SPECTRAL DEVICE FOR INTERROGATION OF BRAGG WAVELENGTHS

Description

Domaine Technique

[0001] The present invention relates to the field of measurements (by time division multiplexing) in optical fibers with Bragg gratings and more particularly, to spectral interrogators of Bragg wavelengths which can be used in optical fiber sensors. Etat de la technique antérieure

[0002] In the field of fiber optic sensors, Bragg gratings are very often used to extract spectral information. One of the measurement extraction techniques is time domain multiplexing (TDM). This technique consists of measuring Bragg peaks from a Bragg grating line one by one. This method involves serial measurement of peaks that are not coincident (each Bragg peak is temporally delayed relative to the previous peak).

[0003] In the state of the art, there are mainly two TDM-based solutions. A first solution consists of making a temporal separation by spatial distancing of the FBG (Fiber Bragg Grating) gratings.

[0004] There Fig. 5A very schematically illustrates a spectral interrogation device with temporal separation by spatial distancing, according to the prior art.

[0005] The spectral interrogation device comprises an optical source 103, a measurement optical fiber 105, a circulator 123 and a detector 109. When a light signal is emitted by the optical source 103, it is directed by the optical circulator 123 towards the measurement optical fiber 105. The latter comprises several Bragg gratings 115 spaced far enough apart in space to create a time gap that can be managed by the electronics of the detector 109. The light peaks reflected by the different Bragg gratings are then directed by the optical circulator 123 towards the detector 109.

[0006] This device is quite simple and does not require additional elements in the acquisition chain. However, it is necessary to impose a minimum distance between two successive Bragg gratings, typically of the order of at least 1 m. For example, for a source emitting a 30 ns light pulse, it is necessary that the time delay be at least 30 ns to be able to distinguish two consecutive light peaks. Taking into account the return path of two peaks reflected by two successive Bragg gratings, the distance between these two successive Bragg gratings must correspond to a delay of 15 ns. Given the speed of light in silica, a delay of 15 ns then corresponds to an inter-grating distance of 3 m. In addition, a typical application generally comprises about fifteen Bragg gratings, therefore requiring a fiber length of at least 45 m.Taking a safety margin of 10% to 20%, we end up with a fiber length of at least 55 m. This makes small footprint applications impossible or very complicated, for example in aircraft engine blades or other relatively small equipment or installations.

[0007] To solve this problem, there is currently a second solution that proposes temporal separation through a spectral delay. This solution consists of imposing a delay dependent on the wavelength or spectral range of the light signal. This method requires the addition of a spectral discrimination element as well as a delay element and can be implemented in two configurations.

[0008] There Fig. 5B very schematically illustrates a spectral interrogation device with time separation by parallel delays, according to the prior art.

[0009] The spectral interrogation device using parallel delays according to this first configuration comprises an optical source 103, two demultiplexing-multiplexing modules 114, 116, a set of optical fibers with parallel delays 118, a measuring optical fiber 105, an optical circulator 123 and a detector 109.

[0010] The light signal emitted by the optical source 103 is decomposed into N spectral channels via the first demultiplexing-multiplexing module 114. Each of the channels is associated with an increasing path length. Channel 1 will travel a length L 1 , channel 2 a length L 2 greater than L 1 , ..., and channel N a length LN greater than L N-1 .

[0011] The difference between two successive lengths is chosen so that the electronics of the detector 109 are capable of discriminating the signals coming from two different spectral channels.

[0012] The N spectral channels are then recombined by the second demultiplexing-multiplexing module 116 before being directed towards the measurement optical fiber 105 comprising several Bragg gratings 115. The light peaks reflected by the Bragg gratings travel the reverse paths before being directed by the circulator 123 towards the detector 109.

[0013] There Fig. 5C very schematically illustrates a spectral interrogation device with time separation by serial delays, according to the prior art.

[0014] The spectral interrogation device by serial delays according to this second configuration comprises an optical source 103, two optical circulators 123, 124, a serial delay optical fiber line 120, a measuring optical fiber 105, and a detector 109.

[0015] The 120-delay fiber optic line in series has 115 Bragg gratings at different wavelengths. Each Bragg grating is a reflector in a certain spectral band. The distance between two Bragg gratings thus ensures the delay between two spectral paths.

[0016] A light signal emitted by the optical source 103 is directed by the first optical circulator 123 to the optical fiber delay line 120 in series. The different paths reflected by the different Bragg gratings are then directed by the second optical circulator 124 to the measurement optical fiber 105 and back to the detector 109.

[0017] However, temporal separation through parallel or serial spectral delay requires relatively long optical fiber lengths to generate a delay between Bragg grating measurements. Typically, several tens of meters are required. In addition, there is a constraint on the spectral positioning of the gratings, requiring the wavelength of each Bragg grating to be precisely matched to a defined spectral range.

