Study on Brillouin Scattering Detection of Distributed Optical Fiber Sensor Based on High Frequency

Li Nan

Abstract： In this paper， the optical fiber sensing technology of brillouin scattering distribution device is introduced in detail， and the scattering signal is detected by correlation detection technology， so that the filtering amplification technology can be effectively improved.

Key words： high frequency microwave technology; distributed optical fiber sensor; brillouin scattering detection

CLC number：R445-4 Document identification code：A

At present， in the field of optical fiber sensing， high frequency microwave technology is a new sensing technology in the scientific community， and it is also a hot technology which is studied by the world. In the application process， the high frequency microwave technology can not only have a long detection distance， but also has the strong anti-radiation， anti-electromagnetic wave interference advantages. And from the physical point of view， its small size and light weight will not be affected by high temperature and high pressure in the working environment， and is widely used in the construction of various buildings.

I. Research Importance

In the 1970s， optical communication technology began to develop， and for this reason， optical fiber sensing technology emerged， which is a brand new sensing technology. Different from other sensing technologies， optical fiber sensing technology is different in the way and mechanism of sensing， signal detection and signal processing is different from ordinary electrical sensors. The specific performance is that the pipeline sensor adopts the form of light wave in the signal processing， uses the optical fiber group as the transmission medium for signal sensing and detection， and the distributed optical fiber sensor has obvious advantages in the system. For example， from a physical point of view， the volume and mass are relatively small， while the material is soft and can be bent. And has the extremely strong anti - electromagnetic interference ability in the concrete use. At the same time， it also has the ability to transmit sensing technology， with a long sensing distance， which is very suitable for some projects requiring long-distance transmission. Because of its main characteristics， it can have a lot of sensing parameters in the transmission summary， and then the temperature and other data for comprehensive detection.This sensor technology with many advantages is widely used in various fields of society， such as the construction of bridges in urban construction projects， the monitoring of railway operation， and even the rocket launching and propulsion in the aviation field.

II. Principle of Optical Fiber Sensing Technology

The principle of the application of optical fiber sensing technology is to make use of the propagation characteristics of the incident tube and make it propagate along the fiber core of the optical fiber. However， due to various physical factors such as temperature and magnetic field in the external environment， the actual length of the optical fiber will be affected， and the refractive index of the optical fiber will change significantly， resulting in a certain degree of fluctuations in the intensity， frequency and other characteristic parameters of the closed optical fiber. In the sensing process， the physical quantity can be sensed effectively by measuring the biomass and chemical quantity.

III. Principle of Brillouin Scattering Signal Detection

Understanding the principle of brillouin scattering is the key to study brillouin scattering. At present， a large number of scholars in China and abroad have conducted in-depth discussions on this issue， and have achieved remarkable results. It is generally believed that the brillouin scattering is related to the density of optical fiber. The density of the material will change in a certain period， and this change is the main factor inducing the scattering. In addition， the pumped light and phonons interact with each other in the fiber. In this process， incident photon energy can appear， its frequency is generally v， and the propagation speed is the speed of light. This photon energy will generate corresponding photons， but the resulting photon will travel in the opposite direction to the photon energy， and the relevant field calls this photon "stokes photon". A large number of experiments and continuous verification results show that there is a certain difference between the stokes photon and the incident photon， mainly reflected in the frequency， and the frequency difference between the two parameters is generally around 11GHz.

After fully grasping the principle of brillouin scattering distributed optical fiber， the sensing measurement can be effectively carried out. As a kind of nonlinear brillouin backscattering signal， the sensing measurement can be fully realized. Once the incident condition of the fiber can reach the specific optical power of the fiber and the brillouin scattering threshold value are the same， the backscattering signal of brillouin can be effectively generated. However， a great deal of experience and data show that strain and temperature are important factors affecting brillouin scattering， for this reason， when the temperature changes to a certain degree or axial strain appears in the propagation path of certain optical fibers， the backward-brillouin scattering light in the optical fibers will have important changes， for example， its frequency and intensity will change significantly， and the actual degree of drift of frequency and the change of intensity are linearly related to the temperature and strain of the environment. For this reason， through the continuous analysis and experiments of researchers， a good linear relationship is finally obtained between the two sides. And the actual temperature and strain change of the fiber distribution can be obtained from the actual frequency shift and intensity change of the brillouin scattering tube of the sensing fiber [1].

