Distributed optical fiber sensing system and optical fiber communication system

Abstract

The invention provides a distributed optical fiber sensing system and an optical fiber communication system, and the system comprises a pulse light unit, a first spatial light modulator, a circulator, a temperature measurement unit, and a controller. The controller is used for receiving the anti-Stokes Raman scattering light and the Stokes Raman scattering light to obtain temperature information of the optical fiber to be measured according to a preset rule, and is also used for obtaining pre-compensated spatial phase distribution through a preset optical fiber mode intrinsic equation, transverse refractive index distribution of the optical fiber to be measured and a coupling effect among modes of the optical fiber; the pre-compensated spatial phase distribution is sent to the first spatial light modulator; and the first spatial light modulator is used for modulating the pre-compensated spatial phase distribution into the pulsed light incident from the pulsed light unit so as to carry out spatial filtering and shaping on the pulsed light, realize mode coupling pre-compensation and inhibit the generation of mode coupling, thereby alleviating the problem of deterioration of the spatial resolution of the multimode distributed temperature sensing system.

Description

Technical Field

[0001] This invention relates to the field of distributed optical fiber sensing technology, and more particularly to a distributed optical fiber sensing system and an optical fiber communication system. Background Technology

[0002] Distributed fiber optic sensing systems can measure information along fiber optic lines and are widely used for long-distance line monitoring. Distributed temperature sensing systems, in particular, obtain fiber temperature information by measuring the spontaneous Raman scattering of light from the fiber. However, the spontaneous Raman scattering coefficient of fiber is very small, limiting the accuracy of temperature measurement and making it difficult to accurately measure fiber temperature in practical applications. To improve the measurement accuracy of the system, existing technologies often use multimode fiber as the medium, that is, multimode fiber as the sensing fiber. This is because multimode fiber can withstand higher incident light power, thus obtaining higher power spontaneous Raman scattering light and improving temperature demodulation accuracy. Summary of the Invention

[0003] Through extensive practical experience, the inventors discovered that when the pulsed light injected into a distributed fiber optic sensing system propagates in a multimode fiber, mode coupling causes distortion broadening of the injected pulsed light, leading to a deterioration in the spatial resolution of the sensing system. Therefore, this multimode distributed sensing system can generally only achieve a measurement distance of up to 10km; beyond this distance, the spatial resolution of the distributed sensing system deteriorates significantly. To alleviate the problem of spatial resolution degradation in multimode distributed sensing systems caused by pulsed light mode coupling, this patent proposes a distributed fiber optic sensing system to solve or partially solve the above-mentioned problem. The technical solution proposed in this invention is as follows:

[0004] A distributed optical fiber sensing system includes a pulsed light unit, a first spatial light modulator, a circulator, a temperature measurement unit, and a controller, wherein:

[0005] The first port of the circulator is connected to the first spatial light modulator, the second port is connected to the external optical fiber under test, and the third port is connected to the temperature measuring unit.

[0006] The pulsed light unit is used to output pulsed light to the first spatial light modulator;

[0007] The temperature measurement unit is used to obtain anti-Stokes Raman scattered light and Stokes Raman scattered light based on the spontaneous Raman scattered light returned by the circulator from the external optical fiber under test, and then send them to the controller.

[0008] The controller is connected to the temperature measurement unit and the first spatial light modulator respectively. It is used to obtain the temperature information of the optical fiber under test according to the received anti-Stokes Raman scattered light and Stokes Raman scattered light according to the preset rules. It is also used to obtain the pre-compensated spatial phase distribution through the preset optical fiber mode eigenvalue equation, the transverse refractive index distribution of the optical fiber under test, and the coupling effect between the optical fiber modes, and send the pre-compensated spatial phase distribution to the first spatial light modulator.

[0009] The first spatial light modulator is used to modulate the pre-compensated spatial phase distribution onto the pulsed light incident by the pulsed light unit, so as to perform spatial filtering and shaping on the pulsed light and realize mode coupling pre-compensation.

[0010] Furthermore, the controller obtains the pre-compensated spatial phase distribution through preset fiber mode eigenvalue equations, the transverse refractive index distribution of the fiber under test, and the coupling effect between different fiber modes, including:

[0011] Based on the intrinsic equation of the fiber mode and using the transverse refractive index distribution of the fiber under test as the boundary condition, the transverse mode field distribution of the fiber under test is calculated.

[0012] Calculate the coupling coefficient of each mode based on the overlap integral between the various modes of the fiber under test.

[0013] Based on the calculated mode coupling coefficient S ij Construct the mode coupling matrix D;

[0014] The eigenvector matrix P is constructed using the eigenvectors of the pattern coupling matrix D.