[0018] Furthermore, the number of Bragg gratings as well as the delay between Bragg gratings are fixed by construction in all state-of-the-art spectral interrogation devices. It is not possible to adapt the delays imposed by construction according to the delays required for different applications. It should be noted that a given application is defined by a certain number of parameters including the number of Bragg gratings, the wavelengths of the Bragg gratings and the spatial spacing between these gratings. Thus, if the application is changed, the entire hardware configuration will have to be modified to have parameters adapted to the new application.

[0019] Zhang et al., "In-Fiber Optic Sensors," the fourth chapter of a 2002 book referenced under the number ISBN: 978-0-8247-0732-3, contains a review of the different possible implementations of measurements using fiber Bragg gratings (FBGs), including in particular multiplexing techniques, for example WDM or TDM. This chapter discloses for example a spectral interrogation device of Bragg wavelengths, comprising: at least one optical source configured to emit a light signal, at least one measuring optical fiber comprising a series of successive Bragg gratings configured to reflect the light signal according to bands of different wavelengths, and at least one wavelength detector.

[0020] The article by Lloyd et al, "Resonant Cavity Time-Division-Multiplexed Fiber Bragg Grating Sensor Interrogator", IEEE Photonics Technology Letters, vol. 16, no. 10, October 2004, pages 2323-2325, ISSN: 1041-1135, DOI: 10.1109 / LPT.2004.834849 describes a TDM measurement device for detecting the return of light signals reflected by FBGs of the same wavelength. This article teaches a TDM-type resonant cavity comprising an arm comprising a total reflector. A light pulse introduced into the cavity induces responses from the FBGs which are then amplified by an optical amplifier SOA; the amplified signals can then be more easily discriminated from the baseline signal and thus detected.

[0021] CN 101 765 031 A, CN 102 879 025 A and JP 2015 031594 A describe a technique combining WDM and TDM for interrogating multiple optical fibers, each comprising a series of FBGs. CN 101 765 031 A uses optical switches to select the group of optical fibers to be interrogated. Within each group of optical fibers, a time delay is introduced to allow discrimination between different fibers and different FBGs. CN 102 879 025 A and JP 2015 031594 A operate in a similar manner with an optical switch and delay loops.

[0022] US 2007 / 024857 A1 operates on a different principle of interrogating a succession of FBGs based on interferometry.

[0023] The article by Dakin et al, "Distributed and Multiplexed Fibre Grating Sensors, including discussion of problem areas", IEICE Transactions on Electronics, Institute of Electronics, vol. E83-C, no. 3, March 3, 2000, pages 391-399, ISSN: 0916-8524, describes a different system for interrogating a single large FBG using a Michelson interferometer, which includes an optical arm with a total reflector.

[0024] The object of the present invention is to overcome the aforementioned drawbacks by proposing a spectral interrogation device making it possible to successively measure Bragg wavelength bands of at least one optical fiber in a precise, robust and flexible manner where the parameters can be adapted to the application. Présentation de l'invention

[0025] The subject of the invention is a device for spectral interrogation of Bragg wavelengths, comprising: at least one optical source configured to emit a light signal, at least one measurement optical fiber comprising a series of successive Bragg gratings configured to reflect the light signal according to bands of different wavelengths, at least one reflection optical fiber comprising a total reflection element, at least one wavelength detector, and at least one optical switch configured to switch the passage of the light signal between said at least one optical source, said at least one measurement optical fiber, said at least one reflection optical fiber, and said at least one detector according to a sequence of three operating modes comprising a first mode, called start mode, in which the light signal emitted by a given optical source is guided from said optical source to a corresponding measurement optical fiber, a second mode, called measurement mode,wherein said light signal is guided to make a predetermined number of round trips in a line formed by a coupling between said measuring optical fiber and a corresponding reflection optical fiber, thus generating a predetermined delay between the successive Bragg gratings, and a third mode, called detection mode, in which said light signal is guided towards a corresponding detector which is configured to successively measure the wavelength bands associated with the Bragg gratings included in said measuring fiber.

[0026] This device allows successively measuring the Bragg wavelength bands of at least one optical fiber in a precise, robust and flexible manner. It allows to significantly reduce the constraint on the distance between two successive Bragg gratings. The choice of the wavelength bands of the Bragg gratings is arbitrary, it is not obligatory that they are located in the center of a given interval. In addition, it does not require much optical fiber length, the measuring optical fiber itself serves as a retarder.

[0027] Advantageously, the device comprises at least one configurable driver configured to control the switching of the optical switch during the start-up, measurement and detection modes according to predetermined parameters.

[0028] Thus, the device is dynamic so that it is easily adaptable to different types of applications.

[0029] Advantageously, said at least one configurable driver is configured to control the predetermined number of round trips of the light signal during the measurement mode as a function of the distance between the Bragg gratings and the measurement rate of the detector.

