies. Indeed, it is not necessary to wait for a return of backscattered wave from the downstream fiber to the upstream fiber to send a new measurement wave to the said upstream fiber.

[0015] Furthermore, the use of high measurement pulse repetition frequencies makes it possible to capture high frequencies of a vibration signal which makes it possible, for a given measurement distance, to improve the accuracy and the detection bandwidth, compared to current state-of-the-art systems.

[0016] The optical repeater according to the invention makes it possible to limit the deterioration or loss of the measured vibration signal.

[0017] The repeater according to the invention may comprise several modulators each producing a frequency shift of a given value, said value being able to be identical or different for two modulators.

[0018] Therefore, it is possible to combine several frequency modulators to choose a desired frequency shift.

[0019] The frequency modulators can be connected in series. In this case, the total frequency shift is equal to the sum of the frequency shifts of each frequency modulator.

[0020] The frequency shift achieved by each frequency modulator can be between 40 MHz and 250 MHz, for example equal to 40 MHz, or 80 MHz, or 110 MHz, or 200 MHz, or 250 MHz.

[0021] The measuring wave may be an infrared wavelength, for example at the central wavelength of 850 nanometers (nm), or 1300 nm, or 1310 nm, or 1500 nm, or preferably 1550 nm. In another variant, the measuring wave may comprise a wavelength in the visible, for example at 700 nanometers or 600 nanometers. In another variant, the measuring wave may comprise a wavelength in the Ultraviolet.

[0022] The frequency modulator may comprise, or may be, an Acousto-Optic Modulator (AOM).

[0023] Consequently, such a modulator retains the optical properties of the light beam (or light beam) and thus limits its degradation during its passage through said repeater according to the invention.

[0024] In one variant, the frequency modulator may comprise an electro-optical modulator such as a silicon Mach-Zehnder modulator.

[0025] The acousto-optic modulator may be arranged to provide a constant offset. This constant offset may be set at the time of manufacturing the acousto-optic modulator.

[0026] The repeater according to the invention may comprise a driver arranged to control an operating mode of the at least one frequency modulator.

[0027] The operating mode of the frequency modulator may comprise a continuous mode and / or a pulse mode.

[0028] Such an arrangement makes it possible to control the operating mode of the frequency modulator.

[0029] When the driver controls the pulse mode of the frequency modulator, the driver can be arranged to: - form pulses at the output of the frequency modulator, i.e. downstream pulses, even if the upstream wave is continuous (i.e. wave at the input of the frequency modulator), and - adjust the width of the downstream pulses and the repetition frequencies of the downstream pulses at the output of the frequency modulator.

[0030] The driver can also put the frequency modulator in continuous mode. In this case there is no modification to the shape or repetition frequency of the upstream pulses. Preferably, the driver is set to control continuous mode.

[0031] . The driver can provide a Radio-Frequency signal (having the offset frequency specified by the manufacturer) at a given power in order to generate the frequency shift of the frequency modulator.

[0032] The driver can control several frequency modulators, preferably when the frequency shift achieved by these frequency modulators is identical. This makes it possible in particular to avoid high consumption of the electrical current supplying the driver and the frequency modulators.

[0033] The optical repeater according to the invention may comprise several drivers, preferably when the frequency shift achieved by these frequency modulators is different.

[0034] The frequency modulator according to the invention can be adjustable so as to adjust the frequency shift that it applies to the upstream measurement wave.

[0035] The adjustment of the offset can be carried out continuously. Alternatively, the adjustment of the offset can be carried out discretely according to a given step, or by selecting a given value from a plurality of predetermined values ​​stored in the repeater according to the invention.

[0036] In a variant, the adjustable frequency modulator may be in the form of an optical comb (frequency comb) tunable using at least one electro-optical modulator, for example with a silicon Mach-Zehnder modulator. By way of non-limiting example, the optical comb may comprise 12 lines spaced 100 MHz apart. In addition, this variant may comprise a polarization maintenance of the at least one electro-optical modulator.

[0037] However, this variant may be more complex and more cumbersome compared to the use of an AOM type frequency modulator in the optical repeater according to the invention.

[0038] The frequency modulator may be arranged to positively and / or negatively shift the frequency of the upstream measurement wave.

[0039] Thus, each frequency modulator can achieve a positive and / or negative shift.

[0040] Consequently, it is possible to modulate, depending on the need, the frequency shift in a positive or negative manner. The optical repeater according to the invention thus makes it possible to achieve multi-frequency shifts.

[0041] The optical repeater according to the invention may comprise several frequency modulators connected in series.

[0042] The frequency shift by each frequency modulator may be different and / or similar.

[0043] Such a repeater makes it possible to modulate the frequency shift to be achieved more precisely.

[0044] In a non-limiting manner, by different frequency shift or different frequency shift is meant a frequency shift or a frequency shift of different absolute value and / or of opposite sign.

[0045] Thus, the optical repeater according to the invention can comprise combinations of frequency modulators having different frequency offsets.

[0046] The first branch of the repeater according to the invention may comprise a first optical wave amplifier. This first amplifier makes it possible to amplify at least an optical wave traveling along the first branch, and in particular the upstream measurement wave or the downstream measurement wave.

[0047] Thus, the device according to the invention can increase the measurement detection length without reducing the maximum frequency of the acoustic bandwidth of the vibration signal. Indeed, in the known state of the art, increasing the detection range involves dividing the repetition frequency of the measurement pulses proportionally to the increase in the range, which also involves a (proportional) reduction in the maximum frequency that can be detected. The device according to the invention makes it possible to overcome this drawback.

