We overview recent advances in visible single- and double-clad fluoride fiber lasers pumped by blue GaN laser diodes. The spectroscopic properties of ZBLAN glasses doped with Pr3+, Ho3+ and Dy3+ ions are revised. Power scalable efficient continuous-wave visible fluoride fiber lasers emitting in the green, yellow, red and deep-red spectral ranges are presented. Pumped by a single-emitter 6-W 443-nm GaN laser diode, a continuous-wave red double-clad Pr:ZBLAN fiber laser delivered 1.51 W at 634.5 nm with a slope efficiency of 31.0%, a laser threshold of 0.63 W and a spatially single-mode output (M2 ~1.02). Employing a high-power fiber-coupled laser module, power scalability up to 4.61 W was achieved at the expense of a lower slope efficiency of 22.8% and an increased laser threshold of 1.74 W. Green Ho:ZBLAN (543 nm) and yellow Dy:ZBLAN (575 nm) fiber lasers with high-brightness core pumping at 450 nm are also reported delivering 100 mW-level output with slope efficiencies of 31.2% and 19.6%, respectively, operating on the fundamental mode. A numerical model to predict the visible laser performance is presented and guidelines for further engineering of visible fiber laser sources are given.
We report on a detailed spectroscopic study of heavily Er3+-doped LiYF4 epitaxial layers with the goal of developing mid-infrared waveguide lasers. Layers with a doping level up to 11 at.% Er3+ were grown on (001) oriented undoped bulk LiYF4 substrates using LiF as a solvent. The absorption spectrum of Er3+ ions was measured. Under excitation at 973 nm, the layers exhibited intense and strongly polarized mid-infrared luminescence spanning from 2.65 to 2.90 μm related to the 4I11/2 → 4I13/2 Er3+ transition. The peak stimulated-emission cross-section at the expected laser wavelength was calculated to be 0.88×10-20 cm2 at 2809 nm for π-polarization. By means of low-temperature (12 K) spectroscopy, the experimental crystal-field splitting of Er3+ multiplets was determined. The luminescence dynamics from Er3+ excited states were studied. For the 11 at.% Er3+ doping, the luminescence lifetimes of the 4I13/2 and 4I11/2 manifolds amounted to 5.54 ms and 2.66 ms, respectively.
We investigated in-band pumping of Tm,Ho,Lu:CaGdAlO4 (CALGO) using a Raman shifted Er-fiber laser (1678 nm) in the continuous-wave (CW) and mode-locked (ML) regimes. The 6-mm long, antireflection-coated, a-cut CALGO was doped with 4.48at.% Tm (sensitizer), 0.54at.% Ho (emission) and 5.51at.% Lu (compositional disorder). For mode-locking we employed a GaSb SESAM and chirped mirrors (round-trip group-delay dispersion: -1250 fs2). Pumping with 5.5 W (unpolarized), the average output power (0.2% output coupler) was 148 mW at ⁓96 MHz. The spectrum was centered at 2071.5 nm with a FWHM of 21.5 nm (sigma-polarization) and the pulse duration was 218 fs (time-bandwidth product: 0.327).
We report on polarized spectroscopic properties of Ho3+ ions in orthorhombic (sp. gr. Pnma) yttrium orthoaluminate YAlO3 crystals for laser development at 2 μm and 3 μm. This includes polarized Raman, absorption and luminescence spectra, fluorescence lifetime measurements and Stark energy-level study. The transition intensities for Ho3+ ions are calculated using the Judd-Ofelt theory. The peak stimulated-emission cross-sections are 2.01×10-20 cm2 at 1977 nm (5 I7 → 5 I8) and 2.31×10-20 cm2 at 2918 nm (5 I6 → 5 I7) for light polarization E || b. For both transitions, pump-induced polarization-switching is expected. The fluorescence lifetimes of the 5 I7 and 5 I6 Ho3+ manifolds are 7.27 and 0.36 ms, respectively (for 1 at.% Ho3+ -doping).
Yb3+,Li+ -codoped monoclinic zinc tungstate (ZnWO4) crystals with optimized Li+ content providing efficient local charge compensation were grown by the Czochralski method. Heavy Li+ codoping makes the Yb3+-doped ZnWO4 crystals less prone to cracking, improves the Yb3+ segregation, reduces the melting point and induces inhomogeneous spectral line broadening. The polarized absorption and stimulated-emission cross-sections of Yb3+ in ZnWO4 were determined. The maximum stimulated-emission cross-section σSE is 2.94×10-20 cm2 at 1055.5 nm corresponding to an emission bandwidth of 12.2 nm for light polarization E || Np. The formation of Yb3+ optical centers in singly Yb3+-doped and Yb3+,Li+ -codoped zinc monotungstate crystals is revealed by low-temperature spectroscopy. The Yb,Li:ZnWO4 laser pumped by a commercial 976-nm Yb-fiber laser generated 2.41 W at approximately 1.06 μm with a slope efficiency of 76.4%, a laser threshold of 143 mW and linear polarization.
We demonstrate a diode-pumped SESAM mode-locked Yb:(Y,Gd)AlO3 laser delivering soliton pulses as short as 28-fs at 1063 nm with an average output power of 21 mW. The maximum average output power is scaled to 135 mW for a pulse duration of 37 fs.
