On Thu, 17 Mar 2011 17:19:09 -0700, Jim Lux
wrote:

Certainly he predicts that the temporal dispersion is going to be 0.1ps
for near IR, which is, shall we say, challenging to measure.

Why?

measuring things to tenths of a picosecond, repeatably, can be tricky..
That's like measuring the phase difference between two 10 GHz signals
to 0.3 degrees. Or, another way to look at it is 1 picolightsecond is
about a third of a millimeter.

A third of a millimeter is no big deal and for an optical (or
sub-optical) signal - trivial. Perhaps, when stated in terms of two
10 GHz signals, "near IR" is being vastly over stated.

You're looking at
a) figuring out how to generate two signals at near IR that has a
frequency offset that can be accurately controlled.

Controlled? This is dreaming in technicolor (or near IR color) if the
source is celestial.

I thought the discussion was about dispersion, the characteristic of
the medium, not sources.

Probably some sort
of heterodyne mixing scheme would be easiest.

Heterodyning is extremely commonplace and accurate - why would it be
pondered as an alternative method?

b) sending those two signals over the optical path through
interplanetary space.

This blurs my understanding of celestial where two signals is a
poverty of what is available from ANY celestial source.

c) recovering the signals,

If there is a problem of recovery, it seems it is more a practical
matter of source selection. Given the billions of celestial sources
available, I don't understand the problem.

measuring the propagation time variation
(say, by looking at the phase difference between the modulation
signals), and then removing atmospheric effects.

Why worry about the atmosphere when you can get above it?

d) it's probably going to be a pretty weak signal, so you'll need to
average. That means your measurement system has to be picosecond stable
over the averaging interval.

OK, so I am lost. This laundry list of difficulties seems to be
prepared to anticipate failure.

Name the near IR source and defend its choice in light (no pun) of
these intractable difficulties.

None of those steps are particularly simple or easy.

I've worked on systems to measure the (microwave) distance to Jupiter
and back with an accuracy of around 1 part in 1E15 at 32 GHz,

32 GHz is what photonics would call far-far IR at roughly 3 to 4
orders of magnitude distant from "near IR."

integrating over 1000 seconds. That's tenths of a picosecond out of 1000
seconds. It's challenging.

No doubt - like trying to push a peanut up Pike's Peak with your nose.
That too has been done with challenge in mind.

How did this slip from "near IR" to 32 GHz?

73's
Richard Clark, KB7QHC