Higher-Index Fiber

We receive from Fiberoptics Technoligy a new batch of fiber. As with our High Index Fiber, the core occupies 83% of the cross-section and the cladding the remaining 17%. This time, the core is made of a proprietary glass of refractive index 1.72, which they call TD5. The cladding is Schott 8250. The core numerical aperture is 0.86.

The fiber is divided into 178 sections, each 9 m long, so that the total length of fiber is around 1600 m. We took a 10-cm sample from each 9-m section and measured its diameter. Of these, 165 had diameter 390-440 μm, and 13 had diameter 130-310 μm. We set aside the thinner fibers and consider the 165 larger fibers. The histogram below shows the distribution of their diameters.


Figure: Distribution of Section Diameter

We were hoping for an average diameter of 440 μm and a range of 400-480 μm. What we have is an average diameter of 411 μm with range 390-440 μm, which is similar to the distribution of our previous high-index fiber.

From the 1600 m of fiber we have 100 m with diameter 430 μm or higher. We need 50 mm to make one 8-mm fiber taper, so we have enough fiber in stock to make two thousand fiber tapers of base diameter 430 μm. According to our calculations, the 430 μm diameter should capture and transport 64% of the light emitted by an EZ500 LED. With 30 mA of LED current and 28 mW of blue light emitted, we are hoping to see 18 mW at the fiber tip.

UPDATE: [28-JUN-13] We find a piece of TD5 fiber with diameter 450 μm. We polish both ends of a 80-mm length. We cover 20 mm of the fiber with nail polish. We run 30 mA through an EZ500 460-nm blue LED and obtain 31 mW emitted power. We lower the fiber onto the LED and obtain 16 mW at the fiber tip, which is 50% capture efficiency, and consistent with our earlier calculations for a fiber of numerical aperture 1.72 and diameter 450 μm.

Diameter Variation

We have of order 300 m of optical fiber with numerical aperture 0.66 (core index 1.63, cladding 1.49). When we first received the fiber, we measured its diameter in one place and obtained a value 390 μm. Since then, we have assumed all fibers we cut from our 300-m batch are 390 μm in diameter. Today we cut 225 sections each roughly 30 cm long from our 300-m batch. We measured the diameter of each section with a micrometer. Our resolution in measuring diameter is 3 μm with our micrometer. We obtain the following histogram.


Figure: Distribution of Diameter Among Samples.
The nominal diameter of the fiber, as supplied by Fiberoptics Technology, was 0.016", or 406 μm. We find the average diameter to be 405 μm, with standard deviation 12 μm. The minimum diameter is 370 μm and the maximum is 440 μm. We measure diameter along the length of a selection of fibers and obtain the following plots. The most rapid change in diameter we observe along a fiber section is 30 μm in 100 mm. Along an 8-mm ISL fiber, we would see no more than a 0.6% change, which is insignificant for our purposes.


Figure: Changes in Diameter With Position.

The following graph uses shows how we expect the coupling efficiency of a fiber to increase with diameter, using the calculation we presented in Diameter and Location, and assuming optimal positioning of the fiber on a blue EZ500 die. We assume that the fiber always has 87% of its area occupied by the core, and we consider only the light entering and captured by the core.


Figure: Calculated Coupling Efficiency versus Outer Diameter of Fiber. We show the light incident upon the core of the fiber base as a function of outer diameter of the base, and we show the light fraction of light carried to the far end for NA = 0.66 and NA = 0.86.

We observed 27% and 30% coupling efficiency with sections of our NA = 0.66 fiber. Until now, we assumed the fiber diameter was uniformly 390 μm, and so we were puzzled with these high coupling efficiencies. But we now see that a diameter of 420 μm would give us close to 27%, and 440 μm would give close to 30%.

We plan to order NA = 0.86 fiber (core index 1.72, cladding 1.49) from Fiberoptics Technology. Variation in diameter as we observe in our NA = 0.66 batch would serve us well, because we would have a range of diameters to choose from. A nominal diameter 440 μm (0.0173") with tolerance ±35 μm (0.0014") should produce a useful batch.

Diameter and Location

We obtain the following picture of 460-nm EZ500 flashing with 80-mA forward current for a few microseconds. We obtain the photograph with a Camera Head (A2075B) and two neutral density filters. Only 0.1% of the light emitted by the LED passes through the filters.


Figure: Intensity of Light At Surface of Blue EZ500

We extract the pixel intensities from this image and find that the intensity at the center is double the intensity near the edges. We write a program that calculates how much light will be incident upon the base of a fiber pressed against the LED surface, as a function of the fiber diameter and its offset from the center of the LED square.


Table: Calculated Fraction of EZ500 Light Incident Upon Fiber Base. We assume the core is 83% of the cross-section, or 91% of the diameter. Also shown are the fraction of light that will be coupled to the core for various fiber numerical apertures.

The table gives the optimal location of the fiber center, as an offset from the center. An NA = 0.66 fiber with outer diameter 390 μm has core diameter 355 μm. The best place for such a fiber is 40-μm from the LED center, in the direction opposite to the bond pad. We apply our capture efficiency relation to the light incident upon the fiber for three values of numerical aperture. We obtain estimates of the fraction of light emitted by the LED that will be transported along the fiber, which we call the coupling efficiency. The NA = 0.66 fiber is the one we have now, the NA = 0.86 fiber is the one we hope to obtain, and the NA = 0.22 is an industry standard. Our calculation suggests that our existing NA = 0.66 fiber, with its 355-μm diameter core placed 40 μm off-center on the LED surface, will transport 23% of the light emitted by the LED.


Figure: Optical Power Output versus Forward Current. We try three C460EZ500 LEDs.

The graph above shows the optical power output of three C460EZ500 LEDs. We see that they emit up to 28 mW at forward current 30 mA, which is far more than the 18 mW emitted by the C470EZ290. The average power output at 30 mA is 27.5 mW. If we combine these diodes with a 440-μm diameter fiber with a 400-μm diameter core of numerical aperture 0.86, our calculation suggests that we will get 15 mW out of the fiber tip.

UPDATE: [10-MAY-13] See Diameter Variation for plots of incidence and efficiency versus the outer diameter of the fiber, using same calculation described above.

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.