Lasers emitting in the visible find applications in biology and medicine. Considering the success of near-infrared fiber lasers, the possibility to optically pump rare-earth-doped fibers in the blue to directly obtain visible emission is attractive. The recent progress in the field of GaN-based blue laser diodes offers new scopes. Dy3+-doped materials have received much interest because of their intense yellow emission originating from the 4F9/2→6H13/2 transition. An involvement of a glass matrix benefiting from enhanced thermo-mechanical properties would ease diode pumping. We report on the synthesis of a series of novel phosphate glasses in the system P2O5-Al2O3–BaO-K2O doped with Dy2O3. The Dy3+ concentrations were 0.05, 0.21, 0.83 and 2.5 [1020 ions/cm3]. The glasses were synthesized by the standard melt-quenching technique and thoroughly characterized in their physical, thermo-mechanical and optical properties. A Dy3+-doped optical fiber was drawn by preform drawing from the developed glasses, with the preform being obtained by rod-in tube technique, combining a cast core and an extruded cladding. Preliminary emission results in the visible from the fabricated fiber will be reported.
Tm-based laser with emission at 2.3 μm could be used to detect atmosphere pollutants and various molecules. Within this work we investigated if oxide compounds could be efficient for this laser emission as complex energy transfers could occur such as non-radiative relaxation and upconversion. Several oxides hosts are investigated, namely CaGdAlO4 (CALGO), Y2O3, CaYAlO4 and CaYAl3O7 to host Tm3+ for laser effect at 2.3 μm. We focus on the optical characterizations and spectroscopic analysis of these materials, determining intrinsic optical features such as lifetime and broadening of the emission bands.
Trivalent titanium ions (Ti3+) are known for their broadband emission in the visible and near-IR. Zinc aluminate spinel, or gahnite (ZnAl2O4) is known as a host matrix for transition-metal ions. We report on the structure and spectroscopic properties of transparent zinc aluminosilicate glass-ceramics (GCs) nucleated by TiO2 and based on Ti3+-doped ZnAl2O4 nanocrystals. The initial glasses were melted under different redox conditions. After heat-treatments at the temperatures in the range of 720 to 1100 °C, transparent GCs were obtained. The materials were studied by the DSC method, XRD analysis, Raman, absorption and luminescence spectroscopy. The main crystalline phase in GCs is ZnAl2O4 with a cubic structure. The crystals are 5 - 21 nm in size. Their unit cell parameters vary with the heat-treatment temperature due to the titanium ions entering the gahnite nanocrystals. The volume fraction of gahnite nanophase increases with the heat treatment temperature. At 1000 – 1100 °C, TiO2 (rutile) crystals with a size of 11 - 37 nm also appear. In GCs obtained from glasses melted under reducing conditions, broadband absorption is observed in the visible and near IR due to the Ti3+ ions in Oh positions in ZnAl2O4 crystals, the absorption of Ti3+ - Ti4+ pairs and the appearance of Ti3+ self-doped rutile. By changing the redox conditions of the glass synthesis, one can control the content of titanium ions in various oxidation states and the spectral properties of GCs.
We report on the synthesis and a detailed spectroscopic study of an Er3+ -doped “mixed” (Lu,Y,La)2O3 transparent sesquioxide ceramic. Nanopowders of the composition (Lu0.63Y0.25La0.05Er0.07)2O3 were obtained by a glycine-nitrate self-propagating high-temperature synthesis. The ceramics were obtained by vacuum sintering at 1750 °C for 3 hours. La2O3 acted as a sintering additive and increased the disorder of the sesquioxide lattice. The ceramic had a cubic (C-type) structure. At 1.05 μm, its transmission was 78.9%. The ceramic exhibited intense mid-infrared emission owing to the 4 I11/2 → 4 I13/2 Er 3+ transition. The corresponding peak stimulated-emission cross-section σSE is 1.12×10-20 cm2 at 2717 nm. The luminescence spectrum exhibited a considerable inhomogeneous broadening as compared to the parent compounds, Er:Lu2O3 and Er:Y2O3. The luminescence lifetimes of the 4 I11/2 and 4 I13/2 manifolds were measured (1.99 ms and 3.35 ms, respectively), representing a favorable ratio for mid-IR laser operation. The crystal-field splitting of Er3+ multiplets in C2 sites was determined at low temperature.
We report on the structure and spectral-luminescent properties of a composite ZnO-Er2O3-Yb2O3 optical ceramic. The ZnO ceramic codoped with 1 wt% Er3+ and 4 wt% Yb3+ was prepared by uniaxial hot pressing of oxide powders at 1180 °C in vacuum. The maximum total transmittance of the ceramic is ~40%. The ceramic is a composite material comprised of hexagonal ZnO microcrystals (mean size: 10-15 μm, a = 3.251 Å and c = 5.201 Å) and cubic sesquioxide Er2O3 and Yb2O3 nanocrystals (mean size: ~130 nm, a = 10.450 Å and 10.555 Å, respectively). Its texture is dominated by the ZnO prism planes (100). The absorption spectrum of the ceramic demonstrates bands characteristic of Er3+ and Yb3+ ions in the Er2O3 and Yb2O3 crystals, respectively. The ceramic exhibits intense red upconversion luminescence. The X-ray induced luminescence spectrum contains two intense emission bands. The more intense one with a maximum at ~390 nm is due to the near-band-edge transitions and its decay time is ~1 ns. The broad band in the green represents defect emission. Its decay deviates from the single-exponential law revealing several characteristic times of ~6, 60 and 250 ns. The free carrier concentration of the composite ceramic is ~7.65×1018 cm-3, which is significantly higher than that for the undoped ZnO one.
