Thin-disk laser technology to power-scale ultrashort pulses

Energy scaling of light sources has always been a driving force in the development of novel laser technologies. High optical power in a beam of coherent radiation is particularly appealing, since it maximizes the nonlinear interaction between light and matter. This fact allows for a broad portfolio of applications, ranging from industrial tasks such as nonthermal cutting and welding to scientific experiments that exploit advanced frequency conversion and compression schemes. Ultimately, the goal is to achieve maximum peak power with optimally stable, compact, and reliable systems. For two decades titanium:sapphire has been the gain medium of choice for the construction of mode-locked oscillators (complemented by high-power amplifiers). The remarkable gain bandwidth of this material enables direct generation of intense pulses that have duration as short as 25fs at kilohertz repetition rates. However, in terms of power scalability, this class of lasers is limited by thermal management within the gain medium, and by the complexity of the Q-switched green pump sources for the amplifiers. Typically, pulse energies are limited to a few millijoules at a 1kHz repetition rate, and the only capacity for improvement is by aggressive cryocooling of the rod-type crystals used for amplification. The search for alternatives focuses on two aspects: finding an active material that supports ultrashort pulses while limiting power dissipation into heat, and selecting a smart geometry of the gain medium that enables efficient cooling while ideally preserving unlimited power scalability. The introduction of ytterbium:yttrium aluminum garnet (Yb:YAG) thin-disk lasers efficiently targeted both issues, providing record-breaking average powers and the possibility of pulsed operation. The geometry of this gain medium—which was pioneered by Adolf Giesen at the University of Stuttgart—exploits effective cooling by means of a large contact area with a heat sink to enable a longitudinal heat flow parallel to the laser beam direction that does Figure 1. Illustration of the laser system, comprising an ultrafast erbium:fiber seed system followed by an ytterbium:fiber pre-amplifier with a chirped-pulse scheme in a thin-disk regenerative cavity. The output pulses centered at 1030nm are 615fs long and contain 17mJ of energy at a repetition rate of 3kHz. This output may be exploited for experiments testing the behavior of condensed matter under extreme sub-cycle electrical bias (which can only be applied optically). I: Intensity.