Optical Fiber Measurement System, Method for Adapting a Communication Optical Fiber to a Measurement System, and Optical Fiber Measurement and Communication System

Abstract

According to the present invention, an optical fiber measurement system is provided with a controlled light generation system (1) and a receiving system (2) connected via an optical path including a directing device (4), and further has a processing unit (9) for controlling the light generation system (1) and for receiving and processing signals from the receiving system (2), characterized by the fact that it has a selective mode device (5) and is adapted to be connected to an optical fiber telecommunication network via the selective mode device (5), and the processing unit (9) is adapted to implement OFDR and / or COTDR measurement techniques for measuring changes in the optical path and processing them into one or more parameters. Furthermore, the present invention also aims at a method for adapting a telecommunication network to a sensor network and an optical fiber measurement and communication system.

Description

Technical Field

[0001] The object of the present invention is an optical fiber measurement system, a method for adapting a communication optical fiber to a measurement system, and an optical fiber measurement and communication system intended for distributed measurements based on Rayleigh scattering.

[0002] State of the Art

[0003] The state of the art includes distributed system solutions for measuring temperature, strain or pressure based on changes in the effective refractive index or optical path. These are systems based on COTDR or OFDR technology. Since it is difficult to detect signals from multimode optical fibers, these solutions are based on single-mode solutions. Single-mode optical fibers have power limitations due to the non-linear effects that increase as the optical power propagating in the fiber increases.

[0004] The COTDR technique (Coherent Optical Time Domain Reflectometer) is also known as fi-OTDR, phi-OTDR and φ-OTDR (Phase-Sensitive Optical Time Domain Reflectometer). All these names refer to a single architecture of the measuring device, where the phase measurement is based on sending coherent pulses and analyzing the signal reflected back in the time domain. The name COTDR emphasizes the coherence of the measurement signal. The name φ-OTDR draws attention to the possibility of performing a quantitative measurement of the phase change in the optical fiber. These two names can be used interchangeably. These terms are used interchangeably, for example, in the following articles: "Coherent Rayleigh time domain reflectometry: novel applications for optical fibre sensing" (Xin LU, EPFL 2016, https: / / infoscience.epfl.ch / record / 221427); and "Bend-insensitive distributed sensing in singlemode-multimode-singlemode optical fiber structure by using Brillouin optical time-domain analysis" by P. Xu, Y. Dong, J. Zhang, D. Zhou, T. Jiang, J. Xu, H. Zhang, T. Zhu, Z. Lu and L. Chen, 9 (2015).

[0005] To determine the effective refractive index optical path as a function of the optical fiber length or, when analyzing signals reflected or scattered along the optical fiber, an optical frequency domain reflectometer is also used in the state of the art: Ding, Zhenyang et al., "Distributed Optical Fibre Sensors Based on Optical Frequency Domain Reflectometry: A review", Sensors (Basel, Switzerland) 18 (2018): 104–127.

[0006] The multimode optical fibers currently in use allow the transmission of more than a hundred modes. The difference in the optical propagation speed between the modes and its spatial distribution are the main factors that make it impossible to perform distributed optical measurements using the COTDR technique in its typical form when using multimode optical fibers.

[0007] Patent application CA2725353C discloses an optical time domain reflectometer (OTDR) system configured to measure backscattering from a multimode optical fiber. The system includes a single spatial mode filtering system for selecting a single Rayleigh backscattering circle generated in response to an optical pulse introduced into the multimode optical fiber. The selected single speckle can be used to perform distributed vibration measurements.

[0008] Application US8520197B2 discloses a distributed optical fiber system, wherein the sensing optical fiber includes at least a first and a second waveguide for independent vibration measurement operations. According to the content of this document, the sensing optical fiber can be a twisted optical fiber with a single-mode core and a multi-mode inner cladding. International Patent Application Publication No. WO2009148824 discloses an optical time domain reflectometer (OTDR) system configured to detect Rayleigh backscattering reflected from a multi-mode sensing optical fiber. The system includes a single spatial mode filtering system to select a single speckle of Rayleigh backscattering generated in response to an optical pulse launched into the multi-mode optical fiber. The detected single speckle can be used for distributed perturbation (vibration) detection. US2009097015 discloses an apparatus for measuring characteristics of a multi-mode optical fiber, wherein an optical pulse source generates optical pulses for transmission into the multi-mode optical fiber. A spatial filter passes a portion of Brillouin backscattering light from the multi-mode optical fiber in response to the optical pulse. Optical detection means detects the portion of Brillouin backscattering light passing through the spatial filter. JP2016080600A discloses a solution to the problem of accurate measurement of strain in a multi-mode optical fiber and provides an optical fiber measurement method for measuring the strain of an optical fiber under test, including the following steps: inputting a test light as a fundamental mode light into the optical fiber under test and measuring the fundamental mode Brillouin scattered light generated by the test light in the optical fiber under test. Preferably, a probe light having a wavelength corresponding to the wavelength of the Brillouin scattered light Br is input into the optical fiber under test, and the test light is input therein as a pump light.

[0009] Single-mode optical fibers are used for long-distance signal transmission in telecommunications - on the order of several kilometers, dozens of kilometers or even hundreds of kilometers. The optical fibers used in such systems are well insulated and protected to avoid signal loss and crosstalk. Due to these protections, they do not work well as sensors.

[0010] Technical problem to be solved

[0011] Known optical fiber measurement systems generally use single-mode sensing optical fibers with a small core diameter, the use of which involves limiting the maximum power of the source due to non-linear phenomena in the fiber structure. Therefore, the maximum measurement distance is limited by the maximum power that does not cause non-linear phenomena. Additionally, due to the reduced visibility of the measurement signal, measurement systems dedicated to single-mode optical fibers do not work well with multi-mode sensing optical fibers. This is inconvenient because multi-mode communication optical fibers are now routinely installed during the construction of buildings and infrastructure. Summary of the invention

[0012] The object of the present invention is to provide a measurement system, a method for adapting a telecommunication network, and a measurement and communication system, which provide the possibility of measuring physical quantities by using a multimode optical fiber used in a short-range telecommunication network, especially in an indoor or inter-building network, as a measurement optical fiber.