We report on the growth, structure and spectroscopy of an Er3+ -doped Na5Y9F32 (5NaF∙9YF3) crystal featuring significant inhomogeneous spectral broadening. Single-crystals of Na5Y9F32 doped with 0.22 – 9.63 at.% Er3+ were grown by the Czochralski method. Er:Na5Y9F32 exhibits a cubic fluorite-type structure (a = 5.4881(2) Å for 5.59 at.% Er3+ doping). The most intense Raman band of this material is found at ~404 cm-1 . Er3+ ions in Na5Y9F32 exhibit a broad and smooth emission band owing to the 4 I11/2 → 4 I13/2 transition with a maximum stimulated-emission cross-section of 0.42×10-20 cm2 at 2708 nm. According to the Judd-Ofelt analysis, the radiative lifetime of the 4 I11/2 multiplet is 10.0 ms and the luminescence branching ratio β( 4 I11/2 → 4 I13/2) is 17.6%. The luminescence lifetimes of the 4 I11/2 and 4 I13/2 Er3+ states were studied as a function of the doping concentration. For 5.59 at.% Er doping, they are 7.72 ms and 6.69 ms, respectively, representing a favorable ratio for mid-infrared laser operation.
Holmium ions (Ho3+) are attractive for generation of green emission according to the 5F4+ 5S2 → 5 I8 transition. We report on the assessment of the potential of Ho3+ -doped fluoride glasses for green fiber lasers exploiting the double-clad fiber geometry. ZBLAN glasses doped with HoF3 (0.1 – 0.9 mol%) were studied. The absorption cross-section for the 5 I8 → 5F1+ 5G6 transition is 1.75×10-20 cm2 at 448.5 nm. Under excitation in the blue, the glasses exhibit intense green luminescence. The stimulated-emission cross-section for the 5F4+ 5S2 → 5 I8 transition is 0.67×10-20 cm2 at 549 nm. With increasing the HoF3 doping level, the luminescence lifetime of the 5F4+ 5S2 states slowly decreases in the range of 291 – 180 μs. The output performance of a diode-pumped green Ho fiber laser was simulated for a fiber geometry with a double D-shaped inner cladding. The variable parameters were the HoF3 doping level, the fiber length, and the output coupling. It is shown that the generation of watt-level green output from such a laser is possible when using relatively short (<1 m) fibers with low doping levels (about 0.5 mol% HoF3), as well as high transmissions of the output coupler (<60%, depending on the passive losses in the fiber).
A comparative study of three disordered calcium niobium gallium garnet (CNGG)-type crystals codoped with Tm3+ and Ho3+ ions is performed: (i) without host modifiers (CNGG), (ii) with Li+ cations added (CLNGG), and (iii) with Li+ and La3+ cations added (LCLNGG), all grown by the Czochralski method. The crystals exhibit inhomogeneously broadened luminescence bands extending beyond 2.1 μm. A diode-pumped Tm,Ho:LCLNGG laser generates 562 mW at 2082 nm with a slope efficiency of 17.4% and a laser threshold of 0.46 W. A continuous wavelength tuning between 1904.1 and 2121.1 nm (tuning range: 217 nm) is achieved with this new garnet compound. The Tm,Ho:LCLNGG crystal is promising for generation of ultrashort pulses from mode-locked lasers emitting above 2 μm.
We report on the Czochralski growth, polarized optical spectroscopy, thermal lensing and diode-pumped laser operation of a Yb3+-doped monoclinic “mixed” calcium rare-earth oxoborate crystal, Gd0.235Y0.672Yb0.093Ca4O(BO3)3 (Yb:GdYCOB). The absorption, stimulated-emission and gain cross-section spectra are determined for light polarizations E || X, Y, Z. The maximum σSE above the zero-phonon line is 0.61 x 10-20 cm2 at 1022.4 nm (for E || X) and the luminescence lifetime of Yb3+ ions is 2.37 ms. The gain spectra of Yb:GdYCOB extend beyond 1.15 μm. The thermal lens is studied for the three principal cuts, X-cut, Y-cut and Z-cut, and it is found to be positive. A continuous-wave diode-pumped X-cut Yb:GdYCOB laser generates 9.27 W at ~1050 nm with a slope efficiency of 68.2%, linear laser polarization E || X and nearly diffraction-limited beam (M2x,y < 1.16). A continuous wavelength tuning from 1020.9 to 1086.3 nm (a tuning range of ~65 nm) is achieved for the Z-cut crystal and the laser polarization E || X.
We report on the continuous-wave (CW) and Kerr-lens mode-locked (KLM) operation of an ytterbium (Yb3+) doped orthorhombic calcium rare-earth borate Yb:Ca3Gd2(BO3)4 (Yb:GdCB) disordered crystal. A high quality 10 at.% Yb:GdCB crystal was grown by the Czochralski method. An X-shaped astigmatically compensated linear cavity was employed for evaluating the CW and KLM laser performance of an a-cut (sp. gr. Pnma) Yb:GdCB crystal. Pumping with a single-transverse mode, fiber-coupled diode laser at 976 nm, a maximum CW output power of 548 mW was obtained at 1049 nm with a slope efficiency of 67.8% and a linear laser polarization (E || b). A broad wavelength tuning range of ~88 nm (1001 – 1089 nm) was achieved in the CW regime. Stable KLM operation was initiated and stabilized by a semiconductor saturable absorber mirror (SESAM). Nearly Fourier-transform-limited pulses as short as 33 fs were generated at a central wavelength of 1055.3 nm with an average output power of 98 mW for a pulse repetition rate of ~67.3 MHz.
