We report on an experimental study of supercontinuum generation in photonic crystal fibers with low-intensity femtosecond pulses, which provides evidence for a novel spectral broadening mechanism. The observed results agree with our theoretical calculations carried out without making the slowly varying envelope approximation. Peculiarities of the measured spectra and their theoretical explanation demonstrate that the reason for the white-light generation in photonic crystal fibers is fission of higher-order solitons into redshifted fundamental solitons and blueshifted nonsolitonic radiation.
High-quality crystals of monoclinic KLu(WO4)2, shortly KLuW, were grown with sizes sufficient for its characterization and substantial progress was achieved in the field of spectroscopy and laser operation with Yb 3+ -and Tm 3+ -doping. We review the growth methodology for bulk KLuW and epitaxial layers, its structural, thermo-mechanical, and optical properties, the Yb 3+ and Tm 3+ spectroscopy, and present laser results obtained in several operational regimes both with Ti:sapphire and direct diode laser pumping using InGaAs and AlGaAs diodes near 980 and 800 nm, respectively. The slope efficiencies with respect to the absorbed pump power achieved with continuous-wave (CW) bulk and epitaxial Yb:KLuW lasers under Ti:sapphire laser pumping were ≈ 57 and ≈ 66%, respectively. Output powers as high as 3.28 W were obtained with diode pumping in a simple two-mirror cavity where the slope efficiency with respect to the incident pump power reached ≈ 78%. Passively Q-switched laser operation of bulk Yb:KLuW was realized with a Cr:YAG saturable absorber resulting in oscillation at ≈ 1031 nm with a repetition rate of 28 kHz and simultaneous Raman conversion to ≈ 1138 nm with maximum energies of 32.4 and 14.4 µJ, respectively. The corresponding pulse durations were 1.41 and 0.71 ns. Passive mode-locking by a semiconductor saturable absorber mirror (SESAM) produced bandwidth-limited pulses with duration of 81 fs (1046 nm, 95 MHz) and 114 fs (1030 nm, 101 MHz) for bulk and epitaxial Projection of the KLu(WO4)2 structure parallel to the b crystallographic direction [010].Yb:KLuW lasers, respectively. Slope efficiency as high as 69% with respect to the absorbed power and an output power of 4 W at 1950 nm were achieved with a diodepumped Tm:KLuW laser. The slope efficiency reached with an epitaxial Tm:KLuW laser under Ti:sapphire laser pumping was 64 %. The tunability achieved with bulk and epitaxial Tm:KLuW lasers extended from 1800 to 1987 nm and from 1894 to 2039 nm, respectively.
We present our recent achievements in the growing and optical characterization of KYb(WO 4 ) 2 ͑hereafter KYbW͒ crystals and demonstrate laser operation in this stoichiometric material. Single crystals of KYbW with optimal crystalline quality have been grown by the top-seeded-solution growth slow-cooling method. The optical anisotropy of this monoclinic crystal has been characterized, locating the tensor of the optical indicatrix and measuring the dispersion of the principal values of the refractive indices as well as the thermo-optic coefficients. Sellmeier equations have been constructed valid in the visible and near-IR spectral range. Raman scattering has been used to determine the phonon energies of KYbW and a simple physical model is applied for classification of the lattice vibration modes. Spectroscopic studies ͑absorption and emission measurements at room and low temperature͒ have been carried out in the spectral region near 1 m characteristic for the ytterbium transition. Energy positions of the Stark sublevels of the ground and the excited state manifolds have been determined and the vibronic substructure has been identified. The intrinsic lifetime of the upper laser level has been measured taking care to suppress the effect of reabsorption and the intrinsic quantum efficiency has been estimated. Lasing has been demonstrated near 1074 nm with 41% slope efficiency at room temperature using a 0.5 mm thin plate of KYbW. This laser material holds great promise for diode pumped high-power lasers, thin disk and waveguide designs as well as for ultrashort ͑ps/fs͒ pulse laser systems.
Single‐walled carbon‐nanotube absorbers are experimentally demonstrated for laser mode‐locking. A saturable absorber device is used to mode‐lock three different bulk solid‐state lasers in a 500 nm‐wide wavelength interval. The devices exhibit a low saturation fluence of <10 µJ cm−2, low scattering losses, and an exceptionally rapid relaxation, with time constants reaching <100 fs. The latter two properties are explained by a decreased curling tendency and increased tube‐to‐tube interactions of the nanotubes, respectively. These properties are the result of an optimized manufacturing procedure in combination with the use of a starting material with a higher microscopic order. The decreased scattering enables universal use of these devices in bulk solid‐state lasers, which tend to be highly sensitive against non‐saturable device losses as caused by scattering. The favorable saturable absorption properties are experimentally verified by mode‐locking the three lasers, which all exhibit near transform‐limited performance with about 100 fs pulse duration. The complete and unconditional absence of Q‐switching side bands verifies the small saturation fluence of these devices.
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