Close look on cubic Tm:KY3F10 crystal for highly efficient lasing on the 3H4 → 3H5 transition

Crystal growth, polarized spectral properties, and 2.3 µm laser performance of Tm:LuVO4 crystal

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 → 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 (12 K) spectroscopy. The maximum stimulated-emission cross-section for the 3H4 → 3H5 transition is 2.48 × 10-20 cm2 at 2363 nm for π-polarization corresponding to an emission bandwidth of 28 nm. Evidence of phonon-assisted emissions of Tm3+ ions above 2 µm is presented. The broadband emission properties of the Tm:LuVO4 crystal make it promising for ultrashort pulse generation. Additionally, pumped by a 796 nm fiber-coupled laser diode, the Tm:LuVO4 laser generated a Watt-level output power at 2279-2295 nm with a slope efficiency of 9.2% and linearly polarized emission (π-polarization).

4.1 W continuous-wave and broadly wavelength tunable Tm-doped laser in 2.1-2.4 µm spectral region based on vibronic and electronic transitions

Efficient diode-pumped continuous-wave (CW) and wavelength tunable Tm:YAP lasers based on the vibronic and electronic transitions are investigated. A total maximum output power of 4.1 W is achieved with multi-wavelength output around 2162 nm and 2274 nm, corresponding to a slope efficiency of 29.8% for a 3 at. % Tm:YAP crystal. A maximum output power of 2.48 W with a slope efficiency of 25.4% is obtained at 2146 nm for a 4 at. % Tm:YAP crystal. Using a birefringent filter (BF), the emission wavelengths of the Tm:YAP laser are tuned over spectral ranges of 59 nm from 2115 nm to 2174 nm and 127 nm from 2267 nm to 2394 nm, respectively, which is the first demonstration of wavelength tunable Tm:YAP laser based on the electronic transition 3H4→3H5 and vibronic transition 3F4→3H6, to the best of our knowledge. The results show great potentials of the Tm:YAP crystal for realizing efficient lasers in the spectral range of 2.1-2.4 µm.

Polarized spectral properties and 2.3 µm laser performance of the Tm:YVO4 crystal

The polarized spectral properties and ∼2.3 µm high-power continuous-wave laser operation of Tm3+-doped yttrium orthovanadate crystal (Tm:YVO4) are reported. For the 3H4 → 3H5 transition, the stimulated-emission cross-section σSE is 1.01 × 10-20 cm2 at 2276 nm corresponding to a large emission bandwidth of 52 nm (for π-polarization). Pumped by a 794 nm laser diode, the 1.5 at.% Tm:YVO4 laser delivered 5.52 W at 2.29 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 → 3H5 and 3F4 → 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 2.3 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 706 ns and 3.65 µJ, respectively, corresponding to a pulse repetition rate of 62.8 kHz.

Upconversion-pumped femtosecond thulium laser at 2309 nm mode-locked by a GaSb-based SESAM

We report on a femtosecond thulium laser operating on the 3H4 → 3H5 transition with upconversion pumping around 1 µm and passively mode-locked by a GaSb-based SEmiconductor Saturable Absorber Mirror (SESAM). This laser employs a 6 at.% Tm:LiYF4 laser crystal and a polarization maintaining Yb-fiber master oscillator power amplifier at 1043 nm as a pump source addressing the 3F4 → 3F2,3 excited-state absorption transition of Tm3+ ions. In the continuous-wave regime, the Tm-laser generates 616 mW at ∼2313 nm with a slope efficiency of 10.0% (vs. the incident pump power) and a linear polarization (π). By implementing a type-I SESAM with a single ternary strained In0.33Ga0.67Sb quantum well embedded in GaSb for sustaining and stabilizing the soliton pulse shaping, the self-starting mode-locked Tm-laser generated pulses as short as 870 fs at a central wavelength of 2309.4 nm corresponding to an average output power of 208 mW at a pulse repetition rate of 105.08 MHz and excellent mode-locking stability. The output power was scaled to 450 mW at the expense of a longer pulse duration of 1.93 ps. The nonlinear parameters of the SESAM are also reported.

