Diode Pumped Alkali Laser Research Articles

Analytical study of the beam quality of a diode-pumped alkali laser with unstable resonator

Analytical model of Cs diode-pumped alkali laser with unstable confocal resonator is reported. Two of the parameters characterizing the quality of the output beam were studied: the axial luminous intensity in the far field and the power in the bucket (PIB). The far-field axial light intensity and PIB were compared with their values for a truncated Gaussian reference beam whose aperture was limited by the aperture of the large resonator mirror and 99% of the Gaussian beam energy passed through the aperture of this mirror. For large values of the unstable resonator magnification factor m > 5, the far-field axial light intensity is approximately 40% larger than that of the Gaussian reference beam. Analysis of the PIB curves showed that for large m > 5, the values of the integrated fractional encircled power within a sufficiently large diffraction angles for the unstable resonator exceed its values for the reference Gaussian beam.

Development and Challenges of High Energy Diode Pumped Alkali Lasers (Invited)

Development and Challenges of High Energy Diode Pumped Alkali Lasers (Invited)

Atmospheric transmission calculation of diode‐pumped alkali laser energy transfer

AbstractWireless power transfer has become a research hotspot in the field of energy transmission, among which laser energy transfer is one of the best approaches for long‐distance wireless transmission. Based on good atmospheric transmittance, especially the wavelengths consistent with the response range of mainstream Si and GaAs photovoltaic cells with relatively high photoelectric efficiency, diode‐pumped alkali laser (DPAL) energy transfer will have huge prospects. To solve the problem of long‐distance laser energy transfer of DPALs, laser atmospheric molecular absorption was calculated by analyzing the atmospheric transmission absorption effects of laser beams with specific wavelengths. The atmospheric transmission of three typical alkalis at different atmospheric modes and altitudes was calculated and compared, and practical suggestions were given.

Analytical model of a diode-pumped cesium laser for investigation of upper-state mixing and quenching reactions.

A one-dimensional analytical model of a diode-pumped cesium vapor laser was constructed, and equations were derived to explain the dependence of the laser power on the hydrocarbon gas partial pressure. By varying the hydrocarbon gas partial pressure over a wide range and measuring the laser power, the corresponding mixing and quenching rate constants were validated. A gas-flow Cs diode-pumped alkali laser (DPAL) was operated with methane, ethane, and propane as buffer gases, with the partial pressures varied from 0 to 2 atmospheres. The experimental results were found to be in good agreement with the analytical solutions, confirming the validity of our proposed method. Separate 3-D numerical simulations were performed, and the output power effectively reproduced the experimental results over the entire buffer gas pressure range.

Kinetics of Metastable Argon Optical Excitation and Gain in Ar/He Microplasmas.

The optically pumped rare-gas metastable laser is capable of high-intensity lasing on a broad range of near-infrared transitions for excited-state rare gas atoms (Ar*, Kr*, Ne*, Xe*) diluted in flowing He. The lasing action is generated by photoexcitation of the metastable atom to an upper state, followed by collisional energy transfer with He to a neighboring state and lasing back to the metastable state. The metastables are generated in a high-efficiency electric discharge at pressures of ∼0.4 to 1 atm. The diode-pumped rare-gas laser (DPRGL) is a chemically inert analogue to diode-pumped alkali laser (DPAL) systems, with similar optical and power scaling characteristics for high-energy laser applications. We used a continuous-wave linear microplasma array in Ar/He mixtures to produce Ar(1s5) (Paschen notation) metastables at number densities exceeding 1013 cm-3. The gain medium was optically pumped by both a narrow-line 1 W titanium-sapphire laser and a 30 W diode laser. Tunable diode laser absorption and gain spectroscopy determined Ar(1s5) number densities and small-signal gains up to ∼2.5 cm-1. Continuous-wave lasing was observed using the diode pump laser. The results were analyzed with a steady-state kinetics model relating the gain and the Ar(1s5) number density.

Open-Path Atmospheric Transmission of Diode-Pumped Alkali Lasers in Maritime and Desert Environments.

