Benchtop Mini Preform Fabrication for Specialty Optical Fibers

W. J. Lai

, L. Zhang

, V. J. J. Yeo

, D. J. M. Ho

and C. H. Tse

Temasek Laboratories, Nanyang Technological University, 50 Nanyang Drive, Singapore 637553

The Photonics Institute, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798

Keywords: Fiber Optics, Fiber Fabrication, Rare Earth Elements.

Abstract: We propose and demonstrate a benchtop version of the mini preform and hence short length fiber fabrication

system. The system is compact, low cost, fast and flexible compared to the standard fabrication systems. We

mimic the recipe used in standard Modified Chemical Vapour Deposition (MCVD) preform fabrication

process. Incorporating with solution doping technique, we have fabricated several short length rare earth

doped silica fibers, including Ytterbium and Erbium. The results obtained serve as a good indication on the

composition of the rare earth elements to be used in the standard processes. The technique is promising and

suitable for rapid specialty optical fiber prototyping.

1 INTRODUCTION

There are several methods currently available for the

fabrication of optical fiber preforms, which can be

broadly classified as either vapor or non-vapor based.

The vapor based methods include vapor axial

deposition (VAD) (Izawa, 2000), outside vapor

deposition (OVD) (Petit et al., 2010), (Blankenship

and Deneka, 1982), (Cho et al., 1998) and modified

chemical vapor deposition (MCVD) (Nagel et al.,

1982). The non-vapor based methods are sol-gel

processing (Matejec et al., 1997), powder sintering

(Auguste et al., 2014), and direct nano-particle

deposition (DND) (Tammela et al., 2002). Among

these, MCVD process is well established and

regularly being employed even for commercial

optical fiber production. The process involves passing

a mixture of gases through a rotating glass tube which

is continuously heated at the surface via a moving

burner. Chemical reactions in the gas results in a layer

of fine soot being formed on the inner surface of the

tube. This subsequently sintered into a clear glass

layer, and finally collapsed into the preform to be

used for fiber fabrication. High precision of this

process enables the production of high quality

preforms and fibers. However, it often requires many

iterations in order to arrive at the intended fiber

design, which increases the cost and time needed for

a successful fabrication. It is not surprising that

sometimes 30 – 50 iterations may be required for a

complete fruitful fabrication. Simply put, preforms

obtained from the earliest iterations are often

unsuitable for applications and therefore wasted.

Realistically, the early stages of fiber research

involve experimenting with different glass hosts,

dopants, mixture of dopants, doping concentrations,

geometries, etc. and only require small volumes of

fiber. In this work, we propose and demonstrate a

scaled-down version of the standard fabrication

process for making the mini preforms that can be

drawn into short length optical fibers. The short

length optical fibers produced are suitable for rapid

prototyping, testing and the first order process

optimization.

The developed setup, which is a large diameter

fiber splicer convertible; is compact and able to

reduce the time and cost investment compared to the

existing standard fiber fabrication processes. To

elaborate, the conventional process yields a 12 mm

thick, 300 – 400 mm long silica preform that is used

to draw into the required fiber. A single iteration from

preform fabrication to fiber pulling takes

approximately 2 working days. In contrast, our

approach produces ~ 1 mm thick, ~ 20 mm long

preforms, which can be drawn into sub-meter length

of fibers sufficient for initial testing and analysis. A

single iteration of this process takes about 2 – 3 hours.

More significantly, the proposed system and process

facilitates the analysis and optimization of initial

iterations for standard preform fabrications.

Conventional preform fabrications can therefore be

shortened to reduce excess time, cost and materials

Lai, W., Zhang, L., Yeo, V., Ho, D. and Tse, C.

Benchtop Mini Preform Fabrication for Specialty Optical Fibers.

DOI: 10.5220/0006614101850189

In Proceedings of the 6th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2018), pages 185-189

ISBN: 978-989-758-286-8

185

for greater research efficacy. We report in this paper

our initial success in fabricating centimeter-long 400

µm size Ytterbium and Erbium doped silica fibers by

using the proposed technique. A short comparison

between the existing fiber fabrication process and our

proposed technique is listed in Table 1.

Table 1: A comparison between the fabrication techniques.

Parameters

Existing

Technique

Proposed

Technique

Heat source

Oxy-

hydrogen

burner

Multi-

electrode

plasma

Isothermic heat zone

Rotating

tube

Ring of fire

Process time

Long

Short

Preform diameter

≥ 12 mm

≤ 2.0 mm, 1

mm for this

study

Fiber length

Long

Short

Precision

High

Low to

moderate

Background loss

Low

Moderate to

high

Cost per iteration

High

Low

Physical footprint

Lab-sized

and multi-

storey high

Benchtop

and modular

units

2 FABRICATION SYSTEM

We converted our existing three-electrode large

diameter splicer (3SAE-LDS) into the main

workhorse for this work. In principle, the proposed

method is not limited to multi-electrode plasma

system, it can also be applied to graphite-based or

laser based large diameter fiber splicers.

