High-gain low-noise distributed fiber vibration sensing system signal processing device and method

Abstract

The application relates to the technical field of distributed optical fiber sensing systems, in particular to a high-gain low-noise distributed optical fiber vibration sensing system signal processing device and method, which is provided with a 2*2 coupler, two PIN photoelectric diodes, two AC filtering circuits, two transimpedance amplification circuits, a signal difference unit and a signal demodulation unit, the two transimpedance amplifiers realize conversion of a current signal into a voltage signal; the input end of the transimpedance amplifier is connected with an AC module, and the output end is connected with the signal difference unit; compared with the prior art, the two transimpedance outputs are combined with the AC filtering circuit, so that the range of the electric signal obtained through photoelectric conversion is increased, the input optical signal is fully utilized, the output signal is increased, the signal-to-noise ratio of the system is significantly improved, and the system measurement capability is enhanced.

Description

Technical fields:

[0001] This invention relates to the field of distributed optical fiber sensing system technology, specifically to a high-gain, low-noise distributed optical fiber vibration sensing system signal processing device and method that can effectively improve the signal-to-noise ratio of the system and thus increase the detection distance of the system. Background technology:

[0002] Distributed fiber optic vibration sensing systems utilize the OTDR (Optical Time-of-Difference Reduction) principle. The light source is highly coherent light, modulated by an acousto-optic modulator into pulsed light, amplified, and then injected into the sensing fiber optic cable. This ensures that the Rayleigh scattering signal received at the fiber's incident end is the result of interference between Rayleigh scattering beams at various points within the pulse width of the light. Due to the coherence of the light source, a clear interference effect is visible in the received pulsed light. When a vibration signal acts on the sensing fiber optic cable at a certain location in the external environment, the phase of the scattered light at that location changes, causing a change in the interference signal. Therefore, by detecting the change in the interference signal, the environmental changes along the entire sensing fiber optic cable can be obtained. This technology is generally used for pipeline safety monitoring, detecting threats along the pipeline route, such as pipeline excavation work.

[0003] Signal processing in distributed fiber optic vibration sensing systems typically employs either direct detection or heterodyne detection. Direct detection involves directly converting the interference signal returned from the fiber optic sensing link into a photoelectric signal, followed by amplification and acquisition. Direct detection results in a relatively poor signal-to-noise ratio (SNR), affecting the detection distance. This is generally addressed by adding an optical amplification module to improve the SNR. Heterodyne detection, on the other hand, involves coherently interfering the interference signal returned from the fiber optic sensing link with a portion of the local oscillator light from a coherent light source to obtain a difference frequency signal. Simultaneously, the light gains energy from the local oscillator light, amplifying the optical signal. Optical heterodyne detection can detect not only amplitude-modulated optical signals but also frequency-modulated and phase-modulated optical signals. Compared to direct detection, heterodyne detection obtains energy from a coherent light source, eliminating the need for additional amplification to improve the SNR, resulting in lower implementation costs. However, the demodulation circuitry is relatively complex.

[0004] Heterodyne detection in existing distributed fiber optic vibration sensing systems. The design of the existing distributed fiber optic vibration sensing heterodyne detection system is attached. Figure 1 As shown: A 2*2 coupler is used to realize two differential frequency optical signals with a phase difference of 180°. The two optical signals are connected to two PIN diodes and operational amplifiers to build a single-channel transimpedance amplifier circuit to realize the construction of a balanced photoelectric detection system, which converts the optical signal into an electrical signal.

[0005] Existing heterodyne detection systems have the following problems: 1. The optical signal is not fully utilized. The output range of a single transimpedance operational amplifier is limited. When the optical power is too high, the output signal of the operational amplifier will saturate. At this time, the optical signal needs to be reduced to ensure that the electrical signal is not distorted, so the signal light cannot be fully utilized. In addition, the output signal can be reduced by reducing the transimpedance value, but compared with the subsequent signal demodulation, it still needs to be proportionally amplified. Reducing the transimpedance value is not conducive to improving the signal-to-noise ratio. In addition, the difference frequency term ω1 of the output signal IF is introduced by the acousto-optic modulator (AOM). The acousto-optic modulator converts coherent DC light into pulsed light and introduces a frequency shift. The frequency of the difference frequency is generally between 80MHz and 200MHz, which is a high-frequency signal. Due to the limitations of current technology, the output signal of a single transimpedance operational amplifier cannot be too high (because when the signal is large, the bandwidth will decrease, resulting in a certain degree of signal distortion), and it needs to be proportionally amplified again.

