handbook of modern sensors

142 4 Optical Components of Sensors
Light rays entering the fiber at angles greater than in(max) will pass through to the
jacket and will be lost. For data transmission, this is an undesirable event, however,
in a specially designed fiber-optic sensor, the maximum entry angle can be a useful
phenomenon for modulating light intensity.
Sometimes, the value in(max) is called the numerical aperture of the fiber. Due
to variations in the fiber properties, bends, and skewed paths, the light intensity does
not drop to zero abruptly but rather gradually diminishes to zero while approaching
in(max) . In practice, the numerical aperture is defined as the angle at which light
intensity drops by some arbitrary number, (e.g., −10 dB of the maximum value).
One of the useful properties of fiber-optic sensors is that they can be formed into
a variety of geometrical shapes depending on the desired application. They are very
useful for the design of miniature optical sensors which are responsive to such stimuli,
as pressure, temperature, chemical concentration, and so forth. The basic idea for use
of fiber optics in sensing is to modulate one or several characteristics of light in a fiber
and, subsequently, to optically demodulate the information by conventional methods.
Astimulus may act on a fiber either directly or it can be applied to a component attached
to the fiber’s outer surface or the polished end to produce an optically detectable signal.
To make a fiber chemical sensor, a special solid phase of a reagent may be formed
in the optical path coupled to the fiber. The reagent interacts with the analyte to
produce an optically detectable effect, (e.g., to modulate the index of refraction or
coefficient of absorption). A cladding on a fiber may be created from a chemical
substance whose refractive index may be changed in the presence of some fluids [3].
When the angle of total internal reflection changes, the light intensity varies.
Optical fibers may be used in two modes. In the first mode (Fig. 4.17A), the same
fiber is used to transmit the excitation signal and to collect and conduct an optical
response back to the processing device. In the second mode, two or more fibers are
(A)
(B)
Fig. 4.17. (A) Single- and (B) dual-fiber-optic sensors.