A time division multiplexing method for improving the upper limit of frequency response of a weak grating array sensing system

Abstract

The application discloses a time division multiplexing method for improving the upper limit of frequency response of a weak light grating array sensing system, which comprises the following steps: a sensing system emits a detection pulse light with a specified pulse width and a pulse period into a front optical fiber connected to the sensing system, so that the reflection signals generated by different detection pulse lights in the weak light grating array are sequentially staggered and have no overlapping area; the reflection signals generated by different detection pulse lights are separated according to the time delay of the reflection signals generated by different detection pulse lights at the same grating position; the reflection signals generated by different detection pulse lights are separated at a receiving end; the reflection signals generated by different detection pulse lights are aligned according to the positions of the reflection signals in the weak light grating array; and the vibration information at each position in the weak light grating array is obtained by phase demodulation of the reflection signals at each position. The application can obtain the vibration information at each position in the weak light grating array at a disturbance sampling rate which is much higher than that of a conventional method.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic sensing technology, specifically relating to a time-division multiplexing method for improving the upper limit of frequency response of a weak grating array sensing system. Background Technology

[0002] Distributed fiber optic sensing technology is an advanced technology that utilizes optical fiber itself as both the sensing and transmission medium for signals. This technology leverages the sensitivity of optical fiber disturbances to the phase and polarization state of light waves to achieve efficient vibration sensing. Its advantages include being uncharged, having a long sensing distance, high sensitivity, fast response speed, and small size and light weight, making it a vibration sensing technology with great potential. Over the past 30 years, distributed fiber optic sensing technology has made significant progress. Its unique distributed sensing characteristics offer unparalleled advantages over traditional point sensors in large-scale object monitoring. With in-depth research and continuous technological development, this technology has achieved exploratory applications in various industries and fields, including oil and gas resource exploration, pipeline condition monitoring, seismic wave detection, anti-vandalism and anti-excavation monitoring, and perimeter protection, demonstrating enormous application potential.

[0003] In distributed optical time-domain sensing technology, the perturbation sampling rate is a key parameter that determines the frequency response range and strain measurement range of the sensing system. Therefore, improving the perturbation sampling rate is crucial for enhancing the performance of the sensing system. However, improving the perturbation sampling rate has always faced two key challenges. First, the perturbation sampling rate is limited by the length of the sensing fiber, as it is equal to the reciprocal of the pulse repetition period, which is limited by the time required for the light to travel to and from the sensing fiber. Second, the perturbation sampling rate is also limited by the length of the front-end fiber, because in practical applications, the sensing end is usually far from the demodulation equipment, and the sensor system always includes a front-end fiber, which can be hundreds of meters or even tens of kilometers long.

[0004] With the maturation of weak grating array manufacturing technology, various distributed sensing time-domain systems based on weak grating arrays have emerged. Compared with traditional single-mode fiber-based systems, weak grating array systems exhibit superior performance in both sensitivity and stability. However, the two aforementioned problems remain unresolved in distributed sensing time-domain systems based on weak grating arrays.

[0005] The paper "Research on Measurement Rate Improvement Technology for Φ-OTDR Quasi-Distributed Sensing" explores a time-division multiplexing method to improve the upper limit of the frequency response of a weak grating array sensing system. By multiplexing time slots between UWFBGs, multiple chirped single pulses are multiplexed within the same measurement period, thereby improving the system's measurement rate. However, this study only discusses the chirped single pulse structure, does not consider the influence of the front-end fiber, and does not address the spread spectrum upper limit. Summary of the Invention

[0006] Technical problem to be solved: In order to solve the technical problem that the perturbation sampling rate of existing distributed optical fiber sensing technology is strictly limited by the length of the sensing fiber and the front-end fiber, thus limiting the frequency response range of the sensing system, this invention provides a time-division multiplexing method to improve the upper limit of the frequency response of a weak grating array sensing system.

[0007] Technical solution:

[0008] A time-division multiplexing method for improving the upper limit of frequency response in a weak grating array sensing system, the time-division multiplexing method comprising the following steps:

[0009] S1, the sensing system is connected to the weak grating array for sensing external information through the front optical fiber. The sensing system emits a detection pulse light with a specified pulse width and pulse period into the front optical fiber connected to it, so that the reflected signals generated by different detection pulse lights in the weak grating array are interleaved in sequence without overlapping areas.

