A multi-point series temperature measurement system based on fiber optic beat frequency sensing technology

Abstract

This invention discloses a multi-point series temperature measurement system based on fiber optic beat frequency sensing technology, belonging to the field of information transmission technology. The system includes: a pump source laser, a wavelength division multiplexer, a resonant cavity module, a photodetector, and a spectrum analyzer. This invention connects DBR laser resonant cavities in series and controls the center wavelengths of the fiber gratings at both ends of the resonant cavities to be different, enabling multiple DBR laser resonant cavities to operate simultaneously without increasing the laser output power. Compared to existing parallel schemes, this effectively reduces the power consumption of the temperature measurement system. Using fiber gratings instead of erbium-doped fiber for sensing achieves high-sensitivity sensing, up to 100 kHz / ℃, and offers smaller size and more flexible sensing. Furthermore, by using a fitting algorithm to improve the sensitivity of multi-mode beat frequency signal temperature measurement, the system can meet the requirements of automated data processing and real-time display of temperature changes.

Description

Technical Field

[0001] This invention relates to a multi-point serial temperature measurement system based on fiber optic beat frequency sensing technology, belonging to the field of information transmission technology. Background Technology

[0002] Fiber optic sensing offers advantages such as resistance to electromagnetic interference, high sensitivity, and ease of long-distance transmission. Because fiber optics are sensitive to signals such as temperature, vibration, and stress, fiber optic sensing technology, especially distributed sensing, is receiving increasing attention. In some application areas, such as aerospace, power systems, and geological exploration, high-sensitivity distributed monitoring is required due to the large number of detection points and long transmission distances.

[0003] To enable distributed multi-point detection using fiber optic sensing technology, much research has been conducted. Currently, traditional reusable fiber optic sensing systems are mainly divided into systems based on optical signal demodulation and systems based on electrical signal demodulation.

[0004] A typical fiber optic sensing system based on optical signal demodulation uses wavelength division multiplexing fiber gratings (FBGs) to achieve multiplexing sensing. Different FBGs have different center wavelengths. By observing the changes in the reflected spectrum wavelength of each FBG with a spectrometer, the sensing demodulation of the FBG is achieved. This sensing system requires an expensive spectrometer, is not portable, and cannot use mature demodulation algorithms to demodulate the data obtained by sensing.

[0005] A typical example of a multiplexed sensing system based on electrical signals is a fiber optic sensing system based on beat frequency signals. Compared to the more complex optical demodulation methods mentioned above, beat frequency sensing technology has gained widespread recognition and affirmation due to its advantages such as simple structure, convenient setup, and low cost. This method can obtain temperature changes at different locations by measuring the frequency offset of different multi-mode beat frequency signals. It only requires a photodetector to convert the optical signal into an electrical signal, greatly reducing the cost of the detection equipment. Large-scale detection using fiber optic beat frequency multiplexing technology is simple in structure, fast in demodulation, and high in measurement accuracy. It can be applied to areas such as grain depot temperature safety, oil storage temperature safety, and residential building fire safety, to monitor temperature changes at different locations in real time.

[0006] However, all currently published or reported related systems involve connecting multiple distributed Bragg reflection (DBR) laser resonators in parallel via couplers. The laser input to the coupler is output as multiple signals, which are then fed into different DBR laser resonators to activate them. The laser generated by the lasing produces a beat frequency signal on a photodetector. However, this common approach has two main problems: First, connecting multiple fiber optic sensing resonators via couplers requires the laser to have a high output power to operate simultaneously. The number of sensors that can be reused is limited by the laser's output power, and a high-power laser undoubtedly brings high cost, high energy consumption, and low heat dissipation to the entire sensing system. Second, all currently proposed fiber optic sensing systems based on beat frequency signals measure temperature through erbium-doped fiber in the DBR laser resonator. This method suffers from low sensitivity (typically 10 kHz / ℃) and large sensor size, requiring the entire erbium-doped fiber to be placed or attached to the object being measured, making it unsuitable for temperature detection of small objects.

