Phase noise rejection optical frequency domain reflectometry method and system with free auxiliary interferometer

Abstract

The present application belongs to the field of distributed optical fiber sensing technology, and relates to a phase frequency noise suppression optical frequency domain reflectometry method and system of an auxiliary-free interferometer. The measurement method comprises: obtaining positive and negative double sideband swept light with the same sweep range and opposite sweep directions; dividing the positive and negative double sideband swept light into double sideband probe light and double sideband local light according to a preset ratio; configuring a pseudo-random noise code and modulating the pseudo-random noise code to the double sideband probe light to obtain spread spectrum double sideband probe light; the spread spectrum double sideband probe light enters the sensing optical fiber to reflect back the back Rayleigh scattering light, the back Rayleigh scattering light interferes with the spread spectrum double sideband local light to output up and down sweep beat signals; the up and down sweep beat signals are separated to obtain up sweep beat signals and down sweep beat signals; after decoding the up sweep beat signals and the down sweep beat signals respectively, the beat signals are separated by band pass filtering, the beat signals are mixed to obtain mixed signals after phase frequency noise suppression, and the frequency spectrum of the mixed signals is analyzed.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of distributed optical fiber sensing. In particular, it relates to a phase frequency noise suppression optical frequency domain reflectometry method and system of an auxiliary-free interferometer. BACKGROUND

[0002] Distributed fiber-optic sensing (DFOS) technology can sense the time and spatial distribution characteristics of various parameters such as refractive index, loss, temperature, strain, etc. along the optical fiber. With its long measurement distance, high spatial resolution, and real-time information acquisition, it has unique advantages in optical fiber link loss detection, infrastructure health monitoring, geological disaster warning, etc. It can be seen that measurement distance and measurement accuracy are the most core technical indicators in DFOS technology. In recent years, the optical frequency domain reflectometry (OFDR) technology based on linear sweep frequency laser and optical coherent demodulation has the potential to achieve millimeter-level positioning accuracy and hundred-kilometer-level measurement length. It can also invert the dynamic changes of environmental physical parameters such as temperature and strain according to the modulation effect of the external environment on the measurement light wave, showing great application potential and attracting widespread attention.

[0003] According to the measurement principle of OFDR, the laser phase and frequency error of the sweep frequency laser are the key to determining the core indicators of OFDR measurement distance and accuracy, which mainly include laser intrinsic phase noise and sweep nonlinearity noise. Therefore, to achieve long-distance, high-precision, high-spatial-resolution distributed measurement and sensing, it is urgent to have a high-coherence, low-phase-noise, and high-linearity wide-range sweep frequency laser. However, due to the mechanism of laser generation, low phase noise and wide sweep frequency range are contradictory and difficult to achieve.

[0004] To this end, using phase-locked, frequency-stabilized, and other technologies to optimize existing laser sources is a direct means to improve phase noise performance and sweep linearity, but its effect is limited by the laser tuning mechanism and its intrinsic frequency modulation response, and can only obtain limited performance improvement, and is easily disturbed by temperature drift and external mechanical vibration, etc.

[0005] With the development of digital signal processing technology, compensation and correction technology based on digital processing algorithm has been widely concerned. At present, the existing mainstream laser phase frequency noise compensation and nonlinear calibration methods mainly include hardware / digital resampling, deskew filter and cascaded phase compensation. All of them need to additionally increase an auxiliary interferometer outside the OFDR main interferometer, obtain the phase frequency error information of the laser through the auxiliary interferometer, and further realize the compensation of the laser phase or frequency error by using different technical means. Therefore, the compensation performance is inevitably limited by the auxiliary interferometer. First, the environmental noise received by the auxiliary interferometer will directly affect the accuracy of the laser noise acquisition, thereby restricting the compensation effect, and additional measures are needed to stabilize its working state. Second, the accuracy and bandwidth of the phase noise acquisition are difficult to be both obtained due to the limitation of the length of the auxiliary interferometer. In addition, most compensation algorithms can only achieve the best effect at the integer multiple of the length of the auxiliary interferometer, and the compensation effect is deteriorated when deviating from the position, resulting in uneven overall compensation effect. It can be seen that the current laser phase noise compensation and nonlinear calibration technology depends on the auxiliary interferometer, and the performance is limited by the auxiliary interferometer in many aspects. SUMMARY

[0006] The present application aims at the problem that the laser phase frequency noise compensation and nonlinear calibration in the existing optical frequency domain reflectometry distributed fiber sensing system depend on the auxiliary interferometer to extract the phase frequency noise, which seriously limits the compensation performance of the auxiliary interferometer, and proposes a phase frequency noise suppression optical frequency domain reflectometry method and system without auxiliary interferometer.

