Continuous light-assisted distributed optical fiber vibration sensing system

Abstract

The invention discloses a distributed optical fiber vibration sensing system assisted by continuous light. The output of a light source is split into pulsed light and continuous light, the pulsed light is used for carrying out distributed vibration positioning on a sensing optical fiber, and the continuous light is used for carrying out high-frequency vibration detection on the sensing optical fiber after frequency shift; pulsed light and continuous light are injected into the sensing optical fiber in the same direction, and frequency band separation is achieved at the tail end of the sensing optical fiber through a continuous light frequency shift module; the photoelectric detection module receives a Rayleigh scattering signal corresponding to the pulsed light and a vibration signal corresponding to the continuous light at the same time; the signal processing module separates and processes the two kinds of signals based on the frequency division multiplexing technology, and vibration positioning and high-frequency vibration response breaking through the limitation of the Nyquist sampling theorem are achieved. According to the invention, the continuous optical signal is used as an auxiliary mode for obtaining high-frequency vibration, and simultaneous measurement of low-frequency and high-frequency vibration signals is realized.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber sensing technology, and in particular to a continuous light-assisted distributed optical fiber vibration sensing system. Background Art

[0002] Optical fiber, as a transmission medium, underpins fiber-optic communications technology. As a sensing medium, new fiber-optic sensing technologies are emerging. Changes in the physical parameters of the optical fiber's environment (such as vibration, temperature, and pressure) can affect the intensity, phase, wavelength, and polarization state of light transmitted through the fiber. Sensing is achieved by detecting these changes in lightwave state parameters. Fiber-optic sensors are resistant to electromagnetic interference, corrosion, and high temperatures, making them widely applicable in a variety of harsh environments.

[0003] Traditional electrical sensors are limited by electromagnetic interference, making point-type fiber optic sensors short-range and difficult to deploy on a large scale. Distributed fiber optic sensing systems combine the advantages of electromagnetic interference resistance with long-distance, large-scale application, resulting in low cost and a high cost-performance ratio. Distributed fiber optic sensing systems use commercially available communication optical fibers as both the sensing element and the transmission medium. They are primarily based on three scattering effects within the optical fiber: Rayleigh scattering, Brillouin scattering, and Raman scattering. Rayleigh scattering is commonly used to detect dynamic signals such as sound waves and vibrations, offering high backscatter power, a high signal-to-noise ratio, and high responsiveness. Brillouin scattering can capture temperature and strain information, and the change in temperature or strain can be derived by demodulating the Brillouin gain spectrum. Raman scattering is only sensitive to temperature changes, and distributed temperature measurement can be achieved by calculating the ratio of light in the Stokes and anti-Stokes directions.

[0004] Phase-sensitive optical time-domain reflectometry, based on the Rayleigh scattering effect, is one of the leading vibration detection technologies. A highly coherent, narrow-linewidth light source continuously transmits probe pulses into a sensing fiber, capturing and demodulating the Rayleigh scattered signal modulated by external vibrations to locate the vibration point and identify the vibration signal. Limited by the Nyquist sampling theorem, the system's maximum response frequency is half the repetition rate, and detection is subject to interference from other physical parameters, limiting high-frequency measurements.

[0005] However, actual application scenarios are becoming increasingly complex and diverse. In some special environments, not only low-frequency vibration signals but also high-frequency vibration signals exist. Conventional distributed systems can increase the system's repetition frequency to improve the maximum frequency response. However, this method sacrifices the system's detection optical power, making it difficult to obtain high-quality Rayleigh scattering signals. This increases the detection difficulty, reduces signal quality, and affects signal demodulation. Therefore, a new distributed sensing system or measurement method is needed to meet the needs of high-frequency signal detection in complex application scenarios, achieving system vibration positioning while also obtaining high-frequency response. Summary of the Invention

[0006] In order to solve the problem that the above-mentioned technology can effectively detect high-frequency signals, the main purpose of the present invention is to provide a distributed optical fiber vibration sensing system based on continuous light assistance. By simultaneously transmitting pulsed light and continuous light in the sensing optical fiber, it can achieve vibration signal positioning while realizing high-frequency vibration signal response, breaking through the limitation of Nyquist sampling theorem on the system's maximum frequency response.