A tunable diode laser absorption spectroscopy (TDLAS) device has been developed to study long-path atmospheric transmission near diode pumped alkali laser (DPAL) emission wavelengths. By employing a single aperture and retro reflector in a mono-static configuration, the noise associated with atmospheric and platform jitter were reduced by a factor of ∼30 and the open-air path length was extended to 4.4 km and over a very broad spectral range, up to 120 cm-1. Water vapor absorption lines near the rubidium (Rb) and cesium (Cs) variants of the DPAL near 795 and 894 nm, oxygen lines near the potassium (K) DPAL near 770 nm, and water vapor absorption in the vicinity of the neodymium-doped yttrium aluminum garnet (Nd:YAG) laser 1.064 μm and chemical oxygen iodine laser (COIL) 1.3 μm lines were studied. The detection limit for path absorbance increases from ΔA = 0.0017 at 100 m path length to 0.085 for the 4.4 km path. Comparison with meteorological instruments for maritime and desert environments yields agreement for the 2.032 km path to within 1.5% for temperature, 4.5% for pressure, and 5.1% for concentration, while agreements for the 4.4 km path are within 1.4% for temperature, 7.7% for pressure, and 23.5% for concentration. An intra cavity output spectroscopy (ICOS) device was also used as a spectral reference to verify location of atmospheric lines. Implications of TDLAS collection system design on signal-to-noise (S/N) are discussed as well as the effect of path turbulence on baseline noise and inform the selection of the DPAL variant least affected by molecular absorption.

Passive mode-locked cesium diode-pumped alkali laser.

A first, to the best of our knowledge, demonstration of passive mode-locking in diode-pumped alkali laser (DPAL) is reported in this paper. An intracavity cesium vapor cell, buffered by atmospheric pressure methane, is used to passively mode lock a continuously pumped cesium DPAL with a static gain medium. A train of short pulses with duration less than 460 ps was observed using a 2.0-GHz bandwidth detector that limited the real time duration measurements. The calculated minimum duration for these pulses is 57 ps.

Dissociative Recombination of K2+ and Rb2+ in Diode-Pumped Alkali Lasers: Potential Energy Curves and Atomic Products.

Dissociative recombination (DR) of K2+ and Rb2+ ions is one of the most important processes in K and Rb diode-pumped alkali lasers (DPALs) strongly affecting their power. We report on the calculations of potential energy curves of the K2+ and Rb2+ molecular ions and of the diabatic 1Σ+, 3Σ+, 1Δ, 3Δ, 1Π, 3Π, 1Φ, and 3Φ valence states of K2 and Rb2 that provide the routes for DR of the ions. These curves are required for subsequent calculations of DR rate constants. It was found that the most likely DR routes for K are along the dissociative states 61Σg+, 53Σu+, and 23Δu and for Rb only 61Σg+. The excited states of K atoms produced by DR are 42P and 52P. Most of the Rb atoms produced by DR are in the 62P excited state, while a small fraction of the atoms produced by the less likely routes along two states of Rb2 are in the 52P state. This conclusion contradicts the kinetic scheme for K and Rb DPAL proposed elsewhere, and thus the kinetic schemes of these DPALs should be modified according to the present results.

Diode pumped alkali laser: current status and prospects

Diode pumped alkali laser: current status and prospects

3D CFD modeling of flowing-gas Rb DPALs: effects of buffer gas composition and of ionization of high lying Rb states

A comprehensive three-dimensional computational fluid dynamics (3D CFD) modeling of flowing-gas Rb diode pumped alkali laser (DPAL) is carried out. The cases of H e / C H 4 and pure He buffer gases are investigated, and the output power and optical efficiency are calculated for various pump powers, mole fractions of methane, buffer gas pressures, and flow velocities. The model considers the processes of excitation of high levels of Rb, ionization, ion-electron recombination, and heating of electrons, which affect the diffusion coefficient of Rb ions. Two types of Rb DPAL were studied: a low-power laboratory-scale device with pump power of several tens of watts and a high-power multi-kilowatt laser. Efficient operation of the Rb laser using pure He as buffer gas can be achieved only in a large-scale laser with a pump beam cross-sectional area of several c m 2 . The calculated results for such a device were compared with those reported by Gavrielides et al. [J. Opt. Soc. Am. B 35, 2202 (2018)JOBPDE0740-322410.1364/JOSAB.35.002202], where a simplified three-level model based on the one-dimensional gas dynamics approach was applied.

Study of Ne:He Plasma of a Periodically Pulsed Discharge for Optically Pumped Rare Gas Laser

Optically pumped rare gas lasers (OPRGL) suggested recently as a chemically inert analog of diode-pumped alkali lasers are under extensive study at present. OPRGLs employ metastable atoms of heavier rare gases (Rg*) in He bath produced in discharge plasma. Ar* OPRGL is the most popular system at present, due to presence of a narrow band diode pump and abundance of Ar. However, Ne* OPRGL is interesting due to its visible lasing wavelength at 703.2 nm nm and presence of channels of energy transfer in Ne-He plasma that facilitate Ne* production. We present the first results of experiments with Ne* OPRGL that include Ne* number density in its active medium, and lasing experiments to determine pumping threshold for s5 → p9 transition in a transverse pumping configuration using a narrow band pulsed dye laser as a pump.

