System for the structural monitoring of structures and corresponding method for the structural monitoring of structures

Abstract

System for the structural monitoring of at least one structure (B), the system comprising: one or more optical fibers (F;F1,F2) coupled at a first end thereof to a node (20;200) of a passive optical network infrastructure for transmitting data (Idata) in said structure (B), said one or more optical fibers (F;F1,F2) extending inside said at least one structure (B) for at least a portion of their length, said length being comprised between said first end and a second end opposite to the first, wherein the length of said portion of optical fiber (F;F1,F2) varies in response to a deformation of said structure (B), wherein, in particular said second end of each optical fiber is coupled to a signal transmitting and receiving device (M) of a user of said network infrastructure, wherein said one or more optical fibers (F;F1,F2) are configured to transmit data (Idata) in a frequency band or in a set of data transmission frequency bands, and an interferometric measurement apparatus (10) configured to: send a laser test light signal (Ip) into at least one optical fiber (F;F1,F2) of said one or more optical fibers (F;F1,F2), wherein said test signal (Ip) propagates in said at least one optical fiber (F;F1,F2), receive a return light signal (Ir) in response to the propagation of said test signal (Ip) in said at least one optical fiber (F;F1,F2), wherein the return light signal (Ir) has a phase that is a function of the length of the at least one optical fiber (F;F1,F2), detect the amplitude of a measurement light signal (Im) obtained by making said return light signal (Ir) interfere with a reference light signal (Iref), in particular obtained from a source laser signal (I0) from whichwhich said test signal is obtained (Ip), the amplitude of said measurement light signal (Im) being a function of the phase of said return light signal (Ir), and obtain (232) values of overall length variations of said at least one optical fiber (F;F1), in particular determined by deformations originating from one or more among low-intensity natural phenomena, seismic phenomena, and anthropogenic stresses, as a function of said detected amplitude of the measurement light signal (Im), analyze (240) time series of said values of overall length variations to obtain (240) parameters (CS) of modal vibrations of said structure (B) or to detect anomalies (AN) with respect to time series (εms(t)) or their parameters under reference conditions.

Description

[0001] "System for the structural monitoring of structures and corresponding method for the structural monitoring of structures"

[0002] TEXT OF THE DESCRIPTION

[0003] Technical field

[0004] The description relates to the monitoring of the structural state of structures , infrastructures and buildings .

[0005] One or more embodiments can be advantageously implemented in a network infrastructure ( for example , a data network infrastructure ) with optical fiber cabling .

[0006] Background

[0007] Structural integrity assessment , or Structural Health Monitoring ( SHM) , has the obj ect of monitoring and veri fying the structural state of buildings and infrastructures ( for example , bridges ) , in order to identi fy potential hazards , caused for example by geological phenomena ( earthquakes , landslides ) or by the deterioration of materials , which in turn can lead to failures or collapses of the structure .

[0008] The structural monitoring o f buildings is a very important topic in the field of civil engineering and civil protection; its obj ect is the prevention of damage to people and infrastructures through the detection of variations in the parameters of the structures , the geometry, or the construction materials that can be indicative of potential threats to the stability of the structures and represent a danger to the population .

[0009] Conventional methods for performing SHM are mainly based on visual assessments of the structural state by expert personnel , or on the installation of speci fic sensors such as accelerometers , strain-meters ( optical fiber or electronic ) . Such monitoring mostly represent targeted interventions , carried out as needed ( for example after an earthquake event ) , with long execution times and high costs .

[0010] Performing monitoring on a large scale and over the long term is therefore currently particularly di f ficult and / or costly in terms of cost .

[0011] Obj ect and summary

[0012] One or more embodiments have the obj ect of facilitating the structural monitoring of structures such as , for example , buildings , bridges , towers , skyscrapers .

[0013] According to one or more embodiments , said obj ect is achieved via a system for the structural monitoring of at least one structure .

[0014] The system comprises : one or more optical fibers coupled at a first end thereof to a node of a passive optical network infrastructure for transmitting data in said structure , said one or more optical fibers extending inside said at least one structure for at least a portion of their length, said length being comprised between said first end and a second end opposite to the first , wherein the length of said portion of optical fiber varies in response to a deformation of said structure , wherein, in particular said second end of each optical fiber is coupled to a signal transmitting and receiving device of a user of said network infrastructure , wherein said one or more optical fibers are configured to transmit data in a frequency band or in a set of frequency bands for data transmission, and an interferometric measurement apparatus configured to : send a laser test light signal into at least one optical fiber of said one or more optical fibers , wherein said test signal propagates in said at least one optical fiber, receive a return light signal in response to the propagation of said test signal in said at least one optical fiber, wherein the return light signal has a phase that is a function of the length of the at least one optical fiber, detect the amplitude of a measurement light signal obtained by making said return light signal interfere with a reference light signal , in particular obtained from a source laser signal from which said test signal is obtained, the amplitude of said measurement light signal being a function of the phase of said return light signal , and obtain values of overall length variations of said at least one optical fiber, in particular determined by deformations originating from one or more among low- intensity natural phenomena, seismic phenomena, and anthropogenic stresses , as a function of said detected amplitude of the measurement light signal , analyze time series of said values of overall length variations to obtain parameters of modal vibrations of said structure or to detect anomalies with respect to time series or their parameters under reference conditions .

[0015] One or more embodiments relate to a corresponding method .

[0016] The claims are an integral part of the technical teaching provided with reference to the embodiments .

[0017] Brief description of the figures

[0018] One or more embodiments wil l now be described, by way of example only, with re ference to the accompanying figures , in which : Figure 1 is a diagram illustrative of a network infrastructure ( for example , a passive optical network) in which a corresponding system for the structural monitoring of buildings according to embodiments of the present description can be implemented,

[0019] Figure 2 is a diagram illustrative of a system for the structural monitoring of buildings as described herein,

[0020] Figures 3A to 3C are diagrams illustrative of possible embodiments of a system for the structural monitoring of buildings as described herein, and

[0021] Figures 4A to 4C are diagrams illustrative of details of an interferometric measurement apparatus included in a system for the structural monitoring of buildings as described herein,

[0022] Figures 5 and 6 are diagrams illustrative of possible embodiments of the present description,

[0023] Figure 7 is a sectional view of a structure monitored via a system for the structural monitoring of buildings , and

[0024] Figure 8 is a block diagram illustrative of steps of a method as described herein .

[0025] The numbers and corresponding symbols in the di f ferent figures generally refer to corresponding parts unless otherwise indicated .

[0026] The figures are drawn to clearly illustrate the relevant aspects of the embodiments and are not necessarily drawn to scale .

[0027] The edges of the features drawn in the figures do not necessarily indicate the termination of the extent of the feature .

[0028] Detailed description

[0029] In the following description, one or more speci fic details are illustrated, for the purpose of providing an in-depth understanding of examples of embodiments of this description . The embodiments can be obtained without one or more of the speci fic details , or with other methods , components , materials , etc . In other cases , known operations , materials or structures are not illustrated or described in detail so that certain aspects of the embodiments will not be obscured .

[0030] A reference to "an embodiment" in the context of the present description is intended to indicate that a particular configuration, structure , or feature described with reference to the embodiment is comprised in at least one embodiment . Therefore , phrases such as " in one embodiment" or simi lar that may be present in one or more points of the present description do not necessarily refer to the same embodiment .

[0031] Furthermore , particular conformations , structures or features can be combined in any suitable way in one or more embodiments .

[0032] The references used herein are provided merely for convenience and therefore do not define the scope of protection or the scope of the embodiments .

[0033] For simplicity and ease of explanation, throughout this description, and unless the context indicates otherwise , similar parts or elements are indicated in the various figures with similar reference symbols and a corresponding description will not be repeated for each figure .

[0034] Optical fiber technology has undergone great development in recent years . Optical fibers are mainly used for digital data transmission, for example for internet traf fic, allowing relatively high data transmission speeds to be achieved .

[0035] Furthermore , fibers can be used as sensors , since their properties are influenced by the external environment , which can therefore be detected optically . Such use of optical fibers as sensors finds multiple applications thanks to advantageous characteristics such as , for example , the high speed of light propagation ( and therefore the low latency of the measurement signal ) and the flexibility and robustness of the optical measurement methods that can be implemented by means of such optical fibers .

[0036] Exemplary detection and / or measurement applications based on optical fibers comprise , for example : remote measurement of temperature and / or concentration of chemical components , earthquake detection, monitoring of marine cetaceans , monitoring of landslides and mudslides , monitoring of structural deformations of buildings and infrastructures , monitoring of vehicular traf fic along roads , and development of systems for smart cities and antiintrusion systems .

