Biomedical Engineering Reference
In-Depth Information
physical quantity, and then measuring the results of the modulation
indirectly by measuring the changes in the physical (chemical)
content. For example, to measure physical qualities, such as the
mechanical measurement of curvature, the bending of the iber
causes lost of light as it escapes from the iber core. The more severe
the bending, the more serious is the loss of light energy. In other
words, light energy is modulated by the curvature, so if the optical
iber is afixed to a mechanical object, the curvature can be measured
indirectly by measuring by the intensity of light through the optical
iber. As another example, a gas will absorb a speciic wavelength of
light, and this absorption increases as the concentration of the gas
increases, with an appropriate light source guided by the iber into
a test area where the light temporarily leaves the iber and is then is
re-captured by the optical iber at the other end, the concentration
of the gas can be deduced by measuring the attenuation of the
speciic wavelength of light energy. A single mechanism for
modulation can occur within the iber, and may also occur in an
external environment around the optical iber. In addition, different
modulation methods can be applied individually or in combination.
Modulation mechanisms include (1) absorption modulation; (2)
chromatic dispersion modulation; (3) scattering based modulation;
(4) luminescence-luorescence based modulation; (5) refractive
index based modulation; (6) geometric effect based modulation; (7)
interferometric and phase modulation; (8) wavelength modulation;
and (9) Doppler's effect modulation.
Depending on sensor positioning, different modulation
mechanisms can be categorized as three types: (1) extrinsic, (2)
intrinsic, and between them (3) evanescent. With extrinsic sensing
methods light is guided to the test area by optical iber, temporarily
leaves the iber, is modulated by the external environment, and is
then captured into the iber and passed to the signal processor for
analysis. The role of optical iber is for signal transmission only,
and is not involved in the sensing function. For intrinsic sensors,
the light does not leave the optical iber; modulation occurs from
changes to the external environment or changes in the internal
characteristics, which affect certain characteristics of the light (such
as wavelength). Evanescent iber optic sensors derive changes to
the parameters tested in the external environment via the escape
or loss of light during transmission. Though both this type and
extrinsic sensors measure changes in light energy, the modulation
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