A forward transmission distributed optical fiber sensing device and method based on few-mode optical fiber

Abstract

The present invention discloses a forward transmission distributed optical fiber sensing device and method based on few-mode optical fiber. The device includes: a transmitting end, a dual-parameter sensing module and a receiving end. The transmitting end is used to generate two laser beams. The two laser beams pass through the dual-parameter sensing module in opposite directions. The dual-parameter sensing module adopts the few-mode optical fiber for forward transmission. The receiving end is used to collect and demodulate the spectral signal and generate a sensing signal. The present invention uses a forward transmission demodulation method to obtain higher optical power and signal-to-noise ratio; by utilizing the different effective refractive indexes of different modes in the few-mode optical fiber, combined with the mode-selective photon lantern technology, it is possible to effectively extract forward optical signals from each mode; by studying the temperature and strain response characteristics of the optical signal phase change under different modes, dual-parameter measurement of temperature and strain can be achieved on the few-mode optical fiber. The device of the present invention has the advantages of strong noise resistance and low R&D cost.

Description

Technical Field

[0001] The present invention relates to the field of optical sensing, and in particular to a forward transmission distributed optical fiber sensing device and method based on few-mode optical fiber. Background Art

[0002] Fiber optic sensors have the characteristics of miniaturization, corrosion resistance, immunity to electromagnetic interference, and long-distance monitoring. They have broad application prospects in oil wells, construction, biomedicine, energy and other fields. Traditional single-mode fiber (SMF) sensors only support single spatial mode transmission, so their sensing characteristics are difficult to adjust flexibly, and it is difficult to distinguish multiple sensing parameters at the same time. They can only sense a single parameter. Although multi-mode fiber (MMF) sensors have multi-mode characteristics, the internal multi-mode coupling phenomenon has become an obstacle to multi-parameter sensing, affecting the sensing effect.

[0003] Therefore, the prior art still needs to be improved and developed. Summary of the invention

[0004] The technical problem to be solved by the present invention is that, in view of the above-mentioned defects of the prior art, a forward transmission distributed optical fiber sensing device and method based on few-mode optical fiber is provided, aiming to solve the problem that single-mode optical fiber sensors in the prior art can only sense a single parameter and the multi-mode coupling problem of multi-mode optical fiber sensors in multi-parameter sensing.

[0005] The technical solution adopted by the present invention to solve the technical problem is as follows:

[0006] In a first aspect, an embodiment of the present invention further provides a double-ended demodulation and positioning device based on a few-mode optical fiber, wherein the device comprises:

[0007] A transmitting end, a dual-parameter sensing module and a receiving end, wherein the transmitting end is used to generate a first laser beam and a second laser beam, the first laser beam and the second laser beam pass through the dual-parameter sensing module in opposite directions, wherein the first laser beam generates a first spectrum signal through the dual-parameter sensing module, and the second laser beam generates a second spectrum signal through the dual-parameter sensing module, the dual-parameter sensing module adopts few-mode optical fiber for forward transmission, and the receiving end is used to collect and demodulate the first spectrum signal and the second spectrum signal and generate a sensing signal.

[0008] In one implementation, the transmitting end includes: a semiconductor laser, a fiber coupler and a 1*2 coupler coupled in sequence, wherein the semiconductor laser is used to emit laser light, the fiber coupler is used to split the laser light into a first laser beam and a second laser beam at the same power, and the 1*2 coupler is used to split the first laser beam into a first signal light and a first reference light, and split the second laser beam into a second signal light and a second reference light.

[0009] In one implementation, the dual-parameter sensing module includes: a first optical fiber circulator, a first mode-selective photon lantern, a few-mode optical fiber, a second mode-selective photon lantern, and a second optical fiber circulator connected in sequence, the first mode-selective photon lantern is used to perform mode separation on the second signal light to obtain a second fundamental mode spectrum and a high-order mode spectrum, the second mode-selective photon lantern is used to perform mode separation on the first signal light to obtain a first fundamental mode spectrum and a high-order mode spectrum, when the first mode-selective photon lantern is used as a mode multiplexer, the second mode-selective photon lantern is used as a mode demultiplexer, and when the second mode-selective photon lantern is used as a mode multiplexer, the first mode-selective photon lantern is used as a mode demultiplexer.

[0010] In one implementation, the first fiber optic circulator is used to receive the first signal light, and transmit the first signal light to the first mode selective photon lantern, the few-mode fiber, the second mode selective photon lantern, the second fiber optic circulator and the receiving end in sequence.

[0011] In one implementation, the second fiber optic circulator is used to receive the second signal light and transmit the second signal light to the second mode selective photon lantern, the few-mode fiber, the first mode selective photon lantern, the first fiber optic circulator and the receiving end in sequence.

[0012] In one implementation, a mixer is used to mix the first fundamental mode spectrum and the higher-order mode spectrum with the first reference light to obtain a first light intensity signal, and to mix the second fundamental mode spectrum and the higher-order mode spectrum with the second reference light to obtain a second light intensity signal;

[0013] A photoelectric detector, used for performing photoelectric conversion on the first light intensity signal and the second light intensity signal respectively to obtain a first sensing signal and a second sensing signal;

[0014] A data acquisition card is used to acquire and demodulate the first sensor signal and the second sensor signal.

