CZ2019628A3 - Distributed optical fibre sensor system - Google Patents

Abstract

The distributed optical fiber sensor system consists of two units - main and remote. The main unit comprises a first coupler (11) and a second coupler (12), the first coupler (11) interfering with signals from the reference and sensing arms propagating in the CCW direction, frequency shifted from the carrier frequency of the laser radiation source (2). A signal from the laser radiation source (2) supplied by the power supply module (5) is connected to both arms of the interferometer via one input port of the first coupler (11). The second coupler (12) serves to obtain the reflected result of the CW interferometer from the reflector (17), to detect it by the first photodetector (3) and to obtain the exact frequency shift generated by the acousto-optical modulator (15) and oscillator (16) in the remote unit. Both signals, filtered from the first photodetector (3) and the demodulated signal from the second photodetector (4) are connected to the control unit (1), where they are digitized and processed.

Description

Distributed optical fiber sensor system

Field of technology

The technical solution relates to a distributed optical fiber system based on the principle of a dual Mach-Zehnder interferometer enabling the localization of events using one pair of optical fibers. The system finds application as a distributed acoustic sensor used to protect perimeters and falls into the field of optoelectric fiber sensors.

Prior art

Distributed optical fiber sensors fall into the category of intrinsic sensors and are characterized in that the modulation of light by the sensed phenomenon takes place directly in the optical fiber while the light is guided through the fiber. At present, techniques based on the principle of reflectometry or the principle of interferometry are mainly used to detect acoustic / mechanical vibrations in the vicinity of optical fibers. In the case of reflectometric systems it is Rayleigh scattering and Brillouin scattering, in interferometric systems they use Mach-Zehnder, Michelson or Sagnac interferometer systems. It is the interferometric systems that are structurally simpler and offer higher sensitivity, however, it is necessary to use three separate fibers or a combination of multiple connections to locate the event, which is very uneconomical and inefficient.

With the right connection, distributed systems enable the detection and localization of events in the immediate vicinity of optical fibers / cables at a distance of up to one hundred kilometers with a resolution of tens of meters, although the storage depth is often up to one meter.

The current research is mainly focused on connection optimization, frequency noise reduction, system range increase, optimization in data processing, etc. The advantage of using scattering phenomena, whether Rayleigh or Brillouin scattering, is in relatively easy event localization, using only one standard telecommunication fiber for measurement and in the possibility of measuring only from one side of the fiber. The disadvantage is the high complexity of the connection and the complex signal processing, which results in high acquisition costs. Typically, the systems operate up to a distance of 40 to 100 km with a spatial resolution of 1 to 100 meters. The fiber pressure / deformation resolution normally reaches values of 0.1 με ³ in the frequency range 0.01 to 50 kHz.

The interferometers work on the principle of interference, which means that the difference in the propagation of the optical beam paths between the measuring and the reference arm is compared. The phase of the light can change under the influence of external influences on the optical fiber, therefore it is possible to use the changes of the light phase for the detection of physical quantities, such as acoustic vibrations or electromagnetic signals. The most common connections are the Mach-Zehnder interferometer and the Michelson interferometer.

The use of a Mach-Zehnder interferometer (MZI) for measuring vibrations has found great use due to its simple connection. MZI is considered a cost-effective solution for distributed optical fiber scanning. The light source is usually a highly coherent laser diode, further the circuit comprises a coupling member for dividing the light beam into two arms. External vibrations are applied to the measuring arm, while the reference arm is isolated from external vibrations. Thus, changes in length and refractive index cause phase modulation between the measuring arm and the reference arm. Subsequently, the phase modulations are converted to intensity modulations by means of a second coupler and converted into an electrical signal by means of a photodetector. In the basic connection, it is only possible to detect events. In order to locate the place of occurrence of the event, it is necessary to combine the MZI with another method. For example, the involvement of a dual MZI is common. In dual MZI, the signal is evenly divided into two paths - CW (clockwise) and CCW (counterclockwise), while in CW the signal is bound directly and in CCW the signal is bound from the opposite end of the fiber. As soon as an external vibration is applied to the measuring arm, a phase change is created at the corresponding point

- 1 CZ 2019 - 628 A3 between both arms. At both ends, signals are received by photodetectors. Due to the fact that two interferometers propagating opposite each other are mounted on the same fibers, the vibration point where external vibrations occur can be calculated. In order for the signal to be coupled from the opposite end of the wave, it is necessary to use a third fiber, which only serves to transport the optical beam to the other end of the interferometer.