[0037] A fiber optic-based detection system exploits the properties of light propagation in order to detect environmental parameters such as strain, vibrations , and variations in pressure and / or temperature .

[0038] The di f ferent fiber optic-based detection or monitoring systems can involve measurements of amplitude , wavelength, polari zation or phase of a probe or test signal sent on the fiber .

[0039] Fiber optic-based measurement systems can be configured to operate as local sensors ( such as , for example , measurement systems based on Fiber Bragg Grating, FBG) or distributed, wherein the measurement is spaced along the entire length of the optical fiber .

[0040] The main measurement techniques in the field, in particular some of them being of the distributed type , comprise : state of polari zation, SOP, measurements , wherein an analysis of the polarization state of a laser signal transmitted through the optical fiber is performed [ Zhan et al , Science 371 , 2021 ] distributed acoustic sensing, DAS , a set of measurement systems wherein laser pulses and back- scattering from impurities distributed along the fiber are exploited ( also known as optical time domain ref lectometry, OTDR, and various variants ) and wherein time-of- f light measurements and interferometry are combined for the purpose of measuring relative phases between coherent pulses [Lindsey et al , JGR Solid Earth 125 , 2020 ; Zhan, Seismological Research Letters 91 , 2020 ] , and laser interferometry, LI , wherein variations in optical phase ( or frequency) of coherent laser signals transmitted through the fiber are measured, observable as variations in amplitude of an interferometric signal in response to constructive or destructive interference phenomena [Marra et al , Science 361 , 2018 ] .

[0041] In one or more embodiments of the present description, laser interferometry techniques based on transmission of laser signals on optical fiber are employed for the structural monitoring of buildings and / or structures .

[0042] In one or more embodiments of the present description an interferometric measurement apparatus is configured to cooperate with an optical fiber network infrastructure already set up for transmitting data to one or more buildings or structures . Said network infrastructure can be , for example , a (passive ) network infrastructure for conventional digital data transmission .

[0043] As mentioned, solutions as described herein exploit network infrastructures equipped with optical fiber cabling already intended for digital data traf fic . In such network infrastructures , the use of the optical fiber in such infrastructures facilitates providing broadband data connection directly to homes and of fices . As per se known to those skilled in the art , such infrastructures can comprise a passive optical network (passive optical network, PON) wherein the optical fibers branch out successively, in order to reach each neighborhood, area and building ( or in general structure ) .

[0044] A further branching can be provided in order to reach with optical fiber cabling the individual modems / routers placed in the homes or rooms of the end users . The term user refers to a user person or more generally to a system or apparatus , also operated automatically without the intervention of physical persons , of the signals transmitted via the optical fiber cabling .

[0045] For a more detailed treatment of some aspects of a network infrastructure as considered herein, one can consult , for example , CISCO, Understand GPON Technology available at : https : / / www . cisco . com / c / en / us / support / docs / switche s / cat al y st-pon- series / 216230 -under st and- gpon- technology . html .

[0046] The optical fibers therefore extend, for at least a part of their length, along the walls and flooring of the structures themselves , for example in dedicated conduits or tubes . A coupling between the structure and the optical fiber is thus obtained so that the length of the optical fiber varies as a function of any deformations of the buildings . In other words , the optical fibers of a network infrastructure as considered herein extend at least for a portion of their length inside structures ( for example , buildings , bridges or similar structures of which structural monitoring is to be performed) , wherein the length of the portion of optical fiber that extends inside the structures can vary in response to a deformation of the structures .

[0047] Figure 1 schematically illustrates a network infrastructure in which systems for the structural monitoring of buildings according to embodiments of the present description can be implemented .

[0048] More in detail , Figure 1 il lustrates a portion of a PON network infrastructure in which solutions as described herein can be implemented . Said portion of the network infrastructure , also called " last mile" portion, comprises the portion of the network infrastructure configured to transmit the s ignal from a cabinet C to a set of buildings or structures B .

[0049] A network infrastructure as considered herein comprises optical fiber cabling arranged between the cabinet C and the individual apartments or houses in the case of a f iber-to-the-home , FTTH infrastructure .

[0050] Such FTTH infrastructures make digital data traf fic ( for example , internet ) available from transmission centers (managed, for example , by the various telephone operators ) to sorting cabinets C where the data traf fic is sorted between buildings and / or homes connected thereto .

[0051] As schematically illustrated in Figure 1 , the signal used for data transmission, indicated here only by two arrows in the propagation directions , is divided several times according to a branched configuration . In the network infrastructure exempli fied in Figure 1 , the signal is divided a first time at the cabinet C, so as to transmit the signal to di f ferent primary nodes 20 . The primary node 20 can comprise optical splitters configured to further divide the signal so as to distribute the signal to the di f ferent buildings or structures, indicated with B, coupled to the primary node 20. In a PON network such secondary nodes comprise passive optical splitters, indicated with the reference 200.

[0052] Once it reaches a building or a structure B, the signal is thus further divided, for example via the splitter 200, which also identifies a node, a secondary node, so as to be distributed to each user of the network (for example, apartments, private houses or offices) .

[0053] As known to those skilled in the art, in network infrastructures PON / FFTH currently in use (and contrary to most long-distance telecommunication cabling) data traffic (for example, internet) can travel back and forth in the same fiber, so as to reduce complexity and costs for the implementation of the network infrastructure. In fact, in network infrastructures of the PON, FTTH type as exemplified in Figure 1, protocols suitable for bidirectional data traffic transmission are normally used, wherein the individual users are reached via multiplexing (for example, time division multiplexing) .

[0054] A network infrastructure as exemplified in Figure 1 is otherwise conventional in the art, and a more detailed treatment is superfluous here.

[0055] Figure 2 is illustrative of a system for monitoring the structural state of structures and / or buildings according to embodiments of the present description.

[0056] As illustrated in Figure 2, an interferometric apparatus 10 can be installed in proximity to a node 20 or 200 of an FTTH network infrastructure (for example an FTTH network infrastructure as described in relation to Figure 1) to which one or more optical fibers F are coupled (at a first end thereof) .

[0057] Said node can comprise: a secondary node 200, arranged in proximity to a single structure B, wherein the one or more optical fibers F couples the secondary node 200 to a corresponding plurality o f signal receiving and transmitting devices (modems and / or routers , for example ) of the individual users , or a primary node 20 , wherein the one or more optical fibers couples the primary node 20 to a corresponding plurality of secondary nodes 200 .

[0058] In one or more embodiments , the interferometric apparatus is coupled to a secondary node 200 , in particular a node therefore that distributes the optical signal in a building or structure B .

[0059] The optical fibers F extend for at least a portion of their length inside the structure B, so as to reach the various housing units , rooms or, in general , portions of the structure itsel f .

[0060] According to an aspect of the present description, such optical fibers F undergo deformations in response to deformations of the structure B within which they extend . In particular, as described in more detail in the following, the interferometric apparatus 10 coupled to the node 200 is configured to detect deformations and in particular variations in the length of the optical fibers F in response to deformations of the structure B within which they extend, so as to obtain information on the structural state of said structure .

[0061] Deformations , or, length variations of such optical fibers due to deformations of the structure within which they extend are therefore indicative of deformations of the structure along di f ferent paths , each detected by a corresponding optical fiber .

[0062] It will be noted how the system described herein exploits a network infrastructure already installed in the structure for which monitoring is to be performed, since the network infrastructure considered herein is a network infrastructure (with optical fibers ) of the type conventionally used for digital data traf fic .

[0063] With reference to Figure 2 , an interferometric apparatus 10 as described herein can comprise : a light source 100 , an interferometer 120 , and a detection device 140 .

[0064] The light source 100 , for example a laser light source , is configured to generate a light signal and to provide it as input to an interferometer 120 . This light signal provides a source signal I o to the interferometer which, as described in the following, can be configured to divide this source signal into a test signal Ip( or probe signal ) and a reference signal Iref , in order to perform the interferometric measurement .

[0065] The interferometer 120 can be configured to receive the source signal I o generated by the light source 100 and to divide the input signal Io into two signals : a test signal Ipprovided as input to one or more optical fibers F via the splitter 20 and which provides the measurement arm of the interferometric measurement system, and a reference signal Iref which is provided as input to a detection device 140 which provides the reference arm of the interferometric measurement system .

[0066] It is noted that , in other embodiments the reference signal Iref can be obtained from : a reference light signal Iref provided as input to an optical fiber F, so as to obtain a dedicated reference arm, or from the optical radiation already present inside the light source itsel f , for example in the laser cavity, in the case of sel f-mixing interferometry ( described below) .

