7.4 Optical Sensors
A biophotonics detection process can involve the sensing of perturbations in a
transmitted optical signal caused by a change in some characteristic of an optical
fiber or of a planar optical waveguide. These characteristics can include variations
in the refractive index of the cladding material due to the adherence of molecules on
the outside of the cladding, relativefiber movements induced by changes in an
external parameter, or changes in the radius of curvature of thefiber axis. Herefirst
Sects.7.4.1and7.4.2describe the sensing of externally induced evanescentfield
perturbations that cause a variation in the transmitted optical power level, which can
be directly correlated to the concentration of specific molecules being measured.
Next Sects.7.4.3and7.4.4illustrate how changes in an external physical parameter
(e.g., pressure, stress, or temperature) can induce relative movements between two
fibers or variations in the bend radius of afiber, respectively. These changes also
result in afluctuation in the transmitted optical power level. The optical power level
variation can be directly correlated to the value of the external physical parameter.
7.4.1 Biorecognition Optical Fiber Sensors
The attractions of opticalfiber-based sensors are their small size, excellent inte-
gration capability within sensor packages, and relatively high sensitivity to detect
diverse analytes. Figure7.13shows the basic operation of a class of biosensors that
use absorbance measurements to detect any variations in the concentrations of
substances that absorb a specific wavelength of light [ 3 , 7 , 23 , 24 ]. First thefiber is
coated with abiorecognition material, which can be an enzyme, an antibody, a
DNA strand, a microorganism, or a cell. Anantibodyis a large Y-shaped molecule
that typically is used by the immune system of a body to identify and neutralize
Illumination
areaFlat-polished
fiber tipsCollection
areaFig. 7.12 Example of dual
flat-polishedfiber distal end
tips
7.4 Optical Sensors 209