Fiber optic sensing system and detection method

Abstract

The embodiment of the application relates to the technical field of optical fiber sensing, and discloses an optical fiber sensing receiving system and a detection method. The system comprises a first signal generating unit, a second signal generating unit, a first signal modulating unit, a second signal modulating unit, a circulator unit and a coherent detection unit. The first signal modulating unit is used for modulating a probe signal generated by the first signal generating unit according to a probe modulation signal generated by the second signal generating unit to generate a target probe signal. The second signal modulator is used for modulating a local oscillator signal generated by the first signal generating unit according to a local oscillator modulation signal generated by the second signal generating unit to generate a target local oscillator signal negatively related to the backscattering signal power. The circulator unit is used for transmitting the target probe signal and outputting the backscattering signal. The coherent detection unit is used for realizing coherent detection of the target local oscillator signal and the backscattering signal to obtain sensing data. The signal-to-noise ratio is improved, the cost is reduced, and the implementation difficulty is reduced.

Description

TECHNICAL FIELD

[0001] Embodiments of the present application relate to the technical field of optical fiber sensing, and particularly relate to an optical fiber sensing system and a detection method. BACKGROUND

[0002] When an optical signal is transmitted in an optical fiber, changes in vibration, temperature, pressure and the like will affect the phase, amplitude, polarization state and the like of the optical signal. Therefore, changes in vibration, temperature, pressure and the like can be reflected by changes in the phase, amplitude, polarization state and the like of the optical signal transmitted in the optical fiber. That is, the optical fiber can be used for sensing detection. Moreover, due to the advantages of the optical fiber itself, such as sensing spatial continuity, sensing and communication integration, anti-electromagnetic interference, low cost and the like, the optical fiber has been applied in the fields of earthquake early warning, bridge health monitoring, security monitoring and the like, and is particularly more widely applied in sensing monitoring scenes of long-distance and large-range sensing. At present, the optical fiber sensing system for optical fiber sensing detection is mainly realized based on the backscattering principle and the coherent detection principle.

[0003] However, the signal-to-noise ratio of the current optical fiber sensing system applied in a long-distance scene is poor, and in order to solve this problem, higher-performance receiving machine devices are required, such as requiring the noise figure of the optoelectronic device to be better, the input signal power range to be larger, and the resolution to be higher, but this will make the cost of the receiver higher and the implementation more difficult. SUMMARY

[0004] The main purpose of the embodiments of the present application is to propose an optical fiber sensing system and a detection method, by generating a local oscillator signal negatively correlated with the power of the backscattering signal, so that the signal power after mixing the backscattering signal and the local oscillator signal in the coherent detection is more stable, and the requirements for the input power range and the resolution of the device are reduced. That is, the functional characteristics of these devices are reduced, and then the cost and the implementation difficulty are reduced.

[0005] To achieve the above object, the embodiment of the present application provides a fiber sensing system, comprising: a first signal generating unit, a second signal generating unit, a first signal modulating unit, a second signal modulating unit, a circulator unit and a coherent detection unit; the first signal generating unit is used for generating a detection signal and a local signal; the second signal generating unit is used for generating a detection modulation signal and a local modulation signal; the first signal modulator is used for pulse modulating the detection signal according to the detection modulation signal to generate a target detection signal; the second signal modulator is used for intensity modulating the local signal according to the local modulation signal to generate a target local signal negatively related to the power of a backscattering signal, the backscattering signal being a signal generated by backscattering of the target detection signal when the target detection signal is transmitted in an optical fiber; the circulator unit is used for transmitting the target detection signal to the optical fiber through a first port and outputting the backscattering signal through a third port; and the coherent detection unit is used for realizing coherent detection of the target local signal and the backscattering signal to obtain sensing detection data.

[0006] To achieve the above object, the embodiment of the present application further provides a detection method applied to the fiber sensing system, the method comprising: generating a detection signal, a local signal, a detection modulation signal and a local modulation signal; pulse modulating the detection signal according to the detection modulation signal to generate a target detection signal; intensity modulating the local signal according to the local modulation signal to generate a target local signal negatively related to the power of a backscattering signal, the backscattering signal being a signal generated by backscattering of the target detection signal when the target detection signal is transmitted in an optical fiber; transmitting the target detection signal to the optical fiber and acquiring the backscattering signal; and coherently detecting the target local signal and the backscattering signal to obtain sensing detection data.

