CN115683387B - Distributed absolute temperature sensing method based on low-birefringence photonic crystal fiber - Google Patents

Abstract

The application provides a distributed absolute temperature sensing method based on a low-birefringence photonic crystal fiber, which comprises the following specific steps: designing and manufacturing a low-birefringence photonic crystal fiber as a sensing fiber; modulating single-frequency pulsed light, controlling the pulsed light to generate x and y polarized state pulsed light through a polarization switch, injecting the x and y polarized state pulsed light into a low-birefringence photonic crystal fiber, collecting corresponding Rayleigh scattering light, and acquiring vector light field information of Rayleigh scattering along the fiber; acquiring vector light field information of Rayleigh scattering light with different frequencies to form a complex Rayleigh scattering pattern in an x polarization state and a y polarization state; performing delay estimation on frequency axis signals of a plurality of Rayleigh scattering patterns at the same position of the optical fiber to acquire frequency shift related to temperature; and obtaining refractive index changes caused by temperature in two polarization directions according to the linear relation between the frequency shift and the refractive index changes, and calculating to obtain absolute temperature information according to different coefficients of the refractive index changes with different concentrations and the temperature changes.

Description

Distributed absolute temperature sensing method based on low birefringence photonic crystal fiber

Technical Field

The invention relates to the field of optical fiber sensing measurement, and in particular to a distributed absolute temperature sensing method based on low-birefringence photonic crystal optical fiber.

Background Art

Distributed optical fiber sensing systems have been widely used in structural health monitoring, geological prospecting, distributed temperature measurement and other fields due to their advantages such as high sensitivity, continuous measurement and application in harsh environments. In the traditional distributed temperature sensing system based on Rayleigh scattering pattern, the detection unit is an ordinary optical fiber, which can only obtain the temperature change by demodulating the frequency shift of the Rayleigh scattering pattern, and cannot obtain the absolute temperature. The method of calibrating the temperature with the Rayleigh scattering pattern is only applicable to the same optical fiber, and it needs to be recalibrated after changing to other optical fibers, which has no practical value. In order to solve the above problems, technicians in this field have tried to replace ordinary optical fibers with high birefringence polarization-maintaining optical fibers, such as China's public patent CN105486425B, but on the one hand, this solution is a point optical fiber temperature sensor, and its sensing structure cannot obtain temperature information along the optical fiber, and cannot be used in large-scale continuous temperature measurement scenarios; on the other hand, its temperature measurement is based on interference spectroscopy, rather than non-Rayleigh scattering.

In addition, there are also distributed birefringence measurement schemes that use traditional high-birefringence polarization-maintaining fibers as sensing fibers. However, due to the large birefringence of traditional polarization-maintaining fibers, a frequency shift range of about 50 GHz is often required to cover the measurement range of two polarization states. There are three ways to achieve such a large frequency shift range: 1. Use a swept-frequency laser with a large sweep range as the system light source; 2. Use a high-stability tunable laser combined with microwave source external modulation; 3. Use an ultra-wide range microwave source combined with an ultra-large modulation bandwidth modulator. The first solution is represented by the US LUNA company, which uses a swept-frequency laser with a large sweep range without mode hopping as the light source, such as the US patent US2006204165A1, but this type of swept-frequency laser is often very expensive; the second solution has been verified by researchers at Rayleigh Federal University of Technology in Lausanne. This solution must strictly lock the frequency of the tunable laser, and its system complexity and cost are also very high; in the third solution, microwave sources with ultra-wide sweep ranges and large bandwidth modulators are relatively rare and expensive.

In addition, there are reports of using probe light with a smaller frequency range to measure the evolution of polarization states during light transmission, such as China's public patent CN110132330A. This method is mainly used to study the evolution of polarization states of light pulses during transmission and cannot be directly used to measure absolute temperature. At the same time, the system requires a very high spatial resolution to ensure that the birefringence of three adjacent optical fiber segments is the same, which requires that the spatial resolution is much smaller than the beat length of the polarization-maintaining fiber and that the detection system has the ability to measure with ultra-high spatial resolution. In this system, the spatial resolution is determined by the frequency sweep range, which means that a very large frequency sweep range is still required as support.

