sensors
Article
Orientation-Dependent Displacement Sensor Using
an Inner Cladding Fiber Bragg Grating
Tingting Yang, Xueguang Qiao *, Qiangzhou Rong * and Weijia Bao
Physics Department, Northwest University, Xi’an 710069, China; [email protected] (T.Y.);
[email protected] (W.B.)
* Correspondence: [email protected] (X.Q.); [email protected] (Q.R.);
Tel.: +86-137-0920-0861 (X.Q.); +86-158-0298-0702 (Q.R.)
Academic Editors: Christophe Caucheteur and Tuan Guo
Received: 18 July 2016; Accepted: 8 September 2016; Published: 11 September 2016
Abstract: An orientation-dependent displacement sensor based on grating inscription over a fiber
core and inner cladding has been demonstrated. The device comprises a short piece of multi-cladding
fiber sandwiched between two standard single-mode fibers (SMFs). The grating structure is fabricated
by a femtosecond laser side-illumination technique. Two well-defined resonances are achieved by the
downstream both core and cladding fiber Bragg gratings (FBGs). The cladding resonance presents
fiber bending dependence, together with a strong orientation dependence because of asymmetrical
distribution of the “cladding” FBG along the fiber cross-section.
Keywords: inner cladding-FBG inscription; femtosecond laser; orientation bending
1. Introduction
Displacement measurement is one of the critical issues in engineering applications (such as
industrial and health monitoring) [1–4]. Fiber Bragg grating, as a smart optical device, has a great
performance on monitoring displacement (bending) [5–8]. For instance, FBG inscription within two
eccentric cores of a polymer fiber has been applied to measure fiber bending [9]. Besides, a bend sensor
based on FBG inscribed in a single-mode fiber within a depressed-index structure has been proposed
and experimentally demonstrated [10]. Recently, Villatoro et al. reported a direction-dependent sensor
based on a fiber within an asymmetric three-core [11]. For these sensors, fiber-bending variation
is retrieved from the wavelength response. Therefore, the temperature perturbation and complex
interrogator are inescapable. A power-referenced interrogation technique is a suggested solution to
those problems. Especially, cladding modes that are converted from the core mode with the method
of core-mismatch [12,13] and post-processing [14,15] significantly lose with fiber bending. Tilted
fiber Bragg grating (TFBG) is another typical device based on the coupling of the core mode to the
amount of backward-propagating cladding modes for bending sensing [16–18]. In addition, long
period gratings (LPGs) have also been widely employed to intrinsically address the coupling of the
core-to-cladding mode whose transmission resonant dip is sensitive to bending [19–21]. However,
when working with cladding modes, the discontinuity technique needs to be provided for ensuring
the coupling of core-to-cladding modes, which may enlarge the light signal-to-noise ratio and make
sensors fabrication complex.
Another alternative technique based on FBG inscription over fiber cladding using a femtosecond
laser side-illumination technique has been demonstrated and successfully utilized for displacement
measurement in our previous work [22]. With the high-intensity and ultrashort pulses of the
femtosecond laser, the nonlinear light-material interaction involving nonlinear multiphoton absorption
and ionization will be induced, and the grating region can be formed in the fiber cladding [22].
The cladding-FBG resonance shows a great response to bending or deflecting on the fiber due to
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cladding [22]. The cladding-FBG resonance shows a great response to bending or deflecting on the
fiber due to induced propagation loss and a good orientation dependence because of the
asymmetrical
distribution
of the
cladding
FBG along
the fiber
cross-section
[23]. Moreover,
power
induced propagation
loss and
a good
orientation
dependence
because
of the asymmetrical
distribution
fluctuations
(originating
the light
source, transmission
lines,
and connectors)
be effectively
of the cladding
FBG alongfrom
the fiber
cross-section
[23]. Moreover,
power
fluctuations can
(originating
from
canceled
out
by
monitoring
the
bend-insensitive
core-mode
reflection.
the light source, transmission lines, and connectors) can be effectively canceled out by monitoring the
In this paper,
we report
the performance for a FBG inscription over a fiber core and inner
bend-insensitive
core-mode
reflection.
depressed-index
cladding
in athe
section
of multi-cladding
(MCF)over
via the
femtosecond
laser
In this paper,
we report
performance
for a FBG fiber
inscription
a fiber
core and inner
side-illumination
technique.
