Integrated Fibre Optics for Sensing based on Whispering-Gallery

Modes

Yazmin Padilla Michel

1,2

, Sigurd Schrader

, Mauro Casalboni

and Patrick Steglich

1,2

Faculty of Engineering and Natural Sciences, Technical University of Applied Sciences Wildau, Wildau, Germany

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

Padilla Michel Y., Schrader S., Casalboni M. and Steglich P..

Integrated Fibre Optics for Sensing based on Whispering-Gallery Modes.

 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.

IntegratedFibreOpticsforSensingbasedonWhispering-GalleryModes

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

eﬀects 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 diﬀerent 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

 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

(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.

IntegratedFibreOpticsforSensingbasedonWhispering-GalleryModes

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.

REFERENCES

Beier, H. T., Coté, G. L., and Meissner, K. E., 2010.

Modelling whispering gallery modes in quantum dot

embedded polystyrene microspheres. In J. Opt. Soc.

Am. B, Vol. 27, 536.

Brambilla, G., Xu, F., Horak, P., Jung, Y., Koizumi, F.,

Sessions, N. P., Koukharenko, E., Feng, X.,

Murungan, G. S., Wilkinson, J. S., and Richardson., D.

J., 2009. Optical fibre nanowires and microwires:

fabrication and applications. In Advances in optics and

photonics, Vol. 1, page 107.

Faustini, L., and Martini, G., 1997. Bend loss in single-

mode fibers. In IEEE Journal of Lightwave

Technology, Vol. 15, No. 4, page 34.

Flaim, T., Wang, Y., and Mercado, R., 2004. High

Refractive Index Polymer Coatings for

Optoelectronics Applications. In Proc. SPIE,

Advances in optical thin films, Vol. 5250, 423.

García-Lorenzo, B., Fuensalida, J. J., Rodriguez-

Hernández, M. A. C., Alonso, A., Barreto, M., Gracia-

Témich, F., Martín, Y., Rodríguez, L.F., Viera, T.,

Padilla-Michel, Y., Fernández, A., Escobar-Romero, J.

F. M., 2008. EDiFiSE: Equalized and diffraction-

limited field spectrograph experiment. In Proc. SPIE

7014, Ground-based and Airborne Instrumentation for

Astronomy II, 70144B.

Good, J. M., Hill, G. J., Mollison, N. T., Vattiat, B. L.,

Murphy, J. D., Kelz, A., Roth, M. M., MacQueen, P.

J., Rafal, M. D., Savage, R. D., Smith, M. P., Bayless,

A. J., 2008. Current status of the HETDEX fiber optic

support system. In Proc. SPIE 7014, Ground-based

and Airborne Instrumentation for Astronomy II,

70147L.

Harris, A. J., and. Castle, P. F., 1986. Bend loss

Measurements on high numerical aperture single-

mode fibers as a function of wavelength and bend

radius. In IEEE Journal of Lightwave Technology,

Vol. LT-4, No. 1, page 34.

Lagerholm, C., Kuntschner, H., Cappellari, M., Krajnovic,

D., McDermid, R. M., and Rejkuba., M., 2012. A way

to deal with the fringe-like pattern in VIMOS-IFU data

(Research Note). In A&A, Vol. 541, A28.

Matsko, A. B., Savchenkov, A. A., Strekalov, D.,

Ilchenko, V. S., Maleki L., 2005. Review of

applications of Whispering-gallery mode resonators in

photonics and nonlinear optics. In IPN Progress

Report 42-162.

Morgan, R., Barton, J. S., Harper, P. G., and. Jones, J. D.

C., 1990. Wavelength dependence of bending loss in

mono-mode optical fibers: effect of the fiber buffer

coating. In Optics Letters, Vol. 15, No. 17, page 947.

Nguyen, N. Q., Gupta, N., Ioppolo, T., Ötügen, M. V.,

2009. Whispering gallery mode-based micro-optical

sensors for structural health monitoring of composite

materials. In J Mater Sci, Vol. 44, 1560.

Padilla Michel, Y., Zoheidi, M., Roth, M. M., Haynes, R.,

Olaya, J.-C., 2012. Applied stress on coated

multimode optical fibres: a different point of view to

bending losses. In Proc. SPIE 8450, Modern

PHOTOPTICS2015-DoctoralConsortium

Technologies in Space- and Ground-based Telescopes

and Instrumentation II, 84503F.

Padilla-Michel, Y., Lucci, M., Casalboni, M., Steglich, P.,

in press (2015a). Mechanical characterisation of the

Four Most Used Coating Materials for Optical Fibres.

In PHOTOPTICS’15, SCITEPRESS.

Padilla-Michel, Y., Steglich, P., Schrader, S., Zoheidi, M.,

in press (2015b). Influence of coating materials and

cladding-to-core ratio on the transmittance of optical

multimode fibres. In nwk16, Tagung im BWV Berliner

Wissenschafts-Verlag.

Sharma, A., Powell-Friend, Y., George, T., and Philips. L.,

1998. Oscillatory light loss within the cladding of a

straight optical fiber: comparison with whispering-

gallery modes. In Optics Letters, Vol. 23, No. 4, page

262.

Su, D., Boechat, A. A. P., Jones, J. D. C., 1992. Beam

delivery by large-core fibers: Effect of launching

conditions on near-field output profile. In Applied

Optics, Vol. 31, No. 27, page 5816.

Sumetsky, M., 2008. Basic Elements for Microfiber

Photonics: Micro/Nanofibers and Microfiber Coil

resonators. In IEEE Journal of Lightwave Technology,

Vol. 26, No. 1, page 21.

Sumetsky, M., Dulashko, Y., Ghalmi, S., 2010a.

Fabrication of miniature optical fibre and microfiber

coils. In Optics and lasers in engineering, Vol. 48,

272.

Sumetsky, M., Dulashko, Y., Windeler. R. S., 2010b

Optical microbubble resonator. In Optics letters, Vol.

35, 898.

Vahala, K. J., 2003. Optical microcavities. In Nature, Vol.

424, 839.

Watts. M. R., 2010. Adiabatic microring resonators. In

Optics letters, Vol. 35, 3231.

IntegratedFibreOpticsforSensingbasedonWhispering-GalleryModes