Benjamin Dannecker

Passively mode-locked Yb:CaF 2 thin-disk laser

We report on the first demonstration of a passively (SESAM) mode-locked Yb:CaF(2) thin-disk laser operating at a repetition rate of 35 MHz with close to diffraction-limited beam quality (M(2) ≈ 1.1) at an average output power of up to 6.6 W. The optical efficiency was 15.3%. Nearly transform limited pulses with a duration of 445 fs and a spectral width of 2.6 nm at full width half maximum (FWHM) were obtained at the maximum output power. This corresponds to a pulse-energy of approximately 0.19 μJ and a peak-power of 0.4 MW.

SESAM-modelocked Yb:CaF2 thin-disk-laser generating 285 fs pulses with 1.78 μJ of pulse energy

We report on a SESAM-modelocked Yb:CaF2 thin-disk oscillator designed to generate more than 1 μJ of pulse energy at a moderate pulse repetition rate. The goal of our experiment was to explore the potential of Yb:CaF2 in a thin-disk laser (TDL) architecture for high power at pulse durations shorter than 300 fs as compared to other Yb-doped crystals exhibiting broad gain bandwidth. At a repetition rate of 10 MHz the laser produced an average output power of up to 17.8 W (1.78 μJ of pulse energy) with a beam quality factor M 2 below 1.2. The pulse duration was measured to be 285 fs, which results in a peak power of 5.5 MW. To the best of our knowledge, this is the highest pulse energy and peak power demonstrated to date with sub-300 fs pulses generated by SESAM-modelocked oscillators, leading to the conclusion that Yb:CaF2 is a very promising crystal for TDL technology.

Thermal behavior of resonant waveguide-grating mirrors in Yb:YAG thin-disk lasers

We present the experimental investigations of different designs of resonant waveguide-grating (RWG) mirrors, used as intracavity folding mirrors in an Yb:YAG thin-disk laser (TDL). 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 laser efficiency, polarization, and wavelength selectivity. It was found that the damage threshold and efficiency can be increased significantly with a proper design of the structure in comparison to the simplest design with a single waveguide layer. 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 TDL. Damage thresholds of 60 kW/cm(2) have been reached where only 63 K of surface temperature increase was observed. This showed that the improved mirrors are suitable for the generation of kW-class narrow linewidth, linearly polarized Yb:YAG TDL.

A 1.78 µJ and 285fs Yb:CaF2 SESAM-modelocked thin-disk oscillator

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.

Radially polarized passively mode-locked thin-disk laser oscillator emitting sub-picosecond pulses with an average output power exceeding the 100 W level

We report on a high-power passively mode-locked radially polarized Yb:YAG thin-disk oscillator providing 125 W of average output power. To the best of our knowledge, this is the highest average power ever reported from a mode-locked radially polarized oscillator without subsequent amplification stages. Mode-locking was achieved by implementing a SESAM as the cavity end mirror and the radial polarization of the LG*01 mode was obtained by means of a circular Grating Waveguide Output Coupler. The repetition rate was 78 MHz. A pulse duration of 0.97 ps and a spectral bandwidth of 1.4 nm (FWHM) were measured at the maximum output power. This corresponds to a pulse energy of 1.6 µJ and a pulse peak power of 1.45 MW. A high degree of radial polarization of 97.3 ± 1% and an M2-value of 2.16 which is close to the theoretical value for the LG*01 doughnut mode were measured.

A 57 W radially polarized SESAM mode-locked thin-disk oscillator

The advantages of radially and azimuthally polarized laser beams for various applications in materi processing, e.g. cutting and drilling, over circularly or linearly polarized beams has been reported amoi others in [1]. Furthermore, for micro material processing ultrafast lasers with pulse durations in the ord of 1 ps have gained tremendous interest. The oscillator we present here is a promising candidate to seed thin-disk multipass amplifier without the need of a complex multi-stages pre-amplification scheme reported in [2].

Thin-disk multipass amplifier delivering 10 GW of peak power

Recent development of thin-disk multipass amplifiers has resulted in several breakthroughs in average power and pulse energy of both ps- and fs-amplifiers [1-3]. These results confirm the suitability of this laser architecture for amplification of ultrafast pulses with high energy and at high average powers thanks to low nonlinearities in the thin-disk and to an efficient cooling scheme. However, as peak powers are reaching or exceeding the GW, even nonlinearities induced by propagation in air become relevant in terms of spectral broadening due to self-phase modulation (SPM).

First demonstration of passively mode-locked Yb:CaF2 thin-disk laser

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.

Investigation on thermal behavior of resonant waveguide-grating mirrors in an Yb:YAG thin-disk laser

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.