Multi-fiber bragg grating temperature inspection system and method based on optoelectronic oscillator

Abstract

The application discloses a multi-fiber Bragg grating temperature inspection system and method based on an optoelectronic oscillator, and belongs to the technical field of optical fiber sensing. The system comprises a broadband light source, an optical circulator, a series FBG group, an optical band-pass filter, an electric spectrum analyzer, a microwave source and an optoelectronic oscillator. The optoelectronic oscillator is formed by a Mach-Zehnder modulator, a dispersion compensation optical fiber, an erbium-doped fiber amplifier, an optical detector, an electric band-pass filter, an electric amplifier and two power dividers which are connected in sequence to form a closed loop. The light of the broadband light source enters the FBG group through the circulator, and the reflected light is selected by the optical band-pass filter to select the reflected light of different FBGs as input. The microwave source generates an injection microwave signal which is coupled to the modulator through the power divider to suppress mode hopping. The wavelength shift of the FBG caused by temperature change is converted into loop delay change through the dispersion compensation optical fiber, so that the frequency of the output microwave signal is changed, the electric spectrum analyzer monitors the frequency change to calculate the temperature of the FBG, and thus multi-point temperature polling detection is realized.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing technology, specifically relating to a multi-fiber Bragg grating temperature monitoring system and method based on an optoelectronic oscillator. Background Technology

[0002] In power systems, the safe operation of transmission lines is crucial. Abnormal increases in line temperature can trigger various safety hazards, making accurate monitoring of transmission cable temperature essential for ensuring the safe operation of the power system. Traditional power industry safety monitoring technologies typically employ electrical sensors, but these are susceptible to electromagnetic interference in practical applications. Fiber Bragg grating (FBG) sensors, with their advantages of electromagnetic interference resistance, small size, and distributed deployment, have been widely used in power systems. FBG sensors are suitable for wavelength division multiplexing (WDM), allowing a single demodulator to monitor multiple sensors, increasing capacity. Typically, FBG sensor demodulation is achieved by directly observing the resonant wavelength of the FBG using equipment such as spectrometers, optical edge filters, and unbalanced fiber interferometers. However, traditional optical demodulation schemes have inherent limitations in measurement resolution, making it difficult to achieve high-precision detection of minute temperature changes.

[0003] Optical oscillators (OEOs) possess hybrid optical and electrical resonant cavities, generating microwave signals with low phase noise. Their unique structure provides a novel method for fiber optic sensing demodulation. Currently, OEOs have been proposed for sensing and demodulating physical quantities such as temperature, voltage, strain, magnetic field, and pressure. By using the optical fiber in the OEO loop as both a temperature sensing element and a delay element, temperature changes can be directly converted into loop delay changes, and subsequently into frequency changes. If the reflected signal from an FBG sensor is used as the OEO light source, the wavelength change of the FBG, under the action of the dispersive element within the loop, can be converted into a frequency shift of the OEO output microwave signal. Furthermore, devices such as PS-FBGs can be used as sensing elements and constructed into microwave photonic filters for frequency selection, achieving high-sensitivity sensing. The temperature sensing systems based on OEO demodulation described above only detect single-point temperatures. Although some systems utilize multiple FBGs to achieve real-time and large-scale monitoring of anomalous temperatures or strains at specific locations, they cannot provide the specific temperature or strain values ​​at the anomalous points. Summary of the Invention

[0004] The purpose of this invention is to provide a multi-fiber Bragg grating (FBG) temperature monitoring system and method based on an optoelectronic oscillator (OEO). The system utilizes an OEO to demodulate multiple cascaded FBGs, converting wavelength changes into frequency changes in the output microwave signal. By automatically polling the reflected light from different FBGs using an optical bandpass filter (OBPF) as the input light for the OEO and observing the frequencies of the corresponding output microwave signals from different FBGs, temperature information at different points can be obtained. Simultaneously, a microwave signal is injected into the loop, resolving the mode-hopping phenomenon that occurs during the continuous reconstruction of the loop without affecting mode hopping. This system utilizes OEO demodulation to achieve multi-point temperature monitoring and improves the resolution and response speed of wavelength division multiplexing (WDM) FBG temperature sensing.

[0005] This invention is achieved through the following technical solution:

[0006] A multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator includes a broadband light source, an optical circulator, an FBG group composed of multiple fiber Bragg gratings with different center wavelengths connected in series, an optical bandpass filter, an electrical spectrum analyzer, a microwave source, and an optoelectronic oscillator (OEO). The optoelectronic oscillator is composed of a Mach-Zehnder modulator, a dispersion compensation fiber, an erbium-doped fiber amplifier, a photodetector, an electrical bandpass filter, an electrical amplifier, a first power divider, and a second power divider connected in sequence to form a closed oscillation loop.

