An optical time-domain reflectometer and a method for eliminating interference fading

Abstract

This invention relates to the field of fiber optic sensing technology, and in particular to an optical time-domain reflectometer and a method for eliminating interference fading, comprising: a laser, a frequency division multiplexing modulator, a circulator, and a signal processing device. The frequency division multiplexing modulator includes a first modulator, a second modulator, a first attenuator, and a second attenuator. The laser is used to generate a coherent light source, which passes sequentially through the first modulator and the first attenuator to obtain a first pulsed light signal. The coherent light source passes sequentially through the first modulator and the second modulator to obtain a second pulsed light signal. The coherent light source passes sequentially through the second attenuator and the second modulator to obtain a third pulsed light signal. By using different combinations of the first modulator, the second modulator, the first attenuator, and the second attenuator, multiple pulsed light signals can be output, saving hardware costs compared to existing technologies.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and in particular to an optical time-domain reflectometer and a method for eliminating interference fading. Background Technology

[0002] Distributed fiber optic sensing technology based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) offers numerous advantages, including high measurement accuracy, fast response speed, long monitoring distance, and resistance to electromagnetic interference. It has been widely applied in perimeter security, rail transit, and oil and gas pipeline monitoring. The principle is that a disturbance signal applied to the optical fiber causes a change in the fiber's refractive index, which in turn leads to changes in the optical path length and the phase of the probe light. Depending on the demodulation method, Φ-OTDRs are classified into intensity-demodulated and phase-demodulated types.

[0003] In phase-demodulated Ф-OTDR systems, interference fading is unavoidable, significantly degrading the signal-to-noise ratio at fading points and causing severe distortion of the sensing information. Specifically, Rayleigh scattering intensity inevitably exhibits minimum points. After superimposing detector intensity noise, the demodulation algorithm produces outliers in the phase result, making it difficult to distinguish between phase jumps caused by interference fading and those caused by actual disturbances. Therefore, Ф-OTDR systems based on a single phase signal suffer from false alarms due to interference fading.

[0004] For coherent detection Ф-OTDR systems, the main methods to eliminate interference fading include: employing frequency division multiplexing technology to reconstruct the phase using probe pulses of different optical frequencies, as detailed in the following schemes: Figure 1 As shown, pulse modulation is performed using three acousto-optic modulators of different frequencies to achieve optical pulse output at three frequencies. Its hardware configuration is relatively complex, requiring multiple frequency-shifting modulator devices.

[0005] Therefore, overcoming the shortcomings of the existing technology is an urgent problem to be solved in this technical field. Summary of the Invention

[0006] The technical problem to be solved by this invention is: how to reduce the number of acousto-optic modulators, achieve the output of multiple optical pulse signals with fewer acousto-optic modulators, and eliminate the effects of interference fading.

[0007] The present invention adopts the following technical solution:

[0008] In a first aspect, an optical time-domain reflectometer is provided, which is connected to a sensing optical fiber and includes: a laser, a frequency division multiplexing modulator, a circulator, and a signal processing device. The frequency division multiplexing modulator includes a first modulator, a second modulator, a first attenuator, and a second attenuator.

[0009] The laser is used to generate a coherent light source, which is then passed sequentially through the first modulator and the first attenuator to obtain a first pulse light signal; the coherent light source is then passed sequentially through the first modulator and the second modulator to obtain a second pulse light signal; and the coherent light source is then passed sequentially through the second attenuator and the second modulator to obtain a third pulse light signal.

[0010] The circulator is used to transmit the first pulse optical signal, the second pulse optical signal, and the third pulse optical signal to the sensing optical fiber, respectively. The sensing optical fiber is used to generate corresponding reflected optical signals according to the first pulse optical signal, the second pulse optical signal, and the third pulse optical signal, respectively, and transmit the reflected optical signals to the signal processing device for processing through the circulator.

[0011] Preferably, the optical time domain reflectometer further includes a first coupler, a first amplifier, and a second amplifier;

[0012] The first coupler is used to split the coherent light source into a first optical signal and a second optical signal. The first optical signal is transmitted to the frequency division multiplexing modulator, and the second optical signal is transmitted to the signal processing device.

[0013] The first amplifier is used to amplify the first pulse optical signal, the second pulse optical signal and the third pulse optical signal, and the amplified optical signal is transmitted to the circulator;

[0014] The second amplifier is used to amplify the reflected light signal, and the amplified light signal is transmitted to the signal processing device.

[0015] Preferably, the frequency division multiplexing modulator further includes a second coupler, a third coupler, a fourth coupler, and a fifth coupler;

[0016] The second coupler is used to split the first optical signal into a third optical signal and a fourth optical signal. The third optical signal passes through the first modulator, the third coupler, the first attenuator and the fourth coupler in sequence to obtain the first pulsed optical signal.

[0017] The third optical signal passes sequentially through the first modulator, the third coupler, the fifth coupler, the second modulator, and the fourth coupler to obtain the second pulsed optical signal;

[0018] The fourth optical signal passes sequentially through the second attenuator, the fifth coupler, the second modulator, and the fourth coupler to obtain the third pulse optical signal.

