Yellow Glass

Yesterday we were mystified when our coupling efficiency dropped from 39% to 21% when we applied glue to the base of a 146-mm higher-index fiber. Today the epoxy is cured. The fiber is attached firmly. There are only a few small bubbles in the glue, towards the surface. We apply 30 mA to the LED. When the glue was curing yesterday, we obtained only 5.3 mW at the top end of the fiber. Today we obtain 8.2 mW. Yesterday the total power output of the LED at 30 mA was 25.7 mW. Our capture efficiency appears to be 32% now the glue has cured, but was only 21% while it was liquid.

Most of our 146-mm fiber is in air. The bottom 4 mm are covered with glue of refractive index 1.5 and a 3-mm section is clamped in steel. It may be that some light is propagating up the fiber in the cladding by total internal reflection at the cladding-air boundary. We apply clear epoxy to 30 mm of the fiber, near the top. The power at the tip remains 8.2 mW. We conclude that the 4-mm coating of epoxy at the base is sufficient to cause the loss of all light except that which is captured by the core.

We cut back the 146-mm fiber to 30 mm and polish the tip. We apply 30 mA and obtain 10.6 mW at the new tip. The fiber and LED are shown below. The conical angle of the emission from the fiber tip is around ±60°, which is consistent with a numerical aperture of 0.86.



Figure: A 30-mm Higher-Index Fiber. Click on image for higher resolution. The base is glued to a C460EZ500. The tip is polished.

The T5 glass used by Fiberoptics Technology is a proprietary glass with index 1.72. The manufacturer tells us the glass is "very yellow", which we take to mean that it absorbs blue light. A similar glass with index 1.72 made by Schott, P-SF69, has transmission over 10 mm of 98.5% at 460 nm. Suppose the T5 glass has transmission η after 10 mm, and the power coupled into our fiber at the base is P, then Pη^3.0 = 10.6 mW and Pη^14.6 = 8.2 mW. From this we conclude that P = 11.3 mW and η = 97.8%.

With 11.3 mW out of 25.7 mW being coupled into the core of our fiber, our coupling efficiency is 44%. Our calculated coupling efficiency for this arrangement is only 38%. It may be that the LED is producing more power today than yesterday. If it is producing 28 mW, which is what we are hoping for from these LEDs, then the coupling efficiency is closer to 40%.

Yesterday, we were mystified because we assumed our fiber in air was not able to transport light in its cladding, because of dirt on its surface, and because of the metal-glass contact in the fiber clamp. And we had forgotten about the absorption of blue light by this high-index glass. Furthermore, there appears to be some problem with the coupling when the glue is liquid, and this we still do not understand.

We have a fiber that produces 10.6 mW at its tip with 30 mA LED current. If we taper the fiber to a total length of 8 mm, which is our intention for the head fixtures, we expect to get 11 mW out of the taper. If we increase the LED current to 50 mW by using shorter steel leads, the power at the taper should increase to around 18 mW.