[0030] Thus, by adapting the number of round trips according to the desired delay between successive Bragg gratings, the device can have a minimal distance between two consecutive Bragg gratings, which can thus be installed in small equipment.

[0031] Advantageously, said at least one optical source is a source of broad spectral band light signals whose width is between approximately 10 nm and approximately 150 nm, the period of a light pulse being between approximately 10 ns and approximately 100 ns.

[0032] Advantageously, said at least one optical source is a broad spectrum source (e.g. SLED or ASE) configured to emit at each emission instant, a light signal comprising all the wavelengths of the spectral band.

[0033] Advantageously, the device comprises additional lengths of optical fibers configured to take into account the propagation time of the light signal as well as the switching time of the optical switch.

[0034] This allows for a margin of uncertainty so as not to be precise to the nanosecond and to be sure that the train is not cut off in the middle.

[0035] According to a first embodiment, the device comprises at least one circulator configured to couple said measuring optical fiber to said corresponding source during the start-up mode and to couple said measuring optical fiber to said corresponding detector during the detection mode.

[0036] Advantageously, said at least one optical switch is configured to switch the passage of the light signal according to two switching operations, including a first switching operation in which said measuring optical fiber is connected to said corresponding source via the corresponding circulator in accordance with the start-up mode, a second switching operation in which said measuring optical fiber is connected to said corresponding reflection optical fiber in accordance with the measurement mode, and a return to the first switching operation, in which said measuring optical fiber is connected to said corresponding detector via said circulator in accordance with the detection mode.

[0037] Advantageously, the device includes: a set of N first optical fiber paths, each of which comprises a source, a circulator, and a detector, a set of N second optical fiber paths, each of which corresponds to a reflection optical fiber, a set of M third optical fiber paths, each of which corresponds to a measurement optical fiber, and said at least one optical switch is a multiple optical switch configured to connect together the different first channels, second channels and third channels according to said start-up, measurement and detection modes and by associating said set of N first channels with said set of N second channels to interrogate the Bragg gratings included in said set of M third channels.

[0038] According to a second embodiment, said at least one optical switch is configured to switch the passage of the light signal according to three connections including a first connection in which said measuring optical fiber is connected to said corresponding source in accordance with the start mode, a second connection in which said measuring optical fiber is connected to a corresponding reflection optical fiber in accordance with the measurement mode, and a third connection, in which said reflection optical fiber is connected to said corresponding detector in accordance with the detection mode.

[0039] Advantageously, the device includes: a set of N first optical fiber paths, each of which comprises a source, a set of N second optical fiber paths, each of which corresponds to a reflection optical fiber, a set of M third optical fiber paths, each of which corresponds to a measurement optical fiber, and a set of N fourth optical fiber paths, each of which comprises a detector, and said at least one optical switch is a multiple switch configured to connect together the different first paths, second paths, third paths and fourth paths according to said start-up, measurement and detection modes and by associating said set of N first paths with said set of N second paths and with said set of N fourth paths to interrogate the Bragg gratings included in said set of M third paths.

[0040] The invention also relates to a sensor comprising a device according to any one of the preceding characteristics, said sensor being configured to measure a physical parameter from variations in the Bragg wavelength bands.

[0041] Other advantages and characteristics of the invention will appear in the detailed non-limiting description below. Brève description des figures

[0042] Embodiments of the invention will now be described, by way of non-limiting examples, with reference to the accompanying drawings, in which: [ Fig. 1 ] very schematically illustrates a device for spectral interrogation of Bragg wavelengths, according to one embodiment of the invention; [ Fig. 2A ] [ Fig. 2B ] [ Fig. 2C ] very schematically illustrate variants of a spectral interrogation device, according to a first preferred embodiment of the invention; [ Fig. 3A ] [ Fig. 3B ] very schematically illustrate variants of a spectral interrogation device, according to a second preferred embodiment of the invention; [ Fig. 4 ] schematically illustrates a sensor for measuring a physical parameter by Bragg gratings, according to one embodiment of the invention; [ Fig. 5A ] very schematically illustrates a spectral interrogation device with temporal separation by spatial distancing, according to the prior art; and [ Fig. 5B ] [ Fig. 5C ] very schematically illustrate spectral interrogation devices with time separation by parallel and serial delays respectively, according to the prior art. Description des modes de réalisation

[0043] The principle of the invention is to achieve a temporal separation by bouncing the optical signal to increase the delay between the Bragg gratings. More particularly, the invention proposes to couple the measurement Bragg gratings with a fixed reflector and to make as many round trips as necessary to artificially increase the distance between the gratings. This increases the temporal delay and thus allows a detector to discriminate the signals coming from the different Bragg gratings.

[0044] There Fig. 1 very schematically illustrates a device for spectral interrogation of Bragg wavelengths, according to one embodiment of the invention.

