Beyond a megawatt of average power in a high-finesse optical enhancement cavity

Optical enhancement cavities are a powerful technical solution for scaling up laser average power, which is essential for a wide range of advanced scientific applications. In this paper, we demonstrate, for the first time, to our knowledge, a four-mirror bow-tie optical enhancement cavity capable of sustaining more than one megawatt intracavity average power at 1064 nm for several minutes, advancing the state of the art in high-power enhancement cavities. A maximum intracavity power of 1.15 MW was achieved, while the cavity beam preserved a clean fundamental Gaussian transverse mode with an effective power enhancement factor above 9000. Optical system modeling permitted the estimation of residual coating absorption, found to be well below a part per million. Particular care was given to the measurement of the phase stability of the system, providing a path for further improvements. These achievements demonstrate that optical enhancement cavities can stably operate beyond megawatt average power, laying a solid foundation for next-generation applications such as steady-state microbunching extreme ultraviolet light sources and photo-neutralization systems for fusion reactors.