Biomedical Engineering Reference
In-Depth Information
EXAMPLE PROBLEM 10.14
Assume that a beam of light passes from a layer of glass with a refractive index
n
1
¼
1.47 into a
second layer of glass with a refractive index of
1.44. Using Snell's law, calculate the critical
angle for the boundary between these two glass layers.
n
2
¼
Solution
¼
n
2
n
1
f
cr
¼
arcsin
arcsin
ð
0
:
9796
Þ
4
f
cr
¼
78
:
Therefore, light that strikes the boundary between these two glasses at an angle greater than
78.4
will be reflected back into the first layer.
The propagation of light along an optical fiber is not confined to the core region. Instead,
the light penetrates a characteristic short distance (on the order of one wavelength) beyond
the core surface into the less optically dense cladding medium. This effect causes the exci-
tation of an electromagnetic field, called the “evanescent-wave,” that depends on the angle
of incidence and the incident wavelength. The intensity of the evanescent-wave decays
exponentially with distance, according to Beer-Lambert's law. It starts at the interface and
extends into the cladding medium.
10.6.2 Sensing Mechanisms
Optical sensors are typically interfaced with an optical module, as shown in Figure 10.38.
The module supplies the excitation light, which may be from a monochromatic source such
as a diode laser or from a broadband source (e.g., quartz-halogen) that is filtered to provide
a narrow bandwidth of excitation. Typically, two wavelengths of light are used: one that is
sensitive to changes in the species to be measured and one that is unaffected by changes in
the analyte concentration. This wavelength serves as a reference and is used to compensate
for fluctuations in source output and detector stability. The light output from the optic
module is coupled into a fiber optic cable through appropriate lenses and an optical
connector.
Several optical techniques are commonly used to sense the optical change across a bio-
sensor interface. These are usually based on evanescent wave spectroscopy, which plays a
major role in fiber optic sensors, and a surface plasmon resonance principle.
In fluorescence-based sensors, the incident light excites fluorescence emission, which
changes in intensity as a function of the concentration of the analyte to be measured. The
emitted light travels back down the fiber to the monitor, where the light intensity is
measured by a photodetector. In other types of fiber optic sensors, the light-absorbing
properties of the sensor chemistry change as a function of analyte chemistry. In the absorp-
tion-based design, a reflective surface near the tip or some scattering material within the
sensing chemistry itself is usually used to return the light back through the same optical
