Patent Description:
The invention preferably relates to distributed or fully distributed sensors, in which an optical fiber is a long uninterrupted sensor, and the measured information are extracted from the analysis of backscattered light.

The backscattered light can typically come from the following scatterings:.

Raman scattering exhibits a large frequency shift of typically 13THz in silica fibers, corresponding to <NUM> at a wavelength of <NUM>. The Raman Anti-Stokes component intensity is temperature dependent whereas the Stokes component is nearly temperature insensitive.

Most of the assets measured or monitored using DFOS (Distributed Fibre Optic Sensing) are linear. This is the case for power cable, for oil and gas pipeline, likewise for water pipeline. In such linear system, the limit is the maximum measurement range.

There are some structures that are similar to star network configuration rather than linear configuration. These are for instance the inter-array cable (IAC) inside windfarm or the gathering lines for the pipeline industry or the fibre-based telecommunication network (point-to-multipoint fiber-to-the-home (FTTH) network, also defined as a PON network) or lines in water / gas/ distribution pipelines or in sewage.

Monitoring of such a star network is difficult.

One possibility is to use multiple channels that are measured one after the other, in sequence. This is only possible when the targeted events have a slow time constant with respect to the sequence. For example, measuring temperature with a Distributed Temperature Sensing (DTS) is compatible with a sequence/channel-based approach. Assuming <NUM> channels and <NUM> to measure one channel, then every channel is measured twice per hour. Given that the time constant of an IAC on thermal variation is in the <NUM> to <NUM> range, there is still a good coverage of the temperature.

This is not the case when looking at short event like anchor drop and short circuit. A short circuit may last for a few <NUM> of milliseconds. In other words, when using a <NUM> measurement and <NUM> sequence, most of the short circuits will not be detected, as they will appear in the non-measured channel. In this case, a sequence/channel-based approach is not possible.

<CIT> discloses an apparatus and method utilizing optical sensors operating in the reflection mode.

The goal of the invention is to present a device and/or method for distributed sensing, preferably arranged for a star network, that is less complex and/or less expensive than prior art but preferably with similar or better performances.

An aspect of the invention concerns a device for distributed sensing comprising:.

The pump signal is preferably a pulsed pump signal, and the controller is preferably arranged and/or programmed so that each gate is, preferably one after the other, in its open state a longer time than the duration of the pulsed pump signal in order to allow the pulsed pump signal to fully pass or go through the gate in its open state.

The controller can be arranged and/or programmed in order to avoid injecting the pump signal in at least two channels at the same time.

The optical receiver can comprise, for each channel, a circulator, arranged for:.

The backscattered signal can comprise a Rayleigh backscattered signal, a Brillouin backscattered signal, or a Raman backscattered signal.

Each channel can comprise an optical fiber monitoring a cable, pipe, branch or string of a star network, preferably monitoring an inter array cable of a star network of a wind farm.

At least one of the channels can be arranged for monitoring several cables, pipes, branches and/or strings departing in several different directions from a central position of the star network, the optical fiber of this at least one channel going back and forth with respect to the central position.

Each channel can comprise an optical fiber monitoring at least one cable, pipe, branch and/or string of at least one among a pipeline network, a transport network such like a road or rail network, a telecommunication network, an information network, or an electrical network.

The optical receiver can comprise a detector shared for all the channels.

The controller can be arranged and/or programmed in order to allow at least two gates to be in the open state at the same time.

Another aspect of the invention concerns a wind farm comprising a device according to the invention.

Another aspect of the invention concerns a method for distributed sensing comprising:.

The pump signal is preferably a pulsed pump signal, and each gate is preferably, preferably one after the other, in its open state a longer time than the duration of the pulsed pump signal in order to allow the pulsed pump signal to fully pass or go through the gate in its open state.

The splitting step can avoid injecting the pump signal in at least two channels at the same time.

The optical receiver can comprise, for each channel, a circulator:.

At least one of the channels can be monitoring several cables, pipes, branches and/or strings departing in several different directions from a central position of the star network, the optical fiber of this at least one channel going back and forth with respect to the central position.

The splitting step can allow at least two gates to be in the open state at the same time.

The method according to the invention can be used for monitoring a wind farm.

Other advantages and characteristics of the invention will appear upon examination of the detailed description of embodiments which are in no way limitative, and of the appended drawings in which:.

These embodiments being in no way limitative, we can consider variants of the invention including only a selection of characteristics subsequently described or illustrated, isolated from other described or illustrated characteristics (even if this selection is taken from a sentence containing these other characteristics), if this selection of characteristics is sufficient to give a technical advantage or to distinguish the invention over the state of the art. This selection includes at least one characteristic, preferably a functional characteristic without structural details, or with only a part of the structural details if that part is sufficient to give a technical advantage or to distinguish the invention over the state of the art.