[0015] In a second aspect, the present invention provides a forward transmission distributed optical fiber sensing method based on few-mode optical fiber, wherein the method comprises:

[0016] Emits laser light through a semiconductor laser, and splits the laser light at the same power through a fiber coupler to obtain a first laser beam and a second laser beam, splits the first laser beam into a first signal light and a first reference light, and splits the second laser beam into a second signal light and a second reference light;

[0017] Performing mode separation on the first signal light through a dual-parameter sensing module to obtain a first spectral signal, wherein the first spectral signal includes a fundamental mode spectrum and a high-order mode spectrum;

[0018] Performing mode separation on the second signal light through a dual-parameter sensing module to obtain a second spectral signal, wherein the second spectral signal includes a fundamental mode spectrum and a high-order mode spectrum;

[0019] The first spectrum signal and the second spectrum signal are transmitted in opposite directions along the few-mode optical fiber through the dual-parameter sensing module, the first spectrum signal and the first reference light are collected and demodulated through the receiving end to obtain a first sensing signal, and the second spectrum signal and the second reference light are collected and demodulated through the receiving end to obtain a second sensing signal;

[0020] According to the first sensing signal and the second sensing signal, the phase change value with temperature and the phase change value with strain are calculated, and the vibration position is located.

[0021] In one implementation, calculating the phase change value with temperature and the phase change value with strain according to the first sensing signal and the second sensing signal includes:

[0022] Obtaining a phase offset according to the first sensing signal and the second sensing signal;

[0023] Calculating a temperature offset and a strain offset according to the phase offset and a preset thermal expansion coefficient and a thermo-optical coefficient of the optical fiber material;

[0024] The phase variation with temperature is calculated based on the temperature offset and a preset temperature coefficient;

[0025] The phase variation with strain value is calculated based on the strain offset and a preset strain coefficient.

[0026] In one implementation, the calculating the phase variation with temperature and the phase variation with strain according to the first sensing signal and the second sensing signal, and locating the vibration position, further includes:

[0027] Mixing a first spectrum signal in the first sensing signal with a first reference light to obtain a first light intensity signal;

[0028] and mixing a second spectrum signal in the second sensing signal with a second reference light to obtain a second light intensity signal;

[0029] The first light intensity signal and the second light intensity signal are analyzed by a phase spectrum time delay positioning method to obtain a vibration position.

[0030] In one implementation, the analyzing the first light intensity signal and the second light intensity signal by a phase spectrum time delay positioning method to obtain a vibration position includes:

[0031] Calculating a phase signal of the first light intensity signal to obtain a first phase signal, and performing Fourier transform on the first phase signal to obtain a first Fourier phase spectrum;

[0032] Calculating a phase signal of the second light intensity signal to obtain a second phase signal, and performing Fourier transform on the second phase signal to obtain a second Fourier phase spectrum;

[0033] Obtaining a phase spectrum time delay difference according to a difference between the first Fourier phase spectrum and the second Fourier phase spectrum;

[0034] The vibration position is located according to the phase spectrum time delay difference.

[0035] In a third aspect, an embodiment of the present invention further provides an intelligent terminal, wherein the intelligent terminal comprises a memory, a processor, and a forward transmission distributed optical fiber sensing program based on few-mode optical fiber stored in the memory and executable on the processor, and when the processor executes the forward transmission distributed optical fiber sensing program based on few-mode optical fiber, the steps of the forward transmission distributed optical fiber sensing method based on few-mode optical fiber as described in any one of the above items are implemented.

[0036] In a fourth aspect, an embodiment of the present invention further provides a computer-readable storage medium, wherein a forward transmission distributed optical fiber sensing program based on few-mode optical fiber is stored on the computer-readable storage medium, and when the forward transmission distributed optical fiber sensing program based on few-mode optical fiber is executed by a processor, the steps of the forward transmission distributed optical fiber sensing method based on few-mode optical fiber as described in any one of the above items are implemented.

[0037] Beneficial effects: Compared with the prior art, the present invention provides a forward transmission distributed optical fiber sensing device and method based on few-mode optical fiber, which uses the forward transmission demodulation method to obtain higher optical power and signal-to-noise ratio; by utilizing the different effective refractive indexes of different modes in the few-mode optical fiber, combined with the mode selection photon lantern technology, the forward optical signal can be effectively extracted from each mode; by studying the temperature and strain response characteristics of the optical signal phase change under different modes, the temperature and strain dual parameter measurement can be realized on the few-mode optical fiber; the optical phase is extracted by the double-end signal delay method and the in-phase orthogonal technology to analyze the vibration position along the few-mode sensing optical fiber. The device of the present invention has the advantages of strong noise resistance and low R&D cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0039] Figure 1 It is a principle block diagram of a forward transmission distributed optical fiber sensing device based on few-mode optical fiber provided in an embodiment of the present invention.

[0040] Figure 2 Schematic diagram of the experimental setup of the dual-parameter measurement sensor system provided in an embodiment of the present invention.

[0041] Figure 3 Schematic diagram of the experimental setup of the double-terminal positioning sensor system provided in an embodiment of the present invention.

[0042] Figure 4 It is a schematic flow chart of a forward transmission distributed optical fiber sensing method based on few-mode optical fiber provided in an embodiment of the present invention.

