The principle and application of distributed fiber optic sensors

Fiber optic temperature sensor, Intelligent monitoring system, Distributed fiber optic manufacturer in China

In recent years, fiber optic sensing technology has developed rapidly and received increasing attention, gradually becoming another major fiber optic application technology industry after the development of fiber optic communication industry. Among them, distributed fiber optic sensing is currently one of the hot research topics both domestically and internationally.

Distributed fiber optic sensing measurement is a technology that utilizes the one-dimensional spatial continuity characteristics of optical fibers for measurement. Fiber optic serves as both a sensing element and a transmission element, allowing for continuous measurement of environmental parameters distributed along the entire length of the fiber, while obtaining the spatial distribution status and temporal information of the measured data. Distributed fiber optic sensing technology mainly includes optical time-domain reflection and frequency-domain reflection technology based on fiber Raman scattering or Brillouin scattering (R/B-OTDR/OFDR), polarized optical time-domain reflection technology based on fiber Rayleigh scattering (P-OTDR), long-distance optical interference technology, and quasi distributed fiber Bragg grating multiplexing technology.

Principles of Distributed Fiber Optic Sensing Technology

Distributed Fiber Optic Sensing Technology Based on Backscattering

When light waves propagate in optical fibers, they generate backscattered light, including Rayleigh scattering, Raman scattering, and Brillouin scattering. By detecting the backscattering generated by various points along the fiber optic cable, the relationship between the backscattered light and the measured (such as temperature, stress, vibration, etc.) can be used to achieve distributed fiber optic temperature sensing based on Raman scattering.

Measuring the anti Stokes Raman reflection signal in optical fibers can achieve distributed temperature sensing. Since the 1980s, extensive research has been conducted on the optical time-domain measurement technology of anti Stokes Raman scattering signals both domestically and internationally.

By utilizing the temperature effect of fiber optic backscattering, the temperature field at each point in the space where the fiber is located modulates the intensity of anti Stokes backscattering light in the fiber. The optical time-domain reflection (OTDR) technology of the fiber optic is used to detect and locate the measured temperature points. This technology has a simple measurement principle and relatively low cost. Currently, it can achieve a measurement distance of over 10 km and has been applied to a certain extent. However, it requires high-power, short pulse light sources and high-speed signal amplification and acquisition devices, and its temperature measurement accuracy and spatial resolution are limited by device performance and cost.

In recent years, optical frequency domain reflection technology (OFDR) has also experienced rapid development. OFDR technology uses a power modulated continuous laser as the light source, so the backward Raman scattering power is nearly 2000 times higher than that of OTDR technology under the same incident conditions. Although the signal is modulated at high speed, the frequency band is narrow and easy to remove noise through filtering, which can greatly improve the signal-to-noise ratio of the sensing signal. It has greater advantages in spatial resolution, detection accuracy, and real-time performance.

Distributed fiber optic temperature/stress sensing based on Brillouin scattering

When using narrow linewidth continuous laser to pump single-mode fibers, Brillouin scattering is a major nonlinear effect. The scattering performance of Brillouin scattering can be described by the magnitude of the Brillouin scattering frequency shift, which is related to the phonon rate of the medium, and this rate depends on temperature and strain. Distributed fiber optic temperature and stress sensing can be achieved by obtaining temperature or stress information through spectral analysis and locating the parameter field distribution using pulsed light.

The distributed fiber optic sensing technology based on stimulated Brillouin scattering has high accuracy and spatial resolution for measuring single distributed parameters such as temperature and stress, and is the most promising and breakthrough technology developed in recent years. It generally adopts a Pump Probe structure, known as Brillouin optical time-domain analysis (BOTDA). At present, distributed fiber optic sensing technologies based on stimulated Brillouin scattering mainly include BOTDA based on pulse laser pumping, BOTDA based on correlated continuous waves, and BOTDA based on dark pulse laser pumping.

Distributed sensing based on polarized light time-domain reflection

Polarized time-domain reflection (POTDR) sensing is a novel sensing technology that achieves distributed fiber sensing by detecting changes in polarization state in optical fibers. POTDR technology is developed on the basis of OTDR technology, and its working principle is that the backward Rayleigh scattering light in the tested single-mode fiber contains additional information about the polarization state changing along the fiber. By coupling linearly polarized light into a fiber optic, Rayleigh scattering occurs when light pulses are transmitted in the fiber. During the scattering process, the polarization state of the light changes with the action of external parameters on the fiber, and the polarization of the light is a function of position. Therefore, by detecting the polarization characteristics of backscattered light, the temporal and spatial distribution of polarization characteristics in the fiber optic can be obtained, thereby obtaining the measured field distribution. Distributed fiber optic sensing technology has excellent measurement accuracy, reliability, and dynamic measurement characteristics, and is inherently safe and easy to lay in engineering. Therefore, it is widely used in civil engineering, aviation, power, petrochemical, medical and other fields.

