RU2783171C1 - Method and apparatus for polling sensor elements of fibre bragg gratings through the end of the fibre using an annular speckle pattern - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: invention relates to measuring equipment, to fibre-optic means of measuring strain, temperature, pressure, and other physical quantities. According to the method, a fibre Bragg grating (FBG) is created in an optical fibre; light from a monochromatic source is supplied to the end of the optical fibre and, following the optical fibre, reflected from the FBG with a diffraction maximum at an angle θ to the axis of the fibre, and at the exit from the fibre, the light beam is shaped as a cone, the angular distribution whereof is further formed on an annular CCD matrix and transmitted to a computing processor wherein the image of the ring is analysed, the size thereof and the temperature associated with the size of the ring is calculated. Apparatus comprises the main components for the implementation of the claimed method, an emission source, a focusing system, an optical fibre with an FBG, and a computing processor.

EFFECT: facilitated operation of the method and creation of a compact circuit of the apparatus with the maintained speed of polling of the system and increased accuracy of measurement.

16 cl, 7 dwg

Description

SUBSTANCE: group of inventions relates to measuring (sensor) technology, in particular to fiber-optic means for measuring deformation, temperature, pressure and other physical quantities.

The invention can find application in such areas as: construction of buildings, structures, bridges and tunnels; in the energy industry - nuclear power plants, thermal power plants, hydroelectric power stations, transformer substations, etc.; in the mining industry during the exploration, construction and operation of wells, mines, galleries; in the oil and gas transport industry during the operation of pipelines, storage facilities, hubs; in the field of helicopter and aircraft construction when testing the strength characteristics of the blades and blades of engines, body elements, as well as the temperature regimes of engines; in the field of space technology when testing engine operating modes; in other areas in the presence of large strength, vibration, radiation and temperature loads.

The measurement of deformations, temperatures, pressures is of great practical importance in various industries, such as food inspection, pharmaceuticals, oil / gas exploration, environmental protection, high voltage power systems, chemical production and oceanographic research. Fiber optic sensors (e.g. strain gauges, temperature gauges, flowmeters, anemometers) have proven attractive as alternatives to their traditional mechanical or electromagnetic counterparts due to their many unique advantages such as small size and weight, resistance to electromagnetic interference, remote sensing capability, stability in radiation environments and opportunities for distributed measurements.

There are many options for manufacturing sensors in optical fibers using various physical principles.

A known method and device for determining deformation and temperature, due to the frequency shift of Rayleigh scattering and Mandelstam-Brillouin scattering using a fiber-optic distributed strain and temperature sensor (EP 2362190, IPC G01B 11/16, G01D 5/353, publ. 2011-08-31 ). This method is proposed to be used, for example, to register leaks in underground gas pipelines.

There is a known method for interrogating a fiber-optic sensor based on a Fabry-Perot resonator (US 10520355, IPC G01F 1/688, G01H 9/00, publ. 2019-12-31), which allows fixing the deformation, temperature or pressure in the medium.

According to the patent (RU 2319988, IPC G01D 5/353, G02B 6/00, G02B 6/34, publ. 2008-03-20), a fiber optic temperature and strain sensor on Bragg gratings located in an optical fiber is known, containing a source distributed over optical fibers fiber Bragg gratings (hereinafter referred to as FBG), a beam splitter and a spectrum analyzer.

Described in (H. Ding, D. Grobnic, C. Hnatovsky, P. Lu, R.B. Walker and S.J. Mihailov, Sapphire fiber Bragg grating coupled with graded-index fiber lens // Photonics North (PN), Quebec City , QC, Canada. - 2019 -. pp.1-1, doi: 10.1109/PN.2019.8819550.) fiber optic temperature sensors based on Bragg gratings in crystalline fibers can be used at temperatures exceeding those of traditional quartz sensors.

Systems that allow recording changes in deformation, temperature, displacement, pressure, etc. are also distinguished by a significant variety of design schemes. Systems that include a source, an optical spectrum analyzer, and a processor that calculates resonant wavelengths, as well as temperatures, strains, pressures, and other physical quantities, are referred to in the literature as interrogators or spectrum analyzers.