We report on sub-100-fs pulse generation from a passively mode-locked laser based on a novel disordered crystal,
lanthanum calcium lithium niobium gallium garnet (LCLNGG) codoped with thulium (Tm3+) and holmium (Ho3+) ions.
In the continuous-wave regime, the Tm,Ho:LCLNGG laser generated a maximum output power of 350 mW at
2080.5 nm with a slope efficiency of 23.8%. By using a Lyot filter, the laser wavelength was continuously tuned over a
broad range of ~210 nm (1904.1 – 2114.1 nm). Soliton mode-locking was initiated and stabilized by a transmission-type
single-walled carbon nanotube saturable absorber. Pulses as short as 63 fs were obtained at a central wavelength of
2072.7 nm with an average output power of 63 mW at a pulse repetition rate of ~102.5 MHz.
We report on a novel approach to fabricate channel (ridge) waveguides (WGs) in bulk crystals using precision diamond saw dicing. The channels feature a high depth-to-width aspect ratio (deep dicing). The proof-of-the-concept is shown for a Tm:LiYF4 fluoride crystal. Channels with a depth of 200 μm and widths of 10–50 μm are diced and characterized with a confocal laser microscopy revealing a r.m.s. roughness of the walls of about 1 μm. The passive waveguiding properties of the channels are proven at ~815 nm showing almost no leakage of the guided mode into the bulk crystal volume. The laser operation is achieved in quasi-CW regime. The maximum peak output power reaches 0.68 W at ~1.91 μm with a slope efficiency of 53.3% (in σ-polarization). The laser mode has a vertical stripe intensity profile. The proposed concept is applicable to a variety of laser crystals with different rare-earth dopants and it is promising for sensing applications.
We systematically study cross-relaxation (CR) and ion clustering in Tm3+:CaF2 crystals using a spectroscopic approach. For this, the luminescence from the 3H4 and 3F4 states was monitored for a broad range of Tm3+ doping concentrations, from 0.01 at.% to 7 at.%. The decay curves were fitted using a model of two ions classes, namely isolated ions showing no energy-transfer processes and ions with neighbors exhibiting both CR and energy-transfer upconversion (ETU), and accounting for energy-migration. The fraction of ions with neighbors and the microscopic concentration-independent CR and ETU parameters are deduced. The critical Tm3+ doping level for which at least half of the active ions are clustered is only 0.7 at.%. The obtained results are relevant for achieving efficient laser operation of Tm3+:CaF2 crystals at the 3F4 → 3H6 (at ~1.9 μm) and the 3H4 → 3H5 (at ~2.3 μm) transitions.
Transparent ceramics of cobalt-doped zinc aluminium spinel (gahnite), Co2+:ZnA2O4, are synthesized by hot pressing at 1520 °C for 4 h in the presence of zinc fluoride, ZnF2, as a sintering additive. The effect of the ZnF2 content (3–10 wt%) on the microstructure, Raman spectra, optical absorption and luminescence of ceramics is studied. The ceramics feature clean grain boundaries, the absence of pores and a narrow grain size distribution (mean grain size: 70-100 μm) resulting in high in-line transparency close to the theoretical limit. The obtained ceramics are suitable for fabrication of saturable absorbers of erbium lasers.
Yttrium orthoaluminate (YAlO3) is an attractive laser host crystal for doping with thulium (Tm3+) ions. We report on the absorption and stimulated-emission (SE) cross-sections of this orthorhombic (sp. gr. Pnma) Tm:YAlO3 crystal for the principal light polarizations, E || a, b and c. Polarized absorption data lead to the Judd-Ofelt parameters Ω2 = 1.46, Ω4 = 2.82 and Ω6 = 1.09 [10-20 cm2]. In particular, for the 3H4 → 3H5 transition, it is found a stimulated emission cross section of 0.86×10-20 cm2 at 2278 nm corresponding to an emission bandwidth of ~12 nm (for E || b). Continuous-wave laser operation on this 3H4 → 3H5 transition is achieved with an 1.8 at.% Tm:YAlO3 crystal under laser-pumping at 776 nm. The mid-infrared Tm:YAlO3 laser generated 0.96 W at ~2274 nm with a slope efficiency of 61.8% and a linear laser polarization (E || b). Tm:YAlO3 is promising for mode-locked lasers at ~2.3 μm.
We report on the first laser operation of a novel double molybdate compound, Yb:KY(MoO4)2. Single-crystals were grown by the Low Temperature Gradient (LTG) Czochralski method. The crystal structure (orthorhombic, sp. gr. Pbna – D142h) was refined with the Rietveld method. Yb:KY(MoO4)2 exhibits a layered structure leading to a strong optical anisotropy and a perfect cleavage along the (100) plane. The stimulated-emission cross-section for Yb3+ ions is 3.70×10-20 cm2 at 1008.0 nm and the emission bandwidth is 37 nm (for light polarization E ||b). Continuous-wave laser operation is achieved in a 3 at.% Yb:KY(MoO4)2 crystal plate (thickness: 286 μm) under diode pumping. The microchip laser generated a maximum output power of 0.81 W at 1021-1044 nm with a slope efficiency of 76.4% and linear polarization. Yb:KY(MoO4)2 crystal films / plates are attractive for sub-ns passively Q-switched microchip lasers and thin-disk lasers.
