CN107741243B - A BOTDR system and method for improving the service life of the system - Google Patents

Abstract

The invention discloses a BOTDR system and a method for improving the service life of the system. The BOTDR system includes a light source module, a first coupler, an electro-optical modulation module, a pulse generation module, an optical pulse amplification module, a circulator, an optical polarization disturbance module, a second A coupler, a third coupler, a first optical detection module, a frequency sweep module, a signal processing module, a second optical detection module, a feedback module and a current control module. By adding a feedback module, the invention measures different parts of the entire sensing fiber by different input powers, and deliberately causes the fiber segment close to the incident end of the light source to generate stimulated Brillouin scattering during the measurement process of the fiber segment away from the light source. , the effective Brillouin peak frequency spectrum of the entire sensing fiber is obtained by fitting and splicing the fractional measurement results separately. The invention solves the problem of reducing the measurement distance caused by the increased loss caused by radiation, and increases the service life of the BOTDR system in the space radiation environment.

Description

A BOTDR system and method for improving the service life of the system

technical field

The invention belongs to the technical field of distributed optical fiber sensors, and particularly relates to a BOTDR system and a method for improving the service life of the system.

Background technique

Brillouin Optical Time Domain Reflectometry (BOTDR) is a distributed optical fiber sensing technology based on spontaneous Brillouin scattering. The system light source injects the pulsed light from one end of the fiber, and detects the back spontaneous Brillouin scattering spectrum of the pulsed light from the same end. The sensing information on the fiber can be obtained by analyzing the spectral line. Brillouin scattering is caused by the interaction of the optical field in the fiber and its induced acoustic field. The Brillouin scattered light has a frequency shift relative to the incident light, called the Brillouin shift. The BOTDR uses the Brillouin frequency shift to analyze the sensing information: the Brillouin frequency shift is related to the fiber temperature and the strain, and the frequency shift is proportional to the temperature change of the fiber and the axial strain. .

With the gradual development of science and technology in the aerospace field, more and more attention has been paid to the monitoring and protection of aerospace devices in harsh space environments. However, traditional sensing technology is difficult to meet the lightweight requirements of aerospace equipment in this field. BOTDR technology has the advantages of anti-electromagnetic interference, light weight, small error, high resolution, simple layout and low cost. It was first used in the aerospace field, and has been successively used in the monitoring of electric power, communication, engineering and other directions in developed countries. Since the 1990s, my country has begun to carry out the application research of optical fiber sensing technology. Among all distributed optical fiber sensing technologies, BOTDR can detect problems and problems in large-scale structures in time by virtue of its ability to measure temperature, strain and other physical quantities, as well as high spatial resolution and long sensing distance. Therefore, it is widely used in safety monitoring.

In the harsh radiation environment of the universe, the radiation effect of ionizing radiation affects the BOTDR system, which can be improved by strengthening the optoelectronic devices, but the optical fiber, as the sensing component of the BOTDR system, must be exposed to the radiation environment for a long time. Under the influence of radiation, a color center will be generated inside the fiber, which will have a certain impact on the sensing ability of the fiber. The more obvious impact is the increase in loss. Although the energy of the incoming optical signal can be increased by increasing the pulse width or power of the probe light, thereby increasing the sensing range of the system, increasing the width of the optical pulse will lead to a decrease in the spatial resolution of the system; The fiber optical power is limited by the existence of the Brillouin excitation threshold. When the detection optical power is less than the Brillouin excitation threshold, the energy of the Brillouin backscattered light has a linear relationship with the energy of the detection light. When the power of the probe light exceeds the stimulated Brillouin excitation threshold, the probe light energy is transferred to the Brillouin Stokes light, which increases the energy of the Brillouin Stokes light, while the probe light energy decays rapidly , eventually leading to a decrease in the dynamic range of the system. Therefore, it is urgent to study and propose a method to improve the service life of BOTDR in radiation environment.

SUMMARY OF THE INVENTION

In order to solve the technical problems raised by the above background technology, the present invention aims to provide a BOTDR system and a method for improving the service life of the system, so as to solve the problem that the color center effect of the optical fiber caused by the space radiation environment causes the transmission loss of the sensing fiber to increase, The sensing distance of the BOTDR system is reduced.

