Μm Laser Transition Research Articles

Dual-wavelength mid-infrared laser operation at 2.8 μm and 3.6 μm in Er3+ doped fluoride fiber

This study presents a pioneering experimental demonstration of dual-wavelength laser operation at 2.8 μm and 3.6 μm in an Er3+-doped ZrF4-BaF2-YF3-AlF3 double-cladding fluoride glass fiber, fabricated in-house. The achievement is realized through the implementation of a novel dual-wavelength pumping scheme utilizing 980 nm and 1150 nm lasers. This innovative scheme enables the excitation of the lower energy level of the 2.8 μm laser transition (4I11/2) to the upper energy level of the 3.6 μm transition (4F9/2) via an excited state absorption process, resulting in simultaneous emission at 2.8 μm and 3.6 μm. The maximum laser output powers at 2.8 μm and 3.6 μm are measured at 0.96 W and 0.32 W, respectively. This groundbreaking work marks the first experimental realization of dual-wavelength laser operation at 2.8 + 3.6 μm in Er3+-doped fluoride fiber, contributing significantly to the advancement of laser technology in this spectral range.

Cryogenically cooled 2.8 μm Er:YAP laser with watt-level output power

We report the first watt-level, cryogenically cooled Er:YAP laser operating in the 3 μm spectral region. The spectroscopic properties of a 5 at% Er:YAP crystal were studied at cooling temperatures ranging from 77–290 K. The fluorescence lifetime of the 4I11/2 level associated with the 3 μm laser transition decreased with an increase in temperature; whereas, the lifetime of the 4I13/2 level increased with an increase in temperature. Moreover, over 1 W of output power at 2798 nm was achieved from the liquid-nitrogen-cooled Er:YAP laser with a slope efficiency of 20%, and the limitations to higher output power were discussed.

Structural and Near-Infrared Properties of Nd3+ Activated Lu3NbO7 Phosphor.

Undoped and Nd3+ doped lutetium niobate phases have been prepared by a conventional solid state reaction method using lutetium acetate and niobium oxide at 1250°C for 6h. X-ray diffraction patterns of the 6mol% Lu3NbO7 sample exhibited a cubic fluorite single phase. Phase structure exhibited interesting crystallization behaviour depending on increasing Nd3+ concentration which led to a Lu3NbO7 single phase formation during the heat treatment process. SEM investigations were also in agreement with the XRD results. Morphologies of Nd3+ doped lutetium niobate powders exhibited oval like shapes and grain sizes varied between 0.3 and 5μm. Near-infrared luminescence properties of Nd3+ doped Lu3NbO7 were also studied. 1.06μm laser transition characteristics of Nd3+ doped lutetium niobate have been observed. Concentration quenching phenomenon was not detected depending on increasing Nd3+ doping concentrations at room temperature.

Energy transfer and energy level decay processes in Tm3+-doped tellurite glass

The primary excited state decay and energy transfer processes in singly Tm3+-doped TeO2:ZnO:Bi2O3:GeO2 (TZBG) glass relating to the 3F4 → 3H6 ∼1.85 μm laser transition have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the 3H4 manifold at 794 nm, the 3H5 manifold at 1220 nm, and 3F4 manifold at 1760 nm has established that the 3H5 manifold is entirely quenched by multiphonon relaxation in tellurite glass. The luminescence from the 3H4 manifold with an emission peak at 1465 nm suffers strong suppression due to cross relaxation that populates the 3F4 level with a near quadratic dependence on the Tm3+ concentration. The 3F4 lifetime becomes longer as the Tm3+ concentration increases due to energy migration and decreases to 2.92 ms when [Tm3+] = 4 mol. % as a result of quasi-resonant energy transfer to free OH− radicals present in the glass at concentrations between 1 × 1018 cm−3 and 2 × 1018 cm−3. Judd-Ofelt theory in conjunction with absorption measurements were used to obtain the radiative lifetimes and branching ratios of the energy levels located below 25 000 cm−1. The spectroscopic parameters, the cross relaxation and Tm3+(3F4) → OH− energy transfer rates were used in a numerical model for laser transitions emitting at 2335 nm and 1865 nm.

