Long-distance telemetering phi-OFDR system based on common-diameter self-reference

Abstract

The invention discloses a long-distance telemetering phi-OFDR system based on common-diameter self-reference, belongs to the technical field of optics, and aims to solve the problem that the spatial resolution of an existing long-distance phi-OFDR system is seriously constrained by a sensing distance and is difficult to break through meter-scale distributed monitoring. A light source outputs linear frequency sweeping laser; the to-be-measured optical fiber is provided with a local reflection source array; a first port of the polarization-maintaining circulator is connected with the light source, a second port is connected with the transmission optical fiber, the transmission optical fiber is connected with the to-be-detected optical fiber, and the photoelectric detector, the data acquisition module and the calculation module are sequentially connected. In the first scheme, the end part of a to-be-detected optical fiber is connected with a Faraday rotating mirror, a third port of a polarization-maintaining circulator is connected with an input end of a polarization beam splitter, and two output ends of the polarization beam splitter are respectively connected with two photoelectric detectors. In the second scheme, all the optical fibers are polarization-maintaining optical fibers, the end of the optical fiber to be detected is connected with a reflector, and a third port of the polarization-maintaining circulator is connected with a photoelectric detector.

Description

Technical Field

[0001] The present invention belongs to the field of optical technologies, and particularly relates to a long-distance telemetry Φ-OFDR system based on common-path self-reference. Background Art

[0002] The Φ-OFDR (phase-sensitive optical frequency-domain reflectometer) technology has shown unique advantages of high spatial resolution, high sensitivity, high signal-to-noise ratio, and low requirement for the detection bandwidth of detectors because it uses a frequency-modulated continuous-wave optical probe to detect the Rayleigh backscattering along the optical fiber and the linear relationship between the phase and physical quantities of interest in the external environment (such as temperature, strain, vibration). It is widely used in fields such as structural health monitoring of large infrastructure, perimeter security, seismic wave detection, and oil pipeline detection.

[0003] However, the existing optical frequency-domain reflection technologies are limited by factors such as the non-linearity of the light source frequency sweep, coherent fading, and polarization fading. The sensing performance decreases significantly with distance, and the spatial resolution is strictly restricted by the sensing distance, making it difficult to achieve high-precision distributed sensing over ultra-long distances of hundreds of kilometers. To realize the application of optical frequency-domain reflection technology over long distances, the conventional approach is to suppress the noise of the laser through various technical means. For example: realizing distributed static strain sensing on a long optical fiber by eliminating the main noise in the system. When the length of the optical fiber is 25 kilometers, the spatial resolution can reach 2.5 m; introducing phase noise compensation in optical frequency-domain reflection to reduce the phase noise of the laser in the entire optical fiber, enabling broadband vibration waveform measurement on a 100-kilometer-long optical fiber, but the spatial resolution can only reach 100 m; using the phase noise term of the measured signal and simultaneously increasing the laser frequency tuning speed to realize an optical frequency-domain reflectometer with a measurement range far exceeding the laser coherence length, enabling Fresnel reflection at a measurement distance of 170 kilometers and backward Rayleigh scattering at 120 kilometers, with a spatial resolution of 200 m; adopting external modulation to obtain better frequency sweep linearity and breaking through the limitation of the laser coherence length by introducing the optical power spectrum method of local backward Rayleigh scattering, enabling distributed temperature sensing with a measurement range of 100 kilometers and a spatial resolution of 25 m.

[0004] However, the spatial resolutions of the above existing solutions are severely restricted by the sensing distance and it is difficult to break through the distributed monitoring at the meter level. Summary of the Invention

[0005] The purpose of the present invention is to solve the problem that the existing Φ-OFDR technology is difficult to achieve long-distance and high-spatial-resolution distributed measurement, and to provide two long-distance telemetry Φ-OFDR systems based on common-path self-reference.

