Journal of Sensor Technology, 2013, 3, 75-83
http://dx.doi.org/10.4236/jst.2013.33013 Published Online September 2013 (http://www.scirp.org/journal/jst)
Role of Flexible Macro Fibre Composite (MFC) Actuator
on Bragg Wavelength Tuning in Microstructure
Polymer Optical Fibre Long Period Grating
for Strain Sensing Applications*
Asok K. Dikshit1,2#, Akhil Raj V. L.2
1BRIC, Inc., New Alipore, Kolkata, India
2Department of Energy Studies, The Glocal University, Delhi-Yamunotri Marg, Saharanpur (UP), India
Received October 8, 2012; revised November 8, 2012; accepted November 16, 2012
Copyright © 2013 Asok K. Dikshit, Akhil Raj V. L. This is an open access article distributed under the Creative Commons Attribu-
tion License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
ABSTRACT
The microstructure polymer optical fibre (mPOF) inscribed long period grating (LPG) offers a wide field of application
in strain sensors arena within the materials elastic limit. Flexible innovative macro fibre composite (MFC) actuator gen-
erates electromechanical force under DC driving voltage. We propose a novel method for Bragg wavelength blue shift-
ing through stretch tuning of mPOF LPG in axial direction under applied DC voltage on attached MFC with LPG. The
grating period of mPOF LPG changes refractive index and causes blue shift of Bragg grating fibre wavelength. The
shifting governs on the values of generated electromechanically strain transfer from flexible MFC to mPOF LPG and
they have potential applications in strain sensor.
Keywords: Microstructure Polymer Optical Fibre (mPOF); Long Period Grating (LPG); Macro Fibre Composite
(MFC); Actuator; Strain Sensing; Blue Shifting
1. Introduction
Much attention has been paid by several researchers to
inscribed microstructure optical fibre (mPOF) long pe-
riod grating (LPG) to develop a new class of optical fi-
bres that have significant potential to get wide variety of
waveguide properties. In general, multiple microscopic
air-holes lattice runs down longitudinally along the leng-
th of the optical fibre with different microstructure pat-
tern. A novel waveguides property can be achieved by
designing various air hole structures, size, shape and dis-
tribution of the holes, which may not be readily achieved
in conventional optical fibres [1-4].
The advantages of using microstructures optical fibre
are more profound than they are in conventional optical
glass fibres. It may fabricate easily in single modes fibres
and also useful in visible window with low loss. It is ap-
plicable for sensing application depending on dispersion
properties of the fibre. Optical loss is a traditional con-
straint for long length uses in polymer optical fibre
(POF). Both POF and mPOF are different from dominant
loss mechanism. Intrinsic losses of acrylic materials in
both POF cases are for molecular absorption and scatter-
ing from in-homogeneities due to local variation in mo-
lecular weight which causes density change. The losses
of mPOF depending on fibre design, are higher than con-
vention POF. The lowest possible loss of mPOF is 0.192
dB/m compare to 0.15 dB/m for POF of same intrinsic
acrylic PMMA materials [5].
Silica based LPG on optical fibre has been explored
mostly, but LPG in polymer fibre has not so much poten-
tial in past due to difficulties in fabrication of single
mode polymer optical fibres. Argos et al. [4] have fabri-
cated successfully single mode mPOF LPG and ventured
a new company Arkema Ltd. to supply it [6]. It has been
established that resonant wavelength decreases with the
increasing of LPG grating period. There was extremely
optimal mode loss that provided side lobe free, 100%
power transfer from core to the cladding mode for a uni-
form LPG [7].
*This work was carried out at CSIR-CGCRI, Kolkata, India.
#Corresponding author.
C
opyright © 2013 SciRes. JST

