<s>
A	O
fiber	B-Device
Bragg	I-Device
grating	I-Device
(	O
FBG	O
)	O
is	O
a	O
type	O
of	O
distributed	B-Device
Bragg	I-Device
reflector	I-Device
constructed	O
in	O
a	O
short	O
segment	O
of	O
optical	B-Architecture
fiber	I-Architecture
that	O
reflects	O
particular	O
wavelengths	O
of	O
light	O
and	O
transmits	O
all	O
others	O
.	O
</s>
<s>
Hence	O
a	O
fiber	B-Device
Bragg	I-Device
grating	I-Device
can	O
be	O
used	O
as	O
an	O
inline	O
optical	B-Architecture
fiber	I-Architecture
to	O
block	O
certain	O
wavelengths	O
,	O
can	O
be	O
used	O
for	O
sensing	O
applications	O
,	O
or	O
it	O
can	O
be	O
used	O
as	O
wavelength-specific	O
reflector	O
.	O
</s>
<s>
This	O
technique	O
uses	O
the	O
interference	O
pattern	O
of	O
ultraviolet	B-Application
laser	O
light	O
to	O
create	O
the	O
periodic	O
structure	O
of	O
the	O
fiber	B-Device
Bragg	I-Device
grating	I-Device
.	O
</s>
<s>
Typically	O
,	O
the	O
refractive	O
index	O
profile	O
can	O
be	O
uniform	O
or	O
apodized	B-Algorithm
,	O
and	O
the	O
refractive	O
index	O
offset	O
is	O
positive	O
or	O
zero	O
.	O
</s>
<s>
Gaussian	O
apodized	B-Algorithm
,	O
</s>
<s>
raised-cosine	O
apodized	B-Algorithm
,	O
</s>
<s>
This	O
supported	O
the	O
development	O
of	O
the	O
first	O
distributed	O
feedback	O
(	O
DFB	O
)	O
fiber	B-Device
lasers	I-Device
,	O
and	O
also	O
laid	O
the	O
groundwork	O
for	O
most	O
complex	O
gratings	O
that	O
followed	O
,	O
including	O
the	O
sampled	O
gratings	O
first	O
made	O
by	O
Peter	O
Hill	O
and	O
colleagues	O
in	O
Australia	O
.	O
</s>
<s>
This	O
is	O
apodization	B-Algorithm
of	O
the	O
refractive	O
index	O
change	O
.	O
</s>
<s>
The	O
term	O
apodization	B-Algorithm
refers	O
to	O
the	O
grading	O
of	O
the	O
refractive	O
index	O
to	O
approach	O
zero	O
at	O
the	O
end	O
of	O
the	O
grating	O
.	O
</s>
<s>
Apodized	B-Algorithm
gratings	O
offer	O
significant	O
improvement	O
in	O
side-lobe	O
suppression	O
while	O
maintaining	O
reflectivity	O
and	O
a	O
narrow	O
bandwidth	O
.	O
</s>
<s>
The	O
refractive	O
index	O
profile	O
of	O
the	O
grating	O
may	O
be	O
modified	O
to	O
add	O
other	O
features	O
,	O
such	O
as	O
a	O
linear	O
variation	O
in	O
the	O
grating	O
period	O
,	O
called	O
a	O
chirp	B-Application
.	O
</s>
<s>
A	O
grating	O
possessing	O
a	O
chirp	B-Application
has	O
the	O
property	O
of	O
adding	O
dispersion	O
—	O
namely	O
,	O
different	O
wavelengths	O
reflected	O
from	O
the	O
grating	O
will	O
be	O
subject	O
to	O
different	O
delays	O
.	O
</s>
<s>
These	O
gratings	O
are	O
called	O
long-period	B-Device
fiber	I-Device
grating	I-Device
.	O
</s>
<s>
Phase-shifted	O
fiber	B-Device
Bragg	I-Device
gratings	I-Device
(	O
PS-FBGs	O
)	O
are	O
an	O
important	O
class	O
of	O
gratings	O
structures	O
which	O
have	O
interesting	O
applications	O
in	O
optical	O
communications	O
and	O
sensing	O
due	O
