CN116907640A - Method for determining ROTDR optimal optical pulse power based on fibonacci number - Google Patents

Abstract

The invention discloses a method for determining ROTDR optimal optical pulse power based on a fibonacci number, and relates to the technical field of optical fiber sensing. The method comprises the following steps: determining the times; calculating coordinates; collecting data; fitting data; adjusting the interval; repeating the calculation; and (5) obtaining the maximum value. In the Raman optical time domain reflectometer, the driving current corresponding to the optimal optical pulse power is determined by utilizing the power value of Stokes light in two different states of spontaneous Raman scattering and stimulated Raman scattering along with different change trends of the sampling position of the optical fiber and combining a data fitting method, and a one-dimensional searching method based on a fibonacci number is introduced for accelerating the searching speed of the optimal optical pulse power; after the steps of determining times, calculating coordinates, collecting data, fitting data, adjusting intervals, repeatedly calculating, obtaining the maximum value and the like are carried out, the driving current corresponding to the optimal light pulse power is rapidly obtained, and the optimal light pulse power is obtained.

Description

Method for determining ROTDR optimal optical pulse power based on fibonacci number

Technical Field

The invention relates to the technical field of optical fiber sensing, in particular to a method for determining ROTDR optimal optical pulse power based on a fibonacci number.

Background

With the rapid development of our economy, the infrastructure construction capacity is also increasing. Correspondingly, the long-distance and large-range infrastructure of China is also increasing. Most of these large infrastructures are in the field, some also in relatively closed environments in the subsurface. These facilities are either weathered or partially damaged due to wind exposure to sunlight or long-term closed moisture. Therefore, monitoring of these large infrastructures is required. Conventional monitoring methods are often point sensors, and would require a large number of sensor probes if such large infrastructure were to be monitored with conventional sensors. Such monitoring methods are inconvenient and uneconomical. The distributed optical fiber sensor can be used as a distributed sensor capable of long-distance continuous measurement and can be used for monitoring large-scale infrastructures.

Temperature is a very important physical parameter. In distributed fiber optic sensing technology, temperature information can be demodulated by measuring back raman scattered power. The device for measuring the distributed temperature based on the back Raman scattering in the optical fiber is generally called a Raman optical time domain reflectometer, and English is called ROTDR for short. For the application of the Raman optical time domain reflectometer, the optical fiber is arranged along the cable, so that the occurrence of fire can be prevented and warned by the optical fiber temperature measurement method; the optical fiber is arranged along the pipeline, so that the leakage point can be monitored by measuring the temperature of the optical fiber and then using the information of the temperature gradient; the optical fiber is laid along the copper strip in the transformer, so that the oil temperature information of the transformer can be monitored. In this way, various kinds of events are predicted directly from temperature information, or the temperature converter is first converted into another physical quantity, and then the event is processed by the physical quantity. However, temperature information must be measured anyway.

Since the temperature information of the raman optical time domain reflectometer is demodulated by raman scattered optical power, the higher the ratio of the optical power to noise, which is generally called signal to noise ratio, the more accurate the temperature information is demodulated. Obviously, the higher the optical power, the greater the value of the signal-to-noise ratio. Therefore, the power values of stokes light and anti-stokes light are to be increased as much as possible in raman optical time domain reflectometry. Thus, the emitted power of the light pulse is increased for a given light pulse width. However, when the emission power of the light pulse increases, stimulated brillouin scattering and stimulated raman scattering are generated. On the one hand, the generation of nonlinear effects such as stimulated brillouin scattering and stimulated raman scattering causes a rapid decay in the pulse power of the light pulse propagating along the optical fiber, which leads to a reduction in the sensing distance. On the other hand, the temperature sensing theory of raman optical time domain reflectometry is basically discussed around stokes and anti-stokes light generated by spontaneous raman scattering, and the temperature demodulation principle of stimulated raman scattering is not clear, which can cause inaccurate temperature information demodulated by the basic theory of spontaneous raman scattering when nonlinear effects are generated. Therefore, in raman optical time domain reflectometry, the pulsed optical power is preferably the maximum power at which the nonlinear effects of stimulated raman scattering and stimulated brillouin scattering are not generated at a given pulse width.