Energy level decay processes in Ho3+-doped tellurite glass relevant to the 3 μm transition

The primary excited state decay processes relating to the 5I6 → 5I7 ∼ 2.9 μm laser transition in singly Ho3+-doped tellurite (TZBG) glass have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the 5I6 energy level at 1151 nm and 5I7 energy level at 1958 nm has established that the rate of energy transfer up-conversion between holmium ions excited to the 5I7 level is negligible for Ho3+ concentrations up to 4 mol. %. Excited state absorption was not observed from either the 5I7 or 5I6 levels and the luminescence from the 5I7 and 5I6 energy levels was measured to peak at ∼2050 nm and ∼2930 nm, respectively. The 5I6 level has a low luminescence efficiency of ∼8.9% due to strong nonradiative multiphonon relaxation. In contrast, decay from the 5I7 level is essentially fully radiative. A linear decrease in the decay time of the 5I6 level with Ho3+ concentration augmentation results from energy transfer to OH− ions in the glass (with NOH ∼ 8.2 × 1017 ions cm−3) and reduces the luminescence efficiency of the 5I6 level to 8% for [Ho3+] = 4 mol. %. Numerical simulation of a fiber laser incorporating 4 mol. % Ho3+ showed that a population inversion of ∼7.8% is reached for square pulses of 100 μs duration and a repetition frequency of 20 Hz at a moderate pump intensity of 418 kW cm−2 if energy transfer to OH− radicals is neglected.

Energy level decay and excited state absorption processes in dysprosium-doped fluoride glass

The primary excited state decay processes relating to the H613/2→H615/2∼3 μm laser transition in singly Dy3+-doped fluoride (ZBLAN) glass have been investigated in detail using time-resolved fluorescence spectroscopy. Selective laser excitation of the F69/2, H67/2 energy levels at 1125 nm and F611/2, H69/2 energy levels at 1358 nm established that the energy levels above the H611/2 level, excluding the F49/2 level, are entirely quenched by multiphonon emission in ZBLAN glass. The H611/2 and H613/2 energy levels emit luminescence with peaks at ∼1700 and ∼2880 nm, respectively, but at low quantum (luminescence) efficiencies. The quantum efficiency of the H611/2 level and H613/2 level is ∼9×10−5 and ∼1.3×10−2, respectively, for [Dy3+]=0.5 mol % based on calculations of the radiative lifetimes using the Judd–Ofelt theory. Excited state absorption (ESA) was detected by monitoring the rise time of the 1700 nm luminescence after tuning the probe wavelength across the spectral range from 1100 to 1400 nm. As a result of nonradiative decay of the higher excited states, ESA contributes to the heating of ∼3 μm fiber lasers based on Dy3+-doped fluoride glass. For [Dy3+] up to 4 mol %, we found no evidence of energy transfer processes between Dy3+ ions that influence the decay characteristics of the H611/2 and H613/2 energy levels.

Low-phonon BaF 2: Ho 3+, Tm 3+ doped crystals for 3.5–4 μm lasing

We study new pumping and sensitization scheme of 3.9 μm mid-IR laser transition ( 5I 5 → 5I 6 of Ho 3+) in the BaF 2 low-phonon crystal doped by Ho 3+ and co-doped by Tm 3+. It includes direct energy transfer from Tm 3+ to Ho 3+ governing by the 3H 4 → 3H 6; 5I 8 → 5I 5 cross-relaxation that gives ninefold increase of absorption of pumping light. We measure and analyze the lifetimes of all participated levels of Ho 3+ and Tm 3+ at low impurity concentrations, as well as energy transfer kinetics of the 3H 4 level of Tm 3+ and the 5I 6 level of Ho 3+ vs. acceptor concentration. We analyze the drawback of sensitization scheme, i.e. a self-quenching of Tm 3+ excitation governing by the 5H 4 → 3F 4; 3H 6 → 3F 4 cross-relaxation. Nevertheless, this drawback compensates by fast depletion of the 5I 6 terminal laser level with energy transfer governing by the 5I 6 → 5I 8; 3H 6 → 3H 5 cross-relaxation. As a result 3.9 μm laser transition of Ho 3+ in BaF 2: Ho 3+: Tm 3+ is not self-terminated. Moreover, efficient depletion of Ho 3+ terminal laser level with energy transfer to the initial level of 3.6 μm laser transition of Tm 3+ may cause cascade lasing in the mid-IR spectral range at two wavelengths of two different ions (Ho 3+ and Tm 3+).

1.06 μm laser transition characteristics of Nd 3+-doped fluorophosphate glasses

The fluorophosphate glasses with the composition (mol%) of (56 − x)P 2O 5 + 17K 2O + (12 − x)MgO + 6MgF 2 + 9Al 2O 3 + xNd 2O 3 and (56 − x)P 2O 5 + 17K 2O + (12 − x)SrO + 6SrF 2 + 9Al 2O 3 + xNd 2O 3 (where x = 0.1, 1.0, and 2.0 mol%) have been prepared and characterized by absorption and luminescence spectra and decay curve analysis. The absorption band positions of Nd 3+ ions have been analyzed using the free-ion Hamiltonian model. From the absorption spectra, Judd–Ofelt intensity parameters have been evaluated which are in turn used to predict radiative properties such as total radiative transition probabilities, radiative lifetimes and branching ratios. The present glasses have high stimulated emission cross-sections, implying that they have potential for laser applications. Lifetime of the 4F 3/2 level decreases with increasing Nd 3+ ions concentration. The non-exponential decay curves have been fitted to the generalized Yokota and Tanimoto model and derived energy transfer and migration parameters, donor–acceptor coupling constants and diffusion coefficients. The free OH content in the 1.0 mol% Nd 2O 3-doped glasses has been estimated from their measured Fourier transform infrared transmittance spectra.

