Advanced laser modeling with BLAZE multiphysics
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![BLAZE Cs DPAL calculated outcoupled laser power (lines) compared to [Beach, 2004] data (points) as a function of outcoupler reflectivity R OC and pump power.](https://www.researchgate.net/profile/Andrew-Palla/publication/312637167/figure/fig1/AS:463924224106496@1487619562375/BLAZE-Cs-DPAL-calculated-outcoupled-laser-power-lines-compared-to-Beach-2004-data_Q320.jpg)
![BLAZE predictions of outcoupled laser power as a function of outcoupler reflectivity and cell temperature using [Beach, 2004] data at 383 K as the baseline.](https://www.researchgate.net/profile/Andrew-Palla/publication/312637167/figure/fig2/AS:463924224106497@1487619562398/BLAZE-predictions-of-outcoupled-laser-power-as-a-function-of-outcoupler-reflectivity-and_Q320.jpg)
![Calculated normalized pump and laser intensity at the detector as a function of time compared to data for the 423 K, 6 cm cell, five-level, 836.7 nm pump baseline case [Readle, 2008].](https://www.researchgate.net/profile/Andrew-Palla/publication/312637167/figure/fig4/AS:463924224106499@1487619562438/Calculated-normalized-pump-and-laser-intensity-at-the-detector-as-a-function-of-time_Q320.jpg)
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The optically pumped rare-gas metastable laser is a chemically inert analogue to three-state optically pumped alkali laser systems. The concept requires efficient generation of electronically excited metastable atoms in a continuous-wave (CW) electric discharge in flowing gas mixtures near atmospheric pressure. We have observed CW optical gain and laser oscillation at 912.3 nm using a linear micro-discharge array to generate metastable Ar(4s, 1s5) atoms at atmospheric pressure. We observed the optical excitation of the 1s5 → 2p9 transition at 811.5 nm and the corresponding fluorescence, optical gain and laser oscillation on the 2p10 ↔ 1s5 transition at 912.3 nm, following 2p9→2p10 collisional energy transfer. A steady-state kinetics model indicates efficient collisional coupling within the Ar(4s) manifold.
Optically pumped lasers that use metastable excited states of Ar have been demonstrated using both pulsed and CW excitation. In terms of Paschen labeling of the states of Ar, the laser system uses excitation of the 2 p 9 - 1 s 5 transition, and lases on the 2 p 10 - 1 s 5 line. Collisional transfer of population from 2 p 9 to 2 p 10 is achieved using He as the buffer gas. For the purpose of modeling and developing this laser, rate constants for state-to-state transfer in Ar ( 2 p i ) + Ar / He mixtures are needed. As the 2 p 10 level can radiate down to 1 s 4 , this lower level also plays a significant role in the laser kinetics. Consequently, rate constants for the relaxation of 1 s 4 by Ar and He are also required. In the present study we have used pulsed laser excitation techniques to measure rate constants of relevance to the optically pumped metastable Ar laser.
Experiments and modeling have led to continued enhancements in the Electric Oxygen-Iodine Laser (ElectricOIL) system. This continuous wave (cw) laser operating on the 1315 nm transition of atomic iodine is pumped by the production of O 2 (a) in a radio-frequency (RF) discharge in an O 2 /He/NO gas mixture. New discharge geometries and increases in gain length, flow rates, discharge power, and resonator mode volume have improved the peak measured gain to 0.26% cm -1 and the outcoupled laser power to 102 W. The BLAZE model has been used to perform end-to-end (discharge though laser resonator) simulations of the new system configuration to help guide this process. Results are in good agreement with data. Additional measurements of gain recovery downstream of an operating laser cavity for five cases of interest are presented and modeled. The deviation of the gain recovery data from calculations based on presently accepted theory is highlighted. Several potential mechanisms to explain this theory are presented and discussed.
Experiments[1] with Electric Oxygen-Iodine Laser (ElectricOIL) heat exchanger technology have demonstrated improved control of oxygen atom density and thermal energy, with minimal quenching of O2(a1Δ), and increasing small signal gain from 0.26% cm-1 to 0.30% cm-1. Heat exchanger technological improvements were achieved through both experimental and modeling studies, including estimation of O2(a1Δ) surface quenching coefficients for select ElectricOIL materials downstream of a radio-frequency discharge-driven singlet oxygen generator. Estimation of O2(a1Δ) quenching coefficients is differentiated from previous studies by inclusion of oxygen atoms, historically scrubbed using HgO[2-4] or AgO[5]. High-fidelity, time-dependent and steady-state simulations are presented using the new BLAZE-VI multi-physics simulation suite[6] and compared to data.