[0007] The method and system combine up-sweeping and down-sweeping with space division phase frequency noise compensation, independently acquire the up-sweeping and down-sweeping beat signals, and extract the beat signals of the same channel. Since the phase frequency noise of the same channel of the up-sweeping and down-sweeping beat signals is common-mode noise, it can be effectively eliminated after mixing. This method eliminates the dependence on the auxiliary interferometer, avoids the limitation of the auxiliary interferometer on the compensation effect, realizes the effective suppression of the laser phase frequency noise, improves the measurement accuracy and measurement distance, and at the same time improves the long-time measurement stability, effectively promotes the development and application of the optical frequency domain reflectometry distributed fiber sensing technology.

[0008] To achieve the above object, the present application adopts the following technical scheme:

[0009] In a first aspect, the present application provides a phase frequency noise suppression optical frequency domain reflectometry method without auxiliary interferometer, comprising the following steps:

[0010] Obtaining positive and negative double sideband swept light, the positive sideband is up-sweeping, the negative sideband is down-sweeping, the swept frequency range of the positive and negative double sideband swept light is the same, and the swept frequency direction is opposite, the positive sideband has a first center frequency, and the negative sideband has a second center frequency;

[0011] As a possible implementation manner, the positive-negative double-sideband swept light is obtained by the following manner:

[0012] Emitting laser;

[0013] Using a radio frequency driving phase modulator with frequency sweeping; the laser is externally modulated by the phase modulator to generate a series of positive sidebands and negative sidebands on both sides of the central optical frequency of the laser;

[0014] Using an optical band-pass filter to obtain the positive sidebands and the negative sidebands corresponding to the order to obtain the positive-negative double-sideband swept light with the same swept frequency range and opposite swept frequency directions.

[0015] Dividing the positive-negative double-sideband swept light into double-sideband probe light and double-sideband local light according to a preset ratio;

[0016] Configuring a pseudo-random noise code and modulating the pseudo-random noise code to the double-sideband probe light to obtain spread spectrum double-sideband probe light;

[0017] As a possible implementation manner, the pseudo-random noise code is any one or a combination of at least two of m sequence, Gold code, Gray code, A1 code and A2 code.

[0018] As a possible implementation manner, the pseudo-random noise code is m sequence, and the driving voltage of the m sequence is V PRN , V PRN is greater than 0 and less than V π , V π is the half-wave voltage of the modulator; the modulation manner is BPSK, and the symbol 0 included in the m sequence corresponds to the driving voltage-V PRN , at this time, the optical field phase of the double-sideband probe light remains the original phase; the symbol 1 included in the m sequence corresponds to the driving voltage V PRN , at this time, the optical field phase of the double-sideband probe light is shifted by π;

[0019] Controlling the chip frequency of the m sequence to be equal to the swept frequency range, at this time, the m sequence separately marks, measures and isolates each channel of the sensing optical fiber, that is, each channel corresponds to a unique m sequence.

[0020] The spread spectrum double-sideband probe light enters the sensing optical fiber to reflect back the back Rayleigh scattering light, the back Rayleigh scattering light interferes with the double-sideband local light to output up and down swept beat frequency signals, and the polarization state of the double-sideband local light is the same as that of the double-sideband probe light;

[0021] Separating the up and down swept beat frequency signals to obtain the up and down swept beat frequency signals;

[0022] The up-sweep beat signal and the down-sweep beat signal are decoded respectively to separate the beat signals corresponding to the same sensing channel of the sensing optical fiber, then the beat signals corresponding to the channels of the up-sweep beat signal and the down-sweep beat signal are extracted through band-pass filtering, then mixed-frequency processing is performed to obtain a mixed-frequency signal after phase noise suppression, and finally the spectrum of the mixed-frequency signal is analyzed.

