Abstract

To date, the most successful coherent XUV sources rely on the process of high-order harmonic generation (HHG) in a noble gas at intensities exceeding 10¹³ W/cm². Passive resonators, excited by a mode-locked laser have become the most prominent HHG sources at repetition rates above 10 MHz. In such enhancement cavities (ECs) a power enhancement of a few orders of magnitude can be reached allowing for the build-up of femtosecond-pulsed beams of average powers of several hundreds of kilowatts. Therefore, ECs lend themselves to driving low efficiency non-linear frequency conversion processes such as HHG. However the rapidly decreasing index of refraction of the gas upon ionization results in phase shifts that alter the constructive interference for the intracavity pulse with the seeding laser pulse at the input coupler. This leads to a saturation behaviour of the intracavity intensity with respect to the input peak power referred to as intensity clamping [1].

© 2015 IEEE