[0007] Patent CN103575313A discloses a frequency division multiplexing device for a multi-longitudinal-mode annular cavity laser sensor based on beat frequency technology, realizing distributed sensing for real-time monitoring of multiple locations. It utilizes a wavelength division multiplexer to achieve parallel multiplexing of multiple resonant cavities. However, this scheme achieves multi-sensor multiplexing by connecting multiple sensors in parallel. Although increasing the number of parallel sensors enables multi-point sensing, the increased number of sensors leads to a continuous decrease in the laser power of individual sensors, failing to reach the laser power threshold required for normal operation of fiber optic laser sensors. This necessitates the use of high-power lasers, undoubtedly causing significant problems in the design cost and power consumption of the entire system. Patent CN103471741A discloses a distributed Raman fiber optic temperature sensing system with a temperature measurement accuracy of 0.5 degrees Celsius and a spatial resolution of less than or equal to 2 meters. This scheme's temperature measurement sensitivity cannot meet current temperature measurement requirements. Furthermore, the large spatial resolution necessitates the use of power amplification circuits to enhance the optical signal, increasing the complexity of the fiber optic system and the difficulty of demodulating the optical signal.

[0008] Yu et al. designed a hybrid multiplexed parallel sensing system using couplers. By designing fiber lasers with different cavity lengths and center wavelengths, they achieved frequency division multiplexing. However, this sensor has the following problems: 1. Depending on the number of parallel resonant cavities, the coupler equally distributes the pump source laser power. The power entering the resonant cavity decreases with the number of parallel resonant cavities, resulting in a significant reduction in the power of the generated beat frequency signal. Therefore, this system requires a high-power pump source to achieve a larger number of resonant cavity multiplexings; 2. Its sensitivity is 6.00 kHz / ℃, which is relatively low in temperature sensitivity (Xiujuan Yu, Xue Dong, Xuefeng Chen, et al. Large-Scale Multilongitudinal Mode Fiber Laser SensorArray With Wavelength / Frequency Division Multiplexing, Journal of Lightwave Technology, vol.35, no.11, Jun.2017.). Summary of the Invention

[0009] To address the problems of low sensitivity, high power consumption, difficult demodulation, and inability to detect the temperature of small objects in temperature monitoring systems based on distributed fiber optic sensing technology, this invention provides a multi-point serial temperature measurement system based on fiber optic beat frequency sensing technology. The system includes: a pump source laser, a wavelength division multiplexer, a resonant cavity module, a photodetector, and a spectrum analyzer.

[0010] The laser is connected to the resonant cavity module via the wavelength division multiplexer, and one end of the wavelength division multiplexer connected to the resonant cavity module is also connected in sequence to the photodetector and the spectrum analyzer.

[0011] The resonant cavity module includes: multiple DBR laser resonant cavities connected in sequence to form a series structure; each DBR laser resonant cavity includes a fiber grating and an erbium-doped fiber, the center wavelength of the fiber grating in different DBR laser resonant cavities is different, and the length of each DBR laser resonant cavity is different.

[0012] Optionally, in the resonant cavity module, each DBR laser resonant cavity includes: two fiber gratings with the same center wavelength and a section of erbium-doped fiber, with the two fiber gratings connected by the erbium-doped fiber.

[0013] Optionally, the wavelength of each channel in each DBR laser resonator is consistent with the center wavelength of its internal fiber grating.

[0014] Optionally, the DBR laser resonator is a multi-longitudinal-mode beat frequency fiber laser cavity.

[0015] Optionally, the system further includes a computer connected to the spectrum analyzer for displaying the spectrum of the temperature-affected multi-mode beat frequency signal.

[0016] Optionally, the center wavelength of the fiber grating is in the range of 1547nm-1556nm.

[0017] Optionally, the pump source laser outputs laser light with a wavelength of 1480 nm.

[0018] The present invention also provides a multi-point temperature measurement method, which is implemented using the above-mentioned multi-point series temperature measurement system, including:

[0019] Step 1: Turn on the pump source laser. The laser output from the pump source laser enters multiple resonant cavities connected in series through a wavelength division multiplexer to generate multi-longitudinal mode laser.

[0020] Step 2: The multi-longitudinal-mode laser enters the photodetector through the wavelength division multiplexer, generating a multi-longitudinal-mode beat frequency signal on the photodetector;

[0021] Step 3: The temperature signal acts on the multiple DBR laser resonant cavities connected in series or the fiber gratings inside the DBR laser resonant cavities, causing a frequency shift in the multi-longitudinal mode beat frequency signal;

[0022] Step 4: The spectrum analyzer demodulates the multi-mode beat frequency signal to obtain the frequency shift of the multi-mode beat frequency signal with temperature, and outputs a spectrum diagram through a computer.

[0023] Optionally, after acquiring the beat frequency signal output by the spectrum analyzer, the computer software is used to perform temperature fitting on the beat frequency signal, and a neural network algorithm is used to fit the linear relationship between temperature and beat frequency signal.

