Efficient room-temperature operation of Cr3+-sensitized,flashlamp-pumped, 2µm lasers

Utilizing the results of Cr³⁺ → Tm³⁺ transfer efficiency studies, we have demonstrated that yttrium aluminium garnet (YAG) is the preferred host for room-temperature, flashlamp-pumped solid-state lasers operating in the 2.0 µm spectral range. We report data on two different sensitizer-activator combinations in YAG and yttrium scandium gallium garnet (YSGG) laser materials: one is doped with Cr:Tm:Ho and operates on the Ho³⁺⁵I₇ →⁵I₈ transition at 2.097 µm; the other is doped only with Cr:Tm, which lases on the Tm³⁺³F₄ →³H₆ transition at 2.014 µm. We have achieved a slope efficiency of 5.1% with the Cr:Tm:Ho:YAG laser, which is the highest slope efficiency yet reported for a room-temperature, flashlamp-pumped, 2 µm solid-state laser. We have measured thresholds as low as 38 J and output energies >1.5 J for that system. We also report the first room-temperature operation of an efficient flashlamp-pumped Cr:Tm:YAG laser at 2.014 µm. Thresholds as low as 43 J, output energies exceeding 2 J, and slope efficiencies as high as 4.5% have been achieved. This is an order of magnitude higher than the efficiency previously reported for a 2.01 µm Cr:Tm:YAG laser operated at cryogenic temperatures. These two efficient 2 µm laser systems (Cr:Tm:Ho:YAG and Cr:Tm:YAG) are discussed in terms of their potential for Q-switched operation.     

Efficient room-temperature operation of Cr3+-sensitized, flashlamp-pumped, 2μm lasers

Utilizing the results of Cr³⁺ Tm³⁺ transfer efficiency studies, we have demonstrated that yttrium aluminium garnet (YAG) is the preferred host for room-temperature, flashlamp-pumped solid-state lasers operating in the 2.0 µm spectral range. We report data on two different sensitizer-activator combinations in YAG and yttrium scandium gallium garnet (YSGG) laser materials: one is doped with Cr:Tm:Ho and operates on the Ho^{3+ 5}I₇ ⁵I₈ transition at 2.097 µm; the other is doped only with Cr:Tm, which lases on the Tm^{3+ 3}F₄ ³H₆ transition at 2.014 µm. We have achieved a slope efficiency of 5.1% with the Cr:Tm:Ho:YAG laser, which is the highest slope efficiency yet reported for a room-temperature, flashlamp-pumped, 2 µm solid-state laser. We have measured thresholds as low as 38 J and output energies >1.5 J for that system. We also report the first room-temperature operation of an efficient flashlamp-pumped Cr:Tm:YAG laser at 2.014 µm. Thresholds as low as 43 J, output energies exceeding 2 J, and slope efficiencies as high as 4.5% have been achieved. This is an order of magnitude higher than the efficiency previously reported for a 2.01 µm Cr:Tm:YAG laser operated at cryogenic temperatures. These two efficient 2 µm laser systems (Cr:Tm:Ho:YAG and Cr:Tm:YAG) are discussed in terms of their potential for Q-switched operation.     

Comparative study of LD-pumped Tm:YAG and Tim:LuAG lasers

Widely tunable performances of Tm:YAG and Tm:LuAG lasers near 2 μm are demonstrated. The emission spectra of Tm³⁺ in these two single crystals are reported and analyzed. The lifetimes of the ³ F ₄ state in thulium-doped YAG and LuAG are 10.1 and 9.2 ms, respectively. In addition, it is proven that the Tm:YAG crystal with two bonded undoped YAG ends is promising for high-efficient operation.     

