We present a SESAM-modelocked Yb:CaF2 thin-disk laser designed for pulse durations below 300 fs and high peak powers of more than 5 MW. A cavity for fundamental mode operation (beam quality factor M2 below 1.2) was set up and modelocked using a SESAM. An average output power of up to 17.8 W was obtained at a repetition-rate of 10 MHz, corresponding to a pulse energy of approximately 1.8 μJ. The pulse duration was measured to be 285 fs, therefore a peak power of 5.5 MW was attained. Our research enables a comparison of the potential of Yb:CaF2 with other Yb-doped crystals with broad gain bandwidth in thin-disk laser technology. We conclude from our results that this gain material is very promising for high pulse energies and high peak powers.
The need for ultra-short (sub-ps) pulsed laser systems with high power and high energy has advanced the mode-locked
Ytterbium-doped thin-disk technology in the last decade. Therefore several research groups have made efforts to explore
new laser crystals e.g. Yb:SSO, Yb:CAlGO or Yb:Lu2O3 for the generation of sub-500 fs pulses in thin-disk oscillators.
Another promising and known candidate for ultra-short pulsed lasers is Yb:CaF2, which has been so far only used in bulk
laser architecture. In this work, we present the first demonstration of a mode-locked Yb:CaF2 laser in thin-disk
configuration. The resonator cavity was designed for eight passes through the disk per roundtrip at a repetition rate of
35 MHz. A saturable absorber mirror (SESAM) was used to obtain the soliton mode-locking. We achieved close-to
transform-limited pulses with a pulse duration of less than 445 fs and an emission spectral width of 2.6 nm at FWHM
(i.e. time-bandwidth product of 0.323). At the average output power of 6.6 W this corresponds to a peak-power of
430 kW and pulse energy of 190 nJ. To the best of our knowledge, this is the highest average output power and pulse
energy using Yb:CaF2 as gain material reported to date. Taking into account the dispersion, self-phase modulation, pulse
energy, output coupling ratio and laser gain, the pulse-duration estimated from the soliton-equation and our numerical
calculations of pulse-propagation is in good agreement with the pulse-duration obtained in the experiment. Higher
powers and shorter pulse-durations with this material are the subject of our future investigations.
We present the experimental investigations of different designs of resonant waveguide-grating mirrors (RWG) which
are used as intracavity folding mirror in an Yb:YAG thin-disk laser. The studied mirrors combine structured fused
silica substrates, a thin-layer waveguide (Ta2O5), a buffer layer (SiO2) and partial reflectors. The grating period was
chosen to be 510 nm to allow resonances at an angle of incidence of ~10° for TE polarization. The waveguide layer has
a thickness of 236 nm. It is followed by the buffer layer with a thickness of 580 nm and the subsequent alternating
Ta2O5/SiO2 layers. The exact coating sequence depends on the two design approaches which were investigated in this
work: either introducing different partial reflectors, i.e. stacks of quarter-wave layers on top of the waveguide while
keeping the groove depth of the grating constant, or increasing the grating depth, while keeping an identical partial
reflector. The investigation was focused on the rise of the surface temperature due to the coupling of the incident
radiation to a waveguide mode, as well as on the laser efficiency, polarization and wavelength selectivity. It is found
that, when compared to the simplest RWG design which consists of only a single Ta2O5 waveguide layer, damage
threshold as well as laser efficiency can be significantly increased, while the laser performances in terms of
polarization- and wavelength selectivity are maintained. So far, the presented RWG allow the generation of linear
polarization with a narrow spectral linewidth down to 25 pm FWHM in a fundamental mode Yb:YAG thin-disk laser.
Damage thresholds of 60kW/cm2 have been reached where only 63K of surface temperature increase was observed.
This shows that the improved mirrors are suitable for the generation of kW-class narrow linewidth, linearly polarized
Yb:YAG thin-disk lasers.