[0019] Preferably, the signal processing device includes a polarization diversity coherent receiving unit and a signal acquisition and processing unit;

[0020] The polarization diversity coherent receiving unit is used to split the reflected optical signal into two orthogonal X-polarized optical signals and Y-polarized optical signals; mix the X-polarized optical signal with the second optical signal to obtain a first beat frequency electrical signal of the X-polarized state; and mix the Y-polarized optical signal with the second optical signal to obtain a second beat frequency electrical signal of the Y-polarized state.

[0021] The signal acquisition and processing unit is used to receive the first beat frequency electrical signal and the second beat frequency electrical signal during the calibration stage of the optical time domain reflectometer, perform frequency band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal to obtain multiple sub-frequency band signals, obtain the intensity information of each sub-frequency band signal according to the first algorithm, and adjust the attenuation ratio of the first attenuator and the second attenuator according to the intensity information.

[0022] The signal acquisition and processing unit is further configured to receive the first beat frequency electrical signal and the second beat frequency electrical signal during the detection phase of the optical time domain reflectometer, perform frequency band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal to obtain multiple sub-frequency band signals, obtain the phase signal of each sub-frequency band signal through the first algorithm, and perform average superposition to obtain the demodulated phase signal sequence.

[0023] Preferably, the operating frequency of the first modulator is f1 and the operating frequency of the second modulator is f2, wherein the sum of f1 and f2 is less than the operating bandwidth of the polarization diversity coherent receiving unit, the sum of f1 and f2 is less than the operating bandwidth of the signal acquisition and processing unit, and f1 is not equal to f2.

[0024] Secondly, a method for eliminating interference fading is provided, wherein the signal processing device includes a polarization diversity coherent receiving unit and a signal acquisition and processing unit; the method for eliminating interference fading includes:

[0025] The polarization diversity coherent receiving unit mixes and balances the reflected light signal with the second light signal separated by the coherent light source according to two orthogonal X-polarization states and Y-polarization states, respectively, to obtain the first beat frequency electrical signal in the X-polarization state and the second beat frequency electrical signal in the Y-polarization state.

[0026] During the calibration phase of the optical time domain reflectometer, the signal acquisition and processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal, performs frequency band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, and then uses a first algorithm to calculate the intensity information of each sub-frequency band signal, and adjusts the attenuation ratio of the first attenuator and the second attenuator according to the intensity information to perform sub-frequency band power equalization.

[0027] During the detection phase of the optical time domain reflectometer, the signal acquisition and processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal. After performing frequency band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, the first algorithm is used to calculate the phase information of each sub-frequency band signal to obtain the phase information of each sub-frequency band signal. The phase information is then averaged and superimposed to eliminate the influence of interference fading.

[0028] Preferably, the step of performing frequency band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal includes:

[0029] The first beat frequency electrical signal and the second beat frequency electrical signal are subjected to frequency band filtering by an intermediate frequency filter with center frequencies of f1 (the operating frequency of the first modulator), f2 (the operating frequency of the second modulator), and f1 + f2 (the operating frequency of the first modulator and the operating frequency of the second modulator), respectively, thereby obtaining multiple sub-frequency band signals.

[0030] Preferably, during the calibration stage of the optical time-domain reflectometer, the signal acquisition and processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal, performs frequency-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, calculates the intensity information of each sub-frequency band signal using a first algorithm, and adjusts the attenuation ratio of the first attenuator and the second attenuator according to the intensity information to perform sub-frequency band power equalization, specifically including:

[0031] During the calibration phase of the optical time-domain reflectometer, the signal acquisition and processing unit acquires a first beat frequency electrical signal {x(k); k = 1, ..., m} and a second beat frequency electrical signal {y(k); k = 1, ..., m}.

[0032] Where K represents the number of sampling points, and m represents the maximum number of sampling points;

[0033] All first beat frequency electrical signals and second beat frequency electrical signals are sorted by time and matched one-to-one;

[0034] Sub-band filtering is performed on the first and second beat frequency electrical signals respectively. Hilbert transform is then applied point-by-point to the first and second beat frequency electrical signals to obtain sub-band signals {H(x(k)); k = 1,....,m} and {H(y(k)); k = 1,....,m}. The intensity information P is then obtained according to Formulas 1, 2, and 3. i :

[0035] Formula 1 is:

[0036] Formula 2 is:

[0037] Formula 3 is:

[0038] Adjust the attenuation ratio of the first and second attenuators according to Formulas 4 and 5 to perform sub-band power equalization:

[0039] Formula four is:

[0040] Formula 5 is:

[0041] Where i takes the values ​​1, 2, and 3; i represents the sequence number of different sub-band signals; m represents the maximum number of sampling points; P1, P2, and P3 represent the intensity information corresponding to different sub-band signals, respectively; V1 represents the attenuation ratio of the first attenuator; and V2 represents the attenuation ratio of the second attenuator.

[0042] Preferably, after performing frequency band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, the first algorithm is used to calculate the phase information of each sub-frequency band signal to obtain the phase information of each sub-frequency band signal, and the phase information is averaged and superimposed to eliminate the influence of interference fading, specifically including:

[0043] The Hilbert transforms of the first beat frequency electrical signal and the second beat frequency electrical signal are calculated point by point to obtain the Hilbert transform sequences of the X state and the Y state, respectively.

