Method and system for coherent differential phase noise and distortion mitigation in optical frequency domain reflectometry

Abstract

This invention belongs to the field of distributed optical fiber sensing technology and relates to a method for synergistic suppression of phase noise and distortion in optical frequency domain reflection using coded differential methods. The method includes: acquiring upper and lower sweep beat frequency signals; separating the upper and lower sweep beat frequency signals to obtain an upper sweep beat frequency signal and a lower sweep beat frequency signal; acquiring the upper and lower sweep beat frequency electrical signals; delaying the upper sweep beat frequency electrical signal by a spatial resolution and then mixing it with the lower sweep beat frequency electrical signal, or delaying the lower sweep beat frequency electrical signal by a spatial resolution and then mixing it with the upper sweep beat frequency electrical signal, to obtain a mixed signal with phase noise and frequency domain distortion suppressed, and analyzing the Fourier frequency domain spectrum of the mixed signal. This invention also provides a system for synergistic suppression of phase noise and distortion in optical frequency domain reflection using coded differential methods, including a positive and negative double-sideband sweep frequency acquisition device, a beam splitter, a spread spectrum device, an interferometer, a beat frequency signal acquisition device, and an electrical delay unit. This invention can effectively suppress phase noise and frequency domain distortion noise.

Description

Technical Field

[0001] This invention belongs to the field of distributed optical fiber sensing technology. In particular, it relates to a method and system for synergistic suppression of optical frequency domain reflection by coded differential phase noise and distortion. Background Technology

[0002] In recent years, Distributed Fiber-Optic Sensing (DFOS) technology has developed rapidly and has been widely used in transportation, industrial manufacturing, national defense and security, and many other fields. The working principle of DFOS is to inject specific detection signals into optical fibers and measure changes in optical parameters such as the phase or intensity of the echo signals to demodulate physical quantities such as strain and temperature distributed along the fiber in real time. This transforms the optical fiber into a distributed sensor that continuously senses external parameters. Within the DFOS technology system, Optical Frequency Domain Reflectometry (OFDR), based on the principle of optical frequency-modulated continuous wave coherent detection, is one of the most promising technologies due to its unique advantages. It can achieve long-distance sensing while possessing the advantages of high sensitivity, high spatial resolution, and low bandwidth requirements, thus playing a crucial role in applications with high comprehensive measurement performance requirements, such as acoustic vibration signal detection and structural health monitoring.

[0003] Based on its principle, OFDR maps backscattered Rayleigh scattering at different locations in the optical fiber onto different frequency components of the interferometric beat frequency signal between the echo and the local oscillator, and uses spectral cross-correlation or phase difference methods to achieve demodulation. Both demodulation methods require Fourier transform frequency domain analysis, measuring spectral shifts or phase changes caused by local delay variations to achieve distributed sensing of the parameter to be measured. Therefore, any noise reflected in the beat frequency spectrum will interfere with the sensing process and affect system performance. The accuracy of spectrum analysis and frequency domain information extraction directly determines key performance indicators such as sensing distance, spatial resolution, and sensing accuracy.

[0004] However, OFDR measurements suffer from two key issues that degrade the quality of the beat frequency spectrum and limit demodulation accuracy: phase frequency noise from the swept laser and frequency domain distortion noise caused by external environmental disturbances or rapidly changing signals. The former leads to a deterioration in the signal-to-noise ratio and spectral broadening of the beat frequency spectrum. Specifically, the sweep nonlinearity and intrinsic phase noise of the swept laser are converted into phase frequency errors in the beat frequency signal. These errors accumulate rapidly with increasing measurement distance, affecting the signal-to-noise ratio of the backscattered Rayleigh signal and degrading the system's spatial resolution and measurement accuracy. The latter, including frequency domain jitter, frequency mismatch, and phase modulation, are significant factors limiting system measurement performance. These primarily originate from the dynamic changes in the optical path difference within a single measurement cycle caused by complex and variable environmental disturbances in real-world scenarios, manifesting as frequency domain distortion in the form of Doppler shift and phase modulation. Due to the physical continuity of optical fibers, the impact of frequency domain distortion accumulates with the phase accumulation as the sensing fiber length increases, making the system extremely sensitive to external interference and severely restricting the widespread application of OFDR in practical scenarios.

[0005] In recent years, researchers have proposed various targeted solutions to address the phase frequency error and frequency domain distortion problems in OFDR systems. Regarding phase frequency noise suppression, optical phase-locked loop (PLL) technology and post-compensation techniques, represented by digital resampling algorithms, have proven to be effective means of compensating for phase frequency errors in swept-frequency lasers. For frequency domain distortion, frequency tracking methods and frequency shift compensation algorithms have effectively improved the system's resistance to environmental interference. However, most existing research focuses on solving only one of these problems individually. In practical applications, phase frequency error and frequency domain distortion often coexist and are coupled, requiring coordinated processing to ensure the overall system performance. Existing techniques addressing only a single problem are insufficient to meet the applicability requirements of real-world environments.

