We report on, to the best of our knowledge, the first fs-laser-written waveguide laser in Yb3+:CaGdAlO4 (Yb:CALGO). With Yb:CALGO crystals grown in our labs, we obtained a slope efficiency of 69% and up to 2.4 W of continuous wave (cw) output power in a waveguide-laser configuration. Moreover, bulk laser experiments with Yb:CALGO were performed, and slope efficiencies up to 73%, optical-to-optical efficiencies of 65%, and maximum cw output powers of 3.3 W were reached. These are the highest efficiencies in the laser configuration with Yb:CALGO.

A neodymium (Nd) doped lithium-niobate-on-insulator (LNOI) was fabricated from a lithium niobate (LN) substrate diffusion-doped with Nd prior to the LNOI fabrication process. Diffusion doping enables the fabrication of differently doped regions on a single LNOI substrate, a key advantage for future applications in highly integrated photonics. Fluorescence spectra, emission cross-section, fluorescence lifetime, and small-signal gain of Nd:LNOI were determined for the first time to our knowledge and compared with data published for diffusion-doped ridge or channel waveguides in LN substrates. For an 11 mm-long sample, we achieve a small-signal gain for π (σ) polarization of about 14 (10) dB when pumping with a Ti:Sapphire laser, which is promising for future active components in Nd:LNOI like amplifiers and lasers.

We report on a systematic investigation of selective etching of fs-laser inscribed microstructures in Y 3 Al 5 O 12 (YAG). The resulting microchannels are up to 8.9 mm long and exhibit cross sections from below 10 µm to more than 100 µm. Aspect ratios of up to 593 were achieved. Investigations with different structuring and etching parameters revealed that the etching process is mainly diffusion determined. The etching depth depends on the square root of time, similar to the well-known Brownian motion. In addition, we could enhance the etching diffusion constant by a factor of two, reducing the time to etch the longest channel by an order of magnitude, using a 1:1 mixture of sulfuric and phosphoric acid instead of pure phosphoric acid. The observed fundamental time dependence in conjunction with diffusion coefficients up to 160 µm/h 1/2 makes the etching behavior highly predictable and paves the way toward arbitrary three-dimensional micro-and nanostructuring over long distances in crystalline materials.

In this Letter, we present high-power continuous wave (CW) and Q-switched femtosecond laser-written Yb: YAG channel waveguide lasers. In Q-switched operation, obtained by a semiconductor saturable absorber mirror (SESAM), as well as in CW operation, the laser generates average output powers of more than 5.6 W and reaches slope efficiencies above 74%. The Q-switched laser operated at a maximum repetition rate of 5.4 MHz with a minimum pulse duration of 11 ns, and with a maximum pulse energy of 1 μJ. This laser has almost an order of magnitude higher average output power than previously reported Q-switched channel waveguide lasers. Dielectric channel waveguide (WG) lasers are among the most promising technologies for compact and reliable laser sources, that combine multiwatt power levels with a high level of integration [1]. Major breakthroughs were achieved thanks to much progress in waveguide fabrication methods such as ionexchange [2], liquid-phase epitaxy [3], and femtosecond-laser (fs-laser) inscription [4][5][6]. In continuous wave (CW) operation, power levels of up to 5.1 W and slope efficiencies up to 73% were reported for fs-laser-inscribed channel waveguide lasers [5]. Several Q-switched and CW modelocked dielectric channel waveguide lasers were demonstrated, and pulse durations down to 285 fs were realized [7,8]. Q-switched operation was demonstrated with average output power levels of up to 680 mW [9].In this Letter, we demonstrate that the combination of femtosecond laser-written crystalline channel waveguides with semiconductor saturable absorber mirrors (SESAMs) enables the realization of a high-power Q-switched channel WG laser.We present the first Q-switched channel WG lasers with multiwatt average output power, achieving pulse energies of up to 1 μJ.The waveguiding microstructures were inscribed into a 7% Yb 3 -doped Y 3 Al 5 O 12 (Yb:YAG) crystal with a fs-laser. They consist of two parallel tracks, which are inscribed by a linear translation of the sample perpendicular to the incident fs-laser beam. These tracks exhibit distances between 22 and 30 μm. Owing to a stress-induced refractive index change, the waveguiding region is in the center between the tracks. Such waveguides are often referred to as type II waveguides [4,5]. Here, we superimposed the linear translation with a velocity of 25 μm/s of the sample with a sine oscillation with an oscillation amplitude of 2 to 4 μm and an oscillation frequency of 70 Hz; the fs-laser writing scheme can be seen in Fig. 1.With this writing scheme a larger refractive index change and a better confinement of the laser mode can be achieved [5]. We modified our previous WG writing scheme [5,6,10] by inserting a pinhole with 600 μm diameter into the beam path of the fslaser to improve the beam quality by mode cleaning. Because of the large distance between the pinhole and the aspheric focusing lens (f 3.1 mm, NA 0.68) used for the laser inscription, only the 0th order of the resulting diffraction pattern is transmitted through the aperture of this lens. The ...

Received Month X, XXXX; revised Month X, XXXX; accepted Month X, XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXX We report on the first continuous wave quasi 3-level lasers emitting in the cyan-blue spectral range in praseodymium doped crystalline materials. Applying Pr 3+ :BaY2F8 as active medium, up to 201 mW of output power at 495 nm could be obtained with a slope efficiency of 27% under pumping with an optically pumped semiconductor laser (2-OPSL) at 480 nm. In the same pumping scheme, utilizing Pr 3+ :LiYF4 output powers up to 70 mW were realized at 491 nm and 500 nm, respectively. With Pr 3+ :BaY2F8 also diode pumped laser operation with up to 11% slope efficiency and 44 mW output power was achieved. In the latter case, detailed investigations on the temperature dependency of the laser output were conducted. [9,10]. Cw three level laser oscillation of trivalent praseodymium at room temperature was so far limited to doped fluoride glass fibers [11,12]. In this letter we report on our first results on cw cyan-blue laser operation of praseodymium doped BaY2F8 (BYF) and LiYF4 (YLF) crystals at room temperature. To operate such lasers on the zero phonon line, very high inversion levels of approximately 50% are necessary, requiring -together with the short effective lifetime of the emitting Pr 3+ -level -highly intense excitation sources. In contrast, transitions terminating in energetically higher lying Stark levels of the 3 H4 ground state are less affected by reabsorption. This should allow for ground state laser operation at comparatively low inversion levels and thus reasonable threshold pump powers for cw laser operation.As host material we chose on the one hand side YLF, which -also due to the high quality of the available crystals -allowed for the highest slope efficiencies of any praseodymium laser so far [13][14][15]. On the other hand side, the Pr:BYF crystal seems, due to the presence of a comparatively strong emission peak at 495 nm, to be a more suitable gain medium for three level laser operation with low laser threshold [16,17]. The effective lifetimes and ground state absorption cross sections of these crystals at comparable doping concentrations are very similar. Therefore it is sufficient to compare their gain spectra to predict their suitability for laser operation into the ground state manifold. Thus, we first determined emission cross sections em applying