The Michelson interferometer (MI), like the Mach-Zehnder interferometer, is a widely used technique. A typical MI circuit uses a signal from a highly coherent laser source, which is split into two beams via a first coupler, which are connected to two fibers - a reference and a measuring. At the end of the fibers, FRM (Farrday rotator mirror) mirrors are placed, on which the signals are reflected back and subsequently recombined in the same coupler. It is important that the difference in optical paths between the two beams be less than the coherence length of the laser. As with the Mach-Zehnder interferometer, it is not possible to determine the location of the event in the basic circuit, so a combination with another method is necessary.

The advantage of using optical fiber interferometers is the relatively easy connection and subsequent signal processing. However, the disadvantage is the complex location of the event, when it is necessary to use three separate telecommunication fibers or combine several methods, which results in high costs. Typically, the systems operate at a distance of 50 to 200 km with a spatial resolution of 1 to 100 meters. The frequency range is limited only by the parameters of the components used.

There are companies in the world offering systems for detecting acoustic / mechanical vibrations, such as:

Silixa - the most important company in the field of distributed optical fiber systems. It offers several types of systems for measuring up to a distance of 100 km. All are mainly intended for the oil and mining industry.

Omnisens - another important representative in the field of long-distance sensing for oil and gas pipelines, etc. It senses stress and temperature using Brillouin scattering.

Sensomet - the company enables the detection of voltage and temperature along the optical fiber at a distance of 30 km. He uses Brillouin's roptyl.

FFT Aura - the company enables the detection of voltage along the optical fiber at a distance of 20 km. It uses Rayleigh scattering.

Optasense - the company enables the detection of voltage along the optical fiber at a distance of up to 50 km. It uses Rayleigh scattering.

The solutions mentioned, for example, in US 8923663 B2 Hill et al use a Rayleigh scattering reflectometric circuit to measure acoustic vibrations in the vicinity of optical fibers. The device works with two different spatial resolutions. The transmitting and receiving parts of the system are modified so that two spatial resolutions can be detected, for example by changing the pulse duration or by using spatial resolution. In addition, the first and second spatial resolutions can be generated simultaneously or sequentially. These systems contain expensive components. The solution uses a complex system of generating two input pulses, which results in complex and expensive processing.

There is also a known solution according to GB 2289331 A Jackson, which is used to measure the temperature and pressure around optical fibers. The solution proposes the use of a pair of fibers in a loop, which de facto creates a Sagnac interferometer. The circuit uses a complex generator of stimulated Brillouin scattering, which generates frequency-shifted pulses propagating in the opposite direction to the main signal. This creates a dual Sagnac interferometer. A significant disadvantage of the Sagnac interferometer is its sensitivity to changes, such as acceleration or rotation, where the motion of the Earth can affect the resulting

-2 CZ 2019 - 628 A3 measurements. Another disadvantage of the solution is the limitation of the minimum fiber length due to the need to generate Brillouin scattering.

The essence of the invention

As already mentioned, the main disadvantage of interferometric systems in the basic embodiment is the impossibility of locating an event in the basic circuit and the necessity of combining several techniques, or the necessity of using three fibers in case of localization requirement. These shortcomings are addressed by a distributed optical fiber sensor system based on the principle of a dual Mach-Zehnder interferometer using the frequency shift of the unmodulated CCW signal, according to the present invention. The system consists of a master and a remote unit. The main device includes first and second optical couplers, the remote unit includes third and fourth optical couplers. The output of the first coupler is connected to the inputs of the second and third couplers by means of a reference and sensing fiber. At the same time, it is connected at the input to the first radiation detector and to a source of highly coherent radiation, which is provided with a wired communication interface containing safety elements for switching off the radiation source in case of interlock loop interruption and is supplied from a stable voltage source. The second input of the second coupler is connected to the second detector and the output is connected to the input of the fourth coupler. A reflector is connected to one output of the fourth coupler, which reflects the output of the CW interferometer back to the main unit. The second output is connected to the output of the acousto-optical modulator, which is controlled by an oscillator of the corresponding frequency and shifts the laser frequency by the frequency of the oscillator. The power supply of the oscillator is taken care of by the power supply module. The input of the acousto-optical modulator is connected to the output of the third coupler. The essence of the new solution is that the signal for the CCW interferometer is not transmitted to the far end by a dedicated fiber, but a part of the signal from the CW interferometer is divided by the third coupler. This signal is then shifted by a defined frequency using an acousto-optical modulator, and this new signal is coupled to the reference and sensing fibers via a fourth coupler. This creates a CCW interferometer.

