The Original Water Vapor Interferometer
This is the original interferometer used for finding the water vapor absorption hole for the Strategic Defense Initiative in 1988.
At the time we thought that the heaviest possible table would isolate the system from external vibrations.
This was true for low frequencies but high frequency acoustic noise was coupled into the table surface which then made its way up through support rods to the Mach-Zehnder mirrors themselves.
Since we were looking for 10^-5 wave fringe shifts it didn't take much acoustic noise to show up on the signal.
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