[0006] A long-distance telemetry Φ-OFDR system based on common-path self-reference of the present invention includes: a light source, a polarization-maintaining circulator, a transmission optical fiber, a fiber under test, a Faraday rotator mirror, a polarization beam splitter, two photodetectors, a data acquisition module, and a calculation module; the light source is used to generate linearly swept laser; the fiber under test has a local reflection source array; the optical signal output end of the light source is connected to the first port of the polarization-maintaining circulator, the second port of the polarization-maintaining circulator is connected to one end of the transmission optical fiber, the other end of the transmission optical fiber is connected to the starting end of the fiber under test, the ending end of the fiber under test is connected to the Faraday rotator mirror, the third port of the polarization-maintaining circulator is connected to the input end of the polarization beam splitter, the two output ends of the polarization beam splitter are respectively connected to the optical signal input ends of the two photodetectors, the electrical signal output ends of the two photodetectors are respectively connected to the two electrical signal input ends of the data acquisition module, and the signal output port of the data acquisition module is connected to the signal input end of the calculation module.

[0007] Optionally, the light source includes an arbitrary function generator, a laser diode controller, and a distributed feedback laser; the electrical signal output end of the arbitrary function generator is connected to the electrical signal input end of the laser diode controller, the electrical signal output end of the laser diode controller is connected to the electrical signal input end of the distributed feedback laser, and the optical signal output end of the distributed feedback laser serves as the optical signal output end of the light source.

[0008] Optionally, the transmission optical fiber is a single-mode optical fiber.

[0009] Optionally, the fiber under test is a single-mode optical fiber.

[0010] Optionally, the local reflection source array of the fiber under test is an array of scattering enhancement points or an array of ultra-weak Bragg gratings inscribed on the fiber under test.

[0011] Optionally, when the input end of the polarization beam splitter is fast-axis input or slow-axis input, both output ends are slow-axis output and the power is equally divided.

[0012] Another long-distance telemetry Φ-OFDR system based on common-path self-reference of the present invention includes: a light source, a polarization-maintaining circulator, a transmission optical fiber, a fiber under test, a reflector, a photodetector, a data acquisition module, and a calculation module; the light source is used to generate linearly chirped laser; the fiber under test has a local reflection source array; both the transmission optical fiber and the fiber under test are polarization-maintaining optical fibers; the optical signal output end of the light source is connected to the first port of the polarization-maintaining circulator, the second port of the polarization-maintaining circulator is connected to one end of the transmission optical fiber, the other end of the transmission optical fiber is connected to the starting end of the fiber under test, the reflector is arranged at the end of the fiber under test, the third port of the polarization-maintaining circulator is connected to the optical signal input end of the photodetector, the electrical signal output end of the photodetector is connected to the electrical signal input end of the data acquisition module, and the signal output port of the data acquisition module is connected to the signal input end of the calculation module.

[0013] Optionally, the light source includes an arbitrary function generator, a laser diode controller, and a distributed feedback laser; the electrical signal output end of the arbitrary function generator is connected to the electrical signal input end of the laser diode controller, the electrical signal output end of the laser diode controller is connected to the electrical signal input end of the distributed feedback laser, and the optical signal output end of the distributed feedback laser is the optical signal output end of the light source.

[0014] Optionally, the local reflection source array of the fiber under test is an array of scattering enhancement points or an array of ultra-weak Bragg gratings written on the fiber under test.

[0015] The long-distance telemetry Φ-OFDR system based on common-path self-reference and its working method of the present invention use the strong reflection generated at the end of the optical fiber as the reference optical path to achieve a common-path self-reference structure; adopt the pre-distortion iterative learning algorithm to obtain the optimal pre-distortion modulation voltage waveform to directly modulate the laser, realize the highly linear chirping of the distributed feedback laser, have the advantage of high frequency response while improving the spatial resolution of the system, and thus realize local high spatio-temporal precision sensing; use the scattering enhancement point array optical fiber as the sensing optical fiber to completely solve the coherent fading problem and improve the signal-to-noise ratio of the system; based on the all-polarization-maintaining optical path, or construct a polarization diversity scheme based on magneto-optical rotation polarization conjugation transmission, cooperate with the rotation vector sum algorithm to solve the polarization fading problem in the self-reference structure, realize polarization-independent coherent detection, and thus realize an ultra-long-distance telemetry sensing system; greatly reduce the size of the sensing system and the complexity of the system, and realize an ultra-compact dynamic sensing system. Description of the Drawings

[0016] Figure 1 is a schematic diagram of the principle of the first long-distance telemetry Φ-OFDR system based on common-path self-reference according to an embodiment of the present application;

[0017] Figure 2 It is a spatio-temporal domain result diagram of multi-vibration measurement using the first co-self-referenced long-distance telemetry Φ-OFDR system according to the embodiment of the present application;

[0018] Figure 3 It is a frequency response result diagram of multi-vibration measurement using the first co-self-referenced long-distance telemetry Φ-OFDR system according to the embodiment of the present application;

[0019] Figure 4 It is a schematic diagram of the principle of the second co-self-referenced long-distance telemetry Φ-OFDR system according to the embodiment of the present application. Detailed implementation manners

[0020] The following uses specific specific examples to illustrate the implementation manners of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific implementation manners. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that, without conflict, the following embodiments and the features in the embodiments can be combined with each other.

