Fiber optic sensing uses optical fibers as sensors, creating thousands of continuous sensor points along the fibers. This is known as distributed fiber optic sensing using distributed fiber optic sensors.
The device used to measure the optical fiber itself is usually called an interrogator. Its purpose is to measure temperature and strain along a standard or specific optical fiber using Raman and Brillouin distributed optical fiber sensor technology.
I. How does fiber optic sensing work?
Optical fiber can serve as a communication path between a test station and external sensors, a process known as external sensing. However, when the optical fiber itself is used as a fiber optic sensing system, this is called intrinsic fiber optic sensing.
The advantage of this type of fiber optic sensing technology is that it eliminates the need for a separate interface between the fiber optic cable and external sensors, thus reducing complexity and cost. For this purpose, external stimuli such as temperature and strain fluctuations need to influence the light source in the cable in a measurable manner to provide useful data.
When photons are randomly scattered after coming into contact with particles in an optical fiber, it is called Rayleigh scattering. This principle has been proven applicable to various types of optical fiber testing techniques, such as OTDR fiber testing, because the volume, wavelength, and position of the light backscattered to the detector can determine the amplitude and location of attenuation events in the fiber.
In a similar manner, Raman scattering in the Stokes band produces temperature-induced changes in the photons scattered back to the source. By measuring the difference in intensity between the backscattered light in the Stokes and anti-Stokes bands, the temperature at any given location along the optical fiber can be precisely determined.
Brillouin scattering is a similar phenomenon in which the wavelength of backscattered light is predictably affected by external temperature and acoustic stimuli. This data, combined with background temperature information at the same point, can be used to accurately determine the strain experienced by the optical fiber and to analyze which regions of the fiber are affected.
II. Distributed Fiber Optic Sensors
Furthermore, Raman and Brillouin scattering have been effectively used in distributed fiber optic sensing (DFS). Raman scattering is used for distributed temperature sensing (DTS), and Brillouin scattering is used for distributed temperature and strain sensing (DTSS). These measurements are unaffected by fiber loss and can be used to accurately monitor temperature and strain over distances of tens of kilometers.
In this context, "distributed" refers only to fiber optic sensing technology that can continuously measure the entire length of the fiber, or distributed fiber optic sensors. Essentially, the fiber itself is a sensor. Since these fiber optic sensing methods are entirely inherent, standard telecommunications fiber optics can be used as the medium if the desired temperature is kept below 100°C (212°F) and the fiber is not subjected to excessive chemical or mechanical damage.
