Work transiestors better vertical?

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Except that the phase shift is independent of the refractive index, so that doesn't work.

Cheers

Phil Hobbs

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=A0 =A0 =A0 ...Jim Thompson

=A0"The Journey is the reward"

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You don't need the coil of fiber to make a Sagnac, just mirrors will work. It's pretty easy to work out the sensitivity to absolute rotation. (At least I did it once.) It goes as the angular velocity time the area enclosed by the beams times the number of 'turns'. (which is why a coil of fiber lets you win big time.)

We've got a Sagnac on a table top with about a 1 square meter area. I think I estimated that we could see rotations of something like 10 rpm.

George H.

Oh, or are you saying that measuring the Earth's rotation is hard. (I'm just wondering how I'm going to stop the Earth so I can find my 'zero'.)

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On a sunny day (Wed, 15 Jun 2011 12:35:36 -0700 (PDT)) it happened George Herold wrote in :

Rotate in same direction at 1 period per true day.

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Flip it over.

A good FOG has bias stability maybe an order of magnitude better than you need to see the earth's rotation.

You completely lost me there - the phase shift should be completely independent of the refractive index, dependent only on rate of rotation, in order to work! And any dependence on index of refraction only needs to be known in order to be compensated for.

That works. Measuring very slow rotations optically is not easy, but is done all the time in both fiber and ring optical gyros. There was a lot published on the details a few decades ago when I still read the SPIE Journal, the most interesting thing being the discovery of a stick-slip friction effect between the counter rotating beams, which would phase lock with each other at very small rotations, making small rotation rates undetectable. The solution was to run the counter-rotating beams at different frequencies, adding a few MHz (IIRC) to one side before injecting it in the loop and then comparing the interference signal from the combined beam detector to the frequency offset signal.

Speaking of interferometry, there was an interesting article in June 9 Laser Focus Workd,

INTERFEROMETRY Emerging applications push receivers to higher speeds OVIDIO ANTON Newport Corp.

Use of autobalanced detectors to monitor the sample and reference beams of an interferometric system delivers high bandwidth with improved gain and reduced noise for applications such as optical coherence tomography. ... We developed the ?Nirvana? autobalanced receiver based on Hobbs? work at IBM. (P. Hobbs, US Patent 5,134,276)

Just in case we were not far enough off topic yet :-).

You have to take them up to the upper atmosphere in an aircraft then dive to simulate zero gravity or they don't work so well.

It would be interesting to take some really sensitive nanovolt-level instrument and see if it's g-sensitive. I think most analog ICs are in fact stress sensitive; I've seen it in DACs and opamps.

John

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Duh, Thanks. I keep thinking of mirrors on a bread board and not fibers.

George H.

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No, it's that the Doppler shift explanation doesn't work because if it did, building a Sagnac in a medium of refractive index n would give n times the fringe shift, and it doesn't.

Cheers

Phil Hobbs

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Doppler shift? It's just a path length difference. (At least that's how I see it.)

I like to think about it with three mirrors and a beam splitter. Wow we even have a picture,

If the whole thing is rotating then the photons going one way hit the mirrors sooner and get around the loop faster (in less time). There's certainly a pathlength difference that can be identified. (But I'll leave that as an exercise :^)

George H.

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If you do the math from that point of view, you get an expression that depends on the speed of light in the surrounding medium, i.e. on the refractive index. This is contradicted by experiment. Try it.

Cheers

Phil Hobbs

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That's wierd! Phase lock around the whole fiber?

Is there a good phrase to search with?

The solution was to run the counter-rotating beams at

Cool, Is that patent still active?

This is the topic of the thread.

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OK I stick pieces of glass between the mirrors, that doesn't look like it makes a lot of difference. Both beams go through the same pieces of glass.

I can still see a path length difference.. I can just do ray optics can't I?

George H.

(don't toy with me Phil, you're a lot smater than I am.)

It expired last October. John L and I are working on an updated version.

Cheers

Phil Hobbs

Late at night, by candle light, Ollie B. Bimmol penned this immortal opus:

Weren't the holes in Albert Hall?

- YD.

So you're being used ?? ...Jim Thompson