Optical time-domain reflectometer, and method for eliminating interference fading

Abstract

The present invention relates to the technical field of optical fiber sensing, and particularly relates to an optical time-domain reflectometer, and a method for eliminating interference fading. The optical time-domain reflectometer comprises: a laser device, a frequency-division multiplexing modulator, a circulator and a signal processing apparatus, wherein the frequency-division multiplexing modulator comprises a first modulator, a second modulator, a first attenuator and a second attenuator; the laser device is used for generating a coherent light source, and the coherent light source sequentially passes through the first modulator and the first attenuator, so as to obtain a first pulsed light signal; the coherent light source sequentially passes through the first modulator and the second modulator, so as to obtain a second pulsed light signal; and the coherent light source sequentially passes through the second attenuator and the second modulator, so as to obtain a third pulsed light signal. Therefore, by means of different combinations of the first modulator, the second modulator, the first attenuator and the second attenuator, multiple pulsed light signals can be outputted; and compared with the prior art, hardware costs are saved.

Description

Optical time domain reflectometer and method for eliminating interference fading

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority from the following patent applications:

[0003] (1) A Chinese patent application entitled “An Optical Time Domain Reflectometer and Method for Eliminating Interference Fading”, filed with the Chinese Patent Office on January 3, 2024, with application number 202410008182.0; Technical Field

[0004] The present invention relates to the field of optical fiber sensing technology, in particular to an optical time domain reflectometer and a method for eliminating interference fading. Background Art

[0005] Distributed fiber optic sensing technology based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) offers numerous advantages, including high measurement accuracy, fast response, long monitoring distance, and resistance to electromagnetic interference. It has been widely used in perimeter security, rail transit, and oil and gas pipeline monitoring. The principle is that a disturbance signal applied to an optical fiber causes a change in the fiber's refractive index, which in turn changes the optical path and the phase of the probe light. Φ-OTDRs are categorized into intensity-demodulation and phase-demodulation types based on their demodulation methods.

[0006] In phase-demodulation Ф-OTDR systems, interferometric fading inevitably occurs, significantly degrading the signal-to-noise ratio at fading points and causing severe distortion of the sensor information. Specifically, this manifests as the inevitable minimum of Rayleigh scattering intensity. When superimposed with detector intensity noise, the demodulation algorithm results in abnormal phase results. It is often difficult to distinguish phase jumps caused by interferometric fading from those resulting from actual disturbances. Consequently, Ф-OTDR systems based on a single phase signal are subject to issues such as false alarms due to interferometric fading.

[0007] For coherent detection Ф-OTDR systems, the main method for eliminating interferometric fading is to employ frequency division multiplexing technology and perform phase reconstruction using probe pulses of different optical frequencies. The specific scheme is shown in Figure 1. Three acousto-optic modulators of different frequencies are used for pulse modulation to achieve optical pulse output at three frequencies. This hardware is relatively complex and requires multiple frequency shift modulators.

[0008] In view of this, overcoming the defects of the prior art is an urgent problem to be solved in this technical field.

[0009] Application Contents

[0010] The technical problem to be solved by the present invention is: how to reduce the number of acousto-optic modulators, realize the output of multiple optical pulse signals with fewer acousto-optic modulators, and eliminate the influence of interference fading.

[0011] The present invention adopts the following technical solutions:

[0012] In a first aspect, an optical time domain reflectometer is provided, the optical time domain reflectometer being connected to a sensing optical fiber and comprising: a laser, a frequency division multiplexing modulator, a circulator, and a signal processing device, the frequency division multiplexing modulator comprising a first modulator, a second modulator, a first attenuator, and a second attenuator;

[0013] The laser is used to generate a coherent light source, the coherent light source sequentially passes through the first modulator and the first attenuator to obtain a first pulse light signal; the coherent light source sequentially passes through the first modulator and the second modulator to obtain a second pulse light signal; the coherent light source sequentially passes through the second attenuator and the second modulator to obtain a third pulse light signal;

[0014] 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, and 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 transmit the reflected light signals to the signal processing device for processing through the circulator.

[0015] Preferably, the optical time domain reflectometer further includes a first coupler, a first amplifier and a second amplifier;

[0016] 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;

[0017] 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;

[0018] The second amplifier is used to amplify the reflected light signal, and the amplified light signal is transmitted to the signal processing device.

