Optoelectronic down conversion: A novel approach for optical beat note stabilisation up to the THz domain

A novel scheme based on optoelectronic down conversion is proposed and demonstrated to obtain an ultrahigh spectral purity and tuneable optical beat note in the GHz and millimetre wave range. The resulting beat note linewidth is measured to be better than 1 Hz over 100 GHz leading to a relative stability lower than 10⁻¹¹. This approach is scalable to the THz domain.