2.1 System Description

The system consists of a three-electrode plasma

discharge that provides a narrow isothermic heat zone

around the circumference of the intended mini

preform. This allows for directed heating of the

preform. Instead of rotating the glass tube to achieve

uniform heating as in the standard process, we create

a ‘Ring of Fire’ around it. Tungsten electrodes, with

melting temperature of 3422 C are selected to give a

maximum processing temperature of about 3000 C,

which is sufficient and comparable to the oxy-

hydrogen burner used in the MCVD lathe. Three

electrodes are orientated in a “Y” configuration so

that the tips form an equilateral triangle. The

electrode spacing can vary depending on the size of

the plasma required for a given tube dimension. Each

electrode is independently modulated 120 degrees out

of phase relative to the other electrodes with a high

voltage high frequency (~ 30 kHz) source. An

example of a tube within the three-electrode plasma

discharge is shown in Figure 1.

Figure 1: Three-electrode plasma discharge for preform

fabrication.

2.2 Process Description

100 mm long synthetic quartz capillaries or tubes

with inner and outer diameters of 0.5 and 1 mm

respectively were used throughout this work. These

tubes were cleaned in an ultrasonic acetone bath and

heat-dried before the start of vapor deposition.

In most MCVD processes, the bubbler technique

is used for vapor deposition. A carrier gas, (typically

oxygen) is fed through liquid precursors such as

SiCl

, GeCl

, BBr

and POCl

to generate the

respective vapors to be further delivered into the

deposition chamber (Oh and Paek, 2012). In our case,

we adopted the evaporator method, where no carrier

gas is intentionally introduced but instead the

pressure inside the precursor container is maintained

at positive level, which is controlled by the

temperature of the container. Due to the formation of

a negative pressure gradient extending towards the

tube, the precursor vapor used for this study, i.e. SiCl

vapor, is channeled through the cleaned and surface

heated tubes. Hence, it is crucial to heat the tube

evenly so as to achieve uniform axial deposition. One

thing to note is that the SiCl

is highly volatile and

expands in volume when changing states. It is also

highly sensitive to moisture, and producing corrosive

HCl fumes. Hence a cool and dry environment is a

must for safety.

To fabricate the active fiber, rare-earth (RE)

dopants need to be deposited on the inner side of the

hollow tube. Conventionally, this is done either by

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

186

solution doping (Townsend et al., 1987) or

nanoparticle deposition (Tammela et al., 2002). We

employed the former method for our mini preform

fabrication for its simplicity.

The RE dopant solutions are prepared by mixing

and dissolving various weight ratios of RE chloride

hydrates in methanol. In this investigation, the

following RE solutions were used: 10 g of

YbCl3.6H2O and 20 g of AlCl3.6H2O dissolved in

200 ml of methanol for Ytterbium doped fiber, and 1

g of ErCl3.6H2O and 20 g of AlCl3.6H2O dissolved

in 200ml of methanol for Erbium doped fiber. High

RE doping concentrations were selected to

demonstrate the feasibility of the proposed technique.

The vapor-deposited tubes were then immersed in

the RE dopant solutions for 30 to 60 minutes, blow-

dried by oxygen gas and heated at about 200 C for 5

minutes to evaporate residual solvent and moisture.

We then replicate the recipe-driven fire polishing,

sintering, collapsing and sealing processes in the

three-electrode plasma system. The process is semi-

automated using National Instruments LabVIEW

software to improve its repeatability and efficiency.

Instead of moving the flame as in the standard

fabrication process, we move the tube back and forth

within the stationary ‘Ring of Fire’. Depending on

factors such as thickness of deposited soot and RE

layers, tube dimensions, and material properties of the

tube, the process requires optimization of variables

(e.g. plasma power, tube traversing speed) for good

performance as well as the elimination of trapped air

bubbles in the preform. For ~ 20 mm long preform

used in this study, the entire sintering, collapsing and

sealing processes took about 30 – 40 minutes. The

average collapsed tube diameter was ~ 870 µm. The

typical recipe used for fabrication, and its

corresponding outer diameter is depicted in Figure 2.

Figure 2: Typical recipe used in the process and its

corresponding outer diameter.

In standard fiber pulling process, the preform is

heated close to the melting point of silica in a furnace

or oven at the top of the fiber drawing tower, a thin

fiber can then be pulled from the lower end of the

preform. The intended fiber diameter can be

controlled by the pulling speed and the furnace

temperature. Before the fiber is wound up, it usually

receives a polymer coating such as acrylate, silicone

or polyimide for mechanical and chemical protection.

Since our fabricated preforms are significantly

smaller, the above mentioned method is not feasible.

