CN118362538A - Polyimide optical fiber-based salinity measuring device and method for optical frequency domain reflectometer - Google Patents
Polyimide optical fiber-based salinity measuring device and method for optical frequency domain reflectometer Download PDFInfo
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- 239000013307 optical fiber Substances 0.000 title claims abstract description 210
- 239000004642 Polyimide Substances 0.000 title claims abstract description 64
- 229920001721 polyimide Polymers 0.000 title claims abstract description 64
- 230000003287 optical effect Effects 0.000 title claims abstract description 44
- 238000000034 method Methods 0.000 title claims abstract description 23
- 238000005259 measurement Methods 0.000 claims abstract description 25
- 230000035559 beat frequency Effects 0.000 claims description 34
- 239000000835 fiber Substances 0.000 claims description 31
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 16
- 230000010287 polarization Effects 0.000 claims description 15
- 238000001228 spectrum Methods 0.000 claims description 14
- 239000012266 salt solution Substances 0.000 claims description 9
- 238000012952 Resampling Methods 0.000 claims description 6
- 239000011248 coating agent Substances 0.000 claims description 6
- 238000000576 coating method Methods 0.000 claims description 6
- 238000010438 heat treatment Methods 0.000 claims description 6
- 239000011259 mixed solution Substances 0.000 claims description 6
- 238000010521 absorption reaction Methods 0.000 claims description 5
- 238000005070 sampling Methods 0.000 claims description 5
- 239000000243 solution Substances 0.000 claims description 4
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 claims description 3
- -1 acrylic ester Chemical class 0.000 claims description 3
- 230000001680 brushing effect Effects 0.000 claims description 3
- 238000001035 drying Methods 0.000 claims description 3
- 238000006068 polycondensation reaction Methods 0.000 claims description 3
- 230000007797 corrosion Effects 0.000 abstract description 5
- 238000005260 corrosion Methods 0.000 abstract description 5
- 238000009826 distribution Methods 0.000 abstract description 3
- 239000000463 material Substances 0.000 description 3
- 238000001514 detection method Methods 0.000 description 2
- 238000002310 reflectometry Methods 0.000 description 2
- 239000000126 substance Substances 0.000 description 2
- 238000009360 aquaculture Methods 0.000 description 1
- 244000144974 aquaculture Species 0.000 description 1
- 230000008602 contraction Effects 0.000 description 1
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- 238000010586 diagram Methods 0.000 description 1
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- 238000009774 resonance method Methods 0.000 description 1
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Abstract
The invention discloses a polyimide optical fiber-based salinity measuring device and a polyimide optical fiber-based salinity measuring method for an optical frequency domain reflectometer. The optical frequency domain reflectometer has the advantages of strong electromagnetic interference resistance, good corrosion resistance and simple structure, and the method can monitor the change of the salinity value in real time and can realize the accurate measurement of the relative salinity distribution.
Description
Technical Field
The invention relates to the technical field of optical fiber sensing, in particular to an optical frequency domain reflectometer salinity measuring device and method based on polyimide optical fibers.
Background
With the rapid development of global industry and technology, salinity sensors are widely applied to the fields of marine fishery and aquaculture, natural environment monitoring and management, industrial production and manufacturing and the like. The current common salinity measurement method comprises a conductivity method, a refractive index method, a microwave remote sensing technology, a surface plasma resonance method and the like. However, these detection methods generally have the disadvantages of large detection equipment volume, complex operation, easy corrosion and poor long-term stability. With the rapid development of optical fiber sensing technology in recent years, there are many irreplaceable advantages over conventional electrical sensing technology. The optical fiber sensor has corrosion resistance, electromagnetic interference resistance during sensing measurement, long-distance and distributed measurement and the like. The optical fiber optical frequency domain reflection technology is a distributed optical fiber sensing technology based on Rayleigh scattering effect, can realize high-precision and distributed measurement of salinity, and provides wide application prospect for industrialization.
Disclosure of Invention
The invention aims at overcoming the defects of the prior art and provides a polyimide optical fiber-based salinity measuring device and a polyimide optical fiber-based salinity measuring method for an optical frequency domain reflectometer. The optical frequency domain reflectometer has the advantages of strong electromagnetic interference resistance, good corrosion resistance and simple structure, and the method can monitor the change of the salinity value in real time and can realize the accurate measurement of the relative salinity distribution.