Disclosure of Invention

The technical problems to be solved by the invention are as follows:

in order to quickly and accurately find the optical pulse power generating spontaneous raman scattering, the invention provides a method for determining the optimal optical pulse power of the ROTDR based on fibonacci number, which comprises the following steps: the implementation steps are as follows:

step one, determining the times: determining the number of times of searching optimal optical pulse power according to the upper limit Imax of the laser driving current and the lower limit Imin of the driving current, wherein Imax and Imin are integers, and the method for determining the number of times sequentially comprises the following steps:

given the accuracy of the search accuracy,

the upper limit fmax= (Imax-Imin)/accuracy of the fibonacci number is calculated,

rounding the Fmax by rounding to obtain Fmax,

calculating the number Fm of the fibonacci number which is not more than Fmax, and solving each fibonacci number fi,

setting 0 for variable n;

step two, calculating coordinates: the coordinate value corresponds to the value of the laser driving current, the initial search range [ a, b ] (a and b are fixed parameters at the time of the initial value giving and are variables in the subsequent calculation) is given for the first time, and for any value j, coordinates x1 (j) and x2 (j):

wherein j ranges from 0 to Fm-1, [ ] represents a rounding off, and the rounded off range falls within the [ a, b ] range;

step three, collecting data: for any value j, the ROTDR curve is acquired twice:

setting the driving current x1 (j) of the laser for the first time, and collecting a Stokes scattered light curve S-ROTDR (x 1 (j)) formed by the back Raman scattered light of the test optical fiber through the photoelectric detector APD,

setting a driving current x2 (j) of a laser for the second time, and collecting a Stokes scattered light curve S-ROTDR (x 2 (j)) formed by the back Raman scattered light of the test optical fiber through an APD (photo-detector);

step four, data fitting: selecting 300 data points at the test optical fiber starting ends of the Stokes light curves S-ROTDR (x 1 (j)) and S-ROTDR (x 2 (j)) respectively for data fitting, wherein the data fitting method adopts a least square method, primary coefficients obtained by fitting are recorded as k1n and k2n, and the value range of n in k1n and k2n is 1 to Fm;

step five, adjusting an interval: the search interval is reduced according to the first order coefficient obtained by fitting, and the search interval is adjusted according to the sizes of the first order coefficients k1n and k2n, and for any value j, the search interval is:

if k1n is less than k2n, then the value of x2 (j) is assigned to b, the value of x1 (j) is assigned to both x2 (j) and variable c, the value of k1n is assigned to k2n,

if k1n is greater than or equal to k2n, then the value of x1 (j) is assigned to a, the value of x2 (j) is assigned to x1 (j) and variable c, and the value of k2n is assigned to k1n;

step six, repeating calculation: adding 1 to a variable n, and sequentially repeating the second, third, fourth and fifth steps, wherein the variable n is larger than Fm, and in the fifth step, if k1n is smaller than k2n, x1 is only calculated when the second step is repeated, in the third step, a first ROTDR curve is only acquired, and if k1n is larger than or equal to k2n, x2 is only calculated when the second step is repeated, and in the third step, a second ROTDR curve is only acquired;

step seven, obtaining the maximum value: when the variable n is larger than Fm, the value of the variable c is recorded, namely the current value corresponding to the optimal optical pulse power.

The method for determining the ROTDR optimal optical pulse power based on the fibonacci number is characterized in that in the first step, the calculation method of the value fi of the fibonacci number is as follows:

wherein, the value range of i in fi is 1 to Fm.

The ROTDR hardware for realizing the method for determining the ROTDR optimal optical pulse power based on the fibonacci number comprises the following basic components: pulse drive, laser, wavelength division multiplexer, optical fiber, first APD, second APD, data acquisition and control circuit; the connection relation of the components is as follows:

the pulse drive is coupled to the laser,

the laser is connected to a wavelength division multiplexer,

the wavelength division multiplexer is connected to the optical fiber and to the first APD by Stokes light and to the second APD by anti-Stokes light,

the first APD is coupled to a first amplifying circuit,

the second APD is coupled to a second amplifying circuit,

the first amplifying circuit and the second amplifying circuit are simultaneously connected with a data acquisition and control circuit,

the data acquisition and control circuit is connected with the pulse driving and the computer.

