We report on a Holmium micro-laser passively Q-switched by a semiconductor saturable absorber (SSA), for the first time to the best of our knowledge. It is based on a 1 at.% Ho:YAG ceramic with good ...energy storage capability and several commercial transmission-type SSAs with 0.24% modulation depth. Under in-band pumping by a Tm fiber laser at 1910 nm, the Ho micro-laser 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. Further shortening of the laser pulses is feasible with the increase of the modulation depth of the SSA while power scaling may lead to Q-switching at MHz-range repetition rates.
Terbium (Tb3+) ions are promising for the development of visible lasers. We report on the spectroscopy of Tb3+ in the monoclinic KYb(WO4)2 (KYbW) crystal, including the Judd-Ofelt (J-O) modelling of ...the transition probabilities, evaluation of the absorption and stimulated-emission (SE) cross-sections with polarized light, and emission lifetime study. Within the strong configuration interaction (SCI) approximation of the J-O theory, the intensity parameters of Tb3+ are Ω2 =0.864, Ω4 =1.657, Ω6 =1.975 10-20cm2 and Δ=38810cm–1. The maximum σabs for the 7F6 → 5D4 transition reaches 2.3×10-21cm2 at 486.7nm and the maximum σSE for the green emission according to the 5D4 → 7F5 transition is 2.3×10-21cm2 at 549.3nm (both for E || Nm). The measured lifetime of the 5D4 state for a 1at% Tb3+ doping is 395±5μs. Under blue excitation, Tb: KYbW exhibits intense green emission with the CIE 1931 coordinates of (0.312; 0.644).
Tm3+-doped monoclinic magnesium monotungstate (MgWO4) is a promising material for lasers emitting at above 2 μm. Stark splitting of 3H6 to 1G4 Tm3+ multiplets in MgWO4 is determined using ...low-temperature spectroscopy, calculated within crystal-field theory modified for the case of an intermediate configuration interaction (ICI) and analyzed in terms of the barycenter plot. Tm3+:MgWO4 features a large Stark splitting of the ground-state (3H6), 633 cm−1, and high transition cross-sections for polarized light due to its low-symmetry structure. A unique feature of Tm3+:MgWO4 to provide a naturally polarized emission at >2 μm, as well as its suitability for broadly tunable and mode-locked lasers in this spectral range are argued. The first tunable and vibronic Tm3+:MgWO4 lasers are demonstrated, yielding a continuous tuning from 1897 to 2062 nm in the former case. A vibronic laser operated at even longer wavelengths up to 2093 nm due to electron-phonon coupling with low-energy phonons of the crystalline host. Moreover, the optical indicatrix axes of high-refractive-index biaxial MgWO4 are assigned.
•Thulium (Tm3+) doped monoclinic magnesium monotungstate crystal (Tm:MgWO4).•Determination of the Stark splitting of Tm3+ multiplets and barycenter plot analysis.•Crystal-field theory modified for intermediate configuration interaction (ICI).•A unique feature of Tm:MgWO4 to provide a naturally polarized emission above 2 μm.•First tunable (165 nm broad tuning range) and vibronic (at 2093 nm) Tm:MgWO4 lasers.
We describe a compact Yb:YAG laser Q-switched by a graphene-based saturable absorber and pumped by a laser diode at 932 or 969 nm. The compact laser generates a maximum average output power value of ...185 mW at 1032 nm with a slope efficiency value of 12%. The shortest duration of the Q-switched pulse achieved is 228 ns at a repetition frequency of 285 kHz. The maximum pulse energy amounts to 0.65 μJ.
Tm3+:Na2La4(WO4)7, a disordered tetragonal scheelite-type tungstate crystal, is grown by the Czochralski method. The polarized absorption, stimulated-emission and gain cross-section spectra are ...determined. The maximum σSE is 1.62 × 10–20 cm2 at 1788.6 nm for σ-polarization. The Judd-Ofelt parameters for Tm3+ are Ω2 = 10.321, Ω4 = 0.183 and Ω6 = 2.122 10–20 cm2. The radiative lifetime of the 3F4 state is 1.63 ms. Raman spectroscopy reveals a maximum phonon energy of 923 cm−1. Laser operation under diode-pumping is achieved with both a-cut and c-cut Tm:Na2La4(WO4)7 crystals, reaching a maximum output power for the a-cut of 715 mW at ~ 1937 nm with a slope efficiency of 34%. Microchip laser operation using the c-cut crystal yields a slope efficiency of 41%. The Tm:Na2La4(WO4)7 crystal is promising for mode-locked lasers due to its broadband emission.
