Aring laser unlocked by the Earth's Sagnac effect has attained a frequency resolution of 1 part in 3 × 10(21) and a rotational resolution of 300 prad. We discuss both theoretically and experimentally the sideband structure of the Earth rotation-induced spectral line induced in the microhertz-hertz region by frequency modulation associated with extra mechanical motion, such as seismic events. The relative sideband height is an absolute measure of the rotational amplitude of that Fourier component. An initial analysis is given of the ring laser record from the Arthur's Pass-Coleridge seismic event of 18 June 1994.
A He-Ne ring laser has given the Earth-rotation-induced Sagnac frequency to a precision of 1 microHz, or 2 parts in 10(8), 2 x 10(-21) of the laser frequency. Because the beat frequency in a ring laser originates in either timereversal or (with a more complicated polarization geometry) parity-violating effects, such systems have the potential of detecting ultrasmall symmetry violations.
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