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 Er~(3+)-,Ho~(3+)-,and Dy~(3+)-doped crystalline lasers.The outlooks and challenges for future development of rare-earth-doped MIR bulk lasers are also discussed.     

Broadband 1.5-um emission of erbium-doped TeO_2-WO_3-Nb_2O_5 glass for potential WDM amplifier

Erbium-doped glass showing the wider 1.5-um emission band is reported in a novel oxide system TeO2-WO3-Nb2O5 and their thermal stability and optical properties such as absorption, emission spectra, cross-sections and fluorescence lifetime were investigated. Compared with other glass hosts, the gain bandwidth properties of Er3+ in TWN glass is close to that of bismuth glasses, and larger than those of tellurite, germanate, silicate and phosphate glasses. The broad and flat 4I13/2-4I15/2 emission and the large stimulated emission cross-section of Er3+ ions around 1.5um can be used as host material for potential broadband optical amplifier in the wavelength-division-multiplexing (WDM) network system.     

Thermal stability and frequency up-conversion properties of Er~(3+)-doped oxyfluoride tellurite glasses

The thermal properties of 68TeO2-15BaF2-5SrF2-10LaF3-2KF glass were measured by different temperature analysis (DTA). Up-conversion luminescence of Er3 ion in the obtained glass was investigated. Mechanism of up-conversion emission was discussed. The result shows that the obtained oxyfluoride tel-lurite glass 68TeO2-15BaF2-5SrF2-10LaF3-2KF has a good thermal stability (△T = 153.6℃) and strong up-conversion green emissions around 527 nm and 549 nm and red emission at 660 nm. This glass can be a promising host material for up-conversion fiber lasers.     

Broadband 1.5-μm emission of erbium-doped TeO2-WO3-Nb2O5 glass for potential WDM amplifier

Erbium-doped glass showing the wider 1.5-/μm emission band is reported in a novel oxide system TeO2-WO3-Nb2O5 and their thermal stability and optical properties such as absorption,emission spectra,cross-sections and fluorescence lifetime were investigated.Compared with other glass hosts,the gain bandwidthproperties of Er3+ in TWN glass is close to that of bismuth glasses,and larger than those of tellurite,germanatc,silicate and phosphate glasses.The broad and flat 4I13/2 → 4I15/2 emission and the largestimulated emission cross-section of Er3+ ions around 1.5 μm can be used as host material for potentialbroadband optical amplifier in the wavelength-division-multiplexing(WDM)network system.     

Blue–green color-tunable emissions in novel transparent Sr_2LuF_7:Eu/Tb glass-ceramics for WLEDs

To introduce ordered nano-structures inside a transparent amorphous matrix with superior optical and mechanical properties bears scientific and technological importance,yet limited success has been achieved.Here,via simple melting-quenching and subsequent thermal activation,we report the successful preparation of transparent nano-structured glass-ceramics embedded in Sr_2Lu F_7 nano-crystals(～26 nm),as evidenced by X-ray diffraction,transmission electron microscopy(TEM),and high resolution TEM.The successful incorporation of dopants into formed Sr_2Lu F_7 nano-crystals with low phonon energy results in highly tunable blue–green photoemission,which depends on excitation wavelength,dopant type,and temperature.We found that Eu~(3+)and Eu~(2+)ions co-exist in this hybrid optical material,accompanied by the broadband blue emission of Eu~(2+)and sharp red emissions of Eu~(3+).A series of optical characterizations are summoned,including emission/excitation spectrum and decay curve measurement,to reveal the reduction mechanism of Eu~(3+)to Eu~(2+).Furthermore,near green–white photoemission is achieved via the enrichment of Tb~(3+)/Eu~(3+)into crystallized Sr_2Lu F_(7 )nano-crystals.The temperature-dependent visible photoemission reveals thermal activation energy increases with the precipitation of Sr_2Lu F_7 nano-crystals in a glass matrix,suggesting better thermal stability of glass-ceramics than precursor glasses.These results could not only deepen the understanding of glass-ceramics but also indicate the promising potential of Eu~(3+)/Tb~(3+)-ions-doped Sr_2Lu F_7 glass-ceramics for UV pumped white light emitting diodes(WLEDs)with good thermal stability.     

Tm^3＋-doped Ga2O3-GeO2-Bi2O3-PbO（PbF2） Glasses for 1.47-μm Optical Amplifications

We report the spectroscopic properties and thermal stability of Tm^3＋-doped Ga2O3-GeO2-Bi2O3-PbO（PbF2） glasses for 1.47-μm optical amplifications. Effects of PbF2 doping on the optical properties and thermal stability of Tm^3＋ -doped gallate-germanium-bismuth-lead glass are inestigated. The measured peak wavelength and full width at half-maximum of the fluorescence are 1465nm and - 120 nm, respectively： Siguificant enhancement of the 1.47-μm emission and the lifetime of a a^3H4 level with increasing PbF2 doping have been observed. The presence of GeO2 provides two potentials of increasing the thermal stability and shortening the ultraviolet cutoff band of host glasses.     

