Cascade Lasing Research Articles

Numerical analysis of cascade lasing in Ho:ZBLAN fiber lasers with efficient output at ∼750 nm

Recent advances in blue-emitting laser diodes (LDs) have enabled visible laser operation in rare-earth-doped fluoride fibers. Therefore, investigation of the power scaling of visible fiber lasers enabled by direct blue diode pumping is significant. In this study, we numerically investigated the performance of a continuous-wave (CW) Ho:ZBLAN fiber laser operating at approximately 750 nm for the 5F4,5S2→5I7 transition using a cascade lasing process. Direct diode pumping at 450 nm is used in conjunction with double-clad fiber geometry. Simultaneous lasing for the mid-IR 5I7→5I8 transition effectively depopulated the 5I7 level and significantly enhanced efficiency and power scalability. A slope efficiency of 49% was associated with the calculated CW output power of 14.2 W at a launched pump power of 30 W. This study demonstrates the significance of depopulating the lower laser level in the development of an efficient Ho:ZBLAN 750 nm fiber laser and offers an alternative approach for developing high-power visible laser sources.

Diode-pumped Tm:YAP laser operating at 2.3 µm with enhanced performance through cascade lasing.

The laser diode (LD)-pumped Tm:YAP (a-cut, 3.5 at.%) laser generated a maximum ∼2.3 µm continuous wave (CW) laser output power of ∼3 W. The higher output power benefited from the positive effect of the cascade lasing (simultaneously operating on the 3H4 → 3H5 and 3F4 → 3H6 Tm3+ transition). It was the highest CW laser output power amongst the LD/Ti:Sapphire-CW-pumped ∼2.3 µm Tm3+-doped lasers reported so far. Under the cascade laser operation, the slope efficiency of the ∼2.3 µm laser emission versus the absorbed pump power increased from 13.0% to 21.4%.

Tm:CALGO lasers at 2.32 µm: cascade lasing and upconversion pumping.

We report on the first laser operation of a disordered Tm:CaGdAlO4 crystal on the 3H4 → 3H5 transition. Under direct pumping at 0.79 µm, it generates 264 mW at 2.32 µm with a slope efficiency of 13.9% and 22.5% vs. incident and absorbed pump power, respectively, and a linear polarization (σ). Two strategies to overcome the bottleneck effect of the metastable 3F4 Tm3+ state leading to the ground-state bleaching are exploited: cascade lasing on the 3H4 → 3H5 and 3F4 → 3H6 transitions and dual-wavelength pumping at 0.79 and 1.05 µm combining the direct and upconversion pumping schemes. The cascade Tm-laser generates a maximum output power of 585 mW at 1.77 µm (3F4 → 3H6) and 2.32 µm (3H4 → 3H5) with a higher slope efficiency of 28.3% and a lower laser threshold of 1.43 W, out of which 332 mW are achieved at 2.32 µm. Under dual-wavelength pumping, further power scaling to 357 mW at at 2.32 µm is observed at the expense of increased laser threshold. To support the upconversion pumping experiment, excited-state absorption spectra of Tm3+ ions for the 3F4 → 3F2,3 and 3F4 → 3H4 transitions are measured for polarized light. Tm3+ ions in CaGdAlO4 exhibit broadband emission at 2.3 - 2.5 µm making this crystal promising for ultrashort pulse generation.

Cascade lasing at ∼2 μm and ∼2.3 μm in a diode-pumped Tm:YVO4 laser.

We report on the cascade continuous-wave operation of a diode-pumped Tm:YVO4 laser on the 3F4 → 3H6 (at ∼2 μm) and 3H4 → 3H5 (at ∼2.3 μm) Tm3+ transitions. Pumped with a fiber-coupled spatially multimode 794 nm AlGaAs laser diode, the 1.5 at.% Tm:YVO4 laser yielded a maximum total output power of 6.09 W with a slope efficiency of 35.7% out of which the 3H4 → 3H5 laser emission corresponded to 1.15 W at 2291-2295 and 2362-2371 nm with a slope efficiency of 7.9% and a laser threshold of 6.25 W.

