Page 38 - Fiber Fracture

FIBER FRACTURE: AN OVERVIEW 23

becoming necessary to write increasingly complex and precise fiber Bragg gratings
(FBGs) on short fiber lengths. An FBG involves an optical fiber along whose core there
occurs a periodic change in refractive index. Fiber Bragg gratings can be made during
fiber drawing by a single laser shot or by using pulsed lasers after fiber drawing. In
the single laser shot technique, fiber coating is applied after the grating is made. These
FBGs are expected to reliably function for 20 to 40 years in a variety of conditions.
Thus their mechanical reliability becomes an important issue. FBGs can be written by
an argon-ion laser system operating in continuous wave (CW) mode at 244 nm or by
KrF pulsed excimer laser operating at 248 nm. Varelas et al. (1997) studied the effects of
laser intensity on the mechanical reliability of FBG. Their main results are summarized
in Fig. 17, which shows the Weibull plots of pulse-irradiated fibers and continuous wave

.+ pristine
--c- CW 0.5 kl/crn2

cw 1 W/crn2
.+ 0.5 W/cm2
-Q- I W/cm'
I I
I 3 5
Fracture strength (GPa)
Fig. 17. Weibull plots of optical glass fibers subjected to pulse-irradiation and continuous wave treatments.
The use of pulsed excimer radiation lowered the fiber fracture strength by as much as a factor of 4 compared
to the fiber that was subjected to a CW argon-ion laser. The lower fracture strength of pulse-irradiated fiber
is a result of the formation of microcracks in the material (after Varelas et al., 1997).