January 15, 1994 / Vol. 19, No. 2 / OPTICS LETTERS
Combined
distributed
temperature
and
strain
sensor
based
on
Brillouin
loss
in an
optical
fiber
X.
Bao, D. J. Webb, and
D. A. Jackson
Applied
Optics
Group, Physics
Laboratory,
The
University,
Canterbury,
Kent CT2 7NR,
UK
Received
July 27, 1993
We present
a novel
distributed
sensor that
utilizes the
temperature
and strain dependence
of
the frequency
at
which
the Brillouin
loss
is maximized
in the interaction
between
a cw
laser and a
pulsed laser.
With a 22-km
sensing
length,
a strain resolution
of 20 uLE
and a temperature
resolution
of 2TC have
been achieved
with a
spatial resolution of 5 m.
Long-range
distributed optical
fiber sensors1-5
exploit the interaction between
laser light and
the
fundamental energy transport
mechanisms in
the scattering
medium. Here
we report on the
use of Brillouin
loss
4
for a combined
distributed
temperature
and strain sensor. Brillouin loss-
based distributed
sensors exploit the Brillouin
interaction
between pulsed
and cw optical
beams
counterpropagating
in an optical fiber.
When the
optical frequency of
the cw beam is greater
than
that of the pulsed beam
by an amount equal
to the
Brillouin frequency shift
VB at some point in
the fiber,
the pulsed beam is amplified through
the Brillouin
interaction, and the cw beam
experiences loss.
Because distributed sensors
based on Brillouin
scattering
are sensitive to both
temperature and
strain, unless
special precautions are taken
when the
fiber is deployed
it will not be possible to determine
whether the system is uniquely
sensing a variation
in temperature
or strain. In
a practical system it
is
virtually impossible to deploy the fiber
such that it
is not sensitive to temperature
variation; however, it
is feasible to install the
fiber such that the effects
of variation in the
strain are minimized. Here
we report
the use of this approach to
recover both
measurands.
Sections of the fiber were deployed
such
that half of its sensing length was subject
to the
influence of variations in both strain
and
temperature, whereas the other half
was isolated
from the effects of strain and used to determine
the
temperature variations of the
first half of the fiber.
The experimental arrangement
is illustrated in
Fig. 1. Both
lasers were solid-state
cw diode-
pumped Nd:YAG ring
lasers emitting close to
1319 nm. The maximum
launched power of the cw
laser was -10
mW. We could adjust the frequency
of the
laser by temperature tuning the cavity.
An
acousto-optic modulator was used to provide
short
optical pulses ranging in time from 50
to 500 ns.
The peak launched pulsed power
from the first-
order diffracted beam was -5
mW. This signal
was
monitored at photodetector
Dl and was also
used
to synchronize the start of the data storage.
The zero-order beam was mixed
with light from
the cw beam
laser by directional coupler DC1 and
detected with a
fast (20-GHz) detector,
the resultant
beat frequency being
monitored with a microwave
spectrum analyzer.
Most of the
pulsed power was launched into one
end
of the 22-km sensing fiber by a 90:10 coupler,
DC2,
whereas the cw beam was injected into
the
other end of the fiber by a variable ratio
coupler,
DC3. The cw light emerging from the
sensing fiber
was detected with a low-noise
photoreceiver with a
125-MHz bandwidth, D2, and monitored
with a stor-
age oscilloscope. Most of the
sensing fiber (22 km)
remained on the original spools
as supplied by the
manufacturer and was subject
to a constant but low-
level tension.
Two separate sections
of the sensing
fiber were placed in thermally
insulated chambers.
Chamber 1 contained
300 m of fiber divided into
three separate
100-m lengths that were subject to
different strain levels. The
first 100-m fiber length
was
wrapped around fiber stretching unit 1, which
contained two quartz cylinders, 1 and
2 (diameters
5.8 cm), separated by 0.5 m. Cylinder
1 was fixed
to a translation stage, so that
its position could be
changed to permit stretching
of this fiber section.
The movement
of cylinder 1 was determined with a
mechanical dial
gauge with 1-Am resolution. The
fiber was also
glued to the quartz cylinders to ensure
SYNC PULSE
Lo
OSCILLOSCOPE
10.1km SENSING
FIBER
n
n
~~~~~Chamber I
LASERI
1st2 0C2
I 1
m (ST) 2
[I- 0th
Stretching
-_
Unit 1
3) OO 1
4
) OOm
AOM
J L_ ~~10.6k.
