Fiber-Optic Transducers

The role of optical fibers in telecommunication systems is now well established.

The recognition of their role in transducers, about two decades ago, led to an explosive number of applications in physical, chemical, and biochemical transducers.101–103 These transducers can be separated into extrinsic and intrinsic types. In extrinsic fiber-optic transducers, the transduction process takes place external to the fiber and the fiber itself plays a passive role as a light conduit.

In intrinsic transducers, however, the transduction process takes place within the optical fiber. The measurand modulates a parameter of the fiber, such as the refractive index, and the fiber in turn modulates the light beam propagating through its core.

Fig 5.43 Total internal reflection in optical fibers.

The optical fiber used in these transducers is of the cladded-core type used in communication links, as shown schematically in Fig. 5.43. The core is made of silica glass, and surrounding the core is a concentric silica cladding.

The light beam in an optical fiber propagates mainly in the core by total internal reflection (TIR), as shown in Fig. 5.43. A light ray, striking the core-cladding interface at S, is partly reflected (A’) and partly refracted (A”) into the cladding.

By Snell’slaw,

A ray from a laser diode propagating through an external medium of refractive index next and striking the fiber end at an angle _ext has to satisfy Snell’s law at that interface, and maintain TIR at the core-cladding interface. Consequently,