[0048] The detection range may depend on the length of the upstream and downstream optical fibers. Each optical fiber may have a similar range, i.e., comprising an identical fiber length or a different range.

[0049] The device according to the invention thus allows an increase in the maximum detection length of the system without deteriorating the measured vibration signal and without losing part of its bandwidth.

[0050] The optical repeater according to the invention therefore has two characteristics: a first allowing it to amplify a light wave and a second allowing it to carry out multi-frequency shifts.

[0051] The frequency modulator is preferably positioned after the first amplifier.

[0052] The frequency modulator may be arranged to eliminate noise in an optical wave traveling along said first branch.

[0053] The second branch may comprise a second optical wave amplifier. This second amplifier makes it possible to amplify at least one optical wave traveling along the second branch, and in particular at least one backscattered optical wave coming from the downstream optical fiber.

[0054] Each amplifier may be arranged to amplify the optical wave power entering said amplifier (for example the upstream wave or the backscattered wave) while retaining its spectral properties, in particular its polarization, its wavelength and its transverse shape. Thus, the deterioration of the optical wave at the output of each amplifier is limited.

[0055] The first optical amplifier and the second optical amplifier may be similar or different.

[0056] The first amplifier and the second amplifier make it possible to limit the linear losses introduced by the optical fibers, for example the upstream and downstream optical fibers.

[0057] The first and second amplifiers, and the driver may be active components. They may thus be powered by electrical sources or power supplies. Direct Current (DC) and / or Alternating Current (AC). The power supply of each active component may be similar or different for all components. The power supply(s) of the active components may be external to said repeater or belonging to the optical repeater according to the invention.

[0058] For example, the first and second optical amplifiers may be powered by a common source delivering an alternating electrical signal. The driver may be powered by a common source delivering a direct electrical signal.

[0059] Each amplifier may preferably be positioned before a frequency modulator or after a frequency modulator.

[0060] The second branch may comprise an optical filter for eliminating noise in an optical wave traveling through said second branch.

[0061] Thus, the repeater according to the invention makes it possible to eliminate, for example, the additive noise introduced by the second amplifier. The backscattered optical wave, which will be studied, will therefore be less distorted. Consequently, the information extracted from the backscattered optical wave will be more reliable and more precise.

[0062] The optical filter may be a passive element, i.e. one which does not require power.

[0063] According to another aspect of the invention, there is provided an optical measuring system comprising: - an optical measuring device comprising a source emitting at least one optical wave, called a measuring wave, and a backscattered wave(s) sensor, and - a first optical fiber, called the first measuring optical fiber, connected to said measuring device, and at least one sensor block, arranged in series with said first optical measuring fiber, and each comprising: - an optical fiber, called the second measuring optical fiber, and - a repeater according to the invention arranged between said second optical fiber measuring and, said first measuring optical fiber, or the second measuring optical fiber of a previous sensor block.

[0064] The system according to the invention makes it possible to propose a vibration measurement system which can contain a multitude of repeaters according to the invention.

[0065] Thus, each sensor block makes it possible to extend the detection zone of said optical measurement system while retaining the advantages provided by the optical repeater(s) according to the invention.

[0066] Thus, such a system makes it possible to cover larger detection zones / distances. The detection range is therefore increased without losing measurement quality because, in particular, the high frequencies of the vibration signal that one seeks to measure are preserved, the bandwidth is not reduced even with a significant increase in the detection range and high pulse frequency waves can be used without disturbing the signals circulating within the system according to the invention.

[0067] The measuring waves and / or the downstream waves and / or the upstream waves can be controlled manually or by a control means controlled by an electronic or computer device.

[0068] The controlled parameters can be: - the widths of the pulses emitted through the upstream and / or downstream fibers, and / or - their repetition frequencies.

[0069] These parameters can be controlled at the level of a DAS system during the formation of the pulse.

[0070] The optical wave source may be multi-frequency, or preferably single-frequency. The optical wave source may be controlled manually or by a control means to adjust the widths of the emitted pulses and / or their repetition frequencies. The optical wave source may have a line width less than or equal to 1 kilohertz centered on the central emission frequency.

[0071] The system according to the invention can therefore detect several events creating vibrations in the detection zone.

[0072] The vibration events can occur simultaneously or at different times. In this case, the system can record a single backscattered signal for each upstream or downstream fiber. The envelope of this backscattered signal for each upstream or downstream fiber varies locally if a vibration occurs at a given position. The vibration does not impact the optical frequency of the backscattered signal but generates localized variations on the instantaneous amplitude and phase of the backscattered signal.

[0073] If the two vibration events are at a very close distance and occur simultaneously, the system according to the invention is capable of distinguishing them because the upstream or downstream optical pulses can be adjusted to be sufficiently narrow (i.e. very high resolution), for example with the driver of the optical repeaters and / or with the control of the optical source of the measuring device, to separate them well locally.

[0074] The system according to the invention may comprise several sensor blocks connected in series to each other. In this case, at least one sensor block may comprise a repeater identical to or different from the repeater of at least one other sensor block.

[0075] Thus, it is possible to combine several different sensor blocks, for example some being able to achieve a positive frequency shift and others being able to achieve a negative frequency shift.

[0076] Therefore, even if the optical measurement system comprises a multitude of sensor blocks each involving a phase shift, it is possible to maintain the same bandwidth of the current optical measurement system.

