On Mar 18, 11:16*am, Richard Clark wrote:
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.

The reference to 10 GHz was to try and relate the problem at Near IR
to something more familiar (since I suspect that most r.r.a.a are more
familiar with RF than light) (since speed of *light* was being
discussed, and the reference cited referred to optical communications
at Near IR)




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.

The figures in the referenced paper showed a manmade source,
presumably with some sort of source which could be designed to make
measuring dispersion easier.



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

Precisely.. but if you're going to measure dispersion, you've got to
have a way to do it, and sending out two signals that are coherent
with each other at a known offset seems to be a fairly straightforward
approach.


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?

For optical signals, there could be other ways to generate multiple
signals at different frequencies that are coherent with each other. I
don't know enough about optical measurement techniques. I *do* know
that you can modulate near IR with RF signals using a variety of
techniques.



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.


Back to the figure reference.
And, of course, getting coherent signals from a celestial source might
be a challenge. I don't know.. maybe some sort of clever correlation
technique in an interferometer would do. Not really my field.
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.

Precision detection of a man made signal from across the solar system
is a challenge.



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?

Indeed. That would make things easier, and is something that people
want to do. But so far, we're stuck with just sending the transmitter
out there.



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.


Not so much failure, but that the original question asked for
experimental data to confirm a fairly well understood effect. My
point is to show that collecting that experimental data is non-
trivial, and not something that you can just rig up in your backyard
with baling wire and sealing wax.


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

There has been more than one optical comm experiment from deep space.


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.

Well.. yes, it *is* hard, but if you want to know the internal
structure of another planet, it does take some work.




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


The example of 32 GHz is to illustrate that I am not just speculating
about the difficulties of doing it at IR. I have personal knowledge
of how hard it is at 32GHz, and I'm pretty sure it's harder at Near
IR.

So someone on a listserv or usenet group who's looking for a "hey I
did the experiment yesterday, and sure enough, dispersive media have
dispersion" isn't going to get it. Nor are they likely to find the
results of someone who might have done it for real without digging a
bit.