Fluorescence studies of the Er ↔ Ho and Er ↔ Tm energy transfer in erbium aluminum garnet

We have studied the flourescence characteristics of Ho³⁺ and Tm³⁺ in ErAlG:Ho, Tm and investigated the Er ? Ho and Er ? Tm energy transfer present in this system. In the infrared, the crystal ErAlG:Ho, Tm presents three groups of sharp lines (centered at ～18 000, ～19 300, and ～21 000 Å), whose intensities are strongly temperature-dependent. The first two groups are attributable to the Tm ³H₄ → ³H₆ transition, while the third group represents the Ho ⁵I₇ → ⁵I₈ transition. In YAlG:Er (50%), Ho(2%), only the Ho emission centered at ～21 000 Å is observed in the infrared. Under pulsed excitation, the emissions at ～17 900, ～19 000, and ～21 000 Å in ErAlG:Ho, Tm present a double exponential decay; the time dependences of the decays of the first two emissions are essentially the same; however, the 19 000 and 21 000 Å emissions present different decay patterns. In YAlG:Er (50%), Ho (2%), on the other hand, the decay of the 21 000 Å emission is purely exponential at all temperatures studied. The above results for ErAlG:Ho, Tm are explained by means of a rate-equation model in which the Ho → Er and Tm → Er transfer are treated as activation-type processes which effect a coupling between (1) the Ho and Tm ions, and (2) the Ho or Tm ions and quenching impurities.     

Optical spectroscopy and efficient continuous-wave operation near 2 μm for a Tm, Ho:KYW laser crystal

The growth, spectroscopy and lasing performance of a novel Tm,?Ho:KY(WO₄)₂ crystal are reported. The peak emission cross-section of the Ho^{3+ 5}I₇→⁵I₈ transition and lifetime of the ⁵I₇ excited state were determined to be 4.8×10^?20 cm² and 1.8 ms, respectively, in a spectral range at around 2060 nm. Using a Ti:sapphire laser as a pump source at 802 nm, a maximum slope efficiency of up to 44% has been achieved with a corresponding output power of 460 mW at 2056 nm during continuous-wave operation of a Tm,?Ho:KY(WO₄)₂ laser at room temperature. A tuning range of 1890–2080 nm has been demonstrated.     

Efficient High-Power Ho:YAG Laser Pumped by Dual-End-Diode-Pumped Tm:YLF Laser

We report a Ho:YAG (Ho-doped yttrium aluminum garnet) laser pumped by a dual-end-diode-pumped Tm:YLF (Tm-doped yttrium lithium fluoride) laser to obtain an efficient experimental device with high output-power characteristics. We study the influence of the specific values of the output coupling mirror transmittance, the resonant cavity length, and the radius of curvature of the output coupling mirror on the Ho:YAG laser output characteristics. Under optimum experimental conditions, under which the output coupling mirror transmittance was 30%, the resonant cavity length was 25 mm and the output coupling mirror radius of curvature was 300 mm, and the maximum pumping power of the dual-end-diode-pumped Tm:YLF laser was 15.2 W. We obtain an efficient high-power 2.122-μm laser output of 7.98 W from the Ho:YAG laser. The optical-to-optical conversion efficiency is 52.5%, and the beam quality factor figures are M _x ² ?=?2.89 and M _y ² ?=?2.97.     

Broadband infrared emission of erbium–thulium-codoped calcium boroaluminate glasses

The near-infrared emission from calcium boroaluminate (CABAL) glasses codoped with Er³⁺ and Tm³⁺ has been investigated. It is shown that by controlling the [Tm]/[Er] concentration ratio a fairly flat emission with a bandwidth of 370 nm can be achieved in the wavelength range from 1.4 to 2.0 μm. The broadband emission is formed by three bands centered at 1.4, 1.5 and 1.8 μm, which are related to the emission from the Tm³⁺: ³ H ₄→³ F ₄, Er³⁺: ⁴ I _13/2→⁴ I _15/2 and Tm³⁺: ³ F ₄→³ H ₆ transitions, respectively. Compared to Er-only doping and Tm-only doping at the same concentration, codoping with both ions leads to a reduction of the intensity and lifetime of the Er-related band at 1.53 μm and to an intensity enhancement of the two Tm-related emission bands at 1.46 and 1.80 μm. This is a result of energy transfer (ET) processes between Er³⁺ and Tm³⁺ ions that are relevant when determining the emission spectrum shape. Two dominant ET processes are identified, both consisting of transferring the energy of the Er³⁺ first excited level (⁴ I _13/2) in the first case to the Tm³⁺ first excited level (³ F ₄), which is excited to the third excited level (³ H ₄), and in the second case to the Tm³⁺ ground state (³ H ₆) which is excited to the first excited level (³ F ₄).     

