CN111637911B - BOTDA distributed optical fiber sensor assisted by common-line double-wave interferometer - Google Patents
BOTDA distributed optical fiber sensor assisted by common-line double-wave interferometer Download PDFInfo
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- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
- G01D5/35383—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using multiple sensor devices using multiplexing techniques
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- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
- G01D5/35306—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement
- G01D5/35309—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement using multiple waves interferometer
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- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
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- G01D5/35354—Sensor working in reflection
- G01D5/35358—Sensor working in reflection using backscattering to detect the measured quantity
- G01D5/35364—Sensor working in reflection using backscattering to detect the measured quantity using inelastic backscattering to detect the measured quantity, e.g. using Brillouin or Raman backscattering
Abstract
The invention discloses a BOTDA distributed optical fiber sensor assisted by a common-row dual-wave interferometer, wherein continuous light output by a tunable laser is divided into an upper branch and a lower branch after passing through an optical coupler; the upper branch enters a sensing optical fiber after passing through a polarization controller, a phase modulator and an optical isolator; the lower branch enters the sensing optical fiber through an acousto-optic modulator, a polarization scrambler, an erbium-doped optical fiber amplifier and a first optical circulator; at a receiving end, the probe light enters the fiber bragg grating through the second optical circulator; the single-frequency Brillouin loss continuous light transmitted by the fiber bragg grating enters an oscilloscope after passing through the optical band-pass filter and the first photoelectric detector; the dual-frequency detection light reflected by the fiber bragg grating enters the oscilloscope after passing through the second photoelectric detector, the low-noise microwave amplifier, the electric band-pass filter, the electric mixer and the electric low-pass filter. The invention can accurately measure and position the dynamic physical quantity received by any position of the optical fiber, and the dynamic sampling rate can be adjusted at will according to the actual requirement.
Description
Technical Field
The invention belongs to the technical field of distributed optical fiber sensing, and particularly relates to a BOTDA distributed optical fiber sensor assisted by a common-line double-wave interferometer.
Background
In recent years, with the rapid development of large infrastructures such as bridges and railways, security has become a focus of attention in various social circles. Distributed sensing capable of achieving high spatial resolution and high dynamic sampling rate over long distances has become a pressing need. As one of the most potential sensing technologies, distributed optical fiber sensing can accurately sense external physical information at various positions of an optical fiber under long distance, complex electromagnetic and severe environments. Among them, the distributed optical fiber dynamic sensor based on stimulated brillouin scattering receives extensive attention and intensive research due to its advantages such as long distance, high spatial resolution and large dynamic measurement range.
At present, the brillouin dynamic sensor mainly makes progress in shortening the brillouin gain spectrum acquisition time, reducing the gain curve average time, acquiring the best polarization fading elimination effect with the least average times and the like, thereby improving the dynamic sampling rate. However, as the length of the fiber increases, the pulse repetition frequency must be decreased to avoid crosstalk of the sensing information, which will cause a corresponding decrease in the dynamic sampling rate. Thus, currently brillouin dynamic sensors are limited to short-range measurements.
On the other hand, although the BOTDA (brillouin optical time domain analysis) distributed optical fiber sensor has the advantages of high positioning accuracy, high dynamic measurement range and the like, the dynamic sampling rate is limited by various factors at the same time, and generally can only be between dozens and hundreds of Hz.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a BOTDA distributed optical fiber sensor assisted by a common-row double-wave interferometer.
The invention discloses a BOTDA distributed optical fiber sensor assisted by a co-traveling double-wave interferometer.
Continuous light output by the tunable laser is divided into an upper branch and a lower branch after passing through the optical coupler; continuous light of an upper branch generates three-frequency probe light through a polarization controller and a phase modulator, then passes through an optical isolator, is injected into a sensing optical fiber, and then enters a receiving end through a first optical circulator; the continuous light of the lower branch path sequentially passes through an acoustic optical modulator to generate up-conversion pulse pump light, then passes through a polarization scrambler and an erbium-doped fiber amplifier and then enters the sensing fiber through a first optical circulator.
At a receiving end, the three-frequency probe light enters the fiber grating through the second optical circulator; the single-frequency Brillouin loss detection light transmitted by the fiber bragg grating sequentially passes through the optical bandpass filter and the first photoelectric detector and then enters the oscilloscope; the dual-frequency detection light reflected by the fiber bragg grating enters the oscilloscope after passing through the second photoelectric detector, the low-noise microwave amplifier, the electric band-pass filter, the electric mixer and the electric low-pass filter in sequence.