to	O
their	O
special	O
filtering	O
characteristics	O
.	O
</s>
<s>
Different	O
coatings	O
of	O
diffractive	O
structure	O
are	O
used	O
for	O
fiber	B-Device
Bragg	I-Device
gratings	I-Device
in	O
order	O
to	O
reduce	O
the	O
mechanical	O
impact	O
on	O
the	O
Bragg	O
wavelength	O
shift	O
for	O
1.1	O
–	O
15	O
times	O
as	O
compared	O
to	O
an	O
uncoated	O
waveguide	O
.	O
</s>
<s>
Addressed	B-Algorithm
fiber	I-Algorithm
Bragg	I-Algorithm
structures	I-Algorithm
(	O
AFBS	O
)	O
is	O
an	O
emerging	O
class	O
of	O
FBGs	O
developed	O
in	O
order	O
to	O
simplify	O
interrogation	O
and	O
enhance	O
performance	O
of	O
FBG-based	O
sensors	O
.	O
</s>
<s>
Fiber	B-Device
Bragg	I-Device
gratings	I-Device
are	O
created	O
by	O
"	O
inscribing	O
"	O
or	O
"	O
writing	O
"	O
systematic	O
(	O
periodic	O
or	O
aperiodic	O
)	O
variation	O
of	O
refractive	O
index	O
into	O
the	O
core	O
of	O
a	O
special	O
type	O
of	O
optical	B-Architecture
fiber	I-Architecture
using	O
an	O
intense	O
ultraviolet	B-Application
(	O
UV	O
)	O
source	O
such	O
as	O
a	O
UV	O
laser	O
.	O
</s>
<s>
Although	O
polymer	O
optic	B-Architecture
fibers	I-Architecture
starting	O
gaining	O
research	O
interest	O
in	O
the	O
2000s	O
,	O
germanium-doped	O
silica	O
fiber	O
is	O
most	O
commonly	O
used	O
.	O
</s>
<s>
To	O
write	O
a	O
high	O
reflectivity	O
fiber	B-Device
Bragg	I-Device
grating	I-Device
directly	O
in	O
the	O
fiber	O
the	O
level	O
of	O
doping	O
with	O
germanium	O
needs	O
to	O
be	O
high	O
.	O
</s>
<s>
This	O
was	O
the	O
first	O
method	O
used	O
widely	O
for	O
the	O
fabrication	O
of	O
fiber	B-Device
Bragg	I-Device
gratings	I-Device
and	O
uses	O
two-beam	O
interference	O
.	O
</s>
<s>
A	O
photomask	B-Algorithm
having	O
the	O
intended	O
grating	O
features	O
may	O
also	O
be	O
used	O
in	O
the	O
manufacture	O
of	O
fiber	B-Device
Bragg	I-Device
gratings	I-Device
.	O
</s>
<s>
The	O
photomask	B-Algorithm
is	O
placed	O
between	O
the	O
UV	O
light	O
source	O
and	O
the	O
photosensitive	O
fiber	O
.	O
</s>
<s>
The	O
shadow	O
of	O
the	O
photomask	B-Algorithm
then	O
determines	O
the	O
grating	O
structure	O
based	O
on	O
the	O
transmitted	O
intensity	O
of	O
light	O
striking	O
the	O
fiber	O
.	O
</s>
<s>
Photomasks	B-Algorithm
are	O
specifically	O
used	O
in	O
the	O
manufacture	O
of	O
chirped	O
Fiber	B-Device
Bragg	I-Device
gratings	I-Device
,	O
which	O
cannot	O
be	O
manufactured	O
using	O
an	O
interference	O
pattern	O
.	O
</s>
<s>
This	O
method	O
is	O
specifically	O
applicable	O
to	O
the	O
fabrication	O
of	O
long	B-Device
period	I-Device
fiber	I-Device
gratings	I-Device
.	O
</s>
<s>
Originally	O
,	O
the	O
manufacture	O
of	O
the	O
photosensitive	O
optical	B-Architecture
fiber	I-Architecture
and	O
the	O
'	O
writing	O
 '	O
of	O
the	O
fiber	B-Device
Bragg	I-Device
grating	I-Device
were	O
done	O
