Evaluation Method for Coherent Fading Suppression Effect of Narrow Linewidth OFDR System

Abstract

This invention discloses a method for evaluating the coherent fading suppression effect of a narrow-linewidth light source OFDR system. The method includes: parameter adjustment of the narrow-linewidth light source to simulate the intrinsic phase noise carried by the laser; parameter adjustment of the coherent fading noise suppression method to quantitatively control its effectiveness; modeling of a one-dimensional fiber OFDR system; using the heterodyne method to demodulate the beat frequency of the backscattered light and the local oscillator light to obtain the required measurement results; generating a quantitative evaluation result of the measurement results, thus completing the simulation and evaluation of the coherent fading suppression effect of the OFDR system. This invention, by independently setting the parameters of the OFDR system and the suppression method, obtains the variation law of the measurement results affected by the coherent fading noise of the non-ideal light source, reflecting the statistical characteristics of coherent fading noise and its suppression effect. The evaluation results are more accurate and comprehensive, enabling multi-variable comparison and multi-angle analysis, providing assistance for system optimization and design in related theoretical research and practical applications.

Description

Technical Field

[0001] This invention belongs to the field of distributed optical fiber sensing and measurement, and particularly relates to a method for evaluating the coherent fading suppression effect of a narrow linewidth light source OFDR system. Background Technology

[0002] With the development of laser and fiber optic technologies, distributed fiber optic sensing and measurement technologies have attracted attention from many scientific research and application fields due to their unique advantages. Among them, distributed fiber optic sensing and measurement technologies based on Rayleigh scattering have the characteristics of high precision and large area coverage, and are widely used in scientific measurement, advanced manufacturing, component monitoring, smart materials and other fields, representing an important development direction for current distributed fiber optic sensing and measurement technologies.

[0003] Distributed fiber optic sensing and measurement technology based on Rayleigh scattering works by injecting a specific probe laser signal into the optical fiber. By measuring the backscattered Rayleigh (RBS) in the echo, physical information along the fiber is obtained. Generally, to improve measurement sensitivity and accuracy, coherent detection is typically used. The beat frequency between the RBS and the local oscillator is acquired within a specific spatial resolution. The intensity and phase of this beat frequency signal are then measured to determine the measured information along the fiber. Typically, probe laser signals can be categorized based on their form, primarily including optical time-domain reflectometry (OTDR) using pulsed probe lasers and optical frequency-domain reflectometry (OFDR) using frequency-modulated continuous-wave probe lasers.

[0004] The refractive index distribution of optical fibers is limited by manufacturing processes and generally exhibits a non-uniform spatial distribution. This directly leads to spatial randomness in the amplitude and phase distribution of the refractive index beams (RBSs) at various points within the fiber. When a large number of RBSs within a finite spatial resolution are coherently superimposed with the local oscillator light at the receiving end, the randomness of the RBS amplitude and phase results in a certain degree of randomness in the superposition of interference. Consequently, within a spatial resolution, the amplitude and phase of the superimposed interference of a large number of RBSs also exhibit a random distribution; this phenomenon is coherent fading.

[0005] Although different systems use different probe laser signals, the presence of coherent fading noise inevitably leads to random fading points in the measurement signal. At these fading points, the amplitude / intensity exhibits local minima, severely degrading the signal-to-noise ratio (SNR) and affecting the accurate demodulation of the intensity and phase at the measurement points. Furthermore, the randomness of the RBS amplitude and phase changes due to external environmental disturbances (such as temperature and strain) on the optical fiber, further exacerbating the impact of coherent fading on SNR.

[0006] Considering the random characteristics of coherent fading noise, the different random distributions of the Relative Shortest Dimensions (RBS) between independent measurements can be utilized to superimpose multiple independent measurement results, thereby effectively suppressing coherent fading noise. Specifically, in distributed fiber optic sensing and measurement systems, methods including polarization diversity, wavelength division multiplexing, multi-core fiber spatial diversity, and multimode fiber mode diversity can be used to achieve multiple independent measurements and superimpose them to suppress coherent fading noise.

