Lasers with sub-hertz line-width and fractional frequency instability around 1×10^-15 for 0.1 s to 10 s averaging time are currently realized by locking onto an ultra-stable Fabry-Perot cavity using the Pound-Drever-Hall method. This powerful method requires tight alignment of free space optical components, precise polarization adjustment and spatial mode matching. To circumvent these issues, we use an all-fiber Michelson interferometer with a long fiber spool as a frequency reference and a heterodyne detection technique with a fibered acousto optical modulator (AOM)¹. At low Fourier frequencies, the frequency noise of our system is mainly limited by mechanical vibrations, an issue that has already been explored in the field of optoelectronic oscillators.^2,3,4

After extensive study of the spools with Finite Element Modeling (FEM), we realize and test a novel spool design (Fig. 1) which is optimized for low vibration sensitivity along all spatial directions and insensitive to the way it is held. We measure a sensitivity of about 10^-11/ms^-2 in all direction for the complete oscillator of 2 km fiber length, limited by the out of spool elements (AOM, coupler, Faraday mirrors). The composed interferometers spool of two symmetrically mounted shows a sensitivity of about 5-8×10^-12/ms^-2. At the conference we will also show frequency noise measurements and the prototype of a simplified oscillator aiming to realize a robust and cost effective very low noise agile laser with acceleration sensitivity below 3×10^-11/ms^-2 in all spatial directions.

Fig. 1:

Low vibration sensitivity spool set up