Cascaded distribution and amplification of ultra-stable laser light

This article presents a proof of concept for a frequency distribution system that delivers ultra-stable laser radiation to multiple user ports through optical links. The system is scalable through power amplification and cascading of distribution stages, without introducing any unstabilized optical paths that could degrade laser stability by adding phase noise. Through out-of-loop measurements, we demonstrate that added noise is negligible compared even to the best state-of-the-art ultrastable lasers, with an added fractional frequency instability of less than 10^-17 at 1 s integration time and averaging down to below 10^-21 at 10⁴ s for the outputs of two cascaded stages including power amplification by a phase-locked laser. The demonstrator setup has been designed for use with strontium optical lattice clocks, i.e., operates at a wavelength of 698 nm, but the scheme can easily be implemented for other wavelengths.