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Physical Review A

Discharge-pumped cw gas lasers utilizing “dressed-atom” gain media

P. P. Sorokin, J. H. Glownia, R. T. Hodgson

Published May 12, 2005• 0 citations
Publisher

Abstract

The possibility of realizing an efficient gaseous laser-beam-generating medium that utilizes $\ensuremath{\Lambda}$-type coherently phased (i.e., ``dressed'') atoms for the active laser species, but that does not inherently require the use of external laser beams for pumping, is explored. Specifically, it is investigated if multiphoton stimulated hyper-Raman scattering (SHRS) processes driven by fluorescence radiation generated in a continuous electrical discharge present within the vapor-containing cell could produce continuous-wave (cw) optical gain at the $\ensuremath{\Lambda}$-atom resonance frequencies ${\ensuremath{\omega}}_{o}$ and ${\ensuremath{\omega}}_{o}^{\ensuremath{'}}$. It is deduced that such gain could result from $n$-photon $(n\ensuremath{\geqslant}4)$ SHRS processes only if absorption of fluorescence pump light occurs in the first three transitions of the $n$-photon sequence representing the process unit step. Estimates of the amount of optical gain that could be produced in such a system indicate that it should be sufficient to allow multiwatt cw laser operation to occur on one set of $\ensuremath{\Lambda}$ transitions connecting levels in a ``double-$\ensuremath{\Lambda}$'' structure, with the pump light being discharge-produced fluorescence centered about the transitions of the other $\ensuremath{\Lambda}$ pair. However, to initiate operation of such a device would require injection into the laser optical cavity of intense ``starter'' laser pulses at both lasing frequencies. What should be an optimal experimental configuration for determining feasibility of the proposed laser device is described. In the suggested configuration, Cs-atom $6{S}_{1∕2}\text{\ensuremath{-}}6{P}_{1∕2}$ transitions form the double-$\ensuremath{\Lambda}$ structure.

DOI
10.1103/physreva.71.053807

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It is well known that driven two-level atoms can exhibit optical amplification.1 This fact has led to the recent demonstration of single-photon lasing in cell and atomic-beam type ensembles of two-level atoms.2 We show that the same simple two-level system can also be utilized as a two- or more-photon amplifying medium. The underlying physics can be understood in terms of the atom field dressed-state picture. When driven by a field detuned from the atomic resonance, significant population inversion between the dressed-state doublets can be created. In the dressed picture, single-photon lasing occurs between inverted sublevels of adjacent dressed-state doublets. Transitions between nonadjacent doublets are still inverted, but require two or more photons. In the degenerate case, the multiphoton emission frequencies differ from the single-photon emission frequency and can hence be selectively enhanced using an optical cavity. Due to the peculiar structure of dressed levels, the multiphoton emissions are all connected through near resonant intermediate states. The multiphoton emissions can also be viewed as a stimulated hyper-Raman scattering of successively higher order.
Driven two-level atoms are shown to exhibit strong, resonantly enhanced, two-photon gain. This gain, unlike that observed in nonlinear mixing processes, is not constrained by a phase-matching condition involving the pump beam. Physically, the gain can be associated with inverted two-photon transitions between dressed states or multiphoton stimulated Raman scattering, and it may be useful in the realization of a two-photon laser. Comparison with the well-known single-photon gain found in the same system is made.
The radiative emission of strongly driven, two-level-like barium atoms into the modes of a confocal optical cavity is studied as a function of barium density. As the barium density is increased, the spatial profile, intensity, and spectrum of light emitted out the ends of the confocal cavity display dramatic changes indicative of a transition from fluorescence to lasing. In this novel two-level system, laser action can be explained in terms of an inversion between the dressed atom-field states or in terms of a type of stimulated hyper-Raman scattering in which atoms lase from the ground to the excited atomic state. A simple dressed-atom rate-equation analysis of this two-level-atom laser is presented. It is pointed out that this same system may provide a means of realizing two-photon-like laser action.
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The narrow-band coherent N-type resonance, promising for the development of advanced atomic clocks, can be considered as a type of three-photon resonance, where a two-photon Raman excitation is combined with a resonant optical pumping field. In this communication, we present an experimental study and a theoretical analysis related to three-photon, bi-chromatic excitation of Cs atomic vapor contained in an 8-mm long cell with 20 Torr of neon. If a coupling laser is fixed at a frequency that is lower by several GHz than the position of the absorption profile of the F g = 4 set of transitions, and a probe laser is tuned over the D 2 line (λ = 852 nm), a narrow high-contrast enhanced absorption N-resonance will be observed in the probe light profile, superimposed on the absorption profile of the F g = 4 set of transitions. We present theoretical modeling aimed to clarify the processes behind the efficiency of the N-resonance preparation for different frequency positions of the coupling laser within the D 2 line of Cs.
Alternative wavelengths for optically pumped alkali vapor lasers have been developed using single photon excitation of higher lying P-states, stimulated Raman processes, two-photon excitation of S and D states, and electric quadruple excitation on S-D transitions. Two photon excitation of Cs 72D leads to competing and cascade lasing producing red and infrared lasers operating on the D-P transitions, followed by ultraviolet, blue, the standard near infrared DPAL transitions operating on P-S transitions. The S-D pump transitions are fully bleached at pump intensities exceeding 1 MW/cm2, allowing for lasing transitions that terminate on the ground state. The kinetics of these systems are complex due to competition for population inversion among the many optical transitions. An optically pumped mid-infrared rubidium pulsed, mirrorless laser has also been demonstrated in a heat pipe along both the 62P3/2 - 62S1/2 transition at 2.73 μm and the 62P1/2 - 62S1/2 transition at 2.79 μm with a maximum energy of ~100 nJ. Performance improves dramatically as the rubidium vapor density is increased, in direct contradiction with the prior work. No scaling limitations associated with energy pooling or ionization kinetics have been observed. Practical application for infrared counter measures depends on the further development of blue diode pump sources. Finally, stimulated electronic Raman scattering and hyper-Raman processes in potassium vapor near the D1 and D2 lines have been observed using a stable resonator and pulsed laser excitation. First and second order Stokes and anti-Stokes lines were observed simultaneously and independently for a pump laser tuning range exceeding 70 cm-1. When the pump is tuned between the K D1 and D2 lines, an efficient hyper-Raman process dominates with a slope efficiency that exceeds 10%. Raman shifted laser may be useful as a target illuminator or atmospheric compensation beacon for a high power diode pumped alkali laser.