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.

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.

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.

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.

Experimental slope efficiencies of 72% to 76% are achieved for a pulsed Rb-methane optically pumped alkali metal vapor laser with pump intensities up to 120kW/cm(2). Measurements characterizing the temporal dynamics, spectral width, beam diameter, and M(2) values of the 795nm laser beam are presented. M(2) values indicate that the 795nm laser beam is 10 to 20 times diffraction limited. The laser system's response to changes in the pump's spectral width, the Rb number density, relaxant concentration, and pump intensity are examined with a broad-band time-dependent one-dimensional rate equation model. The experimental data and the modeling results are shown to be in good agreement for a wide range of experimental conditions. (C) 2011 Optical Society of America

A rubidium laser operating at 795 nm is optically pumped by a pulsed titanium sapphire laser to investigate the dynamics of Diode Pumped Alkali Lasers (DPALs). Linear scaling of output intensity for longitudinal excitation at intensities of 1.3–43 kW/cm2 and as much as 32 times threshold is observed. The slope efficiency depends directly on the number of absorbed photons for alkali concentrations of 0.8–2.0 × 1013 atoms/cm3 with no evidence for second order kinetics. The effective absorption cross section is reduced in part by the broad spectral width of the pump source relative to the pressure broadened lineshape. Spin orbit relaxation between the pumped and upper laser levels is sufficiently fast at 550 Torr of methane to prevent bottlenecking at all but the highest intensities. Comparison of laser characteristics with a quasi-two level analytic model suggests performance near the ideal steady-state limit, with the exception of modest mode matching.

The diode pumped alkali vapor lasers operating at subatmospheric pressure require developing of a new generation of high-power laser diode sources with about 10 GHz wide emission spectrum. The latest achievements in the technology of volume Bragg gratings (VBGs) recorded in photo-thermo-refractive glass opened new opportunities for the design and fabrication of compact external cavity laser diodes, diode bars and stacks with reflecting VBGs as output couplers. We present a diode laser system providing up to 250 W output power and emission spectral width of 20 pm (FWHM) at the wavelength of 780 nm. The stability and position of an emission wavelength is determined by the resonant wavelength of a VBG which is controlled by temperature. Stability of an emitting wavelength is within 5 pm. Thermal tuning of the wavelength provides maximum overlapping of emitting line with absorption spectrum of a Rb (rubidium)- cell. The designed system consists of 7 modules tuned to the same wavelength corresponding to D2 spectral line of Rb87 or Rb85 and coupled to a single output fiber. Analogous systems could be used for other Rb isotopes spectral lines as well as for lasers based on other alkali metal vapors (Cs and K) or any agents with narrow absorption lines.

The performance of several optically pumped rubidium lasers with pump intensities as high as 155kW/cm 2 have been characterized. A quasi CW system with pulse durations of 100ns has been scaled to greater than 32 x threshold, illustrating the effects of bottlenecking due to insufficient spin orbit relaxer concentrations. These lasers scale linearly to peak powers as high as 1 kW. The temporal dynamics of the 100ns pump and laser pulses have been used to study the continuously evolving laser efficiency, and validate a three level DPAL model incorporating a broadband frequency pump.

The diode pumped alkali vapor lasers are significantly beneficent from the developing of a new generation of high-power laser diode sources. The latest achievements in the technology of photo-thermo-refractive volume Bragg gratings opened new opportunities for the design and fabrication of compact external cavity laser diodes and bars with reflecting volume Bragg gratings as output couplers. A new developed fiber coupled 250W source consists from 7 channels of independently stabilized commercially available LD bars with standard AR-coatings at output facets. Using a specially designed reflecting volume Bragg grating, we demonstrated spectral narrowing of a single LD bar spectrum by over two orders of magnitude down to 16-18 pm at 780 nm wavelength. The volume Bragg laser bar output power exceeded 88% of that for the free-running laser bar. The spectral position of each LD precise tuning is made by means of a special method of temperature stabilization of an output volume Bragg coupler. Overall spectral width for whole system was less than 20 pm (<10 GHz). Optical pumping of a rubidium gain medium requires fine-tuning of pumping laser emission to precisely overlap with narrow Rb absorption band. The emission spectrum of the volume Bragg LD bar pumping source was tuned over a 300 pm spectral range without deteriorating the line-width. The absorption of the pump light by Rb atoms was measured in a rubidium vapor mixed with low pressure C2H6 buffer gas. More than 91% power of the pump source was absorbed by the low-pressure Rb vapor.