[0077] Thus the detection range is increased by adding each additional sensor block while maintaining the detection bandwidth of the measurement system according to the invention.

[0078] According to another aspect of the invention, there is provided a railway track comprising at least one optical measuring system according to the invention.

[0079] The optical measurement system may be adapted to cover a detection area greater than 50 kilometers (km), preferably greater than 100 kilometers.

[0080] The optical measurement system can also be used to increase the bandwidth of the detected vibration signal for a detection zone of given length.

[0081] The measurement system also makes it possible to increase the detection zone without deteriorating the bandwidth of the vibration signal. Brief description of the drawings

[0082] Other advantages and characteristics will appear on examining the detailed description of a non-limiting embodiment, and the appended drawings in which:

[0083] [Fig-1] [Fig. 1] is a schematic representation of a first non-limiting example embodiment of an optical repeater according to the invention;

[0084] [Fig.2] [Fig.2] is a first schematic representation of another non-limiting example of an optical repeater according to the invention;

[0085] [Fig.3] [Fig.3] is a schematic representation of another non-limiting exemplary embodiment of an optical repeater according to the invention;

[0086] [Fig.4] [Fig.4] is a schematic representation of another non-limiting exemplary embodiment of an optical repeater according to the invention;

[0087] [Fig.5] [Fig.5] is a representation of an example of an optical measurement system comprising at least two optical repeaters according to the invention;

[0088] [Fig.6] [Fig.6] is a representation of another example of an optical measuring system. Detailed description of the figures

[0089] 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 characteristic preferably functional without structural detail, or with only part of the structural details if this part only is sufficient to confer a technical advantage or to differentiate the invention from the state of the prior art.

[0090] 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.

[0091] In the figures, the elements common to several figures retain the same reference.

[0092] [Fig.l] is a schematic representation of a first non-limiting example embodiment of an optical repeater 100 according to the invention.

[0093] The optical repeater 100 is, in a non-limiting manner, a fiber device, that is to say that all of the optical components of the optical repeater 100 are connected to each other by optical fibers.

[0094] The optical repeater 100 comprises: - a circulator 102, called upstream circulator 102, connected to an optical fiber 104, called upstream optical fiber 104, - a circulator 106, called downstream circulator 106, connected to an optical fiber 108, called downstream optical fiber 108, - a first branch 110 connecting said upstream circulator 102 to said downstream circulator 106 and authorizing the passage of an optical wave A0, called upstream measurement wave A0, from said upstream circulator 102 to said downstream circulator 106, - a second branch 112 connecting said downstream circulator 106 to said upstream circulator 102 and authorizing the passage of an Aldiff optical wave, called Aldiff backscattered wave, from said downstream circulator 106 to said upstream circulator 102.

[0095] The measuring wave A0 comes from the upstream optical fiber 104.

[0096] The first branch of the optical repeater 100 comprises a frequency modulator 114. The frequency modulator 114 is arranged to shift the frequency of the upstream measurement wave, coming from said upstream circulator 102, so as to provide said downstream circulator 106 with an optical measurement wave, called the downstream measurement wave, of a different frequency.

[0097] Alternatively, the first branch of the optical repeater 100 also comprises a first optical amplifier 118. Preferably, the first optical amplifier 118 is positioned before the frequency modulator 114.

[0098] In the optical repeater 100 illustrated in [Fig.l], the first optical amplifier 118 is connected at a first end to the upstream circulator 102 and at a second end to the frequency modulator 114. The frequency modulator 114 is connected to the first amplifier 118 at a first end and to the downstream circulator 106 at a second end.

[0099] The optical amplifier 118 is arranged to amplify the measurement wave AO coming from the upstream circulator 102 before transmitting it to the frequency modulator 114.

[0100] The upstream measurement wave is the wave at the input of the frequency modulator 114, i.e. a wave having the same optical frequency as the wave circulating in the upstream fiber 104.

[0101] Similarly, the downstream measurement wave is called the wave at the output of the frequency modulator 114, i.e. a wave having a frequency different from the wave circulating in the upstream optical fiber 104.

[0102] Thus, in the optical repeater 100 illustrated in [Fig.l], it is considered that the measurement wave A0 and the upstream measurement wave are merged.

[0103] The upstream measurement wave A0 comprises an optical frequency fO. The upstream wave is an optical pulse.

[0104] The upstream measurement wave A0 arrives in the optical repeater 100 by the upstream circulator 102 via the upstream optical fiber 104. The upstream measurement wave A0 then circulates in the first branch 110 of the optical repeater 100 passing through the optical amplifier 118 to amplify the upstream measurement wave A0. The amplified upstream measurement wave A0 is then transmitted to the frequency modulator 114 which will carry out a frequency shift Af of the frequency fO of the amplified upstream measurement wave A0, for example 110 Megahertz (MHz). The wave at the output of the frequency modulator 114 will therefore have a frequency fi = fO + Af.

[0105] At the output of the frequency modulator 114, the frequency-shifted wave, called the downstream AOShift wave, will then reach the downstream circulator 106 to be transmitted to the second optical fiber 108.

[0106] In the downstream optical fiber 108, downstream measurement base A0shift will encounter structural defects internal to the downstream optical fiber 108. Thus, a part of downstream optical base A0Shift will: - be transmitted according to a wave, called in the following the transmitted wave AL The transmitted wave Al will continue its propagation in the downstream optical fiber 108, and - a second part will be backscattered by the structural defects included in the downstream optical fiber 108 and will return in the direction of the downstream circulator 106.

