Page 432 - Fiber Bragg Gratings

Chapter 9

Measurement and

Characterization of

Gratings

The transfer characteristics of a grating are of primary importance for a
number of applications. For example, in high-bit-rate applications, it is
necessary to know if the grating will impart additional dispersion and, if
so, how much. Gratings can be used in a vast number of demanding
applications, such as sensing in harsh environments, or in undersea opti-
cal fiber transmission that requires components to survive the 25-year
design lifespan of the system. For long-term use, it is essential to know
whether or not the grating will maintain its designed characteristics over
the lifetime. It is also important to know, as it is for optical fibers, the
integrity of its mechanical strength for the same reasons. Thus, reliability
is a big issue. The transmission characteristics of certain gratings may
be affected by the out-gassing and annealing processes more than others;
the resonance wavelengths of all gratings drift because of the out-diffusion
of molecular hydrogen in high-pressure sensitized fibers. Stress relaxation
can complicate matters, by altering the induced refractive index modula-
tion. Sensitivity of the Bragg wavelength with temperature and strain
has to be taken into account for such applications as in band-pass filters.
Gratings have to be annealed to stabilize their properties for long-term
use. The bandwidth, reflection profile, and phase response of gratings
require special measurement techniques for proper characterization. In
this chapter we shall consider some of the parameters that are of impor-

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