[0043] Figure 5 It is a block diagram of the internal structure principle of the intelligent terminal provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0044] In order to make the purpose, technical solution and effect of the present invention clearer and more specific, the present invention is further described in detail with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0045] It will be understood by those skilled in the art that, unless expressly stated, the singular forms "one", "said", and "the" used herein may also include plural forms. It should be further understood that the term "comprising" used in the specification of the present invention refers to the presence of the features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof. It should be understood that when we refer to an element as being "connected" or "coupled" to another element, it may be directly connected or coupled to the other element, or there may be intermediate elements. In addition, the "connection" or "coupling" used herein may include wireless connection or wireless coupling. The term "and / or" used herein includes all or any unit and all combinations of one or more associated listed items.

[0046] It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as generally understood by those skilled in the art in the art to which the present invention belongs. It should also be understood that terms such as those defined in general dictionaries should be understood to have meanings consistent with the meanings in the context of the prior art, and will not be interpreted with idealized or overly formal meanings unless specifically defined as herein.

[0047] Fiber optic sensor is a sensor that converts the state of the measured object into a measurable optical signal. Fiber optic sensor sends the incident light beam from the light source into the modulator through the optical fiber. The interaction with the external measured parameters in the modulator changes the optical properties of the light, such as the intensity, wavelength, frequency, phase, polarization state, etc., and becomes a modulated optical signal. The light signal is then sent to the optoelectronic device through the optical fiber and the measured parameters are obtained after the demodulator. Few-mode fiber realizes different sensitivity of different linear polarization modes to external parameters such as temperature and strain by its orthogonality characteristics between different modes. When these parameters change, the transmission characteristics of different linear polarization modes (such as phase shift, loss, etc.) will also change accordingly. Multiple parameters can be measured simultaneously by monitoring the response changes of these modes. In addition, few-mode fiber also has a higher signal-to-noise ratio and a higher nonlinear power threshold, which further improves its sensing performance.

[0048] The present invention designs a dual-parameter distributed optical fiber sensor based on few-mode optical fiber, which uses the forward transmission in-phase / orthogonal demodulation method to replace the traditional back-Rayleigh scattering technology to obtain higher optical power and signal-to-noise ratio. By utilizing the different effective refractive indexes of different modes in the few-mode optical fiber and combining the mode-selective photon lantern technology, the forward optical signal can be effectively extracted from each mode. By studying the temperature and strain response characteristics of the optical signal phase change under different modes, the dual-parameter measurement of temperature and strain can be realized on the few-mode optical fiber, and the double-end signal delay method is used to extract the optical phase for analyzing the vibration position along the few-mode sensing optical fiber, which simplifies the structure of the sensing system and reduces the system complexity.

[0049] Exemplary Devices

[0050] This embodiment provides a forward transmission distributed optical fiber sensing device based on few-mode optical fiber, and this embodiment can be applied to optical fiber sensors. Figure 1 As shown, the device comprises:

[0051] A transmitting end 10, a dual-parameter sensing module 20 and a receiving end 30, wherein the transmitting end 10 is used to generate a first laser beam and a second laser beam, the first laser beam and the second laser beam pass through the dual-parameter sensing module 20 in opposite directions, wherein the first laser beam generates a first spectral signal through the dual-parameter sensing module 20, and the second laser beam generates a second spectral signal through the dual-parameter sensing module 20, the dual-parameter sensing module 20 adopts few-mode optical fiber forward transmission, and the receiving end 30 is used to collect and demodulate the first spectral signal and the second spectral signal and generate a sensing signal.

[0052] The transmitting end 10 comprises:

[0053] A semiconductor laser, an optical fiber coupler and a 1*2 coupler are coupled in sequence, wherein the semiconductor laser is used to emit laser light, the optical fiber coupler is used to split the laser light into a first laser beam and a second laser beam with the same power, the 1*2 coupler is used to split the first laser beam into a first signal light and a first reference light, and split the second laser beam into a second signal light and a second reference light.

[0054] In one embodiment, the dual-parameter sensing module 20 includes: a first optical fiber circulator, a first mode-selective photon lantern, a few-mode optical fiber, a second mode-selective photon lantern, and a second optical fiber circulator connected in sequence, wherein the first mode-selective photon lantern is used to perform mode separation on the second signal light to obtain a second fundamental mode spectrum and a higher-order mode spectrum, and the second mode-selective photon lantern is used to perform mode separation on the first signal light to obtain a first fundamental mode spectrum and a higher-order mode spectrum. When the first mode-selective photon lantern is used as a mode multiplexer, the second mode-selective photon lantern is used as a mode demultiplexer, and when the second mode-selective photon lantern is used as a mode multiplexer, the first mode-selective photon lantern is used as a mode demultiplexer.

[0055] In one embodiment, a laser beam with a wavelength of 1550 nm is emitted by a semiconductor laser 10 and enters a 3 dB fiber coupler 20 to be divided into two beams at a 1:1 optical power ratio. The two beams of light are then respectively divided into a reference light beam and a signal light beam through a 1*2 fiber coupler. The first signal light beam enters the first fiber circulator clockwise, passes through the dual parameter sensing module 40, and is output from the second fiber circulator 50. The other signal light beam is input from the second fiber circulator counterclockwise, passes through the dual parameter sensing module 40, and is output from the first fiber circulator.

[0056] In one embodiment, the first fiber optic circulator is used to receive the first signal light and transmit the first signal light to the first mode selective photon lantern, the few-mode fiber, the second mode selective photon lantern, the second fiber optic circulator and the receiving end in sequence.

