Gold Cylindrical Optical Fiber

Fiber Lab Report

By Aditya Mittal

Spring 2005

Contents:

 

Background

·        Objective

·        Methodology

·        Optical Fiber

·        Mathematical Model

The Experiment

·        Procedure

o       Preparing glass for deposition

o       Preparing the Vacuum Chamber

o       Evaporation System and Deposition Process

o       Fiber Redraw Process

o       Collapsing Process

o       Attaching the hangs

o       Fiber Drawing Process

·        Conclusion and Further Experiment

Credits

·        Acknowledgements

·        References

Background:

 

Objective:

 

Fabricating very thin metal film, specifically, 99.9% pure gold films about 5nm thick for this experiment.

 

Methodology:

 

This shall be done by using a new fabricating process to fabricate a smooth gold film at the core-cladding boundary in fiber.  The process consists of depositing gold onto a clean glass rod which will form the core of the fiber, collapsing a glass tube or ampoule to create the preform, and then drawing the preform into optical fiber with a drawing tower.  In the process of pulling the fiber we also end up pulling the gold into a thin film.

 

Optical Fiber:

 

Optical Fiber is a flexible optically transparent fiber, usually made of glass or plastic, through which light can be transmitted by successive internal reflections.  The fiber consists of a core, a cladding, and a buffer coating as shown:


Parts of a single optical fiber image from http://electronics.howstuffworks.com/fiber-optic1.htm (Tuesday May 10, 2005)

 

 

Mathematical Model:

 

We can think of metals as ‘a sea of mobile electrons’ as electrons flow freely throughout a metal.  So, we can model an extremely thin sheet of metal as free electron gas.  In quantum mechanics the most primitive model of electron gas was constructed by Sommerfield who basically treated the solid as an infinite square well in 3D.  The electrons were free to move around inside, i.e. zero potential inside and infinite potential outside. 

 

Then, Bloch complicated the theory a little by also including the periodic potential inside created by the positively charged nuclei.  Although, neither model considers electron-electron repulsion, they do show the importance of the Pauli Exclusion Principle (no two fermions have identical quantum numbers) in explaining the ‘solidity’ of a metal.  That is since electrons are fermions they do not simply settle in ground state and occupy volume as only 2 electrons can occupy a state. 

 

Assuming there is no excitation of electrons we can then determine the free electron density of a solid, i.e. the number of free electrons per unit volume.  The boundary separating the occupied states from unoccupied ones is called the Fermi Surface and the energy corresponding to the free electron gas is the Fermi Energy. 

 

Analogous to internal thermal energy, quantum mechanical energy also exerts pressure on the walls from inside of the metal and keeps the metal from collapsing.  One main task in our model then is to determine the Fermi energy.  We model the thin gold film on the core-cladding boundary as a ‘metal cylinder fiber’ using collective modes of electrons.  From that it turns out that the Fermi Energy can be determined using the following equation, where N is the surface density of electrons in the thin metal film and n = 1, 2, 3…. The U(k) has the form of kinetic energy with an effective mass.

 

 

 

where and

 

For the metal cylinder fiber model first the relative dielectric constant’s real and imaginary parts are calculated using Kramer Kronig Relation.  This constant depends on the oscillating frequency and wave vector of light.  Two factors contribute to the surface charge density for this thin metal.  One is the electron distribution and the other is the positively charged ion background.  Given the distribution and using normal quantum mechanical means discussed above the Fermi energy is then calculated.  Also, from the equations of the dielectric constant the plasma frequency of the thin metal film is calculated and further the indices of refraction are mapped as a function of wavelength in gold films of different thicknesses such as 2.06 or 4nm.  Following are the graphs for the two thicknesses:

 

Graphs and Fermi Energy equations borrowed from Mathematical Model of Metal Cylinder Fiber
The Experiment

 

Procedure:

 

Preparing glass for deposition:

 

We start with a cutting and cleaning a glass rod of 7056 type glass of 1mm diameter.  The Redraw process explained later is then used to reduce the diameter of this rod to 0.5mm.

 

Preparing the Vacuum Chamber:

 

Vacuum is created in a chamber using a Vacuum Diffusion Pump (Vapor Jet Pump) like the one pictured:

Picture borrowed from Inside a Vacuum Diffusion Pump

 

 

Such a pump can create a vacuum down to 10-10 torr in pressure.  However, before this pump can begin its work, another pump, called the roughing or forepump is used to reduce pressure down to 10-3 torr in the glass chamber.  At this point the vaccum diffusion pump is started and the pressure starts to drop below 10-3 torr.  The reason for this is that the diffusion pump cannot exhaust directly to atmospheric pressure and so the forepump is used to maintain proper discharge pressure conditions. 

