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

Dana Whitlow k8yumdoober at gmail.com
Sat May 12 19:31:46 EDT 2018


Indeed; however, with single mode fiber the limit is not too bad.  At
Arecibo we routinely ran bandwidths in
excess of 1 GHz through fibers of about 1500 ft length with no problems.
For the science fair project a
bandwidth of a few MHz should suffice for lengths of, say, 500 ft.  It's
just that I don't know how bad the
multimode dispersion problem would be when using shorter wavelengths, and
I'm sure not equipped to
make any measurements at home now that I'm retired and far away from the
observatory.

Dana


On Sat, May 12, 2018 at 5:44 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz
> wrote:

> 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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