Method for improving measurement resolution of optical frequency domain reflectometer through dispersion compensation

Abstract

The invention discloses a method for improving the measurement resolution of an optical frequency domain reflectometer through dispersion compensation, belongs to the technical field of frequency modulated continuous wave radars and optical frequency domain reflectometers, and particularly relates to a method for improving the measurement resolution of an optical frequency domain reflectometer through dispersion compensation. According to the method, the bad influence of the dispersion effect of light propagating in the optical fiber on the measurement progress is considered and eliminated, in order to compensate the dispersion effect, a dispersion compensation factor is introduced, a compensation threshold range and an iteration step length are set, and a focusing evaluation function is utilized to evaluate each compensation; after a series of evaluation values are obtained, the dispersion compensation coefficient corresponding to the maximum evaluation value is selected to process the signal, then nonlinear effect elimination processing is carried out, and the measurement resolution is further improved compared with dispersion compensation. The problem that the measurement resolution is reduced due to the fact that the dispersion effect exists in the measurement process of the optical frequency domain reflectometer is solved, the system can carry out accurate dispersion compensation, and the measurement accuracy is improved.

Description

Technical Field

[0001] The invention relates to the technical field of frequency modulated continuous wave radar and optical frequency domain reflectometer, and in particular to a method for improving the measurement resolution of an optical frequency domain reflectometer through dispersion compensation. Background Art

[0002] Distributed fiber optic sensing technology is a sensing technology based on the principle of light scattering. According to the characteristics of scattered light, it can be divided into distributed fiber optic sensors based on Rayleigh scattering, Raman scattering and Brillouin scattering. Rayleigh scattering is a form of interaction between light and matter. Its characteristics are that the wavelength of scattered light is the same as that of incident light and the scattering energy is high. Compared with Raman scattering and Brillouin scattering, Rayleigh scattering is easier to detect, so it has received extensive attention and research in the field of distributed fiber optic sensing. Distributed measurement technologies based on Rayleigh scattering mainly include optical time domain reflectometry (OTDR), coherence domain reflectometry (OCDR) and optical frequency domain reflectometry (OFDR). The differences between them are mainly reflected in the different spatial positioning principles, which also leads to their respective advantages and characteristics in terms of system performance parameters.

[0003] Among the above technologies, OFDR is considered to be a distributed fiber optic sensing technology that can replace OTDR in many fields due to its advantages such as high spatial resolution, high sensitivity and large dynamic range. Spatial resolution is a key performance indicator of OFDR technology and one of the development directions of OFDR technology. Improving the spatial resolution of OFDR is of great significance for improving its measurement accuracy, expanding its application areas and promoting its commercial development. In improving the spatial resolution of OFDR, considering the dispersion effect of long optical fibers and compensating for it is a challenging research direction. After compensating for the dispersion effect, the spatial resolution of OFDR measurement results is significantly improved.

[0004] Therefore, it is necessary to develop a dispersion compensation method for high-resolution optical frequency domain reflectometry to solve the above problems. Summary of the invention

[0005] The purpose of the present invention is to design a method for improving the measurement resolution of an optical frequency domain reflectometer by dispersion compensation in order to solve the above problem.

[0006] The present invention achieves the above-mentioned purpose through the following technical solutions:

[0007] A method for improving the measurement resolution of an optical frequency domain reflectometer by dispersion compensation comprises the following steps:

[0008] Step 1: First, make a prediction without considering the dispersion effect to obtain the preliminary prediction length L;

[0009] Step 2: Substitute the predicted fiber length into the dispersion coefficient estimation formula to generate a preliminary estimated dispersion coefficient. ;

[0010] Step 3: Set the dispersion coefficient threshold transformation range according to the dispersion coefficient, and set the iteration step size;

[0011] Step 4: Substitute the dispersion coefficient obtained in step 3 into the dispersion compensation and obtain the compensated spectrum;

[0012] Step 5: Use the focusing evaluation function to evaluate the dispersion compensation result;

[0013] Step 6: Repeat steps 4 and 5 to obtain the dispersion compensation coefficient corresponding to the highest evaluation value, i.e., the optimal dispersion compensation coefficient;

[0014] Step 7: Perform software nonlinear phase compensation on the signal that has passed the optimal dispersion compensation to obtain a high-resolution measurement result.

