CN113595638B - BOTDA system based on four-frequency-division driving - Google Patents

BOTDA system based on four-frequency-division driving Download PDF

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CN113595638B
CN113595638B CN202110822058.4A CN202110822058A CN113595638B CN 113595638 B CN113595638 B CN 113595638B CN 202110822058 A CN202110822058 A CN 202110822058A CN 113595638 B CN113595638 B CN 113595638B
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intensity modulator
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CN113595638A (en
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魏正军
张祥飞
余佳
郑颖芳
严子凌
孟寅杰
王金东
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South China Normal University
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
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Abstract

The invention discloses a BOTDA system based on four-frequency-division driving, which comprises a narrow-linewidth laser, a first electro-optic intensity modulator, an optical coupler, a polarization controller, a first erbium-doped fiber amplifier, an optical isolator, a single-mode fiber, a first optical filter, a second electro-optic intensity modulator, a second erbium-doped fiber amplifier, a circulator, a second optical filter, an adjustable radio-frequency signal generator, a pulse signal generator and a signal processing unit, wherein the narrow-linewidth laser is connected with the first electro-optic intensity modulator through the first optical filter; the optical coupler is connected with the polarization controller in an upper path, the polarization controller, the first erbium-doped fiber amplifier, the optical isolator and the single-mode fiber starting end are sequentially connected, the optical coupler is connected with the first optical filter in a lower path, the second electro-optical intensity modulator is connected with the second erbium-doped fiber amplifier after being modulated by the pulse signal generator, the second erbium-doped fiber amplifier is connected with the port 1 of the circulator, and the port 2 of the circulator is connected with the tail end of the single-mode fiber. The invention only uses one fourth of the original radio frequency driving signal to realize that the frequency difference of the two beams of light is Brillouin frequency shift.

Description

BOTDA system based on four-frequency-division driving
Technical Field
The invention relates to the technical field of distributed optical fiber sensing, in particular to a BOTDA system based on four-frequency-division driving.
Background
Brillouin Optical Time Domain Analyzers (BOTDAs) based on fiber stimulated Brillouin scattering have great potential application in structural health monitoring of large-scale civil engineering, communication Optical cables, oil and gas pipelines and the like. The Brillouin optical time domain analysis technology utilizes the stimulated Brillouin amplification characteristic to enable two beams of light to oppositely propagate in an optical fiber, wherein one beam of light has the frequency v1Another beam is a pulsed pump light of frequency v2The continuous probe light of (1). When the two beams meet at a certain position of the optical fiber, the frequency difference of the two beams is equal to Brillouin frequency shift vB(about 11GHz), the energy of the optical signal with high frequency is transferred to the optical signal with low frequency, so-called stimulated brillouin amplification. When a certain on the optical fiberWhen a section of strain occurs, the Brillouin frequency shift of the part is formed by vBBecomes vB+ Δ v, causing a sharp change in the brillouin signal in this region. Frequency difference between incident pump light and probe light is made equal to v by frequency sweepingBThe Brillouin scattering signal at the position can be received, the Brillouin Frequency Shift (BFS) is further measured, and the distributed sensing of the temperature and the strain can be realized according to the linear relation between the Brillouin Frequency Shift (BFS) and the temperature and the strain.
As shown in FIG. 1, the conventional BOTDA system has two paths of frequency difference vBThe continuous probe light and the pulse pump light need to be emitted by the laser at a frequency v0The continuous light is divided into two parts by the coupler, and one part of the light passes through a radio frequency signal v on the electro-optical modulationRFAfter modulation, v is obtained0+vRFThe other path of light passes through the electro-optical modulator to obtain v0By scanning v with pulsed pump lightRFFrequency of (d) of the difference v of the two beamsRFBrillouin frequency shift v of proximity optical fiberB(about 11GHz) to generate the stimulated brillouin effect.