[0007] The optical output terminal of the broadband light source is connected to the first port 1 of the optical circulator, the second port 2 of the optical circulator is connected to one end of the series FBG group, and the other end of the series FBG group is in a free state; the third port 3 of the optical circulator is connected to the optical input terminal of the optical bandpass filter, and the optical output terminal of the electrically controlled tunable optical bandpass filter is connected to the optical input terminal of the Mach-Zehnder modulator; one output terminal of the first power divider is connected to the electrospectral analyzer, and the other output terminal is connected to one input terminal of the second power divider; the output terminal of the microwave source is connected to the other input terminal of the second power divider; the output terminal of the second power divider is connected to the radio frequency input terminal of the Mach-Zehnder modulator.

[0008] Furthermore, the optical bandpass filter is used to receive the target center wavelength setting command sent by the host computer to change its center wavelength, and sequentially selects the reflected light of different fiber Bragg gratings in the FBG group as signal light, which serves as the input signal light of the Mach-Zehnder modulator.

[0009] Furthermore, the microwave source is used to generate an injected microwave signal, which is coupled to the radio frequency input of the Mach-Zehnder modulator via a second power divider; the injected microwave signal is used to guide and stabilize the start-up mode of the optoelectronic oscillator, effectively suppress mode jumps, and achieve continuous oscillation of a single selected mode.

[0010] Furthermore, the dispersion compensation fiber is used to introduce a wavelength-delay mapping relationship. When the temperature changes, the wavelength of the fiber Bragg grating shifts, causing a change in the total loop delay, which ultimately leads to a change in the frequency of the microwave signal output by the optoelectronic oscillator.

[0011] Furthermore, the electrospectral analyzer is used to monitor the frequency of the microwave signal output by the photoelectric oscillator, and the change in the microwave signal frequency is used to characterize the temperature change of the corresponding fiber Bragg grating.

[0012] Furthermore, the system also includes a first DC power supply, a second DC power supply, and a third DC power supply. The first DC power supply is connected to the Mach-Zehnder modulator to provide it with a DC bias voltage; the second DC power supply is connected to the photodetector to provide it with an operating voltage; and the third DC power supply is connected to the bandpass filter to provide it with an operating voltage.

[0013] Furthermore, the center wavelength of the broadband light source is 1540~1560nm, and the 3-dB bandwidth is 20~40nm, which can cover the reflection wavelength range of each fiber Bragg grating in the tandem FBG group.

[0014] The center wavelength of each fiber Bragg grating in the cascaded FBG group is 1540~1560nm, the 3-dB bandwidth is 0.1~0.3nm, and there is a wavelength interval of 1~3nm between the reflection spectra of adjacent fiber Bragg gratings, ensuring that the optical bandpass filter can selectively filter out the reflected light of each fiber Bragg grating individually.

[0015] Furthermore, the optical bandpass filter has the following parameters: center wavelength of 1510~1590nm, 3-dB bandwidth of 1~4nm, bandwidth of 20~40MHz, and operating frequency range of 2~26GHz; the dispersion compensation fiber has a length of 3~7km and a dispersion coefficient of -700~-500ps / nm@1545nm; the erbium-doped fiber amplifier has a maximum output power of 15~20dBm; and the photodetector has a bandwidth of 10~30GHz and a responsivity of 0.5~ 1.0 A / W; the operating frequency band of the electric amplifier is 50 kHz to 20 GHz, and the gain is 25 to 30 dBm; the frequency of the first power divider and the second power divider is 0 to 26.5 GHz; the frequency range of the electric spectrum analyzer is 1 to 26.5 GHz, and the resolution bandwidth is 1 Hz to 1 kHz; the frequency of the electric spectrum analyzer is 1 to 26.5 GHz; the frequency range of the microwave source is 250 kHz to 67 GHz; the bandwidth of the Mach-Zehnder modulator is 10 to 40 GHz.

[0016] On the other hand, the present invention provides a method for inspecting a multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator, comprising the following steps:

[0017] S1. The light emitted by the broadband light source is input to the first port 1 of the optical circulator, and then input to the series FBG group via the second port 2 of the optical circulator. The light reflected by multiple FBGs with different center wavelengths is output to the input optical port of the electronically controlled tunable optical bandpass filter through the third port 3 of the optical circulator.

[0018] S2. The host computer sends a target center wavelength setting command to the electronically controlled tunable optical bandpass filter, changes the center wavelength of the electronically controlled tunable optical bandpass filter, and makes the electronically controlled tunable optical bandpass filter select the reflected light of a fiber Bragg grating to be tested in the FBG group as the signal light, and use the signal light as the input signal light of the Mach-Zehnder modulator.

[0019] S3. The input signal light enters the Mach-Zehnder modulator and is modulated by the microwave signal from the second power divider. The modulated optical signal is sequentially introduced into the delay by the dispersion compensation fiber, amplified by the erbium-doped fiber amplifier, and then converted into a microwave signal by the photodetector. The microwave signal is filtered by the electric bandpass filter, amplified by the electric amplifier, and then input to the first power divider.

