Fabrication and Characterization of Flat Fibers l2 13 l24 l K.D. Dambul , , N. Tamchek , , , S.R. Sandoghchi \ M.R. Abu Hassan \ D.C. Tee and F.R. Mahamd Adikan , I
Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia 2 Photonic Research Centre, University of Malaya, 50603 Kuala Lumpur, Malaysia 3 Faculty of Engineering, Multimedia University, 63100 Cyberjaya, Selangor, Malaysia 4 Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia Email:
[email protected]
Abstract- This
paper
reports
the
fabrication
and
II.
preliminary characterization of optical Flat Fibers. Unlike normal
optical
waveguides, opening
the
Flat
fibers
which
Fibers
are
possibility
of
are
basically
ribbon-like
extended
cylindrical
planar
length,
fully
samples, flexible
A.
FABRICATION OF THE FLAT FIBER
Fabrication of the Flat Fiber preform
substrates. The fabrication of the Flat Fiber is performed by
The first stage of the Flat Fiber fabrication is the fabrication
applying vacuum to a hollow silica preform during the fiber
of the Flat Fiber preform that contains both the core and
drawing process. Different vacuum settings are used in order to determine the optimum conditious. Other parameters that affect the fiber drawing process are preform feed speed and fiber drawing speed. Preliminary geometrical characterization
cladding
materials.
The
preform
is
fabricated
using
Modified Vapour Deposition Method (MCVD) to ensure that a high quality Flat Fiber is formed. During the MCVD
results confirmed that the experiment (as a proof of principle)
process,
successfully produced Flat Fibers. Future work includes more
inside a rotating silica tube as it is heated by an external heat
detailed characterization such as determining the optical, mechanical and transmission characteristics of the Flat Fiber.
high temperature oxidation of reagents occurs
source [8]. The heat source, which consists of hydrogen and oxygen flames (with a temperature of 2000 DC) is moved from one end of the silica tube to the other end. This causes
I.
a build-up of soot inside the tube. A thin cladding layer is
INTRODUCTION
first deposited inside the tube to prevent diffusion of
The rapid development of optical communication systems
impurities such as OH [8]. Next, the core material is
has resulted in a growing demand for cheap and reliable
deposited.
integrated optical devices such as splitters, couplers and multiplexers
[1]. In recent years,
improved fabrication
In a normal MCVD process, the silica tube and the core and
methods for integrated optical devices have been reported
cladding deposit are heated again to a temperature that is
[1]. However, most integrated optical devices are commonly
high enough to soften the substrate tube [8]. This results in a
built on rigid substrates such as silicon wafers, which are
collapsed, solid rod which is called a preform. However, for
not mechanically flexible and limited in terms of its length.
a Flat Fiber preform, the process of collapsing the silica tube
Moreover, these optical devices need to be pigtailed to
together with the core and cladding deposit is omitted. Thus,
optical fibers and this incurs unwanted loss. A fully
the Flat Fiber preform is a hollow silica tube that contains
functional integrated optical chip that is fiber-like and
the core and cladding deposit.
mechanically
flexible
has
many
potential
advantages,
particularly in applications such as optoelectronics sensors
B.
Drawing of the Flat Fiber
[2]. This is one of the main motivations for the research on Flat Fibers.
The final stage of the Flat Fiber fabrication is the drawing or pulling of the Flat Fiber preform. Using a conventional fiber
Flat Fibers are ribbon-like planar samples, opening the
drawing tower, the lower end of the preform is placed in a
possibility of extended length, fully flexible substrates. It is
cylindrical heating furnace with a temperature of 2100 DC.
a novel and patented [3] technology that aims to combine
As the temperature increases, it will soften the preform and
the structural advantages of optical fiber and the functional
the melting preform will start to drop. Fiber is then drawn
benefits of planar devices [4]. Not only does the Flat Fiber
from
have a low
also be
stabilized, vacuum is introduced to the upper end of the
potentially used to fabricate multiple functional optical
preform. As a result of the vacuum pressure, the preform is
components.
collapsed into a planar format. The Flat Fiber, which has a
loss over long distances, In
[5],
a
lx3
splitter
it can
device
has
been
demonstrated using a precision micromachining approach
the
furnace
and
once
the
drawing
process
has
bean-shaped profile or cross section, is then produced.
on a Flat Fiber substrate. Bragg gratings [6], waveguide channels [6] and an evanescent field sensor [7] have also
Inside the heating furnace,
been developed and demonstrated using a combination of
heated and drawn along its axial direction [9, 10]. Applying
Flat Fiber substrate and direct-UV writing technology.
the law of conservation of mass, with a known preform feed
978-1-61284-264-6/11/$26.00 ©2011
IEEE
the preform is peripherally
speed (speed of the preform entering the furnace) and a known fiber drawing speed (speed of the fiber exiting the furnace),
the
determined
nominal
diameter
of
the
fiber
can
be
[9]. Assuming that the process occurs at steady [11]:
state, the governing equation for mass is defined as o(pu) + OZ
� o(prv) r
=
or
0
Results and discussions
D.
