Re: [ATM] Large Amateur Telescope

[Thomas Miller <tommy81@comcast.net>]

Matt wrote:

>Optical fiber has a fairly high level of PMD (polarization mode 
>dispersion) which would render your measurement worthless.  Just flex it a 
>little and watch your phase change go all over the map, or rotate the 
>polarizer you've got at the end (you DO have a polarizer on the fiber 
>exit, don't you) and change the phase.

Our device works for any polarization state, including unpolarized 
light.  We are not measuring the phase of light.  We are measuring the time 
delay of a chopped-light waveform.  The phase we refer to is the "phase 
shift" (really a time delay) of the chopped waveform caused by the 300m 
fiber delay loop.  This phase shift is easily seen on the oscilloscope in 
real time in the lab with a bright light source, and it most certainly does 
not jump around when you flex the fiber.  And no, we don't have a polarizer 
on the fiber exit.  This is not only unnecessary, but would attenuate 50% 
of our scarce photons.

We use 200 micron core multimode (MM) fiber.  If we were using single-mode 
fiber, we might have to take PMD into account, but I'm not sure:  I don't 
know much about the quirks of single-mode optics.  I wish we could use 
single-mode (SM) fiber, as it has much less fiber attenuation than MM, but 
it's tough enough stuffing starlight into a 200 micron MM orifice, let 
alone the 8 microns or so for SM.

Perhaps you were assuming that we were using a fairly well-known method of 
lightspeed measurement using opto-electrical choppers (Pockel's cells) for 
which polarization concerns are paramount.  However, we are using a 
mechanical chopper akin to the toothed wheel employed by Fizeau around 1850 
in the first direct lightspeed measurement.  I wish we could use the 
Pockel's cell method, as the chop frequencies are much higher than for 
mechanical choppers, and the fiber delay would be shorter, the fiber 
attenuation less.  The main problem with Pockel's cells, for our purposes, 
is their narrow bandwidth.

The bottom line is that our device is not just a design on a piece of 
paper.  It has been built and tested and it works.  All "theoretical" 
objections seem to have been addressed.

* * *

When someone tells you something can't be done, don't believe it, 
especially if the objection is of a theoretical nature.  The "laws of 
physics" prevented the use of radio waves to create images of the human 
body:  the wavelength was too long, and there was some law about spatial 
resolution being on the order of a wavelength.  Yet MRI uses radio waves to 
create such images.

We were told by engineers at the company that manufactures the fastest 
commercially available mechanical optical chopper that the "laws of 
physics" put a limit on mechanical optical choppers at about 50 kHz.  We 
are now pushing 600 kHz with our latest design.  (We thank this company for 
graciously loaning us their prototype micro-slotted disks.)


>In any case, light goes at different speeds thru different materials, so 
>to measure the speed accurately, you have to know the index of refraction 
>accurately, which is tough to do with optical fiber.

Well, to be truthful, we don't actually measure the speed-of-light in 
absolute terms.  We measure the speed-of-light as a ratio to the 
speed-of-light for a standard lab source.  We assume that our lab source 
has a "normal" lightspeed, but we don't actually measure the absolute speed 
of the lab source or the star:  we just compare them.


Tom-


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