[0019] Preferably, the frequency division multiplexing modulator further includes a second coupler, a third coupler, a fourth coupler and a fifth coupler;

[0020] 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 through the first modulator, the third coupler, the first attenuator and the fourth coupler in sequence to obtain the first pulse optical signal;

[0021] The third optical signal passes through the first modulator, the third coupler, the fifth coupler, the second modulator and the fourth coupler in sequence to obtain the second pulse optical signal;

[0022] The fourth optical signal passes through the second attenuator, the fifth coupler, the second modulator and the fourth coupler in sequence to obtain the third pulse optical signal.

[0023] Preferably, the signal processing device includes a polarization diversity coherent receiving unit and a signal acquisition and processing unit;

[0024] The polarization diversity coherent receiving unit is used to split the reflected optical signal into two orthogonal X-polarization optical signals and Y-polarization optical signals; mix the X-polarization optical signal with the second optical signal to obtain a first beat frequency electrical signal in the X-polarization state, and mix the Y-polarization optical signal with the second optical signal to obtain a second beat frequency electrical signal in the Y-polarization state;

[0025] The signal acquisition and processing unit is configured to receive the first beat frequency electrical signal and the second beat frequency electrical signal during a calibration phase of the optical time domain reflectometer, perform frequency 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 intensity information of each sub-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;

[0026] 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 a 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 a plurality of sub-band signals, obtain a phase signal of each sub-band signal using the first algorithm, and perform average superposition to obtain a demodulated phase signal sequence.

[0027] 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+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 and processing unit, and f1 is not equal to f2.

[0028] In a second aspect, 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:

[0029] 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 a first beat frequency electrical signal in the X polarization state and a second beat frequency electrical signal in the Y polarization state;

[0030] During a 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 sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, calculates each sub-band signal obtained by filtering using a first algorithm to obtain intensity information of each sub-band signal, and adjusts the attenuation ratio of the first attenuator and the second attenuator according to the intensity information to perform sub-band power equalization;

[0031] During the detection phase of the optical time domain reflectometer, a signal acquisition and processing unit receives the first beat frequency electrical signal and the second beat frequency electrical signal, performs frequency sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, and then uses a first algorithm to calculate each sub-band signal to obtain phase information of each sub-band signal. The phase information is averaged and superimposed to eliminate the influence of interference fading.

[0032] Preferably, performing frequency-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal includes:

[0033] The first beat frequency electrical signal and the second beat frequency electrical signal are subjected to frequency band filtering 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, thereby obtaining multiple sub-band signals.

[0034] Preferably, 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, calculates each sub-band signal obtained by filtering using a first algorithm to obtain intensity information of each sub-band signal, and adjusts the attenuation ratio of the first attenuator and the second attenuator according to the intensity information to perform sub-band power balancing, specifically including:

[0035] 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};

[0036] Where K represents the number of sampling points, and m represents the maximum number of sampling points;

[0037] sorting all first beat frequency electrical signals and second beat frequency electrical signals according to time and making a one-to-one correspondence;

[0038] Sub-band filtering is performed on the first beat frequency electrical signal and the second beat frequency electrical signal respectively, and Hilbert transform is performed on the first beat frequency electrical signal and the second beat frequency electrical signal point by point to obtain sub-band signals {H(x(k)); k=1,....,m} and {H(y(k)); k=1,....,m}, and the intensity information P is obtained according to Formula 1, Formula 2 and Formula 3 i :

[0039] Formula 1 is:

[0040] Formula 2 is:

[0041] Formula 3 is:

[0042] The attenuation ratio of the first attenuator and the second attenuator is adjusted according to Formula 4 and Formula 5 to perform sub-band power equalization:

[0043] Formula 4 is:

[0044] Formula 5 is:

[0045] Among them, i takes values ​​of 1, 2, and 3; i represents the serial number of different sub-band signals; m represents the maximum number of sampling points; P1, P2, and P3 respectively represent the intensity information corresponding to different sub-band signals; V1 represents the attenuation ratio of the first attenuator; and V2 represents the attenuation ratio of the second attenuator.

[0046] Preferably, after performing frequency-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, calculating each sub-band signal using a first algorithm to obtain phase information of each sub-band signal, and averaging and superimposing the phase information to eliminate the influence of interference fading, specifically includes:

[0047] calculating the Hilbert transform of the first beat frequency electrical signal and the second beat frequency electrical signal point by point to obtain an X-state Hilbert transform sequence and a Y-state Hilbert transform sequence, respectively;

[0048] The arc tangent of the Hilbert transform sequence of the X state and the arc tangent of the Hilbert transform sequence of the Y state are calculated respectively, and the average superposition is performed to obtain the final acoustic wave signal output.