[0045] The spectral interrogation device 1 comprises at least one optical source 3, at least one measuring optical fiber 5, at least one reflection optical fiber 7, at least one detector 9, at least one optical switch 11, and at least one configurable driver 12.

[0046] Each optical source 3 (the figure 1 shows only one) is configured to emit a light signal (e.g., a pulse of light) 13 with a broad spectral band. Advantageously, the spectral width of a light signal 13 is between approximately 10 nm and approximately 150 nm. The period of a light signal 13 may be between approximately 10 ns and approximately 100 ns.

[0047] For example, the optical source 3 is a SLED (superluminescent diode) or ASE (amplified spontaneous emission source) configured to emit at each emission instant, a light signal 13 comprising all the wavelengths of the spectral band.

[0048] Each optical fiber measures 5 (the figure 1 only shows one) comprises a series of successive FBG Bragg gratings 15 configured to reflect the light signal 13 according to bands 17 or peaks of different wavelengths.

[0049] It will be noted that a Bragg grating 15 is a local modulation of the refractive index forming a wavelength-selective reflector. Each Bragg grating 15 corresponds to a wavelength or rather to an interval or band 17 of wavelengths. Thus, a Bragg grating 15 is a kind of band-pass filter which passes all the bands 17 of wavelengths of the light signal 13 except for one band (or peak) which is reflected. The different bands 17 of wavelengths are multiplexed in the measuring optical fiber 5.

[0050] Furthermore, each reflection optical fiber 7 (the figure 1 only shows one) comprises a total reflection element 19. For example, a totally reflecting mirror 19 (i.e. >99%) is installed on one end of the reflection optical fiber 7.

[0051] Each detector 9 (the figure 1 only shows one) is configured to measure the wavelength bands 17 associated with the different Bragg gratings 15. For example, the detector 9 can be of the optical spectrum analyzer type OSA (Optical Spectrum Analyzer) or a wavelength measuring device (wavemeter) or a photo detector (photo transistor, photodiode, avalanche photodiode) allowing the measurement of the different Bragg peaks.

[0052] The at least one optical switch 11 (the figure 1 only shows one) is electronically controlled by the configurable driver 12 and is advantageously very fast with a switching time of less than approximately 100 ns and whose insertion losses are quite low, of the order of 1 dB or less.

[0053] The optical switch 11 is configured to switch the passage of the light signal 13 between the optical source(s) 3, the measuring optical fiber(s) 5, the reflection optical fiber(s) 7, and the detector(s) 9 according to a sequence of three consecutive operating modes. These operating modes comprise first, second and third modes, called start, measurement, and detection modes, respectively. It will be noted that the switching of the optical switch 11 is controlled by the configurable driver 12 during these start, measurement and detection modes according to predetermined parameters.

[0054] During the start-up mode, the light signal 13 emitted by a given optical source 3 is guided from this optical source 3 to a corresponding measuring optical fiber 5.

[0055] Then, during the measurement mode, the light signal 13 is confined in a line formed by a coupling between the measurement optical fiber 5 and a corresponding reflection optical fiber 7 to make a predetermined number of round trips. The propagation of the light signal 13 according to this back-and-forth movement for a predetermined duration between the series of Bragg gratings 15 of the measurement optical fiber 5 and the totally reflective element 19 at the end of the reflection optical fiber 7 generates a predetermined delay between the successive Bragg gratings 15.

[0056] More particularly, the configurable driver 12 is configured to control the predetermined number of round trips of the light signal 13 (or the predetermined duration) during the measurement mode as a function of the distance between the Bragg gratings 15 and the measurement rate of the detector 9.

[0057] For example, for an interrogation device 1 comprising an optical source 3 emitting a light pulse 13 whose period is 10ns, and a measuring optical fiber 5 whose distance between two successive Bragg gratings is 15 cm which corresponds to a time shift of 1.5ns, the predetermined number is set for at least seven round trips. Indeed, so that the wavelength bands corresponding to two successive Bragg gratings do not overlap, it is necessary for the time shift to be greater than 10ns, that is to say, greater than approximately 7 times 1.5ns. Thus, when the light pulse 13 makes at least seven round trips between each two successive Bragg gratings, the detector 9 can easily discriminate the signals from the different Bragg gratings.

[0058] Advantageously, in order not to be precise to the nanosecond, the interrogation device 1 comprises additional lengths 21 of optical fibers. These additional lengths 21 are configured to take into account the propagation time of the light signal 13 as well as the switching time of the optical switch 11.

[0059] Finally, during the detection mode, the wavelength bands 17 of the light signal 13 are guided towards a corresponding detector 9. This detector 9 is configured to successively measure these wavelength bands 17 associated with the Bragg gratings 15 inscribed in the measuring optical fiber 5.