[0007] Frequency-Shift Averaging (FSAV) is a typical method for suppressing coherent fading. The response of the Resonant Baseline Fiber (RBS) in an optical fiber is a function of the probe light frequency. This method uses probe lasers of different frequencies to obtain a large number of uncorrelated or low-correlation RBS signals. By superimposing or averaging the measurement results at different frequencies, the randomness of signal intensity is reduced, thus suppressing coherent fading noise. Early on, in 1987, P. Healey analyzed the statistical characteristics of the RBS in single-mode fiber based on linear system theory, pointing out the changes in its statistical characteristics when multiple statistically independent component waves are included. Subsequently, in 1992, K. Shimizu et al. presented the principle of suppressing coherent fading noise in a coherent OTDR system using FSAV and conducted preliminary experimental verification. Their research results showed that in the application of the FSAV method, increasing the number of independent frequencies and decreasing the frequency shift interval both help improve its coherent fading suppression effect. Building upon previous theoretical work, and with the continuous development of distributed sensing and measurement technologies, researchers have further derived the impact of coherent fading noise and its suppression capabilities on distributed sensing and measurement by combining different types of measurement systems. In recent years, Z. Wang et al., based on the phase measurement principle of phase-sensitive OTDR (φ-OTDR) systems, provided a preliminary description of the φ-OTDR phase signal-to-noise ratio considering coherent fading noise suppression by using an approximate formula for the probability density function when multiple statistically independent signals are superimposed. Existing research results indicate that the main factors affecting the effectiveness of the FSAV method include: the frequency variation range, the total frequency shift, the number of frequency changes / number of independent frequencies, and the frequency shift interval.

[0008] In practice, besides coherent fading, laser coherence, i.e., laser phase noise, is a crucial factor affecting the performance of distributed fiber optic sensing and measurement systems based on coherent detection. It not only degrades measurement accuracy but also limits the measurement distance range. Therefore, to improve measurement accuracy and distance range, laser sources with high coherence, i.e., low phase noise, are often required. However, laser phase noise influences the evolution of coherent fading, and high coherence can, to some extent, accentuate the impact of coherent fading noise. That is, the combined effect of laser phase noise and coherent fading noise affects the suppression effect of independent measurement superposition on coherent fading noise.

[0009] As mentioned earlier, existing theoretical models and analytical methods mainly consider the suppression effect of independent measurement superposition on coherent fading noise when it acts alone, but lack consideration for laser phase noise. To address this, research on the statistical distribution characteristics and evolution mechanism of coherent fading noise when considering the presence of laser phase noise, as well as the suppression effect of independent measurement superposition on coherent fading noise in this case, can more accurately analyze the noise transmission process of the system and measure the effectiveness of system parameters. This allows for more precise and effective system optimization and design, significantly improving system performance and possessing significant theoretical and practical value. Summary of the Invention

[0010] To meet the needs of preliminary analysis and scheme verification in practical measurements, this invention analyzes the characteristics of coherent fading noise under non-ideal light sources and the effectiveness of suppression methods for specific OFDR systems and suppression methods. It addresses the limitations of existing methods in analyzing coherent fading noise in OFDR systems, the incomplete analysis of the overall impact of narrow-linewidth light sources, and the relatively weak evaluation results. This invention provides an evaluation method for the coherent fading suppression effect of OFDR systems with narrow-linewidth light sources. By independently setting the parameters of the OFDR system and suppression method, and through simulation experiments and data processing of the evaluation system, the variation law of measurement results affected by coherent fading noise from non-ideal light sources is obtained, reflecting the statistical characteristics of coherent fading noise and its suppression effect. The evaluation results are more accurate and comprehensive, enabling multi-variable comparison and multi-angle analysis, providing assistance for system optimization and design in related theoretical research and practical applications.

[0011] To achieve the above objectives, this invention provides a method for evaluating the coherent fading suppression effect of a narrow-linewidth OFDR system, comprising:

[0012] By adjusting the parameters of a narrow linewidth light source, the intrinsic phase noise carried by the laser is simulated.

[0013] The parameters of the coherent fading noise suppression method are adjusted to quantitatively control the effectiveness of the suppression method.

[0014] Based on the intrinsic phase noise and the modulated parameters, a one-dimensional fiber OFDR system model is performed, an OFDR system model to be evaluated is established, and heterodyne measurement is simulated based on the OFDR system model to be evaluated. The required measurement results are obtained by demodulating the beat frequency of the backscattered light and the local oscillator light.

