Higher-Index Taper

In one of our earliest posts, we described how an electric heating element was unable to heat silica-silical fiber to its 1100°C softening point. Our high-index fiber and our higher-index fiber soften at around 430°C. We equip our motor-driven tapering machine with a coil of nichrome wire, potted in high-temperature cement and produce the taper shown below.


Figure: Taper in 410-μm Higher-Index Fiber. Blue 460-nm light. Taper is 2.0 mm long.

The taper shown above is at the top of a 40-mm shaft of higher-index fiber, polished at the base. Its diameter is 410 μm. We lower the base onto a C460EZ500 blue LED die. This particular LED has output power 27 mW (4.85 mA photocurrent through unbiased photodiode) for forward current 30 mA. Because we were in a hurry, we fastened the fiber to the LED with five-minute epoxy. We bake it in our oven at 60°C and within twenty minutes, the epoxy is fully cured.

We drive 30 mA through the LED. We press our 10 × 10 mm² photodiode against the side of the fiber, so that the axis of the fiber is a millimeter from the photodiode window. We expect light emitted by the taper more than 20° from the fiber axis, and in a ±80° slice in the radial direction, to enter the photodiode. We see 3.7 mW (0.67 mW photocurrent). We hold the photodiode 15 mm from the top of the taper, and perpendicular to the fiber axis. We now receive most of the light within ±20° of the axis. We see 1.1 mW (0.2 mA photocurrent). Multiplying the first measurement by 2*9/8 and adding the second, we arrive at 9.4 mW total emission from the taper. Assuming 2.2% loss per 10 mm, the power entering the base of the fiber should be 10.3 mW, which is 38% of 27 mW.

We produced four such tapers from two lengths of higher-index fiber polished at both ends. The one we show above is the shortest. We can make them in the same machine with 6 mm of fiber followed by 2 mm of taper. Such a fiber would emit around 10 mW from its tip for 30 mA of LED current.

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.

Gluing Loss

We take a 150-mm section of our higher-index fiber (NA = 0.86) and polish it at both ends. The diameter of this section is 410 μm.. We lower one end onto a C460EZ500 blue LED die. According to our diameter and location simulation, the best coupling of light from the LED to a 410-μm fiber occurs when the fiber is roughly 40 μm offset from the center of the die, in the direction opposite to the bond pad. We calculate that 58% of the light emitted by the LED will be incident upon the fiber base.

In practice, however, we find we cannot lower the fiber into this optimal position because the wire bond on the bond pad is around 100 μm high. We tried raising the fiber above the wire bond, but we obtained the greatest coupling efficiency with the fiber placed flat on the surface of the LED as close to the center as we can get it. The result is shown below.


Figure: Offset Fiber. This 410-μm diameter fiber is roughly 120 μm from the center of the EZ500.

With the fiber 120 μm from the center, our incidence calculation suggests we will get 52% of the LED light incident upon the base of the fiber. With NA = 0.86, we expect 74% of the light incident upon the base of the fiber to emerge at the other end, for an expected total capture efficiency of 38%.

We run 30 mA of current through the LED. The power emitted by the far end of the fiber is 9.8 mW (photocurrent 1.77 mA and sensitivity 0.18 mA/mW). We remove the fiber and measure 25.2 mW total power output (photocurrent 4.53 mA). Our coupling efficiency is 39%. We re-position the fiber several times. We obtain 38% or 38% coupling efficiency each time.

With the fiber in place, we inject E-30CL clear optical epoxy (RI = 1.5) into the LED package. The epoxy flows around the fiber and LED. The result is shown below, with a few milliamps of current in the LED.


Figure: Offset Fiber with Clear Glue.

As soon as the glue has surrounded the LED, the power at the fiber tip drops from 9.8 mW to 5.3 mW (photocurrent 0.95 mA). We raise up the base of the fiber and remove it from the glue blob and measure the total LED output to be 25.7 mW (photocurrent 4.62 mA). We replace the fiber and place it in its previous position. Once again we obtain 5.3 mW at the fiber tip. Our coupling efficiency has dropped from 39% to 21% with the application of glue.

Right now we have no explanation for this dramatic drop in coupling efficiency with the application of glue. So far as we understand, the a thin layer of transparent material between the fiber and LED will not stop light entering the fiber. Nor will glue on the outside of the fiber reduce its numerical aperture below 0.86. Nevertheless, the effect is real and repeatable. We have observed it many times, today being merely the most detailed observation of the effect to date.