The technical scheme for realizing the aim of the invention is as follows:
an optical frequency domain reflectometer salinity measuring device based on polyimide optical fiber comprises a narrow linewidth laser and a first optical fiber coupler which are connected with each other, wherein:
The output end of the first optical fiber coupler is divided into two paths, wherein one path of output end is connected with the second optical fiber coupler, the output end of the second optical fiber coupler is divided into two paths again, and one path of output end of the second optical fiber coupler is sequentially connected with the polarization controller and the fourth optical fiber coupler; the other output end of the second optical fiber coupler is connected with an a port of the circulator, a b port of the circulator is connected with a polyimide optical fiber, a c port of the circulator is connected with a fourth optical fiber coupler, and the output end of the fourth optical fiber coupler is sequentially connected with a first balance photoelectric detector and a high-speed oscilloscope; the other output end of the first optical fiber coupler is connected with a third optical fiber coupler, the output end of the third optical fiber coupler is divided into two paths again, one output end of the third optical fiber coupler is connected with a fifth optical fiber coupler by adopting a single-mode optical fiber, the other output end of the third optical fiber coupler is sequentially connected with a delay optical fiber and the fifth optical fiber coupler, and the output end of the fifth optical fiber coupler is sequentially connected with a second balanced photoelectric detector and a high-speed oscilloscope;
the circulator and the polarization controller form a main path interferometer unit, the delay optical fiber and the single-mode optical fiber form an auxiliary path interferometer unit, the polyimide optical fiber forms a polyimide optical fiber unit, and the first balanced photoelectric detector and the second balanced photoelectric detector form a balanced photoelectric detector unit;
The polyimide optical fiber is prepared by removing an acrylic ester layer of a common single-mode optical fiber by using a wire stripper, immersing the optical fiber in alcohol, wiping the optical fiber cleanly by using air flow paper, immersing the optical fiber in polyimide solution, placing the optical fiber treated by adopting a pulling method and a brushing method in a heating furnace at 110 ℃ for drying for thirty minutes, raising the temperature to 180 ℃ at a rate of 1 ℃ per minute, and finally heating for thirty minutes at 180 ℃ to obtain the polyimide optical fiber with the coating thickness of 50 um;
the connection is realized by adopting FC/PC optical connectors.
The model of the narrow linewidth laser is TSL-550, the wavelength scanning range is 1540nm-1560nm, and the scanning speed is 20nm/s.
The first photoelectric detector and the second photoelectric detector are of the type PDB430C.
The model of the high-speed oscilloscope is MSOS254A, and the sampling rate is 10MHz/S.
The split ratio of the first optical fiber coupler is 90%:10%, 90% of one light in the first optical fiber coupler enters the second optical fiber coupler, and 10% of one light enters the third optical fiber coupler.
The split ratio of the second optical fiber coupler is 99%:1%, 99% of one light in the second optical fiber coupler enters an a port of the circulator, and 1% of one light enters the polarization controller.
The split ratio of the third optical fiber coupler is 50%:50%, the split ratio of the fourth fiber coupler is 50%:50%; the splitting ratio of the fifth optical fiber coupler is 50%:50%.
The polyimide optical fiber has a coating thickness of 50um and a length of 20m.
The delay optical fiber 5 is a common single-mode optical fiber and has the length of 100m.
The light propagation mode in the technical scheme is as follows: the light source of the narrow linewidth laser enters the first optical fiber coupler, the output light of the first optical fiber coupler is divided into two paths, one path of output light of the first optical fiber coupler is divided into two paths again through the second optical fiber coupler, one path of output light of the second optical fiber coupler is used as reference light to reach the fourth optical fiber coupler through the polarization controller, the other path of output light of the second optical fiber coupler enters the port a of the circulator and is output from the port b and enters the polyimide optical fiber, and the generated Rayleigh backward scattered light is output from the port c of the circulator as signal light to reach the fourth optical fiber coupler, interferes with the reference light to generate beat signals, is converted into electric signals through the first balance photoelectric detector and then is acquired by the high-speed oscilloscope; the other path of output light of the first optical fiber coupler is divided into two paths again through a third optical fiber coupler, one path of output light of the third optical fiber coupler directly reaches the fifth optical fiber coupler, interference is generated after the other path of output light of the third optical fiber coupler passes through a section of delay optical fiber, a beat frequency signal with the frequency of 1.5MHz is generated, the beat frequency signal is converted into an electric signal through a second balanced photoelectric detector, and data is collected by a high-speed oscilloscope and used as an external sampling clock signal.
