Optical Structure with PDMS Microfibre for Displacement
Daniel Kacik and Ivan Martincek
Department of Physics, Faculty of Electrical Engineering, University of Zilina, Univerzitna 1, Zilina, Slovakia
Keywords: Microfiber, Mach-Zehnder Interferometer, PDMS, Displacement, Sensor.
Abstract: We proposed, prepared and demonstrated an optical structure consists of conventional single-mode optical
fibres and a bend PDMS microfiber. The structure forms Mach-Zehnder interferometer with variable length
of an air arm. The structure can be used for sensing of various physical quantities as temperature, humidity,
volatile organic compounds, etc. We demonstrated its usage by displacement measurement. For
determination of a displacement we used a change of interference pattern period. The sensitivity of
proposed structure is 0.027 nm/m.
Optical fibre interferometers are due to their very
high sensitivities and compactness often used for the
measurement of various physical quantities as the
temperature (Li, 2012), (Luo, 2015), the refractive
index (Tian, 2008), the pressure (Xu, 2012), etc.
For purpose of miniaturisation of optical fibre
interferometers it is possible to use a microfibre or a
nanofibre. An advantage of use such fibres is bend
non-sensitivity and a low bending loss due to their
large refractive indices contrast (Chen, 2013).
In recent years a several formation of
interferometers with microfibres have been
published. For example, Mach-Zehnder
interferometer (MZI) structure assembled from silica
and tellurite glass microfibre (Li, 2008) and MZI
structure made of the polymethylmethacrylate (Li,
Displacement fibre sensors can be performed in
different ways. It is often used sensors based on
fibre Bragg grating. A sensitivity of such sensor
proposed in (Shen, 2011) is 0.058 nm/mm in
displacement range of 0 - 20 mm. Another
possibility is the interferometric technique. In (Chen,
2013) a sensitivity of proposed the bent fibre Mach-
Zehnder interferometer structure is 0.835 nm/m in
range of 350 m.
In this manuscript we report a preparation of
polydimethylsiloxane (PDMS) microfiber which
forms one arm of Mach-Zehnder interferometer.
PDMS possess properties such a hydrophobility,
hydrolytic stability non-flammable, high chemical
stability, optically clear and its refractive index is
close to that of glass (Kacik, 2016).
Such optical structure we used for a displacement
determination. When a length of air arm is changed,
the period of interference pattern will change. So we
determine the structure response on the
displacement. The main advantage of such structure
is that consists adjustable air optical line in one arm
and microfibre in second. Such interferometer could
be used in applications where the measurands will
only affect the properties of microfibre (for example
for sensing of volatile organic compounds). A
disadvantage is its sensitivity to many various
physical quantities and the need to differentiate
between them.
For preparation of optical microfibre integrated
between two conventional single-mode optical fibres
(SMFs) we used liquid silicone Sylgard 184 (Dow
Corning) supplied as two-part liquid component kits.
After mixing the pre-polymer and curing agent at
ratio of 10:1, the prepared elastomer was applied on
the cleaved ends of conventional optical fibres
(Figure 1.a). Thereafter, we connected optical fibre
ends covered by PDMS (Figure 1.b). Then PDMS
Kacik D. and Martincek I.
Optical Structure with PDMS Microfibre for Displacement Measurement.
DOI: 10.5220/0006298603650368
2017 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
was cured at room temperature for about 10 hours.
After that time the PDMS elastomer achieved
suitable consistency for microfibre drawing. Axial
alignment of SMFs with PDMS joint was
mechanically adjusted by 3D microstages to obtain
maximal transmitted signal through the PDMS join
at wavelength 1550 nm. PDMS microfibre was
created by gradual distancing of single-mode fibres
ends (Figure 1.c) together with in-situ controlling
the axial position by signal level. When the length
and the diameter of microfibre was sufficient the
PDMS microfibre was cured at the room
temperature for another 40 hours. The prepared
optical microfiber had a diameter of 7 micrometres
and 270 micrometres in length. Then we decreased
the distance between SMFs ends what caused the
bend of microfiber (Figure 1.d).
Figure 1: Illustration of optical microfiber MZI
fabrication. a) PDMS deposition on SMFs ends. b)
connection of fibre ends. c) coaxial drawing. d) MZI
The light transmitted through the input fibre is
coupled to the optical microfibre and then recoupled
to the output fibre of the prepared structure. But for
suitable bend of microfibre it is possible to obtain
the state when the light transmitted through the SMF
is split at the end of input SMF covered by PDMS:
the part of light is still transmitted through the
microfibre and another part is propagated in coaxial
direction to core of input fibre (so the light
propagates in the air). By proper adjusting of output
fibre position it is possible to recoupled the light
transmitted through the air to the output fibre
(Figure 2).
Figure 2: Schematic of Mach-Zehnder interferometer
consists of air arm and optical microfiber in other arm.
If the optical path difference (between light
transmitted through the microfibre and air) is smaller
than coherence length of an optical source one can
observe the interference of light. Example of the
spectral dependence of the interference pattern is
shown in Figure 3.
