Kerr-lens mode-locked 2.3-μm Tm3+:YLF laser as a source of femtosecond pulses in the mid-infrared

High average output power, dual-wavelength synchronously self-mode-locked Ho:LLF laser operating at 2068.5 and 2069.2 nm

A dual-wavelength synchronously self-mode-locked Ho:LLF laser operating at 2068.5 and 2069.2 nm was demonstrated. The maximum average output power was as high as 2.6 W with a pulse repetition frequency of 3.03 GHz. Meanwhile, the output power ratio of the dual-wavelength lasers can be effectively controlled by varying the incident pump power. To the best of our knowledge, this is the first dual-wavelength synchronously self-mode-locked Ho-doped fluoride solid state laser; moreover, our current experimental results represent the highest average output power from a GHz self-mode-locked oscillator in the 2 µm wave band.

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.

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.

Demonstration of a 1 TW peak power, joule-level ultrashort Tm:YLF laser

We report on the demonstration of a diode-pumped, Tm:YLF-based, chirped pulse amplification laser system operating at λ ≈ 1.9 µm that produces amplified pulse energies exceeding 1.5 J using a single 8-pass power amplifier. The amplified pulses are subsequently compressed to sub-300 fs durations by a diffraction grating pair, producing record >1 TW peak power pulses. To the best of our knowledge, this is the highest peak power demonstrated for any solid-state, near-2 µm laser architecture and illustrates the potential of Tm:YLF for the next generation of high-power, diode-pumped ultrashort lasers.

Broadly tunable continuous wave 2.3-µm Tm3+:tellurite bulk glass laser

We report, for the first time to the best of our knowledge, a continuous wave trivalent thulium ion (Tm3+)-doped bulk glass near 2.3 µm. In the experiments, a bulk Tm3+-doped tellurite glass with the stoichiometric composition of 74TeO2-12ZnO-4La2O3-10Na2O (Tm3+:TZLN) was used. Lasing operation was achieved by using an x-fold cavity at the free-running wavelength of 2303 nm. The maximum slope efficiency of 6.2% was obtained with respect to the absorbed pump power with a 1% transmitting output coupler. In this case, as high as 100-mW output power was generated with 2.2 W of absorbed pump power. Continuous, broad tuning was achieved from 2233 nm to 2400 nm. The excitation spectrum of the laser was also investigated and 2.3-µm lasing was obtained by varying the pump wavelength over the 773-809-nm range. The absorption cross section was determined to be 4.4 × 10-21 cm2, based on open-aperture z-scan measurement. By using the laser efficiency data, the emission cross section of the Tm3+:TZLN glass was further determined to be 1.3 × 10-20 cm2 at 2.3 µm.

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.

Continuous-wave Tm:YLF laser with ultrabroad tuning (1772-2145 nm)

We report detailed experimental data aiming for rigorous investigation of Tm:YLF laser performance, especially with a focus on tuning behavior. Continuous-wave (cw) lasing performance of Tm:YLF crystals with thulium dopings in the 2-6% range is investigated under diode and Ti:Sapphire pumping at 792 nm and 780 nm, respectively. While employing the c-axis, we have achieved cw lasing thresholds below 20 mW, laser output power up to 1.42 W, and laser slope efficiencies as high as 70% with respect to absorbed pump power. The passive loss of the Tm:YLF crystal is estimated to be as low as 0.05% per cm, corresponding to a crystal figure of merit above 10000. Via employing this low-loss crystal and a 2-mm thick off-surface optical axis birefringent filter (BRF) with strong sideband rejection, a record cw tuning range covering the 1772-2145 nm interval is demonstrated (except a small gap between 1801-1815nm region). Detailed lifetime and emission cross section measurements have been performed to explain the observed performance, and strategies for further performance enhancement are discussed.

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.

