On Wednesday, February 27, 2013 9:40:25 AM UTC, The Natural Philosopher wrote:
On 27/02/13 09:19, Brian Gaff wrote:

Hmm, so what makes leds have an issue with higher frequencies then, I'd have
thought this was of no concequence , but maybe its to do with the individual
colours of led in the package having different levels on higher
frequencies.

Brian

They are so bad that they are de facto light source for infrared remote
controllers operating at tens or hundreds of khz..and for driving
optical fibres at GHZ rates.

Man, that's really bad.

It's a long while since I was using LEDs to drive optical fibre links, but I recall that they ran out of steam at around 10MHz for most types, with some specialized ones managing up to 100MHz. Almost all the links I've looked at since (and everything at above 100 Mbit/s) use lasers, and a high proportion of those use a separate (even if possibly integrated in the same package) modulator.
Visible LEDs as I recall were something like an order of magnitude slower than infra-red - but I'd agree that this is still more than fast enough not to be visible.