A method for improving the measurement resolution of an optical frequency domain reflectometer by dispersion compensation

Abstract

The application discloses a method for improving measurement resolution of an optical frequency domain reflectometer by 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 measurement resolution of an optical frequency domain reflectometer by dispersion compensation. The method considers eliminating the adverse influence of the dispersion effect of light propagation in an optical fiber on measurement progress. 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 used to evaluate each compensation. After a series of evaluation values are obtained, the dispersion compensation factor corresponding to the maximum evaluation value is selected to process the signal. Then, nonlinear effect elimination processing is performed. Compared with the case without dispersion compensation, the measurement resolution is further improved. The application solves the problem that the dispersion effect influences the measurement resolution and reduces the measurement resolution in the measurement process of the optical frequency domain reflectometer, so that the system can be accurately compensated for dispersion, and the measurement accuracy is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of frequency-modulated continuous wave radar and optical frequency domain reflectometry technology, and in particular to a method for improving the measurement resolution of an optical frequency domain reflectometer through dispersion compensation. BACKGROUND

[0002] Distributed optical fiber sensing technology is a sensing technology based on light scattering principle. According to the characteristics of scattered light, it can be divided into distributed optical fiber sensors based on Rayleigh scattering, Raman scattering and Brillouin scattering. Rayleigh scattering is a form of interaction between light and matter, and 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 optical fiber sensing. Distributed measurement technologies based on Rayleigh scattering mainly include optical time domain reflectometry (OTDR), optical coherence domain reflectometry (OCDR) and optical frequency domain reflectometry (OFDR). The main difference between them is the difference in spatial positioning principle, which also leads to their respective advantages and characteristics in system performance parameters.

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

[0005] The purpose of the present application is to design a method for improving the measurement resolution of an optical frequency domain reflectometer through dispersion compensation to solve the above problems.

[0006] The present application 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, comprising the following steps:

[0008] Step one: first, a preliminary measurement is performed without considering the dispersion effect, to obtain a preliminary predicted length L;

[0009] Step two: the obtained predicted fiber length is substituted into the dispersion coefficient estimation formula to perform calculation, to generate a preliminary estimated dispersion coefficient ;

[0010] Step three: the dispersion coefficient threshold value transformation range is set according to the dispersion coefficient, and the iteration step is set;

[0011] Step four: the dispersion coefficient obtained in step three is substituted into the dispersion compensation to obtain a compensated spectrum diagram;

[0012] Step five: the dispersion-compensated result is evaluated by using a focusing evaluation function;

[0013] Step six: steps four and five are repeated to obtain the dispersion compensation coefficient corresponding to the highest evaluation value, which is the optimal dispersion compensation coefficient;

[0014] Step seven: the signal compensated by the optimal dispersion compensation is subjected to software nonlinear phase compensation to obtain a high-resolution measurement result.

[0015] The present application has the following beneficial effects:

[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. The dispersion compensation coefficient for realizing the optimal dispersion compensation is automatically found. The compensation result is evaluated by using a focusing clarity evaluation function, the dispersion compensation coefficient is iterated in a threshold value range, the dispersion compensation coefficient with the highest evaluation value is finally found, and finally the dispersion compensation is performed by using the dispersion compensation coefficient.

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

[0019] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings, wherein:

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

[0021] Figure 2 An OFDR system diagram used in the present application.

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

[0023] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the drawings can be arranged and designed in various different configurations.

[0024] Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative labor are within the scope of protection of the present application.

[0025] It should be noted that: similar reference numerals and letters represent similar items in the following drawings, therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings.

[0026] In the description of the present application, it should be understood that the terms "upper", "lower", "inner", "outer", "left", "right", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship commonly placed when the product of the present application is used, or the orientation or positional relationship commonly understood by those skilled in the art, and are only for the convenience of describing the present application and simplifying the description, and therefore cannot be understood as indicating or implying that the indicated device or element must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application.

[0027] In addition, the terms "first", "second", etc. are only used for differentiation in description, and cannot be understood as indicating or implying relative importance.

[0028] In the description of the present application, it is also necessary to explain that, unless otherwise explicitly specified and limited, the terms such as "arrange", "connect" should be understood in a broad sense, for example, "connect" can be fixedly connected, or detachably connected, or integrally connected; can be mechanically connected, or electrically connected; can be directly connected, or indirectly connected through an intermediate medium, or the internal communication of two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0029] The specific embodiments of the present application will be described in detail below with reference to the accompanying drawings.