Cascade laser optimization for 3H4 → 3H5 and 3F4 → 3H6 sequent transitions in Tm3+-doped materials

We study a cascade laser scheme involving the 3H4 → 3H5 and 3F4 → 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 3H4→ 3H5 and 3F4→ 3H6 transitions versus the Tm3+ doping level is finally extended to other well-known Tm3+-doped laser materials.

Efficient 1.7 µm pumping of 2 µm thulium lasers

Solid-state 2 µm lasers based on thulium-doped active media Tm:YAG, Tm:YAP, and Tm:YLF were investigated under 1.7 µm resonant diode pumping. In contrast with standard 0.8 µm pump wavelength, a high slope efficiency was achieved, up to 80% in the case of Tm:YAP and Tm:YLF, nearing a quantum limit without relying on Tm3+-Tm3+ cross-relaxation energy transfer. Low thermal load allowed for stable continuous-wave operation with good beam quality and output power up to 6 W (Tm:YAG, Tm:YLF), and 8 W (Tm:YAP).

Diode-pumped efficient high-power cascade Tm:GdVO4 laser simultaneously operating at ∼2 μm and ∼2.3 μm

The laser diode (LD)-pumped efficient high-power cascade Tm:GdVO4 laser simultaneously operating on the 3F4 → 3H6 (at ∼2 μm) and 3H4 → 3H5 (at ∼2.3 μm) Tm3+ transition was first reported in this paper. The cascade Tm:GdVO4 laser generated a maximum total continuous-wave (CW) laser output power of 8.42 W with a slope efficiency of 40%, out of which the maximum ∼2.3 μm CW laser output power was 2.88 W with a slope efficiency of 14%. To our knowledge, 2.88 W is the highest CW laser output power amongst the LD-CW-pumped ∼2.3 μm Tm3+-doped lasers reported so far.

Tm:CALGO lasers at 2.32 µm: cascade lasing and upconversion pumping

We report on the first laser operation of a disordered Tm:CaGdAlO₄ crystal on the ³H₄ → ³H₅ transition. Under direct pumping at 0.79 µm, it generates 264 mW at 2.32 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 → ³H₅ and ³F₄ → 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 585 mW at 1.77 µm (³F₄ → ³H₆) and 2.32 µm (³H₄ → ³H₅) with a higher slope efficiency of 28.3% and a lower laser threshold of 1.43 W, out of which 332 mW are achieved at 2.32 µm. Under dual-wavelength pumping, further power scaling to 357 mW at at 2.32 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 → ³F_2,3 and ³F₄ → 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.

Excited-state absorption and upconversion pumping of Tm3+-doped potassium lutetium double tungstate

We report on a bulk thulium laser operating on the ³H₄ → ³H₅ transition with pure upconversion pumping at 1064 nm by an ytterbium fiber laser (addressing the ³F₄ → ³F_2,3 excited-state absorption (ESA) transition of Tm³⁺ ions) generating 433 mW at 2291 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 2.3 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 1.06 µm are also explored, indicating a positive effect of co-pumping at 0.79 µm on reducing the threshold pump power for upconversion pumping.

Cascade lasing at ∼2 μm and ∼2.3 μm in a diode-pumped Tm:YVO4 laser

We report on the cascade continuous-wave operation of a diode-pumped Tm:YVO₄ laser on the ³F₄ → ³H₆ (at ∼2 μm) and ³H₄ → ³H₅ (at ∼2.3 μm) Tm³⁺ transitions. Pumped with a fiber-coupled spatially multimode 794 nm AlGaAs laser diode, the 1.5 at.% Tm:YVO₄ laser yielded a maximum total output power of 6.09 W with a slope efficiency of 35.7% out of which the ³H₄ → ³H₅ laser emission corresponded to 1.15 W at 2291-2295 and 2362-2371 nm with a slope efficiency of 7.9% and a laser threshold of 6.25 W.

