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).
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 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).
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.
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.
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 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 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.
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 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.
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.
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.
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.
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.
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.
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.
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%.
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.
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.
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 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.
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.
We demonstrate passive Q-switching (PQS) of the Tm-doped BaY2F8 (Tm:BYF) laser for the first time. The Tm:BYF
laser is diode-pumped using an L-shaped hemispherical resonator. In the cw regime, the maximum output power with an
18% Tm-doped BYF crystal reached 1.12 W at ~1920 nm for an absorbed pump power of 3.06 W. In the PQS regime,
maximum pulse energy (720 μJ) and peak power (17.1 kW) were obtained with an 8% Tm-doped BYF crystal and a
Cr:ZnS saturable absorber with 92% low-signal transmission, again near 1920 nm, for a pulse width of ~40 ns and a
repetition rate of 50 Hz.
Laser generation in the 2 μm spectral range is interesting for applications in mid-IR fingerprint applications, including remote sensing, gas detection, high resolution molecular spectroscopy, and medicine. Here, we review the different Tm3+-based waveguide lasers with emission at around 2 μm recently developed in monoclinic potassium double tungstates, including a CW mirrorless and the first Q-switched waveguide laser based on this ion. We also investigated the influence on the waveguide laser performance of the crystallographic plane on which the epitaxial active layer has been grown.
Porous GaN crystals have been grown on Pt- and Au- coated silicon substrates as porous crystals and as porous layers.
By the direct reaction of metallic Ga and NH3 gas in a CVD system, intermetallic metal-Ga alloys formed at the interface
allow the seeding and growth of porous GaN by vapor-solid-solid processes. Current-voltage and capacitance-voltage
measurements confirm that the intermetallic seed layers result in near-ohmic contacts to porous n-GaN with low contact resistivities.
We studied up-conversion emission of triply doped (Ho,Tm,Yb):KLu(WO4)2 (KLuW) nanocrystals at the range of temperature 296-673 K at different excitation wavelengths. The intensity ratio between two emission lines was used for monitoring the temperature. Pumping Yb3+ at 980 nm provides a good response at relatively high temperatures, while pumping Tm3+ at 802 nm provides an excellent sensitivity in the biological range of temperatures., which make the material also attractive for biological temperature sensors.
Passive Q-switching of a diode-pumped (Tm,Yb):KLu(WO4)2 laser, operating between 1910 and 1950 nm, has been
achieved using polycrystalline Cr2+:ZnS as a saturable absorber in a hemispherical L-shaped cavity. The dependence on
the doping levels of Tm and Yb and the low-signal absorption of the saturable absorber has been studied. The highest
average output power reached 272 mW at a repetition rate of 2.04 kHz corresponding to a pulse energy of 133 μJ. This
was achieved with (8.8 at. % Tm, 2.3 at. % Yb):KLu(WO4)2 active element and a saturable absorber with 85% lowsignal
transmission. The highest pulse energy, 148 μJ, was achieved with the same combination at lower (0.94 kHz)
repetition rate. The obtained pulse energy with the different samples is compared with theoretical analysis and the pulse
duration and peak power are estimated from the calculations.
Continuous-wave (CW) lasing of Ho in KRE(WO4)2 (RE=Y, Gd, Lu) crystals has been compared using in-band
pumping by a diode pumped Tm:KLu(WO4)2 laser under identical conditions at room temperature. The three monoclinic
double tungstate hosts perform similarly with the maximum output power obtained for Ho:KY(WO4)2, 406 mW with
slope efficiency of 59.9%. For Ho:KLu(WO4)2 the maximum power reached 392 mW but the slope efficiency was
slightly higher, 61.6%. Ho:KGd(WO4)2, for which the ionic radius difference between dopant and substituted ions is
maximum, generated an output power of 368 mW with slope efficiency of 53.2%.
Monoclinic crystals of Tm-doped KLu(WO4)2 were used to demonstrate pulsed laser operation near 2 μm. Passive Qswitching
and passive mode-locking were the techniques employed to produce such laser pulses. For passive Qswitching
we used an AlGaAs -based diode laser to pump the active elements and Cr:ZnSe and Cr:ZnS crystals as
saturable absorbers. For passive mode-locking we used a Ti:sapphire laser as pump source and single-walled carbon
nanotubes as saturable absorbers. In the former case, maximum pulse energies of 200 μJ for a pulse duration of 70 ns
were achieved at a repetition rate of 3 kHz with Cr:ZnS saturable absorber, while in the latter case, ultrashort pulse
durations of ~10 ps were measured with a maximum average power of 240 mW. In both laser regimes the oscillation
wavelength was ~1945 nm.
We demonstrate here that it is possible to fabricate 1D and 2D diffraction gratings on the (001) surface of RbTiOPO4
(RTP) and KTiOPO4 (KTP) single crystals. We analyzed the linear and nonlinear optical properties of 1D and 2D
nonlinear photonic crystals. We show enhanced second harmonics when the samples were illuminated with a pulsed
Nd:YAG laser, when compared to non-structured surface of the same materials and mainly there exists an asymmetry on
the diffraction patterns of the second harmonic generated light, showing higher intensity in diffraction orders different to
the zero order in the reflection configuration.