Ch~mb-r 2
lo
Stretching
Unit 2
60m
(T)
BEAT
F d'
FREQUENCY
I
(YAG2X
MONITOR
LASER2
SPECTRUM
03 JCI
oC3
Fig. 1. Experimental
arrangement: D's, photodetec-
tors;
AOM, acousto-optic modulator; DC's, directional
couplers. T denotes temperature
sensing, and S denotes
strain sensing. Experimental
details of the unit used to
impose temperature
and strain variations on the fibers
is described in the text.
0146-9592/94/020141-03$6.00/0
©
1994 Optical Society of America
141
142 OPTICS
LETTERS
/ Vol.
19, No.
2 / January
15, 1994
that
it responded
fully
to the applied
strain;
the
second
100-m fiber length
was loosely
coiled in
a
virtually
tension-free
condition outside
the thermal
chamber; the
third 100 m
of fiber was deployed
in
a
similar fashion
as the first
100-m length
on the
two
other quartz
cylinders, 3
and 4, with the
same
diameters
(5.8 cm)
and an initial
separation of
0.5 m.
The
lengths of the
quartz cylinders
were such
that
they
projected
outside the
chamber
and acted as
thermal
insulators
such that the
arrangement
used to
strain
the fiber was
not affected
by the temperature
changes
within the
chambers. The
second thermal
chamber
was arranged
in a fashion
similar to
that
of chamber
1, except
that the fiber
lengths in
this
unit
were 60 mi instead
of 100 m.
Chambers 1 and
2
were installed
along the
sensing fiber
10.3 and 22 km
from the pulsed
end, respectively.
Figure 2(a)
shows one sample
trace measured
with
the storage
oscilloscope monitoring
the
cw beam
intensity, synchronized
with
the launch of
the pulsed
beam
into the fiber
(time scale
50 /s/division),
corresponding
to 5 km
per division.
The pulse width
is 50 ns,
corresponding
to a 5-m spatial
resolution.
This trace
shows the condition
in which
the Brillouin
loss is maximized
for the fiber
section 3-4
subject to
a
low strain at
a temperature
of 40'C; a
similar
signal
is seen from
chamber 2
(38°C), although
the
signal is not fully
matched.
Figure 2(b) is
an
expanded
trace of that
in Fig. 2(a)
in the
region of
chamber
1. Figure
2(c) shows
the case in which
the
beat frequency
has been matched
to fiber
section 1-2,
which is subject
to an additional
100 ,tie
of strain at
40 'C in the
same chamber.
The expanded
time scale
for Figs. 2(b)
and 2(c) is 2 ,us/division,
corresponding
to
200 mi per division.
These
expanded scale
traces
clearly
show that
there are three
different
strain
regions
in the chamber.
To demonstrate
the strain
resolution
of the system,
we reduced the
additional
strain
on fiber
section 1-2
to 50 /ue, while
the
temperature
was maintained
at
40'C. Figure
2(d)
shows
the variations
in the Brillouin
loss signal
when the beat
frequency
has been adjusted
to maxi-
mize
the loss in
fiber section
1-2 at an additional
50 pue of
strain [trace
(i)]. Traces (ii)
and (iii) show
the reduction
in this signal
when the
additional
strains
were set to
20 and 0 ,e,
respectively.
Clearly
this figure
shows that
a change in the
strain of 20 pue
can be
readily detected.
Figure
3(a) shows
the variation
of the Brillouin
frequency
shift VB
as a function of
strain (measured
in chamber
1) at a
fixed temperature
of 18'C.
The
dependence
of the Brillouin
frequency
shift on addi-
tional strain
is seen to be
linear. From
these data
and similar results
measured
at 22 km,
the root-
mean-square
deviations from
the best straight
line
fit
were found to
be near 20
ge. From the
data,
the strain
coefficient of
the Brillouin
frequency shift
was determined
to be 5.4
± 0.7 MHz
per 100 guE,
which
agrees with
the results
of Kurashima
et al.
5
According to
these experimental
data, we
claim that
the
strain resolution
of our system
is ±20
,ue.