[0007] The fiber sensing system provided by the embodiment of the present application generates a detection signal and a local signal in the first signal generating unit, generates a detection modulation signal and a local modulation signal in the second signal generating unit, pulse modulates the detection signal according to the detection modulation signal in the first signal modulator to generate a target detection signal, intensity modulates the local signal according to the local modulation signal in the second signal modulator to generate a target local signal negatively related to the power of a backscattering signal, i.e. provides a low-power target local signal for a high-power backscattering signal and provides a high-power target local signal for a low-power backscattering signal, so that the signal obtained by mixing the backscattering signal and the modulated local signal during coherent detection can be kept within a certain power range, the signal power obtained by mixing the backscattering signal and the target local signal during coherent detection is more stable, the requirements for the input power range and the resolution of some devices are reduced, i.e. the functional characteristics of these devices are reduced, and the cost and implementation difficulty are reduced. Attached Figure Description

[0008] One or more embodiments are illustrated by way of example with reference to the accompanying drawings, and these illustrative descriptions do not constitute a limitation on the embodiments.

[0009] Figure 1 This is a schematic diagram of the structure of the fiber optic sensing system provided in the embodiments of this application;

[0010] Figure 2 This application Figure 1 The illustrated embodiment provides a schematic diagram of the signal flow direction of the fiber optic sensing system.

[0011] Figure 3 This is a schematic diagram of another fiber optic sensing system provided in the embodiments of this application;

[0012] Figure 4 It is an intensity trace diagram of the signal after coherent detection mixing in an existing fiber optic sensing system;

[0013] Figure 5 This is an intensity trace diagram of the signal after coherent detection by the fiber optic sensing system provided in the embodiments of this application;

[0014] Figure 6 This is a flowchart of the detection method provided in the embodiments of this application;

[0015] Figure 7 This is a comparison graph showing the change of local oscillator signal power provided by the existing fiber optic sensing system provided in the embodiments of this application and the target local oscillator signal power of the fiber optic sensing system provided in the embodiments of this application over time;

[0016] Figure 8 This is a schematic diagram of signal synchronization of the target local oscillator signal, backscattered signal, and target detection signal provided in the embodiments of this application. Detailed Implementation

[0017] As can be seen from the background technology, current fiber optic sensing systems suffer from high cost and high implementation difficulty.

[0018] The analysis shows that one of the reasons for the above problems is that in a distributed optical fiber sensing system, the inherent transmission loss of the optical fiber channel causes the attenuation of the sensing signal power. Since the optical fiber at different positions corresponds to different transmission distances, i.e., the signal will experience different transmission losses. Therefore, the sensing signals at different positions in the optical fiber will have different optical powers, i.e., when the receiver directly detects or coherently detects the signal, the detector needs to have a larger input optical power range. Moreover, the amplitude of the electrical signal output by the detector will also have a larger amplitude fluctuation. For example, when monitoring an optical fiber channel with a length of 100 km using a phase-sensitive optical time-domain reflectometer (φOTDR), the signal power fluctuation range caused by the transmission loss of the optical fiber in the sensing signal intensity trace after coherent detection reaches about 35 dB. Considering the random power fluctuation of the sensing signal itself, the overall power fluctuation range is as high as nearly 70 dB. Such a large signal power fluctuation range has higher requirements for the constituent devices of the optical fiber sensing receiver, such as: the detector needs to have a larger input power range and higher receiving sensitivity; the analog-to-digital converter (ADC) and the like need to have a wider input voltage range, and the ADC needs to have higher resolution, otherwise a relatively large quantization noise will be introduced to the small signal, which seriously affects the system performance. Therefore, there is an urgent need for an optical fiber sensing system that can control the signal intensity trace fluctuation range to a smaller range.

