We demonstrate and analyze the use of an ion chamber for measuring laser-induced ionization in cesium gas for the first time, which is of recent interest due to research in diode pumped alkali lasers (DPALs). In this report, the viability of an ion chamber diagnostic with high plasma density and ionization localized to a laser beam is investigated. A simulation of the laser-induced plasma in the ion chamber, based on the Thomson model with diffusion, is developed and will be shown to display similar qualitative behavior to measurements, and bound test results within model uncertainty. The analysis will show that complex processes occur: (1) space-charge limited ion drift, (2) Debye shielding preventing the electric field from penetrating a bulk plasma region, and (3) ambipolar diffusion across the bulk with possibly elevated electron temperature. However, these processes are well understood and do not limit the accuracy of an ion chamber diagnostic for laser-induced ionization rate measurement.

We report direct measurement of the laser induced ionization rate of cesium, relevant for a Diode Pumped Alkali Laser (DPAL), via application of an ion chamber diagnostic. Computer simulation predictions of the multi-step ionization mechanism will be compared against measured ionization rates. The results will be shown to accurately predict the low level of ionization to within an order-of-magnitude, as well as relative trends across pump intensities of 8–100 W/cm ^2 and cesium densities of 0.3–2.2 × 10 ^12 cm ^-3 . Comparison of fluorescence from 7P energy states with known direct excitation pathways and fluorescence from highly-excited 7D states suggests rapid mixing of high energy states. The application of 300 V on the ion chamber electrodes (sufficient to cause current saturation) has minimal impact on fluorescence. This supports the notion that Rydberg states are populated via a neutral particle process, rather than via electron/ion recombination, as has been previously suggested.

Pump modulated laser absorption spectroscopy was used to measure the cesium 5 2 D 5/2 →10 2 F 5/2,7/2 line shape. Alkali density in an optically pumped low pressure cell was increased from 0.5 to 85×10 12 cm −3 , resulting in a factor of 200 increase in Lorentzian width due to Stark broadening (2.76GHz/10 12 cm −3 ). The observed red shift is consistent with literature, and the shift-to-width ratio δ / γ =−0.356±0.015 informs an electron temperature of 1820±100K. A linear trend between alkali density and electron density is seen, suggesting that electron impact is the primary plasma production mechanism. A maximum ionized concentration of 28% was observed.

Pump modulated laser absorption spectroscopy was used to measure the cesium 52 D5/2 -+ 102 F5/2,7/2 line shape. Alkali density in an optically pumped low pressure cell was increased from 0.5 to 85 x 1012 cm-3, resulting in a factor of 200 increase in Lorentzian width due to Stark broadening (2.76 GHz/1012 cm-3). The observed red shift is consistent with literature, and the shift-to-width ratio delta/gamma = -0.356 +/- 0.015 informs an electron temperature of 1820 +/- 100 K. A linear trend between alkali density and electron density is seen, suggesting that electron impact is the primary plasma production mechanism. A maximum ionized concentration of 28% was observed.

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.

Pump modulated laser absorption spectroscopy was used to measure the cesium 62P1/ 2 & RARR; 92S1/ 2 and 52D5/ 2 & RARR; 102F5/ 2, 7/ 2 line shapes with helium, argon, and methane used as collision partners. The broad-ening rates are 99.5 & PLUSMN;3.4, 116.6 & PLUSMN;1.8, 188.2 & PLUSMN;3.3, 133.2 & PLUSMN;1.2, 294 & PLUSMN;4.5, and 710 & PLUSMN;18 MHz/Torr for He, Ar, and CH4 in 6P - 9S and 5D - 10F respectively, while the shift rates are +121 & PLUSMN;21, -29.3 & PLUSMN;21, -61 & PLUSMN;16, +101.4 & PLUSMN;0.8, -273.0 & PLUSMN;8.0, and -231 & PLUSMN;29 MHz/Torr. For those same transitions, the asymmetry rates are +0.18 & PLUSMN;0.15, -3.1 & PLUSMN;0.14, -3.2 & PLUSMN;0.1, +0.14 & PLUSMN;0.04, and -0.32 & PLUSMN;0.22 mrad/Torr, with no reported value for CH4 along 5D - 10F. Broadening rates scale as expected with polarizability, shift direction is to the blue/red with He/other gases, and the direction of the asymmetry correlates to the shift. Published by Elsevier Ltd.

The rubidium 5(2) S-1/2 -> 5(2) D-3/2 and 5(2) S-1/2 -> 5(2) D-5/2 two-color two-photon transitions were examined with a variety of detunings in the region 758-798 nm to reveal the transition rates. Two cw lasers with crossed linear polarizations were overlapped and counterpropagated through a vapor cell, with the transition rate monitored by side fluorescence from 6P. Resonant enhancement via the 5P states results in intensity variations of over 6 orders of magnitude across the detuning range. No destructive interference zeros were seen, as expected from the polarization dependent calculation of the third-order susceptibility. Zeros occur from interference between two intermediate states, but due to the contributions from both symmetric and antisymmetric susceptibility tensors the zero of one is hidden by a non-zero value of the other. The polarization dependence was explicitly seen by fixing the detuning and iterating the linear polarizations from crossed to parallel.