We are now going to describe, in references to <FIG> and <FIG>, the first embodiment of a device <NUM> according to the invention.

Device <NUM> for distributed sensing comprises a pump generator <NUM> arranged for generating a pulsed optical pump signal <NUM>.

"Pulsed" optical signal <NUM> means a pulse regardless of its duration, for example short duration (of the order of a picosecond or femtosecond) or long duration (of the order of a few minutes or longer) or intermediate duration.

Pulse <NUM> duration is typically from one picosecond to <NUM> microseconds.

Pump generator <NUM> comprises typically a laser or a light-emitting diode, arranged for generating or emitting signal <NUM>:.

However other kind of pump generator <NUM> may be employed within the scope of this invention.

Device <NUM> comprises the optical pump splitter <NUM> configured to receive the pump signal <NUM> and split the pump signal <NUM> in a number N of channels <NUM> (N being an integer greater than or equal to <NUM>), each channel <NUM> comprising an optical fiber <NUM> and/or a connector <NUM> arranged for connecting an optical fiber <NUM>.

Each connector <NUM> can also be called "pump connector" <NUM> as it is arranged and used to connect one of the fibers <NUM> to the pump generator <NUM>.

and can be a single optical fiber or can comprise a fiber bundle (typically comprising few tens of individual fibers).

Connector <NUM> comprise a standard connector for optical fiber <NUM> according to prior art or other suitable connection means.

The optical pump splitter <NUM> comprises:.

Each gate <NUM> can also be called "pump gate" <NUM> as it is arranged and used to control if the pump signal <NUM> enters into one of the fibers <NUM> or not.

Each gate <NUM> could be home designed using:.

The pump signal is a pulsed signal, and each gate <NUM> is (preferably one after the other) in its open state a longer time than the duration of the pulsed pump signal <NUM> in order to allow the pulsed pump signal to fully pass or go through the only one gate in its open state while the pump signal <NUM> reaches the gates <NUM>.

Device <NUM> comprises a controller (not illustrated) configured to control the optical pump splitter <NUM>. The controller can include at least one computer, one central processing or computing unit, one analogue electronic circuit (preferably dedicated), one digital electronic circuit (preferably dedicated) and/or one microprocessor (preferably dedicated) and/or software means or other control means known in the art for controlling the optical pump splitter <NUM>.

Device <NUM> comprises an optical receiver <NUM> arranged for receiving a backscattered signal <NUM> from the optical fiber <NUM> or from the connector <NUM> of each channel <NUM>, this backscattered signal <NUM> being generated in fiber <NUM> in response to the pump signal <NUM>.

As illustrated in <FIG>, the optical fiber <NUM> of each channel <NUM> is arranged for monitoring a cable or pipe <NUM> (also referenced <NUM> to <NUM>, and also called branch or string) of a star network, preferably monitoring an inter array cable of a star network of a wind farm.

More precisely, each channel <NUM> comprises an optical fiber <NUM> monitoring at least one cable of a pipeline network, a transport network such like a road or rail network, a telecommunication network, an information network, or an electrical network.

The proposed solution of device <NUM> is thus a time-division multiplexing of the pump signal <NUM>. In this way, each branch (cable or pipe) <NUM> can be covered by a single fiber <NUM> providing also a better signal quality.

At least one channel <NUM> is arranged for monitoring several branches <NUM> departing in several different directions from a central position of the star network; this at least one channel <NUM> going back and forth several times with respect to the central position. For example:.

Thus, for short event measurement and/or simultaneous measurement, it is possible to design the sensing path of fiber <NUM> by following one branch (cable or pipe) <NUM> to the end (following what is known as a string in the windfarm jargon) and come back to the device <NUM> (i.e. to the central position of the star network, known as the offshore substation in the windfarm jargon) before going to the next branch <NUM>, as illustrated in <FIG>.

In case of fiber 31c, the return path is useless. Assuming that each branch <NUM> is <NUM> long and the range of the Distributed Acoustic Sensing (DAS) device <NUM> is <NUM>, then it is possible to measure three strings <NUM> only per fiber <NUM>, with two return paths, each of <NUM> (<NUM> sensing - <NUM> return - <NUM> sensing - <NUM> return - <NUM> sensing). In this specific example of fiber 31c, <NUM>% of the fiber length is not employed for active measurement (the return paths).

As illustrated by fiber 31a in <FIG>, some strings <NUM> (<NUM> and <NUM>) may be connected by a fiber 31a at the far end. In this case the full measurement range can be used.