The optical frequency domain reflectometer salinity measuring method based on the polyimide optical fiber adopts the optical frequency domain reflectometer salinity measuring device based on the polyimide optical fiber, and the method comprises the following steps:
1) The sweep frequency light emitted by the narrow linewidth laser 1 enters the first optical fiber coupler to be divided into two paths of light, one path of light enters the main path interferometer unit, the other path of light enters the auxiliary path interferometer unit, the light entering the main path interferometer unit is divided into two paths of light by the second optical fiber coupler, one path of light entering the circulator is signal light, and the other path of light entering the polarization controller is reference light;
2) In a main path interferometer, after a polyimide optical fiber is immersed in pure water for two hours, the stress change of the polyimide layer caused by water absorption expansion is transmitted to the optical fiber, a Rayleigh scattering signal generated when light waves propagate in the optical fiber is changed, the generated signal returns along a path, enters a fourth optical fiber coupler through a c port of the circulator and is subjected to beat frequency interference with reference light of the main path interferometer at the fourth optical fiber coupler to generate a beat frequency interference signal, and the beat frequency interference signal is converted into an electric signal through a first balance photoelectric detector and is collected by a high-speed oscilloscope;
3) In the auxiliary interferometer, light of the delay optical fiber and the single-mode optical fiber generates interference signals in a fifth optical fiber coupler, the interference signals are converted into electric signals through a second balanced photoelectric detector, the electric signals are collected by a high-speed oscilloscope, the instantaneous frequency of a laser light source is calculated by adopting an arc tangent and phase unfolding algorithm according to the frequency of the interference signals generated by the auxiliary interferometer, interpolation algorithm and resampling are applied to beat frequency interference signals of the main interferometer, and the beat frequency signals with uniform optical frequency intervals are assumed to be reference signals;
4) Then adding salt solution into pure water to enable the salt solution concentration of the mixed solution to be 0.3mol/L, placing a salinity meter in the mixed solution, correcting a sensor according to the salinity acquired by the salinity meter, wherein the salinity is increased, a polyimide layer is subjected to polycondensation, standing for twenty minutes under the concentration of 0.3mol/L, collecting interference beat frequency signals generated by a main interferometer and an auxiliary interferometer by using an oscilloscope at the moment after the polyimide is dehydrated, calculating the instantaneous frequency of a laser light source according to the frequency of the interference signals generated by the auxiliary interferometer by using an arctangent and a phase expansion algorithm, and applying an interpolation algorithm and resampling to the beat frequency interference signals of the main interferometer to obtain beat frequency signals of uniform optical frequency intervals to be assumed as measurement state signals;
5) Respectively performing fast Fourier transform on a reference state signal and a measurement state signal to obtain a distance domain signal, fixedly sliding the distance domain signal by using a window size of 200 points, dividing the distance domain into a plurality of window signals, and performing inverse Fourier transform on the signals of each sliding window to obtain Rayleigh scattering spectrums of the reference state signal and the measurement state signal;
6) Performing cross-correlation operation on the Rayleigh scattering spectrum of each corresponding window reference state signal and the corresponding measured state signal to obtain cross-correlation peak offset of each position, combining the strain frequency shift coefficient to obtain a final position-strain curve graph, and then reversely pushing the strain of each section of the sensing optical fiber obtained by positioning to obtain salinity so as to realize the measurement of the salinity;
7) And then standing for twenty minutes at each concentration interval of 0.5mol/L, collecting interference beat frequency signals generated by a main path interferometer and an auxiliary path interferometer by using an oscilloscope, repeating the operation of the step 4) to obtain uniform optical frequency interval signals which are all measurement state signals, and then sequentially repeating the steps 5) -6) respectively from a reference state signal collected in pure water and the measurement state signals collected under the concentration of other salt solutions;
8) The sensor is calibrated in connection with salinity acquired by a standard commercial salinity meter.