[0021] As used herein, the singular forms "a", "an" and "the" are also intended to include the plural forms unless the context clearly dictates otherwise. It should be further understood that the terms "comprises", "comprising" indicate the presence of the stated features, operations, elements, components, items, kinds, and / or groups, but do not preclude the presence, occurrence or addition of one or more other features, operations, elements, components, items, kinds, and / or groups. The terms "or" and "and / or" used herein are interpreted as inclusive, or meaning any one or any combination. Thus, "A, B or C" or "A, B and / or C" means "any of the following: A; B; C; A and B; A and C; B and C; A, B and C". Exceptions to this definition occur only when the combination of elements, functions or operations are mutually exclusive in some manner.

[0022] Aiming at the problem that the existing Φ-OFDR technology is difficult to achieve long-distance and high-spatial-resolution distributed measurement, the present invention provides two co-self-referenced long-distance telemetry Φ-OFDR systems, which can significantly improve the spatial resolution while realizing telemetry.

[0023] Embodiment 1

[0024] Figure 1 It is a schematic diagram of the principle of the first co-self-referenced long-distance telemetry Φ-OFDR system according to the embodiment of the present application. As Figure 1As shown in the figure, the long-distance telemetry Φ-OFDR system based on common-path self-reference includes a light source, a polarization-maintaining circulator 4, a transmission optical fiber 5, a fiber under test 6, a Faraday rotator mirror 7, a polarization beam splitter 8, a first photodetector 9, a second photodetector 10, a data acquisition module 11, and a calculation module 12.

[0025] The applicant found that there is a non-linear relationship between the frequency of the swept light output by a common light source and time t, resulting in the broadening of the swept light spectrum and reducing the spatial resolution of the Φ-OFDR system. Therefore, in this embodiment, a light source capable of generating linearly swept laser is adopted to provide the spatial resolution of the system.

[0026] In one implementation, the light source includes an arbitrary function generator 1, a laser diode controller 2, and a distributed feedback laser 3. The electrical signal output terminal of the arbitrary function generator 1 is connected to the electrical signal input terminal of the laser diode controller 2, and the electrical signal output terminal of the laser diode controller 2 is connected to the electrical signal input terminal of the distributed feedback laser 3. The distributed feedback laser 3 is equipped with an output optical fiber, and the output optical fiber is a polarization-maintaining optical fiber. The laser output by the distributed feedback laser 3 enters the output optical fiber in a manner of aligning the slow axis, and the optical signal output terminal of the output optical fiber is used as the optical signal output terminal of the light source. The distributed feedback laser is a chirped laser, and the scanning frequency is in the range of [10 -2 , 10 2 GHz, and the repetition frequency is in the range of [10 -1 , 10 5 Hz.

[0027] The control signal of the arbitrary function generator 1 is a pre-distortion modulation voltage waveform. The optimal pre-distortion modulation voltage waveform is obtained by using the pre-distortion iterative learning algorithm and input into the arbitrary function generator 1. Under the action of the pre-distortion modulation voltage waveform, the arbitrary function generator 1 outputs a voltage signal, which is input into the laser diode controller 2 to modulate the output current of the laser diode controller 2. The laser diode controller 2 outputs temperature control information and modulated current information. Under the action of the temperature control information and the modulated current, the root mean square of the non-linear error of the swept light output by the distributed feedback laser 3 reaches the minimum, realizing the output of stable linearly swept light.