Further, the coupling ratio of the optical coupler is 50: 50.
Further, the phase modulator is driven by a microwave signal generated by a microwave generator and split by a microwave splitter.
Further, the acousto-optic modulator is driven by a pulse signal generated by an arbitrary function generator.
Furthermore, a microwave phase shifter is arranged between the microwave beam splitter and the electric mixer, and the initial phase of the local oscillation electric signal is adjusted through the microwave phase shifter, so that the DWI bias point works in the middle of a cosine transformation curve linear region.
Compared with the prior art, the invention has the beneficial technical effects that:
(1) the invention realizes the high-speed measurement of dynamic strain by combining the co-traveling double-wave interferometer with high sampling rate and the Brillouin optical time domain with low sampling rate and high positioning precision.
(2) The dynamic measurement sampling rate of the co-traveling double-wave interferometer assisted Brillouin optical time domain analysis (DWI-BOTDA) distributed optical fiber sensor can be adjusted at will by changing the sampling rate of the acquisition equipment.
(3) The parallel-sharing double-wave interferometer (DWI) provided by the invention has the advantages of large dynamic strain measurement range, flexibility and adjustability.
(4) The cocurrent double-wave interferometer (DWI) provided by the invention is not influenced by polarization fading, laser phase noise and frequency drift, so that the measurement stability is high.
(5) The data acquisition equipment required by the invention has low sampling rate, so the data volume is low, and the potential real-time property and the long-term measurement property are good.
Drawings
FIG. 1 is a schematic view of the structure of the present invention.
The notations in FIG. 1 are: the optical fiber polarization control system comprises a 1-tunable laser, a 2-optical coupler, a 3-polarization controller, a 4-phase modulator, a 5-microwave generator, a 6-microwave beam splitter, a 7-optical isolator, an 8-sensing optical fiber, a 9-arbitrary function generator, a 10-acousto-optic modulator, an 11-polarization scrambler, a 12-erbium-doped optical fiber amplifier, a 13-first optical circulator, a 14-second optical circulator, a 15-optical fiber grating, a 16-optical bandpass filter, a 17-first photoelectric detector, an 18-oscilloscope, a 19-second photoelectric detector, a 20-low-noise microwave amplifier, a 21-electric bandpass filter, a 22-electric mixer, a 23-electric phase shifter and a 24-electric low-pass filter.
Fig. 2 shows a result of a brillouin loss spectrum distribution test in an embodiment of the present invention: (a) is the Brillouin gain spectral distribution of a Brillouin Optical Time Domain Analysis (BOTDA) sensor; (b) the brillouin loss spectrum at the location of the pre-stress applied to the end of the fibre.
FIG. 3 shows the phase-amplitude transition curve test results of the embodiment of the present invention: (a) a three-dimensional plot of phase-amplitude conversion curve over time for a co-traveling Dual Wave Interferometer (DWI) sensor; (b) is a phase-amplitude conversion curve.
FIG. 4 is a graph of the dynamic strain results measured by a Brillouin optical time domain analysis (DWI-BOTDA) distributed optical fiber sensor assisted by a co-traveling dual wave interferometer according to the present invention: (a) a graph of the dynamic strain measured for DWI and BOTDA, and (b) a magnified image in the range of 6 to 11.6 ms.
FIG. 5 is a frequency spectrum diagram corresponding to dynamic strain measured by a co-travelling dual-wave interferometer-assisted Brillouin optical time domain analysis (DWI-BOTDA) distributed optical fiber sensor according to the present invention.
Detailed Description
The invention is described in further detail below with reference to the figures and specific embodiments.
Fig. 1 shows a co-traveling dual-wave interferometer-assisted BOTDA distributed optical fiber sensor of the present invention, which includes an upper branch for generating triple-frequency probe light and a lower branch for generating up-conversion pulse pump light.
Continuous light output by the tunable laser 1 is divided into an upper branch and a lower branch after passing through the optical coupler 2; continuous light of an upper branch generates triple-frequency probe light through the polarization controller 3 and the phase modulator 4, then passes through the optical isolator 7, is injected into the sensing optical fiber 8, and then enters a receiving end through the first optical circulator 13; the continuous light of the lower branch path sequentially passes through an acousto-optic modulator 10 to generate up-conversion pulse pump light, then passes through a polarization scrambler 11 and an erbium-doped fiber amplifier 12, and then enters the sensing fiber 8 through a first optical circulator 13.
