Ring laser for precision measurement of nonreciprocal phenomena

A 0.75 m², square-ring laser operating at the helium-neon wavelength 633 nm (474 THz) was constructed. High-quality mirrors and the absence of media, except for the He-Ne gas mixture in the beam, insure a high-quality factor of the cavity and low lock-in frequency. A net line width of 32±2 MHz is observed. The center of the earth line is determined with a precision of about 1 mHz, that is, with a fractional precision of 1 MHz/474 THz≈2×10^-18 , which outclasses Mossbauer and maser lines by several orders of magnitude. This implies that it is possible to obtain accuracies of such additional effects on the order of millihertz