We demonstrate the first thin disk epitaxial Tm-laser based on the monoclinic KLu(WO4)2 with 15 at. % doping. The
doped epitaxial layer serving as an active medium is only 80 μm thick. The large absorption enables efficient pumping
with only a single double pass of the pump radiation from the diode-laser. For output coupling between 0.4% and 2.8 %,
the slope efficiency in the continuous-wave regime is in the 7-11% range and the laser threshold is 1.5...2.5 W of
absorbed pump power. The laser emission spectra are centered at ~1850, 1915 and 1940 nm for output couples of 2.8%,
1.6% and 0.4% transmission, respectively. In all cases the emission spectra are "structured", consisting of a number
(typically 5...10) of narrow emission lines spread irregularly over 15...30 nm.
The orthovanadate crystals YVO4, GdVO4, and LuVO4 attract much attention as promising host materials for the trivalent Yb-ion since such crystals are characterized by large absorption and emission cross sections, broad absorption
and emission bands, and higher thermal conductivities than most of the other Yb-doped materials. More interestingly,
their laser operation is characterized by optical bistability, apparently a unique feature of Yb-doped vanadates, not found
so far in other Yb-lasers or even in other solid-state lasers. The optically "passive" vanadates, YVO4, GdVO4, and
LuVO4, as well as the stoichiometric YbVO4 exhibit the same zircon structure and continuous isostructural solid
solutions can be expected. The absorption and emission spectra of Yb0.0054:Y0.3481Gd0.6465VO4, a specific compound in the mixed Ybt:YxGd1-x-tVO4 series, inherit the spectroscopic features of both Yb:YVO4 and Yb:GdVO4. We found that this Yb-doped solid solution also displays optical bistability in continuous-wave (cw) laser operation. The strongly pronounced bistability extends from Pabs=1.9 W to Pabs=3.4 W while the output power amounts to 0.98 W at the upthreshold. Distinct from the previously reported Yb:LuVO4 laser, coexistence and switching of the σ and π polarization states occur along with emission wavelength shift in the bistability region upon decreasing the pump power. Increasing the output coupling reduces the bistability region while expanding the coexistence region for the σ and π polarization states.
Monoclinic potassium double tungstates are biaxial laser materials characterized by strong anisotropy of the spectroscopic properties. KLu(WO4)2 is the most attractive of them in the case of Yb-doping because of the close ionic radii of Yb and Lu. In this work we compare crystals of equal dimensions and doping level but different cuts, under the same pumping conditions. Special emphasis is placed on the polarization behavior. We present substantial power scaling with KLu(WO4)2 in the continuous-wave regime by longitudinal fiber-coupled diode laser pumping. Slope efficiency of roughly 80% is achieved while the naturally selected laser polarization is parallel either to the Np or Nm principal optical axes. A maximum output power of 11.0 W was produced from a 2 mm thick uncoated crystal with Ng-cut, the corresponding optical-to-optical efficiency was 68%.
High-quality crystals of KLu(WO4)2, shortly KLuW, were grown with sizes sufficient for characterization of the thermomechanical and optical properties, and substantial progress was achieved in the field of spectroscopy and laser operation with Yb3+- and Tm3+-doping. We review the properties of flux grown KLuW, the Yb3+ and Tm3+ spectroscopy, and present laser results obtained in several operational regimes both with Ti:sapphire and direct diode laser pumping using InGaAs and AlGaAs diodes near 980 and 800 nm, respectively. The slope efficiencies with respect to the absorbed pump power achieved with continuous-wave (CW) bulk and epitaxial Yb:KLuW lasers under Ti:sapphire laser pumping were ≈57 and ≈66%, respectively. Output powers as high as 3.28 W were obtained with diode pumping in a simple two-mirror cavity where the slope efficiency with respect to the incident pump power reached ≈78%. Passively Q-switched laser operation of bulk Yb:KLuW was realized with a Cr:YAG saturable absorber resulting in oscillation at ≈1031 nm with a repetition rate of 28 kHz and simultaneous Raman conversion to ≈1138 nm with maximum energies of 32.4 and 14.4 &mgr;J, respectively. The corresponding pulse durations were 1.41 and 0.71 ns. Passive mode-locking by a semiconductor saturable absorber mirror (SESAM) resulted in bandwidth-limited pulses with duration of 81 fs (1046 nm, 95 MHz) and 114 fs (1030 nm, 101 MHz) for bulk and epitaxial Yb:KLuW lasers, respectively. Slope efficiency as high as 69% with respect to the absorbed power and an output power of 4 W at 1950 nm were achieved with a diode-pumped Tm:KLuW laser. The tunability of this laser, under Ti:sapphire laser pumping, extended from 1800 to 1987 nm. An epitaxial Tm:KLuW laser provided slope efficiency as high as 64% and a tuning range from 1894 to 2039 nm when pumped by a Ti:sapphire laser.
We studied several crystals of Yb-doped LuVO4 with different orientations (a-cut and c-cut) in order to evaluate the
potential of this new laser material for high power continuous-wave operation using simple hemispherical cavities,
longitudinally pumped by a fiber coupled diode laser. We achieved substantial improvement with respect to previous
results in terms of output power and slope efficiency. The highest output power and optical efficiency were obtained for
the &pgr;-polarization using a-cut samples. Bistability of the input-output power characteristics in terms of a hysteresis loop
was also observed. Significant intensity fluctuations were found existing in a small operational region near the critical
point (up-threshold) of the bistability region. The heating of the crystal is reduced in the lasing state when stimulated
emission keeps the part of the radiative relaxation high in comparison to the nonradiative relaxation processes.
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