[0010] S2, based on the time delay of the reflected signals generated by different probe pulses at the same grating position, the reflected signals generated by different probe pulses are separated at the receiving end;

[0011] S3, Align the reflected signals generated by the different probe pulses obtained by separation according to the position of the reflected signals in the weak grating array;

[0012] S4 performs phase demodulation on the reflected signals at each position to obtain the vibration information at each position in the weak grating array.

[0013] Furthermore, the sensing system employs an optical time-domain sensing system.

[0014] Furthermore, the pulse width τ of the probe pulse light satisfies the following condition: τ≤2nΔL / Kc, where L is the fiber length corresponding to the pulse width. τ The following conditions must be met: L τ ≤2ΔL / K; where n is the effective refractive index of the fiber, c is the speed of light in vacuum, ΔL is the grating spacing, and K is an integer representing the number of probe pulse widths that can be accommodated on the grating spacing.

[0015] Furthermore, the pulse period ΔT of the probe pulse light satisfies the following condition: ΔT = 2nL f / Kc+τ+δτ, where L f τ is the length of the weak grating array, n is the effective refractive index of the fiber, τ is the pulse width of the probe pulse, and δτ is the time interval when the reflected signals generated by different probe pulses on the weak grating array interleave sequentially; the corresponding vibration sampling rate of the sensing system is...

[0016] Furthermore, the number K of the probe pulse widths that can be accommodated on the grating spacing is inversely proportional to the pulse width τ of the probe pulse light and directly proportional to the grating spacing.

[0017] Furthermore, in step S4, after demodulating the phase of the reflected signals generated by different probe pulses, the phase information at each alignment position is unwound to obtain the vibration information at each position in the weak grating array.

[0018] Furthermore, the sensing system is connected to an equally spaced array of reflectors for partially reflected light via a front-end optical fiber.

[0019] Beneficial effects:

[0020] First, the time-division multiplexing method of the present invention for improving the upper limit of frequency response of weak grating array sensing system has a perturbation sampling rate that is independent of the length of the front fiber. The longer the front fiber, the greater the improvement in perturbation sampling rate brought about by time-division multiplexing.

[0021] Second, the time-division multiplexing method of the present invention for improving the upper limit of frequency response of weak grating array sensing system means that the perturbation sampling rate is no longer limited by the time required for the optical fiber to travel to and from the sensing fiber, and the frequency response range and strain measurement range of the sensing system are greatly improved.

[0022] Third, the time-division multiplexing method of the present invention for improving the upper limit of frequency response of weak grating array sensing system makes full use of the resources of the acquisition system, minimizes unnecessary signal transmission, and improves signal processing speed by reducing the amount of data. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the time-division multiplexing method for improving the upper limit of frequency response of a weak grating array sensing system according to the present invention;

[0024] Figure 2 This is the time-domain diagram obtained by the traditional method using a 1km front fiber plus a 1km grating fiber;

[0025] Figure 3 This is a time-domain diagram obtained by adding a 1km front fiber and a 1km grating fiber under the time-division multiplexing method;

[0026] Figure 4 This is the demodulation result of 400Hz, 100V vibration under traditional methods;

[0027] Figure 5 This is a schematic diagram of the disturbance location result in the current period of the signal under the time-division multiplexing method;

[0028] Figure 6 This is a schematic diagram of the disturbance location result in the next cycle of the signal under the time-division multiplexing method;

[0029] Figure 7This is a schematic diagram of the demodulation results of 400Hz, 100V vibration under the time-division multiplexing method;

[0030] Figure 8 This is a schematic diagram of the vibration demodulation results at 24.9kHz and 1V using the traditional method;

[0031] Figure 9 This is a schematic diagram of the demodulation results of 99.9kHz, 1V vibration under the time-division multiplexing method. Detailed Implementation

[0032] The following embodiments are provided to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0033] This invention discloses a time-division multiplexing method for improving the upper limit of the frequency response of a weak grating array sensing system. The time-division multiplexing method includes the following steps:

[0034] S1, the sensing system is connected to the weak grating array for sensing external information via a front-end optical fiber. The sensing system emits a probe pulse light with a specified pulse width and pulse period into the front-end optical fiber connected to it, so that the reflected signals generated by different probe pulse lights in the weak grating array are interleaved in sequence without overlapping areas.

