Image: EZ-500 bare die (white) bonded onto a surface mount chip. The die measures 480x480μm, while the chip measures 3.25x3.25mm. Wire-bonded anode and cathode wires are visible connecting to the chip's pads.
We used our stage to abut several kinds of optical fiber to the LED, one at a time, to test the coupling efficiency (the amount of light captured by the fiber, divided by the die's total output). For fibers with cross sectional area smaller than the area of the die, this simple abutment of fiber end to the die is the most efficient method of coupling light into the fiber, as discussed by Hudson, 1974. We abut one end of a 15cm length of fiber to the die. We then measure the coupling efficiency by measuring the light output at the distal end of the optic fiber and dividing this by the total optical power of the LED. We measured the coupling efficiency of four different fibers to range from 0.12% to 17% (for {62.5micron core, NA = .22} and {400micron core, NA=.37}, respectively).
Images: Above, a fiber with a 300μm core, 360μm total diameter. The fiber is not centered over the die, but is instead adjacent to the gold bond pad. Below, a fiber with 62.5μm core (125 μm total) is centered over the die. Click for larger version.
We summarize the coupling efficiency of four different fibers in the table below. The measurements were taken with 72.1mA passing through the EZ-500. Total output was measured to be 56.3mW. We will provide a discussion of these results in our next post.
Image: Light being coupled into a 300μm core fiber.
References:
M. Hudson, "Calculation of the Maximum Optical Coupling Efficiency into Multimode Optical Waveguides," Appl. Opt. 13, 1029-1033 (1974).
Dimitri Kullmann writes, "Thanks Kevan. I gather there are optical fibres with higher NA than 0.37. Do they give any scope to increase the coupling efficiency?"
ReplyDeleteAccording to Shibata et al High Numerical Aperture Multicomponent Glass Fiber, a glass made of 40% PbO by weight will have refractive index 1.65 as compared to usual glass with index 1.55. Using the lead glass as a core we could obtain a numerical aperture of 0.56. In practice, they obtained 0.50. This would give us an acceptance cone of ±34°. Compared to our existing ±22° we could get double the capture efficiency.
But we can't find such fiber on the market. Maybe we have not yet looked hard enough. There is fiber with numerical aperture 0.7, but it has a plastic cladding that we cannot heat for stretching, and it's outer diameter is much greater than its core diameter. It is possible that we could hire a company to make us the special all-glass fiber with lots of lead in it, when we are certain that it's worth doing.
Dennis Kaetzel writes, "see page 30 in the doric catalogue attached for 300um / 0.48NA and 200um / 0.53NA. We could also buy this and ship to you. Maybe one cold ask them for a custom-made 0.53 NA 300um. Of course it is more expensive, but you'll only need tiny amounts, so even an overpriced 1m patch cord will last you very long." He attached a copy of the Doric Lenses catalog.
ReplyDeletePart number 300/330/1000-0.48 has 300 μm core and numerical aperture 0.48. Part number 400/430/1100-0.48 has 400 μm core and numerical aperture 0.48. We will try to obtain samples of both. With aperture 0.48, we expect to capture 23% of LED light, compared to 14% for numerical aperture 0.37.
So far as we can tell, the Doric Lens fiber is of a type we have already considered. The fiber obtains a high numerical aperture with an optically active plastic buffer. The buffer is too thick for us, and we can't taper it. Michael is working on a new post detailing the properties of the high NA fibers and others we have considered and rejected.
ReplyDeleteIn short: it looks as if we did our homework after all. Nevertheless, a fiber with aperture 0.5 made entirely of glass is possible. We just don't know anyone making it.
I've summed up our understanding of the problem here:
ReplyDeletehttp://isldev.blogspot.com/2012/08/high-na-fibers.html