Since the brillouin frequency shift in the optical fiber is high, about 11GHz, in order to obtain an optical signal with a frequency shift of 11GHz by the electro-optical modulator, a radio frequency signal with the same frequency of 11GHz is required to drive according to the frequency shift characteristics of the electro-optical modulator, and a general signal generator is difficult to output such a high-frequency signal, while a dedicated radio frequency signal generator is expensive, so that the system cost is increased sharply.
Disclosure of Invention
In view of this, the present invention provides a BOTDA system based on a quarter-frequency drive, which only uses one quarter of the original rf driving signal to realize the brillouin frequency shift of two beams of optical signals, according to the mechanism of stimulated brillouin in the dual-end BOTDA system and in combination with the frequency shift characteristic of the electro-optical modulator, aiming at the defects of the prior art.
The invention achieves the above purpose through the following technical means:
on one hand, the invention provides a BOTDA system based on four-frequency-division driving, which comprises a narrow-linewidth laser, a first electro-optic intensity modulator, an optical coupler, a polarization controller, a first erbium-doped fiber amplifier, an optical isolator, a single-mode fiber, a first optical filter, a second electro-optic intensity modulator, a second erbium-doped fiber amplifier, a circulator, a second optical filter, an adjustable radio-frequency signal generator, a pulse signal generator and a signal processing unit, wherein the narrow-linewidth laser is connected with the first electro-optic intensity modulator;
the narrow-linewidth laser is connected with a first electro-optic intensity modulator, the first electro-optic intensity modulator is modulated by an adjustable radio frequency signal generator and connected to an optical coupler, an upper path of the optical coupler is connected with a polarization controller, the polarization controller is connected with a first erbium-doped optical fiber amplifier, the first erbium-doped optical fiber amplifier is connected with the optical isolator, the optical isolator is connected with a starting end of a single-mode optical fiber, a lower path of the optical coupler is connected with a first optical filter, the first optical filter is connected with a second electro-optic intensity modulator, the second electro-optic intensity modulator is modulated by a pulse signal generator and then connected with a second erbium-doped optical fiber amplifier, the second erbium-doped optical fiber amplifier is connected with an opening 1 of a circulator, an opening 2 of the circulator is connected with a tail end of the single-mode optical fiber, an opening 3 of the circulator is connected with a second optical filter, the second optical filter is connected with a signal processing unit signal end, and a trigger end of the signal processing unit is connected with the pulse signal generator.
Further, the single mode fiber is a 30km single mode fiber.
Furthermore, the signal processing unit comprises a photoelectric detector and a data acquisition and processor which are sequentially connected, the photoelectric detector is connected with the second optical filter, and the data acquisition and processor is connected with the pulse signal generator.
Furthermore, the data acquisition and processor is an oscilloscope or a data acquisition card and a computer.
On the other hand, the invention provides a method for realizing that the frequency difference of two beams of light is Brillouin frequency shift by only using one fourth of the original radio frequency driving signal, and the BOTDA system based on the quarter-frequency driving comprises the following steps:
generating a ± 2 order double sideband optical signal: continuous optical signals generated by the narrow-linewidth laser pass through a first electro-optic intensity modulator, an electric signal added at a radio frequency end of the first electro-optic intensity modulator is one fourth of Brillouin frequency shift, amplitude values of direct-current bias voltage and radio-frequency modulation voltage of the electro-optic modulator are changed, the intensity of a +/-2-order side wave band in frequency spectrum components of output optical signals is large, other side wave band components can be ignored, and the output optical of the first electro-optic intensity modulator is divided into upper and lower continuous light paths through an optical coupler;
generating continuous detection light: continuous light on the upper path of the optical coupler passes through the polarization controller, the first erbium-doped fiber amplifier and the optical isolator and then is used as continuous detection light to be input into the single-mode fiber to be detected from the initial end of the fiber;
generating pulsed pump light: continuous light of a lower path of the optical coupler is filtered by a first optical filter, only-2 order side band optical signals are reserved, the continuous optical signals output by the first optical filter are modulated by a second electro-optical intensity modulator added with pulse electrical signals to form pulse optical signals, and the pulse optical signals are amplified by a second erbium-doped optical fiber amplifier, enter from an opening of a circulator 1 and exit from an opening of a circulator 2, and then are input to the tail end of an optical fiber from an opening of the circulator 2 so as to be input into a single mode optical fiber to be detected;
