Optical time-domain reflectometer and interference fading rejection method

Abstract

This disclosure relates to the optical fiber sensing technology, and more particularly to an optical time-domain reflectometer and an interference fading rejection method, and includes a laser device, a frequency division multiplex modulator, a circulator and a signal processing device, wherein the frequency division multiplex modulator includes a first modulator, a second modulator, a first attenuator and a second attenuator, the laser device is used to acquire a coherent light source, the coherent light source passes sequentially through the first modulator and the first attenuator to acquire the first pulsed optical signal, the coherent light source passes sequentially through the first modulator and the second modulator to acquire the second pulsed optical signal, the coherent light source passes sequentially through the second attenuator and the second modulator to acquire the third pulsed optical signal, and the output of multiple pulsed optical signals can be realized by different combinations of the first modulator, the second modulator, the first attenuator and the second attenuator, saving hardware costs compared to existing technologies.

Description

Cross-reference to Related Applications

[0001] This disclosure claims the priority of the following patent applications. (1) A Chinese patent application with application number 202410008182.0 and title of invention "A Method for Optical Time Domain Reflectometer and Eliminating Interference Fading", filed with the Chinese Patent Office on January 3, 2024.

Technical Field

[0002] This disclosure relates to the field of optical fiber sensing technology, and particularly to optical time domain reflectometers and interference fading removal methods.

Background Art

[0003] Distributed optical fiber sensing technology based on phase-sensitive optical time-domain reflectometry (hereinafter abbreviated as "Φ-OTDR") has many advantages such as high measurement accuracy, fast response speed, long monitoring distance, and high electromagnetic interference resistance, and is widely applied in fields such as border security, rail transit, and oil and gas pipeline monitoring. Its principle is that the disturbance signal applied to the optical fiber causes a change in the refractive index of the optical fiber, thereby causing a change in the optical path length and the phase of the detected light. Depending on the difference in demodulation methods, Φ-OTDR can be divided into intensity demodulation type and phase demodulation type.

[0004] In a phase demodulation type Φ-OTDR system, the interference fading effect inevitably occurs, significantly deteriorating the signal-to-noise ratio of the fading point, and thus bringing serious distortion to the sensing information. Specifically, the Rayleigh scattering intensity inevitably has a minimum value point, and after the intensity noise of the detector is superimposed and passes through the demodulation algorithm, abnormal values appear in the phase result. Usually, it is difficult to distinguish between the phase jump caused by interference fading and the phase jump caused by an actual disturbance. Therefore, a Φ-OTDR system that performs detection based on a single phase signal has problems such as false alarms of events caused by interference fading.

[0005] Regarding the coherent detection Φ-OTDR system, the method for eliminating interference fading primarily employs frequency division multiplexing (FDM) technology, using detection pulses of different optical frequencies to perform phase reconstruction. Specifically, as shown in Figure 1, pulse modulation is performed using three acousto-optic modulators (AOMs) of different frequencies to achieve three different frequency optical pulse outputs. The hardware configuration is relatively complex, requiring the implementation of multiple frequency shift modulator devices.

[0006] In light of the above circumstances, overcoming the shortcomings of this existing technology is a problem that needs to be solved in this field. [Overview of the project]

[0007] The technical problem that this disclosure aims to solve 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.

[0008] This disclosure employs the following technical solutions.

[0009] In a first embodiment, a time-domain optical reflectometer is provided, which is connected to a sensing optical fiber and includes a laser device, a frequency division multiplex modulator, a circulator, and a signal processing device. The frequency division multiplex modulator includes a first modulator, a second modulator, a first attenuator, and a second attenuator. The laser device is used to generate a coherent light source, and the coherent light source passes sequentially through the first modulator and the first attenuator to acquire a first pulsed optical signal. The coherent light source passes sequentially through the first modulator and the second modulator to acquire a second pulsed optical signal. The coherent light source passes sequentially through the second attenuator and the second modulator to acquire a third pulsed optical signal. The circulator is used to transmit the first pulsed optical signal, the second pulsed optical signal, and the third pulsed optical signal to the sensing optical fiber, respectively. The sensing optical fiber is used to generate reflected optical signals corresponding to the first pulsed optical signal, the second pulsed optical signal, and the third pulsed optical signal, respectively, and transmits the reflected optical signals to the signal processing device via the circulator for processing.

[0010] Preferably, the optical time-domain reflectometer further includes a first coupler, a first amplifier, and a second amplifier. 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 being transmitted to the frequency division multiplex modulator and the second optical signal being transmitted to the signal processing device. 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 signals are transmitted to the circulator. The second amplifier is used to amplify the reflected light signal, and the amplified light signal is transmitted to the signal processing device.

[0011] Preferably, the frequency division multiplex modulator further includes a second coupler, a third coupler, a fourth coupler, and a fifth coupler. 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 pulse 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 acquire the second pulse optical signal. The fourth optical signal passes sequentially through the second attenuator, the fifth coupler, the second modulator, and the fourth coupler to acquire the third pulse optical signal.