How to solve the above problems and provide a low-cost distributed optical fiber temperature sensing system with high adaptability, large-range continuous measurement function, absolute temperature measurement capability, and can be realized using relatively conventional optoelectronic devices is what technicians in this field need to consider.

Summary of the invention

Based on the above problems, the present invention provides a distributed absolute temperature sensing method based on low-birefringence photonic crystal fiber to solve the technical problem that the existing distributed measurement system requires a large-range optical swept frequency source or an electrical swept frequency source.

The embodiment of the present application provides a distributed absolute temperature sensing method based on low birefringence photonic crystal fiber, and the absolute temperature sensing method comprises the following steps:

Step 1: Design and fabricate low birefringence photonic crystal fiber as sensing fiber;

Step 2: modulating a signal light to be modulated emitted by a fixed frequency laser into a single-frequency pulse light through an IQ modulator, controlling the polarization state of the pulse light through a polarization switch, generating an x-polarization state pulse light and a y-polarization state pulse light, wherein the IQ modulator and the polarization switch are driven and synchronized by an arbitrary waveform generator, so that the x-polarization state pulse light and the y-polarization state pulse light have different frequencies and delays;

Step 3: injecting x-polarization state pulse light and y-polarization state pulse light into the fast axis and slow axis of the low-birefringence photonic crystal fiber respectively, collecting the Rayleigh scattered light corresponding to the x-polarization state pulse light and the y-polarization state pulse light respectively through a circulator, and obtaining the vector light field information of Rayleigh scattering along the low-birefringence photonic crystal fiber through a polarization multiplexing 90° optical mixer;

Step 4: stepwise change the frequency of the output electrical signal of the arbitrary waveform generator, and repeat step 2 and step 3 to obtain a series of vector light field information of Rayleigh scattered light of different frequencies, and arrange the vector light field information of the series of Rayleigh scattered light of different frequencies in a form in which the horizontal axis represents the optical fiber distance and the vertical axis represents the matrix, so as to form a complex Rayleigh scattering pattern of the x polarization state and the y polarization state;

Step 5: Use a delay estimation algorithm to estimate the delay of the frequency axis signal of the complex Rayleigh scattering pattern of the x polarization state and the y polarization state at each position on the low birefringence photonic crystal fiber to obtain the frequency offset caused by the temperature change. According to the linear relationship between the frequency offset and the refractive index change, the refractive index change caused by the temperature change in the x and y directions is obtained. Due to the different concentrations of the ethanol solution in the air holes in the x and y directions, the relationship between the refractive index change and the temperature change in the x and y directions is also different. By combining the equations for the refractive index change caused by the temperature in the x and y directions, the absolute temperature information along the optical fiber can be obtained, thereby realizing distributed absolute temperature measurement.

In a possible implementation manner, the distributed absolute temperature sensing method based on low-birefringence photonic crystal fiber, the manufacturing process of the low-birefringence photonic crystal fiber is as follows:

1) Inject ethanol solutions of different concentrations into the innermost air holes of the symmetrical air hole photonic crystal fiber, where the two air holes in the x direction are injected with a solution of concentration c1; and the four air holes in the y direction are injected with a solution of concentration c2, where c1>c2. Ethanol solutions of different concentrations have different refractive indices, which increases the asymmetry of the cross section, thereby transforming the non-polarization-maintaining photonic crystal fiber into a polarization-maintaining photonic crystal fiber.

2) Inject a probe light with a wavelength of 1550nm into the PCF, use a polarization analyzer to measure the polarization state of the projected light, and calculate the birefringence of the polarization-maintaining PCF. Then, adjust the concentration difference of the ethanol solution in the air holes in the x and y directions to reduce the birefringence until the frequency difference corresponding to the birefringence is in the range of 0.8 to 1.5GHz.