Theoretical modelling of a rotating mirror Q switched CW CO laser

Theoretical model for a rotating mirror Q switched CW CO laser is presented. It considers 36 vibration levels including the ground level of CO molecule. Lasing on 20 rotational lines in 18 vibration bands is considered. The lasing on these lines takes place between the rotational sublevels associated with each vibration level. Hence one is required to develop the rate equations for the 36 vibration levels, 397 rotational sublevels and also the equations for the photon densities associated with the360 lasing lines. These equations include the varying loss experienced by the lasing mode as it traverses the gain medium due to rotation of the mirror. The discharge has a thermal gradient. To take this into account we divide the gain medium into two zones, a central one matching with the laser mode and the rest of the discharge zone as the second zone. The two zones are assigned two temperatures as per the calculated average temperatures in these two zones. The calculations of how the above variables vary with time are carried out with the help of a fourth order Runge Kutta routine in the two zones separately. For each time step the model also calculates the intersection areas of the laser mode with the two zones. The actual power profiles of all the individual lasing lines are obtained by combining their powers in these two zones and the corresponding intersection areas. An important application of this laser in additional frequency generation in nonlinear crystals, require time synchronisation among interacting pulses. Our results show that in each vibration band about 12 to –13 lines operate, but only 3 to –4 among them are strong and are well synchronised in time. Their growth is determined by cascade lasing, higher gain coefficient compared to their neighbours and preference of their growth due to rotational relaxation. The results at operation of the rotating mirror at100 Hz, shows that about 222 lines operate and about 65 lines shows this synchronisation property. The results have been validated with experimental data available in the literature.

Efficient cascade lasing on over 17 wavelengths from two-photon excitation of the cesium [formula omitted]D states

A broad survey is taken to assess the agility of the cesium 62D states in the generation of cascade lasing from 455 to 3650 nm. The experiment utilized a heat pipe with 0 to 10 Torr of He, cesium densities of 1014 to 1017 atoms per cm3, and it is pumped by a pulsed dye laser with 1 to 30 mJ/pulse. Cascade lasing, generated by amplified spontaneous emission with no aid of a cavity, is observed on over 17 transitions—nearly every dipole allowed transition from energy levels below 62D. Many lasing lines possess 100 to 229 μJ of energy, slope efficiencies of 4 to 7%, low thresholds favorable for diode pumping, and together have combined optical conversion efficiencies as high as 10%±3%. This agile system could have applications in infrared counter measures, target illuminator lasers, beacon lasers, underwater communication, and other tactical laser applications.

Efficient 2.76 μm continuous-wave laser in extremely lightly Er-doped CaF2 single-crystal fiber

Rare-earth-doped single-crystal fibers are promising for use in developing high-power lasers due to their improved thermal management and excellent optical properties. In this work, Er-doped CaF2 single-crystal fibers of good optical quality were successfully grown by a modified temperature gradient technique method. For the characteristic clustering effect of trivalent rare earth ions within fluorite crystals, a high-efficiency 2759.1 nm laser was obtained in a 1 at.% Er-doped CaF2 crystal fiber. Moreover, even in an extremely lightly doped crystal fiber with a concentration of 0.5 at.%, a continuous-wave (CW) 2758.2 nm laser with a slope efficiency of 20% was achieved. Without the cascade lasing, a ~2.8 μm CW laser operation pumped by a laser diode was first demonstrated in the case of a Er3+ doping concentration below 1 at.%. The results suggest that Er-doped CaF2 single-crystal fibers are promising for mid-infrared lasers.

Experimental and numerical investigation of mid-infrared laser in Pr3+-doped chalcogenide fiber**Project supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 61605095), the Natural Science Foundation of Zhejiang Province, China (Grant No. LY19F050004), the Natural Science Foundation of Ningbo City (Grant No. 2015A610038), the Open Fund of the Guangdong Engineering Technology Research and Development

We report on a chalcogenide glass fiber doped with Pr3+ that can be used for commercialized 1.5- and 2- laser excitations by emitting broadband – fluorescence, which is extruded into a preform and then drawn into a step-index fiber. The spectroscopic properties of the fiber and glass are reported, and the mid-infrared fiber lasers are also numerically investigated. Cascade lasing is employed to increase the inversion population of the upper laser level. The particle swarm approach is applied to optimize the fiber laser parameters. The output power can reach 1.28 W at 4.89- wavelength, with a pump power of 5 W, excitation wavelength at , Pr3+ ion concentration at 4.22 ×1025 ions/m3, fiber length at 0.94 m, and fiber background loss at 3 dB/m.