In the event of an event in the vicinity of the optical cable, the event will be detected on both interferometers, both CW and CCW. Due to the different direction of propagation of both interferometers, the location of the event can be calculated using a formula. The advantage of the proposed solution is also the location of the outputs from both interferometers in the main transmission unit, which ensures the correct synchronization of the outputs.

In a preferred embodiment, the control unit, including the wired communication interface module and the acquisition module, is formed by a single-board industrial computer without moving parts.

The power supply module is preferably redundant.

Advantageously, one pair of optical fibers from the same optical cable can be used.

The advantage of such a dual interferometer for detecting and locating events in the vicinity of optical fibers is that, compared to commonly commercially available systems, it uses only one pair of optical fibers and contains only one source of highly coherent radiation. In addition, the other optical components, i.e. the couplers, the detectors, the reflector and the acousto-optical modulator, can be easily integrated. In the present solution, as in US 8923663 B2 and GB 2289331 A, a source of highly coherent radiation is used. Like US 8923663 B2, the proposed solution uses an acousto-optic modulator, however, the circuit does not include any optical fiber amplifiers that limit spectral applicability and highly sensitive detectors, both of which increase the cost of implementing the system. Unlike GB 2289331 A, the proposed solution is not limited by the minimum fiber length and does not include a complex generator of stimulated Brillouin scattering. In addition, the components used are commercially available from many manufacturers and meet the demanding requirements common to the security industry. Furthermore, the device does not contain any expensive high-frequency electrical circuits and there are no problems with electromagnetic compatibility. The device also allows

-3GB 2019 - 628 A3 deployment on fibers with active data transmission, as both CW and CCW interferometers can be operated in only one DWDM channel even at a minimum channel width of 50 GHz.

Explanation of drawings

The essence of the mentioned solution is further explained and described on the basis of a specific exemplary embodiment with the help of the attached drawing, which shows a block diagram of the device in its most complete embodiment. In the accompanying drawing, the solid lines indicate the optical connections and the dashed lines the electrical connections.

Examples of embodiments of the invention

The connection of the distributed optical fiber sensor system on the principle of a dual Mach-Zehnder interferometer using the frequency shift of the CCW signal is shown in the attached drawing in the form of a block diagram. The device as shown is intended for monitoring acoustic / mechanical vibrations in the immediate vicinity of an optical cable in which both the reference and sensing fibers are located. The remote unit ensures the reflection of the CW interference pattern back into the main unit and at the same time a frequency shift of the laser carrier frequency for the CCW interferometer is created here by means of an acousto-optical modulator. The technique of two interferometers makes it possible to determine the location of the vibration site due to the known speed of signal propagation through the optical fiber. Detection of the outputs of both interferometers in the main unit also ensures accurate synchronization of both signals. The device can then be used for detection, localization, or even classification of events with high accuracy.

The device consists of two interferometers working against each other and using only one source 2 of radiation. The first interferometer, CW, comprises a first coupler 11, which serves to divide and bind the signal from the laser radiation source 2 connected to the first input of the coupler 11 to the two arms 21, 22 of the interferometer. The fourth coupler 13 in the remote unit ensures recombination of signals from the reference and sensing arm fed to the first and second port and thanks to the connected reflector 17 to the third port the resulting modulated signal is reflected back to both arms 21, 22 of the interferometer for further processing in the main control unit 1. To prevent further recombination of the already modulated signal in the first coupler 11, this signal is divided in the second coupler 12 and fed to the first photodetector 3. The basic signal for the second interferometer, CCW, is obtained by dividing part of the signal from one arm of the CW interferometer. by means of a third coupler 14. The obtained signal is frequency shifted in an acousto-optical modulator 15 at a frequency given by an external oscillator 16 supplied from the power supply module 18. The new frequency shifted signal is coupled to the two arms 21, 22 of the interferometer via a fourth port 21, 22. The recombination of the resulting CCW signal of the interferometer takes place in a first coupler 11, to the second port of which a second photodetector 4 is connected, which ensures the conversion of the optical signal into an electrical signal.