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

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

[0032] A main and auxiliary interferometer part for generating a beat signal; the interferometer is a Mach-Zehnder interferometer, 1% of the light from the first optical fiber coupler 2 enters a second optical fiber coupler 3 with a splitting ratio of 50:50 and then enters the auxiliary interferometer, one way of the auxiliary interferometer is introduced into an auxiliary interferometer fiber 5, the exit light of the delay fiber of the auxiliary interferometer fiber 5 and the reference light generate a beat signal in a fourth optical fiber coupler 8 with a splitting ratio of 50:50; 99% of the light from the first optical fiber coupler 2 enters a third optical fiber coupler 4 with a splitting ratio of 50:50 and then enters the main interferometer, one way of the main interferometer is connected with an optical fiber ring 7, the light entering the first inlet of the ring enters, the test optical fiber 16 enters from the second inlet, the backscattered light and the reflected light in the optical fiber come back from the second inlet, and finally the exit light from the outlet of the ring enters a fifth optical fiber coupler 9 with a splitting ratio of 50:50, the exit light and the reference light generate a beat signal;

[0033] A polarization fading suppression part; a polarization controller 6 is introduced in the reference light path of the main interference, and two polarization beam splitters 10, 11 are connected 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 balance detector 12, 13, 14 and data card 15, in the auxiliary interferometer, the beat signal is directly converted into electric signal by photoelectric conversion after passing through the balance detector 12, and then converted into digital signal by data acquisition card 15; for the main interferometer, the reference light and signal light are divided into s light and p light with mutually perpendicular polarization directions after passing through polarizing beam splitter 10, 11, then the s component and p component of the reference light and signal light are interfered respectively and photoelectric conversion is carried out by balance detector 13, 14 to generate beat signals Is and Ip, and then converted into digital signals by data acquisition card 15.

[0035] Signal processing part; the upper computer 17 processes the digital signals collected from the data acquisition card; the features and performance of the application are further described in detail below in conjunction with the embodiments.

[0036] Embodiment:

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

[0038] As shown in Figure 1 , the method for improving the measurement resolution of the optical frequency domain reflectometer by dispersion compensation, comprising the following steps:

[0039] Step one: first, a preliminary measurement is carried out without considering the dispersion effect, and the preliminary predicted length L is obtained;

[0040] Step two: the obtained predicted optical fiber length is substituted into the dispersion coefficient estimation formula to generate the preliminary estimated dispersion coefficient ;

[0041] Step three: set the dispersion coefficient threshold value transformation range according to the dispersion coefficient, and set the iteration step length;

[0042] Step four: substitute the dispersion coefficient obtained in step three into the dispersion compensation to obtain the compensated spectrum diagram;

[0043] Step five: evaluate the result of dispersion compensation using the focusing evaluation function;

[0044] Step six: repeat steps four and five to obtain the dispersion compensation coefficient corresponding to the highest evaluation value, which is the best dispersion compensation coefficient;

[0045] Step seven: the signal compensated by the best dispersion compensation is further subjected to software nonlinear phase compensation to obtain a high-resolution measurement result.

[0046] The instantaneous frequency of the chirped light in step one without considering the dispersion effect The instantaneous frequency of the chirped light in step one without considering the dispersion effect

[0047]

[0048] where γ is the chirp rate, is the initial frequency of the chirp, is the part of the chirp rate variation caused by the nonlinear sweeping of the tunable laser source, t is the time when the light is emitted by the laser source, and t' is a certain time when the light is emitted by the laser source;

[0049] The light intensity without considering the dispersion effect The expression is:

[0050]

[0051] where γ is the chirp rate, is the initial frequency of the chirp, is the nonlinear phase noise introduced by the nonlinear sweeping of the tunable laser source, and its value is equal to where is the initial amplitude of the light field emitted by the tunable laser source, and j is the imaginary unit. However, since the chirped continuous light is composed of different wavelengths of light, the propagation rates of different wavelengths of light are different, and the time delay difference caused by the dispersion effect during propagation needs to be considered. The time delay difference is expressed as:

[0052]

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

[0054]

[0055] In general, the light intensity of the light source is expressed as:

[0056]

[0057] where is the group velocity, is the group velocity dispersion, is the chirp rate, is the delay of the backscattered signal received by the detector, The calculation of can be expressed as:

[0058]

