Greg A. Pitz

Simulation of deleterious processes in a static-cell diode pumped alkali laser

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.

Advancements in flowing diode pumped alkali lasers

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.

An experimental high pressure line shape study of the rubidium D1 and D2 transitions with the noble gases, methane, and ethane

At high pressure the rst resonance lines of rubidium have been observed to broaden asymmetrically. A the- oretical line shape for this asymmetry has been determined via the Anderson-Talman theory and the impact approximation. The broadening and shift rates compared nicely to previous low pressure results and the rates for asymmetry have been measured for the noble gases, methane, and ethane.

Atmospheric propagation properties of various laser systems

Atmospheric propagation properties of various laser systems, including diode pumped alkali lasers (DPALs) and the Chemical Oxygen Iodine Laser (COIL), are of importance. However, there appears to be a lack of highly accurate transmission characteristics of these systems associated with their operating conditions. In this study laser propagation of the rubidium-based DPAL and the COIL has been simulated utilizing integrated cavity output spectroscopy. This technique allowed for the simulation of laser propagation approaching distances of 3 kilometers on a test stand only 35 cm long. The spectral output from these simulations was compared to the HITRAN database with excellent agreement. The spectral prole and proximity of the laser line to the atmospheric absorbers is shown. These low pressure spectral proles were then extrapolated to higher pressures using an in-house hyperne model. These models allowed for the comparison of proposed systems and their output spectral prole. The diode pumped rubidium laser at pressures under an atmosphere has been shown to interact with only one water absorption feature, but at pressures approaching 7 atmospheres the D1 transition may interact with more than 6 water lines depending on resonator considerations. Additionally, a low pressure system may have some slight control of the overlap of the output prole with the water line by changing the buer gases.

Plasma and laser kinetics and field emission from carbon nanotube fibers for an Advanced Noble Gas Laser (ANGL)

A metastable argon laser operating at 912 nm has been demonstrated by optically pumping with a pulsed titanium sapphire laser to investigate the temporal dynamics of an Advanced Noble Gas Laser (ANGL). Metastable argon concentrations on the order of 1011 cm-3 were maintained with the use of a radio frequency (RF) capacitively coupled discharge. The end-pumped laser produced output powers under 2 mW of average power with pulse lengths on the order of 100 ns. A comparison between empirical results and a four level laser model using longitudinally average pump and inter-cavity intensities is made. An alternative, highly-efficient method of argon metastable production for ANGL was explored using carbon nanotube (CNT) fibers.

Field emission excitation of a high pressure noble gas

Electric Hybrid Lasers (EHL) combines the benefits of a solid state laser (SSL) and a gas phase system. EHLs have the electrical capacity of an SSL and the thermal management and beam quality of a gaseous lasing medium. Researchers at Emory University have developed a novel EHL.1 A three-level EHL is being developed that utilizes a capacitively coupled RF discharge to produce metastable excited states of a Noble gas to and from the ground state of the laser. The lowest meta-stable state is optically pumped by employing diodes resonant with the highest energy state and then is spin mixed to transition to the lasing state. The atom then lases back to the beginning meta-stable state. To improve upon the efficiency of the capacitively coupled RF discharge for producing the meta-stable ground state, a new approach for producing meta-stables is investigated utilizing field emission into a high pressure Noble gas. If the electric field to pressure (E/P) ratio is kept sufficiently low, ions and electrons produced via ionization is negligible. The low E/P ratio is achieved due to the low turn-on electric field for the field emitters, thus the majority of the electrons in the gas are due to field emission, resulting in a highly non-neutral plasma. Experimental results have shown that individual field emission fibers can produce relatively high current of greater than a micro-Amp at extremely low electric fields (160 kV/m). In addition, experimental results show that at lower currents, the current-voltage characteristic is consistent with Fowler-Nordheim emission. At higher current levels, the current-voltage characteristic enters into a space charge limited regime where current increases as the square of the voltage. Excitation of the Argon gas using field emission was accomplished and spectroscopic measurements of the optical emission were made showing the lasing state was excited and relaxed to the ground meta-stable state. PIC simulations were able to reproduce the same trends observed in the experimental results. Experimental results showed that Argon meta-stables could be produced at E/P ratios well below what could be used to sustain a standard plasma discharge.

Noble gas meta-stable state excitation using carbon nanotube fiber cathodes

Summary form only given. Electric Hybrid Lasers (EHL) are of great interest for commercial and government application due to their ability to combine the benefits of a solid state laser (SSL) with the benefits of a gas phase system. EHLs have the electrical capacity of an SSL and the thermal management and beam quality of a gaseous lasing medium. Recently, researchers at Emory University have developed a novel EHL.1 The Discharge Assisted Noble Gas Laser (DANGL), is a three-level laser that utilizes a mild electrical discharge to produce metastable excited states of a Noble to form the ground state of the laser. The meta-stables are optically pumped by employing diodes resonant with the highest energy state. After excitation, relaxation via collisions with helium from the highest excited state to the lasing state occurs. The atom then lases back to the metastable state. To improve upon the efficiency of the mild electrical discharge of the original DANGL, a new approach for producing meta-stables is investigated utilizing field emission from Carbon-Nanotube (CNT) fibers into a high pressure Noble gas. If the electric field to pressure (E/P) ratio is kept sufficiently low and pulse widths are short, ionization is significantly reduced. Not allowing for full sustained breakdown allows the majority of the electrons in the gas to result from field emission from the CNT fiber, thus creating a non-neutral plasma. Modeling of the DANGL meta-stable excitation was accomplished with a combined 3-D electromagnetic Particle-in-Cell (PIC) and Monte Carlo Collision (MCC) model. Modeling was performed to optimize the geometry of field emission from the CNT fibers in order to maximize the yield of meta-stable states. Model results showed high yields of Ar meta-stables could be achieved at E/P ratios that could not sustain a standard plasma discharge. Model results enabled the development of optimized experimental set-up and interpretation of the experimental current-voltage characteristics. Experimental results have also shown that CNT fibers can produce relatively high current pulses for extremely low electric field (160 kV/m) at a 5 nanosecond pulse width, thus enabling Noble gas meta-stable excitation without neutral plasma production.