Design of erbium doped double clad ZBLAN Fibre laser

A high powered octagonal double clad ZBLAN (33 μm/330 μm, NA=0.13) glass fibre for mid-infrared light generation is studied using a one dimensional rate equation model. The fibre laser design employs the concept of cascade lasing and includes up-conversion phenomena. The results obtained demonstrate that efficient cascade lasing may be achieved in practice without the need for fibre grating fabrication, as a sufficient level of feedback for laser action is provided by Fresnel light reflection at ZBLAN glass fibre air interfaces. Further enhancement of the laser efficiency can be achieved by terminating one of the fibre ends with a mirror. Simulation results show that the laser operation with 20 W of pump power at 0.98 μm wavelength can be achieved at 2.75 μm operating wavelength with Er3+ ion concentrations of 60,000 ppm.

Theoretical study of population inversion in active doped MIR chalcogenide glass fibre lasers (invited).

We study the mechanism of the population inversion in mid-infrared fibre lasers based on a chalcogenide glass host doped with active lanthanide ions. Three lanthanide dopant ions are considered: terbium, dysprosium and praseodymium. We predict the relevant trivalent ion level populations and gain. The simulation parameters were obtained by fabricating and optically characterising a series of trivalent ion doped chalcogenide glass samples. We also provide simple analytical expressions that aid the design of the cascade lasing process.

Generation of tunable 16 μm radiation from CO2 by cascade lasing

In this paper we propose a scheme to generate tunable 16 μm radiation from CO2 molecules by cascade lasing. The stimulating 9.5 μm radiation is generated internally by the fast rotating mirror Q-switching technique. The optical scheme proposed by us uses an intracavity prism to separate the 9.5 μm and the 16 μm beams. This facilitates independent tuning of the two beams if required. In the present configuration, only the 16 μm cavity is dispersive. The 9.5 μm beam grows spontaneously in a stable semiconfocal resonator. We have developed a theoretical model to simulate the proposed scheme. The model predicts the energy and power of 16 μm radiation. The calculated values are much higher than the previously obtained experimental values. The results point out the feasibility of developing a laser system based on the theoretical design parameters presented in this paper. Such laser systems can find application in uranium isotope separation studies.

Diode Pumped Erbium Cascade Fiber Lasers

Cascading the 4I11/2→4I13/2 transition at 2.8 μm and 4I13/2→4I15/2 transition at 1.6 μm offers a solution to the thermal management of high power Er3+-doped fluoride fiber lasers. We demonstrate an output power of 8.2 W at 2.8 μm from an Er3+-doped fluorozirconate fiber laser with 56 W of launched pump power at 975 nm by cascade laser operation with the 1.6-μm ground-state transition. By careful selection of the Er3+ concentration that prevents significant interaction between Er3+ ions, it is shown that cascade lasing can occur from simple resonator arrangements including Fresnel reflection from the ends of the fiber only. The unsaturated output power suggests that no competing transitions are oscillating which advocates that radiative quenching of the metastable 4I13/2 level provides an elegant way of reducing undesired heat generation. We show that core temperature increases at the tip of the pumped end of the fiber can be an order of magnitude lower than fiber lasers employing single transition oscillation. The potential of further power scaling is demonstrated and the experimental results are verified with the use of a numerical model.

Generation of 13.9 µm radiation from CO2 by cascade lasing or externally applied CO2 laser

13.9 µm radiation from the 1000-0110 transition can be obtained from a CO2 laser by saturating the 0001-1000, 10.6 µm transition with an internally generated q-switched pulse or by the application of an external 10.6 µm pulse. Because of Fermi resonance between the symmetric stretch and the bending modes, decay of population from the 1000 level is fast, and such lasers operate at low power and energies. A theoretical model was developed to study such lasers. The results of the calculations indicate that a large-aperture E-beam-sustained discharge is effective for excitation of the cryogenically cooled gain medium, which uses He rich mixture at low pressure. The system is scalable and capable of generating large powers and energies.

Qualitative improvement in the lasing performance of PbGa2S4 : Dy3+ crystals through Na+ doping

We have studied the lasing properties of lead thiogallate crystals codoped with dysprosium and sodium ions. Sodium doping has been shown to have an advantageous effect on the optical and lasing properties of lead thiogallate crystals. At a pump pulse duration of 1.5 ms, output pulse energies up to 15 mJ have been obtained at a slope efficiency of up to 4%. Cascade lasing on two Dy3+ transitions, at 4.3, 4.5, 4.65, 5.2, 5.3 and 5.4 μm, has been demonstrated.

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+).