[0015] The beneficial effects of the present invention are:

[0016] 1. The dispersion compensation in the optical frequency domain reflectometer is realized, so that the measurement resolution of the optical frequency domain reflectometer system is higher.

[0017] 2. Automatically search for the dispersion compensation coefficient that achieves the best dispersion compensation. Use the focus clarity evaluation function to evaluate the compensation result, use the dispersion compensation coefficient to iterate within a threshold range, and finally find the dispersion compensation coefficient with the highest evaluation value, and finally use the dispersion compensation coefficient for dispersion compensation.

[0018] 3. Compared with the traditional dispersion compensation scheme, the nonlinear phase elimination of the present invention adopts a software compensation method, which requires less hardware than the hardware compensation method of nonlinear phase elimination and is easier to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be regarded as limiting the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative work, among which:

[0020] Figure 1is a flow chart of a method for improving the measurement resolution of an optical frequency domain reflectometer by dispersion compensation according to the present invention;

[0021] Figure 2 is a diagram of the OFDR system used in the present invention.

[0022] Marked in the figure: 1-linear frequency modulation light source, 2-first fiber coupler, 3-second fiber coupler, 4-third fiber coupler, 5-auxiliary interferometer fiber, 6-polarization controller, 7-fiber circulator, 8-fourth fiber coupler, 9-fifth fiber coupler, 10, 11-polarization beam splitter, 12, 13, 14-balanced detector, 15-data acquisition card, 16-test fiber, 17-host computer. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Generally, the components of the embodiments of the present invention described and shown in the drawings here can be arranged and designed in various different configurations.

[0024] Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the invention claimed for protection, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0025] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0026] In the description of the present invention, it should be understood that the terms "upper", "lower", "inside", "outside", "left", "right", etc. indicate directions or positional relationships based on the directions or positional relationships shown in the accompanying drawings, or are directions or positional relationships in which the product of the invention is usually placed when in use, or are directions or positional relationships commonly understood by those skilled in the art. These directions or positional relationships are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and therefore should not be understood as a limitation on the present invention.

[0027] Furthermore, the terms “first”, “second”, etc. are merely used for distinguishing descriptions and are not to be understood as indicating or implying relative importance.

[0028] In the description of the present invention, it is also necessary to explain that, unless otherwise clearly specified and limited, the terms such as "setting" and "connection" should be understood in a broad sense. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or an indirect connection through an intermediate medium, or it can be the internal communication of two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0029] The specific implementation modes of the present invention are described in detail below in conjunction with the accompanying drawings.

[0030] like Figure 2 As shown, an optical frequency domain reflectometer system used in a method for improving the measurement resolution of an optical frequency domain reflectometer by dispersion compensation comprises:

[0031] Linear frequency modulated laser source; the linear frequency modulated light source 1 is connected to the first optical fiber coupler 2 with a 1:99 splitting ratio; at the same time, the light source is connected to the data acquisition card 15 to provide a trigger signal.

[0032] The main and auxiliary interferometer parts generate beat frequency signals; the interferometer adopts Mach-Zehnder interferometer, 1% of the light separated from the first fiber coupler 2 enters the second fiber coupler 3 with a split ratio of 50:50 and then enters the auxiliary interferometer, one path of the auxiliary interferometer introduces the auxiliary interferometer fiber 5, and the output light of the delayed fiber of the auxiliary interferometer fiber 5 beats with the reference light in the fourth fiber coupler 8 with a split ratio of 50:50 to generate a beat frequency signal; 99% of the light separated from the first fiber coupler 2 enters the third fiber coupler 4 with a split ratio of 50:50 and then enters the main interferometer, one path of the main interferometer is connected to the fiber circulator 7, the light enters the first inlet of the circulator, enters the test fiber 16 from the second inlet, the backscattered light and the reflected light in the fiber return from the second inlet, and finally is emitted from the outlet of the circulator to the fifth fiber coupler 9 with a split ratio of 50:50, and the output light of this path beats with the reference light to generate a beat frequency signal;

