Abstract

The interaction of gas-phase molecules with strong electric fields, in the order of 10¹¹ W/cm², over a duration of ~1 ns allows for the adiabatic alignment of the molecules, i.e., to fix them in space. Ensembles of aligned molecules are beneficial, e.g., to enhance the contrast in x-ray diffraction experiments [1]. The alignment of molecules [2] is typically performed with pulsed lasers at repetition rates between 10 Hz and 1 kHz [3]. Such repetition rates are far below the MHz repetition rates of modern accelerator based x-ray sources. The generation of laser pulses at such high repetition rates with pulse durations of nanoseconds and sufficient peak power to achieve focal intensities exceeding 0.1 TW/cm² is very challenging. A continuous wave laser would need an output power of hundreds of kW to achieve the required field intensities in a focal waist of 25 μm, but inside a laser cavity such high powers can be achieved at several orders of magnitude lower pump power levels. To determine the intrinsic losses of the gain material, we set up a linear thin-disk laser resonator [4] with 24 pump passes through a 1.2 mm pump spot and measured the laser performance at several output-coupler transmission rates between 9.5·10⁻⁵ and 4.0 ·10⁻³ (Fig. 1 left). The 55 mm long linear plane-concave resonator allows for efficient multi-mode lasing. The high efficiencies allow for a precise evaluation of an upper limit for the resonator losses. As the active media we compared commercially available Yb(7%):YAG disks against Yb(3%):Lu₂O₃ disks which were grown in our institute. Compared to Yb:YAG, Yb:Lu₂O₃ provides larger cross-sections and a higher thermal conductivity [5]. We demonstrated an Yb:YAG thin-disk laser with an intracavity power (P_int) of 135 kW for an output-coupler transmission rate of 9.5·10^-5 at a pump power (P_p) of only 54 W. At these low output-coupler transmission rates the laser disks heat up to surface temperatures exceeding 100 °C. Such strong heating increases the risk of damaging the crystals. Therefore the pump intensity should not be further increased. Higher intracavity power levels can still be realized by using larger pump spots.

© 2015 IEEE