[0067] Referring again to the interferometric apparatus illustrated in Figure 2 , the reference signal Iref is combined / superimposed with a signal whose phase is indicative of the length of the optical path traveled ( and therefore of the length of the optical fiber along which the signal propagates ) .

[0068] As illustrated, the splitter 200 ( a passive optical splitter, for example ) , which operates as a secondary node , receives as input both the test signal Ipand a data signal I data related to internet traf fic, for example , and to the data transmitted via the network infrastructure in general . The input signals Ip, I data are superimposed and provided as input to one or more optical fibers F .

[0069] Advantageously, the test signal Iptransmitted along the optical fiber F is combined with the data signal I data so as not to interfere with it .

[0070] For this purpose , the test signal Ipcan have a relatively low power, such that it does not interfere with the data signal I data .

[0071] The power of the test light signal Ipcan be determined as a function of the ( digital ) receiving / transmitting device of the end user ( for example , a modem or a router) and, in particular, of the input tolerances or the noise margins of the receiving / transmitting device , in order not to alter or interfere with the data signal Idata . Speci fically, the power of the test light signal Ipis chosen for example to be less than the lower limit of the dynamic range of the data receiver . The data receiver in fact has a maximum signal level , for example to consider the received level as a logical zero . Therefore the power of the test light signal Ipmust not for example exceed this value .

[0072] It has been observed that test signals characteri zed by a launch power of the test signal Ipless than 1 mW can satis fy these conditions and, at the same time , provide a return signal of power suitable for the interferometric measurement ( in terms of signal-to- noise ratio , for example ) . Note that this value , also determined by the reflection level of the interferometric arm, is given here for purely exemplary purposes and is not to be construed in a limiting sense of the possible embodiments .

[0073] In one or more embodiments , the light source 100 and / or the interferometer 120 can be configured to transmit a test signal Iphaving a wavelength outside the portion of the band ( or set of band portions ) used for transmitting the data signal Idata . In network infrastructures of the type conventionally in use operating at wavelengths corresponding to the infrared, IR, the interferometric measurement apparatus can be configured to operate at wavelengths corresponding to the near infrared or in the visible , thus making ( at least notionally) the measurement signals undetectable by the IR photodiodes of the network infrastructure .

[0074] In optical fibers conventionally used for network infrastructures as considered herein, the transmission of data traf fic occurs via signals having frequencies within the ( optimal ) transmission band of the optical fiber, for example via signals with wavelengths between 1535- 1565 nm or between 1490 and 1310 nm .

[0075] The interferometric apparatus can therefore be configured : to send a test signal Iphaving a wavelength comprised in portions of said transmission band of the optical fiber that are not used for transmitting the data signal Idata, and / or to send a test signal Iphaving a wavelength outside said transmission band of the optical fiber .

[0076] It has been observed that the attenuation of a signal at wavelengths outside the ( optimal ) transmission band of the optical fiber does not compromise the interferometric measurement in the case of optical paths having a length of the order of the length of the optical fibers considered herein, for example optical fibers with a length less than 10 km.

[0077] In order to separate the test signals Ip( and in general , the interferometric measurement signals ) and the data signals Idata, one can use for example WDM (Wavelength Division Multiplexing) filters so that the interferometric apparatus receives only interferometric measurement signals and the data traf fic is not disturbed by the interferometric measurement signals .

[0078] Figures 3A to 3C are illustrative of possible embodiments of a system as described herein .

[0079] Figures 3A to 3C are illustrative of embodiments of the present description in which the interferometric apparatus 10 is coupled directly to an optical fiber, downstream of the node 20 , 200 of the network infrastructure . In fact , in one or more embodiments , the interferometric apparatus 10 is coupled subsequently to such nodes , that is to say directly to one or more optical fibers in the final section of the ( PON) network infrastructure .

[0080] For simplicity of description, the interferometric apparatus 10 will be described in relation to embodiments in which the interferometric apparatus is associated with one optical fiber ( for example , an interferometric apparatus installed downstream or upstream of a node 20 , 200 of a network infrastructure ) .

[0081] With reference to Figure 3A, the test signal Ipis provided as input to an optical fiber, at a first end thereof , which extends for at least a part thereof in a building B of which the structural state is to be monitored .

[0082] According to an aspect of the present description, the test signal Ipis characteri zed by a coherence length greater than the length of the optical fiber F on which the interferometric measurement is intended to be performed, in particular of the di f ference in optical path between the measurement path (measurement arm of the interferometer ) and the reference path ( reference arm of the interferometer ) . For example , commercially available low-noise laser light sources can be employed for optical fibers with a length comprised, for example , between tens of meters and a kilometer, as they are suitable for generating test signals ( or source signals ) characteri zed by a suf ficient coherence length .

[0083] As known per se to those skilled in the art , a reflected signal is generated at the second end of the optical fiber F in the structure B, reflecting said test signal Ip. Such passive or parasitic signal reflection normally occurs in optical fibers currently in use for internet traf fic ( for example , at the end thereof connected to a signal receiving and transmitting device M of an apartment - for example , a modem or a router ) , therefore it is not necessary to associate a reflector device to the end of the optical fiber .

[0084] As illustrated, a return signal Ir, that is a reflected signal , from the fiber F propagates in the opposite direction to the test signal Ipand is received by the interferometer 120 and provided to the detection device 140 , superimposed on the reference signal Iref .

[0085] In optical fibers conventionally used in current FTTH / PON-type network infrastructures the intensity of the reflected signal at an end of the optical fiber ( for example , at a common connector thereof when not connected) can be greater than 1 / 100 of the intensity of the incident signal ; for an incident signal ( and therefore a test signal Ip) having a power of about 1 mW or less , a reflected signal ( and therefore a return signal Ir) having an intensity suf ficient for an interferometric measurement is obtained .

[0086] In the embodiment illustrated in Figure 4B, an optical reflector 128 is installed at a second end of the optical fiber F in order to reflect the test signal Ipand generate by reflection a reflected return signal Ir .

[0087] A wavelength filter, for example a WDM filter, or an optical splitter 1200 can be installed at said end where the reflector 128 i s positioned in order to separate the data traf fic signals from the test signal Ipsuch that : the data traf fic signals Idata arrive undisturbed at the end of the optical fiber F connected to a signal receiving / transmitting device M ( a router, for example ) , not shown in the figure , and the test signal Ipis directed to the optical reflector 128 .

[0088] Such embodiment may be desirable , for example , when a higher signal-to-noise ratio is desired since the optical reflector 128 installed at the second end of the optical fiber F facilitates providing a reflected signal Irof greater amplitude compared to a reflected signal obtained by passive / parasitic reflection as described in relation to Figure 3A.

[0089] Note that the configurations discussed in relation to Figures 3A and 3B represent the most easily implementable configurations in FFTH-PON network infrastructures based on simplex cabling, wherein the same optical fiber F is used bidirectionally both for transmitting data and for receiving data .

[0090] In such configurations the return signal Iris obtained by reflection of the measurement signal Ipat the second end of the optical fiber F . The return signal Iris therefore provided as input to the interferometer 120 for the purpose of the interferometric measurement .

[0091] The embodiment illustrated in Figure 3C is usable when two distinct optical fibers Fi, F2 are provided inside a building B for data traf fic and configured to transmit signals in two opposite directions , for example from a cabinet , for example C, or node , for example 20 , to a building, for example B, or, vice versa, from a building to a cabinet or node .

[0092] Such configuration can be representative , for example , of signal transmission architectures based on so-called duplex cabling, wherein a first optical fiber ( TX optical fiber ) is configured to transmit a signal and a second optical fiber (RX optical fiber ) is configured to receive the signal .

[0093] In the configuration illustrated in Figure 3C, a return signal Ircan be obtained simply by connecting the first and second optical fibers Fi, F2 , at their second ends . In other words : a test signal Ipexiting from the interferometer 120 is provided as input to the first optical fiber Fi at a first end thereof , the test signal propagates along the first optical fiber Fi ( TX optical fiber or transmission fiber, for example ) from its first end to its second end, said second end of the first optical fiber Fi being connected to a second end of the second optical fiber F2 , the return signal Irpropagates along the second optical fiber F2 (RX optical fiber, for example ) from the second end to the first end of the second optical fiber F2 , and the return signal Iris provided as input to the interferometer 120 where it is exploited for the purpose of the interferometric measurement .

