Compact high-signal-to-noise-ratio dark pulse BOTDR system for engineering

Abstract

The utility model belongs to the field of distributed optical fiber sensing, and particularly relates to a compact high-signal-to-noise-ratio dark pulse BOTDR (Brillouin Optical Time Domain Reflectometer) system for engineering, which comprises a laser, and the output end of the laser is connected with a first photoelectric coupler which is used for dividing light emitted by the laser into upper light and lower light. The dark pulse modulation system module is arranged, descending pump light is equivalent to a dark pulse through modulation, a computer controls dark pulse time slots so as to change pump pulse characteristics, a corresponding demodulation method is adopted in rear-end data processing, Brillouin frequency shift along a sensing optical fiber is obtained, and the Brillouin frequency shift is converted into Brillouin frequency shift along the sensing optical fiber. The lower limit of the spatial resolution of the method is equal to the fall time of the modulation system, and the spatial resolution can be greatly improved under the condition of improving the signal-to-noise ratio. In addition, due to narrow pulse width and low energy of the traditional bright pulse, an erbium-doped fiber amplifier (EDFA) is generally needed for signal amplification. Dark pulses are close to continuous light in most cases, and EDFA is not needed, so that the system design is simplified, and the cost is reduced.

Description

Technical Field

[0001] The utility model relates to the technical field of distributed optical fiber sensing, in particular to a compact high signal-to-noise ratio dark pulse BOTDR system for engineering use. Background Technique

[0002] With the development of modern engineering technology, structural health monitoring has become particularly important in ensuring the safety of infrastructure and extending its service life. Although traditional monitoring methods can achieve some effects in engineering applications, they often have problems such as a limited number of measurement points, insufficient coverage, and poor anti-interference ability. Distributed optical fiber sensing technology, especially the Brillouin optical time domain reflectometry (BOTDR) system, is gradually becoming an ideal solution to solve these problems.

[0003] As a typical distributed optical fiber sensing technology, Brillouin optical time domain reflectometry (BOTDR) has unique advantages such as being able to measure temperature and strain simultaneously and having a long detection distance (up to hundreds of kilometers). The working principle of the BOTDR system is as follows: A laser beam is injected into the optical fiber. Due to the action of acoustic waves in the optical fiber, Brillouin Stokes light and Brillouin anti-Stokes light will be generated. Changes in the external temperature or stress will cause changes in the parameters of the optical fiber itself, thereby affecting the frequencies of the Stokes and anti-Stokes lights. By demodulating the frequency shift of the Stokes or anti-Stokes light before and after the change in temperature or stress, the magnitude of the change in the external temperature or stress can be obtained. However, since the peak power of the light generated by Brillouin scattering is small, only one-thousandth of the Rayleigh scattered light, in long-distance sensing, the reflected Brillouin scattered light will become increasingly weak, while the noise will increase, resulting in the Brillouin scattered light being submerged in the noise, which means a decrease in the sensing distance. Even if the scattered light is not submerged in the noise, the low peak power will also cause the error of the BOTDR system to increase and the measurement accuracy to decrease, limiting the practicality of the BOTDR system. Summary of the Utility Model

[0004] The utility model aims to provide a compact high signal-to-noise ratio dark pulse BOTDR system for engineering use. By generating dark pulses through the front-end modulation system, local stimulated scattering can be formed in the spontaneous Brillouin scattering in the BOTDR system, obtaining a higher system signal-to-noise ratio. At the same time, the front-end modulation pulse of the system has a high light intensity within the period and can directly enter the optical fiber to be measured without using an optical fiber amplifier and filter, further reducing the system cost and making it more suitable for applications in general engineering fields.

[0005] Therefore, the technical solution adopted by the utility model is as follows:

[0006] Engineering compact high signal-to-noise ratio dark pulse BOTDR system, including a laser, the output end of the laser is connected with a first optoelectronic coupler for splitting the light emitted by the laser into an upper path light and a lower path light, two output ends of the first optoelectronic coupler are respectively connected with a dark pulse modulation system and a frequency-shifted swept-frequency module, the upper path light enters the dark pulse modulation system, and the lower path light enters the frequency-shifted swept-frequency module;

[0007] The output end of the dark pulse modulation system is connected with a circulator, one port of the circulator is connected with a sensing optical fiber, and the other port is connected with a first erbium-doped optical fiber amplifier, the output end of the first erbium-doped optical fiber amplifier is connected with a first fiber Bragg grating, the output end of the first fiber Bragg grating is connected with a second optoelectronic coupler, and the output end of the second optoelectronic coupler is connected with a photodetector.