[0028] The linearly swept light generated by the light source enters the first port 4-1 of the polarization-maintaining circulator 4 and exits through the second port 4-2 of the polarization-maintaining circulator 4, and then enters the transmission optical fiber 5. The polarization-maintaining circulator 4 is a polarization-maintaining circulator with biaxial operation. The linearly swept light generated by the light source can be aligned with the fast axis or the slow axis of the first port 4-1 of the polarization-maintaining circulator 4. In the embodiment of the present application, the slow axis alignment is selected. The transmission optical fiber 5 can be an ordinary single-mode optical fiber, and the length is in the range of [0, 10 5Within the range of m. The linearly chirped optical signal becomes randomly polarized light after entering the transmission optical fiber 5, and the randomly polarized light enters the optical fiber under test 6 after exiting the transmission optical fiber 5. The optical fiber under test 6 can be an ordinary single-mode optical fiber. The optical fiber under test 6 is inscribed with a local reflection source array, for example, an array of scattering enhancement points (Rayleigh scattering) or an array of ultra-weak Bragg gratings is inscribed on the optical fiber under test 6, and the inscription interval between two adjacent local reflection sources is within the range of [1 mm, 10 m]. When the randomly polarized light passes through the local reflection source array of the optical fiber under test 6, reflection occurs, and the reflected light is called the signal light (if the local reflection source is a scattering enhancement point, scattering occurs when the randomly polarized light passes through the scattering enhancement point of the optical fiber under test 6, and the scattered light is called the signal light). The signal light carries the phase information of the light at this position of the ultra-weak Bragg grating. The transmitted part exits the optical fiber under test 6 and enters the Faraday rotator mirror 7 (the reflectivity of the Faraday rotator 7 is within the range of [1%, 100%]). This part of the light is called the reference light. The reference light is phase-shifted by 90 degrees after being reflected by the Faraday rotator mirror 7 and recoupled back into the optical fiber under test 6, and then enters the second port 4-2 of the polarization-maintaining circulator 4 through the transmission optical fiber 5. The reference light propagating in the reverse direction at any position in the transmission optical fiber 5 is orthogonally polarized relative to the forward propagation, so the fast axis is aligned when the reference light enters the second port 4-2. The reference light exits from the third port 4-3 of the polarization-maintaining circulator 4 and enters the polarization beam splitter 8. In the embodiment of the present application, the polarization beam splitter 8 is specially customized. When the light input from its single port is fast-axis light or slow-axis light, both output ports output slow-axis light and the power is equally divided. The reference light input along the fast axis is equally divided into two orthogonally polarized lights, x-polarized and y-polarized, by using this beam splitter. The signal light with a randomly polarized state carrying phase information formed by reflection through the local reflection source array enters the polarization beam splitter 8 together with the reference light. The laser lights output from the two ports of the polarization beam splitter 8 enter the first photodetector 9 and the second photodetector 10 respectively. The bandwidths of the first photodetector 9 and the second photodetector 10 are both within the range of [20 MHz, 1 GHz]. The reference light and the signal light perform beat frequency in the two photodetectors. The data acquisition module 11 collects the beat frequency signals output by the first photodetector 9 and the second photodetector 10, and the collected electrical signals are processed by the calculation module 12 to give the detection result. The calculation module 12 performs Fourier transform on the two beat frequency signals collected by the data acquisition module 11 respectively, and uses the rotating vector sum algorithm to solve its polarization fading problem to obtain a polarization-independent frequency domain signal, and finally uses the phase demodulation algorithm to extract all the dynamic and static information of the area of the optical fiber under test, such as vibration frequency and strain conditions, etc. In the embodiment of the present application, the data acquisition module 11 can be implemented by an oscilloscope, and the calculation module 12 can be an application program embedded in a computer. The network port of the oscilloscope is connected to the network port of the computer.

[0029] In addition, the network port of the computer can also be connected to the network port of any function generator 1. The computer uses the pre-distortion iterative learning algorithm to obtain the optimal pre-distortion modulation voltage waveform and inputs it into any function generator 1.

[0030] The above-mentioned polarization beam splitter 8, first photodetector 9, and second photodetector 10 can achieve polarization diversity. The principle of polarization diversity reception is as follows.

[0031] Since the pre-distortion iterative learning algorithm suppresses the non-linear phenomenon of the frequency sweep of the distributed feedback laser 3, the laser output by the distributed feedback laser 3 is a linearly frequency-swept light, and its sweep frequency is expressed as:

[0032] f = f0 + γt (1)

[0033] where f0 is the initial frequency value, γ is the sweep slope, and t is the time.

[0034] The reference light reflected by the Faraday rotator mirror 7 is expressed as:

[0035]

[0036] where R is the reflectivity of the Faraday rotator mirror, E 01 is the amplitude of the reference light, and φ(t) is the initial phase of the reference light. The initial phase refers to the phase when the laser outputs from the distributed feedback laser 3.