[0107] The transmitted Fonde backscattered wave Al is noted Aldiff. The backscattered wave Aldiff is therefore obtained by downstream Fonde backscattering A0shift, at the output of the modulator of frequency 114, in the downstream optical fiber 108. The backscattered wave Aldiff circulates in the opposite direction to the downstream wave A0shift. The backscattered wave Aldiff then passes into the downstream circulator 106 and into the second branch 112 of the optical repeater 100 before reaching the upstream circulator 102. The backscattered wave Aldiff then passes into the upstream optical fiber 104.

[0108] Thus, the backscattered wave Aldiff circulates in a direction opposite to the upstream measurement wave A0 and the downstream measurement wave A0shift. The backscattered wave Aldiff circulates in a branch of the optical repeater 100 different from that in which the upstream measurement wave A0 circulates.

[0109] In a variant not illustrated in [Fig. 1], the measurement wave A0 coming from the upstream optical fiber 104 may encounter structural defects included in the upstream optical fiber 104 and therefore a portion of this wave may be backscattered. Since the backscattered wave coming from the measurement wave A0 will not have the same frequency as the backscattered wave Aldiff, the two backscattered waves will not be able to interfere with each other.

[0110] The Aldiff backscattered wave may comprise a Raman or Brillouin wave, preferably a Rayleigh wave. By "backscattered wave" is meant a wave resulting from the phenomenon of light scattering.

[0111] According to a non-limiting exemplary embodiment, the upstream measurement wave A0 may be an infrared optical wave, for example at the central wavelength of 1550 nanometers (nm), i.e. a frequency f0 of approximately 193.5 THz (optical frequency of the upstream measurement wave A0 given according to the electromagnetic spectrum). The upstream measurement wave A0 is a laser pulse.

[0112] The first amplifier 118 of the optical repeater 100 is arranged to amplify the power of the measuring wave A0 while preserving its spectral properties, in particular its polarization, its wavelength and its transverse shape.

[0113] Thus, the backscattered wave Aldiff is an infrared optical wave, shifted in frequency from the upstream measurement wave A0 by 110 Megahertz (Af = 110 Megahertz, fi = fO +Af).

[0114] The gain of the first amplifier 118 is between 10 decibels (dB) and 50 decibels (dB). The first amplifier 118 comprises for example an amplification bandwidth of 85 nm centered around the measurement wavelength A0, i.e. 1550 nm.

[0115] The frequency modulator may include a high extinction ratio (about 50 decibels (dB)) around the useful wavelength (1550 nm) so as to suppress the broad spectrum amplified additive noise of the first optical amplifier 118.

[0116] Therefore, the frequency modulator is arranged to limit the noise introduced by the amplification of the measuring wave AO. Therefore, the degradation of the light wave is reduced.

[0117] The first optical amplifier 118 may be an Erbium Doped Optical Amplifier (EDFA). In this way, the length of the downstream fiber 108 may be greater, which increases the detection range. The frequency modulator 114 may comprise an acousto-optic modulator.

[0118] In a non-limiting manner, the first amplifier 118 and the frequency modulator 114 may be powered by an energy source external to said repeater.

[0119] [Fig.2] is a first schematic representation of a second non-limiting example embodiment of a repeater 200 according to the invention.

[0120] The optical repeater 200 comprises the same elements of the optical repeater 100 of [Fig.l].

[0121] The optical repeater 200 of [Fig. 2] further comprises a second optical amplifier 202. The second optical amplifier 202 may be active. In a non-limiting manner, it is powered by the same energy source arranged to power the first optical amplifier 118, said source being external to said repeater 200.

[0122] The second optical amplifier 202 is arranged to amplify the backscattered Aldiff wave at the output of the downstream circulator 106.

[0123] Optionally, the optical repeater 200 of [Fig.2] also comprises an optical filter 204. The second amplifier 202 and the optical filter 204 are both positioned in the second branch 112 of the optical repeater 200.

[0124] The optical filter 204 is preferably positioned at the output of the second optical amplifier 202.

[0125] In the optical repeater 200 illustrated in [Fig.2], the second optical amplifier 202 is connected to the downstream circulator 106 and to the optical filter 204. The optical filter 204 is positioned between the second optical amplifier 202 and the upstream circulator 102.

[0126] In this way, the optical filter 204 is arranged to eliminate noise in the Aldiff backscattered wave traveling along said second branch 112.

[0127] The optical filter 204 is centered on the useful wavelength, which in the case considered is 1550 nanometers.

[0128] The first optical amplifier 118 and the second optical amplifier 202 are each an Erbium Doped Optical Amplifier (EDFA). Thus, the first and second optical amplifiers 118, 202 may be similar. In this case, they may have the same amplification gain.

[0129] In a variant not illustrated, the first and second optical amplifiers 118, 202 are different.

[0130] [Fig. 3] is a schematic representation of a third non-limiting example embodiment of a repeater 300 according to the invention.

[0131] The optical repeater 300 comprises the same elements of the optical repeater 200 of [Fig.2],

[0132] However, the optical filter 204 of [Fig. 2] is integrated with the second optical amplifier 202 of [Fig. 2] to form a single component 302 arranged to both amplify the backscattered wave and denoise this amplified wave. In this case, the component 302 can be custom-made so as to choose or adapt the amplification and / or filtering carried out by the component 302.

[0133] The optical repeater 300 of [Fig. 3] comprises a first power supply 304 composed of a single electrical source 304 arranged to supply voltage to the first amplifier 118 and the component 302. The first power supply 304 is of the 220 volt (V) alternating current (AC) type.