In order to realize the above-mentioned technical purpose, the technical scheme of the present invention is:

A BOTDR system working in a space radiation environment, comprising a light source module, a first coupler, an electro-optical modulation module, a pulse generation module, an optical pulse amplification module, a circulator, an optical polarization disturbance module, a second coupler, and a third coupling The light source module emits continuous light, and the first coupler divides the continuous light into two paths with unequal energies Continuous light, in which one continuous light with higher energy is input to the electro-optical modulation module, and one continuous light with lower energy is used as the coherent local oscillator light to input the optical polarization disturbance module, and the electro-optical modulation module will input the continuous light according to the pulse electrical signal generated by the pulse generation module. The light is modulated into a pulsed light signal, the pulsed light signal is amplified by the optical pulse amplifying module and injected into the fiber, and the backward Brillouin scattering light signal distributed along the fiber is obtained through the circulator, and the second coupler distributes the backward output of the circulator. The Brillouin scattered optical signal is coupled with the randomly distributed local oscillator optical signal output by the optical polarization perturbation signal module, and the Brillouin scattered optical signal is output. The optical signal with higher energy is obtained by the photoelectric conversion of the first optical detection module to obtain a Brillouin scattering electrical signal, which is then swept by the frequency sweep module and filtered and amplified by the signal processing module to obtain the corresponding backward Brillouin scattering electrical signal. The other optical signal with lower energy is input to the feedback module through the second optical detection module, and the feedback module controls the power of the light source signal sent by the light source module by controlling the current control module.

Further, the working process of the feedback module is as follows:

(a) When the power of the light source is not increased, the split point A is obtained: the power of the Brillouin scattering spectrum decreases with the increase of the distance. When the power of the Brillouin scattering spectrum at a certain position is close to the noise floor power, that is, it cannot pass through normally. For Lenz fitting, mark this position as A;

(b) Control the light source current to increase the power of the light source, and in the process, determine whether point A is stimulated: if point A is stimulated, the power near point A is not attenuated by a fixed attenuation coefficient, but violently jitter, when point A is stimulated, it is necessary to adjust the power of the light source until point A is not stimulated;

(c) After increasing the power of the light source, determine whether there is an effective Brillouin scattering signal at the farthest point B of the distance to be measured: if the signal power at B is not submerged by noise, that is, the Lorentz fitting can be performed correctly, it means At this time, there is a valid sensing signal at B.

Further, the first coupler divides the continuous light emitted by the light source module into two continuous lights with an energy ratio of 9:1.

Further, the third coupler divides the input Brillouin scattered optical signal into two optical signals with an energy ratio of 95:5.

Further, the first light detection module adopts an APD avalanche diode.

A method for improving the service life of the above-mentioned BOTDR system, comprising the following steps:

(1) Use the BOTDR system to obtain the spectrum of the split point A and the first segment of the fiber:

Suppose point O is the signal incident end of the optical fiber, the power of the detection light pulse is P ₀ , and point B is the farthest end required to be detected. Sensing effect: The maximum detection range of the system before radiation is OC. When the system is irradiated, the loss of the sensing fiber will increase, resulting in the system not receiving the effective Brillouin scattering signal at point B. At this time, the incident light is maintained first. The power P ₀ remains unchanged, record the farthest point A at which the effective Brillouin scattering signal can be obtained, and at the same time obtain the back Brillouin scattering spectral line of the OA segment fiber;

(2) performing fitting processing on the Brillouin scattering spectral lines of the optical fiber in the OA section obtained in step (1), thereby obtaining the Brillouin peak frequency of the optical fiber in this section;

(3) Increase the power of the incident optical signal, and obtain the spectrum of the second fiber after point A:

Gradually increase the power of the light source until it can ensure that no stimulated Brillouin scattering occurs at point A, and there is an effective Brillouin scattering signal at point B; at this time, stop increasing the power of the light source, and use the power to measure with the BOTDR system, Obtain effective sensing data of the fiber segment between point A and point B;

(4) performing a fitting process on the back Brillouin scattering spectral line of the AB fiber segment obtained in step (3), thereby obtaining the peak frequency of the Brillouin spectral line of the fiber segment;

(5) splicing the peak frequencies of the Brillouin spectral lines obtained in the steps (2) and (4), thereby obtaining the Brillouin peak frequency information of the entire section of the sensing fiber OB;

(6) Adjust the power of the light source to the original power P0, and return to step (1).

Further, in step (3), if the optical fiber in the OA section is not stimulated after the power of the light source is increased, the peak frequency can be measured by using the Brillouin scattering spectral lines of the entire optical fiber OB after the power is increased.

