Optical Structure with PDMS Microfibre for Displacement

Measurement

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.

1 INTRODUCTION

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,

2014).

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.

2 OPTICAL MICROFIBER MZI

FABRICATION

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

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

formation.

3 MACH-ZEHNDER

INTERFEROMETER

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

















2











cos













2









(1)

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.

4 DISPLACEMENT

MEASUREMENT

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.

5 CONCLUSIONS

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.

ACKNOWLEDGEMENTS

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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