[0019] To solve the above problems, the embodiment of the present application provides an optical fiber sensing system, comprising: a first signal generating unit, a second signal generating unit, a first signal modulating unit, a second signal modulating unit, a circulator unit and a coherent detection unit; the first signal generating unit is used to generate a detection signal and a local oscillator signal; the second signal generating unit is used to generate a detection modulation signal and a local oscillator modulation signal; the first signal modulator is used to pulse modulate the detection signal according to the detection modulation signal to generate a target detection signal; the second signal modulator is used to intensity modulate the local oscillator signal according to the local oscillator modulation signal to generate a target local oscillator signal negatively related to the backscattering signal power, the backscattering signal being a signal generated by backscattering of the target detection signal during transmission in the optical fiber; the circulator unit is used to send the target detection signal to the optical fiber through the first port and output the backscattering signal through the third port; and the coherent detection unit is used to realize coherent detection of the target local oscillator signal and the backscattering signal to obtain sensing detection data.

[0020] The optical fiber sensing system provided by the embodiments of the present application can use the local oscillator modulation signal generated by the second signal generator to perform intensity modulation on the local oscillator signal generated by the first signal generator, so that the modulated target local oscillator signal can be negatively correlated with the backscattering signal power, thereby using the target local oscillator signals with different powers to perform non-equivalent amplification on the backscattering signal, so that different power loss compensation can be provided for backscattering signals with different powers, thereby reducing the power fluctuation range of the signal generated after the backscattering signal and the target local oscillator signal are mixed after coherent detection, reducing the requirements on related devices, that is, devices with less high functional characteristics can be used to construct the optical fiber sensing system, and the cost is reduced.

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the embodiments of the present application will be described in detail below with reference to the drawings. However, those skilled in the art can understand that, in the embodiments of the present application, many technical details are proposed to make the readers better understand the present application. However, the technical solutions claimed by the present application can be implemented even without these technical details and various changes and modifications based on the following embodiments. The division of the following embodiments is for the convenience of description, and should not constitute any limitation on the specific implementation modes of the present application, and the embodiments can be combined and referenced with each other on the premise of no contradiction.

[0022] The embodiments of the present application provide an optical fiber sensing system, which is applied in the process of sensing detection by using an optical fiber. Figure 1 As shown in the figure, the optical fiber sensing system at least includes: a first signal generation unit 101, a second signal generation unit 102, a first signal modulation unit 103, a second signal modulation unit 104, a circulator unit 105, and a coherent detection unit 106. Among them, the first signal generation unit 101 is used to generate a detection signal and a local oscillator signal; the second signal generation unit 102 is used to generate a detection modulation signal and a local oscillator modulation signal; the first signal modulator 103 is used to pulse modulate the detection signal according to the detection modulation signal to generate a target detection signal; the second signal modulator 104 is used to intensity modulate the local oscillator signal according to the local oscillator modulation signal to generate a target local oscillator signal negatively correlated with the backscattering signal power, the backscattering signal being a signal generated by backscattering of the target detection signal when transmitting in the optical fiber; the circulator unit 105 is used to send the target detection signal to the optical fiber through the first port and output the backscattering signal through the third port; and the coherent detection unit 106 is used to realize coherent detection of the target local oscillator signal and the backscattering signal to obtain sensing detection data.

[0023] It should be emphasized that the introduction of the circulator unit is to better extract the backscattering signal. The first signal modulation unit is mainly driven by the probe modulation signal generated by the first signal generation unit to realize the electro-optic conversion and modulation of the signal. In order to achieve better results, an Acoustic-Optic Modulator (AOM) with high extinction ratio and good waveform quality can also be used.