[0023] As a possible implementation, the up-sweep beat signal is denoted as , is denoted as:

[0024]

[0025] The down-sweep beat signal is denoted as , is denoted as:

[0026]

[0027] wherein, is the photoelectric current of the up-sweep beat signal; denotes the th sensing channel; is the total number of sensing channels; is the round-trip delay of the th sensing channel; is the reflectivity of the Rayleigh scattering point; is the phase noise of the Rayleigh backscattering beat signal with a delay of ; is the sweep slope; is the first center frequency possessed by the positive sideband, i.e., the starting frequency of the up-sweep light; is the second center frequency possessed by the negative sideband, i.e., the starting frequency of the down-sweep light; is the frequency drift noise caused by the free running of the laser with a center wavelength of ; is the m sequence corresponding to the th channel.

[0028] As a possible implementation, after the up-sweep beat signal and the down-sweep beat signal are decoded respectively, the beat signals corresponding to the same sensing channel of the sensing optical fiber are separated, and the specific flow is as follows:

[0029] ;

[0030] .

[0031] As a possible implementation, the beat signals of the sensing channels corresponding to the two sidebands are band-pass filtered and then mixed-frequency processed to obtain mixed-frequency signals after phase noise suppression of each sensing channel, and the specific process is as follows: ​

[0032] .

[0033] As a possible implementation, decoding all sensing channels obtains a mixed frequency signal after phase noise suppression, denoted as , expressed as:

[0034] .

[0035] In a second aspect, the present application provides a phase noise suppression optical frequency domain reflectometry system without auxiliary interferometer, comprising:

[0036] Positive and negative double sideband swept light acquisition device, laser after sweeping, external modulation and filtering obtains positive and negative double sideband swept light;

[0037] Optical splitter, according to the preset proportion, the positive and negative double sideband swept light is divided into double sideband detection light and double sideband local light;

[0038] Spread spectrum device, including modulator, bias point controller and pseudo random noise code generator, bias point controller is used for providing bias voltage for modulator, bias voltage is equal to half wave voltage of modulator; The modulator receives the pseudo random noise code sent by the pseudo random noise code generator under the control of the bias voltage and modulates the pseudo random noise code to the double sideband detection light, so as to obtain spread spectrum double sideband detection light;

[0039] Interference device, including circulator and optical coupler, the circulator has a port a in communication with the modulator, a port b in communication with the sensing optical fiber, and an output port c; The spread spectrum double sideband detection light enters the circulator through the port a and then enters the sensing optical fiber through the port b, and the backscattered light is reflected back to the backscattered light, and the backscattered light is output to the optical coupler through the port c; After interference with the double sideband local light in the optical coupler, the up and down swept beat frequency signals are output;

[0040] Beat frequency signal acquisition device, including first wavelength division multiplexer, second wavelength division multiplexer, simultaneously connected with optical coupler; It also includes first balanced photodetector and second balanced photodetector connected with first wavelength division multiplexer and second wavelength division multiplexer; The first wavelength division multiplexer and the second wavelength division multiplexer receive the up and down swept beat frequency signals and send them to the first balanced photodetector and the second balanced photodetector after wavelength division multiplexing, that is, the first balanced photodetector receives the up swept beat frequency signal, and the second balanced photodetector receives the down swept beat frequency signal;

[0041] Data acquisition and processing module, after decoding the up and down swept beat frequency signals, the beat frequency signals of each channel of the sensing optical fiber are separated, then the beat frequency signals of the corresponding channels of the up and down swept beat frequency signals are extracted through band pass filtering, then mixed frequency processing is carried out to obtain the mixed frequency signal after phase noise suppression, and finally the frequency spectrum of the mixed frequency signal is analyzed.

[0042] As a possible implementation, the positive and negative double sideband swept light acquisition device comprises:

[0043] a laser for emitting laser light;

[0044] a swept signal generator for generating a swept signal;

[0045] a phase modulator receiving the laser light and the swept signal, modulating the laser light with the swept signal, and generating a series of positive sidebands and negative sidebands on both sides of a preset center optical frequency;

[0046] and an optical bandpass filter for acquiring the positive sidebands and the negative sidebands corresponding to the order to obtain double sideband swept light with the same swept range and opposite swept directions.