[0033] Suppress the polarization fading part; introduce the polarization controller 6 in the reference optical path of the main interferometer, and connect two polarization beam splitters 10 and 11 at the output port of the main interferometer to realize polarization diversity reception of the signal light, so as to suppress the polarization fading effect of the optical fiber sensing system;

[0034] Physical information acquisition part; The physical information acquisition unit includes balanced detectors 12, 13, 14 and a data card 15. In the auxiliary interferometer, the beat signal is directly converted into an electrical signal by the balanced detector 12 for photoelectric conversion and then converted into a digital signal by the data acquisition card 15; for the main interferometer, the reference light and the signal light are divided into s light and p light with polarization directions perpendicular to each other after passing through the polarization beam splitters 10 and 11, and then the s component and p component of the reference light and the signal light interfere with each other and are converted into beat signals Is and Ip by the balanced detectors 13 and 14 for photoelectric conversion, and then converted into digital signals by the data acquisition card 15.

[0035] Signal processing part; host computer 17 processes the digital signal collected from the data acquisition card; the following is a further detailed description of the features and performance of the present invention in conjunction with the embodiments.

[0036] Example:

[0037] This embodiment uses an auxiliary interferometer optical fiber 5 with a length of 90m, the sampling rate of the data acquisition card 15 is set to 65MS / s, the light source sweep rate is set to 800Ghz / s, and the number of sampling points is set to 1M sampling points. After using the method of improving the optical frequency domain reflectometer by dispersion compensation, the system can accurately locate the strong scattering points in the optical fiber, and the measurement accuracy of the optical fiber to be tested is improved by 2-3 times.

[0038] like Figure 1 As shown, the present invention provides a method for improving the measurement resolution of an optical frequency domain reflectometer by dispersion compensation, comprising the following steps:

[0039] Step 1: First, make a prediction without considering the dispersion effect to obtain the preliminary prediction length L;

[0040] Step 2: Substitute the predicted fiber length into the dispersion coefficient estimation formula to generate a preliminary estimated dispersion coefficient. ;

[0041] Step 3: Set the dispersion coefficient threshold transformation range according to the dispersion coefficient, and set the iteration step size;

[0042] Step 4: Substitute the dispersion coefficient obtained in step 3 into the dispersion compensation and obtain the compensated spectrum;

[0043] Step 5: Use the focusing evaluation function to evaluate the dispersion compensation result;

[0044] Step 6: Repeat steps 4 and 5 to obtain the dispersion compensation coefficient corresponding to the highest evaluation value, i.e., the optimal dispersion compensation coefficient;

[0045] Step 7: Perform software nonlinear phase compensation on the signal that has passed the optimal dispersion compensation to obtain a high-resolution measurement result.

[0046] The instantaneous frequency of the linear frequency modulated light in step 1 without considering the dispersion effect It can be expressed as:

[0047]

[0048] Where γ is the frequency modulation rate, is the initial frequency of FM, It represents the frequency modulation rate variation caused by the nonlinear frequency sweep of the tunable laser source, t is the time when the laser source emits the light source, and t' is a certain time when the laser source emits the light source;

[0049] The dispersion effect is not considered. The expression is:

[0050]

[0051] Where γ is the frequency modulation rate, is the initial frequency of FM, is the nonlinear phase noise introduced by the nonlinear frequency sweep of the tunable laser source, and its value is equal to , where represents the initial amplitude of the light field emitted by the tunable laser source, j is an imaginary unit. However, since the frequency modulated continuous light contains lights of different wavelengths, the propagation speeds of lights of different wavelengths are different, and the time delay difference due to the dispersion effect in the propagation process needs to be considered. It is expressed as:

[0052]

[0053] Then the instantaneous frequency considering the dispersion effect is It is expressed as:

[0054]

[0055] Generally speaking, the light intensity of the light source It can be expressed as:

[0056]

[0057] in is the group velocity, is the group velocity dispersion, is the frequency modulation rate, is the time delay of the detector receiving the backscattered signal, The calculation can be expressed as:

[0058]