the stimulated brillouin effect occurs: the continuous detection light of the upper path and the pulse pump light of the lower path respectively enter the single mode optical fiber to be detected from two ends of the optical fiber, when the output frequency of the adjustable radio frequency signal generator added to the first electro-optical intensity modulator is changed, the frequency difference of the two beams of light is also changed, and when the frequency difference of the two beams of light is just Brillouin frequency shift vBWhen the two beams of light are excited, the stimulated Brillouin effect is generated;
generating a backscattered light signal: after the stimulated Brillouin effect occurs, the pulse pump light entering the single-mode fiber can generate a backward Brillouin scattering light signal, the backward Brillouin scattering light signal can be transmitted to the second optical filter from the port 2 of the circulator through the port 3, the Stokes light with the strongest signal is collected by the second optical filter, and data signal processing is carried out on the Stokes light;
data acquisition and processing: the optical signal collected by the second optical filter is processed by the signal processing unit to extract Brillouin frequency shift, and the Brillouin frequency shift is analyzed by an algorithm to calculate the temperature and the strain of a certain position of the optical fiber.
Compared with the prior art, the invention has the beneficial effects that at least:
the BOTDA system based on the quarter-frequency drive has the advantages that the frequency difference of two beams of light is Brillouin frequency shift only by one quarter of the original radio frequency drive signal, an ultrahigh frequency radio frequency signal generator is not needed, a special radio frequency plate is not needed because the required maximum radio frequency does not exceed 3GHz, the cost and the circuit design difficulty are reduced, the system frequency sweep range is narrower, the detection of long-distance dynamic temperature/strain is convenient to realize, and the traditional measurement steps and experimental instruments can be continuously adopted to avoid the increase of extra cost.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.
FIG. 1 is a block diagram of a conventional BOTDA system;
FIG. 2 is a block diagram of the BOTDA system based on the quarter-frequency drive of the present invention;
FIG. 3 is a flow chart of the method for implementing Brillouin frequency shift as the frequency difference between two beams by using only one fourth of the original RF driving signal according to the present invention;
FIG. 4 is a Bessel function J of the first type according to the present inventionn(x) The image of (a);
FIG. 5 is an output spectral plot of the second order sideband maxima of the present invention with the other sidebands suppressed.
Detailed Description
In order to make the aforementioned objects, features and advantages of the present invention comprehensible, embodiments accompanied with figures are described in detail below. It should be noted that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments, and all other embodiments obtained by those skilled in the art without any inventive work based on the embodiments of the present invention belong to the protection scope of the present invention.
Example 1
As shown in fig. 2, the present invention provides a BOTDA system based on a quarter-frequency drive, including: the device comprises a narrow-linewidth laser, a first electro-optic intensity modulator, an optical coupler, a polarization controller, a first erbium-doped fiber amplifier, an optical isolator, a 30km single-mode fiber, a first optical filter, a second electro-optic intensity modulator, a second erbium-doped fiber amplifier, a circulator, a second optical filter, an adjustable radio-frequency signal generator, a pulse signal generator and a signal processing unit.
The narrow-linewidth laser is connected with a first electro-optic intensity modulator, the first electro-optic intensity modulator is modulated by an adjustable radio frequency signal generator and connected to an optical coupler, an upper path of the optical coupler is connected with a polarization controller, the polarization controller is connected with a first erbium-doped optical fiber amplifier, the first erbium-doped optical fiber amplifier is connected with the optical isolator, the optical isolator is connected with a starting end of a 30km single-mode optical fiber, a lower path of the optical coupler is connected with a first optical filter, the first optical filter is connected with a second electro-optic intensity modulator, the second electro-optic intensity modulator is modulated by a pulse signal generator and then connected with a second erbium-doped optical fiber amplifier, the second erbium-doped optical fiber amplifier is connected with a port 1 of a circulator, a port 2 of the circulator is connected with a tail end of the 30km single-mode optical fiber, a port 3 of the circulator is connected with a second optical filter, the second optical filter is connected with a signal end of a signal processing unit, and a trigger end of the signal processing unit is connected with the pulse signal generator.