As illustrated in <FIG>, the controller is arranged and/or programmed in order to avoid injecting the pump signal <NUM> in at least two channels <NUM> at the same time.

Pump generator <NUM> is arranged for generating pulse <NUM> several times, preferably at a temporal period Tp.

The controller is arranged in such a way that, each time pulse <NUM> is generated, this pulse <NUM> is split by the optical pump splitter <NUM> and reach each gate <NUM> of each channel <NUM>, but only one of this gate <NUM> is in its open state <NUM>, the other ones are in the closed state <NUM>.

The controller is arranged in such a way that the gates <NUM> are in their open state <NUM> one after the other, and in such a way that, for a temporal period TN comprising N generated pulses <NUM> for N gates of N channels <NUM>, pulse number i (i an integer equal to <NUM> to N), reaches all the gates <NUM> but only gate <NUM> number i of channel <NUM> number i is in its open state. This way, during a temporal period TN (TN = N x TP), every fiber <NUM> of every channel <NUM> receives one by one pulse <NUM>, one after the other.

Also, as illustrated by <FIG>, the controller is arranged and/or programmed in order to optionally allow at least two gates <NUM> to be in the open state <NUM> at the same time, as long as they are not open at the same time when pulse <NUM> is reaching these at least two gates <NUM>.

This perfectly illustrates technical advantages of device <NUM> according to the invention, compared to prior art (especially compared to a prior art solution using a very fast optical switch):.

The fiber <NUM> reached by signal <NUM> is generating a backscaterred signal <NUM>.

The backscattered signal <NUM> comprises a Rayleigh backscattered signal, a Brillouin backscattered signal, or a Raman backscattered signal. In case of <FIG> and <FIG>, the backscattered signal <NUM> comprises a Rayleigh backscattered signal, a Brillouin backscattered signal (without use of a probe signal for stimulating this Brillouin backscattered signal), or a Raman backscattered signal.

The optical receiver <NUM> typically comprises, for each channel <NUM>, a circulator <NUM>. For each channel <NUM>, the circulator <NUM> of this channel <NUM> is arranged for:.

The optical receiver <NUM> typically also comprises:.

It is also possible to have a single EDFA (not illustrated) at the exit of the 3x1 coupler <NUM> (rather than having three EDFA on each input of the 3x1 coupler <NUM>). Having the 3x1 coupler first is possible for short string only. The EDFA after is thus limited to a small channel number (reasonably up to <NUM>) and short range.

The data or signal <NUM> acquired by optical receiver <NUM> or detector <NUM> is then analyzed according to the classical design of the interrogator , the controller being arranged and/or programmed to know, depending on time, from which channel <NUM> is coming signal <NUM>, based on the channel <NUM> that most recently received the pulse <NUM>.

The analysis is backscattering dependent, typically:.

Thus, in device <NUM>, the pump signal <NUM> is split in N (assuming N channels <NUM>, practically N equal three or four). But as such, the N channels <NUM> are synchronized (they receive the pump <NUM> simultaneously) and measurement by optical receiver <NUM> is done from each channel <NUM>.

To maintain a "in series" measurement, the pump <NUM> is triggered at the max speed corresponding to the length L of the channel <NUM> (assuming for simplicity that the N channels <NUM> have an identical length L), which is N times faster than what is allowed for all the channels and a gating system let one pump <NUM> out of N go through to the relevant channel <NUM>.

Detection is done easily as all channels <NUM> receive the pump <NUM> in series and a single acquisition can be done.

Let's assume three channels <NUM> of <NUM> each, for a total length of <NUM>. The pulse rate would be <NUM> max. Here, the pulse <NUM> is repeated at three times the pulse rate (<NUM>), with pulses i going to the first channel, pulse i+<NUM> going to the second channel <NUM> and pulse i+<NUM> going to the third channel <NUM> in cycles, so that each channel <NUM> receives pulses at <NUM> in the proper timing. Performances are those of <NUM> in term of signal quality, since each pump <NUM> travels <NUM> only instead of <NUM>, with the frequency bandwidth equivalent to <NUM> (<NUM>/<NUM> to take into account sampling theory) because of the total measuring time.

Device <NUM> is directly compatible with any single-based measurement system (like Raman Distributed Temperature Sensing (DTS) for instance) and is not limited to a particular type of Distributed Acoustic Sensing (DAS) interrogator. In fact, it is applicable to all fiber sensing devices working with a single fiber.

Device <NUM> is directly compatible with a Brillouin Optical Time Domain Reflectometry (BOTDR) (temperature or strain).

It is also noted the following various advantages of device <NUM>:.

We are now going to describe, in references to <FIG> and <FIG>, a second embodiment of a device <NUM> according to the invention.

