Touching the limit of FOG angular random walk: Challenges and applications

In this paper, the limit of Angular Random Walk (ARW) performance in FOG (Fiber-Optic Gyroscope) technology will be investigated. First, a theoretical analysis will be carried out, then, an experimental confirmation will be attempted. Corresponding difficulties and solutions to measure low ARW will be highlighted. Finally the measurement will be presented and discussed. Theoretical analysis The scalability of FOG technology is one of its assets. Considering only ARW performance, we address the question of how good it can be theoretically, and practically. Mentioned scalability comes from Sagnac sensitivity which is proportional to length time diameter over wavelength. However the length parameter is limited theoretically. Even in the best case, when sensor is shot noise limited, we can calculate the fiber length after which ARW will no longer improve. Moreover, the diameter is limited practically for field equipment. And finally, the wavelength is part of the technological know-how for each FOG manufacturer. So, despite the famous scalability of FOG technology, an analysis of the physical limitations of the ARW has been carried out. Experimental measurement To validate this analysis, an experiment has been carried out. All the last developments made on iXblue FOG technology to lower the ARW have been implemented for the first time in one setup. 6km of thin coating PM (polarization maintaining) gyro fiber have been spooled over 180mm of mean diameter. The last innovations in FOG optical design have been gathered, including optical RIN subtraction technology. But the achieved ARW still remained stuck more than one order of magnitude higher than expected. We realize that the measure of such a low ARW is a technical issue as such. We develop and compare three different methods to measure real “self-noise” of the sensor, “damped”(use of a damped support), and “buried”(use of calmest location). Result and discussion We finally manage to validate an ARW of 38μ°/√(h). Discussion about how to get even better ARW will be held. Such a unique technology is now ready for innovative applications: scientific for ultra-precise dynamic alignment, spatial for stabilization during distant measurements, or seismic rotational ground motion measurement with a field equipment.