[0006] In view of the above problems, there is an urgent need to propose a technical solution for the coordinated suppression of phase frequency error and frequency domain distortion, so as to overcome the limitations of existing technical methods and comprehensively improve the measurement performance and reliability of OFDR system in real complex environments. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing technologies by proposing a method and system for synergistic suppression of phase noise and distortion in optical frequency domain reflection using coded differential interference. This invention employs a frequency-modulated continuous wave probe light, controlled by Code Division Multiple Access (CDMA) with upward and downward sweeping frequencies of positive and negative sidebands at different wavelengths, to be input into the optical fiber under test. Benefiting from the orthogonal coding characteristics of CDMA, the probe light marks the position information of each backscattering Rayleigh point. Furthermore, by adjusting the relative time delay between the beat frequency signals corresponding to the upward and downward sweeping signals, differential interference between adjacent spatial resolutions can be directly formed, and the sensing signal demodulation can be completed. This method, through spatial differential interference controlled by CDMA, can simultaneously achieve effective suppression of phase noise and frequency domain distortion, significantly improving the measurement reliability and applicability of the system in complex environments.

[0008] In a first aspect, the present invention provides a method for synergistically suppressing optical frequency domain reflection by coded differential phase noise and distortion, comprising the following steps:

[0009] Acquire positive and negative double-sideband sweeping light. The positive sideband is the upper sweeping light and the negative sideband is the lower sweeping light. The positive and negative double-sideband sweeping light are phase-synchronized, have the same sweeping range, and have opposite sweeping directions. The positive sideband has a first initial frequency and the negative sideband has a second initial frequency.

[0010] The positive and negative double-sideband sweep light is divided into double-sideband probe light and double-sideband local oscillator light according to a preset ratio;

[0011] Configure a pseudo-random code and use the pseudo-random code to perform phase modulation on the double-sideband probe light to obtain spread-spectrum double-sideband probe light;

[0012] The spread-spectrum double-sideband probe light enters the sensing fiber and is reflected back into Rayleigh scattered light. The back Rayleigh scattered light interferes with the double-sideband local oscillator light to output up and down sweep beat frequency signals.

[0013] Separate the upper and lower sweep frequency beat signals to obtain the upper sweep frequency beat signal and the lower sweep frequency beat signal;

[0014] The upper sweep frequency beat signal is detected to obtain the upper sweep frequency beat signal, and the lower sweep frequency beat signal is detected to obtain the lower sweep frequency beat signal.

[0015] The upper sweep frequency beat frequency electrical signal is delayed by one spatial resolution and then mixed with the lower sweep frequency beat frequency electrical signal, or the lower sweep frequency beat frequency electrical signal is delayed by one spatial resolution and then mixed with the upper sweep frequency beat frequency electrical signal, so as to obtain the mixed signal after suppressing phase frequency noise and frequency domain distortion. Finally, the Fourier frequency domain spectrum of the mixed signal is analyzed.

[0016] One possible implementation is to achieve a spatial resolution delay by calculating the round-trip delay for propagating a spatial resolution based on the speed of light in the sensing fiber, and adjusting the delay of the electrical delay line to correspond to the calculated delay.

[0017] As one possible implementation, the phase term in the mixing signal includes:

[0018] The phase to be measured in an optical fiber sensing channel is defined as the phase difference between two adjacent spatial resolutions.

[0019] Phase change information caused by all spatial resolutions before the i-th spatial resolution, and the phase change information is phase change information reduced by a preset multiple. The phase change information reduced by a preset multiple characterizes the degree of frequency domain distortion suppression. The preset multiple reduction refers to the phase term relative to the upper and lower sweep beat frequency electrical signals.

[0020] And a phase frequency noise suppression term that approaches zero.

[0021] As one possible implementation, phase frequency noise is greatly suppressed, approaching zero; the frequency domain distortion reduction preset factor is specifically: and ;in, The first initial frequency of the positive sideband. This is the second initial frequency of the negative sideband; The sweep slope; The delay is one spatial resolution.

[0022] As one possible implementation, positive and negative double-sideband swept light can be obtained in the following way:

[0023] Emit laser;

[0024] A radio frequency driven phase modulator with frequency scanning is used; the laser is externally modulated by the phase modulator, generating a series of positive and negative sidebands on both sides of the center optical frequency of the laser.

[0025] Using an optical bandpass filter, positive and negative sidebands of the corresponding order are obtained to produce positive and negative double-sideband swept light with the same sweep range but opposite sweep directions.

[0026] As one possible implementation, the pseudo-random code is any one or a combination of two of the following: m-sequence, Gold code, Gray code, A1 code, and A2 code.

[0027] As one possible implementation, the pseudo-random code is an m-sequence, and the driving voltage of the m-sequence is... V π V πThis represents the half-wave voltage of the modulator; the modulation method is binary phase-shift keying modulation, and the symbol 0 in the m-sequence corresponds to the driving voltage -V. π At this point, the phase of the optical field of the double-sideband probe light remains unchanged; the symbol 1 included in the m-sequence corresponds to the driving voltage V. π At this point, the phase of the optical field of the double-sideband probe light undergoes a phase shift of π;

[0028] By controlling the chip frequency of the m-sequence to be equal to the sweep frequency range, the m-sequence will individually mark, measure, and isolate each channel of the sensing fiber, meaning that each channel corresponds to a unique m-sequence.

[0029] In a second aspect, the present invention provides a coded differential phase noise and distortion synergistic suppression optical frequency domain reflection system, comprising:

[0030] A positive and negative double-sideband swept-frequency light acquisition device obtains positive and negative double-sideband swept-frequency light by driving the laser with radio frequency, external modulation and filtering;

[0031] The beam splitter divides the positive and negative double-sideband swept light into double-sideband probe light and double-sideband local oscillator light according to a preset ratio;

[0032] The spread spectrum device includes an MZM modulator, a bias point controller, and a pseudo-random code generator. The bias point controller provides a bias voltage to the MZM modulator, which is equal to the half-wave voltage of the MZM modulator. Under the control of the bias voltage, the MZM modulator receives double-sideband probe light and modulates the pseudo-random code sent by the pseudo-random code generator onto the double-sideband probe light to obtain spread spectrum double-sideband probe light.

