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The polarized spectral properties and â¼2.3â µm high-power continuous-wave laser operation of Tm3+-doped yttrium orthovanadate crystal (Tm:YVO4) are reported. For the 3H4 â 3H5 transition, the stimulated-emission cross-section σSE is 1.01 × 10-20 cm2 at 2276â nm corresponding to a large emission bandwidth of 52â nm (for π-polarization). Pumped by a 794â nm laser diode, the 1.5 at.% Tm:YVO4 laser delivered 5.52 W at 2.29â µm with a slope efficiency of 19.9%, a laser threshold of 8.70 W, and a linear laser polarization (π). The Tm laser operated on the cascade scheme (on the 3H4 â 3H5 and 3F4 â 3H6 transitions) which was mainly responsible for the observed high laser slope efficiency. We also report on the first passively Q-switched Tm:YVO4 laser at 2.3â µm by employing porous nano-grained cuprous selenide (PNG-Cu2Se) as a saturable absorber. The shortest pulse duration and the highest single pulse energy amounted to 706â ns and 3.65 µJ, respectively, corresponding to a pulse repetition rate of 62.8 kHz.
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We report on the Czochralski crystal growth, polarized optical spectroscopy, and the first continuous-wave laser operation of 1.5 at.% Tm:LuVO4 crystal on the 3H4 â 3H5 transition. The polarized absorption and stimulated-emission properties of Tm3+ ions in LuVO4 were revised and the crystal-field splitting of the Tm3+ multiplets was determined by low-temperature (12â K) spectroscopy. The maximum stimulated-emission cross-section for the 3H4 â 3H5 transition is 2.48 × 10-20 cm2 at 2363â nm for π-polarization corresponding to an emission bandwidth of 28â nm. Evidence of phonon-assisted emissions of Tm3+ ions above 2â µm is presented. The broadband emission properties of the Tm:LuVO4 crystal make it promising for ultrashort pulse generation. Additionally, pumped by a 796â nm fiber-coupled laser diode, the Tm:LuVO4 laser generated a Watt-level output power at 2279-2295â nm with a slope efficiency of 9.2% and linearly polarized emission (π-polarization).
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We report on the growth, polarized spectroscopy and first laser operation of an orthorhombic (space group Pnma) Tm3+,Ho3+-codoped gadolinium orthoscandate (GdScO3) perovskite-type crystal. A single crystal of 3.76 at.% Tm, 0.35 at.% Ho:GdScO3 was grown by the Czochralski method. Its polarized absorption and fluorescence properties were studied revealing a broadband emission around 2â µm. The parameters of the Tm3+ â Ho3+ energy transfer was quantified, P28 = 1.30 × 10-22 cm3µs-1, and P71 = 0.99 × 10-23 cm3µs-1, and the thermal equilibrium lifetime was measured to be 3.5â ms. The crystal-field splitting of Tm3+ and Ho3+ multiplets in Cs symmetry sites of the perovskite structure was determined by low-temperature spectroscopy and the mechanism of spectral line broadening is discussed. The continuous-wave Tm,Ho:GdScO3 laser generated 1.16 W at â¼2.1â µm with a slope efficiency of 50.5%, a laser threshold of 184â mW, a linear laser polarization (E || c) and a spatially single-mode output. The Tm,Ho:GdScO3 crystal is promising for broadly tunable and femtosecond mode-locked lasers emitting above 2â µm.
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We present the growth, spectroscopy, continuous-wave (CW) and passively mode-locked (ML) operation of a novel "mixed" tetragonal calcium rare-earth aluminate crystal, Yb3+:Ca(Gd,Y)AlO4. The absorption, stimulated-emission, and gain cross-sections are derived for π and σ polarizations. The laser performance of a c-cut Yb:Ca(Gd,Y)AlO4 crystal is studied using a spatially single-mode, 976-nm fiber-coupled laser diode as a pump source. A maximum output power of 347â mW is obtained in the CW regime with a slope efficiency of 48.9%. The emission wavelength is continuously tunable across 90â nm (1010 - 1100â nm) using a quartz-based Lyot filter. With a commercial SEmiconductor Saturable Absorber Mirror to initiate and maintain ML operation, soliton pulses as short as 35 fs are generated at 1059.8â nm with an average output power of 51â mW at â¼65.95â MHz. The average output power can be scaled to 105â mW for slightly longer pulses of 42 fs at 1063.5â nm.