[0057] In one embodiment, the second fiber optic circulator is used to receive the second signal light and transmit the second signal light to the second mode selective photon lantern, the few-mode fiber, the first mode selective photon lantern, the first fiber optic circulator and the receiving end in sequence.

[0058] Specifically, achieving temperature and strain separation requires separating the transmitted light signals from different optical modes. Mode-selective photon lanterns can convert multiple fundamental modes to higher-order modes, and vice versa. Few-mode fiber, with its orthogonality between different modes, effectively avoids the influence of inter-modal dispersion and realizes independent and efficient transmission of signals in optical fiber. It has significant advantages in optical sensing due to its low insertion loss, small inter-mode crosstalk and high transmission efficiency.

[0059] In one embodiment, the dual-parameter sensing module is implemented by an experimental setup of a dual-parameter measurement sensing system, such as Figure 2As shown, the laser emitted by the transmitting end laser is divided into one reference light and one signal light of equal power through a 1*2 fiber coupler. The signal light is transmitted through a few-mode fiber, wherein the few-mode fiber passes through a vibration source to change the strain, and then passes through a temperature controller to change the fiber temperature to modulate the reference light. The signal light is sequentially separated into optical modes by a mode multiplexer and a mode demultiplexer in the few-mode fiber. In this embodiment, a mode-selective photon lantern is used as a mode multiplexer and a mode demultiplexer. After the reference light passes through a 1*2 fiber coupler, it is divided into two local oscillator lights of equal power, and is received by the receiving end together with the signal light after mode separation. In one implementation, a water bath thermostat is used as a temperature controller to change the temperature of the few-mode fiber, a phase shifter is used as a vibration source to change the strain of the few-mode fiber, and a 3*3 coupler can also be used to demodulate the phase.

[0060] In one embodiment, the first mode selective photon lantern is coupled to the first optical fiber circulator, and the second mode selective photon lantern is coupled to the second optical fiber circulator, wherein the first mode selective photon lantern and the second mode selective photon lantern serve as a mode multiplexer and a mode demultiplexer for exciting a fundamental mode spectrum and a higher-order mode spectrum.

[0061] In one embodiment, the receiving end includes: mixing the first fundamental mode spectrum and the higher-order mode spectrum with the first reference light to obtain a first light intensity signal, and mixing the second fundamental mode spectrum and the higher-order mode spectrum with the second reference light to obtain a second light intensity signal; a photodetector, used to perform photoelectric conversion on the first light intensity signal and the second light intensity signal, respectively, to obtain a first sensing signal and a second sensing signal; and a data acquisition card, used to acquire and demodulate the first sensing signal and the second sensing signal.

[0062] Specifically, different linear polarization fiber modes in few-mode fiber have different sensitivities to external parameters such as temperature and strain. When these parameters change, the transmission characteristics of different linear polarization fiber modes, such as phase shift and loss, will also change. By monitoring the response changes of these modes, multiple parameters can be measured simultaneously.

[0063] In this embodiment, the forward transmission distributed optical fiber sensing device based on few-mode optical fiber adopts a double-end structure. The external vibration signal experiences an obvious time delay between the two detection ends. The vibration position can be calculated according to the positioning algorithm. However, since the relevant algorithm often encounters the problem of multiple peaks when processing stationary periodic signals, the positioning accuracy is affected. Therefore, this embodiment adopts the phase spectrum time delay method to improve the signal positioning accuracy in the frequency domain. This embodiment can calculate the time delay difference from the two demodulation ends to the vibration signal through the phase spectrum time delay method, and the forward transmission temperature and vibration are simultaneously measured and the FMF double-end forward transmission distributed sensing system.

[0064] In one embodiment, the first signal light and the second signal light pass through the dual-parameter sensing module in opposite directions, thereby realizing dual-end positioning sensing. The experimental setting of the dual-end positioning sensing system of this embodiment is as follows: Figure 3 As shown, the first signal light generated by the transmitting end enters the mode multiplexer, i.e., the first mode selective photon lantern, clockwise, and enters the few-mode sensing fiber for transmission after mode conversion. The few-mode fiber passes through a vibration source to change the strain, and then passes through a temperature controller to change the fiber temperature. After the first signal light is modulated, the first signal light passes through a mode demultiplexer, i.e., the second mode selective photon lantern, for mode demultiplexing to obtain the first fundamental mode spectrum and the high-order mode spectrum and output to the receiving end. Similarly, the second signal light generated by the transmitting end enters the mode multiplexer, i.e., the second mode selective photon lantern, counterclockwise, and enters the few-mode sensing fiber after mode conversion. The few-mode fiber passes through a temperature controller to change the fiber temperature, and then passes through a vibration source to change the strain. After the second signal light is modulated, it passes through a mode demultiplexer, i.e., the first mode selective photon lantern, for multiplexing to obtain the second fundamental mode spectrum and the high-order mode spectrum, and then output to the receiving end, thus realizing double-end positioning sensing.

[0065] Specifically, the laser beam is split into two beams of equal power through a 1*2 coupler, and the photon lantern is used to selectively emit light into different modes of the few-mode fiber. After the pumped fundamental mode mode enters the sensing few-mode fiber, it not only excites the fundamental mode mode, but also generates high-order modes. The forward transmission light trajectory in the fundamental mode mode is captured from the corresponding fundamental mode mode port of the second mode-selective photon lantern, while the forward transmission light trajectory in the high-order mode is captured from the corresponding high-order port of the second mode-selective photon lantern.