[0035] Preferably, the pulse width τ needs to satisfy τ≤2nΔL / Kc, that is, the fiber length L corresponding to the pulse width. τ L must be satisfied τ ≤2ΔL / K, where n is the effective refractive index of the fiber, c is the speed of light in vacuum, ΔL is the grating spacing, and K is an integer representing the number of probe pulse widths that can be accommodated across the grating spacing. The pulse period ΔT must satisfy ΔT=2nL f / Kc+τ+δτ, where L f δτ is the length of the weak grating array, and δτ is the time interval when the reflected signals generated by different probe pulses on the weak grating array interleave sequentially. Because 2nL f / Kc is on the order of microseconds, and τ+δτ is on the order of nanoseconds, so the corresponding vibration sampling rate of the sensing system is... K can be increased by reducing the pulse width and increasing the grating spacing, thereby increasing the perturbation sampling rate and boosting the multiplier.

[0036] The sensing system can be an optical time-domain sensing system with an arbitrary structure based on a weak grating array.

[0037] S2, based on the time delay of the reflected signals generated by different probe pulses at the same grating position, the reflected signals generated by different probe pulses are separated at the receiving end;

[0038] S3, according to the position of the reflected signal in the weak grating array, align the reflected signals generated by the different probe pulses obtained by separation; specifically, align the reflected signals generated by the different probe pulses according to the position of the reflected signal in the grating array.

[0039] S4. Phase demodulation is performed on the reflected signals at each position to obtain the vibration information at each position in the weak grating array. Preferably, after phase demodulation of the reflected signals generated by different probe pulses, the phase information at each aligned position is unwound to obtain the vibration information at each position in the weak grating array.

[0040] This invention can obtain vibration information at various locations in a weak grating array with a perturbation sampling rate that is much higher than that of traditional methods.

[0041] In distributed optical time-domain reflectometry, the frequency response range and strain measurement range depend on the system's perturbation sampling rate, which in turn depends on the pulse repetition period.

[0042]

[0043] Among them, f p It is the perturbation sampling rate, T P It refers to the pulse repetition period. In traditional single-mode fiber-based distributed optical time-domain reflectometers, the constant presence of the scattering signal necessitates that the scattering signals obtained from different pulse repetition periods must not overlap; otherwise, it would be difficult to distinguish them at the receiver.

[0044]

[0045] Therefore, the perturbation sampling rate f p This is limited by the total length L of the front-end fiber and the sensing fiber. However, this limitation does not exist in phase-sensitive optical time-domain reflectometry based on weak grating arrays. The reflected signal intensity at the grating reflection peak is much greater than the scattered signal intensity in the scattering region. Therefore, this invention proposes a highly universal time-division multiplexing technique in optical time-domain reflectometry based on weak grating arrays, the basic principle of which is as follows: Figure 1 As shown. In terms of system structure, taking the dual-pulse self-heterodyne structure as an example, by cleverly involving the pulse repetition period, the beat frequency of each pulse period (excluding the first and last) occupies the non-beat frequency region of other period signals.

[0046] In this embodiment, the optical fiber connected to the sensing system consists of a pre-fiber and a weak grating array for sensing, connected in series. The pre-fiber is used only to transmit optical signals, and the weak grating array is used to sense external information. Alternatively, it can be a pre-fiber and any reflector array that can be used to partially reflect light at equal intervals, with the same principle as the weak grating array.

[0047] In the experiment, a 1km stretch of ordinary single-mode fiber was used as the front-end fiber, and a 1km weak grating array was used as the sensing fiber, with a grating spacing of 10m. Under conventional methods, the pulse repetition period must be at least 20μs, and the resulting time-domain signal is as follows: Figure 2 As shown, the intensity of the scattered light generated by the first 1km single-mode fiber is much weaker than the reflected light generated by the weak grating array in the second 1km. Therefore, when using the time-division multiplexing method, the length of the first fiber does not need to be considered when setting the pulse repetition period.

[0048] According to the time-division multiplexing formula ΔT=2nL f / Kc+τ+δτ, in this example, K is chosen to be 2, the pulse width is set to 30ns, and the pulse repetition period is 5.03us. The resulting time-domain signal is as follows: Figure 3 As shown, the resources of the acquisition system are fully utilized, and unnecessary signal transmissions occur very rarely, thereby reducing the amount of data and improving the signal processing speed.

