Tuesday, April 23, 2013

High-Index Fiber

We receive from Fiberoptics Technology several hundred meters of 0.016" clad rod, F2/8250. We measure its outer diameter as 390 μm. Its core is Schott F2 glass and its cladding is Schott 8250. The core occupies 83% of the fiber cross-sectional area. We have a 355-μm diameter core with a 17-μm layer of cladding. The rod is so thin we can bend it in a 10-cm radius, and the core has refractive index 1.63, so we will call it high-index optical fiber.

We take a 5-cm length of fiber and polish both ends. We try to use hot-glue to hold the fiber in a ferrule, which is our usual method with silica glass, but when we try to remove the hot glue with a propane flame, the glass melts. The F2 glass melts at only 434°C compared to 1100°C for the silica we usually work with. Our flame-based tapering machine will not taper this glass. We will have to develop a lower-temperature heater to take the fiber just above its glass temperature for tapering. We will have to devise a reliable method for polishing the flat end of the fiber without the use of hot glue. For now, we did the best we could holding it between finger and thumb.

We clean the fiber and coat it with NO13685 adhesive. This adhesive has the consistency and appearance of water when applied, and cures to a tacky solid in UV light. Its refractive index is 1.3685. We lower this fiber onto a green EZ500 LED. We pre-cure the coating, then add clear epoxy to hold the fiber in place, and cure further while blowing nitrogen over the exposed coating. (The coating cures to a non-tacky, flexible solid only in the absence of oxygen.) We turn on the LED and take a photograph of the fiber base with our microscope, shown below on the left.

Figure: High-Index Fiber with NO13865 Coating. Left: first attempt, note the epoxy holding the fiber onto the LED. Right: after cleaning the fiber with acetone and re-coating.

In the left-hand picture we see light escaping from the bottom of the fiber, from dirt beneath the coating, and from the meniscus of the epoxy where it encloses the fiber. The fiber tip emits 19% of the LED light (2.6 mW of green light from a total of 13.6 mW generated with 30-mA forward current). We wipe the fiber repeatedly with acetone. The coating comes off and we continue to clean the fiber until hardly any signs of dirt remain. We coat the fiber again, and cure the coating. We obtain the photograph on the right. The fiber tip now emits 24% of the LED light (3.2 mW). The signs of dirt on the coated fiber are greatly reduced, but we still see the same ring of light at the limit of the epoxy.

If we assume light is distributed uniformly across the 480-μ square area of the EZ500, then 43% of its light should enter our 355-μm fiber core and 9% should enter its cladding. The core of index 1.63, combined with the coating of index 1.3865, gives us a numerical aperture of 0.87, so we expect 76% of the light in the core to reach the tip. The cladding has index 1.487, and is combined with the same coating, so it has numerical aperture 0.58 and we expect 34% of the light in the cladding to reach the tip. Adding these up, we expect to get 36% of the LED light at the fiber tip. We see only 24%.

Figure: A 5-cm Coated Fiber on Green EZ500.

The photograph above shows that the light leaking from the exposed length of the fiber is small compared to the light emitted by the tip. Our close-ups of the base of the fiber, however, show a ring of light being lost from the fiber at the limit of the epoxy. It could be that the coating was improperly cured and the epoxy penetrated to the fiber glass along the epoxy meniscus. The epoxy has index 1.5, which reduces the numerical aperture of the cladding to 0.00 and the core to 0.61. A sufficiently lengthy contact between the epoxy and the glass would reduce the fraction of light reaching the tip from 36% to 16%.

Another potential loss of power at the tip is poor polishing of the fiber ends. In the past, we have observed 10% loss of power due to slight imperfections in the polish. The polish on the ends of this fiber is far from perfect.

Nevertheless, we are now seeing 24% of the LED light at the fiber tip, which is better than our previous record of 17%. When combined with our new and more efficient blue LEDs, this fiber would give us 7.2 mW at the fiber tip for a 30-mA LED current.

In the future, we will try the MY132 coating, with refractive index 1.325. This alone will increase our theoretical coupling efficiency from 36% to 42%. We may also find that the LED light is concentrated towards its center, as we did before, where a 300 μm fiber captured 9/5 as much light as we expected. Thus we might obtain up to 75% of the LED light at the tip.

UPDATE: [26-APR-13] Spoke to Adam at Norland Products and decided to try NO1625 and NO164 adhesives to glue the base of the fiber to the LED. These adhesives have refractive index 1.625 and 1.64 respectively, so they should fill scratches in the fiber base and give us a reliable optical connection between the silicon surface and the core glass.

UPDATE: [03-MAY-13] We achieve 30% capture efficiency with the F2-core fiber and a 460-nm EZ500. The LED emits 27 mW at 30 mA forward current and we see 8.2 mW at the far end of an 8-cm fiber coated with black epoxy. Only the 355-μm core carries light. Its numerical aperture with the cladding is 0.67. We expect 43% of the LED light to enter the fiber, and 45% of this light entering to reach the other end, for a total capture efficiency of 19%. With the fiber offset from the LED center, so as to avoid the cathode pad, we get 24%. But when we center the fiber, which requires that we smash down the cathode bond wire, we get 30%.

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