Figure 3(b) is
a plot of the
laser beat
frequency
needed
to maximize
the Brillouin
loss in chamber
1
as a function
of temperature
for fiber
section 3-4.
Since
this part
of the
fiber was
initially
wound
on
the two
5.8-cm-diameter
quartz
cylinders
at 22°C,
there
was
an initial
winding
tension
such
that
the
matched
frequency
wBttotal
corresponded
to
/Bstrain
+
VB,T,
where vBstrain
is the
Brillouin
shift caused
by
the
initial tension
and
vBT is the
frequency
shift that
is
due
to the
temperature.
The
parameter
VB,T
can
be
found
from
the temperature
measurement
when the
fiber
is in
a tension-free
condition
(VBT
= 1.2 ±
0.2
MHz/,C).
3
Hence
as VB,T
is known,
then VBstrami
=
VB,tOt.l
- VB,T
can be
found.
For optimization
of the
Brillouin
loss in
an unstrained
length
of
fiber, the
initial
strain
in section
1-2
was
found to
be -20
Aue.
When
the oven
temperature
is increased,
the
fiber
expands,
and
this tension
will
be reduced.
Hence the
Brillouin
frequency
shift needs
to be
corrected
for this
effect.
The change
in strain
is -a
1
T, where
a,
is the
thermal
expansion
coefficient
for GeO
2
-doped/pure-
silica
cladding
single-mode
fiber
and
a, - 0.62
x
10-
6
/AC. For
a temperature
change of
60'C, a
1
T =
37 pue,
which
is equivalent
to a frequency
change
of
2 MHz,
which from
Fig.
3(b) corresponds
to total
temperature
change
of 1.5°C.
Thus
the maximum
error,
which
occurs
at T = 82TC
(for
the experiment
reported
here)
as a result
of the
thermally
induced
variation
of
the fiber
strain,
is -2°C.
Figure 4
shows the
variation
of the Brillouin
fre-
quency
shift
as a function
of
additional
strain ap-
plied
to fiber section
1-2 in chamber
1 at different
temperatures.
As can
be seen
from the
figure, the
Brillouin
frequency shift
with strain
has a linear
relation
for all temperatures
investigated,
and the
(a)
Q.mb.1
Starting Pulse
_ ..
2 Sensing End
I
_
5km
(b)
(c)
As
_s
_B
.S
_
(d)
2 200m
A, = vB (501PT)
50m -
Fig.
2. Oscilloscope
traces showing
cw beam intensity
monitored
at photodetector
D2,
with the cw frequency
set to maximize
the Brillouin
loss in various
sections of
the fiber
in chamber
1 for different
environmental
condi-
tions:
(a) temperature
40
0
C, time scale
50 ,us/division;
(b) fiber
section 3-4 at a
temperature of
40 SC; time scale
2 As/division;
(c) fiber section
1-2 at
strain of 100
Ae at
40
SC; time scale 2
As/division; (d)
fiber section 1-2
strain
of 50
Ae at 40
0
C;
time scale
0.5 As/division.
mpe
(I _____
.= = o (iii) w
N .. ;
= -1
' I
I
-
January
15, 1994
/ Vol. 19,
No. 2
/ OPTICS
LETTERS
600
500
.5
0
*I
41
400
300
200
too
12.38
12.39
12.40
12.41
12.42
Beat
Frequency
(GHz)
90
(b)
70 ,
60
50
40
g.
40
~ .
En 30
20
1 0
12.36
12.38
12.40 12.42
12.44
12.46
Beat
Frequency
(GHz)
Fig.
3. Laser
beat
frequency
giving
maximum
Brillouin
loss
for the
fibers:
(a)
in fiber
stretching
unit
1 as
a
function
of additional
strain
at
a temperature
of
18'C,
(b) in chamber
1 as
a function
of different
temperatures
at
very low strain.
600
500
C
uR
.5
_
41
400
300
200
100
0 35 T0
o 45 'C
V 48 Tc
V 57 'C
U 62IC
o 66 TC
* 83 c
o aL I.6 I .y w .... . 4. ........ , l. . .
12.38
12.40
12.42 12.44
12.46
12.48
12.50
Beat Frequency
(GHz)
Fig.