[0008] Based on the above technical solution, its working principle and the technical effects produced are as follows:

[0009] In actual use, the laser emits continuous laser light, which is split into two paths by the first optoelectronic coupler, namely the upper path light and the lower path light. The upper path light is modulated into a waveform by the dark pulse modulation system and then input into the optical fiber to be measured through the circulator. In the optical fiber to be measured, spontaneous Brillouin scattering and stimulated Brillouin scattering occur in the optical fiber, generating Stokes light. The returned Stokes light enters the first erbium-doped optical fiber amplifier through the circulator for amplification and is filtered by the first fiber Bragg grating. At this time, the second optoelectronic coupler receives the Stokes light and the lower path light and transports them to the photodetector. The photodetector performs coherent processing on the Stokes light and the lower path light and converts them into electrical signals. The lower path light is used as the reference light. The frequency-shifted swept-frequency module performs step-by-step sweeping within the relevant spectral range of 10.7 GHz to 11.0 GHz, and then the data acquisition and processing system processes these data and outputs the results.

[0010] In a preferred embodiment of the present invention, it can be further configured that: the output end of the photodetector is connected with a data acquisition and processing module, and both the data acquisition and processing module and the frequency-shifted swept-frequency module are connected with a computer.

[0011] In a preferred embodiment of the present invention, it can be further configured that: the laser adopts a narrow linewidth low-noise laser with continuous output, the linewidth of the laser is 30 KHz, and the central wavelength of the laser light emitted by the laser is 1550 nm.

[0012] In a preferred embodiment of the present invention, it can be further configured that: the splitting ratio of the first optoelectronic coupler is 70:30.

[0013] In a preferred embodiment of the present invention, it can be further configured that: the dark pulse modulation system includes a dark pulse generator and a semiconductor optical amplifier, and the extinction ratio of the semiconductor optical amplifier is set to 50 dB.

[0014] In a preferred embodiment, the present utility model can be further configured such that the frequency shift and sweep module performs frequency sweeping around 10.8 GHz.

[0015] In a preferred embodiment, the present utility model can be further configured such that the frequency shift and sweep module (4) is set to perform frequency sweeping in the range of 10.7 GHz to 11.0 GHz with a step size of 0.001 GHz. The frequency shift and sweep module (4) includes a signal generator, the output end of the signal generator is connected to a mixer, and the output end of the mixer is connected to a filter.

[0016] In a preferred embodiment, the present utility model can be further configured such that one input end of the second optoelectronic coupler is connected to the first fiber Bragg grating, the output end is connected to a photodetector, and the splitting ratio of the second optoelectronic coupler is 50:50.

[0017] In a preferred embodiment, the present utility model can be further configured such that the frequency shift and sweep module is connected to a port of the second optoelectronic coupler.

[0018] In a preferred embodiment, the present utility model can be further configured such that spontaneous Brillouin scattering and stimulated Brillouin scattering occur in the sensing optical fiber to generate Stokes light. The photodetector performs coherent processing on the Stokes light and the dropped light input into the second fiber coupler, and then inputs them into the data acquisition and processing module.

[0019] The explanations of the nouns, conjunctions or adjectives involved in the above technical solutions are as follows:

[0020] Fixed connection means that after the parts or components are fixed, there is no relative movement. It is divided into two types: detachable connection and non-detachable connection.

[0021] (1) Detachable connection uses screws, splines, wedge pins, etc. to fix the components together. This connection method can be disassembled during maintenance and will not damage the parts. However, the specifications of the connecting parts used must be correct (such as the length of bolts, keys, wedge pins), and they must be tightened properly.

[0022] (2) Non-detachable connection mainly refers to welding, riveting and mortise fitting, etc. Since it needs to be forged, sawed or oxy-cut to be disassembled during maintenance or replacement, the spare parts generally cannot be used twice. At the same time, during connection, attention should be paid to process quality, technical inspection and remedial measures (such as correction, polishing, etc.);

[0023] Movable connection means that after the parts or components are fixed, there is relative movement.

