Temperature measurement system based on forward Brillouin scattering in multi-core optical fiber

Abstract

The invention discloses a temperature measurement system based on forward Brillouin scattering in a multi-core optical fiber, and relates to the technical field of optical fiber temperature sensors, and the system is characterized in that light emitted by a pump light source is modulated by a pump light modulation system and then enters a polarization controller, so as to increase the gain of a high-order torsion-radial mode; after the light enters the erbium-doped optical fiber amplifier through the polarization controller, one part of the light is injected into a Sgnac ring, and the other part of the light excites a high-order torsion-radial mode through a multi-core optical fiber; light emitted by the detection light source is divided into light beams transmitted in the clockwise direction and the anticlockwise direction through the coupler, and the light beams are injected into the Sagnac ring; the temperature control furnace can continuously and stably control the environment temperature of the multi-core optical fiber; the spectrum analyzer captures the frequency shift of the Brillouin scattering signal; and the signal processing unit processes the frequency shift of the Brillouin scattering signal to obtain temperature data. According to the invention, low-cost, high-precision and high-sensitivity temperature measurement can be realized.

Description

Technical Field

[0001] The present invention relates to the technical field of fiber optic temperature sensors, and particularly to a temperature measurement system based on forward Brillouin scattering in multi-core optical fibers. Background Art

[0002] Current temperature measurement technologies are widely used in fields such as industry, healthcare, and environmental monitoring. However, the temperature sensor technologies currently on the market mainly rely on thermocouples and infrared detectors, and have limitations in terms of high precision, real-time monitoring, and remote measurement.

[0003] The principle of a thermocouple for measuring temperature is the Seebeck effect. A thermocouple consists of two conductors of different materials, which are connected to each other at one end (referred to as the measurement end or hot end) and kept separate at the other end (referred to as the reference end or cold end). When the two conductors form a circuit and the two ends are at different temperatures, a temperature difference will form an electromotive force. The electromotive force in the thermocouple is proportional to the temperature difference, and the temperature difference can be deduced from the electromotive force and then compared with the reference end to obtain the temperature of the measurement end. However, the measurement accuracy of this method is relatively low, and the output electromotive force is small, usually requiring an amplification compensation circuit.

[0004] An infrared detector measures temperature based on the Stefan-Boltzmann law, that is, the total radiation power of an object (i.e., the total energy radiated per unit area) is proportional to the fourth power of its absolute temperature. Although an infrared detector can achieve non-contact measurement and fast response, the measurement range is limited by the sensitivity of the detector and is easily affected by environmental factors, thus affecting the measurement accuracy.

[0005] Fiber optic temperature sensors have been widely used in fields such as industry, healthcare, and environmental monitoring due to their advantages of anti-electromagnetic interference, high temperature resistance, corrosion resistance, high precision, and long-distance sensing, and are expected to achieve high integration, miniaturization, intelligence, and multi-parameter sensing in the future. Existing fiber optic temperature sensors mainly rely on backward Brillouin scattering. However, the Brillouin frequency shift of backward Brillouin scattering is usually on the order of 10 GHz, which requires an expensive broadband signal processing module, increasing the cost and complexity of the demodulation system. However, the forward Brillouin scattering effect in multi-core optical fibers can achieve spatial separation of the pump and the probe to enhance the signal-to-noise ratio, and its Brillouin frequency shift is usually on the order of several hundred Hz, greatly reducing the requirements for broadband detection equipment, providing a new approach for fiber optic temperature sensors.

[0006] Currently, the main temperature sensors on the market are still mainly thermocouples and infrared detectors, and the performance of fiber optic temperature sensors mainly depends on backward Brillouin scattering, whose Brillouin frequency reaches the order of 10 GHz, and there are problems such as high cost, low precision, limited sensitivity, and susceptibility to environmental factors that need to be solved. Summary of the Invention

[0007] An embodiment of the present invention provides a temperature measurement system based on forward Brillouin scattering in a multi-core optical fiber, which can achieve low-cost, high-precision, and high-sensitivity temperature measurement.