[0037] The signal light reflected by the reflection point (such as the scattering enhancement point) on the fiber under test 6 is expressed as:

[0038]

[0039] where n represents the number of reflection points, R m is the reflectivity of the m-th reflection point, E 02 is the amplitude of the signal light, and τ m is the delay of the reference light relative to the m-th reflection point.

[0040] After being split by the polarization beam splitter 8, the reference light power is equally divided, and the signal light is power-distributed according to the angle between its polarization direction and the polarization axes of the two output ports of the polarization beam splitter 8. Therefore, the two paths of light output by the polarization beam splitter 8 are respectively expressed as:

[0041]

[0042] where, θ m is the angle between the polarization direction of the signal light reflected by the m-th reflection point and the polarization axis.

[0043] Accordingly, the two beams of light respectively enter two photodetectors for beat frequency, and the light intensities (beat frequency photocurrent signals) output by the two photodetectors are respectively expressed as:

[0044]

[0045] where Δφ = φ(t + τ m ) - φ(t).

[0046] Since therefore can be ignored. In addition, the first and second terms in formula (5) are DC terms, so formula (5) can be rewritten as:

[0047]

[0048] Formula (6) shows that the polarization diversity scheme of the embodiments of the present application and the above-mentioned long-distance telemetry Φ-OFDR system based on common-path self-reference can effectively suppress the polarization fading phenomenon while retaining the phase information related to vibration, and is applicable to long-distance remote sensing detection.

[0049] Moreover, since there are reflection points engraved on the fiber under test 6, the reflection of light by the reflection points is much stronger than that at other positions. Therefore, it can be considered that within the sensing region corresponding to the theoretical spatial resolution, the result of the coherent superposition of the signal light and the reference light depends on the reflection of the reflection points within this sensing region rather than on the reflection of other positions. And the local reflection source has strong reflection characteristics, avoiding the situation of coherent cancellation caused by random scattering. Therefore, using the fiber with reflection points engraved as the sensing fiber can solve the problem of coherent fading.

[0050] Perform Fourier transform on the above two beat frequency photocurrent signals, and use the rotating vector sum algorithm to perform vector summation on them, so as to efficiently suppress polarization fading, and the obtained signal is:

[0051]

[0052] where k represents the kth measurement, I x (k, f) and I y (k, f) respectively represent the results of the Fourier transform of the beat frequency signals measured in the x polarization and y polarization at the kth measurement. Specifically, the conjugate results of the Fourier transform of the beat frequency signals measured in the x polarization and y polarization in the first measurement (the first sweep period) are used as the reference of the rotating vector, so as to ensure that the rotating vector results of the two orthogonal polarization signals at the same position are aligned in the complex plane.

[0053] The above-mentioned long-distance telemetry Φ-OFDR system based on common-path self-reference is used to test multiple vibration events. Among them, the distributed feedback laser 3 outputs linearly chirped laser with a scanning frequency range of 15.6 GHz and a repetition frequency of 4000 Hz. The length of the transmission fiber 5 is 75.0136 km. A scattering enhancement point array is engraved on the fiber under test 6. The array contains a total of 48 scattering enhancement points. The interval between two adjacent scattering enhancement points is 4 cm. Two adjacent scattering enhancement points form a sensing area. The 48 scattering enhancement points form 47 sensing areas. The connection point of the transmission fiber 5 and the polarization-maintaining circulator 4 is used as the origin. The position of any point on the fiber (transmission fiber 5 and fiber under test 6) is represented by the distance from this point to the origin. The bandwidths of the first photodetector 9 and the second photodetector 10 are both 50 MHz, and the signal acquisition time is 400 ms. The test results are as Figure 2 and Figure 3 shown (the color reflects the magnitude of the vibration intensity). According to Figure 2 it can be known that within the time period of 400 ms, continuous vibrations occur in the 6th and 42nd sensing areas (near the position 75.0136 km from the origin and near the position 75.0152 km from the origin). The above system has a clean noise floor and strong crosstalk suppression ability. According to Figure 3 it can be known that vibrations with a frequency of 200 Hz occur between the two positions 75.0136 km from the origin and 75.0140 km from the origin, and vibrations with a frequency of 200 Hz occur between the two positions 75.0152 km from the origin and 75.0156 km from the origin.

