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The working principle and application of distributed fiber optic sensors

Distributed fiber optic sensing technology has important applications in fiber optic characterization, fault localization, and monitoring of fiber optic environmental temperature, stress, and vibration. Optical time-domain reflection technology, optical time-domain analysis technology, and optical frequency-domain analysis technology are several commonly used technologies in distributed fiber optic sensing technology.

Distributed fiber optic sensors have been widely used in fields such as power, petrochemicals, transportation, civil engineering, and aerospace. However, with the increasing production safety requirements in various industries, single function distributed fiber optic sensors can no longer meet the needs. In order to have a more comprehensive understanding of engineering safety conditions, users often need to simultaneously monitor parameters such as temperature, vibration, and strain in real-time from all angles. Generally, at least two different sets of distributed fiber optic sensors need to be equipped to meet the requirements.

When an optical fiber is affected by external factors such as temperature, stress, vibration, etc., the intensity, phase, frequency, and other parameters of the transmitted light in the fiber will change accordingly. By detecting these parameters of the transmitted light, corresponding physical quantities can be obtained. This technology is called fiber optic sensing technology. The characteristics of the fiber optic itself, such as non electrification, electromagnetic resistance, radiation resistance, high voltage resistance, no spark generation, and good insulation performance, make the fiber optic sensing system the mainstream of sensor systems and gradually replace traditional sensor systems. When the physical quantities on the optical fiber, such as pressure, temperature, humidity, electric field, magnetic field, etc., change, it will cause changes in the physical characteristics of the optical fiber, resulting in various optical effects of the transmitted light waves in the optical fiber, such as scattering, polarization, intensity changes, etc. By detecting changes in light waves in optical fibers, physical quantities such as temperature, pressure, deformation, and water level can be detected. In recent years, the rapid development of optoelectronic devices, especially semiconductor lasers, wavelength division multiplexing and optical coupling technology, detection and processing of optoelectronic signals, and other technologies, has made it a reality for optical fibers to be used as distributed sensor systems.

Distributed fiber optic sensing technology is widely used for monitoring the condition of large substrates such as buildings, bridges, and slopes due to its advantages of distributed measurement, long measurement distance, anti electromagnetic interference, and high insulation strength. It is also applied in the field of electrical engineering to measure temperature and strain of electrical equipment such as submarine cables and overhead transmission lines, and has a very broad application prospect. At present, there are few reports on the detection of transformer winding temperature and strain based on distributed fiber optic sensing technology.
Fiber optic sensors have many advantages such as strong resistance to electromagnetic interference, high sensitivity, good electrical insulation, safety and reliability, corrosion resistance, and the ability to form fiber optic sensing networks. Therefore, they have broad application prospects in various fields such as industry, agriculture, biomedicine, and national defense.
In recent years, the Brillouin optical time-domain analyzer, as a typical representative of distributed fiber optic sensing technology, has received widespread attention. Compared with other fiber optic sensors, the Brillouin optical time-domain analyzer has advantages such as high spatial resolution, ultra long distance sensing, and dynamic measurement. It can simultaneously measure physical quantities such as temperature and microstrain with high precision. Due to the fact that optical fibers serve as both sensor components and signal transmission channels, using optical signals as transmission signals can effectively reduce structural costs.

Distributed fiber optic sensing technology is widely used in pipeline leakage monitoring technology due to its wide sensing space range, the same fiber for sensing and transmission, simple structure, convenient use, low cost of signal acquisition per unit length, and high cost-effectiveness.

Traditional sensors are mostly electric type, with small measurement range and difficult grid connection. Moreover, point sensors have high maintenance costs when measuring large ranges and long distances. In contrast, the sensors of fiber optic sensors are fiber optic, which has a stable structure, resistance to electromagnetic interference, corrosion resistance, small size, and low cost. In addition, the coverage of fiber optic is wide, and it can measure systems with a wide range and spatial distribution. Therefore, since the late 1970s, distributed fiber optic sensing has been widely developed, with the emergence of optical time domain reflection technology (OTDR), Raman optical time domain reflection technology (ROTDR), Brillouin optical time domain reflection technology (BOTDR), and phase sensitive optical time domain reflection technology( Φ- OTDR, etc. At present, Raman optical time-domain reflection (ROTDR) technology based on temperature measurement is relatively mature. Among them, Raman optical time-domain reflection (ROTDR) technology injects pulsed light into the fiber, and the temperature effect of backward Raman scattering spectrum is generated during the propagation of light in the fiber. When the incident light quantum collides with the material molecules in the fiber, elastic and inelastic collisions occur. When elastic collision occurs, there is no energy exchange between the light quantum and the material molecules, and the frequency of the light quantum does not change in any way, resulting in Rayleigh scattering light maintaining the same wavelength as the incident light; In inelastic collisions, energy exchange occurs, and light quanta can release or absorb phonons, resulting in the generation of a longer wavelength Stokes light and a shorter wavelength anti Stokes light. Due to the sensitivity of anti Stokes light to temperature, the system uses the Stokes optical channel as the reference channel and the anti Stokes optical channel as the signal channel. The ratio of the two can eliminate non temperature factors such as light source signal fluctuations and fiber bending, achieving the collection of temperature information.

FJINNO provides distributed fiber optic temperature measurement systems, which are directly sold by manufacturers and can be widely used in comprehensive pipe galleries, cable trenches, oil and gas pipelines, substations, etc.

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