[0015] The mode coupling matrix D and the eigenvector matrix P are used to perform matrix diagonalization to obtain a diagonal matrix A. The diagonal matrix A is used to describe the intensity of each mode component in the case of no mode coupling.

[0016] The pre-compensated mode coupling construction matrix PTx is calculated using the diagonal matrix A and the mode coupling matrix D;

[0017] The spatial phase distribution SP of mode coupling precompensation is calculated based on the precompensation mode coupling construction matrix PTx and the transverse mode field distribution. The final spatial phase distribution is obtained by summing all elements in the spatial phase distribution SP of mode coupling precompensation.

[0018] Furthermore, the temperature measurement unit includes an optical filter, a first detector, a second detector, a data acquisition card, and a controller, wherein:

[0019] The input of the optical filter is connected to the third port of the circulator, and the two outputs of the optical filter are connected to the first detector and the second detector, respectively. The optical filter is used to separate the spontaneous Raman scattered light returned from the third port into anti-Stokes Raman scattered light and Stokes Raman scattered light.

[0020] The first detector and the second detector are used to convert the anti-Stokes Raman scattered light and the Stokes Raman scattered light into electrical signals, respectively.

[0021] The acquisition card is connected to the first detector, the second detector, and the controller, respectively, and is used to convert the anti-Stokes Raman scattered light and Stokes Raman scattered light of the electrical signal into digital signals and send them to the controller.

[0022] The controller is used to calculate the spatial phase distribution SP of the mode coupling pre-compensation and send it to the first spatial light modulator.

[0023] Furthermore, the pulsed light unit includes a laser, a semiconductor optical amplifier, and a pulse signal source, wherein the laser outputs laser light to the semiconductor optical amplifier;

[0024] The semiconductor optical amplifier amplifies the laser and receives control from the pulse signal source to modulate and output pulsed light.

[0025] Furthermore, the pulsed light unit also includes an erbium-doped fiber amplifier, which is used to amplify the input pulsed light before outputting it.

[0026] Furthermore, the distributed optical fiber sensing system also includes: a first beam splitter, a second beam splitter, a first spot analyzer, a second spot analyzer, and a second spatial light modulator, wherein:

[0027] The input end of the first beam splitter is connected to the pulse light unit, one output end is connected to the first spatial light modulator, and the other output end is connected to the first spot analyzer. It is used to split the pulse light output by the pulse light unit into two paths and incident on the first spatial light modulator and the first spot analyzer respectively.

[0028] The second spatial light modulator is connected to the third port of the circulator, the controller, and the second beam splitter, respectively.

[0029] The second beam splitter is also connected to the second spot analyzer and the temperature measurement unit. The second beam splitter is used to split the spontaneous Raman scattering light of the optical fiber under test output by the second spatial light modulator into two paths and output them to the second spot analyzer and the temperature measurement unit respectively.

[0030] The controller is also connected to the first spot analyzer, the second spot analyzer, and the second spatial light modulator, and is used to obtain the spatial intensity modulation matrix to be compensated based on the data monitored by the first spot analyzer and the second spot analyzer, and return it to the second spatial light modulator.

[0031] Furthermore, the controller is also configured to obtain the spatial intensity modulation matrix to be compensated based on the data monitored by the first spot analyzer and the second spot analyzer, including:

[0032] The scaling ratio of the transmit and receive field distribution is obtained using data monitored by the first spot analyzer and the second spot analyzer;

[0033] Determine the largest scaling factor to obtain the target field strength distribution at the receiving end;

[0034] The spatial intensity modulation matrix to be compensated is obtained by utilizing the target field strength distribution at the receiving end.

[0035] Furthermore, temperature information is obtained using the anti-Stokes Raman scattered light and Stokes Raman scattered light obtained from the spontaneous Raman scattered light of the fiber under test.

[0036]

[0037] In the formula, T is the temperature of the fiber under test, T0 is the temperature of the reference fiber, k is the wave vector, h is Planck's constant, Δv represents the Raman frequency shift, and P AS (T0) represents the back-facing anti-Stokes scattering signal power P when the optical fiber is at a reference temperature T0. AS (T) is the back-directed anti-Stokes scattering signal power in the optical fiber when the fiber is at the temperature T to be measured. S (T0) and P S (T) represents the Stokes scattering power at the corresponding temperature.

[0038] On the other hand, the present invention also discloses an optical fiber communication system, including the above-mentioned distributed optical fiber sensing system.