[0008] In a first aspect, the present invention provides a temperature measurement system based on forward Brillouin scattering in a multi-core optical fiber, including: a pump light source, a pump light modulation system, a polarization controller, an erbium-doped fiber amplifier, a multi-core optical fiber, a temperature control furnace, a probe light source, a Sagnac loop, and a signal processing unit;

[0009] The light emitted by the pump light source is modulated by the pump light modulation system and then enters the polarization controller to increase the gain of the high-order twist-radial mode. After passing through the polarization controller and entering the erbium-doped fiber amplifier, a part is injected into the Sgnac loop, and the other part excites the high-order twist-radial mode through the multi-core optical fiber;

[0010] The light emitted by the probe light source is divided into two beams propagating in clockwise and counterclockwise directions through a coupler and injected into the Sagnac loop;

[0011] The temperature control furnace can continuously and stably control the ambient temperature of the multi-core optical fiber;

[0012] The spectrum analyzer is used to capture the frequency shift of the Brillouin scattering signal;

[0013] The signal processing unit is used to process the frequency shift of the Brillouin scattering signal to obtain temperature data.

[0014] In some examples, the pump light modulation system includes: a semiconductor amplifier, an electro-optic modulator, and an arbitrary waveform generator;

[0015] The pump light emitted by the pump light source is modulated by the semiconductor amplifier, the electro-optic modulator, and the arbitrary waveform generator.

[0016] In some examples, a band-pass filter is added to the Sagnac loop to filter out the pump light and only retain the probe light.

[0017] In some examples, the system further includes: a photodetector and an oscilloscope;

[0018] The photodetector detects the output voltage and observes it through the spectrum analyzer or samples it through the oscilloscope.

[0019] In some examples, the signal processing unit is used to perform signal filtering and denoising processing and extract frequency shift data using fast Fourier transform.

[0020] In some instances, the temperature control furnace is used to vary the temperature between 30°C and 100°C with a step size of 5°C, and maintain the temperature for several minutes at each temperature to ensure the stability of the ambient temperature. The magnitude of the forward Brillouin frequency shift at each temperature is recorded, and the forward Brillouin frequency shift and temperature at different temperatures are linearly fitted by the signal processing unit, thereby obtaining the coupling relationship between the forward Brillouin scattering frequency shift and temperature.

[0021] In some instances, the signal processing unit is used to obtain the ambient temperature by utilizing the coupling relationship between the forward Brillouin scattering frequency shift and temperature.

[0022] Generally speaking, compared with the prior art, the above technical solution conceived by the present invention can achieve the following beneficial effects:

[0023] By adopting the form of spatially separating the pump light and the probe light in the multi-core optical fiber, the signal-to-noise ratio of the detection signal is improved, and high-precision and high-sensitivity measurements are realized. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative efforts.

[0025] Figure 1 It is a schematic diagram of the Sagnac loop temperature measurement system provided by an embodiment of the present invention;

[0026] Figure 2 It is a schematic diagram of the structural parameters of the multi-core optical fiber provided by an embodiment of the present invention;

[0027] Figure 3 It is a working principle diagram of the forward Brillouin scattering effect provided by an embodiment of the present invention. Among them, (a) marks the propagation directions of the pump light and the probe light of the forward Brillouin scattering, and (b) is the resonance direction of the acoustic mode and the process of the scattered light signal generated by the acousto-optic interaction;

[0028] Figure 4 It is a simulation principle diagram of measuring temperature by forward Brillouin scattering provided by an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative efforts belong to the protection scope of the present invention.

[0030] In the following description, specific embodiments of the present invention will be described with reference to steps and symbols executed by one or more computers, unless otherwise specified. Therefore, these steps and operations will be mentioned several times as being executed by a computer. What is meant by a computer execution herein includes the operations of a computer processing unit that represents data in a structured form by electronic signals. This operation transforms the data or maintains it at a position in the computer's memory system, which can be reconfigured or otherwise changed in a manner well known to those skilled in the art. The data structure maintained by the data is the physical location of the memory, which has specific characteristics defined by the data format. However, the principles of the present invention are described in the above text, which does not represent a limitation. Those skilled in the art will understand that the following various steps and operations can also be implemented in hardware.

[0031] The term "module" or "unit" used herein can be regarded as a software object executed on the computing system. Different components, modules, engines, and services herein can be regarded as implementation objects on the computing system. The devices and methods herein are preferably implemented in software, and of course, they can also be implemented in hardware, all within the protection scope of the present invention.