[0060] Thus, the interrogation device 1 according to the invention functionally and modulably expands the distance between two successive Bragg gratings 15 by cleverly creating a back-and-forth movement between the Bragg gratings 15 themselves.

[0061] It should be noted that a typical power loss of a fast optical switch is of the order of 0.6 dB, which represents 1.2 dB for a round trip. If we set a power loss limit of approximately 10 dB to 15 dB, this corresponds to a total of approximately 10 round trips. This limit of 10 dB to 15 dB is only indicative; it can be increased if we use a very sensitive detector such as APD and / or if we use a source with greater power.

[0062] For example, considering the application described with reference to the Fig. 5A of the prior art implementing an optical source emitting a 30 ns light pulse. To create a delay of 15 ns corresponding to a total delay of 3 m between two successive Bragg gratings and by making 10 round trips, it is sufficient that the minimum distance between these two successive Bragg gratings is 0.3 m. By taking a safety margin of 10% to 20%, we arrive at an inter-grating distance of approximately 35 cm. By increasing the number of round trips, we can further reduce the distance between the Bragg gratings.

[0063] It should be noted that despite the fact that the Fig. 1 shows only one optical switch, it is of course possible that the interrogation device could include several independent optical switches operating in parallel. In the first case, the optical switches can share the same configurable driver with a common switching sequence. In the second case, each of the switches has its own configurable driver and thus its own switching sequence. The maximum number of switches depends solely on the size of the interrogation device.

[0064] Solely by way of non-limiting examples and for reasons of clarity, the remainder of the description only considers interrogation devices each comprising a single single or multiple optical switch and a single driver.

[0065] THE Figs. 2A-2C very schematically illustrate variants of a spectral interrogation device, according to a first preferred embodiment of the invention.

[0066] According to this first embodiment, the spectral interrogation device 1 comprises at least one optical circulator 23 configured to couple each measurement optical fiber 5 to a corresponding optical source 3 during the start-up mode and to then couple the measurement optical fiber 5 to the corresponding detector 9 during the detection mode.

[0067] The optical switch 11 is configured to switch the passage of the light signal 13 according to two switching operations. In the first switching operation, the measuring optical fiber 5 is connected to the corresponding optical source 3 via the corresponding circulator 23 in accordance with the start mode. In the second switching operation, the measuring optical fiber 5 is connected to the corresponding reflection optical fiber 7 in accordance with the measurement mode. Then, the optical switch 11 returns to the first switching operation, in which the measuring optical fiber 5 is connected to the corresponding detector 9 via the circulator 23 in accordance with the detection mode.

[0068] More specifically, the Fig. 2A shows a block diagram of a spectral interrogation device 1 comprising a single optical source 3, a measurement optical fiber 5, a reflection optical fiber 7, a detector 9, an optical circulator 23, an optical switch 11, and a configurable driver 12.

[0069] During the first switching, the optical switch 11 is by default in position P1. Thus, when a broadband light signal 13 is emitted by the optical source 3, this light signal goes from port C1 to port C2 of the optical circulator 23 and passes via the optical switch 11 to the measurement fiber 5 comprising the series of Bragg gratings 15.

[0070] During the second switching, the optical switch 11 moves to position P2. Then, the light signal is reflected on the Bragg gratings 15 and returns to the reflection fiber 7 comprising the fully reflective element 19. The light signal makes round trips in the cavity formed by the coupling between the measurement 5 and reflection 7 optical fibers. With each round trip, the time delay between two successive Bragg gratings 15 increases. The necessary number of round trips depends on the distance between the Bragg gratings 15 as well as the measurement rate of the detector 9.

[0071] When the minimum number of round trips is reached, the optical switch 11 returns to the first switching by switching back to position P1. The light signal is then directed to port C3 of the optical circulator 23 to be detected by the detector 9.

[0072] There Fig. 2B shows a block diagram of a spectral interrogation device 1 comprising N optical sources 3, N reflection optical fibers 7, N detectors 9, N circulators 23, M measurement optical fibers 5, an optical switch 11, and a configurable driver 12.

[0073] According to this example, the interrogation device 1 comprises a set of N first V11-V1N fiber optic channels, a set of N second V21-V2N fiber optic channels, a set of M third V31-3M fiber optic channels and a multiple optical switch 11 controlled by the configurable driver 12. N is an integer greater than or equal to 1 and M is an integer greater than or equal to N. It will be noted that the multiple optical switch 11 can be replaced by a plurality of independent switches controlled by a common driver or by a plurality of multiple drivers. Of course, it is also possible to have several multiple switches.

[0074] Each channel of the set of N first channels V11-V1N of optical fibers comprises an optical source 3, an optical circulator 23, and a detector 9. The set of N second channels V21-V2N of optical fibers correspond to N reflection optical fibers 7. The set of M third channels V31-V3M of optical fibers correspond to M measurement optical fibers 5.

