axes in the core. Each of these axes will guide light at a different velocity. Crosstalk
between the two axes is suppressed so that polarized light launched into either of
the axial modes will maintain its state of polarization as it travels along thefiber.
Thesefibers are used in special biophotonics applications such asfiber optic sensing
and interferometry where polarization preservation is essential [ 21 ].
The structural arrangement of the bowtie geometry shown in Fig.3.10creates an
extreme birefringence in afiber [ 5 ]. Such a birefringence allows one and only one
polarization state of the fundamental mode to propagate with all other polarization
modes being greatly suppressed. In thesefibers, single-polarization guidance occurs
in only a limited wavelength range of about 100 nm. Outside of that spectral range,
either both of the polarization states or no light at all may be guided. Thesefibers
are used in specialfiber optic biosensing applications where it is desirable to
monitor a single state of polarization.
Example 3.10A single-mode optical fiber has a beat length of 8 cm at
1310 nm. What is the birefringence?
Solution: From Eq. (3.17) and the condition that LB=λ/Bfthe modal bire-
fringence isBf¼k
LB¼
1 : 31 10 ^6 m
8 10 ^2 m¼ 1 : 64 10 ^5
This value is characteristic of an intermediate typefiber, because bire-
fringence can vary from Bf=1× 10 −^3 (for a typical high birefringencefiber)
to Bf=1× 10 −^8 (for a typical low birefringencefiber).3.6 Double-Clad Fibers
A double-cladfiber (DCF) originally was created to help construct opticalfiber
lasers and now is being applied in the medical imagingfield [ 22 – 24 ]. More details on
these imaging applications are given in Chaps. 9 and 10. As indicated in Fig.3.11,a
DCF consists of a core region, an inner cladding, and an outer cladding arranged
concentrically [ 5 ]. Typical dimensions of a commercially available DCF are a 9-μm
core diameter, an inner cladding diameter of 105μm, and an outer cladding with a
125-μm diameter. Light transmission in the core region is single-mode, whereas in
the inner cladding it is multimode. The index of a DCF is decreasingly cascaded
from the core center to the outer cladding boundary. The integration of both
single-mode and multimode transmissions allows using a single opticalfiber for the
delivery of the illumination light (using the single-mode core) and collection of the
tissue-reflected light (using the multimode inner cladding).
3.5 Specialty Solid-Core Fibers 75