[0024] The above technical solutions of the present utility model have the following beneficial technical effects:

[0025] In the present utility model, a dark pulse modulation system module is deployed. Through modulation, the descending pump light is equivalent to a dark pulse. The computer controls the time slot when the dark pulse appears, thereby changing the characteristics of the pump pulse. And by adopting corresponding demodulation methods during the subsequent data processing, the Brillouin frequency shift along the sensing optical fiber can be obtained. The lower limit of the achievable spatial resolution of this measurement method is equal to the fall time of the modulation system. It can achieve a large improvement in spatial resolution while increasing the signal-to-noise ratio of the system. In addition, since the traditional bright pulse has a narrow pulse width and low energy, an erbium-doped fiber amplifier (EDFA) is usually required for signal amplification. In contrast, the dark pulse mostly appears as near-continuous light except in specific cases, so there is no need to use an EDFA, which can simplify the system design and reduce the device cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a schematic diagram of the overall structure of the present utility model;

[0027] Figure 2 is a schematic diagram for comparing the dark pulse and the traditional bright pulse of the present utility model.

[0028] Reference numerals:

[0029] 1. Laser; 2. First optoelectronic coupler; 3. Dark pulse modulation system; 4. Frequency shift and sweep module; 5. First erbium-doped fiber amplifier; 6. First fiber Bragg grating; 7. Circulator; 8. Sensing optical fiber; 9. Second optoelectronic coupler; 10. Photoelectric detector; 11. Data acquisition and processing module; 12. Computer. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] Next, the technical solutions in the embodiments of the present disclosure will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present disclosure. Obviously, the described embodiments are only a part of the embodiments of the present disclosure, rather than all the embodiments. Based on the embodiments in the present disclosure, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present disclosure. It should be noted that, without conflict, the embodiments of the present utility model and the features in the embodiments may be combined with each other.

[0031] A dark pulse is a form of optical pulse with an optical intensity lower than the background optical intensity, presenting as a decrease in optical intensity or a "negative pulse". Due to its characteristic of being close to continuous light, a dark pulse system usually does not require frequent amplification processing. Through specific modulation and demodulation techniques, dark pulses can effectively transmit information and have high sensitivity and accuracy.

[0032] Dark pulses have special advantages compared to traditional bright pulses. Due to the pulse width limitation, bright pulses have a short duration and limited energy, and are more likely to be completely depleted during forward propagation in optical fibers. Therefore, an erbium-doped fiber amplifier (EDFA) needs to be used later to amplify the pulse energy. Dark pulses are close to continuous light in most cases, with sufficient energy within a period, and do not require the use of an EDFA, which simplifies the system design and reduces costs, and is more suitable for ordinary engineering projects. In addition, dark pulses can reduce the nonlinear effects and noise in the system, thereby improving the signal-to-noise ratio and enhancing the signal quality.

[0033] According to the concept of the present application, an embodiment of an engineering compact high signal-to-noise ratio dark pulse BOTDR system for engineering temperature change and strain monitoring is described herein in conjunction with Figure 1 - Figure 2 Specifically, the engineering compact high signal-to-noise ratio dark pulse BOTDR system is configured as an integrated structure, which has components such as a first optoelectronic coupler 2 and a dark pulse modulation system 3. By deploying the dark pulse modulation system 3 module in the present utility model, the descending pump light is made equivalent to a dark pulse through modulation. During actual use, the computer 12 controls the time slot when the dark pulse appears to change the characteristics of the pump pulse, and corresponding demodulation methods are adopted during backend data processing, and thus the Brillouin frequency shift along the sensing optical fiber 8 can be obtained. The lower limit of the achievable spatial resolution of this measurement method is equivalent to the fall time of the modulation system, and a large degree of improvement in spatial resolution can be achieved while improving the system signal-to-noise ratio. In addition, traditional bright pulses have a narrow pulse width and low energy, and usually require an erbium-doped fiber amplifier (EDFA) for signal amplification. In contrast, dark pulses mostly appear as near-continuous light except in specific cases, so the use of an EDFA is not required, which can simplify the system design and reduce the device cost.

[0034] In conjunction with Figure 1 - Figure 2 As shown, the engineering compact high signal-to-noise ratio dark pulse BOTDR system provided by the present utility model includes a laser 1. The output end of the laser 1 is connected to a first optoelectronic coupler 2 for splitting the light emitted by the laser 1 into an upper path light and a lower path light. The two output ends of the first optoelectronic coupler 2 are respectively connected to a dark pulse modulation system 3 and a frequency shift sweep module 4. The upper path light enters the dark pulse modulation system 3, and the lower path light enters the frequency shift sweep module 4;

[0035] The output end of the dark pulse modulation system 3 is connected to a circulator 7. One port of the circulator 7 is connected to a sensing optical fiber 8, and the other port is connected to a first erbium-doped fiber amplifier 5. The output end of the first erbium-doped fiber amplifier 5 is connected to a first fiber Bragg grating 6. The output end of the first fiber Bragg grating 6 is connected to a photodetector 10.

