Speed of light circuits

You'll do a great deal better using CW and a phase measurement. Not as much fun, though.

Cheers,

Phil Hobbs

The LED pulse should be no slower than the response of the photomultiplier. say 2ns risetime 5ns FWHM.

I've used a slight modification of the circuit presented in "A fast timing light pulser for scintillation detectors" Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume

241, Issues 2-3, 1 December 1985, Pages 612-613 J. S. Kapustinsky et al.

It works with some blue LEDs (you've got to match the photocathode)

Not really. If you use a high gain tube. The pulser will kick about 10^7 photons per flash. You can run the photomultiplier straight into the scope.

No idea. Sampling scopes plus photomultipliers are normally a bad idea. You've got 20ns transit time on the photomultiplier plus ~3ns per metre on the optical pathlength - total delay of ~100ns for a reasonable size experiment. Just use an oscilloscope with

We did this to calibrate the Antares neutrino telescope

r-timing.pdf

Spark gaps can be interesting. Charge up a low-inductance cap until it fires a gap. You'll get kilowatts of light with ns risetime. With some moderate optics and a pmt (fresnel or curved mirror for the transmitter, small telescope for the pmt) you can get echoes off clouds.

John

Oh, that sounds so COOL! An excuse to play with high voltage! I'll have to try that as soon as I make it throught the Honey-Do list.

PMTs seemto be in the 7nSec region unless you want to $pend a lot more for (maybe) a X3 improvement... Now if you take that light source and bounce it off the moon (assuming you know the distance) or off a reflector on a rather distant mountain (again assuming you know the distance), then it might be a little more practical.

Some blokes did it a bit back with a vacuum cleaner and a couple of pint glasses.

I'd suggest you are not really an Ancient_Hacker and more like a Lazy_Arse if you can't sort that one out using modern tecknowledgy.

DNA

I was thinking of the Electron Tubes 9813 and similar (sim. Burle 8575) which are ~2ns. OEtech have them for USD 215

I've got a whole load of these looking for a project...

No, it is quite feasible in a largish room. I've done it. I even offered the technology to Ralph S******* as evidence that the speed of light was actually c - but he wasn't interested....

There is also the method of a moderate speed spinning mirror, a hene laser or even a good quality laser pointer, a larger mirror and a football field. Needs far less scope bandwidth and only a simple diode detector, but its a phase method and not so satisfying. You can get the commecial off the shelf kit for ~100$. Just add scope.

Steve Roberts

Are you going to measure the average roundtrip and bounce the light back or go for the realy hard part and try to measure it in a single direction ? if so how will you avoid the propgation delay of the cables between trasmitter/sensor/computer from negating your results ?

If you use a continuos train of pulses you can use a hp5328b to average the difference between two pulses with sub picosecond resolution, with only moderatly fast tramsitter/sensors.

Colin =^.^=

Don't change the cables!

John

use two tubes one with a short optical path (possibly take from the side of the led) through an attenuator (if neccesary)

ajdust the pulse frequency until the phases match.

d=N*lambda Bye. Jasen

Use pulsed light and do time of flight. Rig some contraption with mirrors and corner cube reflectors. In one configuration the light takes a short path (10cm); in the other it goes to the end of the room and hits a corner cube and comes back (20m). You measure the distance with a tape measure to 1mm. You leave all the cables untouched and don't screw with the pulser or photomultiplier supplies between the two configurations. You measure the time delay to a few hundred picoseconds and you have the speed of light to less than a percent accuracy. Its easy.

j

Instead of a "larger mirror" on the far end, you should use a corner reflector/retroreflector. Aiming a flat mirror would be a nightmare.

Cheers! Rich

;-)

Iwo