[0047] Compared with the prior art, the present application has the following beneficial effects:

[0048] 1. The phase frequency noise suppression optical frequency domain reflectometry method of the auxiliary interferometer-free interferometer proposed in the present application does not need to additionally increase an auxiliary interferometer outside the OFDR main interference structure. Therefore, the compensation performance is not limited by the auxiliary interferometer.

[0049] 2. The method and system combine up-sweeping and down-sweeping with space division phase frequency noise compensation, independently collect up-sweeping and down-sweeping beat signals, and extract beat signals of the same channel. Since the phase frequency noise of the same channel of the up-sweeping and down-sweeping beat signals is common-mode noise, it can be effectively eliminated after mixing, the phase frequency noise in the full bandwidth can be accurately compensated, and the compensation effect is uniform.

[0050] 3. Compared with the phase-locked and frequency-stabilized schemes, the system complexity is simplified, the laser frequency drift noise is well eliminated, and the long-time measurement stability of the system is improved. BRIEF DESCRIPTION OF DRAWINGS

[0051] The accompanying drawings, which are included to provide a further understanding of the present application, constitute a part of the present application and illustrate the illustrative embodiments of the present application and their descriptions serve to explain the present application and do not constitute improper limitations on the present application. In the drawings:

[0052] Figure 1 The flowchart of the phase frequency noise suppression optical frequency domain reflectometry method of the auxiliary interferometer-free interferometer proposed in the present application;

[0053] Figure 2 The structure schematic diagram of the phase frequency noise suppression optical frequency domain reflectometry system of the auxiliary interferometer-free interferometer proposed in the present application;

[0054] Figure 3 The structure schematic diagram of the positive and negative double sideband swept light acquisition device proposed in the present application;

[0055] Figure 4 Fig. 2 is a schematic diagram of a processing flow of the data acquisition and processing module in an embodiment of the present application for the up-sweep beat frequency signal and the down-sweep beat frequency signal;

[0056] Figure 5 Fig. 4 is a schematic diagram of a comparison between a measurement result of a method proposed in the present application and a conventional OFDR measurement scheme.

[0057] Reference signs

[0058] 1 - positive and negative double sideband sweep frequency light acquisition device, 10 - laser, 11 - sweep signal generator, 12 - phase modulator, 13 - optical bandpass filter, 2 - optical splitter, 3 - spread spectrum device, 30 - modulator, 31 - bias point controller, 32 - pseudo-random noise code generator, 4 - interference device, 40 - circulator, 41 - optical coupler, 50 - first wavelength division multiplexer, 51 - second wavelength division multiplexer, 52 - first balanced photodetector, 53 - second balanced photodetector, 6 - data acquisition and processing module. DETAILED DESCRIPTION

[0059] In order to clearly describe the technical solutions of the embodiments of the present application, in the embodiments of the present application, the words "first", "second" and the like are used to distinguish the same or similar items or components with basically the same function and role. For example, the first threshold and the second threshold are only used to distinguish different thresholds, and do not limit the order. Those skilled in the art can understand that the words "first", "second" and the like do not limit the number and execution order, and the words "first", "second" and the like do not necessarily mean different.

[0060] It should be noted that in the present application, the words "exemplary" or "for example" are used to represent an example, illustration or description. Any embodiment or design scheme described as "exemplary" or "for example" in the present application should not be interpreted as more preferred or more advantageous than other embodiments or design schemes. Rather, the words "exemplary" or "for example" are intended to present the relevant concept in a specific manner.

[0061] In the present application, "at least one" means one or more, and "multiple" means two or more. The association relationship of "and / or" describing the associated objects means that there can be three kinds of relationships, for example, A and / or B can represent the following three cases: A exists alone, A and B exist together, and B exists alone, wherein A and B can be singular or plural. The character " / " generally represents an "or" relationship between the associated objects before and after it. The following "at least one" or similar expressions mean any combination of these items, including any combination of single item or multiple items. For example, at least one of a, b or c can represent a, b, c, the combination of a and b, the combination of a and c, the combination of b and c, or the combination of a, b and c, wherein a, b and c can be single or multiple.

