Joint demodulation method based on fiber grating array distributed sensing system

Abstract

The invention provides a joint demodulation method based on a fiber grating array distributed sensing system, and the distributed sensing system comprises a sweep frequency light source module, a pulse modulation module, a light beam coupling module, a grating array module, a photoelectric conversion module, and a signal collection and processing module. A radio frequency signal is modulated to sweep frequency laser output by the sweep frequency light source module through the pulse modulation module and then input to the light beam coupling and photoelectric conversion module, a modulated light signal is divided into two paths by a beam splitter, one path serves as local oscillator reference light, and the other path is input to the grating array module. The local oscillator reference light and the reflected light of the grating array are coupled and then input into the photoelectric detector to realize photoelectric conversion, and are acquired and processed by the signal acquisition and processing module to obtain a wavelength-microwave photon time domain impact response set of the distributed sensing system in a scanning period of the frequency sweeping light source; and demodulating the position of the fiber grating corresponding to the sensing quantity corresponding to the time domain impulse response and sensing information corresponding to the wavelength.

Description

Technical Field

[0001] The present invention belongs to the field of fiber Bragg grating sensing, is mainly used for long-distance distributed sensing, and specifically relates to a method for realizing long-distance distributed sensing by using a fiber Bragg grating array as a sensor and a wavelength-time domain joint demodulation technology. Background Art

[0002] Distributed sensing systems based on fiber Bragg gratings have the advantages of anti-electromagnetic interference, corrosion resistance, high temperature resistance, low transmission loss, small size and light weight, and can monitor changes in various physical parameters such as temperature, strain, pressure and refractive index. They are widely used in many fields such as construction and medical treatment. At present, the sensing technology based on fiber Bragg gratings is quite mature, with many perfect sensing and demodulation schemes, and has applications in various sensing scenarios. In the field of fiber Bragg grating sensing, an important indicator is the multiplexing capability of the distributed sensing system. The multiplexing capability of the strong reflection fiber grating is limited by the wavelength range of the light source, and the multiplexing capability is limited. The weak reflection fiber grating has a lower reflectivity and can be reused repeatedly, and the multiplexing capability is stronger. At present, the common distributed sensing network based on weak fiber grating arrays often uses the same weak reflection fiber grating array, which can reuse several identical weak reflection fiber gratings at the Bragg wavelength, but it still needs to be improved for some sensing scenarios that require higher multiplexing capacity. At present, the common demodulation scheme of distributed sensing based on weak reflection fiber grating array generally obtains the time domain signal of the system through the data acquisition card after the output of the photoelectric detector, and then filters and amplifies the time domain signal. It is also necessary to prevent the time domain signal from being aliased or submerged in noise. Summary of the invention

[0003] The purpose of the present invention is to provide a joint demodulation method based on a fiber grating array distributed sensing system without the need for anti-aliasing filtering operations, and for a distributed sensor system using a weakly reflective fiber Bragg grating, the multiplexing capacity can also be increased. The technical solution is as follows:

[0004] A joint demodulation method based on a fiber grating array distributed sensing system is characterized in that the adopted distributed sensing system includes a swept frequency light source module, a pulse modulation module, a beam coupling, a grating array module, a photoelectric conversion module and a signal acquisition and processing module. The swept frequency laser output by the swept frequency light source module modulates the radio frequency signal onto the swept frequency laser through the pulse modulation module and then inputs it into the beam coupling and photoelectric conversion module. The modulated optical signal is divided into two paths by a beam splitter, one path is used as a local oscillator reference light, and the other path is input into the grating array module. The local oscillator reference light and the reflected light of the grating array are coupled and then input into a photoelectric detector to realize photoelectric conversion. The signal acquisition and processing module collects and processes the signal to obtain a wavelength-microwave photon time domain impulse response set of the distributed sensing system within a scanning cycle of the swept frequency light source, and by analyzing the time domain response set, the position of the fiber grating corresponding to the sensing quantity corresponding to the time domain impulse response and the sensing information corresponding to the wavelength are demodulated.

[0005] Furthermore, the signal acquisition and processing module collects and processes the information to obtain a set of wavelength-microwave photon time-domain impulse responses of the distributed sensing system within a scanning cycle of the swept-frequency light source. The specific method is as follows: the signal acquisition and processing module processes the radio frequency signal and the electrical signal output by the photodetector to obtain a spectrum response of the microwave photons, fills zeros at the end of the spectrum response to extend the bandwidth of the spectrum response, and then performs an inverse Fourier transform on the spectrum response to obtain a time domain response, that is, after each change in the wavelength of the swept-frequency light source, the time domain response of each wavelength of the swept-frequency light source within a scanning cycle is obtained in turn, and the set of wavelength-microwave photon time-domain impulse responses of the distributed sensing system within a scanning cycle of the swept-frequency light source is obtained.