[0039] Based on the above technical solution, the beneficial effects of the present invention compared with the prior art are as follows:

[0040] Existing multimode distributed temperature sensing systems typically only excite the fundamental mode of multimode optical fibers, wasting other transmission modes, limiting incident light power, and leading to decreased measurement accuracy. The distributed optical fiber sensing system proposed in this invention includes a pulsed light unit, a first spatial light modulator, a circulator, a temperature measurement unit, and a controller. The controller is connected to both the temperature measurement unit and the first spatial light modulator. It is used to obtain the temperature information of the fiber under test based on received anti-Stokes Raman scattered light and Stokes Raman scattered light according to preset rules. It is also used to obtain a pre-compensated spatial phase distribution through preset fiber mode eigenvalue equations, the transverse refractive index distribution of the fiber under test, and the coupling effect between different fiber modes, and sends the pre-compensated spatial phase distribution to the first spatial light modulator. The first spatial light modulator modulates the pre-compensated spatial phase distribution onto the pulsed light incident from the pulsed light unit to perform spatial filtering and shaping of the pulsed light, achieving mode coupling pre-compensation to suppress mode coupling and thus alleviate the spatial resolution degradation problem of the multimode distributed temperature sensing system. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the structure of a distributed optical fiber sensing system in an embodiment of the present invention;

[0042] Figure 2 In this embodiment of the invention, a graded-index ten-mode optical fiber is used as an example, showing its transverse refractive index distribution.

[0043] Figure 3 In this embodiment of the invention, the distribution of each transverse mode field of the optical fiber is calculated based on the intrinsic equation of the optical fiber mode and the transverse refractive index distribution of the optical fiber as the boundary condition.

[0044] Figure 4 In this embodiment of the invention, the spatial phase distribution of mode coupling pre-compensation is finally calculated based on the matrix PTx and the spatial mode field distribution;

[0045] Figure 5 This is a schematic diagram of the structure of another distributed optical fiber sensing system in some embodiments. Detailed Implementation

[0046] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. For ease of understanding, some technical terms involved in the embodiments of this invention are explained below:

[0047] Multimode fiber: An optical fiber that transmits multiple modes at a given operating wavelength. Based on its refractive index distribution, it is classified into abrupt and graded-index types. A typical multimode fiber has a numerical aperture of 0.2 ± 0.02 and a core / outer diameter of 50 μm / 125 μm. Its transmission parameters are bandwidth and loss. Because multimode fibers transmit hundreds of modes, each with different propagation constants and group rates, the fiber has a narrow bandwidth, high dispersion, and high loss.

[0048] Mode coupling effect: Mode coupling is a power exchange phenomenon between modes in an optical fiber. Mode coupling has the beneficial effect of reducing pulse width broadening. However, it comes at a cost, because any action that couples the propagating mode will also couple the propagating mode with the radiating mode, which will increase the radiation loss of the optical fiber. Mode coupling is widely used in optical fiber communication and optical fiber sensing, and is caused by the axial inhomogeneity of the optical fiber. Bending and microbending of the optical fiber can also produce mode coupling.

[0049] Spatial light modulator: A spatial light modulator is a device that, under active control, modulates a certain parameter of a light field through liquid crystal molecules. For example, it modulates the amplitude of the light field, modulates the phase through the refractive index, modulates the polarization state through the rotation of the polarization plane, or achieves the conversion of incoherent to coherent light, thereby writing certain information into the light wave to achieve the purpose of light wave modulation.

[0050] It is worth noting that the defects mentioned in the background section of the prior art are the result of the inventors' practice and careful research. Therefore, the discovery process of the above problems and the solutions proposed by the embodiments of the present invention in the following text should be considered as contributions made by the inventors to the present invention.

[0051] In some embodiments, such as Figure 1 As shown, a distributed optical fiber sensing system includes a pulsed light unit 10, a first spatial light modulator 20, a circulator 30, a temperature measuring unit 40, and a controller 60. The pulsed light unit 10, the first spatial light modulator 20, and the circulator 30 are all connected via multimode optical fibers, wherein:

[0052] The first port a of the circulator 30 is connected to the first spatial light modulator 20, the second port b is connected to the external optical fiber to be tested, and the third port c is connected to the temperature measuring unit 40.

[0053] The pulsed light unit 10 is used to output pulsed light to the first spatial light modulator 20. In some embodiments, the pulsed light unit 10 includes a laser 11, a semiconductor optical amplifier 12, and a pulse signal source 13. The laser 11 outputs laser light to the semiconductor optical amplifier 12. The semiconductor optical amplifier 12 amplifies the laser light and receives control from the pulse signal source 13 to modulate and output pulsed light. Preferably, the pulsed light unit 10 further includes an erbium-doped fiber amplifier 14, which amplifies the input pulsed light before outputting it.

[0054] The temperature measurement unit 40 is used to obtain the anti-Stokes Raman scattered light and the Stokes Raman scattered light based on the spontaneous Raman scattered light returned by the circulator from the external optical fiber under test, and send them to the controller 60.