[0049] In the experiment, a large-amplitude sinusoidal vibration of 400Hz and 100V was applied near the grating array at a distance of 60m using PZT. The demodulation result of the conventional method at this time is as follows: Figure 4 As shown, due to the excessive vibration intensity, the phase jump value exceeds the strain measurement range, and the demodulation result is clearly not a sinusoidal signal. Furthermore, in the time-division multiplexing method, because signals obtained from different pulse repetition periods are time-division multiplexed, two reflection peaks with drastic amplitude fluctuations are found in the time-domain signal, such as... Figure 5 , Figure 6 As shown.

[0050] Subsequently, phase demodulation was performed on the two reflection peaks with drastic amplitude fluctuations, and the demodulation results were then subjected to DC removal and phase unwinding to obtain... Figure 7 The demodulation results show that using the time-division multiplexing method proposed in this patent, correct demodulation can be achieved when sensing vibrations of the same frequency and magnitude. This verifies the effectiveness of this method in improving the strain measurement range.

[0051] Increasing the perturbation sampling rate not only expands the strain measurement range but also the frequency response range. Therefore, to verify the performance improvement of the proposed time-division multiplexing method, a 24.9 kHz, 1 V vibration was applied using the conventional method. The vibration frequency domain signal demodulated from the time-domain signal obtained using the conventional method is as follows: Figure 8As shown in the figure, since 24.9kHz is very close to the maximum value of the frequency response range under the traditional method, although the frequency peak can be captured in the frequency domain, there is a very obvious spectral folding phenomenon. Continuing to increase the vibration frequency to 99.9kHz, the traditional method can no longer measure it correctly, but the demodulation result obtained using the time-division multiplexing method is as follows... Figure 9 As shown.

[0052] Therefore, under the same system configuration, the maximum frequency response range of the traditional method is 25kHz, while the maximum frequency response range of the proposed time-division multiplexing method is 99kHz. In this embodiment, the maximum frequency response range of the time-division multiplexing method is four times that of the traditional method, thus verifying the fourfold improvement in perturbation sampling rate.

[0053] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A time-division multiplexing method for improving the upper limit of frequency response in a weak grating array sensing system, characterized in that, The time-division multiplexing method includes the following steps: S1, the sensing system is connected to the weak grating array for sensing external information through the front optical fiber. The sensing system emits a detection pulse light with a specified pulse width and pulse period into the front optical fiber connected to it, so that the reflected signals generated by different detection pulse lights in the weak grating array are interleaved in sequence without overlapping areas. S2, based on the time delay of the reflected signals generated by different probe pulses at the same grating position, the reflected signals generated by different probe pulses are separated at the receiving end; S3, Align the reflected signals generated by the different probe pulses obtained by separation according to the position of the reflected signals in the weak grating array; S4, perform phase demodulation on the reflected signals at each position to obtain the vibration information at each position in the weak grating array; The pulse width of the probe pulse light The following conditions must be met: The fiber length corresponding to the pulse width The following conditions must be met: Where n is the effective refractive index of the optical fiber, c is the speed of light in vacuum, and ΔL is the grating spacing. It is an integer representing the number of probe pulse widths that can be accommodated on the grating spacing; The pulse period ΔT of the probe pulse light satisfies the following condition: ,in Where n is the length of the weak grating array, and n is the effective refractive index of the fiber. To detect the pulse width of pulsed light, The time interval is when the reflected signals generated by different probe pulses on the weak grating array are sequentially interleaved; the corresponding vibration sampling rate of the sensing system is... .

2. The time-division multiplexing method for improving the upper limit of frequency response of a weak grating array sensing system according to claim 1, characterized in that, The sensing system is an optical time-domain sensing system.

3. The time-division multiplexing method for improving the upper limit of frequency response of a weak grating array sensing system according to claim 1, characterized in that, The number K of probe pulse widths that can be accommodated on the grating spacing is related to the pulse width of the probe pulse light. It is inversely proportional to the grating spacing and directly proportional to the grating spacing.

4. The time-division multiplexing method for improving the upper limit of frequency response of a weak grating array sensing system according to claim 1, characterized in that, In step S4, after phase demodulation of the reflected signals generated by different probe pulses, the phase information of each aligned position is unwound to obtain the vibration information at each position in the weak grating array.

5. The time-division multiplexing method for improving the upper limit of frequency response of a weak grating array sensing system according to claim 1, characterized in that, The sensing system is connected to an equally spaced array of reflectors for partially reflecting light via a front-end optical fiber.

Images