forward-propagating core and cladding modes in a PCF, which is one form of a
compact inline Mach-Zehnder interferometer.
Thefiber Bragg grating (FBG) that is described in Sect.3.5.1is a popular
precise wavelength-selecting component that was enhanced by the telecom indus-
try. For biophotonics applications, an external force (for example, from the weight
of a person or through some variation in a muscular force) can slightly stretch the
fiber in which the FBG is embedded. This stretching will change the FBG grating
spacing and thus will change the value of the reflected Bragg wavelength.
Applications of FBGs in various healthcare-monitoring disciplines are described in
Sect.7.7.
Next Sect.7.8addresses the concept and function of surface plasmon resonance,
which is implemented in many biosensor applications and lab-on-a-chip sensors to
provide a greatly enhanced sensitivity compared to other sensors. The surface
plasmon resonance effect produces a collective oscillation of electrons when the
surface of a solid or liquid is stimulated by polarized light. A resonance condition
occurs when the frequency of the incident photons matches the natural frequency of
surface electrons that are oscillating against the restoring force of positive nuclei in
the solid or liquid material.
When a pattern of nanoparticles is deposited on the metalized tip of an optical
fiber, surface plasmon waves can be excited by an illuminating optical wave. This
technique is described in Sect.7.9. If the fiber tip is inserted into afluid that
contains ananalyte(a substance being identified and measured), the liquid will
cover the nanoparticle pattern thereby changing the refractive index of the
nanolayer-to-fluid interface. The result is a wavelength shift in the plasmon wave
peak, which then can be used to sense the presence of a substance of interest.
7.2 Optical Fiber Probe Configurations.
Opticalfibers are used widely as biomedical probes for light delivery and collection
functions [ 9 – 15 ]. The diameter of an opticalfiber cable used as a probe is normally
less than half a millimeter, which allows the cable to be inserted into most hollow
medical needles and catheters. Two generic schematics of basic opticalfiber-based
probe systems that use the same optical path for both illumination and light col-
lection functions are shown in Fig.7.3. This optical path could consist of afiber
bundle or it could be selected from one of thefibers described in Chap. 3. In order
to achieve bidirectional lightflow in the samefiber, either a dichroicfilter (see
Sect.5.6), a 3-dB coupler (see Sect.5.7), or an optical circulator (see Sect.5.7)is
used tofirst direct the light from the source to the tissue and then to route the
returning light to a photodetection device. The dichroicfilters and their associated
coupling optics for connecting to light sources, photodetectors, and opticalfibers
are available from a variety of vendors. Optical 3-dB couplers and optical circu-
lators can be purchased asfiber-coupled devices for ease of attaching them to
instrumentation opticalfibers with optical connectors.
200 7 Optical Probes and Biosensors