[0134] The optical repeater 300 of [Fig. 3] further comprises a second power supply 306 comprising two power sources delivering, for a first (not shown), a direct voltage (DC) of 4 volts, and for a second a direct voltage (DC) of 24 volts (not shown). The optical repeater 300 further comprises a driver 308 powered by the second power supply 306, or by the first and second sources of the second power supply 306.

[0135] The optical repeater 300 may comprise a driver 308. The driver may control continuous or pulsed operation of the frequency modulator 114. The bandwidth of the downstream pulse and the shape of the downstream pulse is therefore controlled by the driver 308. The driver 308 illustrated in [Fig.3] controls the optical modulator 114 in continuous mode.

[0136] [Fig.4] is a schematic representation of a fourth non-limiting example embodiment of a repeater 400 according to the invention.

[0137] The optical repeater 400 comprises the same elements of the optical repeater 300 of [Fig. 3] except that the second amplifier 202 and the optical filter 204 are two separate components, as illustrated in [Fig. 2].

[0138] In a non-limiting manner, the optical repeater 400 illustrated in [Fig.4] comprises three frequency modulators 114 connected in series in the first branch 110.

[0139] As a non-limiting example, the three frequency modulators 114 are each controlled by a driver 308. In a variant not illustrated, the optical repeater 400 may comprise a driver controlling all of the frequency modulators 114 of the repeater 400.

[0140] A first frequency modulator 114a positioned directly after the first optical amplifier 118 and arranged to achieve, for example, a positive frequency shift Afa of 80 MHz, so that Afa = 80 MHz.

[0141] A second frequency modulator 114b positioned after the first frequency modulator 114a and arranged to provide, for example, a positive frequency shift Afb of 40 MHz, such that Afb = 40 MHz.

[0142] A third frequency modulator 114c positioned after the second frequency modulator 114b and arranged to achieve, for example, a negative frequency shift Afc of 110 MHz, so that Afc = -110 MHz.

[0143] Thus, in this non-limiting example, downstream frequency base fb at the output of the third frequency modulator 114c, is equal to:

[0144] fj = fO + Afa + Afb + Afc = fO + 80 MHz + 40 MHz - 110 MHz = fO + 10 MHz.

[0145] The optical repeater 400 illustrated in [Fig.4] therefore comprises several modulators of frequency 114 each being able to achieve a different frequency shift, in particular of a different sign. The optical repeater 400 can therefore precisely adjust the frequency shift to be achieved.

[0146] [Fig.5] is a representation of a first optical measurement system 500.

[0147] The optical measurement system 500 is arranged to be positioned along a track railway (not shown), along its entire length. For example, the railway track may be 200 kilometers (Km).

[0148] The optical measurement system 500 comprises at least two optical repeaters 200 according to the second example and an optical measurement device 502.

[0149] The optical measuring device 502 comprises a source of optical measuring wave (not shown) and a backscattered wave(s) sensor (not shown). In particular, the optical measuring device 502 is a vibration measuring device known as a DAS system, from the English Distributed Acoustic Sensing (DAS).

[0150] The optical wave source of the optical measuring device 502 of [Fig. 5] is a laser. The laser may be a semiconductor laser or a fiber laser. The semiconductor laser is for example a laser diode. The optical wave source is arranged to emit upstream measurement base A0 in the infrared frequency band, in particular at the central wavelength of 1550 nanometers (nm), i.e. a frequency of approximately 193.5 THz.

[0151] The frequency of sending the pulses of the measuring waves A0 emitted by the measuring device 502 can be between 1 and 5 kHz, for example it is 4 kHz. The frequency of sending the pulses chosen is a function of the vibration signal that one wishes to record. Consequently, the sending frequency can, depending on the use of the measuring system 500 and the vibration frequencies of interest, be greater than 5 kHz.

[0152] The optical wave source is single-frequency. For example, it has a laser line width less than or equal to 1 kHz, and an average power of the order of ten milliwatts.

[0153] In a non-limiting manner, the optical wave source of the optical measurement system 500 comprises a line width of approximately 30 Hz centered on the central emission frequency, which is 193.5 THz.

[0154] The optical measurement system 500 further comprises a first optical fiber 504, called the first optical measurement fiber, connected to said measurement device 502.

[0155] The optical measurement system 500 also comprises: - a first sensor block 501, comprising a second optical fiber 508 of the first sensor block 501, called second optical measurement fiber connected to a first repeater 200a. - a second sensor block 503, comprising a second optical fiber 510 of the second sensor block 503 connected to a second repeater 200b.

[0156] The first repeater 200a of the first sensor block 501 is connected in series to the second repeater 200b of the second sensor block 503 by the second optical measurement fiber 508 of the first sensor block 501.

[0157] Thus, the first repeater of the first sensor block 501 is directly connected to the first optical measurement fiber 504 while the repeater of the second sensor block 503 is connected to the second measurement fiber 508 of the first sensor block 501.

[0158] In a first variant, the first repeater 200a and the second repeater 200b are identical. In this sense, the first repeater 200a and the second repeater 200b comprise the same optical elements and / or achieve a similar frequency shift.

[0159] The optical measurement system 500 illustrated in [Fig.5] therefore comprises two sensor blocks 501, 503 each comprising two optical repeaters 200a, 200b. Each of the repeaters 200a and 200b is arranged in the form of a loop via two circulators, an upstream circulator 102 and a downstream circulator 106. The first branch 110 of each circulator is arranged to transmit the pulses emitted by the DAS measurement device 502 in a first direction, in particular in a propagation direction similar to the measurement wave A0.