[0013] According to the present invention, an optical fiber measurement system is provided, which is equipped with a controlled light generation system and a receiving system connected via an optical path including a directional device, and further has a processing unit for controlling the light generation system and for receiving and processing signals from the receiving system. It is characterized by the fact that it has a selective mode device and is adapted to be connected to an optical fiber telecommunication network via the selective mode device, and the processing unit is adapted to implement OFDR or COTDR measurement techniques for measuring changes in the optical path and to process them into one or more parameters, especially temperature and / or strain. The selective mode device ensures sufficient visibility of the measurement signal. OFDR and COTDR techniques provide the possibility of measuring physical quantities (such as temperature, pressure, strain) as a function of the length of the measurement optical fiber. The measurement system according to the present invention - connected to an existing network on a multimode optical fiber - enables it to be used as a sensing optical fiber.

[0014] Preferably, the selective mode device is a mode filter, especially a device that selectively increases the loss of higher-order modes, such as those disclosed in US10502897B2.

[0015] Preferably, the selective mode device is a selective mode excitation system. Such a system facilitates the simultaneous measurement of two parameters using different modes.

[0016] A selective mode device is preferably a holographic plate or a sequence of holographic plates. This solution ensures low loss of the light input into the optical fiber and ensures relative freedom regarding the number of addressed modes. There are known solutions for independently addressing from several to dozens of modes. Alternatively, the selective mode device is an asymmetric coupler system.

[0017] Preferably, the processing unit is equipped with a frequency filtering module for filtering the measurement results of the optical path changes. This solution allows the system to separate the optical path changes caused by rapidly changing variables (such as vibrations) from the optical path changes caused by slowly changing variables (such as temperature), and thus allows them to be measured simultaneously.

[0018] Preferably, the selective mode device is controlled and connected to the processing unit and is adapted to perform continuous excitation of different-order modes, while the processing unit is adapted to determine two parameters in the measurement, especially temperature and strain, by means of at least two modes of different orders.

[0019] Even more preferably, the processing unit is adapted to determine two parameters in the measurement, in particular temperature and stress, by means of at least three modes of different orders. This allows an overdetermined system of equations and reduces errors.

[0020] Preferably, the optical fiber measurement system is equipped with a coupler and a section of optical fiber to connect it to a telecommunications network. This allows the system to be inserted into the network even if the free end of the optical fiber is not available.

[0021] The method according to the invention for adapting a telecommunications optical fiber having a core diameter greater than or equal to 20 μm to a measurement system is characterized in that the telecommunications optical fiber is connected to the optical path of the measurement system via a selective mode device, and the measurement system has a controlled light generation system and a receiving system connected by means of a directional device. Using an optical fiber with a core diameter of 20 μm or greater facilitates obtaining a sufficient maximum measurement distance and facilitating the excitation of measurement modes.

[0022] Preferably, a section of optical fiber in an existing optical fiber network is connected to the optical fiber measurement system according to the invention, and the optical fiber of the telecommunications network is cut and connected to a coupler.

[0023] Preferably, an optical fiber measurement and communication system for data transmission and for determining parameters representing physical quantities (especially when selected from the group including temperature and strain) includes at least one optical fiber measurement system according to the invention, which is attached to a telecommunications network equipped with a transmitter and a receiver, the transmitter and receiver being connected to an optical fiber having a core with a diameter greater than 20 μm, and the transmitter wavelength differing from the wavelength of the light generation system by at least 10 nm.

[0024] Preferably, the optical fiber of the telecommunications network has a core diameter greater than 47 μm. This allows a larger mode field and operation at a greater power while avoiding the possibility of nonlinear phenomena.

[0025] Preferably, the transmitter operates at a wavelength below 900 nm, and the light generation system operates at a wavelength above 1000 nm. This allows the use of detectors with different wavelength sensitivity relationships and better separation of measurement and communication signals.

[0026] The sensor according to the invention enables the measurement of changes in refractive index, i.e., changes in optical path length, in a distributed manner. By measuring the change in optical path length, it is possible to determine changes in the temperature or strain of a multimode optical fiber by means of the above-mentioned techniques. This solution adds new functionality to existing and installed multimode optical fibers and is important in the context of the development of SMDM (Spatial Mode Division Multiplexing) technology, which may lead to an increased use of few-mode and multimode optical fibers in long-distance data transmission lines.

[0027] Using the present invention, it is possible to measure the difference in propagation constants between different modes. This enables the quality of an optical fiber to be measured in a distributed manner, i.e., to evaluate whether and to what extent the parameters are maintained along its length, rather than just the average value over the entire length of the segment on which the measurement is made.

[0028] The structure of the measurement system and the measurement and communication systems limits the influence of non-linear phenomena that occur in such measurements. This limitation allows the input power of the system to be increased and thus the maximum distance of the measurement system to be increased. For example, for a single-mode optical fiber with a length exceeding 50 km, the transmission loss is too high and measurement is not possible due to noise, while increasing the input power leads to the occurrence of non-linear phenomena, which also makes measurement impossible. The use of the proposed system allows the power to be increased without generating non-linear phenomena and thus increases the maximum measurable reach.

[0029] Preferably, the optical generation system has an optical power greater than or equal to 5 dBm, and more preferably greater than 20 dBm, which enables sufficient sensitivity to be achieved over the entire area of an in-building or inter-building network. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The object of the present invention is described in the embodiments in the drawings, where Figure 1a a block diagram of an embodiment of an optical fiber measurement system according to the present invention is shown, Figure 1b a block diagram of an alternative embodiment of an optical fiber measurement system according to the present invention is shown, Figure 1c a schematic diagram of a test measurement system is shown, Figure 1d a schematic diagram of an alternative embodiment of a measurement system according to the present invention is shown, Figure 2 a - d show the measurement signals marked with solid lines and their visibility for a 2 m segment marked with dashed lines for optical fibers a) SMF b) Draka OM4 c) Draka 6LP d) InPhoTech 4LP, Figure 3 a - d show the measured values of the frequency shift as a function of the applied strain and linear matching for optical fibers a) Draka OM4 b) Draka 6LP c) InPhoTech 4LP. The determined values of the measurement sensitivity on the fundamental mode based on linear approximation are 138 ± 16, 133 ± 17, and 152 ± 16 MHz / με, respectively, Figure 4 a - c show the measured values of the frequency shift as a function of the applied strain and linear matching for optical fibers a) Draka OM4 b) Draka 6LP c) InPhoTech 4LP, while Figure 5a and b respectively represent the sensitivity R as a function of the wavelength λ of the communication network detector and the measurement system detector in an embodiment of the present invention. DETAILED DESCRIPTION

