A BOTDR-based high signal-to-noise ratio performance improvement system

Abstract

This invention discloses a high signal-to-noise ratio (SNR) performance enhancement system based on BOTDR, relating to the field of distributed fiber optic sensing technology. It includes an optical path unit and a circuit processing unit. The optical path unit comprises a narrow-linewidth laser (Laser), a pulse modulation module, a variable optical attenuator (VOA), a hierarchical optical amplification module, an optical circulator (OC), a polarization scrambler (PS), and a fiber coupler array. The circuit processing unit includes a photodetector pre-amplifier module, a low-noise amplifier, a frequency mixer and down-conversion module, and a frequency selection module. This BOTDR-based high SNR performance enhancement system significantly improves the system's SNR and monitoring accuracy through optimized optical path and circuit structure design, precise selection of core optical components, and innovative optical signal processing links. Through an integrated design of 1550nm band narrow-linewidth continuous optical modulation, hierarchical precise amplification, polarization state optimization, and efficient coherent beat frequency, noise in the Brillouin scattering signal is effectively suppressed during transmission and detection, resulting in a significant improvement in signal strength.

Description

Technical Field

[0001] This invention relates to the field of distributed optical fiber sensing technology, specifically to a high signal-to-noise ratio performance enhancement system based on BOTDR. Background Technology

[0002] As a core carrier of modern information transmission and sensing, the operational stability of optical fiber directly affects the efficiency and security of critical systems such as communication networks and power dispatching. In practical applications, optical fiber is susceptible to environmental factors: on the one hand, extreme temperature changes can alter the physical properties of the fiber, causing fluctuations in signal transmission loss; on the other hand, external mechanical stress (such as line tension and vibration) can cause strain in the fiber, which may lead to link breakage and other failures under long-term effects. These abnormal conditions can significantly threaten the continuity of optical fiber networks, thereby affecting the operation and maintenance of various systems that rely on these networks.

[0003] BOTDR (Optical Time Domain Reflectometer) technology has become a core technology solution for distributed monitoring of optical fibers due to its advantages such as single-end measurement, long-distance coverage, and high spatial resolution. Its core principle is to detect the Brillouin frequency shift (BFS) in the optical fiber and invert the temperature and strain information along the optical fiber to determine whether there are performance abnormalities in the optical fiber.

[0004] However, the optical path design of existing BOTDR systems has many shortcomings: First, the Brillouin scattering signal itself is extremely weak, and it is easily affected by fiber loss during long-distance transmission, which further attenuates the signal strength. Secondly, significant interference factors such as light source noise, polarization state mismatch, and modulation noise in the optical path lead to a low signal-to-noise ratio. Third, the optical amplification module is poorly designed, either failing to meet the signal strength required for long-distance monitoring, or causing nonlinear effects in the optical fiber due to excessive amplification, which in turn affects the signal quality. Fourth, the beat frequency efficiency between the reference light and the scattered light is low, and the polarization mismatch leads to the loss of effective signal. These problems severely limit the monitoring accuracy and applicability of the BOTDR system, making it difficult to meet the high-precision real-time monitoring requirements for scenarios such as fiber optic temperature and strain.

[0005] Therefore, optimizing the optical path structure of the BOTDR system and improving the signal-to-noise ratio during optical signal transmission and detection are key to solving the bottlenecks of existing technologies. To this end, we propose a high signal-to-noise ratio performance improvement system based on BOTDR. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a high signal-to-noise ratio performance enhancement system based on BOTDR, which solves the problems mentioned in the background art, such as weak Brillouin scattering signals, significant noise interference, and insufficient signal-to-noise ratio for long-distance monitoring in existing BOTDR systems.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a high signal-to-noise ratio performance enhancement system based on BOTDR, comprising an optical path unit and a circuit processing unit. The optical path unit includes a narrow linewidth laser, a pulse modulation module, a tunable optical attenuator (VOA), a graded optical amplification module, an optical circulator (OC), a polarization scrambler (PS), and an optical fiber coupler array. The circuit processing unit includes a photodetector preamplifier module, a low-noise amplifier, a frequency mixer and down-conversion module, and a frequency selection module.

