CZ34280U1 - Distributed optical fibre sensor system - Google Patents

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, increasing system range, optimization in data processing, etc. The advantage of using scattering phenomena, whether Rayleigh or Brillouin scattering, the phenomenon of relatively easy event localization 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 is created at the corresponding point

- 1 CZ 34280 U1 change 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:

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

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

- another solution allows the detection of voltage and temperature along the optical fiber at a distance of 30 km using Brillouin's roptyl;

- another solution allows the detection of voltage along the optical fiber at a distance of 20 km using Rayleigh scattering;

- another solution allows the detection of stress along the optical fiber at a distance of up to 50 km using 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 B 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 Sagnacova

-2 CZ 34280 U1 interferometer is the sensitivity to changes such as acceleration or rotation, where the movement of the Earth can affect the resulting measurement. 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 technical solution

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 solved 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 this technical solution. 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 B, a source of highly coherent radiation is used. Like US 8923663 B2, the proposed solution uses an acousto-optical 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 B, 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

-3 GB 34280 U1 electrical circuits and there are no problems with electromagnetic compatibility. The device also allows deployment on fibers with active data transmission, as both CW and CCW interferometers can be operated in only one DWDM channel, even with 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 technical solution

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 place of vibration 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 JT, to the two arms of the interferometer. The fourth coupler 13 in the remote unit ensures the recombination of signals from the reference and sensing arm fed to the first and second ports and thanks to the connected reflector 17 to the third port the resulting modulated signal is reflected back to both arms of the interferometer for further processing in the main control unit 1. to further recombine 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 the third coupler. 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 of the interferometer via the fourth port of the fourth coupler 13. 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 consists of both an optical part and an 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 1, 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 the first photodetector 3 and the second photodetector 4 and are 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, ie in the electrical

-4 CZ 34280 U1 domain. The resulting signal must be filtered from the frequency-shifted signal by means of a second low-pass filter 7 and digitized in the control unit L. Since the frequency of the shifted signal divided from one arm of the sensor system corresponds to the frequency generated by oscillator 16, it is possible to divide part of the signal by electric splitter 10. and preferably this signal after filtering by the first bandpass filter 8 is used as a local oscillator for demodulation of the resulting CCW signal in the frequency mixer 9. The signal from the CCW interferometer must also be filtered from the first interferometer signal by the third bandpass filter 6. demodulation, i.e. a shift to the baseband, for which the frequency mixer 9 is used. The resulting demodulated signal must be digitized in the control unit 1 for subsequent 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 are used, the bandwidth of which corresponds only to the frequency of the oscillator 16.

An embodiment is also preferred in which a suitable selection of the filter bandwidth increases the steepness of the resulting filtering and thus increases the signal-to-noise ratio (SNR) such that the third electric filter 6 and the first filter 8 have the same bandwidth, e.g. 20 MHz and at the same time, the second electric filter 7 has a bandwidth of 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 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 and the second arm 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. The software detects the occurrence of new events in the vicinity of the optical fibers in the following way: if a change in power is detected on the first photodetector 3, then a frequency analysis of the signal is performed. The same applies to the second photodetector 4. If signals of the same frequencies are detected at the input of both photodetectors, 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, the position of the newly generated event can 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 j. Makes the information on outputs and time stamps available via the wired communication interface to the superior 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

-5 CZ 34280 U1 a counter-propagating signal is generated which is frequency-shifted from the laser carrier signal, whereby a second interferometer is formed in the same pair of optical fibers, thus enabling the localization of events. Compared to known solutions, the device uses only one pair of fibers, apart from a laser with a sufficiently long coherence length, it does not contain any expensive and complex components and can be used 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 (8)

CLAIMS FOR PROTECTION A distributed optical fiber sensor system consisting of two units - a main and a remote, wherein the main unit comprises a first coupler (11) and a second coupler (12), a first photodetector (3) and a second photodetector (4), a laser source (2). ) radiation, the first power supply module (5) and the control unit (1) and the remote unit comprising a third coupler (13) and a fourth coupler (14) and the units are connected by a first arm and a second arm of an interferometer, the laser radiation source (2) is connected to the first coupler (11), which is further connected to the input of the second photodetector (4), the second coupler (12) and the first arm of the interferometer, and the second coupler (12) is connected to the input of the first photodetector (3) and the second the interferometer arm and further the first interferometer arm 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 photodetector (3) and the second photodetector (4). ), with the unit (1) and further with a laser radiation source (2), characterized in that the main unit further comprises an electric splitter (10), a first bandpass type electric filter (8), a second low pass electric filter (7), a third an electric bandpass 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). ), wherein 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). ), wherein 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 output ele The frequency mixer (9) is also connected to the control unit (1) and the fourth coupler (14) is connected to the first arm of the interferometer and further to the third coupler (13) and the acousto-optical modulator (15), the third coupler the member (13) is further connected to the reflector (17) and to the acousto-optical modulator (15) and the output of the second power supply module (18) is connected to the input of the oscillator (16) and the output of the oscillator (16) is connected to the input of the acousto-optical modulator (15). -6 CZ 34280 U1 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) being greater than 40 MHz. Distributed optical fiber sensor system according to any one of claims 1 to 6, 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 7, characterized in that the difference in length of the first arm of the interferometer and the second arm of the interferometer is close to the coherence length of the laser source (2).