CN118362538A - Optical frequency domain reflectometer salinity measurement device and method based on polyimide optical fiber - Google Patents

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

Optical frequency domain reflectometer salinity measurement device and method based on polyimide optical fiber

Technical Field

The invention relates to the technical field of optical fiber sensing, and in particular to a salinity measuring device and method of an optical frequency domain reflectometer based on polyimide optical fiber.

Background technique

With the rapid development of global industry and technology, salinity sensors are widely used in marine fisheries and aquaculture, natural environment monitoring and management, industrial production and manufacturing, and other fields. At present, common salinity measurement methods include conductivity method, refractive index method, microwave remote sensing technology, and surface plasmon resonance method. However, these detection methods generally have the disadvantages of large detection equipment, complex operation, easy corrosion, and poor long-term stability. In recent years, with the rapid development of fiber optic sensing technology, it has irreplaceable advantages in many aspects compared with traditional electrical sensing technology. Fiber optic sensors are corrosion-resistant, can resist electromagnetic interference during sensing and measurement, and can perform long-distance and distributed measurements. Fiber optic frequency domain reflectometry technology is a distributed fiber optic sensing technology based on the Rayleigh scattering effect. This technology can achieve high-precision and distributed measurement of salinity, providing broad application prospects for industrialization.

Summary of the invention

The purpose of the present invention is to provide a device and method for measuring salinity using an optical frequency domain reflectometer based on polyimide optical fiber in view of the shortcomings of the prior art. The optical frequency domain reflectometer has strong anti-electromagnetic interference capability, good corrosion resistance, and a simple structure. The method can monitor the change of salinity value in real time and can achieve accurate measurement of relative salinity distribution.

The technical solution for achieving the purpose of the present invention is:

An optical frequency domain reflectometer salinity measurement device based on polyimide optical fiber, comprising an interconnected narrow linewidth laser and a first optical fiber coupler, wherein:

The output end of the first optical fiber coupler is divided into two paths, one of which is connected to the second optical fiber coupler, and the output end of the second optical fiber coupler is divided into two paths again, and one output end of the second optical fiber coupler is connected to the polarization controller and the fourth optical fiber coupler in sequence; the other output end of the second optical fiber coupler is connected to the a port of the circulator, the b port of the circulator is connected to the polyimide optical fiber, and the c port of the circulator is connected to the fourth optical fiber coupler, and the output end of the fourth optical fiber coupler is connected to the first balanced photodetector and the high-speed oscilloscope in sequence; the other output end of the first optical fiber coupler is connected to the third optical fiber coupler, and 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 to the fifth optical fiber coupler using a single-mode optical fiber, and the other output end of the third optical fiber coupler is connected to the delay optical fiber and the fifth optical fiber coupler in sequence, and the output end of the fifth optical fiber coupler is connected to the second balanced photodetector and the high-speed oscilloscope in sequence;

The circulator and the polarization controller constitute a main interferometer unit, the delay optical fiber and the single-mode optical fiber constitute an auxiliary interferometer unit, the polyimide optical fiber constitutes a polyimide optical fiber unit, and the first balanced photodetector and the second balanced photodetector constitute a balanced photodetector unit;

The polyimide optical fiber is a common single-mode optical fiber. After removing the acrylate layer with a wire stripper, the optical fiber is immersed in alcohol and wiped clean with air flow paper. Finally, the optical fiber is immersed in a polyimide solution. The optical fiber treated by the pulling method and the brushing method is placed in a heating furnace at 110°C for 30 minutes, and then the temperature is increased to 180°C at a rate of 1°C per minute. Finally, it is heated at 180°C for 30 minutes to obtain a polyimide optical fiber with a coating thickness of 50um.

The above connections are all made using 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 photodetector and the second photodetector are both of model PDB430C.

The model of the high-speed oscilloscope is MSOS254A, and the sampling rate is 10 MHz/S.