[0055] The controller is connected to the temperature measurement unit 40 and the first spatial light modulator 20 respectively. It is used to obtain the temperature information of the optical fiber under test according to the received anti-Stokes Raman scattered light and Stokes Raman scattered light according to the preset rules. It is also used to obtain the pre-compensated spatial phase distribution through the preset optical fiber mode eigenvalue equation, the transverse refractive index distribution of the optical fiber under test, and the coupling effect between the optical fiber modes, and send the pre-compensated spatial phase distribution to the first spatial light modulator 20.

[0056] The first spatial light modulator 20 is used to modulate the pre-compensated spatial phase distribution onto the pulsed light incident by the pulsed light unit 10, so as to perform spatial filtering and shaping on the pulsed light, solve the problem of spatial resolution degradation of the sensing system caused by the distortion broadening of the injected pulsed light, and realize mode coupling pre-compensation. In this embodiment, the first spatial light modulator 20 is a phase spatial light modulator.

[0057] In some embodiments, the controller 60 may be a computer, used to obtain a pre-compensated spatial phase distribution through a preset fiber mode eigenvalue equation, the transverse refractive index distribution of the fiber under test, and the coupling effect between different fiber modes, including the following steps:

[0058] S101, using the fiber mode eigenvalue equation and the transverse refractive index distribution of the fiber under test as the boundary condition, the transverse mode field distribution Fp(x,y) of the fiber under test is calculated. The fiber mode eigenvalue equation is shown in formula (1):

[0059]

[0060] Where x and y are the spatial coordinates, n is the refractive index of the fiber material under test, and k0 is the wave vector k0 = 2π / λ, where λ is the wavelength of light in the fiber under test. The propagation constants for each mode distribution in the fiber under test are specified above. These parameters are provided by the fiber manufacturer.

[0061] Taking graded-index ten-mode optical fiber as an example, the transverse refractive index distribution provided by the fiber manufacturer is as follows: Figure 2 As shown, based on the fiber mode eigenvalue equation and using the fiber's transverse refractive index distribution as a boundary condition, the distribution of each transverse mode field in the fiber is calculated, resulting in the following: Figure 3 The ten transverse modal field distributions are shown. Figure 3 In the diagram, the horizontal and vertical coordinates of each transverse mode field distribution are represented by a two-dimensional plane coordinate system with the fiber center as the origin (unit: μm; the horizontal axis from left to right is -60, -40, -20, 0, 20, 40, 60; the vertical axis from bottom to top is -60, -40, -20, 0, 20, 40, 60). These coordinates characterize the various positions of the two-dimensional transverse profile of the fiber, and different colors represent the magnitude of the phase. This diagram is not suitable for representation by grayscale, as phase can be positive or negative, and grayscale can only represent absolute values. The calculation process is something that can be performed by those skilled in the art and will not be elaborated upon here.

[0062] S102, calculate the coupling coefficient of each mode based on the overlap integral between each mode of the fiber under test.

[0063] The coupling effect between different modes of the optical fiber under test can be obtained by calculating the coupling coefficient of the corresponding mode from the overlap integral between each mode. The formula for the coupling coefficient is shown in formula (2):

[0064]

[0065] Where Fi and Fj represent different modes of the optical fiber under test. The derivation of formula (2) can be found in the reference: Horak P, Poletti F. Multimode nonlinear fibre optics: theory and applications[J]. Recent progress in optical fiber research, 2012, 3, which will not be repeated here.

[0066] S103, based on the calculated mode coupling coefficient S ij Construct the mode coupling matrix D. The constructed coupling matrix D is:

[0067] D(i,j)=S ij

[0068] Table 1 illustrates the form of the coupling matrix D:

[0069] Table 1

[0070]

[0071] S104, use the eigenvectors of the pattern coupling matrix D to form the eigenvector matrix P.

[0072] Table 2 illustrates the form of the eigenvector matrix P:

[0073] Table 2

[0074]

[0075] S105. Using the pattern coupling matrix D and the eigenvector matrix P, a matrix diagonalization process is performed to obtain a diagonal matrix A.

[0076] A = PDP -1 ;

[0077] Except for the diagonal, the elements in matrix D are the coupling coefficients between each mode, representing the degree of coupling between them. The matrix A after diagonalization is shown in Table 3:

[0078] Table 3

[0079]

[0080] S106, calculate the pre-compensated mode coupling construction matrix PTx using the diagonal matrix A and the mode coupling matrix D.

