First steps for a Giant FOG: Searching for the limits

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Based on recent experiences developing very low noise fiber-optic gyroscopes (FOG), as presented at ISS-2016, first performance results on very large optical fiber coils of up to 1 m diameter are presented. The goal for constructing large FOGs is to evaluate experimentally the physical limits of this kind of technology and to reach the lowest possible noise. While these experiments are probing the fundamental limits of FOG technology, they also serves as a first step for a cost effective very low noise laboratory rotational seismometer and will contribute in a second step to performance improvements on the portable rotational seismometer `blueSeis-3A&B'. This goal has several difficulties: the first one is the winding of the coil, the second concerns the mechanical substrate, and the third is, as usual, related to the measurement. To our knowledge, a winding machine, large enough to wind a 1 meter-diameter coil, does not exist, but thanks to iXblue expertise in the manufacturing of winding machines and calibration tables, a hydride system has been designed, merging these two technologies to fulfill the requirement of winding a large coil on an adequate rotational platform. The characterization of the wobble of the system is presented, since this is a critical parameter for the winding and ultimately the performance. To decrease the sensor sensitivity to environmental noises, unrelated to real ground rotation, is a critical process to achieve the highest attainable measurement sensitivity in seismology. The mechanical substrate of the coil is critical, as it must handle thermal, acoustic, magnetic, and electromagnetic isolation. To demonstrate a very low self-noise, the ideal way is to find an experiment place near a much more precise instrument in a calm enough location. The seismic station collocated to ROMY tetrahedral ring-laser gyroscope [0] is the ideal place to do that. Results of different prototypes during the development process are presented to underline the applicability of each technological response to the Large-FOG requirements. Finally we conclude with presentation of the achieved results with a 1 meter-diameter FOG having a fiber length of 5000 meters.