This
construction
seems
similar
to
the
sensor
proposed
in
the
previous
depressed-index cladding in a section of multi-cladding fiber (MCF) via the femtosecond laser
report
[24]. The technique.
device hasThis
a simple
fabrication
a great
quality. inThe
side-illumination
construction
seems and
similar
to the spectral
sensor proposed
the cladding
previous
mode-assisted
coupling
can
be
used
to
measure
the
fiber
bending
with
high
sensitivity
and
definite
report [24]. The device has a simple fabrication and a great spectral quality. The cladding
orientation
dependence.
mode-assisted coupling can be used to measure the fiber bending with high sensitivity and definite
orientation dependence.
2. Fabrication and Principle of QCF-FBG
2. Fabrication and Principle of QCF-FBG
Figure 1 shows the schematic diagram of the FBG inscription system. The proposed grating is
Figureusing
1 shows
the schematic
thelaser
FBG outputs
inscription
system.
The proposed
fabricated
a Ti:sapphire
laserdiagram
system.ofThe
pulses
of duration
with a grating
1 kHz
is fabricated
using
a Ti:sapphire
laser polarized
system. The
laser
outputs
pulses
of duration
with a 1 kHz
repetition
rate,
which
emits a linearly
light
with
a central
wavelength
of approximately
repetition
which
a linearly polarized
wavelength
of approximately
800
nm. A rate,
section
of 10emits
mm hydrogen-loaded
(atlight
60 °Cwith
andaacentral
H2 pressure
of 10 MPa
for 15 days)
800 nm. A section
10 mm hydrogen-loaded
(at 60
and a Hof
pressure
of
10
MPa
for
multi-cladding
fiber of
(produced
by YOFC), with core
and◦ C
claddings
5
μm
and
14
μm,
20
μm,1536days)
μm,
2
multi-cladding
fiber (produced
byand
YOFC),
with
corea and
claddings
of 5mode
µm and
µm, using
20 µm,
120
μm, is self-aligned
(no-offset)
spliced
with
leading-in
single
fiber14SMF
a
36 µm, 120 µm,
is self-aligned
(no-offset)
and spliced
withThe
a leading-in
single mode
fiberofSMF
using a
commercial
compact
fusion splicer
(Fujikura
FSM-60S).
optical microscope
image
quadruple
cladding
fiber
cross-section
shown(Fujikura
in FigureFSM-60S).
2a. It is seen
the MCF has
a step
refractive
commercial
compact
fusionissplicer
Theclearly
opticalthat
microscope
image
of quadruple
cladding
cross-section
is shown inofFigure
2a. It or
is seen
clearly
that the
thefiber.
MCF The
has acore
stepofrefractive
index
(RI)fiber
profile
via the RI difference
the dopant
material
within
the fiber
is
highly
with
is surrounded
by within
a deeply
cladding.
index
(RI) doped
profile via
thegermanium,
RI differencewhich
of the dopant
or material
the depressed-index
fiber. The core of the
fiber is
Another
cladding
higher RIwhich
wrapsison
the first-layer
and two more depressed
highly doped
withwith
germanium,
surrounded
by acladding,
deeply depressed-index
cladding. external
Another
claddings
are next
to RI
it. wraps on the first-layer cladding, and two more depressed external claddings
cladding with
higher
are next to it.
Sensors 2016, 16, 1473
Figure 1. Schematic diagram of the experimental setup for “cladding” FBG fabrication.
Figure 1. Schematic diagram of the experimental setup for “cladding” FBG fabrication.
