Spectroscopy and diode-pumped laser operation of transparent Tm:Lu3Al5O12 ceramics produced by solid-state sintering.

A transparent Tm:Lu3Al5O12 ceramic is fabricated by solid-state reactive sintering at 1830 °C for 30 h using commercial α-Al2O3 and Lu2O3/Tm2O3 powders and sintering aids - MgO and TEOS. The ceramic belongs to the cubic system and exhibits a close-packed structure (mean grain size: 21 µm). The in-line transmission at ∼1 µm is 82.6%, close to the theoretical limit. The spectroscopic properties of the ceramic are studied in detail. The maximum stimulated-emission cross-section is 2.37×10-21 cm2 at 1749nm and the radiative lifetime of the 3F4 state is about 10 ms. The modified Judd-Ofelt theory accounting for configuration interaction is applied to determine the transition probabilities of Tm3+, yielding the intensity parameters Ω2 = 2.507, Ω4 = 1.236, Ω6 = 1.340 [10-20 cm2] and α = 0.196×10-4 cm. The effect of excited configurations on lower-lying interconnected states with the same J quantum number is discussed. First laser operation is achieved under diode-pumping at 792 nm. A 4 at.% Tm:Lu3Al5O12 ceramic laser generated 3.12 W at 2022-2035nm with a slope efficiency of 60.2%. The ceramic is promising for multi-watt lasers at >2 µm.

Low-loss fs-laser-written surface waveguide lasers at >2 µm in monoclinic Tm3+:MgWO4.

Surface channel waveguides (WGs) based on a half-ring (40-60-µm-diameter) depressed-index cladding (type III) geometry are fabricated in monoclinic Tm3+:MgWO4 by femtosecond (fs) laser writing at a repetition rate of 1 kHz. The WGs are characterized by confocal laser microscopy and µ-Raman spectroscopy. A Tm3+:MgWO4 WG laser generates 320 mW at ∼2.02µm with a slope efficiency of 64.4%. The WG emits a transverse single-mode and linear polarization (E||Nm). A remarkable low loss of <0.1dB/cm is measured for the WG. Vibronic laser emission at ∼2.08µm is also achieved.

Laser operation of cleaved single-crystal plates and films of Tm:KY(MoO4)2.

We report on the crystal growth, spectroscopy and first laser operation of a novel double molybdate compound - Tm:KY(MoO4)2. This orthorhombic (sp. gr. Pbna) crystal exhibits strong anisotropy of the spectroscopic properties due to its layered structure. The maximum stimulated emission cross-section for the 3F4 → 3H6 transition is 2.70×10-20 cm2 at 1856nm with a bandwidth of >110 nm (for E || b). The lifetime of the 3F4 state is 2.29 ms. Crystalline films and plates (thickness down to 70 µm) of high optical quality are obtained by mechanical cleavage along the (100) plane. Continuous-wave diode-pumped laser operation is achieved in such thin films and plates yielding a maximum output power of 0.88 W at ∼1.9 µm with a slope efficiency of 65.8% and a linearly polarized laser output. Vibronic lasing is demonstrated at ∼2.06 µm. Tm:KY(MoO4)2 is promising for microchip and thin-disk lasers.

Spectroscopy and high-power laser operation of a monoclinic Yb3+:MgWO4 crystal.

Monoclinic (wolframite-type) monotungstate crystals are promising for rare-earth doping. We report polarized room- and low-temperature spectroscopy and efficient high-power laser operation of such a ${{\rm Yb}^{3 + }}{:}\,{{\rm MgWO}_4}$Yb3+:MgWO4 crystal featuring high stimulated emission cross section (${\sigma _{\rm SE}}\; = \;{6}.{2}\; \times \;{{10}^{ - 20}}\;{{\rm cm}^2}$σSE=6.2×10-20cm2 at 1056.7 nm for light polarization ${\rm E}\;||\;{N_m}$E||Nm), large Stark splitting of the ground state (${765}\;{{\rm cm}^{ - 1}}$765cm-1), large gain bandwidth (26.1 nm for ${\rm E}\;||\;{N_g}$E||Ng), and strong Raman response (most intense mode at ${916}\;{{\rm cm}^{ - 1}}$916cm-1). A diode-pumped ${{\rm Yb}^{3 + }}{:}\,{{\rm MgWO}_4}$Yb3+:MgWO4 laser generated 18.2 W at ${\sim}{1056}\;{\rm nm}$∼1056nm with a slope efficiency of ${\sim}{89}\% $∼89% and a linearly polarized laser output.