[0020] S4. The first power divider splits the signal into two branches. The first branch is input to the spectral analyzer for real-time spectrum monitoring and recording of the output microwave signal frequency corresponding to the current FBG. The second branch, coupled with the injected microwave signal emitted by the microwave source, is fed back to the radio frequency input terminal of the Mach-Zehnder modulator after being coupled by the second power divider, forming a closed optoelectronic oscillation loop. The injected microwave signal is used to guide and stabilize the start-up mode of the optoelectronic oscillator and suppress mode transitions.

[0021] S5. When the temperature of the environment where the FBG is located changes, the reflected wavelength of the FBG will shift. Through the wavelength-delay mapping of the dispersion compensation fiber, the total delay of the optoelectronic oscillation loop will change, which will eventually lead to a shift in the frequency of the output microwave signal. The temperature of the FBG can be calculated by measuring the change in microwave signal frequency through an electrospectral analyzer and based on the frequency-temperature fitting curve obtained in the experiment.

[0022] S6. The host computer sends the target center wavelength setting command in sequence to control the electronically controlled tunable optical bandpass filter to switch the center wavelength to the center wavelength of the reflection spectrum of the next FBG to be tested, and repeats the above steps S2 to S5; the temperature detection of each FBG in all series FBG groups is completed in sequence to realize the polling inspection of multi-point temperature.

[0023] Furthermore, when the electrically controlled tunable optical bandpass filter switches to select different fiber Bragg gratings, the frequency of the injected microwave signal output by the microwave source is adjusted synchronously to stabilize the oscillation mode of the optoelectronic oscillator loop and prevent mode hopping.

[0024] Compared with the prior art, the advantages of the present invention are as follows:

[0025] (1) By switching the center wavelength of the electrically controlled tunable optical bandpass filter, the reflected light of different fiber Bragg gratings is selected as the input light of the photoelectric oscillator. After passing through the oscillation loop, a photoelectric hybrid oscillation is formed. The output microwave signal spectrum corresponding to different fiber Bragg gratings is observed by an electric spectrum analyzer to realize the polling detection of multi-point temperature.

[0026] (2) The microwave source emits a microwave signal and injects it into the loop through the second power divider. This solves the mode jump phenomenon that occurs in the dynamic reconstruction process of wavelength switching when switching different fiber Bragg grating reflected light as the input light of the oscillation loop. It stabilizes the oscillation mode and solves the influence of mode jump problem on temperature sensing.

[0027] (3) The FBG reflection spectrum is demodulated by an opto-oscillator, and the wavelength change in the optical domain is converted into the frequency change of the microwave signal in the electrical domain. Compared with the traditional multi-point temperature sensing system based on optical demodulation, this system and method significantly improve the resolution and response speed of temperature sensing. Attached Figure Description

[0028] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0029] Figure 1 This is a schematic diagram of the structure of the multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator according to the present invention;

[0030] Among them, the first port is 1, the second port is 2, and the third port is 3;

[0031] Figure 2 The single-mode oscillation spectrum of the photoelectric oscillator after outputting a microwave signal;

[0032] Among them, (a) multimode oscillation spectrum, (b) injection-locked spectrum, and (c) injected microwave signal;

[0033] Figure 3 The spectrum diagram of the output microwave signal corresponding to different FBGs after microwave signal injection;

[0034] Among them, (a) FBG1, (b) FBG2, (c) FBG3, and (d) FBG4;

[0035] Figure 4 Output spectrum of broadband light source;

[0036] Figure 5 Four FGB reflectance spectra;

[0037] Figure 6 The output spectrum of FBG1 at different temperatures during reflection;

[0038] Figure 7 The output spectrum of FBG2 at different temperatures during reflection;

[0039] Figure 8 The output spectrum of FBG3 at different temperatures during reflection;

[0040] Figure 9 The output spectrum of FBG4 at different temperatures during reflection;

[0041] Figure 10 This is a schematic diagram showing the relationship between output frequency and temperature when FBG1 is reflected.

[0042] Figure 11 A schematic diagram showing the relationship between output frequency and temperature when FBG2 is reflected;

[0043] Figure 12 This is a schematic diagram showing the relationship between output frequency and temperature when FBG3 is reflected.

[0044] Figure 13 A schematic diagram showing the relationship between output frequency and temperature when FBG4 is reflected;

[0045] Figure 14 A schematic diagram showing the system measurements and measurement errors when FBG1 is reflected;

[0046] Figure 15 A schematic diagram showing the system measurements and measurement errors when FBG2 is reflected;

[0047] Figure 16 A schematic diagram showing the system measurements and measurement errors when FBG3 is reflected;

[0048] Figure 17 This diagram illustrates the system measurements and measurement errors when FBG4 is reflected. Detailed Implementation

[0049] To clearly and completely describe the technical solution and its specific working process of the present invention, the specific embodiments of the present invention are as follows, in conjunction with the accompanying drawings:

[0050] Example 1

[0051] like Figure 1As shown, this embodiment provides a multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator, including a broadband light source, an optical circulator, four cascaded fiber Bragg gratings (FBG1, FBG2, FBG3, FBG4), an electrically controlled tunable optical bandpass filter, a Mach-Zehnder modulator, a dispersion-compensating fiber, an erbium-doped fiber amplifier, a photodetector, an electrical bandpass filter, an electrical amplifier, a first power divider, a second power divider, an electrical spectrum analyzer, a microwave source, and three DC power supplies; wherein, the Mach-Zehnder modulator, dispersion-compensating fiber, erbium-doped fiber amplifier, photodetector, electrical bandpass filter, electrical amplifier, first power divider, and second power divider are connected in sequence to form a closed optoelectronic oscillator (OEO).

