Measured Capture Efficiency

We have four different types of optical fiber and two different types of light-emitting diode. We make three samples of each fiber, which we name 1 to 3 for each type. Each fiber has two faces, A and B. We lower each face of each fiber onto an LED in turn and measure the power emerging from the other face.



Figure: Fraction of Power Captured By Fibers. The 480-μ square LED is the C460EZ500. The 290-μm square LED is the C470EZ290.

In the above table, we calculate the theoretical capture fraction in two steps. First, we estimate the fraction of light that will enter the fiber, based upon the area of the fiber and the area of the LED. Second, we apply our cosine-distribution solution to the capture efficiency of a fiber of known numerical aperture, which we present here.

In the case of the 400-μm fiber over the 290-μm square LED, we assume all the LED's light enters the fiber. The fiber is larger than the diagonal of the square, and placed within 50 μm of the LED surface by pressing down the bond wire. We observe 15% capture fraction and calculate 14%. Our calculation based upon the fiber numerical aperture appears to be accurate.

In the case of the 300-μm fiber over the 480-μm LED, we assume that only 30% of the light will enter the fiber. But our observed capture fraction is 9% and our calculation is 5%. Given that we already trust our numerical aperture calculation, we suspect that twice as much light is entering the fiber as we expected, which in turn means that the light emitted by the 480-μm square LED is concentrated towards the center.

When we place a 400-μm fiber with NA = 0.37, NA = 0.25, or NA = 0.24 on the 480-μm square LED, the capture fraction we measure is roughly 1.6 times the one we calculate. This result is also consistent with concentration of light towards the center of the light-emitting area.

Our 300-μ NA = 0.41 fiber's capture fraction with the 290-μm LED is 17%. With a 400-μm, NA = 0.37 fiber on the same LED we get 15%. If we assume that all the light from the LED enters both fibers, then the difference in capture fraction is consistent with our calculation due to numerical aperture. This suggests that the light emitted by the 290-μm LED is also concentrated towards the center, so that all of the light enters the 300-μm fiber.

We conclude that our numerical aperture calculation, which assumes a cosine distribution of light emission by the LED, is accurate, but our assumption of uniform distribution of light across the LED is not. The light is concentrated towards the center of the emitting area. Thus we are able to obtain almost double the capture fraction that we would with uniform light distribution.

With the 300-μm, NA = 0.41 fiber on the EZ290 we get 17% of the light emitted by the LED emerging from the tip of our fiber. If we could obtain a C470EZ290 that emitted 40 mW for 30 mA current, as specified by the data sheet, we would obtain 6.8 mW at the fiber tip. As it is, our EZ290s are producing only 17 mW at 30 mA, so we see only 3.0 mW. We do not know why our EZ290s are performing so poorly. The are emitting less than half the light we expect from the calibration sheet supplied with our samples. It may be that they have aged from exposure to air over the past year and a half since we received them. In the case of the EZ500, the LEDs produce 25 mW for 30 mA current, and we get 11% capture efficiency with our 400μm, NA = 0.37 fiber. So we obtain 2.6 mW at the fiber tip.

As things stand today, we can obtain roughly 3 mW with both the EZ290 and the EZ500, using our high numerical aperture 300-μm and 400-μm fibers respectively.