Optically pumped rare gas lasers are being investigated as potential high-energy, high beam quality systems. The lasing medium consists of rare gas atoms (Rg=Ne, Ar, Kr, or Xe) that have been electric discharge excited to the metastable np5(n+1)s P32 state. Following optical excitation, helium (He) at pressures of 200-1000 Torr is used as the energy transfer agent to create a population inversion. The primary technical difficulty for this scheme is the discharge production of sufficient Rg* metastables in the presence of >200  Torr of He. In this Letter, we describe a pulsed discharge that yields >1013  cm-3Ar* in the presence of He at total pressures up to 750 Torr. Using this discharge, a diode-pumped Ar* laser providing 4.1 W has been demonstrated.

Diode-pumped alkali lasers advances are reviewed to include the impact of research toward power scaling, modeling efforts, physical studies, and alternate hybrid laser development.

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 10(11) 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.

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.

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.

Pressure induced broadening and spectral shift rates have been resolved for the two ground state hyperfine lines in the 5(2)S(1/2) -> 6(2)P(1/2) and 5(2)S(1/2) -> 6(2)P(3/2) of Rb-85 and Rb-87 in the presence of 5-50 Tort of helium, argon, methane, and ethane. Broadening rates averaged over the hyperfine components, for the P-1/2 transition for He, Ar, CH4, and C2H6 are 60.2, 45.2, 63.8, and 60.4 MHz/Torr, respectively and 50.4, 42.4, 62.0, and 60.4 MHz/Torr for the P-3/2 transition. Average spectral shift rates for the P-1/2 transition were found to be 15.7, -11.5, -18.1, and -19.9 MHz/Tort for He, Ar, CH4, and C2H6, respectively, and 3.1, - 12.6,-21..8, and -19.9 MHz/Torr for the P-3/2 transition. Deviation in broadening and shift rates between hyperfine lines was found to be as high as 16 and 7.6 MHz/Torr respectively. Published by Elsevier Ltd.

Pressure induced broadening and spectral shift rates have been resolved for the two ground state hyperfine lines in the 52S1/2→62P1/2 and 52S1/2→62P3/2 of 85Rb and 87Rb in the presence of 5–50 Torr of helium, argon, methane, and ethane. Broadening rates averaged over the hyperfine components, for the P1/2 transition for He, Ar, CH4, and C2H6 are 60.2, 45.2, 63.8, and 60.4 MHz/Torr, respectively and 50.4, 42.4, 62.0, and 60.4 MHz/Torr for the P3/2 transition. Average spectral shift rates for the P1/2 transition were found to be 15.7, −11.5, −18.1, and −19.9 MHz/Torr for He, Ar, CH4, and C2H6, respectively, and 3.1, −12.6,−21.8, and −19.9 MHz/Torr for the P3/2 transition. Deviation in broadening and shift rates between hyperfine lines was found to be as high as 16 and 7.6 MHz/Torr respectively.

The pressure broadening and shift rates for the K D2 (4P3/22←4S1/22) transition with N2, CH4, C2H6, C3H8, n-C4H10, and the noble gases were obtained for pressures up to 80 Torr and at a temperature of 55°C by means of laser absorption spectroscopy. Additionally, the broadening and shift rates for the Rb D1 (5P1/22←5S1/22) transition for He, CH4, C2H6, C3H8, and n-C4H10 were obtained using the same techniques and under similar conditions. The K D2 collisional broadening rate, γL, for He, Ne, Ar, Kr, Xe, N2, CH4, C2H6, C3H8, and n-C4H10 are 19.84, 8.88, 18.65, 19.17, 22.19, 18.98, 27.78, 27.60, 27.70, and 33.48MHz/Torr, respectively. The uncertainty in the broadening rates is typically less than 2.1%. The corresponding pressure induced shift rates, δ, are 0.52, −2.06, −5.52, −5.42, −7.01, −5.66, −8.38, −8.04, −9.22, and −9.37MHz/Torr with an uncertainty of less than 1.8%. The Rb D1 collisional broadening rates for He, CH4, C2H6, C3H8, and n-C4H10 are 20.80, 32.78, 30.49, 33.05, and 29.61 with uncertainties typically less than 2.2%. The collisional shift rates for the Rb D1 transition are 5.80, −6.96, −7.88, −8.61, and −9.43 with uncertainties on the order of 1.1%. A comparison with the other alkali broadening and shift cross-sections is presented.

At high pressure the first resonance lines of rubidium have been observed to broaden asymmetrically. A theoretical 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.

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.

The pressure broadening and shift rates for the potassium D1(42P1/2←42S1/2) transition with the noble gases and 3He, H2, HD, D2, N2, CH4, C2H6, C3H8, and n-C4H10 were obtained for pressures up to 80 Torr and at a temperature of 55 °C by means of laser absorption spectroscopy. The collisional broadening rate, γL, for He, 3He, Ne, Ar, Kr, Xe, H2, HD, D2, N2, CH4, C2H6, C3H8, and n-C4H10 are 13.08, 17.46, 6.14, 19.45, 16.64, 20.02, 22.15, 19.36, 17.47, 17.78, 29.35, 26.63, 27.27, and 27.85 MHz/Torr, respectively. The uncertainty in the broadening rates is typically less than 1.6%. The corresponding pressure induced shift rates, δ, are 1.63, 6.82, −1.27, −6.44, −5.42, −6.54, −5.34, −5.10, −4.70, −6.80, −7.41, −8.32, −8.59, and −8.80 MHz/Torr with a uncertainty of less than 2.4%. A comparison with the other alkali D1 broadening cross-sections is presented.

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.