We report on fabrication, structure, spectroscopic and nonlinear properties of a new functional optical material – transparent glass-ceramics (GCs) based on Co2+,Ga3+-codoped ZnO (Co2+:GZO) nanocrystals. The introduction of Ga3+ cations that are smaller than Zn2+ ones and have a different valence state, is expected to modify the crystal field around the Co2+ ions leading to broadband absorption at the 4A2(4F) → 4T1(4F) transition. The glass of the ZnO – K2O – Al2O3 – SiO2 system was doped with 3 mol% Ga2O3 and 0.05 mol% CoO. Transparent GCs were produced by secondary heattreatments at 680 – 860 °C. They contained one crystalline phase - nanosized (8 – 26 nm) hexagonal GZO crystals, Ga3+ ions being distributed between the ZnO nanocrystals and the residual glass. The absorption spectra of GCs contained an intense band at 1.3-1.65 μm related to the 4A2(4F) → 4T1(4F) Co2+ transition in Td sites. A rise of IR losses due to the free charge carrier scattering in GZO was observed. Absorption saturation of transparent GCs was studied at ~1.54 μm. They exhibited low saturation fluence, 0.7–1.3 ± 0.2 J/cm2, and high laser-induced damage threshold, ~25 J/cm2. Co2+,Ga3+- codoped ZnO-based transparent GCs are promising for passive Q-switching of eye-safe erbium lasers emitting at ~1.5- 1.7 μm.
The thermal lensing effect in the Ng-cut Yb:KGW and a-cut Yb:CALGO laser crystals was evaluated and compared using a similar laser cavity and pumping scheme. The Yb:KGW crystal exhibited large anisotropy along the horizontal and vertical axes and the thermal lensing sensitivity factors of Mx=0.52 m-1/W (||Np) and My=0.18 m-1/W (||Nm) were determined. The Yb:CALGO crystal exhibited negligible anisotropy in the thermal lensing power, and thermal lensing sensitivity factors of Mx=0.452 m-1/W (⊥c) and My=0.458 m-1/W (||c) were measured.
Ytterbium-doped transparent ceramics based on cubic garnets are promising for thin-disk lasers. 3.6 at.% Yb:Lu3Al5O12 transparent ceramics were fabricated by a solid-state reaction at 1800 °C in vacuum using Yb:Lu2O3 and Al2O3 nanopowders produced by laser ablation and their spectroscopic properties were studied. The stimulated-emission crosssection is 2.46×10-20 cm2 at 1030.2 nm. The Stark splitting of the Yb3+ multiplets was also determined. A compact CW Yb:Lu3Al5O12 ceramic laser pumped by a fiber-coupled 968 nm InGaAs laser diode generated 5.65 W at ~1031 nm with a slope efficiency of 67.2%. Using quasi-CW pumping, the peak power reached 8.83 W.
Rare-earth-doped calcium niobium gallium garnets (Ca3Nb1.5Ga3.5O12, shortly CNGG) are disordered laser materials attractive for ultrashort pulse generation. We report on the crystal growth by the Czochralski method, spectroscopy and efficient laser operation of Yb3+,Na+ and Yb3+,Na+,Li+-codoped CNGG-type crystals. Their cubic structure is confirmed by X-ray diffraction and Raman spectroscopy. The absorption / stimulated-emission cross-sections and lifetime of Yb3+ are determined. Continuous-wave (CW) laser experiments are performed in a compact cavity using a 968-nm InGaAs pump laser diode. A 11.9 at.% Yb,Na:CNGG crystal generated 3.74 W at 1069.9 nm with a slope efficiency of 56.5%. Yb,Na:CNGG is promising for sub-100-fs mode-locked lasers at ~1 μm.
High power performance of diode-pumped Yb:YAP lasers with different crystal orientation (cuts) was investigated. The crystal samples exhibited similar pump absorption properties. In continuous-wave (CW) regime, the maximum output power of <7 W could be generated using the b-cut and c-cut Yb:YAP crystals with slope efficiency up to 70%, while the laser with the a-cut crystal was limited to 5.6 W of output power most likely due to the thermal lensing effect. Owing to the high thermal conductivity and broad emission bandwidth of Yb:YAP, this crystal is a suitable laser host for high power laser operation in CW and pulsed regimes.
Tetragonal calcium rare-earth aluminates, CaLnAlO4, combine a structural disorder with good thermo-mechanical properties. We report on efficient continuous-wave (CW) and passively Q-switched (PQS) ~2-μm laser operation of a 4 at.% Tm:CaYAlO4 crystal using a compact (6-mm-long) plane-parallel cavity. The pump source was a 791 nm fibercoupled AlGaAs laser diode. The CW output power reached 5.78 W at ~1970 nm with a slope efficiency of 43.6% and a linear laser polarization. Stable PQS operation was achieved using a single-walled carbon nanotube (SWCNT) based transmission-type saturable absorber. The PQS laser generated 2.15 W at ~1945 nm, a record-high average output power for this type of lasers. The best pulse characteristics (energy/duration) were 9.1 μJ/165 ns at a repetition rate of 235 kHz.
Tm,Ho co-doped disordered calcium niobium gallium garnet (CNGG) crystals are investigated as a novel gain medium for mode-locked lasers near 2 μm. With a GaSb-based semiconductor saturable absorber mirror (SESAM) and chirped mirrors for dispersion compensation such a laser is mode-locked at a repetition rate of 89.3 MHz. For a 5% output coupler, a maximum average output power of 157 mW is obtained with a pulse duration of 170 fs (28-nm broad spectrum centered at 2.075 μm, leading to a time-bandwidth product of 0.331). With a 0.5% output coupler, 73-fs pulses are generated at 2.061 μm with a spectral width of 62 nm (time-bandwidth product of 0.320) and an average output power of 36 mW.