[0066] In this embodiment, the signal light (S) from the fundamental mode and the high-order mode are mixed with two reference lights and input to the mixer for signal demodulation. Specifically, the signal demodulation part is mainly composed of a 1*2 fiber coupler, a mixer and a photodetector. The laser passes through a 1*2 coupler to split the laser beam into two beams of equal power. One reference light is input to the reference arm and used as the local oscillator (LO) input of the optical mixer, while the other signal light is input to the sensor arm. The sensor arm signal light (S) is mixed with the reference light of the local oscillator and input to the 90° mixer. Using the 90° mixing algorithm, the internal phase (I) and orthogonal (Q) components of the forward optical signal can be detected, thereby demodulating the phase and amplitude information of the optical signal without an interferometer structure, and then using a photodetector to perform photoelectric conversion on the output light intensity signal. Finally, the data acquisition card collects the sensor signal for demodulation to obtain the sensor signal. The phase analysis unit realizes the measurement of dual parameter coefficients for the response characteristics of temperature and strain. The vibration analysis unit can calculate the time delay difference from the two demodulation ends to the vibration signal through the phase spectrum time delay method, and then calculate the vibration position.

[0067] Exemplary Methods

[0068] like Figure 4 As shown in, this embodiment also provides a forward transmission distributed optical fiber sensing method based on few-mode optical fiber, the method comprising:

[0069] Step S100, emitting laser light through a semiconductor laser, and dividing the laser light at the same power through a fiber coupler to obtain a first laser beam and a second laser beam, dividing the first laser beam into a first signal light and a first reference light, and dividing the second laser beam into a second signal light and a second reference light;

[0070] Step S200: performing mode separation on the first signal light through a dual-parameter sensing module to obtain a first spectrum signal, wherein the first spectrum signal includes a fundamental mode spectrum and a high-order mode spectrum;

[0071] Step S300, performing mode separation on the second signal light through a dual-parameter sensing module to obtain a second spectrum signal, wherein the second spectrum signal includes a fundamental mode spectrum and a high-order mode spectrum;

[0072] Specifically, achieving temperature and strain separation requires separating the transmitted light signal from different optical modes. The mode-selective photon lantern can convert multiple fundamental modes to higher-order modes and vice versa. Therefore, it is mainly used as a mode multiplexer / demultiplexer in space division multiplexing. In this embodiment, the mode-selective photon lantern is used for multiplexing and demultiplexing to achieve the separation of forward light signals of different modes.

[0073] In one embodiment, dual-parameter sensing is achieved by demodulating the phase changes of different modes (fundamental mode and higher-order mode) caused by temperature and strain. Specifically, the few-mode sensing fiber used can support two spatial modes around 1550nm: fundamental mode and higher-order mode. A mode-selective photonic lantern is used as a mode multiplexer / demultiplexer to perform mode selection.

[0074] For example, each mode-selective photon lantern includes three single-mode ends and two optical fibers. The three single-mode ends correspond to the LP01, LP11a, and LP11b modes, respectively. If input is from the LP01 end of the single-mode port 1, it will mainly excite the LP01 mode, and the LP11 mode will also be slightly excited. Similarly, if input is from the other two single-mode ends: LP11a and LP11b, the LP11 mode will be mainly excited, and some LP01 modes will also be excited. In this way, the mode-selective photon lantern can excite the LP01 fundamental mode spectrum and the LP11 high-order mode spectrum.

[0075] Step S400, transmitting the first spectrum signal and the second spectrum signal in opposite directions along the few-mode optical fiber through the dual-parameter sensing module, collecting and demodulating the first spectrum signal and the first reference light through the receiving end to obtain a first sensing signal, and collecting and demodulating the second spectrum signal and the second reference light through the receiving end to obtain a second sensing signal;

[0076] Specifically, the separation of temperature and strain requires the separation of transmitted light signals from different optical modes. The mode-selective photon lantern can convert multiple fundamental modes to higher-order modes and vice versa. Therefore, it is mainly used as a mode multiplexer / demultiplexer in space division multiplexing. In this embodiment, the mode-selective photon lantern is used for multiplexing and demultiplexing to achieve the separation of forward light signals of different modes, and the temperature of the optical fiber is changed through a water bath thermostat, and the strain is changed through a phase shifter to modulate the first spectrum signal and the second spectrum signal.

[0077] Step S500: Calculate the phase change value with temperature and the phase change value with strain according to the first sensing signal and the second sensing signal, and locate the vibration position.

[0078] Specifically, fiber phase modulation is to change the phase of the light wave propagating in the optical fiber through the change of physical quantities, such as temperature and pressure, and then use interference technology to convert this phase change into amplitude change, thereby realizing the measurement of physical quantities. During the transmission of optical signals in optical fibers, their phase will be affected by factors such as the length and refractive index of the optical fiber. The phase of a fixed frequency light wave passing through an optical fiber can be expressed as:

[0079] ,

[0080] in, is the phase of light, is the effective refractive index of the mode, L is the length of the sensing fiber, is the wave number in vacuum, , is the wavelength, is the propagation constant of the optical fiber. Through optical fiber phase modulation, the response characteristics of the phase change of the optical signal to temperature and strain in different modes can be further calculated. In this embodiment, the temperature and strain dual parameter measurements of the first sensing signal and the second sensing signal can be used to analyze the phase change value with temperature and the phase change value with strain, and locate the vibration position along the few-mode sensing optical fiber.