[0012] Preferably, the signal processing device includes a polarization diversity coherent receiving unit and a signal acquisition processing unit. The polarization diversity coherent receiving unit is used to split the reflected light signal into two orthogonal X-polarization state optical signals and Y-polarization state optical signals. The X-polarization state optical signal and the second optical signal are mixed to obtain a first beat frequency electrical signal in the X-polarization state, and the Y-polarization state optical signal and the second optical signal are mixed to obtain a second beat frequency electrical signal in the Y-polarization state. The signal acquisition processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal during the calibration stage of the optical time-domain reflectometer, performs frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal to acquire a plurality of sub-frequency band signals, acquires intensity information for each sub-frequency band signal based on the first algorithm, and adjusts the attenuation ratio of the first attenuator and the second attenuator based on the intensity information. The signal acquisition processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal during the detection stage of the optical time-domain reflectometer, performs frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal to acquire a plurality of sub-frequency band signals, acquires the phase signal of each sub-frequency band signal via the first algorithm, averages and superimposes them to obtain a demodulated phase signal sequence.

[0013] Preferably, the operating frequency of the first modulator is f1 and the operating frequency of the second modulator is f2, where the sum of f1 + f2 is less than the operating bandwidth of the polarization diversity coherent receiving unit, the sum of f1 + f2 is less than the operating bandwidth of the signal acquisition processing unit, and f1 is not equal to f2.

[0014] In a second embodiment, a method for eliminating interference fading is provided, wherein the signal processing device includes a polarization diversity coherent receiving unit and a signal acquisition processing unit. The method for eliminating interference fading is: The polarization diversity coherent receiving unit performs balanced reception by mixing the reflected light signal with a second optical signal separated from the coherent light source according to two orthogonal X-polarization states and Y-polarization states, respectively, and acquires a first beat frequency electrical signal in the X-polarization state and a second beat frequency electrical signal in the Y-polarization state, respectively. In the calibration stage of the optical time-domain reflectometer, the signal acquisition processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal, performs frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, then performs calculations on each sub-frequency band signal obtained by filtering using the first algorithm to obtain intensity information for each sub-frequency band signal, adjusts the attenuation ratio of the first attenuator and the second attenuator based on the intensity information, and equalizes the power of the sub-frequency bands. The detection step of the optical time-domain reflectometer includes the following steps: the signal acquisition processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal, performs frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, performs calculations on each sub-frequency band signal using the first algorithm to obtain phase information for each sub-frequency band signal, averages and superimposes the phase information to remove the effects of interference fading.

[0015] Preferably, frequency band division filtering is performed on the first beat frequency electrical signal and the second beat frequency electrical signal. The method includes performing frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal via an intermediate frequency filter whose center frequencies are the operating frequency f1 of the first modulator, the operating frequency f2 of the second modulator, and the operating frequency f1 of the first modulator + the operating frequency f2 of the second modulator, respectively, thereby obtaining a plurality of sub-frequency band signals.

[0016] Preferably, in the calibration stage of the optical time domain reflectometer, the signal collection and processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal, performs frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, and then uses the first algorithm to perform calculations on each sub-frequency band signal obtained by filtering, obtains the intensity information of each sub-frequency band signal, adjusts the attenuation ratios of the first attenuator and the second attenuator based on the intensity information, and performs power equalization of the sub-frequency band. Specifically, In the calibration stage of the optical time domain reflectometer, the signal collection and processing unit collects the first beat frequency electrical signal {x(k); k = 1,..., m} and the second beat frequency electrical signal {y(k); k = 1,..., m}, where k represents the number of sampling points and m represents the maximum number of sampling points, sort all the first beat frequency electrical signals and the second beat frequency electrical signals according to time and make them correspond one-to-one, perform sub-frequency band filtering on each of the first beat frequency electrical signal and the second beat frequency electrical signal, perform Hilbert transform on each point of the first beat frequency electrical signal and the second beat frequency electrical signal respectively, obtain the sub-frequency band signals {H(x(k)); k = 1,..., m} and {H(y(k)); k = 1,..., m}, and obtain the intensity information Pi based on Equation 1, Equation 2, and Equation 3,

Equation

Equation

Equation

Equation

Number

[0017] Preferably, after performing frequency - band division filtering on the first beat - frequency electrical signal and the second beat - frequency electrical signal, calculations are performed on each sub - frequency band signal using the first algorithm to obtain the phase information of each sub - frequency band signal, and the phase information is averaged and superimposed to remove the influence of interference fading. Specifically, calculating the Hilbert transform for each point of the first beat - frequency electrical signal and the second beat - frequency electrical signal to obtain the Hilbert transform sequence in the X state and the Hilbert transform sequence in the Y state respectively; calculating the arctangent of the Hilbert transform sequence in the X state and the Hilbert transform sequence in the Y state respectively, averaging and superimposing them to obtain the final acoustic signal output.