Mode-locked lasers emitting ultrashort pulses in the 2-μm spectral range at high (100-MHz) repetition rates offer unique opportunities for time-resolved molecular spectroscopy and are interesting as pump/seed sources for parametric frequency down-conversion and as seeders of ultrafast regenerative laser amplifiers. Passively mode-locked lasers based on Tm3+- and Ho3+-doped bulk solid-state materials have been under development for about a decade. In 2009 we demonstrated the first steady-state operation of such a Tm:KLu(WO4)2 laser using a single-walled carbon nanotube (SWCNT) saturable absorber (SA), generating 10-ps pulses at 1.95 μm. In 2012 this laser produced 141-fs pulses at 2.037 μm. More recently, the study of numerous active media with different SAs resulted in the generation of sub-100-fs (sub-10-optical-cycle) pulses. Materials with broad and smooth spectral gain profile were selected, naturally emitting above 2 μm to avoid water vapor absorption/dispersion effects, including anisotropic materials, strong crystal-field distortion in hosts that do not contain rare-earths, crystals with structural or compositional (i.e. mixed compounds) disorder that exhibit inhomogeneous line broadening, mixed laser ceramics, and Tm,Ho-codoping of ordered and disordered crystals and ceramics. A broad absorption band in semiconducting SWCNTs spans from 1.6 to 2.1-μm whereas the absorption of graphene extends into the mid-IR and scales for multilayers, increasing the modulation depth. Compared to GaSb-based semiconductor SA mirrors (SESAMs), the carbon nanostructures exhibit broader spectral response and can be fabricated by simpler and inexpensive techniques. Chirped mirrors were implemented for groupvelocity dispersion compensation, to generate the shortest pulses, down to 52 fs at 2.015 μm.
Demonstration of intracavity loss measurement in a diode-pumped continuous-wave Yb:CALGO laser is presented. The characterization method was based on spectroscopic gain measurements and allowed accurate and reliable determination of intracavity losses above the laser threshold. A comparison with traditional Findlay-Clay analysis was also made and highlighted the advantages of the spectroscopic method.
Monoclinic rare-earth silicates, RE2SiO5, are the promising hosts for Nd3+ doping. We have studied Nd:(Gd,Y)2SiO5, Nd:(Lu,Y)2SiO5 and Nd:Lu2SiO5 crystals for their suitability for ~1.3 μm (4F3/2 → 4I13/2) lasers. The absorption and stimulated-emission cross-section spectra were determined. The continuous-wave laser operation was studied in a compact plano-plano cavity. A b-cut Nd:(Gd,Y)2SiO5 crystal generated up to 0.75 W of linearly polarized emission at 1360.7 nm with a slope efficiency η of 16.9%. For the same crystal operated at the 4F3/2 → 4I11/2 transition, the output power reached 3.84 W at 1077.4 nm with η = 54.5% with a threshold of only 80 mW.
We demonstrated continuous-wave dual-wavelength (DW) operation of a Nd:CALGO laser using a single birefringent filter (BRF) plate. Owing to a new mechanism of the BRF loss action, the equal lasing thresholds at two wavelengths could be provided by a single transmission peak of the filter and wavelength separation was not limited to the free-spectral range (FSR) of the filter. A wide range of DW pairs with wavelength separation from sub-nm up to ~4 nm with adjustable spectral intensity ratio was experimentally demonstrated using a single 2 mm-thick BRF plate.
Energy-transfer processes strongly affect the performance of lanthanide-doped photonic devices. In this work, we introduce a simple stochastic model of energy-transfer processes and successfully apply it to the example of crossrelaxation (CR) and energy-transfer upconversion (ETU) in amorphous Al2O3:Tm3+ waveguides on silicon intended for lasers operating at ~2 μm. The stochastic model is based on the rate-equation formalism and considers two spectroscopically distinct ion classes, namely single ions and ions with neighbours (pairs and clusters), with the corresponding ion fractions being dependent on the doping concentration. We prove that a more accurate description of the luminescence properties of amorphous Al2O3:Tm3+ is obtained when accounting for the presence of these distinct ion classes. Based on the developed model, we derive microscopic CR and ETU parameters of CCR = 5.83×10-38 cm6 s -1 , CETU1 = 0.93×10-40 cm6 s -1 , and CETU2 = 7.81×10-40 cm6 s -1 , and determine the laser quantum efficiency ηq of excitation of Tm3+ ions in the upper laser level. For the maximum Tm3+ concentration of 5.0×1020 cm-3 studied experimentally in this investigation, ηq reaches 1.73. Furthermore, the transition cross-sections at the pump and laser wavelengths are determined. For the 3H6 → 3F4 transition, the maximum stimulated-emission cross-section is σe = 0.47 × 10-20 cm2 at 1808 nm.