[0052] The temperature measurement principle of the photoelectric oscillator in this embodiment is as follows: In the photoelectric oscillation loop, the frequency of the oscillation output microwave signal is... With loop delay Regarding the total loop delay, it includes optical path delay. and circuit delay The electrical delay is much smaller than the optical delay dominated by long optical fibers, therefore the electrical delay can be ignored. The dispersion of dispersive fibers is much greater than that of single-mode fibers, so the optical path delay change caused by single-mode fibers can be ignored. Therefore, the total delay change of the optoelectronic oscillation loop is negligible. Dispersion coefficient of dispersive fiber FBG wavelength offset and the length of the dispersive fiber Related. As a temperature sensor, when the temperature change is... At this time, the wavelength of the FBG will undergo a wavelength shift. The wavelength shift will cause a change in the total delay of the photoelectric oscillation loop. Ultimately, this results in a change in the frequency of the microwave signal output by the photoelectric oscillator. ( Therefore, the temperature of the FBG can be calculated by measuring the frequency change of the microwave signal using an electrospectral analyzer.

[0053] In this embodiment, the broadband light source is the LSM-ASE-CF from Junfeng Technology Co., Ltd., with a center wavelength of 1545nm and a 3-dB bandwidth of 30nm, capable of covering the reflection wavelength range of the four FBGs; the four cascaded FBGs serve as sensing elements, with center wavelengths of FBG1, FBG2, FBG3, and FBG4 of 1545.8nm, 1547.9nm, 1549.9nm, and 1551.8nm, respectively; the electrically controlled tunable optical bandpass filter is the LTF-WE-U from Bofu Optoelectronics Co., Ltd., with a center wavelength of 1... The wavelength range is 510~1590nm, with a 3dB bandwidth of 2nm, capable of matching the FBG reflection spectrum and ensuring effective isolation between adjacent channels; the Mach-Zehnder modulator is AFR's AM20 with a bandwidth of 30GHz; the dispersive fiber, used as the dispersive medium in the optoelectronic oscillator loop, is YOFC's DCMS-SM-C100%-040KM-LCP-05, with a length of 5km and a dispersion of -659ps / nm@1545nm; the erbium-doped fiber amplifier is Pulse Optoelectronics' EDFA-ILA, with a maximum... The output power is 17dBm; the photodetector is OD-PxX000PF-OX00 from Tianqi Technology Co., Ltd., with a bandwidth of 20GHz and a responsivity of 0.93A / W; the electrical bandpass filter is ZLT0226J5 from the Ninth Research Institute of China Electronics Technology Group Corporation, with a bandwidth of 30MHz and an operating frequency range of 2~26GHz; the electrical amplifier is MWLA-000200G28 from Quanbo Electronics Technology Co., Ltd., with a signal gain of 28dB and an operating frequency band of 50kHz~20GHz; the first power divider and the second... The two power dividers are AV81311 from the 41st Research Institute of China Electronics Technology Group Corporation, with a frequency range of 0~26.5GHz and a power division ratio of 50:50; the microwave source is Agilent's E8257D, with a frequency range of 250kHz~67GHz; the spectrum analyzer is Agilent's N9010A, with a measurement signal bandwidth of 10Hz~26.5GHz; the first DC power supply, second DC power supply, and third DC power supply are GW Instek's GPS-2303C, with an adjustable output voltage range of 0V~30V.

[0054] The working principle of the multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator in this embodiment is as follows:

[0055] according to Figure 1 Connect the system described in this embodiment. The FBG group uses four FBGs in series: FBG1, FBG2, FBG3, and FBG4. The temperature of each of the four FBGs can be controlled independently. Turn on all instruments and equipment to ensure all devices are operational.

