Now here's a new idea

John Schutkeker · 2005-02-18T16:31:06+00:00

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

Chris L Peterson 21 years ago

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.

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Chris L Peterson Cloudbait Observatory

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John Schutkeker 21 years ago

snipped-for-privacy@tantalus.cox.net (Greg Hennessy) wrote in news:d1fsos$ia0$1 @tantalus.no-ip.org:

Not money, just time...

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richard schumacher 21 years ago

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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John Schutkeker 21 years ago

richard schumacher wrote in news:no-spam- snipped-for-privacy@news.isp.giganews.com:

DOH!

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Martin Brown 21 years ago

I was referring more to baseline length. The Earth's atmosphere scrambles phases so this is a *lot* harder to do than it sounds.

The baseline lengths that Earth based optical interferometers are likely to be useful at are sufficiently short that you can afford to use any material you like to connect them up to the correlator!

Fibre optics are used for signal transfer in several major interferometer systems but you would need to look carefully to see how they overcame all the practical difficulties. Merlin is moving to fibre optics with a claimed total working bandwidth of 30GB/s.

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It is a lot more tricky than that. At optical frequencies you need a minimum of 3 baselines in a closed loop to get one good observable phase free from atmospheric distortion.

Some of the best engineers and scientists on the planet have just about managed to get these optical systems to work with about two decades of work. To say it is distinctly non-trivial underplays the enormous technical acheivement of getting path compensating correlators to work for optical signals.

At least look at IEEE Transactions and IAU proceeding in Indirect Imaging to avoid making some of the more obvious mistakes.

OLBIN provides the latest news of successes and set backs.

Researching the literature before jumping in feet first will save you a lot of heartache.

Regards, Martin Brown

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