Birgit Weichelt

Enhanced performance of thin-disk lasers by pumping into the zero-phonon line

Pumping Yb:YAG or Yb:LuAG into the zero-phonon line at 969 nm instead of using the common pump wavelength of 940 nm reduces the heat generation by 32%. In addition to the 3% increase of the Stokes efficiency, this significantly reduces the diffraction losses caused by the thermally induced phase distortions leading to a remarkable increase of the overall efficiency especially of fundamental-mode thin-disk lasers. Using this pumping scheme in an Yb:LuAG thin-disk laser, we achieved 742 W of nearly diffraction limited (M2≈1.5) output power at an unprecedented high optical efficiency of 58.5%. For multimode operation (M2≈15) the maximum optical efficiency of an Yb:YAG thin-disk laser was increased to 72%.

Improving the brightness of a multi-kilowatt single thin-disk laser by an aspherical phase front correction

We report on results obtained with an aspherical mirror to compensate for the phase front aberrations of a cw thin-disk laser with a single disk in the resonator. A record output power of 5 kW with a beam quality suitable for laser cutting (beam propagation factor M2=9.2) has been achieved.

Single-layer resonant-waveguide grating for polarization and wavelength selection in Yb:YAG thin-disk lasers.

A single-layer resonant-waveguide grating consisting of a sub-wavelength grating coupler etched into a waveguide is proposed in order to achieve high polarization and high spectral selectivity inside an Yb:YAG thin-disk laser resonator. The designed structure was fabricated with the help of a Lloyds-mirror interference lithography setup followed by reactive ion beam etching down to the desired grating groove depth. The wavelength and polarization dependent reflectivity is measured and compared to the design results. The behaviour of the device at higher temperatures is also investigated in the present work. The device is introduced as the end mirror of an Yb:YAG thin-disk laser cavity. Output powers of up to 123 W with a spectral bandwidth of about 0.5 nm (FWHM) is demonstrated in a multimode configuration (M2~6). In fundamental-mode operation (TEM00 with M2~1.1) 70 W of power with a spectral bandwidth of about 20 pm have been obtained. Moreover, the degree of linear polarization was measured to be higher than 99% for both multimode and fundamental mode operation.

Yb:CaF 2 thin-disk laser

Summary form only given. Novel amplifier systems and laser oscillators generating short pulses and high pulse energies are required in numerous applications, not least in material processing. Currently there are significant efforts to develop such systems [1]. Among other Ytterbium-doped laser gain materials, Yb:CaF<;sub>2<;/sub> is an interesting candidate for highpower cw and passively mode-locked operation for the generation of short pulses because it combines a very broad emission spectrum with a high undoped thermal conductivity (~ 9.7 W/(mK)) [2]. The proof-of principle in ultrafast laser operation was already shown with a bulk crystal by demonstrating pulses shorter than 100 fs in a SESAM passively mode-locked laser system [3]. In the present contribution we report on the first investigation of a Yb:CaF<;sub>2<;/sub> thin-disk laser as this concept is ideally suited for power scaling of ultrafast lasers [4]. For the experimental investigation, two crystals from the same boule with different thicknesses were compared. The nominal doping concentration was specified by the supplier to be around 4 %. The first and non-wedged crystal has a thickness of 200 μm whereas that of the second and wedged (~0.1 °) crystal is 250μm. The thin-disk crystals were glued on copper heat sinks and water cooled from the backside with nominal operating temperatures between 15 and 18 °C. A thin-disk pumping module with a total of 24 pump light passes through the laser crystal was used for sufficient absorption of the pump radiation. The crystals were pumped either with a fiber-coupled laser diode with up to 40 W of power centered at a wavelength of 976 nm with a spectral bandwidth of about 10 nm or at high-powers or, with a 1.2 kW pump source exhibiting a spectral bandwidth of 4 nm centered at a wavelength of 976 nm. In this latter case, only one half of the pump power was used to avoid the damage of the crystal.Figure 1 shows the thermal behavior of the crystal which was examined using a thermal-imaging camera to measure the temperature at its front surface in fluorescence and laser operation. It is recognizable that the temperature at the front surface of the disk in fluorescence operation is significantly lower than in laser operation because of the smaller quantum defect of the shorter centre wavelength of the fluorescence emission. The fact that the temperature is not exceeding 100 °C at power densities of around 8 kW/cm2 represents a relevant and extremely promising information for the further development of high-power Yb:CaF2 thin-disk lasers. After this confirmation, high-power laser operation was demonstrated in a multimode resonator. As shown in Figure 2 a maximum output power of 250 W with an optical efficiency of 47 % was obtained with the 200 μm thick laser crystal. In a further experiment the wedged 250 μm thick crystal was characterized in the perspectives of modelocked operation. The pump spot was set to 1 mm. In a fundamental mode V-shaped resonator (M2 <; 1.1) a maximum of 12.9 W of CW output power with an optical efficiency of 34.2 % has been achieved. The successful demonstration of high power capability and fundamental mode operation were the first steps to set-up a passively mode-locked thin-disk laser. This work is currently under progress and will be presented during the talk.The demonstrated results confirm the potential of Yb:CaF2 for high-power thin-disk laser operation and offer the promising prospect of generating short pulse durations in a passively mode-locked thin-disk laser. Future improvements of the crystal quality, a better control of the doping concentration and a better quality of the polishing will lead to further enhancement of the laser performance.