[0094] Such dual- fiber configuration ( duplex cabling) is widely used for network infrastructures for transmitting signals over relatively large distances ( for example , for signal transmission along submarine cables ) and to a lesser extent in FFTH ( or FFTB ) infrastructures as considered herein . Those skilled in the art will appreciate how the configuration discussed in relation to Figure 3C highlights , despite its lower applicability, the flexibility of a system as described herein, simply implementable also in network infrastructures with duplex cabling .

[0095] Those skilled in the art can appreciate how solutions as described herein are advantageous compared to conventional solutions based on DAS techniques . In fact , the use of low-power test signals Ip( for example , such as not to interfere with the reception of the data signal Idata) , continuous ( at constant power ) , possibly at wavelengths outside the transmission band of the optical fibers ( for example , outside 1535- 1565 nm) , and with relatively narrow bandwidths ( for example , such as to occupy a single WDM channel ) may not be possible or introduce disadvantageous limitations in a measurement system based on DAS techniques , substantially based on the use of pulsed signals that can disturb the transmission and reception of the data signal Idata .

[0096] An interferometric measurement apparatus according to embodiments as described previously represents a relatively simple measurement apparatus compared to DAS measurement apparatuses that require bulky, expensive and energy-consuming hardware, making them unsuitable for large-scale installation and distribution .

[0097] Figures 4A to 4C are illustrative of details of an interferometer 120 according to various embodiments of the present description .

[0098] More in detail , interferometers 120 as described in relation to Figures 4A and 4B can be used in network infrastructures with simplex cabling ( as exempli fied in Figures 3A and 3B, for example ) and interferometers 120 as described in relation to Figure 4C can be used in network infrastructures with duplex cabling ( as exempli fied in Figure 3C, for example ) .

[0099] With reference to Figure 4A, an interferometer 120 as considered herein can comprise a three-port optical splitter 121 , for example a passive optical splitter of the type currently commercially available , having an input port 121a and two output ports 121b, 121c . The first optical splitter 121 is configured to receive at the input port 121a the signal I o generated by the laser device 100 and to divide the received input signal into a reference signal Iref and a test signal Ip.

[0100] The input signal Io can be divided into test signal Ipand reference signal Iref according to a desired power ratio ( for example , 50 : 50 or 90 : 10 ) .

[0101] The reference signal Iref is transmitted to a three- port optical combiner 122 configured to receive said reference signal at an input port 122a thereof . The transmission of the reference signal Iref from the first 121 to the second 122 three-port optical splitter occurs via an optical fiber having a known, relatively small and constant length, so as to provide the reference arm of the interferometer 120 .

[0102] An optical circulator or a splitter 123 is configured to receive the test signal Ipat a first port 123a thereof . A second port 123b of the optical circulator 123 is configured to : transmit the input test signal Ipinto an optical fiber F of which deformations or relative variations in length are to be measured (measurement arms of the interferometer 120 ) , and to receive the return signal Irtransmitted along said optical fiber F .

[0103] The return signal Iris thus extracted from a third port 123c of the optical circulator 123 and provided as input to the three-port optical combiner 122 .

[0104] The three-port optical combiner is thus configured to receive at two input ports 122a, 122b thereof the reference signal Iref and the return signal Irand to provide as output ( at port 122c ) a measurement signal Imobtained by superimposing the input reference signal Iref and return signal Ir. The measurement signal Imis then transmitted to the detection device 140 .

[0105] As illustrated in Figure 4A, a frequency modulator 126 ( for example , an acousto-optic modulator, AOM) can be interposed between the optical splitter 121 and the optical combiner 123 in order to modulate the frequency of the test signal Ip. This frequency modulation of the test signal Ipis reflected in an equal frequency modulation of the return signal Irand, therefore , in a frequency di f ference between the reference signal Iref and the return signal Irprovided as input to the optical combiner 122 .

[0106] Note that a frequency modulation of the test and / or return signals can also be obtained via devices other than an AOM, for example acoustic or piezometric transducers , or even from "parasitic" modulations induced by environmental sources ( such as hums or vibrations of apparatuses adj acent to the fiber ) .

[0107] As known per se to those skilled in the art , introducing or not a frequency di f ference between the reference signal Iref and the return signal Irfacilitates the interferometric measurement in heterodyne or homodyne mode , respectively .

[0108] More in detail , i f the interferometric measurement is operated in homodyne mode ( therefore with an interferometer 120 that does not comprise a frequency modulator 126 ) the reference signal Iref and the return signal Irare combined (via the optical combiner 122 ) into the measurement signal Imwhich in turn is sent to the detection device 140 ( comprising, for example , a pair of detectors , one for each output of the splitter and configured for phase calculation) . This measurement signal Imis maximum in the case of constructive interference and minimum in the case of destructive interference. The interference changes over time as a function of the length of the measurement arm ( i . e . , the optical fiber F arranged in the building B ) .

[0109] Note that in configurations where the reference signal Iref travels a relatively short path ( for example , in the case of local reference optical fibers , as described in relation to Figures 4A to 4C ) which is not influenced by variations induced by deformations or movements of the building to be monitored, the reference signal Iref can be assumed to be phase stable .

[0110] In configurations where this assumption is not veri fied ( for example , in the case of reference arms provided by relatively long optical fiber sections ) the phase measurement is indicative of the relative phase of the non-common sections between the measurement arm and the reference arm .

[0111] The detection device 140 is , in the case of homodyne detection, configured to sample the intensity variations of the measurement signal Improvided thereto and to derive , as a function of said intensity variations , the phase di f ference of the two input signals Iref , Ir and therefore the length variations of the optical fiber inside the building B.

[0112] I f the interferometric measurement is operated in heterodyne mode ( therefore with an interferometer 120 that comprises a frequency modulator 126 as in the embodiments illustrated in Figures 4A to 4C ) , the return signal Iris shi fted in frequency relative to the reference signal Iref . The combination of the reference signal Iref and the return signal Ir, i . e . , the measurement signal Im, is sent to the detector 140 which records a signal at a frequency equal to the di f ference between the frequencies of the two input signals .

[0113] For a more detailed treatment of these aspects of heterodyne detection, one can consult , for example , the publication by Donadello et al . , IEEE Trans . Instrum . Meas . 2005412 , 2023 .

[0114] Heterodyne detection is therefore implemented with a ( slightly) more complicated scheme in terms of hardware , as it involves one more component , and requires sampling at higher frequencies in addition to a demodulation algorithm . This greater hardware complexity is however balanced by better resolution and a better signal-to-noise ratio .

[0115] It can be appreciated how both methods facilitate extracting the spectral information on the type of variation of the optical length of the optical fiber F arranged inside the building B of which structural monitoring is to be performed .

[0116] Figure 4B is illustrative of a possible embodiment in which the interferometer 120 comprises : a four-port optical splitter 124 with ports 124a, 124b, 124c, 124d, and a reflector 128 .

[0117] As illustrated, the laser signal I o generated by the laser generator 100 is provided as input to the optical splitter 124 , at a first port 124a thereof .

[0118] The splitter 124 is configured to divide the input laser signal I o into a reference signal Iref and a test signal Ip, which can be taken at a second port 124b and a third port 124c, respectively, of the optical splitter 124 .

[0119] The reference signal Iref i s taken at the second port 124b of the optical splitter 124 and directed towards a reference optical fiber having a reflector 128 placed at an end thereof . The reflected signal ( indicated for simplicity with the same reference Iref in Figure 4B ) is received by the splitter 124 at the second port 124b thereof . This reference optical fiber with the reflector 126 installed at an end thereof provides the reference arm of the interferometer 120 .

[0120] The reflected signal passes again through the splitter 124 and is provided as output at a fourth port 124d thereof (where it is combined with the return signal Iras described below) .

[0121] The test signal Ipis extracted at the third port 124c of the optical splitter 124 and directed towards the optical fiber F arranged inside the building to be monitored .

[0122] The return signal Ir, received at the same third port 124c of the optical splitter 124 , is directed towards the fourth port of the optical splitter 1224 where it is combined / superimposed with the reference signal Iref . The measurement signal Imthus obtained is then directed towards the detector 140 .

[0123] As illustrated in Figure 4B , in a manner entirely similar to what was described previously, a frequency modulator 126 can be placed downstream of the optical splitter 124 in order to shift the frequency of the test signal Ipor the reference signal Iref ( depending on whether it is configured to receive the signal output from port 124c or 124b, respectively, of the optical splitter 124 ) . Said frequency modulator 126 facilitates heterodyne detection; the di f ference between homodyne and heterodyne detection and the advantages associated with the two modes have been discussed previously .