[0044] The arctangents of the Hilbert transform sequences of the X state and the Y state are calculated respectively, and then averaged and superimposed to obtain the final acoustic signal output.

[0045] Preferably, the step of calculating the arctangents of the Hilbert transform sequences of the X state and the Y state respectively, and then averaging and superimposing them to obtain the final acoustic signal output includes:

[0046] The arctangents of the Hilbert transform sequences of states X and Y are obtained according to formulas six and seven, respectively:

[0047] Formula six is:

[0048] Formula 7 is:

[0049] According to Formula 8, the arctangent of the Hilbert transform sequence of the X state is... arctangent of the Hilbert transform sequence of the Y state The averages are then superimposed to obtain the final sound wave signal output:

[0050] Formula 8 is:

[0051] Where i takes the values ​​1, 2, and 3; i represents the sequence number of the different sub-band signals.

[0052] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0053] On the one hand, the present invention can realize the output of multiple pulse optical signals by different combinations of the first modulator, the second modulator, the first attenuator and the second attenuator, which saves hardware costs compared with the prior art; the present invention only requires 2 acousto-optic modulators to realize the pulse output of 3 frequencies, while the prior art requires 3 acousto-optic modulators, which simplifies the hardware design and reduces costs.

[0054] On the other hand, the Ф-OTDR system, which detects based on the first, second, and third pulse optical signals, can better resist signal degradation and noise, improve signal quality and system performance, and eliminate the effects of interference fading. Attached Figure Description

[0055] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0056] Figure 1 This is a schematic diagram of an existing structure of an optical time-domain reflectometer provided in an embodiment of the present invention;

[0057] Figure 2 This is a schematic diagram of the structure of an optical time-domain reflectometer provided in an embodiment of the present invention;

[0058] Figure 3 This is a schematic diagram of the structure of a frequency division multiplexing modulator for an optical time domain reflectometer provided in an embodiment of the present invention;

[0059] Figure 4 This is a schematic diagram of the specific structure of an optical time-domain reflectometer provided in an embodiment of the present invention;

[0060] Figure 5 This is a schematic flowchart of a method for eliminating interference fading provided in an embodiment of the present invention;

[0061] Figure 6 This is a schematic diagram of the intermediate frequency signal intensity of a method for eliminating optical interference fading provided in an embodiment of the present invention;

[0062] Figure 7 This is a schematic diagram of frequency band filtering of a beat frequency electrical signal, provided by an embodiment of the present invention, to eliminate interference fading;

[0063] Figure 8 This is a schematic diagram of the processing flow of the signal acquisition and processing unit in the detection stage of a method for eliminating interference fading provided in an embodiment of the present invention;

[0064] Figure 9 This is a schematic diagram illustrating the superposition of frequency-division signals in a method for eliminating interference fading provided in an embodiment of the present invention. Detailed Implementation

[0065] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0066] In this invention, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0067] In this invention, unless otherwise explicitly specified and limited, the term "connection" should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral part; it can be a direct connection or an indirect connection through an intermediate medium. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0068] Example 1:

[0069] Phase-demodulated Ф-OTDR systems can quantitatively reconstruct perturbation signals because the phase signal and the perturbation signal are linearly correlated. Most current phase-demodulated Ф-OTDR systems employ coherent detection technology. The basic principle is as follows: a narrow-linewidth light source is modulated into a pulse signal and injected into the sensing fiber. The reflected Rayleigh backscattered light (RBS) signal is mixed with the intrinsic signal for coherent detection, and the phase information of the RBS is obtained using a suitable demodulation algorithm. The demodulated phase information is then input to the signal processing unit for pattern recognition and event detection. In this phase-demodulated Ф-OTDR system, interference fading is unavoidable, significantly degrading the signal-to-noise ratio at fading points and causing severe distortion of the sensing information. Existing methods to eliminate interference fading use frequency division multiplexing (FDM) technology, employing probe pulse signals of different optical frequencies for phase reconstruction, which can effectively eliminate interference fading. However, in order to obtain three detection pulse signals with different optical frequencies, the existing technology uses high-cost hardware and requires three acousto-optic modulators with different frequencies to perform pulse modulation and achieve optical pulse output at three frequencies.

[0070] To reduce costs and simultaneously obtain optical pulse outputs of multiple frequencies, this embodiment provides an optical time-domain reflectometer, such as... Figure 2 As shown, the optical time-domain reflectometer is connected to the sensing fiber. The optical time-domain reflectometer includes: a laser, a frequency division multiplexing modulator, a circulator, and a signal processing device, such as... Figure 3 As shown, the frequency division multiplexing modulator includes a first modulator, a second modulator, a first attenuator, and a second attenuator; the laser is used to generate a coherent light source, which passes sequentially through the first modulator and the first attenuator to obtain a first pulse light signal; the coherent light source passes sequentially through the first modulator and the second modulator to obtain a second pulse light signal; the coherent light source passes sequentially through the second attenuator and the second modulator to obtain a third pulse light signal; the circulator is used to transmit the first pulse light signal, the second pulse light signal, and the third pulse light signal to the sensing optical fiber, respectively; the sensing optical fiber is used to generate corresponding reflected light signals according to the first pulse light signal, the second pulse light signal, and the third pulse light signal, respectively, and transmits the reflected light signals to the signal processing device for processing through the circulator.