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We report on a low-threshold efficient yellow Dy-fiber laser with good beam quality featuring high-brightness pumping. It employs a single-clad 0.2â mol% Dy:ZBLAN fiber pumped by two 450-nm blue GaN laser diodes. The continuous-wave Dy-fiber laser delivers a maximum output power of 109â mW at 575â nm with a laser threshold of 218â mW, a slope efficiency of 19.6%, and beam quality factors M2 x,y â¼ 1.5. The overall optical efficiency versus incident pump power is 13.9%, being record-high, to the best of our knowledge, for this type of laser. The laser performance is simulated based on the spectroscopic data, being in good agreement with the experiment.
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We report on a polarization-resolved study of mid-infrared emission properties of Er3+-doped orthorhombic yttrium aluminum perovskite YAlO3 single crystal. For the 4I11/2 â 4I13/2 Er3+ transition, the stimulated emission cross section is 0.20 × 10-20â cm2 at 2919â nm for light polarization E â c. Pumped by an Yb-fiber laser at 976â nm, the 10â at.% Er:YAlO3 laser delivered 1.36â W at 2919â nm with a slope efficiency of 31.4%, very close to the Stokes limit, a laser threshold as low as 33â mW and a linear polarization. Pump-induced polarization switching between E || b and E || c eigen states was observed and explained by excited-state absorption from the terminal laser level.
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We study a cascade laser scheme involving the 3H4 â 3H5 and 3F4 â 3H6 consecutive transitions in Tm3+-doped materials as a promising technique to favor laser emission at 2.3 µm. We examine the conditions in terms of the Tm3+ doping levels for which the cascade laser is beneficial or not. For this, Tm:LiYF4 lasers based on crystals with several doping levels in the range of 2.5 - 6 at.% with and without cascade laser are studied. For low doping of 2.5 at.% Tm3+, adding the laser emission at 1.9 µm allows to double the output power at 2.3 µm, whereas for high doping of 6 at.%, allowing the laser to operate at 1.9 µm totally suppresses the laser emission at 2.3 µm. An analytical model is developed and confronted with experimental results to predict this doping-dependent phenomenon and forecast the potential benefits. This study of cascade laser emission on the 3H4â 3H5 and 3F4â 3H6 transitions versus the Tm3+ doping level is finally extended to other well-known Tm3+-doped laser materials.
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We report on the first laser operation of a disordered Tm:CaGdAlO4 crystal on the 3H4 â 3H5 transition. Under direct pumping at 0.79 µm, it generates 264â mW at 2.32â µm with a slope efficiency of 13.9% and 22.5% vs. incident and absorbed pump power, respectively, and a linear polarization (σ). Two strategies to overcome the bottleneck effect of the metastable 3F4 Tm3+ state leading to the ground-state bleaching are exploited: cascade lasing on the 3H4 â 3H5 and 3F4 â 3H6 transitions and dual-wavelength pumping at 0.79 and 1.05 µm combining the direct and upconversion pumping schemes. The cascade Tm-laser generates a maximum output power of 585â mW at 1.77â µm (3F4 â 3H6) and 2.32â µm (3H4 â 3H5) with a higher slope efficiency of 28.3% and a lower laser threshold of 1.43 W, out of which 332â mW are achieved at 2.32â µm. Under dual-wavelength pumping, further power scaling to 357â mW at at 2.32â µm is observed at the expense of increased laser threshold. To support the upconversion pumping experiment, excited-state absorption spectra of Tm3+ ions for the 3F4 â 3F2,3 and 3F4 â 3H4 transitions are measured for polarized light. Tm3+ ions in CaGdAlO4 exhibit broadband emission at 2.3 - 2.5 µm making this crystal promising for ultrashort pulse generation.
