Mechanical Characterisation of the Four Most Used Coating

Materials for Optical Fibres

Yazmin Padilla Michel

1,3

, Massimiliano Lucci

, Mauro Casalboni

Patrick Steglich

1,3

and Sigurd Schrader

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

Dept. of Physics, University of Rome “Tor Vergata”, Rome, Italy

Dept. of Industrial Engineering, University of Rome “Tor Vergata”, Rome, Italy

Keywords: Fibre Optics, Coating Materials, Young’s Modulus, Nanoindentation, Attenuation.

Abstract: Optical multimode fibres have a wide variety of applications ranging from industrial to medical use.

Therefore, even if they are just used as waveguides or sensors, it is important to characterise the whole

fingerprint, including the optical and mechanical properties of such fibres. Since the stiffness/elasticity of a

material could influence the optical output of a fibre due to micro-bendings, in this paper we report the

calculated Young’s Modulus of acrylate, fluorinated acrylate, polyimide and silicone, which are the four

most used coating materials for such optical components. The results demonstrate that Young’s Modulus

does have an impact on the attenuation of propagating light along the optical fibre. However, the refractive

index of the coating materials still has a significant impact on the performance of optical fibres.

1 INTRODUCTION

There is a large list of publications from a diverse

scientific area, explaining the relation between

fibre’s throughput and its deformation, e.g. torsion,

strain and bending (Gambling, et al. 1978;

Murakami and Tsuchiya, 1978; Valiente and

Vassallo. 1989; Badar, et al. 1991; Badar and

Maclean, 1991; Boechat, et al. 1991; Renner, 1992;

Faustini and Martini, 1997; Durana, et al. 2003;

Wang, Farrell and Freir, 2005; Kovacevic and

Nikezic, et al. 2006; Wang, et al. 2007; for

mentioning some of them). Fibre characterisation in

the literature has been performed by only

considering the core and cladding of the optical

fibre, while the influence of the coating (i.e. the

protective third polymeric layer of optical fibres) on

the light transmission in the optical fibres has been

fully ignored.

With the increasing number of applications of

fibre-based devices in the chemical industry,

spectroscopy, and life sciences (where the NIR

spectrum is of high importance); besides the

constant emergence of new materials; the

mechanical and optical characterisation of coating

materials is of great importance and must be

included into the theoretical models.

In 2002 Corning Incorporated recognised the

lack of public information about their fibre coatings

and released in a white paper the Young’s Modulus

and performance of their UV-cured dual-layer

optical fibre coatings and its performance (Mitra,

Kouzmina and Lopez, 2010). In Padilla Michel, et al

(2012), it is proved that there is a strong dependency

between coating materials and the optical output of

multimode fibres (MMF) optimised for VIS-NIR

spectrum. In the same proceeding it is concluded

that, regardless of the bending diameter, the

refractive index of the fibre coating is a determining

factor for fibre throughput. Their results also show

that when the coating refractive index is higher than

that of the cladding, the attenuation increases

considerably, while it is negligible when it is lower.

From the experiment reported in the present

paper, we have obtained the Young’s Modulus (E

)

of the same four coating materials tested by Padilla

Michel, et al. (2012), which will be useful to

understand and develop more realistic fibre optics

models. Furthermore, we have proved that the E-

value of the coatings affects the performance of the

optical fibres. However, the refractive index is a

dominant parameter which determines the

throughput of a coated MMF.

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

Mechanical Characterisation of the Four Most Used Coating Materials for Optical Fibres.

DOI: 10.5220/0005336700960102

In Proceedings of the 3rd International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2015), pages 96-102

ISBN: 978-989-758-092-5

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

2 MATERIALS AND METHODS

OF MEASUREMENT

For the present test, the following materials and

devices were used:

 A Nanoindenter (NanoTest from Nano

Materials Ltd., Wrexham, UK) shown in

Figure 1.

 5 fibre holders consisting of a 20 mm x

20 mm x 5 mm plate made of aluminium and

each of them has 5 grooves engraved (see

Figure 2).