A. K. DIKSHIT, AKHIL RAJ V. L. 76
The LPG sensitivity can be manifest itself in two ways:
spectral shift of attenuation band and a change in the
strength of attenuation band. The resonant wavelength of
the mechanically induced LPG can be easily tuned as the
pressure is reversible. The external perturbation by me-
chanical force into the polymer optical fibre LPG can be
induced through electromechanical force generated by
employed flexible PZT-polymer composite actuator na-
mely macro fibre composite (MFC) actuator by applying
direct DC voltage.
The young modulus for PMMA polymers materials
(3.2 GPa) is significantly lower than silica (72 GPa) glass
materials and the elastic limit of PMMA is also an order
of magnitude higher than of silica. It is useful for strain
sensing application at below elastic limit. The strain
sensing can be monitored by shifting of center Bragg
wavelength during external perturbation applied on LPG
mPOF. The linear response is −11.8 nm per % of strain
in mPOF LPG. The high elastic limit of PMMA material
and low loss is unique combination in the visible range in
single mode mPOF for strain sensing applications [8].
Macrofibre composite actuator (MFC) is an innovative
actuator that offers high performance in terms of flexibil-
ity as a cost competitive device. It was made with rec-
tangular PZT-piezo fibres which are sandwiched between
layers of adhesive, electrodes, and polyimide film, and
both sides are covered with polyimide film. The PZT
materials can be considered as mass of minute crystal-
lites and acts as actuator under driving DC voltage.
Crystal lattices of electric dipole in PZT molecules are
caused electric charge distribution in the axis directions
in the chemical bond. Monolithic PZT has restrictions
regarding brittleness and a high mass density and limited
deformation causes vulnerable to accidental breakage
during handling. These restrictions are overcome using
rectangular shape of macrofiber fingers cut from PZT
thin wafer of 178 μm in height and 356 μm in length and
pasted with high shear structural epoxy adhesive, it looks
like discrete piezostack actuators. Two sets of copper
electrode pattern printed on a polyimide film were sand-
wiched through structural epoxy adhesive [9]. Polariza-
tion of piezocrystal produces electromechanical strain
energy density that can be controlled using different
spacing of electrode printed on polyimide film to make
electric field distribution in the plane of PZT-polyimide
composite. The developed composite actuator produces
nearly twice the strain and four times the strain energy
density during polarization in plane. This configuration
of PZT microfiber shows more flexibility, higher per-
formances and durability, compared to a traditional mo-
nolithic PZT actuator [10].
The established empirical relation for mode coupling
occurs in LPG at the resonant wavelength in the follow-
ing equation: λ = (n01 − ncl) Λ, where, Λ is grating period
and n01 and ncl are the effective index of the fundamental
core mode and individual cladding mode of the fibre,
respectively.
The strength of the mode coupling at the resonant
wavelength to the cladding mode oscillates as sinkd,
where k is the coupling coefficient and d is the length of
the grating. The coupling coefficient k is proportional to
the amount of index variation induced into LPG by the
mechanical pressure. It is observed that the blue shift
originated from the high dispersion of the cladding mode
of the mPOF LPG compared with the cladding mode of a
conventional fiber, LPG in mPOF is much more guiding
and dispersive in nature due to the presence air holes,
which might explain the endless single-mode operation.
The important application of single-mode mPOF LPG
gratings in strain sensing, monitors engineering and me-
dical applications [11].
The aim of this paper is to study the effect of innova-
tive flexible macrofibre composite (MFC) actuator gen-
erated electromechanical force under DC driving voltage
experienced on attached mPOF LPG perturbed grating
period causes resonant Bragg wavelength shifting to-
wards decreasing of blue shifting. The potential use of
this flexible MFC actuator attached in mPOF LPG mod-
ule is in strain sensor application.
2. Experiments
Microstructure polymer optical fibre (mPOF) long period
grating (LPG), FM-340 was purchased from Kiriama Pty.
Ltd, Australia. The features of microstructure polymer
LPG as received was given in Table 1.
2.1. MPOF Design
Single mode mPOF have been inscribed with long period
grating (LPG) and possible application is limited for
minimum losses before and after grating, respectively as
shown in Figures 1(c) and (d). The strain of mPOF up to
15% for high elastic limit of PMMA and guide light
through the use of a pattern of tiny holes that run the full
Table 1. Specifications of microstructure polymer optical
fibre (mPOF) long period grating (LPG) FM-340 fabrica-
ted by Kiriama, Pvt. Ltd.
Fibre Diameter (μm) 260
Core Diameter (μm) 15
Core/Cladding Material Polymethylmethacrylate (PMMA)
Grating Strength (dB) 11
Fibre Length (m) 1
Grating Length (cm) 2.5
Grating Wavelength (nm) 687 and 686

