Enhancements in laser interferometry?
16 Mar 2005 20:27:34 UTC
Topic 188564
(moderation:
Hi,
Why don't they just put a cable in the pipe, a number of times forth and back and compare the pulse response time versus a 90-degree alignment of the same construction? That would be much less sensitive to "non-relativistic" effects. Is it about the accuracy? If so, one could still use this very long line for the laser modulation and then do the interferometry more locally.
Where's my fault?
Cheers
Enhancements in laser interferometry?
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I suspect that the propagation of the light in optical fiber is subject to too much variation as the path of internal reflections will not be constant. This dispersion of the wavefront is not too much of a problem for communication purposes, but when you are comparing two coherent signals to a tiny fraction of a wavelenth, this noise would be very difficult to filter out.
I'd also guess that a bundle of optical fiber would have significant response to ground vibration as well, probably at different (higher) frequencies than LIGO's mirrors.
It's easy to filter out -
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It's easy to filter out - however, only at the expense of loosing the coherence ;-)
No, you are definitely right - using fibers would make the instrument extremely sensitive towards any environmental change (and you don't even need a bundle of fibers to measure vibrational effects - you can very well see this in a single fiber already - pressure changes leading to refractive index changes ...), in particular temperature changes.
Most problematic is, that the effects would scale with the fiber length used - not too surprising though, since a 1 m fiber already roughly "contains" 1...2 million wave cycles (talking about visible light around 0,5 ... 1 µm wavelength).