In (Todd M.D., Johnson G.A., Althouse B.L. A novel Bragg grating sensor interrogation system utilizing a scanning filter, a Mach-Zehnder interferometer and a 3×3 coupler // Measurement science and technology. - 2001. - V. 12. - No. 7. - P. 771.) describes a system with a Mach-Zehnder interferometer, which detects by interference the shift of the resonant wavelength of the Bragg grating.

According to the patent (RU 192705 “Multichannel signal analyzer of fiber optic sensors based on fiber Bragg gratings”, IPC G01M 11/00, publ. one or more tunable radiation sources for scanning fiber Bragg gratings controlled by the processor with subsequent reflection analysis.

All of the above systems and methods of their operation, which allow the use of fiber-optic sensors for temperature, strain, pressure, etc., according to the type of radiation source, can be conditionally divided into tunable systems and systems using sources with a wide radiation spectrum. In scanning methods, one or more sources with a narrow spectral generation width and a tunable wavelength “scan” a fiber Bragg grating along the wavelength, having a maximum reflection coefficient at the resonant wavelength λ _Br related to the grating period d by the relation:

where k is the diffraction order,. Other methods use one or more sources with a wide spectrum of radiation, in which the reflection is determined by a wavelength equal to or a multiple of λ _Br . The principle of operation of such a system is illustrated in Fig. 1, where one or more sensor elements 1 based on Bragg gratings, recorded in one or more optical fibers 2, reflects or transmits a light signal 3 with a wide spectrum coming from a source 4 at a resonant wavelength λ _Br , which depends on deformation or temperature sensor. Further, the reflected signal 5 through the fiber through the fiber optic splitter 6, or the transmitted signal 7 directly enters the optical spectrum analyzer 8, which monitors the position of the spectral peak in the reflection spectrum or a dip in the transmission spectrum and allows you to determine the deformation or temperature of the sensor

A common disadvantage of these systems and methods of their operation is the complexity of execution in terms of switching the various devices included in it, the overall bulkiness and complexity of the design. In addition, in order to provide a certain wide range of measurements of temperature, pressure, etc., either a wide spectral range of the radiation source or overlapping radiation ranges of several sources is required, or one or more tunable lasers are needed to scan the Bragg grating with these sources and find the resonant wavelength. The source must be of sufficient brightness to ensure the sensitivity of the method for measuring temperature, pressure, strain, etc. An optical spectrum analyzer must have a certain operating spectral range.

The closest in technical essence to the claimed invention is a method for interrogating the sensor elements of the FBG (patent RU 192705 "Multichannel signal analyzer of fiber-optic sensors based on fiber Bragg gratings", IPC G01M 11/00, publ. 26.09.2019), according to which the means of transmission of light ensure the direction of light from the radiation source to the FBG sensors, and the radiation reflected from the FBG sensors to the interferometer, connected through photodetectors to the processor, which controls the source rearrangement processes by switching optical switches as part of the light transmission means and the detector and processes signals from photodetectors, comparing them with calibration data, gives the value of the current reflection wavelength of each of the FBG sensors in the line (s). A tunable laser source (several laser sources) is used as a source.

The device selected as a prototype (patent RU192705 "Multichannel signal analyzer of fiber optic sensors based on fiber Bragg gratings", IPC G01M 11/00, publ. 09/26/2019), contains a radiation source (tunable laser), a splitter, a system focusing, optical fibers in which fiber Bragg gratings are placed, and a computer processor.

The main disadvantages of the above method and device include the following: the complexity of switching a large number of optical components and elements, a limited wavelength measurement range determined by the operating range of the tunable laser (lasers) and a low system polling rate associated with the limited scanning speed of the tunable laser operating range spectrum.

The problem solved by the present invention is to create a simple compact device for reading the signal reflected from the FBG, as well as a simple way to operate this device while maintaining measurement accuracy and increasing speed compared to the prototype.

The technical result of the invention is aimed at simplifying the operation of the method and creating a compact circuit of the device while maintaining the system polling speed and increasing the measurement accuracy.