The beneficial effects brought by the above technical solutions:

By adding a feedback module in the traditional BOTDR system, the present invention measures different parts of the entire sensing fiber by different input powers, and deliberately makes the fiber segment close to the incident end of the light source generate Stimulated Brillouin scattering phenomenon. The effective Brillouin peak frequency spectrum of the entire sensing fiber can be obtained by fitting and overall splicing of the fractional measurement results. According to the relationship between the Brillouin frequency shift and temperature or strain, the transmission of temperature or strain can be realized. sense.

Description of drawings

Fig. 1 is the system composition block diagram of the present invention;

Fig. 2 is the schematic diagram that the optical fiber attenuation changes with the radiation dose;

Figure 3 is a schematic diagram of the relationship between Brillouin spectral power and sensing distance;

FIG. 4 is a schematic diagram of increasing the power of an incident optical signal.

Detailed ways

The technical solutions of the present invention will be described in detail below with reference to the accompanying drawings.

1 is a schematic structural diagram of a BOTDR system of the present invention, including a light source module, a first coupler, an optical polarization perturbation module, an electro-optical modulation module, a pulse generation module, an optical pulse amplification module, a circulator, a second coupler, and a first optical detector. module, frequency sweeping module, signal processing module, third coupler, second light detection module, feedback module and current control module, wherein the light source module is connected with the first coupler, and the narrow linewidth laser in the light source module emits continuous light, The first coupler is divided into two continuous lights: the first continuous light and the second continuous light (energy ratio is 9:1), the first continuous light is sent to the electro-optic modulation module, and the second continuous light is used as coherent light Local oscillator light; the electro-optical modulation module modulates the first continuous light into a pulsed optical signal according to the pulsed electrical signal generated by the pulse generating module. The pulsed optical signal is amplified by the optical pulse amplifying module and then injected into the optical fiber, and the circulator is used to obtain the distribution along the optical fiber. Backward Brillouin scattering optical signal; the second coupler then couples the backward Brillouin scattering optical signal output by the circulator with the randomly distributed local oscillator optical signal, respectively, to obtain a Brillouin scattering optical signal.

The Brillouin scattered light signal is divided into two channels through the third coupler, and one of the optical signals with 95% energy is obtained by the photoelectric conversion of the first photodetection module to obtain the Brillouin scattered electrical signal, which is then subjected to frequency sweeping, filter amplification, and acquisition. Obtain the corresponding backward Brillouin scattering electrical signal, analyze and process to obtain the Brillouin frequency shift at each point along the fiber distribution, and obtain the temperature or strain change around the fiber according to the Brillouin frequency shift; the other 5% The optical signal of energy enters the second optical detection module, and then the information containing the Brillouin power spectrum along the optical fiber is transmitted to the feedback module, and the feedback module controls the power of the light source by controlling the current control module.

The functions that the feedback control module needs to implement include:

a. When the power of the light source is not increased, obtain the split point A: the power of the Brillouin scattering spectrum decreases with the increase of the distance, when the power of the Brillouin spectrum is close to the noise floor power (the Lorentz fitting cannot be performed normally) , mark this position as point A;

b. Control the light source current to increase the power of the light source, and in this process, determine whether point A is stimulated: if point A is stimulated, the power near point A is not attenuated by a fixed attenuation coefficient, but produces severe jitter. When point A is stimulated, it is necessary to adjust the power of the light source until A is not stimulated;

c. After increasing the power of the light source, judge whether there is a valid Brillouin scattering signal at the farthest point B of the distance to be measured: if the signal power there is not overwhelmed by noise (the Lorentz fitting can be performed correctly), it means this There is a valid sensing signal at time B.

Figure 2 is a schematic diagram of radiation-induced attenuation. The loss of the sensing fiber will increase with the increase of the radiation dose, that is, the maximum detection distance of the sensing system will decrease with the increase of the radiation distance; if the sensing distance is constant Under the premise, when the radiation dose increases to a certain level, the Brillouin scattering signal at the end of the fiber will eventually be overwhelmed by noise, causing the BOTDR to fail to work properly.

The steps of the present invention to improve the service life of the BOTDR system are as follows.

