A table-top femtosecond hard X-ray source at 8 keV provides a flux of up to 1.5 × 10 12 photons/s. Few-cycle 5 μm laser pulses serve as driver that are generated by a multiple-stage optical parametric chirped-pulse amplifier (OPCPA).

Calcium niobium gallium garnet (CNGG) crystals doped with thulium (Tm 3+ ) ions possess disordered structure leading to inhomogeneously broadened emission bands which makes them attractive for generation of ultrashort pulses at ~2 μm [1] . The actual composition of CNGG deviates from the stoichiometry and cationic vacancies are present to ensure charge compensation. They can be eliminated by codoping with univalent alkali cations (Li + , Na + ). The related calcium tantalum gallium garnet (CTGG) shows better thermal properties than CNGG [2] . Here, we report on the growth, spectroscopy and first laser action in a Tm 3+ ,Li + -codoped CTGG (Tm:CLTGG) crystal.

Monoclinic Yb 3+ ,Li + :ZnWO 4 crystals were grown by Czochralski and their structure is refined. The spectroscopic properties of Yb 3+ ions were studied with polarized light and the scheme of Stark splitting of Yb 3+ manifolds in these crystals was determined. The Raman spectra were measured. A diode-pumped 2.90 W laser action at 1059 nm in the crystals was demonstrated.

We report on a model of diode-pumped Thulium lasers passively Q-switched by a graphene saturable absorber applicable also for any other “fast” saturable absorber. It reasonably predicts the dependence of the pulse duration, pulse energy and pulse repetition frequency on the absorbed power. The model is applied in the present work for a Tm: KLuW microchip laser passively Q-switched with a multi-layer graphene saturable absorber. The laser generates ~1 W at 1926 nm with a slope efficiency of 39%. Stable 190 ns /4.1 μJ pulses are achieved at a pulse repetition frequency of 260 kHz. The potential of graphene for the generation of few-ns pulses at ~2 µm is discussed.

We report on the crystal growth, structural and spectroscopic investigation of a novel trigonal calcium vanadate crystal, Tm3+:Ca9La(VO4)7. Polarized absorption, stimulated-emission and gain cross-section spectra are determined for this material. The maximum σSE corresponding to the 3F4 → 3H6 transition amounts to 1.28 × 10−20 cm2 at 1854 nm for σ-polarization. The measured lifetime of the Tm3+ ions in the 3F4 state is 1.19 ms. The Judd-Ofelt analysis performed yielded intensity parameters of Ω2 = 4.682, Ω4 = 0.659 and Ω6 = 0.475 [10−20 cm2]. The polarized Raman spectra indicate a strong and broad band centered at 867 cm−1. Owing to the disordered nature of Ca9La(VO4)7, the Tm3+ ions exhibit a broad and smooth gain profile at ~2 μm. The spectroscopic properties of Tm3+:Ca9La(VO4)7 are very promising for broadly tunable and ultrashort pulse lasers near 2 µm.

We report on the growth, spectroscopic and laser characterization of a novel monoclinic laser crystal, Yb,In:KLu(WO4)2 (Yb,In:KLuW). The absorption, stimulated-emission and gain cross-section spectra of 3.5at% Yb, 5.5at% In-doped KLuW are determined at room temperature with polarized light and compared with those for Yb,In:KYW, as well as singly Yb-doped KLuW and KYW crystals. It is found that the introduction of In results in a decrease of the transition cross-sections and in a spectral broadening of the absorption and emission bands. Such a broadening is more pronounced for light polarization E ||Np. For Yb,In:KLuW, the maximum σabs is 9.9×10–20cm2 at 980.9nm for E ||Nm and the corresponding bandwidth of the absorption peak is 3.7nm. The radiative lifetime for Yb3+ ions is 237±5µs. The stimulated-emission cross-sections are σSE(m)=2.4×10–20cm2 at 1022.4nm and σSE(p)=1.3×10–20cm2 at 1039.1nm corresponding to an emission bandwidth of >30nm and >35nm, respectively. The diode-pumped Ng-cut Yb,In:KLuW microchip laser generated 4.11W at 1042–1048nm with a slope efficiency of 78%. The Yb,In:KLuW crystal is very promising for the generation of sub-100fs pulses in mode-locked lasers due to its broadband emission characteristics.

