Physical Transducers

The most direct application of an optical fiber to measure physical measurands is the microbend transducer, as shown in Fig. 5.44.102,103,105 fiber is deformed by a measurand such as displacement, strain, pressure, force, acceleration, or temperature. In each case, the transduction mechanism is the decrease in light intensity of the beam through the fiber core as a result of the deformation, which causes the core modes to lose energy to the cladding modes. A photodetector is used to provide a corresponding electrical output. This is an intrinsic-type transducer.

Figure 5.45 shows a pressure transducer and a temperature transducer

Fig 5.45 Fiber-optic temperature and pressure transducers.

configured as a Fabry-Perot etalon or interferometer. The spacing between mirrors is critical to the operation of the etalon as a physical transducer.

Any change in that spacing, brought about by a measurand, will cause the transmittance of the etalon to change in proportion to the measurand. These extrinsic transducers are small in size and can be constructed by using integrated circuit techniques.

Optical fibers are also used to measure magnetic and acoustic fields. In a magnetic field transducer as shown in Fig. 5.46, the transduction mechanism consists of the longitudinal strain in the magnetostrictive material caused by the magnetic field and a corresponding strain in the core.

This strain modulates the refractive index of the core and produces a field-dependent phase shift in the output optical beam. An acoustic field transducer can also be made along these lines by replacing the magnetostrictive substrate with an acoustically sensitive coating around the fiber cladding. Consequently, this produces an acoustic field-dependent phase shift of the optical beam.

Fig 5.46 Fiber-optic magnetic field transducer.

A Mach-Zhender interferometer is used to obtain an electrical output proportional to the measurands.