[0006] Furthermore, for each wavelength of the swept-frequency light source corresponding to the wavelength-microwave photon time-domain impulse response set within a scanning cycle of the swept-frequency light source of the distributed sensing system, the corresponding relationship between the time delay corresponding to the peak point and the wavelength is obtained through a peak-finding algorithm, the position of the fiber grating is determined according to the time delay corresponding to the peak, and the change of the sensing quantity on the corresponding fiber grating is demodulated according to the change of the wavelength corresponding to the peak value of the same fiber grating, and the sensing quantity is demodulated based on the calibration of the initial state of the grating array.

[0007] Furthermore, the grating array module is a fiber grating array composed of a plurality of groups of weakly reflective fiber gratings with different Bragg wavelengths that can be reused in series, and each group of weakly reflective fiber gratings with the same Bragg wavelength reuses the band where its Bragg wavelength is located.

[0008] Furthermore, the weakly reflective fiber Bragg grating is a fiber Bragg grating with a reflectivity lower than 10%.

[0009] The present invention adopts the above technical solution, which has the following advantages:

[0010] (1) The present invention proposes a novel joint demodulation method based on a fiber grating array distributed sensing system. By filling the system's spectrum response with zeros and then performing an inverse Fourier transform to obtain the system's time domain response, compared to the directly collected time domain signal, no anti-aliasing filtering is required. Compared to the fiber grating array microwave photon demodulation method using a broadband light source, the sensing grating array can use a weak grating array.

[0011] (2) An embodiment of the present invention, compared with the existing distributed sensing system based on strong reflection fiber grating array and identical weak reflection fiber grating array, can achieve higher multiplexing capacity by serially connecting multiple groups of weak reflection fiber gratings with different Bragg wavelengths. This is at least one order of magnitude higher than the multiplexing capacity of the identical weak reflection fiber grating array, which has higher application value in scenarios with large-capacity sensing requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Attached Figure 1 The present invention is a principle block diagram of a distributed sensing system based on a fiber grating array and a joint demodulation method.

[0013] Attached Figure 2 The present invention is a schematic diagram of the structure of a fiber grating array module in a distributed sensing system based on a fiber grating array and a joint demodulation method.

[0014] Attached Figure 3 This is a working effect diagram of Example 1 of a distributed sensing system based on a fiber grating array and a joint demodulation method involved in the present invention.

[0015] Attached Figure 4 This is a working effect diagram of Example 2 of a distributed sensing system and joint demodulation method based on a fiber grating array involved in the present invention. DETAILED DESCRIPTION

[0016] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0017] The distributed sensor system and joint demodulation method based on fiber grating array of the present invention comprises: a frequency sweeping light source module, a pulse modulation module, a grating array module, a beam coupling and photoelectric conversion module, and a signal acquisition and processing module. The sensing quantity (temperature, strain, vibration, etc.) of the fiber grating is demodulated by the wavelength output by the system, and the position of the fiber grating corresponding to the sensing quantity is demodulated by the time domain impulse response of the microwave photon system.

[0018] In this system, the frequency sweep laser output by the frequency sweep light source module modulates the RF signal output by the signal acquisition and processing module onto the frequency sweep laser through the pulse modulation module, and then inputs into the beam coupling and photoelectric conversion module. The modulated optical signal is divided into two paths by the beam splitter, one of which is input into the grating array module, and after being reflected by the grating array module, it is coupled with the other path and then input into the photodetector. The output electrical signal of the photodetector is processed by the signal acquisition and processing module. Among them, the other path of the modulated light after being split by the beam splitter is used as the local oscillator light as the reference delay of the time domain impulse response of the microwave photonic system. The grating array module is a fiber grating array composed of several groups of fiber gratings with different Bragg wavelengths connected in series, and is composed of several groups of weak-reflection fiber Bragg gratings with different Bragg wavelengths connected in series, wherein the weak-reflection fiber Bragg gratings refer to fiber Bragg gratings with a reflectivity of less than 10% that can be reused, each group of weak-reflection fiber Bragg gratings with the same Bragg wavelength multiplexes the band where its Bragg wavelength is located, and multiple groups of weak-reflection fiber Bragg gratings with different Bragg wavelengths respectively multiplex the band where their Bragg wavelength is located within the wavelength scanning range of the swept light source to improve the multiplexing capacity.

