Biophotonics_Concepts_to_Applications

However, currently special material processing methods, such asfluorine doping
of silica, have resulted infibers with moderate (around 50 % attenuation) to min-
imal (a few percent additional loss) UV sensitivity below 260 nm [ 15 – 17 ]. When
suchfibers are exposed to UV light the transmittancefirst decreases rapidly but then
stabilizes at an asymptotic attenuation value. The specific asymptotic attenuation
level and the time to reach this level depend on thefiber manufacturing process and
on the UV wavelength being used. Shorter wavelengths result in higher attenuation
changes and it takes longer to reach the asymptotic attenuation value.
Solarization-resistantfibers for operation in the 180 to 850-nm range are available
with core diameters ranging from 50 to 1000μm and a numerical aperture of 0.22.

3.5.3 Bend Insensitive Fiber

In many medical applications of opticalfibers within a living body, thefibers need
to follow a sinuous path with sharp bends through arteries that snake around bones
and organs [ 5 ]. Special attention must be paid to this situation, because optical
fibers exhibit radiative losses whenever thefiber undergoes a bend with afinite
radius of curvature. For slight bends this factor is negligible. However, as the radius
of curvature decreases, the losses increase exponentially until at a certain critical
radius the losses become extremely large. As shown in Fig.3.8, the bending loss
becomes more sensitive at longer wavelengths. Afiber with a small bend radius
might be transmitting well at 1310 nm, for example, giving an additional loss of
1 dB at a 1-cm bending radius. However, for this bend radius it could have a
significant loss at 1550 nm resulting in an additional loss of about 100 dB for the
conventionalfiber.
In recent years the telecom industry started installing opticalfiber links within
homes and businesses. Thus bend-loss insensitivefibers were developed because

Bending loss

at 1550 nm

Bending loss

at 1310 nm

Bending loss (dB)

Bending radius (cm)

0.1

100

1

10

0.5

1000

1.0 1.5

100 dB bending

loss at 1550 nm

1 dB bending

loss at 1310 nm

Fig. 3.8 Generic bend loss
sensitivity for conventional
fibers at 1310 and 1550 nm

72 3 Optical Fibers for Biophotonics Applications