[0059] The reference signal light in the main interferometer in step 2, if the dispersion effect is not considered, can be the output light field of the tunable laser source The expression is:

[0060]

[0061] Similarly, if the dispersion effect in the optical fiber is considered, the output light field of the test optical fiber in the main interferometer is The expression is:

[0062]

[0063] in It represents the delay of the incident light being scattered or reflected backward by Rayleigh at the test point located at the test fiber position Z. is the reflection coefficient at the test fiber position Z. Then the beat frequency interference signal of the main interferometer can be expressed as:

[0064]

[0065] And the beat frequency signal without considering the dispersion effect:

[0066]

[0067] By comparison, it is found that the phase of the beat frequency signal considering the dispersion effect contains a quadratic term of time, which will cause the frequency of the beat frequency interference signal collected by the subsequent data acquisition card at equal time intervals to no longer be equally spaced. After fast Fourier transform, the energy of the signal light will be diffused, reducing the spatial resolution of the OFDR system. Therefore, we need to eliminate this quadratic term of time. Therefore, in order to eliminate the quadratic term of time in the phase, the signal is multiplied by a dispersion factor , initially set a dispersion coefficient value:

[0068] ;

[0069] The dispersion coefficient threshold transformation range in step 3 is set to , that is, the dispersion coefficient is ten times higher or lower, and the iterative step size is , ensuring that the best dispersion compensation coefficient can be selected in the iteration;

[0070] In step 4, consider the dispersed beat frequency interference light:

[0071]

[0072] in represents the initial light field amplitude emitted by the tunable laser source, t is the time when the laser source emits the light source, Indicates the time it takes to pass through the delayed optical fiber. , Represents t, Phase noise caused by nonlinear tuning of the light source at each moment;

[0073] Beat frequency interference light without considering dispersion effect:

[0074]

[0075] By comparison, it is found that due to the dispersion effect, there is a quadratic term in the phase of the beat frequency signal. This term will make the frequencies of the beat frequency interference signals collected at equal time intervals not equally spaced, which will lead to a decrease in the resolution of the measurement results.

[0076] Furthermore, the dispersion compensation allows the collected signal to be Multiply, where It is the dispersion coefficient obtained by iteration in step 2, and its calculation formula is:

[0077]

[0078] Where n represents the number of iterations to eliminate the time quadratic term in the phase and achieve the purpose of dispersion compensation.

[0079] In step 5, the focusing evaluation function is used for evaluation, and the dispersion evaluation function is selected as follows:

[0080]

[0081] in is the amplitude corresponding to each sampling point in the power spectrum, N represents the total number of data acquisition cards, and i is the number of sampling points; the larger the evaluation value S is, the higher the resolution of the measurement result after dispersion compensation is.

[0082] In step six, each dispersion compensation coefficient corresponds to an evaluation value. The mapping table between the compensation coefficient and the evaluation value is obtained by setting the step size iteration. The dispersion compensation coefficient corresponding to the maximum evaluation value is obtained by looking up the table. This dispersion compensation coefficient is the optimal dispersion compensation coefficient.

[0083] In step seven, the optical signal after optimal dispersion compensation is subjected to nonlinear phase noise compensation through a de-skewing filtering algorithm.

[0084] The present invention discloses a method for improving the measurement resolution of an optical frequency domain reflectometer by dispersion compensation. The method considers eliminating the adverse effect of the dispersion effect of light propagating in an optical fiber on the measurement progress. In order to compensate for the dispersion effect, a dispersion compensation factor is introduced, a compensation threshold range and an iteration step are set, and each compensation is evaluated by using a focusing evaluation function. After a series of evaluation values ​​are obtained, a dispersion compensation coefficient corresponding to the maximum evaluation value is selected to process the signal, and then a nonlinear effect elimination process is performed. The final measurement accuracy is improved by 2 times compared with the accuracy without dispersion compensation.