The data acquisition and control circuit comprises links such as signal amplification, digital filtering, mathematical operation and the like.

Compared with the prior art, the technical scheme provided by the invention has the following technical effects:

1. the invention adopts a least square method to fit a section of Stokes curve, and judges the optimal optical pulse power by utilizing the change of slope trend caused by nonlinear effect, and the method is simple and reliable;

2. a fibonacci sequence is introduced to conduct one-dimensional random search, so that the search speed of the optimal optical pulse power point is increased.

Drawings

FIG. 1 is a schematic diagram of a method of determining an ROTDR optimal optical pulse power based on a fibonacci number;

FIG. 2 is a schematic diagram of a Raman optical time domain reflectometer;

FIG. 3 is a plot of anti-Stokes light and Stokes light power collected at a drive current of 60 mA;

FIG. 4 is a plot of anti-Stokes light and Stokes light power collected at a drive current of 70 mA;

FIG. 5 is a plot of anti-Stokes light and Stokes light power collected at a drive current of 80 mA;

FIG. 6 is a plot of anti-Stokes light and Stokes light power collected at a drive current of 90 mA;

FIG. 7 is a plot of anti-Stokes light and Stokes light power collected at a drive current of 100 mA;

FIG. 8 is a plot of anti-Stokes light and Stokes light power collected at a drive current of 110 mA;

FIG. 9 is a plot of anti-Stokes light and Stokes light power collected at a drive current of 120 mA;

FIG. 10 is a plot of anti-Stokes light and Stokes light power collected at a drive current of 130 mA;

fig. 11 is a plot of stokes optical power versus fiber sampling point for various drive currents.

Detailed Description

The technical scheme of the invention is further described in detail below with reference to the accompanying drawings:

it will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Fig. 2 is a schematic diagram of a raman optical time domain reflectometer. The data acquisition and control circuit is the control hub of the whole instrument, which provides periodic trigger pulses to the pulse drive. Under the action of pulse driving, the pulse semiconductor laser emits periodic pulse light, and the pulse light is injected into the optical fiber through the wavelength division multiplexer. The incident pulse light collides with the optical fiber molecules in an inelastic way, and energy exchange is carried out to change the frequency of scattered light. Wherein the scattered light with reduced frequency is Stokes light and the scattered light with increased frequency is anti-Stokes light. Stokes light and anti-stokes light caused by raman scattering effect are returned to the optical fiber injection port and then are respectively injected into the first APD and the second APD through the wavelength division multiplexer, wherein APD is acronym of avalanche photodiode Avalanche Photon Diode. Photocurrents generated by the first APD and the second APD respectively pass through the first amplifying circuit and the second amplifying circuit to generate corresponding voltage signals, the two voltage signals enter the data acquisition and control circuit to carry out preliminary demodulation of temperature information, and the two voltage signals can also be sent into a computer for processing, wherein the computer can be an industrial personal computer or a personal computer, and the connection mode between the computer and the data acquisition and control circuit is TCP/IP protocol. In this embodiment, all the components in fig. 2 except the computer are packaged in the same module, and the reference fiber is packaged in the module, and the reference fiber is located between the wavelength division multiplexer and the test fiber, so that the effect of the reference fiber can be ignored when the effect of this embodiment is shown. The computer adopts a personal computer, and two voltage signals entering the computer are the result of accumulation for a plurality of times.

Among the methods of demodulating temperature information using the power curve generated by raman scattering effect, the method of temperature demodulation using the anti-stokes optical power to stokes optical power ratio is one of the most common. The anti-stokes light is more sensitive to temperature information than the stokes light, and thus the anti-stokes light is used as the signal light. Fig. 3 to 10 show curves of anti-stokes light and stokes light power collected at a driving current of 60mA to 130mA, respectively. As can be seen from these power curves, the power of stokes light is generally much greater than the power of anti-stokes light in power values, so stokes light is used as reference light. As the power of the light pulse injected into the fiber increases, spontaneous raman scattering effects are converted into stimulated raman scattering, and stokes light and anti-stokes light produce saturated amplification. Moreover, the threshold power for stokes light to enter saturation amplification is smaller than that of anti-stokes light, i.e. the reference light stokes light enters saturation amplification state first with the increase of the light pulse power. Once this state is entered, the value of the reference light reference is lost, resulting in an error in temperature demodulation information. Therefore, when the laser driving current is set, the stokes light is prevented from entering a saturated amplifying state, so that the anti-stokes light naturally does not enter the saturated amplifying state, the spontaneous raman scattering state is processed by raman scattering, and the temperature information can be correctly demodulated by using a method of performing temperature demodulation by using the ratio of the anti-stokes light power to the stokes light power.