[0054] The long-distance telemetry Φ-OFDR system based on common-path self-reference in this embodiment uses the strong reflected laser generated at the fiber end as the reference optical path, realizing a self-reference structure with a common path for the reference light and the signal light. At the same time, based on magneto-optic rotation polarization conjugate transmission, a polarization diversity scheme based on self-reference is constructed, and the polarization fading problem in the self-reference structure is solved by cooperating with the rotation vector sum algorithm, realizing polarization-independent coherent detection, thereby realizing ultra-long-distance telemetry sensing. At a telemetry distance of 75 km, the spatial resolution can reach 2 cm. The pre-distortion iterative learning algorithm is used to obtain the optimal pre-distortion modulation voltage waveform to directly modulate the laser, realizing the output of highly linearly chirped laser with a high repetition frequency of the distributed feedback laser, having the advantage of high frequency response while improving the spatial resolution of the system, thereby realizing local high spatio-temporal accuracy sensing. Using the fiber engraved with reflection points as the sensing fiber completely solves the coherent fading problem brought by ordinary single-mode fibers and improves the signal-to-noise ratio of the system. At the same time, the complexity and size of the sensing system are greatly reduced, realizing a super-compact dynamic sensing system.

[0055] Embodiment 2

[0056] Figure 4 It is a schematic diagram of the principle of the second long-distance telemetry Φ-OFDR system based on common-path self-reference according to an embodiment of the present application. As Figure 4 shown, the long-distance telemetry Φ-OFDR system based on common-path self-reference includes a light source, a polarization-maintaining circulator 4, a transmission optical fiber 5, a fiber under test 6, a reflector 7, a photodetector 8, a data acquisition module 9, and a calculation module 10.

[0057] The applicant found that there is a non-linear relationship between the frequency of the swept light output by a common light source and time t, resulting in the broadening of the swept light spectrum and reducing the spatial resolution of the Φ-OFDR system. Therefore, in this embodiment, a light source capable of generating linearly swept laser is adopted to provide the spatial resolution of the system.

[0058] In one implementation, the light source includes an arbitrary function generator 1, a laser diode controller 2, and a distributed feedback laser 3. The electrical signal output terminal of the arbitrary function generator 1 is connected to the electrical signal input terminal of the laser diode controller 2, and the electrical signal output terminal of the laser diode controller 2 is connected to the electrical signal input terminal of the distributed feedback laser 3. The distributed feedback laser 3 is equipped with an output optical fiber, and the output optical fiber is a polarization-maintaining optical fiber. The laser output by the distributed feedback laser 3 enters the output optical fiber in a way that the slow axis is aligned. The optical signal output terminal of the output optical fiber is used as the optical signal output terminal of the light source. The distributed feedback laser is a chirped laser, and the scanning frequency is in the range of [10 -2 ,10 2 GHz, and the repetition frequency is in the range of [10 -1 ,10 5 Hz.

[0059] The control signal of the arbitrary function generator 1 is a pre-distortion modulation voltage waveform. The optimal pre-distortion modulation voltage waveform is obtained by using the pre-distortion iterative learning algorithm and input into the arbitrary function generator 1. Under the action of the pre-distortion modulation voltage waveform, the arbitrary function generator 1 outputs a voltage signal, and this voltage signal is input into the laser diode controller 2 to modulate the output current of the laser diode controller 2. The laser diode controller 2 outputs temperature control information and the modulated current information. Under the action of the temperature control information and the modulated current, the root mean square of the non-linear error of the swept light output by the distributed feedback laser 3 reaches the minimum, realizing the output of stable linearly swept light.

