Tuesday, May 7, 2013

Polishing, Cleaning, and Curing

The base of the ISL fiber must be flat and perpendicular to the fiber axis. Light entering a flat base will bend towards the fiber axis. The base must be smooth also: scratches will scatter light that would otherwise enter the fiber. And it must be clean, for dire will absorb light. We can check that a fiber tip is perpendicular, flat, smooth and clean with a specialized microscope we call a fiberscope.

We polish our high-index fiber in the following way. We break off a 5-cm length by crushing both ends with a diamond scribe. We do not scratch and pull nor scratch and bend. These methods produce a longer break. The crushed break leaves only 300 μm of damaged glass. We take the fiber in our fingers and polish it on wet 15-μm grit paper until the damaged glass is gone, which takes about a minute. We do this for both ends, so that both ends are now slightly concave. We place one end in a 440-μm diameter ferrule mounted in a polishing puck. The 390-μm fiber is a loose fit in the ferrule. We press the top end of the fiber to apply polishing pressure, and are now glad that it does not have any sharp spikes of glass left on it from the break. We polish on a flat surface with wet 15-μ grit for thirty seconds. We now have a flat, perpendicular surface. We move to wet 3-μ grit for thirty seconds, then 1-μm grit for thirty seconds. We clean the tip by brushing it along a piece of acetone-soaked lens paper. In the fiberscope, we see the 355-μm core and the 390-μm cladding around it, and a few light scratches. We polish and inspect the other end in the same way.

Now we clean the fiber walls. We hold both ends in acetone-soaked lens paper and clean by stroking away from the center. We do not use alcohol because it leaves a residue. We do not use water because it does not dissolve the oil left upon the fiber by our fingertips.

We place our polished, clean fiber in our alignment fixture and lower it over an LED. For today's experiments we use left-over 290-μm square green EZ290 LEDs with a central bond wire. We press the fiber base onto the bond wire. We turn on the LED. If we have polished the base well, we see no light leaking out of the fiber walls near the base. The only light visible in the neighborhood of the base is the light escaping through the gap between the fiber and the LED. If we have cleaned the walls well, no light emerges from the walls all along the length of the fiber, except where the steel clamp touches the glass. The LED emits 9.0 mW of green light with forward current 30 mA, and we get 6.7 mW out the top end. That's 75% coupling from the LED to a point 5 cm away.

The fiber core has refractive index 1.63 and the air outside has index 1.00, so the numerical aperture of this air-clad fiber is bigger than 1.0. Any light entering the base should propagate to the tip, assuming the walls are in contact only with air. Any dirt on the walls will shine with green light escaping from the fiber. Any residue on the fiber will glow with green light.

Assuming a perfectly-prepared fiber, there remain four sources of loss in our system, and we suppose these add up to 25%. First, there is roughly 4% reflection from the base of the fiber, for light entering at 0-80°. At higher angles, more light is reflected. Let us suppose we lose 6% this way. Some light escapes through the gap between the fiber and LED, and with our photodiode we estimate this to be around 5% also. At the top end of the fiber we have another reflection of 4%. This leaves 10% loss at the steel clamp, which is consistent with how brightly the walls glow inside the clamp. We conclude that our polishing is effective, and our cleaning also.

We apply NO13685 adhesive to the fiber walls above the clamp. This adhesive is runny like water and has refractive index 1.37. No light escapes from the fiber. Power at the tip remains 6.7 mW. We attempt to cure the NO13865 in place, with a UV light. The adhesive evaporates and the walls glow with green light. Power at the tip drops to around 5 mW. It appears that the coating evaporates before it can cure.

We apply NO164 adhesive to a spot on the fiber wall below the clamp. This adhesive has refractive index 1.64. The spot shines brightly with green light escaping from the fiber. As we place dots of NO164 farther down the fiber, they glow brightly and the higher ones go dim. Light from the top of the fiber drops to 4.6 mW.

With the NO164 spots higher up on the fiber, we apply a drop of NO164 between the fiber and the LED surface. This adhesive matches the refractive index of the fiber core, so that scratches in the face of the fiber will no longer act to scatter light. But we see no increase in power at the fiber tip with the NO164 between the base and LED. We repeat the experiment several times, and occasionally we see less power at the tip, which we believe is the result of bubbles trapped between the base and the LED.

We apply adhesive to a freshly-prepared horizontal fiber in a chamber filled with dry nitrogen gas, and illuminate through a thin plastic window with UV light. After two minutes we apply another coat, and continue until we have five coats, which we cure for another ten minutes. We wash with acetone and find that we have removed the adhesive.

We try MY133, another adhesive which is less runny and has refractive index 1.33. We apply one coat to a fiber. It beads up on the fiber and begins to harden. After ten minutes in our curing chamber, it is still tacky to the touch and a layer in a petri dish is still runny. (The lamp intensity is 14 mW/cm2 and this adhesive needs only 2 J/cm2 to cure.) We polish the fiber tip to remove adhesive and lower onto our LED. There are glowing spots in the coating, which suggest dirt embedded in the adhesive. We get 5.9 mW out of the fiber tip.

Figure: A High-Index Fiber Coated with Epoxy. The beads from as a result of surface tension and viscosity. Similar beads appear with MY133 adhesive, but not with the runny NO13685. This epoxy-coated fiber transports 30% of the light emitted by a blue EZ500 (480-μm square die).

The beading up of a coating on a fiber is incompatible with our ISL application. The photograph shows shows beads of epoxy on a length of our high-index fiber. The beads can be double the diameter of the fiber. The beads form in viscous adhesives whether we mount the fiber vertically or horizontally. Runny adhesives do not form beads, but they evaporate in the heat of the UV lamp before they cure.

We spilled our bottle of MY133, which will cost $400 to replace. Even if we can solve the problems of cleaning and curing these adhesives in a thin, uniform layer on our high-index fiber, we are not sure how we can apply a coating to an 8-mm fiber with a tapered tip. None of these adhesives can survive the temperature required to melt glass. We would have to coat the base of a 5-cm fiber, mount it in the stretcher, heat the glass to make the taper, then coat the glass up to the taper.

We conclude that the application of these coatings will be expensive, time-consuming, and unreliable. We will try to obtain cladded fiber with core refractive index 1.7 or greater. Such a fiber would provide us with sufficient numerical aperture on its own, and so greatly simplify the production of the ISL tapered fibers.

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