MM4MPR Presentation

OPTIMISING CORE-CLAD RATIOS IN GLASS EXTRUSIONS FOR OPTICAL FIBRE APPLICATIONSBy Thomas Arnold

4th Year MEng (Materials stream)

Individual project MM4MPR

• Introduction to optical fibres

• Problem statement and objectives

• Overview of previous projects

• Polymer extrusion

• Borosilicate extrusion

• Analysis of results

• Conclusions

Optimising core-clad ratios, Thomas Arnold, 15/05/2014

Outline

Cladding (low refractive index)

Core (high refractive index)

Light

Cladding

Core Light pulse

Input signalOutput signal

Monomode optical fibre

Light redirected into the core.

• Difference in refractive index of cladding and core

• Total internal reflection

Monomode optical fibre

• A core-clad ratio of 60% is optimum


How do optical fibres work?

Multi to single mode…

Ebendorff-Heidepriem, Heike, and Tanya M. Monro. "Analysis of glass flow during extrusion of optical fibre preforms." Optical Materials Express 2.3 (2012)


CoreClad

Die

Bobbin

Pu

nch

Flow of charges

Extrusion

Core charge

Clad charge

Extrusion of the preform(1) (2)

Stable section cut from the extrudate, where the change in cladding thickness is minimal.

Total cross sectional area

Clad cross sectional area

Core-clad ratio (%) = Core area/Total area

Extrudate (3) Preform

Above Tg, the glass charges become viscous liquids and the core charge is forced into the clad charge.

Bhowmick, K., Morvan, H. P., Furniss, D., Seddon, A. B., & Benson, T. M. “Co‐Extrusion of Multilayer Glass Fibre‐Optic Preforms: Prediction of Layer Dimensions in the Extrudate.” Journal of the American Ceramic Society (2013)

~Ø5 mm

Problem statement

• Monomode optical fibres researched at the University of Nottingham are manufactured via extrusion.• This provides a ‘preform’ which is drawn again

to give the final fibre length and diameter.

• Methods for optimising the core clad-ratio of the preform is not fully understood.• This is largely due to the difficulty associated

with analysis of chalcogenide preforms and the expense in carrying out these extrusions.


Seddon, A.B. “Chalcogenide glasses: a review of their preparation, properties and applications.” Journal of Non-Crystalline Solids, 1995

Project objective• Find a suitable material to

replicate the extrusion process – clearly showing core and clad flow patterns.

• Perform two stack and six stack extrusions to understand the flow patterns of the core during extrusion.

• Post analysis of extrusions to determine a method for optimising the core-clad ratio (achieve 60%).


Extruder set-up• Extrusion of the

preform is carried out in a controlled environment The

assembly

Billet Load cell

Furnace

Punch

Cooling coils

Bobbin

BarrelBarrel lining

T/C

Core charge

Extrusion of stack through a Ø4.76 mm die.

Clad charge

Core chargeClad chargeDie

(1) Barrel and die setup

(2) Extrusion flow


Furniss, D., Glass Extruder - Operating manual, University of Nottingham

Previous work – chalcogenide glassesTwo layer extrusion

• Excellent light guiding characteristics.• Commonly used for monomode optical fibres.

But,• Very expensive – limiting experimental work

which can be carried out.

30 50 70 90 110 130 150 170 190 210 2300

20

40

60

80

100

W.H.C (94.03%)

S.D.S (89.44%)

Length along section/mm

Cor

e-cl

ad r

atio

/% A

rea

Three plots of “core-clad ratio vs. length along section” from previous work

Composition, Tg and viscosity effect the curve.


Savage, S. D., Miller, C. A., Furniss, D., & Seddon, A. B. “Extrusion of chalcogenide glass preforms and drawing to multimode optical fibres.” Journal of Non-Crystalline Solids, (2008).Choong, W.C., Seddon, A.B., “Monomode Mid-Infrared Fibre Optics.” University of Nottingham MM4MPR Paper. 2012/13.

S.D Savage W.H Choong

Composition of core Ge17As18Se65 As40Se60

Composition of clad

Ge17As18Se62S3 Ge10As21.4Se66.6

Viscosity (core/clad) /Pa.s

107.2/Not known(320°C)

107.4/107.8 (230°C)

Difference in Tg/°C

35 6

Replicating the extrusion

• Due to high cost of chalcogenide glasses, an alternative low cost glass was to be used for this project.