[0032] Those skilled in the art of the present technology can understand that, unless specifically stated otherwise, the singular forms "a", "an", and "the" used herein may also include the plural forms. It should be further understood that the term "including" used in the specification of the present invention means the presence of features, integers, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or their groups. It should be understood that when we say an element is "connected" or "coupled" to another element, it can be directly connected or coupled to other elements, or there may also be intermediate elements. In addition, the "connection" or "coupling" used herein may include wireless connection or wireless coupling. The phrase "and / or" used herein includes all or any unit and all combinations of one or more related listed items.

[0033] As Figure 1As shown in the figure, it is a temperature measurement system based on forward Brillouin scattering in a multi-core optical fiber provided by an embodiment of the present invention, which shows configurations such as a laser source, a modulator, a multi-core optical fiber, a detector, and a data processing unit. The system includes: a pump light source, a pump light modulation system, a polarization controller, an erbium-doped fiber amplifier, a multi-core optical fiber, a temperature control furnace, a probe light source, a Sagnac loop, and a signal processing unit;

[0034] Among them, the light emitted by the pump light source is modulated by the pump light modulation system and then enters the polarization controller to increase the gain of the higher-order twist-radial mode. After passing through the polarization controller and entering the erbium-doped fiber amplifier, a part is injected into the Sgnac loop, and the other part excites the higher-order twist-radial mode through the multi-core optical fiber;

[0035] The light emitted by the probe light source is divided into two beams propagating in the clockwise and counterclockwise directions through a coupler and injected into the Sagnac loop;

[0036] The temperature control furnace can continuously and stably control the ambient temperature of the multi-core optical fiber;

[0037] A spectrum analyzer is used to capture the frequency shift of the Brillouin scattering signal;

[0038] The signal processing unit is used to process the frequency shift of the Brillouin scattering signal to obtain temperature data.

[0039] In the embodiment of the present invention, a seven-core optical fiber structure is adopted, which includes seven independently operating optical fiber cores. The cores are divided into a central inner core and off-axis outer cores. The inner core is located at the exact center of the optical fiber cross-section, and the six outer cores are evenly distributed in a regular hexagon with the inner core as the center. The core pitch is 35 μm, the core diameter is 9 μm, and the cladding diameter is 125 μm. Each core can independently operate and generate the forward Brillouin scattering effect. The multi-core optical fiber is adopted because the off-axis distribution of its outer cores breaks the circular symmetry of the pump light transmission. Therefore, due to the existence of the electrostriction effect, the pump light transmitted in the off-axis outer cores can excite the higher-order twist-radial mode.

[0040] Forward Brillouin scattering: The forward Brillouin scattering effect is a higher-order nonlinear effect in an optical fiber. When the pump light is injected into the optical fiber to be measured, a transverse acoustic field will be excited. The excited acoustic field oscillates at the entire cross-section of the optical fiber. The probe light propagates in the same direction as the pump light and undergoes acousto-optic interaction with the acoustic field, carrying the forward scattered light signal. The solution of the characteristic equation obtained from the radial stress boundary condition at r = a on the surface of the silica cladding can obtain the vibration frequency f of the forward Brillouin scattering acoustic field nm , and the characteristic equation can be expressed as:

[0041] Det(M)=0 (1)

[0042] Among them, the components of the M matrix are:

[0043]

[0044] Where n and m are integers, representing the order and mode number of the forward Brillouin scattering acoustic field mode, respectively; a is the fiber cladding radius; vs = 3764 m / s represents the shear velocity of the acoustic wave in the optical fiber medium; vd = 6968 m / s represents the longitudinal velocity of the acoustic wave in the optical fiber medium; α = vs / vd represents the velocity ratio; and Jn (x) is the nth-order first-kind Bessel function. The solution ym = Ωn, m*a / vs is a real number, and the resonant frequency of the nth-order torsional-radial mode, i.e., the Brillouin frequency shift, can be obtained as:

[0045]

[0046] The speed of sound vs varies approximately linearly with the temperature of the material:

[0047] v s =v s0 (1+β(T-T0)) (4)

[0048] Here, β is the temperature coefficient. Therefore, the Brillouin frequency shift, i.e., the resonant frequency of each optical mode, is approximately linearly related to temperature. The Brillouin frequency shift of the scattered light signal carried by the probe light, which is spatially separated from the pump light, is linearly related to temperature changes. Therefore, by measuring the Brillouin frequency shift, information about temperature changes in the optical fiber can be obtained:

[0049] f n,m =f n,m0 +βΔT (5)