[0033] The interferometer includes a circulator and an optical coupler. The circulator has port a, which is connected to the MZM modulator, port b, which is connected to the sensing fiber, and port c, which is the output port. The spread spectrum double-sideband probe light enters the circulator through port a, then enters the sensing fiber through port b, and is reflected back to the Rayleigh scattered light. The back Rayleigh scattered light is output to the optical coupler through port c. After interfering with the double-sideband local oscillator light in the optical coupler, the up and down sweep beat frequency signals are output.

[0034] The beat frequency signal acquisition device includes a first wavelength division multiplexer (WDM) and a second WDM, both connected to an optical coupler; it also includes a first detector and a second detector, both connected to the first WDM and the second WDM; the first WDM and the second WDM simultaneously receive the upper and lower sweep beat frequency signals, perform wavelength division multiplexing, and then transmit the signals to the first detector and the second detector respectively. Specifically, the first detector receives the upper sweep beat frequency signal and outputs an upper sweep beat frequency electrical signal, and the second detector receives the lower sweep beat frequency signal and outputs a lower sweep beat frequency electrical signal.

[0035] An electrical delay unit, connected to the first detector or the second detector, delays the upper sweep frequency beat frequency electrical signal by one spatial resolution or the lower sweep frequency beat frequency electrical signal by one spatial resolution;

[0036] An electric mixer mixes a down-sweep beat frequency electrical signal with an up-sweep beat frequency electrical signal delayed by one spatial resolution to obtain a mixed signal; or, it mixes an up-sweep beat frequency electrical signal with a down-sweep beat frequency electrical signal delayed by one spatial resolution to obtain a mixed signal.

[0037] The data acquisition and processing module acquires the mixed signal, performs Fourier transform processing, and extracts the Fourier phase information of the frequency domain signal within each sweep cycle corresponding to the fiber optic sensing channel.

[0038] As one possible implementation, the electrical delay unit is an electrical delay line.

[0039] One possible approach is to calculate the round-trip delay for propagation at a spatial resolution based on the speed of light in the sensing fiber, and then determine the corresponding electrical delay line based on this delay.

[0040] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0041] 1. The method and system for synergistic suppression of phase noise and distortion in optical frequency domain reflection proposed in this invention can effectively suppress phase noise and frequency domain distortion noise through spatial differential interference controlled by code division multiple access, significantly improving measurement accuracy, robustness and signal fidelity, and has good engineering applicability.

[0042] 2. The proposed method and system for synergistic suppression of phase noise and distortion in optical frequency domain reflection using coded differential signaling, through random coding of the double-sideband probe light and pseudo-random code (PRN) phase modulation, ensures that each Rayleigh scattering point is marked with an m-code. Benefiting from the orthogonal coding characteristics of code division multiple access (CDMA), the probe light marks the position information of each backward Rayleigh scattering point. Furthermore, by adjusting the relative time delay between the beat frequency signals corresponding to the upper and lower opposing sweep frequency signals, differential interference between adjacent spatial resolutions can be directly formed, and demodulation of the sensing signal can be completed.

[0043] 3. The optical frequency domain reflection system with phase noise and distortion synergistic suppression proposed in this invention is suitable for complex environments. Experiments show that, under complex external environments, the phase noise and frequency domain distortion noise of this invention are greatly suppressed compared with the prior art. Attached Figure Description

[0044] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and are used to explain the invention, but do not constitute an undue limitation of the invention. In the drawings:

[0045] Figure 1 and Figure 2 This is a flowchart of a method for coordinating phase noise and distortion suppression of optical frequency domain reflection using coded differentials, provided in an embodiment of the present invention.

[0046] Figure 3 This is a schematic diagram of the optical frequency domain reflection system for coordinated suppression of phase noise and distortion using coded differential provided in an embodiment of the present invention;

[0047] Figure 4 This is a schematic diagram of the positive and negative double-sideband swept-frequency optical acquisition device in an embodiment of the present invention;

[0048] Figure 5 This is a comparison chart of the multi-cycle measurement results obtained by the traditional method and the present invention under complex external environments in the embodiments of the present invention.

[0049] Figure 6 In this embodiment of the invention, under complex external environments, both the conventional method and the method of the present invention were used to apply a frequency of 5Hz and an amplitude of approximately 1.6 to a 10cm section of the end of the sensing fiber. Comparison of demodulation results of dynamic strain.

[0050] Figure Labels

[0051] 1-Positive and negative double-sideband swept-frequency optical acquisition device, 10-Laser, 11-Sweep-frequency signal generator, 12-Phase modulator, 13-Optical bandpass filter, 2-Optical splitter, 3-Spread spectrum device, 30-MZM modulator, 31-Bias point controller, 32-Pseudo-random code generator, 4-Interference device, 40-Circulator, 41-Optical coupler, 5-Beat frequency signal acquisition device, 50-First wavelength division multiplexer, 51-Second wavelength division multiplexer, 52-First detector, 53-Second detector, 6-Electrical delay unit, 7-Electrical mixer, 8-Data acquisition and processing module. Detailed Implementation

[0052] To facilitate a clear description of the technical solutions in the embodiments of the present invention, the terms "first" and "second" are used to distinguish identical or similar items with essentially the same function and effect. For example, the first threshold and the second threshold are merely used to distinguish different thresholds and do not limit their order. Those skilled in the art will understand that the terms "first" and "second" do not limit the quantity or execution order, and the terms "first" and "second" are not necessarily different.