Ultrafast laser inscribed waveguide lasers in Tm:CALGO with depressed-index cladding.

Depressed-index buried and surface channel waveguides (type III) are produced in a bulk 3.5 at.% Tm3+: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 (3H6 → 3F4 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 (3H6 → 3H4 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 Tm3+:CALGO waveguides for mode-locked laser operation at ∼2 µm.

"Mixed" Tm:Ca(Gd,Lu)AlO4 - a novel crystal for tunable and mode-locked 2 µm lasers.

We report on the crystal growth, spectroscopy characterization and first laser operation of a new tetragonal disordered "mixed" calcium aluminate crystal, Tm:Ca(Gd,Lu)AlO4. The introduction of Lu3+ leads to an additional inhomogeneous broadening of Tm3+ absorption and emission spectra compared to the well-known Tm:CaGdAlO4. The maximum stimulated-emission cross-section for the 3F4 → 3H6 Tm3+ transition is 0.91 × 10-20 cm2 at 1813 nm for σ-polarization, and the emission bandwidth is more than 200 nm. A continuous-wave diode-pumped Tm:Ca(Gd,Lu)AlO4 laser generates 1.82 W at 1945 nm with a slope efficiency of 29%. Under Ti:Sapphire laser pumping, a continuous tuning of the laser wavelength from 1836 to 2083 nm (tuning range: 247 nm) is demonstrated. The Tm:Ca(Gd,Lu)AlO4 crystal is promising for tunable/femtosecond lasers at ~2 µm.

67-fs pulse generation from a mode-locked Tm,Ho:CLNGG laser at 2083 nm.

We report on a mode-locked Tm,Ho:CLNGG laser emitting in the 2 µm spectral range using single-walled carbon nanotubes (SWCNTs) as a saturable absorber (SA). Pulses with duration of 98 fs are generated at 99.28 MHz repetition rate with an average output power of 123 mW, yielding a pulse energy of 1.24 nJ. Using a 0.5% output coupling, pulses as short as 67 fs, i.e., 10 optical cycles, are produced after extracavity compression with a 3-mm-thick ZnS plate.

Oriented zinc oxide nanorods: A novel saturable absorber for lasers in the near-infrared.

Zinc oxide (ZnO) nanorods (NRs) oriented along the crystallographic [001] axis are grown by the hydrothermal method on glass substrates. The ZnO NRs exhibit a broadband (1-2 µm) near-IR absorption ascribed to the singly charged zinc vacancy VZn -1. The saturable absorption of the ZnO NRs is studied at ≈1 µm under picosecond excitation, revealing a low saturation intensity, ≈10 kW/cm2, and high fraction of the saturable losses. The ZnO NRs are applied as saturable absorbers in diode-pumped Yb (≈1.03 µm) and Tm (≈1.94 µm) lasers generating nanosecond pulses. The ZnO NRs grown on various optical surfaces are promising broadband saturable absorbers for nanosecond near-IR lasers in bulk and waveguide geometries.

Passive Q switching of Yb:CNGS lasers by Cr4+:YAG and V3+:YAG saturable absorbers.