A. K. DIKSHIT, AKHIL RAJ V. L. 77
length of the fibre, one hole is missing in the center re-
gion acts as the core as shown in Figures 1(a) and (b),
the holes pattern changes the optical properties of the
fibre as well materials properties including Young mo-
dulus, greater elastic limit make them desirable sensing
application regarding strain and bending. Microstructure
was observed in optical microscope using software
analysis in Olympus UC-30, Japan.
Figures 1(a) and (b) have shown the reflection spec-
trum characteristics of LPG (Figure 1(c)) and FBG II
(Figure 1(d)). The single mode mPOF made of PMMA
fibre designs has hexagonal arrangements of air holes.
The distance between two holes are 5.5 μm, diameter of
one hole is 3.9 μm, diameter of fibre is 340 μm, and av-
erage hexagonal core diameter is 60.28 μm. There was
different fabrication steps are explored to fabricate mi-
crostructure polymer optical fibre (mPOF). The potential
applications of microstructure polymer fibres are for spe-
cific applications for use in sensing, medicine, and engi-
neering and communication arena. The fabrication con-
sist two main steps. First fabricated a large 8 cm dia per-
form with desired design holes arrangement and
stretched to 6 mm dia cane. They can is sleeved to in-
crease the outer diameter to 12 mm to make secondary
perform from which optical fibre was drawn.
Macrofiber composite (MFC) actuator was purchased
from Smart Materials Inc., Germany. MFC actuators
consist of three primary components: 1) a sheet of alig-
ned rectangular piezoceramic fibers, 2) a pair of thin
polyimide films etched with a conductive electrode pat-
tern and 3 a structural dielectric epoxy adhesive matrix.
Macro fiber composite (MFC) actuator strives to im-
prove current state of the art for a best structural actua-
tion, flexible and replacement of monolithic piezo ce-
ramic predecessors which is shown in Figure 2. The
electric PZT dipole crystals in MFC are randomly ori-
ented throughout the materials domains and no overall
polarization or piezoelectric effect was observed. The
(a) (b)
4000
3500
3000
2500
2000
1500
1000
500
0
Intensity (arb)
400 500 600 700 800 900
Wavelength (nm)
Fibre 6 LPG
Wavelength (nm)
Loss Fibre 6 LPG
0
−2
−4
−6
−8
−10
−12
Loss (dB)
400 500 600 700 800 900
(c) (d)
Figure 1. Optical microscope image of the cleaved end PCF: (a) 260 micron dia cross section view of microstructure fibre; (b)
Core view of hexagonal air holes; (c) Optical loss of microstructure LPG before writing grating and (d) after writing grat-
ing.