[0008] Optionally, the narrow linewidth laser is used to output a continuous optical signal with a center wavelength of 1550nm and a linewidth ≤3kHz. Pulse modulation module: A cascaded dual-optical pulse modulation module (P-SOA) is used as the core modulation device to modulate the continuous optical signal into pulsed light with a high extinction ratio; Tunable optical attenuator (VOA): Connected in series between the narrow linewidth laser and the pulse modulation module, it is used to reduce the optical power incident on the dual-optical pulse modulation module (P-SOA) to below its maximum incident power to avoid damage to the device; Hierarchical optical amplification module: including pulsed erbium-doped fiber amplifier (P-EDFA) and small-signal erbium-doped fiber amplifier (C-EDFA). Optical Circulator (OC): Used to realize unidirectional injection of pulsed light and reverse output of Brillouin scattered light. It injects the pulsed light, which is amplified by the pulsed erbium-doped fiber amplifier (P-EDFA), into the fiber under test and guides the Brillouin scattered light returned by the fiber to the subsequent link. Polarization scrambler (PS): Used to perturb the polarization state of the reference light, so that the polarization state of the reference light matches that of the Brillouin scattered light, thereby improving beat frequency efficiency; Fiber optic coupler group: including a first coupler (splitting ratio 10:90) and a second coupler (splitting ratio 50:50). The first coupler splits the continuous light output from the narrow linewidth laser into 10% reference light and 90% signal light. The second coupler is used to combine the polarization-scrambled reference light with the Brillouin scattered light to achieve coherent beat frequency. The optical detection front-end module uses a photodetector (PD) with a working bandwidth of ≥10GHz to convert the beat frequency optical signal into an electrical signal; Low-noise amplifier: A low-noise differential amplifier (D-LNA) is used to amplify the weak Brillouin signal and increase its strength. Frequency mixing and down-conversion module: Provides a sinusoidal signal of specified frequency and power to be mixed with the electrical signal generated by the photodetector (PD), thereby shifting the Brillouin signal frequency of about 11 GHz down to the bandwidth range of the bandpass filter to facilitate subsequent data acquisition and processing; Frequency selection module: Provides a fixed bandwidth and center frequency to extract the down-frequency Brillouin signal.

[0009] Optionally, the output optical power of the narrow linewidth laser is 23m, and the wavelength stability of the narrow linewidth laser is ≤0.2pm / h, and the power stability is ≤0.02dB / 8h. The adjustable optical attenuator (VOA) has an attenuation adjustment range adapted to the input power requirements of the dual-pulse optical amplifier (P-SOA), ensuring that the optical power incident on the P-SOA does not exceed its rated threshold, the extinction ratio of a single P-SOA is ≥60dB, the pulse width adjustment range is 5-1000ns, and the repetition frequency adjustment range is 100Hz-1MHz. In an ideal state, pulsed light has a base power of 0, a pulse width of τ, and a period of... T Its average power P A0 It can be represented as: In the formula, P P This represents the peak power of the pulsed light. A non-ideal pulsed light, with a non-zero pulse floor power, a pulse width of τ, and a period of... T Its average power P A for: In the formula, P L This refers to the pulsed substrate optical power. Based on the above formula, we can obtain: Therefore, the extinction ratio of the measured pulse can be calculated using the following formula; In the formula, T , τSince the quantity is known, only the average power of the pulse needs to be measured. P A With peak power P P The extinction ratio of the pulsed light can then be determined; High extinction ratio pulse signals are stable. The dual-light pulse modulation module (P-SOA) is a current-sensitive device. When the modulation pulse switches to the "0" state, the gain switch is not turned on, and the gain can be approximated as "0". At this time, there is no light output. Using a cascaded dual-light pulse modulation module (P-SOA) can improve the extinction ratio of the probe pulse light and improve the signal-to-noise ratio of the BOTDR.

[0010] Optionally, the pulsed erbium-doped fiber amplifier (P-EDFA) and the small-signal erbium-doped fiber amplifier (C-EDFA) amplify the signal to ensure signal integrity. The input optical power of the pulsed erbium-doped fiber amplifier (P-EDFA) is adapted to a range of 1-10mW, matching the maximum output power of the dual-optical pulse modulation module (P-SOA). This maximizes the peak power of the pulsed light while avoiding fiber nonlinear effects. The pulsed erbium-doped fiber amplifier (P-EDFA) is used to amplify the modulated pulsed light, with a maximum output power ≥10W and an operating wavelength range of 1527-1565nm. The small-signal erbium-doped fiber amplifier (C-EDFA) is used to amplify the returned Brillouin scattered light, with a gain ≥45dB (operating wavelength range of 1530-1565nm under -45dBm input conditions).

