Fabrication and Characterization of Flat Fibers

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Abstract-This paper reports the fabrication and preliminary characterization of optical Flat Fibers. Unlike normal optical fibers which are basically cylindrical.
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



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

and

J.K.

Sahu,

"Micromachined

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

(kPa) Core dimension

Webb

waveguide devices", Electronics Letters, Vol. 43, No.9, April 2007.

CORE DIMENSIONS Vacuum

AS.

S. Nagel et aI., "An overview of the modified chemical vapor deposition (MCVD) process and performance", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-30, No. 4, April 1982.

[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

J.

Lockwood,

Silicon

Photonics

II:

Components

and

Integration Volume 119 of Topics in Applied Physics, Springer, 2010.