 

The way this pump works is that there are three jet assemblies of diminishing sizes as shown in the picture with an oil at the base.  A heater vaporizes the oil that exits from these jet assemblies and is suddenly cooled by the surrounding coolant, specifically liquid nitrogen in our case which is poured into the system when creating a vacuum.  Liquid nitrogen has a temperature of -196 degrees Celsius.  “The droplets of oil coming down may actually exceed the speed of sound, but there is no sonic boom, largely because the molecules in partial vacuum are too far apart to transmit the sound energy.”  (Joaquim 3)  This very high velocity jet stream conveys a downward motion on the molecules bringing them towards the forepump outlet where the gases are removed by the forepump and the condensed oil begins another cycle.  Advantages of using this pump are that it is one of the most reliable ways of creating a 10-3 to 10-8 torr vacuum, it lasts for more than thirty years and it has no mechanical parts.

 

Using this pumping system we are able to remove all unwanted material from inside our evaporation system.  This evaporation system is used for depositing gold onto our glass rod.

 

Evaporation System and Deposition Process:

 

The evaporation system is depicted in the diagram.  The evaporation occurs inside a vacuum chamber so that there are no impurities in the deposition on the glass rod.  In our experiment a new glass chimney was created by cutting a larger glass beaker in order to achieve a longer length of rod onto which deposition occurs.  This would allow for ease in making the hangs and drawing the fiber in latter steps.  In the chimney, two notches are cut out to allow the glass rod to go through while supporting its cover.  At the bottom glass slides cover any additional openings at the interface of the chimney and the parts that hold the boat in place.  The boat is a small container to hold the gold pill that will be evaporated onto the glass rod.

 

 

 

 

In our experiment the glass rod was held by a pencil lead holder cut out from a mechanical pencil as it is able to grasp such a thin rod well and keep it in place during the deposition.

 

Although, the heater at the bottom ensures that gold will evaporate straight up giving a smooth continuous deposition it is necessary to use the chimney to protect the vacuum chamber from being coated with gold.  As we saw, our chimney also got gold deposition in the process.

 

Although, the spinning of the glass rod was automated, a manual pedal was attached with the system to ensure the glass rod stayed spinning.  In case it stopped it could be restarted with this pedal.  Having to restart the experiment for such a small cause would be time costly.

 

After ensuring a clean vacuum chamber the above described setup is created. Then the chamber is closed and sealed airtight. Then liquid nitrogen is poured into the system to act as the coolant.  After this the forepump is turned on and the pressure is dropped to 10-3 torr.  The foreline might have to be opened and closed many times to achieve this.  Next, the vaccum diffusion pump drops the pressure down further.  After everything is ready, and the vacuum has been created, the deposition process begins by heating the boat by passing a large current through it (120 Amps at about 3V AC).  Within a matter of about 30 seconds the gold evaporates from the boat and a thin film is seen on the glass rod and chimney.  These are the most exciting 30 seconds in the whole deposition process.

 

Fiber Redraw Process:

 

This process is used to reduce the diameter of a glass rod from say 1mm to 0.5mm.  Essentially, a heater called the redraw furnace is used in which the glass rod goes through and at the bottom a weight is hung.  The weight applies the force on the rod by which it is pulled into the desired diameter.  The glass rod is held in a small hole made in a rubber cork to grip the glass rod tightly.  Based on the temperature dependence of the viscosity of the glass the redraw process can be modeled mathematically.  Important factors include the type of glass being used, the initial diameter and rod length, furnace temperature, and amount of weight hung.  Viscosity is given in Pascal-seconds and the following points are defined as the temperature at which the particular glass has a certain viscosity:  Strain Point (1013.5Pa-s), Annealing Point (1012Pa-s), Softening Point (106.6 x Density/(2500kg/m3) Pa-s), and Working Point (103Pa-s).  Although, this process has been explained here, it occurs before deposition to adjust the diameter of the glass rod on which gold is deposited.

 

Collapsing Process:

 

So far, we have been working with just a glass rod.  Collapsing is the process by which we enclose this solid glass rod inside a glass tube.  In our experiment the inside rod has a diameter of .5mm and the tube has a .8mm inner diameter (ID) and 4mm outer diameter (OD).  Stuffing the glass rod inside this tube creates the ampoule that is then collapsed to form the preform that we pull with the drawing tower.  After, the fiber is formed this outer tube forms the cladding and the inner rod forms the core.  Clearly, the gold deposition on the inner rod will then form a smooth gold film at the core-cladding boundary in the fiber.

 

Firstly, a tube is sealed off on one end using a torch to flame the glass.  While using the torch it should always be remembered that the gas should always be opened first before lighting the torch and should not be left on for too long without lighting it.  Sealing one end ensures that the glass rod inside will not fall out.

 

Next, the glass rod is inserted inside the tube and after creating a vacuum in this tube it is “pinched-off” using the torch.  It is important that the interface between the cladding and core is a complete vacuum except the smooth gold film.  During pinch-off the glass tube should be sealed on both ends.

