Biophotonics_Concepts_to_Applications

opticalfibers in such indoor installations must tolerate numerous sharp bends,
These same fibers also can be used for medical applications [ 18 – 20 ]. These
bend-loss insensitivefibers have a moderately higher numerical aperture (NA) than
conventional single-modefibers. Increasing the NA reduces the sensitivity of the
fiber to bending loss by confining optical power more tightly within the core than in
conventional single-modefibers.
Various manufacturers offer a bend-loss insensitivefiber that has a lower
single-mode cutoff wavelength, a nominally 50 % higher index difference valueΔ,
and a 25 % higher NA than conventional telecomfibers [ 5 ]. The higher NA of
low-bend-lossfibers allows an improved coupling efficiency from laser diode
sources to planar waveguides. Generally for bend radii of greater than 20 mm, the
bending-induced loss is negligibly small. Fibers are available in which the maxi-
mum bending induced loss is less than 0.2 dB due to 100 turns on a 10-mm
mandrel. A factor to keep in mind is that at operating wavelengths in the near
infrared, the smaller modefield diameter of low-bend-lossfibers can induce a
mode-mismatch loss when interconnecting thesefibers with standard single-mode
fibers. However, carefully made splices between these differentfibers typically
results in losses less than 0.1 dB.

3.5.4 Polarization-Maintaining Fiber

In a conventional single-mode fiber the fundamental mode consists of two
orthogonal polarization modes (see Chap. 2 )[ 5 ]. These modes may be chosen
arbitrarily as the horizontal and vertical polarizations in the x direction and y
direction, respectively, as shown in Fig.3.9. In general, the electricfield of the light

x

Core

y

Cladding

Horizontal mode sees

an effective index nx

nx

x

Core

y

Cladding

Vertical mode sees

an effective index ny

ny

Fig. 3.9 Two polarization states of the fundamental mode in a single-modefiber

3.5 Specialty Solid-Core Fibers 73