Achievements and Future Perspectives of the Trivalent Thulium-Ion-Doped Mixed-Sesquioxide Ceramics for Laser Applications

This paper is devoted to reviewing the latest results achieved in solid-state lasers based on thulium-doped mixed-sesquioxide ceramics, i.e., (Lu,Sc,Y)2O3. The near- and mid-infrared regions are of interest for many applications, from medicine to remote sensing, as they match molecular fingerprints and cover several atmospheric transparency windows. These matrices are characterized by a strong electron-phonon interaction-which results in a large splitting of the ground state-and by a spectral broadening of the optical transition suitable for developing tunable and short-pulse lasers. In particular, the manuscript reports on the trivalent thulium laser transitions at 1.5, 1.9, and 2.3 µm, along with the thermal and optical characteristics of the (Lu,Sc,Y)2O3 ceramics, including the fabrication techniques, spectroscopic and optical properties, and laser performances achieved in different pumping regimes, such as continuous-wave (CW), quasi-CW, and pulsed modes. A comparison of the performance obtained with these mixed-sesquioxide ceramics and with the corresponding crystals is reported.

Nonlinear optical response of inverse-designed integrated photonic devices

Gradient-based optimization combined with the adjoint method has been demonstrated to be an efficient way to design a nano-structure with a vast number of degrees of freedom. However, most inverse-designed photonic devices are applied as linear photonic devices. Here, we demonstrate the nonlinear optical response in inverse-designed integrated splitters fabricated on a SiN platform. The splitting ratio is tunable under different incident powers. The thermo-optical effect can be used as an effective approach for adjusting the nonlinear optical response threshold and modulation depth of the device. These promising results indicate the great potential of inverse-designed photonic devices in nonlinear optics and optical communications.

Kerr-lens mode-locked Tm-doped sesquioxide ceramic laser

Kerr-lens mode-locked solid-state laser operation at ∼2µm is investigated. Using a Tm³⁺-doped (Lu,Sc)₂O₃ "mixed" sesquioxide ceramic as a gain medium, pulses as short as 58 fs are generated at ∼2081nm via soft-aperture Kerr-lens mode locking. The average output power amounts to 220 mW at a pulse repetition rate of 84.8 MHz. The emitted spectrum at the long-wavelength wing extends to >2.2µm which is attributed to vibronic transitions of the Tm³⁺ ions. The latter is found to be essential for generating pulses with durations in the 50 fs range.

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.

Upconversion pumping of a 2.3 µm Tm3+:KY3F10 laser with a 1064 nm ytterbium fiber laser

We report efficient lasing of the isotropic ${{\rm Tm}^{3 + }}\!:\!{{\rm KY}_3}{{\rm F}_{10}}$Tm³⁺:KY₃F₁₀ crystal near 2.3 µm via upconversion pumping with a 1064 nm ytterbium fiber laser as the pump source. When pumped at 1064 nm, an x-cavity ${{\rm Tm}^{3 + }}\!:\!{{\rm KY}_3}{{\rm F}_{10}}$Tm³⁺:KY₃F₁₀ laser operated at the free-running wavelength of 2344 nm. Lasing was obtained with output couplers having transmissions in the range of 1-3%, and as high as 124 mW of continuous-wave (cw) output power was generated with 604 mW of absorbed pump power by using the 3% output coupler. Broadly tunable cw lasing could be obtained in the 2268-2373 nm wavelength range. An analysis of the experimental power efficiency data shows that nearly all of the absorbed pump photons were converted to 2.3 µm laser output after accounting for the quantum defect of the laser transition and resonator losses. We expect that higher lasing efficiency should be possible by using longer crystals to increase the pump absorption.

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

We report on Czochralski growth, detailed ground- and excited-state absorption and emission spectroscopy and highly-efficient mid-infrared (∼2.3 µm) laser operation of a cubic potassium yttrium fluoride crystal, Tm:KY₃F₁₀. The peak stimulated-emission cross-section for the ³H₄ → ³H₅ transition is 0.34×10^-20 cm² at 2345 nm with an emission bandwidth exceeding 50 nm. The excited-state absorption spectra for the ³F₄ → ³F_2,3 and ³F₄ → ³H₄ transitions are measured and the cross-relaxation is quantified. A continuous-wave 5 at.% Tm:KY₃F₁₀ laser generated 0.84 W at 2331-2346 nm by pumping at 773 nm, with a record-high slope efficiency of 47.7% (versus the incident pump power) owing to the efficient action of energy-transfer upconversion leading to a pump quantum efficiency approaching 2. The first Tm:KY₃F₁₀ laser with ESA-assisted upconversion pumping (at 1048 nm) is also demonstrated. Due to its broadband emission properties, Tm:KY₃F₁₀ is promising for ultrashort pulse generation at ∼2.3-2.4 µm.