[0018] Preferably, calculating the arctangent of the Hilbert transform sequence in the X state and the Hilbert transform sequence in the Y state respectively, averaging and superimposing them to obtain the final acoustic signal output is obtaining the arctangent of the Hilbert transform sequence in the X state and the Hilbert transform sequence in the Y state respectively based on Equation 6 and Equation 7;

Number

Number

number

[0019] To more clearly illustrate the embodiments of this disclosure or the technical solutions in the existing art, the following is a brief introduction to the drawings that may be used in the description of the embodiments or the existing art. Clearly, the drawings in the following description are only a few embodiments of this disclosure, and those skilled in the art can obtain other drawings based on these without any creative work.

[0020] [Figure 1] This is a schematic diagram of an existing configuration of an optical time-domain reflectometer provided in the embodiments of this disclosure. [Figure 2] This is a schematic diagram of the configuration of an optical time-domain reflectometer provided in the embodiments of this disclosure. [Figure 3] This is a schematic diagram of the configuration of a frequency division multiplex modulator for an optical time-domain reflectometer provided in the embodiments of this disclosure. [Figure 4] This is a schematic diagram of the specific configuration of an optical time-domain reflectometer provided in the embodiments of this disclosure. [Figure 5] This flowchart shows a method for eliminating interference fading provided by embodiments of the present disclosure. [Figure 6] This is a schematic diagram of the intermediate frequency signal intensity of a method for eliminating interference fading provided in the embodiments of this disclosure. [Figure 7] This is a schematic diagram of frequency band division filtering for a beat frequency electrical signal, which is a method for removing interference fading provided in the embodiments of this disclosure. [Figure 8] This flowchart shows the processing of the signal acquisition processing unit in the detection step of the method for eliminating interference fading provided in the embodiments of this disclosure. [Figure 9]This is a schematic diagram of the superposition of the interference fading removal method provided in the embodiments of this disclosure onto a frequency band divided signal. [Modes for carrying out the invention]

[0021] To further clarify the purpose, technical solutions, and advantages of this disclosure, the disclosure will be described in more detail below, in conjunction with the drawings and examples. The specific examples described herein are for illustrative purposes only and should not be understood as limiting the disclosure.

[0022] In this disclosure, terms such as “First,” “Second,” etc., are used for illustrative purposes only and should not be understood as indicating or implying relative importance or implicitly specifying the number of technical features being described. Accordingly, features limited by “First,” “Second,” etc., may explicitly or implicitly include one or more such features. In the descriptions of this disclosure, unless otherwise stated, “plural” means two or more.

[0023] In this disclosure, unless otherwise explicitly stated or limited, the term “connection” should be understood broadly, for example, “connection” may be a fixed connection, a detachable connection, or an integral part of a connection; it may be a direct connection or an indirect connection via an intermediate medium; and the technical features of the embodiments of this disclosure described below may be combined with each other insofar as they do not conflict with each other.

[0024] Example 1:

[0025] Phase-demodulated Φ-OTDR systems can achieve quantitative reconstruction of disturbance signals because the phase signal and disturbance signal are linearly correlated. Currently, most phase-demodulated Φ-OTDR systems employ coherent detection technology. The basic principle is to inject a pulsed signal from a narrow-linewidth light source into a sensing optical fiber, mix the reflected Rayleigh backscattering (RBS) signal with the local signal for coherent detection, and obtain the phase information of the RBS using an appropriate demodulation algorithm. The demodulated phase information is input to a signal processing unit for pattern recognition and event detection.

[0026] In this phase-demodulated Φ-OTDR system, interference fading inevitably occurs, significantly degrading the signal-to-noise ratio at the fading point and consequently causing serious distortion in the sensing information. Existing methods to eliminate interference fading effectively use frequency division multiplexing technology to perform phase reconstruction with probe pulse signals of different optical frequencies. However, obtaining probe pulse signals of three different optical frequencies requires relatively high hardware costs in existing technologies, necessitating pulse modulation using three different frequency acousto-optic modulators to achieve three different frequency optical pulse outputs.

[0027] To reduce costs and obtain optical pulse outputs of multiple frequencies simultaneously, this embodiment provides an optical time-domain reflectometer, as shown in Figure 2, which is connected to a sensing optical fiber, and which includes a laser device, a frequency division multiplex modulator, a circulator, and a signal processing device. As shown in Figure 3, the frequency division multiplex modulator includes a first modulator, a second modulator, a first attenuator, and a second attenuator. The laser device is used to generate a coherent light source, which passes sequentially through the first modulator and the first attenuator to acquire a first pulse optical signal, which passes sequentially through the first modulator and the second modulator to acquire a second pulse optical signal, which passes sequentially through the second attenuator and the second modulator to acquire a third pulse optical signal. 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 light signals based on the first pulsed light signal, the second pulsed light signal, and the third pulsed light signal. The reflected light signals are transmitted to the signal processing device via the circulator for processing.

[0028] Here, the laser device may be a narrow-linewidth laser, and the sensing optical fiber may be a non-polarized single-mode optical fiber with a length of 50 kilometers. The first modulator and the second modulator may both be acousto-optic modulators, and acousto-optic modulation is an external modulation technique, and a device that controls and changes the laser beam intensity is usually called a modulator. The modulated signal acts on the transducer in the form of an electrical signal (amplitude modulation), which is further converted into a wave field that changes in the form of an electrical signal, and as the light wave passes through the medium, the optical carrier is modulated to become an intensity-modulated wave that "carries" "information".