In a possible implementation, in the distributed absolute temperature sensing method based on low-birefringence photonic crystal fiber, the IQ modulator modulates the pulse light in a single-sideband modulation manner, so that the pulse light output by the IQ modulator has a frequency shift compared to the signal light to be modulated emitted by the laser, the duration of the pulse light modulated and output by the IQ modulator is limited, and the frequency shift amount is determined by the frequency of the output electrical signal of the arbitrary waveform generator.

In a possible implementation, the distributed absolute temperature sensing method based on low-birefringence photonic crystal fiber is applied to a distributed absolute temperature sensing system based on low-birefringence photonic crystal fiber, the distributed absolute temperature sensing system based on low-birefringence photonic crystal fiber includes a laser and a coupler, the coupler is used to divide the light emitted by the laser into a local oscillator signal light and a signal light to be modulated, and the distributed absolute temperature sensing system based on low-birefringence photonic crystal fiber also includes:

The IQ modulator is used to perform single-sideband pulse modulation of different frequencies on the modulated signal light to obtain a single-frequency pulse light;

The polarization switch is connected to the IQ modulator, and the polarization switch is used to modulate the pulse light to obtain the x-polarization state pulse light and the y-polarization state pulse light;

The low birefringence photonic crystal fiber is used to transmit x-polarization state pulse light and y-polarization state pulse light and generate corresponding Rayleigh scattered light;

The circulator couples the light beam into the low birefringence photonic crystal through the lens, and the circulator is used to collect Rayleigh scattered light generated by the x-polarization state pulse light and the Rayleigh scattered light generated by the y-polarization state pulse light;

The polarization-multiplexed 90° optical mixer is connected to the circulator and receives a local oscillator signal, and the polarization-multiplexed 90° optical mixer is used to perform coherent detection on Rayleigh scattered light to obtain vector light field information of Rayleigh scattering along the low-birefringence photonic crystal fiber; and a detector is connected to the polarization-multiplexed 90° optical mixer, and is used to receive the in-phase signal and the orthogonal phase signal of the Rayleigh scattered signal corresponding to the x-polarization state pulse light and the y-polarization state pulse light, respectively, so as to obtain the Rayleigh scattering vector light field information corresponding to the x-polarization state pulse light and the y-polarization state pulse light, respectively.

In a possible embodiment, the distributed absolute temperature sensing method based on low-birefringence photonic crystal fiber further includes an arbitrary waveform generator, which is connected to the IQ modulator. The arbitrary waveform generator is used to change the frequency shift of the x-polarization state pulse light and the y-polarization state pulse light modulated by the IQ modulator compared to the local oscillator signal light, and to generate pulsed light.

In a possible implementation, the distributed absolute temperature sensing method based on low-birefringence photonic crystal fiber further includes a polarization controller, wherein the polarization controller is connected to the coupler, and the polarization controller is connected to the polarization multiplexing 90° optical mixer, and the local oscillator signal light is connected to the polarization multiplexing 90° optical mixer via the polarization controller.

Compared with the prior art, the beneficial effects of the present invention are as follows: on the one hand, the present invention designs and manufactures a special low-birefringence photonic crystal fiber, and during the manufacturing process, the concentration of the ethanol solution in the two vertical air holes is adjusted to adjust the asymmetry of the optical fiber cross section, thereby realizing the manufacture of the low-birefringence photonic crystal fiber, so that the birefringence at the corresponding frequency difference is about 1 GHz. Since the transverse mode of the photonic crystal fiber is determined by the cross-sectional distribution of the optical fiber, even if the difference in the refractive index of the two polarization states is relatively small, it can still maintain good stability and uniformity, thereby reducing the system error of the absolute temperature measurement and avoiding an additional calibration process. Using the optical fiber as a sensing optical fiber for absolute temperature measurement, due to the low birefringence, only about 1 GHz is needed to cover the frequency sweep range of the two polarization states. Therefore, the present invention does not need to introduce a frequency sweep laser, a tunable laser and a frequency stabilization device, a large-range microwave source and a large-bandwidth modulator, and can only realize its measurement function and ensure the necessary accuracy through the commonly used light source, IQ modulator and signal source in communication.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of the structure of a system used in a distributed absolute temperature sensing method based on low-birefringence photonic crystal fiber provided in an embodiment of the present application.