[0081] P Tx =AD -1 ;

[0082] A is a diagonal matrix representing that, apart from the individual effects of each mode, the coupling effect between modes is zero. The pre-compensated mode coupling matrix PTx is calculated, as shown in Table 4:

[0083] Table 4

[0084]

[0085] S107. The spatial phase distribution SP of mode coupling pre-compensation is calculated based on the pre-compensation mode coupling construction matrix PTx and the transverse mode field distribution. The final spatial phase distribution is obtained by summing all elements in the spatial phase distribution SP of mode coupling pre-compensation.

[0086] SP(x,y)=F(x,y)P Tx ;

[0087] At this point, SP is in one-dimensional vector form, where each element represents the spatial phase distribution required for pre-compensation of each mode. Finally, all elements are summed to obtain the final spatial phase distribution. Using a first spatial light modulator, this spatial phase distribution is modulated onto the incident multimode pulse, thereby spatially filtering and shaping the injected pulse to achieve mode coupling pre-compensation. Finally, the spatial phase distribution for mode coupling pre-compensation is calculated based on the matrix PTx and the spatial mode field distribution, as shown below. Figure 4As shown in the figure. The horizontal and vertical axes are two-dimensional plane coordinates (unit: μm) with the fiber center as the origin, representing the various positions of the two-dimensional cross-section of the fiber, and the different colors represent the magnitude of the phase.

[0088] In some embodiments, the temperature measuring unit 40 includes an optical filter 41, a first detector 421, a second detector 422, and a data acquisition card 43, wherein:

[0089] The input terminal of the optical filter 41 is connected to the third port c of the circulator 30, and the two output terminals of the optical filter 41 are connected to the first detector 421 and the second detector 422, respectively. The optical filter 41 is used to separate the spontaneous Raman scattered light returned from the third port c into anti-Stokes Raman scattered light and Stokes Raman scattered light. The first detector 421 and the second detector 422 are used to convert the anti-Stokes Raman scattered light and the Stokes Raman scattered light into electrical signals, respectively. The acquisition card 43 is connected to the first detector 421, the second detector 422, and the controller 60, respectively, and is used to convert the electrical signals of the anti-Stokes Raman scattered light and the Stokes Raman scattered light into digital signals and send them to the controller 60.

[0090] Specifically, this invention uses the ratio of the anti-Stokes signal to the Stokes signal to demodulate the temperature:

[0091]

[0092] In the formula, T0 is the reference fiber temperature, k is the wave vector, h is Planck's constant, Δv represents the Raman frequency shift (which depends on the material properties. For silica fiber, the Raman frequency shift is approximately 13.2 THz), and P... AS (T0) represents the back-facing anti-Stokes scattering signal power P when the optical fiber is at a reference temperature T0. AS (T) is the back-stokes signal power in the optical fiber when the optical fiber is at the temperature T to be measured. S (T0) and P S (T) represents the Stokes scattered light power at the corresponding temperature. Temperature information along the line can be demodulated by detecting the Stokes light and anti-Stokes light signals.

[0093] This invention calculates the spatial phase distribution of mode coupling pre-compensation in multimode fiber and uses a first spatial light modulator 20 to spatially filter and shape the pulsed light, thereby solving the problem of spatial resolution degradation of the sensing system caused by distortion broadening of the injected pulsed light and achieving mode coupling pre-compensation. This invention can suppress the mode coupling effect of pulsed light in multimode fiber, improve temperature demodulation accuracy, and enable monitoring over longer distances.

[0094] Based on the above embodiments, the inventors further discovered that the mode coupling matrix of multimode fiber is not constant in many cases. Affected by environmental factors, pulsed light is disturbed during propagation in multimode fiber, and this disturbance exacerbates the coupling effect between modes, causing a certain degree of change in the coupling matrix. Therefore, it is necessary to compensate for the mode coupling caused by fiber disturbance at the receiving end. Thus, in some other embodiments, in conjunction with… Figure 5 As shown, the distributed optical fiber sensing system also includes: a first beam splitter 51, a second beam splitter 52, a first spot analyzer 53, a second spot analyzer 54, and a second spatial light modulator 55, wherein:

[0095] The input end of the first beam splitter 51 is connected to the pulse light unit 10, one output end is connected to the first spatial light modulator 20, and the other output end is connected to the first spot analyzer 53. It is used to split the pulse light output by the pulse light unit 10 into two paths and respectively incident on the first spatial light modulator 20 and the first spot analyzer 53.

[0096] The second spatial light modulator 55 is connected to the third port c of the circulator 30, the controller 60, and the second beam splitter 52, respectively. It is used to change the amplitude of the receiving spatial mode according to the spatial intensity modulation matrix to be compensated returned by the controller 60, so that the spatial field distribution at the transmitting and receiving ends is basically consistent.

