Suspended Core Microstructured Polymer Optical Fibre:
Connecting to Reality
Richard Lwin1,2, Geoff Barton2, Trungta Keawfanapadol2, Maryanne Large1, Leon Poladian3,
Roger Tanner3, Martijn van Eijkelenborg1, and Shicheng Xue3
1. Optical Fibre Technology Centre, Australian Photonics CRC, University of Sydney,
206 National Innovation Centre, Australian Technology Park, Eveleigh NSW 1430, Australia,
Phone: +61 (02) 9351 1929, Fax: +61 (02) 93511911, [email protected]
2. School of Chemical Engineering, University of Sydney NSW, Australia
3. School of Mathematics and Statistics, University of Sydney NSW, Australia
4. School of Aerospace, Mechanical and Mechatronics Engineering, University of Sydney NSW,
Australia
Abstract
We use Suspended Core mPOF to experimentally test rheological predictions, and optically
characterize fibres in terms of numerical aperture and loss. This fibre has thin bridges supporting the
core, which help produce high NA, and low loss. We have obtained our lowest loss to date with this
design at 0.2dB/m – a figure competitive with conventional POF.
Introduction: Microstructured Polymer Optical Fibres (mPOFs), [1]-[3] are rapidly developing as
alternative to conventional polymer optical fibre. They are potentially, cheaper and simpler to make,
and offer a wide variety of new optical properties through tailorising the hole structure.
The draw process for these fibres however is critical, because deformation of the hole structure can lead
to a change in the optical properties. In order to study this, and allow a comparision of theoretical and
experimental results, we chose to fabricate simple Suspended Core or ‘Grapefruit’ fibres (Figure 1).
The simplicity of this structure makes it ideal for 3D viscoelastic rheological hole deformation analysis
during the necking down to fibre [4]. This has been important to fabrication, since a calibrated
rheological computational model of the draw process is required, in the aim to understand and predict
the behaviour of hole deformation and hence determine the initial hole array drilled in the preform for a
desired microstructure. The optical properties of such fibres have also been studied theoretically [5] and
their large holes make them ideal for microfluidics and bio-sensing applications [6].
This paper will reveal the development of Suspended Core mPOF, from the challenges during
fabrication, to the acceptance of the 3D rheological model.
Fabrication Challenge: Suspended Core
mPOF consists of a straightforward array of
large holes with thin bridges, which is
structurally weak for fabrication, since the
large air fraction leads to the natural
tendency for the microstructure to collapse
due to surface tension and viscous forces
[4]. Also the close proximity of the large
holes lends itself to a high degree of hole
size and shape deformation.
Initial Suspended Core mPOFs were drawn
Figure 1: SEM images of Suspended Core mPOFs.
a) Left fibre from old convective heating tower. b)
Right fibre from new radiative heating tower.
in the convective heating draw tower, which was unable to give radially isothermal heating [7]. The
large air fraction only heightened the temperature gradient, by insulating the core and thus restricting
deformation. The Scanning Electron Microscope (SEM) image in Figure 1a shows holes still close to
circular and the bridges only moderately thinning.
Suspended Core mPOF drawn in the new commercial draw tower uses radiative IR heating, which
produces a uniform temperature profile in the radial direction. With higher temperature at the core,
more substantial hole structure deformation is experienced, resulting in hole expansion and bridge
thinning (Figure 1b). Table 1 lists the bridge thickness for 400µm fibre drawn from both towers,
revealing the bridges for the new tower around 3 times thinner. In some cases sleeving of the fibre has
been used to increase the outer diameter [3].
Modeling Microstructure Behaviour: Recent investigation of mPOFs has centered at the behavior of
the hole structure deformation as the preform transforms into fibre in the neckdown region [4]. A
computational finite element analysis program, PolyFlow, has been used to predict both the neckdown
profile and the various forms of hole deformation, due to complicated
interactions between optimised fabrication conditions to the non-linear
viscoelastic polymer properties.
Figure 2: PolyFlow
model of Suspended Core
The uniform radial temperature profile associated with the radiative
heating means a radially isothermal profile – a more accurate axial
temperature profile of the furnace will be incorporated in later models.
The isothermal profile greatly improves the computational efficiency,
yet is still capable of determining the degree of deformation accurately.
As seen in Figure 2 and comparing against Figure 1b, employing an
isothermal profile to the Suspended Core mPOF model, agrees
qualitatively well to the real fibre hole deformation due to radiative
heating.
Optical Performance: The large air fraction gives Suspended Core mPOFs the potential of exceptional
loss and NA performance. For loss, the thin bridges reduce leakage through confinement losses to a
minimal value. Thin bridges are expected to create high NA. However it should be noted that the NA
for the simulation models consisted of bridges shaped as rectangular guides [5], rather than the circular
hole profile associated with Suspended Core mPOFs.
Suspended Core mPOFs produced from the
convective heating tower had relatively thick bridges,
so confinement losses became the dominant
mechanism. By contrast, the fibre from the new tower
had much thinner and straighter bridges, hence
restricting confinement losses. Figure 3 shows the
loss from the new tower being substantially lower
than from the old tower. As indicated in Table 1, the
loss for the new tower is over 20 times better than that
of the old tower at 650nm. However it is interesting to
note that slight increase in bridge thickness for fibre
of around 12% can produce losses almost 90% higher,
even though the fibre came from the same preform
and drawn under similar temperature and tension
conditions. The confinement loss argument is further
emphasised since the unusually high loss profile for
Figure 3: Loss curve for Suspended
Core MPOF drawn in different towers,
including conventional POF.
the old tower occurs at low wavelength, where the effects of confinement losses are most felt. These
loss figures are the lowest loss yet achieved for mPOFs, and a loss of 0.192dB/m for 650nm, makes
mPOFs serious competitors to conventional POFs. The loss of a conventional POF at 650nm is
0.15dB/m [8]. The relationship of bridge thickness and loss in these fibres is such that it strongly
suggests confinement loss is the dominant loss mechanism rather than surface scattering.
Table 1: The optical performance for Suspended Core MPOFs produced using the old convective and
new radiative furnaces
Min. Bridge
Loss (dB/m) @
Predicted NA @ Measured NA @
Thickness (µm)
650nm
650nm
650nm
1.79
3.85
0.23
0.18
Convective
0.55
0.192
0.38
0.27
Radiative e.g. 1
0.62
0.360
0.37
0.21
Radiative e.g. 2
Table 1 shows that using a Suspended Core mPOF of greater than 2m lengths, the measured NA varied
from the predicted high NA when just thin bridges were considered. Issues concern confinement loss
due to the single ring of holes. The hole geometry has an impact, since the fibre holes have rounded
profiles with thicknesses varying along the bridge length, which is in contrast to straight constant
thickness sides of waveguides used in the model. The latter factor is evident since the discrepancies in
NA, has occurred for fibre from both towers, even though the bridges in the new tower are much
straighter.
Conclusion: Suspended Core mPOFs have been useful in attaining good qualitative agreement for
rheological modeling, in the future aim to have a simulation model that predicts hole deformation
behaviour prior to designing the microstructure array. This fibre has proved that it is acceptable to
employ an isothermal profile for the model, which helps the computational efficiency. For NA, the
measurements have varied from the model, however differences between the geometry of the real holes
compared to the model have been the main reasons for this discrepancy. With regards to loss though,
the thin bridges of Suspended Core mPOFs significantly reduced mPOF loss to now be competitive
with conventional POF.
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