• Initially, a polymer material was selected.

• Following problems with the polymer extrusion, an oxide glass was melted for the extrusions in this project.

Polymer Tg/°C Tm/°C CTE/10-6°C-

1

Optical transmissibility

PMMA 100 - 122 250 - 260 70 -150 Excellent

PS 92 - 100 240 - 260 10 – 150 Excellent

PVC 80 180 - 210 50 Good

Selecting the polymer…

Optical quality necessary for post extrusion analysis.

Non toxic and heat resistant

Optimising core-clad ratios, Thomas Arnold, 15/05/2014Tsao, Chia-Wen, and Don L. DeVoe. "Bonding of thermoplastic polymer microfluidics." Microfluidics and Nanofluidics (2009)

PMMA extrusion – two stack

• The extrusion was carried out at 137°C (~15°C above Tg).

• The extrudate showed significant die swell.

• We were unable to mitigate this problem, thought to be due to the thermal expansion and elastic characteristics of polymers.20.69 m

m

Die (Ø4.76 mm)

Bobbin

PMMAcharges

Barrel

Applied force from punch

Furnace

Heat

10 m

m

~Ø14 mm

Extrusion of stack (24 mm in height) through a Ø4.76 mm die.

2 charges with identical geometry

The samples – transparent and blue charges

Extrusion set-up

The resultant extrudate


Den Doelder, C. F. J., and R. J. Koopmans. "The effect of molar mass distribution on extrudate swell of linear polymers." Journal of Non-Newtonian Fluid Mechanics (2008)

PMMA extrusion – six stack

20 mm

9 mm

Thickness/mm

1.97 2.10 1.90 2.11 1.93 1.99

100 105 110 115 120 125 130 135 140

Clear PMMA

Blue PMMA

Temperature/°C

Vis

cosi

ty/P

a.s/

m 4×107

4×106

4×105

The resultant extrudate• Die swell, contraction and

amalgamation of the PMMA layers was observed.

• Replication of fibre optic extrusion was not achieved.

Viscosity graph for the transparent and coloured PMMA samples

• Viscosity measurements showed clear differences between Tg values.

• Factor in the failure of the PMMA extrusion.


Ebendorff-Heidepriem, H., Monro, T. M., van Eijkelenborg, M. A “Extruded high-NA microstructured polymer optical fibre.”(2007).

Oxide glass – selection and batching

• Borosilicate was selected because of its high water solubility – allowing simple cleaning of components.

• High borate content – water solubility.

• Silicate – strengthening of the glass.

• Sodium dioxide – network modifier, to encourage chemical bonding between the borate and silicate.

Molecular structure of borosilicate

Borate

molecules

Sodium dioxide atoms

Oxygen atomsSilicon atoms

Boron atom

Sil

icat

e m

olec

ule


Glass samples melted

• 25g Batches of borosilicate were prepared with the composition: 5Na2O-75B2O3-20SiO2.

• This was divided into two melts, the second melt having 0.5wt% cobalt oxide added, giving a blue colour.

• Viscosity between glass melts was matched.

Manara, D., A. Grandjean, and D. R. Neuville. "Advances in understanding the structure of borosilicate glasses: A Raman spectroscopy study." American Mineralogist (2009)

Borosilicate extrusion – two stack

• The extrusion was carried out at a temperature of 535°C (above the Tg of borosilicate).

• The transparent borosilicate was the cladding charge and the cobalt oxide doped (blue) glass the core.


15 sections cut as shown for post processing

20 60 100 140 1800

20

40

60

80

100

W.H.C (94.03%)

S.D.S (89.44%)

Borosillicate

Length along section (mm)

Cor

e-cl

ad r

atio

/% A

rea

Two stack extrudate and cross sections

Core-clad ratio of borosilicate extrusion vs length along section, including data from previous studies for comparison.

Borosilicate extrusion – six stack


• Transparent and cobalt oxide doped glass showed clear core entry.