[0075] For example, the optical switch 11 is a multiple switch with 2NxM arms, 2N arms of which are included in a first side of the switch and the other M arms are included in a second side of the multiple switch 11. This multiple optical switch 11 is configured to connect together the different first channels V11-V1N, second channels V21-V2N and third channels V31-V3M according to the three modes of starting, measurement and detection. More particularly, the multiple switch associates the set of N first channels V11-V1N with the set of N second channels V21-V2N to interrogate the Bragg gratings 15 included in the set of M third channels. Each first channel is associated with a corresponding second channel to interrogate a subset of the set of M third channels V31-V3M so that the entire set of M third channels V31-V3M ends up being interrogated.This makes it possible to carry out M consecutive series of the three operating modes, each series of which includes a start mode, a measurement mode and a detection mode.

[0076] Of course, other configurations or hybrid combinations can be produced according to this first embodiment depending on the number of arms on the first and second sides of the optical switch 11 used.

[0077] There Fig. 2C is an example showing the operating steps of a spectral interrogation device 1 in the case where N=1 and M=2.

[0078] Thus, according to this example, the spectral interrogation device 1 comprises a first channel V1 comprising an optical source 3, an optical circulator 23, and a detector 9; a second channel V2 corresponding to a reflection optical fiber 7; a multiple 2x2 optical switch 11; and two third channels V31, V32 corresponding to first 51 and second 52 measurement optical fibers comprising first and second series of Bragg gratings 15 respectively. The 2x2 optical switch is configured to connect the first V1 and second V2 channels on the first side of the optical switch 11 to the two third channels V31, V32 on the second side. It will be noted that when the optical switch 11 is said to be in position Pij this means that the ith channel on the first side of the optical switch 11 is connected to the jth channel on the second side of the optical switch 11 knowing that in this example, i=1 or 2, and j=1 or 2.

[0079] The operation takes place in two periods according to two consecutive series of the three operating modes of start-up, measurement and detection. In the first period (corresponding to a first series of the three operating modes), the first measurement optical fiber 51 comprising the first series of Bragg gratings is interrogated according to the following steps E11-E13: In step E11 (corresponding to a first start-up mode), the optical switch 11 is in position P11 to guide the light signal onto the first measurement optical fiber 51 comprising the first series of Bragg gratings.

[0080] In step E12 (corresponding to a first measurement mode), the optical switch 11 is switched to position P21 so that the light signal travels back and forth between the mirror 19 included in the reflection optical fiber 7 and the first series of Bragg gratings included in the first measurement optical fiber 51.

[0081] In step E13 (corresponding to a first detection mode), the optical switch 11 is switched back to position P11 so that the light signal goes to the detector 9 via the circulator 23.

[0082] In a second step (steps E21-E23), the second measurement optical fiber 51 comprising the second series of Bragg gratings is interrogated according to the following steps E21-E23: In step E21 (corresponding to a second start-up mode), the optical switch 11 is in position P12 to guide the light signal onto the second measurement optical fiber 52 comprising the second series of Bragg gratings.

[0083] In step E22 (corresponding to a second measurement mode), the optical switch 11 is switched to position P22 so that the light signal travels back and forth between the mirror 19 included in the reflection optical fiber 7 and the second series of Bragg gratings included in the second measurement optical fiber 52.

[0084] In step E23 (corresponding to a second detection mode), the optical switch 11 is switched back to position P12 so that the light signal goes to the detector via the circulator.

[0085] This operating mode therefore makes it possible to interrogate two Bragg grating fibers with a 2x2 switch. In general, we can have any number N greater than or equal to 1 of optical sources 3 and reflection optical fibers 7, as well as any number M of measurement optical fibers 5 with M>N, with a 2NxM multiple optical switch as illustrated in the Fig. 2A .

[0086] THE Figs. 3A et 3B very schematically illustrate variants of a spectral interrogation device, according to a second preferred embodiment of the invention.

[0087] According to this second embodiment, the spectral interrogation device 1 does not include a circulator. Indeed, one of the arms of the optical switch 11 is used as a detection arm, thus making it possible to do without the use of a circulator.

[0088] In this case, the optical switch 11 is configured to switch the passage of the light signal according to three connections. In the first connection, the measuring optical fiber 5 is connected to the corresponding optical source 3 in accordance with the start mode. In the second connection, the measuring optical fiber 5 is connected to a corresponding reflection optical fiber 7 in accordance with the measurement mode. In the third connection, the reflection optical fiber 7 is connected to the corresponding detector 9 in accordance with the detection mode.

[0089] More specifically, the Fig. 3A shows a block diagram of a spectral interrogation device 1 comprising a single optical source 3, a measurement optical fiber 5, a reflection optical fiber 7, a detector 9, and a multiple 2x2 optical switch 11 controlled by a configurable driver 12.