Intense 2.7 μm and broadband 2.0 μm emission from diode-pumped Er3+/Tm3+/Ho(3+)-doped fluorophosphate glass

This Letter reports intense emission at 2.7 μm and broadband emission at 2.0 μm from Er(3+)/Tm(3+)/Ho(3+)-doped fluorophosphate glass. The fluorescence characteristics and energy transfer upon excitation of a conventional 980 nm laser diode are investigated. Based on the fluorescence spectra and lifetime measurement, the effect of Tm(3+) and Ho(3+) ions on intense 2.7 μm emission in fluorophosphate glass is demonstrated. It is also found that the effective bandwidth of 2.0 μm emission due to Tm(3+) and Ho(3+) ions can reach as high as 196 nm. These results indicate that the advantageous spectroscopic characteristics of Er(3+)/Tm(3+)/Ho(3+) triply doped fluorophosphate glass together with the outstanding thermal properties may become an attractive host for the mid-IR solid state lasers.     

Energy transfer and thermalization in LiYF4:Tm, Ho

Energy transfer processes are very important in solid-state laser systems because they can cause an enhancement of the luminescence emission resulting in a reduction of the laser threshold. In this work, a detailed investigation to understand the basic processes of energy transfer between Tm and Ho ions in LiYF₄, a solid-state laser crystal, was made. Data includes absorption, luminescence excitation and response to pulsed excitation. Dynamics of the energy transfer was analyzed by considering the kinetic evolution of the emissions of both ions. It was found that the energy transfer process between the ³ F ₄ spectral manifold of Tm and the ⁵ I ₇ spectral manifold of Ho results in thermal equilibration of these two manifolds. Received: 20 May 1999 / Revised version: 20 August 1999 / Published online: 27 January 2000     

Ho:LaF3 single crystal as potential material for 2 μm and 2.9 μm lasers

Optical properties of a Ho-doped LaF₃ single crystal have been detailed investigated as a promising material for 2 μm and 2.9 μm lasers for the first time. Judd–Ofelt theory was applied to analyze the absorption spectrum to determine the J–O intensity parameters Ω_t(t=2,4,6), based on which the emission probabilities, branching ratio and radiative lifetime for the as-grown crystal were all calculated. The stimulated emission cross-sections of the ⁵I₇ → ⁵I₈ and ⁵I₆ → ⁵I₇ transitions were obtained by using the Fuchtbauer–Ladenburg method. The gain cross-section for 2 μm emission becomes positive once the population inversion level reaches 30%. The Ho:LaF₃ crystal shows long fluorescence lifetime of ⁵I₇ manifold (25.81 ms) as well as ⁵I₆ manifold (10.37 ms) compared with other Ho³⁺-doped crystals. It can be proposed that the Ho:LaF₃ crystal may be a promising material for 2 μm and 2.9 μm laser applications.     

Q-switched operation of an in-band-pumped Ho:LuAG laser with kilohertz pulse repetition frequency

We report continuous-wave (CW) and repetitively Q-switched operation of an in-band-pumped Ho:LuAG laser at room temperature. End-pumped by a Tm:YLF solid-state laser with emission wavelength of 1.91 μm, the CW Ho:LuAG laser generated 5.4-W output at 2100.7 nm with beam quality factor of M ²～1.03 for an incident pump power of 14.1 W, corresponding to slope efficiency of 67% with respect to absorbed pump power. Up to 1.5-mJ energy per pulse at pulse repetition frequency (PRF) of 3 kHz and 4.5-W average power with FWHM pulse width of 28 ns at 5 kHz were demonstrated in repetitively Q-switched operation.     