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The laser beam is precisely focused along one side of the MCF core-cladding interface (~2 μm
core offset) before inscription. The average pulse energy of the laser output is fixed at 0.65 mJ
(controlled by an optical attenuator), which is optimized by trial and error. The exposure time lasts
60 s (i.e., 60,000 laser pulses) and then a 5 mm grating region in the fiber core and cladding can be
achieved simultaneously. As in the zoomed photographic images shown in Figure 2b, the grating
inscription region is located along one side of interface of the fiber core and inner cladding. The
formation of the uniform periodic patterns is based on nonlinear light-material interactions
involving nonlinear multiphoton absorption and ionization because of the high-intensity and
Figure
2.2.(a)(a)
Microscope
image
of theofMCF
Inset shows
theshows
refractive
cross-section
Figurepulses,
Microscope
image
the cross-section.
MCF
cross-section.
Inset
theindex
refractive
index
ultrashort
which is different
from
the UV-induced
color-center
photosensitivity
[25,26].
of
the
MCF.
(b)
Photomicrograph
of
the
gratings;
(c)
Schematic
diagram
of
mode-coupling
inside fiber.
cross-section
of
the
MCF.
(b)
Photomicrograph
of
the
gratings;
(c)
Schematic
diagram
of
In addition, those index changes are mediated by a densification from the nonlinear multiphoton
mode-coupling
inside fiber.
ionization
that causes
local melting and rapid quenching in the dielectric material after
The laser beam is precisely focused along one side of the MCF core-cladding interface (~2 µm
the optical breakdown. Furthermore, the formation of cladding-FBG is consistent with type-II
core Figure
offset) 2c
before
inscription.
The average
of grating
the laser
output is
fixed at 0.65The
mJ
shows
the schematic
diagram pulse
of theenergy
created
structure
configuration.
damage gratings [25,27].
(controlled
anmismatch
optical attenuator),
which
optimized
by trial
and error.
The exposure
time lasts 60 s
interface ofby
the
core between
theis SMF
and QCF
is used
to forward
the core-to-cladding
mode coupling and the backward cladding-to-core recoupling. The cladding modes coupled and the
core mode will get reflected by the downstream cladding and core FBG, the cladding mode
resonance will partially be recoupled back to the upstream SMF, and eventually returns to the
interrogation system. Therefore, two well-defined resonances in the reflections have been achieved.
What is special is that the inner index-depressed cladding cannot confine the cladding modes well
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(i.e., 60,000 laser pulses) and then a 5 mm grating region in the fiber core and cladding can be achieved
simultaneously. As in the zoomed photographic images shown in Figure 2b, the grating inscription
region is located along one side of interface of the fiber core and inner cladding. The formation of
the uniform periodic patterns is based on nonlinear light-material interactions involving nonlinear
multiphoton absorption and ionization because of the high-intensity and ultrashort pulses, which is
Figure
(a) UV-induced
Microscope color-center
image of thephotosensitivity
MCF cross-section.
InsetInshows
the those
refractive
different
from2. the
[25,26].
addition,
indexindex
changes
cross-section
the MCF. from
(b) Photomicrograph
of the gratings;
(c) Schematic
diagram
of
are mediated
by a of
densification
the nonlinear multiphoton
ionization
that causes
local melting
mode-coupling
inside
fiber.
and rapid quenching in the dielectric material after the optical breakdown. Furthermore, the formation
of cladding-FBG is consistent with type-II damage gratings [25,27].
Figure 2c
2c shows
thethe
created
grating
structure
configuration.
The
Figure
shows the
the schematic
schematicdiagram
diagramof of
created
grating
structure
configuration.
interface
of the
mismatch
core
between
thethe
SMF
the core-to-cladding
core-to-cladding
The
interface
of the
mismatch
core
between
SMFand
andQCF
QCFisisused
usedto
to forward
forward the
mode
coupling
and
the
backward
cladding-to-core
recoupling.
The
cladding
modes
coupledand
andthe
the
mode coupling and the backward cladding-to-core recoupling. The cladding modes coupled
coremode
modewill
will
reflected
bydownstream
the downstream
cladding
and
core
the mode
cladding
mode
core
getget
reflected
by the
cladding
and core
FBG,
the FBG,
cladding
resonance
resonance
will
partially
be
recoupled
back
to
the
upstream
SMF,
and
eventually
returns
to
the
will partially be recoupled back to the upstream SMF, and eventually returns to the interrogation
interrogation
system.