[0024] Alternatively, methods for fitting the linear relationship between temperature and multi-mode beat frequency signals include wavelet transform algorithms and support vector machine algorithms.

[0025] The beneficial effects of the present invention are:

[0026] The multi-point series temperature measurement system based on fiber beat frequency signals of the present invention connects DBR laser resonators in series and controls the center wavelength of the fiber gratings at both ends of each different DBR laser resonator to be different, so that different DBR sensors can work at different wavelengths. Since the laser output is a broadband laser with a certain wavelength range, as long as the wavelength of the DBR laser resonator is controlled within the laser output wavelength range, multiple DBR laser resonators can work simultaneously without the need for additional high-power pump sources and couplers. Therefore, compared with the existing parallel scheme, the power consumption of the temperature measurement system is effectively reduced.

[0027] By using fiber gratings in each DBR laser resonator instead of erbium-doped fiber for sensing, high-sensitivity sensing can be achieved, with a sensitivity of up to 100 kHz / ℃, which is 10 times the sensitivity of traditional temperature sensing based on erbium-doped fiber.

[0028] Sensing based on fiber gratings in a DBR laser resonator has a smaller size and more flexible sensing capabilities compared to traditional erbium-doped fiber sensing methods.

[0029] This invention fits the acquired multi-mode beat frequency signal data and temperature data to improve the sensitivity of multi-mode beat frequency signal temperature measurement, which can meet the performance requirements of automated data processing and real-time display of temperature changes in fiber optic beat frequency temperature laser sensing systems. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below 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 effort.

[0031] Figure 1 This is a schematic diagram of the resonant cavity temperature measurement structure of the multi-point series temperature measurement system based on fiber optic beat frequency sensing technology provided in Embodiment 2 of the present invention.

[0032] Figure 2 This is a schematic diagram of the fiber optic grating temperature measurement structure of the multi-point serial temperature measurement system based on fiber optic beat frequency sensing technology provided in Embodiment 2 of the present invention.

[0033] Figure 3 This is a schematic diagram of the principle of a fiber laser resonator.

[0034] Figure 4 This is a fiber grating spectrum diagram of multiple resonant cavities connected in series in Embodiment 2 of the present invention.

[0035] Figure 5 This is a simulation diagram of the demodulation temperature sensitivity of the multi-longitudinal mode beat frequency signal of each resonant cavity in Embodiment 2 of the present invention.

[0036] Figure 6 This is a simulation diagram of the stability of the multi-longitudinal mode beat frequency signal of each resonator in Embodiment 2 of the present invention.

[0037] Figure 7 This is a graph showing the surface temperature changes of three parts of the human body according to an embodiment of the present invention. Detailed Implementation

[0038] To make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0039] Example 1:

[0040] This embodiment provides a multi-point serial temperature measurement system based on fiber optic beat frequency sensing technology, including: a pump source laser, a wavelength division multiplexer, a resonant cavity module, a photodetector, and a spectrum analyzer;

[0041] The laser is connected to the resonant cavity module via the wavelength division multiplexer, and one end of the wavelength division multiplexer connected to the resonant cavity module is also connected in sequence to the photodetector and the spectrum analyzer.

[0042] The resonant cavity module includes: multiple DBR laser resonant cavities connected in sequence to form a series structure; each DBR laser resonant cavity includes a fiber grating and an erbium-doped fiber, the center wavelength of the fiber grating in different DBR laser resonant cavities is different, and the length of each DBR laser resonant cavity is different.

[0043] Example 2:

[0044] This embodiment provides a multi-point series temperature measurement system based on fiber optic beat frequency sensing technology. (See also...) Figure 1 It includes a pump source laser, a wavelength division multiplexer, five DBR laser resonators connected in series, a photodetector, a spectrum analyzer, and a computer. The pump source laser is connected to the five DBR laser resonators connected in series via the wavelength division multiplexer. One end of the wavelength division multiplexer connected to the DBR laser resonators is also connected to the photodetector, the spectrum analyzer, and the computer in series.

[0045] Each DBR laser resonator includes two fiber gratings with the same center wavelength and one erbium-doped fiber, such as... Figure 1 , 2 As shown, the two fiber gratings are connected by erbium-doped fiber. Figure 3 The diagram shows a structure of a DBR laser resonator, which includes two fiber gratings (FBGs) with the same center wavelength, a section of erbium-doped fiber (EDF), and the two FBGs connected by the erbium-doped fiber.