[0006] Furthermore, in existing technologies, the optical signal gains of the two PIN diodes cannot be strictly identical, which introduces noise. See Figure 2 As shown, when the optical signal gains from the two PIN diodes are identical, the signal noise from the two PIN diodes will be canceled out by differential coupling. When the gains of the two optical signals differ significantly, noise will be introduced. However, in actual use, due to factors such as the length of the fiber optic cable integrated with the coupler, the consistency of splice quality, and the consistency of PIN diode responsivity, the gains of the two optical signals cannot be completely identical. The coherent signal obtained by the 2*2 coupler includes a DC signal and a difference frequency signal, where the difference frequency signal is 180° out of phase. In the figure, I3 = I2 - I1. When the optical signal gains from the two PIN diodes are identical, the AC signal will double, and the DC signal will become 0. At this time, the optical noise will also be greatly reduced due to subtraction. When the gains of the two signals are inconsistent, a DC signal will be introduced, and the optical noise will increase like the DC signal. Summary of the Invention:

[0007] This invention addresses the shortcomings and deficiencies of existing technologies by proposing a high-gain, low-noise distributed fiber optic vibration sensing system signal processing device and method that can effectively improve the system's signal-to-noise ratio and thus increase the system's detection distance.

[0008] This invention achieves its purpose through the following measures:

[0009] A signal processing device for a high-gain, low-noise distributed fiber optic vibration sensing system is characterized by comprising a 2*2 coupler, two PIN photodiodes, two AC filter circuits, two transimpedance amplifier circuits, a signal differential unit, and a signal demodulation unit. The 2*2 coupler realizes the difference frequency of the two optical signals. The two inputs of the 2*2 coupler are respectively connected to the local oscillator light output from the light source and the signal light returned from the fiber optic sensing link. The two output signal lights with difference frequency signals are respectively connected to the two PIN photodiodes.

[0010] The photodiode is mainly used for converting optical signals to current signals. The photodiode's pigtail is connected to the two outputs of the 2*2 coupler, and the photodiode's output is connected to two AC filter modules.

[0011] The AC filter module is used to filter out the DC signal output by the PIN photodiode. The AC module is implemented by building a high-pass filter circuit with capacitors and resistors. The input of the AC filter module is directly connected to the PIN photodiode, and the output is connected to the inverting input of the transimpedance operational amplifier.

[0012] The two transimpedance amplifiers convert current signals into voltage signals; the input of the transimpedance amplifier is connected to the AC module, and the output is connected to the signal differential unit.

[0013] The signal differential unit is used for subtraction of two transimpedance amplified voltage signals; the two input terminals of the signal differential unit are respectively connected to the output terminal of the transimpedance amplification, and the output terminal is connected to the signal demodulation unit.

[0014] The signal demodulation unit is used to demodulate the envelope of the signal. The input of the signal demodulation unit is connected to the signal differential unit, and the output of the signal demodulation unit is generally connected to the acquisition device.

[0015] In this invention, the transimpedance amplifier is a typical transimpedance amplifier structure (also known as TIA) composed of an operational amplifier and a feedback circuit; the operational amplifier is a dedicated transimpedance amplifier, such as OPA847 or OPA657.

[0016] The signal differential unit in this invention includes an operational amplifier and resistor networks R3, R4, R5, and R6 for performing subtraction and amplification operations. The amplification factor is between 5 and 10 times, depending on the requirements of the subsequent acquisition circuit. The signal differential unit also includes a potentiometer VR1, which is used for fine-tuning of a single signal to ensure the suppression of the amplification factor of the entire signal link of the two signals being subtracted. During use, it is necessary to observe the noise level in real time and adjust accordingly.

[0017] The signal demodulation unit of this invention can use a high-speed diode, such as 1N4148, and capacitors and resistors to build a diode envelope demodulation circuit to achieve signal demodulation and finally obtain the DVS interference signal. Alternatively, it can use an envelope detection chip, such as ADL5910 or ADL5904, for envelope detection. The demodulated carrier frequency of the signal demodulation unit is generally between 80MHz and 200MHz, and the output signal bandwidth is between 0 and 20MHz.