Blue upconversion luminescence in Tm3+/Yb3+ co-doped new oxyfluoride tellurite glass

The thermal stability, Raman spectrum and upconversion properties of Tm3+/Yb3+ co-doped new oxyfluoride tellurite glass are investigated. The results show that Tm3+/Yb3+ co-doped oxyfluoride tellurite glass possesses good thermal stability, lower phonon energy, and intense upconversion blue luminescence. Under 980-nm laser diode (LD) excitation, the intense blue (475 nm) emission and weak red (649 nm) emission corresponding to the 1G4 → 3H6 and 1G4 → 3F4 transitions of Tm3+ ions respectively,were simultaneously observed at room temperature. The possible upconversion mechanisms are evaluated.The intense blue upconversion luminescence of Tm3+/Yb3+ co-doped oxyfluoride tellurite glass can be used as potential host material for the development of blue upconversion optical devices.     

Influence of Tm~(3+) ions on the amplification of Ho~(3+):~5I_7→~5I_8 transition in fluoride glass modified by Al(PO_3)_3 for applications in mid-infrared optics

In this work, we investigate a new type of fluoride glasses modified by Al(PO_3)_3 with various Tm~(3+)∕Ho~(3+) doping concentrations. The introduced PO_-~3 plays an effective role in improving the glass-forming ability and thermal stability. Besides, 1.47, 1.8, and 2.0 μm emissions originating from Tm~(3+)and Ho~(3+), respectively, are observed.The spectroscopic properties and energy transfer mechanisms between Tm~(3+)and Ho~(3+)are analyzed as well. It is noted that the higher predicted spontaneous transition probability(118.74 s-1) along with the larger product of measured decay lifetime and the emission cross section(σemi×τ) give evidence of intense 2.0 μm fluorescence.     

Electronic structure and photoluminescence property of a novel white emission phosphor Na_3MgZr(PO_4)_3:Dy~(3+) for warm white light emitting diodes

To explore suitable single-phase white emission phosphors for warm white light emitting diodes, a series of novel phosphors Na_3MgZr(PO_4)_3:xDy~(3+)(0 ≤ x ≤ 0.03) is prepared, and their phase purities as well as photoluminescence properties are discussed in depth via x-ray diffraction structure refinement and photoluminescence spectrum measurement.The electronic structure properties of the Na_3MgZr(PO_4)_3host are calculated. The results reveal that Na_3MgZr(PO_4)_3 possesses a direct band gap with a band gap value of 4.917 e V. The obtained Na_3MgZr(PO_4)_3:Dy~(3+) phosphors are all well crystallized in trigonal structure with space group Rc, which has strong absorption around 365 nm and can generate warm white light emissions peaking at 487, 576, and 673 nm upon ultraviolet excitation, which are attributed to the transitions from ~4F_(9/2) to ~6H_(15/2),~6H_(13/2), and ~6H_(11/2) of Dy~(3+) ions, respectively. The optimal doping content, critical distance, decay time, and Commission International de L'Eclairage(CIE) chromaticity coordinates are investigated in Dy~(3+) ion-doped Na_3MgZr(PO_4)_3. The thermal quenching analysis shows that Na_3MgZr(PO_4)_3:Dy~(3+) has a good thermal stability, and the thermal activation energy is calculated. The performances of Na_3MgZr(PO_4)_3:Dy~(3+) make it a potential single-phase white emission phosphor for warm white light emitting diode.     

Blue Upconversion Luminescence in Tm^3＋／Yb^3＋-Codoped Lead Chloride Tellurite Glass

The upconversion properties of Tm^3 /Yb3 -codoped lead chloride tellurite glass under 980hm excitation were investigated. The intense blue (476nm) emission and weak red (649 nm) emission corresponding to the ^1G4→4 ^3H6 and ^1G4→^3H4 transitions of Tm^3 ions, respectively, were simultaneously observed at room temperature. The dependence of upconversion intensities on excitation power and the possible upconversion mechanisms are evaluated. The intense blue upconversion luminescence of Tm^3 /Yb^3 -codoped lead chloride tellurite glass can be used as potential host material for the development of blue upconversion optical devices.     