Spectroscopic properties of Er3+:YAG at 300–550 K and their effects on the 1.6 μm laser transitions

The temperature dependent lifetimes, absorption, and emission spectra of an Er3+:YAG at 300–550 K are investigated and compared to the expected evolution using a theoretical description of the spectra. Population and linewidth dependent changes in the spectra can be separated, allowing conclusions on pump and laser efficiency at elevated temperatures. The temperature sensitivity of Er3+:YAG is for instance much less than that of Yb3+:YAG.

New fluoride glasses for laser applications

A new fluoride glass composition revealing a high stability, with a weak nucleation tendency, has been studied. The erbium spectroscopic properties have been investigated in these glasses in order to determine the parameters relevant to the 3 μm 4I11/2→4I13/2 laser transition. A Judd–Ofelt analysis was performed to obtain radiative lifetimes and emission cross-sections. Lifetime measurements have also been performed to optimize Er3+ concentration, taking into account the self-terminating behavior of the 3 μm laser transition. Also, Er:ZBLALiP microsphere 1550 nm lasers have been made with pumping by a half fiber taper at 1480 nm.

Multiphonon relaxation of mid-IR transitions of rare-earth ions in the crystals with fluorite structure

Fluorescence kinetics decay of the 4I 9/2 initial level for the 4.6 μm laser transition of erbium in the fluoride crystals with fluorite structure (CaF 2, SrF 2, CdF 2, BaF 2, and PbF 2) with short phonon spectra was measured at room temperature and 77 K. It was found that in the row of these crystals doped with erbium, the governing factor that influenced multiphonon relaxation (MR) rates is the number of phonons n, which is determined by the frequency of effective longitudinal optical phonon. A decrease of effective phonon frequency raises the number of phonons n. This decreases the MR rate and increases the fluorescence lifetimes for PbF 2 and BaF 2. The latter fact could be favorable in laser generation in the 4–5 μm spectral region where multiphonon relaxation usually dominates. Influence of the unit dimension of crystal lattice (rare-earth ion to ligand distance R 0) on the MR rate is discussed.

Population of lasing erbium levels in YLF : Er 3+ crystals under upconversion cw LD pumping

Formation of inverse population of working levels of 3 μm and green laser transitions in LiY 1− x Er x F 4 ( x=0.003–1) crystals under IR cw InGaAs laser-diode pumping is studied. Populations of studied levels were experimentally controlled by luminescence spectra. Dependencies of steady-state population on the dopant concentration and pump power density are calculated from a system of rate balance equations for 7 lowest excited erbium states. The energy-transfer mechanisms are determined and the predominant mechanisms elucidated. Microparameters and concentration dependence of the energy-transfer rates and non-linear coupling are obtained from theoretical and experimental rates of intra- and intercenter relaxation. Good agreement between the theory and experiment is obtained.

Theoretical comparison of Er3+-doped crystal lasers

The theoretical results are presented from a model that has been developed to simulate the 3 μm laser transition in Er3+-doped laser crystals. The rate equations for the seven lowest energy levels of Er:YAG, Er:YSGG, Er:YLF and Er:BAYF have been solved numerically for both continuous wave (cw) and pulsed (Q-switched and gain switched) laser operation with direct optical pumping into the 4I11/2 energy level. The dependence of slope efficiency on the Er3+ concentration for each laser crystal was investigated for cw operation and the relative performance of Er(15%):YLF, Er(15%):BAYF, Er(50%):YAG and Er(50%):YSGG was compared for each mode of operation. The change in the slope efficiency of Er:YLF at high Er3+ concentration, due to additional multi-ion processes, was calculated for a wide range of rate coefficients. It was determined that the slope efficiency could be reduced by as much as 12% by these processes and thus could explain the reduction in the slope efficiency as determined experimentally for lasers using highly doped fluoride crystals as the gain medium.

Limiting factors in PRF scaling of barium vapour lasers

We have studied the factors limiting the performance of a barium vapour laser (BVL) as the pulse repetition frequency (PRF) is elevated. By measuring the population densities of key barium species, we have found that increasing the PRF leads to progressively larger depletion of the axial pre-pulse Ba ground-state density (up to 85% at 15 kHz PRF), due to the combined effects of ion pumping and gas heating. A direct consequence of this Ba density depletion is a proportional decrease in the peak upper laser-level density of the 1.50 μm and 1.13 μm laser transitions (i.e. Ba1P1) during the excitation pulse. The results suggest that ground-state depletion is the dominant factor limiting laser output at high PRFs, and that the effects of the pre-pulse lower laser-level and electron densities play a secondary role. The implications to the PRF scaling of BVLs and other metal vapour lasers are discussed.

