Even with single mode fiber its finite group delay dispersion will likely restrict the usable light source bandwidth.
Bruce > On 13 May 2018 at 03:38 Dana Whitlow <k8yumdoo...@gmail.com> wrote: > > > It may be that a nicely-written request to Corning could yield the loan of > a big spool of fiber > for the duration of a science fair project. > > Another alternative, perhaps easier to implement, might be an > electrically-driven light modulator > at the detector end. For the source, an LED or diode laser is easy to > modulate at respectable > rates. This approach should allow use of such high frequencies that an > open optical path using > mirrors might even suffice. > > Or here's an intermediate scheme: > If one were to use two modulated sources (or one with a beamsplitter), with > one path delayed > by the long(ish) fiber and the other by a minimal-length local fiber, > something resembling a streak > camera (implemented with a rotating mirror) might permit use of > substantially higher pulse rates > than with a rotating disk, without incurring the need for anything very > fancy in the way of mechanics. > Only the modulated source should require a reasonably accurate drive > frequency- the "detector" > would be essentially self-calibrating. A small mirror, say of cm size, > could probably be safely > rotated at Dremel speeds approaching 500 rev/s, and if 1 mrad angular > resolution is attained, > this would yield a resolution of ~160 ns. So a fiber length of 500 ft > (approx 750 ns one-way delay) > should yield an angular separation of nearly five "dots" between delayed > and undelayed dots. > And if the sources are modulated at a rate such that a few pulse > repetitions are visible in the > field of view, the scheme is self-calibrating as long as the PRF and the > velocity factor in the fiber > are known. Probably the only precision work would be the optics required > to focus a reasonable > amount of light from the source(s) onto the two fibers., and I believe this > requirement could be > adequately met with microscope objectives borrowed from one's school's > biology lab. > > A fly in the ointment is that if ordinary (read, inexpensive) IR fiber is > used at convenient visible > wavelengths, propagation will occur in more than one spatial mode, with > different modes propagating > at different speeds. I don't know how much of a problem this would > raise. But it may be that if > tweaking of the transmitting end illumination is done, both in angle and > transverse position, most > of the propagating light could be confined to a single mode. I speak of > visible wavelengths simply > because using these avoids the cost of electronic detectors, oscilloscopes, > etc, potentially saving > a lot on the cost of the experiment as well as making for a more satisfying > presentation. > > Dana > > > On Sat, May 12, 2018 at 9:45 AM, Bob kb8tq <kb...@n1k.org> wrote: > > > Hi > > > > > > > > > On May 12, 2018, at 7:01 AM, jimlux <jim...@earthlink.net> wrote: > > > > > > On 5/11/18 9:08 PM, Jeff Woolsey wrote: > > >> David.vanhorn wrote: > > >>> Measuring the speed of light (Fizeau or Michelson method? Other ways) > > >>> > > >>> > > >>> I saw a great demo of this at the Exploratorium in SF. They had a > > long spool of fiber optic, a disc with holes, and a light source. When > > static, if the light shines through the hole in the disc into the fiber, > > then you can see the light coming out the other end of the fiber through a > > different hole. When rotating, you increase speed and the fiber output > > gets dimmer and dimmer till it's gone. At that point, the light going > > into the fiber arrives when the other end is blocked, and vice versa. High > > tech, but simple. > > >>> > > >> My favorite exhibit that we never see anymore. IIRC it was a quarter > > >> mile of fiber and a green laser. And ISTR that the disc had one hole on > > >> one arm and two radially on the other, but I can't remember why. I > > >> thought that the light would pass through the same hole twice, once on > > >> the way in and on the way out when that same hole rotated 180 degrees to > > >> the other end of the fiber. The disk spun somewhere around 50 rps (60 > > >> with an AC motor?). > > > > > > > > > 1km in free space would be 6 microseconds round trip. I'm not sure a > > disk spinning at 3600 rpm would work. you'd need to have the "hole > > spacing" be on the order of 6 microseconds - and at 100 rps (6000 RPM), 10 > > ms/rev, you'd need the sending and receiving hole 6/10000 of a rev apart > > (about 0.2 degrees). > > > > > > if you had 10 km of fiber, it would be a bit easier. > > > > I think the term “long fiber” in this case should really be “very very > > long”. Exactly how the typical student > > funds the acquisition of something in the “many miles” range, I have no > > idea. > > > > You could use an optical grating of some sort as your “spinning disk”. The > > end of the fiber is going to be > > mighty small. The spacing on the grating could be quite tight. Where you > > get a circular part like that …. > > again no idea. > > > > Bob > > > > > > > _______________________________________________ > > > time-nuts mailing list -- time-nuts@febo.com > > > To unsubscribe, go to https://www.febo.com/cgi-bin/ > > mailman/listinfo/time-nuts > > > and follow the instructions there. > > > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to https://www.febo.com/cgi-bin/ > > mailman/listinfo/time-nuts > > and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
