Biomedical Engineering Reference
In-Depth Information
FIGURE 2-80
Microbalance odor sensor and the transfer function for amylacetate gas. (a)
Construction schematic. (b) Relationship between gas concentration and frequency change.
[Adapted from (Fraden 1996).]
Optical sensors are usually based on optical fibers or planar waveguides, and there
are three main methods of sensing at the surfaces of these devices:
• The molecule directly affects the optical properties of the waveguide such as evanes-
cent waves
1
or surface plasmons.
2
• An optical fiber is used to convey light to and from the sample. Changes in the
optical properties of the medium containing the molecule of interest are sensed by
an external spectrophotometer.
• An indicator or chemical reagent placed near the tip of the optical fiber reacts with
the molecule of interest to produce an optical signature that can be detected. These
include absorption spectroscopy and fluorimetry.
Light sources range from a wide variety of coherent laser sources from LEDS extend-
ing from the infrared (IR) to the visible as well as broad spectral incandescent lamps.
Optical elements include optical fiber, lenses, prisms, beam splitters, polarizers, and
diffraction gratings. Optical fibers and lenses convey light to and from the test area,
whereas prisms and diffraction gratings provide wavelength selection for frequency se-
lective processing. Polarizers are useful in identifying or selecting the light polarization
for transmission and reception.
Photodetection is most often performed using photodiodes that are sensitive to the
frequency band of interest. Most systems use a pair of identical diodes: one in the active
channel; and the other to function as a reference.
2.4.17.1 Evanescent Wave Spectroscopy
When light propagates along an optical fiber, it is not completely confined to the core region
but penetrates a short distance (typically one wavelength) into the cladding. Therefore,
1
Electromagnetic waves generated in the medium outside the optical waveguide when light is reflected
from within.
2
Resonances induced by an evanescent wave in a thin film deposited on a waveguide surface.
