Kolja Beil

Thermal and laser properties of Yb:LuAG for kW thin disk lasers

Thin disk laser experiments with Yb:LuAG (Yb:Lu(3)Al(5)O(12)) were performed leading to 5 kW of output power and an optical-to-optical efficiency exceeding 60%. Comparative analyses of the laser relevant parameters of Yb:LuAG and Yb:YAG were carried out. While the spectroscopic properties were found to be nearly identical, investigations of the thermal conductivities revealed a 20% higher value for Yb:LuAG at Yb(3+)-doping concentrations of about 10%. Due to the superior thermal conductivity with respect to Yb:YAG, Yb:LuAG offers thus the potential of improved performance in high power thin disk laser applications.

Efficient high-power continuous wave Er:Lu 2 O 3 laser at 2.85 μm

We report on crystal growth, spectroscopy, thermal conductivity and (4)I(11/2)→(4)I(13/2)-laser performance of Er:Lu(2)O(3). Pumping with an optically pumped semiconductor laser at 971 nm, 1.4 W of cw output power with a slope efficiency of ∼36% at 2.85 µm was obtained at room temperature. This exceeds the Stokes efficiency due to an upconversion process recycling population of the lower laser level back into the upper laser level, yielding the highest efficiency of any Er-sesquioxide laser around 3 µm. Under diode pumping, 5.9 W of output power with 27% of slope efficiency was achieved with an M(2) of 1.2 to 1.4.

SESAMs for high-power femtosecond modelocking: power scaling of an Yb:LuScO 3 thin disk laser to 23 W and 235 fs

We report on power scaling of a modelocked thin disk laser based on the broadband mixed sesquioxide material Yb:LuScO₃. One of the key elements to achieve this result was an improved SESAM design with reduced two-photon-absorption (TPA) and high damage threshold. In a first experiment, using a standard antiresonant SESAM with no topcoating, we could demonstrate record short pulse durations of 195 fs at a moderate average power of 9.5 W. Furthermore, we were able to power scale our thin disk laser while keeping the pulses short reaching 23 W at a pulse duration of 235 fs. This was made possible by designing a new SESAM with multiple quantum wells (QW) and a suitable dielectric topcoating. We will present SESAM optimization guidelines for short pulse generation from high-power modelocked oscillators.

Self-referenceable frequency comb from an ultrafast thin disk laser

We present the first measurement of the carrier envelope offset (CEO) frequency of an ultrafast thin disk laser (TDL). The TDL used for this proof-of-principle experiment was based on the gain material Yb:Lu(2)O(3) and delivered 7 W of average power in 142-fs pulses, which is more than two times shorter than previously realized with this material. Using only 65 mW of the output of the laser, we generated a coherent octave-spanning supercontinuum (SC) in a highly nonlinear photonic crystal fiber (PCF). We detected the CEO beat signal using a standard f-to-2f interferometer, achieving a signal-to-noise ratio of >25 dB (3 kHz resolution bandwidth). The CEO frequency was tunable with the pump current with a slope of 33 kHz/mA. This result opens the door towards high-power frequency combs from unamplified oscillators. Furthermore, it confirms the suitability of these sources for future intralaser extreme nonlinear optics experiments such as high harmonic generation and VUV frequency comb generation from compact sources.

Yb:CaGdAlO 4 thin-disk laser with 70% slope efficiency and 90 nm wavelength tuning range

In the last decades high power ultrafast lasers have attracted considerable attention for many applications. The most promising approach to get high power ultrashort pulses directly from an oscillator is a modelocked thin disk laser (TDL) using a broadband gain material. For this purpose, Yb-doped materials are very suitable since their simple energy-level scheme allows for highly efficient laser operation and the strong electron-phonon coupling results in broad spectra. In addition, to maintain efficient cooling a high thermal conductivity is required which often stands in contradiction to a very broad spectrum. This paper reports on improved crystal growth and highly efficient thin disk laser operation of Yb:CALGO, using a 5.9% Yb-doped CALGO in a TDL setup with 24 pump passes through the gain medium and a pump spot diameter of 1.2 mm. A slope efficiency of 70% and 57% of optical-to-optical efficiency with respect to the incident pump power could be obtained with an output coupler transmission of 1.6%, representing the highest efficiency obtained with this material so far. The laser output power of 30 W was limited by the available pump power from the 50 W fiber coupled pump diode laser.