In this embodiment, the signal processing unit includes a photodetector and a data acquisition and processor connected in sequence; the photoelectric detector is connected with the second optical filter, and the data acquisition and processor is connected with the pulse signal generator. The data acquisition and processor is an oscilloscope or a data acquisition card plus a computer.
Example 2
Referring to fig. 3, the invention provides a method for realizing that the frequency difference of two beams of light is brillouin frequency shift by using only one fourth of the original radio frequency driving signal, and the adoption of the quarter-frequency driving based BOTDA system comprises the following steps:
1) generating a +/-2-order double-sideband optical signal: continuous optical signals generated by the narrow-linewidth laser pass through the first electro-optic intensity modulator, an electric signal added at a radio frequency end of the first electro-optic intensity modulator is about one fourth of Brillouin frequency shift, amplitude values of direct-current bias voltage and radio-frequency modulation voltage of the electro-optic modulator are changed, the intensity of a +/-2-order side wave band in frequency spectrum components of the output optical signals is large, other side wave band components can be ignored, and the output optical of the first electro-optic intensity modulator is divided into upper and lower continuous light paths through the optical coupler;
2) and generating continuous detection light: after passing through the polarization controller, the first erbium-doped fiber amplifier and the optical isolator, the continuous light on the upper path of the optical coupler is input into a 30km single-mode optical fiber to be detected from the initial end of the optical fiber as continuous detection light;
3) generating pulse pump light: continuous light of a lower path of the optical coupler is filtered by a first optical filter, only-2 order side band optical signals (forming a loss type BOTDA system) are reserved, the continuous optical signals output by the first optical filter are modulated by a second electro-optical intensity modulator added with pulse electrical signals to form pulse optical signals, the pulse optical signals are amplified by a second erbium-doped optical fiber amplifier, enter from an opening of a circulator 1 and exit from an opening of the circulator 2, and then are input to the tail end of an optical fiber from an opening of the circulator 2 so as to be input into a 30km single-mode optical fiber to be detected;
4) generating a stimulated Brillouin effect: the continuous detection light of the upper path and the pulse pump light of the lower path respectively enter the 30km single-mode optical fiber to be detected from two ends of the optical fiber, when the output frequency of the adjustable radio frequency signal generator added on the first electro-optical intensity modulator is changed, the frequency difference of the two beams of light is also changed, and when the frequency difference of the two beams of light is just Brillouin frequency shift vBWhen the two beams of light are excited, the stimulated Brillouin effect is generated;
5) generating a backscattered light signal: after the stimulated Brillouin effect occurs, the pulse pump light entering the 30km single-mode fiber can generate a backward Brillouin scattering light signal which is transmitted to the second optical filter from the port 2 of the circulator through the port 3, and the second optical filter is used for collecting Stokes light with the strongest signal and processing data signals of the Stokes light;
6) data acquisition and processing: the optical signal collected by the second optical filter is processed by the signal processing unit to extract the Brillouin frequency shift, and the temperature and the strain of a certain part of the optical fiber are calculated by analyzing the Brillouin frequency shift through an algorithm.
The invention utilizes the frequency shift characteristic of the electro-optical intensity modulator to maximize the intensity of the +2 order and-2 order side wave band signals in the output light, and other side wave bands such as carrier waves (0 order), plus or minus 1 order and the like are inhibited or even disappear, so that signal interference is not generated, and therefore, the output light wave expression of the electro-optical intensity modulator needs to be analyzed.
