In the past years, fluoride glass fibers (ZBLAN) doped with various rare-earth ions enabled a breakthrough in the development of coherent light sources directly emitting in the visible under blue diode pumping. Holmium ions $(\text{Ho}^{3+})$ offer intense absorption around 450 nm (as the corresponding transition is spin-allowed) and enable green emission at 550 nm (the ${ }^{5} \mathrm{S}_{2}+{ }^{5} \mathrm{F}_{4} \rightarrow{ }^{5} \mathrm{I}_{8}$ transition). Li et al. reported on a low-brightness GaN diode-pumped Ho:ZBLAN laser delivering 376 mW at 544 nm, however with a low optical efficiency of ~12% [1]. High-brightness core pumping enables the development of compact laser sources with high overall optical efficiency [2]. In the present work, we report on an efficient green Ho:ZBLAN fiber laser with high-brightness diode-pumping.

We report on a continuous-wave (CW) praseodymium fluoride fiber laser delivering 10-watt level output power in the red spectral range. It employs a double-clad 0.8 mol% PrF3-doped ZBLAN fiber as a gain medium and a high-power fiber-coupled 442-nm GaN laser diode module as a pump source. The CW Pr-laser delivers 9.1 W at 635 nm with a slope efficiency of 27.0% (versus launched pump power), a laser threshold of 1.32 W, and a single-mode output. In the quasi-continuous-wave regime, its output is further scaled to 10.32 W. The spectrum broadening in this laser is explained by four-wave mixing and is well described by the square root law. This result represents a new milestone in developing visible fluoride fiber lasers. The temperature-dependent spectroscopy of Pr3+ ions in the ZBLAN glass is studied, and the limits for further power scaling of red fluoride fiber lasers are discussed.

We report on the fabrication and characterization of Dy3+-doped phosphate glass fibers for applications in yellow lasers. The fibers (core/cladding diameters: 11/125 mu m) were fabricated in the system P2O5-Al2O3-Ba2O-K2O with their core doped with 0.12 at.% Dy3+. For bulk glass, the absorption cross-section sigma(abs) is 0.11x10(-20) cm(2) at 452.4 nm (absorption bandwidth: 8.4 nm), in the range well addressed by blue GaN laser diodes. The transition probabilities for Dy3+ ions are calculated using the Judd-Ofelt theory. The stimulated-emission cross-section in the yellow sigma(SE) is 0.36x10(-20) cm(2) at 574.1 nm, and the luminescence lifetime for the F-4(9/ 2) manifold is 818 mu s, indicating a luminescence quantum efficiency close to unity. mu-Raman and mu-luminescence mapping was performed over the fiber end-facet indicating a homogeneous core composition and a uniform distribution of Dy3+ ions.

We report on the first visible orange samarium laser directly pumped by a blue 465-nm GaN semiconductor laser diode. The diode-pumped continuous-wave Sm:LiYF4 laser delivers 23.9 mW at 605 nm (4G5/2 -> 6H7/2 transition) with a slope efficiency of 9.4%, a laser threshold of 385 mW and a linear polarization (pi). We investigate the impact of pump beam quality on laser performance and revisit the polarized spectroscopic properties of Sm3+ ions in the LiYF4 crystal. The stimulated-emission cross-section sigma SE amounts to 1.27 x 10-20 cm2 at 604.8 nm for pi-polarized light, and the luminescence lifetime of the 4G5/2 level is 4.02 ms diode-pumped orange and red samarium lasers.

Currently, Praseodymium (Pr3+) doped fluoride crystals (LiYF4) and glasses (ZBLAN) are recognized for the development of multi-colour laser sources directly emitting visible radiation under pumping by blue GaN-based laser diodes or semiconductor lasers [1], [2]. Monolithic cavities employing composite gain media (i.e., those consisting of an undoped and a rare-earth doped part) benefit from reduced intracavity losses and improved thermal management. Such monolithic micro-lasers emitting in the visible are relevant for applications in biology, e.g., super-resolution microscopy, or as seed sources for laser amplifiers. We report on an efficient and power scalable diode-pumped monolithic red laser employing a composite gain medium based on Pr3+-doped LiYF4 (Pr:YLF) crystal.