[0036] For the technical solution of this embodiment, the dark pulse modulation system 3 is electrically connected to the computer 12. The dark pulse modulation system 3 includes a dark pulse generator and a semiconductor optical amplifier, which are mainly used to modulate the pump light to generate a waveform. The semiconductor optical amplifier is mainly used to enhance and expand the optical signal to maintain the stability and availability of the signal during long-distance transmission. The extinction ratio of the semiconductor optical amplifier is set to 50 dB. During use, the set dark pulse modulation system 3 is used to modulate the descending pump light to be equivalent to a dark pulse, and the computer 12 is used to control the time slot when the dark pulse appears, so as to change the characteristics of the pump pulse, and corresponding demodulation methods are adopted during the subsequent data processing, and the Brillouin frequency shift along the sensing optical fiber 8 can be obtained.

[0037] For the technical solution of this embodiment, the output end of the photodetector 10 is connected to the data acquisition and processing module 11, and both the data acquisition and processing module 11 and the frequency shift and sweep module 4 are connected to the computer 12. In the sensing optical fiber 8, spontaneous Brillouin scattering and stimulated Brillouin scattering occur in the optical fiber to generate Stokes light. The photodetector 10 performs coherent processing on the Stokes light and the dropped light input by the second optical fiber coupler, and then inputs them into the data acquisition and processing module 11.

[0038] It should be noted that when the pump light is incident on the sensing optical fiber 8, three types of scattering will occur due to the inhomogeneity and nonlinear effects of the optical fiber material, namely Rayleigh scattering, Brillouin scattering, and Raman scattering. Among them, Brillouin scattering includes spontaneous Brillouin scattering and stimulated Brillouin scattering; when the input optical power is low, due to molecular thermal motion, spontaneous acoustic fields will be generated in the sensing optical fiber 8. These acoustic fields will generate refractive index gratings in the optical fiber that are the same as the acoustic wave transmission direction and speed. When the incident light meets the refractive index grating, it will be reflected, that is, spontaneous Brillouin scattering. Due to the Doppler effect, the scattered light will have a certain frequency shift compared to the incident light. The light with an increased frequency is called anti-Stokes light, and the light with a decreased frequency is called Stokes light; when the incident optical power continuously increases and exceeds the stimulated threshold, the incident light will interfere with the backward-transmitted Stokes light, causing the refractive index in the optical fiber to change, and then resulting in the electrostrictive effect, generating a large number of acoustic waves. The acoustic waves generate refractive index gratings, reflecting a large amount of incident light and generating a lot of Brillouin scattered light, which is stimulated Brillouin scattering.

[0039] For the technical solution of this embodiment, the laser 1 uses a narrow linewidth and low-noise laser 1 with continuous output. The linewidth of the laser 1 is 30 KHz, and the central wavelength of the laser emitted by the laser 1 is 1550 nm.

[0040] For the technical solution of this embodiment, the splitting ratio of the first optoelectronic coupler 2 is 70:30, which is convenient for splitting the laser emitted by the laser 1 into two paths, and the lower path light is used as the reference light.

[0041] For the technical solution of this embodiment, the frequency shift and sweep module (4) is set to sweep frequencies in the range of 10.7 GHz to 11.0 GHz, with a step size of 0.001 GHz. The frequency shift and sweep module (4) includes a signal generator. The output end of the signal generator is connected to a mixer, and the output end of the mixer is connected to a filter. The signal generator generates a signal with an adjustable frequency, which serves as the local oscillator (LO) signal of the mixer. The mixer receives this signal and the signal to be measured (RF signal), performs frequency mixing and outputs an intermediate frequency (IF) signal. The filter is connected to the output end of the mixer, selects the required frequency components and suppresses noise, and outputs a clean intermediate frequency signal. By adjusting the frequency of the signal generator, the frequency mixing process and the bandwidth and center frequency of the filter can be controlled, thereby realizing the comprehensive control and data acquisition of the frequency shift and sweep process.

[0042] For the technical solution of this embodiment, a second optoelectronic coupler 9 is also connected between the first fiber Bragg grating 6 and the photodetector 10. One input end of the second optoelectronic coupler 9 is connected to the first fiber Bragg grating 6, and the output end is connected to the photodetector 10. The frequency shift and sweep module 4 is connected to one port of the second optoelectronic coupler 9.