[0053] It should be noted that in this invention, the terms "exemplary" or "for example" are used to indicate examples, illustrations, or descriptions. Any embodiment or design described as "exemplary" or "for example" in this invention should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of terms such as "exemplary" or "for example" is intended to present the relevant concepts in a concrete manner.

[0054] In this invention, "at least one" refers to one or more, and "more than one" refers to two or more. "And / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, or B alone, where A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects are in an "or" relationship. "At least one" or similar expressions refer to any combination of these items, including any combination of singular or plural items. For example, "at least one of a, b, or c" can represent: a, b, c, a combination of a and b, a combination of a and c, a combination of b and c, or a, b, and c, where a, b, and c can be single or multiple.

[0055] This invention aims to provide a method and system for synergistic suppression of phase noise and distortion in optical frequency domain reflection using coded differential modulation. It employs a frequency-modulated continuous wave probe light, controlled by Code Division Multiple Access (CDMA) with upward and downward sweeping frequencies of positive and negative sidebands at different wavelengths, connected to the optical fiber under test. Benefiting from the orthogonal coding characteristics of CDMA, the probe light marks the position information of each backscattering Rayleigh point. By adjusting the relative time delay between the beat frequency signals corresponding to the upward and downward sweeping signals, differential interference between adjacent spatial resolutions can be directly formed, and the sensing signal can be demodulated. This method, through spatial differential interference controlled by CDMA, can simultaneously achieve effective suppression of phase noise and frequency domain distortion, significantly improving the measurement reliability and applicability of the system in complex environments.

[0056] In a first aspect, embodiments of the present invention provide a method for coordinating phase noise and distortion suppression of optical frequency domain reflection using coded differentials, see [link to previous section]. Figure 1 and Figure 2 It includes the following steps:

[0057] Acquire positive and negative double-sideband sweeping light. The positive sideband is the upper sweeping light and the negative sideband is the lower sweeping light. The positive and negative double-sideband sweeping light are phase-synchronized, have the same sweeping range, and have opposite sweeping directions. The positive sideband has a first initial frequency and the negative sideband has a second initial frequency.

[0058] As one possible implementation, positive and negative double-sideband swept light can be obtained in the following way:

[0059] Emitting a laser; for example, a RIO external cavity laser is used to emit a laser, and the center frequency of the emitted laser is... The linewidth is 5kHz;

[0060] A radio frequency driven phase modulator with frequency scanning is used; the laser is externally modulated by the phase modulator, generating a series of positive and negative sidebands on both sides of the center optical frequency of the laser.

[0061] Using an optical bandpass filter, positive and negative sidebands of the corresponding order are obtained to produce positive and negative double-sideband swept light with the same sweep range but opposite sweep directions.

[0062] As an example, an external modulation sweep technique is employed, utilizing the coordinated operation of an external cavity laser and a phase modulator to achieve the linearly swept frequency-modulated continuous wave signal required for measurement. When the phase modulation depth is large, a series of higher-order modulation sidebands can be generated. Each sideband has a strict frequency and phase synchronization relationship with the carrier and the modulation signal, thus it can be used to extend the effective sweep range of the phase modulator drive signal. Specifically, when the modulation drive signal performs a linear sweep, and its sweep range is F... m At this time, for the k-th and -k-th phase modulation sidebands, their corresponding sweep frequency range will be k times that of the original modulation signal, but the sweep frequency directions are opposite. By selecting these two higher-order sidebands through an optical bandpass filter, positive and negative sideband sweep frequency light with phase synchronization, the same sweep frequency range, but opposite sweep frequency directions can be obtained.

[0063] The positive and negative double-sideband sweep light is divided into double-sideband probe light and double-sideband local oscillator light according to a preset ratio;

[0064] As an example, an optical coupler is used to split the positive and negative double-sideband sweep light into double-sideband probe light and double-sideband local oscillator light, with the following ratio: 90% of the part is used as double-sideband probe light and 10% of the part is used as double-sideband local oscillator light.

[0065] Configure a pseudo-random code and use the pseudo-random code to perform phase modulation on the double-sideband probe light to obtain spread-spectrum double-sideband probe light;

[0066] As one possible implementation, the pseudo-random code is any one or a combination of two of the following: m-sequence, Gold code, Gray code, A1 code, and A2 code. For example, a combination of m-sequence and Gold code, or a combination of Gold code, Gray code, and A1 code, etc.

[0067] As one possible implementation, the pseudo-random code is an m-sequence, and the driving voltage of the m-sequence is... V π V πThis is the half-wave voltage of the Mach-Zehnder modulator (MZM); the modulation method is binary phase-shift keying modulation, and the symbol 0 in the m-sequence corresponds to the driving voltage -V. π At this point, the phase of the optical field of the double-sideband probe light remains unchanged; the symbol 1 included in the m-sequence corresponds to the driving voltage V. π At this point, the phase of the optical field of the double-sideband probe light undergoes a phase shift of π; the spread-spectrum double-sideband probe light can then be represented as:

[0068]

[0069] in, The amplitude of the optical field of the positive sideband probe light. The initial frequency of the positive sideband probe light. The amplitude of the optical field of the negative sideband probe light. The initial frequency of the negative sideband probe light. and Indicates the opposite sweep slope, This represents the intrinsic phase noise of the laser. This represents an m-sequence.

