MAGNETICALLY-ACTUATED VARIABLE OPTICAL ATTENUATORS USING FERROFLUID-DOPED ELASTOMER IMPLEMENTED BY

MAGNETICALLY-ACTUATED VARIABLE OPTICAL ATTENUATORS
USING FERROFLUID-DOPED ELASTOMER IMPLEMENTED BY
COMBINATION OF SOFT LITHOGRAPHY AND INKJET PRINTING
TECHNOLOGIES
S. de Pedro1, V. J. Cadarso2, X. Muñoz-Berbel1,J.A. Plaza1, J. Sort3, J. Brugger2, S. Büttgenbach4, and
A. Llobera1
1
CNM-IMB-CSIC, Esfera UAB,Campus UAB, Bellaterra, Spain
2
Microsystems Laboratory, EPFL, Switzerland
3
ICREA and Physics Department, UAB, Barcelona, Spain
4
Institut für Mikrotechnik, Germany
ABSTRACT
This paper reports the implementation of magnetic
variable optical attenuators (M-VOA) by soft lithography
(SLT) and using polydimethylsiloxane (PDMS) as
constituent material. Two different fabrication protocols
are used and compared. In the first case, a two-layer
structure containing a clean PDMS layer on a magnetic
PDMS (M-PDMS) layer is fabricated by SLT. M-PDMS is
obtained by doping clean PDMS with different ferrofluid
(FF) amounts. The second protocol consists of selectively
dispensing droplets of FF by the inkjet printing technique
(IPT) on a clean and non-cured PDMS structure previously
defined by SLT. The optical and mechanical properties of
structures fabricated using both protocols and containing
similar ferrofluid amounts are compared.
INTRODUCTION
Variable optical attenuators (VOAs), with a dynamic
control of the optical power, have been positioned as one of
the most relevant microoptoelectromechanical systems
(MOEMS) in optical sensor technology and
telecommunications. This interest may associate to the
notable evolution that VOA systems have been suffered
last years. For instance, traditional silicon VOAs (Young
modulus = 100-200 GPa) have been often substituted by
more polymeric VOAs (Young modulus = 1-4 GPa) with
higher sensitivities and dynamic ranges.
On the other hand, light attenuation has also evolved
from high voltage electrical actuation to magnetic
actuation, which, apart from being much simpler and safer,
can be actuated remotely reducing the complexity of the
VOA structure. Additionally, magnetically-actuated or
magnetic VOAs (M-VOAs) can be actuated from large
distances (few millimeters) [1] producing large
displacements (from tens to hundreds of mm) [2]. The
main disadvantage of M-VOAs is that they are limited to
magnetic materials, for example magnetic polymers.
Magnetic polymers are commonly obtained by mixing
conventional polymers with magnetic nanoparticles. Since
nanoparticles may tend to aggregate [3], the presence of the
surfactant is in most of cases essential to guarantee their
homogeneous distribution on the polymer [4]. It is
important to note that the presence of aggregates may alter
the optical and mechanical properties of the polymer [5].
In this article, two protocols for the fabrication of
M-VOA
using
the
elastomeric
material
polydimethylsiloxane (PDMS) [6] as constituent material
are presented and compared. The same M-VOA structure
is used in both cases. The differences are in the fabrication
protocol. In the first approach, a two-layer structure
containing a clean PDMS layer on a magnetic PDMS
(M-PDMS) layer is fabricated by SLT. M-PDMS is
obtained by doping clean PDMS with different amounts of
ferrofluid (FF), a colloidal suspension of Fe3O4
superparamagnetic nanoparticles with surfactant.
The second protocol consists of selectively
dispensing droplets of FF by the inkjet printing technique
(IPT) on a clean and non-cured PDMS structure previously
defined by SLT. The FF fluid drops are finally trapped in a
second clean PDMS layer. In this case, ferrofluid is not
homogeneously distributed along the cantilever but
concentrated on specific areas.