Pulsed lasing from optically pumped rare gas metastable atoms (Ne, Ar, Kr, and Xe) has been demonstrated previously. The laser relies on a three-level scheme, which involves the ( n + 1 ) p [ 5 / 2 ] 3 and ( n + 1 ) p [ 1 / 2 ] 1 states from the n p 5 ( n + 1 ) p electronic configuration and the metastable ( n + 1 ) s [ 3 / 2 ] 2 level of the n p 5 ( n + 1 ) s configuration (Racah notation). Population inversions were achieved using relaxation from ( ( n + 1 ) p [ 5 / 2 ] 3 to ( n + 1 ) p [ 1 / 2 ] 1 induced by collisions with helium or argon at pressures near 1 atm. Pulsed lasing was easily achieved using the high instantaneous pump intensities provided by a pulsed optical parametric oscillator excitation laser. In the present study we examine the potential for the development of a continuous wave (CW) optically pumped Ar laser. We report lasing of the 4 p [ 1 / 2 ] 1 → 4 s [ 3 / 2 ] 2 (912.547 nm) transition following CW diode laser excitation of the 4 p [ 5 / 2 ] 3 ← 4 s [ 3 / 2 ] 2 line (811.754 nm). A pulsed discharge was used to generate Ar 4 s [ 3 / 2 ] 2 , and the time-resolved lasing kinetics provide insights concerning the radiative and collisional relaxation processes.
Multiple variants of the Diode Pumped Alkali Laser (DPAL) have recently been demonstrated at the Air Force Research Laboratory (AFRL). Highlights of this ongoing research effort include: a) a 571W rubidium (Rb) based Master Oscillator Power Amplifier (MOPA) with a gain (2α) of 0.48 cm⁻¹, b) a rubidium-cesium (Cs) Multi-Alkali Multi-Line (MAML) laser that simultaneously lases at both 795 nm and 895 nm, and c) a 1.5 kW resonantly pumped potassium (K) DPAL with a slope efficiency of 50%. The common factor among these experiments is the use of a flowing alkali test bed.
In the last four years, a few research groups worked on the feasibility of compressive sampling (CS) in ultrasound medical imaging and several attempts of applying the CS theory may be found in the recent literature. In particular, it was shown that using iotap-norm minimization with p different from 1 provides interesting RF signal reconstruction results. In this paper, we propose to further improve this technique by processing the reconstruction in the Fourier domain. In addition, alpha -stable distributions are used to model the Fourier transforms of the RF lines. The parameter p used in the optimization process is related to the parameter alpha obtained by modelling the data (in the Fourier domain) as an alpha -stable distribution. The results obtained on experimental US images show significant reconstruction improvement compared to the previously published approach where the reconstruction was performed in the spatial domain.
The complex interactions in a diode pumped alkali laser (DPAL) gain cell provide opportunities for multiple deleterious processes to occur. Effects that may be attributable to deleterious processes have been observed experimentally in a cesium static-cell DPAL at the United States Air Force Academy [B.V. Zhdanov, J. Sell, R.J. Knize, "Multiple laser diode array pumped Cs laser with 48 W output power," Electronics Letters, 44, 9 (2008)]. The power output in the experiment was seen to go through a "roll-over"; the maximum power output was obtained with about 70 W of pump power, then power output decreased as the pump power was increased beyond this point. Research to determine the deleterious processes that caused this result has been done at the Air Force Research Laboratory utilizing physically detailed simulation. The simulations utilized coupled computational fluid dynamics (CFD) and optics solvers, which were three-dimensional and time-dependent. The CFD code used a cell-centered, conservative, finite-volume discretization of the integral form of the Navier-Stokes equations. It included thermal energy transport and mass conservation, which accounted for chemical reactions and state kinetics. Optical models included pumping, lasing, and fluorescence. The deleterious effects investigated were: alkali number density decrease in high temperature regions, convective flow, pressure broadening and shifting of the absorption lineshape including hyperfine structure, radiative decay, quenching, energy pooling, off-resonant absorption, Penning ionization, photoionization, radiative recombination, three-body recombination due to free electron and buffer gas collisions, ambipolar diffusion, thermal aberration, dissociative recombination, multi-photon ionization, alkali-hydrocarbon reactions, and electron impact ionization.
In recent studies, an optically pumped Ar*/He laser has been demonstrated using the Ar 4p[1/2]1→4s[3/2]2 transition at 912.55 nm. Time-resolved data for this system, recorded using CW laser excitation and pulsed discharge production of Ar* 4p[3/2]2, yielded laser output pulses that were of unexpectedly short duration. It was speculated that radiative relaxation from the upper laser level to the 4s[3/2]1 state (607 cm-1 above 4s[3/2]2) caused termination of the laser pulse. In the present study this hypothesis has been tested by observing the energy transfer kinetics of the 4s[3/2]2 and 4s[3/2]1 states in Ar/He gas mixtures. Following pulsed laser excitation out of 4s[3/2]2, population recovery was observed on a μs time scale. Energy transfer from 4s[3/2]1 to 4s[3/2]2, induced by collisions with He, was characterized. The rate constant was found to be (1.0±0.5)x10-13 cm3 s-1. These observations confirmed that radiative transfer to 4s[3/2]1 was responsible for the short duration laser pulses. Modeling of a fully CW optically pumped Ar* laser shows that radiative transfer to 4s[3/2]1 reduces the number density of the Ar* atoms involved in lasing, but is otherwise benign.