[0024] As Figure 2As shown, when the optical fiber sensing system is used for sensing detection, the first signal generating unit 101 will generate a probe signal 1 and a local oscillator signal 2. In addition to directly sending the probe signal 1 into the optical fiber 107, the probe signal 1 is also pulse-modulated to generate a target pulse optical signal with specific intensity, pulse width, frequency and other parameters. That is, the second signal generating unit 102 also generates a probe modulation signal 3 for pulse-modulating the probe signal 1. After the first signal generating unit 101 sends the generated probe signal 1 into the first signal modulation unit 103 and the second signal generating unit 102 sends the generated probe modulation signal 3 into the first signal modulation unit 103, the first signal modulation unit 103 pulse-modulates the probe signal 1 according to the probe modulation signal 3 to generate a pulse optical signal, i.e., a target probe signal 4. Then, the target probe signal 4 generated by the first signal modulation unit 103 is sent into the optical fiber 107 through the first port of the circulator unit 105. Thus, the target probe signal 4 will be transmitted in the optical fiber 107 and backscattered to obtain a backscattered signal 5 opposite to the propagation direction of the target probe signal 4, and output from the third port of the circulator unit 105 and sent into the coherent detection unit 106. According to different backscattering principles, the backscattered signal 5 can be a backscattered signal based on Rayleigh scattering, a backscattered signal based on Brillouin scattering, a backscattered signal based on Raman scattering, etc. At the same time, in order to non-equivalently amplify the backscattered signal 5, a target local oscillator signal 7 negatively correlated with the backscattered signal 5 needs to be provided. Therefore, the second signal generating unit 102 also generates a local oscillator modulation signal 6 for modulating the local oscillator signal 2, and sends the local oscillator modulation signal 6 into the second signal modulation unit 104, so that the second signal modulation unit 104 can intensity-modulate the local oscillator signal 2 sent by the first signal generating unit 101 into the second signal modulation unit 104 according to the local oscillator modulation signal 6 to obtain the target local oscillator signal 7, so as to achieve the purpose of providing a target local oscillator signal 7 with relatively low power when the power of the backscattered signal 5 is high, and providing a target local oscillator signal 7 with relatively high power when the power of the backscattered signal 5 is low. Then, the second signal modulation unit 104 sends the target local oscillator signal 7 into the coherent detection unit 106, and the coherent detection unit 106 realizes coherent detection of the backscattered signal 5 and the target local oscillator signal 7. During the coherent detection process, a signal with a power range stably in a relatively small fluctuation range is obtained through control of the target local oscillator signal 7.

[0025] It should be noted that the backscattering signal is generated from the moment the target detection signal enters the optical fiber from one end of the optical fiber until the target detection signal reaches the other end of the optical fiber, and the power of the target detection signal is continuously lost during this process. Correspondingly, the power loss of the backscattering signal obtained by backscattering is also continuously accumulated, that is, the power of the backscattering signal gradually decreases from the moment the target detection signal enters the optical fiber from one end to the moment the target detection signal leaves the optical fiber from the other end. That is, in order to keep the power of the mixed signal within a certain range without large fluctuations, the target local oscillator signal which is inversely related to the power of the backscattering signal should be gradually increased during the above process. Therefore, the power variation of the local oscillator modulation signal generated by the second signal generating unit is not specifically limited in this embodiment. In the case that the power of the local oscillator signal generated by the first signal transmitting unit is stable, the power of the local oscillator modulation signal generated by the second signal generating unit can be dynamically modulated as long as it gradually increases during the process from the moment the target detection signal enters the optical fiber to the moment the backscattering signal completely returns and leaves the optical fiber. By setting a reasonable power of the target local oscillator signal, the power of the signal obtained by mixing the target local oscillator signal and the backscattering signal in the coherent detection process is flexibly controlled within a reasonable fluctuation range.

[0026] In order to ensure that the signal obtained by mixing the target local oscillator signal and the backscattering signal in the coherent detection process is more stable, in some examples, the relationship between the target local oscillator signal and the backscattering signal can be further constrained to be inversely proportional to the power.

[0027] In order to keep the signal obtained by mixing the target local oscillator signal and the backscattering signal in the coherent detection process within the smallest range, that is, to require the signal trace to be flat in the coherent detection process, the target local oscillator signal can be set to just compensate for the power loss of the backscattering signal on the basis of the inverse proportion between the target local oscillator signal and the backscattering signal. Therefore, in some examples, when the power of the backscattering signal is determined based on the attenuation of the optical fiber, the power trace of the backscattering signal should be related to the loss coefficient and the transmission time of the optical fiber, which is: The power trace of the target local oscillator signal can be: wherein t ∈ [0, T], T is the time length of a detection period of the target detection signal, t0 is the starting time of the detection period of the target detection signal, p s (t0+t) is the power of the backscattering signal at t0+t, p LO(t0+t) is the power of the target local oscillator signal at t0+t, P0 is the power of the target probe signal entering the optical fiber, a is the loss coefficient of the optical fiber, c is the speed of light, n is the refractive index of the optical fiber, r(t0+t) is the backscattering intensity at each position in the optical fiber at t0+t, P Lo is a preset target local oscillator signal power constant. It should be noted that the probe period can be understood as a period in which the target probe signal is sent into the optical fiber.

