Integrated Fibre Optics for Sensing based on Whispering-Gallery
Modes
Yazmin Padilla Michel
1,2
, Sigurd Schrader
1
, Mauro Casalboni
2
and Patrick Steglich
1,2
1
Faculty of Engineering and Natural Sciences, Technical University of Applied Sciences Wildau, Wildau, Germany
2
Dept. of Industrial Engineering, University of Rome “Tor Vergata”, Rome, Italy
1 RESEARCH PROBLEM
In the field of photonics, optical fibres have shown
to be a very versatile tool for multiple applications.
Due to their high transmission, single-mode (SMF)
and multimode fibres (MMF) have been widely used
for manufacturing devices useful for different areas.
Especially MMFs have been preferred for
assembling fibre bundles because of their relatively
easy light feeding and coupling compared to SMF;
in addition to their low cost and strength. These fibre
bundles manufactured with MMF are used for
spectroscopy in areas such as Astrophysics (García-
Lorenzo, et al 2008; Good, et al 2008) and Medicine
(for optical coherent tomography). In both fields,
having as lower losses as possible in a constantly
changing medium is mandatory.
When the modes going along the fibre core reach
the coating, due to bending losses or cladding
coupling, the transmission of standard SMF (Harris
& Castle, 1986; Faustini & Martini, 1997; Morgan,
et al 1990; Sharma, et al 1990) and MMF (Padilla-
Michel, et al in preparation) show an oscillatory
behaviour. This behaviour is comparable to the
Whispering-Gallery Modes (WGM) observed in
photonic nano-cavities such as micro-resonators
(Matsko, et al 2005). These WGMs have shown to
be a very efficient tool for multiple applications
covering from sensing (stress, temperature,
refractive index and pH) to filtering (photonic
filters).
WGMs in MMF have a double impact in the
performance of finished devices. On one hand, the
oscillatory effect on the spectrum has been an
undesirable instrument footprint, for applications
where a flat and stable reference spectrum is
required (e.g. Astrophysics, Medicine or Pharmacy).
On the other hand, with this thesis we want to prove
that the WGM produced in MMF can be the base for
a potential low-cost and profitable resonator. This
will be achieved exploiting the helicoidal modes
propagating along the coating of MMF, which
resemble the WGM observed in the spectrum of a
coil resonator.
2 OUTLINE OF OBJECTIVES
The present thesis project has two main objectives.
First objective is to provide a solution to the so-
called “fringing-like pattern” observed for example
in fibre bundles made with circular-core MMF
(Lagerholm, et al 2012). This will be achieved doing
a complete characterization of the most used coating
materials, including:
Bending losses (Padilla-Michel, et al 2012);
Intrinsic absorption bands of coating
materials;
Mechanical properties (accepted paper in
Photoptics 2015);
Effect of cladding-coating ratios (Presented
abstract in EOSAM 2014, and accepted
conference paper in NKW16 2015);
Modelling the mode propagation in squared
and circular fibre-core geometries.
These models will be of great interest to find the
optimal geometrical properties, eliminate the
fringing-like pattern mentioned formerly, and
improving the performance of the MMF used for
spectroscopy, tomography and laser surgery.
The second objective is to develop an
innovative, efficient and profitable resonator,
exploiting the WGM produced in MMF. This will be
achieved characterizing the physicochemical
properties of the coating material to produce a high
quality WGM-based resonator.
All these results will be very useful for a better
understanding of mode behaviour in MMF. For
example, modelling the fringing produced by the
modes propagation in an ideal MMF will be useful
for detecting coating-thickness variations, turning
into a quality test during the drawing of a MMF.
This model will also be useful for explaining and
solve the problem of fibre bundles performance such
68
Padilla Michel Y., Schrader S., Casalboni M. and Steglich P..
Integrated Fibre Optics for Sensing based on Whispering-Gallery Modes.
Copyright
c
2015 SCITEPRESS (Science and Technology Publications, Lda.)
as the already mentioned fringing-like pattern
reported in astronomical instruments such as the
VIMOS-IFU (Lagerholm, et al 2012).
3 STATE OF THE ART
3.1 Scientific Background
According to Harris & Castle, 1986; when a SMF is
bent, the fundamental mode is shifted outwards the
fibre core and the outer portion of the evanescent
field of this mode is guided through the cladding.
This radiation bounces in the cladding-air interface
and around the fibre bending, producing a WGM
(see Figure 1).
Figure 1: Schematic of a bent SMF showing the
generation of a WGM in the cladding. The schematic is
taken from Harris & Castle 1986.
When the WGM is in phase with the
fundamental mode, synchronized coupling occurs
producing a maximum in the transmission, and vice
versa.