[0015] The measurement results are quantitatively evaluated to complete the evaluation of coherent fading suppression and its effectiveness in the OFDR system.

[0016] Optionally, parameter adjustment for narrow linewidth light sources includes:

[0017] By inputting the predetermined linewidth value of the OFDR system laser to be evaluated, the laser phase noise level in one evaluation is set, and the overall laser phase noise level of the OFDR system is adjusted according to the usage requirements.

[0018] Optionally, parameter tuning of the coherent fading noise suppression method includes:

[0019] The method for suppressing coherent fading noise based on frequency shift averaging quantitatively controls the effectiveness of the suppression method by setting key parameters, including the number of independent frequencies, frequency shift interval, and total frequency shift amount.

[0020] Optionally, the local oscillator light is:

[0021]

[0022] Among them, e LO (t,0) is the analytical representation of the local oscillator light in complex form, where t is time and E is the distance from the oscillator. s0 c represents the amplitude of the light source. s To apply the heterodyne frequency offset, v m Let γ be the m-th independent frequency, and γ be the sweep slope. Laser phase noise in phase form.

[0023] Optionally, the backscattered light is:

[0024]

[0025] Among them, R r Let τ' be the backscattering Rayleigh coefficient at point r. r The actual delay of the backscattered Rayleigh light generated at point r relative to the local oscillator light.

[0026] Optionally, obtaining the desired measurement results includes:

[0027] The OFDR system uses the heterodyne method to obtain the total signal intensity measured by the sensing unit as the sum of the beat frequencies between the backscattered light and the local oscillator light generated at different scattering elements of the same frequency;

[0028] Perform a Fourier transform on the sum of the beat frequencies to obtain the amplitude and phase angle at the preset frequency point, and obtain the required measurement results.

[0029] Optionally, the sum of the beat frequencies is:

[0030]

[0031] Among them, i A The sum of beat frequencies is N, where N is the number of backscattering Rayleigh elements within Δz, and n is the number of independent frequencies.

[0032] Compared with the prior art, the present invention has the following advantages and technical effects:

[0033] This invention, through the independent setting of parameters for the OFDR system and suppression method, and by evaluating the simulation experiments and data processing of the system, obtains the variation law of the measurement results affected by the coherent fading noise of non-ideal light sources, reflects the statistical characteristics of coherent fading noise and its suppression effect, and provides more accurate and comprehensive evaluation results. It can compare multiple variables and analyze from multiple perspectives, providing assistance for system optimization and design in relevant theoretical research and practical applications. Attached Figure Description

[0034] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings:

[0035] Figure 1 This is a schematic diagram of the process for evaluating the coherent fading suppression effect of a narrow-linewidth OFDR system according to an embodiment of the present invention.

[0036] Figure 2 This is a schematic diagram illustrating the principle of coherent fading noise suppression in the OFDR system according to an embodiment of the present invention.

[0037] Figure 3 This is a schematic diagram showing the results of evaluating the effectiveness of the intensity suppression method according to an embodiment of the present invention;

[0038] Figure 4 This is a schematic diagram showing the results of evaluating the effectiveness of the intensity suppression method according to an embodiment of the present invention;

[0039] Figure 5 This is a schematic diagram showing the results of evaluating the effectiveness of the intensity suppression method according to an embodiment of the present invention;

[0040] Figure 6 This is a schematic diagram showing the results of evaluating the effectiveness of the intensity suppression method according to an embodiment of the present invention. Detailed Implementation

[0041] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0042] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0043] like Figure 1As shown, the method for evaluating the coherent fading suppression effect of the narrow linewidth light source OFDR system proposed in this embodiment includes:

[0044] Step 1. Narrow linewidth light source parameter adjustment: Set the laser phase noise level in one evaluation process by inputting the predetermined linewidth value of the laser in the OFDR system to be evaluated, so as to simulate the intrinsic phase noise carried by the laser in the experiment.

[0045] Step 2. Suppression method parameter adjustment: The OFDR system to be evaluated is based on the principle of suppressing coherent fading noise by frequency shift averaging. By setting key parameters such as the number of independent frequencies, frequency shift interval, and total frequency shift amount, the effectiveness of the suppression method can be quantitatively controlled.