FIG2 is a schematic cross-sectional view of a low-birefringence photonic crystal fiber in a system to which a distributed absolute temperature sensing method based on a low-birefringence photonic crystal fiber is applied according to an embodiment of the present application.

FIG3 is a schematic flow chart of a distributed absolute temperature sensing method based on low-birefringence photonic crystal fiber provided in an embodiment of the present application.

Main component symbols

Distributed absolute temperature sensing system based on low birefringence photonic crystal fiber 1

Laser 11

Coupler 12

IQ Modulator 13

Arbitrary Waveform Generator 14

Polarization switch 15

Circulator 16

Low Birefringence Photonic Crystal Fiber 17

Polarization Controller 18

90° Optical Mixer 19

Coupling lens 20

Detector 10

The following specific implementation methods will further illustrate the present application in conjunction with the above-mentioned drawings.

DETAILED DESCRIPTION

The following description will refer to the accompanying drawings to more fully describe the content of the present application. Shown in the accompanying drawings are exemplary embodiments of the present application. However, the present application can be implemented in many different forms and should not be interpreted as being limited to the exemplary embodiments set forth herein. These exemplary embodiments are provided to make the present application thorough and complete, and to fully convey the scope of the present application to those skilled in the art. Similar reference numerals represent identical or similar components.

The terms used herein are only used for the purpose of describing specific exemplary embodiments and are not intended to limit the present application. As used herein, unless the context clearly indicates otherwise, the singular forms "a", "an" and "the" are intended to include the plural forms as well. In addition, when used herein, "includes" and/or "comprises" and/or "has", integers, steps, operations, components and/or components, but do not exclude the existence or addition of one or more other features, regions, integers, steps, operations, components and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs. In addition, unless explicitly defined herein, terms such as those defined in general dictionaries should be interpreted as having a meaning consistent with their meaning in the relevant technology and the content of this application, and will not be interpreted as an idealized or overly formal meaning.

The following will describe exemplary embodiments in conjunction with the accompanying drawings. It should be noted that the components depicted in the reference drawings are not necessarily shown to scale; and the same or similar components will be given the same or similar reference numerals or similar technical terms.

The specific implementation methods of the present application are further described in detail below with reference to the accompanying drawings.

As shown in FIG1 , the embodiment of the present application provides a distributed absolute temperature sensing system 1 based on low-birefringence photonic crystal fiber. The distributed absolute temperature sensing system 1 based on low-birefringence photonic crystal fiber includes a laser 11, a coupler 12, an IQ modulator 13, an arbitrary waveform generator 14, a polarization switch 15, a circulator 16, a coupling lens 20, a low-birefringence photonic crystal fiber 17, a polarization controller 18, a 90° optical mixer 19 for polarization multiplexing, and a detector 10.

The coupler 12 is used to divide the light emitted by the laser 11 into local oscillator signal light and the signal light to be modulated. The polarization controller 18 is connected to the coupler 12, and the polarization controller 18 is connected to the 90° optical mixer 19 of polarization multiplexing. The local oscillator signal light is connected to the 90° optical mixer 19 of polarization multiplexing via the polarization controller 18.

The IQ modulator 13 is used to perform single-sideband pulse modulation of different frequencies on the modulated signal light to obtain a single-frequency pulse light. The polarization switch 15 is connected to the IQ modulator 13, and the polarization switch 15 is used to modulate the pulse light to obtain an x-polarization state pulse light and a y-polarization state pulse light. The low-birefringence photonic crystal fiber 17 is used to transmit the x-polarization state pulse light and the y-polarization state pulse light and generate corresponding Rayleigh scattered light.