[0056] When switching different photoelectric bandpass filters (FBGs) as sensing channels using an electronically controlled tunable optical bandpass filter, a significant jump phenomenon occurs in the output oscillation mode corresponding to the same FBG; that is, the output oscillation mode cannot be stably repeated during wavelength switching. To solve this problem, the photoelectric oscillator is first adjusted as follows: Figure 2 The multimode oscillation state is shown in (a). In this state, the frequency of one of the oscillation modes is selected as the injection signal frequency output by the microwave source to achieve injection locking. Figure 2 Figure (b) shows the output spectrum of the injection-locked photoelectric oscillator. Due to the significant suppression of side modes, the output signal quality is noticeably improved. However, in the injection-locked state, the frequency of the oscillation mode is pulled to near the injection signal frequency, causing the output spectrum to shift within a certain temperature range, thus affecting the temperature sensing function. Therefore, the injection signal frequency is adjusted to bring the output spectrum to a position similar to... Figure 2 The single-mode oscillation state is shown in (c). In this state, because the power of the original locked mode dominates in mode contention, it remains dominant in the spectrum, and the system can still maintain single-mode oscillation. Injecting a signal ensures that the oscillation mode remains stable and does not jump modes when repeatedly switching between different FBG channels, effectively solving the mode-hopping problem.

[0057] Before conducting temperature sensing tests, the frequency of the injected signal required by the microwave source when selecting different FBGs as sensing channels is determined. In the initial state, microwave signals are injected into the photoelectric oscillator loops corresponding to the four FBGs, and the injected signal frequency is adjusted to the right of the selected oscillation mode. The output oscillation signal of the photoelectric oscillator corresponding to each FBG is recorded, such as... Figure 3 As shown in (a)-(d). In actual operation, when the electronically controlled tunable optical bandpass filter is switched to different center wavelengths to select the corresponding FBG, the frequency of the corresponding injected signal is adjusted synchronously to avoid mode hopping and ensure the accuracy of the observed signal.

[0058] The inspection method of the multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator in this embodiment includes the following:

[0059] For temperature sensing, the temperature of FBG1 is initially set to 20℃. Light emitted from a broadband light source is input to the first port 1 of the optical circulator, and the spectrum is as follows: Figure 4 As shown, light is input to four cascaded FBGs via the second port 2 of the optical circulator. The light reflected by the four FBGs is output from the third port 3 of the optical circulator, and the spectrum is as follows. Figure 5As shown, the third port 3 of the circulator is connected to the input optical port of the optical bandpass filter. The host computer sends a setting command corresponding to the center wavelength of FBG1, so that the center wavelength of the optical bandpass filter is aligned with the center wavelength of the reflection spectrum of FBG1, which is 1545.8 nm. The reflected light from FBG1 is filtered out and used as the input signal light for the Mach-Zehnder modulator, which is then modulated by the microwave signal output from the second power divider. The modulated optical signal is input into the dispersion-compensating fiber, introducing a 2.4 × 10⁻⁶ ohmmeter. -5 After a delay of s, the delayed optical signal is amplified by an erbium-doped fiber amplifier (EDBFA). The EDBFA operates in constant power output mode with an output power of 6dBm. The signal is then converted into a microwave signal by a photodetector. An electrical bandpass filter with a center frequency of 2GHz filters the signal output from the photodetector, completing the frequency selection function of the photoelectric oscillation loop. The frequency-selected microwave signal is amplified by an electrical amplifier providing a gain of 28dB. The amplified microwave signal is split into two branches by a first power divider. The first branch microwave signal is input to an electrospectral analyzer for real-time spectrum observation. The second branch microwave signal has the same frequency as the microwave source. The injected microwave signal at 2.001461355 GHz, after being coupled by the second power divider, is fed back to the RF input of the Mach-Zehnder modulator, modulating the optical signal reflected by FBG1 to form a closed photoelectric oscillation loop. Because the microwave signal injected by the microwave source can assist in frequency selection, guide and stabilize the oscillation mode of the photoelectric oscillator, effectively suppress mode transitions, and achieve continuous oscillation of a single selected mode, a stable microwave signal output can be observed on the spectral analyzer, with a frequency value of [missing value]. 2.001435349 GHz. The temperature of FBG1 was gradually increased from 20°C to 32°C in 2°C increments. The output spectrum of the photoelectric oscillator at different temperatures was monitored using an electro-spectral analyzer, yielding the following results: Figure 6 The output curves of the photoelectric oscillator at different temperatures are shown.