IV. Temperature Strain Characteristics of Brillouin Scattering Signals

Based on the analysis and discussion of the practical principle of brillouin scattering distributed optical fiber sensor， it is found that the actual change is found between the brillouin frequency shift， the scattered light power， the temperature and the strain， however， it is necessary to make a detailed comparison and analysis of the optical fiber material so as to obtain the relationship between the temperature strain and the optical fiber material.

Based on the study of brillouin frequency shift， it can be determined that the optical fiber itself has elastic and thermal optical effects. For this reason， the refractive index of the optical fiber will be significantly fluctuated by the temperature of the optical fiber and the actual vibrations changes， and thus generate induction. However， the effect of temperature and strain on the frequency shift of brillouin is limited and independent. Therefore， we need to use control variables to discuss the influence.

In the discussion of the relationship between brillouin power and temperature and strain， it is important for researchers to pay attention to the important influence on brillouin scattering tube power. Through the experiment of temperature and stress on the refractive index of the optical fiber， it is found that the refraction of both hands of the optical fiber in the thin slice is also affected by the ambient temperature， and has a significant change. And the intensity of reflected light can reach the maximum under the same phase matching condition.

V. Brillouin High Precision Distributed Optical Fiber Sensing System Parameters

The study of brillouin scattering shows that the actual system parameters of its high-precision distributed optical fiber sensor include three aspects.

（i） Spatial Resolution

In fact， the spatial resolution of optical fiber distributed sensor in the system refers to the effective discrimination of the minimum spatial distance in the sensing process and the effective representation of the minimum spatial distribution to be measured.

（ii） Detection Sensitivity

As discussed above， the gain of brillouin can satisfy Lorentz distribution， so the expression formula of the minimum detection amount of temperature and strain can be obtained by calculation and formula.

（iii） Dynamic Range

The specific definition of dynamic range in the sensing system has been clearly defined， which is essentially the difference between the backscatter power and the noise power. Therefore， in the actual detection process， the maximum range of optical fiber loss can be easily detected， and the measurement range can be determined， and the better the performance of the dynamic range， the more clear and larger the measurement distance of the system will be. Such data calculation and derivation can accurately grasp the actual situation of optical fiber and help researchers to understand the variation parameters of its distributed sensing system.

VI. Distributed Pipeline Sensing Technology Based on Brillouin Scattering

In the process of researching the distributed sensing system of the pipeline， it should be clear that the most obvious factors affecting the fiber material are the changes of temperature and strain， which will cause the scattering characteristics to change to a certain extent， which will further change the brillouin frequency shift and its light power significantly. In view of the characteristics of the brillouin fiber sensing system， researchers need to use the theory of distributed optical fiber sensing and specify the sensing parameter data， and know the influence of temperature and strain on brillouin scattering. At the same time， in the brillouin scattering distributed optical fiber sensing， the detection of brillouin scattering signal are also needed to pay much attention， such as brillouin optical time domain reflectionoptical time domain analysis， optical frequency domain analysis， therefore， in various distributed sensing technologies， it is necessary to make clear that the backward reflection sensing technology in the time domain can be detected by single end， so as to be able to carry out remote sensing and avoid interference from pipeline link breakpoints [2].

To sum up， in the process of studying brillouin scattering distributed optical fiber sensing， it is necessary to clarify its specific principle， at the same time， to clarify the technical points in each link， and to pay attention to the actual influence of brillouin scattering signal temperature strain on the optical fiber material.

References：

[1] 陳福昌 ， 胡佳成 ， 张承涛 . 基于高频微波技术的分布式布里渊光纤温度传感器 [J]. 中国激光 ，2012（06）：131-135.

[2] 崔良端 ， 朱忠翰 ， 贾亮 . 高频微波印制板技术及发展前景 [J]. 电子技术与软件工程 ，2017（15）：88.

基于高频微波技术的分布式光纤传感器布里渊散射信号检测研究

李 楠

（辽宁邮电规划设计院大连办事处，辽宁 大连 116001）

摘要：本文对布里渊散射分布设的光纤传感技术进行了详细的介绍，同时利用相关检测技术对其散射信号进行了检测，这样就可以有效地改进滤波的放大技术。

关键词：高频微波技术;分布式光纤传感器;布里渊散射信号检测