The modulation voltage applied to two electrodes (respectively, a direct current electrode and a radio frequency electrode) of the electro-optical intensity modulator is set as follows:
V＝VDC+VRFcos(wRFt） (1)
wherein, VDCFor a DC bias voltage applied to the DC electrode, VRFFor the radio-frequency modulation voltage applied to the radio-frequency electrode, wRFFor the angular frequency of the rf signal, the expression of the output light wave is:
Figure BDA0003172248650000071
wherein E is0Is the amplitude of the incident light, w0The frequency of the incident light is the frequency of the incident light,
Figure BDA0003172248650000072
to modulate depth, VπIs a half-wave voltage, and is,
Figure BDA0003172248650000073
the phase resulting from the dc bias voltage.
Expanding the formula (2) according to a first type Bessel function:
Figure BDA0003172248650000074
output light as shown in formula (3)Except for the optical frequency w of the incident light0In addition, there are w0±wRF，w0±2wRF，w0±3wRFAnd (3) equal light frequency components. For simplicity, these optical frequency components are referred to as … sidebands of 0 th order, 1 st order and 2 nd order, respectively, according to the corresponding order of the Bessel function. From equation (3), the intensities of the 0 th order and 1 to 3 th order sideband are:
0 order:
Figure BDA0003172248650000075
1, order:
Figure BDA0003172248650000076
2, stage:
Figure BDA0003172248650000077
3, stage:
Figure BDA0003172248650000078
as shown in the formula (4), the relative value of the light intensity of each spectrum in the spectrum depends on each order value J of the Bessel function of the first kindn(C) Square of and phase caused by DC bias voltage
Figure BDA0003172248650000081
I.e. the intensity of each order of the sideband is subject to a dc bias voltage VDCAnd a radio frequency modulation voltage VRFThe combined effect of (a). Drawing a first class Bessel function J through MATLAB (matrix laboratory) of mathematical simulation softwaren(x) The curve when n is 0,1,2,3 is shown in fig. 4.
Fig. 4 shows that when the modulation depth C is 3.1, the intensity of the 2 nd order sideband is maximum, but the intensity of the 1 st and 3 rd order sidebands cannot be ignored, so that the dc bias voltage V needs to be changed againDCSuppresses the intensity of the 1 st and 3 rd order sideband. By derivation, C ═ 3.1, VDCAt 0V, the second order sideband is maximized and the other sidebands are sufficiently suppressed, as shown in fig. 5.
It was analyzed that although the carrier wave (0 th order sideband) was not sufficiently suppressed, its intensity was small compared to the second order sideband intensity, and the frequency difference between the two beams was large (about 5.5GHz), which also caused signal interference with difficulty. The analysis shows that the second-order sideband intensity in the output spectrum can be maximized by calculating and accurately setting the values of the radio frequency modulation voltage and the direct current bias voltage.
The BOTDA system based on the quarter-frequency drive has the advantages that the frequency difference of two beams of light is Brillouin frequency shift only by one quarter of the original radio frequency drive signal, an ultrahigh frequency radio frequency signal generator is not needed, a special radio frequency plate is not needed because the required maximum radio frequency does not exceed 3GHz, the cost and the circuit design difficulty are reduced, the system frequency sweep range is narrower, the detection of long-distance dynamic temperature/strain is convenient to realize, and the traditional measurement steps and experimental instruments can be continuously adopted to avoid the increase of extra cost.