[0046] The wavelength of each channel in the DBR laser resonator is the same as the center wavelength of its respective internal fiber grating, and the center wavelength of the fiber gratings in different resonators is different.

[0047] In this embodiment, the five DBR laser resonator cavities have fiber gratings with center wavelengths of 1556nm, 1552nm, 1548nm, 1554nm, and 1550nm, respectively. The fiber gratings have a 3dB bandwidth of 240pm, wavelengths of 1547nm-1556nm, and reflectivity of 93% for all wavelengths. The erbium-doped fiber has an absorption coefficient of 36dB / m at 1532nm.

[0048] In this embodiment, the pump source outputs a 1480nm wavelength laser, which enters multiple cascaded DBR laser resonators through the 1480nm port of a wavelength division multiplexer. The laser generates multi-mode laser light through each DBR laser resonator and is output to a photodetector through the 1550nm port of the wavelength division multiplexer, generating a multi-mode beat frequency signal on the photodetector. When a temperature signal is applied to each DBR laser resonator or the fiber grating in the DBR laser resonator, the multi-mode beat frequency signal will undergo a frequency shift. The multi-mode beat frequency signal is input to a spectrum analyzer to complete signal acquisition. The signal output by the spectrum analyzer is displayed on a computer, and the frequency change corresponding to the temperature signal is obtained, that is, the beat frequency signal is affected by temperature and its frequency shifts.

[0049] After acquiring the beat frequency signal output by the spectrum analyzer, the computer software is used to perform temperature fitting on the beat frequency signal, and a neural network algorithm is used to fit the linear relationship between temperature and beat frequency signal.

[0050] In practical applications, the pump laser source and wavelength division multiplexer can use other parameters, such as a 980nm pump laser source, a 980nm & 1550nm wavelength wavelength multiplexer, etc.; the resonant cavity can use devices with other parameters, such as using an optical mirror to replace another fiber grating with the same center wavelength in the last DBR laser resonant cavity, or a ring cavity structure, or other structures that can generate multi-longitudinal mode lasers are all applicable; the center wavelength of the fiber grating can be 1558nm, 1560nm, etc., and the absorption coefficient of erbium-doped fiber is 6dB / m@1532nm, 13dB / m@1532nm, etc., which are not limited in this application.

[0051] like Figure 4 As shown, five DBR laser resonators were connected in series, and a spectrometer was connected to the output of the 1550nm port of the wavelength division multiplexer to display the center wavelength of each resonator. The measured spectra of the DBR laser resonators were 1548nm, 1550nm, 1552nm, 1554nm, and 1556nm, respectively.

[0052] Example 3:

[0053] This embodiment provides a multi-point temperature measurement method, which is implemented using the multi-point series temperature measurement system described in Embodiment 2, and includes the following steps:

[0054] Step 1: Turn on the pump source laser. The laser output from the pump source laser enters multiple DBR laser resonant cavities connected in series through a wavelength division multiplexer to generate multi-longitudinal mode laser.

[0055] Step 2: The multi-longitudinal-mode laser enters the photodetector through the wavelength division multiplexer, generating a multi-longitudinal-mode beat frequency signal on the photodetector;

[0056] Step 3: The temperature signal acts on the multiple DBR laser resonant cavities connected in series or the fiber gratings inside the DBR laser resonant cavities, causing a frequency shift in the multi-longitudinal mode beat frequency signal;

[0057] Step 4: The spectrum analyzer demodulates the multi-mode beat frequency signal to obtain the frequency shift of the multi-mode beat frequency signal with temperature, and outputs a spectrum diagram through a computer.

[0058] After acquiring the beat frequency signal output from the spectrum analyzer, computer software is used to fit the temperature to the beat frequency signal. A neural network algorithm is then used to fit the linear relationship between temperature and the beat frequency signal. Methods for fitting the linear relationship between temperature and multi-mode beat frequency signals include: neural network algorithm, wavelet transform algorithm, and support vector machine algorithm.

[0059] To further illustrate the beneficial effects achievable by the system and method of the present invention, a series of experimental tests were conducted on the system of Embodiment 2, and the experimental results are as follows:

[0060] The fitting results for the data pairs of multi-longitudinal mode beat frequency signal and temperature signal for each fiber Bragg grating are as follows: Figure 5 As shown, a linear neural network algorithm was used to fit the data, and the temperature sensitivities of each resonant cavity were 96.3196 kHz / ℃, 109.1802 kHz / ℃, 66.0034 kHz / ℃, 109.9055 kHz / ℃, and 72.4899 kHz / ℃, respectively. Compared with the sensitivity of 10 kHz / ℃ in the prior art, the present invention has significant advantages.

