Distributed fiber-optic sensors and their applications

Optical fibers are ubiquitous in communication primarily because of its high information-carrying capacity, low loss, small size, negligible electromagnetic interference, low maintenance, and relatively smaller operating expense. Though the deployment of optical communication was initiated for long-distance applications, the increasing bandwidth requirements have resulted in a great reduction in these distant ranges of installation, to include metropolitan and access networks, data 310centers, and possibly even within a semiconductor processor chip. Along with the increasing utilization of optical fibers for communication, there has been a significant progress in its use for sensing physical parameters. Jacketed fibers with different layers of protection are used for communication applications in order to minimize the influence of environmental perturbations such as temperature, stress, strain, vibrations, and humidity on the propagating signals. In contrast, a sensing application is focused around precisely measuring the changes in amplitude, frequency, phase, or polarization of the propagating optical signal to quantify the desired physical parameter that has created the change. The other parameters that could be sensed in general through optical sensing include rotation, torque, acceleration, voltage, current, pH concentration, and nuclear radiation-this list is definitely not exhaustive. A detailed review of optical fiber sensors can be found in [1]. Distributed sensing refers to such an application where the fiber is itself the sensing element [2]. The desired measurand is monitored continuously along a given length of the fiber and as a function of time. Like in communication, there are some unique advantages presented by the optical fibers as compared to the electronic counterparts, when used for sensing. The 1D nature of the fiber is probably the biggest advantage, since the parameter to be sensed can now be estimated in the desired direction, without getting influenced by the changes in other directions. The other key advantages due to its electrically passive operation include the possibility of a safe operation in a chemically or electrically hostile environment, lighter weight, amenability to bending, and minimal susceptibility to electromagnetic interference.