Theoretical study on amplified spontaneous emission (ASE) in a V-pumped thin-disk alkali laser

In the paper, a mathematical model was systematically constructed to evaluate the amplified spontaneous emission (ASE) effects and temperature distribution of a vapor cell of the V-pumped thin-disk alkali lasers. There have been few literatures on studying ASE in a diode-pumped alkali laser (DPAL) although such a phenomenon is normally serious especially in a thin-disk configuration. By reducing the number of the outputted conditions and adopting a parallel computation algorithm, the computation speed has been demonstrated to be greatly improved by comparing to the method we ever used before. The results show that the ASE effects in a thin-disk DPAL cannot be ignored especially when the size of a pump beam becomes big. It has also been proved that ASE in a thin-disk DPAL tightly relates to the pump power, thickness and temperature of a thin-disk cell, and reflectivity of the output coupler (OC). The calculation approaches in this paper are thought to be valuable in the design of a high-powered DPAL and can also be applied to the other kinds of high-powered lasers.

Improving the beam quality of DPALs by refractive index gradients induced by the pump beam in the heated gain medium: experimental verification of the theoretical prediction

We recently predicted [J. Opt. Soc. Am. B 35, 558 (2018)JOBPDE0740-322410.1364/JOSAB.35.000558] that large radial gradients of the refractive index induced by the pump beam in the heated gain medium of diode-pumped alkali lasers (DPALs) could lead to improved DPAL beam quality. This effect depends on the composition of the buffer gas in the cell. We report here on verifying this counterintuitive finding by careful measurements of the beam quality factor M 2 in static Cs DPALs with different compositions of the buffer gases.

Peak-power enhancement of a cavity-dumped cesium-vapor laser by using dual longitudinal-mode oscillations.

We have performed cavity dumping of a diode-pumped alkali laser (DPAL) and have observed a saw-like structure in the pulse waveform that appears to be caused by interference between two or more longitudinal modes. We have confirmed that multimode oscillations are caused by spatial hole burning, and the largest peak was seen when only two oscillation modes were present. This phenomenon may be useful for enhancing the cavity dumping of the DPAL, but it was not always observed. Therefore, we developed a numerical simulation to predict the number of longitudinal modes excited under a given set of conditions and provides guidelines to facilitate dual-mode oscillation. Using these guidelines, we have obtained a pulse with a peak power of 250 W, which is higher than the average power circulating in the cavity and is a 38-fold enhancement of the continuous-wave (CW) output.

Satellites of the resonance transitions of alkali atoms in mixtures with CF4

The absorption and luminescence excitation spectra of gas phase mixtures of alkali metals atoms A (A = K, Rb, Cs) with carbon tetrafluoride molecules CF4 are studied in the region of transitions from the ground state A(2S1/2) to the lowest resonance states A(2P1/2, 3/2). It is shown that these transitions have intense satellites corresponding to the simultaneous vibrational excitation of the carbon tetrafluoride molecule, A(2S1/2) + CF4(v3=0) + hv → A(2P1/2, 3/2) + CF4(v3=1), where v3 is the IR active mode of CF4 with a vibrational quantum energy of 1281 cm–1. The satellites are relatively narrow bands (FWHM ~ 30 cm–1) slightly (< 10 cm–1) shifted to the red from the energies corresponding to the asymptotes A(2PJ) + CF4(v3=1), J=1/2, 3/2. The spectral width and position of the satellites indicate that these transitions occur at relatively large atom-molecule distances. In contrast to the majority of the studied before pair excitation processes in the gas phase, the intensity borrowing mechanism responsible for the observed satellite bands cannot be explained by the dipole-dipole interaction. Possible alternative mechanisms are discussed. It is also established that mixtures with CF4 are chemically stable including at elevated temperatures up to at least 200°C and their optical excitation results in resonance luminescence of alkali atoms. As compared to Ar, collisions with CF4 provides a higher rate of the population transfer between the A(2P1/2) and A(2P3/2) states. These results indicate that CF4 may be used as a component of the active medium of diode pumped alkali lasers.

Controlling the beam quality in DPALs by changing the resonator parameters

The beam quality of diode-pumped alkali lasers (DPALs) is of major concern in the studies of these lasers. We report on experimental studies and modeling of the beam propagation factor M2 in flowing-gas Cs DPALs with stable optical resonators and of its dependence on the resonator geometry. The measured values of M2 are in agreement with those calculated using the optical model of multi-transverse mode operation (Auslender et al. in Opt Express 25:19767, 2017). Conditions for substantial improvement of the output laser beam quality, reducing M2 to close to unity, are found. Changing the length of the resonator, and/or the radius of curvature of the high reflection mirror, leaving all other parameters of the laser unchanged, makes it possible to control the beam quality. Techniques for controlling M2 in DPALs with wide-aperture beams have not been studied elsewhere and are very important for the applications of this laser. In addition to the optical model, a simple analytical method based on the model suggested by Siegman (IEEE J Quantum Electron 29:1212, 1993) is applied to the experimental results and predicts a simple scaling law for M2 as a function of the size of the resonator fundamental mode.

