Stabilizing the free spectral range of a large ring laser

Large ring lasers employed in geodesy and fundamental physics require stability of the perimeter at or below the parts-per-billion level. We present two complementary approaches to actively control the perimeter length of such ring lasers, reaching a relative length stability of $4\times 10^{-10}$. These methods can readily be implemented and bring the stability of heterolithic devices on par with monolithic designs.