Drawing of the Optical Fibre

The EDZF reform is drawn in a similar manner as to any other optical fibre. The schematic of the fibre drawing process is shown in Figure 11. The completed preform is mounted onto the top of a drawing tower, and secure in a heating furnace. The furnace will heat the preform to between 2000 to 2200^oC, allowing it to become viscous enough that the fibre strand may be extracted by a capstan puller from the preform. A pyrometer, together with a temperature regulator, is used to measure and control the heating of the preform. The drawn fibre is then measured using a diameter measuring device, which is connected to a speed regulator. Therefore, if the diameter of the fibre becomes too thin or thick, the drawing speed is adjusted until the diameter is once again at the desired value.

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Figure 11: Fibre drawing tower configuration, with dual-coating cups.

Once the optimum diameter has been achieved, the fibre is then coated on-line with a protective polymer coating. In order to do this, the fibre is channeled through the first coating cup, which contains the primary resin coating, and then subsequently through the second coating cup which contains the secondary resin coating¹². As the fibre continues along the drawing path, it will now pass through a UV curing oven, which will cure the primary and secondary resin coatings. It must be noted that until the fibre is drawn and coated with its protective resin or polymer cover, it risks being contaminated. Therefore, special care must be taken in designing the furnaces so that they themselves do not become a source of particle contamination, which will significantly degrade the fibre strength. The final product is then wound along a drum by a winding machine.

12 The deposition of the acrylate can be done using either the ‘Wet-on-Wet’ method or ‘Wet-on-Dry’ method. The ‘Wet-on-Wet’ method, as the name implies, involves the deposition of the second polymer layer on top of the first layer before it has been cured, thus the two layers are cured together at the same time. The ‘Wet-on-Dry’ method on the other hand entails the deposition and curing of the first layer, before the second layer is deposited and cured. The ‘Wet-on-Wet’ approach allows for a quicker deposition process, although it carries an inherent risk, whereby if the inner layer does not cure properly, it would affect the overall integrity of the protective polymer coating. On the other hand, the ‘Wet-on-Dry’ method is slower, but guarantees a stronger coating. In this work however, the ‘Wet-on-Wet’ method is applied as the risk of the inner polymer coating failing is deemed negligible.

54 Control of the fibre diameter is important in a steady-state drawing process where mass-conservation is necessary. During steady-state process, the following relation is derived from mass-conservation rule:

= ………..……..…….….(7)

where and are the preform and fibre diameters, while and are preform feed down and fibre drawing speeds. Due to local fluctuations, a steady state can sometimes not be achieved at all; this can result non-uniformities in the preform diameter. As this will affect the overall performance of the EDZF, thus it is important to ensure that the diameter of the drawn fibre is measured and controlled precisely, with very tight dimensional tolerances. The fibre drawing speed is typically between 30 m/min to 600 m/min, although in industrial production setups higher drawing speeds can be obtained¹³.

The Ultraviolet (UV) curable resins used for the fabrication of the EDZF are acrylate resins, and are necessary to protect the fibre from contact and foreign particles, as these will affect the life of the fibre in terms of strength and static fatigue.

Furthermore, the fibre needs to be protected from microbending, and as such the coating must be concentric and bubble free around the fibre as well as having stable performance under different environments. For the primary coating, Desolite DP-1004 resin is used, while the secondary coating is fabricated from Desolite DS-2015 resin. The primary coating, which is the softer inner layer, protects the fibre and a hard outer layer ensures good mechanical properties. The uniformity of the coating is maintained by adjusting the flow pressure of the inlet gases into the primary and secondary coating resin cups and also by properly aligning the position of primary and secondary coating cup units during the drawing process. A laser concentricity monitor controls the coating process. Bubbles may be formed at the interface of the fibre surface and the hard coating because of the way the coating liquid moves surrounding the surface of the fibre, although this is easily overcome by controlling the pressure and keeping the viscosity of the liquid constant during the drawing

13 Industrial fibre drawing techniques use drawing speeds of up to 2000 m/min in a 20 m high tower with proper drawing tension and fibre cooling.

55 process before it enters the UV curing oven. As a final step, the fibres are wound on a precision spooling machine. The fabrication of the EDZF can be summarized as in Figure 12.

Figure 12: Summarized process flowchart for EDZF fabrication

The following section details the characterization of the EDZF’s morphology and spectral characteristics.

Preparation

•Tube selection

•Tube cleaning, fixing and joining with delivery tube and the collector tube on two ends

•Fixing of composite tube to the lathe

MCVD Process

•Fire polishing and etching

•Deposition of cladding layers

•Deposition of unsintered core layer/layers

•Sintering of the right hand portion of the tube at the end

•Separation of the reaction tube from the collector tube and then from the lathe

Solution Doping Process

•Dipping of the reaction tube into an aqueous or alcoholic solution of the precursor of the dopants

•Lifting of the tube from the solution

Sintering and Collapsing

Process

•Fixing of the tube to the lathe and joining with the collector tube

•Oxidation of the unsintered but soaked core layer

•Dehydration of the unsintered but soaked core layer

•Sintering of the unsintered but soaked core layer

•Collapsing of the deposited tube at high temperature to form final preform

Fibre Drawing Process

•Drawing of fibre from preform

•Coating of fibre with protective polymer

•Ploymer Curing

•Winding of drawn fibre

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