Graphene mode-locked operation of Tm3+:YLiF4 and Tm3+:KY3F10 lasers near 2.3 µm

We report experimental demonstration of graphene mode-locked operation of ${{\rm Tm}^{3 + }}\!:\!{{\rm YLiF}_4}$Tm³⁺:YLiF₄ (YLF) and ${{\rm Tm}^{3 + }}\!:\!{{\rm KY}_3}{{\rm F}_{10}}$Tm³⁺:KY₃F₁₀ (KYF) lasers near 2.3 µm. To scale up the intracavity pulse energy, the cavity was extended, and double-end pumping was employed with a continuous-wave, tunable ${{\rm Ti}^{3 + }}\!:\!{\rm sapphire}$Ti³⁺:sapphire laser delivering up to 1 W near 780 nm. The extended ${{\rm Tm}^{3 + }}\!:\!{\rm KYF}$Tm³⁺:KYF laser cavity was purged with dry nitrogen to eliminate pulsing instabilities due to atmospheric absorption lines, but this was not needed in the case of the ${{\rm Tm}^{3 + }}\!:\!{\rm YLF}$Tm³⁺:YLF laser. Once initiated by graphene, stable uninterrupted mode-locked operation could be maintained with both lasers. With the extended cavity ${{\rm Tm}^{3 + }}\!:\!{\rm YLF}$Tm³⁺:YLF laser, 921 fs pulses were generated at a repetition rate of 17.2 MHz at 2304 nm. 739 fs pulses were obtained at the repetition rate of 54 MHz from the ${{\rm Tm}^{3 + }}\!:\!{\rm KYF}$Tm³⁺:KYF laser at 2340 nm. The corresponding pulse energy and peak power were 2.4 nJ and 2.6 kW for the ${{\rm Tm}^{3 + }}\!:\!{\rm YLF}$Tm³⁺:YLF laser, and 1.2 nJ and 1.6 kW for the ${{\rm Tm}^{3 + }}\!:\!{\rm KYF}$Tm³⁺:KYF laser. We foresee that it should be possible to generate shorter pulses at higher pump levels.

High power simultaneous dual-wavelength CW and passively-Q-switched laser operation of LD pumped Tm:YLF at 1.9 and 2.3 µm

Simultaneous dual-wavelength laser oscillation of Tm:YLF at 1.9 and 2.3 µm were successfully realized. The three-mirror cavity was exploited to study dual-wavelength laser performance, which is formed by a shared input mirror and two independent output couplers for the two laser wavelengths. Under an absorbed pump power of 15.2 W, the maximum CW output powers of 5.49 W around 1908nm and 1.12 W around 2305 nm were simultaneously obtained, corresponding to a total optical-to-optical conversion efficiency of 43.5%. A Cr²⁺:ZnSe was further used to passively Q-switch the dual-wavelength laser, generating pulses with pulse widths of 554 ns at 1.9 µm and 4 µs at 2.3 µm. To the best of our knowledge, this is the first report on the dual-wavelength laser operation of Tm³⁺-single-doped solid-state laser at 1.9 and 2.3 µm. The higher optical conversion efficiency, smaller wavelength competition effect and simultaneous dual-wavelength output make this Tm-doped solid-state laser have potential application in medical surgery.

Watt-level passively mode-locked Tm:YLF laser at 1.83 µm

Passive, continuous-wave mode-locked (CWML) operation of a 1.83 µm Tm:YLF laser is experimentally demonstrated for the first time, to the best of our knowledge. Two specially selected output couplers are used to realize this operation. Stability of the CWML laser is obtained with a commercial semiconductor saturable absorber mirror. The maximum average output power is 1.04 W with a pulse duration of 107 ps and repetition rate of 54.1 MHz. Further, a 0.1 mm fused-quartz Fabry-Perot etalon is used to tune the central wavelength of the stable CWML laser at 1827.2 nm, 1829.5 nm, 1831.9 nm, and 1833.5 nm.

