crystal fiber

© The Author(s) 2009. This article is published with open access at Springerlink.com Abstract High harmonic generation (HHG) of intense in-frared laser radiation [1, 2] enables coherent vacuum-UV (VUV) to soft-X-ray sources. In the usual setup, energetic femtosecond laser pulses are strongly focused into a gas jet, restricting the interaction length to the Rayleigh range of the focus. The average photon flux is limited by the low con-version efficiency and the low average power of the com-plex laser amplifier systems [3–6] which typically operate at kilohertz repetition rates. This represents a severe limitation for many experiments using the harmonic radiation in fields such as metrology or high-resolution imaging. Driving HHG with novel high-power diode-pumped multi-megahertz laser systems has the potential to significantly increase the aver-age photon flux. However, the higher average power comes at the expense of lower pulse energies because the repeti-tion rate is increased by more than a thousand times, and efficient HHG is not possible in the usual geometry. So far, two promising techniques for HHG at lower pulse en-ergies were developed: external build-up cavities [7, 8] an