[0046] Step 3. Heterodyne measurement: After the parameters are set, the OFDR system to be evaluated will use the heterodyne method to demodulate and obtain the required measurement results based on the beat frequency of the backscattered light and the local oscillator light.

[0047] Step 4. Statistical analysis and evaluation: This system will perform statistical analysis on the measurement results, resulting in more accurate and comprehensive results. It can perform multi-variable comparisons and multi-angle analysis for quantitative evaluation.

[0048] In a specific embodiment, by setting the parameters of the OFDR system to be evaluated in steps 1 and 2, the swept frequency signal generated by the narrow linewidth light source can be analytically represented as:

[0049]

[0050] Among them, E s0 v0 is the amplitude of the light source, v0 is the initial frequency of the light source, and γ is the sweep slope. The laser phase noise is in phase form. The parameters of the swept frequency signal are determined according to the settings in steps 1 and 2, and simulation is performed in the model in step 3.

[0051] By inputting the predetermined linewidth value Δv of the OFDR system laser to be evaluated. FWHM The laser phase noise level is set during an initial evaluation, and the overall laser phase noise level of the OFDR system is adjusted according to usage requirements. In distributed sensing and measurement, at a certain linewidth, the incremental change increases with the increase of measurement distance. The corresponding time delay τ x As the distance increases, the effect of laser phase noise will gradually become apparent. When the measurement distance exceeds the coherence length, the laser phase noise tends to stabilize at a certain level, and the system will be unable to perform measurements.

[0052] The OFDR system model to be evaluated is described below, specifically corresponding to step 3, obtaining the beat frequency signal.

[0053] In one specific embodiment, for a segment of optical fiber under test, the measurement signal is analyzed in units of spatial resolution Δz, where Δz is determined by the frequency sweep signal parameters of the OFDR system.

[0054]

[0055] Where c is the speed of light in a vacuum, n e F is the refractive index of the optical fiber. s The frequency sweep range is defined as follows: the fiber under test is divided into a series of sensing units of equal length based on the length of Δz.

[0056] In a specific embodiment, let the initial optical frequency be v0. When the frequency shifting averaging method is used, the incident light will contain n different frequency components, which change at equal intervals from v0 to v0+v, with the interval between changes being Δv = v / n.

[0057] Because the photodetector at the receiver of an OFDR system has a limited response bandwidth, the total signal i A The final effective form at the receiving end is

[0058]

[0059] Where n is the number of independent frequencies in the system, Δv is the frequency shift interval, N is the number of backscattering Rayleigh elements within Δz, p and q represent the numbers of any two scattering elements arranged along the fiber, and R p R q Let τ' be the backscattering Rayleigh coefficients at points p and q, respectively. p 、τ' q These represent the actual delay of the backscattered Rayleigh light generated at points p and q relative to the local oscillator light.

[0060] like Figure 2 As shown, in a specific embodiment, the OFDR system obtains the intensity result measured by a sensing unit using the heterodyne method. At the receiving end, a large number of independent backscattered Rayleigh beams beat with the local oscillator beam, where the local oscillator beam can be analytically represented as:

[0061]

[0062] Among them, v s To apply the heterodyne frequency offset, v m For the m-th independent frequency, v m = v0 + mΔv, where m = 0, 1, ..., n-1. The backscattered Rayleigh light produced by the r-th scattering element is:

[0063]

[0064] Among them, R rLet τ' be the backscattering Rayleigh coefficient at point r. r The actual delay of the backscattered Rayleigh light generated at point r relative to the local oscillator light.

[0065] In one specific embodiment, the OFDR system obtains the intensity result measured by a sensing unit using the heterodyne method. The final total signal intensity is the sum of the beat frequencies between the backscattered light and the local oscillator light generated at different scattering elements at the same frequency.

[0066]

[0067] to i A Perform a Fourier transform and calculate the frequency v. s The amplitude and phase angle at a given point are used to obtain the intensity result of a single measurement.

[0068] In a specific embodiment, for a given OFDR system and suppression method parameter set in an evaluation, the parameters are obtained by performing a large number of independent repeated measurements in the evaluation system. In each measurement, the specific value sequence of the random offset δL of the actual position of the scattering element or the laser phase noise sequence is changed under specific parameters. This yields a large number of measurement results with identical statistical characteristics, from which the statistical features of the measurement result samples are calculated. This evaluation method selects the variance of the intensity results. Used to measure its randomness.