[0029] The first port of the circulator is used to receive the first pulsed optical signal, the second pulsed optical signal, and the third pulsed optical signal, respectively. The second port of the circulator is used to transmit the first pulsed optical signal, the second pulsed optical signal, and the third pulsed optical signal to the sensing optical fiber. The reflected optical signals of the first pulsed optical signal, the second pulsed optical signal, and the third pulsed optical signal are acquired in the sensing optical fiber. The second port of the circulator is also used to receive the reflected optical signal, and transmits the reflected optical signal to the signal processing device via the third port of the circulator, where the signal processing device performs the corresponding processing on the reflected optical signal. The main processes of the corresponding processing performed by the signal processing device on the reflected optical signal will be described later.

[0030] Compared to existing technologies, the beneficial effects of this embodiment are, on the one hand, that this disclosure can realize the output of multiple pulsed optical signals by different combinations of the first modulator, second modulator, first attenuator, and second attenuator. Compared to existing technologies, hardware costs are reduced, as this disclosure can realize pulse output of three frequencies with only two acousto-optic modulators, whereas existing technologies require three acousto-optic modulators, thus simplifying hardware design and reducing costs. On the other hand, the Φ-OTDR system that performs detection based on the first pulsed optical signal, second pulsed optical signal, and third pulsed optical signal can withstand signal degradation and noise better, improving signal quality and system performance, and eliminating the effects of interference fading.

[0031] Next, the configuration of the optical time-domain reflectometer will be described in detail. In a preferred embodiment, as shown in Figure 4, the optical time-domain reflectometer further includes a first coupler, a first amplifier, and a second amplifier. The input terminal of the first coupler is connected to the laser device. One of the output terminals of the first coupler is connected to the input terminal of a frequency division multiplex modulator. The other output terminal of the first coupler is connected to the signal processing device. The input terminal of the first amplifier is connected to the output terminal of the frequency division multiplex modulator. The output terminal of the first amplifier is connected to the first port of the circulator. The input terminal of the second amplifier is connected to the third port of the circulator. The output terminal of the second amplifier is connected to the signal processing device.

[0032] 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 multiplex modulator, and the second optical signal is transmitted to the signal processing device. 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 signals are transmitted to the circulator. The second amplifier is used to amplify the reflected optical signal, and the amplified optical signal is transmitted to the signal processing device.

[0033] The role of the first coupler is to split the coherent light source into two parts. The first optical signal is transmitted to a frequency division multiplexer, which is used to modulate the optical signal, enabling the transmission of data of multiple different frequencies within the optical fiber. The second optical signal is transmitted to a signal processing unit, which is used to analyze the second optical signal and the reflected optical signal together, enabling the analysis of the characteristics of the optical fiber link and the identification of fault locations. The role of the first amplifier is to amplify the first pulse optical signal, the second pulse optical signal, and the third pulse optical signal, and the amplified signal is transmitted to a circulator. The circulator separates the incoming and outgoing signals, ensuring that the signals flow in the correct direction. The second amplifier is used to amplify the reflected optical signal that has been reflected back, improving the detection sensitivity of the signal processing unit and enabling further analysis. The Φ-OTDR can identify fault locations, losses, and other important characteristics within the optical fiber link by transmitting pulse optical signals and analyzing the reflected optical signals that have been reflected back.

[0034] In order to obtain the first pulse optical signal, the second pulse optical signal, and the third pulse optical signal, in a preferred embodiment, referring to Figure 3, the frequency division multiplex modulator further includes a second coupler, a third coupler, a fourth coupler, and a fifth coupler.

[0035] 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.

[0036] 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 pulse 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 pulse 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 pulse optical signal.

[0037] Here, the path for acquiring 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 acquire the first pulsed optical signal.

[0038] Path for acquiring the second pulse optical signal: The third optical signal passes sequentially through the first modulator and the third coupler, then through the fifth coupler, is further modulated by the second modulator, and finally passes through the fourth coupler to acquire the second pulse optical signal.

[0039] The path for acquiring 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 acquire the third pulse optical signal.

[0040] The frequency division multiplexer acquires multiple pulsed optical signals having different frequencies. This is crucial for achieving efficient frequency division multiplexed optical communication, enabling the simultaneous transmission of multiple different data streams on the same optical fiber, with each stream corresponding to different optical signal characteristics. This is used to eliminate the effects of subsequent interference fading. After the frequency division multiplexer acquires multiple pulsed optical signals having different frequencies, the corresponding reflected optical signals for each pulsed optical signal (i.e., the first pulsed optical signal, the second pulsed optical signal, and the third pulsed optical signal) are acquired via a circulator. Subsequently, the reflected optical signals are processed by the signal processing device to eliminate the effects of interference fading, and at the same time, by analyzing the reflected optical signals that have been reflected back, fault locations, losses, or other important characteristics in the optical fiber link can be identified. Specific analysis methods will be described later.