[0049] Preferably, the calculating the arc tangent of the Hilbert transform sequence of the X state and the Hilbert transform sequence of the Y state respectively and performing average superposition to obtain the final acoustic wave signal output comprises:

[0050] The inverse tangent of the Hilbert transform sequence of the X state and the Hilbert transform sequence of the Y state are obtained according to Formula 6 and Formula 7 respectively:

[0051] Formula 6 is:

[0052] Formula 7 is:

[0053] According to formula 8, the arc tangent of the Hilbert transform sequence of the X state and the arc tangent of the Hilbert transform sequence of the Y state Perform average superposition to obtain the final sound wave signal output:

[0054] Formula 8 is:

[0055] The value of i is 1, 2, or 3; i represents the sequence number of different sub-band signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0057] FIG1 is a schematic diagram of the existing structure of an optical time domain reflectometer provided by an embodiment of the present invention;

[0058] FIG2 is a schematic structural diagram of an optical time domain reflectometer provided by an embodiment of the present invention;

[0059] 3 is a schematic structural diagram of a frequency division multiplexing modulator of an optical time domain reflectometer provided by an embodiment of the present invention;

[0060] FIG4 is a schematic diagram of the specific structure of an optical time domain reflectometer provided by an embodiment of the present invention;

[0061] FIG5 is a flow chart of a method for eliminating interference fading provided by an embodiment of the present invention;

[0062] FIG6 is a schematic diagram of intermediate frequency signal strength of a method for optically eliminating interference fading provided by an embodiment of the present invention;

[0063] 7 is a schematic diagram of performing frequency band filtering on a beat frequency electrical signal in a method for eliminating interference fading provided by an embodiment of the present invention;

[0064] 8 is a schematic diagram of a processing flow of the signal acquisition and processing unit in the detection phase of a method for eliminating interference fading provided by an embodiment of the present invention;

[0065] FIG9 is a schematic diagram of superimposing sub-band signals in a method for eliminating interference fading provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0066] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present 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 only used to explain the present invention and are not intended to limit the present invention.

[0067] The terms "first," "second," etc., used in this disclosure are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referenced. Thus, a feature identified with "first," "second," etc., may explicitly or implicitly include one or more of such features. In the description of this disclosure, unless otherwise specified, "plurality" means two or more.

[0068] In the present invention, unless otherwise specified or limited, the term "connection" should be understood broadly. For example, "connection" can mean fixed connection, detachable connection, or integration; it can mean direct connection or indirect connection through an intermediary. Furthermore, the technical features involved in the various embodiments of the present invention described below may be combined as long as they do not conflict with each other.

[0069] Example 1:

[0070] Phase-demodulated Φ-OTDR systems can quantitatively restore the disturbance signal because the phase signal is linearly correlated with the disturbance signal. Currently, the vast majority of phase-demodulated Φ-OTDR systems employ coherent detection technology. The basic principle is to modulate a narrow-linewidth light source into a pulse signal and inject it into the sensing fiber. The reflected Rayleigh backscattering (RBS) signal is mixed with the intrinsic signal for coherent detection, and a suitable demodulation algorithm is used to obtain the RBS phase information. The demodulated phase information is input into the signal processing unit for pattern recognition and event detection. In such phase-demodulated Φ-OTDR systems, interferometric fading inevitably occurs, significantly degrading the signal-to-noise ratio at the fading point and causing severe distortion of the sensor information. Existing methods for eliminating interferometric fading employ frequency-division multiplexing technology, which effectively eliminates interferometric fading by reconstructing the phase of detection pulse signals at different optical frequencies. However, in order to obtain detection pulse signals of three different optical frequencies, the hardware cost used in the existing technology is relatively high, and three acousto-optic modulators of different frequencies are required to perform pulse modulation to achieve optical pulse output of three frequencies.