[0007] The present invention includes a light source, a pulse light modulation module, a continuous light frequency shift module, a sensing optical fiber, a photoelectric detection module and a signal processing module;

[0008] The light source output is split into pulsed light and continuous light, the pulsed light is used to perform distributed vibration positioning on the sensing optical fiber, and the continuous light is used to perform high-frequency vibration detection on the sensing optical fiber after frequency shift;

[0009] The pulse light and the continuous light are injected into the sensing optical fiber in the same direction, and the frequency bands are separated at the end of the sensing optical fiber through the continuous light frequency shift module;

[0010] The photoelectric detection module simultaneously receives the Rayleigh scattering signal corresponding to the pulse light and the vibration signal corresponding to the continuous light;

[0011] The signal processing module separates and processes the two signals based on frequency division multiplexing technology, thereby achieving vibration positioning and high-frequency vibration response that breaks through the limitations of the Nyquist sampling theorem.

[0012] Compared with the prior art, the present invention has the following beneficial effects:

[0013] 1. This invention supplements the transmission of continuous light signals in a distributed fiber-optic sensing system using pulsed light transmission, overcoming the limitation of the maximum response frequency of pulsed light systems imposed by the Nyquist sampling theorem. By using continuous light signals as an auxiliary method for acquiring high-frequency vibrations, it achieves simultaneous measurement of low- and high-frequency vibration signals.

[0014] 2. Distributed fiber optic vibration sensing based on pulsed light and high-frequency response based on continuous light are integrated into one solution and configured in the same light source. They transmit and detect simultaneously, and the signal separation does not affect each other, achieving vibration positioning and high-frequency response at a low cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 Schematic diagram of the structure of the continuous light-assisted distributed optical fiber sensing system of this embodiment.

[0016] Figure 2 This is a schematic diagram of the two key signal controls in this system. DETAILED DESCRIPTION

[0017] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with examples and drawings. The exemplary embodiments of the present invention and their descriptions are only used to explain the present invention and are not intended to limit the present invention.

[0018] like Figure 1 As shown, this embodiment provides a distributed fiber optic sensing system assisted by continuous light, including a narrow-linewidth laser 1. The narrow-linewidth laser 1 emits laser light as a light source, which is split into two beams by a 2×1 coupler 2. One beam of light contains the majority of the light source's energy. Under signal ① set by a signal generator 15, this beam of light is modulated into pulsed light by an acousto-optic modulator 3. The pulsed light is amplified to the required optical power by a pulsed light amplifier 4 (the acousto-optic modulator 3 and the pulsed light amplifier 4 constitute a pulsed light modulation module), enters a fiber circulator 6 through a 2×1 coupler 5, and is then injected into a sensing fiber 7. The laser 1 is a wavelength-tunable narrow-linewidth continuous fiber laser.

[0019] The other light beam is split into two beams again by the 2×1 coupler 10, serving as continuous light and local oscillator light respectively. The continuous light enters the other input port of the coupler 5 and is injected into the sensing optical fiber by the circulator 6 together with the pulsed light. The local oscillator light and the Rayleigh scattered signal returned by the optical fiber circulator 6 beat each other after passing through the 2×2 coupler 11.

[0020] The distributed sensing system features a unique frequency-shifting structure (i.e., a continuous optical frequency-shifting module) at the end of the sensing fiber. This structure consists of a second fiber circulator 8 and an acousto-optic modulator 9. The optical signal transmitted to the end of the sensing fiber is returned to the sensing fiber by the fiber circulator 8. This optical signal undergoes a frequency shift under the action of the acousto-optic modulator 9, and is not in the same frequency band as the Rayleigh backscattered signal generated by the pulsed light. Notably, the optical signal ② set by the acousto-optic modulator 9 is completely opposite to the pulsed optical signal ①. Therefore, the pulsed light in the optical signal transmitted thereto is suppressed, while the continuous optical signal is allowed to pass through and return to the sensing fiber.

[0021] At this point, the optical signal returning from the sensing fiber primarily consists of the Rayleigh backscattered signal and the frequency-shifted continuous optical signal. The balanced detector 12 not only captures the beat-frequency Rayleigh backscattered signal but also converts the continuous optical signal into an electrical signal. The data acquisition card 13 captures these signal components and outputs them to the computer 14 for data processing and calculation. (The data acquisition card 13 and computer 14 constitute the signal processing module.)

[0022] Furthermore, the key signal parameters set by the signal generator in the system, signal ① and signal ② respectively control the pulse light and continuous light, and their periods are set to the same value T according to the time it takes for light to travel along the optical fiber length. Figure 2As shown, the pulse width of signal ① is set to a small value t1 to meet the system's spatial resolution. Signal ② is set to be mostly high, with a low-level region t2 greater than the pulse width t1 of signal ①. A delay is set so that the low-level region covers the pulses of signal ①. This setting allows the continuous optical signal to pass while completely isolating the pulsed light, ensuring that circulator 8 only returns the continuous optical signal.