[0124] As already mentioned, the interferometer 120 schematically illustrated in Figure 4C is usable in embodiments in which the network infrastructure comprises duplex cabling as already discussed in relation to Figure 3C .

[0125] As illustrated, the source s ignal Io is provided as input to an optical splitter 122 , at a first input port 121a thereof . The optical splitter 121 is configured to divide the source signal I o received as input into a reference signal Iref and a test signal Iphaving a desired amplitude ratio .

[0126] The reference signal Iref is provided as input to an optical combiner 122 , at a first input 122a thereof .

[0127] The test signal Ipis sent towards a first optical fiber Fi ( a TX optical fiber , for example ) , at a first end thereof . The test signal Ippropagates along the first optical fiber Fi connected at a second end thereof opposite to the first to a second optical fiber F2 ( an RX optical fiber, for example ) . The return signal Irpropagates along the second optical fiber F2 and provided as input to the optical combiner 122 , at a second input 122b thereof .

[0128] The optical combiner 122 is configured to superimpose / combine the reference signal Iref and the return signal Irreceived as input into a measurement signal Imwhich is provided to the detector 140 .

[0129] Also in this configuration, a frequency modulator 126 can be placed downstream of the optical splitter 122 in order to frequency shi ft the test signal Ip. Said frequency modulator 126 facilitates heterodyne detection; the di f ference between homodyne and heterodyne detection and the advantages associated with the two modes have been discussed previously .

[0130] In one or more embodiments , the interferometric apparatus 10 can comprise an interferometric apparatus of the type currently called sel f-mixing interferometer, SMI ( also known as feedback interferometer or sel fmixing interferometer ) . In this configuration, a measurement signal Imis obtained by superimposing (making interfere) the source signal Io inside the source itself (for example, inside the laser cavity in the case where the light source comprises a laser source) and a return signal Irreceived in response to the reflection of the test signal Ip(obtained, for example, via a reflector placed at the second end of the optical fiber) .

[0131] The amplitude of the measurement signal Imobtained inside the laser cavity by introducing the modulation can be detected simply by sampling the monitor photodiode present in the source. In other words, the detection device can be provided by a detection device (for example, a photodiode) already incorporated in the light source (for example, a laser source) set up to monitor the power of the light source.

[0132] In other words, in such embodiments, the interferometric measurement apparatus 10 is configured to detect the amplitude of a measurement light signal Imobtained by making the return light signal Irinterfere with a reference light signal Iref obtained from a source laser signal Io from which said test signal is obtained Ip, inside the laser source that generates said source laser signal, in particular in a laser cavity.

[0133] Such embodiments can be advantageous as they involve a significant simplification of the interferometric scheme (as they make optical splitters and / or modulators as described previously superfluous) and, consequently, a lower price and a more compact interferometric apparatus.

[0134] For further information on self-mixing interferometry, one can consult, for example, the document by Donati and Norgia, Optical Engineering 57, 2013 or the document by Taimre et al, Adv. Opt. Photon. 7, 2013. As already mentioned, a system for the structural monitoring of buildings and / or structures as described herein can exploit the presence of a plurality of optical fibers coupled to the same node , for example a ( secondary) node 200 in order to perform multiple interferometric measurements simultaneously .

[0135] For this purpose , a corresponding plurality of interferometric apparatuses ( for example , installed downstream of the node 200 ) as described previously can be set up in proximity to a node 200 to which a plurality of optical fibers is coupled, each configured to perform an interferometric measurement on a corresponding optical fiber .

[0136] In one or more embodiments , a single interferometric apparatus 10 is configured to be installed downstream of the node of the network infrastructure 20 , 200 and configured to send a plurality of test signals Ipinto a corresponding plurality of optical fibers associated with the node 20 , 200 .

[0137] In such embodiments , a source signal is divided ( for example , via an optical spl itter ) into a plurality of test signals Ip.

[0138] Each test signal Ipis then frequency modulated (via an AOM, for example ) at a respective test frequency and, subsequently, sent on one corresponding optical fiber in the plurality of optical fibers .

[0139] A plurality of test signals Ipat a corresponding test frequency is therefore superimposed on a data signal I data transmitted via the respective optical fiber . In a manner similar to what was described before , such test signals Ipcan be characteri zed by a power and / or a frequency such that they do not interfere with the data signals I data transmitted via the optical fibers .

[0140] The return signals Ir( generated by passive reflection or via a reflector as described previously) received from each optical fiber are superimposed on a reference signal Iref (possibly obtained via splitting of the source signal Io ) and sent to a detection device 140 .

[0141] The signal received by the detection device 140 will therefore be indicative of the superposition of the return signals Ir( each characteri zed by a corresponding test frequency) received from each optical fiber and of the reference signal Iref .

[0142] In the signal analysis phase , the contribution of each return signal Ircan be isolated by demodulating the frequencies of the signal of interest , thus facilitating an interferometric measurement indicative of the length variation of each optical fiber of the plurality of optical fibers .

[0143] Figure 5 illustrates a poss ible implementation of a system configured to perform an interferometric measurement on a plurality of optical fibers Fi, F2 , F3 associated with an optical splitter at a node 20 , 200 of a network infrastructure .

[0144] A test signal Ipis provided to the optical splitter 20 , 200 , superimposed on the data signal Idata and distributed on the plurality of optical fibers Fi, F2 , F3 associated therewith .

[0145] Referring, for example , to the optical fiber indicated with the reference Fi, an optical splitter 1200 (possibly provided with DWDM filters ) is installed in proximity to the second end of the optical fiber Fl connected to a signal receiving / transmitting device M of the end user .

[0146] The test signal Ipis extracted from the optical fiber and sent to a frequency modulator 1261 ( for example , an AOM, or any frequency or phase modulator suitable for the purpose ) and then to a reflector 128 .

[0147] A first return signal Ircharacteri zed by a first frequency is thus generated and sent towards the first end of the first optical fiber Fi .

[0148] Proceeding similarly for the plurality of fibers Fi, F2, F3 associated with the node 20, 200, a corresponding plurality of return signals Ir,i, Ir,2, Ir,3 is obtained by modulating the test signal Ipat a corresponding frequency (via a corresponding frequency modulator 1261, 1262, 1263) .

[0149] As illustrated, a return signal Irobtained by superimposing the return signals Ir,i, Ir,2, Ir,3 from each optical fiber Fi, F2, F3 is received by the interferometric apparatus 10 associated with the node 20, 200.

[0150] Similarly to what was described before, during signal analysis, the contribution of each return signal Ir, i, Ir,2, Ir,3 can be isolated starting from the (sum) return signal Ir, so as to distinguish the length variations (strain) of each optical fiber on which the interferometric measurement is performed.

[0151] In one or more embodiments, the interferometric measurement on a plurality of optical fibers can be performed without the addition of frequency modulators. The interferometric apparatus 10 can be installed upstream of a node 20, 200 of the network infrastructure (as illustrated in Figure 2, for example) and configured to perform an interferometric measurement of any deformations (length variation) of one or more optical fibers associated with said node 20, 200.

[0152] The interferometric apparatus can be configured to provide as input to the optical splitter 20, 200 of the node a test signal Ipwhich is distributed into all the optical fibers F associated with the optical splitter.

[0153] A return signal Iris obtained by superimposing the return signals coming from each associated optical fiber F of the node 20, 200. The return signal Iris thus superimposed on a reference signal Iref obtaining a measurement signal Imwhich is sent to a detection device (comprising a photodiode, for example) .

[0154] The contributions to the sum return signal Irby each fiber associated with the node 20, 200 (and therefore the contributions of each optical fiber to the measurement signal Im) can be distinguished during signal analysis, for example via modal analysis of characteristic frequencies as described below.

[0155] In one or more embodiments, the data signal Idata can be used to perform the interferometric measurement. In other words, the test signal Ipis obtained from the data signal Idata . Such embodiments can be advantageous in so far as they do not require the installation of an interferometric measurement apparatus including a light source .

[0156] A possible implementation of such embodiments is exemplified in Figure 6.

[0157] As illustrated in the figure, the interferometric measurement apparatus 10 can be installed upstream of a node 20, 200 of the network infrastructure (for example, a PON) and can comprise three optical splitters (or optical circulators) 1201, 1202, 1203 and a detection device 140 (comprising one or more photodiodes, for example) .

[0158] A first optical splitter 1201 can be configured to take a portion of the data signal Idata so as to obtain a reference signal Iref . The data signal Idata is thus provided as input to the optical splitter 20, 200 of the node of the network infrastructure (in this sense the second optical splitter 1202 is "transparent" to the data signal Idata directed towards the node 20, 200) and distributed onto the plurality of optical fibers F associated with the node.