 

The ampoule is now ready to be collapsed.  Firstly, it is enclosed inside a so-called bomb.  It is this bomb that is placed first inside the pre-heat furnace and then pushed into the collapsing furnace.  The pre-heat furnace heats the bomb to about 360 degrees and then the bomb is pushed further in where it rests until it gets to about 630 degrees Celsius in the collapsing furnace.  It is kept at 630 degrees for about 30 seconds and then pulled back into the pre-heat furnace for annealing that is to subject the glass to a process of heating and slow cooling in order to toughen and reduce brittleness.  At this time the preheat furnace is set to about 450 degrees Celsius.  The bomb is allowed to sit in the pre-heat furnace for 8 to 10 hours and is then taken out.  At this point, we have a single piece of glass rod which has the gold film trapped in between the core and cladding.  After cooling it to room temperature, the collapsing process should be repeated a few more times in order to get an extremely stress resistant fiber.

 

Attaching the hangs:

 

Before the preform can be attached to the drawing tower to be pulled, handles or hangs must be attached to it to hold it on the tower.  For the purpose of attaching the hangs there is a special machine that holds the preform and handle being attached and rotates them and allows horizontal movement by spinning the wheel to bring them closer or move them farther apart.  The machine is depicted below.

 

 

The two main things to keep in mind for this part are that the glass used for the handle must be the same as the glass with which the fiber is being made.  Otherwise, the glass cracks in the process of melting the handle and fiber together.  The other thing is that the torch is hottest when a short hard blue oxidizing flame is seen.  This is the kind of flame that should be used for this part in order to prevent blackening of the rods and a strong attachment.

 

Fiber Drawing Process:

 

This is the final step in the experiment.  The drawing tower is depicted in the diagram.  The preform is hung at the top and comes down through the furnace.  Time must be taken to carefully set up the fiber in the center so that it would come straight down through the furnace at the top of the drawing tower and not touch the heating coil of the furnace.  The furnace is set at about 640 degrees Celsius which is the temperature at which the glass we used melts.  The melting point of 24 carat gold used is higher so that wouldn’t melt and accumulate in one place when using this temperature for drawing the fiber.  While the upper handle is attached to the top to hang the preform, the lower handle has a small mass attached to it to help the fiber get pulled down by the force of gravity.  This process will extrude the gold which will be of nanometer thickness.

 

Slowly, the fiber begins to come down and the rate at which the fiber came down increases consistently.  In about an hour and a half we had some length of fiber with gold at the core-cladding interface.  The fiber is then attached to the pulley to maintain constant pressure and thus an even diameter and the fiber can then be collected on the spools at the bottom.  Attached with the tower is a laser operated monitor that determines the diameter of the falling fiber.  In addition to that the furnace temperature is also constantly monitored.

 

 

Conclusion and Further Experiment:

 

In our experiment, a problem occurred that the thin gold tore in places giving a gold deposited region in the fiber and then a region where clear glass could be seen and the pattern continued through the thin fiber.  It was contemplated by Dr. Kornreich that perhaps the problem could be resolved by pulling a glass with a higher melting point at a higher temperature as the glass would flow more slowly and smoothly.  Melting at a higher temperature also implied using a metal that would melt at a higher temperature than pure gold.  Gold had been chosen because it is highly malleable.  Therefore, it was determined that the new metal could be a gold alloy that has a higher melting point than that of pure gold.  That is the next experiment in line.

Credits:

 

Acknowledgements:

 

Special thanks to Dr. Phillip Kornreich for involving me with this gorgeous project of his, sharing his ideas and research with me, supporting my interests in the learning of science and mathematics, and maintaining an extremely caring approach.

 

Also, special thanks to graduate student Ramesh Narayanan for all the insightful discussions and help with gathering information for this paper, and to Zheng-Xuan Lai for getting me started on things in lab.  It has also been nice knowing Akshob Bangle and Jim Flattery while working in the lab.

 

References and Readings:

 

Sharma, Halme, and Butusov.  1981.  Optical Fiber Systems and Their Components.  Berlin: Springer-Verlag Berlin Heidelberg.

 

Griffiths, David J.  2005.  Introduction to Quantum Mechanics.  New Jersey: Pearson Education, Inc.

 

Shankar, R.  1994.  Principles of Quantum Mechanics.  New York: Plenum Press.

 

Redraw Process.  Revised: 24 March 2003.

 

Mathematical Model of Metal Cylinder Fiber.

 

Joaquim and Foley.  Inside a Vaccum Diffusion Pump.  http://xtronix.com/Diffusion%20Pump.pdf#search='operation%20of%20a%20vacuum%20chamber%20and%20pump'.  Santovac Fluids, Inc. and Varian, Inc., Vacuum Technologies.  8 May 2005.

 

Extra Pictures of Our Lab: