Fibre-Optic Sagnac Interferometer in a FOG Minimum Configuration as Instrumental Challenge for Rotational Seismology

Rotational seismology caused high interest in the investigation on rotational movements generated by earthquakes, mines, and existing in engineering structures. The most oppressive aspects of the research in this field are technical requirements for sensors. They have to be extremely sensitive, as well as possess an extremely wide dynamic range of operation from 0.01 to 100 Hz. This paper points out that solutions basing on the optical technology are the most promising devices in comparison to existing mechanical and electromechanical ones. Especially, instruments basing on the von Laue-Sagnac effect seem to be the most appropriate to investigate rotational phenomena due to the fact that they are entirely insensitive to a translational motion, operate without inertia mass, and are able to measure rotation rate in wide frequency and amplitude band. The paper presents a new device named FOSREM that, based on the FOG minimum configuration, possesses special solutions that makes it perfect, in our knowledge, for any rotation sensing. It enables to measure only a rotational component in a wide range of signal amplitude of 2·10
^-8
-10 rad/s, as well as in a wide frequency band from DC to 328.12 Hz. The presented performance characteristics, as well as a laboratory investigation, confirm FOSREM's opportunity to measure rotation both in seismological observatories and engineering constructions.