[0071] In order to reduce costs and obtain optical pulse outputs of multiple frequencies at the same time, an optical time domain reflectometer is provided in this embodiment. As shown in Figure 2, the optical time domain reflectometer is connected to a sensing optical fiber. The optical time domain reflectometer includes: a laser, a frequency division multiplexing modulator, a circulator and a signal processing device. As shown in Figure 3, 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, and the coherent light source passes through the first modulator and the first attenuator in sequence to obtain a first pulse light signal; the coherent light source passes through the first modulator and the second modulator in sequence to obtain a second pulse light signal; the coherent light source passes through the second attenuator and the second modulator in sequence 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, and 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, and transmit the reflected light signals to the signal processing device for processing through the circulator.

[0072] The laser can be a narrow-linewidth laser, and the sensing fiber can be a non-polarization-maintaining single-mode fiber with a length of 50 kilometers. Both the first and second modulators can be acousto-optic modulators (AOMs). AOM is an external modulation technique, and the device that controls the intensity variation of the laser beam is generally referred to as a modulator. The modulated signal acts on the transducer in the form of an electrical signal (amplitude modulation), which is then converted into a wave field that varies 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 that "carries" information.

[0073] The first port of the circulator is used to receive the first, second, and third pulsed light signals, respectively, and the second port of the circulator is used to transmit the first, second, and third pulsed light signals to the sensing optical fiber. The first, second, and third pulsed light signals generate reflected light signals in the sensing optical fiber. The second port of the circulator is further used to receive the reflected light signals and transmit them to the signal processing device via the third port of the circulator. The signal processing device performs corresponding processing on the reflected light signals. The main process of the signal processing device performing the corresponding processing on the reflected light signals will be described below.

[0074] Compared with the prior art, the present embodiment has the following advantages: First, the present invention can output multiple pulsed optical signals through different combinations of the first modulator, the second modulator, the first attenuator, and the second attenuator, thus reducing hardware costs compared to the prior art; the present invention only requires two acousto-optic modulators to achieve pulse output at three frequencies, while the prior art requires three acousto-optic modulators, simplifying hardware design and reducing costs. Second, the Ф-OTDR system, which performs detection based on the first, second, and third pulsed optical signals, can better resist signal degradation and noise, improve signal quality and system performance, and eliminate the effects of interference fading.

[0075] Next, the structure 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 of the first coupler is connected to the laser, one of the outputs 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 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.

[0077] The function of the first coupler is to split the coherent light source into two parts: the first optical signal is transmitted to the frequency division multiplexing modulator, which is used to modulate the optical signal so as to transmit data of multiple different frequencies in the optical fiber. The second optical signal is transmitted to the signal processing device, which is used to combine the second optical signal and the reflected optical signal for analysis in order to analyze the characteristics of the optical fiber link or locate the fault point. The function 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 signals are transmitted to the circulator. The circulator plays the role of separating the incoming and outgoing signals to ensure that the signals flow in the correct direction. The second amplifier is used to amplify the reflected optical signal to improve the detection sensitivity of the signal processing device for further analysis. By sending pulse optical signals and analyzing the reflected optical signals, the OTDR can determine the fault location, loss and other important characteristics in the optical fiber link.

[0078] In order to obtain the first pulse light signal, the second pulse light signal and the third pulse light signal, in a preferred embodiment, referring to FIG3 , the frequency division multiplexing modulator further includes a second coupler, a third coupler, a fourth coupler and a fifth coupler.

[0079] The input end of the second coupler is connected to the first coupler, one output end of the second coupler is connected to the input end of the first modulator, the input end of the third coupler is connected to the output end of the first modulator, one output end of the third coupler is connected to the input end of the first attenuator, the output end of the first attenuator is connected to an input end of the fourth coupler, and the output end of the fourth coupler is connected to the input end of the first amplifier. The other output end of the second coupler is connected to the input end of the second attenuator, the output end of the second attenuator is connected to one input end of the fifth coupler, the other output end of the third coupler is connected to the other input end of the fifth coupler, the output end of the fifth coupler is connected to the input end of the second modulator, and the output end of the second modulator is connected to the other input end 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 through the first modulator, the third coupler, the first attenuator, and the fourth coupler in sequence to obtain the first pulse optical signal; the third optical signal passes through the first modulator, the third coupler, the fifth coupler, the second modulator, and the fourth coupler in sequence to obtain the second pulse optical signal; the fourth optical signal passes through the second attenuator, the fifth coupler, the second modulator, and the fourth coupler in sequence to obtain the third pulse optical signal.