[0071] The laser can be a narrow-linewidth laser, and the sensing fiber is a non-polarization-maintaining single-mode fiber with a length of up to 50 kilometers. Both the first and second modulators can be acousto-optic modulators. Acousto-optic modulation is an external modulation technique; the device that controls the intensity change of the laser beam is usually called a modulator. The modulation signal acts on the transducer in the form of an electrical signal (amplitude modulation), and is then converted into a wave field that changes in the form of an electrical signal. When the light wave passes through the medium, the optical carrier is modulated, becoming an intensity-modulated wave "carrying" information.

[0072] The first port of the circulator is used to receive the first pulse optical signal, the second pulse optical signal, and the third pulse optical signal, respectively. The second port of the circulator is used to transmit the first pulse optical signal, the second pulse optical signal, and the third pulse optical signal to the sensing optical fiber. The first pulse optical signal, the second pulse optical signal, and the third pulse optical signal are reflected in the sensing optical fiber. The second port of the circulator is also used to receive the reflected optical signal and transmit the reflected optical signal to the signal processing device through the third port of the circulator. The signal processing device performs corresponding processing on the reflected optical signal. The main process of the signal processing device performing corresponding processing on the reflected optical signal will be described below.

[0073] Compared with existing technologies, the beneficial effects of this embodiment are as follows: On the one hand, the present invention can achieve the output of multiple pulse optical signals through different combinations of the first modulator, the second modulator, the first attenuator, and the second attenuator, thus saving hardware costs compared with existing technologies; the present invention only requires two acousto-optic modulators to achieve pulse output at three frequencies, while existing technologies require three acousto-optic modulators, simplifying hardware design and reducing costs. On the other hand, the Ф-OTDR system based on the detection of the first pulse optical signal, the second pulse optical signal, and the third pulse optical signal can better resist signal degradation and noise, improve signal quality and system performance, and eliminate the effects of interference fading.

[0074] Next, the structure of the optical time domain reflectometer will be described in detail.

[0075] In a preferred embodiment, such as Figure 4As shown, the optical time-domain reflectometer further includes a first coupler, a first amplifier, and a second amplifier. The input of the first coupler is connected to the laser, one output of the first coupler is connected to the input of a frequency division multiplexing modulator, and the other output of the first coupler is connected to the signal processing device. The input of the first amplifier is connected to the output of the frequency division multiplexing modulator, and the output of the first amplifier is connected to the first port of the circulator. The input of the second amplifier is connected to the third port of the circulator, and the output of the second amplifier is connected to the signal processing device.

[0076] The first coupler is used to split the coherent light source into a first optical signal and a second optical signal. The first optical signal is transmitted to the frequency division multiplexing modulator, and the second optical signal is transmitted to the signal processing device. The first amplifier is used to amplify the first pulsed optical signal, the second pulsed optical signal, and the third pulsed optical signal. The amplified optical signal is transmitted to the circulator. The second amplifier is used to amplify the reflected optical signal. The amplified optical signal is transmitted to the signal processing device.

[0077] The first coupler splits the coherent light source into two parts: the first optical signal is transmitted to a frequency division multiplexing modulator (FDM) for modulation, enabling the transmission of multiple different frequencies of data within the optical fiber. The second optical signal is transmitted to a signal processing unit, which combines the second and reflected optical signals for analysis to assess the characteristics of the fiber optic link or locate fault points. The first amplifier amplifies the first, second, and third pulsed optical signals, which are then transmitted to a circulator. The circulator separates the incoming and outgoing signals, ensuring they flow in the correct direction. The second amplifier amplifies the reflected optical signal, improving the detection sensitivity of the signal processing unit for further analysis. By transmitting pulsed optical signals and analyzing the reflected signals, the OTDR can determine the location of faults, losses, and other critical characteristics within the fiber optic link.

[0078] To obtain the first pulse optical signal, the second pulse optical signal, and the third pulse optical signal, in a preferred embodiment, refer to... Figure 3 The frequency division multiplexing modulator further includes a second coupler, a third coupler, a fourth coupler, and a fifth coupler;

[0079] The input terminal of the second coupler is connected to the first coupler. One output terminal of the second coupler is connected to the input terminal of the first modulator. The input terminal of the third coupler is connected to the output terminal of the first modulator. One output terminal of the third coupler is connected to the input terminal of the first attenuator. The output terminal of the first attenuator is connected to one input terminal of the fourth coupler. The output terminal of the fourth coupler is connected to the input terminal of the first amplifier. The other output terminal of the second coupler is connected to the input terminal of the second attenuator. The output terminal of the second attenuator is connected to one input terminal of the fifth coupler. The other output terminal of the third coupler is connected to the other input terminal of the fifth coupler. The output terminal of the fifth coupler is connected to the input terminal of the second modulator. The output terminal of the second modulator is connected to the other input terminal of the fourth coupler.