[0028] It should be noted that the amplitude, frequency, phase, etc. of the local oscillator modulation signal are generated by a controller to achieve the purpose of controlling the power of the target local oscillator signal.

[0029] In some examples, the modulation period of the local oscillator signal is the same as the sending period of the target probe signal.

[0030] It should be particularly noted that the modulation period of the local oscillator signal should be no less than the time required for the target probe signal to be transmitted from one end of the optical fiber to the other end and for the backscattering signal to be transmitted from the other end of the optical fiber to the one end, that is, no less than the time for the optical signal to be transmitted in the optical fiber for one round trip. Specifically, the modulation period of the local oscillator signal should be no less than (2*n*L) / c, where n is the refractive index of the optical fiber, c is the speed of light, and L is the radial length of the optical fiber.

[0031] Also, in some examples, the local oscillator modulation signal is synchronized with the target probe signal, so that through the synchronization of the target probe signal and the local oscillator modulation signal, the backscattering signals transmitted back from positions close to the position where the target probe signal enters the optical fiber and from positions far from the position where the target probe signal enters the optical fiber correspond to different target local oscillator signal powers, respectively.

[0032] It can be understood that in coherent detection, the local oscillator signal and the received probe signal are optically mixed to realize down-conversion of the signal from an optical carrier frequency to a microwave carrier frequency, and then the center frequency of the signal is detected by a photodetector, that is, the difference between the frequency of the probe signal and the frequency of the local oscillator signal, and then the intermediate frequency signal passes through a demodulation and compensation algorithm to obtain a baseband signal output. Specifically, in the case where the probe signal is and the local oscillator signal is , the heterodyne signal after coherent reception is where P s is the power of the probe signal, and P lo is the power of the local oscillator signal. It can be seen that by increasing the power P lo of the local oscillator signal, the output signal i(t) can be amplified. That is, the optical fiber sensing system provided in the embodiment also realizes amplitude adjustment of the coherent reception signal by intensity modulation of the local oscillator signal, improves the signal power, reduces the requirement for system reception sensitivity, and realizes high signal-to-noise ratio.

[0033] It can also be understood that intensity modulation of the local oscillator signal by the second signal modulation unit does not affect the phase of the local oscillator signal, as follows: taking a Mach-Zehnder modulator (MZM) as an example to analyze the modulation process of the local oscillator signal: the transfer function of the MAM can be represented as where φ1(t) and φ2(t) represent the phase shifts of the upper arm and the lower arm of the MZM, respectively, E out is the output, and E in is the input. When the phase shifts between the two arms are opposite, the signal obtained at the output end is the target local oscillator signal after intensity modulation, at which time the relationship between the local oscillator signal (E in (t)) and the target local oscillator signal (E out (t)) can be represented as Therefore, there is only amplitude change from the local oscillator signal to the target local oscillator signal, i.e., the coefficient and there is no phase change, i.e., intensity modulation of the local oscillator signal does not affect the phase of the local oscillator signal. Therefore, the optical fiber sensing system provided in the embodiment can realize phase-based optical fiber sensing detection.

[0034] In order to facilitate those skilled in the art to better understand the optical fiber sensing system provided in the embodiment, different components of the optical fiber sensing system will be specifically illustrated as follows.

[0035] As shown in Figure 3 , the first signal generation unit 101 can include a laser 1011 and a coupler 1012, wherein the coupler 1012 is used to split the continuous laser generated by the laser 1011 to generate a probe signal and a local oscillator signal. That is, the laser generates a continuous laser signal, which is sent into the coupler 1012, and the coupler 1012 splits the laser signal into two paths to obtain the probe signal and the local oscillator signal. Wherein the power size of different signals occupied during splitting can be flexibly set according to needs. Further, in order to provide a high-coherent light source, the laser can be set as a narrow-line-width laser.