[0059] The reference signal light in the main interferometer in step two, if the dispersion effect is not considered, the light field emitted by the tunable laser source The expression is:

[0060]

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

[0062]

[0063] wherein represents the time delay of the back Rayleigh scattering or reflection of the incident light at the test point located at the position Z of the test fiber, is the reflection coefficient at the position Z of the test fiber. 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 is:

[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 signals collected by the subsequent data acquisition card at equal time intervals to be no longer equal intervals, and after fast Fourier transform, the energy of the signal light will be spread, reducing the spatial resolution of the OFDR system, and therefore we need to eliminate the 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 , and a dispersion coefficient is preliminarily set as the value:

[0068] ;

[0069] The dispersion coefficient threshold value transformation range in step three is set as , i.e. the dispersion coefficient is ten times higher and lower, and the iteration step is , which ensures that the best dispersion compensation coefficient can be selected in the iteration;

[0070] The beat frequency interference light considering dispersion in step four is:

[0071]

[0072] wherein represents the initial light field amplitude of the tunable laser source exit, t is the time of the laser source exit light source, represents the time through the delay fiber, , respectively represent t, ​Phase noise caused by nonlinear tuning of light source;

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

[0074]

[0075] It is found that due to the existence of dispersion effect, there is a quadratic term of time in the phase of beat frequency signal, which makes the frequency of beat frequency interference signal collected under equal time interval not equal interval, and causes the resolution of measurement result to be reduced.

[0076] Further, the dispersion compensation makes the collected signal multiplied by dispersion factor , wherein is the dispersion coefficient obtained by iteration in step two, and the calculation formula is:

[0077]

[0078] In the formula, n represents the iteration number, so that the quadratic term of time in the phase is eliminated, thereby achieving the purpose of dispersion compensation.

[0079] In step five, the dispersion evaluation function is evaluated by using the focusing evaluation function, and the dispersion evaluation function is selected as follows:

[0080]

[0081] , wherein 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 sampling point number; 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, and a mapping table of compensation coefficients and evaluation values is obtained by setting a step length and iteration, and the dispersion compensation coefficient corresponding to the maximum evaluation value is obtained according to the table, and the dispersion compensation coefficient is the best dispersion compensation coefficient.

[0083] In step seven, the light signal after the best dispersion compensation is compensated for nonlinear phase noise by using the de-chirp filtering algorithm.

[0084] The method for improving the measurement resolution of the optical frequency domain reflectometer by dispersion compensation, the method considers eliminating the adverse effects of the dispersion effect of the light propagating in the optical fiber on the measurement progress, 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 used to evaluate each compensation, a series of evaluation values are obtained, the dispersion compensation coefficient corresponding to the maximum evaluation value is selected, the signal is processed, nonlinear effect elimination processing is performed, and finally the measurement accuracy is improved by 2 times compared with the accuracy without dispersion compensation.

[0085] The above merely describes the preferred embodiments of the present application, and it should be pointed out that those skilled in the art can make several improvements and refinements without departing from the technical principles of the present application, and these improvements and refinements should also be considered as falling within the protection scope of the present application.

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, perform 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 to obtain the compensated spectrum; Step 5: Use the focusing evaluation function to evaluate the dispersion compensation results; 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 high-resolution measurement results; In 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 for the detector to receive the backscattered signal, The calculation can be expressed as: ， Where L is the length estimated in step 1, n is the refractive index corresponding to the center wavelength of the swept 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 1 into the predicted dispersion coefficient. In step 3, the dispersion coefficient threshold conversion range is set to , that is, the dispersion coefficient is ten times higher or lower, and the iterative step size is ; In step 4, the dispersion compensation step is: Beat frequency interference light considering dispersion: ， 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. 、 Represent t, t- Phase noise caused by nonlinear tuning of the light source at each moment; Beat frequency interference light without considering the dispersion effect: ， 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.

2. 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 1, the pre-measurement is to measure the optical fiber to be measured using an optical frequency domain reflectometer system that only considers eliminating nonlinear phase noise, and obtain a preliminary estimated length L.

3. The method for improving the measurement resolution of optical frequency domain reflectometry by dispersion compensation according to claim 1, characterized in that: In step 5, the evaluation method is to use the 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.

4. The method for improving the measurement resolution of optical frequency domain reflectometry 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 iteratively setting a step size. 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.

5. The method for improving the measurement resolution of optical frequency domain reflectometry by dispersion compensation according to claim 1, characterized in that: In 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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