Now here's a new idea

J

John Schutkeker

Guest
Somebody on this group recently told me that scientists are building an
optical interferometer, with two sensors linked by fiber optic cable. Does
anybody have any thoughts on how to step-down the EM frequency so that the
signal could be transmitted by twisted pair, without losing synchronization
between the two signal sources?










--
"I prefer to detour the repository of tradition, wherever possible."
--James Steranko
 
"John Schutkeker" <jschutkeker@sbcglobal.net.nospam> wrote in message
news:Xns960175317BE88lkajehoriuasldfjknak@151.164.30.44...
Somebody on this group recently told me that scientists are building an
optical interferometer, with two sensors linked by fiber optic cable.
Does
anybody have any thoughts on how to step-down the EM frequency so that the
signal could be transmitted by twisted pair, without losing
synchronization
between the two signal sources?
Click to expand...
Heterodyne the light with a laser. (Hint: it's
not a new idea.)

George
 
Actually, the real reason why it wouldn't work is because, having down-
shifted the light, you'd destroy synchronization by running it through a
circuit switched phone system. That's the "new" part that I didn't
mention in my OP. Even if you had a tunable delay circuit that self-
activated every time you redialed, the signal integrity would completely
disintegrate the minute your waveform hit the telco switching machine.

And another headline making idea bites the dust.
 
USNO built one at Anderson Mesa aobut 6 years ago and it has been in
use for several years. Go to their web site for info.
 
"jdb_mars@yahoo.com" <jdb_mars@yahoo.com> wrote in
news:1110892561.129007.289870@o13g2000cwo.googlegroups.com:

USNO built one at Anderson Mesa aobut 6 years ago and it has been in
use for several years. Go to their web site for info.
Click to expand...
Was it circuit switched or just a frequency step-down?
 
I really don't know since I was at USNO in DC and only hear rumors
about the Flagstaff operation. Sorry. There was someone I knew out
there who occasionally posted on newsgroups, but I've been retired
for 4 years now and forget everything about that place. Will look his
name up in my empty brain.
 
Try this: http://www.nofs.navy.mil/projects/npoi/
 
In article <1110932461.407528.202460@o13g2000cwo.googlegroups.com>,
jdb_mars@yahoo.com <jdb_mars@yahoo.com> wrote:
I really don't know since I was at USNO in DC and only hear rumors
about the Flagstaff operation. Sorry. There was someone I knew out
there who occasionally posted on newsgroups, but I've been retired
for 4 years now and forget everything about that place. Will look his
name up in my empty brain.
Click to expand...
If someone can repeat the question (I only see replies, not the head
article) I can try to answer the question. I am an employee of USNO.
 
greg.hennessy@tantalus.cox.net (Greg Hennessy) wrote in news:d17vv2$in5$1
@tantalus.no-ip.org:

If someone can repeat the question (I only see replies, not the head
article) I can try to answer the question. I am an employee of USNO.
Click to expand...
I wanted to build an optical imaging interferometer that worked over a
phone line.
 
"John Schutkeker" <jschutkeker@sbcglobal.net.nospam> wrote in message
news:Xns961CAF8E9E119johnschutkeker@151.164.30.48...
greg.hennessy@tantalus.cox.net (Greg Hennessy) wrote in news:d17vv2$in5$1
@tantalus.no-ip.org:

If someone can repeat the question (I only see replies, not the head
article) I can try to answer the question. I am an employee of USNO.

I wanted to build an optical imaging interferometer that worked over a
phone line.
Click to expand...
Why didn't you?
 
In article <Xns961CAF8E9E119johnschutkeker@151.164.30.48>,
John Schutkeker <jschutkeker@sbcglobal.net.nospam> wrote:
If someone can repeat the question (I only see replies, not the head
article) I can try to answer the question. I am an employee of USNO.

I wanted to build an optical imaging interferometer that worked over a
phone line.
Click to expand...
How were you planning on downconverting the terahertz frequency
optical frequencies to a bandwidth a phone line could handle?
 
John Schutkeker wrote:
greg.hennessy@tantalus.cox.net (Greg Hennessy) wrote in news:d17vv2$in5$1
@tantalus.no-ip.org:

If someone can repeat the question (I only see replies, not the head
article) I can try to answer the question. I am an employee of USNO.

I wanted to build an optical imaging interferometer that worked over a
phone line.
Click to expand...
Why? What makes you think you would *ever* see any fringes?

BTW A single interferometer pair cannot form an image. So your claimed
objective is impossible or an oxymoron. You need a several different
baselines and a range of PA angles to stand any chance of forming even a
primitive image from interferometer data. Look up aperture synthesis
radio telescopes for more information at much easier wavelengths.

State of the art research optical interferometers like COAST use
relatively modest path lengths and devious path compensation measures in
climate controlled bunkers. I am unsure what the longest optical
interferometry baseline to see useful fringes is to date but I would be
surprised if it was more than 500m (the longest working optical baseline
I know of is 330m at CHARA).

ADS abstracts has some of the technical details online. Try the JPL
OLBIN coordinating site for an explanation of what you are up against:

http://olbin.jpl.nasa.gov/intro/

Regards,
Martin Brown
 
Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote in news:d1e4mt$tfj
$1@news8.svr.pol.co.uk:

Why? What makes you think you would *ever* see any fringes?
Click to expand...
Twisted pair is radially symmetrical. If that's too demanding, I can
relax the constraints to shielded-twisted pair or coax. However, I'm
following this idea on someone else's word that fiber optic can do it.
Was that guy telling me the truth?

