Active and passive resonant Sagnac cavities: A comparison

Space geodesy has a great interest in real-time observations of the rotational motion of the Earth. Apart from the rotation rate, this includes the orientation of the rotation axis, usually referred to as the polar motion. Large ring lasers and passive gyroscopes, both have a resonant traveling wave cavity in common, and this cavity solely defines the rotation sensing ability. In this paper, we examine and compare the advantages and disadvantages of the active and the passive operation. Apart from some similarities, each technique suffers from a different set of limitations, and it is not yet clear which approach will eventually go further.