[0031] Figure 1a A block diagram of an optical fiber measurement system adapted to be connected to a telecommunications network socket according to an embodiment is shown. Figure 1a The optical fiber measurement system illustrated therein includes a controlled light generation system 1 and a receiving system 2 connected via an optical path. The optical path includes a directional device 4, which is a circulator connected to the light source 1 and the receiving system 2 through a single-mode optical fiber 3. The circulator output is connected to a selective mode device 5 via the single-mode optical fiber 3. The selective mode device is equipped with a plug 12a connected to a telecommunications network socket 12b. The connection of the plug and the socket results in the connection of the optical fiber measurement device according to an embodiment of the present invention to the telecommunications network. Connecting to an existing network with a multimode optical fiber enables the optical fiber to be used as a sensing optical fiber 7.

[0032] This solution is particularly convenient when the optical fiber as part of the network is placed in an accessible location and terminated with a standard optical fiber connector (such as FC, SC, E200, LC or another standard connector).

[0033] In this case, the output of the selective mode device can be directly connected to the available end of the optical fiber as part of the network, or to a suitable socket in the existing network infrastructure. If the optical fiber 7 is used for data transmission, a WDM coupler can be used to separate the measurement signal generated by the light generation system 1 and the telecommunications signal. Alternatively, separation can be achieved by using other detectors with different characteristics, as long as the wavelength of the telecommunications network transmitter and the wavelength of the light generation system 1 are sufficiently different, which means that one of them is significantly higher than 1000 nm and the other is significantly lower than 1000 nm.

[0034] Additional separation of measurement and communication signals can be achieved by addressing an OM4 multimode optical fiber with a mode multiplexer, enabling independent access to, for example, the LP01 and LP11 modes. In this case, the mode multiplexer can be placed at different ends of the optical fiber. The transmission system should be connected to a pair of appropriate multiplexer outputs corresponding to the LP01 mode. The measurement system should be connected to the input corresponding to the LP11 mode. Transmission using a data communication device in mode LP01 and measurement using the measurement system according to the present invention in mode LP11 can be carried out simultaneously.

[0035] The multimode optical fiber of the telecommunications network 7 is a measurement optical fiber, in which light from the light generation system 1 is scattered. The scattering result returns to the optical fiber measurement system, passes through the selective mode device 5 providing sufficient visibility W, and is disturbed and detected in the receiving system 2.

[0036] When connecting the optical fiber measurement device according to the present invention to the telecommunication network as described above, standard cleaning of the optical fiber connectors and inspection of the network condition are required. The network inspection can be carried out with the aid of an optical reflectometer. This inspection allows detection of damaged or inoperative connectors. In addition, as with any connection of a COTDR device, it may be necessary to adjust the output power level to avoid non-linear phenomena in the optical fiber. The power adjustment can be carried out by modifying the power supply parameters of the optical amplifier or by changing the settings of other parts of the system. Based on the visibility inspection of the measurement signal recorded as a function of time at the detector of the receiving system 2, an appropriate power level can be selected.

[0037] The measurement system is equipped with a processing unit 9 for controlling the light generation system 1 and receiving and processing signals from the receiving system 2. As described in "Fiber-Optic Distributed Strain and Temperature Sensing With Very High Measurand Resolution Over Long Range Using Coherent OTDR" by Y. Koyamada, M. Imahama, K. Kubota and K. Hogari (J. Light. Technol. 27, 1142–1146), the COTDR measurement technique for measuring changes in the optical path and converting them into temperature changes is implemented using the processing unit. This solution can be particularly used for temperature measurement in server rooms. The server room is equipped with a network infrastructure. At the same time, changes in the optical path within the optical fiber infrastructure in the server room are mainly caused by temperature changes due to the presence of edges of other environmental exposures, which cause changes such as pressure or strain changes. When tuning the light generation system 1 with a frequency less than or equal to 500 MHz, a wavelength step is required for a measurement with an accuracy of 1°. This step is understood as the minimum applied change in the pulse frequency.

[0038] The optical fiber measurement system according to the present invention can also be inserted into the telecommunication network in a situation where the multimode telecommunication optical fiber is not terminated with a convenient connector. In this case, the optical fiber measurement system is equipped with a coupler 6 attached to a section of the optical fiber 8 through the selective mode device 6. The coupler 6 is inserted into the optical fiber 7 of the telecommunication network, as Figure 1b shown.

[0039] According to the present invention, a method for adapting the multimode optical fiber of an optical fiber telecommunication network to a measurement system is provided. To enable multimode operation, the network should be equipped with optical fibers having a core diameter of 20 μm or greater for each measurement and communication system. This adaptation includes Figure 1a , Figure 1b orFigure 1d The fiber optic measurement system according to an embodiment of the present invention shown in is connected to the optical fiber 7 of a telecommunication network. Thus, the optical fiber 7 is a sensing optical fiber, which together with the measurement system forms a sensor. In particular, matching connectors can be connected, and a solution can be obtained in which the fiber optic measurement system according to the present invention is attached to the end of the optical fiber 7 - as Figure 1a shown in, or a coupler 6 can be inserted into the optical fiber 7, thereby obtaining Figure 1b the configuration shown in. In both cases, the optical fiber 7 of the telecommunication network becomes a sensing optical fiber. The measurement optical fiber 7 is connected to the optical path of the fiber optic measurement system via a selective mode device 5, which is equipped with a controlled light generation system 1 and a receiving system 2 connected by means of a directional device 4.

[0040] For testing with a typical multimode telecommunication optical fiber, a fiber optic measurement system has been created according to the present invention, the schematic diagram of which is shown in Figure 1c . The light generation system 1 includes a laser, a polarization controller PC, an electro-optic modulator EOM, and a semiconductor optical amplifier SOA and an optical amplifier EDFA in a pulse generation system.

[0041] The receiving system 2 includes an oscilloscope with a detector PD, which is equipped with a spectral filter and an optical amplifier EDFA at the input.

