Optical Fibre Drawing Methods

How to Prepare a Preform without Cladding:

Wash the preform with soap and water, then wash it with isopropanol and dry it off with a lint-free tissue. Do not use any other solvent (i.e. hexane, acetone or ethanol), as they will dissolve the preform.

Bring the preform to the workshop to drill two lateral holes at the top and bottom of the preform. Put on the safety lab coat and eyewear. Use a 2 mm drill bit on the drill press. Use the wash bottle filled with water to cool down the preform and reduce dust. Slowly drill the two holes perpendicularly through the preform. These holes will be used to pass a metal wire through the preform to attach it to the pole to feed it into the oven and to attach the weight. Make sure to go slow so as to avoid creating cracks in the preform. If cracks are created next to the perpendicular holes, the preform will not be able to be put in the drawing oven. New holes can be drilled lower. If cracks are created along the preform, it will not draw into proper fibre. They will have higher transmission losses.

The preform needs to be annealed to remove any residual stress and any trapped moisture or solvent in the material. Put the preform in the chemistry laboratory’s oven for 24h at around 90ºC (turn the dial at approximately 2.5).

How to Prepare a Preform with Cladding:

Choose the PMMA tube with the right diameter for the preform used. Cut the tube with a saw in the workshop. Rinse the PMMA tube with deionized water to ensure no water marks remain after drying. Wash the preform with soap and water to take off any dust.

The bottom of the PMMA tube needs to be sealed. To do so, heat up the drawing oven at around 155ºC and insert the bottom of the PMMA tube into the oven. Align it with the oven’s hotspot (approximately 12 cm lower than the top of the metal claw). Use pliers to seal about 2 cm of the bottom of the PMMA tube. Take the PMMA tube back to the workshop and drill a hole in the middle of the sealed end. If the PMMA tube used is thin, holes need to be made to let out the air at the bottom during the vacuum, so the cladding does not fold over itself. Heat up a needle using the lighter, in the optics lab, and pierce four holes at each corner of the PMMA tube, near the sealed edge. If the PMMA tube used is thick, this step is not required.

The PMMA used for the cladding and the preform need to be annealed under vacuum. Put the PMMA tubes and the preforms in the chemistry laboratory’s oven for 24h at around 90ºC (turn the dial at approximately 2.5). Put the preforms and the PMMA tube in the oven and seal the door. To create the vacuum, turn on the pump located on the ground next to the oven and turn the vacuum dial on the oven to 25 Hg. When the level of vacuum desired is reached, turn off the pump. Later, before opening the door of the oven, turn the vacuum dial back to 0, to let the air back into the oven and equilibrate the pressure.

Wash the preform and the cladding tube with isopropanol and dry them off with a lint-free tissue. Do not use any other solvent (i.e. hexane, acetone or ethanol), as they will dissolve the preform and the cladding tube. Also, do not rinse with soap and water, as it will bring back moisture into the material and they will have to be annealed again. Insert the preform in the cladding tube.

How to Use the Oven:

The temperature controller needs to be turned on and will need to reach the temperature of approximately 155ºC to melt the polystyrene, depending on the dopant concentration. It might be required to heat it to 170ºC in extreme cases. Turn on the oven by pressing the power button. The oven needs to be heated slowly, so turn it up using the button with arrow pointing up in increments of 20ºC, until 150ºC is reached.

After the drawing process is done, bring the temperature controller back to 20ºC using the arrow pointing downwards. To turn it off, press the power button.

How to Set up the Preform in the Oven:

On the bottom hole of the preform or cladding, use the metal wire to hang the weight. If there is no cladding, use the top hole to attach the preform to the long metal rod with another metal wire.

Screw the metal rod or the PMMA cladding tube in the metal claw to hold the preform in the oven. Determine where to start drawing the fibre on the preform. Measure 12 cm above that point and align it with the top of the metal claw holding the metal rod or the PMMA cladding tube. The hotspot is located 12 cm deep in the oven when measured from the top of the metal claw. As it is the hottest part of the oven, the fibre will start to draw from there. The diaphragm at the top of the oven can be used to centre the preform in the oven.

If cladding is used, put the rubber cap on the top of the cladding tube and create a vacuum inside the cladding. To do so, use the current controller on a low setting to create vacuum.

How to Pull the Fibre:

When the weight starts to slowly drop, carefully pull on it straight down. When fibre starts showing, pull slowly and constantly on the lowest part of the fibre, to avoid burning your fingers and deforming the fibre. Always pull the fibre perpendicular to the ground. Pulling at a constant speed will keep the diameter of the fibre constant. If need be, push the metal rod or the PMMA cladding down in the oven at a constant speed.

If the drawing of the fibre shows resistance, close the diaphragm at the top of the oven to keep the heat inside the oven. The temperature of the oven can also be turned up a little. If the fibre starts heating up too fast, open the diaphragm and lower the temperature back to around 150ºC.

The fibre can then be cut off of the preform and the weight. Afterwards, it should be rolled on a drum or stored properly.