Biophotonics_Concepts_to_Applications

resulting observed constructive and destructive interference patterns at the sensor
output then allow an assessment of the external parameter being evaluated.
Similarly, as a second example, cascaded segments offibers with different core
diameters can be used to couple light from the core in onefiber into the cladding of
an adjoinedfiber. This is illustrated in Fig.7.21wherefiber type 1 could be a
multimodefiber andfiber type 2 a single-modefiber. The cladding modes that now
travel in the insertedfiber segment (type 2) then interact with an external parameter
through evanescent wave coupling. At the secondfiber junction, the cladding
modes are coupled back into the core offiber type 1 to create an interference pattern
at thefiber sensor output.
A third example is the use of photonic crystalfibers. Theflexibility of photonic
crystalfiber structures enables them to be used in a wide variety of biosensors. In
this case,fiber type 2 can be a PCF. More details on this structure and some
applications are discussed in Sect.7.6.

Cladding

Core

Cladding light-analyte interaction

Analyte

Core-to-cladding

light coupling

in grating-1

Cladding-to-core

light coupling

in grating-2

Grating-1 Grating-2

1

2

3

4

Fig. 7.20 Sensor structure with two long-period gratings embedded in afiber core

Cladding

Core

Cladding

Fiber type 1

Core-to-cladding

light coupling

Cladding-to-core

light coupling

Fiber type 2 Fiber type 1

Analyte

Fig. 7.21 Sensor using cascaded segments of differentfibers

218 7 Optical Probes and Biosensors