[0062] The embodiment of the present application aims to provide a phase frequency noise suppression optical frequency domain reflectometry method and system without auxiliary interferometer, which combines external modulation and phase noise compensation to eliminate phase noise, eliminates the dependence on auxiliary interferometer, realizes phase noise suppression of low-cost semiconductor laser (SL), and further promotes the application of low-cost SL in distributed optical fiber sensing.

[0063] In the first aspect, the embodiment of the present application provides a phase frequency noise suppression optical frequency domain reflectometry method without auxiliary interferometer, referring to Figure 1 , comprising the following steps:

[0064] Obtaining positive and negative double sideband swept light, the positive sideband is up-swept, the negative sideband is down-swept, the swept frequency ranges of the positive and negative double sideband swept light are the same, and the swept frequency directions are opposite, the positive sideband has a first center frequency, and the negative sideband has a second center frequency;

[0065] Specifically, the positive and negative double sideband swept light is obtained by the following way:

[0066] Emitting laser, exemplarily, using a semiconductor laser to emit laser, the center optical frequency of the emitted laser is , and the line width is 500 kHz;

[0067] Using a frequency scanning radio frequency to drive a phase modulator; the laser passes through the phase modulator for external modulation, and a series of positive sidebands and negative sidebands are generated on both sides of the center optical frequency of the laser;

[0068] Using an optical band-pass filter to obtain positive sidebands and negative sidebands corresponding to the order to obtain positive and negative double sideband swept light with the same swept frequency range and opposite swept frequency directions.

[0069] Exemplarily, the center optical frequency of the laser is a series of positive sidebands and negative sidebands are generated on both sides of the two sidebands, the range of the positive sidebands and the negative sidebands is the same, both are 1GHz, the directions are opposite, the center frequency of the positive sideband is the upper sweep, the center frequency of the negative sideband is the lower sweep.

[0070] The positive and negative double sideband swept light is divided into double sideband probe light and double sideband local oscillator light according to a preset ratio.

[0071] As an example, the optical coupler is used to divide the positive and negative double sideband swept light into double sideband probe light and double sideband local oscillator light, and the division ratio is: 90% of the part as double sideband probe light, and 10% of the part as double sideband local oscillator light.

[0072] The pseudo-random noise code is configured and modulated to the double sideband probe light to obtain the spread spectrum double sideband probe light.

[0073] As an example, the signal generator is used to send the pseudo-random noise code, and the pseudo-random noise code is any one or a combination of at least two of m sequence, Gold code, Gray code, A1 code and A2 code. For example, m sequence and Gold code combination, Gold code, Gray code and A1 code combination, etc.

[0074] As another example, the pseudo-random noise code uses m sequence, and the driving voltage of the m sequence is V PRN , V PRN The voltage value is greater than 0 and less than V π , V π is the half-wave voltage of the modulator; the modulation mode is BPSK, and the symbol 0 included in the m sequence corresponds to the driving voltage-V PRN , at this time, the optical field phase of the double sideband probe light remains the original phase; the symbol 1 included in the m sequence corresponds to the driving voltage V PRN , at this time, the optical field phase of the double sideband probe light is shifted by π; at this time, the spread spectrum double sideband probe light is represented as:

[0075]

[0076] wherein, is the optical field amplitude of the positive sideband probe light, is the initial frequency of the positive sideband probe light, is the optical field amplitude of the negative sideband probe light, is the initial frequency of the negative sideband probe light, and represent opposite sweep slopes, represents the intrinsic phase noise of the laser, is the center wavelength Frequency drift noise caused by the free operation of the laser. This represents an m-sequence.

[0077] 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.

[0078] This application achieves signal crosstalk by randomly encoding the double-sideband probe light and using pseudo-random noise code (PRN) phase modulation, so that each Rayleigh scattering point is marked with an m-code, and the signal mixing under the same fiber delay is recovered.

[0079] The spread-spectrum double-sideband probe light enters the sensing fiber and is reflected back into Rayleigh scattered light. The back-scattered Rayleigh scattered light interferes with the double-sideband local oscillator light to output up-and-down sweep beat frequency signals. The polarization state of the double-sideband local oscillator light is the same as that of the double-sideband probe light.

