  Laser gyroscopes making use of the Sagnac effect have been used as highly
accurate rotation sensors for many years. First used in aerospace and defense
applications, these devices have more recently been used for precision
seismology and in other research settings. In particular, mid-sized (~1
m-scale) laser gyros have been under development as tilt sensors to augment the
adaptive active seismic isolation systems in terrestrial interferometric
gravitational wave detectors. The most prevalent design is the "active"
gyroscope, in which the optical ring cavity used to measure the Sagnac
degeneracy breaking is itself a laser resonator. In this article, we describe
another topology: a "passive" gyroscope, in which the sensing cavity is not
itself a laser but is instead tracked using external laser beams. While subject
to its own limitations, this design is free from the deleterious lock-in
effects observed in active systems, and has the advantage that it can be
constructed using commercially available components. We demonstrate that our
device achieves comparable sensitivity to those of similarly sized active laser
gyroscopes.