The arbitrary waveform generator 14 is connected to the IQ modulator 13 , and is used to change the frequency shift of the x-polarization state pulse light and the y-polarization state pulse light modulated by the IQ modulator 13 compared to the local oscillator signal light, and to generate pulse light.

The arbitrary waveform generator 14 is also connected to the polarization switch 15 , and the IQ modulator 13 and the polarization switch 15 are driven and synchronized by the arbitrary waveform generator 14 .

The circulator 16 is connected to the low-birefringence photonic crystal fiber 17 via a coupling lens 20 . The circulator 16 is used to collect the Rayleigh scattered light corresponding to the x-polarization state pulse light and the y-polarization state pulse light respectively.

The low-birefringence photonic crystal fiber 17 is a low-birefringence polarization-maintaining fiber.

The polarization-multiplexed 90° optical mixer 19 is connected to the circulator 16 . At the same time, the polarization-multiplexed 90° optical mixer 19 receives the local oscillator signal. The polarization-multiplexed 90° optical mixer 19 is used to demodulate the Rayleigh scattered light to obtain the vector light field information of Rayleigh scattering along the low birefringence photonic crystal fiber 17 .

The detector 10 is connected to the 90° optical mixer 19, and the two spaced detectors 10 are respectively used to receive the in-phase signal and the orthogonal phase signal of the Rayleigh scattering corresponding to the x-polarization state pulse light and the y-polarization state pulse light, thereby obtaining the Rayleigh scattering vector light field information corresponding to the x-polarization state and the y-polarization state pulse light.

The laser 11 emits a single-frequency laser, which is divided into two parts by the coupler 12. One part passes through the polarization controller 18 and is connected to the 90° optical mixer 19 as the local oscillator signal for local oscillator coherent detection; the other part is connected to the IQ modulator 13 for single-sideband pulse modulation of different frequencies, and then enters the polarization switch 15 to adjust to the appropriate polarization state, and then enters the low-birefringence photonic crystal fiber 17 for sensing through the circulator 16. The back Rayleigh scattered signal is connected to the signal end of the polarization multiplexed 90° optical mixer 19 through the circulator 16 for demodulation; the signals of the x polarization state and the y polarization state are received by two different detectors 10 respectively.

In one embodiment, the laser 11 can be a fixed-frequency narrow-linewidth light source commonly used in coherent communications.

As shown in FIG2 , in one embodiment, the low birefringence photonic crystal fiber 17 is a symmetrical air hole photonic crystal fiber, and the asymmetry of the fiber cross section is achieved by distributing and injecting ethanol solutions of different concentrations in the x and y directions, thereby converting the non-polarization-maintaining photonic crystal fiber into a low birefringence polarization-maintaining photonic crystal fiber. The refractive index change and temperature change coefficients in different polarization directions are different, so the frequency change caused by temperature is measured in a distributed manner in the x and y polarization directions, and the relationship between the refractive index change and the temperature change of the two polarization states can be obtained according to the corresponding relationship between the frequency change and the refractive index change, as shown in the following formula.

和

为折射率随温度变化的系数，可以通过实验标定得到，且由于光子晶体光纤横截面的稳定性，仅需对其中一小段光纤进行标定，即可反应整个传感光纤的参数。联立两个方程，即可得到T₁和T₂，即得到温度变化前后的绝对温度的大小，实现绝对温度的分布式测量。in

and

is the coefficient of refractive index changing with temperature, which can be obtained through experimental calibration. Due to the stability of the cross section of the photonic crystal fiber, only a small section of the fiber needs to be calibrated to reflect the parameters of the entire sensing fiber. By combining the two equations, _T1 and _T2 can be obtained, that is, the absolute temperature before and after the temperature change, and the distributed measurement of absolute temperature can be realized.

As shown in FIG. 3 , an embodiment of the present application further provides a distributed absolute temperature sensing method based on low birefringence photonic crystal fiber.