[0060] The host computer sequentially sends center wavelength setting commands for FBG2, FBG3, and FBG4 at 1547.9nm, 1549.9nm, and 1551.8nm, respectively. This sets the center wavelength of the electrically controlled tunable optical bandpass filter to the reflection spectrum of FBG2, FBG3, and FBG4. The reflected light from the three FBGs is sequentially filtered out and used as the input signal light for the Mach-Zehnder modulator, which is then modulated sequentially by the microwave signal output from the second power divider. The modulated optical signals are then sequentially input into dispersion-compensating optical fibers, each with a wavelength of 2.4 × 10⁻⁶. -5After a delay of s, the delayed optical signal is sequentially amplified by an erbium-doped fiber amplifier. The erbium-doped fiber amplifier operates in constant power output mode, with an output power of 6dBm. The signal is then sequentially converted into corresponding microwave signals by a photodetector. The center frequency and passband of the electrical bandpass filter are fixed at 2GHz and 30MHz, respectively, filtering the signal output from the photodetector to complete the frequency selection function of the photoelectric oscillation loop. The frequency-selected output electrical signal is sequentially amplified by an electrical amplifier, which consistently provides a gain of 28dB. The amplified microwave signal is split into two branches by a first power divider. The first branch microwave signal is sequentially input to an electrospectral analyzer for real-time spectrum observation. The second branch microwave signal has frequencies corresponding to those emitted by the microwave source. 2.001437327GHz 2.001273375GHz The 2.001255380 GHz microwave signal, after being coupled by the second power divider, is sequentially fed back to the RF input of the Mach-Zehnder modulator. This modulates the optical signals reflected by FBG2, FBG3, and FBG4 in sequence, forming a closed photoelectric oscillation loop. A stable microwave signal output can be observed on the spectral analyzer, with the frequency values ​​being... 2.001411232GHz 2.001247280GHz 2.001229662 GHz. The temperatures of FBG2, FBG3, and FBG4 were gradually increased from 20°C to 32°C in 2°C increments. The output spectra of the photoelectric oscillator at different temperatures were monitored using an electro-spectral analyzer, yielding the following results: Figure 7 , Figure 8 and Figure 9 The output curves of the photoelectric oscillator at different temperatures are shown, which complete the polling detection of temperature.

[0061] Experimental Results and Performance Analysis:

[0062] Further Figure 6 , Figure 7 , Figure 8 , Figure 9 Linear fitting was performed on the temperature-frequency offset data shown, and the temperature sensitivities of the output spectra corresponding to the four FBGs were obtained as follows: 584.22 Hz / ℃, 608.33 Hz / ℃, 567.60 Hz / ℃, and 591.01 Hz / ℃, with correlation coefficients of 0.9995, 0.9991, 0.9983, and 0.9992, respectively. Figure 10 , Figure 11 , Figure 12 , Figure 13 As shown, this indicates a very high linear correlation between frequency shift and temperature change.

[0063] Through the Figure 10 , Figure 11 , Figure 12 , Figure 13 By analyzing the fitted curve, the temperature measurement value corresponding to each FBG can be calculated. Then, the measurement error can be obtained by comparing the true value with the measured value, and the corresponding curve can be plotted as follows. Figure 14 , Figure 15 , Figure 16 , Figure 17 As shown, the temperature measurement errors corresponding to the four FBGs are controlled within ±0.18℃, ±0.23℃, ±0.28℃, and ±0.19℃, respectively, proving that the multi-fiber Bragg grating temperature monitoring system based on photoelectric oscillators in this embodiment has good temperature measurement accuracy under different sensing channels.

[0064] The resolution of the temperature monitoring system is determined by the sensitivity and the resolution of the electrospectral analyzer. By setting the electrospectral analyzer's resolution within the 1Hz-1kHz range and combining this with the average sensitivity of the four FBGs, the system's temperature resolution can be calculated to be approximately 1.7 × 10⁻⁶. -3 The temperature range of this temperature monitoring system is primarily limited by the free spectrum range of the photoelectric oscillator. Dividing the free spectrum range by the system sensitivity yields a maximum temperature variation range of 69℃. The single-turn delay of the photoelectric oscillator loop is approximately 25 microseconds, and the response time of the output signal is on the order of milliseconds.

[0065] Example 2

[0066] This embodiment provides a method for monitoring the temperature of a multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator, including the following steps:

[0067] S1. After the system is connected, turn on all the instrument switches to ensure all devices are operational. The light emitted by the broadband light source is input to the first port 1 of the optical circulator, and then input to the n FBGs in the series FBG group via the second port 2 of the optical circulator. The center wavelengths of the reflection spectra corresponding to FBG1, FBG2, ..., FBGn are respectively... , … The bandwidth of each light is B. The light reflected by the n FBGs is output from the third port 3 of the optical circulator and connected to the input optical port of the electrically controlled tunable optical bandpass filter.

[0068] S2. The host computer sends the corresponding center wavelength of FBG1 to the electronically controlled tunable optical bandpass filter. The setting command changes the center wavelength of the electronically controlled tunable optical bandpass filter to the center wavelength of the reflection spectrum of FBG1. The reflected light from FBG1 is filtered out as signal light, and the signal light is used as the input signal light for the Mach-Zehnder modulator.

[0069] S3. The input signal light enters the Mach-Zehnder modulator and is modulated by the microwave signal from the second power divider. The modulated optical signal is input into the dispersion-compensating fiber, introducing a corresponding delay. The delayed optical signal is then amplified by an erbium-doped fiber amplifier and converted into a microwave signal by a photodetector. The center frequency and passband width of the electrical bandpass filter are set to filter the signal output from the photodetector, completing the frequency selection function of the photoelectric oscillation loop. The frequency-selected output electrical signal is determined and amplified by the electrical amplifier. The amplified microwave signal is input to the first power divider.