The splitting ratio of the first optical fiber coupler is 90%:10%, 90% of the light in the first optical fiber coupler enters the second optical fiber coupler, and 10% of the light enters the third optical fiber coupler.

The splitting ratio of the second optical fiber coupler is 99%:1%, and 99% of the light in the second optical fiber coupler enters the a port of the circulator, and 1% of the light enters the polarization controller.

The splitting ratio of the third optical fiber coupler is 50%:50%, the splitting ratio of the fourth optical fiber coupler is 50%:50%, and the splitting ratio of the fifth optical fiber coupler is 50%:50%.

The polyimide optical fiber has a coating thickness of 50 um and a length of 20 m.

The delay optical fiber 5 is a common single-mode optical fiber with a 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 fiber coupler, the output light of the first fiber coupler is divided into two paths, one of the output lights of the first fiber coupler is divided into two paths again through the second fiber coupler, one of the output lights of the second fiber coupler is used as a reference light and passes through the polarization controller to reach the fourth fiber coupler, the other output light of the second fiber coupler enters from the a port of the circulator, outputs from the b port, enters the polyimide fiber, and the Rayleigh backscattered light generated is output from the c port of the circulator as a signal light and reaches the fourth fiber coupler, interferes with the reference light, generates a beat signal, and is converted into an electrical signal by the first balanced photodetector and then collected by a high-speed oscilloscope; the other output light of the first fiber coupler is divided into two paths again through the third fiber coupler, one of the output lights of the third fiber coupler directly reaches the fifth fiber coupler, interferes with the other output light of the third fiber coupler after passing through a delay fiber, generates a beat signal with a frequency of 1.5 MHz, and is converted into an electrical signal by the second balanced photodetector and then collected by a high-speed oscilloscope as an external sampling clock signal.

A method for measuring salinity using an optical frequency domain reflectometer based on polyimide optical fiber, using the above-mentioned optical frequency domain reflectometer salinity measuring device based on polyimide optical fiber, the method comprising the following steps:

1) The frequency-sweeping light emitted by the narrow linewidth laser 1 enters the first fiber coupler and is divided into two paths of light, one path of light enters the main interferometer unit, and the other path enters the auxiliary interferometer unit. The light entering the main interferometer unit is divided into two paths of light by the second fiber coupler, one path entering the circulator is the signal light, and the other path entering the polarization controller is the reference light;

2) In the main interferometer, after the polyimide optical fiber is immersed in pure water for two hours, the stress change of the polyimide layer due to water absorption and expansion is transmitted to the optical fiber, and the Rayleigh scattering signal generated when the light wave propagates in the optical fiber changes. The generated signal returns along the way and enters the fourth optical fiber coupler through the c port of the circulator, and generates beat frequency interference with the reference light of the main interferometer at the fourth optical fiber coupler to generate a beat frequency interference signal. The beat frequency interference signal is converted into an electrical signal by the first balanced photodetector and collected by a high-speed oscilloscope;

3) In the auxiliary interferometer, the light of the delayed optical fiber and the single-mode optical fiber generates an interference signal in the fifth optical fiber coupler, and the interference signal is converted into an electrical signal by the second balanced photodetector and collected by a high-speed oscilloscope. According to the frequency of the interference signal generated by the auxiliary interferometer, the instantaneous frequency of the laser light source is calculated by using the inverse tangent and phase unwrapping algorithms, and the beat frequency interference signal of the main interferometer is applied with an interpolation algorithm and re-sampling to obtain a beat frequency signal with a uniform optical frequency interval, which is assumed to be a reference state signal;

4) Then, a salt solution is added to the pure water to make the salt solution concentration of the mixed solution 0.3 mol/L, a salinity meter is placed in the mixed solution, and the sensor is corrected according to the salinity obtained by the salinity meter. At this time, the salinity increases, and the polyimide layer will undergo polycondensation. After standing for twenty minutes at a concentration of 0.3 mol/L to allow the polyimide to lose water, an oscilloscope is used to collect the interference beat frequency signals generated by the main interferometer and the auxiliary interferometer. According to the frequency of the interference signal generated by the auxiliary interferometer, the instantaneous frequency of the laser light source is calculated by the inverse tangent and phase unwrapping algorithms, and the beat frequency interference signal of the main interferometer is applied with an interpolation algorithm and re-sampled to obtain a beat frequency signal with a uniform optical frequency interval, which is assumed to be a measurement state signal;