[0011] Optionally, the coupled light output from the second coupler is directly incident on the input end of the photodetector (PD), with no additional attenuation devices in the optical path link, reducing optical signal loss.

[0012] Optionally, all devices in the optical path unit are dedicated devices for the 1550nm band, and the device interface is a standard SMA interface to ensure low loss and high stability of the optical path connection.

[0013] Optionally, the photodetector (PD) has a bandwidth of 10-12 GHz, a peak responsivity of ≥0.8 A / W, and a maximum output swing of 900 mV. It can eliminate Rayleigh signals and other signals while converting Brillouin signals into electrical signals for convenient subsequent processing.

[0014] Optionally, the low-noise differential amplifier (D-LNA) has a bandwidth of 1-18GHz, a gain of 35dBm, and a gain flatness of ±2dBm. It amplifies the weak Brillouin signal converted into an electrical signal and suppresses common-mode signals, thereby further improving the signal-to-noise ratio of the BOTDR.

[0015] Optionally, the frequency selection module downshifts the high-frequency Brillouin signal to facilitate subsequent signal extraction and data processing. The local oscillator of the mixer in the frequency mixing and downsampling module operates at a frequency of 8-16 GHz, the local oscillator input signal must be greater than 13 dBm, and the output signal range is DC-2.5 GHz. The sweep frequency source operates at a frequency of 8-12 GHz, with a minimum frequency step of 1 Hz, an output power of 23 dBm, and supports non-sweep mode, externally triggered sweep mode, and free sweep mode.

[0016] Optionally, the bandpass filter has a center frequency of 400MHz and a bandwidth of 90MHz to extract the down-frequency Brillouin signal, facilitating subsequent data acquisition and processing.

[0017] This invention provides a high signal-to-noise ratio performance enhancement system based on BOTDR, which has the following beneficial effects: This BOTDR-based high signal-to-noise ratio (SNR) performance enhancement system significantly improves the system's SNR and monitoring accuracy through optimized optical path and circuit structure design, precise selection of core optical components, and innovative optical signal processing links. Its optical path unit comprises a narrow-linewidth laser, an optical pulse modulation module (dual P-SOA cascade), a graded EDFA (erbium-doped fiber amplifier) ​​amplification module, a polarization scrambler, a coupler, and a photodetector pre-module. Through an integrated design of 1550nm band narrow-linewidth continuous optical modulation, graded precise amplification, polarization state optimization, and efficient coherent beat frequency, noise in the Brillouin scattering signal is effectively suppressed during transmission and detection, resulting in a significant improvement in signal strength. It is particularly suitable for distributed online monitoring of fiber temperature and strain in scenarios such as power communication links and long-distance fiber optic transmission networks, providing reliable technical support for the safe operation and maintenance of infrastructure such as power communication links and long-distance fiber optic transmission networks. Attached Figure Description

[0018] Figure 1 This is a structural block diagram of the present invention; Figure 2 This is a diagram showing the high extinction ratio after the dual SOA cascade of the present invention; Figure 3 The Brillouin time-domain image after down-conversion and bandpass filtering according to this invention; Figure 4 This is the power-frequency diagram of Bourniu. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0020] Please see Figures 1 to 4This invention provides a technical solution: a high signal-to-noise ratio (SNR) performance enhancement system based on BOTDR. This system consists of an optical path unit and a circuit processing unit. The optical path unit is the core module for high SNR performance enhancement, and the circuit processing unit is used for subsequent processing of the electrical signal after photodetection, such as filtering, frequency reduction, data acquisition, and analysis. The optical path unit includes a narrow linewidth laser, a pulse modulation module, a tunable optical attenuator (VOA), a graded optical amplification module, an optical circulator (OC), a polarization scrambler (PS), and an optical fiber coupler array. The circuit processing unit includes a photodetector preamplifier module, a low-noise amplifier, a frequency mixer and down-conversion module, and a frequency selection module.

