Читать книгу Global Navigation Satellite Systems, Inertial Navigation, and Integration - Mohinder S. Grewal - Страница 85

There are two essential designs for optical gyroscopes, both of which depend on the Sagnac effect, a phenomenon studied by Franz Harress in 1911 [6] and Georges Sagnac in 1913. The effect has to do with the relative delay of two light beams from the same source traveling in opposite directions around the same closed loop, and how their relative delay depends on the rotation rate of the apparatus in the plane of the loop. The effect has been named for Sagnac, who showed that the delay difference was proportional to the rotation rate, and the effect scaled as the area of the loop. The effect was not used for sensing rotation until after a working laser was demonstrated in 1960, first with the lasing cavity in the closed optical path – the ring laser gyroscope (RLG) – and later using a kilometers‐long coil of optical fiber – the fiber optic gyroscope (FOG).

Ring laser gyroscopes are rate integrating gyroscopes. Their output interferometric phase rate is proportional to rotation rate, so each output phase shift represents an incremental inertial angular rotation angle. To minimize temperature and pressure sensitivities, their closed‐loop optical paths are typically machined into very stable materials. Early designs exhibited a “lock‐in” problem near zero input rates, due to backscatter off the mirrors. Later designs avoided this by using out‐of‐plane optical paths and multi‐frequency lasing cavities.

Fiber optic gyroscopes were first developed after single‐mode optical fibers became available, about a decade after the first laser. Unlike RLGs, FOGs are rate gyros. Their output is proportional to the input rotation rate, and must be integrated to get rotation angles. The optical loop in this case is a very long coil of optical fiber with an external laser source.