The recent advances in the development of Holmium monoclinic double tungstate thin-disk lasers are reviewed. The thin-disk is based on a 250-μm-thick 3 at. % Ho:KY(WO4)2 active layer grown on a (010)-oriented KY(WO4)2 substrate. When pumped by a Tm-fiber laser at 1960 nm with a single-bounce pump geometry, the continuous-wave Ho:KY(WO4)2 thin-disk laser generates an output power of 1.01 W at 2057 nm corresponding to a slope efficiency η of 60% and a laser threshold of only 0.15 W. The thin-disk laser is passively Q-switched with a GaSb-based quantum-well semiconductor saturable absorber mirror. In this regime, it generates an average output power of 0.551 W at ~2056 nm with η = 44%. The best pulse characteristics are 4.1 μJ / 201 ns at a repetition rate of 135 kHz. The laser performance, beam quality and thermo-optic aberrations of such lasers are strongly affected by the Ho3+ doping concentration. For the 3 at.% Ho3+-doped thin-disk, the thermal lens is negative (the sensitivity factors for the two principal meridional planes are -1.7 and -0.6 m-1/W) and astigmatic. The Ho:KY(WO4)2 epitaxial structures are promising as active elements in mode-locked thin-disk lasers at ~2060 nm.
An optimized design of a birefringent filter with an off-plane optical axis is presented to enable multi-wavelength operation of diode-pumped Yb-ion solid-state lasers. The simulation results indicate that such birefringent filters can be used for the development of powerful tunable multi-wavelength Yb-ion laser sources. A comparison with performance of a standard birefringent filter that has its optical axis lying in plane of a plate is also given.
A buried depressed-index channel waveguide with a circular cladding and a core diameter of 40 μm is fabricated in a bulk monoclinic 3 at.% Tm:KLu(WO4)2 crystal by femtosecond direct laser writing. In the continuous-wave regime, the Tm waveguide laser generates ∼210 mW at 1849.6 nm with a slope efficiency η of 40.8%. Passively Q-switched operation is achieved by inserting transmission-type 2D saturable absorbers (SAs) based on few-layer graphene and MoS2. Using the graphene-SA, a maximum average output power of ∼25 mW is generated at 1844.8 nm. The pulse characteristics (duration/energy) are 88 ns/18 nJ at a repetition rate of 1.39 MHz.
We report on the first application of a topological insulator based on antimony telluride (Sb2Te3) as a saturable absorber (SA) in a bulk microchip laser. The transmission-type SA consisted of a thin film of Sb2Te3 (thickness: 3 nm) deposited on a glass substrate by pulsed magnetron sputtering. The saturable absorption of the Sb2Te3 film was confirmed for ns-long pulses. The microchip laser was based on a Tm:GdVO4 crystal diode-pumped at ∼802 nm. In the continuous-wave regime, this laser generated 3.54 W at 1905-1921 nm with a slope efficiency η of 37%. The Q-switched laser generated a maximum average output power of 0.70 W at 1913 nm. The pulse energy and duration were 3.5 μJ and 223 ns, respectively, at a repetition rate of 200 kHz. The Sb2Te3 SAs are promising for passively Q-switched waveguide lasers at ∼2 μm.
Multi-watt dual-wavelength operation of a diode-pumped Yb:CALGO laser was achieved by introducing a thin birefringent filter (BRF) plate into a standard laser cavity. A BRF plate with a thickness of 0.5 mm was capable of generating a dual-wavelength output with a frequency offset of 1.31 THz (equivalent to a wavelength separation of 4.8 nm). The output power of the dual-wavelength laser at 1044.7 nm and 1049.5 nm was 2.6 W. The developed dual-wavelength Yb:CALGO laser with multi-watt output power is an attractive and cost-effective candidate for generation of THz radiation and dual-wavelength mode-locked lasers.
Tetragonal calcium rare-earth aluminates, CaLnAlO4, are attractive laser host crystals. The emission of Nd3+ ions at 1.3- 1.4 μm due to the 4F3/2 → 4I13/2 transition is of interest for medicine, fiber optics, and light conversion. We report on compact Nd:CaLnAlO4 lasers using a plane-plane cavity. With an a-cut 0.8 at.% Nd:CaYAlO4 crystal diode-pumped at 802 nm, a maximum continuous-wave output power of 365 mW was achieved at 1.365 & 1.390 μm corresponding to the σ-polarization. The 4F3/2 → 4I13/2 laser performance of the Nd:CaLnAlO4 crystals was compared to that from a monoclinic Nd:KGd(WO4)2. At the 4F3/2→ 4I11/2 transition (1.08 μm), a Nd:CaYAlO4 micro-laser generated multi-watt output (>4 W) with a slope efficiency of 39%.
Alexandrite is a well-known material for broadly tunable and power-scalable near-IR lasers. We measured the thermal coefficients of the optical path (TCOP) and thermo-optic coefficients (TOCs) of Alexandrite at 632.8 nm for three principal light polarizations, E || a, E || b and E || c. All TOCs are positive and show a notable polarization-anisotropy, dna/dT = 5.5, dnb/dT = 7.0 and dnc/dT = 14.9×10-6 K-1. We also characterized thermal lensing in a continuous-wave Alexandrite laser which used a Brewster-oriented c-cut 0.16 at.% Cr3+ doped BeAl2O4 crystal pumped at 532 nm and emitted at 750.9 nm (E || b). The measured thermal lens was positive and astigmatic. The sensitivity factors of the thermal lens (Mx,y = dDx,y/dPabs) were found to be Mx = 1.74 and My = 2.38 [m-1/W].