[0021] The narrow linewidth laser is used to output a continuous optical signal with a center wavelength of 1550nm and a linewidth ≤3kHz. Pulse modulation module: A cascaded dual-optical pulse modulation module (P-SOA) is used as the core modulation device to modulate the continuous optical signal into pulsed light with a high extinction ratio; Tunable optical attenuator (VOA): Connected in series between the narrow linewidth laser and the pulse modulation module, it is used to reduce the optical power incident on the dual-optical pulse modulation module (P-SOA) to below its maximum incident power to avoid damage to the device; Hierarchical optical amplification module: including pulsed erbium-doped fiber amplifier (P-EDFA) and small-signal erbium-doped fiber amplifier (C-EDFA). Optical Circulator (OC): Used to realize unidirectional injection of pulsed light and reverse output of Brillouin scattered light. It injects the pulsed light, which is amplified by the pulsed erbium-doped fiber amplifier (P-EDFA), into the fiber under test and guides the Brillouin scattered light returned by the fiber to the subsequent link. Polarization scrambler (PS): Used to perturb the polarization state of the reference light, so that the polarization state of the reference light matches that of the Brillouin scattered light, thereby improving beat frequency efficiency; Fiber optic coupler group: includes a first coupler (splitting ratio 10:90) and a second coupler (splitting ratio 50:50). The first coupler splits the continuous light output from the narrow linewidth laser into 10% reference light and 90% signal light. The second coupler is used to combine the polarization-scrambled reference light with the Brillouin scattered light to achieve coherent beat frequency.

[0022] The optical detection front-end module uses a photodetector (PD) with a working bandwidth of ≥10GHz to convert the beat frequency optical signal into an electrical signal; Low-noise amplifier: A low-noise differential amplifier (D-LNA) is used to amplify the weak Brillouin signal and increase its strength. Frequency mixing and down-conversion module: Provides a sinusoidal signal of specified frequency and power to be mixed with the electrical signal generated by the photodetector (PD), thereby shifting the Brillouin signal frequency of about 11 GHz down to the bandwidth range of the bandpass filter to facilitate subsequent data acquisition and processing.

[0023] Frequency selection module: Provides a fixed bandwidth and center frequency to extract the down-frequency Brillouin signal.

[0024] The output optical power of the narrow linewidth laser is 23m, and the wavelength stability of the narrow linewidth laser is ≤0.2pm / h, and the power stability is ≤0.02dB / 8h.

[0025] The adjustable optical attenuator (VOA) has an attenuation adjustment range adapted to the input power requirements of the dual-pulse optical amplifier (P-SOA), ensuring that the optical power incident on the P-SOA does not exceed its rated threshold, the extinction ratio of a single P-SOA is ≥60dB, the pulse width adjustment range is 5-1000ns, and the repetition frequency adjustment range is 100Hz-1MHz. In an ideal state, pulsed light has a base power of 0, a pulse width of τ, and a period of... T Its average power P A0 It can be represented as: In the formula, P P This represents the peak power of the pulsed light.

[0026] A non-ideal pulsed light, with a non-zero pulse floor power, a pulse width of τ, and a period of... T Its average power P A for: In the formula, P L This represents the pulsed substrate optical power.

[0027] Based on the above formula, we can obtain: Therefore, the extinction ratio of the measured pulse can be calculated using the following formula.

[0028] In the formula, T , τSince the quantity is known, only the average power of the pulse needs to be measured. P A With peak power P P The extinction ratio of the pulsed light can then be determined.

[0029] It is evident that pulse signals with high extinction ratios are stable. However, the dual-light pulse modulation module (P-SOA) is a current-sensitive device. When the modulation pulse switches to the "0" state, the gain switch is not turned on, and the gain can be approximated as "0". At this time, there is almost no light output. Using a cascaded dual-light pulse modulation module (P-SOA) can further improve the extinction ratio of the probe pulse, thereby further improving the signal-to-noise ratio of the BOTDR.

[0030] The pulsed erbium-doped fiber amplifier (P-EDFA) and the small-signal erbium-doped fiber amplifier (C-EDFA) amplify the signal to ensure signal integrity. The input optical power of the pulsed erbium-doped fiber amplifier (P-EDFA) is adapted to a range of 1-10mW, matching the maximum output power of the dual-optical pulse modulation module (P-SOA). This maximizes the peak power of the pulsed light while avoiding fiber nonlinear effects. The pulsed erbium-doped fiber amplifier (P-EDFA) amplifies the modulated pulsed light, with a maximum output power ≥10W and an operating wavelength range of 1527-1565nm. The small-signal erbium-doped fiber amplifier (C-EDFA) amplifies the returned Brillouin scattering light, with a gain ≥45dB (operating wavelength range of 1530-1565nm under -45dBm input conditions).

[0031] The coupled light output from the second coupler is directly incident on the input end of the photodetector (PD), and there are no additional attenuation devices in the optical path link, reducing optical signal loss.