In fig. 3 to 10, the power values before and after the sampling point are from the reference fiber, and data fitting is performed for stokes optical power at 251 to 550 three hundred fiber sampling points at the beginning of the test fiber. The variation of stokes optical power with the sampling point of the optical fiber is an exponential distribution. However, when there are only 300 fiber sampling points, the variation value of stokes optical power with the fiber sampling points can be considered to be linear distribution. Then, at the three hundred sampling points, data fitting is carried out on the Stokes optical power values by using a linear function with parameters, so that a primary coefficient term and a constant term are obtained by fitting, and the primary coefficient term is the fitting coefficient in FIG. 11. When the power of the optical pulse injected into the optical fiber is not very high, the power value of the optical pulse injected into the optical fiber is mainly the power value affecting the stokes optical power value, and the larger the value is, the larger the absolute value of the fitting coefficient is. Since the optical power at this time decreases with an increase in the number of optical fiber sampling points, the value of the fitting coefficient decreases with an increase in the optical power. When the power of the light pulse injected into the optical fiber reaches a threshold value, the stokes light enters an excited amplified state, at which time the stokes light is mainly expressed by a gain coefficient, and there are two exponential terms related to the optical fiber sampling position in the expression expressing the optical power, one being that the stokes light power decreases with an increase in the optical fiber sampling position and one being that the stokes light power increases with an increase in the optical fiber sampling position. As a result of the two exponential contributions, the ratio of stokes optical power over fiber sampling position changes becomes larger as the optical pulse power injected into the fiber increases. Therefore, the fitting coefficient in fig. 11 shows a trend of change that decreases and increases with increasing optical pulse power injected into the optical fiber, and the inflection point of the trend of change is exactly the driving current corresponding to the best optical pulse power to be found. The trend in fig. 11 is a result of collecting data point by point in steps of 10mA over the range of 60mA to 120mA of the driving current and processing. A total of 12 data acquisitions are performed, each time data is acquired, the computer transmits a driving current setting command to the data acquisition and control circuit through the TCP/IP protocol, and then transmits a data reading command, and the data read once by the computer undergoes several light pulses for the data acquisition and control circuit, so that it takes much time for each current setting point to set current reading data. In order to save time, i.e. reduce the number of steps required to search for the drive current corresponding to the optimum optical pulse power, a fibonacci-based one-dimensional search method is introduced in the present invention. The detailed implementation steps are shown in fig. 1, and include:

step one, determining the times: in this embodiment, the upper limit and the lower limit of the laser driving current are respectively 60mA and 120mA, the searching precision is set to 5mA, the upper limit Fmax of fibonacci numbers is 12, and since 12 is just an integer, F [ max ] is 12 after Fmax is rounded. Further, the values 1, 2, 3, 5, 8 of fibonacci numbers are calculated. While the variable n is set to 1 for use in the iterative computation phase.

Step two, calculating coordinates: the calculated coordinate value is the set value of the laser driving current. The first time the calculation of step two is performed, the initial search range is set to [60mA,120mA ]. Setting a variable j, if the variable j is calculated in the first calculation step two, j=0, and the value of j is consistent with n in other times of calculation, j=0 corresponds to the whole process in fig. 1, and j >0 mainly corresponds to the part shown by the dotted line in fig. 1. For the jth calculation, the calculation of the two coordinate values is performed as follows:

the rounding calculation is performed on the calculated values of x1 (j) and x2 (j). Further, the set values of x1 (j) and x2 (j) are constrained to values acceptable at the time of setting the laser driving current.