Continuous-wave Tm:YAlO3 laser at ∼2.3 μm

The orthorhombic Tm³⁺:YAlO₃ crystal is promising for laser operation at the H₄3→F₄3 (1.5 μm) and H₄3→H₅3 (2.3 μm) transitions. Stimulated-emission cross-sections for these transitions are determined with polarized light. Peak values of 0.59×10^-20  cm² and 0.80×10^-20  cm² are found at 1438 nm and 2275 nm, respectively, both for E∥b. The cross-relaxation defining the upper-laser level lifetime is quantified. Continuous-wave lasing at the H₄3→H₅3 transition is achieved with an a-cut 1.5 at.% Tm:YAlO₃ crystal. The laser generated 254 mW at 2273 nm with a slope efficiency of 17.8% and linear polarization (E∥b).

Thulium laser at ∼2.3 μm based on upconversion pumping

We report on novel upconversion (UC) pumping schemes for 2.3 μm thulium (Tm) lasers (the H₄3→H₅3 transition) based on a photon avalanche mechanism populating the intermediate metastable level (F₄3) acting as an effective ground state. The proposed pump wavelengths are ∼1 and ∼1.5  μm, each one corresponding to a resonant excited-state absorption transition F₄3→F_2,33 and F₄3→H₄3, respectively. UC pumping at 1040, 1055, and 1451 nm of 2.3 μm Tm:LiYF₄ lasers is demonstrated. In the former case, the laser generates 102 mW at 2302 nm with a slope efficiency of 14.6% (versus the incident pump power). The laser dynamics is studied. UC pumping is promising for reaching high efficiencies in 2.3 μm Tm lasers.

Continuous-wave mid-infrared laser operation of Tm3+:KY3F10 at 2.3 μm

We report continuous-wave lasing demonstration of a Tm³⁺:KY₃F₁₀ crystalline gain medium at 2343 nm. A narrow-linewidth, tunable, continuous-wave Ti³⁺:sapphire laser was used to end pump a Tm³⁺:KY₃F₁₀ crystal with thulium doping of 8 at. %. With 1 W of pump power, the resonator with a 1% output coupler generated output powers of 31 and 122 mW in single-pumping and double-pumping configurations, respectively. Excitation efficiency of the laser was investigated for pump wavelengths in the 765-806 nm range. Four pump bands were identified in this wavelength range, with the two most efficient pump bands centered at 773 and 778 nm. The output of the laser could be tuned smoothly and continuously over a spectral width of 125 nm from 2260 to 2385 nm. The lifetime of the H₄3 level was further measured to be 16 μs, and the emission cross section was determined based on lasing threshold measurements.

Efficient Tm:YAG and Tm:LuAG lasers pumped by 681 nm tapered diodes

In this paper, we present highly efficient continuous-wave (cw) laser operation of Tm:YAG and Tm:LuAG lasers pumped by high-brightness red tapered diodes. The single-emitter tapered diode lasers (TDLs) provide up to 1 W of pump power around 680 nm. By adjusting the operation temperature of the TDL, the pump central wavelength could be matched to the strong absorption peak of Tm³⁺ ions in this region (H₆3→F3₃ excitation). This absorption peak is around threefold stronger than the usually employed 785 nm transition (H₆3→H3₄). In the cw laser experiments, we have achieved slope efficiencies exceeding 55% at room temperature, which is far above the Stokes limited slope efficiency (34%), indicating presence of a strong two-for-one cross-relaxation process. Pumping with high-brightness tapered diode lasers further facilitated usage of smaller pump spots (enabling quite low lasing thresholds) and generation of near-diffraction limited output beam profiles from standard z-type cavities. To the best of our knowledge, this is the first report of diode pumping of Tm-doped solid-state lasers around 680 nm as well as the first usage of TDLs as pump sources in Tm-doped laser systems.

Sub-80 fs mode-locked Tm,Ho-codoped disordered garnet crystal oscillator operating at 2081 nm

We demonstrate a mode-locked (ML) femtosecond laser based on the disordered garnet crystal Tm,Ho:CNGG. Employing a single-walled carbon nanotube saturable absorber, pulses as short as 83 and 76 fs at 2081 nm are achieved without and with external compression, respectively. The latter represents, to the best of our knowledge, the shortest pulse duration obtained from any Ho-doped or Tm,Ho-codoped laser. The average power amounts to 67 mW at a repetition rate of 102 MHz. By analyzing the soliton ML regime, the nonlinear refractive index of Tm,Ho:CNGG is estimated to be ∼1.1×10^-19  m²/W.