Absorption, stimulated-emission and gain cross-sections are determined for 3 at.% Tm:CaGdAlO4. This crystal is employed in a microchip laser diode-pumped at 802 nm. In the continuous-wave (CW) regime, this laser generates 1.16 W at 1883-1893 nm with a slope efficiency of 32% with respect to the absorbed pump power. Using a special "bandpass" output coupler, vibronic CW laser operation up to 2043 nm is achieved. For passive Q-switching of the Tm:CaGdAlO4 laser-saturable absorbers (SAs) based on CVD-grown graphene and randomly-oriented arc-discharge single-walled carbon nanotubes (SWCNTs) in a PMMA film. The SWCNT-SA demonstrates superior performance. The laser produced a maximum average output power of 245 mW at 1844 nm with a slope efficiency of 8%. The latter corresponds to a pulse energy and duration of 6 μJ and 138 ns, respectively, at a repetition rate of 41 kHz. Using the graphene-SA, 2.8 μJ, 490 ns pulses are obtained at a repetition rate of 86 kHz.
A trigonal 5.6 at.% Yb:YAl3(BO3)4 (Yb:YAB) crystal is employed in continuous-wave (CW) and passively Q-switched microchip lasers pumped by a diode at 978 nm. Using a 3 mm-thick, c-cut Yb:YAB crystal, which has a higher pump absorption efficiency, efficient CW microchip laser operation is demonstrated. This laser generated a maximum output power of 7.18 W at 1041–1044 nm with a slope efficiency η of 67% (with respect to the absorbed pump power) and an almost diffraction-limited beam, M2x,y < 1.1. Inserting a Cr:YAG saturable absorber, stable passive Q-switching of the Yb:YAB microchip laser was obtained. The maximum average output power from the Yb:YAB/Cr:YAG laser reached 2.82 W at 1042 nm with η = 53% and a conversion efficiency with respect to the CW mode of 65% (when using a 0.7 mm-thick Cr:YAG). The latter corresponded to a pulse duration and energy of 7.1 ns / 47 μJ at a pulse repetition rate (PRR) of 60 kHz. Using a 1.3 mm-thick Cr:YAG, 2.02 W were achieved at 1041 nm corresponding to η = 38%. The pulse characteristics were 4.9 ns / 83 μJ at PRR = 24.3 kHz and the maximum peak power reached 17 kW. Yb:YAB crystals are very promising for compact sub-ns power-scalable microchip lasers.
We report on the passive Q-switching of a compact diode-side-pumped Er,Yb:glass laser by a novel saturable absorber (SA) based on transparent glass-ceramics (GC) containing Co2+Mg(Al,Ga)2O4 nanocrystals. To prepare the GC, an initial magnesium aluminosilicate glass doped with Ga2O3 containing 0.1 mol% CoO was synthesized by a conventional melt-quenching technique and heat-treated at 850–950 °C. The X-ray diffraction analysis of the GC confirmed the precipitation of Co2+Mg(Al,Ga)2O4 crystals with spinel structure (6-7 nm in size). Depending on the heat-treatment temperature, the saturation intensity for the GC measured at 1540 nm was in the 0.5...0.7 J/cm2 range and the recovery time was in the 240...335 ns range. Using the SA based on GC prepared by the heat-treatment at 950 °C with an initial transmission of 84.7%, we generated stable Q-switched pulses 1.14 mJ in energy and 7.2 ns in duration. The peak power reached 160 kW, the repetition rate was 1 Hz and the laser wavelength was 1535 nm. The developed GCs are promising for Q-switching of erbium lasers emitting at 1.5-1.7 μm.
Tetragonal rare-earth calcium aluminates, CaLnAlO4 where Ln = Gd or Y (CALGO and CALYO, respectively), are attractive laser crystal hosts due to their locally disordered structure and high thermal conductivity. In the present work, we report on highly-efficient power-scalable microchip lasers based on 8 at.% Yb:CALGO and 3 at.% Yb:CALYO crystals grown by the Czochralski method. Pumped by an InGaAs laser diode at 978 nm, the 6 mm-long Yb:CALGO microchip laser generated 7.79 W at 1057–1065 nm with a slope efficiency of η = 84% (with respect to the absorbed pump power) and an optical-to-optical efficiency of ηopt = 49%. The 3 mm-long Yb:CALYO microchip laser generated 5.06 W at 1048–1056 nm corresponding to η = 91% and ηopt = 32%. Both lasers produced linearly polarized output (σ- polarization) with an almost circular beam profile and beam quality factors M2x,y <1.1. The output performance of the developed lasers was modeled yielding a loss coefficient as low as 0.004-0.007 cm-1. The results indicate that the Yb3+- doped calcium aluminates are very promising candidates for high-peak-power passively Q-switched microchip lasers.
We report on the growth, spectroscopic and laser characterization of a novel monoclinic laser crystal, 3.5 at.% Yb, 5.5 at.% In:KLu(WO4)2 (Yb,In:KLuW). Single-crystals of high optical quality are grown by the TSSG method. The absorption, stimulated-emission and gain cross-sections are determined for this material at room temperature with polarized light. The maximum σabs is 9.9×10-20 cm2 at 980.8 nm for light polarization E || Nm. The radiative lifetime of Yb3+ in Yb,In:KLuW is 237±5 μs. The stimulated-emission cross-sections are σSE(m) = 2.4×10-20 cm2 at 1022.4 nm and σSE(p) = 1.3×10-20 cm2 at 1039.1 nm corresponding to an emission bandwidth of >30 nm and >35 nm, respectively. A diode-pumped Ng-cut Yb,In:KLuW microchip laser generates 4.11 W at 1047-1052 nm with a slope efficiency of 78%. Passive Q-switching of a Yb,In:KLuW laser is also demonstrated. The Yb,In:KLuW crystal seems very promising for sub-100 fs mode-locked lasers.
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