The distributed absolute temperature sensing method based on low birefringence photonic crystal fiber comprises the following steps:

Step 1: Design and fabricate low birefringence photonic crystal fiber as sensing fiber.

In one embodiment, the low birefringence photonic crystal fiber is manufactured as follows:

1) Inject ethanol solutions of different concentrations into the innermost air holes of the symmetrical air hole photonic crystal fiber, where the two air holes in the x direction are injected with a solution of concentration c1; and the four air holes in the y direction are injected with a solution of concentration c2, where c1>c2. As shown in the abstract figure, the refractive index of ethanol solutions of different concentrations is different, thereby increasing the asymmetry of the cross section, thereby transforming the non-polarization-maintaining photonic crystal fiber into a polarization-maintaining photonic crystal fiber.

2) Inject a probe light with a wavelength of 1550nm into the PCF, use a polarization analyzer to measure the polarization state of the projected light, and calculate the birefringence of the polarization-maintaining PCF. Then, adjust the concentration difference of the ethanol solution in the air holes in the x and y directions to reduce the birefringence until the frequency difference corresponding to the birefringence is around 1GHz.

Step 2: modulate the signal light to be modulated emitted by the laser into a single-frequency pulse light through the IQ modulator, control the polarization state of the pulse light through the polarization switch, generate x-polarization state pulse light and y-polarization state pulse light, and the IQ modulator and the polarization switch are driven and synchronized by the arbitrary waveform generator so that the x-polarization state pulse light and the y-polarization state pulse light have different frequencies and delays.

In one embodiment, the frequency difference between the x-polarization state pulse light and the y-polarization state pulse light is determined by the low birefringence parameter of the low birefringence photonic crystal fiber, and the delay is equal to the round trip time of a single pulse.

Step three: Inject the x-polarization state pulse light and the y-polarization state pulse light into the fast axis and slow axis of the low birefringence photonic crystal fiber respectively, collect the Rayleigh scattered light corresponding to the x-polarization state pulse light and the y-polarization state pulse light respectively through a circulator, and obtain the vector light field information of Rayleigh scattering along the optical fiber through the 90° optical mixer of polarization multiplexing.

In one embodiment, the IQ modulator modulates the pulse light in a single-sideband modulation manner, and the duration of the IQ modulator modulating the pulse light is limited, so that the pulse light output by the IQ modulator has a frequency shift compared to the signal light to be modulated emitted by the laser, and the frequency shift amount is determined by the frequency of the output electrical signal of the arbitrary waveform generator.

Step 4: Step by step changing the frequency of the output electrical signal of the arbitrary waveform generator, and repeating steps 1 and 2 to obtain a series of vector light field information of Rayleigh scattered light of different frequencies, and arranging the vector light field information of the series of Rayleigh scattered light of different frequencies in a form in which the horizontal axis represents the optical fiber distance and the vertical axis represents the matrix, so as to form a complex Rayleigh scattering pattern of the x polarization state and the y polarization state.

Step 5: Use a delay estimation algorithm to estimate the delay of the frequency axis signal of the complex Rayleigh scattering pattern of the x polarization state and the y polarization state at each position on the low birefringence photonic crystal fiber to obtain the frequency offset caused by the temperature change. According to the linear relationship between the frequency offset and the refractive index change, the refractive index change caused by the temperature change in the x and y directions is obtained. Due to the different concentrations of the ethanol solution in the air holes in the x and y directions, the relationship between the refractive index change and the temperature change in the two directions is also different. By combining the equations for the refractive index change caused by the temperature in the x and y directions, the absolute temperature information along the optical fiber can be obtained, thereby realizing distributed absolute temperature measurement.

In one embodiment, the delay estimation method is a cross-correlation algorithm, and may further be a least mean square cross-correlation algorithm.