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

[0082] Path for generating the second pulsed optical signal: The third optical signal passes through the first modulator and the third coupler in sequence, then passes through the fifth coupler, is further modulated by the second modulator, and finally passes through the fourth coupler to obtain the second pulsed optical signal.

[0083] Path for generating the third pulse optical signal: the fourth optical signal first passes through the second attenuator, then passes 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 pulse light signals with different frequencies. It is very important to realize efficient frequency division multiplexing optical communication, and realize the simultaneous transmission of multiple different data streams on the same optical fiber, and each stream corresponds to a different optical signal characteristic. So as to facilitate the subsequent elimination of the influence of interference fading. After the frequency division multiplexing modulator generates multiple pulse light signals with different frequencies, the reflected light signals corresponding to each pulse light signal (i.e., the first pulse light signal, the second pulse light signal and the third pulse light signal) are obtained through the circulator. The reflected light signal is then processed by the signal processing device to eliminate the influence of interference fading. At the same time, by analyzing the reflected light signal, the fault location, loss or other important characteristics in the optical fiber link can be determined. The specific analysis method will be described below.

[0085] In a preferred embodiment, referring to FIG4 , the polarization diversity coherent receiving unit is used to divide the reflected light signal into two orthogonal X-polarization state light signals and Y-polarization state light signals; the X-polarization state light signal is mixed with the second light signal to obtain a first beat frequency electrical signal in the X-polarization state, and the Y-polarization state light signal is mixed with the second light signal to obtain a second beat frequency electrical signal in the Y-polarization 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 phase of the optical time domain reflectometer, and to process the first beat frequency electrical signal and the second beat frequency electrical signal. The signal is subjected to frequency band filtering to obtain a plurality of sub-band signals, and intensity information of each sub-band signal is obtained according to a first algorithm, and the attenuation ratio of the first attenuator and the second attenuator is adjusted 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 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 a plurality of sub-band signals, obtain a phase signal of each sub-band signal respectively through the first algorithm, and perform average superposition to obtain a demodulated phase signal sequence.

[0086] 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 performs coherent mixing with the input intrinsic light (i.e., the second light signal) for balanced reception. Its operating bandwidth is AC-400M to obtain the first beat frequency electrical signal and the second beat frequency electrical signal.

[0087] The optical time domain reflectometer includes a calibration phase and a detection phase. During the calibration phase, 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, uses a Hilbert intensity demodulation algorithm (i.e., a first algorithm) to obtain intensity information of each sub-band signal, and adjusts the first attenuator and the second attenuator in the frequency division multiplexing modulator based on the intensity information of each sub-band signal to perform power equalization on the intensity information of each sub-band signal. The specific process will be described in detail below.

[0088] During the detection phase, 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, uses the Hilbert phase demodulation algorithm to obtain phase signals of the two polarization states, and performs average superposition to obtain a demodulated phase signal sequence. The specific process is described below. The sampling rate of the signal acquisition and processing unit is 245 to 255 MSPS, and the quantization resolution is 12 to 16 bits.

[0089] 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+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 and processing unit, and f1 is not equal to f2.

[0090] Based on the structure of the frequency-division multiplexing modulator shown in Figure 3, this configuration aims to effectively distinguish the two signals passing through the first modulator and the second modulator 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 signals can be accurately parsed and processed. Furthermore, 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.

[0091] Example 2:

[0092] In embodiment 1, an optical time domain reflectometer is proposed. In this embodiment, a method for eliminating interference fading is proposed, as shown in FIG5 , including: the signal processing device includes a polarization diversity coherent receiving unit and a signal acquisition and processing unit;

[0093] Step 101: The polarization diversity coherent receiving unit mixes and balances the reflected optical signal with the second optical signal separated by the coherent light source according to two orthogonal X polarization states and Y polarization states, respectively, to obtain a first beat frequency electrical signal in the X polarization state and a second beat frequency electrical signal in the Y polarization state.

[0094] 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 performs coherent mixing with the input intrinsic light (i.e., the second light signal) for balanced reception. Its operating bandwidth is AC-400M to obtain the first beat frequency electrical signal and the second beat frequency electrical signal.

[0095] Step 102: 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 sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, and then uses a first algorithm to calculate each sub-band signal obtained by filtering to obtain intensity information of each sub-band signal. The attenuation ratios of the first attenuator and the second attenuator are adjusted based on the intensity information to perform sub-band power balancing.

