An optical frequency domain reflectometer with environmental disturbance resistance and large strain measurement capability

Abstract

The application discloses an optical frequency domain reflectometer with environmental disturbance resistance and large strain measurement capability, and belongs to the field of distributed optical fiber sensing.The optical frequency domain reflectometer comprises a frequency-modulated continuous wave signal generation module, a light splitting device, a modulation device, a circulator, a polarization controller, an optical coupling device, an optoelectronic conversion module and a data acquisition and processing module.The frequency-modulated continuous wave signal generation module generates an optical signal, which is split into probe light and local oscillator light by the light splitting device.The probe light is modulated by the modulation device and is sent to an optical fiber by the circulator.The Rayleigh backscattering light signal generated by the optical fiber and the local oscillator light interfere with each other in the optical coupling device to obtain interference light.The optoelectronic conversion module mixes beat signals of the interference light, and the data acquisition and processing module analyzes and processes the beat signals.The probe light is modulated to mark the position information of the Rayleigh backscattering point, and the Rayleigh scattering signal corresponding to the position is restored, so that the crosstalk between channels is suppressed, and high-precision measurement with large dynamic range and environmental disturbance resistance is realized.

Description

An optical frequency domain reflectometer that is resistant to environmental disturbances and has large strain measurement capabilities Technical Field

[0001] This invention belongs to the field of distributed optical fiber sensing. Specifically, it relates to an optical frequency domain reflectometer that is resistant to environmental disturbances and has the capability to measure large strain. Background Technology

[0002] Distributed fiber optic sensing uses optical fiber as the sensing medium and features long-distance, multi-channel continuous coverage, high spatial resolution, lightweight design, and real-time monitoring capabilities. It can infer changes in external information, such as loss, strain, and temperature, by sensing the influence of the external environment on the parameters of the light waves propagating in the sensing fiber. Optical Frequency Domain Reflectometry (OFDR) uses linearly swept light as the probe light. By analyzing the amplitude and phase of the Rayleigh Backscattering (RBS) signal in the sensing fiber, it can accurately locate and sense external disturbances at various points along the fiber. Specifically, when the external environment (such as temperature, stress, and vibration) changes, it alters the local refractive index of the fiber through the photoelastic or thermo-optical effects, or causes changes in fiber length through mechanical deformation, thereby altering the amplitude, phase, or frequency characteristics of the RBS in the fiber. By acquiring these RBS signals through optoelectronic devices and combining them with signal processing algorithms, the spatiotemporal distribution and changes of external information along the sensing fiber can be inferred with high precision. Compared to traditional distributed fiber optic sensing technology that relies primarily on optical temporal reflectance, this technology has the potential for high sensitivity, high spatial resolution, and long-distance monitoring. It has attracted widespread attention in applications such as structural health monitoring, geological disaster early warning, and marine environmental observation, and is an important development direction in the field of distributed fiber optic sensing technology.

[0003] Phase-sensitive OFDR technology, which has emerged in recent years, can obtain environmental parameter transformation information by directly demodulating the phase changes of the RBS signal. Benefiting from its optical wave phase-level sensitivity, it provides an effective technical approach to improve measurement sensitivity. However, while offering high sensitivity, it also faces a series of scientific problems and technical challenges. On the one hand, the 2π-periodic phase winding means that the demodulation range of the sensor usually cannot exceed the wavelength of a single light wave. Considering the ultrashort wavelength characteristics of light, this problem severely limits the effective measurement range. On the other hand, due to the physical continuity of optical fibers, various external environmental noise interferences accumulate with increasing fiber distance, specifically manifested as additional phase modulation or Doppler frequency shift applied to the RBS signal. The resulting frequency distortion will directly damage the measurement accuracy, introduce large errors, or even cause demodulation failure for OFDR based on frequency-distance mapping. These problems pose a serious challenge to phase-sensitive OFDR technology.

[0004] The most direct solution is to directly correct the frequency shift caused by the Doppler effect using frequency tracking. However, this method only tracks the Doppler frequency shift and cannot address frequency distortion caused by increasing accumulated errors. Based on the fundamental mechanism of wavelength-limited range, researchers proposed a dual-wavelength phase-sensitive OFDR measurement method. This method uses dual-wavelength swept signals with different starting wavelengths for measurement. By demodulating the RBS phases obtained from the two wavelengths separately and employing phase differential or frequency differential processing methods, it can effectively overcome the measurement accuracy degradation or demodulation failure caused by frequency distortion due to additional phase modulation and Doppler frequency shift, thereby improving the dynamic range.

