TECHNOLOGIES
OPTICAL FIBERS AS SENSORS
A bundle of optical fibers
Optical fiber is a waveguide of light signals made in silica (SiO2) which is mainly used in optical communications.
Optical fiber has following characteristics: broad bandwidth (12.5 THz), low loss (0.2 dB/km), light in weight (1 km of fiber weighs around 27 g), small in size (volume of 1 km of fiber is similar to that of a pen), flexible (radius of curvature of 1 cm), EMI immunity, cheap (around 2 Cent per 1 m).
Optical fibers can be used as sensors by using the fibers as optical signal guides or the sensing probes. Various fiber sensors exist as interferometric sensor, fiber Bragg grating (FBG) sensor, fiber gyroscope, distributed fiber sensor, semiconductor-based temperature sensor, phase-detector etc. These fiber sensors are used to sense various physical variable such as temperature, strain, acoustic signals, angular velocity etc.
Fiber sensors are utilized in many fields as constructions, aerospace, energy, transportation, communications etc.
DISTRIBUTED SENSORS
NSPX’s professional field is the distributed sensor. Distributed sensor is a linear shaped, continuous distribution of many sensors.
Concept of fiber sensor
Distributed sensors use optical fiber as the sensing probe (sensing body). By accumulating the backscattering signal from each fragment of the fiber (sensing point), distributed map of the total sensing probe is acquired. The range of ordinary distributed sensor gets up to 10s of kilometers with spatial resolution of sub-meters. One distributed sensor can be seen as more than 10,000 point sensors. Distributed sensing is realized in time-domain, frequency-domain, and correlation-domain techniques.
Nonlinear light scattering process
As light interacts with media, small proportion of the light is experiences nonlinear light scattering process. Distributed sensors use nonlinear light scattering process to sense physical variables.
There are four major three types of nonlinear light scattering processes which are Rayleigh scattering, Brillouin scattering, Raman scattering. Rayleigh scattering is the caused by light interacting with the polarization of the atom of media. Brillouin scattering is the phenomenon of light interacting with the vibrations of array of atoms of the media. Raman scattering is the phenomenon of light interacting with the energy level of the atom of media.
There are many types of distributed sensors as OTDR (optical time-domain reflectometry), OFDR (optical frequency-domain reflectometry), BOTDA (Brillouin optical time-domain analysis), BOCDA (Brillouin optical correlation-domain analysis), BDG-OTDA (Brillouin dynamic grating optical time-domain analysis) etc. They are categorized by which nonlinear light process they use and by the technique of distributed measurement.