DEVICE FOR FIBER OPTICAL MEASURING AND TRANSPORTING MEASURING SIGNALS

Description

[0001] The present invention relates to a device for fiber-optical measurement and transmission of measurement signals.

[0002] The present metrological task requires a decentralized measurement technique ( distributed sensingThis type of system is used to measure in a spatially distributed manner and then aggregate the distributed measurement results at a single measuring station. Due to the spatial distribution of the measurement process, there is a possibility that unauthorized persons could gain access to the measurement data, read it out, or even delete relevant measurement results from the data stream. An example of this is surveillance systems for premises that warn of unauthorized activities on the grounds, such as industrial sites or railway property. Interfering with or eavesdropping on such surveillance systems involves various aspects: On the one hand, the measurement results obtained from the surveillance can be falsified to conceal unauthorized access to the monitored area. On the other hand, the measurement results can also be read out to determine whether people, such as security personnel, are present on the monitored premises.

[0003] A highly successful example of decentralized measurement technology utilizes telecommunications fiber optic cables and pulsed or amplitude-modulated laser light, which is sent through the fiber and exhibits specific temporal intensity sequences and frequency components. The system measures the small power components that are backscattered against the intended beam direction by statistically distributed defects in the fiber's glass. If the fiber stretches in a region between two defects, the time interval between two backscattered pulses increases. Conversely, if the fiber is compressed in the same region, the interval between the two backscattered pulses decreases. The fiber should have good frictional contact with the surrounding soil. In this case, stretching and compression of the fiber indicate acoustic vibrations in the environment. Such a system is called a DAS system (Distributed Acoustic Sensing System).A disadvantage of the known system is that, in principle, the fiber optic cable can be tapped and the backscattered light diverted via an optical diode, thereby attenuating it as desired. In this case, the measurement signal would fall into the hands of an unauthorized person.

[0004] A fiber-optic DAS measuring device is known from GB 2,442,745. It is intended, for example, for use in burglar alarms. The device is designed to send groups of light pulses of different frequencies through conventional optical fibers and to observe the interference of light components that have been backscattered at different points along the fiber. A disadvantage is that the measurement signals can be intercepted by an unauthorized person without being noticed. Existing signals can also be deleted without being detected.

[0005] A fiber-optic DAS measuring device is known from WO 2010 / 020795. The known decentralized DAS measurement technology is intended, for example, for monitoring oil and gas pipelines. P- and S-waves in solid media, such as soil, are used for measurement. If a source triggers both P-waves (primary or pressure waves) and S-waves (secondary or shear waves), these propagate at different speeds in the medium. Both types of waves cause the solid medium to expand and contract at the frequency of the disturbance. The optical fiber, which is in contact with the solid medium, expands or contracts accordingly. The different propagation speeds of P-waves and S-waves allow them to be distinguished. A disadvantage here, too, is that the measurement signals can be intercepted by an unauthorized person without being noticed. Existing signals can also be erased without being detected.

[0006] A DAS system for monitoring a railway network is known from WO 2017 / 093741 A1. Conventional optical fibers from the telecommunications sector are used as the measuring waveguide. The fibers are simply laid along a transport network, such as a railway line or road, in a narrow duct. The optical fiber is housed in a protective casing, so no maintenance is required. Optical pulses with a specific frequency pattern are fed into the measuring fiber from a measuring station. The backscatter of these pulses is evaluated, focusing on Rayleigh backscatter. Spatial resolution for the acoustic disturbance is achieved using frequencies modulated onto the optical pulse sequences. A disadvantage here, too, is that the measurement signals can be intercepted by an unauthorized person without being noticed. Existing signals can also be deleted without being detected.

[0007] The evaluation of Brillouin scattering of optical pulses in fibers is known from US 2016 / 0025524 A1.

[0008] WO 2011 / 153018 discloses a method for reducing crosstalk between waveguides in an interferometric measuring system with more than one waveguide.

