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

Dana Whitlow k8yumdoober at gmail.com
Sat May 12 11:38:57 EDT 2018


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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