[0050] Sagnac ring interference system: The Sagnac ring is an optical device based on the interference effect of light waves with high sensitivity and good stability. The monochromatic light emitted by the light source is divided into two beams of light, which propagate in opposite directions along the loop formed by the same optical fiber, forming a Sagnac ring. When the Sagnac ring does not rotate, the optical path of the two beams of light is equal, and interference at the combiner will form a maximum or completely eliminated interference signal, so a small phase change can be obtained, that is, the information of the forward Brillouin scattered light carried by the detection light. The system of the present invention modulates the pump light into a short pulse and injects it into an off-axis outer core of the multi-core optical fiber in the same direction as the detection light to excite a high-order torsional-radial mode TR with a larger gain. 18,8 A bandpass filter is added to the Sagnac loop to filter out the pump light signal and retain only the detection light. The signal is received by a photodetector at the output end, analyzed by a spectrometer, and then post-processed.

[0051] Optical signal post - processing method: The optical signals directly obtained in the experiment usually carry a lot of interfering noise signals. Therefore, the present invention uses a differential signal processing algorithm and a low - pass filtering algorithm to process the signals so as to remove the influence of high - frequency noise. Before the experiment starts, the data without optical signals is pre - saved. After obtaining the optical signal data, the two are subtracted to obtain the signal with noise removed, and then through the low - pass filtering algorithm, the high - frequency noise can be further removed, thereby improving the sensing accuracy and sensitivity.

[0052] As Figure 2 shown, it shows the arrangement of the cores of the multi - core optical fiber in the cross - section; the system uses a seven - core optical fiber, with the central inner core located exactly in the middle, and the off - axis outer cores evenly distributed in a regular hexagon. The core diameter d0 = d1 = 9μm, the cladding diameter is 125μm, and the core pitch is 35μm.

[0053] As Figure 3 shown, it marks the propagation directions of the pump light and the probe light of the forward Brillouin scattering, the resonance direction of the acoustic mode, and the process of the scattered light signal generated by the acousto - optic interaction.

[0054] As Figure 4 shown, it shows the simulation results of the good linear relationship between the Brillouin frequency shift and the temperature change.

[0055] Specifically, the system of the present invention consists of the following main parts:

[0056] Pump light source: A laser source with a wavelength of 1553nm is used, the output power is 10mW, and the line width of the light source is very narrow, 100kHz.

[0057] Pump light modulation system: Using a semiconductor amplifier, an electro - optic modulator, and an arbitrary waveform generator to modulate the pump light into a pulsed light with a period of 1μs and a duration of 0.5ns. The semiconductor amplifier provides a high modulation extinction ratio that cannot be achieved in the electro - optic modulator, but it cannot support short pulses. First, a pulse with a duration of 5ns and a period of 1μs is generated in the semiconductor amplifier, and then it is further modulated by the electro - optic modulator into a pulse with a duration of 0.5ns and a period of 1μs.

[0058] Polarization controller: Repeatedly adjust the polarization controller so that the modulation depth of the probe light by a specific acoustic mode reaches the maximum, that is, increase the gain of the high - order torsional - radial mode TR18,8.

[0059] Erbium - doped fiber amplifier: Amplify the power of the pump light modulated by the modulation system to a level between 20mW - 60mW and inject it into the Sagnac loop.

[0060] Multi - core optical fiber: A 30 - m - long commercial seven - core optical fiber is used, and its cross - section is shown in Figure 2, The pump light is injected into an off-axis outer core of a seven-core optical fiber to excite a high-order twist-radial mode.

[0061] Detection light source: A laser source with a wavelength of 1550 nm and a line width of 10 kHz is used. The light beam is divided into two beams transmitted in the clockwise and counterclockwise directions through a coupler and injected into the Sagnac loop. The light transmitted in the clockwise direction is transmitted in the same direction as the pump light. Since the phase matching condition for forward Brillouin scattering is satisfied, the inter-core phase modulation of the detection light transmitted in the clockwise direction caused by forward Brillouin scattering accumulates over the entire fiber length. The detection of the non-linear phase perturbation transmitted in the counterclockwise direction can be ignored.

[0062] Temperature control furnace: It can continuously and stably control the ambient temperature of the optical fiber to be measured.

[0063] Signal processing unit: The function of the band-pass filter in the Sagnac loop is to filter out the pump light and only retain the detection light. The output voltage detected by the photodetector is observed through a radio frequency spectrum analyzer or sampled through a real-time digital oscilloscope with a 6 GHz bandwidth. The traces recorded by the oscilloscope are averaged over 1024 to 4096 repetitions. After the data acquisition is completed, the signal is first filtered and denoised, and the frequency shift data is extracted using the fast Fourier transform (FFT) technology.