[0009] From EP 3 680 638 A1 a method has become known to be able to check the integrity (correct functioning) of the fiber sensor (the source of the Rayleigh backscattering).

[0010] WO 2015 / 170355 A1 discloses a method for obtaining information on fiber strain and fiber temperature changes by evaluating Rayleigh backscattering and stimulated Brillouin scattering at high spatial resolution and low cost.

[0011] The present invention is based on the objective of securing the reading of data from a decentralized measurement system for an authorized person against others.

[0012] According to the invention, the problem is solved by a device having the features of claim 1. Advantageous embodiments are the subject of the dependent claims.

[0013] The device according to the invention is designed and intended for fiber-optical measurement and transmission of measurement signals. The device comprises a measuring station that has at least one measuring beam guided in a waveguide and a reference light beam. This means that a waveguide for the measuring beam and a waveguide for the reference light beam are provided. The reference light beam runs within the measuring station and is thus protected against external, unauthorized access. Both light beams are connected to a

[0014] The interferometer unit is connected to at least one, preferably two, photodetectors. According to the invention, the waveguide-guided measuring beam is fed back into the measuring station. The invention is based on the insight that all the light transmitted through the fibers can be used for the measurement, and not merely a small backscattered portion of the light. This results in a potentially significantly higher signal for the same light power in the fiber. In particular, the measurement of acoustic signals in a DAS system is no longer dependent on the size and frequency of defects in the fiber, which are used as scattering centers. By feeding the light-carrying fiber back into the measuring station, the transmitted light can be evaluated there.

[0015] In an alternative version of the invention, a fiber loop is provided through which the measuring beam is guided back to the measuring station. The waveguide carrying the light is designed as a ring or a loop, with both ends located in the measuring station. The measuring beam exits the measuring station, passes through the waveguide, and re-enters the measuring station. Technically preferred, an independent light beam is sent in the opposite direction through the same fiber or through a fiber laid parallel to the same measuring points, and this light is also measured. The time shift of the same signals on the two measuring beams allows the location of the measurement to be determined. Besides using a second fiber, light of different wavelengths, different polarizations, or with differences in other parameters such as modulation, etc., can also be guided in opposite directions in a common fiber for spatial resolution.If the opposing rays are distinguishable, unwanted scattered light from one direction of travel to the other can be easily discriminated against.

[0016] In an alternative embodiment of the invention, which can also be used in addition to the fiber loop, the light travels through a fiber strand and is reflected as completely as possible at its end, so that it travels through the same fiber in the opposite direction. The fiber strand preferably has a waveguide that guides the measuring beam, the waveguide originating at one end from the measuring station and having a reflector at its free end. The reflector is part of a local measuring unit that is read out optically. In a more compact embodiment, the reflective measuring unit is directly connected to the fiber end, so that the light beam does not first have to exit the fiber.

[0017] One advantage of this fiber strand design is that no independent light beam is needed to spatially resolve an acoustic disturbance.

[0018] According to the invention, a continuous-wave laser light is split into the measuring beam and the reference beam. The continuous-wave laser light has a low average power, so that the expected measurement signal just barely protrudes from the photon shot noise (quantum noise). The laser light is modulated randomly or quasi-randomly in power, phase, and / or frequency. Due to the random, non-repeating modulation, the transit time of one half of the light can be uniquely measured relative to the other half. One half serves as the measuring beam, which collects the decentralized measurement signal and transports it to the measuring station. The other half of the light remains in the measuring station and serves as the reference beam, which is not influenced by measurement signals. Preferably, squeezed light is used for the continuous-wave laser light. By splitting the squeezed laser light into two beams, quantum entanglement arises with respect to the amplitude (or...The intensity and phase (or frequency) of the two beams are measured. Any attempt to eavesdrop or delete the measurement data irreversibly leads to a decrease in entanglement. This decrease is measured and quantified in addition to the signal when comparing the reflected measurement beam and the reference beam, thus revealing the eavesdropping attempt. If the unauthorized person only intercepts a small portion of the light, the eavesdropping attempt is more difficult for the authorized person to detect, but the unauthorized person receives a correspondingly worse signal-to-noise ratio. If the amount of intercepted signals exceeds a self-imposed threshold, the readout of the decentralized measurement can be preferentially interrupted. This ensures that an authorized person always has a significantly better signal-to-noise ratio than the signal available to an eavesdropper.Further safeguarding of the measurement data can be achieved by a rapid in-situ adjustment of the light power, ensuring, based on just acquired measurement data, that signal amplitudes are never much larger than the noise power of the entangled states.