Therefore,
two
well-defined
resonances
in
the
reflections
have
been
achieved.
system. Therefore, two well-defined resonances in the reflections have been achieved. What is special
that the inner index-depressed
cannot
confine
the cladding
modes
isWhat
that is
thespecial
inner is
index-depressed
cladding cannotcladding
confine the
cladding
modes
well because
of well
the
because
of
the
special
RI
profile
of
the
MCF.
The
fiber
deformation
not
only
influences
the
modes
special RI profile of the MCF. The fiber deformation not only influences the modes coupling at the
couplingjunction
at the splicing
but also loss
the of
propagation
lossmodes
of theincladding
in the
inner
splicing
but also junction
the propagation
the cladding
the innermodes
cladding.
Hence,
cladding.
Hence,sensor
the investigated
sensor
has ato
great
to fiber
bending.because
In addition,
because
the
investigated
has a great
response
fiberresponse
bending.
In addition,
the effective
the
effective
refractive
index
(RI)
difference
between
the
core
and
inner
cladding
is
0.032,
the
refractive index (RI) difference between the core and inner cladding is 0.032, the resonant modes
resonant
modes
present a clear
wavelength
ofcenter
1.88 nm,
and the center
of the
present
a clear
wavelength
separation
of 1.88 separation
nm, and the
wavelengths
of thewavelengths
reflection spectra
reflection
spectra
are
1548.97
nm
and
1547.09
nm,
as
shown
by
the
red
line
in
Figure
3.
are 1548.97 nm and 1547.09 nm, as shown by the red line in Figure 3.
Figure3.3.Spectra
Spectraof
ofQCF-FBG
QCF-FBGwith
withand
andwithout
withoutbending.
bending.
Figure
Ingeneral,
general,the
theRI
RIof
ofsilica
silicamaterials
materialsisismodified
modifiedby
bythe
thechange
changeof
ofthe
thegeometrical
geometricalcross-section
cross-sectionof
of
In
the
fiber
which
is
caused
by
bending-induced
anisotropic
strain.
Therefore,
once
this
configuration
the fiber which is caused by bending-induced anisotropic strain. Therefore, once this configurationis
bending
or or
deflecting
thethe
fiber
introduces
refractive
index
variations
across
the fiber
that
isachieved,
achieved,
bending
deflecting
fiber
introduces
refractive
index
variations
across
the fiber
influence
the
reflection
spectrum
in
several
ways:
the
forward
core-to-cladding
mode
coupling
at
the
that influence the reflection spectrum in several ways: the forward core-to-cladding mode coupling
SMF-MCF
splicing
junction
(forward
coupling
loss);
the
propagation
loss
of
the
cladding
modes
at the SMF-MCF splicing junction (forward coupling loss); the propagation loss of the cladding
between
the splicing
junctionjunction
and downstream
FBG (bend
and
the reflection
loss of cladding
modes
between
the splicing
and downstream
FBGloss);
(bend
loss);
and the reflection
loss of
modes
between
the
first
depressed-index
cladding
and
second
cladding
because
can
cladding modes between the first depressed-index cladding and second cladding because bending
bending can
introduce
a
strong
first-to-second
cladding
coupling
due
to
the
downside
of
the
depressed
RI.
introduce a strong first-to-second cladding coupling due to the downside of the depressed RI. Finally,
Finally,
the backward
cladding-to-core
recoupling
at the SMF-MCF
splicing
junction
have a
the
backward
cladding-to-core
recoupling
at the SMF-MCF
splicing junction
will
have a will
significant
significant
fluctuation.