[0085] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for improving the measurement resolution of an optical frequency domain reflectometer by dispersion compensation, characterized in that: The following steps are involved: Step 1: First, make a prediction without considering the dispersion effect to obtain the preliminary prediction length L; Step 2: Substitute the predicted fiber length into the dispersion coefficient estimation formula to generate a preliminary estimated dispersion coefficient. ; Step 3: Set the dispersion coefficient threshold transformation range according to the dispersion coefficient, and set the iteration step size; Step 4: Substitute the dispersion coefficient obtained in step 3 into the dispersion compensation and obtain the compensated spectrum; Step 5: Use the focusing evaluation function to evaluate the dispersion compensation result; Step 6: Repeat steps 4 and 5 to obtain the dispersion compensation coefficient corresponding to the highest evaluation value, i.e., the optimal dispersion compensation coefficient; Step 7: Perform software nonlinear phase compensation on the signal that has passed the optimal dispersion compensation to obtain a high-resolution measurement result.

2. The method for improving the measurement resolution of an optical frequency domain reflectometer by dispersion compensation according to claim 1, characterized in that: In the step 1, the predicted measurement is to measure the optical fiber to be measured using an optical frequency domain reflectometer system that only considers the elimination of nonlinear phase noise, so as to obtain a preliminary estimated length L.

3. The method for improving the measurement resolution of an optical frequency domain reflectometer by dispersion compensation according to claim 1, characterized in that: In the step 2, the dispersion coefficient estimation formula is: , in represents the initial dispersion coefficient estimate, is the group velocity, is the group velocity dispersion, is the frequency modulation rate, is the time delay of the detector receiving the backscattered signal, The calculation can be expressed as: ， Wherein L is the length estimated in step one, n is the refractive index corresponding to the central wavelength of the swept frequency optical signal in the optical fiber, and c is the speed of light. The predicted dispersion coefficient is obtained by substituting the measured optical fiber length obtained in step one into the predicted dispersion coefficient.

4. The method for improving the measurement resolution of an optical frequency domain reflectometer by dispersion compensation according to claim 3, characterized in that: In step 3, the dispersion coefficient threshold change range is set to , that is, the dispersion coefficient is ten times higher or lower, and the iterative step size is .

5. The method for improving the measurement resolution of an optical frequency domain reflectometer by dispersion compensation according to claim 4, characterized in that: In the step 4, the dispersion compensation step is: Consider the dispersion of the beat frequency interference light: ， in represents the initial light field amplitude emitted by the tunable laser source, t is the time when the laser source emits the light source, Indicates the time it takes to pass through the delayed optical fiber. , Represents t, Phase noise caused by nonlinear tuning of the light source at each moment; Beat frequency interference light without considering the dispersion effect: ， is the initial frequency of the frequency modulation, and the dispersion compensation makes the collected signal and the dispersion factor Multiply, where is the dispersion coefficient obtained by iteration, and its calculation formula is: ， Where n represents the number of iterations. The subsequent dispersion factor is multiplied by the collected signal to eliminate the time quadratic term in the phase, thereby achieving the purpose of dispersion compensation.

6. The method for improving the measurement resolution of an optical frequency domain reflectometer by dispersion compensation according to claim 1, characterized in that: In step 5, the evaluation method is to use a focusing evaluation function for evaluation, and the dispersion evaluation function is selected as follows: ， in is the amplitude corresponding to each sampling point in the power spectrum, N represents the total number of sampling points, i represents the i-th sampling point, and the larger the evaluation value S is, the higher the resolution of the measurement result after dispersion compensation is.

7. The method for improving the measurement resolution of an optical frequency domain reflectometer by dispersion compensation according to claim 1, characterized in that: In step six, the method for obtaining the optimal dispersion coefficient is as follows: each dispersion compensation coefficient corresponds to an evaluation value, and a mapping table of compensation coefficients and evaluation values ​​is obtained by setting a step size iteration, and the dispersion compensation coefficient corresponding to the maximum evaluation value is obtained by looking up the table. The dispersion compensation coefficient is the optimal dispersion compensation coefficient.

8. The method for improving the measurement resolution of optical frequency domain reflectometry by dispersion compensation according to claim 1, characterized in that: In the step seven, the nonlinear phase compensation method is to perform nonlinear phase noise compensation on the optical signal after optimal dispersion compensation through a processing algorithm, and the processing algorithm includes interpolation, nonlinear Fourier transform, and de-skewing filtering.

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