Overview Of Distributed Fiber Optic Sensors

Distributed optical fiber sensor is a sensor that uses unique distributed optical fiber detection technology to measure or monitor the spatial distribution and time-varying information along the optical fiber transmission path. It arranges the sensing optical fiber along the field, and can obtain the spatial distribution of the measured field and the change information over time at the same time, which is attractive for many industrial applications. The principle of the distributed optical fiber sensing system is to use optical fiber as the sensing sensitive element and transmission signal medium at the same time, and adopt advanced otdr technology and ofdr technology to detect changes in temperature and strain at different positions along the optical fiber to realize truly distributed measurement .　The principle of micronoptics temperature measurement is a distributed temperature sensing system based on raman scattering effect; the principle of strain measurement is a distributed temperature and strain sensing system based on brillouin scattering, which can measure temperature and strain at the same time. It uses a sensing fiber that is sensitive to a specific measured field to measure the basic loss or scattering along the length of the fiber. Usually adopt otdr (optical time domain reflectometer) technology, obtain the spatial change information of the measured field from the output information. Therefore, this continuous distribution sensor can obtain the distribution of the measured field along the length of the optical fiber with a certain spatial resolution. The otdr technology is currently an indispensable device for fault (such as breakpoint) location and diagnosis in optical fiber communication.　The most basic form of distributed optical fiber sensor is to directly use otdr to detect excessive local loss along the length of the optical fiber.　Distributed optical fiber temperature sensing initially demonstrated. It utilizes the characteristic that the backscattering coefficient changes with temperature. In order to improve the measurement sensitivity, a liquid core fiber is used. The disadvantage of this scheme is that the sensitivity of the solid-core fiber is extremely low, the liquid-core fiber is impractical, and the received signal is related to the mode structure. Because the birefringence parameters in single-mode fibers are sensitive to many physical quantities, such as strain, pressure, electric field, and magnetic field. Therefore, this derivative otdr technology has broad application potential.　The basic otdr technology is essentially an optical radar. The principle of optical ranging between ordinary radar and distributed optical fiber sensor is similar. In order to improve the spatial resolution of the measurement, various technologies have been derived, such as continuous wave range adjustment (fmcw), which are essentially optical frequency domain reflection technologies (ofdr).　Several researchers have reported using the relationship between Raman scattering and temperature to form distributed temperature sensing. One is to use the improved otdr to analyze the ratio of Stokes and anti-Stokes backscattered components. Recently, a distributed temperature sensor that only measures the anti-Stokes component and double-ended Raman otdr has been reported, with a measurement length of 950m and a temperature resolution.　The main disadvantage of this scheme is that the Raman scattering coefficient is very small, almost 3 orders of magnitude lower than Rayleigh scattering, so it requires a high-power laser and a high-gain low-noise amplifier.　Recently, some people have studied the relationship between the temperature and absorption or fluorescence of rare earth fiber to form distributed temperature sensing. However, the use of fluorescence characteristics requires rare earth fibers to have a short fluorescence lifetime.