Pressure-induced fractional changes of 10(-7) in the geometry of a large He-Ne ring laser gyroscope induce backscatter phase changes and thus a fractional pulling of the Sagnac frequency of ~5 x 10(-3). To counter this, the optical frequency was stabilized against an iodine-stabilized laser with a high-finesse Fabry-Perot interferometer and piezoelectric control of the ring perimeter. This scheme, although limited in principle by residual geometric asymmetry and in practice by low beam powers (10 pW), stabilized the perimeter to 2.4 nm (6 x 10(-10) or 300 kHz for the optical frequency) and the Sagnac frequency to 100 parts per million over several days.
The design and initial operation of a vertical square He-Ne ring laser G0 with a perimeter of 14 m is discussed. This builds on earlier demonstrations of the feasibility of large ring lasers (perimeter approximately 4 m) for single-mode gyroscope operation and with lesser pulling than navigation gyroscopes. With servoing of the rf excitation to yield single-mode operation, G0 gave a quality factor 1 x 10(12) and a Sagnac line with a frequency of 287.8 +/- 1.0 Hz induced by Earth rotation Omega(E). This has confirmed some vital questions over the feasibility of very large gyroscopes for geodetic measurements at the level of 10(-9) Omega(E).
An historic and simple experiment has been revitalised through the availability of supercavity mirrors and also through a heightened interest in interferometry as a test of physical theory. We describe our helium-neon ring laser, and present results demonstrating a fractional frequency resolution of 2�1x10-18 (1�0 mHz in 474 THz). The rotation of the earth unlocks the counterrotating beams. A new field of spectroscopy becomes possible, with possible applications to geophysical measurements such as seismic events and earth tides, improved measurements of Fresnel drag, detection of ultraweak nonlinear optical propert~es of matter, and also searches for preferred frame effects in gravitation and for pseudoscalar particles.
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