[0005] Chinese invention patent CN115235367A discloses a high-precision dual-frequency optical frequency domain reflectometer with a large strain measurement range, comprising: an optical modulation module for combining continuous light beams and modulating dual-frequency continuous light into swept-frequency continuous light; an optical interference module for interfering backscattered light emitted from the optical fiber under test with the swept-frequency continuous light, and adjusting the polarization state of the backscattered light to obtain interference light; a photoelectric conversion module for converting the interference light into an electrical signal; and an acquisition and processing module connected to the photoelectric conversion module for acquiring data and analyzing and processing the data. While this scheme possesses large strain measurement capabilities, its resistance to environmental disturbances still needs improvement.

[0006] Chinese invention patent CN113804299A discloses a distributed bidirectional polarization measurement device for fiber optic devices based on optical frequency domain interferometry, belonging to the field of fiber optic measurement technology. It includes a tunable laser source module, a device under test (DUT) module, an optical frequency domain interferometric reflection measurement module, an optical frequency domain interferometric transmission measurement module, and a signal acquisition and analysis module. Its key feature is the use of a linearly swept laser as the interrogation light. The interrogation light passes through a polarizer and is injected into the DUT. The Rayleigh scattering light returns after passing through the polarizer and is injected into the optical frequency domain interferometric reflection measurement module, while the forward propagation light and polarization-coupled light are injected into the optical frequency domain interferometric transmission measurement module. The distributed Rayleigh scattering spectrum and distributed polarization crosstalk spectrum of the fiber optic device are measured respectively, and the data are fused using a cross-correlation algorithm. To improve anti-interference capability, this patent solution uses a combination of OFDR and OFDP, resulting in a relatively complex structure.

[0007] Therefore, existing phase-sensitive OFDR distributed fiber optic sensing technology faces challenges such as limited maximum measurement range and frequency distortion damaging measurement accuracy and causing measurement failures, which restricts its practical application in real and complex environments. Summary of the Invention

[0008] This invention provides an optical frequency domain reflectometer that is resistant to environmental disturbances and has the ability to measure large strain, in order to at least partially solve the above-mentioned technical problems.

[0009] As a first aspect of the present invention, an optical frequency domain reflectometer with resistance to environmental disturbances and large strain measurement capability is provided, comprising:

[0010] The opposing frequency sweep optical frequency modulated continuous wave signal generation module generates opposing frequency sweep optical frequency modulated continuous wave signals composed of beams with the same sweep range but opposite directions.

[0011] The beam splitter divides the opposing frequency-sweeping optical frequency-modulated continuous wave signal into probe light and local oscillator light.

[0012] The modulation device modulates the probe light using a pseudo-random noise code via binary phase-shift keying.

[0013] The circulator sends the modulated probe light to the sensing fiber and outputs the Rayleigh backscattered light signal generated in the sensing fiber.

[0014] A polarization controller controls the polarization state of the local oscillator light, making the polarization state of the local oscillator light match the Rayleigh backscattered light signal output by the circulator.

[0015] An optical coupling device interferes the local oscillator light with the Rayleigh backscattered light signal to obtain interference light;

[0016] The photoelectric conversion module converts the interference light into an electrical signal, obtains the beat frequency signals of the upper and lower frequency sweep light, and mixes the beat frequency signals.

[0017] The data acquisition and processing module is connected to the photoelectric conversion module to analyze and process the mixed beat frequency signal.

[0018] The counter-sweeping frequency-modulated continuous wave signal generation module includes a first tunable laser, a second tunable laser, and a first wavelength division multiplexer; the first tunable laser and the second tunable laser are combined through the first wavelength division multiplexer; the starting optical frequency of the first tunable laser... Greater than the starting optical frequency of the second tunable laser .

[0019] The counter-sweeping frequency-modulated continuous wave signal generation module includes a single-frequency laser, a phase modulator, a first signal generator, and an optical bandpass filter. The first signal generator generates a sweeping signal to modulate the laser emitted by the single-frequency laser through the phase modulator, and the optical bandpass filter extracts the corresponding positive and negative sidebands.

