Experimental Gravitational Physics

Laser Gyroscope for Rotation Sensing

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 (~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.

The first use of a ring laser cavity to detect rotational motion was demonstrated by Macek and Davis in 1963, and their design remains essentially unchanged in most current implimentations. The operating principle for all optical gyroscopes is the Sagnac effect: in a ring geometry, if the system is rotating in the optical plane, the roundtrip optical path lengths traversed by two counter-propagating beams are unequal. This earliest design was itself an improvement of a non-resonant, phase-sensitive interferometer introduced by Sagnac himself. In that instrument, an interferometric fringe shift was produced at the output proportional to the rotation rate, and its sensitivity was therefore limited by the achievable fringe resolution. Another type of interferometric optical gyroscope, the fiber optic gyroscope (FOG), is not discussed here. It is similar to Sagnac's original interferometer, but with the free-space system replaced by many windings of a fiber, increasing sensitivity. For an excellent contemporary review of all optical gyroscope technologies, see Chow:1985. Conversion of the ring into a laser cavity created a bidirectional resonator, wherein the supported modes in each of the two directions---their frequencies being dependent on the respective roundtrip phases---are non-degenerate in the presence of rotation. This allowed Macek and Davis to use far more sensitive heterodyne techniques to measure the frequency splitting caused by rotation. As reported in their original paper, the Sagnac-induced frequency shift is given by delta f = 4 / (lambda S) x A x omega, where A is the vector of area enclosed by the cavity, S is the cavity perimeter, lambda is the laser wavelength, omega is the angular velocity, and delta f is the optical frequency splitting.

The concept of an externally illuminated (``passive'') laser gyroscope was first presented by 'Ziggy' Ezekiel and Balsamo in 1977. Previously, a major issue with the common active design had been discovered: at small rotation rates, backscatter-induced crosstalk effects caused the counter-propagating modes to lock to one another in frequency, leading to a null output. It was believed that this effect was caused by the presence of the gain medium within the gyroscope cavity. Ezekiel and Balsamo sought therefore to circumvent this effect by locking an external laser to a passive optical ring cavity. In their setup, acousto-optic modulators (AOMs) were used to shift the laser frequency up macroscopically in common mode for the two counter-propagating beams. A primary loop locked the cavity length to one upshifted beam, and a secondary loop adjusted the frequency of the other beam's AOM to lock it to the counter-propagating mode. Ultimately, it was found that even passive designs exhibit this ``lock-in'' effect, which was determined to be the result of back-scattering from one beam to the other.