A. K. DIKSHIT, AKHIL RAJ V. L. 78
Figure 2. 2D picture of soft macrofiber composite (MFC)
actuator shows flexibility and durability.
poling of the materials, the neighboring dipoles are aligns
with each other to form regions of local alignment known
as Weiss domains. Henceforth, all the dipoles are aligned
during application of voltage and producing a net dipole
moment into the Weiss domain and show a net polariza-
tion as dipoles per unit volume increases. The dipolar
orientations aligned in a Weiss domain grow during
voltage application changes the polarization and dipoles
are oriented towards opposite electric field. The material
stretches in the direction of the applied field. The dipoles
are locked in that alignment after power is removed, giv-
ing the ceramic PZT crystals remnant polarization and a
permanent deformation. The shifting of Bragg wave-
length with the application of DC driving at particular
voltage is proportional to the changes of dipoles orienta-
tion towards poling direction at that time. The changes of
wavelength is fast at initial time of voltage application as
dipoles are forced to orient in the opposite direction and
produces Weiss domain into the preferred direction into
the crystallites.
The developed composite generated twice the strain
and four times strain energy density during polarization
in the plane. To mitigate all these limitation regarding
monolithic PZT actuators, macrofibre composite actuator
is best solve for high performance soft flexible actuator.
The PZT dipole molecules are oriented into the Weiss
domain in the poling axis line direction. Applications of
DC voltage on macro fiber composite where electric di-
poles are oriented in the lateral direction and produces
electromechanical force due to alignment of electric di-
poles towards the poling axis line of PZT [12].
The changes are more for bigger sizes of composite
sample 85-28P1 compare to small size of 28-07P1. The
big dimension of composite has more electric PZT crys-
tal dipoles which will have more movement under par-
ticular voltage for same time period. The numbers of
PZT crystal dipole domains are more for big size of sam-
ples rather than small size of composite. The Bragg
wavelength changes are more for larger sizes of sample
rather than small one at a fixed voltage due to more PZT
crystal dipoles involvements. It is observed that changes
are rapid at initial time period of DC driving voltage and
total changes are very small. The displacement was
measured using LVDT sensor device in test rig consists
two clamps at two ends.
The displacement was governed with the following
equation; δ = α N d33V, where α is a constant of correct-
ing coefficient from the weak field d33 value to strong
field strain constant, and is about 1.5 for multilayer ac-
tuators. d33 is piezoelectric strain constant and V is the
applied voltage. N is (2n-1), where n is the number of
electrode lines on one layer [13].
It was reported that the central Bragg lasing peak
wavelength tuning was 1.25 nm using MFC sizes 2807P1,
1.82 nm for 4010P1, 2.15 nm for 8507P1 and 3.47 nm
for 8528P1. The lasing line width is around 0.04 nm at
20 dB which corresponds less than 0.006 nm at 3 dB.
The output power does not change with application of
voltage as well as for different time duration [14].
2.2. Experiment Set-Up
In this proposed experimental model, we have explored
two separates POF LPG with Bragg wavelengths at 687
and 686, which are depicted in Table 1 with loss ~11 dB
each. These LPG were placed in separate experiments.
LPG was mounted on different size of MFC’s actuators.
The actuator dimensions are 28 mm × 07 mm, 40 mm ×
10 mm, and 85 mm × 28 mm, respectively. They were
mounted with special adhesive DP-460 supplied by 3 M,
USA which has high shear and good elastic in nature.
In this setup, the POF LPG was mounting on MFC
actuator and blue shifting of Bragg wavelength is achie-
ved through electromechanical stress generated into
MFC attached with LPG under driving DC voltage as
shown in Figure 3.
In each set of experiment, LPG is mounted on differ-
ent sizes of macro composite (MFC) actuator with zero
shear high elastic dielectric glue under certain strain con-
dition. The driving of DC voltage on MFC generates
electromechanical force and stretches mPOF LPG in ax-
ial direction due to d33 effect of MFC actuator. This
changes the pitch length in LPG and modulates refractive
index which in turn shifts the center Bragg wavelength of
LPG. The amount of axial strain generates under DC
driving voltage into piezo actuator MFC transfer to the
LPG and changes the grating pitch lengths which are
directly proportional with blue shifting of Bragg wave-
length.
The strain testing of MFC was done using LVDT sen-
sor using test rig consists of two clamps at two ends. The
real displacement was measured by the active length/
clamp distance. The total displacement is not possible to
measure by maximum active length. It may be calculate

A. K. DIKSHIT, AKHIL RAJ V. L. 79
Light Source
(LED) POF LPG
MFC Actuator
OSA
+ve −ve
Figure 3. Schematic representation of experimental set up using PZT-polyimide macrofibre composite (MFC) actuator for
pitch modulator of LPG mounted on for strain sensor.
through following equation. Max displacement = real
strength × max. active length.
3. Results and Discussion
3.1. Theory of LPG
The fabrication of LPG was done by inscribing periodic
refractive index variation by photo induced into polymer
optical fibre (POF). The transmission mode spectrum
consists of distinct resonant loss bands that are coupled
with fundamental core modes. At phase matching condi-
tion, it will give resonant transmission wavelength at
res

.
(1)

eff eff
rescore m,clad
nn Λ   

Where and are the effective index of fun-
eff
core
neff
m,clad
n
damental core mode and mth cladding mode, respectively
and Λ, is the period of LPG [15].
3.2. Strain Sensitivity
The resonant wavelength (res

) of the POF LPG will
shift under axial strain due to index modulation period (Λ)
of the LPG increases. The effective index of core and
clad modes will decreases due to photo elastic effect of
the fibre material [16],

res
res strain
d=1Γ
d


 