[0042] The light generation system 1 and the receiving system 2 are connected to the selective mode device via a circulator 4, and the third port of the selective mode device is connected to 5 via a single-mode optical fiber 3. The selective mode device 5 is connected to the sensing optical fiber 7, where a strain application system is provided in the form of a micrometer stage 10, which has an adjustment range of 25 mm and a handle 300 mm away from the stage. A 10 m long free section 11 is left at the end of the sensing optical fiber.

[0043] The light generation system 1 is controlled by a processing unit 9. This unit is also used to receive and process data from the oscilloscope.

[0044] To check the sensitivity of strain measurements performed on three few-mode and one multimode optical fibers, a high-resolution φ-OTDR system has been constructed, similar to that presented in "Fiber-Optic Distributed Strain and Temperature Sensing With Very High Measurand Resolution Over Long Range Using Coherent OTDR" by Y. Koyamada, M. Imahama, K. Kubota, and K. Hogari (J. Light. Technol. 27, 1142–1146).

[0045] As the sensing optical fiber 7, 4 km of OM4 optical fiber, 1 km of Draka 6LP optical fiber, and 100 m of InPhoTech 4LP optical fiber were tested.

[0046] The system is additionally extended with a selective mode device 5 in the form of a higher order mode filter (HOMF), which is implemented as described in “Passive higher order mode filter for 850 nm multimode fiber transmission” by L. Chorchos, J. P. Turkiewicz, L. Szostkiewicz, M. Napierala, L. Ostrowski, B. Bienkowska and T. Nasilowski (Microw. Opt. Technol. Lett. 59, 1959–1962 (2017)).

[0047] In the optical generation system 1, a DFB laser operating at 1550 nm is used. Before the start of the measurement, the wavelength tuning characteristics of the laser are measured by changing the power supply current. The tuning range is 29 GHz, and a higher resolution can be achieved using a wider range. To correctly measure the change in the radiation intensity of each point of the optical fiber as a function of laser tuning, the wavelength step during tuning is 92 MHz. This provides sufficient wavelength scanning, which is necessary for correctly measuring temperature or strain and for more precisely determining the spectral shift, and thus, measurements can be obtained with a smaller step. However, the measurement time increases with the decrease in the laser length step. For example, a step of 92 MHz translates to a temperature measurement accuracy of 0.2°. To obtain optical pulses of 2 ns, an electro-optic modulator EOM is used. A semiconductor optical amplifier SOA - a pulse generation system - used as an optical gate is synchronized with the EOM in order to obtain an extinction coefficient of 60 dB, which allows us to obtain a spatial resolution of 20 cm on a 5 km long optical fiber. Typically, to generate pulses, a continuous source, a short electrical pulse generator, and an element that modulates the light intensity at the output according to the set pulses are required. Such an element can be an SOA or an EOM, or both simultaneously. The SOA and EOM in the “off” state let some light pass through. The ratio of the power transmitted in the “on” state to the power transmitted in the “off” state is called the extinction coefficient. The higher this value, the more distinguishable the pulse is from the noise. The series connection of the SOA and EOM modulators provides a higher combined extinction coefficient than each of them individually. These devices must be synchronized so that the pulses emitted by one device are not cut off by the other device. To increase the pulse power, an optical amplifier EDFA is used.

[0048] The optical pulse is fed into the sensing fiber via the directional device 4 - a circulator and the selective mode device 5. In this embodiment, the selective mode device 5 is a mode filter. In the absence of a mode filter, the pulse propagating in the multimode fiber would excite all modes. To avoid this effect, a high - order mode filter (HOMF) is used, which allows us to filter out the high - order modes and only excite the fundamental mode. The M 2 test is performed to check the proper operation of the filter. For each fiber, the value obtained is less than 1.1. In the absence of strong perturbations in the fiber structure, the optical power propagates only in the selected mode; due to their orthogonality, there is no power dissipation from the fundamental mode to the high - order modes. As a result of Rayleigh scattering, a portion of the power of the pulse propagating in the fiber is reflected back from each point into all the modes propagating in the measurement fiber. The returned signal is filtered again by the mode filter, which results in a higher visibility W.

[0049] Then, the returned signal is amplified by a second optical amplifier to increase the signal - to - noise ratio. To filter out the noise of the amplified spontaneous emission (ASE), a tunable filter with a spectral width of 1 nm is used in the system. The signal is recorded by the receiving system 2, which is implemented in the form of an oscilloscope equipped with a 1 GHz DC detector. The detector bandwidth allows maintaining an appropriate spatial resolution of the measurement; better resolution can be achieved by using a more broadband detector. Generally, a detector with a bandwidth greater than 0.5 GHz operates properly. The oscilloscope operates at a sampling rate of 4 GHz.

[0050] To apply a known strain to the fiber, a system based on a micrometer stage 10 is constructed, to which the sensing fiber 7 is connected approximately 10 m from its free end. The other end attached to the mode filter has been fixed.

[0051] The measurement of the fiber strain consists of recording data from two laser scans. The first scan is performed on the unstrained fiber and the second scan is performed after the fiber is strained. For each point of the fiber, the cross - correlation value between the wavelength and the intensity has been calculated. Based on the maximum cross - correlation value, the laser wavelength shift is determined, which is linearly correlated with the strain of the fiber.

[0052] The tests were carried out on three different optical fibers. The first one is a commercially available multimode fiber OM4 (Draka), in which 34 LP mode groups can propagate at a wavelength of 1550 nm. The next fiber is a 6 LP graded index from Draka. The last fiber tested is a 4 LP fiber manufactured by InPhoTech, with a core diameter of 24 μm. All these fibers are characterized by a parabolic distribution of the refractive index. These fibers were chosen to test single-mode measurements on both few-mode and multimode fibers that are commonly used for indoor communication and serve as short-distance telecommunication lines.

[0053] A series of optical fibers with different diameters were used as sensing fibers to test the present invention. The best results were obtained in the range from 45 to 55 μm, especially 50 μm. Using fibers with a larger diameter, a longer range can be achieved due to the larger mode field, which allows for a greater power within the linear range of the sensing fiber 7 below the power value that causes non-linear phenomena. The operation of a fiber with a 62.5 μm core was also tested. A range sufficient for measurements within a single building has already been achieved using a fiber with a 20 μm core.