[0097] The second beam splitter 52 is also connected to the second spot analyzer 54 and the optical filter 41, and is used to split the spontaneous Raman scattering light of the optical fiber under test output by the second spatial light modulator 55 into two paths and output them to the second spot analyzer 54 and the temperature measurement unit 40 respectively.

[0098] The controller 60 is also connected to the first spot analyzer 51, the second spot analyzer 52, and the second spatial light modulator, and is used to obtain the spatial intensity modulation matrix to be compensated based on the data monitored by the first spot analyzer 51 and the second spot analyzer 52, and return it to the second spatial light modulator.

[0099] Specifically, controller 60 also performs the following tasks:

[0100] S201, the scaling ratio of the transmit and receive field distribution is obtained using the data monitored by the first spot analyzer 53 and the second spot analyzer 54.

[0101] The emitting light field F monitored by the first spot analyzer 53 Tx (x,y) represents each originating mode F Txi Linear superposition of (x,y):

[0102]

[0103] The receiving end light field F monitored by the second spot analyzer 54 Rx(x,y) represents the various receiver modes F Rxi Linear superposition of (x,y):

[0104]

[0105] The scaling factor R(x,y) of the transmit / receive field distribution is:

[0106] R(x,y)=F Tx (x,y) / F Rx (x,y)

[0107] Where x and y are spatial distribution coordinates.

[0108] S202, determine the largest scaling factor to obtain the target field strength distribution at the receiving end.

[0109] Let Rmax(x,y) be the largest scaling factor. Then the target field strength distribution at the receiving end is:

[0110] F Tx (x,y) / R max (x,y)

[0111] S203, the spatial intensity modulation matrix to be compensated is obtained by utilizing the target field strength distribution at the receiving end.

[0112] Finally, the spatial intensity modulation matrix SI of the second spatial light modulator 55 can be obtained as follows:

[0113] SI(x,y)=(F Tx (x,y) / R max (x,y)) / F Rx (x,y)

[0114] Thus, the second spatial optical modulator 55 is used to change the amplitude of the spatial mode at the receiving end, so that the spatial field distribution at the transmitting and receiving ends is basically the same.

[0115] This invention calculates the spatial phase distribution of mode coupling pre-compensation in multimode fiber and uses a first spatial light modulator 20 to spatially filter and shape the pulsed light, thus solving the problem of spatial resolution degradation in the sensing system caused by distortion broadening of the injected pulsed light and achieving mode coupling pre-compensation. Furthermore, by comparing the mode field distributions at the receiver and transmitter, a second spatial light modulator 55 is used to intensity-modulate the received optical signal, further suppressing mode coupling caused by fiber disturbances. This method suppresses mode coupling while injecting multiple modes without affecting measurement accuracy. This invention can suppress the mode coupling effect of pulsed light in multimode fiber, improve temperature demodulation accuracy, and achieve monitoring over longer distances.

[0116] In addition, the spontaneous Raman scattered light in the fiber under test returns through the second port b of the circulator 30 and enters the second spatial light modulator 55 through the third port c, then enters the optical filter 41. The optical filter 41 separates the Stokes Raman scattered light (1660nm) and the anti-Stokes Raman scattered light (1450nm). The Stokes Raman scattered light and the anti-Stokes Raman scattered light are detected by the first detector 421 and the second detector 422, and the temperature can still be demodulated using the ratio of the anti-Stokes and Stokes signals as described above.

[0117] In some embodiments, the laser 11 operates at a wavelength of approximately 120 nm. The pulse signal source 13 has a pulse width of 10 ns to 1000 ns. The semiconductor optical amplifier 12 has an extinction ratio greater than 40 dB and a rise time of less than 10 ns. The first detector 421 has a bandwidth of 125 MHz, and the second detector 422 has a bandwidth of 125 MHz. The acquisition card 43 has a sampling rate of 250 MSa / s. The first spatial light modulator 20 is a phase-type spatial light modulator with a spectral range of 1400 nm to 1700 nm and an adjustable maximum phase of 4.1 π. The second spatial light modulator 55 is an intensity-type spatial light modulator with a spectral range of 1400 nm to 1700 nm and an adjustable intensity ratio range of 0 to 1. The first spot analyzer 53 and the second spot analyzer 54 have a resolution of 1624*1224 pixels, a spacing of 60 micrometers, and operate at 15 frames per second at full resolution.

