The scattering based sensors are all intrinsic interferometric fiber optic sensors, meaning that they depend on changes in the interference signal to obtain strain and temperature measurements. What makes scattering technologies unique is that they utilize naturally occurring backscattering light to create the interference signal. The scattering occurs due to random imperfections in the lattice structure of the fiber optic cable created during manufacturing. There are three different types of signals that are used in scattering based fiber optic sensors: Raman, Rayleigh and Brillouin. Since fiber Bragg gratings or other engineered reflective surfaces are not used, scattering based systems can use telecom grade fiber with special fiber terminations.
Generally speaking, scattering based sensors have spatial resolution on the order of meters and refresh rates of 10-30 seconds at best. As the sensing length increases, the refresh rate significantly decreases. One limitation of scattering techniques is a low signal to noise ratio. By comparison, fiber Bragg grating based sensors have much higher signal to noise ratios. Applications that are well suited for scattering technologies are ones that do not require real time measurements, require kilometers of sensing length, and do not need the spatial resolution to be less than a meter. The three different types of scattering technologies have some key differences that will be discussed below.
Raman signals are composed of stokes and anti-stokes components, however, only the anti-stokes component is used to obtain temperature measurements. Unlike Brillouin and Rayleigh components, Raman backscattering is a result of thermally induced vibrations. As a result, Raman based systems, commonly referred to as Distributed Temperature Sensing (DTS) systems, are sensitive to temperature, but not to strain. Raman scattering is the most common type of scattering based systems on the market. The most common uses include oil and gas pipeline monitoring for security and leak detection, perimeter fence security, and linear heat detection systems.
Rayleigh signals have more intensity than Raman signals and are more sensitive to strain than to temperature. This makes Rayleigh scattering systems most effective for measuring strain. Due to the manufacturing process, the density and composition varies throughout the fiber. This alters the refractive index of the lattice structure. Strain and temperature cause spectral shifts in Rayleigh backscattering, which can be identified by an interrogator. Rayleigh scattering based systems are not as common as Raman systems, but have been used in similar use cases. The biggest application for Rayleigh scatter based systems to date is distributed acoustic sensing (DAS). DAS systems show the most promise in downhole oil and gas solutions.
Brillouin scattering systems are sensitive to both strain and temperature and have the longest measurement distance of the scattering technologies. Unlike the other two techniques, there are few commercially available Brillouin based sensing systems. The majority of the work done on this type of technology is research based. Applications that show the most promise for Brillouin scattering systems are similar to Raman and Rayleigh scattering applications. The main advantage Brillouin scattering will offer is the ability to cover longer distances and monitor both strain and temperature.
There are many use cases for fiber optic sensors, whether scattering based, FBG based, or otherwise. Understanding the unique advantages of each type is essential for choosing the right technology for your application. Contact Sensuron today to learn more about how fiber optic sensing can make your project successful.