as Multi-Sensors as Multi-Sensors
are insensitive to perturbations that do not reach the core.
Although FBGs have a multitude of potential applications,
especially for in situ and remote sensing, they are limited in
that they cannot be used to detect pure bending and changing in the medium adjacent to the cladding boundary.
Hence, researchers have made many e;orts to get core
light to interact with the fiber surroundings. ;is is usually
accomplished by etching or polishing the cladding away,
tapering or using special microstructured fibers with holes
or cladding slots, etc. In all the resulting devices, the goal is
to make the e;ective index of the core mode depend on more
than just the diameter of the core and the refractive indices
of the core and cladding.
Using modes guided by the cladding of the fiber
Of course, many more sensing modalities become accessible
if the grating can couple light to cladding modes. ;ese
modes are guided by the interface between the cladding
and the outside medium, a structure that can support several
hundred guided modes in conventional single-mode fiber. As
it turns out, forward-coupling gratings, the so-called “long-
period gratings” (LPGs), can be used to couple light into
select cladding modes resonantly and e;ciently. ;e change
of sign in the phase-matching equation causes LPGs to
benefit from two sensitivity enhancements: the much larger
period (which multiplies any e;ective index change) and the
fact that the di;erence in e;ective indices can be optimized
to shift rapidly in response to certain perturbations by wave-
guide design.
1047-6938/11/10/28/6-$15.00 ©OSA
October 2011 | 29