[0020] The beam splitting device includes a first optical coupler, which distributes the probe light and the local oscillator light into different energies.

[0021] The energy ratio of the probe light to the local oscillator light is 90:10.

[0022] The modulation device includes a Mach-Zehnder modulator, which operates in carrier suppression mode and uses BPSK modulation mode. The pseudo-random noise code is output from a second signal generator to the Mach-Zehnder modulator.

[0023] The pseudo-random noise code is an M-sequence, Gold code, Gray code, A1 code, or A2 code.

[0024] The optical coupling device includes a second optical coupler, and the photoelectric conversion module includes a second wavelength division multiplexer, a third wavelength division multiplexer, a first balanced photodetector, a second balanced photodetector, and an electric mixer.

[0025] The second wavelength division multiplexer and the third wavelength division multiplexer are connected to the second optical coupler;

[0026] The first balanced photodetector is connected to the second wavelength division multiplexer and the third wavelength division multiplexer to receive the beat frequency signal of the up-sweep light.

[0027] The second balanced photodetector is connected to the second wavelength division multiplexer and the third wavelength division multiplexer to receive the beat frequency signal of the down-sweep light.

[0028] The bandwidth of the electric mixer is 1~3600MHz.

[0029] The sensing fiber is a common single-mode fiber, a polarization-maintaining fiber, a weak reflection grating array fiber, or a Rayleigh scattering enhanced fiber.

[0030] Based on the above solution, it can be seen that the optical frequency domain reflectometer of the present invention, which is resistant to environmental disturbances and has large strain measurement capabilities, has at least the following advantages over the prior art:

[0031] This invention generates opposing frequency-modulated continuous wave (FM) signals with the same sweep range but opposite directions using a counter-sweeping frequency-modulated (CSLM) signal generation module. These signals are then split into probe light and local oscillator light using a beam splitter. The probe light is then modulated using pseudo-random spread spectrum coding with binary phase shift keying (BPSK), and subsequently used for sensing. The BPSK-modulated probe light marks the position information of each Rayleigh backscattering point. Finally, the beat frequency signals of the two complementary CSLM signals are mixed. Due to the correlation characteristics of the pseudo-random sequence, the mixing process only restores the Rayleigh scattering signal at the corresponding position of the matched coded sequence, greatly suppressing crosstalk between channels. This process is equivalent to forming a radio frequency carrier with a frequency difference between the two light waves for measurement and sensing, improving the maximum dynamic range and suppressing the influence of frequency distortion on phase demodulation. Attached Figure Description

[0032] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the embodiments will be briefly described below.

[0033] Figure 1 is a schematic diagram of the optical frequency domain reflectometer of the present invention, which is resistant to environmental disturbances and has large strain measurement capabilities.

[0034] Figure 2 is a schematic diagram of one implementation of the counter-sweeping frequency-modulated continuous wave signal generation module of the present invention;

[0035] Figure 3 is a schematic diagram of another implementation of the counter-sweeping frequency-modulated continuous wave signal generation module of the present invention;

[0036] Figure 4 shows a comparison of the traces of an existing single-wavelength optical frequency domain reflectometer and the optical frequency domain reflectometer of this invention.

[0037] Figure 5 shows the dynamic large strain demodulation results of the traditional dual-wavelength optical frequency domain reflectometer and the optical frequency domain reflectometer of the present invention.

[0038] In the above figures, the meanings of the reference numerals are as follows:

[0039] In the diagram: 1. First tunable laser; 2. Second tunable laser; 3. First wavelength division multiplexer; 4. Single-frequency laser; 5. Phase modulator; 6. First signal generator; 7. Optical bandpass filter; 8. Optical sweep frequency modulated continuous wave signal generation module; 9. First optical coupler; 10. Mach-Zehnder modulator; 11. MZM bias point controller; 12. Second signal generator; 13. Circulator; 14. Sensing fiber; 15. Polarization controller; 16. Second optical coupler; 17. Second wavelength division multiplexer; 18. Third wavelength division multiplexer; 19. First balanced photodetector; 20. Second balanced photodetector; 21. Electrical mixer; 22. Data acquisition and processing module. Detailed Implementation

[0040] To better understand the technical solutions of the embodiments of the present invention, the embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0041] It should be understood that the described embodiments are merely some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0042] Referring to Figure 1, this embodiment of the invention discloses an optical frequency domain reflectometer that is resistant to environmental disturbances and has large strain measurement capabilities, including a counter-sweeping frequency-modulated continuous wave signal generation module 8, a beam splitter, a modulation device, a circulator 13, a polarization controller 15, an optical coupling device, a photoelectric conversion module, and a data acquisition and processing module.