[0060] The linearly chirped light generated by the light source enters the first port 4-1 of the polarization-maintaining circulator 4, exits from the second port 4-2 of the polarization-maintaining circulator 4, and then enters the transmission optical fiber 5. The polarization-maintaining circulator 4 is a polarization-maintaining circulator operating in a biaxial manner. The linearly chirped light generated by the light source can be aligned with the fast axis or the slow axis of the first port 4-1 of the polarization-maintaining circulator 4. In the embodiment of the present application, the slow axis is selected for alignment. Both the transmission optical fiber 5 and the fiber under test 6 are all polarization-maintaining optical fibers, and the length is in the range of [0, 10 5 m. The linearly chirped light is injected into the fiber under test 6 after passing through the transmission optical fiber 5. The fiber under test 6 is inscribed with a local reflection source array. For example, an array of scattering enhancement points (Rayleigh scattering) or an array of ultra-weak Bragg gratings is inscribed on the fiber under test 6. The inscription interval between two adjacent local reflection sources is in the range of [1 mm, 10 m]. When the randomly polarized light passes through the local reflection source array of the fiber under test 6, reflection occurs, and the reflected light is called the signal light (if the local reflection source is a scattering enhancement point, the randomly polarized light is scattered when passing through the scattering enhancement point of the fiber under test 6, and the scattered light is called the signal light). The signal light carries the phase information of the light at this position of the ultra-weak Bragg grating. The transmitted part is called the reference light. The reference light is reflected on the surface of the reflector 7 (the reflectivity of the reflector 7 is in the range of [1%, 100%]), and the reflected reference light is recoupled back into the fiber under test 6, enters the second port 4-2 of the polarization-maintaining circulator 4 together with the signal light, exits from the third port 4-3 of the polarization-maintaining circulator 4, and finally enters the photodetector 8 (the bandwidth of the photodetector 8 is in the range of [20 MHz, 1 GHz]), and beat frequency occurs inside the photodetector 8. The beat frequency signal output by the photodetector 8 is collected by the data acquisition module 11, and the collected signal is processed by the calculation module 12 to give the detection result.

[0061] In the embodiment of the present application, the reflector 7 can be any device with a reflection function. In one implementation, a reflective film, such as a gold film, is deposited on the end face of the fiber under test 6, and the gold film is used to reflect the linearly chirped light.

[0062] The calculation module 10 performs Fourier transform on the beat frequency signal collected in real time by the data acquisition module 9, directly obtains a polarization-independent frequency-domain signal, and uses a phase demodulation algorithm to extract all the dynamic and static information of the fiber under test area, such as vibration frequency and strain conditions, etc.

[0063] In the embodiment of the present application, the data acquisition module 11 can be implemented by an oscilloscope, and the calculation module 12 can be an application program embedded in a computer. The network port of the oscilloscope is connected to the network port of the computer.

[0064] In addition, the network port of the computer can also be connected to the network port of any function generator 1. The computer uses a pre-distortion iterative learning algorithm to obtain the optimal pre-distortion modulation voltage waveform and inputs it to the any function generator 1.

[0065] The long-distance telemetry Φ-OFDR system based on common-path self-reference in this embodiment can suppress the polarization fading phenomenon, and the principle is as follows.

[0066] Since the pre-distortion iterative learning algorithm is used to suppress the non-linearity of the laser frequency sweep, the laser outputs linearly frequency-swept light, and its scanning frequency is expressed as:

[0067] f = f0 + γt (8)

[0068] where f0 is the initial frequency value, γ is the scanning slope, and t is the time.

[0069] The reference light reflected by the reflector 7 is expressed as:

[0070]

[0071] where R is the reflectivity of the reflector 7, E 01 is the amplitude of the reference light, φ(t) is the initial phase of the reference light, and the initial phase refers to the phase when the laser outputs from the distributed feedback laser 3.

[0072] The signal light reflected by the reflection points (such as scattering enhancement points) on the fiber under test 6 is expressed as:

[0073]

[0074] where n represents the number of reflection points, R m is the reflectivity of the m-th reflection point, E 02 is the amplitude of the signal light, τ m is the delay of the reference light relative to the m-th reflection point.

[0075] The signal light and the reference light enter the photodetector 8 for beating, and the light intensity of the beating signal is expressed as:

[0076]

[0077] where Δφ = φ(t + τ m ) - φ(t).

[0078] Since it can be ignored. In addition, the first and second terms in formula (11) are DC terms, so formula (11) can be rewritten as:

[0079]

[0080] Perform Fourier transform on the photocurrent signal obtained by the above beating, and then use the frequency-domain phase demodulation algorithm to obtain all the information of the fiber region under test.