Step 1, use the BOTDR system to obtain the spectrum of the split point A and the first segment of the fiber:

Figure 3 is a schematic diagram of the relationship between Brillouin spectral power and sensing distance. Point O is the signal incident end of the optical fiber, the power of the detection light pulse is P ₀ , and point B is the farthest end required to be detected. The initial light source power P0 should be increased as much as possible on the premise of ensuring no excitation phenomenon, so as to improve the sensing effect. The maximum detection range of the system before radiation is OC, but when the system receives a certain dose of radiation, the loss of the sensing fiber will increase, resulting in the system not receiving the effective Brillouin scattering signal at point B (overwhelmed by noise). At this time, keep the incident optical power P ₀ unchanged, and record the farthest sensing position A at this time (the farthest point where the effective Brillouin scattering signal can be obtained); at the same time, the back Brillouin scattering of the OA fiber is obtained. spectral lines.

It should be noted that as the radiation dose increases, the determination of point A changes dynamically according to the radiation dose received by the system. The specific performance is as follows: point A is the farthest effective sensing distance when the light source power is P ₀ (without boosting power).

Point A is obtained by the feedback control module of the system; the specific performance is: before the power of the light source is increased, the power of the Brillouin scattering spectrum decreases with the increase of the distance, when the power of the Brillouin spectrum is close to the noise floor power (cannot pass through the Lenz fit), mark this location as point A.

Step 2, performing fitting processing on the Brillouin spectral line of the optical fiber in the OA section obtained in step 1, so as to obtain the Brillouin peak frequency of the optical fiber in this section.

Step 3, increase the power of the incident optical signal, and obtain the spectrum of the second fiber after point A:

FIG. 4 is a schematic diagram of increasing the power of an incident optical signal. Gradually increase the power of the light source until it can be ensured that: (1) no stimulated Brillouin scattering occurs at point A (2) there is an effective Brillouin scattering signal at point B. At this time, stop increasing the power of the light source, and use the power to measure with the BOTDR system to obtain effective sensing data of the fiber segment between point A and point B. Considering that the radiation environment may have an impact on the threshold of the fiber, if the OA section of the fiber is not stimulated after the light source power is increased in step 3, you can use the Brillouin scattering spectrum of the entire fiber OB after the increased power to peak. Frequency measurements to get better Brillouin scattering lines.

It should be noted that with the increase of the radiation dose, the increase of the incident light power is dynamically changed according to the specific power of the reflected spectral line. The specific performance is as follows: the power of the light source should be increased to ensure that point A is not stimulated, and that there is an effective sensing signal at point B.

The boost of light source power is regulated by the current control module controlled by the feedback module.

Whether point A is stimulated is judged by the feedback control module; the specific performance is as follows: in the process of increasing the power of the light source, if point A is stimulated, the power near point A is not attenuated by a fixed attenuation coefficient, but produces a violent jitter. When point A is stimulated, it is necessary to adjust the power of the light source until point A is not stimulated.

Whether there is an effective Brillouin scattering signal at point B is judged by the feedback control module; the specific performance is as follows: when the power of the light source is increased, if the signal power at point B is not submerged by noise (the Lorentz fitting can be performed correctly), it means At this time, there is an effective sensing signal at point B.

In addition, considering that the power of stimulated scattering is relatively large, in order to ensure the normal operation of the system, the optical signal divided into two channels by the third coupler is detected by different optical detection modules. It is recommended that the first optical detection module adopts APD avalanche diodes.

In step 4, the back-Brillouin scattering spectral line of the AB fiber segment obtained in step 3 is fitted, so as to obtain the peak frequency of the Brillouin spectral line of the fiber.

Step 5: Perform splicing processing on the peak frequencies of the Brillouin spectral lines obtained in steps 2 and 4, so as to obtain the Brillouin peak frequency information of the entire sensing fiber OB.

Step 6, adjust the power of the light source to the original power P0, and repeat step 1 for measurement.

The embodiment is only to illustrate the technical idea of the present invention, and cannot limit the protection scope of the present invention. Any changes made on the basis of the technical solution according to the technical idea proposed by the present invention all fall within the protection scope of the present invention. .