[0096] The frequency-division filtering of the first beat frequency electrical signal and the second beat frequency electrical signal comprises: performing frequency-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, thereby obtaining a plurality of sub-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 FIG6 , the center frequencies of the intermediate frequency filters are 40 MHz, 80 MHz, and 120 MHz, respectively.

[0097] As shown in FIG7 , the first beat frequency electrical signal and the second beat frequency electrical signal are subjected to frequency band filtering. Each signal is decomposed into multiple sub-bands, each sub-band covering a different frequency range. This is done to perform a more detailed analysis of the signal at different frequencies. The specific method is as follows:

[0098] 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};

[0099] Where K represents the number of sampling points and m represents the maximum number of sampling points.

[0100] All first beat frequency electrical signals and second beat frequency electrical signals are sorted according to time and are in one-to-one correspondence.

[0101] Sub-band filtering is performed on the first beat frequency electrical signal and the second beat frequency electrical signal respectively, and Hilbert transform is performed on the first beat frequency electrical signal and the second beat frequency electrical signal point by point to obtain sub-band signals {H(x(k)); k=1,....,m} and {H(y(k)); k=1,....,m}, and the intensity information P is obtained according to Formula 1, Formula 2 and Formula 3 i :

[0102] Formula 1 is:

[0103] Formula 2 is:

[0104] Formula 3 is:

[0105] The attenuation ratio of the first attenuator and the second attenuator is adjusted according to Formula 4 and Formula 5 to perform sub-band power equalization:

[0106] Formula 4 is:

[0107] Formula 5 is:

[0108] Among them, i takes values ​​of 1, 2, and 3; i represents the serial number of different sub-band signals; m represents the maximum number of sampling points; P1, P2, and P3 respectively represent the intensity information corresponding to different sub-band signals; V1 represents the attenuation ratio of the first attenuator; and V2 represents the attenuation ratio of the second attenuator.

[0109] Among them, the sampling rate of the signal acquisition unit is 250MSPS, the unit length of each sampling point is about 0.4m, and the total sampling length is 50km.

[0110] By adjusting the attenuation ratio of the first and second attenuators, the power levels of signals across different frequency bands are maintained consistent, meeting specific requirements. This ensures that the optical time domain reflectometer operates with the same efficiency and accuracy across all frequency bands. This calibration process ensures accurate and consistent OTDR measurement results across all frequency bands, thereby improving the performance and reliability of the entire system. This is crucial for subsequent fiber inspection and troubleshooting, as signals in different frequency bands may exhibit different reflection characteristics due to the varying properties of the fiber.

[0111] 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, performs frequency sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, and then uses a first algorithm to calculate each sub-band signal to obtain phase information of each sub-band signal. The phase information is averaged and superimposed to eliminate the influence of interference fading.

[0112] During the detection phase, the signal acquisition and processing unit first collects the first beat frequency electrical signal {x(k); k = 1, ...., 125000} and the second beat frequency electrical signal {y(k); k = 1, ...., 125000}. K represents the number of sampling points, m is 125000, the sampling rate of the signal acquisition unit is 250MSPS, the unit length of each sampling point is approximately 0.4m, and the total sampling length is 50km. All first beat frequency electrical signals and second beat frequency electrical signals are sorted by time and mapped one to one.

[0113] In a preferred embodiment, as shown in FIG8 and FIG9 , step 103 specifically includes:

[0114] Step 1031: Calculate the Hilbert transform of the first beat frequency electrical signal and the second beat frequency electrical signal point by point, obtaining the Hilbert transform sequence of the X state and the Hilbert transform sequence of the Y state, respectively. The first beat frequency electrical signal and the second beat frequency electrical signal are subjected to sub-band filtering, with the center frequencies of the intermediate frequency filters being 40 MHz, 80 MHz, and 120 MHz, respectively. The value of m is 125,000, and the Hilbert transform is calculated point by point, obtaining the Hilbert transform sequence of the X state {H(x(k)); k = 1, ...., 125,000} and the Hilbert transform sequence of the Y state {H(y(k)); k = 1, ...., 125,000}.

[0115] Step 1032: Calculate the arctangent of the Hilbert transform sequence of the X state and the arctangent of the Hilbert transform sequence of the Y state respectively, and perform average superposition to obtain the final acoustic wave signal output.