[0160] The second branch 112 of each circulator is arranged to receive the backscattered signals in a second direction, opposite to the direction of propagation of the measuring wave A0.

[0161] Optionally, the optical repeaters 200a and 200b illustrated in [Fig.5] may each comprise: - the first power supply 304 (not shown in [Fig.5]), supplying the first and second amplifiers 118, 202, and - the second power supply 306 (not shown in [Fig.5]), powering the driver 308 (not shown) via the two power sources included in the second power supply 306. In another variant, the optical repeaters 200a and 200b may be powered by a power source external to said repeaters, for example present along the railway line.

[0162] Each repeater 200a, 200b may comprise a driver 308.

[0163] The optical measurement system 500 illustrated in [Fig.5] is arranged to detect vibrations along the first fiber 504 of the measurement system 500 and / or along each of the second optical fibers 508, 510 of each sensor block 501, 503 of the optical measurement system 500.

[0164] The vibrations correspond to an event occurring on the railway track, for example the movement of a vehicle or the passage of an object on the railway track.

[0165] The principle used by the optical measurement system 500 is as follows. To detect vibrations along the first optical fiber 504 and / or along the second optical fibers 508, 510, a very brief light pulse, called the measurement wave A0, and of given temporal width is sent within the first fiber 504 by the laser of the optical measurement device 502, also called the DAS device 502. The aim then being to record at least one backscattered wave which travels through the first fiber 504 and / or the second fiber 508, 510 in an opposite direction (to the light pulse emitted by the laser) and which therefore returns to the DAS device 502 via the first fiber 504. The backscattered wave can come from each scattering point included along the entire length of the first and second fibers 504, 508, 510.

[0166] Each time a light pulse is sent (i.e. emitted) into the first fiber 504, at least one backscattered wave is recorded. If the first fiber 504 and the second fibers 508 or 510 do not undergo any vibration, the backscattered wave received is always the same. When a vibration, corresponding, for example, to an event on the railway, reaches the first fiber 504 and / or the second fibers 508, 510, it causes the backscattered wave of the first fiber or the second fiber 504, 508, 510 to vary and it is the exploitation of this variation, by signal processing, which makes it possible to trace back to the source vibration.

[0167] Each sensor block 501, 503 of the optical measurement system 500 carries out a frequency shift Af of the wave arriving at the input of its sensor block 501, 503, that is to say either by the first fiber 504 for the case of the first sensor block 501, or by the second fiber 508 for the case of the second sensor block 503.

[0168] Each repeater, of the first sensor block 501 and second sensor block 503, amplifies the wave arriving at the input of its respective sensor block and carries out a frequency shift of this wave, which allows the backscattered optical waves present in the first and second fibers 504, 508, 510 to circulate without interfering with each other. With each addition of a sensor block along the path of the optical wave, an amplification and an additional frequency shift are carried out, allowing the optical wave to propagate further without reducing the maximum frequency of the acoustic bandwidth of the vibration that one wishes to detect.

[0169] In the first branch 110 of each repeater 200, the measurement wave A0 is amplified first by the first optical amplifier 118 of the first sensor block 501. The optical frequency f0 is retained. Then, the amplified measurement wave is shifted by the frequency modulator 114 of the first sensor block 501 to an optical frequency fl by a difference Afi of 200 MHz, for example, relative to the frequency f0 (fi = f0 + Afi) of the measurement wave A0.

[0170] The downstream wave, amplified relative to the measurement base A0 and having an optical frequency fl, is then transmitted to the second fiber 508 via the downstream circulator 106 of the first sensor block 501.

[0171] As a non-limiting example, the optical fibers of the measurement system 500 comprise a length of 25 Kilometers (Km) so as to maintain an acceptable bandwidth for the measurement device 504.

[0172] In the second fiber 508 of the first sensor block 501, Fonde downstream will produce: - a backscattered wave Aldiff, in the opposite direction to Fonde upstream of the repeater 200a of the first sensor block 501, and - a transmitted optical wave Al, in the same direction as the downstream end of the first sensor block and which will circulate along the entire length of the second fiber 508 of the first sensor block 501 to reach the second sensor block 503.

[0173] The backscattered wave Aldiff will return to the optical repeater 200a of the first sensor block 501 and pass into the second branch 112 of the optical repeater 200a of the first sensor block 501 via the downstream circulator 106 of the repeater 200a of the first sensor block 501 before reaching the DAS measuring device 502.

[0174] The transmitted optical wave Al will enter the second sensor block 503. The operation of the second sensor block 503 is similar to the first sensor block.

[0175] The transmitted optical wave Al is amplified and frequency-shifted to a frequency f2 by a difference Af2 of 200 MHz, for example, relative to the frequency fl of Fonde downstream of the first sensor block 501 (f2 = fi + Af2). The wave frequency-shifted by the second sensor block 503 is called the downstream wave of the second sensor block 503.

[0176] The downstream wave of the second sensor block 503 is then transmitted to the second fiber 510 of the second sensor block 503 via the downstream circulator 106 of the second sensor block 503. As for the first sensor block, a portion of the downstream wave of the second sensor block 503 will be transmitted and another part will be backscattered into a backscattered wave A2diff.