[0018] This invention also proposes a signal processing method for a high-gain, low-noise distributed optical fiber vibration sensing system, characterized by the following steps:

[0019] Step 1: The local oscillator light split from the narrow linewidth is connected to the Rayleigh scattered light returned by the distributed fiber optic vibration sensing system and to the two input terminals of a 2*2 coupler. Mixing is performed in the 2*2 coupler to obtain two difference frequency signals. The frequency of the difference frequency signals depends on the modulation frequency of the pulse modulator AOM. Step 2: The two difference frequency signals output by the 2*2 coupler are connected to two PIN photodiodes. The current signals output by the two photodiodes are filtered by an AC filter circuit to remove DC signals before entering two transimpedance operational amplifiers respectively.

[0020] Step 3: Two transimpedance operational amplifiers generate two difference frequency voltage signals, with the two signals having a phase difference of 180°.

[0021] Step 4: The difference frequency voltage signal is differentially divided by the signal differential unit to increase the voltage signal and output it to the demodulation circuit;

[0022] Step 5: The signal demodulation circuit extracts the envelope of the difference frequency signal to obtain the interference signal required for the analysis of the distributed optical fiber vibration sensing system.

[0023] The invention also includes, in order to ensure that the system has a high signal-to-noise ratio, observing the noise floor of the interference signal and adjusting the differential unit potentiometer in real time, stopping modulation when the noise floor is at its minimum value, and ensuring that the amplification coefficients of the two signals are consistent; the obtained interference signal is directly used for algorithm analysis in the distributed fiber optic vibration sensing system.

[0024] Compared with existing technologies, this invention employs a dual-channel transimpedance output combined with an AC filter circuit to increase the range of the electrical signal obtained from photoelectric conversion, thereby fully utilizing the input optical signal and increasing the output signal. A signal differential processing unit is designed, and a subtraction circuit is built to perform differential operations on the two transimpedance signals, increasing the voltage signal. The signal differential processing unit also incorporates a potentiometer for fine-tuning to ensure consistent gain coefficients between the two signal links, reducing noise output and improving the signal-to-noise ratio. An envelope detection circuit is added to extract the high-frequency signal envelope. The obtained envelope signal can be directly used for signal analysis in a distributed fiber optic vibration sensing system, significantly improving the system's signal-to-noise ratio and enhancing its measurement capabilities. Attached image description:

[0025] Appendix Figure 1 This is a schematic diagram of the heterodyne structure of a distributed fiber optic vibration sensing system in existing technology. (Attached) Figure 2 This is a schematic diagram of a PIN diode transresistance switching circuit in the existing technology.

[0026] Appendix Figure 3 This is a structural block diagram of the present invention.

[0027] Appendix Figure 4 This is a comparative schematic diagram of the photoelectric conversion and transimpedance operational amplifier section in Embodiment 1 of the present invention with the prior art, wherein... Figure 4(1) is a principle block diagram in Example 1. Figure 4 (2) is a principle block diagram of the prior art. Detailed implementation method:

[0028] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0029] Example 1:

[0030] This example addresses the shortcomings of existing technologies by proposing a high-gain, low-noise signal processing method for distributed fiber optic vibration sensing systems. Based on existing heterodyne detection, the photoelectric conversion section is first designed with one PIN diode corresponding to one transimpedance operational amplifier, resulting in two transimpedance outputs. Simultaneously, an AC filter circuit is added to the transimpedance operational amplifier section to remove DC current, effectively utilizing the optical signal and increasing the magnitude of the electrical signal after photoelectric conversion. Secondly, to ensure consistent gain between the two signals, a signal differential unit is added. By fine-tuning the amplification coefficient of each electrical signal using a potentiometer, balanced output of the two electrical signals is achieved, further reducing noise.

[0031] In this example, the signal processing device of the high-gain, low-noise distributed fiber optic vibration sensing system includes a 2*2 coupler, two PIN photodiodes, two AC filter circuits, two transimpedance amplifier circuits, a signal differential unit, and a signal demodulation unit. The 2*2 coupler realizes the difference frequency of the two optical signals. The two inputs of the 2*2 coupler are respectively connected to the local oscillator light output from the light source and the signal light returned from the fiber optic sensing link. The two output signal lights with difference frequency signals are respectively connected to the two PIN photodiodes.