At a receiving end, the three-frequency probe light enters the fiber grating 15 through the second optical circulator 14; the single-frequency Brillouin loss detection light transmitted by the fiber grating 15 enters an oscilloscope 18 after passing through an optical bandpass filter 16 and a first photoelectric detector 17 in sequence; the dual-frequency probe light reflected by the fiber bragg grating 15 enters the oscilloscope 18 after passing through the second photoelectric detector 19, the low-noise microwave amplifier 20, the electric band-pass filter 21, the electric mixer 22 and the electric low-pass filter 24 in sequence.
Further, the coupling ratio of the optical coupler 2 is 50: 50.
Further, the phase modulator 4 is driven by a microwave signal generated by a microwave generator 5 and split by a microwave splitter 6.
Further, the acousto-optic modulator 10 is driven by a pulse signal generated by an arbitrary function generator 9.
Further, a microwave phase shifter 23 is arranged between the microwave beam splitter 6 and the electric mixer 22, and the microwave phase shifter 23 is used for adjusting the initial phase of the local oscillation electric signal, so that the DWI bias point works in the middle of the linear region of the cosine transform curve.
When in use, the continuous light output by the tunable laser 1 is divided into two paths after passing through the optical coupler 2; in the upper branch, the continuous light enters the phase modulator 4 after the polarization state of the light is adjusted by the polarization controller 3 to generate three-frequency detection light. The phase modulator 4 is generated by a microwave generator 5 and driven by a microwave signal split by a microwave splitter 6. The triple-frequency probe light is then injected into the sensing fiber 8 through the optical isolator 7. The three-frequency detection light enters the receiving end through the first optical circulator 13 after passing through the sensing optical fiber 8
In the lower branch, the continuous light passes through the acousto-optic modulator 10 to generate up-converted pulsed pump light. The acousto-optic modulator is driven by an electrical pulse signal generated by an arbitrary function generator 9. Then, the up-conversion pulse pump light is disturbed by a deflector 11 and amplified by an erbium-doped fiber amplifier 12, and then injected into the sensing fiber 8 through a first optical circulator 13.
At the receiving end, the three-frequency probe light enters the lower branch fiber grating 15 through the second optical circulator 14. Through the single-frequency Brillouin loss transmitted by the fiber grating 15, the three-frequency detection light enters the first photoelectric detector 17 after being further filtered by the optical band-pass filter 16 and is converted into an electric signal. The electrical signal finally enters an oscilloscope 18. The dual-frequency detection light reflected by the fiber grating 15 enters the second photodetector 19 of the upper branch circuit through the second optical circulator 14 and is converted into an electrical signal. Then, the electric signal is amplified by the low-noise microwave amplifier 20 and filtered by the electric band-pass filter 21, and then enters the electric mixer 22 to be mixed with the electric signal generated by the microwave generator 5, split by the microwave beam splitter 6 and phase-shifted by the microwave phase shifter 23, so as to generate the electric signal containing the baseband and the radio frequency. The radio frequency part is then filtered out by a low pass filter 24, leaving only the baseband signal. Finally, the baseband signal enters the oscilloscope 18.