The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is more specific and detailed, but not construed as limiting the scope of the present invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims (4)

1. A BOTDA system based on four-frequency-division driving is characterized by comprising a narrow-linewidth laser, a first electro-optic intensity modulator, an optical coupler, a polarization controller, a first erbium-doped fiber amplifier, an optical isolator, a single-mode fiber, a first optical filter, a second electro-optic intensity modulator, a second erbium-doped fiber amplifier, a circulator, a second optical filter, an adjustable radio-frequency signal generator, a pulse signal generator and a signal processing unit;
the narrow-linewidth laser is connected with a first electro-optic intensity modulator, an adjustable radio frequency signal generator generates Brillouin frequency-shifted radio frequency signals with the frequency of one fourth, namely a quarter-frequency driving signal, the adjustable radio frequency signal generator is connected with the first electro-optic intensity modulator, the first electro-optic intensity modulator is modulated by the adjustable radio frequency signal generator and connected with an optical coupler, the upper path of the optical coupler is connected with a polarization controller, the polarization controller is connected with a first erbium-doped fiber amplifier, the first erbium-doped fiber amplifier is connected with the first erbium-doped fiber amplifier, the optical isolator is connected with the initial end of a single-mode optical fiber, the lower path of the optical coupler is connected with a first optical filter, the first optical filter is connected with a second electro-optic intensity modulator, the second electro-optic intensity modulator is modulated by a pulse signal generator and then connected with a second erbium-doped optical fiber amplifier, the second erbium-doped optical amplifier is connected with a port 1 of an optical isolator, a port 2 of the optical isolator is connected with the tail end of the single-mode optical fiber, the port 3 of the circulator is connected with a second optical filter, the second optical filter is connected with the signal end of the signal processing unit, and the trigger end of the signal processing unit is connected with the pulse signal generator;
the signal processing unit comprises a photoelectric detector and a data acquisition and processor which are sequentially connected, the photoelectric detector is connected with the second optical filter, and the data acquisition and processor is connected with the pulse signal generator.
2. The BOTDA system based on quarter-frequency drive of claim 1, wherein the single mode fiber is a 30km single mode fiber.
3. The BOTDA system based on the quarter-frequency drive as claimed in claim 1, wherein the data acquisition and processor is an oscilloscope or a data acquisition card + computer.
4. A method for realizing the brillouin frequency shift of two beams by using only one fourth of the original rf driving signal, which adopts the BOTDA system based on divide-by-four driving as claimed in any one of claims 1-3, comprising the following steps:
generating a ± 2-order double-sideband optical signal: continuous optical signals generated by the narrow-linewidth laser pass through a first electro-optic intensity modulator, an electric signal added at a radio frequency end of the first electro-optic intensity modulator is one fourth of Brillouin frequency shift, amplitude values of direct-current bias voltage and radio-frequency modulation voltage of the electro-optic modulator are changed, the intensity of a +/-2-order side wave band in frequency spectrum components of output optical signals is large, other side wave band components can be ignored, and the output optical of the first electro-optic intensity modulator is divided into upper and lower continuous light paths through an optical coupler;
generating continuous detection light: continuous light on the upper path of the optical coupler passes through the polarization controller, the first erbium-doped fiber amplifier and the optical isolator and then is used as continuous detection light to be input into the single-mode fiber to be detected from the initial end of the fiber;
generating pulsed pump light: continuous light of a lower path of the optical coupler is filtered by a first optical filter, only-2 order side band optical signals are reserved, the continuous optical signals output by the first optical filter are modulated by a second electro-optical intensity modulator added with pulse electrical signals to form pulse optical signals, and the pulse optical signals are amplified by a second erbium-doped optical fiber amplifier, enter from an opening of a circulator 1 and exit from an opening of a circulator 2, and then are input to the tail end of an optical fiber from an opening of the circulator 2 so as to be input into a single mode optical fiber to be detected;
the stimulated brillouin effect occurs: the continuous detection light of the upper path and the pulse pump light of the lower path respectively enter the single mode optical fiber to be detected from two ends of the optical fiber, when the output frequency of the adjustable radio frequency signal generator added to the first electro-optical intensity modulator is changed, the frequency difference of the two beams of light is also changed, and when the frequency difference of the two beams of light is just Brillouin frequency shift vBWhen the two beams of light are excited, the stimulated Brillouin effect is generated;
generating a backscattered light signal: after the stimulated Brillouin effect occurs, the pulse pump light entering the single-mode fiber can generate a backward Brillouin scattering light signal, the backward Brillouin scattering light signal can be transmitted to the second optical filter from the port 2 of the circulator through the port 3, the Stokes light with the strongest signal is collected by the second optical filter, and data signal processing is carried out on the Stokes light;
data acquisition and processing: the optical signal collected by the second optical filter is processed by the signal processing unit to extract the Brillouin frequency shift, and the temperature and the strain of a certain part of the optical fiber are calculated by analyzing the Brillouin frequency shift through an algorithm.
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