[0080] The second coupler is used to split the first optical signal into a third optical signal and a fourth optical signal. The third optical signal passes sequentially through the first modulator, the third coupler, the first attenuator, and the fourth coupler to obtain the first pulsed optical signal. The third optical signal passes sequentially through the first modulator, the third coupler, the fifth coupler, the second modulator, and the fourth coupler to obtain the second pulsed optical signal. The fourth optical signal passes sequentially through the second attenuator, the fifth coupler, the second modulator, and the fourth coupler to obtain the third pulsed optical signal.

[0081] The path for generating the first pulsed optical signal is as follows: the third optical signal first passes through the first modulator, the modulated third optical signal passes through the third coupler, then through the first attenuator, and finally through the fourth coupler to obtain the first pulsed optical signal.

[0082] The path for generating the second pulsed optical signal is as follows: the third optical signal passes sequentially through the first modulator and the third coupler, then through the fifth coupler, and is further modulated by the second modulator. Finally, it passes through the fourth coupler to obtain the second pulsed optical signal.

[0083] The path for generating the third pulse optical signal is as follows: the fourth optical signal first passes through the second attenuator, then through the fifth coupler, is further modulated by the second modulator, and finally passes through the fourth coupler to obtain the third pulse optical signal.

[0084] The frequency division multiplexing modulator generates multiple pulsed optical signals with different frequencies. This is crucial for achieving efficient frequency division multiplexing optical communication, enabling the simultaneous transmission of multiple different data streams on the same optical fiber, each corresponding to a different optical signal characteristic. This facilitates subsequent elimination of the effects of interference fading.

[0085] After multiple pulsed optical signals with different frequencies are generated by the frequency division multiplexing modulator, they are passed through a circulator to obtain the reflected optical signals corresponding to each pulsed optical signal (i.e., the first pulsed optical signal, the second pulsed optical signal, and the third pulsed optical signal). The reflected optical signals are then processed by the signal processing device to eliminate the effects of interference fading. Furthermore, by analyzing the reflected optical signals, the location of faults, losses, or other important characteristics in the optical fiber link can be determined. Specific analysis methods will be described below.

[0086] In a preferred embodiment, refer to Figure 4 The polarization diversity coherent receiving unit is used to divide the reflected optical signal into two orthogonal X-polarized optical signals and Y-polarized optical signals; mix the X-polarized optical signal with the second optical signal to obtain a first beat frequency electrical signal in the X-polarized state, and mix the Y-polarized optical signal with the second optical signal to obtain a second beat frequency electrical signal in the Y-polarized state; the signal acquisition and processing unit is used to receive the first beat frequency electrical signal and the second beat frequency electrical signal during the calibration stage of the optical time domain reflectometer, perform frequency band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal to obtain multiple sub-frequency band signals, and obtain the intensity information of each sub-frequency band signal according to a first algorithm, and adjust the attenuation ratio of the first attenuator and the second attenuator according to the intensity information; the signal acquisition and processing unit is also used to receive the first beat frequency electrical signal and the second beat frequency electrical signal during the detection stage of the optical time domain reflectometer, perform frequency band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal to obtain multiple sub-frequency band signals, obtain the phase signal of each sub-frequency band signal respectively through the first algorithm, and perform average superposition to obtain a demodulated phase signal sequence.

[0087] The polarization diversity coherent receiving device uses a polarization beam splitter to split the received RBS signal (i.e., the reflected light signal) into two orthogonal X-polarization states and Y-polarization states, and then coherently mixes them with the input intrinsic light (i.e., the second optical signal) for balanced reception. Its operating bandwidth is AC-400M, so as to obtain the first beat frequency electrical signal and the second beat frequency electrical signal.

[0088] The optical time-domain reflectometer includes a calibration stage and a detection stage. During the calibration stage, the signal acquisition and processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal from the polarization diversity coherent receiving unit. It uses the Hilbert intensity demodulation algorithm (i.e., the first algorithm) to obtain the intensity information of each sub-band signal. Based on the intensity information of each sub-band signal, it adjusts the first attenuator and the second attenuator in the frequency division multiplexing modulator to perform power equalization on the intensity information of each sub-band signal. The specific process will be described in detail below.

[0089] The signal acquisition and processing unit receives a first beat frequency electrical signal and a second beat frequency electrical signal from the polarization diversity coherent receiving unit during the detection phase. It then uses the Hilbert phase demodulation algorithm to obtain the phase signals of the two polarization states, and averages and superimposes them to obtain the demodulated phase signal sequence. The specific process will be described in detail below. The sampling rate of the signal acquisition and processing unit is 250 MSPS, and the quantization resolution is 14 bits.

[0090] In a preferred embodiment, the operating frequency of the first modulator is f1 and the operating frequency of the second modulator is f2, wherein the sum of f1 and f2 is less than the operating bandwidth of the polarization diversity coherent receiving unit, the sum of f1 and f2 is less than the operating bandwidth of the signal acquisition and processing unit, and f1 is not equal to f2.

[0091] Depend on Figure 3 The frequency division multiplexing modulator structure shown is designed to effectively distinguish the two signals processed by the first and second modulators in the frequency domain, avoiding interference caused by frequency overlap. By ensuring that the sum of f1 and f2 is less than the operating bandwidth of the polarization diversity coherent receiving unit and the signal acquisition and processing unit, frequency aliasing can be avoided, ensuring that the signal can be accurately resolved and processed. Simultaneously, the condition that f1 is not equal to f2 ensures that the two signals have different frequency characteristics, further facilitating their distinction in the frequency domain.