The connection of the distributed optical fiber sensor system is supported by both the optical part and the electrical part. The basis is that both units contain the power supply modules of the necessary power supply for the individual optoelectric and electrical components. The first power supply module 5 supplies power to the main control unit of the laser radiation source 2 and the first photodetector 3 and the second photodetector 4, while the second power supply module serves to supply only the oscillator.

Optical signals from both interferometers are detected on photodetectors 3, 4 and subsequently converted into electrical form. In the case of a CW interferometer, the signal on the first photodetector 3 also contains a frequency-shifted signal of the interferometer CCW, which is shifted from the main frequency in the range of tens of MHz. Therefore, it is not possible to perform the filtration in the optical domain, and it is necessary to perform the filtration only after the first photodetector 3, i.e. in the electrical domain. The resulting signal is necessary

-4GB 2019 - 628 A3 filter from the frequency-shifted signal using the second low-pass filter 7 and digitize in the control unit 1.

Since the frequency of the shifted signal divided from one arm 21 of the sensor system corresponds to the frequency generated by the oscillator 16. it is possible to divide part of the signal by the electric splitter 10 and preferably use this signal as a local oscillator after demodulation by the first bandpass filter 8 in the frequency mixer 9. The signal from the CCW interferometer must also be filtered from the signal of the first interferometer by means of a third bandpass filter 6, in addition it is necessary to perform demodulation, ie shift to the baseband, which is used by the frequency mixer 9. The resulting demodulated signal must be digitized in the control unit 1 for further processing.

The control unit 1 is supplied from the power supply module 5, which can be redundant in one possible preferred embodiment. The control unit 1 is equipped with a wired communication interface module and can, as already mentioned, preferably be formed by a single-board industrial computer without moving parts.

In one preferred embodiment, the reflector 17 rotates the polarization of the reflected signal, thus minimizing the effect of polarization on the useful signal. In another preferred embodiment, the oscillator 16 is thermally stabilized, thereby reducing fluctuations in its output frequency. In yet another preferred embodiment, inexpensive photodetectors 3 and 4 are used, the bandwidth of which corresponds only to the frequency of the oscillator 16.

An embodiment is also preferred in which, by suitable selection of the bandwidth of the filters 8, 7 and 6, the slope of the resulting filtering is increased, and thus the signal-to-noise ratio (SNR) is increased so that the third electric filter 6 and the first filter 8 have the same bandwidth. , for example 20 MHz and at the same time the second electrical filter 7 has a bandwidth equal to exactly 20 MHz, the frequency of the oscillator not being less than 40 MHz.

In a preferred embodiment, the electric signal mixer 9 and the electric splitter 10 operate on the frequency of the oscillator 16 and the frequency of the vibrations acting on one or both arms 21, 22 of the interferometer, in order to be able to demodulate the signal properly.

Also advantageous is an embodiment which minimizes the demands on the laser source 2 and thus its cost, in which the difference between the length of the first arm 21 and the second arm 22 of the interferometer is as close as possible, in the extreme case equal to the coherence length of the laser source 2.

The control unit 1 contains software that measures the voltage levels obtained on the individual photodetectors 3 and 4. The software detects the occurrence of new events in the vicinity of the optical fibers as follows: if a change in power is detected on the first photodetector 3, then a frequency analysis is performed. The same applies to the second photodetector 4. In the case of detecting signals of the same frequencies at the input of both photodetectors 3, 4, a time difference between the detected signals is detected. From the known speed of the light propagating through the optical fiber and from the time difference between the detected signals from the individual photodetectors 3,4, the position of the newly generated event can then be determined. A new event can also be classified from the frequency response. Thanks to the condition of event detection by both interferometers at the same time, the minimization of false positive alarms is ensured. The software of the control unit 1 makes the power and time stamp information available via a wired communication interface to the master system.