separately	O
.	O
</s>
<s>
As	O
well	O
as	O
reducing	O
associated	O
costs	O
and	O
time	O
,	O
this	O
also	O
enables	O
the	O
mass	O
production	O
of	O
fiber	B-Device
Bragg	I-Device
gratings	I-Device
.	O
</s>
<s>
Mass	O
production	O
is	O
in	O
particular	O
facilitating	O
applications	O
in	O
smart	O
structures	O
utilizing	O
large	O
numbers	O
(	O
3000	O
)	O
of	O
embedded	O
fiber	B-Device
Bragg	I-Device
gratings	I-Device
along	O
a	O
single	O
length	O
of	O
fiber	O
.	O
</s>
<s>
The	O
primary	O
application	O
of	O
fiber	B-Device
Bragg	I-Device
gratings	I-Device
is	O
in	O
optical	O
communications	O
systems	O
.	O
</s>
<s>
They	O
are	O
also	O
used	O
in	O
optical	O
multiplexers	B-Protocol
and	O
demultiplexers	O
with	O
an	O
optical	O
circulator	O
,	O
or	O
optical	B-Protocol
add-drop	I-Protocol
multiplexer	I-Protocol
(	O
OADM	B-Protocol
)	O
.	O
</s>
<s>
A	O
demultiplexer	O
can	O
be	O
achieved	O
by	O
cascading	O
multiple	O
drop	O
sections	O
of	O
the	O
OADM	B-Protocol
,	O
where	O
each	O
drop	O
element	O
uses	O
an	O
FBG	O
set	O
to	O
the	O
wavelength	O
to	O
be	O
demultiplexed	O
.	O
</s>
<s>
Conversely	O
,	O
a	O
multiplexer	B-Protocol
can	O
be	O
achieved	O
by	O
cascading	O
multiple	O
add	O
sections	O
of	O
the	O
OADM	B-Protocol
.	O
</s>
<s>
FBG	O
demultiplexers	O
and	O
OADMs	B-Protocol
can	O
also	O
be	O
tunable	O
.	O
</s>
<s>
In	O
a	O
tunable	O
demultiplexer	O
or	O
OADM	B-Protocol
,	O
the	O
Bragg	O
wavelength	O
of	O
the	O
FBG	O
can	O
be	O
tuned	O
by	O
strain	O
applied	O
by	O
a	O
piezoelectric	O
transducer	O
.	O
</s>
<s>
The	O
sensitivity	O
of	O
a	O
FBG	O
to	O
strain	O
is	O
discussed	O
below	O
in	O
fiber	B-Device
Bragg	I-Device
grating	I-Device
sensors	O
.	O
</s>
<s>
This	O
means	O
that	O
fiber	B-Device
Bragg	I-Device
gratings	I-Device
can	O
be	O
used	O
as	O
sensing	O
elements	O
in	O
optical	B-Algorithm
fiber	I-Algorithm
sensors	I-Algorithm
.	O
</s>
<s>
Also	O
,	O
is	O
the	O
coefficient	O
of	O
temperature	O
,	O
which	O
is	O
made	O
up	O
of	O
the	O
thermal	O
expansion	O
coefficient	O
of	O
the	O
optical	B-Architecture
fiber	I-Architecture
,	O
,	O
and	O
the	O
thermo-optic	O
coefficient	O
,	O
.	O
</s>
<s>
Fiber	B-Device
Bragg	I-Device
gratings	I-Device
can	O
then	O
be	O
used	O
as	O
direct	O
sensing	O
elements	O
for	O
strain	O
and	O
temperature	O
.	O
</s>
<s>
They	O
can	O
also	O
be	O
used	O
as	O
transduction	O
elements	O
,	O
converting	O
the	O
output	O
of	O
another	O
sensor	O
,	O
which	O
generates	O
a	O
strain	O
or	O
temperature	O
change	O
from	O
the	O
measurand	O
,	O
for	O
example	O
fiber	B-Device
Bragg	I-Device
grating	I-Device
gas	O
sensors	O
use	O
an	O
absorbent	O
coating	O
,	O
which	O
in	O
the	O
presence	O
of	O
a	O
gas	O
expands	O
generating	O
a	O
strain	O
,	O
which	O
is	O
measurable	O
by	O
the	O
grating	O
.	O
</s>
<s>
The	O
Bragg	B-Device