The mechanical properties (vertical displacement
under different permanent magnetic fields) and optical
losses of structures containing similar FF amounts are
compared.
MATERIALS AND METHODS
Reagents
PDMS (Sylgard Silicon Elastomer 184, Dow Coring Corp)
prepolymer solution was prepared by mixing the elastomer
and the curing agent (Sylgard Curing Agent 184, Dow
Coring Corp) in 10:1 ratio (v:v), respectively. FF was
purchased from Liquids Research Limited. The FF
presents a uniform size distribution and low Fe304 particle
agglomeration (< 20 nm) dispersed into isoparafin with 10
nm as the mean particle diameter and a saturation of 400 G.
M-PDMS was obtained by adding the FF suspension to
previously prepared prepolymer solution.
Design
The proposed M-VOA is illustrated on Figure 1. It is
composed by an input self-alignment system that fixes the
input fiber optics in the optimal position to couple the light
into the waveguide cantilever (4000 µm length, 250 µm
wide, 250 µm high). The waveguide cantilever includes
two sets of parallel air mirrors distributed along the
cantilever length to guide the light to the cylindrical
microlens positioned at the free end cantilever. The
cylindrical microlens focuses the guided light to the output
self-alignment system, where it is collected. Additionally,
the M-VOA design includes two reservoirs connected by a
microchannel surrounding the cantilever to trap either the
M-PDMS or the FF drops depending on the fabrication
4000 µm
protocol.
output self-alignment
system
cylindrical microlens
waveguide cantilever
air mirrors
microchannel
input self-alignment
system
reservoir
Figure 1: Design top view of the M-VOAs designed
showing all its components in detail: reservoirs, the
microchannel, the self-alignment systems, waveguide
cantilever, air mirrors and cylindrical microlens.
M-VOA’s fabrication
M-VOAs are fabricated using either conventional SLT
or a combination of SLT and IPT [6]. The same two-level
SU-8 master, with reservoirs and microchannel in the first
level (50 µm-thick) and the input/output self-alignment
system and the waveguide cantilever (including air mirrors
and microlens) in the second level (250 µm-thick), is used
in both cases.
In the M-VOA prepared by conventional SLT
(M-VOASLT), M-PDMS is obtained by mixing 0.016,
0.025 or 0.033 µl of FF into PDMS prepolymer solution.
The first level of the master is filled with M-PDMS
avoiding overflowing. After a short curing step (5 minutes
at 80ºC), the second level of the master is filled with clean
PDMS and cured for 20 minutes at 80ºC. With this
protocol, both PDMS layers become bonded in a single
M-VOA structure, Figure 2 a).
On the other hand, the M-VOA obtained by a
combination of SLT and IPT (M-VOASLT+IPT) is prepared
as follows. The first level of the master is filled with clean
PDMS and partially cured (5 minutes at 80ºC). Next, FF
drops are dispensed by IPT in the microchannel area close
to the end of the waveguide cantilever (see Figure 2 b). The
final volumes of FF in the microchannel are 0.018, 0.026 or
0.030 µl, depending on the case. Finally, the second level is
filled with clean PDMS prepolymer and cured for 20
minutes at 80ºC. Following this protocol, FF drops remain
stably trapped on a single M-VOA structure.
(a)
(b)
Figure 2: Lateral section scheme and top view picture of
the fabricated (a) M-VOASLT and (b) M-VOASLT+IPT. In (a)
scheme, light grey corresponds to clean (non-doped
PDMS) and dark grey region to M-PDMS. In (b) scheme,
black spots represents IPT dispensed FF microdrops.
4000 µm
Setup
For the mechanical characterization, the M-VOA is
placed on a micropositioning platform on top of a magnet
of variable magnetization. The waveguide cantilever
deflection at each magnetic field is measured by following
the protocol detailed below. Firstly, the 635 nm
wavelength laser beam (Laser source, 633 nm, 10 mW,
model 1137P, JDS Uniphase) is focused at the free end
waveguide cantilever without external magnetization. This
value is taken as reference. Next, a known magnetization is
applied and the cantilever is deflected. The laser beam is
again focused at the free cantilever end and this
displacement is used to determine the deflection.