Compact diode-pumped continuous wave and passively Q switched Tm:YAG laser at 2.33 µm

Compact diode-pumped continuous wave (CW) and passively Q switched Tm:YAG lasers operating on the ³H₄ → ³H₅ transition are demonstrated. Using a 3.5-at.% Tm:YAG crystal, a maximum CW output power of 1.49 W is achieved at 2330 nm with a slope efficiency of 10.1%. The first Q switched operation of the mid-infrared Tm:YAG laser around 2.3 µm is realized with a few-atomic-layer MoS₂ saturable absorber. Pulses as short as 150 ns are generated at a repetition rate of 190 kHz, corresponding to a pulse energy of 1.07 µJ. Tm:YAG is an attractive material for diode-pumped CW and pulsed mid-infrared lasers emitting around 2.3 µm.

Watt-level diode-pumped Tm:YVO4 laser at 2.3 µm

In this Letter, a watt-level laser diode (LD)-pumped ∼2.3-µm (on the ³H₄→³H₅ quasi-four-level transition) laser is reported based on a 1.5 at.% a-cut Tm:YVO₄ crystal. The maximum continuous wave (CW) output power obtained is 1.89 W and 1.11 W with the maximum slope efficiency of 13.6% and 7.3% (versus the absorbed pump power) for the 1% and 0.5% transmittance of the output coupler, respectively. To the best of our knowledge, the CW output power of 1.89 W we obtained is the highest CW output power amongst the LD-pumped ∼2.3-µm Tm³⁺-doped lasers.

Excited-state absorption in thulium-doped materials in the near-infrared

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 → ³F_2,3 (at ∼1 µm) and ³F₄ → ³H₄ (at ∼1.5 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.

Dual-wavelength-pumping of mid-infrared Tm:YLF laser at 2.3 µm: demonstration of pump seeding and recycling processes

Upconversion pumping of thulium lasers emitting around 2.3 µm (the ³H₄ → 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 → ³F_2,3 loop was also observed. The pump absorption seeding is a viable route for the development of low-threshold upconversion pumped thulium lasers.

Third-order optical nonlinearity measurements and optical limiting experiment in Tm: YAG crystal

In this paper, we investigate the third-order nonlinearities and optical limiting effect of Tm: YAG crystal at a wavelength of 1064 nm. We experimentally measure different energy densities (6.4, 12.8, and 19.2J/cm²) and obtain the nonlinear absorption coefficient, nonlinear refractive index, and third-order nonlinear susceptibility of Tm: YAG crystal. Z-scan results show that Tm: YAG crystal exhibits a large nonlinear absorption coefficient (3.34×10^-9m/W) at the wavelength of 1064 nm. We also measure the transmittance of Tm: YAG crystals of three different lengths (7, 15, and 20 mm) to evaluate its nonlinear optical limiting performance. For the 20 mm Tm: YAG crystal, the maximum transmittance without optical limiting effect and minimum transmittance with nonlinear optical limiting effect at a 1064 wavelength nm are 84.2% and 47.8%, respectively, which indicates that Tm: YAG crystal may be a solid material for nonlinear optical limiting at 1064 nm.

Er:KY3F10 laser at 2.80 µm

We report on the mid-infrared laser operation of a cubic 15 at.% Er³⁺:KY₃F₁₀ crystal. In the quasi-continuous-wave regime, the peak power reaches 255 mW at 2.80 µm (the ⁴I_11/2→⁴I_13/2 transition) with a slope efficiency of 10.9% and a laser threshold of 58 mW. Two pumping schemes (to the ⁴I_11/2 and ⁴I_9/2 states) are compared. The emission properties of the Er³⁺ ions in KY₃F₁₀ are studied, indicating high stimulated-emission cross-section of 0.57×10^-20cm² at 2.80 µm, a large gain bandwidth of 40 nm, and a long ⁴I_11/2 state lifetime of 4.64 ms.