[0043] Specifically, in actual use, the laser 1 emits continuous laser light, which is split into two paths by the first optoelectronic coupler 2, namely the upper path light and the lower path light. The upper path light is modulated into a waveform by the dark pulse modulation system 3, and then enters the fiber under test through the circulator 7. In the fiber under test, spontaneous Brillouin scattering and stimulated Brillouin scattering occur in the fiber, generating Stokes light. The returned Stokes light enters the first erbium-doped fiber amplifier 5 through the circulator 7 and is filtered by the first fiber Bragg grating 6. At this time, the second optoelectronic coupler 9 receives the Stokes light and the lower path light and transports them to the photodetector 10. The photodetector 10 performs coherent processing on the Stokes light and the lower path light and converts them into electrical signals. The lower path is used as the reference light. The frequency shift and sweep module 4 performs step-by-step frequency sweeping in the relevant spectral range of 10.7 GHz to 11.0 GHz, and then the data acquisition and processing system processes these data and outputs the results.

[0044] Next, in combination with the drawings and embodiments, the compact high signal-to-noise ratio dark pulse BOTDR system for engineering provided by the present utility model will be further described.

[0045] Engineering compact high signal-to-noise ratio dark pulse BOTDR system, including a laser 1, the output end of the laser 1 is connected with a first optoelectronic coupler 2, which is used to divide the light emitted by the laser 1 into an upper path light and a lower path light. The two output ends of the first optoelectronic coupler 2 are respectively connected with a dark pulse modulation system 3 and a frequency shift sweeping module 4. The upper path light enters the dark pulse modulation system 3, and the lower path light enters the frequency shift sweeping module 4;

[0046] The output end of the dark pulse modulation system 3 is connected with a circulator 7. One port of the circulator 7 is connected with a sensing optical fiber 8, and the other port is connected with a first erbium-doped fiber amplifier 5. The output end of the first erbium-doped fiber amplifier 5 is connected with a first fiber Bragg grating 6. The output end of the first fiber Bragg grating 6 is connected with a photodetector 10.

[0047] The working principle and usage process of the present utility model: In actual use, the laser 1 emits continuous laser light, which is divided into two paths by the first optoelectronic coupler 2, namely the upper path light and the lower path light. The upper path light is modulated into a waveform by the dark pulse modulation system 3, and then input into the fiber to be measured through the circulator 7. In the fiber to be measured, spontaneous Brillouin scattering and stimulated Brillouin scattering occur in the optical fiber, generating Stokes light. The returned Stokes light enters the first erbium-doped fiber amplifier 5 through the circulator 7 for amplification, and is filtered by the first fiber Bragg grating 6. At this time, the second optoelectronic coupler 9 receives the Stokes light and the lower path light and transports them to the photodetector 10. The photodetector 10 performs coherent processing on the Stokes light and the lower path light and converts them into electrical signals. The lower path light serves as a reference light. The frequency shift sweeping module 4 performs step-by-step sweeping within the relevant spectral range of 10.7 GHz to 11.0 GHz, and then the data acquisition and processing system processes these data and outputs the results.

[0048] In summary, in the technical solution of this embodiment, the dark pulse modulation system 3 module is deployed. By modulation, the descending pump light is equivalent to a dark pulse. The computer 12 controls the time slot when the dark pulse appears to change the characteristics of the pump pulse. And corresponding demodulation methods are adopted during the subsequent data processing, and the Brillouin frequency shift along the sensing optical fiber 8 can be obtained. The lower limit of the spatial resolution that can be achieved by this measurement method is equivalent to the falling time of the modulation system. While improving the signal-to-noise ratio of the system, a large degree of improvement in spatial resolution is achieved. In addition, due to the narrow pulse width and low energy of traditional bright pulses, an erbium-doped fiber amplifier (EDFA) is usually required for signal amplification. In contrast, dark pulses mostly appear as near-continuous light except in specific cases. Therefore, EDFA is not required, which can simplify the system design and reduce the device cost.

[0049] In the present utility model, the term "a plurality of" refers to two or more, unless otherwise clearly defined. The term "and / or" as used herein includes any and all combinations of one or more of the related listed items. Terms such as "mounted", "connected", "coupled", "fixed", etc. shall be construed in a broad sense. For example, "connected" can be a fixed connection, a detachable connection, or an integral connection; "coupled" can be a direct connection or an indirect connection through an intermediate medium. For those of ordinary skill in the art, the specific meanings of the above terms in the present utility model can be understood according to specific circumstances.