[0070] By controlling the chip frequency of the m-sequence to be equal to the sweep frequency range, the m-sequence will individually mark, measure, and isolate each channel of the sensing fiber, meaning that each channel corresponds to a unique m-sequence.

[0071] This application employs random coding and pseudo-random code (PRN) phase modulation on the double-sideband probe light, ensuring that each Rayleigh scattering point is marked with an m-code. Benefiting from the orthogonal coding characteristics of code division multiple access (CDMA), the probe light marks the position information of each backward Rayleigh scattering point. Furthermore, by adjusting the relative time delay between the beat frequency signals corresponding to the up and down opposing sweep frequency signals, differential interference between adjacent spatial resolutions can be directly formed, and demodulation of the sensing signal can be achieved.

[0072] The spread-spectrum double-sideband probe light enters the sensing fiber and is reflected back into Rayleigh scattered light. The back Rayleigh scattered light interferes with the double-sideband local oscillator light to output up and down sweep beat frequency signals.

[0073] As an example, the sensing fiber can be one of the following: ordinary single-mode fiber, polarization-maintaining fiber, weak reflection grating array fiber, or Rayleigh scattering enhanced fiber. Regardless of the fiber used, the impact of frequency domain distortion noise on OFDR is consistent, mainly manifested as Doppler frequency shift and phase modulation caused by external environmental disturbances or the strain signal being measured, which gradually worsens with increasing sensing fiber distance.

[0074] Separate the upper and lower sweep frequency beat signals to obtain the upper sweep frequency beat signal and the lower sweep frequency beat signal;

[0075] As an example, wavelength division multiplexers are used to separate the upper and lower sweep frequency beat signals to obtain upper and lower sweep frequency beat signals. Typically, phase noise gradually increases with the increase of fiber distance, causing the OFDR trace signal-to-noise ratio to deteriorate with distance.

[0076] The upper sweep frequency beat signal is detected to obtain the upper sweep frequency beat signal, and the lower sweep frequency beat signal is detected to obtain the lower sweep frequency beat signal.

[0077] As an example, detectors are used to detect the upper sweep frequency beat signal and the lower sweep frequency beat signal respectively to obtain the upper sweep frequency beat signal and the lower sweep frequency beat signal.

[0078] The up-sweep frequency beat frequency electrical signal is denoted as , Represented as:

[0079]

[0080] The down-sweep frequency beat frequency electrical signal is denoted as , Represented as:

[0081]

[0082] in, This refers to the photocurrent of the up-sweep frequency beat signal; The total number representing spatial resolution. Indicates the first Index of Rayleigh scattering points Represents the reflectivity of the Rayleigh scattering point. For delay Phase noise of the Rayleigh backscattered beat frequency signal; The sweep slope; This is the first initial frequency of the positive sideband, i.e., the initial frequency of the up-sweep light; The negative sideband has a second initial frequency, which is the initial frequency of the down-sweep light; For the first The m-sequence corresponding to each channel.

[0083] The upper sweep frequency beat frequency electrical signal is delayed by one spatial resolution and then mixed with the lower sweep frequency beat frequency electrical signal, or the lower sweep frequency beat frequency electrical signal is delayed by one spatial resolution and then mixed with the upper sweep frequency beat frequency electrical signal, so as to obtain the mixed signal after suppressing phase frequency noise and frequency domain distortion. Finally, the Fourier frequency domain spectrum of the mixed signal is analyzed.

[0084] As an example, delaying the upsweep frequency beat frequency electrical signal The signal obtained later for:

[0085]

[0086] As another example, delaying the downsweep frequency beat frequency electrical signal The signal obtained later for:

[0087]

[0088] in Indicates the first The next spatial resolution of the first spatial resolution +1 corresponds to the delay. Due to the correlation characteristics of the m-code, only Rayleigh scattering signals with consistent coding sequences will be recovered after mixing, while mixing between signals with inconsistent coding sequences will be greatly suppressed. The degree of suppression depends on the autocorrelation coefficient of the m-code, i.e., the length of the m-code.

[0089] One possible implementation is to achieve a spatial resolution delay by calculating the round-trip delay for propagating a spatial resolution based on the speed of light in the sensing fiber, and adjusting the delay of the electrical delay line to correspond to the calculated delay.

[0090] As an example, the spatial resolution of this embodiment is 10cm. Based on the speed of light propagation in the sensing fiber, the round-trip delay for propagating one spatial resolution is calculated to be 1ns. Therefore, the electrical delay line should also be adjusted to delay by 1ns.

[0091] As one possible implementation, the phase term in the mixing signal includes:

[0092] The phase to be measured in an optical fiber sensing channel is defined as the phase difference between two adjacent spatial resolutions.

[0093] No. Phase change information caused by all spatial resolutions before a given spatial resolution, and this phase change information is phase change information reduced by a preset factor. The phase change information reduced by a preset factor characterizes the degree of frequency domain distortion suppression. The preset factor refers to the phase term relative to the upper and lower sweep beat frequency electrical signals.

[0094] And a phase frequency noise suppression term that approaches zero.

[0095] As one possible implementation, phase frequency noise is greatly suppressed, approaching zero; the frequency domain distortion reduction preset factor is specifically: and ;in, The first initial frequency of the positive sideband. This is the second initial frequency of the negative sideband; The sweep slope; The delay is one spatial resolution.

