MM4MPR Presentation

OPTIMISING CORE-CLAD RATIOS IN
GLASS EXTRUSIONS FOR OPTICAL FIBRE
APPLICATIONS
By 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 signal
Output 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)

Core
Clad
Die
Bobbin
Punch
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
Barrel
Barrel lining
T/C
Core
charge
Extrusion of stack
through a Ø4.76 mm die.
Clad
charge
Core charge
Clad charge
Die
(1) Barrel and die setup
(2) Extrusion flow
Furniss, D., Glass Extruder - Operating manual, University of Nottingham

Previous work – chalcogenide glasses
Two layer extrusion
• Excellent light guiding characteristics.
• Commonly used for monomode optical
fibres. But,
• Very expensive – limiting experimental work
which can be carried out.
0
20
40
60
80
100
30 80 130 180 230
Core-cladratio/%Area
Length along section/mm
W.H.C (94.03%)
S.D.S (89.44%)
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
Tsao, 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.
20.69mm
Die (Ø4.76 mm)
Bobbin
PMMA
charges
Barrel
Applied force from punch
Furnace
10mm
~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
20mm
9 mm
Thickness/mm
1.97
2.10
1.90
2.11
1.93
1.99
100 110 120 130 140
Viscosity/Pa.s/m
Temperature/°C
Clear PMMA
Blue PMMA
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
Silicate
molecule
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
0
20
40
60
80
100
20 60 100 140 180
Length along section (mm)
W.H.C (94.03%)
S.D.S (89.44%)
Borosillicate
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
100
0 20 40 60 80 100
Core-cladratio/%
Core 1
Core 2
Core 3
Core 4
Core 5
2 layer
stack
Core-clad ratio of six stack borosilicate extrusion vs length along section,
including two stack borosilicate extrusion.
Core
Core charge
height/mm
Clad charge
height/mm
Core height
percentage /%
Stable core-
clad ratio /%
1 15 3 83% 95
2 12 6 67% 89
3 9 9 50% 80
4 6 12 33% 68
5 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 4
Core 3
Core 2
Core 1
Core 5
Clad
Core
Core 4
Core 3
Core 2
Core 1
Core 5
Clad
Core
Core 4
Core 3
Core 2
Core 1
Core 5
Clad
Core 4
This equates
to a stable
core/clad
ratio of
~67%
~6mm~12mm

Core<Clad Core>Clad
Equal core and
clad charge
height
0
10
20
30
40
50
60
40
50
60
70
80
90
100
10 30 50 70 90
Coreentryposition/mm
Core charge height% of total stack (17.88 mm)
Core-clad ratio (Max) Core entry
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
Core
Max 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 …
𝜋𝐷 𝑝𝑟𝑒𝑓𝑜𝑟𝑚²
4
× 𝑙 𝑝𝑟𝑒𝑓𝑜𝑟𝑚 =
𝜋𝐷𝑓𝑖𝑏𝑟𝑒²
4
× 𝑙 𝑓𝑖𝑏𝑟𝑒
∴
4.792
× 16
1²
= 370 𝑚𝑚 𝑓𝑖𝑛𝑎𝑙 𝑓𝑖𝑏𝑟𝑒 𝑙𝑒𝑛𝑔𝑡ℎ
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
Core-cladratiobyarea

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.
0
20
40
60
80
100
20 60 100 140 180
Length along section (mm)
W.H.C (94.03%)
S.D.S (89.44%)
J.B
Borosillicate
Core-clad ratio of borosilicate extrusion vs
length along section, including data from
previous studies for comparison.
0
20
40
60
80
100
0 50 100
Core-cladratio/%
Core-clad ratio of six stack borosilicate
extrusion vs length along section, including
two stack borosilicate extrusion.
Core charge
Clad charge
Die
Static
material of
clad charge

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