Thermal properties of borate crystals for high power optical parametric chirped-pulse amplification

The potential of borate crystals, BBO, LBO and BiBO, for high average power scaling of optical parametric chirped-pulse amplifiers is investigated. Up-to-date measurements of the absorption coefficients at 515 nm and the thermal conductivities are presented. The measured absorption coefficients are a factor of 10-100 lower than reported by the literature for BBO and LBO. For BBO, a large variation of the absorption coefficients was found between crystals from different manufacturers. The linear and nonlinear absorption coefficients at 515 nm as well as thermal conductivities were determined for the first time for BiBO. Further, different crystal cooling methods are presented. In addition, the limits to power scaling of OPCPAs are discussed.

Continuous Wave and Mode-locked Yb3+:Y2O3 Ceramic Thin-disk Laser

CW and mode-locked laser operation based on an Yb3+:Y2O3 ceramic thin disk is reported. In CW laser operation, an output power of 70 W with an optical-to-optical efficiency of 57.4% was achieved. A higher slope efficiency of 70% was also obtained with a 50-W pump laser diode with more suitable emission characteristics. In mode-locked laser operation, pulses as short as 547 fs with an average power of 7.4 W were obtained. To our knowledge, this is the first demonstration of a high power CW and mode-locked laser operation based on Yb3+:Y2O3 ceramic thin disks.

Dual-gain SESAM modelocked thin disk laser based on Yb:Lu 2 O 3 and Yb:Sc 2 O 3

We present for the first time a SESAM-modelocked thin-disk laser (TDL) that incorporates two gain materials with different emission spectra in a single TDL resonator. The two gain media used in this experiment are the sesquioxide materials Yb:Lu2O3 and Yb:Sc2O3, which have their spectral emission peak displaced by ≈7 nm. We can benefit from a combined gain bandwidth that is wider than the one provided by a single gain material alone and still conserve the excellent thermal properties of each disk. In these first proof-of-principle experiments we demonstrate pulse durations shorter than previously achieved with the single gain material Yb:Lu2O3. The oscillator generates pulses as short as 103 fs at a repetition rate of 41.7 MHz and a center wavelength of around 1038 nm, with an average output power of 1.4 W. A different cavity layout provides pulses with a duration of 124 fs at an output power of 8.6 W. This dual-gain approach should allow for further power scaling of TDLs and these first results prove this method to be a promising new way to combine the record output-power performance of modelocked TDLs with short pulse durations in the sub-100 fs regime.

SESAMs for high-power femtosecond modelocking: power scaling of an Yb:LuScO3 thin disk laser to 23 W and 235 fs

We report on power scaling of a modelocked thin disk laser (TDL) based on the broadband mixed sesquioxide material Yb:LuScO3 (22 nm full width half maximum (FWHM) emission bandwidth). In a first experiment, we could demonstrate pulse durations as short as 195 fs at a moderate average power of 9.5 W. Furthermore, we were able to power scale our TDL while keeping the pulses short reaching 23 W at a pulse duration of 235 fs. A key element to achieve this result was the design of new SESAM structures with multiple quantum wells (QW) and a suitable dielectric topcoating, resulting in SESAMs with appropriate parameters for short pulse geneartion, low two-photon absorption (TPA) and high damage thresholds. We will present SESAM optimization guidelines for short pulse generation from high-power modelocked oscillators.

Efficient OPSL-pumped mode-locked Yb:Lu2O3 laser with 67% optical-to-optical efficiency.

We present a mode-locked Yb:Lu2O3 laser with up to 67% of optical-to-optical efficiency. By utilizing a high brightness optically pumped semiconductor laser (OPSL) as a pump source and using a semiconductor saturable absorber mirror (SESAM) we obtained self-starting mode locking. A pulse duration of 571 fs at 4.73 W of average output power with an optical-to-optical efficiency of 67% was achieved. In a slightly different cavity configuration the pulse duration was reduced to 313 fs at 2.16 W of average output power. In both cases the pulse duration was longer than the Fourier limit and the spectrum supports significantly shorter pulse durations. The laser wavelength is centered at 1034 nm and the repetition rate is 100.76 MHz in both cases. In continuous wave fundamental mode operation the optical-to-optical efficiency was as high as 78% with output powers exceeding 5 W.