Among
these
effects,
the
change
in
the
cladding-to-core
mode
recoupling
at
fluctuation. Among these effects, the change in the cladding-to-core mode recoupling at the splicing
the splicing
junctiontois be
thought
to be dominant,
view
of the behavior
the recoupled
junction
is thought
dominant,
especially especially
in view ofinthe
behavior
that the that
recoupled
power
decreases and increases around its unbent stage, as shown in Figure 3. In addition, the transverse
intensity distribution of the mode in the MCF will be altered as fiber bending [28], caused by the RI
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16, 1473 and increases around its unbent stage, as shown in Figure 3. In addition,4 the
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power
decreases
transverse intensity distribution of the mode in the MCF will be altered as fiber bending [28], caused
by the RI change of the fiber. It will reduce the recoupling efficiency from the backward propagating
change of the fiber. It will reduce the recoupling efficiency from the backward propagating cladding
cladding modes to the core of the upstream fiber. Therefore, the recoupled cladding mode will have
modes to the core of the upstream fiber. Therefore, the recoupled cladding mode will have an
an extremely high sensitivity to fiber bending. As a result, bending the fiber will induce a strong
extremely high sensitivity to fiber bending. As a result, bending the fiber will induce a strong intensity
intensity modulation over the recoupled cladding mode but will have no effect on the core mode,
modulation over the recoupled cladding mode but will have no effect on the core mode, and thus
and thus the power of the reflected core mode can be used as a reference to compensate for the
the power of the reflected core mode can be used as a reference to compensate for the unwanted
unwanted power fluctuations. In addition, the cladding resonance presents high orientation
power fluctuations. In addition, the cladding resonance presents high orientation dependence as the
dependence as the asymmetrical distribution of the cladding-FBG over the fiber cross-section.
asymmetrical distribution of the cladding-FBG over the fiber cross-section.
3.
3. Experiment
ExperimentResults
Resultsand
andDiscussion
Discussion
The
displacement sensing
shown in
in Figure
Figure 4.
4. The
The schematic
schematic diagram
diagram for
for the
the displacement
sensing system
system is
is shown
The light
light
from
an
amplified
spontaneous
emission
(ASE)
is
launched
into
the
fabricated
sensor
through
from an amplified spontaneous emission (ASE) is launched into the fabricated sensor through aa
circulator.
circulator. The
The reflection
reflection light
light from
from the
the sensor
sensor is
is monitored
monitored by
by an
an optical
optical spectrum
spectrum analysis
analysis (OSA)
(OSA)
with
a
wavelength
resolution
of
0.02
nm.
In
the
experiment,
one
side
of
the
sensing
probe
heldonona
with a wavelength resolution of 0.02 nm. In the experiment, one side of the sensing probe isisheld
arotator
rotatoratata afixed
fixedstage
stagewhich
whichcan
canchange
changethe
thebending
bendingdirection,
direction, and
and the
the other
other free
free end
end is
is fixed
fixed to
to aa
translation
μm resolution
resolution providing
providing displacement
displacement along
vertical. The
translation stage
stage with
with aa 10
10 µm
along the
the vertical.
The free-fiber
free-fiber
length
downstream
is
carefully
selected
to
ensure
that
fiber
bending
can
achieve
the
maximum
length downstream is carefully selected to ensure that fiber bending can achieve the maximum effect
effect
on
the
cladding
resonance
mode
power.
The
power
of
the
reflected
cladding
mode
decrease
with
on the cladding resonance mode power. The power of the reflected cladding mode decrease with the
the
increasing
fiber bending,
bending,while
whilethe
theresonance
resonancewavelength
wavelength
stays
unchanged,
as shown
by blue
the blue
increasing fiber
stays
unchanged,
as shown
by the
line
line
in Figure
3. The
device
shows
response
bending
with
the
highestsensitivity
sensitivityofof27.7
27.7dB/mm
dB/mm at
in Figure
3. The
device
shows
thethe
response
to to
bending
with
the
highest
at
60°
ranging
from
−60
to
+60
µ
m
(like
an
inverted
V-shape),
as
shown
in
Figure
5.
