Line shapes for the Rb D1 (5S1/22↔5P1/22) and D2 (5S1/22↔5P3/22) transitions with 4He and 3He collisions at pressures of 500–15,000Torr and temperatures of 333–533K have been experimentally observed and compared to predictions from the Anderson–Talman theory. The ground XΣ1/2+2 and excited AΠ1/22, AΠ3/22, and BΣ1/2+2 potential energy surfaces required for the line shape predictions have been calculated using a one-electron pseudo-potential technique. The observed collision induced shift rates for 4He are dramatically higher for the D1 line, 4.60±0.12MHz/Torr, than the D2 line, 0.20±0.14MHz/Torr. The asymmetry is somewhat larger for the D1 line and has the same sign as the shifting rate. The 3He broadening rate for the D2 line is 4% larger than the 4He rate, and 14% higher for the D1 line, reflecting the higher relative speed. The calculated broadening rates are systematically larger than the observed rates by 1.1–3.2MHz/Torr and agree within 14%. The primary focus of the current work is to characterize the high pressure line shapes, focusing on the non-Lorentzian features far from line center. In the far wing, the cross-section decreases by more than 4 orders of magnitude, with a broad, secondary maximum in the D2 line near 735nm. The potentials do not require empirical modification to provide excellent quantitative agreement with the observations. The dipole moment variation and absorption Boltzmann factor is critical to obtaining strong agreement in the wings.

Doppler free two photon absorption spectroscopy was employed to measure the pressure broadening and frequency shift rates of the 5S1/2 (F = 3) → 5D5/2 (F = 5, 4, 3, 2, 1) (778.105 nm) and the 5S1/2 (F = 2) → 7S1/2 (F = 2) (760.126 nm) two photon transitions in 85Rb by the noble gases and N2. To our knowledge, these rates are reported on for the first time. The self-broadening and shift rate of the 5S1/2 (F = 3) → 5D5/2 (F = 5, 4, 3, 2, 1) transition and self -broadening rate of the 5S1/2 (F = 2) → 7S1/2 (F = 2) transition were also measured. The temperature dependence of the self-frequency shift (Rb-Rb collisions) of these transitions is presented. Helium diffusion rates through Quartz and Pyrex cells are also calculated and the implication of helium diffusion through glass vapor cells is discussed in regards to atomic frequency standards based on these transitions. Experimental pressure broadening and shift rates are compared to theoretically calculated rates assuming a 6, 8 or 6, 8, 10 difference potential and pseudo potential model. Reasonable agreement is achieved between experimental and theoretical values.

Line shapes for the Cs D1 (6 2S1/2–6 2P1/2) and D2 (6 2S1/2–6 2P3/2) transitions for He, Ne, and Ar collisions at pressures of 100–2280Torr and temperatures of 294–448K have been experimentally observed and compared to predictions from the Anderson–Talman theory. Asymmetry in the core of the line shape is generally correlated with shift rates, except for the Ne D1 line, which is red shifted, but blue shaded. There is a dramatic difference between the D1 and D2 lines for the shift and asymmetry parameters, particularly for He and Ne. A blue satellite is observed in the far wing of the D2 line at 827.2, 833.4, and 834.9nm for He, Ne, and Ar, respectively. The amplitude of the blue satellite scales linearly with pressure, exceeding 0.014% of the peak cross-section at 2280Torr. Modest red shoulders are observed for both the D1 and D2 lines associated with extrema in the difference potentials. Existing ab initio potential surfaces require empirical modification to adequately describe the observed spectra. However, the long-range dipole and quadrupole polarizabilities are sufficient to establish the observed broadening and shifting rates. Two distinct sets of difference potentials are developed that adequately represent the line shape observations. Predictions for the temperature dependence of the collision induced shift are significantly different for the two sets of empirically modified potentials.

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.

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 effects of pump laser spectral bandwidth on the performance of longitudinally pumped diode-pumped alkali lasers is explored by extending the analytic, three-level model using longitudinally averaged number densities. By assuming a statistical distribution between the upper two levels, the limiting solution for the quasi-two level system is achieved. A second limiting solution is identified for strongly bleached conditions where the atom recycle rate, limited by spin–orbit relaxation, fully specifies the output power. Performance in the intermediate regime depends significantly on the pump bandwidth relative to the atomic absorption line width and requires numerical simulation. The ratio of populations for the two excited, 2 P 3/2,1/2 states completes an analytic solution and depends primarily on pump laser bandwidth, threshold, and alkali concentration. Absorption well into the wings on the atomic profile can be utilized by increasing alkali concentration, but imposes increased pump intensity threshold.

A mathematical method is described to compute the pressure dependent spectrum of the D1 and D2 lines of atomic cesium in the presence of argon. The method is based on the Anderson Tallman unified theory of pressure broadening in which the spectrum is determined form the Fourier transform of the auto-correlation function. The method uses modified potential energy surfaces of the ground and excited states that correlate to the 2S1/2 ground state and the 2P1/2 and 2P3/2excited states at large inter-nuclear separation. These surfaces are used to form interaction difference potentials to determine the auto-correlation function. In addition to being able to compute pressure dependent spectra that exhibit symmetry and far wing structure the method also allows us to compute the low pressure shift and broadening rates of the Lorentzian line core.