[0023] Furthermore, the acousto-optic modulators 3 and 9 have different frequency shift characteristics, resulting in the Rayleigh backscatter signal and the returned continuous optical signal being in different frequency bands. Based on frequency division multiplexing technology and utilizing different intermediate frequency characteristics, digital signal processing techniques are employed by computer 14 to separate the Rayleigh backscatter signal and the modulated continuous optical signal in different frequency bands. Vibration signal localization is then achieved through methods such as moving average and moving difference, and signal demodulation is achieved using algorithms such as the inverse tangent algorithm and a 3×3 demodulation algorithm.

[0024] In summary, the continuous light-assisted distributed fiber optic sensing system described in this embodiment can implement distributed fiber optic vibration sensing for external vibration detection, wide-frequency vibration response, and intelligent identification of vibration signals. It has two main functions: one is to use pulsed light to detect the sensing fiber to achieve vibration signal positioning and frequency response within the Nyquist sampling theorem limit, with a specific value of 1 / 2 repetition rate. The other is to transmit a continuous light signal through the sensing fiber to continuously detect the vibration source, breaking through the sampling customization limit, achieving a system frequency response greater than 1 / 2 repetition rate, and obtaining a high-frequency signal. This can be applied to fields such as pipeline leakage monitoring, border security monitoring, and large-scale infrastructure structure monitoring.

[0025] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A continuous light-assisted distributed optical fiber vibration sensing system, comprising a light source, a pulse light modulation module, a continuous light frequency shift module, a sensing optical fiber, a photoelectric detection module, and a signal processing module, characterized in that: The light source output is split into pulsed light and continuous light, the pulsed light is used to perform distributed vibration positioning on the sensing optical fiber, and the continuous light is used to perform high-frequency vibration detection on the sensing optical fiber after frequency shift; The pulse light and the continuous light are injected into the sensing optical fiber in the same direction, and the frequency bands are separated at the end of the sensing optical fiber through the continuous light frequency shift module; The photoelectric detection module simultaneously receives the Rayleigh scattering signal corresponding to the pulse light and the vibration signal corresponding to the continuous light; The signal processing module separates and processes the two signals based on frequency division multiplexing technology, thereby achieving vibration positioning and high-frequency vibration response that breaks through the limitations of the Nyquist sampling theorem.

2. The system according to claim 1, wherein: The pulse light modulation module includes a first acousto-optic modulator and a pulse light amplifier. The first acousto-optic modulator modulates continuous light into pulse light under the control of a first signal output by a signal generator, and the pulse light amplifier amplifies the pulse light to a required optical power.

3. The system according to claim 1 or 2, characterized in that The continuous light frequency shift module includes a fiber circulator and a second acousto-optic modulator. The second acousto-optic modulator performs frequency shift on the continuous light reaching the end of the sensing optical fiber under the control of a second signal output by a signal generator, and re-injects the frequency-shifted continuous light into the sensing optical fiber through the fiber circulator.

4. The system according to claim 3, characterized in that The second signal output by the signal generator has the same period as the first signal and has a pulse-low level complementary to the first signal, so that the low level region of the second signal covers the pulse region of the first signal, thereby suppressing reflection of the pulse light at the optical fiber circulator.

5. The system according to claim 1, wherein: The photoelectric detection module includes a balanced light detector for performing beat frequency detection on the Rayleigh scattering signal and the frequency-shifted continuous light signal, and converting the light signal into an electrical signal.

6. The system according to claim 1 or 5, characterized in that The signal processing module includes a high-speed signal acquisition card and a computer. The high-speed signal acquisition card performs analog-to-digital conversion on the electrical signal output by the balanced light detector, and the computer uses digital signal processing technology to separate signals of different frequency bands and perform vibration positioning and high-frequency vibration demodulation.

7. The system according to claim 6, characterized in that The digital signal processing technology includes at least one of a moving average, a moving difference, an arc tangent algorithm or a 3×3 demodulation algorithm.

8. The system according to claim 7, characterized in that The moving average or moving difference is used to realize vibration signal positioning; the arc tangent algorithm or 3×3 demodulation algorithm is used to realize signal demodulation.

9. The system according to claim 1, wherein: The light source is a wavelength-tunable narrow-linewidth continuous fiber laser.

10. The system according to claim 1, wherein: The sensing fiber is a single-mode communication fiber, and its length matches the signal period T set by the signal generator, and T satisfies T ≥ 2nL / c, where n is the fiber refractive index, L is the sensing fiber length, and c is the speed of light.

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