[0043] The opposing frequency-sweeping optical frequency-modulated continuous wave signal generation module 8 is used to generate upper and lower frequency-sweeping lights with the same sweeping range but opposite sweeping directions. The upper and lower frequency-sweeping lights are combined to form an opposing frequency-sweeping optical frequency-modulated continuous wave signal, wherein the starting frequency of the upper frequency-sweeping light is greater than the starting frequency of the lower frequency-sweeping light.

[0044] As shown in Figure 2, in one embodiment, the opposing frequency-sweeping optical frequency-modulated continuous wave signal generation module 8 includes a first tunable laser 1, a second tunable laser 2, and a first wavelength division multiplexer 3. The first tunable laser 1 and the second tunable laser 2 are combined by the first wavelength division multiplexer 3; the starting optical frequency of the first tunable laser 1... The starting light frequency is greater than that of the second tunable laser 2. .

[0045] The sweep frequency range of these two tunable lasers can be set as needed, for example, the sweep frequency range can be set to 100MHz. These two tunable lasers can be set as C-band lasers, for example, the starting light frequency of the first tunable laser 1 is 194.05THz (wavelength 1546nm), and the starting light frequency of the second tunable laser 2 is 193.05THz (wavelength 1554nm).

[0046] The laser frequency emitted by the first tunable laser 1 varies continuously between 194.05 THz and 194.05 THz + 100 MHz, hence it is called up-sweeping laser.

[0047] The laser frequency emitted by the second tunable laser 2 varies between 193.05 THz and 193.05 THZ-100 MHz, thus becoming a downscan laser.

[0048] The first wavelength division multiplexer combines the light from two tunable lasers to form a counter-sweeping frequency-modulated continuous wave signal with the same sweep range but opposite sweep directions.

[0049] As shown in Figure 3, this invention also discloses a counter-sweeping frequency-modulated continuous wave signal generation module 8 with another structure, including a single-frequency laser 4, a phase modulator 5, a first signal generator 6, and an optical bandpass filter 7. The first signal generator 6 generates a sweeping signal to modulate the laser emitted by the single-frequency laser 4 through the phase modulator 5, and the optical bandpass filter 7 extracts the corresponding positive and negative sidebands.

[0050] The center optical frequency of the single-frequency laser 4 is After modulation by phase modulator 5, the optical carrier will be in the spectrum. A series of sidebands (i.e., positive and negative sidebands) are generated on both sides. The corresponding positive and negative sidebands are extracted by the optical bandpass filter 7. In this embodiment, the frequency sweep range of both positive and negative sidebands is 100MHz, but the frequency sweep direction is opposite.

[0051] The beam splitter is used to split the opposing frequency-sweeping optical frequency-modulated continuous wave signal into probe light and local oscillator light. In this embodiment, the beam splitter is a first optical coupler 9. When the first optical coupler 9 splits the opposing frequency-sweeping optical frequency-modulated continuous wave signal into probe light and local oscillator light, the two can have different energy ratios, for example, the energy ratio of probe light to local oscillator light is 90:10.

[0052] The modulation device is used to perform binary phase-shift keying modulation on the probe light using pseudo-random noise code. In this embodiment, the modulation device is a Mach-Zehnder modulator 10, which is generally abbreviated as MZM. The model used in this embodiment is MXAN-LN-20. Other types of MZMs can also be used. The MZM-LN-20 can operate in the 1550nm band and has a modulation bandwidth of 20GHz. Its half-wave voltage V π Approximately 5V. The MZM bias point controller 11 is used to provide the bias voltage V for the Mach-Zehnder modulator 10. B Its magnitude is equal to the MZM half-wave voltage V. π This maintains the Mach-Zehnder modulator 10 at the zero (Null) operating point.

[0053] At this operating point, the carrier wave and even harmonics from the laser are completely suppressed, hence this mode is also called carrier suppression. The second signal generator 12 is used to transmit pseudo-random noise (PRN) codes. PRN codes include, but are not limited to, m-sequences, and can also employ Gold codes, Gray codes, A1 codes, A2 codes, etc.