5) The reference state signal and the measurement state signal are respectively converted into distance domain signals by fast Fourier transform, and then the distance domain signal is fixedly slid with a window size of 200 points, and the distance domain is divided into multiple window signals, and then the signal of each sliding window is converted into the wavelength domain by inverse Fourier transform to obtain the Rayleigh scattering spectra of the reference state signal and the measurement state signal;

6) Perform cross-correlation calculation on the Rayleigh scattering spectra of the reference state signal and the measurement state signal of each corresponding window to obtain the cross-correlation peak offset of each position, and combine it with the strain frequency shift coefficient to obtain the final position-strain curve. Then, the strain size of each section of the sensing optical fiber obtained by positioning is reversed to obtain the salinity, thereby realizing the measurement of salinity;

7) After that, the concentration interval is 0.5 mol/L each time, and the mixture is allowed to stand for 20 minutes. The interference beat frequency signals generated by the main interferometer and the auxiliary interferometer are collected by an oscilloscope, and step 4) is repeated to obtain uniform optical frequency interval signals, which are all measurement state signals. Then, the reference state signals collected in pure water and the measurement state signals collected at other salt solution concentrations are repeated in steps 5) to 6) respectively;

8) The sensor is calibrated based on the salinity obtained using a standard commercial salinometer.

The intensity of Rayleigh scattered light is inversely proportional to the fourth power of the wavelength of the incident light, which can be expressed as shown in formula (1):

Where _I0 is the intensity of the incident light; λ is the wavelength of the incident light; θ is a scattering angle,

The Rayleigh scattering coefficient is expressed as shown in formula (2):

Where n represents the effective optical fiber refractive index; K is the Boltzmann constant; T represents temperature; p represents the elastic coefficient of the material; β _T represents the isothermal compressibility at temperature T. It can be seen from formula (2) that when the temperature or strain changes, the Rayleigh scattering coefficient will change and the Rayleigh scattering spectrum will shift. The Rayleigh scattering spectrum under temperature or strain is cross-correlated with the initial Rayleigh scattering spectrum without external disturbance to obtain the offset of the spectrum. The offset corresponds to the change in the temperature or strain of the optical fiber.

Assume that the light fields of the signal light _Es (t) and the reference light _Er (t) satisfying the coherence condition of the main interferometer are as shown in formula (3) and formula (4) respectively:

where _f0 is the initial frequency of the tunable laser, represents the random phase of the reference light, represents the reflectivity at the time delay position, represents the random phase of the backscattered Rayleigh signal at time t,

According to the square law characteristics of the photodetector, the photocurrent output by the photodetector is assumed to be I(t). Considering that the photodetector does not respond to the part above the cutoff frequency and the DC part will be filtered out, the output signal I(t) of the first balanced photodetector is as shown in formula (5):

in, The term represents the phase difference between the two beams of light at time t, which varies nonlinearly with time. _fb represents the beat frequency of the two beams of light, which varies linearly with time and is a function of the position of the scattering point on the sensing optical fiber.

This technical solution realizes distributed relative salinity measurement based on polyimide sensitive materials and optical frequency domain reflection technology. Polyimide material is coated on the single-mode optical fiber. The characteristics of polyimide swelling when absorbing water and shrinking when losing water are used to convert the change of salinity into stress change on the optical fiber. These changes will cause the parameters characterizing the characteristics of light waves in the single-mode optical fiber to change. Fourier transform and cross-correlation operation are performed on these signal parameters to obtain the optical fiber strain change information, and then the salinity value of the solution is measured by reverse calculation.