[0019] In a further preferred embodiment, a random interference signal is superimposed on the measured and the reference laser beam in such a way that the interference signal is removed in the interferometer unit with the reference laser beam.

[0020] In a preferred embodiment, the interference signal covers at least half, and preferably more than 80%, of a signal frequency band. The random number generator for generating the random interference signal can preferably be a quantum random number generator.

[0021] In a preferred embodiment, the measurement and reference beams superimposed in the interferometer unit have quantum uncertainties entangled with each other due to the squeezed light, wherein the squeeze factor with respect to the quantum noise of the measurement signal is at least 2 (corresponding to 3dB).

[0022] In a preferred further development, the interferometer unit is designed to read out a noise-free signal amplitude with a first detector and a noise-free signal phase with a second detector.

[0023] The interferometer unit is fed with a local oscillator for evaluating the superimposed measurement beams and reference beams.

[0024] In a further preferred embodiment, one or more sensors are provided in the waveguide of the measuring beam, which modulate a measured value onto the measuring beam. The modulation of the measured values ​​can be performed for both the fiber loop and fiber strand configurations. The modulated measured values ​​can, for example, originate from sensors for detecting local temperature or local humidity.

[0025] All versions measure the total light transmitted through a fiber, not just a small backscattered portion. This results in a potentially much stronger signal for the same light power in the fiber. In particular, the measurement of acoustic signals with a DAS system is no longer dependent on the size and frequency of defects in the fiber, which are used as scattering centers.

[0026] In one embodiment, the light travels from a measuring station back to the station through a fiber loop. It is generally advantageous to send an independent light beam in the opposite direction through the same fiber, or through a fiber laid parallel to the same measuring points, and to measure this light as well. The time shift of the same signals on the two measuring beams allows the location of the measurement to be determined, similar to how this is possible with the known DAS system based on backscattered signals.

[0027] In another embodiment, the light travels through a fiber strand and is ultimately reflected back through the same fiber as completely as possible. This embodiment is particularly suitable in combination with an additional, remote, specialized pressure, temperature, or humidity sensor that only functions for reflected light. In this embodiment, no independent light beam is required to determine the location of an acoustic disturbance.

[0028] All versions utilize continuous-wave laser light, the power, amplitude, phase, and / or frequency of which is randomly or quasi-randomly modulated. This random, non-repeating modulation allows for the unambiguous measurement of the transit time of one half of the light relative to the other half. One half serves as the measuring beam, collecting the decentralized measurement signal and transmitting it to the measuring station. The other half of the light remains in the central measuring station, serving as a reference beam and not collecting a measurement signal.

[0029] In a particularly relevant implementation, squeezed light is used. By superimposing and splitting two beams of squeezed light, two beams are created that are quantum mechanically entangled with respect to their amplitudes (or intensities) and phases (or frequencies). Similar to quantum cryptography, any attempt to eavesdrop on the measurement data irreversibly leads to a decrease in entanglement. This decrease is measured and quantified in addition to the signal when comparing the reflected measurement beam and the reference beam. If the unauthorized person only intercepts a small portion of the light, the available signal-to-noise ratio is significantly worse because they do not have access to the entangled beam. If the amount of intercepted signal exceeds a self-imposed threshold, the measurement would be interrupted.This ensures that the authorized person always has a significantly better signal-to-noise ratio than other people. Further validation of the measurement data can be achieved by adjusting the light intensity, ensuring that signal amplitudes are never much larger than the noise power of the entangled state.

