The Fotonic sensor which started the process was based upon
what now would be termed an extrinsic modulation scheme—in
other words, the fiber is used to link the optical signal to the
sensing point. In addition to the many variations on the basic
Fotonic sensor, where the signal clearly depends upon attenuation,
a number of other architectures have been investigated and
some exploited using attenuation independent modulation onto
the optical carrier.
Three basic approaches have been examined:
1) color modulation, for example, using spectral slicing
of the radiation from a light-emitting diode;
2) interferometric measurements of the distance between
the end of the optical fiber and the reflective surface;
3) configuring the parameter to be measured into a modulation
scheme, which introduces an amplitude modulation
at a frequency dependent upon the measurand.
The basic principles of these approaches are illustrated in
Fig. 6, and again the initial demonstrations of these principles
date back to the period between 1975 and 1985 [17]–[19]. Digital
shaft encoding or longitudinal displacement measurement
operating on a spectral slicing principle proved to be robust and
capable of up to 12-bit accuracy. Mechanical resonators—variations
on the vibrating wire gauge—are well established as precision
measurement concepts. The principle is simple. A mechanical
strain applied along the axis of the wire changes the resonant
frequency and through measuring this frequency, the strain
may be inferred with the usual proviso concerning temperature
sensitivity. The optical fiber versions of these resonators
use thermo-optic excitation and with careful design can operate
through fiber links several kilometers in length. The mechanical
engineering of the sensor head is demanding but these sensors
have now found their place in very demanding applications,
notably the measurement of pressure “down hole” in oil wells.
Interferometric measurement systems for diaphragm displacement
have been based upon both using the space between the
diaphragm and the fiber end as a Fabry–Perot cavity and measuring
the spectral transmission characteristic, and on exploiting
white-light interferometry to replicate the displacement of the
diaphragm within the receiver. In white-light interferometry, an
incoherent source is used to illuminate the measurement interferometer
and fringes are detected at the receiver only when the
path difference of a reference interferometer exactly matches
that of the measurement. The first demonstration of this principle
[20] highlighted the potential precision with which this
measurement could be made in sensing, and subsequently the
basic principle has been extensively exploited.