[0036] The second signal generation unit 102 can include a signal generator 1021 and a controller 1022, and the signal generator 1021 generates a probe modulation signal and a local oscillator modulation signal, respectively, wherein the signal generator 1021 generates a local oscillator modulation signal capable of modulating the local oscillator signal to obtain a target local oscillator signal negatively correlated with the backscattering signal under the control of the controller 1022.

[0037] The first signal modulation unit 103 can be an AOM, so as to obtain a target probe signal with high extinction ratio and good waveform quality.

[0038] The second signal modulation unit 104 can be an intensity modulator, and the target local oscillator signal is generated by driving the local oscillator modulation signal generated by the second signal generation unit 102.

[0039] The circulator unit 105 can be an optical circulator, and the target probe signal is input into the optical circulator from the first port, and is output from the second port and sent into the sensing optical fiber. From the moment the target probe signal is sent into the sensing optical fiber, until the target probe signal is transmitted to the other end of the optical fiber, a backscattering signal opposite to the propagation direction of the target probe signal will be continuously generated. The optical circulator will receive the backscattering signal through the second port, output the backscattering signal from the third port, and send it into the coherent detection unit 106 to realize extraction of the backscattering signal.

[0040] The coherent detection unit 106 can include a coupler 1061, a coherent detector 1062, and an ADC 1063, wherein the coupler 1061 is used to combine the target local oscillator signal and the backscattering signal to obtain a mixed signal. That is, the target local oscillator signal and the backscattering signal will be mixed and received by the coupler 1061 to realize variable gain amplification of the backscattering signal; the coherent detector 1062 is used to coherently detect the target sensing signal and perform photoelectric conversion to obtain a sensing analog electrical signal, and the ADC 1063 is used to perform analog-to-digital conversion on the sensing analog electrical signal to obtain sensing detection data.

[0041] In particular, the optical fiber sensing system further includes an amplifier 108, which is used to amplify the target probe signal generated by the first signal modulation unit. At this time, the circulator unit 105 is used to send the amplified target probe signal into the optical fiber.

[0042] It should be noted that in the scenario of sensing detection by using the optical fiber providing communication services, since the use of a Raman amplifier in the communication optical fiber cable will cause certain impact on the service signal in the channel, therefore, the amplifier cannot be directly set as a Raman amplifier. The difference is that at this time, the amplifier can use an erbium-doped optical fiber amplifier (EDFA), which directly amplifies the optical signal in a single end, and the signal is no longer affected by the amplifier after being output.

[0043] As can be seen from the above description, the optical fiber sensing system provided in the embodiment is based on an optical frequency domain reflectometer (OFDR), a phase-sensitive optical time domain reflectometer (φOTDR), a coherent optical time domain reflectometer (COTDR), a Brillouin optical time domain reflectometer (BOTDR), a Brillouin optical time domain analyzer (BOTDA), a Raman optical time domain reflectometer (ROTDR), and the like, and additionally introduces an intensity modulation function for the local signal, that is, an intensity modulator is introduced, and the signal generator provides a probe modulation signal generation function and additionally provides a local modulation signal generation function, so as to provide a target local signal negatively related to the power of the backscattering signal, so as to stabilize the power change range of the mixed signal in a relatively small range. Specifically, when a 100 km long optical fiber is used for sensing, the intensity trace of the signal obtained by mixing the backscattering signal and the local signal without intensity modulation of the conventional optical fiber sensing system is as shown in FIG. 8, the signal power fluctuation range caused by the transmission loss of the optical fiber is about 35 dB, and the overall amplitude fluctuation range is as high as nearly 70 dB after the amplitude fluctuation of the signal itself is superimposed. When the optical fiber sensing system provided in the embodiment is used, the intensity trace of the mixed signal is as shown in FIG. 9 when the power trace of the target local signal is the aforementioned expression, the signal power fluctuation range caused by the transmission loss of the optical fiber is still about 35 dB, but the overall fluctuation range is basically consistent with the fluctuation range, that is, about 35 dB. Figure 4 Figure 5

[0044] Another aspect of the embodiment of the present application also provides a detection method, as shown in FIG. 10, applied to the optical fiber sensing system described in the above embodiment, the detection method at least includes the following steps. Figure 6

[0045] Step 601, generating a probe signal, a local signal, a probe modulation signal, and a local modulation signal.