In the optical characterization, M-VOAs are placed on
a support with a magnet (as before). A 125 µm multimodal
fiber optic located at the input self-alignment system
couples the light from the 635 nm wavelength laser (Laser
source, 635 nm, 2.5 mW, Model S1FC, Thorlabs GMBH)
to the waveguide cantilever. The cylindrical microlens at
the end of the cantilever focalizes the propagating light to
the fiber optics located at the output self-alignment system
connected to a power meter (Newport Power Meter, Model
1930F-SL). The relative optical losses are determined as a
function of the magnetic field.
RESULTS AND DISCUSSION
Mechanical characterization
For the mechanical characterization of the M-VOAs,
the deflection of the cantilever is measured by using the
setup described in the previous section. Figure 3 shows the
variation of the deflection with the applied magnetic field
for M-VOASLT and M-VOASLT+IPT containing similar FF
amount.
In all cases, deflection linearly increases with the
applied magnetic field until saturation around 0.29-0.57
kG. However, the deflection magnitude at saturation
depends on two factors: (i) the FF amount and (ii) on the
fabrication protocol. As expected, larger deflection
magnitudes are obtained when increasing the FF volume in
both M-VOASLT and M-VOASLT+IPT. When comparing
M-VOA with similar FF volumes, larger deflection
magnitudes are always obtained by M-VOASLT+IPT (see
Table 1). This result may be due to the different
distribution of FF in both structures. That is, whereas for
M-VOASLT FF is homogeneously distributed along the
cantilever waveguide, in the case of M-VOASLT+IPT, FF is
more concentrated in the free end of the cantilever
waveguide becoming more sensitive to the magnetic field.
(a)
0,36
0,32
4000 µm M-VOASLT VFF= 0,016 µl
4000 µm M-VOASLT+IJT VFF= 0,018 µl
∆y/∆y0 (mm)
0,28
0,24
0,20
∆y
= 0,162 mm
max
0,16
0,12
∆y
= 0,138 mm
max
0,08
0,04
0,0
0,1
0,2
0,3
0,4
0,5
0,6
B (kG)
Optical characterization
Optical characterization is performed as indicated in
previous section. The variation of the relative optical losses
with the applied magnetic field of both M-VOASLT and
M-VOASLT+IPT structures containing similar FF volumes
are represented in Figure 4. As for the mechanical
properties, in all cases the relative optical losses increase
with the applied magnetic field until saturation. Again,
larger relative optical losses are recorded when increasing
the FF volume in the M-VOA structure. Additionally,
M-VOASLT+IPT structures also show larger relative optical
losses when compared with M-VOASLT of similar FF
volume (Table 2).
(b)
(a)
-13
0,32
4000 µm M-VOASLT VFF= 0,025 µl
4000 µm M-VOASLT+IJT VFF= 0,026 µl
∆y/∆y0 (mm)
0,28
∆y
= 0,203 mm
max
0,24
0,20
0,16
∆y
= 0,156 mm
max
0,12
0,08
0,04
0,0
0,1
0,2
0,3
∆ROLmax=1,03 dB
-14
0,4
0,5
Relative optical losses (dB)
0,36
0,6
-15
-16
-23
∆ROLmax=0,67 dB
-24
-25
B (kG)
4000 µm M-VOASLT VFF= 0,016 µl
4000 µm M-VOASLT+IJT VFF= 0,018 µl
0,0
0,1
0,2
0,3
0,4
0,5
0,6
B (kG)
(c)
(b)
0,32
4000 µm M-VOASLT VFF= 0,033 µl
4000 µm M-VOASLT+IJT VFF= 0,030 µl
∆y/∆y0 (mm)
4000 µm M-VOASLT VFF= 0,025 µl
4000 µm M-VOASLT+IJT VFF= 0,026 µl
-13
0,28
0,24
∆ymax= 0,211 mm
0,20
0,16
∆ymax=0,168 mm
0,12
0,08
0,04
0,0
0,1
0,2
0,3
0,4
0,5
0,6
Relative optical losses (dB)
0,36
∆ROLmax=1,94 dB
-14
-15
-19
-20
B (kG)
∆ROLmax=1,03
0,0
0,1
0,2
0,3
0,4
0,5
0,6
B (kG)
Table 1. ∆ymax for M-VOASLT and VOASLT+IPT for different
FF volumes.