The intensity of the Rayleigh scattered light is inversely proportional to the fourth power of the wavelength of the incident light, and can be expressed as shown in equation (1):
Wherein I 0 is the intensity of the incident light; lambda is the wavelength of the incident light; θ is a kind of scattering angle and,
The Rayleigh scattering coefficient is expressed as shown in formula (2):
wherein n represents the effective optical fiber refractive index; k is Boltzmann constant; t represents the temperature; p represents the elasto-optical coefficient of the substance; beta T represents isothermal compressibility at temperature T, and as can be seen from equation (2), when temperature or strain changes, the rayleigh scattering coefficient changes, and the rayleigh scattering spectrum shifts; the Rayleigh scattering spectrum when being subjected to temperature or strain is subjected to cross-correlation with the initial Rayleigh scattering spectrum when no external disturbance exists, the offset of the frequency spectrum is obtained, the offset corresponds to the change amount of the optical fiber when being subjected to temperature or strain,
Assuming that the optical fields of the signal light E s (t) and the reference light E r (t) satisfying the main interferometer coherence condition are shown in the formula (3) and the formula (4), respectively:
Where f 0 is the initial frequency of the tunable laser, Representing the random phase of the reference light,Representing the reflectivity at the time delay location,Representing the random phase of the back rayleigh scattering signal at time t,
According to square law characteristics of the photo-detector, the photo-current output by the photo-detector is assumed to be I (t), and considering that the photo-detector does not respond to a part higher than the cut-off frequency and a direct current term part is filtered, the first balanced photo-detector output signal I (t) is shown in a formula (5):
Wherein, The term denotes the phase difference of the two light beams at time t, which varies nonlinearly with time, and f b denotes the beat frequency of the two light beams, which varies linearly with time and is a function of the position of the scattering point on the sensing fiber.
According to the technical scheme, distributed relative salinity measurement is realized based on polyimide sensitive materials and optical frequency domain reflection technology, polyimide materials are coated on a single-mode fiber, the change of salinity is converted into stress change on the fiber by utilizing the characteristics of water absorption expansion and water loss contraction of polyimide, parameters which characterize the optical wave characteristics of the single-mode fiber are changed by the changes, fourier transformation and cross-correlation operation are carried out on the signal parameters to obtain fiber strain change information, and then solution salinity value is reversely measured.
The optical frequency domain reflectometer has the advantages of strong electromagnetic interference resistance, good corrosion resistance and simple structure, and the method can monitor the change of the salinity value in real time and can realize the accurate measurement of the relative salinity distribution.
Drawings
Fig. 1 is a schematic structural diagram of an embodiment.
In the figure, a narrow linewidth laser 2, a first optical fiber coupler 3, a third optical fiber coupler 4, a second optical fiber coupler 5, a delay optical fiber 6, an optical fiber circulator 7, a polarization controller 8, a single-mode optical fiber 9, a polyimide optical fiber 10, a fourth optical fiber coupler 11, a fifth optical fiber coupler 12, a second photoelectric detector 13, a first photoelectric detector 14 and a high-speed oscilloscope.
Detailed Description
The present invention will now be further illustrated, but not limited, by the following figures and examples.
Examples:
Referring to fig. 1, an optical frequency domain reflectometer salinity measuring apparatus based on polyimide optical fiber comprises a narrow linewidth laser 1 and a first optical fiber coupler 2 interconnected, wherein:
The output end of the first optical fiber coupler 2 is divided into two paths, wherein one path of output end is connected with the second optical fiber coupler 4, the output end of the second optical fiber coupler 4 is divided into two paths again, and one path of output end of the second optical fiber coupler 4 is sequentially connected with the polarization controller 7 and the fourth optical fiber coupler 10; the other output end of the second optical fiber coupler 4 is connected with an a port of the circulator 6, a b port of the circulator is connected with a polyimide optical fiber 9, a c port of the circulator is connected with a fourth optical fiber coupler 10, and the output end of the fourth optical fiber coupler 10 is sequentially connected with a first balance photoelectric detector 13 and a high-speed oscilloscope 14; the other output end of the first optical fiber coupler 2 is connected with a third optical fiber coupler 3, the output end of the third optical fiber coupler 3 is divided into two paths again, one output end of the third optical fiber coupler 3 is connected with a fifth optical fiber coupler 11 by adopting a single-mode optical fiber 8, the other output end of the third optical fiber coupler 3 is sequentially connected with a delay optical fiber 5 and the fifth optical fiber coupler 11, and the output end of the fifth optical fiber coupler 11 is sequentially connected with a second balanced photoelectric detector 12 and a high-speed oscilloscope 14;
In the embodiment, the circulator 6 and the polarization controller 7 form a main path interferometer unit, the delay optical fiber 5 and the single-mode optical fiber 8 form an auxiliary path interferometer unit, the polyimide optical fiber 9 forms a polyimide optical fiber unit, and the first balance photoelectric detector 13 and the second balance photoelectric detector 12 form a balance photoelectric detector unit;
In the example, after an acrylic ester layer is removed by using a wire stripper for a common single-mode fiber, immersing the optical fiber into alcohol, wiping the optical fiber cleanly by using air flow paper, finally immersing the optical fiber into polyimide solution, placing the optical fiber treated by adopting a pulling method and a brushing method in a heating furnace at 110 ℃ for drying for thirty minutes, then raising the temperature to 180 ℃ at a rate of 1 ℃ per minute, and finally heating for thirty minutes at 180 ℃ to obtain the polyimide optical fiber with the coating thickness of 50 um;
in this example, FC/PC optical connectors are used for connection.