Spectral and laser properties of Yb and Ho co-doped（YLa）2O3 transparent ceramic

Highly transparent Yb,Ho doped(YLa)2O3 ceramic was fabricated by conventional ceramic processing with nanopowders.The absorption and emission spectra of the ceramic was investigated.The energy transfer mechanism between Yb3+ and Ho3+ was also discussed.The strong emission band around 2 μm indicated that the Yb-Ho:(Y 0.90 La 0.10)2O3 transparent ceramic is a promising gain medium for the generation of 2 μm laser emissions.The laser operation of Yb-Ho co-doped(YLa)2O3 ceramic at 2.1 μm is first reported.     

2μm emission performance in Ho3+ doped fluorophosphate glasses sensitized with Er3+ and Tm3+ under 800 nm excitation

The spectroscopic properties and energy transfer mechanisms in Ho³⁺ doped fluorophosphate glasses sensitized by Er³⁺ and Tm³⁺ at the 2 μm region are investigated. The absorption spectra and fluorescence spectra of the Tm–Ho doubly-doped glass system and Er–Tm–Ho triply-doped glass system are measured. According to the absorption spectra, the Judd–Ofelt parameters and spontaneous transition probability are calculated and compared with those of other glass hosts. From the fluorescence spectra, the fluorescence intensity of Er–Tm–Ho glass system at 2.0 μm is 0.95 and larger than that of the Tm–Ho glass system, which is 0.69. Meanwhile, the 2.0 μm to 1.8 μm and 2.0 μm to 1.53 μm peak intensity ratios in the Er–Tm–Ho glass system are 8.63 and 22.79, respectively, suggesting sufficient energy transfer between Er³⁺, Tm³⁺ and Ho³⁺ ions. In addition, the pumping schemes and energy transfer mechanisms of Tm–Ho doubly-doped and Er–Tm–Ho triply-doped glasses are discussed. The study indicates that the Er–Tm–Ho triply-doped glass system is a significant sensitized way pumped at 800 nm in fluorophosphate glasses for 2 μm applications.     

In-band pumped highly efficient Ho:YAG ceramic laser with 21 W output power at 2097 nm

We report on the high-power and high-efficiency operation of a polycrystalline Ho:YAG ceramic laser in-band pumped by a Tm fiber laser at ~1907 nm. Lasing characteristics of a 1.5 at.% and a 2.0 at.% Ho3?-doped YAG ceramic were investigated and compared. Using an output coupler of 6% transmission, over 21.4 W of cw output power at 2097 nm has been generated with the 1.5 at.% doped Ho:YAG ceramic under 35 W of incident pump power, corresponding to an average slope efficiency with respect to the incident pump power of 63.6% and an optical-to-optical conversion efficiency of 61.1%.     

Short-term frequency characteristics of 2 µm single frequency Solid-state lasers

The Short term frequency stability characteristics of 2 μm single frequency Solid-state lasers was investigated. The two laser systems of 2 μm single longitudinal mode oscillation Tm, Ho:YLF microchip laser and Ho:YAG NPRO laser were designed and constructed. The Short term frequency stability of these two laser were measured with the fiber delay self-beating heterodyne method. The 3dB width of the relative frequency fluctuation of Tm, Ho:YLF microchip laser and Ho:YAG NPRO laser were measured to be 895 and 736 Hz with 500 m fiber optical (2.5 μs delay). The proportional relation between the lasing fluctuation and the delay time were 358 and 263 Hz/μs, respectively. The vibration experiment was presented and it indicated that the NPRO Ho:YAG was more terrible to the influence of vibration, which is important in the practical application of wind measurement lidar.     