Step three, collecting data: corresponding to the jth calculation in the second step, two ROTDR curves, i.e., a stokes scattered light curve S-ROTDR (x 1 (j)) and S-ROTDR (x 2 (j)) formed by the fiber back raman scattered light through the photodetector APD when the drive current is set to x1 (j) and x2 (j), are collected. It should be noted here that if x1 (j) and x2 (j) occur in the previous number of calculations, there is no need to re-acquire the stokes optical power curve at the corresponding set current.

Step four, data fitting: 300 data points were selected at the test fiber start of Stokes light curves S-ROTDR (x 1 (j)) and S-ROTDR (x 2 (j)) respectively for data fitting. The fitted region is selected at the beginning of the test fiber because the stokes light threshold power increases with increasing fiber sampling point, i.e., the closer the fiber sampling point is to the fiber injection port, the more prone the stimulated raman scattering phenomenon occurs. The data fitting method adopts a least square method, 300 data points are directly fitted by using a linear function with parameters, and primary term coefficients obtained by fitting are recorded as k1n and k2n, wherein the value range of n in k1n and k2n is 1 to Fm. It should also be noted that if x1 (j) and x2 (j) in step three in the same calculation occur in the previous calculation times, no corresponding fitting calculation is required. And the first time the value of x1 (j) or x2 (j) occurs, the fitting coefficients calculated by fitting are saved together with the corresponding set current values for use in a later search. Thus, the calculation amount and the time of data transmission can be reduced, thereby improving the searching efficiency.

Step five, adjusting an interval: the search interval is reduced according to the first order coefficient obtained by fitting, and the search interval is adjusted according to the sizes of the first order coefficients k1n and k2n, and for any value j, the search interval is:

if k1n is less than k2n, then the value of x2 (j) is assigned to b, the value of x1 (j) is assigned to both x2 (j) and variable c, the value of k1n is assigned to k2n,

if k1n is greater than or equal to k2n, then the value of x1 (j) is assigned to a, the value of x2 (j) is assigned to x1 (j) and variable c, and the value of k2n is assigned to k1n;

the values of a and b for the 0 th calculation, i.e., the 0 th search or the first search, are determined by the range in which the laser can be set, and may be determined based on human experience.

Step six, repeating calculation: adding 1 to a variable n, and sequentially repeating the second, third, fourth and fifth steps, wherein the variable n is larger than Fm, and in the fifth step, if k1n is smaller than k2n, x1 is only calculated when the second step is repeated, in the third step, a first ROTDR curve is only acquired, and if k1n is larger than or equal to k2n, x2 is only calculated when the second step is repeated, and in the third step, a second ROTDR curve is only acquired;

step seven, obtaining the maximum value: when the variable n is greater than Fm, the value of the variable c, i.e. the current value corresponding to the optimal light pulse power to be obtained, is recorded, and in this embodiment, the final value of c is 90mA, which is consistent with the results shown in fig. 11. This illustrates the effectiveness of the present invention.

In summary, in the method for determining the optimal optical pulse power of the ROTDR based on the fibonacci number, in the Raman optical time domain reflectometer, the driving current corresponding to the optimal optical pulse power is determined by utilizing the different variation trends of the power values of Stokes light in two different states of spontaneous Raman scattering and stimulated Raman scattering along with the sampling position of the optical fiber and combining a data fitting method, and a one-dimensional searching method based on the fibonacci number is introduced for accelerating the searching speed of the optimal optical pulse power. After the steps of determining times, calculating coordinates, collecting data, fitting data, adjusting intervals, repeatedly calculating, obtaining the maximum value and the like are carried out, the driving current corresponding to the optimal light pulse power is rapidly obtained, and the optimal light pulse power is obtained.

The foregoing is only a partial embodiment of the present invention, and it should be noted that it will be apparent to those skilled in the art that modifications and adaptations can be made without departing from the principles of the present invention, and such modifications and adaptations are intended to be comprehended within the scope of the present invention.