The model of the narrow linewidth laser in the example is TSL-550, the wavelength scanning range is 1540nm-1560nm, and the scanning speed is 20nm/s.
The first photodetector 13 and the second photodetector 12 in this example are both PDB430C.
The high-speed oscilloscope 14 in this example is model MSOS254A with a sampling rate of 10MHz/S.
The splitting ratio of the first fiber coupler 2 in this example is 90%:10%, 90% of one light in the first optical fiber coupler 2 enters the second optical fiber coupler 4, and 10% of one light enters the third optical fiber coupler 3.
The split ratio of the second fiber coupler 4 in this example is 99%:1%, 99% of one light in the second optical fiber coupler 4 enters the a-port of the circulator 6, and 1% of one light enters the polarization controller 7.
The splitting ratio of the third fiber coupler 3 in this example is 50%:50%, the split ratio of the fourth fiber coupler 10 is 50%:50%; the splitting ratio of the fifth optical fiber coupler 11 is 50%:50%.
The polyimide optical fiber in this example has a coating thickness of 50um and a length of 20m.
The delay fiber 5 in this example is a common single mode fiber with a length of 100m.
The light propagation in this example is as follows: the light source of the narrow linewidth laser 1 enters the first optical fiber coupler 2, the output light of the first optical fiber coupler 2 is divided into two paths, one path of output light of the first optical fiber coupler 2 is divided into two paths again through the second optical fiber coupler 4, one path of output light of the second optical fiber coupler 4 is used as reference light to reach the fourth optical fiber coupler 10 through the polarization controller 7, the other path of output light of the second optical fiber coupler 4 enters from the port a of the circulator 6 and is output from the port b and enters into the polyimide optical fiber, so that the generated Rayleigh back scattering light is output from the port c of the circulator 6 as signal light to reach the fourth optical fiber coupler 10, interference is generated with the reference light, beat frequency signals are generated, the beat frequency signals are converted into electric signals through the first balance photoelectric detector 13, and then the data are collected by the high-speed oscilloscope 14; the other path of output light of the first optical fiber coupler 2 is divided into two paths again through the third optical fiber coupler 3, one path of output light of the third optical fiber coupler 3 directly reaches the fifth optical fiber coupler 11, interference occurs after the other path of output light of the third optical fiber coupler 3 passes through the one-section delay optical fiber 5, beat frequency signals with the frequency of 1.5MHz are generated, the beat frequency signals are converted into electric signals through the second balance photoelectric detector 12, and data are collected through the high-speed oscilloscope 14 and used as external sampling clock signals.