In one embodiment, the delay estimation algorithm can be a cross-correlation curve or a weighted cross-correlation curve of the frequency axis at different positions of the Rayleigh scattering reference pattern and the Rayleigh scattering measurement pattern; the movement information corresponding to the maximum value of the cross-correlation curve is obtained; according to the frequency shift relationship corresponding to the Rayleigh scattering temperature or strain, the temperature or strain information along the optical fiber can be quantitatively obtained. In the weighted cross-correlation algorithm, the weight can be obtained by a Roth processor or a coherent smoothing transform or a phase transform or an Eckart filter or an HT processor. The quantization error in the estimation process can be reduced by interpolation.

In one embodiment, the minimum mean square error of the Rayleigh scattering reference pattern and the Rayleigh scattering measurement pattern at different positions on the frequency axis is calculated to obtain the movement information corresponding to the minimum value of the minimum mean square error curve; according to the frequency shift relationship corresponding to the Rayleigh scattering temperature or strain, the temperature or strain information along the optical fiber can be quantitatively obtained. The quantization error in the estimation process can be reduced by interpolation.

In one embodiment, for the absolute temperature sensing method, during the modulation process, at the initial moment ( t=t0 ₎ , the signal output frequency of the arbitrary waveform generator is set to be _a pulse signal with a duration of τ, and after inputting the IQ modulator, a pulse signal with a frequency of v+f1x _is output; the polarization switch is adjusted so that the pulse signal is injected into the low birefringence photonic crystal fiber along the x polarization direction; at the moment t=t1 _, the signal output frequency of the arbitrary waveform generator is set to be _a pulse signal with a duration of τ, and after inputting the IQ modulator, a pulse signal with a frequency of v+f1y _is output; the polarization switch is adjusted so that the pulse signal is injected into the low birefringence photonic crystal fiber along the y polarization direction. Wherein _t1 is greater than the time it takes for the pulse to go back and forth in the optical fiber. Then the _{frequencies f2x} and f2y are changed _and the above process is repeated N times, that is, one measurement is completed.

Claims (6)