[0032] The photodiode is mainly used for converting optical signals to current signals. The photodiode's pigtail is connected to the two outputs of the 2*2 coupler, and the photodiode's output is connected to two AC filter modules.

[0033] The AC filter module is used to filter out the DC signal output by the PIN photodiode. The AC module is implemented by building a high-pass filter circuit with capacitors and resistors. The input of the AC filter module is directly connected to the PIN photodiode, and the output is connected to the inverting input of the transimpedance operational amplifier.

[0034] The two transimpedance amplifiers convert current signals into voltage signals; the input of the transimpedance amplifier is connected to the AC module, and the output is connected to the signal differential unit.

[0035] The signal differential unit is used for subtraction of two transimpedance amplified voltage signals; the two input terminals of the signal differential unit are respectively connected to the output terminal of the transimpedance amplification, and the output terminal is connected to the signal demodulation unit.

[0036] The signal demodulation unit is used to demodulate the envelope of the signal. The input of the signal demodulation unit is connected to the signal differential unit, and the output of the signal demodulation unit is generally connected to the acquisition device.

[0037] During operation, the local oscillator light split from the narrow linewidth beam and the Rayleigh scattered light returned by the distributed fiber optic vibration sensing system are connected to the two input terminals of a 2*2 coupler. Mixing is performed in the 2*2 coupler to obtain two difference frequency signals. The frequency of the difference frequency signals depends on the modulation frequency of the pulse modulator AOM. The two difference frequency signals output by the 2*2 coupler are connected to two PIN photodiodes. The current signals output by the two photodiodes are filtered by an AC filter circuit to remove DC signals before entering two transimpedance operational amplifiers. The two transimpedance operational amplifiers produce two difference frequency voltage signals with a 180° phase difference. The difference frequency voltage signals are differentially divided by a signal differential unit to amplify the voltage signal, and then output to the demodulation circuit. The signal demodulation circuit extracts the envelope of the difference frequency signals to obtain the interference signal required for analysis by the distributed fiber optic vibration sensing system.

[0038] To ensure a high signal-to-noise ratio, the noise floor of the interference signal was observed, and the differential unit potentiometer was adjusted in real time. Modulation was stopped when the noise floor was at its minimum value to ensure that the amplification coefficients of the two signals were consistent. The obtained interference signal was directly used for algorithm analysis in the distributed fiber optic vibration sensing system.

[0039] As attached Figure 4 As shown, this example connects the two signal beams output from the 2*2 coupler to two PIN diodes. The PIN diode outputs are then connected to an AC filter unit and two transimpedance operational amplifiers to achieve transimpedance output of the two difference frequency signals. This method eliminates the DC input while increasing the output range of the photoelectric conversion signal, theoretically doubling the signal output range. When the signal beam is large, avoiding reducing the transimpedance value ensures effective utilization of the optical signal output.

[0040] Due to the limitations of operational amplifier devices, assuming Figure 4 The maximum output range of the right-hand signal is 0 to A. In this example, two signals with an output range of 0 to A can be output. As shown in the figure, this example adds AC filtering to remove the influence of DC. Based on the application requirements of the distributed fiber optic vibration sensing system, the DC signal is not used for subsequent signal analysis, so it can be directly filtered out. A signal differential unit is added to ensure that the gain of the two signals is consistent and to reduce noise. Due to issues such as the length of the optical fiber, splice quality, and PIN diode responsivity in actual use, the two signals cannot be completely identical. Therefore, a subtraction proportional circuit is designed by adding a signal differential unit. In the proportional circuit, a high-precision potentiometer is used to adjust the gain of a single PIN diode. That is, by adjusting the potentiometer, the single-channel amplification factor of the subtraction circuit is adjusted, ensuring signal consistency and further reducing noise.

[0041] In summary, the method described in this invention, when applied to a distributed optical fiber vibration sensing system, effectively improves the system's signal-to-noise ratio, thereby increasing the system's detection range.