[0054] In this embodiment, the PRN code used is an m-sequence, and the voltage range of the m-sequence output by the second signal generator 12 is controlled within -V. π ~ +V π This modulation mode belongs to the BPSK method, which is characterized by a π-phase flip of the optical field when a symbol flip occurs.

[0055] Specifically, in the m-sequence, the symbol "0" corresponds to the driving voltage -Vπ, at which point the optical field phase remains unchanged, and the symbol "1" corresponds to the driving voltage Vπ. π At this point, the phase of the light field undergoes a phase shift of π. The light signal can then be expressed as:

[0056]

[0057] Where E1, And E2, These represent the optical field amplitude and initial frequency of the up-sweep and down-sweep light, respectively. and Indicates the opposite sweep slope, This represents the m-sequence (±1) output by the second signal generator 12, where i is a complex number, t is time, and exp is the complex exponent. When the chip frequency of the m-code equals the sweep frequency range, Based on the specific time delay of each spatial resolution, it will ensure that each channel is individually labeled, measured and isolated.

[0058] Circulator 13 transmits the modulated probe light to sensing fiber 14 and outputs the Rayleigh backscattered light signal generated within sensing fiber 14. Circulator 13 has ports a, b, and c. Probe light enters circulator 13 through port a and then enters sensing fiber 14 connected to port b. Rayleigh backscattered light signal enters circulator 13 through port b and exits through port c. Sensing fiber 14 is a standard single-mode fiber, polarization-maintaining fiber, weak-reflection grating array fiber, or Rayleigh scattering enhanced fiber. The Rayleigh backscattered light signal exiting through port c enters an optical coupling device. In this embodiment, the optical coupling device is a second optical coupler 16.

[0059] The polarization controller 15 is used to control the polarization state of the local oscillator light, matching it with the Rayleigh backscattered light signal output by the circulator 13. This allows the local oscillator light and the Rayleigh backscattered light signal to interfere with each other.

[0060] The optical coupling device is a second optical coupler 16, used to interfere the local oscillator light with the Rayleigh backscattered light signal to obtain interference light.

[0061] The photoelectric conversion module converts the interference light into electrical signals, obtaining the beat frequency signals of the upper and lower frequency swept light, and then mixes the beat frequency signals. The data acquisition and processing module 22 is connected to the photoelectric conversion module and is used to analyze and process the mixed beat frequency signals.

[0062] In this embodiment, the photoelectric conversion module includes a second wavelength division multiplexer 17, a third wavelength division multiplexer 18, a first balanced photodetector 19, a second balanced photodetector 20, and an electric mixer 21.

[0063] The second wavelength division multiplexer 17 and the third wavelength division multiplexer 18 are connected to the second optical coupler 16 for wavelength division multiplexing.

[0064] The first balanced photodetector 19 is connected to the second wavelength division multiplexer 17 and the third wavelength division multiplexer 18 to receive the beat frequency signal of the upsweep light.

[0065] The second balanced photodetector 20 is connected to the second wavelength division multiplexer 17 and the third wavelength division multiplexer 18, and is used to receive the beat frequency signal of the down-sweep light. The second wavelength division multiplexer 17, the third wavelength division multiplexer 18, the first balanced photodetector 19, and the second balanced photodetector 20 are all existing devices. The first balanced photodetector 19 and the second balanced photodetector 20 are abbreviated as BPD.

[0066] The photocurrent signal output by the BPD is the sum of the Rayleigh backscattered signals along the optical fiber. Under normal circumstances, unwanted environmental disturbances and dynamic signals of interest can occur within a single scan cycle. It is the i-th spatial resolution position on the sensing fiber. The round-trip delay at the location, the time-varying delay caused by the accumulation of previous environmental disturbances. It can be represented as The two photocurrents can be expressed as follows:

[0067]

[0068]

[0069] Ip represents the upper-sweep beat frequency signal, In represents the lower-sweep beat frequency signal, N represents the total number of spatial resolutions of the Rayleigh scattering signal, i represents the index of the i-th Rayleigh scattering point, and R represents the reflectivity of the Rayleigh scattering point. Frequency distortion caused by environmental disturbances is respectively... and Given. Clearly, this time-varying delay will cause frequency distortion, including Doppler shift or harmful phase modulation. Represents m-sequence After time delay The signal after.

