We did a unit at my last job called 'velocity of light'.

It was pretty simple and you cud accurately measure distance down to 1cm.

Infrared led emitter switched at 50MHz, focused using a lens to a 2 inch beam,
reflected back by a mirror to an infrared detector (5 inches away from the
emitter).

The tx 50MHz was generated using a 50MHz xtal which drove the tx led.

Their was a 50.025MHz second xtal osc which was used to mix down both the tx
signal and the rx signal to 25KHz IF (we now have 2 25KHz waveforms), these two
25KHz carriers were then phase compared - so easy to see the smallest of
movements in the mirror on a basic scope.

So all the hard work is done at 25KHz (phase measuring) - one of todays little
mcpu's will do this easily (ATmega16 for example).

You could just as easily use laser or maybe rf in place of the IR led's, though
directing RF at such low freq's would be somewhat difficult.

Obviously at 50MHz, the phase difference would cycle every 6 meters (total
reflected path), but if your a bit cleverer (though not hard to do) you could
get the freq to sweep from a low freq (say 5MHz) upto wot ever you like - using
a pair of single xtal referenced PLL's to generate the two oscillator freq's
(whilst maintaining the 25KHz difference) and then calculate an exact distance
in the cpu.

This method is simple and relatively cheap to judge distances very accurately
with no need for very short pulses (high bandwidths) and very fast logic.

Clive