grating	I-Device
then	O
transduces	O
the	O
strain	O
to	O
the	O
change	O
in	O
wavelength	O
.	O
</s>
<s>
Specifically	O
,	O
fiber	B-Device
Bragg	I-Device
gratings	I-Device
are	O
finding	O
uses	O
in	O
instrumentation	O
applications	O
such	O
as	O
seismology	O
,	O
pressure	O
sensors	O
for	O
extremely	O
harsh	O
environments	O
,	O
and	O
as	O
downhole	O
sensors	O
in	O
oil	O
and	O
gas	O
wells	O
for	O
measurement	O
of	O
the	O
effects	O
of	O
external	O
pressure	O
,	O
temperature	O
,	O
seismic	O
vibrations	O
and	O
inline	O
flow	O
measurement	O
.	O
</s>
<s>
In	O
the	O
1990s	O
,	O
investigations	O
were	O
conducted	O
for	O
measuring	O
strain	O
and	O
temperature	O
in	O
composite	O
materials	O
for	O
aircraft	O
and	O
helicopter	B-Application
structures	O
.	O
</s>
<s>
Recently	O
the	O
development	O
of	O
high	O
power	O
fiber	B-Device
lasers	I-Device
has	O
generated	O
a	O
new	O
set	O
of	O
applications	O
for	O
fiber	B-Device
Bragg	I-Device
gratings	I-Device
(	O
FBGs	O
)	O
,	O
operating	O
at	O
power	O
levels	O
that	O
were	O
previously	O
thought	O
impossible	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
a	O
simple	O
fiber	B-Device
laser	I-Device
,	O
the	O
FBGs	O
can	O
be	O
used	O
as	O
the	O
high	O
reflector	O
(	O
HR	O
)	O
and	O
output	O
coupler	O
(	O
OC	O
)	O
to	O
form	O
the	O
laser	O
cavity	O
.	O
</s>
<s>
The	O
gain	O
for	O
the	O
laser	O
is	O
provided	O
by	O
a	O
length	O
of	O
rare	O
earth	O
doped	O
optical	B-Architecture
fiber	I-Architecture
,	O
with	O
the	O
most	O
common	O
form	O
using	O
Yb3+	O
ions	O
as	O
the	O
active	O
lasing	O
ion	O
in	O
the	O
silica	O
fiber	O
.	O
</s>
<s>
These	O
Yb-doped	O
fiber	B-Device
lasers	I-Device
first	O
operated	O
at	O
the	O
1kW	O
CW	O
power	O
level	O
in	O
2004	O
based	O
on	O
free	O
space	O
cavities	O
but	O
were	O
not	O
shown	O
to	O
operate	O
with	O
fiber	B-Device
Bragg	I-Device
grating	I-Device
cavities	O
until	O
much	O
later	O
.	O
</s>
<s>
The	O
major	O
advantage	O
of	O
these	O
all	O
fiber	O
systems	O
,	O
where	O
the	O
free	O
space	O
mirrors	O
are	O
replaced	O
with	O
a	O
pair	O
of	O
fiber	B-Device
Bragg	I-Device
gratings	I-Device
(	O
FBGs	O
)	O
,	O
is	O
the	O
elimination	O
of	O
realignment	O
during	O
the	O
life	O
of	O
the	O
system	O
,	O
since	O
the	O
FBG	O
is	O
spliced	O
directly	O
to	O
the	O
doped	O
fiber	O
and	O
never	O
needs	O
adjusting	O
.	O
</s>
<s>
However	O
,	O
the	O
packaging	O
and	O
optimization	O
of	O
the	O
splices	O
and	O
FBGs	O
themselves	O
are	O
non-trivial	O
at	O
these	O
power	O
levels	O
as	O
are	O
the	O
matching	O
of	O
the	O
various	O
fibers	O
,	O
since	O
the	O
composition	O
of	O
the	O
Yb-doped	O
fiber	O
and	O
various	O
passive	O
and	O
photosensitive	O
fibers	O
needs	O
to	O
be	O
carefully	O
matched	O
across	O
the	O
entire	O
fiber	B-Device
laser	I-Device
chain	O
.	O
</s>
