14.1   INTRODUCTION

Although the most important application of optical fibers is in the field of communication,
optical fibers are finding more and more applications in the area of sensing.
The use of optical fibers for such applications offers the same advantages as in the
field of communication: lower cost, smaller size, rugged, higher accuracy, greater
flexibility with multifunctional capabilities, wide range of sensor gauge lengths, and
greater reliability. Compared to conventional electrical sensors, such fiber optic sensors
are immune to external electromagnetic interference and can also be used in
hazardous and explosive environments. A very important attribute of fiber optic sensors
is the possibility of having distributed (i.e., measuring over a continuous region)
or quasi-distributed (i.e., measuring at a large number of discrete points in some
region) sensing geometries, which would otherwise be too expensive or complicated
using conventional sensors. Using fiber optic sensors, it is possible to measure almost
any external parameter, such as pressure, temperature, electric current, magnetic field,
rotation, acceleration, strain, and chemical and biological parameters, with greater
precision and speed. These advantages lead to increased integration of such fiber
optic sensors into such civil structures as bridges and tunnels, process industries,
medical instruments, aircrafts, missiles, and even cars.

Figure 14.1 is a schematic of a fiber optic sensor system. Light from a suitable
source is coupled into an optical fiber. The external disturbance modifies some property
of the light beam,which is then guided through the optical fiber to a detector. Fiber
optic sensors can be classified broadly into two categories: extrinsic and intrinsic. In
extrinsic sensors, the optical fiber simply acts as a device to transmit and collect light
from a sensing element that is external to the fiber. The sensing element responds to
the external perturbation, and the change in the characteristics of the sensing element
is transmitted by the return fiber for analysis. The optical fiber here plays no role
other than transmitting the light beam to and from the sensing region. Such fiber optic
sensors are easy to design and fabricate and relatively inexpensive. Examples of such
sensors are Doppler anemometers, noncontact vibration measurement, and pressure
sensors which find wide applications in automobiles and aerospace, for example.