Gordon D. Hager

A study of collisional quenching and radiation-trapping kinetics for Rb(5p) in the presence of methane and ethane using time-resolved fluorescence

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 °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 87Rb 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 (σ) of methane and ethane at 40 °C to be σ ≤ 0.019 A2 and σ ≤ 0.033 A2, respectively. These values are nearly two orders of magnitude smaller than previously reported.

Experimental and numerical modeling studies of a pulsed rubidium optically pumped alkali metal vapor laser

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 120 kW/cm2. Measurements characterizing the temporal dynamics, spectral width, beam diameter, and M2 values of the 795 nm laser beam are presented. M2 values indicate that the 795 nm 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 broadband 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.

Kinetics of an optically pumped metastable Ar laser

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.

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.

Pressure broadening and frequency shift of the 5S1/2 → 5D5/2 and 5S1/2 → 7S1/2 two photon transitions in 85Rb by the noble gases and N2

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.

A quasi-two level analytic model for end pumped alkali metal vapor laser

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.

Extended saturation analysis and analytical model of diode-pumped alkali lasers

An analytic model for the cw diode pumped alkali laser is developed by considering the longitudinally averaged number densities of the ground 2S1/2 and first excited 2P3/2, and 2P1/2 states. The pump intensity to reach threshold requires fully bleaching the pump transition and exceeding optical losses, typically about 200 Watts/cm2. 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 bottlenecking. By assuming a statistical distribution between the upper two levels, the limiting analytic solution for the quasitwo level system is achieved. For properly designed gain conditions, the quasi two level solution is usually achievable and represents ideal performance.

Influence of Broadband Excitation on the Performance of Diode Pumped Alkali Lasers

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.

A Three Level Analytic Model for Alkali Vapor Lasers

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 2S1/2 and first excited 2P1/2 and 2P3/2 states. The threshold pump intensity includes both the requirements to fully bleach the pump transition and exceed optical losses, typically about 200 W/cm2. 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.

Measurement of Rubidium Number Density under Optically Thick Conditions

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.