[0096] As an example, mixing signals The phase terms in the text include , and . The item contains the first +1 spatial resolution relative to the first Phase change information at a spatial resolution, i.e., the phase to be measured in this sensing channel. Due to the phase accumulation effect caused by fiber continuity, Includes the first Phase change information caused by all spatial resolutions up to a given spatial resolution, relative to the up-sweep beat frequency electrical signal. and down-scan frequency beat frequency electrical signal The middle phase term and The phase change of this term has decreased. and This indicates that the present invention directly obtains the phase to be measured while greatly suppressing the influence of frequency domain distortion noise, thereby improving the robustness and high fidelity of the system. The system supports use in complex environments. The terms indicate that phase noise is greatly suppressed, due to Much smaller than the laser coherence length, this term is close to 0, which greatly improves the system's accuracy.

[0097] Therefore, the mixing signal Represented as:

[0098]

[0099] or

[0100]

[0101] Finally, the mixed signal After Fourier transform processing, Fourier phase information of the frequency domain signal within each sweep cycle corresponding to the sensing channel is extracted.

[0102] Secondly, embodiments of the present invention provide a coded differential phase noise and distortion co-suppressing optical frequency domain reflection system, see [link to relevant documentation]. Figure 3 ,include:

[0103] Positive and negative double-sideband swept light acquisition device 1: The laser is driven by radio frequency scanning, externally modulated and filtered to obtain positive and negative double-sideband swept light;

[0104] Beam splitter 2 splits the positive and negative double-sideband swept light into double-sideband probe light and double-sideband local oscillator light according to a preset ratio;

[0105] The spread spectrum device 3 includes an MZM modulator 30, a bias point controller 31, and a pseudo-random code generator 32. The bias point controller 31 provides a bias voltage to the MZM modulator 30, which is equal to the half-wave voltage of the modulator. Under the control of the bias voltage, the MZM modulator 30 receives double-sideband probe light and modulates the pseudo-random code sent by the pseudo-random code generator 32 onto the double-sideband probe light to obtain spread spectrum double-sideband probe light.

[0106] Interference device 4 includes circulator 40 and optical coupler 41. Circulator 40 has port a connected to MZM modulator 30, port b connected to sensing optical fiber, and output port c. Spread spectrum double-sideband probe light enters circulator 40 through port a, then enters sensing optical fiber through port b, and is reflected back to Rayleigh scattered light. Back Rayleigh scattered light is output to optical coupler 41 through port c. After interfering with double-sideband local oscillator light in optical coupler 41, up and down sweep beat frequency signals are output.

[0107] The beat frequency signal acquisition device 5 includes a first wavelength division multiplexer 50 and a second wavelength division multiplexer 51, which are connected to an optical coupler 41. It also includes a first detector 52 and a second detector 53, which are connected to the first wavelength division multiplexer 50 and the second wavelength division multiplexer 51. After receiving the upper and lower sweep beat frequency signals, the first wavelength division multiplexer 50 and the second wavelength division multiplexer 51 perform wavelength demultiplexing and then send them to the first detector 52 and the second detector 53 respectively. That is, the first detector 52 receives the upper sweep beat frequency signal and outputs the upper sweep beat frequency electrical signal, and the second detector 53 receives the lower sweep beat frequency signal and outputs the lower sweep beat frequency electrical signal.

[0108] The electrical delay unit 6 is connected to the first detector 52 or the second detector 53 to delay the upper sweep frequency beat frequency electrical signal by one spatial resolution or the lower sweep frequency beat frequency electrical signal by one spatial resolution.

[0109] As one possible implementation, electrical delay unit 6 is an electrical delay line.

[0110] One possible approach is to calculate the delay that light takes to propagate to a spatial resolution based on the speed of light in the sensing fiber, and then determine the corresponding electrical delay line based on that delay.

[0111] The electric mixer 7 mixes the down-sweep beat frequency electrical signal with the up-sweep beat frequency electrical signal delayed by one spatial resolution to obtain a mixed signal; or, it mixes the up-sweep beat frequency electrical signal with the down-sweep beat frequency electrical signal delayed by one spatial resolution to obtain a mixed signal.

[0112] The data acquisition and processing module 8 acquires the mixed signal, performs Fourier transform processing, and extracts the Fourier phase information of the frequency domain signal within each sweep cycle corresponding to the fiber optic sensing channel.

[0113] See Figure 4 As one possible implementation, the positive and negative double-sideband swept-frequency optical acquisition device 1 includes:

[0114] Laser 10 is used to emit laser light;

[0115] Sweep frequency signal generator 11 is used to generate sweep frequency signals;

[0116] Phase modulator 12 receives laser light and a frequency sweep signal. After modulating the laser light with the frequency sweep signal, a series of positive and negative sidebands are generated on both sides of the preset center optical frequency.

[0117] And an optical bandpass filter 13, to obtain positive and negative sidebands of the corresponding order to obtain double-sideband swept light, with the same sweep range and opposite sweep directions.

[0118] See Figure 5 The graph shows a comparison of multi-cycle measurement results obtained by traditional methods and the present invention under complex external environments. The horizontal axis represents sensing distance, and the vertical axis represents energy.

