Rare-earth ions-doped mid-infrared(2.7-3 μm)bulk lasers:a review[Invited]

Mid-infrared (MIR) laser sources operating in the 2.7–3μm spectral region have attracted extensive attention for many applications due to the unique features of locating at the atmospheric transparency window,corresponding to the"characteristic fingerprint"spectra of several gas molecules,and strong absorption of water.Over the past two decades,significant developments have been achieved in 2.7–3μm MIR lasers benefiting from the sustainable innovations in laser technology and the great progress in material science.Here,we mainly summarize and review the recent progress of MIR bulk laser sources based on the rare-earth ions-doped crystals in the 2.7–3μm spectral region,including Er3+-,Ho3+-,and Dy3+-doped crystalline lasers.The outlooks and challenges for future development of rare-earth-doped MIR bulk lasers are also discussed.

Tm^(3+) and Nd^(3+) singly doped LiYF_4 single crystals with 3-5 μm mid-infrared luminescence

Mid-infrared（MIR） emissions of 2.4 and 3.5 μm from Tm3＋：LiYF4 single crystals attributed to3H4 →3H5 and3H5 →3F4 transitions as well as MIR emissions of 4.2,4.3,and 4.5 μm from Nd3＋：LiYF4 lasers attributed to4I15/2 →4I13/2,4I13/2 →4I11/2,and4I11/2 →4I9/2 transitions,respectively,are observed.LiYF4 single crystals possess high transmittance of over 85% in the 2.5-6 μm range.The large emission crosssections of Tm-doped crystals at 2.4 μm（1.9×10-20cm2） and Nd-doped crystals at 4.2 μm（0.84×10-20 cm2） as well as the high rare-earth doping concentrations,excellent optical transmission,and chemicalphysical properties of the resultant samples indicate that Nd3＋and Tm3＋singly doped crystals may be promising materials for application in MIR lasers.

Enhanced 1-5 μm near-and mid-infrared emission in Ho/Ybcodoped TeO-ZnF oxyfluorotellurite glasses

Intense 1-5 μm infrared emission from near-to mid-infrared was obtained from Ho3+/Yb3+codoped TeO2-ZnF2 oxyfluorotellurite glasses which were prepared by melt-quenching method under the 980 nm LD excitation,and the emission intensity can be enhanced with the increase of ZnF2 content.Judd-Ofelt analysis was used to evaluate the radiative transition parameters of the excited levels according to the absorption spectra.The stimulated emission cross section of 5 I6→5 I8(1.2 μm),5 I7→5 I8(2.0μm),5 I6→5 I7(2.85 μm) and 5 I5→5 I6(4.0 μm) transitions were calculated to reach 0.639 × 10-20,0.760 ×10-20,0.985×10 20 and 0.484 × 10-20 cm2,respectively.The energy transfer coefficients(CDA) are enhanced with the increase of ZnF2 content and phonon contribution ratios of phonon assisted energy transfer process between Ho3+ and Yb3+were figured out.Our results demonstrate that these TeO2-ZnF2 glasses,which possess good thermal stability and transparency,low phonon energy(about 600 cm-1),excellent near-and mid-infrared emission in the range of 1-5 μm wavelength,would be promising material for infrared optical fibers and infrared lasers.

GSA and ESA dual-wavelength pumped 2.3μm Tm:YLF laser on the ^(3)H_(4)→^(3)H_(5) transition

The population trapping effect of the 3F4 level is an important factor limiting the power scaling of the 2.3 μm thulium(Tm)laser on the 3H4→3 H5 transition.In this Letter,we demonstrate a novel scheme of ground state absorption(GSA)(3 H6→3H4) and excited state absorption(ESA)(3F4→3H4] dual-wavelength pumped 2.3 μm Tm lasers.Introducing an ESA pumping process can accurately excite the Tm3+ ions accumulated in the 3F4 level to the 3H4 level,constructing a double populating mechanism for the upper laser level 3H4.A proof-of-principle experimental demonstration of the GSA(785 nm) and ESA(1470 nm) dual-wavelength pumped 2.3 μm Tm:LiYF4(Tm:YLF) laser was realized.A maximum continuous-wave output power of 1.84 W at 2308 nm was achieved under 785 and 1470 nm dual-wavelength pumping,increased by60% compared with the case of 785 nm single-wavelength pumping under the same resonator condition.Our work provides an efficient way to achieve higher output power from 2.3 μm Tm-doped lasers on the 3H4→3 H5 transition.