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We report on the first, to the best of our knowledge, in-band pumped T m 3+ , H o 3+ codoped waveguide (WG) laser. A depressed-index surface channel WG (type III) with a 50 µm half-ring cladding is fabricated in a 5 at. % T m 3+ , 0.5 at. % H o 3+ : K L u ( W O 4 ) 2 crystal by femtosecond pulse direct laser writing. Under in-band pumping by a 1679 nm Er Raman fiber laser, T m 3+ and H o 3+ colasing is observed in the WG and explained by bidirectional energy transfer. The maximum total output power at ∼1942 n m ( T m 3+ ) and 2059 nm ( H o 3+ ) is 448 mW with a slope efficiencyM of 40.6%, which is a record high for this type of WG lasers. The maximum output power of the Ho laser reaches 144 mW.

Surface channel waveguides (WGs) based on a half-ring (40-60-µm-diameter) depressed-index cladding (type III) geometry are fabricated in monoclinic Tm 3+ :MgWO 4 by femtosecond (fs) laser writing at a repetition rate of 1 kHz. The WGs are characterized by confocal laser microscopy and μ -Raman spectroscopy. A Tm 3+ :MgWO 4 WG laser generates 320 mW at ∼2.02µm with a slope efficiency of 64.4%. The WG emits a transverse single-mode and linear polarization ( E || N m ). A remarkable low loss of <0.1dB/cm is measured for the WG. Vibronic laser emission at ∼2.08µm is also achieved.

Monoclinic (wolframite-type) monotungstate crystals are promising for rare-earth doping. We report polarized room- and low-temperature spectroscopy and efficient high-power laser operation of such a ${{\rm Yb}^{3 + }}{:}\,{{\rm MgWO}&#95;4}$Yb 3+ :MgWO 4 crystal featuring high stimulated emission cross section (${\sigma &#95;{\rm SE}}\; = \;{6}.{2}\; \times \;{{10}^{ - 20}}\;{{\rm cm}^2}$σ SE =6.2×10 -20 cm 2 at 1056.7 nm for light polarization ${\rm E}\;||\;{N&#95;m}$E||N m ), large Stark splitting of the ground state (${765}\;{{\rm cm}^{ - 1}}$765cm -1 ), large gain bandwidth (26.1 nm for ${\rm E}\;||\;{N&#95;g}$E||N g ), and strong Raman response (most intense mode at ${916}\;{{\rm cm}^{ - 1}}$916cm -1 ). A diode-pumped ${{\rm Yb}^{3 + }}{:}\,{{\rm MgWO}&#95;4}$Yb 3+ :MgWO 4 laser generated 18.2 W at ${\sim}{1056}\;{\rm nm}$∼1056nm with a slope efficiency of ${\sim}{89}\% $∼89% and a linearly polarized laser output.

A surface channel waveguide (WG) laser is produced by diamond saw dicing of a 15 μm thick 10 at. % Tm:KY 1-x-y Gd x Lu y (WO 4 ) 2 monoclinic double tungstate thin film grown by liquid phase epitaxy on an undoped KY(WO 4 ) 2 substrate. The WG propagation losses are 1.1±0.5  dB/cm. When pumped at 802 nm, laser operation is achieved with a maximum output power of 262 mW at 1833 nm with a record slope efficiency of 82.6% (versus the absorbed pump power) in a TE 10 spatial mode (linear laser polarization, E‖N m ). Diamond saw dicing of double tungstate epitaxies is a promising technology for manufacturing WGs for sensing applications.

We report on efficient laser operation of the first holmium monoclinic double tungstate waveguide laser fabricated by femtosecond direct laser writing. A depressed-index buried channel waveguide with a 60 μm diameter circular cladding was inscribed in 5 at.% Ho 3+ :KGd(WO 4 ) 2 . It was characterized by confocal microscopy and μ-Raman and μ-luminescence spectroscopy, indicating well-preserved crystallinity of its core. Pumped by a thulium bulk laser, the holmium waveguide laser generated 212 mW at 2055 nm with a slope efficiency of 67.2%. The waveguide propagation losses were 0.94±0.2  dB/cm.

We report on an efficient diode-pumped continuous-wave erbium-doped monoclinic double tungstate laser. It is based on a 1 at. % Er 3+ :KLu(WO 4 ) 2 (Er:KLuW) crystal cut along the N g optical indicatrix axis. The Er:KLuW microchip laser, diode pumped at 0.98 μm, generates 268 mW at 1.610 μm with a slope efficiency of 30%. The output is linearly polarized (E||N m ), and the laser beam is nearly diffraction limited (Mp,m2<1.1). Spectroscopic properties of Er 3+ in KLuW are also presented. The stimulated-emission cross-section σ SE is 0.46×10 -20   cm 2 at ∼1.609  μm for E||N m . The microchip Er:KLuW laser outperforms the commercial Er,Yb:glass.