Figure 3: Example of spectral dependence of interference
pattern of prepared microfibre MZI.
According to Figure 3 we assume that the
interference pattern is formed by two beams, one
guided through the microfibre and second one
through the air. For this case the output intensity can
be expressed as
where I
and I
are intensity of light that coupled to
the core of the output SMF from the microfibre and
from air arm, respectively,
is phase constant of the
beam (mode) guided through the microfibre, z
is the
length of the microfibre, z
is length of air arm, n is
refractive index of air and
is the wavelength of the
propagating light in vacuum.
Similarly, as it was mentioned before, the prepared
structure is sensitive to various physical quantities.
So, for the measurement of the displacement the
temperature, the pressure and the humidity in the
room were monitored and keep constant. For the
spectral dependence measurement of the interference
pattern of the prepared microfiber MZI the
broadband light source coupled to the fibre (SLED
Safibra OFLS-6) with central wavelength at 1500
nm and 100 nm spectral range was used. The output
SMF was connected to and the spectrum recorded by
an optical spectrum analyser (Anritsu MS9710B),
with a resolution of 0.07 nm. The structure
consisting from SMFs ends covered by PDMS and
optical microfibre was placed on differential
micrometre stages in order to adjust the relative
positions of SMFs and increasing the length of air
arm. The change of the length of air arm was
detected by inductance probe, which allow one to
distinguish variations of the length with resolution of
hundredth of a micrometre.
At first, the reproducibility was investigated. We set
the length of air arm to value 80 micrometres and
measured 10 times spectral dependences with time
difference about 5 minutes between each
measurement. The spectral shift of interference
pattern was observed. It could be caused by very
slightly change in temperature. But we were
interested if it also occurs the change in interference
pattern period. The determination of interference
pattern period was done for two neighbouring
maxims at (close to) wavelength 1550 nm. The
average value of period was 7.491 nm and its root
mean square was determined to 0.005 nm.
The structure is characterised by possibility to
change the length of air arm while the length of the
second arm is constant. For this reason the prepared
structure can be used for displacement measurement.
We were measured spectral dependencies of
interference pattern for lengths of the air arm from
45 m to 130 m. Example of spectral dependencies
(for the lengths of air arm 45 m, 60 m and 70 m)
in the investigated wavelength range from 1500 nm
to 1600 nm are shown in Figure 4.
As it can be seen from dependencies shown in
Figure 4 for particular length of air arm the power
level is changed. We assume that the intensity of
light propagated through the microfibre was the
same (for small change of length of air arm) but
what change was the intensity of light recoupled to
the output fibre due to change of the microfibre
bend. It means that there was a change in
emission/excitation of conditions. In addition, there
is also a change in period of interference pattern.
Figure 4: Spectral dependencies of interference pattern for
lengths of air arm 45 m, 60 m and 70 m. The spectral
dependencies are not corrected to spectral characteristic of
light source.
In Figure 5 there is shown the values of period of
interference pattern for particular length of air arm.
Determined values of period of interference pattern
obtained from measured spectral dependencies of
interference pattern can be fitted by linear function.
The function is also shown in Figure 5. There is
good correlation between determined values of
interference period and linear function. The
discrepancy could be caused by inaccurate
determination of maxims wavelength, small
fluctuation of temperature. Also it could be caused
by misalignment of input and output SMFs.
From fitting function it is possible to determine
sensitivity of the proposed structure. The period of
interference pattern changes the value 0.027 nm
when the length of air arm changes about 1 m. So
the resolution is 0.027 nm/m.
Figure 5: Determined values of interference pattern period
for particular length of air arm and its fitting function.
We prepared and demonstrated an optical structure
consists of conventional single-mode optical fibres
connected by bend optical PDMS microfibre. The
optical structure forms Mach-Zehnder interferometer
with variable lengths of air arm. The wavelengths
and distances of maximums and minimums of the
transmitted interference spectra depends on length of
air arm. For the operation of the sensor a broadband
light source can be used and an optical spectral
analyser. For determination of displacement we used
a change of the period of interference pattern. The
sensitivity of proposed structure is 0.027 nm/m.
The sensitivity could be improved by determination
of a phase of interference pattern instead of a period.
An advantage of such interferometer is its adjustable
air optical line. Such interferometer could be used in
applications where the measurands will only affect
the properties of microfibre (for example for sensing
of volatile organic compounds). A disadvantage of
the optical structure is its sensitivity to many
physical quantities.
This work was supported by Slovak National Grant
Agency No. VEGA 1/0491/14, 1/0278/15 and
Slovak Research and Development Agency under
the project No. APVV-0395-12, APVV-15-0441 and
the R&D operational program Centre of excellence
of power electronics systems and materials for their
components I. No. OPVaV-2008/2.1/01-SORO,
ITMS 26220120003 funded by European regional
development fund (ERDF) and the project ITMS
2610120021, co-funded from EU sources and
European Regional Development Fund.
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