The following results were obtained from the fabrication of the Flat Fiber, performed in the Flat Fiber Laboratory, Department of
Electrical Engineering, Faculty of Engineering, University of Malaya, Malaysia.
(1)
0.18
____ Highest --e-- A",rage
0.16 -
where
r is the radial coordinate or radius (in m), u is the
axial velocity component (m/s),
is the radial velocity
v
component, z is the axial coordinate (m) and p is the density 3 (kg/m ). The governing equations for momentum and energy transport can be found in
[11].
E .s
�
� �
0.140.120.1
"0
�
0.08-
As the preform proceeds through the heating zone of the
g
0.06
furnace, its temperature and viscosity varies
'"
"
[9] and its
shape narrows down sharply to yield a neck-down profile
e
i§
0.04 0.0
[10]. The neck-down profile is affected by the fiber-drawing conditions and it also impacts the diameter uniformity, strength and transmission loss of the fiber
o ------�----�--� o 0.5 1.5 2 2.5
[9,10].
Vacuum (kPa)
Fig. 1. Difference in diameter (mm) versus vacuum (kPa)
The necking process starts when the surface temperature of the preform is higher than the softening point of the material
[12]. For a fixed necking shape, the surface temperature [12]. It
0.4 ---*""-
decreases as the external gas flow velocity increases
should be noted that a change of a few degrees in surface
0.35
temperature can significantly affect the viscosity of the preform and the gas flow
[12]. Further analysis on the neck [9-12].
down profile can be found in C.
E .s
0.3
Ii; 1ii
Experimental setup
Table
�
0.25-
1 shows the experimental parameters for the
fabrication of the Flat Fiber.
0.2-
In this experiment, Flat Fibers are drawn using different vacuum
pressures
in
order to
determine the optimum
o
vacuum pressure.
,
,
0.5
1.5
Vacuum (kPa)
2
2.5
Fig. 2. Diameter (mm) versus vacuum (kPa)
Based on the measurement unit (Zumbach unit) of the fiber drawing tower, for the proposed Flat Fiber dimension, the width is defined by the y diameter and the height is defined by the
A\Srage Diameter y
--e-- A\erage Diameter x
x diameter.
Fig.
1 and Fig. 2 show that the fiber becomes collapsed once
vacuum is applied to the hollow preform. A collapsed fiber or a bean-shaped fiber can be determined by the large difference between the
x and y diameters. The average x and y diameters increases as the vacuum pressure increases to 2 kPa. However, it is observed that at a vacuum pressure of 2.5 kPa, the difference between difference between the
TABLE I EXPERIMENTAL PARAMETERS
the two diameters decreases. This can be due to several Parameter Vacuum pressure Preform size
reasons.
Value
0,1,1.5,2.0 and 2.5 kPa 60 cm Outer diameter 25 mm 0.025 mmlmin 1 m/min 2100°C 300 flIll (width) x 125 flIll Length
=
=
Preform feed speed Fiber drawing speed Furnace temperature Proposed dimension of Flat Fiber
(height)
At the time of the experiment, the neck-down profile of the preform, which occurs inside the heating furnace, cannot be observed. It may be possible that in the neck-down region, certain region of the fiber had collapsed before it was pulled. This premature collapse could be due to the interaction between the various parameters affecting the neck-down region,
such
distribution
as
a
non-uniform
surface
temperature
[12]. This could cause an accumulation of the
collapsed region at the preform tip, which could lead to non uniformity in the fiber collapse. At this point, the vacuum
pressure values could have also been averaged, and cause a
0.16 ---�----�---.
circular collapse at the vacuum pressure of 2.5 kPa.
0.14 -
The following force balance equation describes the forces that act on the fiber during the fiber drawing process [12]:
«
0.04 0.0 o
Equation (2) shows that when the preform is fully collapsed and the hollow tube is sealed, the vacuum pressure could produce a pulling effect in the opposite direction. This force increases as the vacuum pressure increases. Thus, it is important that the preform speed rate and fiber drawing
o
0.5
2
1.5 Vacuum (kPa)
2.5
Fig. 4. Average feed rate (mm1min) versus vacuum (kPa)
III.
CHARACTERIZATION OF THE FLAT FIBER
speed are controlled correctly at high vacuum pressure, to ensure that the fiber diameter does not decrease and become prone to breakage. For a normal optical fiber drawing, the fiber diameter decreases as the drawing speed increases [11]. This is because as the draw speed increases, the viscosity decreases due to a lower glass temperature (since the preform spends less time inside the furnace)
and the neck-down rate
(m
(I)
increases [11]. However, in the fabrication of Flat Fibers, the fiber diameter is not only a function of the drawing speed, but is also a function of the vacuum pressure. Fig. 2 shows that the fiber diameter decreases as the vacuum pressure increases. Fig. 3 and Fig. 4 show that the fiber drawing speed is inversely proportional to the preform feed rate.
