<s>
Hydrogen	B-Device
darkening	I-Device
is	O
a	O
physical	O
degradation	O
of	O
the	O
optical	O
properties	O
of	O
glass	O
.	O
</s>
<s>
The	O
problem	O
is	O
particularly	O
relevant	O
to	O
fiber	B-Architecture
optic	I-Architecture
cables	O
—	O
particularly	O
in	O
oil	O
and	O
gas	O
wells	O
where	O
fiber	B-Architecture
optic	I-Architecture
cables	O
are	O
used	O
for	O
distributed	B-Algorithm
temperature	I-Algorithm
sensing	I-Algorithm
(	O
DTS	O
)	O
.	O
</s>