[0081] In one implementation, step S500 in this embodiment includes the following steps:

[0082] Step S501, obtaining a phase offset according to the first sensing signal and the second sensing signal;

[0083] Specifically, when there is an external disturbance, the phase shift caused is:

[0084] ,

[0085] In the formula, is the axial expansion of the optical fiber, is the change in fiber diameter caused by axial stretching. The first term is the change in fiber length caused by elastic deformation. and They represent the photoelastic effect and waveguide effect of optical fiber respectively. Since the phase change caused by the waveguide effect is two or three orders of magnitude smaller than the first two terms, it can be ignored.

[0086] The phase offset can be approximately expressed as:

[0087] ,

[0088] The relationship between the phase shift and the strain can be obtained from the above formula:

[0089] ,

[0090] in , is the effective elastic-optical coefficient, and is the axial and radial strains. From the above formula, we can see that the phase shift is proportional to the optical fiber strain caused by the disturbance. In this way, the phase offset of the first sensing signal and the second sensing signal can be obtained: .

[0091] Step S502, calculating a temperature offset and a strain offset according to the phase offset and a preset thermal expansion coefficient and thermo-optic coefficient of the optical fiber material;

[0092] Step S503, calculating the phase change value with temperature according to the temperature offset and a preset temperature coefficient;

[0093] Step S504: Calculate the phase variation with strain according to the strain offset and a preset strain coefficient.

[0094] Specifically, when the temperature changes, the phase of the optical signal shifts, and the temperature sensitivity of the sensor can be expressed as:

[0095] ,

[0096] In the formula, and Respectively represent the thermal expansion coefficient and thermo-optical coefficient of the optical fiber material, so that the phase shift can be obtained .

[0097] Specifically, when there are external factors such as temperature or strain, the refractive index of different modes leads to different phases in different modes. In order to achieve dual-parameter sensing, two optical modes with different strain and temperature sensitivities are required. The phase changes of the two optical modes with temperature and strain can be expressed as:

[0098] ,

[0099] in , and , denote the gauge coefficient and temperature coefficient of modes LP01 and LP11a respectively. If , is the phase variation with strain is the phase change with temperature. From the above formula, the phase change with temperature and the phase change with strain are:

[0100] ,

[0101] Step S505: Mix the first spectrum signal in the first sensing signal with the first reference light to obtain a first light intensity signal;

[0102] Step S506: Mix the second spectrum signal in the second sensing signal with the second reference light to obtain a second light intensity signal;

[0103] In this embodiment, the laser beam is split into two beams of equal power by passing through a 1*2 coupler. One beam is the reference arm, which is used as the local oscillator (LO) input of the optical mixer, and the other is the sensor arm. The sensor arm signal light (S) is mixed with the LO and input into the 90° mixer. The internal phase (I) and orthogonal (Q) components of the forward optical signal can be detected by using the 90° mixing algorithm. The signal light (S) from the LP01 and LP11a modes is mixed with the two local oscillators and input into the mixer. Then, the photoelectric detector is used to perform photoelectric conversion on the output first light intensity signal and the second light intensity signal. Finally, the data acquisition card collects the sensor signal for demodulation.

[0104] Step S507: Analyze the first light intensity signal and the second light intensity signal by using a phase spectrum time delay positioning method to obtain a vibration position.

[0105] In one implementation, step S507 in this embodiment includes the following steps:

[0106] Step S5071: Calculate the phase signal of the first light intensity signal to obtain a first phase signal, and perform Fourier transform on the first phase signal to obtain a first Fourier phase spectrum;

[0107] Step S5072: Calculate the phase signal of the second light intensity signal to obtain a second phase signal, and perform Fourier transform on the second phase signal to obtain a second Fourier phase spectrum;

[0108] Step S5073, obtaining a phase spectrum time delay difference according to a difference between the first Fourier phase spectrum and the second Fourier phase spectrum;

[0109] Step S5074: locate the vibration position according to the phase spectrum time delay difference.

[0110] Specifically, the phase spectrum time delay positioning method is as follows: Assume is the phase change caused by external vibration, is the time it takes for light to propagate from the vibration position to the second fiber circulator, is the time it takes for light to propagate from the vibration position to the first optical fiber circulator, then the delay difference is:

[0111] ,

[0112] In one embodiment, the first phase signal and the second phase signal are calculated by IQ demodulation. The phase signals received and demodulated by the second fiber optic circulator and the first fiber optic circulator can be written as:

[0113] First phase signal: ,

[0114] Second phase signal: ,

[0115] Obviously, ,

[0116] For the phase signal and , after Fourier transformation, we get the first Fourier phase spectrum and the second Fourier phase spectrum , respectively expressed as:

[0117] ,

[0118] ,

[0119] After phase subtraction, the phase spectrum time delay difference is ,

[0120] Finally, the vibration position can be determined by:

[0121] ,

[0122] in, is the refractive index of the optical fiber, The speed of light.

[0123] In one implementation, the phase spectrum time delay positioning method can be implemented by the cross-correlation (xcorr) algorithm and the fast Fourier transform FFT algorithm of MATLAB.