 Fast curing glue (cyanoacrylate).

 5 aluminium posts (see Figure 1, right).

 5 fibre samples of each material listed in

Table 1. The length of each fibre sample is

1 cm (total 25 fibres samples of 1 cm length).

Table 1: Fibre parameters provided by FibreTech GmbH

(Berlin, Germany). The material of the fibre’s core was

fused silica and that of the cladding was fluorinated fused

silica. The refractive indices of the preform were given by

the manufacturer (Heraeus Suprasil

) at λ=600 nm.

Coating

Materials

Core

)

Clad

)

Coating

)

Core/Clad/Coating

outer diameters

[μm]

Single-coated

acrylate

1.458 1.442 1.49 200/240/380

Double-coated

acrylate

1.458 1.442

1.505 (I)

1.541 (II)

200/240/375

Fluorinated

acrylate

1.458 1.442 1.414 200/240/350

Polyimide 1.458 1.442 1.7 200/240/264

Silicone 1.458 1.442 1.409 200/240/306

Considering that the thickness of the studied

coatings in this paper range from 12 µm to 70 µm

and that the materials are elastic (E in the order of

MPa for polymers) compared to the cladding (71.7

GPa) the measurement of E was not straightforward

and several parameters had to be considered.

For the indenter, a Berkovich tip, i.e. a

three-sided diamond, was used. Thermal effects

were minimised using a temperature controller set at

23 °C. The Nanoindenter was mounted on an

anti-vibrational table to minimize the disturbances.

In Olson (2002), it is shown that the lower the E-

values to be measured, the greater the contact depth

should be. Hence for materials where E is in the

order of MPa, the indent should have at least 1 µm

of depth to consider the measurement to be accurate.

Also considering all the errors and trials reported in

Olson (2002), and the dimensions of the outer

diameter of our samples, we decided not to polish

the coatings (i.e. the side-polishing of the fibres), to

keep the coating surface as smooth and uniform as

possible.

Figure 1: On the left, a schematic showing the main

components of the Nanoindenter (the schematic was taken

from the NanoTest specifications brochure). On the right,

a picture of the machine with a mounted sample is shown.

Batches of five fibres of the same coating were

glued onto an aluminium plate (see Figure 2) using a

minimum amount of cyanoacrylate. Once the sample

was mounted on the XYZ-stage of the nanoindenter

(see Figure 1), it is important to carefully select the

area to be used for indentation, which for our

experiment is an array of 3x10 indents. The indents

were made very close to each other (separation of 15

µm), so that, the entire array was clearly focused

with the microscope. Thus, we minimise

miscalculations of the maximum depth of the

indents.

Figure 2: Schematic of the aluminium plate used for

holding the fibres and a cross-section of an optical fibre

mounted on a groove. The indentation area is on top of the

fibre centre.

After choosing the area for indentation, the

machine creates a “Contact Compliance vs Voltage”

curve to show where the inflexion point occurs. This

is an important parameter because it is taken as the

zero-point to measure the indent depth and calculate

its contact area. From the accuracy of this

measurement depends the over- or underestimation

of E. The measurements were repeated two or three

times and a total of 330 indents were done on the

samples mentioned in Table 1.

MechanicalCharacterisationoftheFourMostUsedCoatingMaterialsforOpticalFibres

2.1 Available Public Data

As mentioned in section 1, there are few

publications regarding E of the fibre’s coating

materials. Some available public data for acrylate

coatings are listed in Table 2. These data were used

as reference to verify the validity of our results.

Table 2: Published E-values of acrylate coatings.

Author

Young’s Modulus of double and single

coated acrylate [MPa]

Outer

Coating

Inner

Coating

Single

Coating

Mitra, et al. (2010) 650 – 950 1 – 1.7 ~ 1500

Olson (2002) 763 – 1043 3.29 – 5.06 36.8 – 60.4

3 ANALYSIS AND RESULTS

Every time a measurement is finished, the software

of the Nanoindenter (NanoTest NT1) shows an

indentation review with a “Load vs Depth” curve

together with the calculated Reduced Modulus (E

)

and other parameters such as Hardness (H), Elastic

Recovery, Contact Compliance and the respective

statistical errors.