Temperature dependence of the fine structure mixing induced by 4He and 3He in K and Rb Diode Pumped Alkali Lasers

A critical evaluation of the experimental and theoretical literature regarding the rates for fine structure mixing in the first excited 2P3/2,1/2 states of potassium and rubidium induced by collision with 4He is performed. The evaluation is used to develop a recommendation for the fine structure mixing rates as a function of temperature. The rate coefficient for transitions from Rb 5 2P3/2 to 52P1/2 induced by helium ranges from 1.2 to 4.6 × 10–12 cm3 s−1 as temperature increases from 300–600 K. Remarkable agreement for the thermally average cross-sections is observed between the empirical, semi-classical perturbative, and quantum scattering theory approaches. In addition to collisions with 4He, new energy-dependent cross-sections for the K 42P mixing induced by 3He are reported using time-dependent channel packet scattering theory. The 3He rates are faster than for 4He due to both increased relative speed and more impulsive interactions. The fine-structure mixing rates in high-power Diode Pumped Alkali Lasers limit intensity scaling and are the primary source of heating. This heating induces spatial and temporal optical phase variations and accurate values for mixing rates as a function of temperature are critical for high fidelity laser modeling and simulation.

Modeling of diode-pumped cesium vapor laser by combination of computational fluid dynamics and wave-optics

We have developed a numerical simulation method for diode-pumped alkali lasers (DPALs). We previously reported the use of a simplified gas-flow model, whose adaptability was restricted to forced gas-flow DPALs. In the present study, we have employed an incompressible, three-dimensional computational fluid dynamics (CFD) model. The natural convective flow is modeled using a modified Boussinesq approximation. The results of the simulations were found to be in good agreement with those for a compressible CFD model for a heat-induced flow problem. As a result, good agreement with the experimental results was achieved for a sealed DPAL. On the other hand, the results for the forced gas-flow DPAL were the same as those for the simplified gas-flow model, which confirms the validity of the latter model. The dependence of the beam quality on the pump power for the sealed DPAL was consistent with previously reported results.

Analytic non-adiabatic derivative coupling terms for spin-orbit MRCI wavefunctions. I. Formalism.

Analytic gradients of electronic eigenvalues require one calculation per nuclear geometry, compared to at least 3n + 1 calculations for finite difference methods, where n is the number of nuclei. Analytic nonadiabatic derivative coupling terms (DCTs), which are calculated in a similar fashion, are used to remove nondiagonal contributions to the kinetic energy operator, leading to more accurate nuclear dynamics calculations than those that employ the Born-Oppenheimer approximation, i.e., that assume off-diagonal contributions are zero. The current methods and underpinnings for calculating both of these quantities, gradients and DCTs, for the State-Averaged MultiReference Configuration Interaction with Singles and Doubles (MRCI-SD) wavefunctions in COLUMBUS are reviewed. Before this work, these methods were not available for wavefunctions of a relativistic MRCI-SD Hamiltonian. Calculation of these terms is critical in successfully modeling the dynamics of systems that depend on transitions between potential energy surfaces split by the spin-orbit operator, such as diode-pumped alkali lasers. A formalism for calculating the transition density matrices and analytic derivative coupling terms for such systems is presented.

Dependence of static K DPAL performance on addition of methane to He buffer gas: 3D CFD modeling and comparison with experimental results

Recently it was observed that adding a small amount of methane to the He buffer gas of a static potassium diode pumped alkali laser (K DPAL) increases considerably the laser power. Further increase in the amount of methane leads to a moderate decrease in power. In the present work the effect of methane addition was investigated theoretically applying a 3D computational fluid dynamics (CFD) and potassium kinetics model, which was supplemented by the analysis of the electron temperature and K ion ambipolar diffusion. It was found that for a pure He buffer the K DPAL power is lower than for ${\rm He}/{{\rm CH}_4}$He/CH4 mixtures due to slow ion-electron recombination and high electron temperature exceeding 3000 K. The high electron temperature in pure He results in fast ambipolar diffusion of K ions to the wall and depletion of the neutral K atoms in the lasing region. These effects are mitigated when methane is added to the buffer gas. The calculated results for the normalized laser power are in satisfactory agreement with the experimental ones.