In the invention, three-frequency detection light and up-conversion pulse pump light are transmitted oppositely to form a concurrent dual-wave interferometer and Brillouin optical time domain analysis. By combining the co-traveling dual-wave interferometer with high sampling rate and the Brillouin optical time domain analysis with low sampling rate and high positioning precision, the dynamic strain on each position of the optical fiber can be accurately measured and positioned. The principle analysis is as follows:
although the Brillouin optical time domain analysis distributed optical fiber sensor has the advantages of high positioning accuracy, high dynamic measurement range and the like, the dynamic sampling rate of the sensor is limited by various factors at the same time and can only be between dozens of Hz and hundreds of Hz generally. In order to improve the dynamic sampling rate of the optical fiber sensor, the invention provides a Brillouin optical time domain analysis distributed optical fiber sensor assisted by a co-traveling double-wave interferometer. First, the present invention proposes a novel common row Dual Wave Interferometer (DWI). The DWI adopts double-frequency light waves to simultaneously transmit in the optical fiber and sense the dynamic physical quantity suffered by the optical fiber. Due to the difference of the frequencies, the phase variation of the dual-frequency optical wave caused by the dynamic physical quantity is different. After passing through the sensing fiber, the dual-frequency light wave enters a photoelectric detector for beating. The phase change caused by dynamic strain in the microwave signal generated by beat frequency is:
in the formula (I), the compound is shown in the specification,
c、Δf、neff、ν、P11、P12、LDand epsilon respectively represent phase change caused by dynamic strain, light speed in vacuum, double-frequency probe light frequency difference, Poisson's ratio, Pockel coefficient 1, Pockel coefficient 2, dynamic length (length of optical fiber subjected to dynamic strain) and strain. As can be seen from equation (1), the phase change due to strain can be changed by changing the frequency difference Δ f between the two-frequency probe lights. Δ f is set to 10.98GHz (Brillouin frequency Shift attachment) in the present invention. Meanwhile, other coefficients in the formula (1) adopt common parameters of standard single-mode optical fibers. The phase change due to the dynamic strain can be obtained as follows:
since the maximum and linear measurement ranges of DWI are determined by the maximum available and linear amplitude ranges of the cosine transform curve, respectively. According to the phase-amplitude correspondence of the cosine transform curve, the phase variation ranges corresponding to the maximum usable and linear amplitude ranges are about 1.25rad and 2.82rad, respectively. When this 2.82rad is substituted into formula (2), the maximum measurement range of DWI is about 10000 microstrain at 1m dynamic length. The dynamic measurement range is improved by about 17555 times compared with a Mach-Zehnder interferometer under the same condition, and is similar to that of BOTDA. Meanwhile, the dynamic sampling rate of the DWI is determined only by the sampling rate and the average number of times of the acquisition equipment, so that the DWI can simultaneously realize high dynamic sampling rate and low data volume (high real-time performance). For example, when the sampling rate of the acquisition device is only 100kSa/s and the average number of times is 10, the dynamic sampling rate of the DWI can reach 10 kHz. In addition, because the polarization state of the dual-frequency probe light is always kept unchanged and the dual-frequency probe light has almost the same time delay when the dual-frequency probe light is transmitted in the optical fiber, the DWI is not influenced by polarization fading, laser phase noise and frequency drift. This allows the DWI to have a higher measurement stability. These advantages make DWI well suited for combination with BOTDA to achieve high speed dynamic strain measurements.
Subsequently, the invention provides a parallel double-wave interferometer-assisted Brillouin optical time domain analysis (DWI-BOTDA) distributed optical fiber sensor system. In this system, three-frequency probe light (each two frequencies having a frequency interval of about 10.98 GHz) is obtained by phase modulation. Meanwhile, the up-conversion pulse pump light is obtained through acousto-optic modulation. The three-frequency probe light and the up-conversion pulse pump light are transmitted in opposite directions in the optical fiber. 200MHz up-conversion introduced by acousto-optic modulation enables the pulse pumping light to generate stimulated Brillouin scattering only with one sideband of the three-frequency probe light, and Brillouin Optical Time Domain Analysis (BOTDA) is formed. While the other sideband and the optical carrier form a co-row Dual Wave Interferometer (DWI). Therefore, by combining the DWI with high sampling rate and the BOTDA with low sampling rate and high positioning precision, the time/frequency domain and the position information of the dynamic strain can be accurately measured.
A bias point adjusting method of a common-row double-wave interferometer (DWI) adjusts the initial phase of a local oscillation microwave signal through a microwave phase shifter between a microwave generator and an electric mixer. Therefore, the bias point (initial phase) of the baseband DWI signal after the frequency mixing filtering is adjusted in the middle of the linear slope of the cosine transform curve so as to achieve the optimal linear relation.
The effects of the present invention are illustrated by the following tests.
Fig. 2(a) is a three-dimensional graph showing the distribution of the Brillouin Optical Time Domain Analysis (BOTDA) loss spectrum along the optical fiber. The change of the Brillouin loss spectrum center frequency (Brillouin frequency domain) caused by the prestress at the tail end of the optical fiber is clearly visible (as shown by a dotted circle in FIG. 2 (a)), which shows that the BOTDA can accurately locate the external physical change of the optical fiber. Fig. 2(b) is a brillouin loss spectrum at the stress loading position. It can be seen from the figure that the measured brillouin loss spectrum matches well with the standard lorentz curve. The linear region range of the Brillouin loss spectrum slope obtained by linear fitting is about 20MHz, which shows that the measurement range of the slope auxiliary BOTDA is about 400 mu epsilon.