[0032] All devices in the optical path unit are dedicated 1550nm band devices, and the device interface is a standard SMA interface, ensuring low loss and high stability of the optical path connection.

[0033] The photodetector (PD) has a bandwidth of 10-12 GHz, a peak responsivity of ≥0.8 A / W, and a maximum output swing of 900 mV. It can eliminate Rayleigh signals and other signals while converting Brillouin signals into electrical signals for convenient subsequent processing.

[0034] The low-noise differential amplifier (D-LNA) has a bandwidth of 1-18GHz, a gain of 35dBm, and a gain flatness of ±2dBm. It amplifies the weak Brillouin signal converted into an electrical signal and suppresses common-mode signals, further improving the signal-to-noise ratio of the BOTDR.

[0035] The frequency selection module downshifts the high-frequency Brillouin signal to facilitate subsequent signal extraction and data processing. The local oscillator of the mixer in the frequency mixing and down-conversion module operates at a frequency of 8-16 GHz, the local oscillator input signal must be greater than 13 dBm, and the output signal range is DC-2.5 GHz. The sweep frequency source operates at a frequency of 8-12 GHz, with a minimum frequency step of 1 Hz, an output power of 23 dBm, and supports non-sweep frequency mode, externally triggered sweep frequency mode, and free sweep frequency mode.

[0036] The bandpass filter has a center frequency of 400 MHz and a bandwidth of 90 MHz. The down-frequency Brillouin signal is extracted to facilitate subsequent data acquisition and processing.

[0037] The working principle of the optical path unit is as follows. Optical signal splitting and modulation link: The narrow linewidth laser outputs continuous light in the 1550nm band, which is split into 10% reference light and 90% signal light by the first coupler (10:90). The signal light is first adjusted to the input power of the P-SOA through the VOA, and then modulated into pulse light with a high extinction ratio by the dual P-SOA cascaded modulation module. The high extinction ratio of the P-SOA can effectively suppress noise interference in the modulation process and ensure the integrity of the pulse signal.

[0038] Signal amplification and transmission link: The modulated pulsed light is amplified by a pulsed EDFA to an optimal power value close to the threshold of fiber nonlinear effects, avoiding excessive signal attenuation, and then injected into the fiber under test through an optical circulator. The high-power output characteristics of the pulsed EDFA ensure that the signal light can still generate a detectable Brillouin scattering signal after long-distance transmission (≥50km).

[0039] Reference light optimization link: The 10% reference light split from the first coupler is polarized by a polarization scrambler (PS) to eliminate the problem of low beat frequency efficiency caused by the fixed polarization state of the reference light, so that the polarization state of the reference light is matched with that of the Brillouin scattered light that returns later, thereby improving the signal strength of coherent beat frequency.

[0040] Coherent beat frequency and optical detection link: The Brillouin backscattered light generated in the fiber under test returns through an optical circulator and is combined with the polarization-scrambled reference light through a second coupler (50:50) to achieve coherent beat frequency. The beat-frequency optical signal is then incident on a high-bandwidth photodetector, converted into an electrical signal, and signals other than those from the Brillouin backscatter are initially filtered out.

[0041] Signal amplification link: The electrical signal output by the photodetector, which contains the Brillouin signal, is amplified by a low-noise differential amplifier circuit to amplify the weak Brillouin scattered light. At the same time, it suppresses a large amount of electromagnetic interference in the working environment (such as power supply noise, space electromagnetic radiation) and dark current noise generated by the photodetector itself. This effectively suppresses the common-mode noise of the two input ports, greatly improves the signal-to-noise ratio, avoids the useful signal being drowned out by noise, and allows the subsequent demodulation algorithm to accurately identify the Brillouin frequency shift changes.

[0042] Signal mixing and downconversion link: The 10.3GHz-10.5GHz sinusoidal signal output by the sweep frequency source and the signal output by the low noise differential amplifier circuit are downconverted to the bandwidth of the bandpass filter by the mixer, which facilitates subsequent ADC acquisition for Brillouin signal analysis.

[0043] Signal frequency selection and acquisition link: The Brillouin signal output by the mixer has been down-converted to 300MHz-700MHz. Therefore, a bandpass filter with a center frequency of 400MHz and a bandwidth of 90MHz is used to extract the Brillouin signal. Then, an oscilloscope with a bandwidth of 800MHz is used to observe the Brillouin signal and the ADC is used for data acquisition and processing. Example 2:

[0044] like Figure 1 As shown, a high signal-to-noise ratio performance enhancement system based on BOTDR is disclosed. The system includes a distributed Brillouin fiber optic sensing optical path, a signal-to-electric conversion module, and a Brillouin down-conversion acquisition and collection module.