1. Application in Civil Engineering Structures

Distributed fiber optic sensing technology is widely used in safety detection of civil engineering structures such as bridges, rock deformation measurement, road and site measurement, and perimeter security monitoring. It can provide important data for monitoring the speed, load capacity, and type of transportation vehicles. The measurement accuracy of this type of sensor can reach several microstrain levels, with good reliability, and can achieve dynamic measurement. By using distributed embedding, it can also monitor the health status of the entire building, thereby preventing the occurrence of engineering and traffic accidents.

2. Applications in the aerospace field

In the field of aerospace, flight safety is a highly concerned aspect. Fiber optic sensors have the advantages of small size, light weight, and high sensitivity. Distributed fiber optic sensing technology was successfully used for non-destructive testing in the aerospace field as early as 1988. Embedding fiber optic sensors into aircraft or launch tower structures to form a distributed intelligent sensing network can enable real-time monitoring of the internal mechanical performance and external environment of the aircraft and launch tower. Boeing has conducted a lot of research in this area. At present, distributed fiber optic sensing technology can be used to achieve strain and displacement monitoring at aircraft wings, wings, stabilizer shafts, support rods, and other locations, as well as real-time online measurement of operating temperatures at connection points such as motors and circuits.

3. Applications in the Shipbuilding Industry

Fiber optic sensing technology is also widely used in the shipbuilding industry, such as strain monitoring at critical positions of the ship, damage assessment, and early warning under overload conditions. Structural defects in ships often affect their safety performance. A large-scale structural health monitoring system based on distributed fiber optic sensing technology can monitor the health status of the ship in real time, thereby preventing accidents from occurring. Large scale application of fiber optic sensing technology for real-time detection of damage in ships and submarines.

4. Applications in the power industry

The rapid expansion of the power grid and the continuous improvement of voltage levels have put forward higher requirements for the reliability and safe operation of power equipment. However, high-voltage detection technology cannot keep up with the development of the situation, and conventional detection equipment can no longer meet the current needs. At present, distributed fiber optic sensors are an ideal detection technology and have important applications in the safety monitoring of high-voltage power systems. For example, it can be used for monitoring cable temperature and cable conductor current carrying capacity. fiber optic temperature sensors can be used to monitor the surface temperature of long-distance transmission lines in real time, calculate the allowable load and current carrying capacity of conductor temperature, and provide comprehensive and effective solutions for fault monitoring and load management of transmission lines, ensuring the safety of transmission lines, improving asset utilization, discovering potential faults, and achieving preventive maintenance.

5. Applications in the petrochemical industry

Leakage is the main fault in the operation of oil pipelines, often resulting in huge losses. Therefore, oil pipeline leakage detection is an important issue that urgently needs to be solved in the petroleum industry. By using sensors installed near pipelines, pressure and vibration signals generated by incidents such as leaks, nearby mechanical construction, and human damage can be picked up. Furthermore, pipeline leaks can be detected and located through sensing related technologies. Distributed fiber optic sensing technology is very suitable for long-distance pipeline leakage detection due to its ability to obtain continuous spatial and temporal distribution information of the measured physical field. In addition, distributed fiber optic sensing technology can also be used to monitor the strain and bending status of high-pressure pipelines in real time.

6. Applications in Medicine

Fiber optic sensors are soft, compact, with high degrees of freedom, մեկուսացում, and are not affected by radio frequency and microwave interference, with high measurement accuracy. They have obvious advantages in medical applications, such as detecting human blood vessels, human surgical correction, and ultrasonic field measurement. Fiber optic endoscopy makes it feasible to examine almost all parts of the human body, and the operation does not cause pain or discomfort to patients. Among them, fiber optic vascular endoscopy has been applied in human cardiac catheterization. Fiber optic endoscopy is not only used for diagnosis, but is also currently entering the field of treatment, such as polypectomy surgery. Microwave heating treatment technology is currently an effective way of treatment, but the temperature of microwave heating treatment technology is difficult to grasp, and fiber optic temperature sensors can precisely monitor the effective temperature of microwave heating treatment technology. The research and application of fiber optic temperature sensors in cancer treatment are increasingly emerging.