[time-nuts] Anybody have suggestions for time related science fair projects?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat May 12 18:44:36 EDT 2018


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 <k8yumdoober at 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 <kb8tq at n1k.org> wrote:
> 
> > Hi
> >
> >
> >
> > > On May 12, 2018, at 7:01 AM, jimlux <jimlux at 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
> >
> >
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