[0069] To make the content of this invention clearer and easier to understand, a specific embodiment will be provided, setting a particular set of OFDR system and suppression method parameters, wherein the local oscillator power is 10 dBm, the sweep period is 50 μs, the sweep slope is 16 GHz / s, the frequency shift interval is 1.5 MHz, and the number of independent measurements m is 1000. In this evaluation system, simulation experiments and data processing are performed according to the above method to obtain several results of a single evaluation. Examples will be given to illustrate the main functions and application ideas of this invention. The features of this invention will be pointed out in conjunction with other accompanying drawings.

[0070] Figures 3 to 6 This diagram illustrates the results of evaluating the effectiveness of the suppression method measured in this embodiment of the invention. It includes all perspectives of the evaluation results from this system, showcasing a comprehensive view of the evaluation results from multiple angles. The diagram presents the evaluation results in a multi-variable format, improving the information concentration and analytical efficiency of the results and facilitating quantitative comparisons between different variables.

[0071] Depend on Figure 3It is evident that as the linewidth increases, regardless of the value of the number of independent frequencies n (i.e., whether or not the frequency-shifting averaging method is applied), the randomness of the total signal intensity decreases. Further analysis reveals that as the linewidth increases, the laser phase noise contained in the total signal obtained within a spatial resolution increases, disrupting the coherent superposition between signals. This results in a decrease in the coherent superposition component and an increase in the intensity superposition component in the total signal, thus reducing its randomness. This low level of randomness, caused by laser phase noise, is detrimental to accurate measurement, and the downward trend of the curve indicates a deterioration in the overall signal quality.

[0072] Depend on Figure 4 As can be seen, the randomness of the total signal decreases with increasing n, regardless of the linewidth value. This indicates that, considering the actual situation of laser phase noise, the FSAV method can still effectively suppress coherent noise. Quantitative data can be obtained from the evaluation results as needed.

[0073] because Figure 3 and Figure 4 The presentation of all data uses a multivariate approach, containing more information. Figure 3 In this study, while observing the variation of intensity randomness with increasing line width, it is possible to obtain the curves obtained under different n values ​​and make quantitative comparisons. Figure 4 Similarly, although increasing the linewidth generally reduces the randomness of the total signal strength, comparing the curves obtained under different n values ​​shows that when the linewidth increases, compared to the original case where n is 1, the rate of decrease in the randomness of the total signal strength after using the frequency-shifting averaging method gradually slows down with the increase of n. The application of the FSAV method resists the influence of laser phase noise on the total signal to a certain extent, and the effect of FSAV becomes more obvious with the increase of n.

[0074] Depend on Figure 5 and Figure 6It is evident that as the measurement distance increases, regardless of the linewidth and the value of n, the randomness of the total signal intensity decreases. When the measurement distance increases to a certain extent, this downward trend gradually flattens out until it reaches a stable state. At this point, the randomness of the total signal intensity fluctuates around a certain value but no longer changes significantly. Further analysis reveals that as the measurement distance increases, the laser phase noise introduced into the intensity signal increases. Affected by this laser phase noise, the coherent superposition between signals is disrupted, resulting in a decrease in the coherent superposition component and an increase in the intensity superposition component in the total signal. Consequently, the randomness of the total signal decreases. This low level of randomness, caused by laser phase noise, is detrimental to accurate measurement, and the downward trend of the curve indicates a deterioration in the overall signal quality. When the measurement distance exceeds the coherent length of the light source, the signals become incoherent, and the coherent superposition component in the total signal is completely replaced by intensity superposition. The randomness of the total signal thus decreases to a limit value, at which point the system becomes unmeasurable.

[0075] Figure 5 and Figure 6 It also adopted a multivariate form, by Figure 5 It is evident that as the measurement distance increases, the rate of decrease in the randomness of the total signal intensity gradually slows down with the increase of n, until the measurement distance exceeds the coherence length of the light source, at which point the randomness of the total signal intensity drops to a limit and tends to stabilize. This indicates that the application of the frequency-shifting averaging method, to a certain extent, resists the influence of laser phase noise on the total signal, and the effect of the frequency-shifting averaging method becomes more obvious with the increase of n. Figure 6 It can be seen that, with different line width values, the intensity randomness decreases as the line width increases; however, when the measurement distance is very large, the signals are almost uncorrelated, and the intensity randomness will eventually show a law that is negatively correlated with the laser phase noise level.