[0118] In this embodiment of the invention, multimode optical fiber is used as the medium. The light emitted from laser 11 is modulated into pulsed light by semiconductor optical amplifier 12 and controlled by pulse signal source 13. After being amplified by erbium-doped fiber amplifier 14, the pulsed light passes through first beam splitter 51. One path is emitted to first spot analyzer 53 for monitoring the incident light spot. The other path is spatially filtered and shaped by first spatial light modulator 20, passes through first port a of circulator 30, and enters the fiber under test through port b. Spontaneous Raman scattered light in the fiber under test returns through second port b of circulator 30, enters second spatial light modulator 55 through third port c, and passes through second beam splitter 52. One path is emitted to second spot analyzer 54 for monitoring the emitted light spot. The other path enters optical filter 41, which separates anti-Stokes Raman scattered light and Stokes Raman scattered light. These optical signals are converted into electrical signals by first detector 421 and second detector 422, and the data is collected by acquisition card 43 and transmitted to controller 60 (or computer). This invention uses a spatial light modulator to shape and filter the injected and scattered light, realizing a multimode mode coupling pre-compensation and mode coupling compensation scheme caused by fiber disturbance in multimode fiber sensing. This improves the temperature demodulation accuracy and enables monitoring over longer distances.

[0119] On the other hand, the present invention also discloses an optical fiber communication system, which includes at least the aforementioned distributed optical fiber sensing system, and can utilize the distributed optical fiber sensing system to monitor temperature information along the optical fiber under test. Other structures of this optical fiber communication system can be found in the prior art, and will not be described in detail here.

[0120] In the detailed description above, various features are combined together in a single embodiment to simplify the invention. This approach to disclosure should not be construed as reflecting an intention that embodiments of the claimed subject matter require more features than are explicitly stated in each claim. Rather, as reflected in the appended claims, the invention is presented with fewer features than all of the features in the single disclosed embodiment. Therefore, the appended claims are hereby explicitly incorporated into the detailed description, with each claim representing a separate preferred embodiment of the invention.

[0121] The foregoing description includes examples of one or more embodiments. It is certainly impossible to describe all possible combinations of components or methods in order to describe the above embodiments, but those skilled in the art will recognize that further combinations and arrangements of the various embodiments are possible. Therefore, the embodiments described herein are intended to cover all such changes, modifications, and variations that fall within the scope of the appended claims. Furthermore, the term “comprising” as used in the specification or claims is interpreted in a manner similar to the term “including,” just as “including,” is interpreted as a conjunction in the claims. Additionally, the use of any term “or” in the specification of the claims is intended to mean “non-exclusive or.”

Claims

1. A distributed optical fiber sensing system, characterized in that, It includes a pulsed light unit (10), a first spatial light modulator (20), a circulator (30), a temperature measuring unit (40), and a controller (60), wherein: The first port a of the circulator (30) is connected to the first spatial light modulator (20), the second port b is connected to the external optical fiber to be tested, and the third port c is connected to the temperature measuring unit (40). The pulsed light unit (10) is used to output pulsed light to the first spatial light modulator (20); The temperature measuring unit (40) is used to obtain anti-Stokes Raman scattered light and Stokes Raman scattered light based on the spontaneous Raman scattered light returned by the circulator (30) through the external optical fiber under test, and send them to the controller (60). The controller (60) is connected to the temperature measuring unit (40) and the first spatial light modulator (20) respectively. It is used to obtain the temperature information of the optical fiber under test according to the received anti-Stokes Raman scattered light and Stokes Raman scattered light according to the preset rules. It is also used to obtain the pre-compensated spatial phase distribution through the preset optical fiber mode eigen equation, the transverse refractive index distribution of the optical fiber under test, and the coupling effect between the optical fiber modes, and send the pre-compensated spatial phase distribution to the first spatial light modulator (20). The first spatial light modulator (20) is used to modulate the pre-compensated spatial phase distribution onto the pulse light incident by the pulse light unit, so as to perform spatial filtering and shaping on the pulse light and realize mode coupling pre-compensation.

2. The distributed optical fiber sensing system as described in claim 1, characterized in that, The controller (60) obtains a pre-compensated spatial phase distribution through preset fiber mode eigenvalue equations, the transverse refractive index distribution of the fiber under test, and the coupling effect between different fiber modes, including: Based on the intrinsic equation of the fiber mode and using the transverse refractive index distribution of the fiber under test as the boundary condition, the transverse mode field distribution of the fiber under test is calculated. Calculate the coupling coefficient of each mode based on the overlap integral between the various modes of the fiber under test. Construct the mode coupling matrix based on the calculated mode coupling coefficients; An eigenvector matrix is ​​constructed using the eigenvectors of the pattern coupling matrix. By using the pattern coupling matrix and the eigenvector matrix, a matrix diagonalization process is performed to obtain a diagonal matrix; The pre-compensation mode coupling construction matrix is ​​calculated using the diagonal matrix and the mode coupling matrix; The spatial phase distribution of mode coupling precompensation is calculated based on the precompensation mode coupling construction matrix and the transverse mode field distribution. The final spatial phase distribution is obtained by summing all elements in the spatial phase distribution of mode coupling precompensation.