[0124] Based on the above embodiments, the present invention further provides an intelligent terminal, whose principle block diagram can be shown as follows: Figure 5 As shown. The intelligent terminal includes a processor, a memory, a network interface, a display screen, and a temperature sensor connected through a system bus. Among them, the processor of the intelligent terminal is used to provide computing and control capabilities. The memory of the intelligent terminal includes a non-volatile storage medium and an internal memory. The non-volatile storage medium stores an operating system and a computer program. The internal memory provides an environment for the operation of the operating system and the computer program in the non-volatile storage medium. The network interface of the intelligent terminal is used to communicate with an external terminal through a network connection. When the computer program is executed by the processor, a forward transmission distributed optical fiber sensing method based on few-mode optical fiber is implemented. The display screen of the intelligent terminal can be a liquid crystal display screen or an electronic ink display screen, and the temperature sensor of the intelligent terminal is pre-set inside the intelligent terminal to detect the operating temperature of the internal device.

[0125] Those skilled in the art will understand that Figure 5The principle block diagram shown in the figure is only a block diagram of a partial structure related to the scheme of the present invention, and does not constitute a limitation on the smart terminal to which the scheme of the present invention is applied. A specific smart terminal may include more or fewer components than those shown in the figure, or combine certain components, or have a different arrangement of components.

[0126] In one embodiment, a smart terminal is provided, the smart terminal comprising a memory, a processor, and a forward transmission distributed optical fiber sensing program based on few-mode optical fiber stored in the memory and executable on the processor, and when the processor executes the forward transmission distributed optical fiber sensing program based on few-mode optical fiber, the following operation instructions are implemented:

[0127] Emits laser light through a semiconductor laser, and splits the laser light at the same power through a fiber coupler to obtain a first laser beam and a second laser beam, splits the first laser beam into a first signal light and a first reference light, and splits the second laser beam into a second signal light and a second reference light;

[0128] Performing mode separation on the first signal light through a dual-parameter sensing module to obtain a first spectral signal, wherein the first spectral signal includes a fundamental mode spectrum and a high-order mode spectrum;

[0129] Performing mode separation on the second signal light through a dual-parameter sensing module to obtain a second spectral signal, wherein the second spectral signal includes a fundamental mode spectrum and a high-order mode spectrum;

[0130] The first spectrum signal and the second spectrum signal are transmitted in opposite directions along the few-mode optical fiber through the dual-parameter sensing module, the first spectrum signal and the first reference light are collected and demodulated through the receiving end to obtain a first sensing signal, and the second spectrum signal and the second reference light are collected and demodulated through the receiving end to obtain a second sensing signal;

[0131] According to the first sensing signal and the second sensing signal, the phase change value with temperature and the phase change value with strain are calculated, and the vibration position is located.

[0132] A person of ordinary skill in the art can understand that all or part of the processes in the above-mentioned embodiment methods can be implemented by instructing the relevant hardware through a computer program, and the computer program can be stored in a non-volatile computer-readable storage medium. When the computer program is executed, it can include the processes of the embodiments of the above-mentioned methods. Among them, any reference to memory, storage, operating database or other media used in the embodiments provided by the present invention may include non-volatile and / or volatile memory. Non-volatile memory may include read-only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable programmable ROM (EEPROM) or flash memory. Volatile memory may include random access memory (RAM) or external cache memory. By way of illustration and not limitation, RAM is available in many forms, such as static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDRSDRAM), enhanced SDRAM (ESDRAM), synchronous link (Synchlink) DRAM (SLDRAM), memory bus (Rambus) direct RAM (RDRAM), direct memory bus dynamic RAM (DRDRAM), and memory bus dynamic RAM (RDRAM), etc.

[0133] In summary, the present invention discloses a forward transmission distributed optical fiber sensing device and method based on few-mode optical fiber, the device includes: a transmitting end, a dual-parameter sensing module and a receiving end, the transmitting end is used to generate two laser beams, the two laser beams pass through the dual-parameter sensing module in opposite directions, the dual-parameter sensing module adopts few-mode optical fiber for forward transmission, and the receiving end is used to collect and demodulate the spectral signal and generate a sensing signal. The present invention uses a forward transmission demodulation method to obtain higher optical power and signal-to-noise ratio; by utilizing the different effective refractive indices of different modes in the few-mode optical fiber, combined with the mode-selective photon lantern technology, it is possible to effectively extract forward optical signals from each mode; by studying the temperature and strain response characteristics of the optical signal phase change under different modes, dual-parameter measurement of temperature and strain can be achieved on the few-mode optical fiber. The device of the present invention has the advantages of strong noise resistance and low R&D cost.

[0134] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A forward transmission distributed optical fiber sensing device based on few-mode optical fiber, characterized in that: The device comprises: a transmitting end, a dual-parameter sensing module and a receiving end, wherein the transmitting end is used to generate a first laser beam and a second laser beam, the first laser beam and the second laser beam pass through the dual-parameter sensing module in opposite directions, wherein the first laser beam generates a first spectrum signal through the dual-parameter sensing module, and the second laser beam generates a second spectrum signal through the dual-parameter sensing module, the dual-parameter sensing module adopts few-mode optical fiber for forward transmission, and the receiving end is used to collect and demodulate the first spectrum signal and the second spectrum signal and generate a sensing signal; The transmitting end comprises: A semiconductor laser, a fiber coupler and a 1*2 coupler coupled in sequence, wherein the semiconductor laser is used to emit laser light, the fiber coupler is used to split the laser light into a first laser beam and a second laser beam at the same power, the 1*2 coupler is used to split the first laser beam into a first signal light and a first reference light, and the second laser beam into a second signal light and a second reference light; The dual-parameter sensing module includes: a first optical fiber circulator, a first mode-selective photon lantern, a few-mode optical fiber, a second mode-selective photon lantern, and a second optical fiber circulator connected in sequence, the first mode-selective photon lantern is used to perform mode separation on the second signal light to obtain a second fundamental mode spectrum and a high-order mode spectrum, the second mode-selective photon lantern is used to perform mode separation on the first signal light to obtain a first fundamental mode spectrum and a high-order mode spectrum, when the first mode-selective photon lantern is used as a mode multiplexer, the second mode-selective photon lantern is used as a mode demultiplexer, and when the second mode-selective photon lantern is used as a mode multiplexer, the first mode-selective photon lantern is used as a mode demultiplexer.