[0116] The inverse tangent of the Hilbert transform sequence of the X state and the Hilbert transform sequence of the Y state are obtained according to Formula 6 and Formula 7 respectively:

[0117] Formula 6 is:

[0118] Formula 7 is:

[0119] According to formula 8, the arc tangent of the Hilbert transform sequence of the X state and the arc tangent of the Hilbert transform sequence of the Y state Perform average superposition to obtain the final sound wave signal output:

[0120] Formula 8 is:

[0121] The value of i is 1, 2, or 3; i represents the sequence number of different sub-band signals.

[0122] 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, thereby improving signal quality and reliability. Leveraging the advantages of multi-band processing and phase information analysis, the accuracy and efficiency of optical time domain reflectometers in fiber link detection and fault diagnosis are enhanced. This advanced signal processing allows for more accurate location of damage, breaks, or other problems within the optical fiber.

[0123] The specific structure of the optical time domain reflectometer is as described in Example 1 and will not be described in detail in this embodiment.

[0124] 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 in the scope of protection of the present invention.

Claims

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

2. The optical time domain reflectometer according to claim 1, characterized in that, The first port of the circulator is used to respectively receive the first pulsed optical signal, the second pulsed optical signal, and the third pulsed optical signal. The second port of the circulator is used to send the first pulsed optical signal, the second pulsed optical signal, and the third pulsed optical signal to the sensing optical fiber; The first pulsed optical signal, the second pulsed optical signal, and the third pulsed optical signal obtain reflected optical signals in the sensing optical fiber. The second port of the circulator is also used to receive the reflected optical signals and send the reflected optical signals to the signal processing device through the third port of the circulator. The signal processing device performs corresponding processing on the reflected optical signals.

3. The optical time domain reflectometer according to claim 1, characterized in that, The optical time domain reflectometer further includes a first coupler, a first amplifier, and a second amplifier; The first coupler is used to divide 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.

4. The optical time domain reflectometer according to claim 3, characterized in that, The input end of the first coupler is connected to the laser. One output end of the first coupler is connected to the input end of the frequency division multiplexing modulator, and the other output end of the first coupler is connected to the signal processing device; The input end of the first amplifier is connected to the output end of the frequency division multiplexing modulator, and the output end of the first amplifier is connected to the first port of the circulator; The input end of the second amplifier is connected to the third port of the circulator, and the output end of the second amplifier is connected to the signal processing device.

5. The optical time domain reflectometer according to claim 3, characterized in that, The frequency division multiplexing modulator further includes a second coupler, a third coupler, a fourth coupler, and a fifth coupler; The second coupler is configured to divide the first optical signal into a third optical signal and a fourth optical signal. The third optical signal sequentially passes 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 sequentially passes 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 sequentially passes through the second attenuator, the fifth coupler, the second modulator, and the fourth coupler to obtain the third pulsed optical signal.

6. The optical time domain reflectometer according to claim 5, characterized in that, The input end of the second coupler is connected to the first coupler. One output end of the second coupler is connected to the input end of the first modulator. The input end of the third coupler is connected to the output end of the first modulator. One output end of the third coupler is connected to the input end of the first attenuator. The output end of the first attenuator is connected to one input end of the fourth coupler. The output end of the fourth coupler is connected to the input end of the first amplifier; The other output end of the second coupler is connected to the input end of the second attenuator. The output end of the second attenuator is connected to one input end of the fifth coupler. The other output end of the third coupler is connected to the other input end of the fifth coupler. The output end of the fifth coupler is connected to the input end of the second modulator. The output end of the second modulator is connected to the other input end of the fourth coupler.

7. The optical time domain reflectometer according to claim 5, characterized in that, Path for generating 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 obtain the first pulsed optical signal; Path for generating the second pulsed optical signal: The third optical signal sequentially passes 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; Path for generating the third pulsed optical signal: 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 pulsed optical signal.

8. The optical time domain reflectometer according to claim 3, characterized in that, The signal processing device includes a polarization diversity coherent reception unit and a signal acquisition and processing unit; The polarization diversity coherent reception unit is configured to divide the reflected optical signal into two orthogonally polarized optical signals in the X polarization state and the Y polarization state; mix the X polarization state optical signal with the second optical signal to obtain a first beat frequency electrical signal in the X polarization state, and mix the Y polarization state optical signal with the second optical signal to obtain a second beat frequency electrical signal in the Y polarization state; The signal acquisition and processing unit is configured to receive the first beat frequency electrical signal and the second beat frequency electrical signal during the calibration phase 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 the intensity information of each sub-band signal according to a first algorithm, and adjust the attenuation ratios 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 during the detection phase 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 the phase signals of each sub-band signal respectively through the first algorithm, and perform average superposition to obtain a demodulated phase signal sequence.