[0019] Its working principle is as follows: the central wavelength of the swept-frequency laser light source drifts continuously within its scanning range. At each central wavelength of the swept-frequency laser light source, the signal acquisition and processing module processes the RF signal output by the swept-frequency laser light source and the electrical signal output by the photodetector after the beat frequency to obtain the spectrum response of the system. Then, zeros are added at the end of the spectrum response to extend the bandwidth of the spectrum response, and then the spectrum response is inversely Fourier transformed to obtain the time domain response of the system, wherein the purpose of zero addition is to improve the resolution of the time domain response. Then, after each change in the wavelength of the swept-frequency light source, the time domain response of the swept-frequency light source at each wavelength in a scanning cycle is obtained in turn, and the time domain response set of the sensor system in a scanning cycle of the swept-frequency light source is obtained. The time domain response set is plotted into a wavelength-time domain pseudo-color diagram to show the same number of bright spots as the number of gratings, and the bright spots correspond to the gratings one by one in the order of the bright spots in the grating array, and the peak point of each bright spot is obtained by the peak-finding algorithm. The change in the sensing quantity on the fiber grating is demodulated according to the wavelength change corresponding to the bright spot peak, and the position of the fiber grating is determined according to the time delay corresponding to the bright spot peak. The positions of the bright spots corresponding to different fiber Bragg gratings are determined by the grating spacing and the grating wavelength, and no overlap will occur (even if the grating wavelength and reflectivity are the same).

[0020] Attached Figure 1It is a principle block diagram of the fiber grating array distributed sensing system and joint demodulation method proposed in the present invention. The swept frequency light source module outputs tunable laser, which is modulated and split by the pulse modulation module. One path is used as the local oscillator reference light, and the other path is input into the grating array module. The local oscillator reference light and the reflected light of the grating array are coupled and input into the photoelectric detector to realize photoelectric conversion. Finally, the signal acquisition and processing module collects and processes the signal to obtain the time domain response set of the sensing system within a scanning cycle of the swept frequency light source, and the final demodulation result of the sensing information is obtained by analyzing the time domain response set.

[0021] Attached Figure 2 The invention is a schematic structural diagram of two fiber Bragg grating array modules in a fiber Bragg grating array distributed sensing system and a joint demodulation method proposed in the present invention. The fiber Bragg grating array module is a weak reflection fiber Bragg grating array, which is composed of a plurality of groups of weak reflection fiber Bragg gratings with different Bragg wavelengths connected in series, wherein the weak reflection fiber Bragg gratings refer to fiber Bragg gratings with weak reflectivity that can be reused, each group of weak reflection fiber Bragg gratings with the same Bragg wavelength multiplexes the band where its Bragg wavelength is located, and a plurality of groups of weak reflection fiber Bragg gratings with different Bragg wavelengths multiplex different bands within the wavelength scanning range of a swept light source to improve the multiplexing capacity.

[0022] Embodiment 1: In order to more clearly introduce the sensing and demodulation principle of the present invention, the attached Figure 2 The weak reflection fiber Bragg grating array shown in the figure is simplified as an embodiment. This embodiment uses three groups of weak reflection fiber Bragg gratings with different Bragg wavelengths. The Bragg wavelengths of the three groups of weak reflection fiber Bragg gratings are 1548nm, 1550nm and 1552nm respectively. This embodiment uses each group of weak reflection fiber Bragg gratings composed of 3 same Bragg wavelengths. The local oscillator reference light and the reflected light of the grating array are coupled and converted into an electrical signal by a photodetector, and the wavelength-time domain pseudo-color image of the system within a scanning cycle (1546nm to 1554nm) of the swept frequency light source is obtained by the signal acquisition and processing module, as shown in the attached figure. Figure 3 As shown in the figure on the left, the wavelength of the frequency-sweeping light source corresponding to the maximum value of each time-domain pulse peak is the Bragg wavelength of the corresponding weak-reflection fiber Bragg grating. When the wavelength of the fifth weak-reflection fiber Bragg grating in the grating array changes from 1550nm to 1551nm under external disturbance, the signal acquisition and processing module obtains the wavelength-time domain pseudo-color image of the system within a scanning cycle of the frequency-sweeping light source (1546nm to 1554nm), as shown in the attached figure. Figure 3 As shown in the figure on the right, the wavelength of the swept-frequency light source corresponding to the maximum value of the time-domain pulse peak value corresponding to the fifth weak-reflection fiber Bragg grating drifts to 1551 nm, thereby realizing the demodulation of the sensing information.