2. The forward transmission distributed optical fiber sensing device based on few-mode optical fiber according to claim 1 is characterized in that: The first optical fiber circulator is used to receive the first signal light, and transmit the first signal light to the first mode selective photon lantern, the few-mode optical fiber, the second mode selective photon lantern, the second optical fiber circulator and the receiving end in sequence.

3. The forward transmission distributed optical fiber sensing device based on few-mode optical fiber according to claim 1, characterized in that: The second optical fiber circulator is used to receive the second signal light, and transmit the second signal light to the second mode selective photon lantern, the few-mode optical fiber, the first mode selective photon lantern, the first optical fiber circulator and the receiving end in sequence.

4. The forward transmission distributed optical fiber sensing device based on few-mode optical fiber according to claim 1, characterized in that: The receiving end comprises: A mixer, configured to mix the first fundamental mode spectrum and the higher-order mode spectrum with the first reference light to obtain a first light intensity signal, and to mix the second fundamental mode spectrum and the higher-order mode spectrum with the second reference light to obtain a second light intensity signal; A photoelectric detector, used for performing photoelectric conversion on the first light intensity signal and the second light intensity signal respectively to obtain a first sensing signal and a second sensing signal; A data acquisition card is used to acquire and demodulate the first sensor signal and the second sensor signal.

5. A forward transmission distributed optical fiber sensing method based on few-mode optical fiber, characterized in that: The method is based on the forward transmission distributed optical fiber sensing device based on few-mode optical fiber according to any one of claims 1 to 4, comprising: Emits laser light through a semiconductor laser, and splits the laser light at the same power through a fiber coupler to obtain a first laser beam and a second laser beam, splits the first laser beam into a first signal light and a first reference light, and splits the second laser beam into a second signal light and a second reference light; Performing mode separation on the first signal light through a dual-parameter sensing module to obtain a first spectral signal, wherein the first spectral signal includes a fundamental mode spectrum and a high-order mode spectrum; Performing mode separation on the second signal light through a dual-parameter sensing module to obtain a second spectral signal, wherein the second spectral signal includes a fundamental mode spectrum and a high-order mode spectrum; The first spectrum signal and the second spectrum signal are transmitted in opposite directions along the few-mode optical fiber through the dual-parameter sensing module, the first spectrum signal and the first reference light are collected and demodulated through the receiving end to obtain a first sensing signal, and the second spectrum signal and the second reference light are collected and demodulated through the receiving end to obtain a second sensing signal; According to the first sensing signal and the second sensing signal, the phase change value with temperature and the phase change value with strain are calculated, and the vibration position is located.

6. The forward transmission distributed optical fiber sensing method based on few-mode optical fiber according to claim 5 is characterized in that: The calculating, according to the first sensing signal and the second sensing signal, a phase variation value with temperature and a phase variation value with strain, comprises: Obtaining a phase offset according to the first sensing signal and the second sensing signal; Calculating a temperature offset and a strain offset according to the phase offset and a preset thermal expansion coefficient and a thermo-optical coefficient of the optical fiber material; The phase variation with temperature is calculated based on the temperature offset and a preset temperature coefficient; The phase variation with strain value is calculated based on the strain offset and a preset strain coefficient.

7. The forward transmission distributed optical fiber sensing method based on few-mode optical fiber according to claim 5, characterized in that: The method of calculating the phase variation with temperature and the phase variation with strain according to the first sensing signal and the second sensing signal, and locating the vibration position, further includes: Mixing a first spectrum signal in the first sensing signal with a first reference light to obtain a first light intensity signal; Mixing a second spectrum signal in the second sensing signal with a second reference light to obtain a second light intensity signal; The first light intensity signal and the second light intensity signal are analyzed by a phase spectrum time delay positioning method to obtain a vibration position.

8. The forward transmission distributed optical fiber sensing method based on few-mode optical fiber according to claim 7 is characterized in that: The step of analyzing the first light intensity signal and the second light intensity signal by a phase spectrum time delay positioning method to obtain a vibration position includes: Calculating a phase signal of the first light intensity signal to obtain a first phase signal, and performing Fourier transform on the first phase signal to obtain a first Fourier phase spectrum; Calculating a phase signal of the second light intensity signal to obtain a second phase signal, and performing Fourier transform on the second phase signal to obtain a second Fourier phase spectrum; Obtaining a phase spectrum time delay difference according to a difference between the first Fourier phase spectrum and the second Fourier phase spectrum; The vibration position is located according to the phase spectrum time delay difference.

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