Figure 2: Schematic of the WGM interference model from
Morgan et al 1990. The dashed line is the fundamental
mode traveling along the fibre core, the continues line is
radiation traveling from the core to the cladding until
strike the second coating-air interface, and then going
back to the cladding.
Morgan, et al 1990; noticed that when the
refractive index of the coating is higher than that of
the cladding, the evanescent field escaping from the
core reaches the coating producing a WGM
bouncing along the coating-air interface (see Figure
2).
In Sharma, et al 1998 it is reported the same
oscillatory effect in straight SMF. In their
experiment, the light source was coupled directly
into the cladding. Therefore, when the cladding
modes strike the cladding-coating interface these are
refracted into the coating, because the refractive
index of the cladding is lower than that of the
coating. Afterwards, the coating modes are reflected
from the coating-air interface and partially refracted
into the cladding (see Figure 3).
Figure 3: A ray-tracing model (taken from Sharma, et al
1998) which explains the spectral oscillations produced by
modes interference along the cladding.
In Sharma, et al 1998; is also showed that
removing the fibre coating, the oscillations are
vanished (see Figure 4), proving that the oscillatory
losses are produced in the coating.
Figure 4: The plot taken from Sharma, et al 1998 is
showing the spectrum of (a) the SMF with coating and (b)
without coating.
This model is reported as useful for measuring
the refractive index of the coating at different
wavelengths as well as for sensing applications.
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3.2 Existing Types of Resonators
In nature we can find WGM resonances in liquid
droplets, but there is also a variety of fabricated
resonators such as Fabry-Perot, micro-spheres, ring,
disk, cylindrical, capillary resonators and those
made based on optical fibre nanowires (for recent
reviews, see e.g. Vahala, 2003, Matsko, et al 2005
and Brambilla, et al 2009).
Knot, loop and coil resonators, which are of most
interest to this thesis project, are fabricated from
optical fibres nanowires or microwires. This type of
resonator produces the oscillatory losses exploiting
its large evanescent field. Since the diameter of the
nanowire is similar or even smaller than the
wavelength of the feeding light, the evanescent field
exceeds the physical boundary of the nanowire.
Therefore, when the nanowire is coiled (see Figure
5), the mode propagating along the wire interferes
with its own evanescent field, resulting in a
resonator
(Brambilla, et al 2009).
The input and
output couplings of these resonators are similar to
those of a SMF. There is a wide range of techniques
to manufacture nanowires, and every of them have
challenging problems
(Sumetsky, et al 2010a).
Figure 5: Variations of transmission of a micro-fibre coil
resonator immersed in water and ethanol. The inset is the
picture of the tested micro-fibre coil. On the right hand of
the picture is a close-up showing the shift of the
oscillations due to refractive index change of the measured
sample. Picture adapted from Sumetsky, et al 2010a.
In analogy to coil resonators, the WGM observed
in our MMF are produced when the modes trapped
in the fibre coating bounce in the same manner as in
a ring cavity, allowing the WGMs propagate along
this layer (see Figure 6).
As well as in Figure 5, the way that a MMF will
tell us if there is a change of stress or pH, it will be
with a shift of the oscillatory spectrum as shown in
Figure 7.
Up to the knowledge of the group, there is no
public literature reporting, characterizing or
modelling WGM in big MMF, i.e. with a core
diameter bigger than 50µm.
Figure 6: Schematics of helicoidal (skew) modes bouncing
along a fibre coating. Figure modified from Su, et al 1992.
Figure 7: The fringe-like pattern observed in the VIMOS-
IFU. On top, spectrum of one MMF clearly showing the
eects of the WGM (black). On bottom, the corresponding
normalized correction function for the same MMF (black).
In grey, it is shown the spectrum and corrections function
from the same MMF but observed in a dierent night,
meaning that the MMF is being stressed due to the rotation
of the telescope. The plot is taken from Lagerholm, et al
2012.
4 METHODOLOGY
As mentioned above, this thesis is divided in two
parts. The experiments conducted in this first part
will cover:
Characterisation of changes in the fibre
spectrum when using different type of light
(monochromatic and broadband) for feeding
fibres. This is to model and understand the
interference process into the fibre coating;
Transmission characterisation of square-core
fibres to be compared with the performance of
circular-core ones;
Spectral characterisation of different coating
materials;
Theoretical modelling to develop equations
that provide calibration curves for MMF
performance and devices made with them;
PHOTOPTICS2015-DoctoralConsortium
70
Method to minimize the modal dispersion
characteristic of MMF.