Here, the horizontal tapering method was adopted

instead. Similarly, the pulling speed and the power of

the flame require optimization for desired fiber

diameters.

In principle, the fabricated preforms can be drawn

into fibers of any diameter narrower than that of the

preforms. The maximum fiber length that can be

drawn from a given preform dimension is calculated

based on the following relationship, where l

is the

length of preform, r

and r

are the radii of the preform

and fiber respectively.















(1)

3 RESULTS AND DISCUSSION

3.1 Physical Dimensions

Using the proposed method, we have successfully

fabricated several ~ 20 mm long ~ 870 µm diameter

preforms doped with either Ytterbium or Erbium.

These preforms were pulled to fibers with a final

diameter of 400 µm and lengths ranging from 50 to

75 mm. The typical fiber end face of the fiber is

depicted in Figure 3. The core is clearly seen, albeit

not perfectly circular in shape. This is mainly due to

the unevenness in the solution doping or slight mis-

alignment of the tube in the ‘Ring of Fire’. As we

pointed out at the beginning of this article, the focus

of our proposed technique is rapid prototyping at

early research phases, prior to scale-up and / or

further development for applications. The typical

cladding size of the fibers is shown in Figure 4. The

maximum cladding size variation is estimated to be ±

10 µm, or about ± 2.5 % for the 400 µm fiber. The

refractive index profiles of the fabricated fibers vary

from one to another, depending on the solution

composition and recipe used in the development.

Nonetheless, a typical index profile is depicted in

Figure 5 for illustration purpose.

Benchtop Mini Preform Fabrication for Specialty Optical Fibers

187

Figure 3: Typical fiber end face.

Figure 4: Cladding diameter showing uniformity

throughout the axial length.

Figure 5: Typical refractive index profile of the fabricated

fiber.

3.2 Spectral Performance

To analyze the spectra behavior of the fabricated

fiber, the fiber was spliced to a piece of passive fiber

for ease of connection and analysis. It was illuminated

by a tungsten halogen light source covering the

spectral range from 360 nm to 2000 nm, and pumped

either by 915 nm or 976 nm laser sources, depending

on the fibers’ RE dopants. The spectra were captured

using the optical spectrum analyzer (OSA) AQ-6315.

Reference spectrum was also taken using the similar

manner, however with only passive fiber.

The spectra behavior of the fabricated ~ 50 mm

Ytterbium doped silica fiber when pumped at 915 nm

and 976 nm are illustrated in Figure 6 (a) and (b). The

emission spectra of the fiber (with respect to the

reference) can be clearly seen even for such a short

length of fiber. We believe that this is mainly due to

the high doping concentration of the RE element,

which in turn demonstrated the capability of our

fabrication technique and its potential with other RE

dopant choices. More importantly, this marks the

initial success of the method.

Figure 6: Transmission spectra of the fabricated Ytterbium

doped silica fiber when pumped at (a) 915 nm; (b) 976 nm.

We further investigate the method using Erbium

as another active element. The spectra of the fiber

when pumped at 976 nm were obtained as follows.

Once again the amplified spontaneous emission

(ASE) spectra of the Erbium doped silica fiber can be

observed, indicating the successful incorporation of

Erbium dopants within the fiber.

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

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Although the fibers fabricated show promising

spectral results, they suffer from high background

loss, this is mainly due to the purity and preparation

environment and method of the samples.

Furthermore, clean room environment was not

utilized during the process.

Nonetheless, we would like to perceive this

shortcoming in a positive manner: i.e. clean room

environment is not necessary for this prototyping

process, although it would be good for potential better

results. To further improve the fabrication precision,

tighter control of the temperature is essential. This

“quick and dirty” process can be adopted until one is

satisfy with the doping solution composition before

switching it to the standard fabrication for good

quality fibers.

Figure 7: Spectra behavior of the fabricated Erbium doped

silica fiber when pumped at 976 nm.

4 CONCLUSIONS

In conclusion, we have demonstrated a concept of

miniaturizing the standard fiber fabrication processes

to a benchtop version with repeatable results. We

have also successfully fabricated rare earth doped, i.e.

Ytterbium and Erbium doped silica fibers using the

method incorporating with solution doping technique,

and obtained reasonable spectra results. Undeniably,

the length and quality of the fibers are traded-off by

the time and cost involved. This process is beneficial

for those who are requiring small quantity of the

specialty optical fiber for rapid prototyping purposes,

especially for the less mature optical fiber

technologies, such as soft-glasses fibers. It also opens

up the possibilities of exploring other glass hosts with

various materials and compositions in a smaller scale.

ACKNOWLEDGEMENTS

This work is funded by Temasek Laboratories @

NTU (Grant No: 9016100134). We acknowledge the

technical support by the laboratory managers and

technical support officers of OPTIMUS and COFT,

NTU

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