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We report on a bulk thulium laser operating on the 3H4 â 3H5 transition with pure upconversion pumping at 1064â nm by an ytterbium fiber laser (addressing the 3F4 â 3F2,3 excited-state absorption (ESA) transition of Tm3+ ions) generating 433â mW at 2291â nm with a slope efficiency of 7.4% / 33.2% vs. the incident / absorbed pump power, respectively, and linear laser polarization representing the highest output power ever extracted from any bulk 2.3â µm thulium laser with upconversion pumping. As a gain material, a Tm3+-doped potassium lutetium double tungstate crystal is employed. The polarized ESA spectra of this material in the near-infrared are measured by the pump-probe method. The possible benefits of dual-wavelength pumping at 0.79 and 1.06â µm are also explored, indicating a positive effect of co-pumping at 0.79â µm on reducing the threshold pump power for upconversion pumping.
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We propose an experimental method for the determination of the transfer function linewidth of a laser amplifier. This technique is based on a transfer function scan using frequency modulation to amplitude modulation (FM-to-AM) temporal modulation measurement as a function of wavelength. Using this method, we show that the output spectrum of a laser amplifier in Q-switch mode is not representative of the transfer function of the amplifier.
Assuntos
Amplificadores Eletrônicos , Lasers , Desenho de Equipamento , LuzRESUMO
We report on the first, to the best of our knowledge, continuous-wave laser operation of a Tm3+,Ho3+-codoped calcium fluoride crystal at â¼2.1 µm. Tm,Ho:CaF2 crystals were grown by the Bridgman method, and their spectroscopic properties were studied. The stimulated-emission cross section for the 5I7 â 5I8 Ho3+ transition is 0.72 × 10-20 cm2 at 2025 nm, and the thermal equilibrium decay time is 11.0â ms. A 3 at. % Tm, 0.3 at. % Ho:CaF2 laser generated 737â mW at 2062-2088 nm with a slope efficiency of 28.0% and a laser threshold of 133â mW. Continuous wavelength tuning between 1985 and 2114â nm (tuning range: 129â nm) was demonstrated. The Tm,Ho:CaF2 crystals are promising for ultrashort pulse generation at â¼2 µm.
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We report on the first deep-red laser operation of a heavily Eu3+-doped cesium gadolinium double molybdate crystal with a perfect cleavage. A 17 at. % Eu:CsGd(MoO4)2 laser based on cleaved single-crystal plates generated a maximum continuous-wave output power of 212â mW at 703.1â nm (5D0 â 7F4 transition) with a slope efficiency of 30.1%, a low laser threshold of 51â mW, linear polarization, and a beam quality factor M2 = 1.6-1.7. This monoclinic crystal is promising for deep-red microchip lasers.
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We report on the first, to our knowledge, mid-infrared laser operation of two Er3+-doped barium-containing fluorite-type crystals, BaF2 and (Sr,Ba)F2, featuring a low-phonon energy behavior. A continuous wave 4.9 at.% Er:(Sr,Ba)F2 laser generated 519â mW at 2.79â µm with a slope efficiency of 25.0% and a laser threshold of 27â mW. The vibronic and spectroscopic properties of these crystals are determined. The phonon energy of (Sr,Ba)F2 is as low as 267â cm-1. The Er3+ ions in this crystal feature a broadband emission owing to the 4I11/2 â 4I13/2 transition and a long luminescence lifetime of the 4I11/2 level (10.6â ms) making this compound promising for low-threshold, broadly tunable, and pulsed 2.8-µm lasers.
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We report on the first laser operation of a Sm3+-doped monoclinic KGd(WO4)2 double tungstate crystal in the red spectral range. Pumped by a frequency-doubled optically pumped semiconductor laser (2ω-OPSL) at 479.1â nm, the 0.8 at. % Sm:KGd(WO4)2 laser generated an output power up to 17.6â mW at 649.1â nm (the 4G5/2 â 6H9/2 transition) with a slope efficiency of 16.9%, a laser threshold down to 29â mW and a linear polarization. The laser exhibited a self-pulsing behavior, delivering µs-long pulses with a repetition rate of a few kHz. The polarized spectroscopic properties of Sm3+ ions were determined as well.
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Depressed-cladding surface channel waveguides were inscribed in a 0.5 at.% Pr:LiYF4 crystal by femtosecond Direct Laser Writing. The waveguides consisted of a half-ring cladding (inner diameter: 17â µm) and side structures ("ears") improving the mode confinement. The waveguide propagation loss was as low as 0.14 ± 0.05â dB/cm. The orange waveguide laser operating in the fundamental mode delivered 274â mW at 604.3â nm with 28.4% slope efficiency, a laser threshold of only 29â mW and linear polarization (π), representing record-high performance for orange Pr waveguide lasers.