[0064] In the embodiment of the present invention, the coupling relationship between the forward Brillouin frequency shift and temperature is obtained in the following manner:

[0065] The temperature is changed between 30 °C and 100 °C using a temperature control furnace, with a change step of 5 °C. It is maintained for ten minutes at each temperature to ensure the stability of the ambient temperature, and the magnitude of the forward Brillouin frequency shift at each temperature is recorded. Then, the forward Brillouin frequency shift and temperature at different temperatures are linearly fitted to obtain the coupling relationship between the forward Brillouin scattering frequency shift and temperature, that is, the coupling relationship between the forward Brillouin frequency shift and temperature change is realized.

[0066] In the embodiment of the present invention, the temperature magnitude is decoupled using the coupling relationship in the following manner:

[0067] Using the temperature sensitivity and temperature coupling relationship of the forward Brillouin scattering of the outer core of the seven-core optical fiber obtained above, the ambient temperature is detected. The ambient temperature is changed by the temperature control furnace in the range of 30 °C - 100 °C, with a change step of 5 °C, and the forward Brillouin scattering frequency shift at different temperatures is monitored and recorded. Then, the ambient temperature is decoupled by substituting it into the temperature coupling relationship, and this is repeated ten times for different temperatures.

[0068] In the embodiment of the present invention, the error analysis is carried out in the following manner:

[0069] The temperature using forward Brillouin scattering frequency shift decoupling is compared with the actual temperature of the temperature control furnace to obtain the measurement error, so as to evaluate the temperature measurement ability of the present invention.

[0070] The above has introduced in detail a temperature measurement system based on forward Brillouin scattering in multi-core optical fiber provided by the embodiments of the present invention. Specific examples are used in this article to elaborate on the principle and implementation manner of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those skilled in the art, according to the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A temperature measurement system based on forward Brillouin scattering in a multi-core optical fiber, characterized in that Including: A pump light source, a pump light modulation system, a polarization controller, an erbium-doped fiber amplifier, a multi-core fiber, a temperature control furnace, a detection light source, a Sagnac loop, and a signal processing unit; The light emitted by the pump light source is modulated by the pump light modulation system and then enters the polarization controller to increase the gain of the high-order twist-radial mode. After passing through the polarization controller and entering the erbium-doped fiber amplifier, a part is injected into the Sgnac loop, and the other part excites the high-order twist-radial mode through the multi-core fiber; The light emitted by the detection light source is divided into two beams propagating in the clockwise and counterclockwise directions by a coupler and injected into the Sagnac loop; The temperature control furnace can continuously and stably control the ambient temperature of the multi-core fiber; The spectrum analyzer is used to capture the frequency shift of the Brillouin scattering signal; The signal processing unit is used to process the frequency shift of the Brillouin scattering signal to obtain temperature data.

2. The system according to claim 1, wherein The pump light modulation system includes: a semiconductor amplifier, an electro-optic modulator, and an arbitrary waveform generator; The pump light emitted by the pump light source is modulated by the semiconductor amplifier, the electro-optic modulator, and the arbitrary waveform generator.

3. The system according to claim 1 or 2, characterized in that, A band-pass filter is added to the Sagnac loop to filter out the pump light and only retain the detection light.

4. The system according to claim 3, characterized in that, The system further includes: a photodetector and an oscilloscope; The photodetector detects the output voltage and observes it through the spectrum analyzer or samples it through the oscilloscope.

5. The system according to claim 4, characterized in that, The signal processing unit is used to perform signal filtering and denoising processing and extract frequency shift data using fast Fourier transform.

6. The system according to claim 5, wherein The temperature control furnace is used to change the temperature between 30°C and 100°C with a step size of 5°C, and maintain it for several minutes at each temperature to ensure the stability of the ambient temperature. Record the magnitude of the forward Brillouin frequency shift at each temperature, and linearly fit the forward Brillouin frequency shift and temperature at different temperatures through the signal processing unit to obtain the coupling relationship between the forward Brillouin scattering frequency shift and temperature.

7. The system according to claim 6, wherein The signal processing unit is used to obtain the ambient temperature using the coupling relationship between the forward Brillouin scattering frequency shift and temperature.

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