The optical frequency domain reflectometer salinity measuring method based on the polyimide optical fiber adopts the optical frequency domain reflectometer salinity measuring device based on the polyimide optical fiber, and the method comprises the following steps:
1) The sweep frequency light emitted by the narrow linewidth laser 1 enters the first optical fiber coupler 2 to be divided into two paths of light, one path of light enters the main path interferometer unit, the other path of light enters the auxiliary path interferometer unit, the light entering the main path interferometer unit is divided into two paths of light by the second optical fiber coupler 4, one path of light entering the circulator 6 is signal light, and the other path of light entering the polarization controller 7 is reference light;
2) In the main path interferometer, after the polyimide optical fiber 9 is immersed in pure water for two hours, the stress change of the polyimide layer in water absorption expansion is transmitted to the optical fiber, the Rayleigh scattering signal generated when light waves propagate in the optical fiber is changed, the generated signal returns along the path, enters the fourth optical fiber coupler 10 through the c port of the circulator 6, and beat frequency interference is generated at the fourth optical fiber coupler 10 with reference light of the main path interferometer, so that beat frequency interference signals are generated, converted into electric signals through the first balance photoelectric detector 13 and collected by the high-speed oscilloscope 14;
3) In the auxiliary interferometer, the light of the delay optical fiber 5 and the single-mode optical fiber 8 generates interference signals in a fifth optical fiber coupler 11, the interference signals are converted into electric signals through a second balanced photoelectric detector 12 and are collected by a high-speed oscilloscope 14, the instantaneous frequency of a laser light source is calculated by adopting an arctangent and phase unfolding algorithm according to the frequency of the interference signals generated by the auxiliary interferometer, an interpolation algorithm and resampling are applied to beat frequency interference signals of the main interferometer, and the beat frequency signals with uniform optical frequency intervals are assumed to be reference state signals;
4) Then adding salt solution into pure water to enable the salt solution concentration of the mixed solution to be 0.3mol/L, placing a salinity meter in the mixed solution, correcting a sensor according to the salinity acquired by the salinity meter, wherein the salinity is increased, a polyimide layer is subjected to polycondensation, standing for twenty minutes under the concentration of 0.3mol/L, collecting interference beat frequency signals generated by a main interferometer and an auxiliary interferometer by using an oscilloscope 14 at the moment after polyimide is dehydrated, calculating the instantaneous frequency of a laser light source according to the frequency of the interference signals generated by the auxiliary interferometer through an arctangent and a phase expansion algorithm, and applying an interpolation algorithm and resampling to the beat frequency interference signals of the main interferometer to obtain beat frequency signals with uniform optical frequency intervals to be assumed as measurement state signals;
5) Respectively performing fast Fourier transform on a reference state signal and a measurement state signal to obtain a distance domain signal, fixedly sliding the distance domain signal by using a window size of 200 points, dividing the distance domain into a plurality of window signals, and performing inverse Fourier transform on the signals of each sliding window to obtain Rayleigh scattering spectrums of the reference state signal and the measurement state signal;
6) Performing cross-correlation operation on the Rayleigh scattering spectrum of each corresponding window reference state signal and the corresponding measured state signal to obtain cross-correlation peak offset of each position, combining the strain frequency shift coefficient to obtain a final position-strain curve graph, and then reversely pushing the strain of each section of the sensing optical fiber obtained by positioning to obtain salinity so as to realize the measurement of the salinity;
7) Then, standing for twenty minutes at each concentration interval of 0.5mol/L, collecting interference beat frequency signals generated by a main path interferometer and an auxiliary path interferometer by using an oscilloscope 14, repeating the operation of the step 4) to obtain uniform optical frequency interval signals which are all measurement state signals, and then sequentially repeating the steps 5) -6) respectively from a reference state signal collected in pure water and the measurement state signals collected under the concentration of other salt solutions;
8) The sensor is calibrated in connection with salinity acquired by a standard commercial salinity meter.
The intensity of the Rayleigh scattered light is inversely proportional to the fourth power of the wavelength of the incident light, and can be expressed as shown in equation (1):
Wherein I 0 is the intensity of the incident light; lambda is the wavelength of the incident light; θ is a kind of scattering angle and,
The Rayleigh scattering coefficient is expressed as shown in formula (2):
wherein n represents the effective optical fiber refractive index; k is Boltzmann constant; t represents the temperature; p represents the elasto-optical coefficient of the substance; beta T represents isothermal compressibility at temperature T, and as can be seen from equation (2), when temperature or strain changes, the rayleigh scattering coefficient changes, and the rayleigh scattering spectrum shifts; the Rayleigh scattering spectrum when being subjected to temperature or strain is subjected to cross-correlation with the initial Rayleigh scattering spectrum when no external disturbance exists, the offset of the frequency spectrum is obtained, the offset corresponds to the change amount of the optical fiber when being subjected to temperature or strain,
Assuming that the optical fields of the signal light E s (t) and the reference light E r (t) satisfying the main interferometer coherence condition are shown in the formula (3) and the formula (4), respectively:
Where f 0 is the initial frequency of the tunable laser, Representing the random phase of the reference light,Representing the reflectivity at the time delay location,Representing the random phase of the back rayleigh scattering signal at time t,
According to square law characteristics of the photo-detector, the photo-current output by the photo-detector is assumed to be I (t), and considering that the photo-detector does not respond to a part higher than the cut-off frequency and a direct current term part is filtered, the first balanced photo-detector output signal I (t) is shown in a formula (5):
Wherein, The term denotes the phase difference of the two light beams at time t, which varies nonlinearly with time, and f b denotes the beat frequency of the two light beams, which varies linearly with time and is a function of the position of the scattering point on the sensing fiber.