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We report on thermal, spectroscopic, and laser properties of transparent 5 at.% Tm3+-doped yttria and "mixed" yttria-scandia ceramics fabricated by vacuum sintering at 1750°C using nanoparticles produced by laser ablation. The solid-solution (Tm0.05Y0.698Sc0.252)2O3 ceramic features a broadband emission extending up to 2.3 µm (gain bandwidth, 167â nm) and high thermal conductivity of 4.48 W m-1 K-1. A Tm:Y2O3 ceramic laser generated 812â mW at 2.05 µm with a slope efficiency η of 70.2%. For the Tm:(Y,Sc)2O3 ceramic, the output power was 523â mW at 2.09 µm with η = 44.7%. These results represent record-high slope efficiencies for any parent or "mixed" Tm3+-doped sesquioxide ceramics.
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We report on the first, to the best of our knowledge, laser operation on the 4I11/2 â 4I13/2 transition of erbium-doped disordered calcium lithium niobium gallium garnet (CLNGG) crystals with broadband mid-infrared emission properties. A 41.4â at.% Er:CLNGG continuous-wave laser generated 292â mW at 2.80â µm with 23.3% slope efficiency and a laser threshold of 209â mW. Er3+ ions in CLNGG feature inhomogeneously broadened spectral bands (σSE = 1.79 × 10-21â cm2 at 2.79â µm; emission bandwidth, 27.5â nm), a large luminescence branching ratio for the 4I11/2 â 4I13/2 transition of 17.9%, and a favorable ratio of the 4I11/2 and 4I13/2 lifetimes, exhibiting values of 0.34â ms and 1.17â ms (for 41.4â at.% Er3+), respectively.
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A compact Tm:GdVO4 laser pumped by a 794â nm laser diode generated 6.09â W at 2.29â µm (3H4 â 3H5 Tm3+ transition) with a high slope efficiency of 30.8% and linear laser polarization (π). The polarized spectroscopic properties of Tm3+ in GdVO4 were also revised. The peak stimulated-emission cross section of Tm3+ is 2.97 × 10-20 cm2 at 2280â nm, corresponding to an emission bandwidth of 42â nm for π-polarized light.
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Excited-state absorption (ESA) is a key process for upconversion pumping schemes of thulium (Tm3+) doped laser materials. We have systematically studied two ESA transitions in the near-infrared spectral range, namely 3F4 â 3F2,3 (at â¼1â µm) and 3F4 â 3H4 (at â¼1.5â µm), in various Tm3+-doped fluoride (ZBLAN glass, cubic KY3F10 and CaF2, tetragonal LiYF4 and LiLuF4, monoclinic BaY2F8 crystals) and oxide (cubic Y3Al5O12, orthorhombic YAlO3 crystals) laser materials, using a pump-probe method with a polarized light. An approach to calculate the constants of energy-transfer upconversion (ETU) is also presented.
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Upconversion pumping of thulium lasers emitting around 2.3 µm (the 3H4 â 3H5 transition) has recently attracted a lot of attention as it is compatible with the mature Yb-laser technology. To explore this possibility, we built a mid-infrared Tm:LiYF4 laser pumped by an Yb:CaF2 laser at 1.05 µm delivering an output power of 110 mW at 2.31 µm for a maximum incident pump power of 2.0 W. A strong absorption issue appeared in the Tm laser: the slope efficiency vs. the incident pump power was 7.6% while that vs. the absorbed pump power reached 29%. To overcome this issue, a dual-wavelength pumping at 0.78 µm and 1.05 µm was explored (combining both the direct and upconversion pumping schemes). The reciprocal interplay between the two pumps was studied to evaluate their benefits in terms of the pump absorption and laser efficiency. We observed an interesting decrease of the laser threshold for upconversion pumping when adding a small fraction of the direct pump revealing a seeding effect for the excited-state absorption from the metastable 3F4 level. A recycling process of this manifold by excited-state absorption in the 3F4 â 3F2,3 loop was also observed. The pump absorption seeding is a viable route for the development of low-threshold upconversion pumped thulium lasers.