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

10. The optical time domain reflectometer according to claim 8, characterized in that, 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 reception unit, the sum of f1 + f2 is less than the operating bandwidth of the signal acquisition and processing unit, and f1 is not equal to f2.

11. A method for eliminating interference fading, characterized in that, The method is applied to the optical time domain reflectometer according to any one of claims 1 - 10, and the signal processing device includes a polarization diversity coherent reception unit and a signal acquisition and processing unit; The method for eliminating interference fading includes: The polarization diversity coherent reception unit performs mixing and balanced reception on the reflected optical signal with the second optical 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 in the X polarization state and the second beat frequency electrical signal in the Y polarization state respectively; 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. After performing sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, a first algorithm is used to calculate each sub-band signal obtained by filtering to obtain the intensity information of each sub-band signal, and the attenuation ratios of the first attenuator and the second attenuator are adjusted according to the intensity information to perform sub-band power equalization; 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 sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, a first algorithm is used to calculate each sub-band signal to obtain the phase information of each sub-band signal, and the phase information is subjected to average superposition to eliminate the influence of interference fading.

12. The method for eliminating interference fading according to claim 11, wherein Performing sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal includes: The first beat frequency electrical signal and the second beat frequency electrical signal are subjected to sub-band filtering through intermediate frequency filters with center frequencies of 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, thereby obtaining a plurality of sub-band signals.

13. The method for eliminating interference fading according to claim 11, characterized in that, 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 sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, the first algorithm is used to calculate each sub-band signal obtained by filtering to obtain the intensity information of each sub-band signal, and the attenuation ratios of the first attenuator and the second attenuator are adjusted according to the intensity information to perform sub-band power equalization. Specifically, it includes: During the calibration stage of the optical time domain reflectometer, the signal acquisition and processing unit acquires 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; All the first beat frequency electrical signals and the second beat frequency electrical signals are sorted according to time and are in one-to-one correspondence; Perform sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal respectively, perform Hilbert transform on the first beat frequency electrical signal and the second beat frequency electrical signal point by point to obtain sub-band signals {H(x(k)); k = 1,....,m} and {H(y(k)); k = 1,....,m}, and obtain the intensity information P according to Formula 1, Formula 2 and Formula 3 i : Formula 1 is as follows: Formula 2 is as follows: Formula three is as follows: The attenuation ratios of the first attenuator and the second attenuator are adjusted according to Formula 4 and Formula 5 to perform sub-band power equalization: Equation 4 is as follows: Formula Five is as follows: where i takes values 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 respectively represent the intensity information corresponding to different sub-band signals; V1 represents the attenuation ratio of the first attenuator; V2 represents the attenuation ratio of the second attenuator.

14. The method for eliminating interference fading according to claim 11, wherein After performing sub-band filtering on the first beat frequency electrical signal and the second beat frequency electrical signal, the first algorithm is used to calculate each sub-band signal 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, it includes: The Hilbert transform is calculated point by point for the first beat frequency electrical signal and the second beat frequency electrical signal, respectively obtaining the Hilbert transform sequence in the X state and the Hilbert transform sequence in the Y state; The arctangent of the Hilbert transform sequence in the X state and the Hilbert transform sequence in the Y state are calculated respectively, and are averaged and superimposed to obtain the final acoustic signal output.

15. The method for eliminating interference fading according to claim 14, characterized in that, The calculating the arctangent of the Hilbert transform sequence in the X state and the Hilbert transform sequence in the Y state respectively, and averaging and superimposing to obtain the final acoustic signal output includes: The arctangent of the Hilbert transform sequence in the X state and the Hilbert transform sequence in the Y state are obtained respectively according to Formula 6 and Formula 7: Equation six is as follows: Formula VII is as follows: The arctangent of the Hilbert transform sequence of the X state according to Formula VIII Arctangent of the Hilbert transform sequence of the Y state Perform averaging and superimposing to obtain the final acoustic signal output: Formula VIII is as follows: where i takes values of 1, 2, 3; i represents the serial number of different sub-band signals.

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