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Single crystals of Yb 3+ -doped NaGd͑WO 4 ͒ 2 with up to 20 mol % ytterbium content have been grown by the Czochralski technique in air or in N 2 +O 2 atmosphere and cooled to room temperature at different rates ͑4-250°C/h͒. Only the noncentrosymmetric tetragonal space group I4 accounts for all reflections observed in the single crystal x-ray diffraction analysis. The distortion of this symmetry with respect to the centrosymmetric tetragonal space group I4 1 / a is much lower for crystals cooled at a fast rate. Na + , Gd 3+ , and Yb 3+ ions share the two nonequivalent 2b and 2d sites of the I4 structure, but Yb 3+ ͑and Gd 3+ ͒ ions are found preferentially in the 2b site. Optical spectroscopy at low ͑5 K͒ temperature provides additional evidence of the existence of these two sites contributing to the line broadening. The comparison with the 2 F 7/2 ͑n͒ and 2 F 5/2 ͑nЈ͒ Stark energy levels calculated using the crystallographic Yb-O bond distances allows to correlate the experimental optical bands with the 2b and 2d sites. As a novel uniaxial laser host for Yb 3+ , NaGd͑WO 4 ͒ 2 is characterized also with respect to its transparency, band-edge, refractive indices, and main optical phonons. Continuous-wave Yb 3+ -laser operation is studied at room temperature both under Ti:sapphire and diode laser pumping. A maximum slope efficiency of 77% with respect to the absorbed power is achieved for the polarization by Ti:sapphire laser pumping in a three-mirror cavity with Brewster geometry. The emission is tunable in the 1014-1079 nm spectral range with an intracavity Lyot filter. Passive mode locking of this laser produces 120 fs long pulses at 1037.5 nm with an average power of 360 mW at Ϸ97 MHz repetition rate. Using uncoated samples of Yb: NaGd͑WO 4 ͒ 2 at normal incidence in simple two-mirror cavities, output powers as high as 1.45 W and slope efficiencies as high as 51% are achieved with different diode laser pump sources.

Lasing of Yb3+ in a disordered single crystal host, NaGd(WO4)2, is reported. Pump efficiencies as high as 20% and slope efficiencies as high as 30% are achieved for both sigma- and pi-polarizations with Ti:sapphire laser pumping. The emission of Yb:NaGd(WO4)2 is centered near 1030 nm. Tunability between 1016 and 1049 nm is obtained with a Lyot filter.

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Passive Q-switching performance was found to be greatly improved by use of a new Nd-doped mixed vanadate crystal Nd:Gd0.64Y0.36VO4 compared with that achieved with Nd:YVO4 and Nd:GdVO4. At an absorbed pump power of 12 W, an average output power of 2.78 W was obtained at a pulse repetition frequency of 15.4 kHz with an optical conversion efficiency of 23.2%, and the slope efficiency was determined to be 45.5%. The resulting pulse energy, peak power, and pulse width were 181 microJ, 26.6 kW, and 6.8 ns, respectively.

A single crystal of NaGd(WO 4) 2 with disordered structure was grown by the Czochralski method with 5 mol % of Tm 3+-doping in the melt. This crystal host belongs to the I 4 tetragonal space group. The Tm 3+ optical absorption and emission spectroscopy revealed strong polarization dependence in accordance with the S 4 local site symmetry of the uniaxial host. The Stark energy levels of Tm 3+ in this host were determined from 5 K optical absorption and photoluminescence measurements. The 300 K lifetime of the upper laser level for the 3 F 4 → 3 H 6 transition amounts to 1.35±0.20 ms. The maximum ground state absorption cross sections for the 3 H 4 level of interest for diode pumping near 795 nm are 2.9×10-20 and 1.18×10-20 cm 2 for the σ and π polarization, respectively. The maximum emission cross sections amount to 10.6×10-21 cm 2 at 1796 nm and 9.5×10-21 cm 2 at 1847 nm, for the σ and π polarization, respectively. Laser operation in the cw regime has been achieved at room temperature under Ti:sapphire and diode laser pumping using an uncoated sample. A maximum output power of ≈300 mW was obtained at 1925 nm for the σ polarization under Ti:sapphire laser pumping. Slightly higher powers were obtained with diode pumping but the threshold pump power also increased. In this case the laser naturally selected the σ polarization. The optical bands of Tm 3+ in this crystal have inhomogeneous contribution and their FWHM reaches, e.g. for the 3 H 6 → 3 F 4 transition at 5 K, about 60 cm-1. This is related to Na and Gd (or Tm) local disorder between the two nonequivalent lattice sites of the host. The full cw laser tunability range at 300 K, from 1813 to 2025 nm, corresponds to ≈17 THz, and is one of the broadest achieved with a Tm 3+-doped crystalline material, despite the relatively low efficiency obtained with this first sample of Tm:NaGd(WO 4) 2 .

Direct sub-50-fs pulse generation is demonstrated with a mode-locked Yb:YCa4O(BO3)3 laser. With external compression, pulses as short as 35 fs are generated at 1055 nm. The oscillator operating at a repetition rate of 95 MHz is pumped by a two-section distributed Bragg reflector tapered diode laser and mode locked by a semiconductor saturable absorber mirror. The onset of self-Raman-conversion for pulse spectral bandwidths exceeding 40 nm (FWHM) is observed.