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.