Claims (4)

1. A method for determining the optimal optical pulse power of a ROTDR based on fibonacci numbers, comprising the steps of, for a test fiber at a given temperature:

step 1, determining the times: determining the times and the numbers of optimal optical pulse power searches according to the upper limit Imax of the laser driving current and the lower limit Imin of the driving current, wherein Imax and Imin are integers, and the method for determining the times sequentially comprises the following steps:

step 1-1: given the accuracy of the search accuracy,

step 1-2: the upper limit fmax= (Imax-Imin)/accuracy of the fibonacci number is calculated,

step 1-3: rounding the Fmax by rounding to obtain Fmax,

step 1-4: calculating the number Fm of the fibonacci number which is not more than Fmax, and solving each fibonacci number fi,

step 1-5: setting 0 for variable n;

step 2, calculating coordinates: the coordinate value corresponds to the value of the laser driving current, an initial search range [ a, b ] is given for the first time, wherein a and b are constant parameters at the time of the initial value giving and are variables in the subsequent calculation, and for any value j, coordinates x1 (j) and x2 (j):

wherein j ranges from 0 to Fm-1, [ ] represents a rounding off, and the rounded off range falls within the [ a, b ] range;

step 3, collecting data: for any value j, the ROTDR curve is acquired twice:

setting the driving current x1 (j) of the laser for the first time, and collecting a Stokes scattered light curve S-ROTDR (x 1 (j)) formed by the back Raman scattered light of the test optical fiber through the photoelectric detector APD,

setting a driving current x2 (j) of a laser for the second time, and collecting a Stokes scattered light curve S-ROTDR (x 2 (j)) formed by the back Raman scattered light of the test optical fiber through an APD (photo-detector);

step 4, data fitting: selecting 300 data points at the test optical fiber starting ends of the Stokes light curves S-ROTDR (x 1 (j)) and S-ROTDR (x 2 (j)) respectively for data fitting, wherein the data fitting method adopts a least square method, primary coefficients obtained by fitting are recorded as k1n and k2n, and the value range of n in k1n and k2n is 1 to Fm;

step 5, adjusting the interval: the search interval is reduced according to the first order coefficient obtained by fitting, and the search interval is adjusted according to the sizes of the first order coefficients k1n and k2n, and for any value j, the search interval is:

if k1n is less than k2n, then the value of x2 (j) is assigned to b, the value of x1 (j) is assigned to both x2 (j) and variable c, the value of k1n is assigned to k2n,

if k1n is greater than or equal to k2n, then the value of x1 (j) is assigned to a, the value of x2 (j) is assigned to x1 (j) and variable c, and the value of k2n is assigned to k1n;

step 6, repeating calculation: adding 1 to a variable n, and sequentially repeating the steps 2, 3, 4 and 5, wherein the variable n is larger than Fm, and in the step 5, if k1n is smaller than k2n, only x1 is calculated when repeating the step 2, in the step 3, only a first ROTDR curve is acquired, and if k1n is larger than or equal to k2n, only x2 is calculated when repeating the step 2, and in the step 3, only a second ROTDR curve is acquired;

step 7, obtaining the maximum value: when the variable n is larger than Fm, the value of the variable c is recorded, namely the current value corresponding to the optimal optical pulse power.

2. The method for determining the optimal optical pulse power of the ROTDR based on the fibonacci number as recited in claim 1, wherein in step 1, the calculation method of the value fi of the fibonacci number is as follows:

wherein, the value range of i in fi is 1 to Fm.

3. A method of determining a ROTDR optimum optical pulse power based on fibonacci numbers as in claim 1 wherein the ROTDR hardware implementing the method consists essentially of: the device comprises a pulse drive, a laser, a wavelength division multiplexer, a test optical fiber, a first APD, a second APD and a data acquisition and control circuit; wherein:

the pulse drive is coupled to the laser,

the laser is connected to a wavelength division multiplexer,

the wavelength division multiplexer is connected with the test optical fiber, is connected with the first APD through Stokes light, is connected with the second APD through anti-Stokes light,

the first APD is coupled to a first amplifying circuit,

the second APD is coupled to a second amplifying circuit,

the first amplifying circuit and the second amplifying circuit are simultaneously connected with a data acquisition and control circuit,

the data acquisition and control circuit is connected with the pulse driving and the computer.

4. A method of determining the optimal optical pulse power for a ROTDR based on fibonacci numbers as in claim 3 wherein said data acquisition and control circuitry includes signal amplification, digital filtering, and mathematical operations.