This optical frequency domain reflectometer has strong anti-electromagnetic interference ability, good corrosion resistance and simple structure. This method can monitor the changes in salinity values in real time and can achieve accurate measurement of relative salinity distribution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic structural diagram of an embodiment.

In the figure, 1. narrow linewidth laser 2. first fiber coupler 3. third fiber coupler 4. second fiber coupler 5. delay fiber 6. fiber circulator 7. polarization controller 8. single mode fiber 9. polyimide fiber 10. fourth fiber coupler 11. fifth fiber coupler 12. second photodetector 13. first photodetector 14. high speed oscilloscope.

Detailed ways

The content of the present invention is further described below in conjunction with the drawings and embodiments, but the present invention is not limited thereto.

Example:

1 , a polyimide optical fiber-based optical frequency domain reflectometer salinity measurement device includes an interconnected narrow linewidth laser 1 and a first optical fiber coupler 2, wherein:

The output end of the first fiber coupler 2 is divided into two paths, one of which is connected to the second fiber coupler 4, and the output end of the second fiber coupler 4 is divided into two paths again, and one output end of the second fiber coupler 4 is sequentially connected to the polarization controller 7 and the fourth fiber coupler 10; the other output end of the second fiber coupler 4 is connected to the a port of the circulator 6, the b port of the circulator is connected to the polyimide fiber 9, and the c port of the circulator is connected to the fourth fiber coupler 10, and the output end of the fourth fiber coupler 10 is sequentially connected to the first balanced photodetector 13 and the high-speed oscilloscope 14; the other output end of the first fiber coupler 2 is connected to the third fiber coupler 3, and the output end of the third fiber coupler 3 is divided into two paths again, one output end of the third fiber coupler 3 is connected to the fifth fiber coupler 11 using the single-mode fiber 8, and the other output end of the third fiber coupler 3 is sequentially connected to the delay fiber 5 and the fifth fiber coupler 11, and the output end of the fifth fiber coupler 11 is sequentially connected to the second balanced photodetector 12 and the high-speed oscilloscope 14;

In this example, the circulator 6 and the polarization controller 7 constitute a main interferometer unit, the delay fiber 5 and the single-mode fiber 8 constitute an auxiliary interferometer unit, the polyimide fiber 9 constitutes a polyimide fiber unit, and the first balanced photodetector 13 and the second balanced photodetector 12 constitute a balanced photodetector unit;

In this example, the polyimide optical fiber is a common single-mode optical fiber. After removing the acrylate layer with a wire stripper, the optical fiber is immersed in alcohol and wiped clean with airflow paper. Finally, the optical fiber is immersed in a polyimide solution. The optical fiber treated by the pulling method and the brushing method is placed in a heating furnace at 110°C for 30 minutes, and then the temperature is increased to 180°C at a rate of 1°C per minute. Finally, it is heated at 180°C for 30 minutes to obtain a polyimide optical fiber with a coating thickness of 50um.

In this example, FC/PC optical connectors are used for connection.

In this example, the model of the narrow linewidth laser is TSL-550, the wavelength scanning range is 1540nm-1560nm, and the scanning speed is 20nm/s.

In this example, the first photodetector 13 and the second photodetector 12 are both of model PDB430C.

In this example, the model of the high-speed oscilloscope 14 is MSOS254A, and the sampling rate is 10 MHz/S.

In this example, the splitting ratio of the first optical fiber coupler 2 is 90%:10%, and 90% of the light in the first optical fiber coupler 2 enters the second optical fiber coupler 4 , and 10% of the light enters the third optical fiber coupler 3 .

In this example, the splitting ratio of the second optical fiber coupler 4 is 99%:1%, and 99% of the light in the second optical fiber coupler 4 enters the port a of the circulator 6 , and 1% of the light enters the polarization controller 7 .

In this example, the splitting ratio of the third optical fiber coupler 3 is 50%:50%, the splitting ratio of the fourth optical fiber coupler 10 is 50%:50%, and the splitting ratio of the fifth optical fiber coupler 11 is 50%:50%.