Further
research
work
will
be
done
to
further
investigate these effects.
(III)
(N)
Fig. 5: Cross section of fiber (magnification of 5x) for vacuum settings of 0, 1, 1.5 and 2.0 shown in Fig. 5 (I) - (N) respectively
0.4!*----�---�--____,
Fiber drawing speed (mlmin) 0.35
(I)
(II)
0.3
0.25 0L-
-0�.5
--
� Vacuum
---
1.� 5
---
-'2
---
�2.5
---
Fig. 3. Fiber drawing speed (m/min) versus vacuum (kPa)
(llI)
(N)
Fig. 6: Cross section of fiber (magnification of lOx) for vacuum settings of 0, 1, 1.5 and 2.0 shown in Fig. 6 (I) - (N) respectively
Fig. 5 and Fig. 6 confirm that the application of vacuum to the hollow preform during the fiber drawing process has caused the fiber to collapse, thus producing Flat Fibers. Geometrical measurements of the samples using a microscope were undertaken and compared with the dimension measurements from the Zumbach unit. The comparison shows a reasonable match between the two measurements, with the average percentage error of 8%. From Fig. 6, the core dimensions are also measured. The results are shown in Table 2. In this experiment, it shows that the core dimensions of a useful operating and working region can be produced by the Flat Fiber at a vacuum pressure of 1.5 kPa, where the ratio of the core dimensions (x and y) is 5.2. In some arrayed waveguide grating filters, 2 waveguide core dimension of 750 x 200 nm (with a core dimension ratio of 3. 75) is used to reduce phase errors caused by the variation of the core width [13].
IV.
CONCLUSION AND FuTURE WORK
Preliminary geometrical characterization results confirmed that the experiment (as a proof of principle) successfully produced Flat Fibers. Future work includes more detailed characterization such as determining the optical, mechanical and transmission characteristics of the Flat Fiber. ACKNOWLEDGMENT
The authors would like to express their gratitude to TM R&D (MCVD lab) for the fabrication of the Flat Fiber preform. REFERENCES [1]
R. G. Hunsperger, Integrated Optics Theory and Technology, Sixth
[2]
The Engineer, pp. 12, 12-25 March 2007.
Edition, Springer, 2009. [3]
W02008/035067 Al A Method of Fabricating a Planar Substrate having
Waveguide
Channels
(The
Flat
Fibre
Technology);
PCTGB2007003552 Method of Fabricating a Planar Substrate having Optical Waveguides.
In conclusion, the experiment as a proof of principle has successfully produced Flat Fibers.
[4]
'Flat Fibre: The Best of Both Worlds', Materials World, May 2007.
[5]
S. Ambran, C. Holmes, J.C. Gates, AS. Webb, F.R. Mahamd Adikan, P.G.R.
Smith
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J.K.
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Multimode
Interference Device in Flat-fiber", Photonics Global Conference, Singapore, 14-16 December 2010. [6]
TABLE 2 Measured from microscope images
[7]
lOx magnification
(in�)
(x)
et.
al,
"MCVD
planar
substrates
for
UV-written
sensing in novel flat fiber," wsers and Electro-Optics, 2008 and 2008 Conference on Quantum Electronics and wser Science. CLEOIQELS 2008. Conference on , vol., no., pp.I-2, 4-9 May 2008.
Core dimension
(y)
[8]
(in�) 0
75
68
1.0
132
125
1.5
130
25
2.0
140
12
2.5
90
4
Holmes, C.; Adikan, F.R.M.; Webb, AS.; Gates, J.C.; Gawith, C.B.E.; Sahu, J.K.; Smith, P.G.R.; Payne, D.N.; , "Evanescent field
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Webb
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CORE DIMENSIONS Vacuum
AS.
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[9]
Paek, U. C. and Runk, R. B., "Physical behavior of the neck-down region during furnace drawing of silica fibers," Journal of Applied Physics, vo1.49, no.8, pp.4417-4422, Aug 1978.
[10] Xu Cheng and Yogesh Jaluria, "Feasibility of High Speed Furnace Drawing of Optical Fibers", Journal of Heat Transfer, 126, 852, 2004. [11] Mawardi, A and Pitchumani, R., "Optical Fiber Drawing Process Model Using an Analytical Neck-Down Profile," Photonics Journal, IEEE, vol.2, noA, pp.620-629, Aug. 2010. [12] Roy Choudhury, S., and Jaluria, Y., "Practical Aspects in the Drawing of an Optical Fiber," Journal of Materials Research, 13, pp. 483-493, 1998. [13] David
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