[0054] For correct measurements using a ϕ-OTDR system on a single-mode fiber, the expected signal visibility should be at least 0.75. The visibility of the signal is defined as:

[0055]

[0056] where I max and I min are the maximum and minimum signal intensities, respectively. To correctly characterize the system, the visibility of every 2 m of the sensing fiber has been calculated. As you can see, for each section of the fiber, the visibility reaches a value above 0.75, which proves the correctness of the measurement. Figure 2 For each measured fiber, the fiber signal is depicted as a solid line, and its visibility is marked with a dashed line. The results for few-mode and multimode fibers are similar to those for single-mode fibers.

[0057] Figure 2 Shown are the measured signals marked with solid lines and their visibility for 2 m sections marked with dashed lines recorded using a detector for fibers a) SMF b) Draka OM4 c) Draka 6LP d) InPhoTech 4LP.

[0058] The visibility obtained for few-mode and multimode fibers is similar to that obtained for single-mode fibers.

[0059] The third step of the measurement is to analyze the frequency shift introduced by changing the fiber strain on the fiber test section. For each fiber, three measurements were collected for different strains. To verify the constancy of the measurement conditions, two measurements were carried out for each given fiber and strain. For each pair of such measurements, the wavelength shift at each point of the fiber was close to 0 GHz, indicating the correctness and repeatability of the measurement. Figure 3 A graph showing the cross-correlation value as a function of fiber length and frequency shift is presented. Figure 3 The graph presented in (a) shows the measurement data for the entire Draka OM4 fiber. Figure 3 The data presented in (b), (c), and (d) show only the data from the strained segments in Draka OM4, Draka LP, and InPhoTech 4 LP, respectively. For all fibers, the shift in the maximum cross-correlation value of the strained segments is clearly visible. At the same time, it can be seen that this shift only concerns the strained segments and does not occur in the rest of the fiber. By measuring the frequency shift between three different values of the stress set by the measurement setup and adjusting a straight line to the obtained values, it is possible to determine the measurement sensitivity of the fundamental mode for all the tested fibers. Due to the laser length adjustment step size, measurement errors were identified in the measurement. It is not possible to measure changes less than half of the tuning step size. All the obtained values differ by no more than the measurement error value from the sensitivity of the single-mode fiber (150 MHZ / με).

[0060] Figure 4 The measured values of the frequency shift as a function of the applied strain and linear matching of the fibers a) Draka OM4 b) Draka 6LP c) InPhoTech 4LP are presented. The determined values of the measurement sensitivity on the fundamental mode based on linear approximation are 138±16, 133±17, and 152±16 MHz / με, respectively.

[0061] The above embodiments show distributed strain measurement using COTDR technology on multimode fibers, which is unknown in the state of the art. The selective excitation of the fundamental mode and the detection of the return signal from the single mode allow for ϕ-OTDR measurements to be carried out on existing and future multimode communication lines.

[0062] The above configuration allows for obtaining a sensor system that includes an optical generation system with a tunable wavelength, which is controlled by a processing unit connected to a receiving system. The processing unit, the optical generation system, and the receiving system can be positioned close to each other, while the sensing fiber is placed further away7. This only requires the use of a longer segment of single-mode fiber 3.

[0063] This also allows for obtaining a scalable system using multiple sensors and multiple sensing fibers placed at different locations.

[0064] Using a wavelength different from that of the telecommunication network in the optical fiber measurement system according to the present invention makes it easier to ensure the coexistence of the sensor system in the existing telecommunication network. Attributable to this, measurements can be carried out without interrupting the transmission in the telecommunication network.

[0065] By connecting the telecommunication network according to the present invention to the optical fiber measurement system according to the present invention, the adaptation of the telecommunication network according to the present invention results in a measurement system that uses the telecommunication optical fiber specifically as a sensing optical fiber during measurement - if the telecommunication equipment is turned off during measurement.

[0066] On the other hand, if coexistence between the measurement system, the transmitter and the receiver of the telecommunication network is ensured, an optical fiber measurement and communication system is created for data transmission and parameter (especially temperature and strain) determination. Then, the network has at least one operational telecommunication data transmitter and receiver, which is connected to an optical fiber having a core diameter exceeding 20 μm, thereby allowing the use of multiple modes and higher power. The larger numerical aperture of the optical fiber with a larger core diameter allows signals to be more easily input into the optical fiber, which simplifies the transmission system. At the same time, the larger mode field allows the use of more power, which increases the maximum measurement distance of the system. This is preferred because it makes it possible to measure more than one parameter simultaneously using higher-order modes. If a core with a diameter of 25 μm, and preferably 47 μm or larger, is used, this is even easier.

[0067] The wavelength of the transmitter differs from the wavelength of the light generation system 1 by at least 10 nm.

[0068] If the transmitter (like a typical transmitter) operates at a wavelength below 900 nm and the light generation system 1 is designed to operate at a wavelength greater than 1000 nm, the separation of the measurement and communication signals is easy. This arrangement is suitable for a large proportion of communication networks.

[0069] If the difference between λ1 and λ2 is greater than 10 nm, the fiber optic measurement system according to the invention operating at wavelength λ1 can use an existing multimode fiber operating at wavelength λ2 used in building and inter-building telecommunication networks. Connecting the measurement system to such a fiber allows obtaining a sensor where the communication fiber acts as a sensing fiber. If λ2 is a wavelength shorter than 900 nm and λ1 is a wavelength longer than 1000 nm, the measurement signal will not be detected by the telecommunication system and the telecommunication signal will not be detected by the sensor system because of the need to use receiving systems made of different materials. If λ1 and λ2 are greater than 1000 nm, it is possible to use a WDM coupler used in telecommunications to separate signals of different wavelengths, thus separating the measurement and telecommunication signals. The minimum difference required for such a system of λ1 and λ2 is 10 nm. By the simultaneous coexistence and interaction of the sensor and communication system according to the invention, the measurement and communication systems are realized.