We overview recent advances in visible single- and double-clad fluoride fiber lasers pumped by blue GaN laser diodes. The spectroscopic properties of ZBLAN glasses doped with Pr3+, Ho3+ and Dy3+ ions are revised. Power scalable efficient continuous-wave visible fluoride fiber lasers emitting in the green, yellow, red and deep-red spectral ranges are presented. Pumped by a single-emitter 6-W 443-nm GaN laser diode, a continuous-wave red double-clad Pr:ZBLAN fiber laser delivered 1.51 W at 634.5 nm with a slope efficiency of 31.0%, a laser threshold of 0.63 W and a spatially single-mode output (M-2 similar to 1.02). Employing a high-power fiber-coupled laser module, power scalability up to 4.61W was achieved at the expense of a lower slope efficiency of 22.8% and an increased laser threshold of 1.74 W. Green Ho:ZBLAN (543 nm) and yellow Dy:ZBLAN (575 nm) fiber lasers with high-brightness core pumping at 450 nm are also reported delivering 100 mW-level output with slope efficiencies of 31.2% and 19.6%, respectively, operating on the fundamental mode. A numerical model to predict the visible laser performance is presented and guidelines for further engineering of visible fiber laser sources are given.

We report on a low-threshold efficient yellow Dy-fiber laser with good beam quality featuring high-brightness pumping. It employs a single-clad 0.2 mol% Dy:ZBLAN fiber pumped by two 450-nm blue GaN laser diodes. The continuous-wave Dy-fiber laser delivers a maximum output power of 109 mW at 575 nm with a laser threshold of 218 mW, a slope efficiency of 19.6%, and beam quality factors M2 x,y ∼ 1.5. The overall optical efficiency versus incident pump power is 13.9%, being record-high, to the best of our knowledge, for this type of laser. The laser performance is simulated based on the spectroscopic data, being in good agreement with the experiment.

We report on polarized spectroscopy and orange laser operation under 2ω-OPSL and GaN-diode pumping of Sm:LiYF4 crystals. The Samarium laser delivers 12 mW at 605 nm with a threshold of 51 mW and a linear polarization.

A continuous-wave 442-nm blue diode-pumped double-clad Pr:ZBLAN fiber laser yields 9.1 W at 635 nm with 27.0% slope efficiency and a single-mode output and in the quasi-continuous-wave regime, it is scaled to 10.32 W.

Holmium ions (Ho3+) are attractive for generation of green emission according to the 5F4+ 5S2 → 5 I8 transition. We report on the assessment of the potential of Ho3+ -doped fluoride glasses for green fiber lasers exploiting the double-clad fiber geometry. ZBLAN glasses doped with HoF3 (0.1 – 0.9 mol%) were studied. The absorption cross-section for the 5 I8 → 5F1+ 5G6 transition is 1.75×10-20 cm2 at 448.5 nm. Under excitation in the blue, the glasses exhibit intense green luminescence. The stimulated-emission cross-section for the 5F4+ 5S2 → 5 I8 transition is 0.67×10-20 cm2 at 549 nm. With increasing the HoF3 doping level, the luminescence lifetime of the 5F4+ 5S2 states slowly decreases in the range of 291 – 180 μs. The output performance of a diode-pumped green Ho fiber laser was simulated for a fiber geometry with a double D-shaped inner cladding. The variable parameters were the HoF3 doping level, the fiber length, and the output coupling. It is shown that the generation of watt-level green output from such a laser is possible when using relatively short (<1 m) fibers with low doping levels (about 0.5 mol% HoF3), as well as high transmissions of the output coupler (<60%, depending on the passive losses in the fiber).

A Tm,Ho:LiYF4 planar waveguide grown by Liquid Phase Epitaxy is in-band pumped by a Raman fiber laser at 1679 nm. The waveguide laser generates 540 mW at 2051 nm with a slope efficiency of 32.6%.

We report on a deep-red fiber laser delivering 0.71 W at 716.7 nm with a slope efficiency of 9.0%. It is based on a Pr3+-doped ZBLAN double-clad fiber pumped by a 442-nm GaN diode.