The optical frequency shift reflectometer is combined with the sensitive material polyimide, stress changes of polyimide during water absorption expansion and water loss shrinkage are transmitted to the optical fiber, the strain is obtained by demodulating the change condition of an optical signal of the optical fiber, and then a corresponding salinity value is obtained by reverse thrust.
Claims (10)
1. The utility model provides an optical frequency domain reflectometer salinity measuring device based on polyimide optic fibre which characterized in that, including interconnected narrow linewidth laser instrument and first fiber coupler, wherein:
The output end of the first optical fiber coupler is divided into two paths, wherein one path of output end is connected with the second optical fiber coupler, the output end of the second optical fiber coupler is divided into two paths again, and one path of output end of the second optical fiber coupler is sequentially connected with the polarization controller and the fourth optical fiber coupler; the other output end of the second optical fiber coupler is connected with an a port of the circulator, a b port of the circulator is connected with a polyimide optical fiber, a c port of the circulator is connected with a fourth optical fiber coupler, and the output end of the fourth optical fiber coupler is sequentially connected with a first balance photoelectric detector and a high-speed oscilloscope; the other output end of the first optical fiber coupler is connected with a third optical fiber coupler, the output end of the third optical fiber coupler is divided into two paths again, one output end of the third optical fiber coupler is connected with a fifth optical fiber coupler by adopting a single-mode optical fiber, the other output end of the third optical fiber coupler is sequentially connected with a delay optical fiber and the fifth optical fiber coupler, and the output end of the fifth optical fiber coupler is sequentially connected with a second balanced photoelectric detector and a high-speed oscilloscope;
the circulator and the polarization controller form a main path interferometer unit, the delay optical fiber and the single-mode optical fiber form an auxiliary path interferometer unit, the polyimide optical fiber forms a polyimide optical fiber unit, and the first balanced photoelectric detector and the second balanced photoelectric detector form a balanced photoelectric detector unit;
The polyimide optical fiber is prepared by removing an acrylic ester layer of a common single-mode optical fiber by using a wire stripper, immersing the optical fiber in alcohol, wiping the optical fiber cleanly by using air flow paper, immersing the optical fiber in polyimide solution, placing the optical fiber treated by adopting a pulling method and a brushing method in a heating furnace at 110 ℃ for drying for thirty minutes, raising the temperature to 180 ℃ at a rate of 1 ℃ per minute, and finally heating for thirty minutes at 180 ℃ to obtain the polyimide optical fiber with the coating thickness of 50 um;
the connection is realized by adopting FC/PC optical connectors.
2. The polyimide fiber-based optical frequency domain reflectometer salinity measuring device according to claim 1, wherein the narrow linewidth laser is of a model TSL-550, has a wavelength scanning range of 1540nm-1560nm and has a scanning speed of 20nm/s.
3. The polyimide fiber-based optical frequency domain reflectometer salinity measuring device of claim 1, wherein the first and second photodetectors are each PDB430C.
4. The polyimide fiber-based optical frequency domain reflectometer salinity measuring device according to claim 1, wherein the high-speed oscilloscope is model number MSOS254A and has a sampling rate of 10MHz/S.
5. The polyimide fiber-based optical frequency domain reflectometer salinity measuring device of claim 1, wherein the first fiber coupler has a split ratio of 90%:10%, 90% of one light in the first optical fiber coupler enters the second optical fiber coupler, and 10% of one light enters the third optical fiber coupler.
6. The polyimide fiber-based optical frequency domain reflectometer salinity measuring device of claim 1, wherein the second fiber coupler has a split ratio of 99%:1%, 99% of one light in the second optical fiber coupler enters an a port of the circulator, and 1% of one light enters the polarization controller.
7. The polyimide fiber-based optical frequency domain reflectometer salinity measuring device of claim 1, wherein the third fiber coupler has a split ratio of 50%:50%, the split ratio of the fourth fiber coupler is 50%:50%; the splitting ratio of the fifth optical fiber coupler is 50%:50%.