[0159] At the second end of one (or more) optical fiber F associated with the node 20, 200, a further optical splitter 1204 and a reflector can be installed ( in a manner entirely similar to that described in relation to Figure 3B ) configured to take a portion of the data signal I data and generate (by reflection) a return signal I r .

[0160] As illustrated, the return signal Iris directed (via the second optical splitter 1202 of the measurement apparatus 10 ) towards the third optical splitter 1203 , where it is combined with the reference signal Iref into a measurement signal Imsent to a detection device 140 .

[0161] The fractions of the data signal I data that are taken (via the splitters 1201 and 1204 ) are such that the reception of the data s ignal I data by the signal receiving / transmitting device M is not altered .

[0162] In other words , in embodiments as described in relation to Figure 6 , the test light signal Ipsent into at least one optical fiber F coincides with the data signal Idata . The interferometric apparatus is configured to : reflect at the second end of said at least one optical fiber F a first fraction of said data signal I data to generate said return signal Ir, and detect the amplitude of a measurement light signal Im obtained by making said return light signal Irinterfere with a reference light signal Iref , for example obtained by taking at the first end of the at least one optical fiber F a second fraction of said data signal I data .

[0163] Figure 7 exempli fies an application of an interferometric apparatus 10 as described herein configured to detect deformations or length variations of a plurality of fibers Fi, F2 , F3 that extend in the same building B in order to transmit a data signal I data to a corresponding plurality of end users .

[0164] In the embodiment illustrated by way of example only in Figure 7 , three optical fibers extend at various points of the building starting from the same node 200 to which an interferometric apparatus 10 configured to perform an interferometric measurement via the three fibers is coupled . In particular : a first optical fiber Fi extends hori zontally on a first floor of the building B, a second optical fiber F2 extends vertically up to a second floor of the building B and subsequently hori zontally at said second floor, a third optical fiber F3 extends vertically up to a third floor of the building B and subsequently hori zontally at said third floor .

[0165] As described previously, the detected values of deformation of the optical fibers , that is , the variation of their length ( indicated with the references All, AI2 and AI3 in figure 7 ) , are indicative of an overall length variation of each of the optical fibers .

[0166] Starting from these detected values of overall deformations of the optical fibers , locali zed or segmented information on the deformations of the building B can be obtained .

[0167] For example , the detected length variation All of the first optical fiber Fi is indicative of a deformation of the first optical fiber Fi and therefore of the building B, in a substantially hori zontal direction, at the first floor of the building B relative to the segment Si .

[0168] From the comparison of the length variations All, AI2 of the first optical fiber Fi and the second optical fiber F2 , information on deformations of the building B in the substantially vertical portion or segment S2 thereof extending from the f irst to the second floor can be derived .

[0169] From the comparison of the length variations AI2 , Ala of the second optical fiber F2 and of the third optical fiber F3, information can be derived on deformations of the building B in the substantially vertical portion or segment S3 thereof extending from the second to the third floor .

[0170] Such di f ferential analysis between the di f ferent detected signals therefore facilitates locating or segmenting the source of deformation of the building B and, in particular, identi fying in which portion / segment of the building B a fracture / failure occurs or facilitating a more complete modal analysis ( described below) of the vibrations of the building B . As said, it allows to exclude signals from non-relevant areas .

[0171] For a more detailed treatment of such a di f ferential analysis , one can consult , for example , the publication by Marra et al . , Science 376 , 2022 .

[0172] In one or more embodiments , the interferometric apparatus 10 can be placed at a (primary) node 20 of the network infrastructure configured to transmit the data signal I ata to and from a plurality of structures and / or buildings coupled thereto ( for example , a neighborhood) .

[0173] In such a configuration, a monitoring system as described herein is therefore configured to monitor the structural state of the plurality of structures B coupled to said primary node 20 . Said plurality of structures B can be monitored simultaneously via an apparatus as described previously, performing loop-back and segmentations of the PON network, from the cabinet to the FTTH end users .

[0174] Such implementation may require , depending on the total length of the optical path, a more powerful laser source in terms of power and coherence length, and a more complex data analysis .

[0175] In such embodiments , the system operates entirely similarly to what is described in relation to Figure 7 and a further description will not be repeated here .

[0176] Those skilled in the art can appreciate how performing a multi- fiber interferometric measurement has multiple advantages , including, for example , reducing the contribution due to noise generated by the non- relevant portion of the optical fiber ( for example , along the common stretches of two or more optical fibers ) while maintaining the information on the di f ferential segments relevant for structural monitoring .

[0177] In one or more embodiments , the reference signal Iref is represented by the return signal Irfrom one of the plurality of optical fibers to which the test signal Ipis provided ( for example , the optical fiber Fi of Figure 7 ) , and the measurement signal Imrepresents the relative phase between said reference arm and a measurement arm represented by one of the other fibers ( for example , the optical fiber F2 of Figure 7 ) .

[0178] It can be appreciated how in such configurations the noise sources of the stretches common to Fi and F2 are compensated, and the signal Imis representative of the phase di f ference between the return signals coming from the optical fibers Fi and F2 , corresponding to the deformations of only the non-common stretches , for example the stretches reaching di f ferent parts of the building .

[0179] With reference to the block diagram of Figure 8 , a method for deriving a signal representative of the strain or deformation, indicated overall with the numerical reference 205 , will now be described .

[0180] In a first step 210 it is provided to detect , via the detector 140 comprising for example one or more photodiodes , the measurement signal Im, representative of the optical interference , and trans form it into an electrical measurement signal Iem.

[0181] This electrical measurement signal Iemin a step 220 is ampli fied and sampled, employing for example an analog-to-digital converter and a digital acquisition system, for example with a sampling rate of the order of 100 Hz in the case of homodyne detection, or up to several kHz or MHz in the case of heterodyne detection, obtaining as output a digital measurement signal Idm-

[0182] This digital measurement signal Idm is analyzed in an analysis procedure 230 for example via a local software system, i . e . , located in proximity to the detector 140 , which operates on a corresponding processing system .

[0183] In the case of heterodyne detection, the analysis procedure 230 first comprises an optional step of demodulating the digital measurement signal Idm which operates at radio- frequency (RF) , for example via I / Q demodulation, i . e . , in-phase and quadrature . In the case of homodyne detection, this step is omitted . The phase can be extracted from the continuous signal or from the pair of signals by analyzing their intensity .

[0184] In a step 231 , a reconstruction of the continuous phase of the digital measurement signal Idm is therefore applied, for example through a fringe unwrapping algorithm, compensating for phase j umps ( caused by wrappings every 2n ) . There fore step 231 produces a continuous measurement phase signal m.

[0185] In a step 232 , this measurement phase signal m is normali zed to a phase value B representative of the structural strain, i . e . , a baseline measurement phase signal value m, in the absence of anomalous events , obtaining a normali zed measurement signal £msrepresentative of the length variation of the fiber, for example F, due to the current deformation, or strain, a deformation or strain which is measured on the entirety of the fiber F, i . e . , an integral strain . In general , one can operate with a normal i zed signal i f the reference rest length is known; when it is not known, one operates with a non-normalized measurement, in units of length (optical path variation) .

[0186] In the case of any segmented fibers, an optional step is provided wherein the localized deformation or strain for each segment is extracted via differential measurements, as illustrated for example with reference to Figure 7. By way of example, the system can be configured to probe different fibers with different paths, for example reaching different buildings or different floors of a building. Given a location, for example in the building, for example the basement where the system 10 is installed, each signal coming from each fiber provides the measurement of the deformation, spatially integrated between the location of system 10 and the different fiber paths, in the example between the basement and the different floors. By performing a differential comparison between the different signals, it is possible to locate the source of deformation, since the common path cancels out and the measurement along the different fibers can be isolated [Marra et al, Science 376, 2022] . This allows, for example, identifying where (in which segment) a fracture or failure occurs along the building, or providing multiple frequency measurements that can enable a more complete modal analysis of the building's vibrations, this also in particular by implementing data fusion techniques, also improving the quality of the input for the digital twin model of the building itself.