[0177] The transmitted optical wave A2 can enter a third sensor block (not shown) operating like the first and second sensor blocks 501, 503. In a non-limiting manner, the frequency shift produced by the optical repeater of the third sensor block can be a difference Af3 of 200 MHz, for example, relative to the frequency f2 of the second sensor block 503.

[0178] Each wave backscattered by the first, the second sensor block and etc. will return to the optical device 502. Each backscattered wave will therefore take the second branches 112 of each optical repeater crossed to reach the measuring device 502. Each backscattered wave comprises a Rayleigh type wave. Preferably, each of the optical repeaters of the optical measuring system illustrated in [Fig.5], comprises a second optical amplifier 202 arranged to amplify the backscattered waves at the various frequencies f1, f2, f3, etc. circulating in this branch and an optical filter 204 to eliminate the amplified additive noise of the second amplifier 202. In a variant not illustrated, the optical repeaters do not comprise an optical filter 204.

[0179] The backscattered optical waves Aldiff and A2diff from each of the sensor blocks 501, 503 do not interfere since they do not have the same optical frequency (fl and f2) and are detectable by the optical measuring device DAS 502 because they have been amplified by a second optical amplifier 202 at each passage in a second branch 112 of an optical repeater of a sensor block.

[0180] To increase the maximum detectable frequency of the acoustic band of a vibratory signal to be recorded, it is sufficient to insert additional sensor blocks comprising frequency modulators making it possible to obtain a downstream wave of a different frequency (f3, f4, etc.) than the previous sensor blocks. Each additional block produces a frequency shift of the wave arriving at the input of its sensor block.

[0181] To increase the detection range with a high repetition frequency and the maximum frequency of the acoustic band of a vibratory signal to be recorded, it is sufficient to insert in the same way additional sensor blocks comprising frequency modulators making it possible to obtain a downstream wave of different frequency (f3, f4 ...) than the previous sensor blocks and at least one optical amplifier to amplify the signal of the wave entering each sensor block. The additional sensor blocks may be composed, in a non-limiting manner, of any of the optical repeaters 100, 200, 300, 400 illustrated respectively in FIGURES 1, 2, 3, and 4.

[0182] The more sensor blocks the optical measurement system 500 comprises, the more the number of frequency shifts increases. Consequently, the bandwidth of the DAS device 502 necessary for acquiring the vibration signal can increase. Preferably, to avoid increasing the bandwidth of the DAS device 502, the sensor blocks of the optical measurement system 500 are arranged to produce positive or negative frequency shifts Af depending on the chosen frequency modulators. Thus, some sensor blocks will produce positive shifts, while others will produce negative shifts.

[0183] In a non-limiting manner, the measurement system 500 is arranged to detect vibration frequencies of 2 kilohertz (KHz) with 4 sensor blocks according to the invention or more than 2 KHz for a greater number of sensor blocks.

[0184] The plurality of backscattered waves received by the DAS device 502 are then processed by data processing and separation.

[0185] In a first variant, the processing and separation of the data can be carried out digitally. The data around each optical frequency corresponding to a given section of fiber, i.e. corresponding to the first fiber 504 or the second fibers 508, 510 are extracted digitally. Thus, the structure of the optical measurement system 500 can be preserved without the need to make structural modifications such as hardware modifications.

[0186] In a second variant, the processing can also be carried out in an electronic analog manner. Upon reception of the backscattered optical wave and after conversion of the backscattered optical wave into an electrical signal (i.e. photo-detection), the electrical signal is separated by analog separators (not shown). There are as many analog separators as there are frequencies emitted by the different sensor blocks. Consequently, each backscattered wave of different frequency is analyzed separately. This method makes it possible to reduce the data processing time and save analysis time. However, this involves the use of an acquisition system with multiple reception channels.

[0187] In a variant, not illustrated, it is possible for several events to occur in the detection zone of the measurement system 500. Thus, each event will produce a localized variation in the instantaneous amplitude and phase of the backscattered signal. It is therefore possible to detect and separate these vibrational events in space and time in a very precise manner. However, the backscattered waves will not be able to interfere with each other and since they will be of a different frequency than the measurement wave A0, they will not interfere with the backscattered wave of the measurement wave A0 either.

[0188] [Fig.6] is a representation of a second optical measuring system 600.

[0189] The optical measuring system 600 is arranged to be positioned along a railway track (not shown).

[0190] The measuring system 600 illustrated in [Fig.6] comprises, in a non-limiting manner, two sensor blocks 601, 603.

[0191] The measurement system 600 comprises two optical repeaters, - a first repeater 200a belonging to the first sensor block 601 and identical to the optical repeater 200a of the first sensor block 501 of [Fig.5], and - a second optical repeater 602 belonging to the second sensor block 603.

[0192] The optical measurement system 600 comprises the same elements as the optical measurement system 500 of [Fig.5], apart from the fact that the optical repeater 602 of the second sensor block 603 is different from the optical repeater 200a of the first sensor block 601.

[0193] Indeed, the optical repeater 602 of the second sensor block 603 comprises, in a non-limiting manner, two frequency modulators 114a and 114b connected in series, each capable of producing a different frequency shift Af2> a, Af2> b.

[0194] In a variant not illustrated, the optical repeater 602 of the second sensor block 603 may comprise more than two frequency modulators 114, as illustrated with the optical repeater 400 of [Fig.4].

[0195] The two frequency modulators 114a, 114b of the optical repeater 602 of the second sensor block 603 are each controlled by the driver 308 (not shown). The optical repeaters 200a and 602 of the first and second sensor blocks 601, 603 each comprise the first and second power supplies 304, 306.