Device <NUM> will be described only for its differences compared to device <NUM>.

In case of <FIG> and <FIG>, the backscattered signal <NUM> comprises a Brillouin backscattered signal, with use of a probe signal <NUM> for stimulating this Brillouin backscattered signal <NUM>.

As illustrated by <FIG>, device <NUM> according to the invention is directly compatible with a Brillouin Optical Time Domain Reflectometer (BOTDA) (loop configuration) by simply splitting the probe <NUM> in N (three in this case of <FIG>) and looping on a per channel <NUM> basis. This could be advantageous to maintain very short spatial resolution (<NUM>) over the inter array on a string basis (<NUM>) that would not be possible on the in-series length (<NUM>).

The pump gating does not have to be symmetric and could be adapted to match the actual length of each string, provided that it is possible to generate pump signals <NUM> with different time interval.

Compared to device <NUM>, device <NUM> further comprises a probe generator <NUM> for generating the optical probe signal <NUM>.

Probe generator <NUM> comprises typically a laser or a light-emitting diode.

However other kind of probe generator <NUM> may be employed within the scope of this invention.

Compared to device <NUM>, device <NUM> further comprises an amplifier <NUM>, preferably an Erbium-Doped Fiber Amplifier (EDFA), arranged for amplifying the probe signal <NUM> before being the optical probe splitter <NUM>, <NUM> described below.

Each channel <NUM> comprises an optical fiber <NUM> and/or:.

Compared to device <NUM>, device <NUM> further comprises the optical probe splitter <NUM>, <NUM> arranged for splitting the probe signal <NUM> in the N channels <NUM>.

The optical probe splitter <NUM>, <NUM> for splitting the probe signal <NUM> comprise:.

Each probe gate <NUM> could be home designed using:.

The controller (not illustrated) are arranged and/or programmed to keep the gate <NUM> of a given channel <NUM> in its open state <NUM> during all time signal <NUM> is acquired from this same channel <NUM> by optical receiver <NUM> and/or detector <NUM>, while all the other gates <NUM> are in the closed state.

The gates <NUM> can be in the open state <NUM> only one by one.

The gates <NUM> and the gates <NUM> are respectively at two opposite ends of the fibers <NUM>.

An embodiment of wind farm according to the invention comprises device <NUM> or <NUM>.

We are now going to describe, in reference to <FIG>, steps of an embodiment of a method according to the invention implemented by device <NUM> or <NUM>.

This method for distributed sensing comprises:.

The optical fiber <NUM> of each channel <NUM> is monitoring:.

At least one channel <NUM> is monitoring several cables departing in several different directions from the central position of the star network, the optical fiber <NUM> of this at least one channel <NUM> going back and forth with respect to the central position.

Device <NUM> and/or <NUM> is preferably used for monitoring a wind farm.

Of course, the invention is not limited to the examples which have just been described and numerous amendments can be made to these examples without exceeding the scope of the invention.

In particular, at least one fiber <NUM> or each fiber <NUM> can comprise a Fibre Bragg Grating (FBG). In this case, signal <NUM> is not necessary a pulsed signal <NUM> but can be a pulsed signal <NUM> or a continuous signal <NUM>. Nevertheless, this embodiment is not a preferred embodiment, because if signal <NUM> is a continuous signal <NUM>, then the gating is more complex, because the controller are then arranged and/or programmed in order to avoid at least two gates <NUM> to be in the open state <NUM> at the same time.

Claim 1:
Device (<NUM>, <NUM>) for distributed sensing comprising:
- A pump generator (<NUM>) arranged for generating an optical pump signal (<NUM>),
- An optical pump splitter (<NUM>) configured to receive the pump signal (<NUM>) and split the pump signal in a number N of channels (<NUM>), each channel comprising an optical fiber (<NUM>) or a connector (<NUM>) arranged for connecting an optical fiber (<NUM>), N being an integer greater than or equal to <NUM>,
- A controller configured to control the pump splitter (<NUM>),
- An optical receiver (<NUM>) arranged for receiving a backscattered signal (<NUM>) from the optical fiber (<NUM>) or from the connector (<NUM>) of each channel,
characterized in that:
- The optical pump splitter (<NUM>) comprises a gating system comprising a gate (<NUM>) for each channel among the N channels, each gate associated to a given channel being arranged for having:
∘ an open state (<NUM>) allowing the pump signal to go from the pump generator (<NUM>) to the optical fiber (<NUM>) or the connector (<NUM>) of the associated channel, and
∘ a closed state (<NUM>) for which the pump signal (<NUM>) cannot go from the pump generator (<NUM>) to the optical fiber (<NUM>) or the connector (<NUM>) of the associated channel.