We report on the first Tm3+-doped double tungstate waveguide laser passively Q-switched by a graphene saturable absorber using a 12.4 mu m-thick 3 at.% Tm:KY0.58Gd0.22Lu0.17(WO4)(2) epitaxial layer grown on a (0 1 0)-oriented pure KY(WO4)(2) substrate. This laser generated 5.8 nJ/195 ns pulses at 1831.8 nm corresponding to a pulse repetition frequency of 1.13 MHz. These are the shortest pulses achieved in passively Q-switched Tm waveguide lasers. The laser slope efficiency was 9% and the Q-switching conversion efficiency reached 45%. Graphene is promising for the generation of ns pulses at similar to 2 mu m in Tm3+-doped double tungstate waveguide lasers operating in the MHz-range.

A novel monoclinic magnesium tungstate crystal, Tm:MgWO 4 , was grown and characterized in terms of structure, spectroscopy and laser generation. The output power was 772 mW at 2017-2029 nm with a slope efficiency of 39%.

A diode-pumped Yb:KLu(WO4)2 microchip laser passively Q-switched by a Cr4+:YAG saturable absorber generated a maximum average output power of 590 mW at 1031 nm with a slope efficiency of 55%. The pulse characteristics were 690 ps/47.6 μJ at a pulse repetition frequency of 12.4 kHz. The output beam had an excellent circular profile with M2<1.05. Yb:KLu(WO4)2 is very promising for ultrathin sub-ns microchip lasers.

A 2 μm chirped pulse amplification source generates 55 mJ picosecond pulses at a 1 kHz repetition rate. The system consists of a high-gain Ho:YLF regenerative amplifier (RA) operating in the single-energy regime and a dual-rod Ho:YLF power amplifier. Pulses of ∼10 mJ energy from the RA are linearly scaled up to 55 mJ in the power amplifier, corresponding to a high overall extraction efficiency of >20%. The system displays an exceptional high stability with a pulse-to-pulse rms as low as 0.3%. Pulse compression is performed up to the 25 mJ energy level, resulting in pulses close to the Fourier-transform limit with a duration of 4.3 ps and a peak power of 4.4 GW.

A lattice-matched, 5 at.% Tm, 0.5 at.% Ho-codoped, 5.77 mu m-thick KY0.58Gd0.22Lu0.20(WO4)(2) active layer with optimized refractive index contrast is grown by liquid phase epitaxy on the (3 1 0) face of pure KY(WO4)(2) substrate. Laser operation at 2051 nm (I-5(7) -> I-5(8) transition of the Ho3+ ion) is demonstrated with this waveguide pumped at 794 nm. The maximum continuous wave output power amounts to 1.9 mW at 2051 nm corresponding to a slope efficiency of 10.5%. The laser threshold is as low as 1.5 mW of absorbed pump power. The developed structure is promising for single-transverse-mode channel holmium waveguide lasers. Laser operation in 3 at.% Tm-singly doped 4.41 mu m thick layer grown on the (3 1 0)-oriented substrate is also demonstrated at 1841 nm with a slope efficiency of 31%.

Monoclinic Ho:KY(WO4)2 crystals doped with up to 7.5 at.% Ho are grown by the Top Seeded Solution Growth-Slow Cooling method. The evolution of their unit cell parameters in dependence on the Ho doping and temperature is studied. The polarized low-temperature (6 K) optical absorption of the Ho3+ ion is investigated in detail to determine the energy of the Stark sub-levels. Room-temperature absorption, stimulated-emission and gain cross-section spectra of Ho:KY(WO4)2 crystals are derived for polarizations parallel to the principal optical axes, E||Np, Nm and Ng. The maximum absorption cross-section for the 5I8→5I7 transition is 1.60×10−20 cm2 at 1961.0 nm and the maximum stimulated-emission cross-section for the 5I7→5I8 transition is 2.65×10−20 cm2 at 2056.3 nm (for E||Nm). The radiative lifetime of the upper laser level of the Ho3+ ion (5I7) amounts to 4.8 ms. Continuous-wave Ho3+ laser operation is achieved under in-band pumping by a Tm laser at 1946 nm. In the microchip configuration, the maximum output power reached 205 mW at 2105 nm with a slope efficiency as high as 85%.

A 2 μm chirped pulse amplification source generates 55 mJ picosecond pulses at a 1 kHz repetition rate. The system consists of a high-gain Ho:YLF regenerative amplifier (RA) operating in the single-energy regime and a dual-rod Ho:YLF power amplifier. Pulses of ∼10  mJ energy from the RA are linearly scaled up to 55 mJ in the power amplifier, corresponding to a high overall extraction efficiency of >20%. The system displays an exceptional high stability with a pulse-to-pulse rms as low as 0.3%. Pulse compression is performed up to the 25 mJ energy level, resulting in pulses close to the Fourier-transform limit with a duration of 4.3 ps and a peak power of 4.4 GW.