[0050] It should be noted that when an element is referred to as being "assembled on", "mounted on", "fixed to", or "disposed on" another element, it can be directly on the other element or there can also be an intermediate element. When an element is considered to be "connected" to another element, it can be directly connected to the other element or there may be an intermediate element at the same time. The terms "vertical", "horizontal", "upper", "lower", "left", "right", and similar expressions used herein are for illustrative purposes only and do not represent the only embodiments.

[0051] In the description of this specification, the descriptions of terms such as "one embodiment", "some embodiments", "specific embodiments", etc. mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present utility model. In this specification, the schematic descriptions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0052] Although the embodiments of the present utility model have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present utility model. The scope of the present utility model is defined by the claims and their equivalents.

Claims

1. A compact high signal-to-noise ratio dark pulse BOTDR system for engineering use, comprising a laser (1), characterized in that: The output end of the laser (1) is connected to a first photoelectric coupler (2) for dividing the light emitted by the laser (1) into upstream light and downstream light, and the two output ends of the first photoelectric coupler (2) are respectively connected to a dark pulse modulation system (3) and a frequency shift scanning module (4), the upstream light enters the dark pulse modulation system (3), and the downstream light enters the frequency shift scanning module (4); The output end of the dark pulse modulation system (3) is connected to a circulator (7), one port of the circulator (7) is connected to a sensing optical fiber (8), and the other port is connected to a first erbium-doped optical fiber amplifier (5), the output end of the first erbium-doped optical fiber amplifier (5) is connected to a first optical fiber Bragg grating (6), the output end of the first optical fiber Bragg grating (6) is connected to a second photoelectric coupler (9), and the output end of the second photoelectric coupler (9) is connected to a photodetector (10).

2. The compact high signal-to-noise ratio dark pulse BOTDR system for engineering use according to claim 1, characterized in that: The output end of the photoelectric detector (10) is connected to a data acquisition and processing module (11), and the data acquisition and processing module (11) and the frequency shift and frequency scanning module (4) are both connected to a computer (12).

3. The compact high signal-to-noise ratio dark pulse BOTDR system for engineering use according to claim 2, characterized in that: The laser (1) adopts a continuously output narrow line width low noise laser (1), the line width of the laser (1) is 30KHz, and the central wavelength of the laser emitted by the laser (1) is 1550nm.

4. The compact high signal-to-noise ratio dark pulse BOTDR system for engineering use according to claim 2, characterized in that: The splitting ratio of the first photoelectric coupler (2) is 70:

30.

5. The compact high signal-to-noise ratio dark pulse BOTDR system for engineering use according to claim 4, characterized in that: The dark pulse modulation system (3) comprises a dark pulse generator and a semiconductor optical amplifier, and the extinction ratio of the semiconductor optical amplifier is set to 50 dB.

6. The compact high signal-to-noise ratio dark pulse BOTDR system for engineering use according to claim 5, characterized in that: The frequency shift and sweep module (4) performs frequency sweep at about 10.8 GHz.

7. The compact high signal-to-noise ratio dark pulse BOTDR system for engineering use according to claim 6, characterized in that: The frequency shift and sweep module (4) is configured to sweep the frequency within the range of 10.7 GHz to 11.0 GHz, with a step length of 0.001 GHz. The frequency shift and sweep module (4) comprises a signal generator, the output end of the signal generator is connected to a mixer, and the output end of the mixer is connected to a filter.

8. The compact high signal-to-noise ratio dark pulse BOTDR system for engineering use according to claim 6, characterized in that: An input end of the second photoelectric coupler (9) is connected to the first fiber Bragg grating (6), and an output end is connected to the photodetector (10), and the splitting ratio of the second photoelectric coupler (9) is 50:

50.

9. The compact high signal-to-noise ratio dark pulse BOTDR system for engineering use according to claim 8, characterized in that: The frequency shift and frequency scanning module (4) is connected to a port of the second photoelectric coupler (9).

10. The compact high signal-to-noise ratio dark pulse BOTDR system for engineering use according to claim 8, characterized in that: Spontaneous Brillouin scattering and stimulated Brillouin scattering occur in the sensing optical fiber (8) to generate Stokes light. The photodetector (10) performs coherent processing on the Stokes light and the downstream light input by the second photoelectric coupler (9), and then inputs the resultant light into a data acquisition and processing module (11).

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