Laser characteristics of 2-µm microchip Tm,Ho:Lu<Subscript>2</Subscript>SiO<Subscript>5</Subscript> laser

Cryogenic temperature operation of a 〈010〉-cut microchip Tm,Ho:Lu₂SiO₅ laser end-pumping by a fiber-coupled laser-diode is presented. A 2.73 W absorbed pump power is used to generate a maximum laser output of 470 mW, representing a 17.2% optical-to-optical conversion efficiency and a 61.7% slope efficiency corresponding to absorbed power. In the experiment, the oscillating wavelength shifting from 2.03 to 2.08 μm has been observed and a single 2.03 μm wavelength oscillation has obtained. To our knowledge, this work is the first time to investigate the Cryogenic temperature operation of microchip Tm,Ho:Lu₂SiO₅.     

Spectroscopic characteristics of YSGG:Cr,Ho, YSGG:Cr,Tm and YSGG:Cr,Tm,Ho

In this paper some spectroscopic properties of Y₃Sc₂Ga₃O₁₂ (YSGG):Cr,Ho, YSGG:Cr,Tm and YSGG:Cr,Tm,Ho are presented. The three lowest energetic manifolds of Ho³⁺ and Tm³⁺ in YSGG have been determined at 10 K. Time-resolved fluorescence build-up and decay of the laser active ions was measured in three crystals with different dopant concentrations. The transitions investigated were⁵I₇ →⁵I₈ for Ho³⁺ near 2000 nm and³H₄ →³H₆ for Tm³⁺ near 1800 nm. The data found on time-resolved fluorescence and energy level positions are useful for the discussion of the energy transfer characteristics of the system.     

Compact diode end-pumped c-cut Tm,Ho:YAlO3 laser

Cryogenic temperature operation of a c-cut Tm(5%),Ho(0.3%):YAlO₃ laser end-pumping by a fiber-coupled laser-diode is reported. A 2.55 W incident pump power is used to generate 160 mW of laser output, representing a 7.1% optical-to-optical conversion efficiency respect to the incident pump power. In the experiment, three wavelengths—2.04, 2.10, and 2.13 μm oscillation have been observed simultaneously, which is agreement with the inherent a-axis polarization fluorescence spectra of Tm,Ho:YAlO₃. Also single wavelength oscillation performance has been observed which is centered at 2.13 μm.     

2.1μm Ho：LuAG ceramic laser intracavity pumped by a diode-pumped Tm：YAG laser

We report a compact Ho：LuAG ceramic laser intracavity pumped by a diode-pumped Tm：YAG ceramic laser. The laser oscillation is accomplished by using a common linear cavity configuration containing Tm：YAG and Ho：LuAG ceramics. The 1.0 at.% Ho：LuAG ceramic laser yields 1.15 W of maximum output simultaneously at 2094 and 2100 nm with a beam quality factor of M2-2.8.     

Simultaneous dual-wavelength operation at 1.06 and 1.34 μm in Nd-vanadate laser crystals

Output performances of Nd-vanadate lasers with simultaneous dual-wavelength emission on the 1.06-μm ⁴ F _3/2 → ⁴ I _11/2 transition and the ⁴ F _3/2 → ⁴ I _13/2 transition at 1.34 μm are discussed. The design uses a linear resonator for emission at 1.06 μm and an L-type folded resonator for the 1.34-μm wavelength, and the ratio between the power of a single wavelength and the total power is adjusted by the choice of the output mirror transmissions. A continuous-wave (CW) Nd:GdVO₄ laser with total output power in the range of 3.9 to 6.8 W and the corresponding ratio of the output power at 1.06 μm to the total output power between 0.26 and 0.97 is realized. It is also shown that in comparison with the pump at 808 nm, the pump directly into the ⁴ F _3/2 emitting level at 879 nm improves the total output power. Furthermore, a Nd:GdVO₄ laser with simultaneous emission at 1.06 and 1.34 μm and that generates also green light at 0.53 μm by intracavity frequency-doubling with LiB₃O₅ (LBO) nonlinear crystal is demonstrated.