Industrial applicability

This technical solution is industrially well usable especially for monitoring acoustic / mechanical vibrations in the vicinity of telecommunication optical cables, on fibers with active data traffic. Thanks to the use of low-power continuous radiation, there is no interaction with data traffic in the side channels. Using a remote unit

-5GB 2019 - 628 A3 is a counter-propagating signal frequency-shifted from the laser carrier signal, thereby creating a second interferometer in the same pair of optical fibers, thereby enabling event localization. Compared to known solutions, the device uses only one pair of fibers, except for a laser with a sufficiently long coherence length, it does not contain any expensive and complex components and is usable in all working wavelengths of the fibers used. The technical solution also includes remote monitoring, including monitoring of processed optical signals. The proposed solution is based on commonly available components.

Claims (5)

PATENT CLAIMS A distributed optical fiber sensor system consisting of two units - a main and a remote, wherein the main unit comprises first and second couplers (11, 12), first and second photodetectors (3, 4), laser source (2) illuminated, first power supply the module (5) and the control unit (1) and the remote unit comprises third and fourth coupling members (13, 14) and the units are connected by a first arm (21) and a second arm (22) of an interferometer, the laser radiation source (2) being connected with a first coupler (11) further connected to the input of the second photodetector (4), the second coupler (12) and the first arm (21) of the interferometer, and the second coupler (12) is connected to the input of the first photodetector (3) and the second arm (22) of the interferometer and further the first arm (21) of the interferometer is connected to the third coupling member (13) and the control unit (1) is connected to the laser radiation source (2) and the power supply module (5) is connected to the first and second photodetector (3,4), with control unit (1) and further with laser source radiation main (2), characterized in that the main unit further comprises an electric splitter (10), a first bandpass type electric filter (8), a second low pass filter (7), a third bandpass electric filter (6) and an electric frequency mixer (9) and the remote unit further comprises a fourth coupler (14), a reflector (17), an acousto-optical modulator (15), an oscillator (16) and a second power supply module (18), the output of the first photodetector (3) ) is connected to the input of the electric splitter (10), its first output is connected to the input of the second electric filter (7) and the second output of the electric splitter (10) is connected to the first input of the electric frequency mixer (9), where the output of the second photodetector (4) ) is connected to the input of the third electric filter (6) and its output is connected to the second input of the electric frequency mixer (9) and the output of the second electric filter (7) is connected to the control unit (1) and the output of the electric frequency mixer (9) e is also connected to the control unit (1) and further the fourth coupler (14) is connected to the first arm (21) of the interferometer and further to the third coupler (13) and the acousto-optical modulator (15), and the third coupler (13). ), the third coupling member (13) being further connected to the reflector (17) and the acousto-optical modulator (15) and the output of the second power supply module (18) being connected to the oscillator input (16) and the oscillator output (16) being connected to the input of the acousto-optical modulator (15). The distributed optical fiber sensor system according to claim 1, characterized in that the reflector (17) is set to rotate the polarization of the reflected light by 90 °. Distributed optical fiber sensor system according to claim 1 or 2, characterized in that the first power supply module (5) and the second power supply module (18) are redundant. Distributed optical fiber sensor system according to any one of claims 1 to 3, characterized in that the oscillator (16) is thermally stabilized. Distributed optical fiber sensor system according to any one of claims 1 to 4, characterized in that the bandwidth of the first photodetector (3) and the second photodetector (4) corresponds to the frequency generated by the oscillator (16). Distributed optical fiber sensor system according to any one of claims 1 to 5, characterized in that the first electrical filter (8) and the third electrical filter (6) have the same bandwidth of 20 MHz and at the same time the second electrical filter (7) has the same width band of 20 MHz, the frequency of the oscillator (16) not being less than 40 MHz. Distributed optical fiber sensor system according to any one of claims 1 to 6, characterized in that the electric frequency mixer (9) and the electric splitter (10) operate on the frequency of the oscillator (16) and the vibration frequency acting on one or both arms (21). 22) interferometer. -7 CZ 2019 - 628 A3 Distributed optical fiber sensor system according to any one of claims 1 to 7, characterized in that the control unit (1) is formed by a single-board industrial computer without moving parts. Distributed optical fiber sensor system according to any one of claims 1 to 8, characterized in that the difference in length of the first arm (21) and the second arm (22) of the interferometer approaches the coherence length of the laser source (2).