[0119] Figure 5 In Figure (a), the measurement traces obtained by the traditional method in a portion of the sensing range are shown. It can be seen that the results are significantly affected by the phase frequency error of the swept laser, resulting in an extremely low overall signal-to-noise ratio, spectral distortion of the measurement traces, and severe damage to the quality of the beat frequency spectrum. Figure 5 (b) is a magnified view of a part of (a), where obvious frequency domain distortion can be observed at the tail reflection peak, specifically manifested as Doppler frequency shift and phase modulation phenomena.

[0120] Figure 5 Figure (c) shows the corresponding interval traces measured using the present invention under the same complex environment. The results show that the system and method provided in this application effectively suppress phase frequency noise, with a signal-to-noise ratio significantly better than traditional methods, and good beat frequency spectrum quality. Figure 5 Image (d) is a magnified view of a portion of image (c), and... Figure 5 Compared with (b), the frequency domain distortion noise is greatly suppressed, and the position of the reflection peak at the end of each cycle is stable.

[0121] In summary, the solution provided by this invention can effectively suppress phase noise and frequency domain distortion noise, significantly improve the measurement accuracy, robustness and signal fidelity of the system in complex environments, and has good engineering applicability.

[0122] See Figure 6 This demonstrates how, under complex external environments, both traditional methods and the present invention apply a 5 Hz frequency and an amplitude of approximately 1.6 to a 10 cm long section of the end of a sensing fiber optic cable. The demodulation results of dynamic strain. Figure 6 In Figure (a), the demodulated strain signal time-domain curve is shown, with time on the horizontal axis and strain value on the vertical axis. It can be seen that the solution provided by this invention can accurately restore the applied dynamic strain waveform, while the traditional solution fails to demodulate due to the severe influence of phase frequency noise and frequency domain distortion noise. Figure 6 In Figure (b), the corresponding strain power spectral density is shown, with the horizontal axis representing frequency and the vertical axis representing strain power spectral density. This invention exhibits a distinct strain response peak at 5 Hz, with an amplitude of -8.76 dB. The corresponding strain magnitude is 1.62. The actual strain applied is highly consistent with the strain actually applied. Simultaneously, this invention exhibits excellent noise performance, with a noise floor of -50 dB. (Corresponding strain sensitivity reaches 3.2) In contrast, traditional phase demodulation schemes cannot effectively identify strain signals, and the target frequency components are completely submerged by noise.

[0123] Although the invention has been described herein in conjunction with various embodiments, those skilled in the art will understand and implement other variations of the disclosed embodiments by reviewing the accompanying drawings, disclosure, and other materials. In this specification, the word "comprising" does not exclude other components or steps, and "a" or "an" does not exclude multiple components. A single processor or other unit can implement several functions listed in the specification. While certain measures are described in different embodiments, this does not mean that these measures cannot be combined to produce good results.

[0124] Although the invention has been described in conjunction with specific features and embodiments, it is obvious that various modifications and combinations can be made therein without departing from the spirit and scope of the invention. Accordingly, this specification and drawings are merely illustrative of the invention and are considered to cover any and all modifications, variations, combinations, or equivalents within the scope of the invention. Clearly, those skilled in the art can make various alterations and modifications to the invention without departing from its spirit and scope. Thus, if such modifications and modifications fall within the scope of the invention and its equivalents, the invention is also intended to include such modifications and modifications.

Claims

1. A method for synergistically suppressing optical frequency domain reflection by coded differential phase noise and distortion, characterized in that, Includes the following steps: Acquire positive and negative double-sideband sweeping light. The positive sideband is the upper sweeping light and the negative sideband is the lower sweeping light. The positive and negative double-sideband sweeping light are phase-synchronized, have the same sweeping range, and have opposite sweeping directions. The positive sideband has a first initial frequency and the negative sideband has a second initial frequency. The positive and negative double-sideband sweep light is divided into double-sideband probe light and double-sideband local oscillator light according to a preset ratio; Configure a pseudo-random code and use the pseudo-random code to perform phase modulation on the double-sideband probe light to obtain spread-spectrum double-sideband probe light; The spread-spectrum double-sideband probe light enters the sensing fiber and is reflected back into Rayleigh scattered light. The back Rayleigh scattered light interferes with the double-sideband local oscillator light to output up and down sweep beat frequency signals. Separate the upper and lower sweep frequency beat signals to obtain the upper sweep frequency beat signal and the lower sweep frequency beat signal; The upper sweep frequency beat signal is detected to obtain the upper sweep frequency beat signal, and the lower sweep frequency beat signal is detected to obtain the lower sweep frequency beat signal. The upper sweep frequency beat frequency electrical signal is delayed by one spatial resolution and then mixed with the lower sweep frequency beat frequency electrical signal, or the lower sweep frequency beat frequency electrical signal is delayed by one spatial resolution and then mixed with the upper sweep frequency beat frequency electrical signal, so as to obtain the mixed signal after suppressing phase frequency noise and frequency domain distortion. Finally, the Fourier frequency domain spectrum of the mixed signal is analyzed.

2. The method for synergistic suppression of phase noise and distortion in optical frequency domain reflection using coded differential as described in claim 1, characterized in that, A spatial resolution delay is achieved by calculating the round-trip delay for propagating a spatial resolution based on the speed of light in the sensing fiber, and adjusting the delay of the electrical delay line to correspond to the calculated delay.