[0041] In a preferred embodiment, referring to Figure 4, the polarization diversity coherent receiving unit is used to split the reflected light signal into two orthogonal X-polarization state light signals and Y-polarization state light signals, mix the X-polarization state light signal and the second light signal to obtain a first beat frequency electrical signal in the X-polarization state, and mix the Y-polarization state light signal and the second light signal to obtain a second beat frequency electrical signal in the Y-polarization state. The signal acquisition processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal during the calibration stage of the optical time-domain reflectometer, performs frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal to obtain a plurality of sub-frequency band signals, obtains intensity information of each sub-frequency band signal based on a first algorithm, and adjusts the attenuation ratio of the first attenuator and the second attenuator based on the intensity information. The signal acquisition processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal during the detection stage of the optical time-domain reflectometer, performs frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal to acquire a plurality of sub-frequency band signals, acquires the phase signal of each sub-frequency band signal via the first algorithm, and averages and superimposes them in order to obtain a demodulated phase signal sequence.

[0042] The polarization diversity coherent receiving device employs a polarization beam splitter to acquire the first beat frequency electrical signal and the second beat frequency electrical signal. It splits the received RBS signal (i.e., reflected light signal) into two orthogonal X-polarization states and Y-polarization states, and coherently mixes each with the input local light (i.e., the second light signal) to perform balanced reception. Its operating bandwidth is AC-400M.

[0043] The optical time-domain reflectometer includes a calibration stage and a detection stage. In the calibration stage, the signal acquisition processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal from the polarization diversity coherent receiving unit, acquires intensity information for each sub-frequency band signal using a Hilbert intensity demodulation algorithm (i.e., the first algorithm), adjusts the first and second attenuators in the frequency division multiplex modulator based on the intensity information for each sub-frequency band signal, and performs power equalization for the intensity information of each sub-frequency band signal. The specific process will be described later.

[0044] In the detection stage, the signal acquisition processing unit receives a first beat frequency electrical signal and a second beat frequency electrical signal from the polarization diversity coherent receiving unit, acquires phase signals of the two polarization states using the Hilbert phase demodulation algorithm, averages and superimposes them, and obtains a demodulated phase signal sequence. The specific process will be described later. Here, the sampling rate of the signal acquisition processing unit is 245-255 MSPS, and the quantization resolution is 12-16 bits.

[0045] In a preferred embodiment, the operating frequency of the first modulator is f1 and the operating frequency of the second modulator is f2, where the sum of f1 + f2 is less than the operating bandwidth of the polarization diversity coherent receiving unit, the sum of f1 + f2 is less than the operating bandwidth of the signal acquisition processing unit, and f1 is not equal to f2.

[0046] As can be seen from the configuration of the frequency division multiplexer shown in Figure 3, the purpose of this setting is to effectively distinguish between the signals of the two paths, which pass through the first modulator and the second modulator, in the frequency domain, and to avoid interference due to frequency overlap. By ensuring that the sum of f1 and f2 is smaller than the operating bandwidth of the polarization diversity coherent receiving unit and the signal acquisition processing unit, frequency aliasing can be avoided and it can be guaranteed that the signals are accurately analyzed and processed. At the same time, the condition that f1 is not equal to f2 ensures that the signals of the two paths have different frequency characteristics, making it easier to distinguish them in the frequency domain.

[0047] Example 2:

[0048] While Example 1 proposed an optical time-domain reflectometer, this embodiment proposes a method for eliminating interference fading, and as shown in Figure 5, the signal processing device includes a polarization diversity coherent receiving unit and a signal acquisition processing unit.

[0049] Step 101: The polarization diversity coherent receiving unit performs balanced reception by mixing 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, and obtains the first beat frequency electrical signal for the X-polarization state and the second beat frequency electrical signal for the Y-polarization state, respectively.

[0050] The polarization diversity coherent receiving device employs a polarization beam splitter to acquire the first beat frequency electrical signal and the second beat frequency electrical signal. It splits the received RBS signal (i.e., reflected light signal) into two orthogonal X-polarization states and Y-polarization states, and coherently mixes each with the input local light (i.e., the second light signal) to perform balanced reception. Its operating bandwidth is AC-400M.

[0051] Step 102: In the calibration stage of the optical time-domain reflectometer, the signal acquisition processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal, performs frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, then performs calculations on each sub-frequency band signal obtained by filtering using the first algorithm to obtain intensity information for each sub-frequency band signal, adjusts the attenuation ratio of the first attenuator and the second attenuator based on the intensity information, and equalizes the power of the sub-frequency bands.

[0052] Here, performing frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal involves performing frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal through intermediate frequency filters whose center frequencies are the operating frequency f1 of the first modulator, the operating frequency f2 of the second modulator, and the operating frequency f1 of the first modulator + the operating frequency f2 of the second modulator, respectively, thereby obtaining multiple sub-frequency band signals. That is, the first beat frequency electrical signal is filtered through intermediate frequency filters with different center frequencies, and the second beat frequency electrical signal is also filtered through intermediate frequency filters with different center frequencies. As shown in Figure 6, the center frequencies of the intermediate frequency filters are 40 MHz, 80 MHz, and 120 MHz, respectively.

