Our current taper machine was designed to produce long tapers (20mm) out of 500 micron fiber. It burns propane, fed through a small torch nozzle, to heat the fiber. This torch tends to heat more than one millimeter of the fiber at a time, making it challenging to create very short tapers. Additionally, the flow of burning propane and associated air turbulence perturbs the fiber, often making the taper slightly asymmetric.
An ideal heat source for our project would be able to heat a very small section of fiber (<1mm) to the temperature at which it becomes malleable (~1800K) without inducing air currents that perturb the fiber. Some fiber heaters use electricity to generate heat, though few or none are designed to heat very small sections of fiber. We decided to try building a simple electric heater.
Most wire heating elements are made from resistance wire alloys such as nickel-chrome. We didn't have any resistance wire on hand in the lab, so we wound a coil out of 0.02" diameter, 7 strand, stainless steel wire. We attached a power supply and began increasing the current through the coil. The coil began to glow visibly red. We gradually increased the current and the coil glowed brighter and became yellow-orange.
We laid a 300 micron silica optic fiber through the center of the coil. We bent the fiber twenty degrees while still touching the coil, and it kept its bent shape after returning to room temperature. The coil burned out in this process, but not before it had transferred enough heat to the fiber to soften it and, in principle, make a taper.
Unfortunately, it is impractical for our fiber to touch the coil directly while making tapers. Can this simple heater soften glass with radiation only, or did the glass heat principally due to to conduction?
Our stainless steel coil can be maintained at a yellow glow without failure, which correlates to approximately 1270K. We model the heated wire as a black body and use Wien's displacement law to calculate the peak emission wavelength to be ~2.3 microns. Unfortunately, at this wavelength, a 300 micron thick section of our fiber absorbs far under 1% of radiation (as inferred from the datasheet). However, absorption of radiation increases rapidly with wavelength (silica glass absorbs nearly 100% of radiation over λ=5 micron). By roughly integrating Planck's law we see that up to 40% of radiation from the coil may be absorbed by the fiber. Of course, only a fraction of radiation will strike the fiber to begin with, most will radiate into the surrounding area or into other parts of the coil.
We positioned the coil in place of the torch in the fiber-stretching machine. We inserted a fiber through its axis, ensuring that it did not touch the coil. We applied 10V to the coil and gradually increased the current flowing through the fiber until it reached 1 amp, glowing yellow hot. After one minute, tension was applied to the fiber by the machine. The fiber failed to stretch, and instead, was simply pulled through the coil at a rate of 4mm/minute. The polyimide coating cleanly burned away, but the glass did not reach a temperature sufficient for tapering.
Above: 300 micron fiber in position to be tapered, inside of a stainless steel heating element. The element's temperature gradient is apparent by the color transition from red (~960K) near the alligator clips to yellow (~1270K) near the coil center.
We increased the current to 1.1A, which was still insufficient to soften the fiber. This corresponds to 11W of power. If we assume that 10% of radiated power intersects the fiber, and 40% is absorbed, the fiber receives energy at a rate of about 0.4W. This is insufficient to heat the fiber, but we can estimate the amount of power that would be necessary. We assume that the majority of heat leaves the hot zone via conduction along the fiber, and use Fourier's law to calculate the rate at which heat is dissipated. For the fiber to exist in thermal equilibrium at 1800K, we calculate that heat is dissipated at a rate of 1.5W per side of the hotzone. We must transfer 3W into the fiber to soften it.
We increased the current further, and by 1.2A, the coil failed by a combination of oxidation and melting.
Above: Coil after failure. Melted area apparent near right side. Note the polyimmide coating (left of coil) and bare silica (right of coil) where the fiber had moved through the coil.
Stainless steel is not an ideal material for the job since it oxidizes quickly and can't be maintained at a temperature much over 1270K. Alloys such as Kanthal A-1 are designed for use at temperatures up to 1670K. Since total power radiated goes as T^4 (Stefan-Boltzmann), this increase in temperature represents a three time increase in power radiated over stainless steel. Materials such as MoSi2 can radiate 7 times as much power as our steel coil. We can investigate these materials to see whether they're capable of softening glass. It may turn out that because the overall efficiency is so poor that it is impossible to use a conventional heating element to heat a small section of fiber to the softening point by radiation alone. In that case, we'll proceed with our original plan of upgrading to a thin-flamed oxy-hydrogen torch.
Additional Pages: Silica glass properties
Well, you can use the co2 laser for heating the fiber in ultra small region for tapering very sharp taper. In the past I have used it for tapering fiber or making on fiber structures. It can heating the fiber in small region as small as below 100 microns(If you have very good lens, you can focus the heating spot to ~10microns)
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