[0046] In some examples, the probe signal and the local signal are obtained by branching a laser beam according to a certain power distribution ratio.

[0047] Step 602, pulse modulating the probe signal according to the probe modulation signal to generate a target probe signal.

[0048] Step 603, intensity modulating the local signal according to the local modulation signal to generate a target local signal negatively related to the power of a backscattering signal, the backscattering signal being a signal generated by backscattering of the target probe signal in the optical fiber.

[0049] In some examples, the target local signal negatively related to the power of the backscattering signal is generated.

[0050] ​​​​In some other examples, the power trace of the backscattered signal within one detection period of the target detection signal is... Accordingly, the generated power trace is The target local oscillator signal, where t∈[0,T], T is the duration of the detection period of the target detection signal, t0 is the start time of the detection period of the target detection signal, p s (t0+t) represents the power of the backscattered signal at time t0+, P0 represents the power of the target detection signal entering the optical fiber, α represents the loss coefficient of the optical fiber, c represents the speed of light, n represents the refractive index of the optical fiber, r(t0+t) represents the backscattering intensity at each position in the optical fiber at time t0+t, and P Lo This is the preset power constant. At this time, as... Figure 7 As shown, compared to the traditional fiber optic sensing method (solid line) where the local oscillator signal remains at 0dB, the target local oscillator signal power provided by this embodiment (dashed line) exhibits a logarithmic linear increase. Furthermore, the signal obtained after coherent detection mixing is: At this point, the target's local oscillator signal is just enough to compensate for the power loss of the backscattered signal.

[0051] Furthermore, in some cases, the local oscillator signal is modulated periodically according to the transmission period of the target detection signal, so that the transmission period of the target detection signal is the same as the modulation period of the local oscillator signal.

[0052] The local oscillator signal is periodically modulated with a period duration of not less than (2*n*L) / c, where n is the refractive index of the optical fiber, c is the speed of light, and L is the radial length of the optical fiber.

[0053] In some examples, step 601 generates a local oscillator modulation signal synchronized with the target detection signal in order to determine the local oscillator modulation signal used at different times and improve the accuracy of modulation.

[0054] For example, the signal can be arranged as follows Figure 8 Synchronization is shown in the diagram. The solid line represents the backscattered signal, the dashed line represents the target local oscillator signal, and the filled area represents the pulsed target detection signal. The synchronization period for all three is (2*n*L) / c. Within this period, as the propagation time of the backscattered signal in the optical fiber increases, the power of the backscattered signal gradually decreases, while the power of the generated target local oscillator signal gradually increases. Here, τ is the pulse response duration of the target local oscillator signal, T = (2*n*L) / c, and t0 is the start time of the period.

[0055] Step 604: Send the target detection signal to the optical fiber and acquire the backscatter signal.

[0056] In some examples, the target probe signal is amplified before being sent into the optical fiber to increase the power after backscattering.

[0057] At step 605, the target local oscillator signal and the backscattered signal are coherently detected to obtain sensing detection data.

[0058] In some examples, the coherently detecting the target local oscillator signal and the backscattered signal can be implemented by the following way: combining the target local oscillator signal and the backscattered signal to obtain an interfered target sensing signal, then optoelectronically detecting the interfered target sensing signal to obtain a sensing analog electrical signal, and then analog-digital converting the sensing analog electrical signal to obtain the sensing detection data.

[0059] In addition, it should be understood that the step division of the above various methods is only for the purpose of clear description, and when implemented, one step can be combined or some steps can be split and decomposed into multiple steps, as long as the same logical relationship is included, and it is within the protection scope of the patent; adding irrelevant modifications or introducing irrelevant designs in the algorithm or process, but not changing the core design of the algorithm and process, are within the protection scope of the patent.

[0060] It can be found that the embodiments of the present application are method embodiments corresponding to the system embodiments, and the embodiments of the present application can be implemented in cooperation with the system embodiments. The related technical details mentioned in the system embodiments are still valid in the present embodiments. In order to reduce repetition, they will not be described here. Correspondingly, the related technical details mentioned in the present embodiments can also be applied in the system embodiments.

[0061] The above embodiments are provided to those skilled in the art to implement and use the present application, and those skilled in the art can make various modifications or changes to the above embodiments without departing from the inventive idea of the present application, so the protection scope of the present application is not limited by the above embodiments, but should conform to the maximum scope of the innovative features mentioned in the claims.

Claims

1. A fiber optic sensing system, characterized in that, include: The system comprises a first signal generating unit, a second signal generating unit, a first signal modulation unit, a second signal modulation unit, a circulator unit, and a coherent detection unit; The first signal generating unit is used to generate a probe signal and a local oscillator signal; The second signal generating unit is used to generate a probe modulation signal and a local oscillator modulation signal; The first signal modulation unit is used to pulse modulate the detection signal according to the detection modulation signal to generate a target detection signal; The second signal modulation unit is used to modulate the intensity of the local oscillator signal according to the local oscillator modulation signal to generate a target local oscillator signal that is negatively correlated with the power of the backscattered signal. The backscattered signal is the signal generated by backscattering when the target detection signal is transmitted in the optical fiber. The power of the target local oscillator signal is inversely proportional to the power of the backscattered signal. The modulation period of the local oscillator signal is the same as the transmission period of the target detection signal. The circulator unit is used to send the target detection signal to the optical fiber through the first port and output the backscatter signal through the third port; The coherent detection unit is used to perform coherent detection of the target local oscillator signal and the backscattered signal to obtain sensor detection data.

2. The fiber optic sensing system according to claim 1, characterized in that, The power trace of the backscattered signal is In the case of this, the power trace of the target local oscillator signal is ,in, , The duration of one detection cycle of the target detection signal. This is the start time of the detection period of the target detection signal. for The power of the backscattered signal at that time. for The power of the target local oscillator signal at the given time. The power of the target detection signal when it enters the optical fiber. This represents the loss coefficient of the optical fiber. At the speed of light, Let be the refractive index of the optical fiber. for The backscattering intensity at each location in the optical fiber at time t. The local oscillator power constant is preset when the target detection signal enters the optical fiber.

3. The fiber optic sensing system according to claim 1, characterized in that, The modulation period of the local oscillator signal is not less than ,in, Let be the refractive index of the optical fiber. At the speed of light, This represents the radial length of the optical fiber.

4. The fiber optic sensing system according to any one of claims 1 to 2, characterized in that, The local oscillator modulation signal is synchronized with the target detection signal.

5. The fiber optic sensing system according to any one of claims 1 to 2, characterized in that, The first signal generating unit includes a laser and a coupler. The coupler is used to split the continuous laser light generated by the laser to generate the detection signal and the local oscillator signal.

6. The fiber optic sensing system according to any one of claims 1 to 2, characterized in that, The fiber optic sensing system further includes: an amplifier, which amplifies the target detection signal generated by the first signal modulation unit, and a circulator unit, which transmits the amplified target detection signal to the optical fiber through the first port.

7. The fiber optic sensing system according to any one of claims 1 to 2, characterized in that, The coherent detection unit includes a coupler, a coherent detector, and an analog-to-digital converter (ADC). The coupler is used to combine the target local oscillator signal and the backscattered signal to obtain a target sensing signal. The coherent detector is used to coherently detect the target sensing signal and perform photoelectric conversion to obtain a sensing analog electrical signal. The ADC is used to perform analog-to-digital conversion on the sensing analog electrical signal to obtain sensing detection data.

8. A detection method, characterized in that, The method, applied to the fiber optic sensing system as described in any one of claims 1 to 7, comprises: Generate a detection signal, a local oscillator signal, a detection modulation signal, and a local oscillator modulation signal; The detection signal is pulse-modulated according to the detection modulation signal to generate a target detection signal; The local oscillator signal is intensity modulated according to the local oscillator modulation signal to generate a target local oscillator signal that is negatively correlated with the power of the backscattered signal. The backscattered signal is the signal generated by backscattering when the target detection signal is transmitted in the optical fiber. The power of the target local oscillator signal is inversely proportional to the power of the backscattered signal. The modulation period of the local oscillator signal is the same as the transmission period of the target detection signal. The target detection signal is sent to an optical fiber and the backscattered signal is acquired; Coherent detection is performed on the target local oscillator signal and the backscattered signal to obtain sensor detection data.

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