3. The method for synergistic suppression of phase noise and distortion in optical frequency domain reflection using coded differential as described in claim 1, characterized in that, The phase term in the mixer signal includes: The phase to be measured in an optical fiber sensing channel is defined as the phase difference between two adjacent spatial resolutions. Phase change information caused by all spatial resolutions before the i-th spatial resolution, and the phase change information is phase change information reduced by a preset multiple. The phase change information reduced by a preset multiple characterizes the degree of frequency domain distortion suppression. The preset multiple reduction refers to the phase term relative to the upper and lower sweep beat frequency electrical signals. And a phase frequency noise suppression term that approaches zero.

4. The method for synergistic suppression of phase noise and distortion in optical frequency domain reflection using coded differential as described in claim 3, characterized in that, Phase frequency noise is greatly suppressed, approaching zero; frequency domain distortion is reduced by the preset factor as follows: and ;in, The first initial frequency of the positive sideband. This is the second initial frequency of the negative sideband; The sweep slope; The round-trip delay is given by a spatial resolution.

5. The method for synergistic suppression of phase noise and distortion in optical frequency domain reflection using coded differential as described in claim 1, characterized in that, Positive and negative double-sideband swept light is obtained in the following way: Emit laser; A radio frequency driven phase modulator with frequency scanning is used; the laser is externally modulated by the phase modulator, generating a series of positive and negative sidebands on both sides of the center optical frequency of the laser. Using an optical bandpass filter, positive and negative sidebands of the corresponding order are obtained to produce positive and negative double-sideband swept light with the same sweep range but opposite sweep directions.

6. The method for synergistic suppression of phase noise and distortion in optical frequency domain reflection using coded differential as described in claim 1, characterized in that, The pseudo-random code is any one or a combination of two of the following: m-sequence, Gold code, Gray code, A1 code, and A2 code.

7. The method for synergistic suppression of phase noise and distortion in optical frequency domain reflection using coded differential as described in claim 2, characterized in that, The pseudo-random code is an m-sequence, and the driving voltage of the m-sequence is... V π V π This represents the half-wave voltage of the Mach-Zehnder modulator; the modulation method is binary phase-shift keying modulation, and the symbol 0 in the m-sequence corresponds to the driving voltage -V. π At this point, the phase of the optical field of the double-sideband probe light remains unchanged; the symbol 1 included in the m-sequence corresponds to the driving voltage V. π At this point, the phase of the optical field of the double-sideband probe light undergoes a phase shift of π; By controlling the chip frequency of the m-sequence to be equal to the sweep frequency range, the m-sequence will individually mark, measure, and isolate each channel of the sensing fiber, meaning that each channel corresponds to a unique m-sequence.

8. A coded differential phase noise and distortion synergistic suppression optical frequency domain reflection system, characterized in that, include: A positive and negative double-sideband swept-frequency light acquisition device obtains positive and negative double-sideband swept-frequency light by driving the laser with radio frequency, external modulation and filtering; The beam splitter divides the positive and negative double-sideband swept light into double-sideband probe light and double-sideband local oscillator light according to a preset ratio; The spread spectrum device includes an MZM modulator, a bias point controller, and a pseudo-random code generator. The bias point controller provides a bias voltage to the MZM modulator, which is equal to the half-wave voltage of the MZM modulator. Under the control of the bias voltage, the MZM modulator receives double-sideband probe light and modulates the pseudo-random code sent by the pseudo-random code generator onto the double-sideband probe light to obtain spread spectrum double-sideband probe light. An interferometer includes a circulator and an optical coupler. The circulator has an a port connected to an MZM modulator, a b port connected to a sensing optical fiber, and an output port c. The spread spectrum double-sideband probe light enters the circulator through port a, then enters the sensing fiber through port b, and is reflected back to the Rayleigh scattered light. The back Rayleigh scattered light is output to the optical coupler through port c. In the optical coupler, it interferes with the double-sideband local oscillator light and outputs the up and down sweep beat frequency signal. The beat frequency signal acquisition device includes a first wavelength division multiplexer (WDM) and a second WDM, both connected to an optical coupler; it also includes a first detector and a second detector, both connected to the first WDM and the second WDM; the first WDM and the second WDM simultaneously receive the upper and lower sweep beat frequency signals, perform wavelength division multiplexing, and then transmit the signals to the first detector and the second detector respectively. Specifically, the first detector receives the upper sweep beat frequency signal and outputs an upper sweep beat frequency electrical signal, and the second detector receives the lower sweep beat frequency signal and outputs a lower sweep beat frequency electrical signal. An electrical delay unit, connected to the first detector or the second detector, delays the upper sweep frequency beat frequency electrical signal by one spatial resolution or the lower sweep frequency beat frequency electrical signal by one spatial resolution; An electric mixer mixes a down-sweep beat frequency electrical signal with an up-sweep beat frequency electrical signal delayed by one spatial resolution to obtain a mixed signal; Alternatively, the up-sweep beat frequency electrical signal can be mixed with the down-sweep beat frequency electrical signal delayed by one spatial resolution to obtain a mixed signal; The data acquisition and processing module acquires the mixed signal, performs Fourier transform processing, and extracts the Fourier phase information of the frequency domain signal within each sweep cycle corresponding to the fiber optic sensing channel.

9. The optical frequency domain reflection system for coordinated suppression of phase noise and distortion by coded differential according to claim 8, characterized in that, The electrical delay unit is an electrical delay line.

10. The optical frequency domain reflection system for coordinated suppression of phase noise and distortion by coded differential according to claim 8, characterized in that, The round-trip delay for propagating to one spatial resolution is calculated based on the speed of light in the sensing fiber, and the corresponding electrical delay line is determined based on this delay.

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