In this example, the polyimide optical fiber has a coating thickness of 50 um and a length of 20 m.

In this example, the delay optical fiber 5 is a common single-mode optical fiber with a length of 100 m.

In this example, the propagation mode of the light is as follows: the light source of the narrow linewidth laser 1 enters the first fiber coupler 2, the output light of the first fiber coupler 2 is divided into two paths, one of the output lights of the first fiber coupler 2 is divided into two paths again through the second fiber coupler 4, one of the output lights of the second fiber coupler 4 is used as a reference light to reach the fourth fiber coupler 10 through the polarization controller 7, and the other output light of the second fiber coupler 4 enters from the a port of the circulator 6, outputs from the b port, and enters the polyimide fiber, thereby generating Rayleigh backscattered light as a signal light, which is output from the c port of the circulator 6 to the fourth fiber coupler 10. The first fiber coupler 10 interferes with the reference light to generate a beat frequency signal, which is converted into an electrical signal by the first balanced photodetector 13 and then collected by a high-speed oscilloscope 14; the other output light of the first fiber coupler 2 is divided into two paths again by the third fiber coupler 3, and one of the output lights of the third fiber coupler 3 directly reaches the fifth fiber coupler 11, and interferes with the other output light of the third fiber coupler 3 after passing through a delay fiber 5, generating a beat frequency signal with a frequency of 1.5 MHz, which is converted into an electrical signal by the second balanced photodetector 12 and then collected by a high-speed oscilloscope 14 as an external sampling clock signal.

A method for measuring salinity using an optical frequency domain reflectometer based on polyimide optical fiber, using the above-mentioned optical frequency domain reflectometer salinity measuring device based on polyimide optical fiber, the method comprising the following steps:

1) The frequency-sweeping light emitted by the narrow linewidth laser 1 enters the first fiber coupler 2 and is divided into two paths of light, one path of light enters the main interferometer unit, and the other path enters the auxiliary interferometer unit. The light entering the main interferometer unit is divided into two paths of light by the second fiber coupler 4, one path of light entering the circulator 6 is the signal light, and the other path of light entering the polarization controller 7 is the reference light;

2) In the main interferometer, after the polyimide optical fiber 9 is immersed in pure water for two hours, the stress change of the polyimide layer due to water absorption and expansion is transmitted to the optical fiber, and the Rayleigh scattering signal generated when the light wave propagates in the optical fiber changes. The generated signal returns along the way and enters the fourth optical fiber coupler 10 through the c port of the circulator 6, and generates beat frequency interference with the reference light of the main interferometer at the fourth optical fiber coupler 10, generating a beat frequency interference signal, which is converted into an electrical signal by the first balanced photodetector 13 and collected by the high-speed oscilloscope 14;

3) In the auxiliary interferometer, the light of the delayed optical fiber 5 and the single-mode optical fiber 8 generates an interference signal in the fifth optical fiber coupler 11, and the interference signal is converted into an electrical signal by the second balanced photodetector 12 and collected by the high-speed oscilloscope 14. According to the frequency of the interference signal generated by the auxiliary interferometer, the instantaneous frequency of the laser light source is calculated by using the inverse tangent and phase unwrapping algorithms, and the beat frequency interference signal of the main interferometer is applied with the interpolation algorithm and re-sampling to obtain a beat frequency signal with uniform optical frequency interval, which is assumed to be a reference state signal;

4) Then, a salt solution is added to the pure water to make the salt solution concentration of the mixed solution 0.3 mol/L, a salinity meter is placed in the mixed solution, and the sensor is corrected according to the salinity obtained by the salinity meter. At this time, the salinity increases, and the polyimide layer undergoes polycondensation. After the polyimide is left to stand for 20 minutes at a concentration of 0.3 mol/L to lose water, an oscilloscope 14 is used to collect the interference beat frequency signals generated by the main interferometer and the auxiliary interferometer. According to the frequency of the interference signal generated by the auxiliary interferometer, the instantaneous frequency of the laser light source is calculated by the inverse tangent and phase unwrapping algorithms, and the beat frequency interference signal of the main interferometer is applied with an interpolation algorithm and re-sampled to obtain a beat frequency signal with a uniform optical frequency interval, which is assumed to be a measurement state signal;

