Non-planar femtosecond enhancement cavity for VUV frequency comb applications

TL;DR

This paper introduces a novel non-planar femtosecond enhancement cavity design that produces circular beams, enabling tighter focusing and higher power for VUV frequency comb applications, improving efficiency and mirror safety.

Contribution

A new three-dimensionally folded cavity geometry that ensures circular beam profiles and allows for higher power focusing in femtosecond enhancement cavities.

Findings

Achieved circular beam profiles with the new cavity design

Enabled tighter focus and larger mirror beam cross-section

Improved performance for VUV frequency comb generation

Abstract

External passive femtosecond enhancement cavities (fsECs) are widely used to increase the efficiency of non-linear conversion processes like high harmonic generation (HHG) at high repetition rates. Their performance is often limited by beam ellipticity, caused by oblique incidence on spherical focusing mirrors. We introduce a novel three-dimensionally folded variant of the typical planar bow-tie resonator geometry that guarantees circular beam profiles, maintains linear polarization, and allows for a significantly tighter focus as well as a larger beam cross-section on the cavity mirrors. The scheme is applied to improve focusing in a Ti:Sapphire based VUV frequency comb system, targeting the 5th harmonic around 160 nm (7.8 eV) towards high-precision spectroscopy of the low-energy isomer state of Thorium-229. It will also be beneficial in fsEC-applications with even higher seeding and intracavity power where the damage threshold of the mirrors becomes a major concern.