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We review the recent progress in passively mode-locked Tm- and Ho-lasers operating near 2 μm, comparing saturable absorbers based on carbon nanostructures (SWCNTs and graphene) with SESAMs, employed in lasers based on different active media.

Inband-pumped Ho:YAG ceramic lasers emitting near 2.09 μm are passively Q-switched by mono- and multi-layered graphene, SWCNTs and Cr:ZnSe saturable absorbers. With SWCNTs, 85-ns, 4.9-μJ pulses are generated at a repetition rate of 165 kHz.

Depressed-index channel waveguides are produced in Tm:CALGO by femtosecond direct-laser-writing. Under in-band-pumping at 1679 nm by a Raman fiber laser, the waveguide laser generates 0.81 W at 1866-1947 nm with a slope efficiency of 50.8%.

Cobalt ions (Co2+) embedded in tetrahedral (Td) sites are known for their intense absorption around 1.5 μm. Crystals such as spinel, MgAl 2 O 4 , doped with Co2+ ions in Td sites are recognized as excellent saturable absorbers (SAs) for eye-safe erbium lasers. Due to the relative difficulty to grow high-quality Co:MgAl 2 O 4 crystals and fixed position of the Co2+ absorption band in spinel single-crystals, other Co2+-doped materials were proposed such as transparent glass-ceramics (GCs) and ceramics based on various spinels [1]. Transparent GCs offer a size-scalable synthesis method based on glass melting technology and variable spectroscopic properties as compared to crystals, and much higher laser induced damage threshold (LIDT) as compared to ceramics. GCs based on MgAl 2 O 4 spinel nanocrystals modified by the addition of Ga3+ cations allow one to extend the SA properties until 1.7 μm [2].

Tm:KLuW and Ho:KLuW microchip lasers are Q-switched by SWCNTs generating 40-ns/4.0-μJ and 82-ns/0.17-μJ pulses, respectively. The Tm-laser produces average/peak power of ∼1 W/100 W at 1.919 μm with a slope efficiency of 40%.

Mode-locked (ML) lasers emitting in the spectral range of ∼2 μm are based on Thulium (Tm3+) or Holmium (Ho3+) ions. The former one typically emits slightly below 2 μm where the atmosphere water vapour absorption can constrain the emission bandwidth of a ML laser. A disordered host material can provide inhomogeneous broadening of the emission spectra beyond 2 μm: recently, a Tm3+-doped disordered CNGG-type garnet generated 78 fs pulses at 2017 nm [1]. Another route is the use of Tm3+,Ho3+ codoped materials for further spectral broadening.

A Ho:YAG ceramic microchip laser Q-switched by a semiconductor saturable absorber generated 450 mW at 2089 nm with 37% slope efficiency. Stable 89-ns, 3.2-µJ pulses are achieved at a repetition rate of 141 kHz.

Tm-doped monoclinic double tungstate crystals, KRE(WO 4 ) 2 or shortly KREW, are suitable for efficient multi-watt thin-disk lasers emitting at ∼1.9 μm [1] due to the high available doping levels and high transition cross-sections of the Tm3+ ions. Holmium (Ho3+) ions are attractive for laser emission above 2 μm due to the 5I 6 ↔5I 8 transition which is of interest for medical applications. KREW crystals are very suitable for Ho3+ doping [2] as well and efficient inband-pumped bulk Ho:KREW lasers have been demonstrated [3]. In the present work, we report on the first Ho:KREW thin-disk laser.

We have developed a device for measuring of parameters of thermal lens (TL) in laser active elements under longitudinal diode pumping. The measurements are based on the probe beam method. This device allows one to determine sign and optical power of the lens in the principal meridional planes, its sensitivity factor with respect to the absorbed pump power and astigmatism degree, fractional heat loading which make it possible to estimate integral impact of the photoelastic effect to the formation of TL in the laser element. The measurements are performed in a linearly polarized light at the wavelength of 532 nm. Pumping of the laser element is performed at 960 nm that makes it possible to study laser materials doped with Yb3+ and (Er3+, Yb3+) ions. The precision of measurements: for sensitivity factor of TL – 0,1 m-1/W, for astigmatism degree – 0,2 m-1/W, for fractional heat loading – 5 %, for the impact of the photoelastic effect – 0,5 × 10-6 K-1. This device is used for characterization of thermal lens in the laser active element from an yttrium vanadate crystal, Er3+,Yb3+:YVO .

[EN]We report on the first erbium (Er3+) doped double tungstate waveguide laser. As a gain material, we studied a monoclinic Er3+:KLu(WO4)2 crystal. A depressed-index buried channel waveguide formed by a 60 μm-diameter circular cladding was fabricated by 3D femtosecond direct laser writing. The waveguide was characterized by confocal laser microscopy, μ-Raman and μ-luminescence mapping, confirming that the crystallinity of the core is preserved. The waveguide laser, diode pumped at 981 nm, generated 8.9 mW at 1533.6 nm with a slope efficiency of 20.9% in the continuous-wave regime. The laser polarization was linear (E, Nm). The laser threshold was at 93 mW of absorbed pump power.

We report on the first Tm3+-doped double tungstate waveguide laser passively Q-switched by a graphene saturable absorber using a 12.4 µm-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 ~2 µm in Tm3+-doped double tungstate waveguide lasers operating in the MHz-range.