An ultra-low noise and high-gain servo controller for ultra-stable laser systems.

State-of-the-art ultra-stable laser systems exhibit fractional frequency stability on the order of 10-17. The imminent challenge lies in advancing this stability to the 10-18 range, thereby approaching the fundamental limit imposed by thermal noise. To achieve such a milestone, it is necessary that all technical noise sources, particularly electrical noise, be suppressed well below the level of thermal noise. As an initial stride toward 10-18 stability, we have improved an ultra-stable laser system through the redesign of its frequency stabilization feedback electronics, introducing an innovative servo controller architecture. This innovative servo controller incorporates a pre-amp stage comprising 16 parallel operational amplifiers, which feeds into a high-gain stage constituted by four cascaded integrators. This design yields an impressive input noise floor of 1.4 nV/Hz at 1 Hz and delivers a substantial servo gain of 230 dB at 1 Hz. Consequently, the upgraded ultra-stable laser system exhibits a residual in-loop error noise that contributes a mere fractional frequency stability of 4.7 × 10-20, signifying an improvement of two orders of magnitude. This significant advancement not only paves the way for addressing other technical noise challenges but also holds immense appeal for applications demanding utmost precision, including ultra-stable laser systems, gravitational wave detectors, and optical clocks.