The set task and the technical result are achieved by the fact that the method of interrogating the sensor elements of fiber Bragg gratings through the end of the fiber using an annular speckle pattern is characterized by the fact that at least one fiber Bragg grating (FBG) is created in the optical fiber, light, at least At least one monochromatic radiation source is fed to the end of the optical fiber through a splitter, which then, following the optical fiber, is reflected from the FBG with a diffraction maximum at an angle θ to the fiber axis, and at the output of the fiber the light beam has the shape of a cone, the angular distribution which is further formed on a CCD matrix in the form of a ring and transmitted to a computing processor, in which the image of the ring is analyzed, its size is calculated, and the temperature associated with the size of the ring is calculated.

The set task and the technical result are also achieved in the device for implementing the claimed method, which contains a radiation source connected via optical communication through a splitter with a focusing system, an optical fiber with an FBG and a computer processor. At least one monochromatic radiation source or a broadband radiation source with light filters is used as a radiation source, and a CCD array is installed between the focusing system and the computer processor.

In addition, the task and the technical result is achieved in the interrogation method by simultaneously using several rings formed by the reflection of one laser source from several FBGs with different periods.

In addition, in the interrogation method, several rings are formed simultaneously in one sensor point, as a result of the reflection of several laser sources with different wavelengths from one or more FBGs with different periods. FBGs may differ in period and may be located at the same or different measurement points. When placed in one measuring point, the accuracy of measurements increases. When several FBGs are placed at a distance from each other, it becomes possible to measure temperature/tension and other values distributed. The use of several laser radiation sources at different wavelengths to use the reflected ring signal from one or more FBGs is necessary to improve the accuracy of measuring temperature/tension and other physical quantities and to expand the dynamic range of such a measurement.

In addition, the interrogation method can use a chirped FBG with a variable period to obtain a thick ring in order to improve the measurement accuracy. If there are several close periods in the FBG, the range of diffraction angles expands, which leads to an increased ring width, which also improves the measurement accuracy.

In the interrogation method, a ring image can be formed in transmitted light, and not in reflected light, and in the device for implementing this method, the CCD array is installed at the end of the fiber opposite from the radiation input.

Also, in the interrogation method, the analyzed image in the light passing through the optical fiber has a dark ring cut out from the speckle pattern, while in the device for implementing this method, the CCD array is installed at the end of the fiber opposite from the radiation input.

In addition, in the interrogation method, the light from the radiation source may be applied at an angle to the fiber axis. Changing the angle of input of radiation into the fiber is necessary to control the optical path difference during diffraction from FBG elements and control the angle of this diffraction, i.e. the size of the ring on the CCD matrix. This option will expand the dynamic range of temperature/tension measurements, etc.

In addition, the interrogation method can use several FBGs at one measuring point with different periods, providing adjacent measurement ranges of temperature, pressure, tension and other physical quantities in a wide range due to the overlapping of measurement ranges by each ring. For each ring there is a limit temperature/tension, etc., determined by the limit numerical aperture of the fiber. At the moment the ring goes beyond the numerical aperture, the ring providing measurement in the next temperature range should already appear in the numerical aperture, which provides an expansion of the dynamic range of temperature measurements.

In addition, the interrogation method can use a monochromatic light source with different linear polarizations for reflection from different FBGs, it is possible to encode reflection from them with different linear polarizations while maintaining the polarization state in the fiber, which makes it possible to increase the number of measurement points in one optical fiber.

In addition, the interrogation method can use a monochromatic light source with circular polarization, which will make it possible to achieve greater stability of the ring structure, and, consequently, improve the accuracy of temperature measurement.

In particular, the device uses a modal mixer in the area between the source and the FBG. In particular, a mode filter is installed in the device in the area between the FBG and the CCD array. To vary the mode composition of the radiation, it is possible to use a mode mixer and a mode filter. A modal mixer can be used in the region between the source and the FBG grating to enrich the modal spectrum and increase its homogeneity. A modal filter can be used after reflection from the FBG to improve accuracy by reducing the ring thickness in the form of a depleted mode spectrum.