[0053] As shown in Figure 7, frequency band division filtering is performed on the first beat frequency electrical signal and the second beat frequency electrical signal, respectively. Each signal is decomposed into multiple sub-frequency bands, each covering a different frequency range. The purpose of this is to analyze the signal in more detail at different frequencies.

[0054] The specific method involves the signal acquisition processing unit acquiring a first beat frequency electrical signal {x(k); k=1,...,m} and a second beat frequency electrical signal {y(k); k=1,...,m} during the calibration stage of the optical time-domain reflectometer.

[0055] Here, k represents the number of sampling points, and m represents the maximum number of sampling points.

[0056] All first-beat frequency electrical signals and second-beat frequency electrical signals are sorted in time and matched one-to-one.

[0057] Sub-frequency band filtering is performed on the first beat frequency electrical signal and the second beat frequency electrical signal, and a Hilbert transform is performed on each of the first beat frequency electrical signal and the second beat frequency electrical signal point by point to obtain sub-frequency band signals {H(x(k));k=1,...,m} and {H(y(k));k=1,...,m}, and the intensity information is obtained based on equations 1, 2, and 3.

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[0058] Based on equations 4 and 5, the attenuation ratio of the first and second attenuators is adjusted to equalize the power in the sub-frequency band.

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[0059] Here, the possible values ​​of i are 1, 2, and 3, where i represents the sequence number of a different sub-frequency band signal, m represents the maximum number of sampling points, P1, P2, and P3 represent intensity information corresponding to different sub-frequency band signals, V1 represents the attenuation ratio of the first attenuator, and V2 represents the attenuation ratio of the second attenuator.

[0060] Here, 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.

[0061] By adjusting the attenuation ratio of the first and second attenuators, the power levels of the signals are matched across different frequency bands, ensuring that specific needs are met. This is used to ensure that the optical time-domain reflectometer operates with the same efficiency and accuracy across different frequency bands. Through such a calibration process, the Φ-OTDR can ensure that the measurement results across each frequency band are accurate and consistent, thereby improving the overall performance and reliability of the system. This is extremely important for subsequent optical fiber detection and fault diagnosis work, because signals in different frequency bands may exhibit different reflection characteristics due to different properties in the optical fiber.

[0062] Step 103: In the detection stage of the optical time-domain reflectometer, the signal acquisition processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal, performs frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, then performs calculations on each sub-frequency band signal using the first algorithm to obtain phase information for each sub-frequency band signal, averages and superimposes the phase information to eliminate the effects of interference fading.

[0063] In the detection stage, the signal acquisition processing unit first similarly acquires a first beat frequency electrical signal {x(k); k=1,...,125000} and a second beat frequency electrical signal {y(k); k=1,...,125000}. Here, k represents the number of sampling points, the value of 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 first beat frequency electrical signals and second beat frequency electrical signals are sorted in time and matched one-to-one.

[0064] In a preferred embodiment, as shown in Figures 8 and 9, step 103 specifically includes steps 1031 and 1032.

[0065] Step 1031: A point-by-point Hilbert transform is calculated for the first beat frequency electrical signal and the second beat frequency electrical signal to obtain the Hilbert transform sequence for state X and the Hilbert transform sequence for state Y, respectively.

[0066] Here, sub-band filtering is performed on the first beat frequency electrical signal and the second beat frequency electrical signal, respectively, with the center frequencies of the intermediate frequency filters being 40MHz, 80MHz, and 120MHz. The value of m is taken as 125000, and its Hilbert transform is calculated for each point to obtain the Hilbert transform sequence {H(x(k));k=1,...,125000} for the X state and the Hilbert transform sequence {H(y(k));k=1,...,125000} for the Y state.

[0067] Step 1032: Calculate the arctangent of the Hilbert transform sequence for state X and the Hilbert transform sequence for state Y, average them, and superimpose them to obtain the final acoustic signal output.

[0068] Based on equations 6 and 7, the arctangents of the Hilbert transform sequences for state X and state Y are obtained, respectively.

number

number

[0069] Based on equation 8, the arctangent φ of the Hilbert transform sequence of the X state is obtained. ix and the arctangent φ of the Hilbert transform sequence of the Y state iy The signals are averaged and superimposed to obtain the final acoustic signal output.

number

[0070] Here, the possible values ​​of i are 1, 2, and 3, where i represents the sequence number of a different sub-frequency band signal.

[0071] By further analyzing and processing the final acoustic signal output, the characteristics of the signal detected by the optical time-domain reflectometer can be identified. Simultaneously, by averaging and superimposing phase information from different frequency bands, the effects of interference fading can be effectively reduced, improving signal quality and reliability. Leveraging the advantages of multi-frequency band processing and phase information analysis, the accuracy and efficiency of the optical time-domain reflectometer in optical fiber link detection and fault diagnosis can be improved. Through such advanced signal processing, damage, breaks, or other problems in the optical fiber can be identified more accurately.

[0072] The specific configuration of the aforementioned optical time-domain reflectometer is described in Example 1, and will not be explained in this example.