[0070] S4. The first power divider splits the signal into two branches. The first branch is input to a spectrum analyzer for real-time spectrum monitoring and recording of the current output microwave signal frequency corresponding to the FBG. The second branch corresponds to the frequency emitted by the microwave source. The injected microwave signal is coupled through the second power divider and fed back to the radio frequency input of the Mach-Zehnder modulator, forming a closed optoelectronic oscillation loop; wherein, the injected microwave signal is used to guide and stabilize the oscillation mode of the optoelectronic oscillator and suppress mode jumps;

[0071] S5. The microwave signal from the oscillation output can be observed on the electron spectrometer, with a frequency of... When the temperature changes, the wavelength of FBG1 will shift, which will cause a change in the total loop delay, and ultimately cause a change in the frequency of the output microwave signal of the photoelectric oscillator measured by the electrospectral analyzer. By changing the temperature of FBG1, the curve of the frequency of the output microwave signal of the photoelectric oscillator as a function of temperature can be obtained.

[0072] S6. A continuous scanning command is sent from the host computer, causing the optical bandpass filter to automatically and continuously switch its center wavelength to the center wavelength of the reflection spectrum of FBG2, FBG3, ..., FBGn. The reflected light from each FBG is sequentially filtered out as the input signal light for the Mach-Zehnder modulator, and then sequentially modulated by the microwave signal output from the second power divider. The modulated optical signals are sequentially input into the dispersion compensation fiber, introducing a corresponding delay. The delayed optical signals are then sequentially amplified by the erbium-doped fiber amplifier, and finally sequentially converted into corresponding microwave signals by the photodetector. The signal output from the photodetector is filtered sequentially using an electric bandpass filter to achieve frequency selection in the photoelectric oscillation loop. The frequency-selected output electrical signal is determined and amplified sequentially by an electric amplifier. The amplified microwave signal is split into two branches by a first power divider. The microwave signal from the first branch is sequentially input to an electrospectral analyzer for real-time spectrum observation, outputting microwave signals of different frequencies in sequence. The microwave signal from the second branch is coupled with microwave signals emitted by a microwave source at frequencies f2, f3, ..., fn (for injection) after being coupled through a second power divider and then sequentially fed back to the RF input of a Mach-Zehnder modulator, forming a closed photoelectric oscillation loop. When the temperature changes, the wavelength of each FBG shifts, causing a change in the total loop delay, which ultimately leads to a change in the frequency of the photoelectric oscillator output microwave signal measured by the electrospectral analyzer. By changing the temperature of each FBG, a curve showing the frequency of the photoelectric oscillator output microwave signal as a function of temperature can be obtained, thus completing the temperature polling detection.

[0073] The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0074] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0075] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator, characterized in that, It includes a broadband light source, an optical circulator, an FBG group composed of multiple fiber Bragg gratings with different center wavelengths connected in series, an optical bandpass filter, an electrical spectrum analyzer, a microwave source, and an optoelectronic oscillator; wherein, the optoelectronic oscillator is composed of a Mach-Zehnder modulator, a dispersion-compensating fiber, an erbium-doped fiber amplifier, a photodetector, an electrical bandpass filter, an electrical amplifier, a first power divider, and a second power divider connected in sequence to form a closed oscillation loop. The optical output terminal of the broadband light source is connected to the first port of the optical circulator, the second port of the optical circulator is connected to one end of the series FBG group, and the other end of the series FBG group is in a free state; the third port of the optical circulator is connected to the optical input terminal of the optical bandpass filter, and the optical output terminal of the electrically controlled tunable optical bandpass filter is connected to the optical input terminal of the Mach-Zehnder modulator; one output terminal of the first power divider is connected to the electrospectral analyzer, and the other output terminal is connected to one input terminal of the second power divider; the output terminal of the microwave source is connected to the other input terminal of the second power divider; the output terminal of the second power divider is connected to the radio frequency input terminal of the Mach-Zehnder modulator.

2. The multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator as described in claim 1, characterized in that, The optical bandpass filter is used to receive the target center wavelength setting command sent by the host computer to change its center wavelength, and sequentially selects the reflected light of different fiber Bragg gratings in the FBG group as the signal light, which serves as the input signal light of the Mach-Zehnder modulator.

3. The multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator as described in claim 1, characterized in that, The microwave source is used to generate an injected microwave signal, which is coupled to the radio frequency input of the Mach-Zehnder modulator via a second power divider. The injected microwave signal is used to guide and stabilize the start-up mode of the optoelectronic oscillator, effectively suppress mode jumps, and achieve continuous oscillation of a single selected mode.

4. The multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator as described in claim 1, characterized in that, The dispersion compensation fiber is used to introduce a wavelength-delay mapping relationship. When the temperature changes, the wavelength of the fiber Bragg grating shifts, causing a change in the total loop delay, which ultimately leads to a change in the frequency of the microwave signal output by the optoelectronic oscillator.

5. The multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator as described in claim 1, characterized in that, The electrospectral analyzer is used to monitor the frequency of the microwave signal output by the photoelectric oscillator, and the change in the microwave signal frequency is used to characterize the temperature change of the corresponding fiber Bragg grating.