Growth Peculiarities and Properties of KR3F10 (R = Y, Tb) Single Crystals

Cubic KR3F10 (R = Y, Tb) single crystals have been successfully grown using the Bridgman technique. Growth of crystals of this type is complicated due to the hygroscopicity of potassium fluoride and melt overheating. The solution to the problem of oxygen-incorporated impurities has been demonstrated through the utilization of potassium hydrofluoride as a precursor. In this study, the crystal quality, structure features, and optical, thermal and electrophysical properties of KR3F10 were examined. Data on the temperature dependences of conductivity properties of KTb3F10 crystals were obtained for the first time. These crystals indicated thermal conductivity equal to 1.54 ± 0.05 Wm−1K−1 at room temperature caused by strong phonon scattering in the Tb-based crystal lattice. Ionic conductivities of KY3F10 and KTb3F10 single crystals were 4.9 × 10−8 and 1.2 × 10−10 S/cm at 500 K, respectively, and the observed difference was determined by the activation enthalpy of F− ion migration. Comparison of the physical properties of the grown KR3F10 crystals with the closest crystalline analog from the family of Na0.5−xR0.5+xF2+2x (R = Tb, Y) cubic solid solutions is reported.

Watt-level efficient 2.3 µm thulium fluoride fiber laser

We report on an efficient mid-infrared thulium (Tm) fiber laser operating on the ³H₄→³H₅ transition and featuring an upconversion pumping scheme. This laser comprises a heavily Tm³⁺-doped (2.50 mol. %) zirconium fluoride glass fiber pumped by a tunable Yb fiber laser around 1.05 µm corresponding to the ³F₄→³F_2,3 excited-state absorption transition. The laser generates 1.24 W at 2269-2282 nm with a slope efficiency of 37% in the quasi-continuous-wave regime. The Tm-glass fiber exhibits a broadband ³H₄→³H₅ emission with a bandwidth of 173 nm, making it very promising for femtosecond fiber oscillators at ∼2.3µm.

Watt-level diode-pumped thulium lasers around 2.3 µm

We report on efficient diode-pumped mid-infrared lasers based on Tm:LiYF₄, Tm:Y₃Al₅O₁₂, and Tm:YAlO₃ crystals. These lasers operate in the continuous-wave (CW) regime and deliver watt-level output power at the wavelengths of 2.2-2.3 µm (the ³H₄→³H₅Tm³⁺ transition). In particular, a 1.8 at. % Tm:YAlO₃ laser pumped at 789 nm generates a maximum CW output power of 1.32 W at 2272-2277 nm with a slope efficiency of 33.1% (with respect to the absorbed pump power), a linear laser polarization (E∥b), and a fundamental transverse output mode (the measured Mx2=1.26, My2=1.68). In the quasi-CW regime, the output peak power is scaled up to 2.69 W. The pump quantum efficiency and the fractional heat loading are estimated and discussed.

Tunable continuous-wave laser operation of Tm3+:BaY2F8 near 2.3 µm

We report, for the first time to our knowledge, tunable continuous-wave laser action in the Tm³⁺:BaY₂F₈ (BYF) crystal near 2.3 µm. In the experiments, a BYF crystal doped with 3 at. % thulium was end pumped with a narrow-linewidth, tunable Ti³⁺:sapphire laser with up to 920 mW of incident power. Lasing was achieved for the two pump polarizations of E//x and E//y. The best power performance was obtained in the case of E//x, double-end pumping, where 100 mW of output power was obtained at 2290 nm with 920 mW of pump power and 1% output coupler. The laser could be continuously tuned from 2233 to 2385 nm. Excitation spectra for E//x and E//y pumping were measured in the 760-810 nm range, and the optimum pumping wavelength was determined to be 779 nm for E//x. By using the lifetime and lasing threshold data, the stimulated emission cross section at 2290 nm was further determined to be (0.66±0.06)×10^-24m².