[0073] The foregoing are merely preferred embodiments of the Disclosure and do not limit the Disclosure. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the Disclosure should also be included within the scope of the Disclosure.

Claims

1. A time-domain optical reflectometer, wherein the time-domain optical reflectometer is connected to a sensing optical fiber and includes a laser device, a frequency division multiplex modulator, a circulator, and a signal processing device, wherein the frequency division multiplex modulator includes a first modulator, a second modulator, a first attenuator, and a second attenuator. The laser device is used to generate a coherent light source, the coherent light source sequentially passes through the first modulator and the first attenuator to acquire a first pulsed optical signal, the coherent light source sequentially passes through the first modulator and the second modulator to acquire a second pulsed optical signal, and the coherent light source sequentially passes through the second attenuator and the second modulator to acquire a third pulsed optical signal. The circulator is used to transmit the first pulsed optical signal, the second pulsed optical signal, and the third pulsed optical signal to the sensing optical fiber, the sensing optical fiber is used to generate reflected optical signals corresponding to the first pulsed optical signal, the second pulsed optical signal, and the third pulsed optical signal, and the reflected optical signals are transmitted via the circulator to the signal processing device for processing. A time-domain optical reflectometer characterized by the following features.

2. The first port of the circulator is used to receive the first pulsed optical signal, the second pulsed optical signal, and the third pulsed optical signal, respectively, and the second port of the circulator is used to transmit the first pulsed optical signal, the second pulsed optical signal, and the third pulsed optical signal to the sensing optical fiber. The sensing optical fiber acquires the reflected light signals of the first pulse light signal, the second pulse light signal, and the third pulse light signal. The second port of the circulator is also used to receive the reflected light signals. The reflected light signals are transmitted to the signal processing device via the third port of the circulator, and the signal processing device performs the corresponding processing on the reflected light signals. The optical time-domain reflectometer according to feature 1.

3. The aforementioned time-domain optical reflectometer further includes a first coupler, a first amplifier, and a second amplifier. 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 multiplex 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, and the amplified optical signals are transmitted to the circulator. The second amplifier is used to amplify the reflected light signal, and the amplified light signal is transmitted to the signal processing device. The optical time-domain reflectometer according to feature 1.

4. The input terminal of the first coupler is connected to the laser device, one of the output terminals of the first coupler is connected to the input terminal of the frequency division multiplex modulator, and the other output terminal of the first coupler is connected to the signal processing device. The input terminal of the first amplifier is connected to the output terminal of the frequency division multiplex modulator, and the output terminal of the first amplifier is connected to the first port of the circulator. The input terminal of the second amplifier is connected to the third port of the circulator, and the output terminal of the second amplifier is connected to the signal processing device. The optical time-domain reflectometer according to feature 3.

5. The frequency division multiplex modulator further includes a second coupler, a third coupler, a fourth coupler, and a fifth coupler. The second coupler is used to split the first optical signal into a third optical signal and a fourth optical signal, and the third optical signal passes sequentially through the first modulator, the third coupler, the first attenuator, and the fourth coupler to acquire the first pulse 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 acquire the second pulse optical signal. The fourth optical signal passes sequentially through the second attenuator, the fifth coupler, the second modulator, and the fourth coupler to acquire the third pulse optical signal. The optical time-domain reflectometer according to feature 3.

6. 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, and 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, and the output terminal of the second modulator is connected to the other input terminal of the fourth coupler. The optical time-domain reflectometer according to feature 5.

7. The path for acquiring the first pulsed optical signal: 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 acquire the first pulsed optical signal. Path for acquiring the second pulse optical signal: The third optical signal passes sequentially through the first modulator and the third coupler, then through the fifth coupler, is further modulated by the second modulator, and finally passes through the fourth coupler to acquire the second pulse optical signal. The path for acquiring the third pulsed 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 acquire the third pulsed optical signal. The optical time-domain reflectometer according to feature 5.

8. The signal processing device includes a polarization diversity coherent receiving unit and a signal acquisition processing unit. The polarization diversity coherent receiving unit is used to split the reflected light signal into two orthogonal X-polarized state light signals and Y-polarized state light signals, mix the X-polarized state light signal and the second light signal to obtain a first beat frequency electrical signal in the X-polarized state, and mix the Y-polarized state light signal and the second light signal to obtain a second beat frequency electrical signal in the Y-polarized state. The signal acquisition processing unit, in the calibration stage of the optical time-domain reflectometer, receives the first beat frequency electrical signal and the second beat frequency electrical signal, performs frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal to acquire a plurality of sub-frequency band signals, acquires intensity information for each sub-frequency band signal based on the first algorithm, and adjusts the attenuation ratio of the first attenuator and the second attenuator based on the intensity information. The signal acquisition processing unit, in the detection stage of the optical time-domain reflectometer, receives the first beat frequency electrical signal and the second beat frequency electrical signal, performs frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal to acquire a plurality of sub-frequency band signals, acquires the phase signal of each sub-frequency band signal via the first algorithm, averages and superimposes them to obtain a demodulated phase signal sequence. The optical time-domain reflectometer according to feature 3.