Claims (7)

1. a BOTDR system working under the space radiation environment is characterized in that: comprising a light source module, a first coupler, an electro-optical modulation module, a pulse generating module, an optical pulse amplifying module, a circulator, an optical polarization disturbance module, a second a coupler, a third coupler, a first light detection module, a frequency sweep module, a signal processing module, a second light detection module, a feedback module and a current control module; the light source module emits continuous light, and the first coupler divides the continuous light into Two paths of continuous light with unequal energies, one with higher energy is input into the electro-optic modulation module, and one with lower energy is used as the coherent local oscillator light input into the optical polarization perturbation module, and the electro-optic modulation module is based on the pulse generated by the pulse generation module. The electrical signal modulates the input continuous light into a pulsed light signal. The pulsed light signal is amplified by the optical pulse amplifying module and injected into the fiber through the circulator, and the backward Brillouin scattering light signal distributed along the fiber is obtained through the circulator. The second coupling The device couples the backward Brillouin scattered light signal output by the circulator with the randomly distributed local oscillator light signal output by the optical polarization perturbation signal module, and outputs the Brillouin scattered light signal. The third coupler will couple the coherent output signal. The Brillouin scattering optical signal is divided into two optical signals with different energies. The optical signal with higher energy is converted into a Brillouin scattering electrical signal after photoelectric conversion by the first optical detection module, and then the frequency sweeping module sweeps the signal to obtain a Brillouin scattering electrical signal. The processing module filters and amplifies to obtain the corresponding backward Brillouin scattering electrical signal. The other optical signal with lower energy is input to the feedback module through the second light detection module, and the feedback module realizes the light source signal sent to the light source module by controlling the current control module. power control. 2. the BOTDR system that works under the space radiation environment according to claim 1, is characterized in that: the working process of described feedback module is as follows: (a) When the power of the light source is not increased, the split point A is obtained: the power of the Brillouin scattering spectrum decreases with the increase of the distance. When the power of the Brillouin scattering spectrum at a certain position is close to the noise floor power, that is, it cannot pass through normally. For Lenz fitting, mark this position as A; (b) Control the light source current to increase the power of the light source, and in the process, determine whether point A is stimulated: if point A is stimulated, the power near point A is not attenuated by a fixed attenuation coefficient, but violently jitter, when point A is stimulated, it is necessary to adjust the power of the light source until point A is not stimulated; (c) After increasing the power of the light source, determine whether there is an effective Brillouin scattering signal at the farthest point B of the distance to be measured: if the signal power at B is not submerged by noise, that is, the Lorentz fitting can be performed correctly, it means At this time, there is a valid sensing signal at B. 3. The BOTDR system according to claim 1 or 2, wherein the first coupler divides the continuous light emitted by the light source module into two continuous lights with an energy ratio of 9:1. 4. The BOTDR system according to claim 1 or 2, wherein the third coupler divides the input Brillouin scattering optical signal into two optical signals with an energy ratio of 95:5. 5 . The BOTDR system according to claim 1 or 2 , wherein the first light detection module adopts an APD avalanche diode. 6 . 6. a kind of method that promotes the described BOTDR system life of claim 2, is characterized in that, comprises the following steps: (1) Use the BOTDR system to obtain the spectrum of the split point A and the first segment of the fiber: Suppose point O is the signal incident end of the optical fiber, the power of the detection light pulse is P ₀ , and point B is the farthest end required to be detected. Sensing effect: The maximum detection range of the system before radiation is OC. When the system is irradiated, the loss of the sensing fiber will increase, resulting in the system not receiving the effective Brillouin scattering signal at point B. At this time, the incident light is maintained first. The power P ₀ remains unchanged, record the farthest point A at which the effective Brillouin scattering signal can be obtained, and at the same time obtain the back Brillouin scattering spectral line of the OA segment fiber; (2) performing fitting processing on the Brillouin scattering spectral lines of the optical fiber in the OA section obtained in step (1), thereby obtaining the Brillouin peak frequency of the optical fiber in this section; (3) Increase the power of the incident optical signal, and obtain the spectrum of the second fiber after point A: Gradually increase the power of the light source until it can ensure that no stimulated Brillouin scattering occurs at point A, and there is an effective Brillouin scattering signal at point B; at this time, stop increasing the power of the light source, and use the power to measure with the BOTDR system, Obtain effective sensing data of the fiber segment between point A and point B; (4) performing a fitting process on the back Brillouin scattering spectral line of the AB fiber segment obtained in step (3), thereby obtaining the peak frequency of the Brillouin spectral line of the fiber segment; (5) splicing the peak frequencies of the Brillouin spectral lines obtained in the steps (2) and (4), thereby obtaining the Brillouin peak frequency information of the entire section of the sensing fiber OB; (6) Adjust the power of the light source to the original power P0, and return to step (1). 7. method according to claim 6, is characterized in that: in step (3), if after raising light source power, OA segment optical fiber also does not have excited phenomenon, then can use the bridging of the whole segment optical fiber OB after raising power. The measurement of the peak frequency of the Yuan scattering spectrum is performed.