In particular, in the device, in the area between the FBG and the CCD array, a system is installed that causes mechanical vibrations of a part of the optical fiber. To combat ring speckling, a mechanical speckle homogenizer can be used, which is a device that causes slight mechanical vibrations of any part of the optical fiber on the way from the FBG to the CCD array with a frequency exceeding the frame rate of the CCD array, which also affects the measurement accuracy.

In addition, the interrogation method can create FBGs in a planar waveguide, which will simplify image analysis and improve system performance.

The essence of the invention is illustrated by the following graphics: Fig. 2 is a block diagram of the device, FIG. 3 is a block diagram of the operation of the method, FIG. 4a - image of the ring at a temperature of 20°C, fig. 4b - image of the ring at a temperature of 100°C, fig. 5 is a block diagram of a device in which a CCD camera is installed at the end of the fiber opposite from the radiation input, FIG. 6a - image of the ring on the CCD matrix, fig. 6b - image of a ring on a CCD matrix using a device in the circuit that causes mechanical vibrations of the fiber, fig. 7a is an image of a ring formed by reflection from a chirped grating, FIG. 7b is an image of a ring formed by reflection from a grating with a constant period.

The task is achieved by the device for implementing the method shown in Fig. 2. The device contains, for example, one laser source of optical monochromatic radiation 9, a dividing element, for example, a translucent mirror 10, an optical fiber 11 with one or more FBGs 12, a focusing system 13, a CCD array 14, a processor 15.

The technical essence of the method is illustrated in Fig. 3. In the present invention, it is proposed to use a monochromatic laser radiation source 9 or a broadband radiation source - a laser diode with a constant wavelength λ ₀ and an appropriate light filter as a source. The radiation of this source through a translucent mirror 10 is fed to the input of optical fiber 11 (shown in Fig. 3 by a dotted line). When light from the source radiation hits the FBG 12, there is a reflection back with a diffraction maximum at an angle θ to the fiber axis, the value of which is related to the FBG period d and the source wavelength λ ₀ by formula (2)

where k is the order of diffraction and n is the refractive index of the medium.

Thus, the reflected signal will propagate predominantly at an angle θ to the fiber axis, exciting the meridional and sagittal modes of the fiber, and, at the exit from the fiber, will propagate at an angle θ' upon refraction.

The image of the angular distribution of light in the beam coming out of the end of the fiber on the matrix of the CCD camera 14 will have the shape of a ring, due to the coaxial symmetry of the problem.

The dependence of the ring radius R on the grating period d, fiber refractive index n, wavelength λ ₀ and diffraction order k is expressed by formula (3):

When the period d changes, due to a change in temperature, deformation or pressure, the angle θ will also change and, consequently, the radius of the ring on the CCD camera. By calculating the radius of the ring, the value of the temperature or deformation of the sensor element (SBR) is determined.

The refractive index n and period d depend on temperature almost linearly, according to formulas (4) and (5)

where α is the coefficient of thermal expansion of the material and β is the thermo-optical coefficient of the material (for quartz fibers α=5.5*10 ^-7 K ^-1 , β=10*10 ^-6 K ^-1 ), n ₀ and d ₀ are calibration values refractive index and lattice period at a known temperature. Thus, combining formulas (3), (4) and (5), we obtain a relationship between the radius of the ring and the change in temperature ΔT, for example, expressed by relation (6):

where R is the ring radius, ΔT are temperature changes, n ₀ and d ₀ are the calibration values of the refractive index and grating period at a known temperature, λ ₀ is the wavelength, k is the order of diffraction, α is the thermal expansion coefficient of the material, β is the thermo-optical coefficient material.

The presented scheme of the device is compact due to the use of such simple components as a CCD array and a laser radiation source. In contrast to the prototype device, which uses a tunable laser and a complexly connected system of optical dividers and photodiodes. In addition, by reducing the number of elements and simplifying them, the cost of the device is significantly reduced.

The proposed method for interrogating the system has a speed limited only by the speed of the CCD array, while the speed of the prototype is determined by the speed of scanning the spectral window by a tunable laser.

Example 1. At wavelength λ ₀ =532 nm, grating period d=1098 nm, diffraction order k=6 and refractive index of quartz optical fiber n=1.4607 at room temperature t=20°C, the ring has a radius R≈0.208 ( Fig. 4a) and at R≈0.22 at a temperature of t=90°C (Fig. 4b).