Modelling of a simple Dy3+ doped chalcogenide glass fibre laser for mid-infrared light generation

A simple Dy3+-doped chalcogenide glass fibre laser design for mid-infrared light generation is studied using a one dimensional rate equation model. The fibre laser design employs the concept of cascade lasing. The results obtained demonstrate that efficient cascade lasing may be achieved in practice without the need for fibre grating fabrication, as a sufficient level of feedback for laser action is provided by Fresnel light reflection at chalcogenide glass fibre–air interfaces. Further enhancement of the laser efficiency can be achieved by terminating one of the fibre ends with a mirror. A numerical analysis of the effect of the Dy3+ doping concentration and fibre loss on the laser operation shows that with 5 W of pump power, at 1.71 μm wavelength, output powers above 100 mW at ∼ 4.5 μm wavelength can be achieved with Dy3+ ion concentrations as low as 3 × 1019 cm−3, when fibre loss is of the order 1dB/m.

Cascade lasing of molecular HBr in the four micron region pumped by a Nd:YAG laser

Due to a narrow window of high atmospheric transmission near 4 /spl mu/m there is a great deal of interest for a scalable laser energy source in this spectral region. We propose a concept combining the advantages of solid-state and gas laser technology. It takes advantage of a coincidence of a Nd:YAG laser line and an overtone transition of the molecule HBr. Tuning a Q-switched Nd:YAG laser to 1.3391 /spl mu/m allows us to excite the v(0 /spl rarr/ 3), J(4 /spl rarr/ 5) vibrational-rotational transition of HBr. To stabilize the pump frequency, a diode laser locked to this HBr transition seeds the Nd:YAG laser. Once excited, HBr can potentially lase in three subsequent steps to the ground state, two of which were observed experimentally, emitting up to three photons in the 4-/spl mu/m region. We present theoretical and experimental results demonstrating the operational principle of this laser system. The comparison of experiment and theory suggests: 1) that intracavity CO/sub 2/ can force the system to lase on only those transitions that are within the atmospheric transmission window, and 2) the existence of amplified spontaneous emission driven by pure rotational transitions.

Spectral characteristics of nonchain HF and DF electric-discharge lasers in efficient excitation modes

The spectral characteristics of efficient nonchain HF and DF chemical lasers are studied. It is found that the emission spectra of nonchain lasers operating with high efficiency are strongly broadened. Almost 30 emission lines of an HF laser and cascade lasing on the v(3-2) → v(2-1) → v(1-0) vibrational transitions of HF molecules for a number of rotational lines are obtained. It is shown that the development of discharge inhomogeneities significantly reduces the number of lasing lines in the spectra of nonchain chemical lasers. For an SF6 — D2 mixture excited by a generator with an inductive storage, about 40 lasing lines are observed on four vibrational transitions of DF molecules and the v(4-3) → v(3-2) → v(2-1) → v(1-0) cascade lasing is obtained at several rotational lines. Nonchain HF and DF electric-discharge lasers with a total and intrinsic efficiency of up to 6% and 10%, respectively, pumped from capacitive and inductive generators are developed.

Efficient oscillation regimes of an HF laser pumped by a nonchain chemical reaction initiated by a self-sustained discharge

The amplitude — time and spectral characteristics of laser radiation and discharge characteristics in mixtures of SF6 with hydrogen and hydrocarbons at a high lasing efficiency are investigated. Lasing efficiencies of ~10 % with respect to the deposited energy are obtained under the pumping from capacitive and inductive energy storage units. It is shown that, during a pump pulse, the maximum efficiencies are achieved at high values of the parameter E/p in the laser gap. When profiled electrodes and UV preionisation are used, a specific output energy of a nonchain HF laser of ~140 J L-1 atm-1 and a lasing efficiency of ~4.5 % with respect to the stored energy were obtained. It is shown that the emission spectrum of a nonchain HF laser operating at high lasing efficiencies becomes significantly wider and cascade lasing is realised on the ν(3–2) → ν(2–1) → ν(1–0) vibrational transitions at several rotational lines.

Cascade lasing with spherical microparticles

Lasing characteristics of dye-doped spherical solid microparticles produced by photopolymerization in aerosols are investigated. It is well known that dielectric spherical microparticles are optical resonators with very high quality. The resonant internal field is not completely confined to the interior of the particle. An evanescent wave extends a couple of micrometers into the surrounding area. This evanescent field decays exponentially. We demonstrate that cascade lasing can be excited by coupling of two spherical microresonators through this evanescent field.