We report on the first holmium (Ho 3+ ) monoclinic double tungstate thin-disk laser. It is based on a 250 μm thick 3 at. % Ho:KY(WO 4 ) 2 active layer grown on a (010)-oriented KY(WO 4 ) 2 substrate. When pumped by a Tm-fiber laser at 1960 nm with a single-bounce (single double-pass) pump geometry, the CW Ho:KY(WO 4 ) 2 thin-disk laser generated 1.01 W at 2057 nm, corresponding to a slope efficiency η of 60% and a laser threshold of only 0.15 W. Implementing a double-bounce (second double-pass) for the pump, the output of this laser was scaled to 1.57 W with η=55%. The maximum stimulated emission cross section σ SE of the Ho 3+ ions in the epitaxial layer reaches 2.5×10 -20   cm 2 at 2056.5 nm for E‖N m . The Ho:KY(WO 4 ) 2 epitaxial structures are promising for multi-watt mode-locked thin-disk lasers at ∼2.06  μm.

Multi-watt continuous-wave (CW) operation of tetragonal rare-earth calcium aluminate Yb:CaLnAlO 4 (Ln=Gd,Y)) crystals in plano-plano microchip lasers was demonstrated with an almost quantum-defect-limited slope efficiency. Pumped at 978 nm by an InGaAs laser diode, a 3.4 mm long 8 at. % Yb:CaGdAlO 4 laser generated 7.79 W at 1057-1065 nm with a slope efficiency of η=84% (with respect to the absorbed pump power). An even higher η=91% was achieved with a 2.5 mm long 3 at. % Yb:CaYAlO 4 laser, from which 5.06 W were extracted at 1048-1056 nm. Both lasers produced linearly polarized output (σ-polarization) with an almost circular diffraction-limited beam (Mx,y2<1.1). The output performance of the developed lasers was modeled, yielding an internal loss coefficient as low as 0.004-0.007  cm -1 . In addition, their spectroscopic properties were revisited.

Monoclinic thulium-doped magnesium monotungstate, Tm 3+ :MgWO 4 , is promising for efficient power-scalable continuous-wave (CW) and passively Q-switched lasers at >2  μm. Under diode pumping at 802 nm, a compact CW laser based on Z-cut Tm:MgWO 4 generated 3.09 W at 2022-2034 nm with a slope efficiency of 50% which represents the highest output power ever achieved with this type of laser host. Stable passive Q-switching of the Tm:MgWO 4 laser is demonstrated for the first time, to the best of our knowledge, using single-walled carbon nanotubes, graphene, and Cr 2+ :ZnS saturable absorbers. Using the latter, the best performance is achieved with 16.1 μJ/13.6 ns pulses at 2017.8 nm with a maximum average output power of 0.87 W and a peak power of 1.18 kW.

Depressed-index channel waveguides with a circular and photonic crystal cladding structures are prepared in a bulk monoclinic Tm:KLu(WO 4 ) 2 crystal by 3D direct femtosecond laser writing. The channel waveguide structures are characterized and laser operation is achieved using external mirrors. In the continuous-wave mode, the maximum output power of 46 mW is achieved at 1912 nm corresponding to a slope efficiency of 15.2% and a laser threshold of only 21 mW. Passive Q-switching of a waveguide with a circular cladding is realized using single-walled carbon nanotubes. Stable 7 nJ/50 ns pulses are achieved at a repetition rate of 1.48 MHz. This first demonstration of ∼2  μm fs-laser-written waveguide lasers based on monoclinic double tungstates is promising for further lasers of this type doped with Tm 3+ and Ho 3+   ions.

A diode-pumped Yb:KLu(WO 4 ) 2 microchip laser passively Q-switched by a Cr 4+ :YAG saturable absorber generated a maximum average output power of 590 mW at 1031 nm with a slope efficiency of 55%. The pulse characteristics were 690 ps/47.6 μJ at a pulse repetition frequency of 12.4 kHz. The output beam had an excellent circular profile with M 2 <1.05. Yb:KLu(WO 4 ) 2 is very promising for ultrathin sub-ns microchip lasers.