[0090] According to this example, the first and second arms of a first side of the switch are connectable to the optical source 3 and to the reflection optical fiber 7, respectively. The first and second arms of a second side of the switch are connectable to the measurement optical fiber 5 and to the detector 9, respectively. Thus, by using one arm of the optical switch 11 as a detection arm, the assembly is simplified and the number of components is reduced.

[0091] When first connected, the optical switch 11 is by default in position P11. Thus, when a broadband light signal 13 is emitted by the optical source 3, this light signal passes via the optical switch 11 to the measuring fiber 5 comprising the series of Bragg gratings 15.

[0092] At the second connection, the optical switch moves to position P21. Then, the light signal 13 is reflected on the Bragg gratings 15 and returns to the reflection fiber 7 comprising the fully reflective element 19. The light signal makes round trips in the coupling line between the measurement 5 and reflection 7 optical fibers. Once the minimum number of round trips is reached, the device 1 switches to the configuration of the third connection.

[0093] During this third connection, the optical switch 11 moves to position P22 so that the bands 17 of different wavelengths of the light signal are directed towards the detector 9.

[0094] There Fig. 3B shows a block diagram of a spectral interrogation device comprising N optical sources 3, N reflection optical fibers 7, N detectors 9, M measurement optical fibers 5, an optical switch 11, and a configurable driver 12.

[0095] More particularly, this device comprises a set of N first V11-V1N fiber optic channels, a set of N second V21-V2N fiber optic channels, a set of M third V31-V3M fiber optic channels, a set of N fourth V41-V4N fiber optic channels, and a multiple optical switch 11 controlled by a configurable driver 12. As previously, N is an integer greater than or equal to 1 and M is an integer greater than or equal to N.

[0096] Each of the channels of the set of N first channels V11-V1N of optical fibers comprises an optical source 3. The set of N second channels V21-V2N of optical fibers correspond to N reflection optical fibers 7. The set of M third channels V31-V3M of optical fibers correspond to M measurement optical fibers 5. The set of N fourth channels V41-V4N of optical fibers comprises N detectors 9, respectively.

[0097] For example, the set of N first optical fiber paths V11-V1N and the set of N second optical fiber paths V21-V2N are connectable to a first side of the optical switch 11. The set of M third optical fiber paths V31-V3M are connectable to a second side of the optical switch 11. The set of N fourth optical fiber paths V41-V4N can be connected to the first or second side of the optical switch 11. Alternatively, a portion of the set of N fourth optical fiber paths V41-V4N can be connected to the first side of the optical switch 11 and the other portion to the second side of the optical switch. Of course, other configurations or combinations are possible depending on the number of arms on the first and second sides of the optical switch used.

[0098] The multiple optical switch 11 (or several switches) is configured to connect together the different first channels, second channels, third channels and fourth channels according to the three operating modes of starting, measuring and detecting and by associating the set of N first channels with the set of N second channels and with the set of N fourth channels to interrogate the Bragg gratings included in the set of M third channels.

[0099] There Fig. 4 schematically illustrates a sensor for measuring a physical parameter by Bragg gratings, according to one embodiment of the invention.

[0100] The sensor 61 comprises a spectral interrogation device 1 according to the embodiments described above. This sensor 61 is configured to measure a physical parameter (for example, temperature, deformation, displacement, etc.) from the variations of the Bragg wavelength bands 15.

[0101] Indeed, a variation in the physical parameter causes a modulation of the Bragg grating and therefore a variation in the positions of the wavelength bands relative to a reference position. The amplitude of this variation is representative of the physical parameter to be measured.

[0102] Thus, the invention has several advantages over the state of the art. A first advantage concerns the fact that the spectral position of the Bragg gratings is completely free or arbitrary. It is not necessary to fix the wavelengths in specific intervals. A constraint that is found in the time division multiplexing methods based on spectral delay according to the prior art.

[0103] A second advantage is that the required optical fiber length is shorter, since the measurement optical fiber is used as the element for generating the delay between the Bragg gratings. It should be noted that in time division multiplexing methods based on spectral delay, several tens of meters of optical fiber are required. This long length complicates integration and assembly.

[0104] A third advantage is the fact that the minimum possible distance between two consecutive Bragg gratings can be reduced by adapting the number of round trips. The limit lies only in the fact that with each round trip, the optical power is reduced. Thus, to maintain a sufficient optical level, one can either limit oneself to about ten round trips or use a more powerful optical source, in which case one can increase the number of round trips.

[0105] A fourth advantage is the fact that the device according to the invention is dynamic. It is possible to very simply adapt the number of round trips of the light pulse. This is not possible according to the state of the art since the delay is fixed by the length of the fiber, which is fixed.

[0106] Of course, various modifications may be made by those skilled in the art to the invention which has just been described, solely by way of non-limiting examples.