Example 2. For a chirped grating, in which the period changes smoothly from 1097 to 1099 nm (Fig. 7-a), in comparison with a non-chirped grating with a period of 1098 nm (Fig. 7-b), the image of the ring is wider, which allows using more information when image analysis and allows you to improve the accuracy of measurements.

Example 3. When the device is made with a generator of mechanical vibrations, the speckling decreases, the contrast in the speckles decreases, the speckles are destroyed and “blurred”. When analyzing the image, this leads to an increase in the accuracy of determining the radius of the ring and, consequently, the temperature (Figs. 6a, 6b). On (Fig. 6b) shows the image of the ring, obtained in the device with the generator of mechanical vibrations, and compared with a conventional ring (Fig. 6a). The use of this approach in calculating the temperature made it possible to reduce the measurement error from 3 to 1 degree.

Example 4. It is possible to form a dark ring image in transmitted light, as well as a light ring at diffraction angles close to 180 degrees. With this embodiment of the method, the splitting device is excluded from the device, and the CCD camera is placed at the end of the fiber opposite from the radiation input - Fig. 5, which also leads to a simplification of the device design while maintaining the measurement accuracy and system speed.

Thus, the problem of creating a simple compact device for reading a signal reflected from an FBG has been solved, as well as a simple way to operate this device while maintaining accuracy and speed in comparison with the prototype, while reducing the cost of the device.

The technical result of the invention is aimed at simplifying the operation of the method and creating a compact circuit of the device while maintaining the system polling speed and increasing the measurement accuracy.

Claims (21)

1. A method for interrogating the sensor elements of fiber Bragg gratings through the fiber end using an annular speckle pattern, characterized in that - at least one fiber Bragg grating (FBG) is created in the optical fiber, - light from at least one monochromatic radiation source is fed to the end of the optical fiber through a splitter, which then diffracts from the FBG at an angle θ to the fiber axis and at the output of the fiber the light beam has the shape of a cone, - which is further focused on a CCD matrix in the form of a ring and transmitted to a computing processor, - in which the image of the ring is analyzed and its size is calculated, - calculate the temperature based on the size of the ring. 2. The interrogation method according to claim 1, characterized in that several rings are simultaneously used, formed by the reflection of one laser source from several FBGs with different periods. 3. The interrogation method according to claim 1, characterized in that several rings are formed simultaneously in one sensor point, as a result of the reflection of several laser sources with different wavelengths of radiation from one or more FBGs with different periods. 4. The polling method according to claim 1, characterized in that a chirped FBG is used. 5. The interrogation method according to claim 1, characterized in that the annular image after diffraction is formed in transmitted light. 6. The interrogation method according to claim 1, characterized in that the analyzed image, formed after diffraction in transmitted light, has a dark ring. 7. The interrogation method according to claim 1, characterized in that the source radiation direction makes an angle with the fiber axis. 8. The polling method according to claim 1, characterized in that several FBGs are used at one measuring point with a different period. 9. The interrogation method according to claim 1, characterized in that a monochromatic light source with different linear polarization is used to reflect from different FBGs. 10. The interrogation method according to claim 1, characterized in that a monochromatic light source with circular polarization is used. 11. The interrogation method according to claim 1, characterized in that the FBG is used in a planar waveguide. 12. A device for implementing the method according to claim 1, containing a radiation source connected by optical communication through a splitter with a focusing system, an optical fiber with an FBG and a computer processor, characterized in that at least one monochromatic a radiation source or a broadband radiation source with light filters, and a CCD camera is installed between the focusing system and the computing processor. 13. The device according to claim 12, characterized in that the CCD camera is installed at the end of the fiber opposite from the radiation input. 14. The device according to claim 12, characterized in that a modal mixer is used in the area between the source and the FBG. 15. The device according to claim 12, characterized in that a mode filter is installed in the area between the FBG and the CCD camera. 16. The device according to claim 12, characterized in that a system is installed in the area between the FBG and the CCD camera, causing mechanical vibrations of a part of the optical fiber.

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