Biophotonics_Concepts_to_Applications

Incorporation of such features into an opticalfiber is achieved through either
material or structural variations [ 5 ]. For biophotonics applications the main spe-
cialty solid-corefiber types are photosensitivefibers for creating internal gratings,
fibers resistant to darkening from ultraviolet light, bend-loss insensitivefibers for
circuitous routes inside bodies, and polarization-preserving optical fibers for
imaging and forfluorescence analyses in spectroscopic systems. The following
subsections describe the characteristics of these opticalfibers.

3.5.1 Photosensitive Optical Fiber

A photosensitivefiber is designed so that its refractive index changes when it is
exposed to ultraviolet light [ 5 ]. For example, doping thefiber core material with
germanium and boron ions can provide this sensitivity. The main application for
such afiber is to create a shortfiber Bragg grating(FBG) in thefiber core, which is
a periodic variation of the refractive index along thefiber axis [ 12 – 14 ].
This index variation is illustrated in Fig.3.7, where n 1 is the refractive index of
the core of thefiber, n 2 is the index of the cladding, andΛis the period of the
grating, that is, the spacing between the maxima of the index variations [ 5 ]. If an
incident optical wave at a wavelengthλB(which is known as theBragg wavelength)
encounters a periodic variation in refractive index along the direction of propaga-
tion,λBwill be reflected back if the following condition is met:

kB¼2neffK ð 3 : 16 Þ

Here neffis the effective refractive index, which has a value falling between the
refractive indices n 1 of the core and n 2 of the cladding. When a specific wavelength
λBmeets this condition, the grating reflects this wavelength and all others will pass
through. Fiber Bragg gratings are available in a selection of Bragg wavelengths
with spectral reflection bandwidths at a specific wavelength varying from a few
picometers to tens of nanometers.

Input spectrum

Transmission

spectrum

Core

Cladding

Bragg grating

Wavelength

Intensity

typically a few mm or cm

Fig. 3.7 A periodic index variation in the core of a single-modefiber creates afiber Bragg grating
(J. Biomed. Opt. 19(8), 080902 (Aug 28, 2014). doi:10.1117/1.JBO.19.8.080902)

70 3 Optical Fibers for Biophotonics Applications