A three level model for optically pumped alkali metal vapor lasers is developed by considering the steady state rate equations for the longitudinally averaged number densities of the ground 2 S1/2 and first excited 2 P3/2, and 2 P1/2 states. For efficient operation, the collisional relaxation between the two upper levels should be fast relative to stimulated emission. By assuming a statistical distribution between the upper two levels, the limiting analytic solution for the quasi-two level system is achieved. A second limiting solution is identified for strongly bleached conditions where the atom recycle rate, limited by spin-orbit relaxation, fully specifies the output power. Performance in the intermediate regime depends significantly on pump laser bandwidth and requires numerical simulation. The ratio of populations for the two excited, 2 P3/2,1/2 states completes an analytic solution and depends primarily on pump laser bandwidth, threshold and alkali concentration.

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

An analytic model for the cw diode pumped alkali laser is developed by considering the longitudinally averaged number densities of the ground (2)S(1/2) and first excited (2)P(3/2), and (2)P(1/2) states. The pump intensity to reach threshold requires fully bleaching the pump transition and exceeding optical losses, typically about 200 Watts/cm(2). Slope efficiency depends critically on the fraction of incident photons absorbed and the overlap of pump and resonator modes, approaching the quantum efficiency of 0.95 - 0.98. For marginal cavity transmission losses, peak performance is achieved for low output coupling. For efficient operation, the collisional relaxation between the two upper levels should be fast to prevent bottle-necking. By assuming a statistical distribution between the upper two levels, the limiting analytic solution for the quasi-two level system is achieved. For properly designed gain conditions, the quasi two level solution is usually achievable and represents ideal performance.

A measurement of rubidium number density under optically thick conditions has been demonstrated by measuring the wings of the D1 absorption spectra using a laser with a 0.16 nm (75 GHz) fine tuning range. This technique can measure the absolute concentration in rubidium under conditions where the absorption coefficient and path length product yield conditions where the central region of the line is opaque. The laser was tuned to a region sufficiently far into the short wavelength wing of the absorption where transmission through the cell was possible. The laser was then scanned through the central opaque region of the line to the adjacent long wavelength wing. The wavelength of the scan was calibrated by using a 1.5 GHz etalon and a cell containing only naturally occurring rubidium as a frequency reference. The measured absorption spectra for various cell conditions of temperature and pressure were then fit to a pressure broadened Voigt profile thereby allowing the determination of the rubidium number density.

A three level analytic model for optically pumped alkali metal vapor lasers is developed considering the steady-state rate equations for the longitudinally averaged number densities of the ground (2)S(1/2) and first excited (2)P(1/2) and (2)P(3/2) states. The threshold pump intensity includes both the requirements to fully bleach the pump transition and exceed optical losses, typically about 200 W/cm(2). Slope efficiency depends critically on the fraction of incident photons absorbed and the overlap of pump and resonator modes, approaching the quantum efficiency of 0.95-0.98, depending on alkali atom. For efficient operation, the collisional relaxation between the two upper levels should be fast relative to stimulated emission. By assuming a statistical distribution between the upper levels, the limiting analytic solution for the quasi-two level system is achieved. Application of the model and comparisons to recent laser demonstrations is 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.

We experimentally demonstrate a high efficiency potassium laser using a 0.15nm bandwidth alexandrite laser as the pump source. The laser uses naturally occurring helium as the buffer gas. We achieve a 64% slope efficiency and a 57% optical to optical conversion. A pulsed laser model shows good agreement with the data.

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.

Optically pumped alkali vapor lasers have been constructed using excitation of the D-2 line (P-2(3/2)-S-2(1/2)) followed by lasing on the D-1 line (P-2(1/2)-S-2(1/2)). Collisional relaxation is used to transfer population from P-2(3/2) to the P-2(1/2) level. The collision partner used for this step must have a large cross section for inducing transfer between the P-2(J) levels, combined with a very small cross section for electronic quenching of the form M(P-2(J))+Q -> M(S-2(1/2))+Q (where M is an alkali metal and Q is the energy transfer agent). Ethane has proved to be an effective energy transfer agent for optically pumped Rb and Cs lasers. However, modeling of data for the Rb/C2H6 laser with the literature value for the quenching rate constant was unsuccessful. We have reexamined the quenching of Rb(P-2(J)) by C2H6 using time-resolved fluorescence techniques. Radiation trapping was significant under the conditions of our measurements, and an analysis of the interplay between the kinetics of trapping and quenching was carried out. It was found that quenching of Rb(P-2(J)) by C2H6 was very inefficient. The upper bound established for the quenching cross section was two orders of magnitude lower than that indicated by the previous determination.

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, S-2(1/2), P-2(3/2), and (2)p(1/2), energy states for the three level laser system. The approximation is then made that the relaxation between the two upper levels, P-2(3/2) and P-2(1/2) 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, P-2(3/2) and P-2(1/2), 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.

: Planar Laser-Induced Fluorescence (PLIF) was used to visualize the flow of a supersonic nozzle with a single supersonic injector. The nozzle simulates Chemical Oxygen Iodine Laser (COIL) flow conditions with non-reacting, cold flows, where the injected flow is seeded with iodine. A laser sheet near 565nm excites the iodine, and the florescence is imaged with a gated, CCD camera. Streamwise and semi-spanwise (oblique-view) images were taken, where the presence of injected flow is highlighted. With these images, the flow structures are identifiable and the mixing quality between the primary and injected flow can be quantitatively measured. Histograms of image ensembles were taken at varying downstream locations to quantify the mixing quality of the flow.