[0070] The electric mixer 21 has a bandwidth of 1~3600MHz and is used to mix the photocurrent signals output by the first balanced photodetector 19 and the second balanced photodetector 20. Utilizing the autocorrelation properties of the m-sequence, only the product of Rayleigh backscattered signals from the same location is considered, i.e., they have the same... Only signals with the same spatial resolution can be recovered. Products originating from different locations are suppressed due to the spread spectrum effect; the degree of suppression depends on the autocorrelation coefficient of the m-code, i.e., the length of the m-code. The resulting photocurrent signal I after mixing... mix for:

[0071]

[0072] It can be seen that this system is based on While suppressing frequency distortion, the dynamic range was improved. This significantly reduces frequency distortion caused by environmental disturbances while improving sensing capabilities. The mixed signal is received and processed by the data acquisition and processing module 22, which extracts the Fourier phase information of the frequency domain signal within each sweep cycle. By using the difference on the distance axis and the slowly varying time axis, the entanglement value of the phase change caused by the measured signal at each moment is obtained. .

[0073] in,

[0074]

[0075] In the formula, n is the refractive index of the optical fiber. The fiber strain coefficient Let L be the strain value, L be the length of the strain region, and c be the speed of light in a vacuum. As can be seen from the above equation, compared to single-frequency light, the phase change caused by the same strain magnitude in a dual-frequency strategy system is significantly greater. This reduces the frequency, thus allowing for the measurement of a wider range of dynamic strains. Utilizing a frequency of... and When continuous light is used for detection, the measurable dynamic strain of the dual-frequency measurement system is increased to that of a single-frequency system. Measurement system / Times (or single frequency) Measurement system / times).

[0076] Referring to Figure 4, it can be seen that interference from complex external environments accumulates over distance in optical fibers, causing phase modulation or Doppler shift in the phase term of the Rayleigh backscattered signal. The prior art shown in part (a) of Figure 4 exhibits significant frequency distortion. However, the optical frequency domain reflectometer provided by this invention, which is resistant to environmental disturbances and possesses large strain measurement capabilities, greatly suppresses frequency distortion. This manifests as a stable Rayleigh scattering point in the frequency-distance domain. As can be seen from the Rayleigh scattering distribution in Figure 4 (b), this method not only suppresses frequency distortion but also doubles the spatial resolution; that is, within the same sweep time, the sweep slope is twice as fast, resulting in a doubled sweep range.

[0077] Referring to Figure 5, in part (a) of Figure 5, it can be seen that traditional dual-wavelength phase-sensitive optical frequency domain reflectometers improve dynamic range by demodulating the phases of the Rayleigh backscattered signals obtained from the two wavelengths separately and then subtracting the two phases. However, due to frequency distortion caused by complex environmental noise, single-wavelength demodulation fails, resulting in the inability to obtain the correct phase and introducing significant errors, which can completely mask the true sensing information in severe cases. Part (b) of Figure 5 shows that the method of the present invention has the ability to suppress frequency distortion while also measuring dynamic signals with large strain. The signal in the figure is a dynamic strain with a strain magnitude of 1309 με and a frequency of 0.8 Hz.

[0078] The optical frequency domain reflectometer of this invention achieves high-precision measurement with a large dynamic range, resistant to environmental disturbances, compared to traditional distributed fiber optic sensing technology. Compared to discrete sensors such as FBG sensors, this method possesses distributed fiber optic sensing capabilities (the capabilities of OFDR technology), offering higher accuracy and a greater number of channels.

[0079] This invention generates opposing frequency-modulated continuous wave (FM) signals with the same sweep range but opposite directions using a frequency-sweeping optical signal generation module. After splitting the signals using a beam splitter, the probe light is used to modulate the two optical signals with binary phase-shift keying (BPSK) using pseudo-random spread spectrum coding. The probe light is then used for sensing. The BPSK-modulated probe light marks the position information of each Rayleigh backscattering point. Finally, the beat frequency signals of the two complementary FM continuous waves are mixed. Due to the correlation characteristics of the pseudo-random sequence, only the Rayleigh scattering signal at the corresponding position of the coded sequence is restored after mixing, greatly suppressing crosstalk between channels. This process is equivalent to forming a radio frequency carrier with a frequency difference between the two optical waves for measurement and sensing, improving the maximum dynamic range and suppressing the influence of frequency distortion on phase demodulation. The beat frequency signal of the radio frequency carrier generated by mixing the beat frequency signals of the dual-frequency light is demodulated using the phase method. By extracting the phase change, the sensing information is retrieved. This system achieves high-precision measurement with a large dynamic range and strong resistance to environmental disturbances.