The parameters H and E

are calculated from the

tangents slope of the resulting unloading curve. The

following equation development is taken from Olson

(2002). On one hand, H is defined as: H=P/A

where P is the Maximum Load and A

is the

Residual Indentation Area. For our measurements,

P=5±0.2 mN was used. On the other hand, the

quantity E

relates the E-values of the indenter and

the sample as follows:





















1













1







(1)

where ν is the Poisson’s Ratio, E

and E

are the E-

values of the sample and indenter, respectively.

From equation 1 we isolate E

, as follows:



















1

















1







(2)

The ν-values range normally between 0 and 0.50

(for majority of materials), being 0.50 the value for

perfectly elastic materials such as rubber, hence we

have taken ν=0.50 for silicone coating. The ν-values

of single coating acrylate (ν=0.49) and double

coating acrylate (ν=0.44) are taken by averaging the

values from Olson (2002). The value ν=0.34 of

polyimide was provided by the manufacturer

DuPont™. For fluorinated acrylate the used value is

ν=0.37. This procedure is to extrapolate E from E

Finally, from equation 2, we obtained the final E

of each material as shown in the following

subsections.

3.1 Double-coated Acrylate

Double coated acrylate coating is a typical UV-cured

dual layer of urethane-acrylate oligomers consisting

of a very elastic inner acrylate and a second outer

layer of stiffer acrylate (see Table 2). Since we have

not polished the coatings, only the E

of the outer

coating was measured. As shown in table 1, the fibre

manufacturer has provided only the specification of

the outer diameter of the fibre, while there is no

information about the inner coating diameter being

provided. Therefore we have made the very

superficial indents of approximately 1 µm of depth,

assuming that the outer coating thickness is much

thicker than 1 µm.

Figure 3: On the left, snapshot of the 20 best loading

curves. On the right, a picture of the area selected after

indentation, the material is so elastic that the indents are

not visible.

Figure 3 shows the results of the best

nanoindentations made on this coating. We

discarded those curves affected by error, i.e.

scratches or contaminants on the surface. The

average of the 20 selected curves (Figure 3, left) is

=2572±57 MPa and the average Maximum Depth

(MD) is 1507±27 nm, these values are comparable to

that given in Table 2. Substituting this value into

equation 2 we obtained E

=2079 MPa for the outer

acrylate coating.

3.2 Single-coated Acrylate

Single-coated acrylate is an epoxy acrylate (2-

phenoxyethyl acrylate), which is less stiff than the

outer coating acrylate mentioned in section 3.1, but

it must be approximately ten times stiffer than the

inner coating acrylate (based on Table 2). As well as

with the double coated acrylate, we have carried out

three series of measurements to ensure the

PHOTOPTICS2015-InternationalConferenceonPhotonics,OpticsandLaserTechnology

repeatability of our measurements.

Figure 4: On the left, snapshot of the 30 best loading

curves. On the right, a picture of the area selected after

nanoindentation, where indents are not visible.

The calculated E

of the curves on Figure 4 is

9±1 MPa at a MD of 25455±548 nm. Substituting

this value into equation 2 we obtain E

=7 MPa for

the single-coated acrylate.

3.3 Fluorinated Acrylate

Fluorinated acrylate is a fluorinated diacrylate

oligomer, which can be used either for coating or

cladding material of an optical fibre. This coating is

normally used for guiding high power lasers, and is

the only one compatible for medical applications,

according to the United States Pharmacopeia (USP).

Figure 5: On the left, snapshot of the 17 best loading

curves. On the right, a picture of the area selected after

nanoindentation.

In Figure 5 (right) we can see the indents, which

are tiny triangles inside the red rectangle. The fact

that the indentations could be marked, might

indicate that this material is less elastic than the

other two acrylates mentioned in sections 3.1 and

3.2; however the result does not confirm this

hypothesis.

