QUESTION: I am interested in efficiently coupling a 125 or 200um core-diameter fiber onto an LED die. My understanding is that the coupling efficiency is limited by the NA and core diameter of the fiber in most circumstances. Is it possible to use a tapered fiber, e.g. starting at 1000um core and moving to 125 or 200 um core to increase the irradiance at the small diameter fiber side?
ANSWER:
Transmission light from the LED die into any fiber is controlled by both the core and NA as well as the NA of the light coming out of the source. Obviously, the larger the fiber NA and core the better the coupling efficiency. We’ve done a lot of work using LED dies in conjunction with 1000um POF fiber which is a perfect match. Due to most application requirements, we’ve been forced to develop methods of coupling to 200um fibers with a .37NA and .48NAs. Inexpensive tapers did not work unless they were very long which are very expensive. Also, tapers had to be manufactured with a gl/gl fiber of .22NA which defeat the purpose of the design criteria. The shorter the taper, the more severe the internal angle thus the coupling efficiency is very poor. This can be easily modeled by performing a ray trace on zemax. Here is what we’d recommend, assuming the power is fairly low, a plastic lens would be the most cost effective as well as having the best coupling efficiency. There will be other issues that will have to be dealt with here related to the output pattern of the LED die.
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Oval or rectangular shape.
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Hot spots
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Cladding modes of the fiber.
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Fiber output, if the length is short will be very strange as well.
In short, there are quite a few issues to consider and we are more than happy to put you in contact with someone to discuss further.
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