[0196] The first frequency modulator 114a of the second sensor block 603 is positioned directly after the first optical amplifier 118 and is arranged to achieve, for example, a positive frequency shift Af2> a of 110 MHz, so that Af2> a = 110 MHz.

[0197] The second frequency modulator 114b of the second sensor block 603 is positioned after the first frequency modulator 114a of the second sensor block 603 and is arranged to achieve, for example, a negative frequency shift Af2> b of 80 MHz, so that Af2> b = - 80 MHz.

[0198] Thus, the downstream wave of frequency f2, at the output of the second frequency modulator 114b of the second sensor block 603 is equal to:

[0199] f2 = fl+ Af2> a + Af2> b = fl+ 110 MHz - 80 MHz = fl+ 30 MHz.

[0200] The two sensor blocks 601, 603 each achieve a different frequency shift. Thus, a sensor block positioned between two sensor blocks may be different from the previous sensor block or different from the following sensor block. A multitude sensor blocks can thus be connected in series from the first optical measurement fiber 504. Each additional sensor block makes it possible to increase the detection range of the optical measurement system 600 and to increase the maximum frequency of the acoustic bandwidth of the vibration signal.

[0201] The combination of different sensor blocks illustrated in [Fig.6] makes it possible to avoid modifying the current bandwidth of the DAS 502 measuring device.

[0202] Thus, to minimize the frequency offset deviation relative to the frequency of the optical Found pulse fO emitted by the DAS measuring device 502 and to keep the same acquisition system with an assembly of a plurality of sensor blocks, the optical measuring system 600 illustrated in [Fig.6] comprises a combination of sensor blocks each capable of achieving either a positive or negative frequency offset, for example of 30 and 10 MHz or of - 30 and - 10 MHz respectively.

[0203] The additional optical insertion losses introduced by each frequency modulator (typically 6 dB per frequency modulator 114) are compensated by the second optical amplifier 202 of the current sensor block or the first optical amplifier 118 of the next sensor block.

[0204] Consequently, the optical measurement system 600 illustrated in [Fig.6] allows an increase in the maximum detection length of the measurement system and an increase in the maximum frequency of the acoustic band of the vibration signal via a combination of sensor blocks connected in series, which leads to an increase in the length of the measurement fibers (combination between the first fiber and the second fiber of each sensor block) without deteriorating the vibration signal that it is desired to record and without losing part of the bandwidth of said optical measurement system 600.

[0205] Of course, the invention is not limited to the examples which have just been described. Numerous modifications can be made to these examples without departing from the scope of the present invention as described.

Claims

Claims

1. Optical repeater (100, 200, 300, 400, 602) comprising: - a circulator (102), called upstream, provided for coupling said repeater (100, 200, 300, 400) to an optical fiber (104), called upstream, - a circulator (106), called downstream, provided for coupling said repeater (100, 200, 300, 400, 602) to an optical fiber (108), called downstream, - a first branch (110) connecting said upstream circulator (102) to said downstream circulator (106) and allowing the passage of an optical wave, called a measurement wave, from said upstream circulator (102) to said downstream circulator (106), - a second branch (112) connecting said downstream circulator (106) to said upstream circulator (102) and allowing the passage of an optical wave, called a backscattered wave, from said downstream circulator (106) to said upstream circulator (102), characterized in that said first branch (110) comprises at least one frequency modulator (114) arranged to shift in frequency an optical wave, called an upstream measurement wave,coming from said upstream circulator (102) so as to supply said downstream circulator (106) with an optical wave, called downstream measurement wave, of different frequency, in that the first branch (110) comprises a first optical wave amplifier (118) and in that the second branch (112) comprises a second optical wave amplifier (202, 302).,

2. Repeater (100, 200, 300, 400, 602) according to claim 1, characterized in that the frequency modulator (114) comprises, or is, an acousto-optic modulator (114).

3. Repeater (200, 300, 400, 602) according to any one of the preceding claims, characterized in that it comprises a driver (308) arranged to control an operating mode of the at least one frequency modulator (114).

4. Repeater (200, 300, 400, 602) according to any one of the preceding claims, characterized in that the frequency modulator (114) is arranged to positively and / or negatively shift the frequency of the upstream measuring wave.

5. Repeater (200, 300, 400, 602) according to any one of the preceding claims, characterized in that the second branch (112) comprises an optical filter (204, 302) for eliminating noise in an optical wave traveling through said second branch (112).

6. Optical measurement system (500, 600) comprising: - an optical measurement device (502) comprising a source emitting at least one optical wave, called a measurement wave, and a backscattered wave(s) sensor, and - a first optical fiber (504), called the first measurement optical fiber, connected to said measurement device (502), and at least one sensor block (501, 503, 601, 603) comprising: - an optical fiber (508, 510), called the second measurement optical fiber, - an optical repeater (100, 200, 300, 400, 200a, 200b, 602) according to any one of the preceding claims arranged between said second measurement optical fiber and said first measurement optical fiber (504), or said second measurement optical fiber (508, 510) of a previous sensor block (501, 503, 601, 603).

7. Optical measuring system (500, 600) according to the preceding claim, characterized in that it comprises several sensor blocks (501, 503, 601, 603) connected in series to each other, at least one sensor block comprising a repeater (100, 200, 300, 400, 200a, 200b, 602) identical to or different from the repeater of at least one other sensor block (501, 503, 601, 603).

8. Railway track comprising at least one optical measuring system (500, 600) according to any one of claims 6 to 7.

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