[0023] Embodiment 2: In order to more clearly introduce the sensing and demodulation principle of the present invention, the attached Figure 2 The weak reflection fiber Bragg grating array shown in the figure is simplified as an embodiment. This embodiment uses three groups of weak reflection fiber Bragg gratings with different arrangement orders. The arrangement orders of the three groups of weak reflection fiber Bragg gratings are 1548-1550-1552nm, 1552-1550-1548nm and 1548-1552-1550nm respectively. This embodiment uses each group of weak reflection fiber Bragg gratings with 3 different Bragg wavelengths and arranged in different orders. The local oscillator reference light and the reflected light of the grating array are coupled and converted into an electrical signal by a photoelectric detector, and the wavelength-time domain pseudo-color image of the system within a scanning cycle of the swept frequency light source (1546nm to 1554nm) is obtained by the signal acquisition and processing module, as shown in the attached Figure 4 As shown in the figure on the left, the wavelength of the frequency-sweeping light source corresponding to the maximum value of each time-domain pulse peak is the Bragg wavelength of the corresponding weak-reflection fiber Bragg grating. When the wavelength of the fifth weak-reflection fiber Bragg grating in the grating array changes from 1550nm to 1551nm under external disturbance, the signal acquisition and processing module obtains the wavelength-time domain pseudo-color image of the system within a scanning cycle of the frequency-sweeping light source (1546nm to 1554nm), as shown in the attached figure. Figure 4 As shown in the figure on the right, the wavelength of the swept-frequency light source corresponding to the maximum value of the time-domain pulse peak value corresponding to the fifth weak-reflection fiber Bragg grating drifts to 1551 nm, thereby realizing the demodulation of the sensing information.

Claims

1. A joint demodulation method based on a fiber grating array distributed sensing system, characterized in that: The adopted distributed sensing system includes a swept-frequency light source module, a pulse modulation module, a beam coupling, a grating array module, a photoelectric conversion module and a signal acquisition and processing module. The swept-frequency laser output by the swept-frequency light source module modulates the radio frequency signal onto the swept-frequency laser through the pulse modulation module and then inputs into the beam coupling and photoelectric conversion module. The modulated optical signal is divided into two paths by a beam splitter, one path is used as a local oscillator reference light, and the other path is input into the grating array module. The local oscillator reference light and the reflected light of the grating array are coupled and then input into the photoelectric detector to realize photoelectric conversion. The signal is collected and processed by the signal acquisition and processing module to obtain a wavelength-microwave photon time domain impulse response set of the distributed sensing system within a scanning cycle of the swept-frequency light source, and by analyzing the time domain response set, the position of the fiber grating corresponding to the sensing quantity corresponding to the time domain impulse response and the sensing information corresponding to the wavelength are demodulated.

2. The distributed sensing system joint demodulation method according to claim 1, characterized in that: The specific method for acquiring and processing the signal by the signal acquisition and processing module to obtain the wavelength-microwave photon time domain impulse response set of the distributed sensing system within a scanning cycle of the swept-frequency light source is as follows: the signal acquisition and processing module processes the radio frequency signal and the electrical signal output by the photodetector to obtain the spectrum response of the microwave photons, fills zeros at the end of the spectrum response to extend the bandwidth of the spectrum response, and then performs an inverse Fourier transform on the spectrum response to obtain the time domain response, that is, after each change in the wavelength of the swept-frequency light source, the time domain response of each wavelength of the swept-frequency light source within a scanning cycle is obtained in turn, and the wavelength-microwave photon time domain impulse response set of the distributed sensing system within a scanning cycle of the swept-frequency light source is obtained.

3. The distributed sensing system joint demodulation method according to claim 1, characterized in that: For each wavelength of the swept-frequency light source corresponding to the wavelength-microwave photon time-domain impulse response set within a scanning cycle of the swept-frequency light source of the distributed sensing system, the corresponding relationship between the time delay and the wavelength corresponding to the peak point is obtained through a peak-finding algorithm, and the position of the fiber grating is determined according to the time delay corresponding to the peak. The change of the sensing quantity on the corresponding fiber grating is demodulated according to the change of the wavelength corresponding to the peak value of the same fiber grating, and the sensing quantity is demodulated based on the calibration of the initial state of the grating array.

4. The distributed sensing system joint demodulation method according to claim 1, characterized in that: The grating array module is a fiber grating array composed of a plurality of groups of weakly reflective fiber gratings with different Bragg wavelengths that can be reused in series, and each group of weakly reflective fiber gratings with the same Bragg wavelength reuses the band where its Bragg wavelength is located.

5. The distributed sensing system joint demodulation method according to claim 1, characterized in that: The weak reflection fiber Bragg grating is a fiber Bragg grating with a reflectivity lower than 10%.

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