Efficient pump beam shaping for high-power thin-disk laser systems

We report a beam-shaping technique whereby the output power from a high-power laser-diode stack is efficiently coupled, reconfigured, and transmitted to a thin-disk laser by means of a compact optical fiber bundle. By using this technique, the power density is increased by a factor of 2 when compared to direct coupling with a octagonal fused silica rod while the numerical aperture is kept constant. Transmission efficiency of 80% was measured for the beam shaper without antireflection coating. The top-hat distribution is numerically calculated at the thin-disk laser crystal.

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.

Experiments towards resolving the proton charge radius puzzle

We review the status of the proton charge radius puzzle. Emphasis is given to the various experiments initiated to resolve the conflict between the muonic hydrogen results and the results from scattering and regular hydrogen spectroscopy.

Yb 3+ -doped ceramic thin-disk lasers of Lu-based oxides

CW laser operations of thin-disk lasers with Lu-based oxide ceramics are reported. An output power of 166 W and a slope efficiency of 72.2% were obtained with an Yb:LuAG ceramic disk. We have also successfully demonstrated thin-disk lasers with Yb:Lu2O3 ceramics which were bonded by our soldering and gluing techniques. Slope efficiencies of 60.6% and 55.6% were obtained from a soldered disk and a glued disk, respectively.

Demonstration of a Yb 3+ -doped Lu 3 Al 5 O 12 ceramic thin-disk laser

Yb:LuAG ceramic is very promising for thin-disk laser and amplifier architectures since it exhibits a higher thermal conductivity at high doping concentrations and a larger emission cross section than Yb:YAG. In this Letter, we present what we believe to be the first demonstration of a thin-disk laser based on Yb:LuAG ceramic. A maximum output power of 101 W with an optical efficiency of 56% and a slope efficiency of 64% was obtained with a multimode laser resonator. Fundamental-mode laser operation with near diffraction limited beam quality (M2≈1.22) was also achieved. The fundamental-mode laser resonator showed the output power of 49 W, an optical efficiency of 31%, and a slope efficiency of 44%. A linearly polarized output beam was demonstrated in multimode operation using an intracavity Brewster window. The depolarization loss was measured to be as low as 0.15% per round trip.

Improving the brightness of a multi-kW thin disk laser with a single disk by an aspherical phase-front correction

Multi-kW beams with high brightness offer advantages in material processing applications with large distance beam propagation such as remote welding. To achieve the combination of high power and low beam parameter product, the thin disk laser concept is widely used due to its power scalability. Nevertheless, the efficient generation of several kilowatts of output power per disk at beam parameter products below ~3 mm·mrad is limited by an aspherical wavefront distortion in the disk and air turbulences in front of it. In the present paper the limiting factors are discussed and a novel method for compensation is presented. The compensating mirror consists of a silica substrate with a top-hat-shaped layer of 100 nm height to generate the desired phase-front correction and a conventional HR-coating on top. To prevent air convection in front of the thin disk crystal, the laser resonator was filled with helium. The experimental results yield a maximum output power of 3.4 kW and an optical efficiency of 49 % with a beam parameter product of ~2.6 mm•mrad (M2 ~ 8) at a cooling water temperature of 30 °C.