The experiments conducted in the second part of
the thesis will cover:
Identifications of the optimal coupling
parameters to excite WGM of high sensitivity;
Increase the quality Q factor by doping the
coating. In WGM-based sensors, the Q factor
can reach the order of 10
6
(Nguyen, et al
2009), producing very narrow peaks in the
transmission spectrum;
Characterisation of the sensitivity of our
WGM-based sensor under pH and stress
changes;
Modelling the sensitivity of the WGM-based
resonator.
Finally, we will make test of our first prototypes
in the laboratories of our industrial collaborator.
This will give us the main measurable and
quantifiable result of the project.
5 EXPECTED OUTCOME
5.1 Characterisation of Fibre Samples
The characterization tests will cover all the
mechanical and optical properties. The mechanical
properties tests will include environmental
sensitivity (stress, temperature and pH), geometry
(comparison between circular- and squared-core
fibres) and coating texture (scattering control).
Regarding optical properties tests, these will include
focal ratio degradation, butt-coupling (refractive
index), and transmission at different wavelength
ranges.
5.2 Modelling of Fibre Components
For the present thesis, several types of MMF will be
tested. On one hand, MMF with graded-index
cladding to avoid cladding modes reaching the
coating. As mention in section 3, coating modes are
responsible of the WGM effect; therefore these
fibres will have a flat and stable transmission
spectrum. These fibres will be used to manufacture
devices with high quality transmission and flat
spectrum.
On the other hand, we will develop special
MMFs to exploit the WGM effect. This will be
achieved using step-index MMF coated with a
polymer which refractive index must be as higher
than that of the cladding as possible. The goal is to
find the ideal refractive index difference between
cladding and coating (Flaim, et al 2004). These
fibres will be used as WGM-based resonators, for
sensing purposes.
During this phase the group will make the
corresponding simulations and modelling of the
different types of fibres using COMSOL
Multiphysics, which is a simulation software based
on finite element analysis. Afterwards our industrial
partner will take charge of the manufacturing of the
prototype.
5.3 Simulations of WGM Propagation
and Model Development
During this phase, we will develop a new model
based on the existing WGM theory to explain the
experimental results obtained from the fibres with
doped coating. This model will be a milestone for
the design and development of our MMF-based
resonator.
This model will also be of great importance for
all the research and industrial areas using MMF
coated with high refractive index. These types of
coatings are the most used in the industry because of
their stripping properties. Also, this will be the first
model explaining mode propagation in big-core
MMF carrying thousands of modes.
5.4 Design and Modelling of
MMF-based Resonator
Once the model obtained in section 5.3 give us the
bases to understand and control WGM, we will find
out the optimal fibre configuration to create our
MMF-based resonator. As well as in the case of a
coil or knot resonator, we will test different coil and
knot configurations (see Figure 8) in order to
increase the Q-factor and the finesse. Also we will
control the free spectral range, exploiting the
evanescent field of the fibres.
The advantage of our MMF-based resonator over
these micro resonators is the very low price, easy
manufacturing, in addition to its easy coupling due
to the big core of the fibres used.
5.5 Characterisation of the First
WGM-based Resonator
As well as in section 5.1, this MMF-based circuit
will be characterized mechanically and optically. We
will do the same test mentioned in that section.
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Figure 8: On top, example of a micro coil resonator taken
from Sumetzki, 2008. On bottom, picture of a knot
resonator made using an optical fibre microwire
(Brambilla, et al 2009).
6 STAGE OF THE RESEARCH
During the first year of this thesis project, several
characterisation tests on coating materials have been
done. Some of the results have been already
presented and published in conference papers (see
references of section 2).
Based on the applied stress results presented in
Padilla-Michel, et al 2012, we decided to make a
Mechanical characterization of the same four
coating materials reported in that paper. These
materials are: fluorinated acrylate, acrylate, silicone
and polyimide. The experiment consisted in
calculate the Young’s Modulus of these four coating
materials using a Nanoindenter (Padilla-Michel, et al
in press 2015a). Based on the obtained results, it is
also discussed the impact of the Young’s Modulus
of these coating materials on the attenuation. In the
same paper is also compared the impact of the
Young’s Modulus and the impact of the refractive
index of these coating materials on the fibre
performance.
We also carried out the spectral characterization
of the four coating materials in the near infrared
range (0.8 µm - 2.5 µm) using a Fourier Transform
spectrometer, optimised for NIR. The results of this
experiment are part of a paper in preparation
regarding WGM in MMF.
The next step of the thesis is the modelling of
modes propagation in the cladding-coating interface
using COMSOL Multyphysics. The results will be
compared with a ray tracing model made with the
optical design program ZEMAX.
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