[0061] Secondly, this application conducted experiments to simultaneously measure the system stability for different resonant cavities. During the experiments, the system was maintained at a constant temperature of 20°C, and results were recorded every minute for 60 minutes. Figure 6 As shown, the frequency shifts of the multi-longitudinal mode beat frequency signals of each resonant cavity are 14.6kHz, 16.4kHz, 14.6kHz, 13.6kHz, and 12.8kHz, respectively. This demonstrates that the multi-longitudinal mode beat frequency-based cascaded multiplexed temperature measurement system provided in this application has high stability and can demodulate a stable temperature signal.

[0062] To assess the temperature measurement performance of fiber Bragg gratings in different resonant cavities, multi-point temperature measurement of human body surface temperature was conducted. Fiber Bragg gratings from three resonant cavities were selected as sensing elements and attached to the forehead, back of the left hand, and back of the right hand, respectively. A standard body thermometer was used as a reference device. The frequency of the beat frequency sensing signal and the corresponding temperature value from the thermometer were recorded every minute. Figure 7 The figure shows the temperature change results of three parts of the human body. The curves show that the present invention can be flexibly applied to small-scale temperature measurement scenarios such as body temperature measurement.

[0063] Some steps in the embodiments of the present invention may be implemented using software, and the corresponding software program may be stored in a readable storage medium, such as a CD or a hard disk.

[0064] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A multi-point series temperature measurement system based on fiber optic beat frequency sensing technology, characterized in that, The system includes: a pump source laser, a wavelength division multiplexer, a resonant cavity module, a photodetector, and a spectrum analyzer; The laser is connected to the resonant cavity module via the wavelength division multiplexer, and one end of the wavelength division multiplexer connected to the resonant cavity module is also connected in sequence to the photodetector and the spectrum analyzer. The resonant cavity module includes: multiple DBR laser resonant cavities connected in sequence to form a series structure; each DBR laser resonant cavity includes two fiber gratings with the same center wavelength and a section of erbium-doped fiber, the two fiber gratings are connected by the erbium-doped fiber, the center wavelengths of the fiber gratings in different DBR laser resonant cavities are different, and the length of each DBR laser resonant cavity is different, the system uses fiber gratings for sensing.

2. The multi-point series temperature measurement system according to claim 1, characterized in that, The wavelength of each channel in each DBR laser resonator is consistent with the center wavelength of the fiber grating inside it.

3. The multi-point series temperature measurement system according to claim 2, characterized in that, The DBR laser resonator is a multi-longitudinal mode beat frequency fiber laser cavity.

4. The multi-point series temperature measurement system according to claim 3, characterized in that, The system also includes a computer connected to the spectrum analyzer for displaying the spectrum of the temperature-affected multi-mode beat frequency signal.

5. The multi-point series temperature measurement system according to claim 1, characterized in that, The center wavelength of the fiber grating is in the range of 1547nm-1556nm.

6. The multi-point series temperature measurement system according to claim 1, characterized in that, The pump source laser outputs laser light with a wavelength of 1480nm.

7. A multi-point temperature measurement method, characterized in that, The method is implemented using the multi-point series temperature measurement system described in claim 4, including: Step 1: Turn on the pump source laser. The laser output from the pump source laser enters multiple DBR laser resonant cavities connected in series through a wavelength division multiplexer to generate multi-longitudinal mode laser. Step 2: The multi-longitudinal-mode laser enters the photodetector through the wavelength division multiplexer, generating a multi-longitudinal-mode beat frequency signal on the photodetector; Step 3: The temperature signal acts on the fiber gratings inside the multiple DBR laser resonant cavities connected in series, causing a frequency shift in the multi-longitudinal mode beat frequency signal; Step 4: The spectrum analyzer demodulates the multi-mode beat frequency signal to obtain the frequency shift of the multi-mode beat frequency signal with temperature, and outputs a spectrum diagram through a computer.

8. The multi-point temperature measurement method according to claim 7, characterized in that, After acquiring the beat frequency signal output by the spectrum analyzer, the computer software is used to perform temperature fitting on the beat frequency signal, and a neural network algorithm is used to fit the linear relationship between temperature and beat frequency signal.

9. The multi-point temperature measurement method according to claim 8, characterized in that, Methods for fitting the linear relationship between temperature and multi-mode beat frequency signals also include wavelet transform algorithm and support vector machine algorithm.

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