A novel fiber-coupled indium tin oxide (ITO) coated glass slide sensor for performing surface plasmon polariton chemical monitoring in the ∼3.5  μm mid-infrared (IR) range is reported. Efficient mid-IR fiber coupling is achieved with 3D laser written waveguides, and the coupling of glass waveguide modes to ITO surface plasmon polaritons (SPPs) is driven by the varying phase matching conditions of different aqueous analytes across the anomalous dispersion range determined by their molecular fingerprints. By means of using both a mid-IR fiber supercontinuum source and a diode laser, the excitation of SPPs is demonstrated. The sensor sensitivity is tested by discriminating CH from OH features of ethanol in water solutions, demonstrating an instrumental ethanol limit of detection of 0.02% in a wide concentration range of at least 0%-50%. The efficient optical monitoring of mid-IR SPPs in smart glass could have a broad range of applications in biological and chemical sensing.

We report on the direct laser fabrication of step-index waveguides in fused silica substrates for operation in the 3.5 μm mid-infrared wavelength range. We demonstrate core-cladding index contrasts of 0.7% at 3.39 μm and propagation losses of 1.3 (6.5) dB/cm at 3.39 (3.68) μm, close to the intrinsic losses of the glass. We also report on the existence of three different laser modified SiO₂ glass volumes, their different micro-Raman spectra, and their different temperature-dependent populations of color centers, tentatively clarifying the SiO₂ lattice changes that are related to the large index changes.

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

We report on a continuous-wave Ho:KLu(WO4)2 (KLuW) microchip laser with a record slope efficiency of 84%, the highest value among the holmium inband-pumped lasers, delivering 201 mW output power at 2105 nm. The Ho laser operating at room temperature on the (5)I8→(5)I7 transition is in-band-pumped by a diode-pumped Tm:KLuW microchip laser at 1946 nm. Ho:KLuW laser operation at 2061 and 2079 nm is also demonstrated with a maximum slope efficiency of 79%. The microchip laser generates an almost diffraction-limited output beam with a Gaussian profile and a M2<1.1. The laser performance of the Ng-cut Ho:KLuW crystal is very similar for pump light polarizations ‖Nm and Np. The positive thermal lens plays a key role in the laser mode stabilization and proper mode-matching. The latter, together with the low quantum defect under in-band-pumping (∼0.08), is responsible for the extraordinary high slope efficiency.

A diode-pumped microchip laser containing a quasi-monolithic plano-plano cavity is realized on the basis of a Tm:KLu(WO₄)₂ crystal. The maximum CW output power is 3.2 W (at an absorbed pump power of 6.8 W) and the slope efficiency as high as 50.4%. The laser is operating at 1946 nm in the TEM₀₀ mode with a M²<1.05. Microchip operation with Tm:KLu(WO₄)₂ is, in principle, due to a special crystal cut along the N(g) optical indicatrix axis. This crystal cut possesses positive near-spherical thermal lens that provides the required mode stabilization in the plano-plano cavity. Sensitivity factors of the thermal lens, "generalized" thermo-optic coefficients and constants describing the photoelastic effect are determined for the monolithic Tm:KLu(WO₄)₂ crystal.

A diode-pumped thin-disk laser based on Tm:KLu(WO4)2/KLu(WO4)2 epitaxies is realized. The emission is in the 1850-1945 nm spectral range for Tm-doping between 5 and 15 at. %. The maximum slope efficiency of 47% with respect to the absorbed power obtained with 5 at. % Tm:KLu(WO4)2/KLu(WO4)2 corresponds to a maximum output power of ~6 W in cw operation.

Single-pass pumping of a thin disk consisting of an only 50 microm thick epitaxial layer of 32 at.% Yb-doped KLu(WO(4))(2) grown on a 0.35 mm thick undoped KLu(WO(4))(2) substrate is demonstrated. The thin-disk laser delivered 9 W of continuous wave output power near 1030 nm with a slope efficiency of 77%.

Mode locking of an Yb-doped bulk laser in the 1 microm spectral range using a single-walled carbon nanotube saturable absorber (SWCNT-SA) is demonstrated for the first time, to our knowledge. Passive mode locking of an Yb:KLuW laser resulted in nearly transform-limited pulses as short as 115 fs at 1048 nm. In addition, the nonlinear response of the SWCNT-SA was measured, yielding a modulation depth of 0.25% and a relaxation time of 750 fs.

We report on continuous-wave lasing of Yb(3+) at room temperature in the noncentrosymmetric RbTiOPO(4) crystal, codoped with Nb(5+), for all three possible polarizations. A maximum output power of 154 mW at 1050 nm was obtained for an absorbed power of 386 mW. The highest slope efficiency reached approximately 60% and the lowest threshold (with respect to the absorbed power) was 18 mW. The laser was tunable from 1009 to 1081 nm.