VFF (µl)
∆ymax (mm)
M-VOASLT
M-VOASLT+IPT
M-VOASLT
M-VOASLT+IPT
0.016
0.018
0.138
0.162
0.025
0.026
0.156
0.203
(c)
-12
4000 µm M-VOASLT VFF= 0,033 µl
4000 µm M-VOASLT+IJT VFF= 0,030 µl
-13
Relative optical losses (dB)
Figure 3: Variation of the deflection magnitude with the
applied magnetic field for similar FF volumes. (a) 0.016µl
and 0.018
 µl for M-VOASLT and M-VOASLT+IPT,
respectively. (b) 0.025 µl and 0.026 µl for M-VOASLT and
M-VOASLT+IPT, respectively. (c) 0.033µl and 0.030µl for
M-VOASLT and M-VOASLT+IPT.
-14
-15
∆ROLmax=3,46 dB
-16
-17
∆ROLmax=1,39 dB
-18
-19
0,0
0,1
0,2
0,3
0,4
0,5
0,6
B (kG)
0.033
0.030
0.168
0.211
Figure 4: Variation of the relative optical losses with the
applied magnetic field for similar FF volumes. (a) 0.016µl
and 0.018
 µl for M-VOASLT and M-VOASLT+IPT,
respectively. (b) 0.025 µl and 0.026 µl for M-VOASLT and
M-VOASLT+IPT, respectively. (c) 0.033µl and 0.030µl for
M-VOASLT and M-VOASLT+IPT.
Table 2. ∆ROL for M-VOASLT and VOASLT+IPT for different
FF volumes.
∆ROL (dB)
VFF (µl)
M-VOASLT
M-VOASLT+IPT
M-VOASLT
M-VOASLT+IPT
0.016
0.018
0.67
1.03
0.025
0.026
1.03
1.94
0.033
0.030
1.39
3.46
CONCLUSIONS
Two different fabrication protocols based on either
conventional SLT or the combination of SLT and IPT are
used for the fabrication of M-VOA using PDMS as
constituent material. PDMS doped with FF (M-PDMS) and
trapped FF microdrops are respectively responsible of the
magnetic properties of M-VOASLT and M-VOASLT+IPT.
M-VOASLT+IPT always show larger deflection magnitudes
and relative optical losses when compared with M-VOASLT
with a similar FF volume. The FF distribution, much more
concentrated to the free end of the waveguide cantilever,
seems to be the cause of the enhanced sensitivity recorded
by M-VOASLT+IPT.
ACKNOWLEDGEMENTS
The research leading to these results has received funding
from the European Research Council under the European
Community's
Seventh
Framework
Programme
(FP7/2007-2013) / ERC grant agreement n° 209243. The
authors would like to acknowledge the Ramon y Cajal
grant, the Ministerio de Educación, Cultura y Deportes for
the student mobility grant and the German Research
Foundation (DFG) for supporting this work in the
framework of the Collaborative Research Group
mikroPART FOR 856 (Microsystems for particulate
life-science products).
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CONTACT
*Sandra de Pedro, Dept. of Micro and Nano Systems,
CNM-IMB-CSIC
Barcelona,
Spain.
Tel:
+34-93-5947700-2128; Fax: +34-93-580-1496; E-mail:
sandra.depedro@imb-cnm.csic.es