Claims

1. Bragg wavelength spectral interrogation device, having: - at least one optical source (3) designed to emit a light signal (13), - at least one measurement optical fibre (5) comprising a series of successive Bragg gratings (15) designed to reflect the light signal in different wavelength bands, - at least one reflective optical fibre (7) comprising a total reflection element (19), - at least one wavelength detector (9), and - at least one optical switch (11) designed to switch the passage of the light signal between said at least one optical source (3), said at least one measurement optical fibre (5), said at least one reflective optical fibre (7), and said at least one detector (9) according to a sequence of three operating modes comprising a first mode, known as start-up mode, in which the light signal emitted by a given optical source (3) is guided from said optical source to a corresponding measurement optical fibre (5), a second mode, known as measurement mode, in which said light signal is guided to make a predetermined number of return trips in a line formed by a coupling between said measurement optical fibre (5) and a corresponding reflective optical fibre (7), thus generating a predetermined delay between the successive Bragg gratings, and a third mode, known as detection mode, in which said light signal is guided to a corresponding detector (9) which is designed to successively measure the wavelength bands associated with the Bragg gratings comprised in said measurement fibre.

2. Device according to Claim 1, characterized in that it includes at least one parameterizable driver (12) designed to control the switching of the optical switch (11) during start-up, measurement and detection modes according to predetermined parameters.

3. Device according to Claim 2, characterized in that said at least one parameterizable driver (12) is designed to control the predetermined number of return trips of the light signal during the measurement mode as a function of the distance between the Bragg gratings (15) and the measurement rate of the detector (9).

4. Device according to any one of the preceding claims, characterized in that said at least one optical source (3) is a source of light signals with a wide spectral band whose width is between approximately 10 nm and approximately 150 nm, the period of a light pulse being between approximately 10 ns and approximately 100 ns.

5. Device according to Claim 4, characterized in that said at least one optical source (3) is a broadband source designed to emit, at each emission time, a light signal comprising all the wavelengths of the spectral band.

6. Device according to any one of the preceding claims, characterized in that it includes additional lengths (21) of optical fibres designed to take account of the propagation time of the light signal as well as the switching time of the optical switch.

7. Device according to any one of the preceding claims, characterized in that it includes at least one circulator (23) designed to couple said measurement optical fibre to said corresponding source during the start-up mode and to couple said measurement optical fibre (5) to said corresponding detector (9) during the detection mode.

8. Device according to Claim 7, characterized in that said at least one optical switch (11) is designed to switch the passage of the light signal in two switching operations, including a first switching operation in which said measurement optical fibre (5) is connected to said corresponding source (3) via the corresponding circulator (23) in accordance with the start-up mode, a second switching operation in which said measurement optical fibre (5) is connected to said corresponding reflective optical fibre (7) in accordance with the measurement mode, and a return to the first switching operation, in which said measurement optical fibre (5) is connected to said corresponding detector (9) via said circulator (23) in accordance with the detection mode.

9. Device according to Claim 7 or 8, characterized in that it includes: - a set of N first optical fibre channels (V11-V1N), each of which includes a source (3), a circulator (23), and a detector (9), - a set of N second optical fibre channels (V21-V2N), each of which corresponds to a reflective optical fibre (7), - a set of M third optical fibre channels (V31-V3M), each of which corresponds to a measurement optical fibre (5), and in that said at least one optical switch (11) is a multiple optical switch designed to connect together the different first channels, second channels and third channels according to said start-up, measurement and detection modes and associating said set of N first channels with said set of N second channels to interrogate the Bragg gratings comprised in said set of M third channels.

10. Device according to any one of Claims 1 to 6, characterized in that said at least one optical switch (11) is designed to switch the passage of the light signal according to three connections, including a first connection in which said measurement optical fibre (5) is connected to said corresponding source (3) in accordance with the start-up mode, a second connection in which said measurement optical fibre (5) is connected to a corresponding reflective optical fibre (7) in accordance with the measurement mode, and a third connection, in which said reflective optical fibre (7) is connected to said corresponding detector (9) in accordance with the detection mode.

11. Device according to Claim 10, characterized in that it includes: - a set of N first optical fibre channels (V11-V1N), each of which includes a source, - a set of N second optical fibre channels (V21-V2N), each of which corresponds to a reflective optical fibre, - a set of M third optical fibre channels (V31-V3M), each of which corresponds to a measurement optical fibre (5), and - a set of N fourth optical fibre channels (V41-V4N), each of which includes a detector, and in that said at least one optical switch (11) is a multiple switch designed to connect together the different first channels, second channels, third channels and fourth channels according to said start-up, measurement and detection modes and associating said set of N first channels with said set of N second channels and said set of N fourth channels to interrogate the Bragg gratings comprised in said set of M third channels.

12. Sensor including a device according to any one of the preceding claims, said sensor (61) being designed to measure a physical parameter from variations in Bragg wavelength bands.