[0092] Example 2:

[0093] An optical time-domain reflectometer was proposed in Example 1. In this example, a method for eliminating interference fading will be proposed, such as... Figure 5 As shown, the signal processing device includes a polarization diversity coherent receiving unit and a signal acquisition and processing unit.

[0094] Step 101: The polarization diversity coherent receiving unit mixes and balances the reflected light signal with the second light signal separated from the coherent light source according to two orthogonal X-polarization states and Y-polarization states, respectively, to obtain the first beat frequency electrical signal in the X-polarization state and the second beat frequency electrical signal in the Y-polarization state.

[0095] The polarization diversity coherent receiving device uses a polarization beam splitter to split the received RBS signal (i.e., the reflected light signal) into two orthogonal X-polarization states and Y-polarization states, and then coherently mixes them with the input intrinsic light (i.e., the second optical signal) for balanced reception. Its operating bandwidth is AC-400M, so as to obtain the first beat frequency electrical signal and the second beat frequency electrical signal.

[0096] Step 102: During the calibration stage of the optical time domain reflectometer, the signal acquisition and processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal. After performing frequency band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, the first algorithm is used to calculate the intensity information of each sub-frequency band signal. The attenuation ratio of the first attenuator and the second attenuator is adjusted according to the intensity information to perform sub-frequency band power equalization.

[0097] The step of performing frequency-band filtering on the first and second beat frequency electrical signals includes: using intermediate frequency (IF) filters with center frequencies of f1 (the operating frequency of the first modulator), f2 (the operating frequency of the second modulator), and f1 + f2 (the operating frequency of the first modulator + the operating frequency of the second modulator), respectively, to perform frequency-band filtering on the first and second beat frequency electrical signals, thereby obtaining multiple sub-frequency band signals. That is, the first beat frequency electrical signal is filtered by IF filters with different center frequencies, and the second beat frequency electrical signal is also filtered by IF filters with different center frequencies. Figure 6 As shown, the center frequencies of the intermediate frequency filters are 40MHz, 80MHz, and 120MHz, respectively.

[0098] like Figure 7 As shown, the first and second beat frequency electrical signals are subjected to frequency-band filtering. Each signal is decomposed into multiple sub-bands, each covering a different frequency range. This is done to allow for more detailed analysis of the signal at different frequencies. Specifically:

[0099] During the calibration phase of the optical time-domain reflectometer, the signal acquisition and processing unit acquires a first beat frequency electrical signal {x(k); k = 1, ..., m} and a second beat frequency electrical signal {y(k); k = 1, ..., m}.

[0100] Where K represents the number of sampling points and m represents the maximum number of sampling points.

[0101] All first beat frequency electrical signals and second beat frequency electrical signals are sorted according to time and matched one-to-one.

[0102] Sub-band filtering is performed on the first and second beat frequency electrical signals respectively. Hilbert transform is then applied point-by-point to the first and second beat frequency electrical signals to obtain sub-band signals {H(x(k)); k = 1,....,m} and {H(y(k)); k = 1,....,m}. The intensity information P is then obtained according to Formulas 1, 2, and 3. i :

[0103] Formula 1 is:

[0104] Formula 2 is:

[0105] Formula 3 is:

[0106] Adjust the attenuation ratio of the first and second attenuators according to Formulas 4 and 5 to perform sub-band power equalization:

[0107] Formula four is:

[0108] Formula 5 is:

[0109] Where i takes the values ​​1, 2, and 3; i represents the sequence number of different sub-band signals; m represents the maximum number of sampling points; P1, P2, and P3 represent the intensity information corresponding to different sub-band signals, respectively; V1 represents the attenuation ratio of the first attenuator; and V2 represents the attenuation ratio of the second attenuator.

[0110] The signal acquisition unit has a sampling rate of 250 MSPS, the unit length of each sampling point is about 0.4m, and the total sampling length is 50km.

[0111] By adjusting the attenuation ratio of the first and second attenuators, the power level of the signal is ensured to be consistent across different frequency bands, meeting specific requirements. This ensures that the optical time domain reflectometer (OTDR) operates with the same efficiency and accuracy across different frequency bands. This calibration process ensures that the OTDR's measurement results are accurate and consistent across all frequency bands, thereby improving the overall system performance and reliability. It is crucial for subsequent fiber optic testing and fault diagnosis, as signals in different frequency bands may exhibit different reflection characteristics due to varying properties within the fiber.

[0112] Step 103: During the detection phase of the optical time domain reflectometer, the signal acquisition and processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal. After performing frequency band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, the first algorithm is used to calculate the phase information of each sub-frequency band signal to obtain the phase information of each sub-frequency band signal. The phase information is averaged and superimposed to eliminate the influence of interference fading.

[0113] During the detection phase, the signal acquisition and processing unit first acquires the first beat frequency electrical signal {x(k); k = 1, ..., 125000} and the second beat frequency electrical signal {y(k); k = 1, ..., 125000}. Here, K represents the number of sampling points, m is 125000, the sampling rate of the signal acquisition unit is 250 MSPS, the unit length of each sampling point is approximately 0.4 m, and the total sampling length is 50 km. All the first and second beat frequency electrical signals are sorted by time and matched one-to-one.