[0045] The entire optical path of the project is constructed. The distributed Brillouin fiber optic sensing optical path converts the Brillouin signals in the fiber into electrical signals for subsequent acquisition and processing. The distributed Brillouin fiber optic sensing optical path includes a narrow-linewidth laser (Laser), coupler, tunable optical attenuator (VOA), optical pulse modulation module (SOA), pulsed erbium-doped fiber amplifier (P-EDFA), optical circulator (OC), continuous optical erbium-doped fiber amplifier (EDFA), polarization scrambler (PS), and photodetector (PD). The narrow-linewidth laser (Laser) emits continuous light with a wavelength of 1550nm, which is split into two paths by a 90:10 coupler. One path (90%) serves as the probe path, used to generate the probe pulse entering the sensing fiber, while the other path serves as the reference path, used to generate reference light. The continuous light entering the probe path first needs to pass through the tunable optical attenuator (VOA) to reduce its power below the maximum incident power of the optical pulse modulation module (P-SOA), and then modulates it into pulsed light. Because the peak power of the pulse output from the pulse modulation module (P-SOA) is relatively low, it needs to be amplified to a certain power by a pulsed erbium-doped fiber amplifier (P-EDFA). Then, the probe pulse from the amplifier's spontaneous emission (ASE) is filtered out by a dense wavelength division multiplexer 1 (DWDM1) and enters the sensing fiber via an optical circulator (OC). Due to the weak Brillouin scattering signal, the Brillouin scattering signal generated in the fiber needs to pass through the output end of the optical circulator before entering an erbium-doped fiber amplifier (C-EDFA) for amplification. It then passes through a dense wavelength division multiplexer 2 (DWDM2) to filter out the amplifier's spontaneous emission (ASE) noise, and then enters a 50:50 coupler to beat with the reference light. The reference light needs to pass through a polarization scrambler (PS) to continuously change its polarization state to eliminate polarization fading noise caused by different polarization states. The beat-frequency signal then enters a photodetector (PD) and is converted into two electrical signal outputs.

[0046] The optical path for the entire project is constructed. The distributed Brillouin fiber optic sensing circuit down-converts the Brillouin signal in the fiber and extracts the useful Brillouin signal for acquisition by the AD acquisition module and subsequent data processing. The distributed Brillouin fiber optic sensing circuit includes a low-noise differential amplifier (D-LNA), a mixer, an electromagnetic local oscillator (ELO), a bandpass filter (BPF), an ADC, and an oscilloscope. The electrical signal output from the photodetector first needs to be amplified by the low-noise differential amplifier (D-LNA), and then mixed with the sinusoidal signal generated by the electromagnetic local oscillator (ELO) in the mixer to achieve down-conversion. The down-converted electrical signal is then frequency-selectively filtered by the bandpass filter (BPF) to extract the Brillouin signal, and then acquired by the ADC for subsequent temperature and strain demodulation.

[0047] like Figure 2 As shown, the ideal pulsed light (pulse substrate power is 0) has a pulse width of τ and a period of... T Its average power P A0 It can be represented as: In the formula, P P This represents the peak power of the pulsed light.

[0048] A non-ideal pulsed light (pulse substrate power is not zero), with a pulse width of τ and a period of... T Its average power P A for: In the formula, P L This represents the pulsed substrate optical power.

[0049] Based on the above formula, we can obtain: Therefore, the extinction ratio of the measured pulse can be calculated using the following formula.

[0050] In the formula, T , τ Since the quantity is known, only the average power of the pulse needs to be measured. P A With peak power P P The extinction ratio of the pulsed light can then be determined.

[0051] It is evident that pulse signals with high extinction ratios are stable. P-SOA is a current-sensitive device. When the modulation pulse switches to the "0" state, the gain switch is not turned on, and the gain can be approximated as "0". At this time, there is almost no light output. Using cascaded P-SOA can improve the extinction ratio of the probe pulse light, thereby further improving the signal-to-noise ratio of the BOTDR.