[0187] The normalized measurement signal £msrepresentative of the integrated or overall strain thus extracted is in a step 240 acquired in time series, i.e., £ms(t) , and analyzed to obtain 240 parameters CS, such as natural frequencies, modal shapes and strain distribution, of modal vibrations of said structure B, for example via spectral analysis, e.g., FFT (Fast Fourier Transform) , or operational modal analysis, or via machine learning systems trained to classify, based on training time series of strain sms(t) referred to predetermined CS parameters, the current modal CS parameters of the structure B, or structure B-fiber F system, which originates the temporal strain signal, £ms(t) . Alternatively, the analysis step 240 can perform analysis of such time series, i.e., temporal strain signal, £ms(t) , via methods that identify portions of signal representative of an anomaly AN with respect to time series £ms(t) or their parameters under reference conditions. For example, series with anomalous amplitudes, e.g., above a threshold, can simply be detected, or series with a portion of signal that presents a pattern different from the signal under reference conditions (i.e., subjected to allowed vibrations, such as from road traffic, or anthropogenic, or in any case environmental) , recognized via pattern recognition algorithms, for example implemented via neural networks .

[0188] It can be appreciated how, the structural integrity assessment (SHM) is thus based on the principle that, structural failures or modifications of the structural properties also lead to changes in the natural frequencies of the structural vibrations, measured via analysis in the frequency domain of the strain signals. Any variations or alterations of these vibrational modes can represent signals of structural risk. Furthermore, events such as collapses can be determined as localized anomalies directly from the time series of strain.

[0189] In a block 250 then the modal parameters CS and / or the anomalous signal portions AN are analyzed and if their values or characteristics indicate a type of danger or damage of the structure B, a corresponding alarm AL can be issued, in particular transmitted in real time via a telecommunications network, for example the Internet , in particular together with the related measured data, to a remote centrali zed system 300 that performs structural risk assessments for the structure B . The block 250 can, alternatively or additionally, also signal the alarm AL locally, for example inside the structure B . Note that the transmission can occur using the same optical fiber network, that is the network that exchanges the data Idata with the building, towards remote centers , e . g . 300 , and / or towards the users .

[0190] Speci fically, regarding the measurement of the integrated strain, i . e . , measurement signal £msrepresentative of the integrated strain, representative of the structural strain for a given fiber-structure coupling, the order of magnitude of the natural fundamental frequencies for a reinforced concrete building usually varies in the spectral region between 1 and 10 Hz , which are associated with integrated strain values of the order of 10~8- l Cr7in the case of small local earthquakes . The amplitude of the accelerations in such buildings due to ambient vibrations is measured in a range of about 1 CT3- 1 CT5g, g being the acceleration of gravity, depending on the amplitude of such ambient vibrations . The described laser interferometry system can detect frequency deviations of the optical carrier up to an order of milliHertz , mHz , in a spectral field that varies from 0 . 01Hz up to several kHz . The laser interferometry system described herein can measure ground velocity below I CT4m / s , and integrated strain below about 10~9under typical environmental noise conditions . Therefore it appears feasible to obtain strain data via the laser interferometry system described herein useful for SHM analysis as described below in the presence of environmental excitations . For example, the integrated strain data, measurement signal smsrepresentative of the integrated strain, can be used in a technique called Bayesian Operational Modal Analysis (BAYOMA) , a probabilistic method that is employed to extract dynamic characteristics from the collected data such as parameters OS .

[0191] Thus the system in general receives a measurement signal Imrepresentative of the deformation to which the fiber F is subjected. This measurement signal Imis in general acquired over time, giving rise to a time series of strain values £ms(t) , which in general can identify vibrations of the structure B.

[0192] For example, the analysis procedure 240 can comprise, in the time domain, detecting anomalies AN by isolating the anomalous observations in a time series of strain values £ms(t) , for example via methods such as Isolation Forest, or operate via pattern recognition systems via neural networks such as autoencoders, which detect anomalies based on reconstruction errors.

[0193] The anomaly detection procedure AN can use a CLASP (Classification Score Profile) technique, which models the problem as time series segmentation, reducing it to a binary classification problem of the sub-sequences. The CLASP technique creates sub-sequences of the time series and trains a binary k-NN classifier for each possible division point to distinguish the sub-sequences to the left and right of the point. The performance of the classifier is used to construct a classification score profile (ClaSP) , where the local maxima indicate potential change points.

[0194] According to an aspect of the solution described herein, once changes, i.e., anomalies AN, are detected in the measurement signals sms(t) , pattern recognition algorithms can be applied to identify vibration patterns associated with earthquakes , micro-seismicity or structural failures .

[0195] More generally, possible approaches usable by the analysis step 240 include : nearest neighbor-based methods : such as Dynamic Time Warping (DTW) , which compares time sequences allowing temporal deformations ;

[0196] Kernel methods : such as Global Alignment Kernel ( GAK) , which uses kernels to measure the similarity between time series in a non-linear way; shapelet-based methods : such as Shapelet Trans form, which extracts discriminative shapelets from the time series ; tree-based methods : such as Time Series Forest , already mentioned, which uses subsequences of the time series to feed a random forest , and Proximity Forest , which exploits speci fic metrics for time series to determine the node splitting criteria ;

[0197] Deep learning : such as Inception Time , a convolutional neural network optimi zed for time series classi fication .

[0198] Therefore , based on the above , the described solution refers to a system for the structural monitoring of at least one structure , for example the building B, the system comprising : one or more optical fibers F ; FI , F2 coupled at a first end thereof to a node 20 , 200 of a passive optical network infrastructure for transmitting data I ata in said structure B, said one or more optical fibers F, Fi, F2 extending inside said at least one structure B for at least a portion of their length, said length being comprised between said first end and a second end opposite to the first , wherein the length of said portion of optical fiber F, Fi, F2 varies in response to a deformation of said structure B, wherein, in particular said second end of each optical fiber is coupled to a signal transmitting and receiving device M of a user of said network infrastructure , wherein said one or more optical fibers F, Fi, F2 are configured to transmit data Idata in a frequency band or in a set of data transmission frequency bands , and an interferometric measurement apparatus 10 configured to : send a laser test light signal Ipinto at least one optical fiber F, Fi, F2 of said one or more optical fibers F, Fi, F2 , wherein said test signal Ippropagates in said at least one optical fiber F, Fi, F2 , receive a return light signal Irin response to the propagation of said test signal Ipin said at least one optical fiber F, Fi, F2 , wherein the return light signal Irhas a phase that is a function of the length of the at least one optical fiber F, Fi, F2 , detect the amplitude of a measurement light signal Im obtained by making said return light signal Irinterfere with a reference light signal Iref , in particular obtained from a source laser signal I o from which said test signal is obtained Ip, the amplitude of said measurement light signal Imbeing a function of the phase of said return light signal Ir, and obtain ( for example via a step 232 ) values of overall length variations of said at least one optical fiber, in particular determined by deformations originating from one or more among low-intensity natural phenomena, seismic phenomena, and anthropogenic stresses , as a function of said detected amplitude of the measurement light signal Im, analyze ( for example via a step 240 ) time series of said values of overall length variations to obtain parameters CS of modal vibrations of said structure B or to detect anomalies AN with respect to time series £ms( t ) or their parameters under reference conditions.

[0199] Furthermore, said system for structural monitoring is configured, for example in step 250, to analyze said parameters, CS, of modal vibrations of said structure, for example B, or said anomalies, AN, evaluate the emission of an alarm signal, AL, based on said analysis, and if positive emit said alarm signal, for example AL, in particular transmit said alarm signal to a remote center, for example 300. The analysis step 250 can also be performed elsewhere, by simply sending the data from step 240.

[0200] Furthermore, said system for structural monitoring is configured to identify in said values of overall strain, for example £ms(t) , values associated with stresses in determined positions or areas of said optical fiber F, Fi, for example, in particular via differential analysis .

[0201] Furthermore, said system for structural monitoring is configured to compare said parameters of modal vibrations, for example CS, with respective reference values of modal vibration parameters, for example CS, of the structure, for example B, for example calculated on the real structure or on a simulation thereof, e.g. digital twin, to identify anomalies, for example AN, or damage in said structure, for example B. A variation of a modal frequency or other modal parameter can indicate either damage or another type of anomaly that changes the modal behavior of the structure, in particular a building .

[0202] Thus, the advantages of the described solution are clear from the previous description.

[0203] The described monitoring system is capable of detecting length variations locally, and of transmitting in real time over the network (directly via the local fiber or wireless connection) the data and data labels corresponding to the alarm to a centrali zed service of public / private entities set up for damage mitigation and prevention .

[0204] The system requires simple installation on par with any other telecommunications component , without the need for invasive interventions by speciali zed personnel and reuses the common fibers already installed by internet providers on the buildings or infrastructures themselves . It therefore allows the coexistence of the sensing service with data, since the sensing signal does not interfere with the internet flow or the digital communication devices ( e . g . , modems ) .