3. The distributed optical fiber sensing system as described in claim 1, characterized in that, The temperature measurement unit (40) includes an optical filter (41), a first detector (421), a second detector (422), and a data acquisition card (43), wherein: The input end of the optical filter (41) is connected to the third port c of the circulator (31), and the two output ends of the optical filter (41) are connected to the first detector (421) and the second detector (422) respectively; the optical filter (41) is used to separate the spontaneous Raman scattered light returned from the third port c into anti-Stokes Raman scattered light and Stokes Raman scattered light; The first detector (421) and the second detector (422) are used to convert the anti-Stokes Raman scattered light and the Stokes Raman scattered light into electrical signals, respectively; The acquisition card is connected to the first detector (421), the second detector (422), and the controller (60) respectively, and is used to convert the anti-Stokes Raman scattered light and Stokes Raman scattered light of the electrical signal into digital signals and send them to the controller (60).

4. The distributed optical fiber sensing system as described in claim 1, characterized in that, The pulsed light unit (10) includes a laser (11), a semiconductor optical amplifier (12), and a pulse signal source (13), wherein the laser (11) outputs laser light to the semiconductor optical amplifier (12); The semiconductor optical amplifier (12) amplifies the laser and receives control from the pulse signal source (13) to modulate and output pulsed light.

5. The distributed optical fiber sensing system as described in claim 4, characterized in that, The pulsed light unit (10) further includes an erbium-doped fiber amplifier (14), which is used to amplify the input pulsed light and output it.

6. The distributed optical fiber sensing system as described in claim 1, characterized in that, The distributed optical fiber sensing system further includes: a first beam splitter (51), a second beam splitter (52), a first spot analyzer (53), a second spot analyzer (54), and a second spatial light modulator (55), wherein: The input end of the first beam splitter (51) is connected to the pulse light unit (10), one output end is connected to the first spatial light modulator (20), and the other output end is connected to the first spot analyzer (53). It is used to split the pulse light output by the pulse light unit (10) into two paths and be incident on the first spatial light modulator (20) and the first spot analyzer (53) respectively. The second spatial light modulator (55) is connected to the third port c of the circulator (30), the controller (60), and the second beam splitter (52), respectively; The second beam splitter (52) is also connected to the second spot analyzer (54) and the temperature measuring unit (40) to split the spontaneous Raman scattering light of the fiber under test output by the second spatial light modulator (55) into two paths and output them to the second spot analyzer (54) and the temperature measuring unit (40) respectively. The controller (60) is also connected to the first spot analyzer (53), the second spot analyzer (54), and the second spatial light modulator (55) to obtain the spatial intensity modulation matrix to be compensated based on the data monitored by the first spot analyzer (53) and the second spot analyzer (54), and return it to the second spatial light modulator (55). The second spatial light modulator (55) is used to change the amplitude of the receiving spatial mode according to the spatial intensity modulation matrix to be compensated, so that the spatial field distribution of the transmitting and receiving ends is consistent.

7. The distributed optical fiber sensing system as described in claim 6, characterized in that, The controller is also used to obtain the spatial intensity modulation matrix to be compensated based on the data monitored by the first spot analyzer (53) and the second spot analyzer (54), including: The scaling factor of the transmit and receive field distribution is obtained using data monitored by the first spot analyzer (53) and the second spot analyzer (54); Determine the largest scaling factor to obtain the target field strength distribution at the receiving end; The spatial intensity modulation matrix to be compensated is obtained by utilizing the target field strength distribution at the receiving end.

8. The distributed optical fiber sensing system as described in claim 1, characterized in that, The preset rule for obtaining the temperature information of the optical fiber under test according to a preset rule includes: In the formula, T is the temperature of the fiber under test, T0 is the temperature of the reference fiber, k is the wave vector, h is Planck's constant, Δv represents the Raman frequency shift, and P AS (T0) represents the back-facing anti-Stokes scattering signal power P when the optical fiber is at a reference temperature T0. AS (T) is the back-directed anti-Stokes scattering signal power in the optical fiber when the fiber is at the temperature T to be measured. S (T0) and P S (T) represents the Stokes scattering power at the corresponding temperature.

9. The distributed optical fiber sensing system as described in claim 1, characterized in that, The pulsed light unit (10), the first spatial light modulator (20), and the circulator (30) are all connected by multimode optical fiber.

10. An optical fiber communication system, characterized in that, It includes at least the distributed optical fiber sensing system described in any one of claims 1-9 above.

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