Efficient 2.7-μm emission in Er3＋-doped bismuth germanate glass pumped by 980-nm laser diode

An Er 3+-doped lead-free bismuth germanate glass is synthesized. Its 2.7-μm emission property is analyzed, and the efficient 2.7-μm emission from the glass is observed under 980-nm laser diode excitation. The prepared glass possesses high spontaneous transition probability (64.8 s 1 ) and a large calculated emission cross section (6.61×10 21 cm 2 ) corresponding to the 4I11/2→4I13/2 transition. The multiphonon relaxation rate for the excited state 4I11/2 is only 236 s 1 . Therefore, the excellent spectroscopic properties and outstanding thermal stability suggest that this glass is a suitable host for developing solid-state lasers operating in the mid-infrared.     

Structural evolution study of additions of Sb_2S_3 and CdS into GeS_2 chalcogenide glass by Raman spectroscopy

The structures of pseudo-binary GeS_2–Sb_2S_3, GeS_2–CdS, Sb_2S_3–CdS, and pseudo-ternary GeS_2–Sb_2S_3–CdS chalcogenide systems are systematically investigated by Raman spectroscopy. It is shown that a small number of [S_3Ge–GeS_3]structural units(SUs) and-S-S-/S8 groups exist simultaneously in GeS_2 glass which has a three-dimensional continuous network backbone consisting of cross-linked corner-sharing and edge-sharing [GeS_4] tetrahedra. When Sb_2S_3 is added into GeS_2 glass, the network backbone becomes interconnected [GeS_4] tetrahedra and [SbS_3] pyramids. Moreover, Ge atoms in[S_3Ge–GeS_3] SUs tend to capture S atoms from Sb_2S_3, leading to the formation of [S_2Sb–SbS_2] SUs. When CdS is added into GeS_2 glass, [Cd_4GeS_6] polyhedra are formed, resulting in a strong crystallization tendency. In addition, Ge atoms in[S_3Ge–GeS_3] SUs tend to capture S atoms from CdS, resulting in the dissolution of Ge–Ge bond. Co-melting of Sb_2S_3 or CdS with GeS_2 reduces the viscosity of the melt and improves the homogeneity of the glass. The GeS_2 glass can only dissolve up to 10-mol% CdS without crystallization. In comparison, GeS_2–Sb_2S_3 glasses can dissolve up to 20-mol% CdS,implying that Sb_2S_3 could delay the construction of [Cd_4GeS_6] polyhedron and increase the dissolving amount of CdS in the glass.     

Optical transitions in Er^3＋ doped lead germanate glasses

Er^{3+}- and Er^{3+}/Yb^{3+}-doped lead germanate glasses that are suitable for use in fibre lasers and optical amplifiers as well as optical waveguide devices have been fabricated and characterized. The absorption spectra from near-infrared to visible were obtained and the Judd－Ofelt parameters were determined from the absorption band. Intense and broad 1.53μm infrared fluorescence and visible upconversion luminescence were observed under 976 nm diode laser excitation. For 1.53μm emission band, the full widths at half-maximum are 36, 37, 51 nm for GPE, GPYE and GPFE samples, respectively. For frequency upconversion emission, the intense bands centred at around 524, 545, 657nm are due to the {}^4S_{3/2}+{}^2H_{11/2}→{}^4I_{15/2} and {}^4F_{9/2}→{}^4I_{15/2} transitions of Er^{3+} ions. The quadratic dependence of the green and red emissions on excitation power indicates that the two-photon absorption process occurs under the 976nm excitation.     

Composition Dependence of Spectroscopic Properties of Er^3＋ Doped TeO2-WO3-ZnO Glasses

Er^3 -doped TeO2-WO3-ZnO glasses were prepared and the absorption spectra, emission spectra and fluorescence lifetimes were measured. With more Te02 content in the glasses, the emission full width at half maximum (FWHM) increases while the lifetime of the ^4I13/2 level of Er^3 decreases. The stimulated emission cross-sectionof Er^3 calculated by the McCumber theory is as large as 0.86pm^2. The product of the FWHM and the emissioncross-section σe of Er^3 in TeO2-WO3-ZnO glass is larger than those in other glasses, which indicates that the glasses are promising candidates for Er^3 -doped broadband amplifiers. The Judd-Otfelt parameter Ω6 shows close composition dependence of the 1.5μm emission bandwidth. The more the TeO2 content is, the larger thevalues of Ω6 and FWHM.     

The 2-μm to 6-μm mid-infrared supercontinuum generation in cascaded ZBLAN and As_2Se_3 step-index fibers

Fiber-based mid-infrared(MIR) supercontinuum(SC) sources benefit from their spectral brightness and spatial coherence that are needed for many applications, such as spectroscopy and metrology. In this paper, an SC spanning from 2 μm to 6 μm is demonstrated in cascaded ZrF_4–BaF_2–LaF_3–AlF_3–NaF(ZBLAN) and As_2Se_3 step-index fibers. The pump source is a ZBLAN fiber-based MIR SC laser with abundant high-peak-power soliton pulses between 3000 nm and 4200 nm. By concatenating the ZBLAN fiber and the As_2 Se_3 fiber, efficient cascading red-shifts are obtained in the normal dispersion region of the As_2 Se_3 fiber. The spectral behavior of cascaded SC generation shows that the long-wavelength proportion of MIR SC generated in the ZBLAN fiber plays a critical role for further spectral extension in the As_2 Se_3 fiber.     

