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Modal content in a 50 µm core diameter photonic bandgap Bragg fiber is studied with respect to the launching condition at a wavelength around 1860 nm by spatially and spectrally resolved imaging.

We report for the first time, to the best of our knowledge, an innovative design concept for intracavity pulse stretching in a regenerative amplifier, employing a single “grating-mirror” based on a leaky-mode grating-waveguide design. The very compact and flexible layout allows for femtosecond pulses to be in principle easily stretched up to nanosecond durations. The design has been tested in a diode-pumped Yb:CALGO regenerative amplifier followed by a standard transmission grating compressor. Sub-200-fs pulses (stretched pulses ≈110 ps) with 205-μJ energy at 20-kHz repetition rate have been demonstrated. In order to prove the robustness and potential for energy scaling of leaky-mode grating-waveguide intracavity stretcher, we generated stretched pulses with energies of up to ≈700 μJ (400-ps long) at a lower repetition rate of 10 kHz. A simple model is proposed for the study of the cavity in presence of induced spatial chirp.

We demonstrated for the first time, to the best of our knowledge a new intracavity pulse stretching design, employing a single grating-mirror based on a leaky-mode grating-waveguide design. The extremely compact and flexible layout allows for femtosecond pulses to be easily stretched up to nanosecond durations. The stretcher was implemented in a diode-pumped Yb:CALGO regenerative amplifier followed by a standard transmission grating compressor. We demonstrated sub-200 fs long pulses (stretched pulses ≈ 110 ps) with a maximum energy of 205 μJ at 20 kHz repetition rate. As a proof of the robustness and potential energy scaling of leaky-mode grating-waveguide intracavity stretcher, energies up to 700 μJ and 400 ps long pulses before compression at a lower repetition rate of 10 kHz, have been achieved. A simple model is proposed to investigate the cavity behavior in presence of induced spatial chirp.

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 2 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 2 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

In this contribution, the authors present, the first laser experiments ever demonstrated with Yb:Gd2O3 and Yb:Y2O3 crystals grown by this new flux method. The demonstrated the first diode-pumped Yb:Gd2O3 laser. Furthermore the new and advantageous flux method allows for the growth of highly Yb-doped sesquioxides which is very promising for the elaboration of cubic Re2O3 dedicated to thin-disk laser applications.

The present contribution reports on the modelling, fabrication, as well as the intra-cavity characterization of fully dielectric grating-mirrors for Yb:YAG thin-disk lasers. By integrating a low-index leaky-mode waveguide and a sub-wavelength diffraction grating onto an HR mirror, such devices achieve high diffraction efficiencies, in our case >99.8% in the - 1st order. Used as simple one-to-one replacement for laser cavity end mirrors, they allow a highly efficient generation of beams with very narrow spectral bandwidth and high-polarization purity. Since the grating-waveguide-mirrors (GWM) are used in Littrow configuration, they also permit to tune the wavelength of the emitted laser beam. Therefore they can replace a combination of etalons, birefringent filters, Brewster plates and/or thin-film polarizers usually employed for these tasks.

The development of the thin-disk laser [1] marks a major improvement in terms of power scaling at good beam quality when compared to classical solid-state laser concepts. The highly efficient cooling scheme leads to a significant reduction of thermally induced wave front distortions. The reduced spherical component of the distortions, often referred to as “thermal lensing” [2], enables the realization of brilliant lasers with a broad dynamic stability range. When it comes to fundamental-mode operation, however, the aspherical components of the wave-front distortions introduced by the strong temperature gradient at the edge of the pump spot in the laser crystal lead to diffraction losses and therefore severely limit the power scalability. As stable fundamental-mode operation is an important prerequisite for the efficient generation of ultra-short pulses by mode-locking, these losses are also limiting the scalability of the average power of thin-disk based femtosecond oscillators. This limitation can be overcome by means of intra-cavity deformable mirrors which are used to compensate for the aspherical components of the wavefront distortions [3].

We present Yb:SSO as promising material for femtosecond thin-disk oscillators confirmed by 80 nm of tuning range, 280 W of multimode cw output, and 298 fs mode-locked pulses at 27.8 W of average output power.

Laser sources with tuneable or stabilized emission wavelength and/or reduced spectral bandwidth have many applications in various fields, like second-harmonic generation [1], spectroscopy, wavelength multiplexing etc. The required wavelength control can be accomplished by several means, e.g. by introducing wavelength-selective elements such as prisms, gratings, etalons, birefringent filters, etc. into the laser resonator. Some of these elements (e.g. conventional gratings and multi-plate birefringent filters) introduce significant additional losses to the resonator. This limits their application to high-power lasers, especially to thin-disk lasers which are, due to the low gain of the disk, very sensitive to intra-cavity losses. Most of these elements (e.g. prisms, birefringent filters, and etalons) are used in transmission, which can lead to a reduced dynamic stability and to beam quality deteriorations due to thermal lensing effects which occur under high power levels.