In Figure 5 (left) the 17 best loading curves of

this material are presented. The calculated E

929±17 MPa at a MD of 2665±30 nm. By

substituting this E

value into equation 2 we obtain

=803 MPa for the fluorinated acrylate.

3.4 Polyimide

Polyimide (n-methyl-2-pyrrolidone) is a heat-cured

coating, unlike the acrylates which are UV-curing.

Since the heat temperature used for curing can vary,

any difference could change the mechanical/optical

properties of the cured coating; hence the final

product can be different from manufacturer to

manufacturer. As an example, the polyimide-coated

fibres from FiberTech GmbH (like our sample) have

a brilliant golden colour (see Figure 6). On the other

hand, the polyimide coating from Polymicro

Technologies™ has a toasted brown colour, and the

refractive index is 1.78. Therefore, the E

for this

coating is expected to be different.

Figure 6: Picture of the selected area for indentation of the

polyimide-coated fibre under test.

Noteworthy is that, even if this was the stiffest

material, curiously there were no visible indents

after the measurement.

Since polyimide coatings are usually thin (our

sample has 12 µm of thickness) and it is expected to

be the stiffest one, we made a shallower

nanoindentation with a MD=901±22 nm. In figure 7

the 20 best loading curves are shown with an

average E

=5470±205 MPa. Finally, from equation 2

we obtain E

=4861 MPa. This confirms that

polyimide is the stiffest coating material that was

tested in our experiments.

3.5 Silicone

Polydimethylsiloxane, also known as silicone, is a

very rubberlike material. Hence, the resulting

loading/unloading curve had a very small depth

difference (see Figure 8), contrary to the curves of

MechanicalCharacterisationoftheFourMostUsedCoatingMaterialsforOpticalFibres

the previous materials. In Figure 8, the 30 best

indents obtained from this material are shown. The

curves are together in a very well-defined group.

Since it is the most elastic coating among the all

mentioned materials in this paper; we increased the

depth of the nanoindentation to MD=21248±270 nm

and the average E

was 7 MPa.

Figure 7: The best 20 loading curves of polyimide coating

are shown.

Figure 8: The best 30 loading curves obtained from

silicone coating.

Finally, the obtained E

is 5 MPa, which is

surprisingly similar to that of the single-coated

acrylate, even if it feels gummy to the touch.

3.6 Comparing All Coatings

Table 3 presents a comparison between the best

results obtained from each coating. We have

confirmed that polyimide is the stiffest material of

the all tested materials while silicone is the most

elastic one. There are differences between the

existing data (Table 2) and our results (Table 3).

Table 3: Comparative table of the best results obtained

from Load/Unload curves and equation 2.

Material MD [nm] H [MPa] E

[MPa] E [MPa]

Double-coated

acrylate

1507±27 123±5 2572±57 2079±46

Single-coated

acrylate

25455±548 0.44±0.0 8.80±1.00 6.70±0.70

Fluorinated

acrylate

2665±30 37.7±1.0 929±17 803±15

Polyimide 901±22 393±24 5470±205 4861±182

Silicone 21248±270 1.1±0.01 6.60±0.01 4.90±0.01

For example, E of the double-coated acrylate

turns out to be twice than the reported value and the

single coating acrylate is five times lower than the

reported one. However, considering the high

sensitivity of the Nanoindenter, the limitations of the

method for measuring elastic materials of the order

of MPa (see section 2), and curing variations

between manufacturers (see section 3.4); the

agreement can be considered good and, therefore,

the measurements are reliable.

As mentioned in section 1, the purpose of this

experiment is to find out how the E-value and the

attenuation are related. Therefore, we have taken the

attenuation data from Padilla Michel, et al. (2012)

and our data from Table 3, the resulting graph is

shown in Figure 9. In agreement with

Padilla Michel, et al. (2012), we have noticed a

considerable difference between when the coating

refractive index is higher and lower than that of the

cladding, which forms two groups.

