Highly coherent two-color laser and its application for low-noise microwave generation

Two-color lasers with high coherence are essential for precision measurements and low-noise photonic microwave generation. However, conventional two-color lasers often suffer from reduced coherence when the frequency spacing is large. Here, we leverage the Pound-Drever-Hall technique to synchronize two lasers to a common ultra-stable optical reference cavity to break through the thermal noise constraint, achieving a highly coherent two-color laser. By overcoming non-common mode noise, we achieve an exceptional fractional frequency instability of 2.7 × 10^-17 at 1 second, normalized to the optical frequency. To characterize coherence across large frequency spacings, we use electro-optical frequency division to transfer the stability of a 0.5 THz spaced two-color laser to a 25 GHz microwave signal. The resulting 25 GHz signals exhibit remarkable phase noise of - 74 dBc Hz^-1 at 1 Hz and - 120 dBc Hz^-1 at 100 Hz. Our results pave the way for a new era in precision measurement and light-matter interaction.