Trigonal langasite-type ordered silicate crystal Yb:Ca3NbGa3Si2O14 (Yb:CNGS) is a promising material for efficient ∼1 µm lasers. We report on the first passively Q-switched Yb:CNGS laser using Cr4+:YAG and V3+:YAG saturable absorbers (SAs) with a 976 nm volume-Bragg-grating-stabilized diode as a pump source. The laser crystal was a c cut 3 at.% Yb:CNGS grown by the Czochralski method. It was placed in a compact microchip-type laser cavity. With a Cr4+:YAG SA, very stable 62.2 µJ/4.4 ns pulses were achieved at a repetition rate of 22.5 kHz. The average output power was 1.40 W at 1015.3 nm, corresponding to a Q switching conversion efficiency of 90%. With the V3+:YAG SA, the pulse characteristics were 13.3 µJ/11.1 ns at a higher repetition rate of 68.4 kHz. The performance of the Yb:CNGS/Cr4+:YAG was numerically modeled showing a good agreement with the experiment.

Sub-80 fs mode-locked Tm,Ho-codoped disordered garnet crystal oscillator operating at 2081 nm.

We demonstrate a mode-locked (ML) femtosecond laser based on the disordered garnet crystal Tm,Ho:CNGG. Employing a single-walled carbon nanotube saturable absorber, pulses as short as 83 and 76 fs at 2081 nm are achieved without and with external compression, respectively. The latter represents, to the best of our knowledge, the shortest pulse duration obtained from any Ho-doped or Tm,Ho-codoped laser. The average power amounts to 67 mW at a repetition rate of 102 MHz. By analyzing the soliton ML regime, the nonlinear refractive index of Tm,Ho:CNGG is estimated to be ∼1.1×10-19 m2/W.

78 fs SWCNT-SA mode-locked Tm:CLNGG disordered garnet crystal laser at 2017 nm.

A passively mode-locked Tm:CLNGG laser using single-walled carbon nanotubes (SWCNT) as a saturable absorber (SA) is demonstrated at 2017 nm. Pulses as short as 78 fs are generated at an 86 MHz repetition rate with an average output power of 54 mW. By increasing the output coupling from 0.5% to 1.5%, a higher power of 100 mW is achieved for slightly longer pulses with a duration of 105 fs at 1996 nm.

Efficient diode-pumped Er:KLu(WO4)2 laser at ∼1.61 µm.

We report on an efficient diode-pumped continuous-wave erbium-doped monoclinic double tungstate laser. It is based on a 1 at. % Er3+:KLu(WO4)2 (Er:KLuW) crystal cut along the Ng optical indicatrix axis. The Er:KLuW microchip laser, diode pumped at 0.98 µm, generates 268 mW at 1.610 µm with a slope efficiency of 30%. The output is linearly polarized (E||Nm), and the laser beam is nearly diffraction limited (Mp,m2<1.1). Spectroscopic properties of Er3+ in KLuW are also presented. The stimulated-emission cross-section σSE is 0.46×10-20 cm2 at ∼1.609 µm for E||Nm. The microchip Er:KLuW laser outperforms the commercial Er,Yb:glass.

Continuous-wave and passively Q-switched cryogenic Yb:KLu(WO4)2 laser.

We study cryogenic laser operation of an Yb-doped KLu(WO4)2 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.

Microchip Yb:CaLnAlO4 lasers with up to 91% slope efficiency.

Multi-watt continuous-wave (CW) operation of tetragonal rare-earth calcium aluminate Yb:CaLnAlO4(Ln=Gd,Y)) crystals in plano-plano microchip lasers was demonstrated with an almost quantum-defect-limited slope efficiency. Pumped at 978 nm by an InGaAs laser diode, a 3.4 mm long 8 at. % Yb:CaGdAlO4 laser generated 7.79 W at 1057-1065 nm with a slope efficiency of η=84% (with respect to the absorbed pump power). An even higher η=91% was achieved with a 2.5 mm long 3 at. % Yb:CaYAlO4 laser, from which 5.06 W were extracted at 1048-1056 nm. Both lasers produced linearly polarized output (σ-polarization) with an almost circular diffraction-limited beam (Mx,y2<1.1). The output performance of the developed lasers was modeled, yielding an internal loss coefficient as low as 0.004-0.007 cm-1. In addition, their spectroscopic properties were revisited.