[0030] In another embodiment, light is emitted via a fiber optic loop through special sensors positioned at specific locations. These sensors could measure a local parameter, such as temperature or humidity, and encode it into transmitted light at a specific frequency. Another possibility is the reading and transmission of a camera image.

[0031] In a similar embodiment, the light is guided via a fiber optic strand to a special sensor and reflected back into the fiber. Possible sensors measure a local quantity, such as temperature, humidity, or a camera image, and encode this value at a specific frequency by reflecting it back onto the light.

[0032] A preferred embodiment of the invention is explained in more detail using two exemplary embodiments: Figure 1: A measuring station with a ring-shaped closed waveguide and Figure 2: A measuring station with a waveguide that has a free end of high reflectivity.

[0033] Figure 1Figure 1 shows a measuring station 10 from which a light beam L1 emerges within a fiber F1, passes over the distributed measuring points M1 to M4, and re-enters measuring station 10. The optical fiber F1 is closed in a ring and has no reflective end. A second light-carrying optical fiber F2 remains within measuring station 10. The light beams in F1 and F2 originate from the same laser and exhibit correlated and anticorrelated quasi-random noise. F1 leads to Figure 1The measuring beam, which receives acoustic signals along its entire path, receives further signals from special sensors in the area of ​​measuring points M1 to M4. These acoustic signals across the entire length of waveguide F1 cause stretching and compression of the light-carrying fibers, resulting in phase modulation of the measuring laser beam. The measuring laser beam, together with a reference laser beam guided in the reference waveguide F2, is directed to a beam splitter S1 in an interferometer unit. The output beams of beam splitter S1 each strike a photoelectric detector D1 and D2. A light beam is also directed via a waveguide F0 to a second beam splitter S2, which directs the split light beam to detectors D1 and D2. The light beams L0, L1, and L2 originate from the same laser source or from multiple laser sources that are stabilized and controlled relative to each other.

[0034] An important aspect of the aforementioned measuring device is that the laser light L1, originating from an authorized, secure measuring station 10, is distributed for decentralized measurements, and the predominant proportion, more than 50%, preferably more than 90%, returns to the transmitter after the decentralized measurement has been completed. Thus, the measurement of backscattered laser light, e.g., Rayleigh backscattering, is no longer used.

[0035] Another important aspect of the described device is to superimpose interference signals onto the laser light L1 before it is emitted, within the trusted measuring station, over a large portion of the signal frequency band. These interference signals are random and therefore unpredictable. Preferably, they are chosen to overpower the expected measurement signals with the highest possible amplitude, so that very little information about the measurement data can be gleaned from the light signal L1 after the data has been acquired. By superimposing the interference signal, a correlated laser light L2 can be generated, which makes it possible to subtract the superimposed interference from the laser light L1 and thus read the results of the decentralized measurement. The true randomness of the interference signals can be generated, for example, by a quantum random number generator.

[0036] When the two laser beams L1 and L2 are superimposed at the beam splitter S1, the splitter is adjusted so that all intentionally applied random amplitude interference signals destructively interfere at one output, and all intentionally applied random phase interference signals destructively interfere at the other output. The light corresponding to the amplitude signal is directed to photodetector D1, while the light corresponding to the phase interference signals is directed to photodetector D2. The light L0 in the optical path F0 serves as a local oscillator field, which optically amplifies the measurement and acts as a microscopically precise time reference. The local oscillator field is split in the beam splitter, allowing the interference-free signal amplitude to be read out at detector D1 and the interference-free signal phase to be read out at detector D2. Despite destructive interference, a minimal residual interference always remains.If the residual interference is greater than a predetermined value, this indicates the proportion of light that was intercepted. Based on this, appropriate measures can be taken to rectify the security vulnerability.