[0114] In a preferred embodiment, such as Figure 8 and Figure 9 As shown, step 103 specifically includes:

[0115] Step 1031: Calculate the Hilbert transform point by point for the first beat frequency electrical signal and the second beat frequency electrical signal to obtain the Hilbert transform sequence of the X state and the Hilbert transform sequence of the Y state, respectively.

[0116] In this process, sub-band filtering is performed on the first and second beat frequency electrical signals, with the center frequencies of the intermediate frequency filters being 40MHz, 80MHz, and 120MHz, respectively. The value of m is taken as 125000, and its Hilbert transform is calculated point by point to obtain the Hilbert transform sequence of X state {H(x(k)); k=1,....,125000} and the Hilbert transform sequence of Y state {H(y(k)); k=1,....,125000}.

[0117] Step 1032: Calculate the arctangent of the Hilbert transform sequence of the X state and the Hilbert transform sequence of the Y state respectively, and then average and superimpose them to obtain the final sound wave signal output.

[0118] The arctangents of the Hilbert transform sequences of states X and Y are obtained according to formulas six and seven, respectively:

[0119] Formula six is:

[0120] Formula 7 is:

[0121] According to Formula 8, the arctangent of the Hilbert transform sequence of the X state is... arctangent of the Hilbert transform sequence of the Y state The averages are then superimposed to obtain the final sound wave signal output:

[0122] Formula 8 is:

[0123] Where i takes the values ​​1, 2, and 3; i represents the sequence number of the different sub-band signals.

[0124] By further analyzing and processing the final acoustic signal output, the characteristics of the signal detected by the optical time-domain reflectometer (OTDR) can be identified. Simultaneously, by averaging and superimposing phase information from different frequency bands, the effects of interference fading can be effectively reduced, thereby improving signal quality and reliability. Utilizing the advantages of multi-band processing and phase information analysis improves the accuracy and efficiency of the ORT in fiber optic link detection and fault diagnosis. This advanced signal processing allows for more accurate localization of damage, breaks, or other problems in the optical fiber.

[0125] The specific structure of the optical time domain reflectometer is described in Example 1, and will not be repeated in this example.

[0126] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An optical time domain reflectometer, characterized by, The optical time domain reflectometer is connected with a sensing optical fiber, and comprises a laser, a frequency division multiplexing modulator, a circulator and a signal processing device. The laser is used to generate a coherent light source, which sequentially passes through the first modulator and the first attenuator to obtain a first pulsed light signal; the coherent light source sequentially passes through the first modulator and the second modulator to obtain a second pulsed light signal; and the coherent light source sequentially passes through the second attenuator and the second modulator to obtain a third pulsed light signal. The circulator is used to transmit the first pulsed light signal, the second pulsed light signal and the third pulsed light signal to the sensing optical fiber, respectively; the sensing optical fiber is used to generate corresponding reflected light signals according to the first pulsed light signal, the second pulsed light signal and the third pulsed light signal, respectively, and transmit the reflected light signals to the signal processing device through the circulator for processing.

2. The optical time domain reflectometer of claim 1, wherein, The optical time domain reflectometer further comprises a first coupler, a first amplifier and a second amplifier. The first coupler is used to divide the coherent light source into a first light signal and a second light signal, the first light signal is transmitted to the frequency division multiplexing modulator, and the second light signal is transmitted to the signal processing device. The first amplifier is used to amplify the first pulsed light signal, the second pulsed light signal and the third pulsed light signal, and the amplified light signals are transmitted to the circulator. The second amplifier is used to amplify the reflected light signals, and the amplified light signals are transmitted to the signal processing device.

3. The optical time domain reflectometer of claim 2, wherein, The frequency division multiplexing modulator further comprises a second coupler, a third coupler, a fourth coupler and a fifth coupler. The second coupler is used to divide the first light signal into a third light signal and a fourth light signal, the third light signal sequentially passes through the first modulator, the third coupler, the first attenuator and the fourth coupler to obtain the first pulsed light signal; The third light signal sequentially passes through the first modulator, the third coupler, the fifth coupler, the second modulator and the fourth coupler to obtain the second pulsed light signal; The fourth light signal sequentially passes through the second attenuator, the fifth coupler, the second modulator and the fourth coupler to obtain the third pulsed light signal.

4. The optical time domain reflectometer of claim 2, wherein, The signal processing device comprises a polarization diversity coherent receiving unit and a signal acquisition processing unit. The polarization diversity coherent receiving unit is used to divide the reflected light signals into two orthogonal X-polarization state light signals and Y-polarization state light signals; mix the X-polarization state light signals with the second light signal to obtain an X-polarization state first beat frequency electric signal, and mix the Y-polarization state light signals with the second light signal to obtain a Y-polarization state second beat frequency electric signal; The signal acquisition and processing unit is configured to receive the first beat frequency electrical signal and the second beat frequency electrical signal in a calibration stage of the optical time domain reflectometer, perform sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal to obtain a plurality of sub-band signals, and obtain intensity information of each sub-band signal according to a first algorithm, and adjust an attenuation ratio of the first attenuator and the second attenuator according to the intensity information. The signal acquisition and processing unit is further configured to receive the first beat frequency electrical signal and the second beat frequency electrical signal in a detection stage of the optical time domain reflectometer, perform sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal to obtain a plurality of sub-band signals, obtain phase signals of each sub-band signal respectively through the first algorithm, and perform average superposition to obtain a demodulated phase signal sequence.