Yb 3+ to Er 3+ energy transfer and rate-equations formalism in the eye safe laser material Yb:Er:Ca 2Al 2SiO 7

Rate equations formalism is used to predict the population ratio of the Er 3+ 4 I 13 2 levels involved in the 1.55 μm laser transition in the Yb:Er:CAS laser materials. An effective Yb → Er energy transfer, favourable to the Er 3+ 1.55 μm laser emission, is demonstrated in this laser host. Indeed, the Yb → Er transfer and the Er → Yb back transfer rates are calculated to be 6 x 10 −16 and 0.45 x 10 −16 cm 3 s −1, respectively. Attempts of codoping the system with Nd 3+, Eu 3+ and Ce 3+ have been realised in order to increase the population of the Er 3+ 4 I 13 2 laser emitting level. Best results are obtained with Ce 3+ ion since in the sample containing 6 x 10 20 Ce 3+/cm 3, the Er 3+ 4 I 11 2 level lifetime is divided by a factor of 3 while the Er 3+ 4 I 13 2 fluorescence lifetime remains unaffected. On the contrary, codoping with Nd 3+ or Eu 3+ ions simultaneously decreases the Er 3+ 4 I 11 2 and 4 I 13 2 kinetics parameters. The role of the other parameters such as Yb/Er concentrations ratios is also discussed.

Hydroxil and iron ions in YAP: Er laser crystals

4S3/2→4I9/2 (1.663Μm) and4I11/2→4I13/2 (2.731Μm) laser transitions in YAP∶Er were studied using a material containing Fe, hydroxil impurities and other codopants. Iron impurity causes luminescence quenching of both the4S3/2 and4I11/2 levels whereas OH− ions enlarge losses, particularly at 2.731Μm. The effect of iron and hydroxil impurities may be easily diminished by adding Mg2+ ions, whereas the admixture of Ce3+ or Zr4+ brings no positive effects.

The solution of radiative transfer problems in molecular bands without the LTE assumption by accelerated lambda iteration methods

An iterative method based on the use of approximate transfer (or Λ) operators, which was designed initially to solve multilevel NLTE line formation problems in stellar atmospheres, is adapted and applied to the solution of the NLTE molecular band radiative transfer in planetary atmospheres. The matrices to be constructed and inverted are much smaller than those used in the traditional Curtis matrix technique, which makes possible the treatment of more realistic problems (including rotational NLTE, overlapping of lines in the bands and overlapping of bands with continuua) using relatively small computers. This technique converges much more rapidly than straightforward iteration between the transfer equation and the equations of statistical equilibrium (Λ-iteration). A test application of this new technique to the solution of NLTE radiative transfer problems for optically-thick and thin bands (the 4.3 μm CO 2 band in the Venusian atmosphere and the 4.7 and 2.3 μm CO bands in the Earth's atmosphere) is described. The necessity for careful treatment of rotational NLTE in weak lines of the 4.3 μm CO 2 fundamental band is shown. The possible influence of neglecting or treating approximately the CO 2 10 μm laser transitions and also effects of the overlap of the 4.3 μm CO 2 with the continuum are investigated. The rotational NLTE effects in the 4.7 μm CO band in the upper Earth's atmosphere are estimated.

The 3 μm Erbium laser

The characteristic properties of flashlamp- and laser excited erbium doped crystal or glass lasers are reviewed. The excitation mechanisms are discussed in detail. In particular it is shown that with moderate pumping inversion in a heavily doped flashlamp-pumped YAG:Er or YAlO 3:Er crystal laser can only be achieved between suitable Stark-sublevels of the 3 μm laser transition but not between the entire 4 I 11 2 and 4 I 13 2 bands.

Output power and gain obtained for a laser transition in pure neon at 5.4 μm

The output power and the gain were determined for the 5.4 μm (3p1–3s'1) laser transition in a compact laser containing pure neon. The length of the active medium was 60 cm. The measurements were made at different neon pressures and for different discharge currents. The results obtained were used to draw the conclusion on the suitability of a pure neon laser emitting at λ = 5.4 μm for the detection of traces of nitric oxide.

New wavelengths of the YAlO<inf>3</inf>: Er laser

In addition to the 1.662 μm laser transition, stimulated emission at three new wavelengths at 1.677, 1.706, and 1.729μm have been observed at 300 K in the YAlO 3 :Er crystal with an activator concentration of 1.25 weight percent.