[0037] The advantage of the invention is particularly evident when the correlated interference signals on the light fields L1 and L2 exist in the form of entangled quantum states. Entangled quantum states of the light fields L1 and L2 exist when squeezed laser light is split into the two beams L1 and L2 using a beam splitter. If phase changes are measured during the decentralized measurement, entanglement exists with respect to the amplitude and phase-squared modulation of the light fields from L1 and L2. In this embodiment, the returning light is superimposed with the entangled light at a beam splitter in the measuring station, and the two outputs are used to read out one amplitude modulation with squeezed quantum uncertainty and the other a phase-squared modulation with squeezed quantum uncertainty. The squeeze factor in relation to the signal strength indicates the maximum amount of information that has been lost in total.If the amount of information transmitted via the decentralized measurement is too high, the emission of light can be stopped as a reaction.

[0038] Figure 2 corresponds to the structure in Figure 1 The difference is that the light-carrying waveguide F1 is guided outwards to the measuring point M5 via a Faraday rotator R. A reflector can be provided at the measuring point M5, or at a remote point, which reflects the light beam guided in the waveguide F1 back so that it is directed via the Faraday rotator R to the beam splitter S1 in the interferometer unit. The reflector can also be designed as a pressure-sensitive, temperature-sensitive, or humidity-sensitive sensor that varies the optical path length depending on these measured quantities at selected frequencies.

Claims

1. Device for fibre-optical measurement and transport of measurement signals, comprising a measuring station which has one or more laser sources and at least two waveguides (F1, F2), which are configured to guide at least one measurement beam and one reference light beam in a respective waveguide, wherein the waveguide (F2) guiding the reference light beam runs within the measuring station and the at least two light beams are brought together at an interferometer unit which has at least one photodetector (D1, D2), wherein the waveguide (F1) guiding the measurement beam is formed as a ring in which both ends are arranged in the measuring station and the measurement beam, coming from the measuring station, passes through the waveguide and re-enters the measuring station and / or the waveguide (F1) guiding the measurement beam originates with one end in the measuring station and has a reflector at its free end, characterized in that a continuous-wave laser light is split into the measurement beam and the reference beam, wherein the continuous-wave laser light is randomly or quasi-randomly modulated in power, phase and / or frequency.

2. Device according to claim 1, characterized in that entangled laser light is provided for the continuous-wave laser light.

3. Device according to one of claims 1 to 2, characterized in that a random interference signal (N) is superimposed on the measurement and reference laser beams (L1, L2) in such a way that the interference signal (N) is removed from the measurement data in the interferometer unit with the reference laser beam.

4. Device according to claim 3, characterized in that the interference signal covers at least half, preferably more than 80%, of a signal frequency band.

5. Device according to claim 3 or 4, characterized in that a quantum random generator is provided for generating the random interference signal.

6. Device according to claims 3 to 5, characterized in that the measurement and reference beams superimposed in the interferometer unit provide beams with squeezed quantum uncertainties.

7. Device according to claim 6, characterized in that the squeezing factor is at least two (corresponds to 3dB).

8. Device according to one of claims 1 to 7, characterized in that the interferometer unit is configured to read out an interference-free signal amplitude with a first detector and an interference-free signal phase with a second detector.

9. Device according to one of claims 1 to 7, characterized in that the interferometer unit is fed by a local oscillator for evaluating the superimposed measurement and reference beams.

10. Device according to one of claims 1 to 9, characterized in that one or more sensors are provided in the waveguide of the measurement beam, which modulate a measurement value onto the measurement beam.

11. Device according to claim 10, characterized in that the sensor is configured to detect a local temperature or a local air humidity or a camera image.

12. Device according to one of claims 1 to 11, characterized in that the interferometer unit has two photodetectors (D1, D2).