[0052] like Figure 3 As shown, the backscattered light from the Leyborg and the reference light are coupled together via a coupler and beat at the photodetector PD, with the output center frequency being [value missing]. The high-frequency Brillouin scattering signal is mixed with a sinusoidal signal output from a microwave source (frequency: ( Figure 3Mixing at 10.4 GHz can shift the high-frequency Brillouin scattering spectrum to the mid-frequency band, followed by a bandpass filter (center frequency 10.4 GHz). With a bandwidth of B), frequency-selective filtering allows the data acquisition system to capture a specific frequency point in the Brillouin scattering spectrum. ( The time-domain power curve under the Brillouin scattering spectrum is obtained by stepping the output frequency of the microwave source. The power-distance distribution curve at each frequency point in the Brillouin scattering spectrum can be scanned and measured. Finally, after curve fitting and data processing, the Brillouin scattering spectrum at each sampling point along the optical fiber can be obtained.

[0053] like Figure 4 As shown, since the ELO output frequency of 10.4 GHz is mixed with the original Brillouin signal, the theoretical Brillouin center frequency seen on the oscilloscope at this time is... The final signal is then passed through a bandpass filter and subjected to Fourier transform to obtain the Brillouin spectrum shown in the figure. The figure shows that the center frequency is around 400MHz, which is consistent with the theory.

[0054] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A high signal-to-noise ratio performance enhancement system based on BOTDR, characterized in that: Includes optical path unit and circuit processing unit, The optical path unit includes a narrow linewidth laser, a pulse modulation module, a tunable optical attenuator (VOA), a graded optical amplification module, an optical circulator (OC), a polarization scrambler (PS), and an optical fiber coupler array. The circuit processing unit includes a photodetector preamplifier module, a low-noise amplifier, a frequency mixer and down-conversion module, and a frequency selection module.

2. The high signal-to-noise ratio performance enhancement system based on BOTDR according to claim 1, characterized in that: The narrow linewidth laser is used to output a continuous optical signal with a center wavelength of 1550nm and a linewidth ≤3kHz. Pulse modulation module: A cascaded dual-optical pulse modulation module (P-SOA) is used as the core modulation device to modulate the continuous optical signal into pulsed light with a high extinction ratio; Tunable optical attenuator (VOA): Connected in series between the narrow linewidth laser and the pulse modulation module, it is used to reduce the optical power incident on the dual-optical pulse modulation module (P-SOA) to below its maximum incident power to avoid damage to the device; Hierarchical optical amplification module: including pulsed erbium-doped fiber amplifier (P-EDFA) and small-signal erbium-doped fiber amplifier (C-EDFA). Optical Circulator (OC): Used to realize unidirectional injection of pulsed light and reverse output of Brillouin scattered light. It injects the pulsed light, which is amplified by the pulsed erbium-doped fiber amplifier (P-EDFA), into the fiber under test and guides the Brillouin scattered light returned by the fiber to the subsequent link. Polarization scrambler (PS): Used to perturb the polarization state of the reference light, so that the polarization state of the reference light matches that of the Brillouin scattered light, thereby improving beat frequency efficiency; Fiber optic coupler group: including a first coupler (splitting ratio 10:90) and a second coupler (splitting ratio 50:50). The first coupler splits the continuous light output from the narrow linewidth laser into 10% reference light and 90% signal light. The second coupler is used to combine the polarization-scrambled reference light with the Brillouin scattered light to achieve coherent beat frequency. The optical detection front-end module uses a photodetector (PD) with a working bandwidth of ≥10GHz to convert the beat frequency optical signal into an electrical signal; Low-noise amplifier: A low-noise differential amplifier (D-LNA) is used to amplify the weak Brillouin signal and increase its strength. Frequency mixing and down-conversion module: Provides a sinusoidal signal of specified frequency and power to be mixed with the electrical signal generated by the photodetector (PD), thereby shifting the Brillouin signal frequency of about 11 GHz down to the bandwidth range of the bandpass filter to facilitate subsequent data acquisition and processing; Frequency selection module: Provides a fixed bandwidth and center frequency to extract the down-frequency Brillouin signal.