6. The multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator as described in claim 1, characterized in that, The system further includes a first DC power supply, a second DC power supply, and a third DC power supply. The first DC power supply is connected to the Mach-Zehnder modulator to provide it with a DC bias voltage; the second DC power supply is connected to the photodetector to provide it with an operating voltage; and the third DC power supply is connected to the bandpass filter to provide it with an operating voltage.

7. The multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator as described in claim 1, characterized in that, The center wavelength of the broadband light source is 1540~1560nm, and the 3-dB bandwidth is 20~40nm, which can cover the reflection wavelength range of each fiber Bragg grating in the series FBG group. The center wavelength of each fiber Bragg grating in the cascaded FBG group is 1540~1560nm, the 3-dB bandwidth is 0.1~0.3nm, and there is a wavelength interval of 1~3nm between the reflection spectra of adjacent fiber Bragg gratings, ensuring that the optical bandpass filter can selectively filter out the reflected light of each fiber Bragg grating individually.

8. The multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator as described in claim 1, characterized in that, The optical bandpass filter has the following parameters: center wavelength of 1510~1590nm, 3-dB bandwidth of 1~4nm, bandwidth of 20~40MHz, and operating frequency range of 2~26GHz; the dispersion compensation fiber has a length of 3~7km and a dispersion coefficient of -700~-500ps / nm@1545nm; the erbium-doped fiber amplifier has a maximum output power of 15~20dBm; and the photodetector has a bandwidth of 10~30GHz and a responsivity of 0.5~1. 0A / W; the operating frequency band of the electric amplifier is 50kHz~20GHz, and the gain is 25~30dBm; the frequency of the first power divider and the second power divider is 0~26.5GHz; the frequency range of the electric spectrum analyzer is 1~26.5GHz, and the resolution bandwidth is 1Hz~1kHz; the frequency of the electric spectrum analyzer is 1~26.5GHz; the frequency range of the microwave source is 250kHz~67GHz; the bandwidth of the Mach-Zehnder modulator is 10~40GHz.

9. The inspection method of the multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator as described in claim 1, characterized in that, Includes the following steps: S1. The light emitted by the broadband light source is input to the first port of the optical circulator, and then input to the series FBG group through the second port of the optical circulator. The light reflected by multiple FBGs with different center wavelengths is output from the third port of the optical circulator to the input port of the electronically controlled tunable optical bandpass filter. S2. The host computer sends a target center wavelength setting command to the electronically controlled tunable optical bandpass filter, changes the center wavelength of the electronically controlled tunable optical bandpass filter, and makes the electronically controlled tunable optical bandpass filter select the reflected light of a fiber Bragg grating to be tested in the FBG group as the signal light, and use the signal light as the input signal light of the Mach-Zehnder modulator. S3. The input signal light enters the Mach-Zehnder modulator and is modulated by the microwave signal from the second power divider; The modulated optical signal is sequentially introduced into a dispersion-compensating fiber for delay, amplified by an erbium-doped fiber amplifier, and then converted into a microwave signal by a photodetector. The microwave signal is filtered by an electrical bandpass filter and amplified by an electrical amplifier before being input to the first power divider. S4. The first power divider splits the signal into two branches. The first branch is input to the spectral analyzer for real-time spectrum monitoring and recording of the output microwave signal frequency corresponding to the current FBG. The second branch, coupled with the injected microwave signal emitted by the microwave source, is fed back to the radio frequency input terminal of the Mach-Zehnder modulator after being coupled by the second power divider, forming a closed optoelectronic oscillation loop. The injected microwave signal is used to guide and stabilize the start-up mode of the optoelectronic oscillator and suppress mode transitions. S5. When the temperature of the environment where the FBG is located changes, the reflected wavelength of the FBG will shift. Through the wavelength-delay mapping of the dispersion compensation fiber, the total delay of the optoelectronic oscillation loop will change, which will eventually lead to a shift in the frequency of the output microwave signal. The temperature of the FBG can be calculated by measuring the change in microwave signal frequency through an electrospectral analyzer and based on the frequency-temperature fitting curve obtained in the experiment. S6. The host computer sends the target center wavelength setting command in sequence to control the electronically controlled tunable optical bandpass filter to switch the center wavelength to the center wavelength of the reflection spectrum of the next FBG to be tested, and repeats the above steps S2 to S5. The temperature of each FBG in all tandem FBG groups is detected sequentially, realizing multi-point temperature polling and inspection.

10. The inspection method of the multi-fiber Bragg grating temperature monitoring system based on an optoelectronic oscillator as described in claim 9, characterized in that, When the electrically controlled tunable optical bandpass filter switches to select different fiber Bragg gratings, the frequency of the injected microwave signal output by the microwave source is adjusted synchronously to stabilize the oscillation mode of the optoelectronic oscillator loop and prevent mode hopping.

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