[0076] This invention discloses an evaluation method that allows for customized design of OFDR system and suppression method parameters based on practical application needs. It is applicable to the analysis of common practical application scenarios, and has the ability to analyze the impact of coherent fading noise on measurement results and typical suppression methods. It has sufficient theoretical and practical application value.

[0077] The evaluation method fully considers the non-ideal light source conditions in reality, and can reflect the characteristics of coherent fading noise under the influence of laser phase noise and the changing law of the effect of its suppression method. It provides a more comprehensive evaluation of the effect of the suppression method, and more accurately observes and describes the relevant laws and conclusions. The data can be used for quantitative analysis, and provide a certain reference for parameter design and instrument selection in practical applications.

[0078] The evaluation results assess the level of coherent fading noise by evaluating the randomness of the total signal strength / power, reflecting the signal quality under actual conditions affected by light source performance. Under specific OFDR system and suppression method parameter settings, the variation of randomness with the frequency-shift averaging method parameters and the magnitude of laser phase noise can be determined. The overall level of laser phase noise can be set by adjusting the linewidth of the narrow-linewidth laser in the OFDR system. In simulations of distributed sensing and measurement, the variation of laser phase noise with measurement distance can be more accurately reflected. The evaluation results can be flexibly adjusted at the display angle as needed, and multiple parameters can be adjusted simultaneously in a single evaluation, yielding multivariate joint analysis results.

[0079] The above are merely preferred embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for evaluating the effect of coherent fading suppression in a narrow linewidth light source OFDR system, characterized in that, include: By adjusting the parameters of a narrow linewidth light source, the intrinsic phase noise carried by the laser is simulated. The parameters of the coherent fading noise suppression method are adjusted to quantitatively control the effectiveness of the suppression method. Based on the intrinsic phase noise and the modulated parameters, a one-dimensional fiber OFDR system model is performed, an OFDR system model to be evaluated is established, and heterodyne measurement is simulated based on the OFDR system model to be evaluated. The required measurement results are obtained by demodulating the beat frequency of the backscattered light and the local oscillator light. Obtaining the required measurement results includes: The OFDR system uses the heterodyne method to obtain the total signal intensity measured by the sensing unit as the sum of the beat frequencies between the backscattered light and the local oscillator light generated at different scattering elements at the same frequency; Perform a Fourier transform on the sum of the beat frequencies to obtain the amplitude and phase angle at the preset frequency point, and obtain the required measurement results; The sum of the beat frequencies is: in, The sum of beat frequencies for The number of backscattering Rayleigh elements in the array. The number of independent frequencies; The measurement results are quantitatively evaluated to complete the evaluation of coherent fading suppression and its effectiveness in the OFDR system.

2. The method for evaluating the coherent fading suppression effect of a narrow-linewidth OFDR system according to claim 1, characterized in that, Parameter adjustment for narrow linewidth light sources includes: By inputting the predetermined linewidth value of the OFDR system laser to be evaluated, the laser phase noise level in one evaluation is set, and the overall laser phase noise level of the OFDR system is adjusted according to the usage requirements.

3. The method for evaluating the coherent fading suppression effect of a narrow-linewidth OFDR system according to claim 1, characterized in that, Parameter tuning is used to suppress coherent fading noise, including: The method for suppressing coherent fading noise based on frequency shift averaging quantitatively controls the effectiveness of the suppression method by setting key parameters, including the number of independent frequencies, frequency shift interval, and total frequency shift amount.

4. The method for evaluating the coherent fading suppression effect of a narrow-linewidth OFDR system according to claim 1, characterized in that, The local oscillator light is: in, For the analytic representation of a local oscillator in complex form, For time, For the amplitude of the light source, To apply heterodyne frequency offset, For the first Each independent frequency, The sweep slope, Laser phase noise in phase form.

5. The method for evaluating the coherent fading suppression effect of a narrow-linewidth OFDR system according to claim 1, characterized in that, The backscattered light is: in, for The backscattering Rayleigh coefficient of a point. for The actual delay of the backscattered Rayleigh light generated by the point relative to the local oscillator light.

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