[0080] As an example, the sensing fiber is one of the following: ordinary single-mode fiber, polarization-maintaining fiber, weak reflection grating array fiber, and Rayleigh scattering enhanced fiber.

[0081] Separate the upper and lower sweep frequency beat signals to obtain the upper sweep frequency beat signal and the lower sweep frequency beat signal;

[0082] As an example, a wavelength division multiplexer (WDM) is used to separate the upper and lower sweep beat signals to obtain an upper sweep beat signal and a lower sweep beat signal. Typically, phase noise increases with fiber distance, causing the OFDR trace's signal-to-noise ratio to deteriorate with distance. The upper sweep beat signal is denoted as... , Represented as:

[0083]

[0084] The down-sweep frequency beat frequency signal is denoted as , Represented as:

[0085]

[0086] in, This refers to the photocurrent of the up-sweep frequency beat signal; Indicates the first One sensing channel; This represents the total number of sensor channels; For the first Round-trip delay of each sensor channel; The reflectance at the Rayleigh scattering point; For delay Phase noise of the Rayleigh backscattered beat frequency signal; The sweep slope; This is the first center frequency of the positive sideband, i.e., the starting frequency of the up-sweep light; The negative sideband has a second center frequency, which is the starting frequency of the down-sweep light; center wavelength Frequency drift noise caused by the free operation of the laser; For the first The m-sequence corresponding to each channel.

[0087] After decoding the upper and lower sweep frequency beat signals respectively, the beat signal corresponding to the same sensing channel of the sensing fiber is separated. The specific process is as follows:

[0088]

[0089]

[0090] Then, the beat frequency signals of the corresponding channels of the upper and lower sweep frequency beat frequency signals are extracted by bandpass filtering, and then mixed to obtain the mixed signal after phase frequency noise suppression, specifically:

[0091]

[0092] Finally, the spectrum of the mixing signal is analyzed, and the mixed signal after phase frequency noise suppression is denoted as: , is represented as:

[0093]

[0094] It can be seen that the phase frequency noise of the mixed signal is completely eliminated. Then, the Fourier phase information of the frequency domain signal within each sweep cycle is extracted. By using the difference on the distance axis and the slowly varying time axis, the entanglement value of the phase change caused by the signal under test at each moment is obtained. .

[0095] Secondly, embodiments of the present invention provide a phase-frequency noise-suppressing optical frequency domain reflectance measurement system without an auxiliary interferometer, see [link to relevant documentation]. Figure 2 It includes: a positive and negative double-sideband sweep frequency acquisition device 1, a beam splitter 2, a spread spectrum device 3, an interferometer, a beat frequency signal acquisition device, and a data acquisition and processing module 6;

[0096] Positive and negative double-sideband swept light acquisition device 1: The laser is swept, externally modulated and filtered to obtain positive and negative double-sideband swept light;

[0097] See Figure 3 As one possible implementation, the positive and negative double-sideband swept-frequency optical acquisition device 1 includes:

[0098] Laser 10 is used to emit laser light;

[0099] The sweep signal generator 11 is used to generate a sweep signal;

[0100] The phase modulator 12 receives the laser and the sweep signal, modulates the laser with the sweep signal, and generates a series of positive sidebands and negative sidebands on both sides of the preset central optical frequency;

[0101] The optical band-pass filter 13 obtains the positive sidebands and the negative sidebands corresponding to the order to obtain the double-sideband sweep light, and the sweep ranges are the same and the sweep directions are opposite.

[0102] The optical splitter 2 splits the positive and negative double-sideband sweep light into double-sideband probe light and double-sideband local light according to a preset ratio;

[0103] The spread spectrum device 3 includes a modulator 30, a bias point controller 31, and a pseudo-random noise code generator 32. The bias point controller 31 is used to provide a bias voltage for the modulator 30, and the bias voltage is equal to the half-wave voltage of the modulator. The modulator 30 receives the double-sideband probe light under the control of the bias voltage, modulates the pseudo-random noise code sent by the pseudo-random noise code generator 32 to the double-sideband probe light, and obtains spread spectrum double-sideband probe light;