[0070] For example, in a data transmission network in a multimode fiber equipped with a VCSEL (Vertical Cavity Surface Emitting Laser) light source and a silicon detector at wavelength λ2 = 850 nm, a sensor with a multimode fiber 7 is used, which is equipped with a system for generating radiation, which is equipped with a laser with a wavelength of 1550 nm. Thus, the lasers of the data network and the measurement system have wavelengths below 900 nm and above 1000 nm respectively. In this embodiment, in the data transmission network and in the measurement system, appropriate detectors for their respective bands are used in the data transmission network, namely a silicon detector at 850 nm and an InGaAs (Indium Gallium Arsenide) detector at 1550 nm.

[0071] Figure 5a and 5b A plot of the sensitivity R as a function of the λ wavelength of the communication network detector and the measurement system 2 is shown. The silicon detector has an operating range of approximately 400 - 1100 nm, and the InGaAS detector has an operating range of 800 - 1700 nm. The operating ranges of the detectors partially overlap, but the maxima of their responses exist at separate wavelength values. As a result, the source of the data transmission network operating at 850 nm has little effect on the response of the InGaAS detector in the receiving system 2, and the light generation system 1 does not interfere with the silicon detector of the telecommunication network. As a result, it is possible to obtain an undisturbed simultaneous operation of the sensor and data transmission. By using a 1300 nm high-pass filter in the sensor (not shown in the figure), additional noise reduction can be achieved, which is connected in series with the selective mode device 5 to the path.

[0072] An alternative embodiment uses a wavelength λ1 of the light generation system 1 that is closer to the wavelength λ2 used in the data transmission network. When the wavelength difference When it is at least 10 nm, preferably 50 nm, the measurement signal is successfully filtered from the transmitted signal.

[0073] If both λ1 and λ2 are greater than 1000 nm, it is difficult to use a detector that enables easy differentiation. It is possible to use a WDM coupler used in telecommunications to separate signals of different wavelengths, thereby separating the measurement and telecommunications signals.

[0074] In this configuration, both the sensor and the data transmission device are connected to the sensing optical fiber 7 via a coupler or a WDM filter that filters the signal into a separate channel. In the case of the sensor, the WDM coupler is placed between the selective mode device and the sensing optical fiber 7. The minimum wavelength difference required between the sensor and the data transmission system is .

[0075] For example, a multimode optical fiber with a wavelength λ2 = 1310 nm in the data transmission system can be used as the sensing optical fiber 7 of a sensor according to the present invention, and the sensor is equipped with a system for generating radiation 2 with a wavelength λ1 = 1550 nm. For such wavelengths, the difference between the light source used for transmission and the light source used in the sensor is 240 nm, and both operate in the range above 1000 nm. In such a case, a WDM filter is required for normal operation. Such a filter should be applied at both ends of the optical fiber used. For the specified wavelengths, good results are obtained when the transmission device is connected to channel 31 and the sensor is connected to channel 55 - the channel designations are adopted according to ITU-T G.694.2. This configuration allows the sensor and the devices of the data communication network to operate without interruption.

[0076] The hybrid system can also be constructed using multimode multi-core optical fibers and exciting the sensor and telecommunications signals in separate modes and / or separate cores.

[0077] The solution according to the present invention is compatible with multimode networks and can be used as a sensor that operates in only one optical fiber mode.

[0078] Mode filters can be used to selectively increase the loss of higher-order modes. As the selective mode device 5, a holographic plate or a sequence of holographic plates can also be used. An embodiment of a good mode filter is also the subject of US Patent US10502897B2.

[0079] In some cases, due to more than one parameter, the sensing optical fiber is exposed to a change in the optical path. For example, this is the case when the optical fiber is exposed to both vibration and temperature changes. This makes the measurement more difficult because a linear equation with two unknown parameters then needs to be solved, and this is algebraically impossible. The situation where vibration and temperature occur simultaneously can be solved by using frequency domain discrimination. Mechanical vibration is associated with a change in the optical path, which changes much faster than the change caused by temperature fluctuations. Frequency filtering of the signal representing the change in the optical path as a function of time by means of a low-pass filter allows the change related to temperature to be obtained, and frequency filtering by means of a high-pass filter allows the change related to mechanical vibration to be obtained.

[0080] Using a fixed-wavelength light generation system operating in pulse mode with a pulse repetition rate of 1 kHz or higher, it is possible to make measurements at a rate sufficient to determine the mechanical vibration frequencies in the frequency range typically present in a building. In such a system, the change in the behavior of the measurement signal on the detector over time is detected in successive measurements by successive pulses. The vibration frequency at a point on the sensing optical fiber 7 corresponding to a given delay in time is determined by analyzing the changes in successive measurements with successive pulses. Measuring the mechanical vibration frequency is important for assessing the hazard of construction work carried out near a building. Such measurements generally do not require good spatial resolution, and one meter is sufficient.

[0081] For example, when measuring strain in a building, this method is not always possible, in which case both the length change related to temperature and the optical path change related to strain are slow changes. Using frequency filtering thus cannot solve the problem of solving one equation for two unknown parameters. The number of equations needs to be increased. This can be done by using more than one mode in the measurement. Using different order modes with different propagation coefficients enables two parameters to be measured simultaneously.

[0082] When measuring with two different modes, in order to measure two parameters simultaneously - namely temperature and strain, a system of equations is obtained:

[0083]

[0084] where the specific quantities mean:

[0085] The change in the effective refractive index of the j-th mode measured using the change in the optical path;

[0086] The coefficient determined in calibration, representing the change in the effective refractive index of the j-th mode affected by temperature;

[0087] The coefficient determined in calibration, representing the change in the effective refractive index of the j-th mode affected by strain;

[0088] Temperature change;

[0089] Relative length change representing strain.

[0090] In a properly calibrated measurement system, this system of equations is a system of two equations with two unknown parameters, which can be solved when the determinant is not 0:

[0091]

[0092] The solution of the system of equations allows the two unknown parameters to be tested.

[0093] In an embodiment where more parameters are set and more modes are used - specifically: J parameters and J modes, having different effective refractive indices and reacting differently to environmental factors representing the parameters to be measured (e.g., strain, pressure, temperature, radiation) - the condition for solving the system of equations is that the measurement matrix M p[J×J] has a non - zero determinant.

[0094]

[0095] where is the change in the effective refractive index of the j - th mode under the influence of the k - th physical quantity. Meanwhile and .