[0080] The optical frequency domain reflectometer of this invention can withstand the interference of environmental noise, suppress the influence of frequency distortion on phase demodulation, and support distributed fiber optic sensing in complex environments.

[0081] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A frequency domain reflectometer with resistance to environmental disturbances and large strain measurement capability, characterized in that, include: A counter-sweeping frequency-modulated continuous wave signal generation module generates upper and lower sweeping light beams with the same sweeping range but opposite sweeping directions, which are then combined to form a counter-sweeping frequency-modulated continuous wave signal. The module includes a first tunable laser, a second tunable laser, and a first wavelength division multiplexer. The first and second tunable lasers are combined using the first wavelength division multiplexer. The starting frequency of the first tunable laser is... Greater than the starting optical frequency of the second tunable laser Alternatively, the opposing frequency-sweeping optical frequency-modulated continuous wave signal generation module includes a single-frequency laser, a phase modulator, a first signal generator, and an optical bandpass filter. The first signal generator generates a sweeping signal to modulate the laser emitted by the single-frequency laser through the phase modulator. The optical bandpass filter extracts the corresponding positive and negative sidebands. A beam splitter separates the opposing frequency-sweeping optical frequency-modulated continuous wave signal into a probe beam and a local oscillator beam. A modulation device modulates the probe beam using a pseudo-random noise code via binary phase-shift keying. A circulator transmits the modulated probe beam to a sensing fiber and outputs a Rayleigh backscattered light signal generated within the sensing fiber. A polarization controller controls the polarization state of the local oscillator beam to match the polarization state of the local oscillator beam with the Rayleigh backscattered light signal output by the circulator. An optical coupling device causes the local oscillator beam to interfere with the Rayleigh backscattered light signal to obtain interference light. A photoelectric conversion module converts interference light into electrical signals, obtaining beat frequency signals of upper-sweep and lower-sweep light, and then mixes the beat frequency signals. A data acquisition and processing module, connected to the photoelectric conversion module, analyzes and processes the mixed beat frequency signals. The optical coupling device includes a second optical coupler, and the photoelectric conversion module includes a second wavelength division multiplexer, a third wavelength division multiplexer, a first balanced photodetector, a second balanced photodetector, and an electrical mixer. The second and third wavelength division multiplexers are connected to the second optical coupler. The first balanced photodetector is connected to the second and third wavelength division multiplexers to receive the beat frequency signal of the upper-sweep light. The second balanced photodetector is connected to the second and third wavelength division multiplexers to receive the beat frequency signal of the lower-sweep light. The chip frequency of the m-code is equal to the sweep frequency range.

2. The optical frequency domain reflectometer with environmental disturbance resistance and large strain measurement capability according to claim 1, characterized in that, The beam splitter includes a first optical coupler that distributes the probe light and the local oscillator light into different energies.

3. The optical frequency domain reflectometer with environmental disturbance resistance and large strain measurement capability according to claim 1, characterized in that, The energy ratio of the probe light to the local oscillator light is 90:

10.

4. The optical frequency domain reflectometer with environmental disturbance resistance and large strain measurement capability according to claim 1, characterized in that, The modulation device includes a Mach-Zehnder modulator, which operates in carrier suppression mode and uses BPSK modulation mode. The pseudo-random noise code is output to the Mach-Zehnder modulator by a second signal generator.

5. The optical frequency domain reflectometer with environmental disturbance resistance and large strain measurement capability according to claim 4, characterized in that, The pseudo-random noise code is an M-sequence, Gold code, Gray code, A1 code, or A2 code.

6. The optical frequency domain reflectometer with environmental disturbance resistance and large strain measurement capability according to claim 1, characterized in that, The bandwidth of the electric mixer is 1~3600MHz.

7. The optical frequency domain reflectometer with environmental disturbance resistance and large strain measurement capability according to claim 1, characterized in that, The sensing fiber is a common single-mode fiber, a polarization-maintaining fiber, a weak reflection grating array fiber, or a Rayleigh scattering enhanced fiber.

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