Besides,
both
the
◦
60 ranging from −60 to +60 µm (like an inverted V-shape), as shown in Figure 5. Besides, both the
intensity
the core
core mode
mode remain
remain unchanged.
unchanged.
intensity and
and the
the Bragg
Bragg wavelength
wavelength of
of the
Figure
Figure 4.
4. Schematic
Schematic diagram
diagram of
of MCF-FBG
MCF-FBG as
as aa displacement
displacement sensing
sensing system.
system.
Figure 5. Cladding
Cladding resonance
resonance mode
mode power
power and wavelength versus displacements.
Figure
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In order
to1473
characterize
Sensors
2016, 16,
the bending orientation dependence of sensor, the fiber is rotated5 of
from
7
0° to In
360°
with
step of 20°,the
and
the bending-induced
intensityofloss
is recorded
angle.
order
to acharacterize
bending
orientation dependence
sensor,
the fiberatiseach
rotated
fromThe
0◦
◦
◦
order
to characterize
theorientations
bending orientation
dependence
of
sensor,
fiber
is rotated
from
bending
sensitivity
is calculated,
and isa recorded
strong
angular
dependence
of the
to
360 In
with
a step
ofof
20 different
, and the
bending-induced
intensity
loss
atthe
each
angle.
The bending
0°
to
360°
with
a
step
of
20°,
and
the
bending-induced
intensity
loss
is
recorded
at
each
angle.
The
bending
response
has
been
achieved,
as
shown
in
Figure
6.
It
is
caused
by
the
asymmetrical
sensitivity of different orientations is calculated, and a strong angular dependence of the bending
bending
sensitivity
of different
orientations
calculated,
andbya the
strong
angular
dependence
of of
thethe
distribution
of
theachieved,
cladding
grating
over
theisfiber
cross-section,
which
is similar
to our previous
response
has
been
as
shown
in Figure
6.
It is
caused
asymmetrical
distribution
bending
response
has
been
achieved,
as
shown
in
Figure
6.
It
is
caused
by
the
asymmetrical
work [23].
The maximum
sensitivity
is realized
thetobending
axis is
parallel
to the
grating
cladding
grating
over the fiber
cross-section,
which when
is similar
our previous
work
[23]. The
maximum
distribution
of
the cladding
gratingresults
over the
fiber
cross-section,
which
is
similar
to our previous
plates,
and
the
minimum
sensitivity
from
the
bending
axis
being
at
the
orthogonal
direction.
sensitivity is realized when the bending axis is parallel to the grating plates, and the minimum
work [23]. The maximum sensitivity is realized when the bending axis is parallel to the grating
It is
important
to from
note that
the orientation
function
not completely
symmetrical.
This to
behavior
is
sensitivity
results
the bending
axis being
at the is
orthogonal
direction.
It is important
note that
plates, and the minimum sensitivity results from the bending axis being at the orthogonal direction.
mainly
due
to
that
the
fact
that
the
change
of
the
effective
RI
for
the
cladding
mode
caused
by
the orientation function is not completely symmetrical. This behavior is mainly due to that the fact
It is important to note that the orientation function is not completely symmetrical. This behavior is
different
fiber bending
orientations
is
different
from
thebyasymmetrical
distribution
of the
that
the change
the effective
RI for
thechange
cladding
mode
caused
different
fiber bending
orientations
mainly
due toofthat
the fact that
the
of the
effective
RI for
the cladding
mode caused
by
cladding-FBG
over
the
fiber cross-section.
is
different
from
the
asymmetrical
distribution
of
the
cladding-FBG
over
the
fiber
cross-section.
different fiber bending orientations is different from the asymmetrical distribution of the
cladding-FBG over the fiber cross-section.
Figure
Figure 6.
6. Angular
Angular dependence
dependence of
of the
the displacement
displacement responsivity
responsivity of
of the
the sensor.
sensor.
Figure 6. Angular dependence of the displacement responsivity of the sensor.