5) The reference state signal and the measurement state signal are respectively converted into distance domain signals by fast Fourier transform, and then the distance domain signal is fixedly slid with a window size of 200 points, and the distance domain is divided into multiple window signals, and then the signal of each sliding window is converted into the wavelength domain by inverse Fourier transform to obtain the Rayleigh scattering spectra of the reference state signal and the measurement state signal;

6) Perform cross-correlation calculation on the Rayleigh scattering spectra of the reference state signal and the measurement state signal of each corresponding window to obtain the cross-correlation peak offset of each position, and combine it with the strain frequency shift coefficient to obtain the final position-strain curve. Then, the strain size of each section of the sensing optical fiber obtained by positioning is reversed to obtain the salinity, thereby realizing the measurement of salinity;

7) After that, the concentration interval is 0.5 mol/L each time, and the mixture is left to stand for 20 minutes. The interference beat frequency signals generated by the main interferometer and the auxiliary interferometer are collected by using an oscilloscope 14, and the operation of step 4) is repeated to obtain uniform optical frequency interval signals, which are all measurement state signals. Then, the reference state signals collected in pure water and the measurement state signals collected at other salt solution concentrations are respectively repeated in steps 5) to 6);

8) The sensor is calibrated based on the salinity obtained using a standard commercial salinometer.

The intensity of Rayleigh scattered light is inversely proportional to the fourth power of the wavelength of the incident light, which can be expressed as shown in formula (1):

Where _I0 is the intensity of the incident light; λ is the wavelength of the incident light; θ is a scattering angle,

The Rayleigh scattering coefficient is expressed as shown in formula (2):

Where n represents the effective optical fiber refractive index; K is the Boltzmann constant; T represents temperature; p represents the elastic coefficient of the material; β _T represents the isothermal compressibility at temperature T. It can be seen from formula (2) that when the temperature or strain changes, the Rayleigh scattering coefficient will change and the Rayleigh scattering spectrum will shift. The Rayleigh scattering spectrum under temperature or strain is cross-correlated with the initial Rayleigh scattering spectrum without external disturbance to obtain the offset of the spectrum. The offset corresponds to the change in the temperature or strain of the optical fiber.

Assume that the light fields of the signal light _Es (t) and the reference light _Er (t) satisfying the coherence condition of the main interferometer are as shown in formula (3) and formula (4) respectively:

where _f0 is the initial frequency of the tunable laser, represents the random phase of the reference light, represents the reflectivity at the time delay position, represents the random phase of the backscattered Rayleigh signal at time t,

According to the square law characteristics of the photodetector, the photocurrent output by the photodetector is assumed to be I(t). Considering that the photodetector does not respond to the part above the cutoff frequency and the DC part will be filtered out, the output signal I(t) of the first balanced photodetector is as shown in formula (5):

in, The term represents the phase difference between the two beams of light at time t, which varies nonlinearly with time. _fb represents the beat frequency of the two beams of light, which varies linearly with time and is a function of the position of the scattering point on the sensing optical fiber.

In this example, an optical frequency shift reflectometer is used in combination with the sensitive material polyimide to transmit the stress changes of polyimide when it absorbs water and expands, and loses water and shrinks to the optical fiber. The strain size is obtained by demodulating the changes in the optical fiber optical signal, and then the corresponding salinity value is obtained by reverse calculation. This method effectively realizes the continuous measurement of salinity. In addition, this optical fiber sensor has the advantages of small size, no reaction with electrolyte, and distributed measurement, which can realize accurate monitoring of salinity. The experimental results are in good agreement with the salinity meter.

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.