(2)
where,  is waveguide dispersion factor, it plays an im-
portant role on the applied strain and temperature sensi-
tivity on the POF LPG. The strain sensitivity (res
d
d


), of
an LPG in the POF depends on the following four pa-
rameters, the elasto-optic coefficients of the core and
clad materials, waveguide dispersion factor (), the index
modulation (Λ), and mode order (m).
The waveguide dispersion factor,
 
res
d
d
ncorenm, clad


  (3)
The dispersion curve of single mode hexagonal micro-
structure design air holes in mPOF LPG has shown in
Figure 4. These values were obtained for the mPOF LPG
fibre with Λ = 5.5 μm, diameter of one hole is 3.9 μm,
diameter of fibre is 340 μm, and average hexagonal core
diameter is 60.28 μm. The design has done for low loss
confinement by inserting optimum air holes surrounded
to the core of the fibre.
Figure 4 shows the variation of refractive index at
different wavelength (λ) and pitch length (Λ) are 0.8. 0.9,
1.0. 1.1, or 1.2. The refractive index (n) of PMMA is
1.48998 at 632 nm wavelength. The sensitivity was pre-
dominant influences by central air-hole diameter (dc) and
the distance between consecutive air holes (Λ) over the
dispersion characteristics. It was observed that dispersion
factor (λ) is always negative, and this causes blue shifting
of the resonant wavelength under axial strain through
perturbation with flexible actuator [17].
3.3. MFC Displacement Measurements
Figure 5 showed displacement of different dimensions
MFC actuator under DC driving voltage in test rig con-
sists of two clamps using LVDT sensor device. MFC
with dimension of 85-28P1 (Figure 5(c)) will produce
maximum 92.5 micron displacement with clamp distance
65 mm whereas it is more which 120.7 micron for 85
mm clamp distance. This value is much less for actuator
dimension of 40-10P1 (Figure 5(b)) and 28-07P1 (Fig-
ure 5(a)), respectively. It was observed that displacement
of 40-10P1 MFC with clamp distance 20 mm was 31.6
micron and 63.2 micron for clamp distance 40 mm, re-
spectively. Similarly the maximum displacement was
58.8 micron of 28-07P1 MFC for clamp distance 28 mm
whereas it was 27.3 micron for clamp distance 13 mm.
There was a hysteresis observed in MFC which is an in-
herent property of PZT materials [12] as shown in Fig-
ures 5(a)-(c).
The MFC is unique design innovative actuator that of-
fers flexible, soft, durable, and increases strain actuator
efficiency, directional sensing and damage tolerant. The
displacement of the rectangular piezoceramic aligned
fibres sandwiched between layer of adhesive, electrode
and polyimide film can be measured using following

A. K. DIKSHIT, AKHIL RAJ V. L. 80
Figure 4. Schott dispersion of acrylic PMMA matrix of mPOF LPG.
(a) (b)
(c)
Figure 5. Displacement vs. voltage for different dimension of MFC under driving voltage of 0 V to 1500 V: (a) 28-07P1, (b)
40-10P1 and (c) 85-28P1.

