Abstract

Chirped-pulse oscillators providing high-energy output pulses at high repetition rates are of special interest for a number of applications, such as micromachining or writing of waveguide structures in transparent materials and as seed oscillators for enhancement cavities [1], The exact shape of the intracavity dispersion is predicted to have a large influence on the mode-locking characteristics and spectral shape of the output pulses of Ti: sapphire chirped-pulse oscillators [2], We have experimentally investigated the influence of the exact shape of the intracavity dispersion on the operation of such oscillators. Our experimental results are in good agreement with the theoretical predictions, and provide us with useful design criteria for high-energy chirped-pulse oscillators. In Fig. 1, we show the output power in cw and mode-locked operation for two different shapes of the intra-cavity dispersion (mirror combinations) vs. pump power. Since the mirror losses for both combinations are the same, the maximum cw output power is equal as well. In agreement with theoretical predictions, the mirror combination that provides moderate positive fourth-order dispersion (FOD) around the gain maximum provides much more stable mode-locked operation, and can thus be operated at higher pump powers. With an optimized shape of the intra-cavity dispersion, we demonstrate generation of 60 nJ pulses at 70 MHz repetition rate, with a bandwidth supporting compression down to 33 fs. With such a system, writing of waveguide structures in transparent materials can be done at much higher speeds as demonstrated up to now. In a system in which dispersion compensation is done with chirped mirrors and intra-cavity prisms, we demonstrate central wavelength tunability, without introducing considerable loss in output power, as shown in Fig. 2. Such a system is very interesting for spectroscopic applications that require high average power and wavelength tunability at a high repetition rate. For example the combination of such a system with an enhancement cavity [1] allows tunability of XUV frequency comb.

© 2007 IEEE