3. The high signal-to-noise ratio performance enhancement system based on BOTDR according to claim 1, characterized in that: The output optical power of the narrow linewidth laser is 23m, and the wavelength stability of the narrow linewidth laser is ≤0.2pm / h, and the power stability is ≤0.02dB / 8h. The adjustable optical attenuator (VOA) has an attenuation adjustment range adapted to the input power requirements of the dual-pulse optical amplifier (P-SOA), ensuring that the optical power incident on the P-SOA does not exceed its rated threshold, the extinction ratio of a single P-SOA is ≥60dB, the pulse width adjustment range is 5-1000ns, and the repetition frequency adjustment range is 100Hz-1MHz. In an ideal state, pulsed light has a base power of 0, a pulse width of τ, and a period of... T Its average power P A0 It can be represented as: In the formula, P P This represents the peak power of the pulsed light. A non-ideal pulsed light, with a non-zero pulse floor power, a pulse width of τ, and a period of... T Its average power P A for: In the formula, P L This refers to the pulsed substrate optical power. Based on the above formula, we can obtain: Therefore, the extinction ratio of the measured pulse can be calculated using the following formula; ; In the formula, T , τ Since the quantity is known, only the average power of the pulse needs to be measured. P A With peak power P P The extinction ratio of the pulsed light can then be determined; High extinction ratio pulse signals are stable. The dual-light pulse modulation module (P-SOA) is a current-sensitive device. When the modulation pulse switches to the "0" state, the gain switch is not turned on, and the gain can be approximated as "0". At this time, there is no light output. Using a cascaded dual-light pulse modulation module (P-SOA) can improve the extinction ratio of the probe pulse light and improve the signal-to-noise ratio of the BOTDR.

4. The high signal-to-noise ratio performance enhancement system based on BOTDR according to claim 2, characterized in that: The pulsed erbium-doped fiber amplifier (P-EDFA) and the small-signal erbium-doped fiber amplifier (C-EDFA) amplify the signal to ensure signal integrity. The input optical power of the pulsed erbium-doped fiber amplifier (P-EDFA) is adapted to a range of 1-10mW, matching the maximum output power of the dual-optical pulse modulation module (P-SOA). This maximizes the peak power of the pulsed light while avoiding fiber nonlinear effects. The pulsed erbium-doped fiber amplifier (P-EDFA) amplifies the modulated pulsed light, with a maximum output power ≥10W and an operating wavelength range of 1527-1565nm. The small-signal erbium-doped fiber amplifier (C-EDFA) amplifies the returned Brillouin scattering light, with a gain ≥45dB (operating wavelength range of 1530-1565nm under -45dBm input conditions).

5. A high signal-to-noise ratio performance enhancement system based on BOTDR according to claim 2, characterized in that: The coupled light output from the second coupler is directly incident on the input end of the photodetector (PD), and there are no additional attenuation devices in the optical path link, reducing optical signal loss.

6. The high signal-to-noise ratio performance enhancement system based on BOTDR according to claim 2, characterized in that: All devices in the optical path unit are dedicated 1550nm band devices, and the device interface is a standard SMA interface, ensuring low loss and high stability of the optical path connection.

7. A high signal-to-noise ratio performance enhancement system based on BOTDR according to claim 2, characterized in that: The photodetector (PD) has a bandwidth of 10-12 GHz, a peak responsivity of ≥0.8 A / W, and a maximum output swing of 900 mV. It can eliminate Rayleigh signals and other signals while converting Brillouin signals into electrical signals for convenient subsequent processing.

8. A high signal-to-noise ratio performance enhancement system based on BOTDR according to claim 2, characterized in that: The low-noise differential amplifier (D-LNA) has a bandwidth of 1-18GHz, a gain of 35dBm, and a gain flatness of ±2dBm. It amplifies the weak Brillouin signal converted into an electrical signal and suppresses common-mode signals, further improving the signal-to-noise ratio of the BOTDR.

9. A high signal-to-noise ratio performance enhancement system based on BOTDR according to claim 2, characterized in that: The frequency selection module downshifts the high-frequency Brillouin signal to facilitate subsequent signal extraction and data processing. The local oscillator of the mixer in the frequency mixing and down-conversion module operates at a frequency of 8-16 GHz, the local oscillator input signal must be greater than 13 dBm, and the output signal range is DC-2.5 GHz. The sweep frequency source operates at a frequency of 8-12 GHz, with a minimum frequency step of 1 Hz and an output power of 23 dBm. It supports non-sweep frequency mode, externally triggered sweep frequency mode, and free sweep frequency mode.

10. A high signal-to-noise ratio performance enhancement system based on BOTDR according to claim 2, characterized in that: The bandpass filter has a center frequency of 400MHz and a bandwidth of 90MHz, which extracts the down-frequency Brillouin signal to facilitate subsequent data acquisition and processing.

Images