[0096] In an embodiment where more parameters (K parameters) are determined and more modes (J modes) are used - having different effective refractive indices and reacting differently to environmental factors representing the parameters to be measured - the measurement matrix is not square. The condition for the solvability of the system of equations is J≥K, and the non - zero determinant of the matrix, which is the product of the measurement matrix and its transpose .

[0097]

[0098] where is the change in the effective refractive index of the j - th mode under the influence of the k - th physical quantity, while and and .

[0099] A measure of the quality of the measurement fiber is the value of the determinant of the matrix where the element is the change in the effective refractive index of the j - th mode under the influence of the k - th parameter. Maximizing the determinant of the presented matrix minimizes the error in distinguishing individual parameters, which is generated by the numerical propagation of the measurement error in determining the change in the effective refractive index of individual modes.

[0100] Measurements using many measurement modes require the use of a controlled selective mode device 5.

[0101] An example of such a device can be a mode multiplexer. The device has one output carried out by means of a few-mode or multi-mode optical fiber. In addition, it has several or a dozen inputs. The number of inputs depends on the number of modes addressed by the device. The input optical fibers can be single-mode optical fibers. The operation of the mode multiplexer consists in introducing light from a given input into an appropriate mode channel of the multi-mode optical fiber. With a certain accuracy, it can be assumed that the channels are independently addressed and the energy is introduced only into the selected modes. The device works similarly in the other direction, filtering the signal from the multi-mode optical fiber, splitting it into appropriate modes, the energy of which is supplied by the corresponding inputs.

[0102] In Figure 1d the embodiment shown, the selective mode device 5 is controlled and connected to the processing unit 9 and adapted for the sequential excitation of different order modes. The processing unit is adapted to determine temperature and strain in the measurement by means of at least two modes of different mode orders and to control the selective mode device 5. It is preferable to use a programmable processing unit 9, and then the task of controlling and determining the object under test based on the measurement signals and calibration data can be solved by a computer program running on the processing unit 9.

[0103] Optical fibers of building telecommunication networks usually pass through a large part of the building height and are rigidly bound to its structure at least at several points. Therefore, the adaptation of the telecommunication network to the measurement network and its use as a sensing optical fiber 7 and the elimination of the influence of temperature allow obtaining measurements of strain in the building.

[0104] Even better results can be achieved if the optical fiber is also designed to perform the sensing function from the very beginning. Then, the optical fiber can be attached and pre-stretched at convenient design points, thus allowing measurements even when the distance between the points decreases (loosens).

[0105] During changes in the building geometry, the sensing optical fiber changes its length together with the structure, which directly translates into a measurable change in the fiber strain. Using the known distribution of the optical fiber in the building and the possibility of localizing changes in the fiber strain, distributed measurements enable us to correctly interpret the data and simulate the building strain.

[0106] Due to this effect, the device can be used to replace a mode filter. In this case, the measurement system is connected to the selected input of the multiplexer (e.g., the input corresponding to the fundamental mode LP01), and the sensing optical fiber is connected to the multiplexer output. In this case, the multiplexer works like a mode filter, enabling correct measurements to be achieved. After performing a measurement on the first selected mode, it is possible to reconnect the measurement system to another input corresponding to another mode (one of the higher-order modes, e.g., LP11). After such a configuration change, the measurement can be performed again.

[0107] The switching can be done manually by the user or can be automated by using an automatic optical fiber switch together with a control system. For example, a standard MEMS-based switch in a 1x2 configuration (one input, two outputs) can be used. In this case, the measurement system is connected to the switch input, and the two selected multiplexer inputs are connected to the switch outputs. Using the control system of the switch, the switch can be set to one of the positions for fundamental mode measurement and then switched to the position for higher-order mode measurement. The system can be coupled to the pulse generation and processing system of the measurement device.

[0108] The use of a mode multiplexer enables the measurement of two optical fiber modes and then the comparison of the obtained results. If the selected modes belong to different mode groups, then their effective refractive indices change. The coefficients of temperature sensitivity and stress sensitivity are also different. By measuring the same physical change - a change in temperature or strain - in two optical fiber modes, it is possible to create a system of equations. By solving this system of equations, it is possible to independently determine each of the mentioned quantities.

[0109] Another application of the system described in the embodiments is the measurement of the difference in the effective refractive indices of modes, which can be used, for example, during the characterization of optical fibers.

[0110] An embodiment of the technical implementation of the mode multiplexer idea is the use of a sequence of holographic plates. It is possible to create a mode multiplexer that enables independent access to six modes of the Draka 4 LP multimode optical fiber manufactured by the Prysmian group. An important parameter of the mode multiplexer is the energy level of the modes introduced outside the mode addressed by a given input. In the case of holographic plate technology, it is possible to obtain a ratio of the power of the modes introduced other than the selected mode (e.g., PROTEUS-S) to the power of the selected mode at -15 dB. In practice, this means that it is possible to perform correct measurements on the addressed mode using a multiplexer made using this technology.

[0111] Another embodiment of selective mode excitation is the use of an asymmetric coupler, among other things, as disclosed in Q. Huang, Y. Wu, W. Jin and K. S. Chiang's "Mode Multiplexer With Cascaded Vertical Asymmetric Waveguide Directional Couplers" (in Journal of Lightwave Technology, vol. 36, no. 14, pp. 2903-2911, 15 July 15, 2018, doi: 10.1109 / JLT.2018.2829143). In this case, the device is based on a sequence of fiber couplers such that they are able to independently excite modes in such a way that, although in the simplest case, one would be sufficient. The device can have a single-mode fiber input and two independent multi-mode fiber outputs. One mode, such as the LP01 fundamental mode and the LP11 higher-order mode, for example from KS PHOTONICS, is excited in each output. When using this solution, the measurement system is connected to the input port. The sensing fiber should be connected to one of the output ports. After the measurement of the selected mode, the sensing fiber must be switched to the second multiplexer output.

[0112] The advantage of using this solution is the possibility of measuring several modes using a relatively simple multiplexer.

[0113] The disadvantages of this solution include a low coupling efficiency of 80% compared to 97% for the example solution with a holographic plate, and the need to use a much more difficult-to-access multi-mode switch in terms of measurement automation.

[0114] Depending on the application, a processing unit 9 adapted to implement various measurement techniques and dedicated components of the measurement system are used.