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.

In recent years, there is a growing interest to thulium (Tm) lasers operating at ~2.3 µm according to the 3 H 4 → 3 H 5 transition [1] , [2] . Such short-wavelength infrared emission is of practical importance for sensing of atmospheric species (HF, CO, CH 4 and H 2 CO). Guillemot et al . reported on a continuous-wave (CW) Tm:KY 3 F 10 laser delivering 0.84 W at 2331-2346 nm with a slope efficiency of 53.8% [1] . Canbaz et al . developed a Kerr-lens mode-locked Tm:LiYF 4 laser emitting femtosecond (514 fs) pulses at 2303 nm [2] . However, so far, little attention was paid to passively Q-switched (PQS) Tm lasers at ~2.3 μm. In the present work, we report on a diode-pumped PQS Tm:LiYF 4 laser operating on the 3 H 4 → 3 H 5 transition and delivering stable pulsed output without any colasing at ~1.9 μm.

We report on a red praseodymium fiber laser delivering 1.07 W at 634.5 nm with a slope efficiency of 20.7% (versus the incident pump), a laser threshold of 0.55 W, and a single-mode output ( M x , y 2 < 1.1 ) in the quasi-continuous-wave regime. It is based on a 0.6 mol.% P r 3 + -doped ZBLAN double-clad fiber with a 5.5 µm core, a double D-shaped (diameters, 115/125 µm) inner cladding, and a length of 5.0 m. The fiber is pumped by a multimode 443 nm GaN diode. The laser design is optimized using a numerical model. The proposed concept is suitable for the development of diode-pumped high brightness watt-level visible praseodymium fiber lasers.

We report on highly efficient and power-scalable laser operation in a thulium-doped high-phonon-energy crystal [monoclinic double tungstate, K L u ( W O 4 ) 2 ] on the 3 H 4 → 3 H 5 T m 3 + transition giving rise to the short-wave infrared emission at ∼ 2.3 µ m . A 3 at. % Tm-doped crystal generated a maximum continuous-wave output power of 1.12 W at ∼ 2.22 and 2.29 µm with a record-high slope efficiency of 69.2% (versus the absorbed pump power), a slightly multimode beam ( M x , y 2 = 2.2 and 2.6), and a linear laser polarization. The ∼ 2.3 µ m laser outperformed the one operating on the conventional 3 F 4 → 3 H 6 transition (at ∼ 1.95 µ m ). The effect of the Tm concentration on the ∼ 2.3 µ m laser performance indicates a gradually increasing pump quantum efficiency for the 3 H 4 upper laser level with the Tm doping. For the 3 at. % Tm-doped crystal, it reached 1.8 ± 0.1 (almost two-for-one pump process), which is attributed to efficient energy-transfer upconversion. We discuss the physical nature of the laser emissions occurring at intermediate wavelengths between the electronic 3 H 4 → 3 H 5 and 3 F 4 → 3 H 6 transitions and highlight the role of electron-phonon coupling (vibronic processes) in the appearance of such laser lines. This allowed us to better understand the near- and mid-infrared emission from thulium ions, which can be used in broadly tunable and fs mode-locked 2–2.3 µm lasers.

We report on a diode-pumped Tm:LiYF4 laser operating on the H-3(4) -> H-3(5) transition (at similar to 2.3 mu m) passively Q-switched by a Cr2+:ZnSe saturable absorber. This laser delivers a maximum average output power of 130 mW at 2304.6 nm with a nearly diffraction limited beam, a linear polarization (pi) and no colasing at similar to 1.9 mu m. The corresponding pulse characteristics (duration/energy) are 1.24 mu s/3.6 mu J at a repetition rate of 36 kHz. By power scaling under quasi-CW pumping, even shorter pulse durations of 870 ns and higher pulse energies of 6.1 mu J are achieved. The performance of the laser is simulated using a model of a quasi-four-level gain medium and a "slow" saturable absorber showing a good agreement with the experiment. The effect of the thermal lens of the saturable absorber on the pulse characteristics is discussed.