Claims

1. A signal processing method for a high-gain, low-noise distributed optical fiber vibration sensing system, wherein the high-gain, low-noise distributed optical fiber vibration sensing system signal processing device includes a 2*2 coupler, two PIN photodiodes, two AC filter circuits, two transimpedance amplifier circuits, a signal differential processing unit, and a signal demodulation unit, wherein the 2*2 coupler realizes the difference frequency of the two optical signals, the two inputs of the 2*2 coupler are respectively connected to the local oscillator light output from the light source and the signal light returned from the optical fiber sensing link, and the two output signal lights with difference frequency signals are respectively connected to the two PIN photodiodes; The photodiode is used to convert optical signals into current signals. The photodiode's pigtail is connected to the two outputs of the 2*2 coupler, and the photodiode's output is connected to two AC filter circuits. The AC filter circuit is used to filter out the DC signal output by the PIN photodiode. The AC filter circuit is implemented by building a high-pass filter circuit with capacitors and resistors. The input of the AC filter circuit is directly connected to the PIN photodiode, and the output is connected to the inverting terminal of the transimpedance amplifier circuit. The two-channel transimpedance amplifier circuits convert current signals into voltage signals; the input of the transimpedance amplifier circuit is connected to an AC filter circuit, and the output is connected to a signal differential processing unit. The signal differential processing unit is used for subtraction of the two transimpedance amplified voltage signals; the two input terminals of the signal differential processing unit are respectively connected to the output terminal of the transimpedance amplifier circuit, and the output terminal is connected to the signal demodulation unit. The signal demodulation unit is used to perform envelope demodulation of the signal. The input of the signal demodulation unit is connected to the signal differential processing unit, and the output of the signal demodulation unit is connected to the acquisition device. The transimpedance amplifier circuit uses an operational amplifier and a feedback circuit to form a typical transimpedance amplifier structure. A dedicated transimpedance amplifier is selected for implementation. The signal differential processing unit includes an operational amplifier and a resistor network R3, R4, R5, and R6 to perform subtraction and amplification operations. The amplification factor is 5 to 10 times, depending on the requirements of the subsequent acquisition circuit. The signal differential processing unit also includes a potentiometer VR1, which is used for fine-tuning a single signal to ensure the suppression of the amplification factor of the entire signal link for the two signals being subtracted. During use, the noise level needs to be observed and adjusted in real time. The signal demodulation unit uses high-speed diodes and capacitors / resistors to build a diode envelope demodulation circuit to achieve signal demodulation and ultimately obtain a DVS interference signal. An envelope detection circuit is used for envelope detection. The demodulated carrier frequency of the signal demodulation unit is between 80MHz and 200MHz, and the output signal bandwidth is between 0 and 20MHz. Its characteristics are... The use of dual transimpedance outputs combined with an AC filter circuit increases the range of the electrical signal obtained from photoelectric conversion, ensuring full utilization of the input optical signal and increasing the output signal. A signal differential processing unit is included, employing a subtraction circuit to perform differential operations on the two transimpedance signals, thereby increasing the voltage signal. A potentiometer is added to the signal differential processing unit for fine-tuning, ensuring consistent gain coefficients in both signal links, reducing noise output, and improving the signal-to-noise ratio. An envelope detection circuit is added to extract the high-frequency signal envelope; the obtained envelope signal can be directly used for signal analysis in the distributed fiber optic vibration sensing system. The specific steps of the high-gain, low-noise distributed fiber optic vibration sensing system signal processing method are as follows: Step 1: The local oscillator light split from the narrow linewidth light source and the Rayleigh scattered light returned by the distributed fiber optic vibration sensing system are connected to the two input terminals of the 2*2 coupler. The two difference frequency signals are obtained by mixing in the 2*2 coupler. The frequency of the difference frequency signal depends on the modulation frequency of the pulse modulator AOM. Step 2: The 2*2 coupler outputs two difference frequency signals, which are connected to two PIN photodiodes. The current signals output by the two photodiodes are filtered by the AC filter circuit to remove the DC signal before entering the two transimpedance amplifier circuits respectively. Step 3: Two transimpedance amplifier circuits produce two difference frequency voltage signals with a phase difference of 180° between them; Step 4: The differential frequency voltage signal is differentially processed by the signal differential processing unit to increase the voltage signal and output it to the demodulation circuit; Step 5: The signal demodulation circuit extracts the envelope of the difference frequency signal to obtain the interference signal required for the analysis of the distributed fiber optic vibration sensing system; It also includes observing the noise floor of the interference signal to ensure a high signal-to-noise ratio, and adjusting the potentiometer of the signal differential processing unit in real time. When the noise floor is at its minimum value, modulation is stopped to ensure that the amplification coefficients of the two signals are consistent. The obtained interference signal is directly used for algorithm analysis in the distributed fiber optic vibration sensing system.

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