[0104] The interference device 4 includes a circulator 40 and an optical coupler 41. The circulator 40 has a port a in communication with the modulator 30, a port b in communication with the sensing optical fiber, and an output port c. The spread spectrum double-sideband probe light enters the circulator 40 through the port a, then enters the sensing optical fiber through the port b, and then reflects back to the backscattering light. The backscattering light is output to the optical coupler 41 through the port c, and then interferes with the double-sideband local light in the optical coupler 41 to output the up and down sweep beat signals;

[0105] The beat signal acquisition device includes a first wavelength division multiplexer 50 and a second wavelength division multiplexer 51, which are connected to the optical coupler 41 at the same time. The first wavelength division multiplexer 50 and the second wavelength division multiplexer 51 are connected to the first balanced photodetector 52 and the second balanced photodetector 53 at the same time. The first wavelength division multiplexer 50 and the second wavelength division multiplexer 51 receive the up and down sweep beat signals at the same time, and then perform wavelength division multiplexing to send the signals to the first balanced photodetector 52 and the second balanced photodetector 53, respectively. That is, the first balanced photodetector 52 receives the up sweep beat signal, and the second balanced photodetector 53 receives the down sweep beat signal;

[0106] The data acquisition and processing module 6 decodes the up and down sweep beat signals, respectively, separates the beat signals of each channel of the sensing optical fiber, extracts the beat signals of the corresponding channels of the up and down sweep beat signals through band-pass filtering, then performs mixing processing to obtain the mixed signal after suppressing the phase frequency noise, and finally analyzes the frequency spectrum of the mixed signal. Figure 2 Figure 4 The data acquisition and processing module 6 decodes the up and down sweep beat signals, respectively, separates the beat signals of each channel of the sensing optical fiber, extracts the beat signals of the corresponding channels of the up and down sweep beat signals through band-pass filtering, then performs mixing processing to obtain the mixed signal after suppressing the phase frequency noise, and finally analyzes the frequency spectrum of the mixed signal.​

[0107] See Figure 5 In figure (a), the test results of the traditional OFDR measurement scheme are shown. Due to the influence of phase noise, the Rayleigh scattering point exhibits significant broadening in the OFDR trace, leading to a decrease in measurement accuracy. In contrast, Figure 5 Figure (b) illustrates the measurement results of the method in this embodiment, which effectively eliminates laser phase noise, significantly improves the broadening problem of Rayleigh scattering points, and clearly distinguishes the tail-end reflection peak, fully demonstrating the superiority of this method. This improvement significantly enhances the measurement accuracy and reliability of the OFDR system.

[0108] 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.

[0109] 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 measuring optical frequency domain reflectance with phase frequency noise suppression without an auxiliary interferometer, characterized in that, Includes the following steps: Obtain positive and negative double-sideband swept light. The positive sideband is the upper sweep frequency and the negative sideband is the lower sweep frequency. The sweep frequency range of the positive and negative double-sideband swept light is the same and the sweep frequency direction is opposite. The positive sideband has a first center frequency and the negative sideband has a second center frequency. According to a preset ratio, the positive and negative double-sideband sweep light is divided into double-sideband probe light and double-sideband local oscillator light; Configure a pseudo-random noise code and modulate the pseudo-random noise code onto the double-sideband probe light to obtain a 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-scattered Rayleigh scattered light interferes with the double-sideband local oscillator light to output up-and-down sweep beat frequency signals. The polarization state of the double-sideband local oscillator light is the same as that of the double-sideband probe light. Separate the upper and lower sweep frequency beat signals to obtain the upper sweep frequency beat signal and the lower sweep frequency beat signal; After decoding the upper and lower sweep beat signals respectively, the beat signals of the same sensing channel of the sensing fiber are separated. Then, the beat signals of the corresponding channels of the upper and lower sweep beat signals are extracted by bandpass filtering. Subsequently, the mixing process is performed to obtain the mixed signal after phase frequency noise suppression. Finally, the spectrum of the mixed signal is analyzed.

2. The method for phase frequency noise suppression optical frequency domain reflectance measurement without auxiliary interferometer 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.

3. The method for phase frequency noise suppression optical frequency domain reflectance measurement without auxiliary interferometer as described in claim 1, characterized in that, The pseudo-random noise code is any one or a combination of at least two of the following: m-sequence, Gold code, Gray code, A1 code, and A2 code.