1. A distributed absolute temperature sensing method based on low birefringence photonic crystal fiber, characterized in that the absolute temperature sensing method comprises the following steps: Step 1: Design and fabricate low birefringence photonic crystal fiber as sensing fiber; Step 2: modulating a signal light to be modulated emitted by a fixed-frequency laser into a single-frequency pulse light through an IQ modulator, controlling the polarization state of the pulse light through a polarization switch, generating an x-polarization state pulse light and a y-polarization state pulse light, wherein the IQ modulator and the polarization switch are driven and synchronized by an arbitrary waveform generator, so that the x-polarization state pulse light and the y-polarization state pulse light have different frequencies and delays; Step 3: injecting x-polarization state pulse light and y-polarization state pulse light into the fast axis and slow axis of the low-birefringence photonic crystal fiber respectively, collecting the Rayleigh scattered light corresponding to the x-polarization state pulse light and the y-polarization state pulse light respectively through a circulator, and obtaining the vector light field information of Rayleigh scattering along the low-birefringence photonic crystal fiber through a polarization multiplexing 90° optical mixer; Step 4: stepwise change the frequency of the output electrical signal of the arbitrary waveform generator, and repeat step 2 and step 3 to obtain a series of vector light field information of Rayleigh scattered light of different frequencies, and arrange the vector light field information of the series of Rayleigh scattered light of different frequencies in a form in which the horizontal axis represents the optical fiber distance and the vertical axis represents the matrix, so as to form a complex Rayleigh scattering pattern of the x polarization state and the y polarization state; Step 5: Use a delay estimation algorithm to estimate the delay of the frequency axis signal of the complex Rayleigh scattering pattern of the x polarization state and the y polarization state at each position on the low birefringence photonic crystal fiber to obtain the frequency offset caused by temperature change. According to the linear relationship between the frequency offset and the refractive index change, the refractive index change caused by temperature change in the x and y directions is obtained. Due to the different concentrations of ethanol solution in the air holes in the x and y directions, the relationship between the refractive index change and the temperature change in the x and y directions is different. By combining the equations for the refractive index change caused by temperature in the x and y directions, the absolute temperature information along the optical fiber is obtained. 2. The distributed absolute temperature sensing method based on low birefringence photonic crystal fiber according to claim 1, characterized in that the manufacturing process of the low birefringence photonic crystal fiber is as follows: (1) Ethanol solutions of different concentrations are injected into the innermost air holes of the symmetrical air hole photonic crystal fiber, wherein the two air holes in the x direction are injected with a solution of concentration c1; and the four air holes in the y direction are injected with a solution of concentration c2, where c1>c2; (2) A probe light with a wavelength of 1550 nm was injected into the PCF. The polarization state of the projected light was measured using a polarization analyzer. The birefringence of the polarization-maintaining PCF was calculated. The birefringence was reduced by adjusting the concentration difference of the ethanol solution in the air holes in the x and y directions until the frequency difference corresponding to the birefringence was 0.8 to 1.5 GHz. 3. The distributed absolute temperature sensing method based on low-birefringence photonic crystal fiber as described in claim 1 is characterized in that the modulation mode of the IQ modulator to modulate the pulse light is single-sideband modulation, so that the pulse light output by the IQ modulator has a frequency shift compared to the signal light to be modulated emitted by the laser, and the frequency shift amount corresponding to the frequency shift is determined by the frequency of the output electrical signal of the arbitrary waveform generator. 4. The distributed absolute temperature sensing method based on low birefringence photonic crystal fiber according to claim 1, characterized in that the distributed absolute temperature sensing method based on low birefringence photonic crystal fiber is applied to a distributed absolute temperature sensing system based on low birefringence photonic crystal fiber, the distributed absolute temperature sensing system based on low birefringence photonic crystal fiber comprises a laser and a coupler, the coupler is used to divide the light emitted by the laser into a local oscillator signal light and a signal light to be modulated, the distributed absolute temperature sensing system based on low birefringence photonic crystal fiber also comprises: The IQ modulator is used to perform single-sideband pulse modulation of different frequencies on the modulated signal light to obtain a single-frequency pulse light; The polarization switch is connected to the IQ modulator, and the polarization switch is used to modulate the pulse light to obtain the x-polarization state pulse light and the y-polarization state pulse light; The low birefringence photonic crystal fiber is used to transmit x-polarization state pulse light and y-polarization state pulse light and generate corresponding Rayleigh scattered light; The circulator couples the light beam into the low birefringence photonic crystal through a coupling lens, and the circulator is used to collect Rayleigh scattered light generated by the x-polarization state pulse light and the Rayleigh scattered light generated by the y-polarization state pulse light; The polarization multiplexing 90° optical mixer is connected to the circulator and receives a local oscillator signal, and the polarization multiplexing 90° optical mixer is used to perform coherent detection on Rayleigh scattered light to obtain vector light field information of Rayleigh scattering along the low birefringence photonic crystal fiber; and The detector is connected to the polarization multiplexed 90° optical mixer and is used to receive the in-phase signal and the orthogonal phase signal of the Rayleigh scattering signal corresponding to the x-polarization state pulse light and the y-polarization state pulse light, respectively, so as to obtain the Rayleigh scattering vector light field information corresponding to the x-polarization state pulse light and the y-polarization state pulse light, respectively. 5. The distributed absolute temperature sensing method based on low-birefringence photonic crystal fiber as described in claim 4 is characterized in that it also includes an arbitrary waveform generator, which is connected to the IQ modulator, and is used to change the frequency shift of the x-polarization state pulse light and the y-polarization state pulse light modulated by the IQ modulator compared to the local oscillator signal light, and is used to generate pulse light. 6. The distributed absolute temperature sensing method based on low-birefringence photonic crystal fiber as described in claim 4 is characterized in that it also includes a polarization controller, wherein the polarization controller is connected to the coupler, and the polarization controller is connected to the polarization multiplexing 90° optical mixer, and the local oscillator signal light is connected to the polarization multiplexing 90° optical mixer via the polarization controller.

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