FIG. 3(a) is a three-dimensional graph showing the amplitude of a co-traveling Dual Wave Interferometer (DWI) baseband signal at different initial phases as a function of time. As can be seen from the figure, the output DWI baseband signal is very stable at each initial phase, which means that it is not affected by polarization fading, laser phase noise and frequency drift, and thus has very high measurement stability. Fig. 3(b) is a phase-intensity conversion curve of the DWI baseband signal, and it can be seen that the consistency between the measured value and the cosine curve is good. The maximum and linear measurement ranges correspond to 1.52rad and 2.82rad, respectively.
Subsequently, a dynamic strain was applied at the pre-stress of the fiber tail end to test the DWI-BOTDA performance. FIG. 4(a) shows the dynamic strain measured for BOTDA and DWI. Fig. 4(b) is an enlarged view of fig. 4(a) at 6 to 11.6 ms. The dynamic sampling rate of BOTDA is 200Hz, while the DWI dynamic sampling rate is 10 kHz. As can be seen from fig. 2 and 4, the DWI with high sampling rate can measure the dynamic strain more quickly and accurately, and the BOTDA with low sampling rate can accurately locate the dynamic strain position. Therefore, the DWI-BOTDA system provided by the invention can accurately measure and position the dynamic strain.
Finally, the dynamic strains measured by DWI and BOTDA were subjected to frequency domain analysis, as shown in fig. 5. As can be seen from the figure, 1) the dynamic strain signal frequency spectrum measured by DWI and BOTDA is basically the same; 2) the applied dynamic strain comprises a fundamental frequency of 29.75Hz and a harmonic frequency of 49.5 Hz; 3) DWI signals have a higher signal-to-noise ratio.
The measuring sampling rate of the sensor to the dynamic physical quantity is not limited by factors such as the length of the optical fiber and the like. The random adjustment of the dynamic sampling rate can be realized by increasing the sampling rate of the acquisition equipment. The DWI is not influenced by polarization fading, laser phase noise and frequency drift, so that the measurement stability is high. Due to the fact that the required sampling rate of the acquisition equipment is low, the DWI-BOTDA is low in data volume and good in potential real-time performance and long-term measurement performance.
Claims (5)
1. A BOTDA distributed optical fiber sensor assisted by a common-line double-wave interferometer is characterized in that the common-line double-wave interferometer comprises an upper branch for generating three-frequency probe light and a lower branch for generating up-conversion pulse pump light;
continuous light output by the tunable laser (1) is divided into an upper branch and a lower branch after passing through the optical coupler (2); continuous light of an upper branch generates three-frequency probe light through a polarization controller (3) and a phase modulator (4), then passes through an optical isolator (7), is injected into a sensing optical fiber (8), and then enters a receiving end through a first optical circulator (13); the continuous light of the lower branch path sequentially passes through an acoustic optical modulator (10) to generate up-conversion pulse pump light, then passes through a polarization scrambler (11) and an erbium-doped fiber amplifier (12) and then enters a sensing fiber (8) through a first optical circulator (13);
at a receiving end, the three-frequency probe light enters the fiber grating (15) through the second optical circulator (14); the single-frequency Brillouin loss detection light transmitted by the fiber grating (15) enters an oscilloscope (18) after sequentially passing through an optical bandpass filter (16) and a first photoelectric detector (17); the dual-frequency detection light reflected by the fiber bragg grating (15) enters the oscilloscope (18) after passing through the second photoelectric detector (19), the low-noise microwave amplifier (20), the electric band-pass filter (21), the electric mixer (22) and the electric low-pass filter (24) in sequence.
2. A co-traveling dual wave interferometer assisted BOTDA distributed fiber sensor according to claim 1, characterized in that the coupling ratio of the optical coupler (2) is 50: 50.
3. A co-traveling dual wave interferometer assisted BOTDA distributed fiber optic sensor according to claim 1, characterized in that the phase modulator (4) is driven by a microwave signal generated by a microwave generator (5) and split by a microwave splitter (6).
4. A co-traveling dual wave interferometer assisted BOTDA distributed fiber optic sensor according to claim 1, characterized in that the acousto-optic modulator (10) is driven by a pulse signal generated by an arbitrary function generator (9).
5. A co-traveling dual-wave interferometer assisted BOTDA distributed optical fiber sensor according to claim 1, characterized in that a microwave phase shifter (23) is further provided between the microwave beam splitter (6) and the electric mixer (22), and the microwave phase shifter (23) is used to adjust the initial phase of the local oscillator electric signal, so that the DWI bias point operates in the middle of the linear region of the cosine transform curve.
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