[0115] The COTDR technique requires a narrow spectral source - with the possibility of tuning the central wavelength - for measurements. To perform a complete measurement, it is necessary to send several or dozens - depending on the required temperature or strain measurement range - of pulses with different central wavelengths. The tuning occurs between pulse generations and does not have to be continuous.

[0116] The disadvantage of this solution is the low measurement speed, with a single measurement lasting up to one minute. Due to the fact that at least several scattered pulses must be collected for a single measurement, this technique is not very convenient for monitoring rapidly changing phenomena such as vibrations or temperature changes of several K / min.

[0117] Measurements using the COTDR technique can be carried out with a relatively high spatial resolution on the order of 10 cm, and are even single-digit cm at distances on the order of several kilometers or even tens of kilometers.

[0118] Measurements using this technique allow for reference measurements, so that measurements can be carried out periodically. Continuous measurements of the building are not required to determine its strain. For measuring the strain of a building and the temperature in a server room, since the changes in these two parameters are rather slow, this technique is the best choice.

[0119] The Chirp-OTDR technique is a special case of COTDR. This measurement uses optical pulses, where the central wavelength is tuned during the emission of a single pulse. The advantage of this technique lies in the possibility of quantitatively measuring the selected parameter in a single emission. The disadvantage is that COTDR has a lower spatial resolution of several meters, because the pulse cannot be too short since it must last long enough to perform the wavelength tuning process. Tuning the wavelength of the source during the pulse also requires a more complex source layout, because synchronization, measurement, and linear correction of the tuning are needed.

[0120] The OFDR technique is based on frequency-domain data analysis. It requires a linearly tunable laser to operate. In practice, an additional reference interferometer should be used to compensate for the minor source tuning non-linearity, which makes the system more complex and vulnerable to environmental vibrations. The disadvantage of this technique also lies in the duration of the measurement. During one measurement, the laser must adjust its central wavelength, which is done at a finite speed. This means that the measurement can take up to several seconds. This makes this technique only suitable for measuring slowly changing phenomena, without vibrations or other environmental disturbances. The advantage of this technique is that the measurement has a very high spatial resolution on the order of millimeters.

[0121] The present invention allows for the functional expansion of multimode fiber optic networks used in buildings beyond data transmission and uses them as sensors. The results of the tests conducted allow us to use the COTDR technique to measure the strain of building structures exposed to high strain due to strong winds - especially in high-rise buildings - or to measure the temperature in a data center.

[0122] A fiber optic telecommunication network means a fiber optic infrastructure that can be used to transmit information by changing the modulation of the light transmitted through it. This group particularly includes telecommunication cables and optical fibers permanently installed in the facility, as well as cables and optical fibers that can be used or are intended for telecommunication applications. A telecommunication optical fiber is understood to mean any optical fiber that can be used to transmit signals.

[0123] Those skilled in the art will note that the scope of the present invention includes not only devices for making measurements based on optical signals generated by Rayleigh scattering, but also solutions using Brillouin or Raman scattering.

Claims

1. An optical fiber measurement system comprising a controlled light generation system (1), and a receiving system (2) optically connected via an optical path including a directing device (4), and further comprising a processing unit (9) for receiving and processing signals from the receiving system (2) and for implementing OFDR and / or COTDR measurement techniques for measuring changes in optical path and processing them into at least one parameter, and a selective mode device (5) for single-mode detection of backscattered light, and being adapted to be connected to an optical fiber infrastructure via the selective mode device (5), Characterized in that the processing unit (9) being further adapted to control the controlled light generation system (1), and the selective mode device being adapted to selectively excite a single mode in a telecommunication network, and the selective mode device (5) being controlled and connected to the processing unit (9), and being adapted to continuously excite different order modes, while the processing unit (9) is adapted to determine at least two parameters in the measurement by means of at least two modes of different orders.

2. The optical fiber measurement system according to claim 1, characterized in that, The selective mode device (5) is a selective mode excitation system for continuously exciting different modes.

3. The optical fiber measurement system according to claim 1 or 2, characterized in that, The selective mode device (5) is a holographic plate or a sequence of holographic plates.

4. The optical fiber measurement system according to claim 1 or 2, characterized in that, The selective mode device (5) is an asymmetric coupler system.

5. The optical fiber measurement system according to claim 1 or 2, characterized in that The processing unit (9) has a frequency filtering module for filtering the measured optical path.

6. The optical fiber measurement system according to claim 1 or 2, characterized in that, The processing unit (9) is adapted to determine two parameters in the measurement by means of at least three modes of different orders.

7. The optical fiber measurement system according to claim 1 or 2, characterized in that, It further has a coupler (6) and an optical fiber section (8) for connection to a telecommunication network.

8. A method for adapting a telecommunication optical fiber (7) having a core diameter greater than or equal to 20 μm to a measurement system, characterized in that, A telecommunication optical fiber (7) is connected to the optical path of the measurement system via a selective mode device (5) for single-mode detection of backscattered light and as a selective excitation of a single mode, the measurement system having a controlled light generation system (1) and a receiving system (2) connected by means of a directing device (4), and the selective mode device (5) being controlled and connected to the processing unit (9), and being adapted to continuously excite different order modes, while the processing unit (9) is adapted to determine at least two parameters in the measurement by means of at least two modes of different orders.

9. The method according to claim 8, wherein The telecommunication optical fiber (7) is connected to the optical fiber measurement system as specified in claim 7, and the optical fiber (7) of the telecommunication network is cut and connected to the coupler (6).

10. An optical fiber measurement and communication system for data transmission and for determining a parameter representing a physical quantity, characterized in that, It includes at least one optical fiber measurement system as specified in any one of claims 1 to 7, the optical fiber measurement system being connected to a telecommunication network equipped with a transmitter and a receiver, the transmitter and the receiver being connected to a telecommunication optical fiber (7) having a core with a diameter of 20 μm or greater, and the transmitter wavelength differing from the wavelength of the controlled light generation system (1) by at least 10 nm.

11. The optical fiber measurement and communication system according to claim 10, characterized in that, The telecommunication optical fiber (7) has a core diameter greater than 47 μm.

12. The optical fiber measurement and communication system according to claim 10 or 11, characterized in that, The transmitter operates at a wavelength shorter than 900 nm, and the controlled light generation system (1) operates at a wavelength longer than 1000 nm.

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