The temperature response for the sensor is also investigated by placing the MCF-FBG in a
The
temperature
response
forfor
the the
sensor
is also
investigated
by placing
the MCF-FBG
in a heating
The
temperature
response
sensor
is also
investigated
by placing
the MCF-FBG
in a
heating oven with an accuracy◦ of ±0.1 °C. The temperature is varied from
20 to 65 °C. For every
◦
oven
with oven
an accuracy
±0.1 C.ofThe
temperature
is varied from
20 tofrom
65 C.
every
heating
with anofaccuracy
±0.1
°C. The temperature
is varied
20For
to 65
°C. temperature
For every
temperature point, the temperature is kept constant for 20 min in order to ensure a well-distributed
point,
the temperature
kept constant
for 20
min infor
order
to ensure
a well-distributed
temperature
temperature
point, theistemperature
is kept
constant
20 min
in order
to ensure a well-distributed
temperature around the sensor probe before each record. We plot the cladding resonance
temperature
around
thebefore
sensoreach
probe
before
We plot
the cladding
resonance
around
the sensor
probe
record.
Weeach
plot record.
the cladding
resonance
wavelength
as the
wavelength as the function of temperature, as shown in Figure 7. It is seen that the wavelength shift
wavelength
as the function
of temperature,
in Figure
7. Itwavelength
is seen that the
function
of temperature,
as shown
in Figureas7.shown
It is seen
that the
shiftwavelength
presents ashift
linear
presents
a linear
sensitivity
ofof6.9
the intensity
intensity of
of thereflected
reflectedcladding
cladding
mode
◦ C whereas
presents
a 6.9
linear
sensitivity
6.9pm/°C
pm/°C whereas
whereas
the
mode
is is
sensitivity
of
pm/
the
intensity
of the reflected
claddingthe
mode is almost
kept unchanged
almost
kept
unchanged
with
the
temperature
rising,
as
shown
in
Figure
7.
Therefore,
almost
kept unchanged
with
temperature
as shownthein displacement
Figure 7. Therefore,
thethe
with
the temperature
rising,
as the
shown
in Figurerising,
7. Therefore,
measurement
displacement
measurement
is
temperature
independent,
meanwhile
giving
it
a
potential
displacement independent,
measurement meanwhile
is temperature
independent,
meanwhile
giving it measurements
a potential forfor
is temperature
giving
it a potential
for simultaneous
of
simultaneous
measurements
of displacement and temperature.
simultaneous
measurements
displacement
and
temperature.of displacement and temperature.
Figure
7. 7.Temperature
of “cladding”
“cladding” FBG
FBGreflection
reflectionresonance
resonanceincluded
included
Figure
Temperatureresponse
response performances
performances of
Figure
7.
Temperature
response
performances
of
“cladding”
FBG
reflection
resonance
included
wavelength
and
power
wavelength
and
powerfluctuation
fluctuationwith
withincreasing
increasing temperature.
temperature.
wavelength and power fluctuation with increasing temperature.
Sensors 2016, 16, 1473
6 of 7
4. Conclusions
In this paper, a novel FBG inscribed in a multi-cladding single-mode fiber over the core and
depressed-index cladding by a femtosecond laser is proposed and experimentally demonstrated.
A reflection spectrum with two defined resonant modes is obtained corresponding to the core mode
and cladding mode. The FBG-based device is employed for displacement measurement, and its
sensitivity shows high orientation dependence. Moreover, the construction can provide remote sensing
as a reflection probe, and the fabrication is simple and effective, making it a good candidate for
structural health monitoring.
Acknowledgments: This work was supported by the National Natural Science Foundation of China
(Nos. 60727004, 61077060, 61205080), the National High Technology Research and Development Program 863
(Nos. 2007AA03Z413, 2009AA06Z203), the Ministry of Education Project of Science and Technology Innovation
(No. Z08119), the Ministry of Science and Technology Project of International Cooperation (No. 2008CR1063),
the Shanxi Province Project of Science and Technology Innovation (Nos. 2009ZKC01-19, 2008ZDGC-14).
Author Contributions: Tingting Yang and Weijia Bao conducted the experiment. Tingting Yang prepared the
paper. Xueguang Qiao, Qiangzhou Rong and Weijia Bao proposed the idea and revised the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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