Electronic – What are the tradeoffs for the various options coupling a LED to an optical fiber


I'm trying to couple a white LED to a fiber bundle that's comprised of several 30 uM silica fibers. The bundle outer diameter is on the order of .5 mm. The NA of the fiber is .87 and the whole bundle is placed in a stainless steel ferrule that's .125" in diameter. I've read about different methods of coupling — everything from a ball lens to gluing the fiber directly over the LED die.

I'm curious if anyone has experience doing something like this. Can anyone delineate the advantages/disadvantages of the various methods? Are the main tradeoffs cost vs transmission efficiency? If so what methods are on which end of the spectrum?

It's worth stating that I'm not sold on any particular LED, but I have experimented with simply holding the ferrule over a LXML-PWC2 — tons of light comes in (enough for my application) with iF ~400mA. This would be a reasonable solution except that the efficiency of transfer is terrible (obviously) so I have to use more current than I'd like. The result is a rather large heat sink, which I'd like to shrink or ditch all together.

Best Answer

"Typical" fiber for communication is 9, 50, or 62.5 um diameter in the core, but there is a 125 um cladding that is also necessary for the fiber to work. There is also 900 um core plastic optical fiber. What is out there for illumination, I'm not sure. Clearly you're not working with one of the types that I'm familiar with.

But, the key point is that the core is smaller than the cladding. And the light coupled into the fiber will be (best-case) what falls on the core. With .87 NA you'll probably get pretty close to this ideal. So I'll just make up some numbers. Say you have a 30 um core in a 60 um cladding. Overall the maximum coupling efficiency you could expect from uniformly illuminating the end of the fiber bundle is 302/602 or 25%.

Then there's a factor for the packing density of round objects into a an area (the gaps between the fibers in the bundle) which I believe is about 78% best-case.

And a reflection loss of about 4% for light entering the glass from air.

Add all these up, (.25) x (.78) x (.96), and you have about 19% best-case coupling efficiency. (You'll need to re-calculate this knowing the correct factor for the first term)

If you are getting near this, I'd say you are actually doing pretty well.

Of course it wouldn't hurt to find an LED that emits in a narrowish cone instead of over half of all space, or even to add some kind of lens to focus the light on the area of the fiber bundle. But generally your best case coupling is still not going to be all that great.