BTW A single interferometer pair cannot form an image. So your claimed
objective is impossible or an oxymoron. You need a several different
baselines and a range of PA angles to stand any chance of forming a
primitive image from interferometer data.
Click to expand...
If each baseline is cheap enough, you can afford to install several
detectors.

State of the art research optical interferometers like COAST use
relatively modest path lengths, devious path compensation measures and
climate controlled bunkers.
Click to expand...
I'm gambling that by the time I finish this, an enabling technology for
phase preserving delay lines will emerge. Hopefully somebody smart is
working on that, but if they're not, then I have yet another hard
problem to work on.

Climate controlled signal pipes don't seem like too much of a reach.

I am unsure what the longest optical interferometry
baseline to see useful fringes is to date but I would be
surprised if it was more than 500m (the longest working optical
baseline I know of is 330m at CHARA).
Click to expand...
Let me know if you can find any hard numbers about this. For proof of
concept, we just need to match the state of the art.

ADS abstracts has some of the technical details online. Try the JPL
OLBIN coordinating site for an explanation of what you are up against:
Click to expand...
I guess my homework this weekend is to read up on ADS, OLBIN, COAST and
aperture synthesis.
 
John Schutkeker <jschutkeker@sbcglobal.net.nospam> wrote in
news:Xns961CB0F05C55Bjohnschutkeker@151.164.30.48:

http://www.nofs.navy.mil/projects/npoi/

Vacuum pipes suggest a laser interferometer, where I wanted to use wires.
Click to expand...
These guys just have beam conduits.
 
In article <Xns961DB34E1C8EAlkajehoriuasldfjknak@151.164.30.42>,
John Schutkeker <jschutkeker@sbcglobal.net.nospam> wrote:
I wanted to build an optical imaging interferometer that worked over a
phone line.

How were you planning on downconverting the terahertz frequency
optical frequencies to a bandwidth a phone line could handle?

I'm not a heterodyne savant, but somebody told me that lasers might do it,
although I'm still open to suggestions.
Click to expand...
You apparently missed my point, which is no one knows how to
downconvert an optical frequency to something lower.

Radio interferometers such as the VLA have detectors that make use of
the wave nature of the radiation, so they mix with with a local
oscillator, convert to voltages, and then combine the signals in a
correlator.

Optical interferometers have vaccuum delay lines, mix the light beams
physically, and then have APD or CCD detectors to measure the fringes.

The reason I asked you how you were planning on downconverting the
signal was a hint that the technology of doing what you asked for
doesn't exist.

I'm willing to sacrifice bandwidth for twisted pair (or shielded twisted
pair) because the heart of the idea is to gain distance (which translates
to aperture size) cheaply.
Click to expand...
What is the bandwidth you think you can get out of twisted pair? If
you express that as a fraction bandwidth of an optical signal (a few
hundred terahertz) what sort of fractional bandwidths are you seeing?
If too low you'd need the Keck telescope to measure Sirius.
 
In article <Xns961DB9DBCF9D2lkajehoriuasldfjknak@151.164.30.42>,
John Schutkeker <jschutkeker@sbcglobal.net.nospam> wrote:
I'm gambling that by the time I finish this, an enabling technology for
phase preserving delay lines will emerge.
Click to expand...
I hope you aren't putting much money on that bet.
 
On Sat, 19 Mar 2005 00:43:25 +0000 (UTC), greg.hennessy@tantalus.cox.net
(Greg Hennessy) wrote:

You apparently missed my point, which is no one knows how to
downconvert an optical frequency to something lower.
Click to expand...
I wouldn't agree with that. The OP was talking about using a heterodyne
technique, and of course you can do that optically- indeed, it is
regularly done in optical signal processing. I think the point here is
that usually when you talk about down converting, the assumption is that
you are down converting a _carrier_ that is somehow modulated with much
lower frequency information. In this case, however, there is no
modulation- the carrier itself is the signal of interest. While you can
technically mix that with another optical signal and produce a new
signal in a very low frequency range (low enough to be transmitted
electrically on wire), there is no point in doing so since you have
destroyed the essential original information in the process.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
 
greg.hennessy@tantalus.cox.net (Greg Hennessy) wrote in news:d1fsos$ia0$1
@tantalus.no-ip.org:

I hope you aren't putting much money on that bet.
Click to expand...
Not money, just time...
 
In article <fp1n31hf5l8jl65pj7lv1hmld9u1fgr22j@4ax.com>,
Chris L Peterson <clp@alumni.caltech.edu> wrote:

On Sat, 19 Mar 2005 00:43:25 +0000 (UTC), greg.hennessy@tantalus.cox.net
(Greg Hennessy) wrote:

You apparently missed my point, which is no one knows how to
downconvert an optical frequency to something lower.

I wouldn't agree with that. The OP was talking about using a heterodyne
technique, and of course you can do that optically- indeed, it is
regularly done in optical signal processing. I think the point here is
that usually when you talk about down converting, the assumption is that
you are down converting a _carrier_ that is somehow modulated with much
lower frequency information. In this case, however, there is no
modulation- the carrier itself is the signal of interest. While you can
technically mix that with another optical signal and produce a new
signal in a very low frequency range (low enough to be transmitted
electrically on wire), there is no point in doing so since you have
destroyed the essential original information in the process.
Click to expand...
Never mind transmitting the information: I wonder what he imagines will
be used to detect the optical phase at each point in the image plane.
 
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