9. The sampling rate of the signal acquisition processing unit is 245 to 255 MSPS, and the quantization resolution is 12 to 16 bits. The optical time-domain reflectometer according to feature 8.

10. Let f1 be the operating frequency of the first modulator and f2 be the operating frequency of the second modulator, where the sum of f1 + f2 is less than the operating bandwidth of the polarization diversity coherent receiving unit, the sum of f1 + f2 is less than the operating bandwidth of the signal acquisition processing unit, and f1 is not equal to f2. The optical time-domain reflectometer according to feature 8.

11. A method for removing interference fading, wherein the method is applied to an optical time-domain reflectometer according to any one of claims 1 to 10, and the signal processing device includes a polarization diversity coherent receiving unit and a signal acquisition processing unit, and the method for removing interference fading is: The polarization diversity coherent receiving unit performs balanced reception by mixing the reflected light signal with a second optical signal separated from the coherent light source according to two orthogonal X-polarization states and Y-polarization states, respectively, and acquires a first beat frequency electrical signal in the X-polarization state and a second beat frequency electrical signal in the Y-polarization state, respectively. In the calibration stage of the optical time-domain reflectometer, the signal acquisition processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal, performs frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, then performs calculations on each sub-frequency band signal obtained by filtering using the first algorithm, obtains intensity information for each sub-frequency band signal, adjusts the attenuation ratio of the first attenuator and the second attenuator based on the intensity information, and equalizes the power of the sub-frequency bands. The detection step of the optical time-domain reflectometer includes the following steps: the signal acquisition processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal, performs frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, performs calculations on each sub-frequency band signal using the first algorithm to obtain phase information for each sub-frequency band signal, averages and superimposes the phase information to remove the effects of interference fading. A method for eliminating interference fading, characterized by the following features.

12. Performing frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal is, This includes performing frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal through an intermediate frequency filter whose center frequencies are the operating frequency f1 of the first modulator, the operating frequency f2 of the second modulator, and the operating frequency f1 of the first modulator + the operating frequency f2 of the second modulator, respectively, thereby obtaining a plurality of sub-frequency band signals. The method for eliminating interference fading according to feature 11.

13. In the calibration stage of the optical time-domain reflectometer, the signal acquisition processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal, performs frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, then performs calculations on each sub-frequency band signal obtained by filtering using the first algorithm, obtains intensity information for each sub-frequency band signal, and adjusts the attenuation ratio of the first attenuator and the second attenuator based on the intensity information to equalize the power of the sub-frequency bands. Specifically, this involves: In the calibration step of the optical time-domain reflectometer, the signal acquisition 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}, Here, k represents the number of sampling points, and m represents the maximum number of sampling points. The first beat frequency electrical signals and the second beat frequency electrical signals are sorted in time and made to correspond one-to-one, Sub-frequency band filtering is performed on the first beat frequency electrical signal and the second beat frequency electrical signal, and a Hilbert transform is performed on each of the first beat frequency electrical signal and the second beat frequency electrical signal point by point to obtain sub-frequency band signals {H(x(k)); k=1, ..., m} and {H(y(k)); k=1, ..., m}, and the intensity information Pi is obtained based on equations 1, 2, and 3. [Math 1] [Math 2] [Math 3] Based on equations 4 and 5, the attenuation ratio of the first and second attenuators is adjusted to equalize the power in the sub-frequency band. [Math 4] [Math 5] Here, the possible values ​​of i are 1, 2, and 3, i represents the sequence number of a different sub-frequency band signal, m represents the maximum number of sampling points, and P １ , P ２ , and P ３ Each of these indicates intensity information corresponding to a different sub-frequency band signal, V １ V represents the attenuation ratio of the first attenuator, ２ This indicates the attenuation ratio of the second attenuator. The method for eliminating interference fading according to feature 11.

14. Specifically, after performing frequency band division filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, calculations are performed on each sub-frequency band signal using the first algorithm to obtain phase information for each sub-frequency band signal, and the phase information is averaged and superimposed to remove the effects of interference fading. The Hilbert transform is calculated point by point for the first beat frequency electrical signal and the second beat frequency electrical signal, and the Hilbert transform sequence for state X and state Y are obtained, respectively. This includes calculating the arctangent of the Hilbert transform sequence of state X and the Hilbert transform sequence of state Y, averaging them, superimposing them, and obtaining the final acoustic signal output. The method for eliminating interference fading according to feature 11.

15. Calculating the arctangents of the Hilbert transform sequences for state X and state Y, averaging them, and superimposing them to obtain the final acoustic signal output is: Based on equations 6 and 7, the arctangents of the Hilbert transform sequence of state X and state Y are obtained, respectively. [Math 6] [Number 7] Based on equation 8, the arctangent φ of the Hilbert transform sequence of the X state is obtained. ix and the arctangent φ of the Hilbert transform sequence of the Y state iy The steps include averaging and superimposing the signals to obtain the final acoustic signal output, [Number 8] Here, the possible values ​​of i are 1, 2, and 3, and i represents the sequence number of a sub-frequency band signal with a different value. The method for eliminating interference fading according to feature 14.

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