8. The polyimide fiber-based optical frequency domain reflectometer salinity measuring device of claim 1, wherein the polyimide fiber has a coating thickness of 50um and a length of 20m.
9. The polyimide fiber-based optical frequency domain reflectometer salinity measuring device according to claim 1, wherein the delay fiber is a common single-mode fiber with a length of 100m.
10. A polyimide optical fiber-based optical frequency domain reflectometer salinity measuring method, which adopts the polyimide optical fiber-based optical frequency domain reflectometer salinity measuring device according to any one of claims 1-9, and is characterized in that the method comprises the following steps:
1) The method comprises the steps that sweep frequency light emitted by a narrow linewidth laser enters a first optical fiber coupler to be divided into two paths of light, one path of light enters a main path interferometer unit, the other path of light enters an auxiliary path interferometer unit, the light entering the main path interferometer unit is divided into two paths of light by a second optical fiber coupler, one path of light entering an circulator is signal light, and the other path of light entering a polarization controller is reference light;
2) In a main path interferometer, after a polyimide optical fiber is immersed in pure water for two hours, the stress change of the polyimide layer caused by water absorption expansion is transmitted to the optical fiber, a Rayleigh scattering signal generated when light waves propagate in the optical fiber is changed, the generated signal returns along a path, enters a fourth optical fiber coupler through a c port of the circulator and is subjected to beat frequency interference with reference light of the main path interferometer at the fourth optical fiber coupler to generate a beat frequency interference signal, and the beat frequency interference signal is converted into an electric signal through a first balance photoelectric detector and is collected by a high-speed oscilloscope;
3) In the auxiliary interferometer, light of the delay optical fiber and the single-mode optical fiber generates interference signals in a fifth optical fiber coupler, the interference signals are converted into electric signals through a second balanced photoelectric detector, the electric signals are collected by a high-speed oscilloscope, the instantaneous frequency of a laser light source is calculated by adopting an arc tangent and phase unfolding algorithm according to the frequency of the interference signals generated by the auxiliary interferometer, interpolation algorithm and resampling are applied to beat frequency interference signals of the main interferometer, and the beat frequency signals with uniform optical frequency intervals are assumed to be reference signals;
4) Then adding salt solution into pure water to enable the salt solution concentration of the mixed solution to be 0.3mol/L, placing a salinity meter in the mixed solution, correcting a sensor according to the salinity acquired by the salinity meter, wherein the salinity is increased, a polyimide layer is subjected to polycondensation, standing for twenty minutes under the concentration of 0.3mol/L, collecting interference beat frequency signals generated by a main interferometer and an auxiliary interferometer by using an oscilloscope at the moment after the polyimide is dehydrated, calculating the instantaneous frequency of a laser light source according to the frequency of the interference signals generated by the auxiliary interferometer by using an arctangent and a phase expansion algorithm, and applying an interpolation algorithm and resampling to the beat frequency interference signals of the main interferometer to obtain beat frequency signals of uniform optical frequency intervals to be assumed as measurement state signals;
5) Respectively performing fast Fourier transform on a reference state signal and a measurement state signal to obtain a distance domain signal, fixedly sliding the distance domain signal by using a window size of 200 points, dividing the distance domain into a plurality of window signals, and performing inverse Fourier transform on the signals of each sliding window to obtain Rayleigh scattering spectrums of the reference state signal and the measurement state signal;
6) Performing cross-correlation operation on the Rayleigh scattering spectrum of each corresponding window reference state signal and the corresponding measured state signal to obtain cross-correlation peak offset of each position, combining the strain frequency shift coefficient to obtain a final position-strain curve graph, and then reversely pushing the strain of each section of the sensing optical fiber obtained by positioning to obtain salinity so as to realize the measurement of the salinity;
7) And then standing for twenty minutes at each concentration interval of 0.5mol/L, collecting interference beat frequency signals generated by a main path interferometer and an auxiliary path interferometer by using an oscilloscope, repeating the operation of the step 4) to obtain uniform optical frequency interval signals which are all measurement state signals, and then sequentially repeating the steps 5) -6) respectively from a reference state signal collected in pure water and the measurement state signals collected under the concentration of other salt solutions;
8) The sensor is calibrated in connection with salinity acquired by a standard commercial salinity meter.
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