In order to see clearly how E, refractive index

and attenuation are related, in Figure 10 we present a

graph of the coatings refractive indices as a function

of the attenuation. In both graphs (Figures 9 and 10)

the group of the higher refractive index (right side)

and the group of lower refractive index (left side) are

marked. Although, for the group with lower

refractive index, E does not influences the

attenuation, for the group with higher refractive

index, it is observed that the higher value of E, leads

to the lower attenuation value. This is completely

contrary to our expectations.

PHOTOPTICS2015-InternationalConferenceonPhotonics,OpticsandLaserTechnology

100

Figure 9: Resulting E of the five coatings tested in this

experiment, compared with the attenuation values of

Padilla Michel, et al. (2012). The error bars are too small

(see Table 3) to be visible in the graph. Plotting E

instead

of E does not change the trend of the graph.

Figure 10: Refractive index versus attenuation. The data

were obtained from Padilla Michel, et al. (2012).

Figures 9 and 10 suggest that there are other side

effects produced by coating elasticity. Analysing Fig

10, indicates that when the coating material, like

acrylate, has a high refractive index value, and a low

value of E, micro-bendings are produced in the

cladding-coating interface leading to an increase in

the attenuation (private communication with Dr.

Roberto Montanari) while the fibre is bent as

reported in Padilla Michel, et al (2012).

4 DISCUSSION

The loading curves selected to obtain the E

reported

in Table 3, were those forming a well-defined group.

However, in some cases the loading curves from

different series of nanoindentation had a

considerable difference, even if the same procedure

was followed (see Figure 11). These curves were

discarded for the calculation of Er and the

measurements were repeated. There are different

factors that could contribute to these differences:

 The coating area used for indentation, was not

perfectly homogenous.

 The 3x10 array of indents could have been

made off-side the fibre centre.

 Since the cyanoacrylate used for gluing the

fibres and the coatings, both, are translucent; it

might be that some indentations were made on

an area with debris of glue, which could not

be noticed at the moment of selecting the area

for indentation.

Figure 11: On the left, loading curves of double coating

acrylate, showing a group of 10 indents dispersed from the

other group. On the right, loading curves of fluorinated

acrylate, showing some dispersed curves, as well.

As mentioned in section 3.6, since the micro-

bendings produced in the cladding-coating interface

increase the attenuation, this parameter should be

considered carefully in the case of acrylate.

Therefore an alternative method should be used in

order to calculate the attenuation without micro-

bending losses. As a good alternative we may feed

the optical fibres directly through the cladding, thus

some cladding modes will interact with the coating

without bending the fibre. This will be the next test

to corroborate our results and hypothesis.

5 CONCLUSIONS

In this paper, we presented the Young’s Modulus of

the four most used coating materials for NIR fibres.

As mentioned above, these mechanical properties

are seldom published by the manufacturers;

therefore, these results are helpful for a better

understanding of the optical fibres performance.

The principle of a waveguide is that the

surrounding media (in this case the cladding) must

have a refractive index lower than that of the

waveguide core in order to keep the modes within it.

However, when a waveguide is bent the high order

MechanicalCharacterisationoftheFourMostUsedCoatingMaterialsforOpticalFibres

101

modes leak into the cladding producing the so-called

bending losses. Like explained in Padilla Michel, et

al (2012), when the refractive index of the coatings

is lower than that of the cladding, the leaky modes

go back to the core, due to refractive index

differences between layers; therefore, the fibre

seems to be insensitive to bending losses

independently of the E of the coating. However,

when the coating refractive index is higher than that

of the cladding, the leaky modes are trapped in the

coating producing a fibre which is very sensitive to

bending losses. In this condition, the E of the coating

plays an important role in the so-called bending

losses.

ACKNOWLEDGEMENTS

The authors would like to thank the AiF Projekt

GmbH (project number KF2014158NT3), the

Interne ZV-Cooperation Rome project, for the funds

granted, and Dr. Mohamad Zoheidi for providing the

fibre samples.

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