[0205] The described monitoring system allows the realtime transmission of alarms or reports in case of potential dangers ,

[0206] The described monitoring system allows performing SHM on a large scale and continuously .

[0207] Through the possibility of segmenting, and therefore locali zing, it i s possible to perform the measurement on di f ferent sectors of the building or structure and / or reduce the impact on the measurement of noise generated in non-relevant sections / segments of the optical fiber .

[0208] While the underlying principles remain, the details and embodiments can vary, even appreciably, from what has been described, purely by way of example , without departing from the scope of the embodiments .

[0209] The scope of protection is defined by the attached claims .

Claims

1. CLAIMS1. System for the structural monitoring of at least one structure (B) , the system comprising: one or more optical fibers (F; FI, F2) coupled at a first end thereof to a node (20;200) of a passive optical network infrastructure for transmitting data (Idata) in said structure (B) , said one or more optical fibers (F; FI, F2) extending inside said at least one structure (B) for at least a portion of their length, said length being comprised between said first end and a second end opposite to the first, wherein the length of said portion of optical fiber (F; FI, F2) varies in response to a deformation of said structure (B) , wherein, in particular said second end of each optical fiber is coupled to a signal transmitting and receiving device (M) of a user of said network infrastructure, wherein said one or more optical fibers (F; FI, F2) are configured to transmit data (Idata) in a frequency band or in a set of data transmission frequency bands, and an interferometric measurement apparatus (10) configured to: send a laser test light signal (Ip) into at least one optical fiber (F; Fi, F2) of said one or more optical fibers (F; Fi, F2) , wherein said test signal (Ip) propagates in said at least one optical fiber (F; Fi, F2) , receive a return light signal (Ir) in response to the propagation of said test signal (Ip) in said at least one optical fiber (F; Fi, F2) , wherein the return light signal (Ir) has a phase that is a function of the length of the at least one optical fiber (F; Fi, F2) , detect the amplitude of a measurement light signal (Im) obtained by making said return light signal (Ir) interfere with a reference light signal (Iref) , inparticular obtained from a source laser signal (Io) from which said test signal is obtained (IP) , the amplitude of said measurement light signal (Im) being a function of the phase of said return light signal (Ir) , and obtain (232) values of overall length variations of said at least one optical fiber (F; Fi, F2) , in particular determined by structural deformations of the structure (B) , originating from one or more among low-intensity natural phenomena, seismic phenomena, hydrogeological instability, environmental phenomena, subsidence, ageing of materials and anthropogenic stresses, as a function of said detected amplitude of the measurement light signal , analyze (240) time series of said values of overall length variations to obtain (240) parameters (CS) of modal vibrations of said structure (B) or to detect anomalies (AN) with respect to time series (£ms(t) ) or their parameters under reference conditions.

2. System for structural monitoring according to claim 1, wherein said laser test light signal (Ip) has a coherence length at least equal to the length over which said values of overall length variations are measured.

3. System for structural monitoring according to claim 1 or 2, wherein said system is configured (250) to analyze said parameters (CS) of modal vibrations of said structure (B) or said anomalies (AN) , evaluate the emission of an alarm signal (AL) based on said analysis, and if positive emit said alarm signal (AL) , in particular transmit said alarm signal to a remote center (300) .

4. System for structural monitoring according to one of the preceding claims, wherein said system (10) is configured to identify in said values of overall strain ( £ms ( t ) ) values associated with stresses in determined positions or areas of said optical fiber (F; Fi, F2) , inparticular via differential analysis of measurement signals obtained by more than one fiber.

5. System for structural monitoring according to one of the preceding claims, wherein said system is configured to compare said parameters of modal vibrations (CS) with respective reference values of modal vibration parameters (CS) of the structure (B) to identify anomalies (AN) or damage in said structure (B) .

6. System for structural monitoring according to one of the preceding claims, wherein one or more reflective elements (280) are applied on the optical fibers (F; Fi, F2) to reflect said return signal (Ir) .

7. System for structural monitoring according to one of the preceding claims, wherein said system is configured to reflect said return signal (Ir) via parasitic reflections in the fiber (F) , in particular using as reflective elements receiving or transmitting devices and / or passive components already present in the optical fibers (F; Fi, F2) .

8. System for the structural monitoring according to one of the preceding claims, wherein said interferometric measurement apparatus (10) is coupled to said node (200) of the network infrastructure both colocated at said structure (B) , in particular in proximity to a beam splitter (200) .

9. System for the structural monitoring according to one of the preceding claims, wherein said system is configured to operate the detecting of the phase of said return light signal (Ir) via heterodyne phase measurements in the frequency domain.

10. System for the structural monitoring according to one of claims 1 to 8, wherein said system is configured to operate the detecting of the phase of said return light signal (Ir) via homodyne phase measurements in the continuous domain.

11. System for the structural monitoring according to one of the preceding claims, wherein in said system the power of the test signal (Ip) is selected to be less than the lower limit of the input dynamic range of the signal transmitting and receiving device (M) , in particular less than a maximum signal level for considering the received level as a logical zero.

12. System for the structural monitoring according to one of the preceding claims, wherein said test light signal (Ip) sent into at least one optical fiber (F; Fi, F2) of said one or more optical fibers (F; Fi, F2) is obtained from the data signal (Idata) , and wherein the interferometric apparatus (10) is further configured to: reflect at the second end of said one or more optical fibers (F; Fi, F2) a first fraction of said data signal (Idata) to generate said return signal (Ir) , and detect the amplitude of a measurement light signal (Im) obtained by making said return light signal (Ir) interfere with a reference light signal (Iref) , in particular obtained by drawing at the first end of said one or more optical fibers (F; Fi, F2) a second fraction of said data signal (Idata) .

13. System for the structural monitoring according to one of the preceding claims, wherein said one or more optical fibers (F; Fi, F2) are configured to transmit data (Idata) in a frequency band or in a set of data transmission frequency bands.

14. System for the structural monitoring according to one of the preceding claims, wherein the interferometric measurement apparatus (10) comprises a self-mixing interferometric measurement apparatus, configured to detect the amplitude of a measurement light signal (Im) obtained by making the return light signal (Ir) interfere with a reference light signal (Iref) obtained from a source laser signal (Io) from which saidtest signal is obtained (Ip) inside the laser source that generates said source laser signal, in particular in a laser cavity.

15. Method for the structural monitoring of at least one structure (B) by means of the monitoring system (10) according to one or more of claims 1 to 12, comprising: sending a test light signal (Ip) into at least one optical fiber (F;Fi, F2) of said one or more optical fibers (F;Fi, F2) , wherein said test signal (Ip) propagates in said at least one optical fiber (F;Fi, F2) , receiving a return light signal (Ir) in response to the propagation of said test signal (Ip) in said at least one optical fiber (F; Fi, F2) , wherein the return light signal (Ir) has a phase that is a function of the length of the at least one optical fiber (F; Fi) , detecting the amplitude of a measurement light signal (Im) obtained by making said return light signal (Ir) interfere with a reference light signal (Iref) , in particular obtained from a source laser signal (Io) from which said test signal is obtained (IP) , the amplitude of said measurement light signal (Im) being a function of the phase of said return light signal (Ir) , obtaining (232) values of overall length variations of said at least one optical fiber (F; Fi, F2) , in particular determined by structural deformations of the structure (B) , originating from one or more among low- intensity natural phenomena, seismic phenomena, hydrogeological instability, environmental phenomena, subsidence, ageing of materials and anthropogenic stresses, as a function of said detected amplitude of the measurement light signal, and analyzing (240) time series of said values of overall length variations to obtain (240) parameters (CS) of modal vibrations of said structure (B) or to detect anomalies (AN) with respect to time series( Sms ( t ) ) or their parameters under reference conditions.

16. Method for the structural monitoring according to claim 15, comprising analyzing said parameters (CS) of modal vibrations of said structure (B) or said anomalies (AN) , evaluating the emission of an alarm signal (AL) based on said analysis, and if positive emitting said alarm signal (AL) , in particular transmitting said alarm signal to a remote center (300) .

17. Method for the structural monitoring according to claim 15 or 16, comprising identifying in said values of overall strain (£ms(t) ) values associated with stresses in determined positions or areas of said optical fiber (F; Fi) , in particular via differential analysis.

18. Method for the structural monitoring according to one of claims 15 to 17, comprising comparing said parameters of modal vibrations (CS) with respective values of reference modal vibration parameters (CS) of the structure (B) to identify anomalies (AN) or damage in said structure (B) .

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