4. The method for phase frequency noise suppression optical frequency domain reflectance measurement without auxiliary interferometer as described in claim 2, characterized in that, The pseudo-random noise code is an m-sequence, and the driving voltage of the m-sequence is... V PRN V PRN Voltage value greater than 0 and less than V π V π The half-wave voltage of the modulator; the modulation method is BPSK, and the symbol 0 in the m-sequence corresponds to the driving voltage -V. PRN 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. PRN 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.

5. The method for phase frequency noise suppression optical frequency domain reflectance measurement without auxiliary interferometer according to claim 1, characterized in that, The up-sweep frequency beat frequency signal is denoted as , Represented as: The down-sweep frequency beat frequency signal is denoted as , Represented as: in, For the up-sweep frequency beat frequency signal photocurrent, The photocurrent for the down-sweep beat frequency signal; Indicates the first One sensing channel; This represents the total number of sensor channels; For the first Round-trip delay of each sensor channel; The reflectance at the Rayleigh scattering point; For delay Phase noise of the Rayleigh backscattered beat frequency signal; The sweep slope; This is the first center frequency of the positive sideband, i.e., the starting frequency of the up-sweep light; The negative sideband has a second center frequency, which is the starting frequency of the down-sweep light; center wavelength Frequency drift noise caused by the free operation of the laser; For the first The m-sequence corresponding to each channel.

6. The method for phase frequency noise suppression optical frequency domain reflectance measurement without auxiliary interferometer as described in claim 5, characterized in that, After decoding the upper and lower sweep frequency beat signals respectively, the beat signal corresponding to the same sensing channel of the sensing fiber is separated. The specific process is as follows: ； 。 7. The method for phase-frequency noise suppression optical frequency domain reflectance measurement without auxiliary interferometer as described in claim 6, characterized in that, The beat frequency signals of the two sidebands corresponding to the sensing channels are bandpass filtered and then mixed to obtain the mixed signals of each sensing channel after phase frequency noise suppression. Specifically: 。 8. The method for phase-frequency noise suppression optical frequency domain reflectance measurement without auxiliary interferometer as described in claim 7, characterized in that, The mixed signal obtained after decoding all sensor channels and suppressing phase frequency noise is denoted as , is represented as: 。 9. A phase-frequency noise-suppressing optical frequency domain reflectance measurement system without an auxiliary interferometer, characterized in that, include: A positive and negative double-sideband swept light acquisition device, in which laser light is obtained after sweeping, 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 a modulator, a bias point controller, and a pseudo-random noise code generator. The bias point controller is used to provide a bias voltage to the modulator, which is equal to the half-wave voltage of the modulator. After receiving the double-sideband probe light under the control of the bias voltage, the modulator modulates the pseudo-random noise code sent by the pseudo-random noise 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 a 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 balanced photodetector and a second balanced photodetector, both connected to the first and second WDM. The first and second WDM simultaneously receive the upper and lower sweep beat frequency signals, perform wavelength division multiplexing, and then transmit the signals to the first and second balanced photodetectors respectively. Specifically, the first balanced photodetector receives the upper sweep beat frequency signal, and the second balanced photodetector receives the lower sweep beat frequency signal. The data acquisition and processing module decodes the upper and lower sweep frequency beat signals to separate the beat signals of each channel of the sensing fiber. Then, it extracts the beat signals of the corresponding channels of the upper and lower sweep frequency beat signals through bandpass filtering. Subsequently, it performs mixing processing to obtain the mixed signal after phase frequency noise suppression. Finally, it analyzes the spectrum of the mixed signal.

10. The phase-frequency noise suppression optical frequency domain reflectance measurement system without auxiliary interferometer according to claim 9, characterized in that, The positive and negative dual-sideband swept-frequency optical acquisition device includes: A laser, used to emit laser light; A sweep frequency signal generator is used to generate sweep frequency signals. The phase modulator receives laser light and a frequency sweep signal. After modulating the laser light with the frequency sweep signal, it generates a series of positive and negative sidebands on both sides of the preset center optical frequency. And an optical bandpass filter, to obtain the positive and negative sidebands of the corresponding order to obtain double-sideband swept light, with the same sweep range and opposite sweep directions.

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