Sellmeier equations, group velocity dispersion, and thermo-optic dispersion formulas for CaLnAlO4 (Ln = Y, Gd) laser host crystals.

We studied the refractive index and dispersive properties of the tetragonal rare-earth calcium aluminates, CaLnAlO4 (Ln=Gd or Y). Sellmeier equations were derived for the spectral range of 0.35-2.1 µm. The group velocity dispersion (GVD) in CaGdAlO4 is positive at ∼1 µm, 95 fs2/mm and negative at ∼2 µm, -40 fs2/mm. The GVD values for CaYAlO4 are similar. In addition, thermo-optic coefficients, dn/dT, and thermal coefficients of the optical path were determined for CaYAlO4. dn/dT is negative at ∼1 µm, dno/dT=-7.8, and dne/dT=-8.7×10-6 K-1. Thermo-optic dispersion formulas were constructed. The obtained data are of key importance to the design of high-power mode-locked oscillators at ∼1 and ∼2 µm based on such laser hosts.

Semiconductor saturable absorber Q-switching of a holmium micro-laser.

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.

Crystal growth, optical spectroscopy and laser action of Tm3+-doped monoclinic magnesium tungstate.

We report on the crystal growth, spectroscopic investigation and laser performance of Tm3+-doped monoclinic magnesium tungstate (Tm:MgWO4), for the first time, to the best of our knowledge. A high-quality crystal has been grown by the top seeded solution growth method. The relevant spectroscopic properties are characterized in terms of absorption, luminescence and Raman spectroscopy. Judd-Ofelt (J-O) analysis is performed to evaluate the spontaneous emission probabilities and the radiative lifetimes. The absorption, stimulated-emission and gain cross-section spectra are determined for the principal light polarizations. The first laser action in the 2 µm spectral range is demonstrated in the regime of continuous-wave operation with a maximum output power of 775 mW and a slope efficiency of 39%.

Monoclinic Tm3+:MgWO4: a promising crystal for continuous-wave and passively Q-switched lasers at ∼2 µm.

Monoclinic thulium-doped magnesium monotungstate, Tm3+:MgWO4, is promising for efficient power-scalable continuous-wave (CW) and passively Q-switched lasers at >2 µm. Under diode pumping at 802 nm, a compact CW laser based on Z-cut Tm:MgWO4 generated 3.09 W at 2022-2034 nm with a slope efficiency of 50% which represents the highest output power ever achieved with this type of laser host. Stable passive Q-switching of the Tm:MgWO4 laser is demonstrated for the first time, to the best of our knowledge, using single-walled carbon nanotubes, graphene, and Cr2+:ZnS saturable absorbers. Using the latter, the best performance is achieved with 16.1 µJ/13.6 ns pulses at 2017.8 nm with a maximum average output power of 0.87 W and a peak power of 1.18 kW.

Subnanosecond Tm:KLuW microchip laser Q-switched by a Cr:ZnS saturable absorber.

Passive Q-switching of a compact Tm:KLu(WO(4))(2) microchip laser diode pumped at 805 nm is demonstrated with a polycrystalline Cr(2+):ZnS saturable absorber. This laser generates subnanosecond (780 ps) pulses with a pulse repetition frequency of 5.6 kHz at 1846.6 nm, the shortest pulse duration ever achieved by Q-switching of ~2 µm lasers. The maximum average output power is 146 mW with a slope efficiency of 21% with respect to the absorbed power. This corresponds to a pulse energy of 25.6 µJ and a peak power of 32.8 kW.

Tm:KLu(WO(4))(2) microchip laser Q-switched by a graphene-based saturable absorber.

We report on the first Tm-doped double tungstate microchip laser Q-switched with graphene using a Tm:KLu(WO4)2 crystal cut along the Ng dielectric axis. This laser generates a maximum average output power of 310 mW with a slope efficiency of 13%. At a repetition rate of 190 kHz the shortest pulses with 285 ns duration and 1.6 µJ energy are achieved.