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.     

Sensitization of Sn~(2+) on Tb~(3+) luminescence for deep UV excitation in phosphate glasses

Tb~(3+)and Sn~(2+)co-doped strontium phosphate glasses are prepared and their unique photoluminescence(PL)properties for deep UV excitation are investigated. With the co-doped Sn~(2+)ions, Tb~(3+)keeps the original PL behaviors under near UV excitation while its PL action for deep UV excitation is enhanced tremendously.PL emission and excitation spectra demonstrate the sensitization role of Sn~(2+)on the Tb~(3+)emissions for deep UV excitation that is associated with the strong deep UV absorption of Sn~(2+)for greatly enhancing the resonance of the Tb~(3+)excitation with the deep UV light source. The decay curves of Sn~(2+)and Tb~(3+)emissions for both singly doped and co-doped samples are single exponentially well fitted with almost the same emission lifetime(τ) values in the microsecond and millisecond time regimes, respectively, confirming that Sn~(2+)and Tb~(3+)act as an independent activator in the present phosphate glass matrix while an involved energy transfer from Sn~(2+)to Tb~(3+)is radiative. Moreover, Sn~(2+)and Tb~(3+)can be co-excited with deep UV light to emit tunable light from blue to green with the definite CIE chromaticity coordinate for different applications.     

Bismuth-doped germanate glass fiber fabricated by the rod-in-tube technique

Bismuth(Bi)-doped laser glasses and fiber devices have aroused wide attentions due to their unique potential to work in the new spectral range of 1 to 1.8 μm traditional laser ions, such as rare earth, cannot reach. Current Bi-doped silica glass fibers have to be made by modified chemical vapor deposition at a temperature higher than2000°C. This unavoidably leads to the tremendous loss of Bi by evaporation, since the temperature is several hundred degrees Celsius higher than the Bi boiling temperature, and, therefore, trace Bi(～50 ppm) resides within the final product of silica fiber. So, the gain of such fiber is usually extremely low. One of the solutions is to make the fibers at a temperature much lower than the boiling temperature of Bi. The challenge for this is to find a lower melting point glass, which can stabilize Bi in the near infrared emission center and, meanwhile, does not lose glass transparency during fiber fabrication. None of previously reported Bi-doped multicomponent glasses can meet the prerequisite. Here, we, after hundreds of trials on optimization over glass components,activator content, melting temperature, etc., find a novel Bi-doped gallogermanate glass, which shows good tolerance to thermal impact and can accommodate a higher content of Bi. Consequently, we successfully manufacture the germanate fiber by a rod-in-tube technique at 850°C. The fiber exhibits similar luminescence to the bulk glass, and it shows saturated absorption at 808 nm rather than 980 nm as the incident power becomes higher than 4 W. Amplified spontaneous emissions are observed upon the pumps of either 980 or 1064 nm from germanate fiber.     

Er~(3+)-doped fluorogallate glass for mid-infrared applications

We report an Er3t-doped fluorogallate glass with good thermal and chemical stability. The low maximum phonon energy and high mid-infrared(IR) transmittance of the glass are confirmed by Raman and IR spectra,respectively. Based on Judd–Ofelt theory, intensity parameters and radiative properties are determined from the absorption and emission spectra. The proposed glass possesses a large fluorescence branching ratio β(21.71%) and a maximum stimulated emission cross-section σemof Er3t:4I11∕2→4I13∕2transition at 2.71 μm(1.04 × 10-20cm2). The results indicate that it can be potentially applied in high-power 2.7 μm fiber lasers.     

Frequency Upconversion Emission of Er^3＋-Doped Strontium-Lead-Bismuth Glasses

Er^3 -doped strontium-lead-bismuth glasses for developing potential upconversion lasers have been fabricated and characterized. Under 975 nm excitation, intense green and red emissions centred at 525, 546, and 657nm,corresponding to the transitions ^2H11/2 → ^4I15/2, ^4S3/2 → ^4I15/2, and ^4F9/2 → ^4I15/2, respectively, were observed at room temperature. The upconversion mechanisms are discussed based on the energy matching and quadratic dependence on excitation power, and the dominant mechanisms are excited state absorption and energy transfer upconversion for 525 and 546nm emissions, and energy transfer upconversion for 657nm emission.