5. The optical time domain reflectometer of claim 4, wherein, The working frequency of the first modulator is f1, and the working frequency of the second modulator is f2, wherein the sum of f1 and f2 is less than the working bandwidth of the polarization diversity coherent receiving unit, the sum of f1 and f2 is less than the working bandwidth of the signal acquisition and processing unit, and f1 is not equal to f2.

6. A method of mitigating against fading due to interference, characterized by, The method is applied to the optical time domain reflectometer according to any one of claims 1-5, and the signal processing device comprises a polarization diversity coherent receiving unit and a signal acquisition and processing unit. The method for eliminating interference fading comprises: The polarization diversity coherent receiving unit performs balanced mixing reception on the reflected light signal according to two orthogonal X polarization states and Y polarization states respectively, and the second light signal is divided from the coherent light source, to obtain a first beat frequency electrical signal of the X polarization state and a second beat frequency electrical signal of the Y polarization state respectively. In the calibration stage of the optical time domain reflectometer, the signal acquisition and processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal, performs sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, adopts a first algorithm to calculate each sub-band signal obtained by filtering, obtains intensity information of each sub-band signal, and adjusts an attenuation ratio of the first attenuator and the second attenuator according to the intensity information to perform sub-band power equalization. In the detection stage of the optical time domain reflectometer, the signal acquisition and processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal, performs sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, adopts a first algorithm to calculate each sub-band signal, obtains phase information of each sub-band signal, and performs average superposition on the phase information to eliminate the influence of interference fading.

7. The method of canceling interference fading of claim 6, wherein, The sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal comprises: The first beat frequency electrical signal and the second beat frequency electrical signal are filtered by intermediate frequency filters with center frequencies of f1, f2 and f1+f2, where f1 is the working frequency of the first modulator, f2 is the working frequency of the second modulator, and f1+f2 is the sum of the working frequencies of the first modulator and the second modulator.

8. The method of canceling interference fading of claim 6, wherein, The signal acquisition and processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal in the calibration stage of the optical time domain reflectometer, performs sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, and calculates each sub-band signal by using a first algorithm to obtain intensity information of each sub-band signal, and adjusts the attenuation ratios of the first attenuator and the second attenuator according to the intensity information to perform sub-band power equalization, specifically comprising: In the calibration stage of the optical time domain reflectometer, the signal acquisition and processing unit acquires first beat frequency electrical signals {x(k); k=1,....,m} and second beat frequency electrical signals {y(k); k=1,....,m}; Wherein, K represents the number of sampling points, and m represents the maximum number of sampling points. All first beat frequency electrical signals and second beat frequency electrical signals are sorted by time and one-to-one corresponding; respectively, and performing point-by-point Hilbert transform on the first beat frequency electrical signal and the second beat frequency electrical signal to obtain sub-band signals {H(x(k)); k=1,....,m} and {H(y(k)); k=1,....,m}, and obtaining the intensity information P according to Formula One, Formula Two and Formula Three i : Equation one is: Equation two is: Equation three is: The attenuation ratios of the first attenuator and the second attenuator are adjusted according to formula four and formula five to perform sub-band power equalization: Equation Four is: Equation five is: Wherein, i takes the value of 1, 2, 3; i represents the serial number of different sub-band signals; m represents the maximum number of sampling points; P1, P2 and P3 represent the intensity information corresponding to different sub-band signals respectively; V1 represents the attenuation ratio of the first attenuator; V2 represents the attenuation ratio of the second attenuator.

9. The method of canceling interference fading of claim 6, wherein, After the first beat frequency electrical signal and the second beat frequency electrical signal are filtered by sub-band, each sub-band signal is calculated by using a first algorithm to obtain the phase information of each sub-band signal, and the phase information is averaged and superimposed to eliminate the influence of interference fading, specifically comprising: The Hilbert transform of the first beat frequency electrical signal and the second beat frequency electrical signal is calculated point by point to obtain the Hilbert transform sequence of X state and the Hilbert transform sequence of Y state respectively; The arctangent of the Hilbert transform sequence of X state and the Hilbert transform sequence of Y state is calculated respectively, and the final sound wave signal output is obtained by averaging and superimposing.

10. The method of canceling interference fading of claim 9, wherein, The arctangent of the Hilbert transform sequence of X state and the Hilbert transform sequence of Y state is calculated respectively, and the final sound wave signal output is obtained by averaging and superimposing, comprising: The arctangent of the Hilbert transform sequence of X state and the Hilbert transform sequence of Y state is obtained according to formula six and formula seven respectively: Equation Six is: Equation Seven is: The arctangent of the Hilbert transform sequence of the X state according to equation eight and the arctangent of the Hilbert transform sequence of the Y state are averaged and superimposed to obtain the final acoustic signal output: Equation Eight is: Wherein, i takes the value of 1, 2, 3; i represents the serial number of different sub-band signals.

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