4. Laser
beat
frequency
giving
maximum
Brillouin
loss
for the fibers
in fiber
stretching
unit
1 as a function
of additional
strain at
different
chamber
1 temperatures.
average
strain
coefficient
(the
slope of
the straight
line)
is 5.7
± 0.4
MHz
per 100
l.te for
the temperature
range
investigated
(18-82
'C).
This average
value
is
slightly
higher
than
the value
measured
at 18
'C, but
it is within
measurement
errors
of the
system.
To use
the system
to
measure
both temperature
and
strain
at a specific
location,
one
will have
to
deploy
the
fiber such
that the
first half
of the
sensing
is tightly
bonded
to the
structure
to be monitored;
hence
it will instantaneously
sense
both
temperature
and
strain. The
second
half of
the fiber
will be
colo-
cated and
deployed
such that
the effects
of structural
changes
on the
fiber are
minimized,
permitting
the
temperatures
of both
fiber sections
to
be determined.
Given
that
the
calibration
curves
of
Figs.
3(b) and
4
have been
measured
for a specific
fiber
and
that
the
fiber is
uniform,
it is now
possible
to determine
both
the temperature
and the
strain of
any specified
location
along
the fiber
by
the following
procedure.
Step
1 is
to measure
the
Brillouin
frequency
cor-
responding
to the
maximum
Brillouin
loss
at the
specified
location
in the fiber
section
sensitive
only
to temperature
change
and
then
use the
Brillouin
frequency
to temperature
coefficient
from
Fig. 3(b)
to determine
the
temperature.
A similar
procedure
is required
for
step 2,
in which
the Brillouin
loss
is
maximized
for the colocated
fiber
section
sensitive
to
both temperature
and
strain.
The data
presented
in Fig.
4 are
then
used
to
deduce
the strain.
As it
is feasible
to measure
only
the
strain-versus-frequency
shift
curve
for a finite
number
of temperatures,
some
form of
extrapolation
is
required.
One
approach
is
to assume
that
the
slope
of the
frequency-versus-strain
curve
is inde-
pendent
of temperature;
hence,
from the
measured
value of
the maximized
Brillouin
loss frequency
at
zero additional
strain and
the value
of
the tem-
perature
acquired
from step
1, the
strain
can
be
obtained.
The more
accurate
procedure
would
be (as
indicated
in Fig.
4) to
measure
the actual
additional
strain-versus-frequency
curves for
a large
number
of
temperatures
and
then
extrapolate
over
a smaller
range-this
would
give a
better accuracy.
To summarize,
we
have realized
a combined
dis-
tributed
temperature
and strain
sensor
based
on
Brillouin
loss
that has
a 22-km
sensing
length
with
a
20-tte resolution
and 2 C
temperature
resolution
combined
with a spatial
resolution
of
5 m. For
this
sensor
to
be used
in a practical
application,
it will
be necessary
to
deploy it
such that
half the
sensor
detects
temperature
and
strain
while the
other
half
senses
temperature,
giving a
sensor
with a length
equal
to half
the physical
length
of the
fiber.
It is
envisaged
that this
sensor
could be
used to
moni-
tor stress
levels
continuously
in major
installations
such as
dams, nuclear
reactors,
oil pipelines,
and
buildings.
The
authors
gratefully
acknowledge
the financial
support
of
the UK
Science and
Engineering
Research
Council
through
the
Optoelectronics
Interdisciplinary
Research
Centre,
Southampton,
UK.
References
1.
J. P. Dakin,
D. J.
Partt, G.
W. Bibby,
and J.
N. Ross,
Electron.
Lett.
21, 569
(1985).
2.
T. Kurashima,
T.
Horiguchi,
and M.
Tateda, Opt.
Lett.
15,
1038 (1990).
3. X.
Bao, D.
J. Webb,
and
D. A. Jackson,
Opt.
Lett.
18,
552 (1993).
4. X. Bao,
D. J.
Webb, and
D. A. Jackson,
in
Proceedings
of the
Ninth International
Conference
on
Optical Fiber
Sensors
(Associazione
Elettrotecnica
ed Elettronica
Italiana,
Firenze,
Italy,
1993),
postdeadline
paper
3.
5. T. Kurashima,
T. Horiguchi,
and
M. Tateda,
IEEE
Photon.
Technol.
Lett. 2,
352 (1990).
143