Depressed-index buried and surface channel waveguides (type III) are produced in a bulk 3.5 at.% Tm
:CALGO crystal by femtosecond direct-laser-writing at kHz repetition rate. The waveguides are ...characterized by confocal microscopy and µ-Raman spectroscopy. Under in-band-pumping at 1679 nm (
H
→
F
transition) by a Raman fiber laser, the buried channel waveguide laser with a circular cladding (diameter: 60 µm) generated a continuous-wave output power of 0.81 W at 1866-1947 nm with a slope efficiency of 71.2% (versus the absorbed pump power) and showed a laser threshold of 200 mW. The waveguide propagation losses were as low as 0.3 ± 0.2 dB/cm. The laser performance under in-band pumping was superior compared pumping at ∼800 nm (
H
→
H
transition), i.e., the convetional pump wavelength. Vibronic laser emission from the WG laser above 2 µm is also achieved. The low-loss behavior, the broadband emission properties and good power scaling capabilities indicate the suitability of Tm
:CALGO waveguides for mode-locked laser operation at ∼2 µm.
We study cryogenic laser operation of an Yb-doped KLu(WO
)
crystal pumped with a volume Bragg grating (VBG) stabilized diode laser at 981 nm. In the continuous wave laser regime, a maximum output ...power of 4.31 W is achieved at 80 K with a slope efficiency of 44.0% with respect to the incident pump power. Using a 85% initial transmission Cr:YAG crystal for passive Q-switching, an average output power of 2.11 W is achieved at 100 K for a repetition rate of 19 kHz. The pulse energy, pulse duration and peak power amount to 111 µJ, 231 ns and 0.48 kW, respectively.
We report on the crystal growth, structure, room- and low-temperature (6K) absorption and emission spectroscopy of a 3at% Yb3+-doped trigonal ordered langasite-type silicate crystal, Ca3NbGa3Si2O14 ...(Yb: CNGS). The Rietveld refinement data are presented. Absorption, stimulated-emission and gain cross-sections are determined for the Yb3+ ion in CNGS for π and σ polarizations. The maximum stimulated emission cross-section amounts to σSE = 0.97 × 10−20cm−2 at 1014nm (σ-polarization). The Stark splitting of the Yb3+ multiplets is resolved. Microchip laser operation is obtained with both a-cut and c-cut 3at% Yb: CNGS samples under end-pumping at 976nm by a Volume Bragg Grating-stabilized diode laser. For the latter cut, the maximum output power reaches 7.61W at 1048–1060nm with a slope efficiency of 59%. By varying the output coupling, multi-watt emission over a 50nm-broad spectral range is achieved.
Continuous-wave microchip laser operation and thermal lensing are studied for Yb-doped gallium garnets, Yb:LuGG, Yb:YGG, Yb:CNGG and Yb:CLNGG under diode-pumping at ~932 and 969 nm. It is shown that ...although thermal the conductivity of Ga garnets is lower than that of Yb:YAG, the compromised thermo-optic properties, high absorption in the zero-phonon line and low internal loss make the ordered Yb:YGG and Yb:LuGG crystals to be promising for compact highly efficient microchip lasers. In particular, Yb:LuGG microchip laser generated 8.97 W of output power with a slope efficiency eta= 75% and 9.31 W with eta= 65%, for pumping at 932 and 969 nm, respectively. Multi-watt output in the range 1039-1078 nm is emitted for different transmission of the output coupler. The sensitivity factor of the thermal lens for this crystal is 2.1 m super(?1)/W (pumping at 969 nm with a pump waist radius of 100 mum) and the estimated thermal conductivity is 5.8 + or - 0.5 W/mK. Power scaling of Yb:CNGG and Yb:CLNGG microchip lasers is limited by poor thermo-optic properties and high internal losses. Ordered Ga garnets show good prospects for the development of passively Q-switched microchip lasers with high pulse energies.
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