[0081] The long-distance telemetry Φ-OFDR system based on common-path self-reference in this embodiment uses the strong reflected laser generated at the fiber end as the reference optical path, realizing a self-reference structure with a common path for the reference light and the signal light. At the same time, all polarization-maintaining transmission fibers and fibers to be measured are used to solve the polarization fading problem, thereby realizing an ultra-long-distance telemetry sensing system. The pre-distortion iterative learning algorithm is used to obtain the optimal pre-distortion modulation voltage waveform to directly modulate the laser, realizing a high-repetition-rate and highly linear swept laser output of the distributed feedback laser, having the advantage of high frequency response while improving the spatial resolution of the system, thereby realizing local high spatio-temporal accuracy sensing. Using the fiber engraved with reflection points as the sensing fiber completely solves the coherent fading problem brought by ordinary single-mode fibers, improves the signal-to-noise ratio of the system, and at the same time greatly reduces the complexity and size of the sensing system, realizing an ultra-compact dynamic sensing system.

Claims

1. A long-distance telemetry Φ-OFDR system based on common-path self-referencing, characterized in that Comprising: A light source, a polarization-maintaining circulator, a transmission optical fiber, a fiber under test, a Faraday rotator mirror, a polarization beam splitter, two photodetectors, a data acquisition module, and a calculation module; The light source is used to generate linearly swept laser light; The fiber under test has a local reflection source array; The optical signal output end of the light source is connected to the first port of the polarization-maintaining circulator, the second port of the polarization-maintaining circulator is connected to one end of the transmission optical fiber, the other end of the transmission optical fiber is connected to the starting end of the fiber under test, the ending end of the fiber under test is connected to the Faraday rotator mirror, the third port of the polarization-maintaining circulator is connected to the input end of the polarization beam splitter, the two output ends of the polarization beam splitter are respectively connected to the optical signal input ends of the two photodetectors, the electrical signal output ends of the two photodetectors are respectively connected to the two electrical signal input ends of the data acquisition module, and the signal output port of the data acquisition module is connected to the signal input end of the calculation module.

2. The system according to claim 1, wherein The light source includes an arbitrary function generator, a laser diode controller, and a distributed feedback laser; The electrical signal output end of the arbitrary function generator is connected to the electrical signal input end of the laser diode controller, the electrical signal output end of the laser diode controller is connected to the electrical signal input end of the distributed feedback laser, and the optical signal output end of the distributed feedback laser serves as the optical signal output end of the light source.

3. The system according to claim 1, wherein The transmission optical fiber is a single-mode optical fiber.

4. The system according to claim 1 or 3, characterized in that, The fiber under test is a single-mode optical fiber.

5. The system according to claim 4, wherein The local reflection source array of the fiber under test is an array of scattering enhancement points inscribed on the fiber under test.

6. The system according to claim 4, wherein The local reflection source array of the fiber under test is an array of ultra-weak Bragg gratings inscribed on the fiber under test.

7. The system according to claim 1, characterized in that, When the input end of the polarization beam splitter is a fast-axis input or a slow-axis input, both output ends are slow-axis outputs and the power is equally divided.

8. A long-distance telemetry Φ-OFDR system based on common-path self-referencing, characterized in that Comprising: A light source, a polarization-maintaining circulator, a transmission optical fiber, a fiber under test, a reflector, a photodetector, a data acquisition module, and a calculation module; The light source is used to generate linearly swept laser light; The fiber under test has a local reflection source array; Both the transmission optical fiber and the fiber under test are fully polarization-maintaining optical fibers; The optical signal output end of the light source is connected to the first port of the polarization-maintaining circulator, the second port of the polarization-maintaining circulator is connected to one end of the transmission optical fiber, the other end of the transmission optical fiber is connected to the starting end of the fiber under test, the reflector is arranged at the ending end of the fiber under test, the third port of the polarization-maintaining circulator is connected to the optical signal input end of the photodetector, the electrical signal output end of the photodetector is connected to the electrical signal input end of the data acquisition module, and the signal output port of the data acquisition module is connected to the signal input end of the calculation module.

9. The system according to claim 8, wherein The light source includes an arbitrary function generator, a laser diode controller, and a distributed feedback laser; The electrical signal output end of the arbitrary function generator is connected to the electrical signal input end of the laser diode controller, the electrical signal output end of the laser diode controller is connected to the electrical signal input end of the distributed feedback laser, and the optical signal output end of the distributed feedback laser is the optical signal output end of the light source.

10. The system according to claim 8 or 9, characterized in that, The local reflection source array of the optical fiber to be measured is a scattering enhancement point array or an ultra-weak Bragg grating array inscribed on the optical fiber to be measured.

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