• Core entry for cores 1 –

5 was observed at 5.3, 14.3, 24.5, 39.8 and 58.5 mm respectively

Multi-stack extrudate

Cross section showing core entries

Multi-stack arrangement

Core 1

Clad

Core 3

Core 2

Core 5

Core 4

Alternating colours were used to allow cores to be identified in the preform.Each charge was ~3 mm in height.

Results


0 20 40 60 80 1000

20

40

60

80

100Core 1

Core 2

Core 3

Core 4

Core 5

2 layer stack


Cor

e-cl

ad r

atio

/%

Core-clad ratio of six stack borosilicate extrusion vs length along section, including two stack borosilicate extrusion.

CoreCore

charge height/mm

Clad charge

height/mm

Core height percentage /

%

Stable core-clad

ratio /%1 15 3 83% 952 12 6 67% 893 9 9 50% 804 6 12 33% 685 3 15 17% 44

1 2 3 4 5

Core 4 Core 3 Core 2 Core 1

Core 5

Clad

Core

Actual stack Equivalent stack


Core 5

Clad

Core



Core 5

Clad

Core



Core 5

Clad

Core 4


This equates to a stable core/clad ratio of ~67%

~6 mm

~12 mm

Core<Clad Core>Clad

Equal core and clad charge height

10 20 30 40 50 60 70 80 9040

50

60

70

80

90

100

0

10

20

30

40

50

60Core-clad ratio (Max)Polynomial (Core-clad ratio (Max))

Core charge height% of total stack (17.88 mm)

Cor

e-cl

ad r

atio

/% A

rea

Cor

e en

try

pos

itio

n/m

m

Further analysis A polynomial relationship exists between peak core-clad ratio and core entry with the absolute stack height.

From this, an exact core-clad ratio of 60% can be expected from a clad charge height of 12.6 mm and a core charge height of 5.4 mm

A stable length of ~16 mm can be interpolated from the table for a core-clad ratio of 60%


Peak core-clad ratio and entry of core along section of six stack borosilicate extrusion vs absolute stack height

CoreMax core/clad ratio/%

Position/mm

Stable length/mm

1 95.3 5.3 ~60.0

2 88.7 14.3 ~40.0

3 80.4 24.5 ~35.0

4 67.6 39.8 ~20.0

5 43.6 58.5 ~7.5

Core Core height percentage /%

Stable core-clad ratio /%

1 83% 95

2 67% 89

3 50% 80

4 33% 68

5 17% 44

Preform to fibre optic

If a fibre preform of Ø4.76 mm with a stable region of Ø16 mm is drawn to a fibre with a final of Ø1 mm …


Core

Clad16 mm

370 mm

Preform

Final fibre

• Extrusions to understand how to optimise core-clad ratios can be successfully carried out using borosilicate glasses.

• A polynomial relationship exists between peak core-clad ratio and core entry with the absolute stack height.o Further experiments are required with charges of

different geometries to validate the polynomial relationship established in this project.

• From this, an exact core-clad ratio of 60%:o A clad charge height 70% of the overall

stack height (12.6 mm). o A core charge height 30% of the overall

stack height (5.4 mm)• A resultant preform length of ~16 mm giving a final

fibre length of 370 mm.

Conclusions


↑Clad charge (thickness) Core charge ↓ (thickness)

Position along extrudate

Cor

e-cl

ad r

atio

by

area

Observations• Equal viscosities of core and clad

charges, leading to a steeper gradient, was validated.

• A core-clad ratio of ~67.6% with a stable length of ~20 mm is achievable, with a clad charge height of ~12 mm and a core charge height of ~6 mm.

• Reduction in core-clad ratio for cores 4 and 5 was observed following peak core-clad ratio.


20 60 100 140 1800

20

40

60

80

100

W.H.C (94.03%)S.D.S (89.44%)J.BBorosillicate

Length along section (mm)

Cor

e-cl

ad r

atio

/% A

rea

Core-clad ratio of borosilicate extrusion vs length along section, including data from previous studies for comparison.

0 20 40 60 80 1000

20

40

60

80

100


Cor

e-cl

ad r

atio

/%

Core-clad ratio of six stack borosilicate extrusion vs length along section, including two stack borosilicate extrusion.

Core chargeClad chargeDie

Static material of clad charge

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MM4MPR Presentation