Developments in diode pumped alkali laser (DPAL) systems have been impeded because of the catastrophic failure of laser windows. The window’s failure is caused by localized laser-induced heating of window material. This heating is believed to occur due to increases in absorption on or near the surface of the window. This increase is believed to be caused by either adsorption of carbon-based soot from the collisional gas or by the diffusion of rubidium into the bulk material. The work presented here will focus on the diffusion of Rb into the bulk window materials and will strive to identify a superior material to use as windows. The results of this research indicate that aluminum oxynitride (ALON), sapphire, MgAl2O4 (spinel), and ZrO2 are resistant to alkali-induced changes in optical properties.
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.
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.
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.
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.
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
The absolute absorption and stimulated emission cross-sections, including the effects of hyperfine splitting
and pressure broadening at low to moderate pressures are computed and compared with experimental
results. The comparison is excellent and requires no fit parameters. An analysis of the degree to which the
lineshape can be approximated by a single Lorentzian profile is provided as a function of background
In this paper we describe a quasi-two level analytic model for end pumped Alkali metal vapor lasers. The
model is developed by considering the steady state rate equations for the number densities of the, 2S1/2,
2P3/2, and 2P1/2, energy states for the three level laser system. The approximation is then made that the
relaxation between the two upper levels, 2P3/2 and 2P1/2, caused by collisions with additive ethane is much
faster, in fact infinitely fast, by comparison with any other process in the system including stimulated
emission. With this assumption the ratio of the number densities for the upper two levels, 2P3/2 and 2P1/2, is
given by its statistical equilibrium value and the mathematical description becomes that of a quasi-two level
system from which an analytic solution can be extracted. The analytic model description gives expressions
for the threshold pump power and the slope efficiency including intra-cavity losses. Applications of the
model and comparisons with the steady state three level model developed by Beach et al. will be presented.
Chemical lasers offer the highest powers necessary for many weapons applications, but require significant logistical support in the delivery of specialized fuels to the battlefield. In the Chemical Oxygen-Iodine Laser (COIL), which is the weapon aboard the Airborne Laser (ABL), gaseous chlorine and liquid basic hydrogen peroxide are used to generate the singlet oxygen energy reservoir. The goal of the current multi-university research program is to demonstrate an oxygen-iodine laser with electrical discharge production of singlet oxygen. Typically, oxygen discharges are limited to about 15% yield for singlet oxygen. The electron excitation cross-sections as a function of E/N are well established. However, the kinetics for electron and singlet oxygen interactions is considerably more difficult to study. Optical diagnostics for O2(a, b), and O, have been applied to a double microwave discharge flow tube. By examining the difference in singlet oxygen kinetics between the two discharges in series, considerable information regarding the excited-state, excited-state interactions is obtained. Under certain discharge conditions, the O2(a) concentration significantly increases outside of the discharge, even after thermal effects are accounted.