Since Krupke et.al. proposed and demonstrated pumping alkali atoms using diode lasers in 2003, there has been lot of interest in the diode pumped alkali laser (DPAL) systems. Several researchers have been able to scale the DPAL system to powers in the tens of watts. We have conducted a systems-level, weight-scaling study of a notional medium power, CW DPAL system. Three different modes of operation are considered: (i) very high pressure operation (over 25 atmospheres of He) in which the absorption and emission lines of the alkali atoms are broadened sufficiently to allow for efficient pumping with off-the-shelf diodes that have line width of 2 to 3 nm, (ii) intermediate pressure regime (~ 5 atmospheres) that requires diodes that are line narrowed to ~0.4 nm, and (iii) low pressure operation (~ 1 atmosphere) that requires diodes that are line narrowed to < 0.1 nm for efficient pumping of pump radiation into the alkali vapor. In the latter two cases some amount of methane, ethane, or some other gas would be needed to mix the two upper states rapidly; while in the first case, helium is used to broaden the transition and to mix the upper states. We have considered closed-cycle transverse flowing systems with the transverse length limited by medium inhomogeneity caused by heat deposition into the gas. Weight models have been developed for each of the following sub-systems: Pump Diodes, Fluid Flow System, Thermal Management System, Optics and Diagnostics System, Instrumentation & Control System, and Electrical Power system. The results of our weight estimates for a notional 100 kW DPAL system are presented.

The pressure broadening and shift rates of the rubidium D2 absorption line 52S1/2→52P3/2 (780.24nm) with CH4, C2H6, C3H8, n-C4H10, and He were measured for pressures ≤80 Torr using high-resolution laser spectroscopy. The broadening rates γB for CH4, C2H6, C3H8, n-C4H10, and He are 28.0, 28.1, 30.5, 31.3, and 20.3 (MHz/Torr), respectively. The corresponding shift rates γS are −8.4, −8.8, −9.7, −10.0, and 0.39 (MHz/Torr), respectively. The measured rates of Rb for the hydrocarbon buffer gas series of this study are also compared to the theoretically calculated rates of a purely attractive van der Waals difference potential. Good agreement is found to exist between measured and theoretical rates.

An experimental study using time-resolved fluorescence techniques together with theoretical simulations has been conducted and used to determine the quenching cross-sections of rubidium-methane and rubidium-ethane. Radiation trapping was significant under many of the experimental conditions (temperatures 40-130 degrees C and pressures 50-700 Torr) and a detailed analysis of the interplay between radiation trapping and quenching kinetics was carried out. Modifications of the Holstein equation for radiation trapping were implemented to account for the quasi-2 level behaviour of the Rb atom for high buffer gas pressures, the absolute frequency-dependent absorption cross-section for Rb with variable buffer gas pressures which accounts for the hyperfine splitting of Rb-87 and modification of the trapping factors so that radiation trapping and quenching by an additive quenching gas could be treated simultaneously. Experimental results supported by theoretical simulations bound the quenching cross-sections (s) of methane and ethane at 40 degrees C to be sigma <= 0.019 angstrom(2) and sigma <= 0.033 angstrom(2), respectively. These values are nearly two orders of magnitude smaller than previously reported.

: A master oscillator power amplifier (MOPA) with variable amplifier gain lengths was built to demonstrate power enhancement of an alkali vapor laser. A small signal gain of 0.91 / cm for two different gain lengths was observed. For a 2 cm long amplifier gain length an amplification of 7.9 dB was observed.