A. K. DIKSHIT, AKHIL RAJ V. L. 81
equation [13], displacement, δ = α × Nd33 × V, where α is
constant and is about 1.5 and N is 2n-1, where n is the
number of electrode lines on one layer, d33 is piezoelec-
tric strain constant and V is the applied voltage.
Big dimension MFC has more piezo electric dipoles
crystal which is able to produce large electromechanical
force under certain driving applied DC voltage. Similar
trend was also observed in Bragg wavelength blue shift-
ing for larger dimension of MFC. MFC of 85-28P1 has
more Bragg wavelength shifting compare to small size
MFC of 2807P1 as shown in Figure 6(b). It was ob-
served in the Table 2 that the shifting is 0.672 nm for
2807P1 MFC, 1.058 nm for 4010P1 MFC and 2.255 nm
for 8528P1 MFC, respectively. The strain optic co-effi-
cient of plastic optical fibre is about twice that of silica
optical fibre, that will produce twice the Bragg wave-
length shifting of microstructure LPG silica glass fibre
under same strain produces into MFC actuator .The ther-
mo optic coefficient of polymers is negative owing to the
predominant effect of the density change with tempera-
ture. The experimental operational temperature is in be-
tween 80˚C to 120˚C of polymer fibres, they are good
enough for structural analysis [18].
Figure 7 shows wavelength shifting with different ap-
plied DC voltage on MFC which is attached with mPOF
LPG which causes electromechanical force on its
changes periodic index (Λ). The Bragg peak position
moves lower visible length, peak shape and line width
does not vary with applied DC voltage. The above Fig-
ure 7 shows that shifted Bragg wavelength peak width is
remain same. This represents that the PZT electric di-
poles are oriented into the Weiss domain in axis d33 di-
rection. There was no broadening of peak observed and
reveals that dipoles are not align into the perpendicular
180
160
140
120
100
80
60
40
20
0
MFC 2807 P1
MFC 4010 P1
MFC 8528 P1
0 250 500 750 1000 1250 1500
Voltage (V)
Displacement (µm)
688.0
687.5
687.0
686.5
686.0
685.5
685.0
MFC 2807 P1
MFC 4010 P1
MFC 8528 P1
0 250 500 750 1000 1250 1500
Voltage (V)
Wavelength (nm)
(a) (b)
Figure 6 (a) Voltage (V) vs. displacement (μm) and, (b) Voltage (V) vs. wavelength (nm) shift for different dimension of
MFC: (▪) 2807P1, (●) 4010P1 and (▲) 8528P1.
Figure 7. Bragg wavelength shift at different voltages: (a) 0 V, (b) 500 V, (c) 1000 V, and (d) 1500 V.

A. K. DIKSHIT, AKHIL RAJ V. L. 82
Table 2. Strain %, changes periodic index (Λ) and wavelength shift (Δλ) under applied DC voltage on mPOF LPG attached
MFC.
Dimension
of MFC (P1)
Applied DC
Voltage, (V) Strain % Resonance Wavelength
(nm) min
res

at 0 V
Wavelength Changes
(nm), max
res

Wavelength shift,
Δλ (nm)
28-07 1500 0.067 687.3 686.958 −0.672
40-10 1500 0.096 687.3 686.572 −1.058
85-28 1500 0.205 687.3 685.958 −2.255
direction of d33 effect. Similarly it was shown in Figure
7 that peak power remains same for different voltage to
tune Bragg wavelength intensity. It represent that peak
intensity variation was not observed during application of
voltage on macro fiber composite where electric dipoles
are orient in the lateral direction and produces electro-
mechanical force due to alignment of electric dipoles
[12-14]. The Bragg peak changes are more for big di-
mension of MFC sample rather than small one at par-
ticular voltages.
4. Conclusion
This aim of this work is to make strain sensor device
using mPOF LPG mounted on flexible innovative MFC
actuators. The microstructure polymer optical fibre
(mPOF) LPG makes single mode operations in visible
wavelength window, low insertion loss, small backward
reflection, and low cost and large diameter possible high
potential for strain sensing applications. The blue shifting
of Bragg wavelength is effectively more for using either
big size of MFC or at applied higher DC voltage. They
were 0.672 nm and 2.255 nm for 28-07P1 and 85-28P1
MFC under applied 1.5 KV, respectively, keeping line
peak shape and line width unchanged with applied DC
voltage. The sensitivity of LPG can be tailored by con-
trolling microstructure design through controlling disper-
sion factor with mode matching of core and clad. This
proposes model on mPOF LPG that has strong potential
for sensor tuning device. It is simple, low-cost, flexible
and efficient for future Bragg wavelength tuning in sen-
sor device applications.
5. Acknowledgements
The authors wish to acknowledge Council of Scientific
Industrial Research (CSIR) for financial support through
CSIR EMPOWER project OLP 0285. Akhil Raj V. L.,
Project student wishes to thanks all Research fellows of
Central Glass and Ceramic Research Institute (CG &
CRI), Kolkata 700 032 for cooperation during MS thesis
work. The Authors also wish Dr. Alexander Argyros, the
University of Sydney, Australia for providing micro-
structure long period grating (mPOF) to carry out this
work.
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