The focus of this research program was the investigation of XPAL properties, and new pumping schemes, as well as modeling, and measuring critical photoionization and excited state-excited state reaction rates in order to improve the performance of XPALs.

Figure 1. Schematic diagram of the laser system.

The first several experiments were conducted to measure the smallest energy defect that can exist between the pumped and lasing states in a three-level laser system. This is a problem critical to the cooling of the laser medium by pumping it through the lower lying states. In short, Rb-Xe gas mixtures were used to demonstrate the collapse of a three-level laser when the thermal energy kT reaches approximately the energy defect between the laser and pumped levels.

It has been shown through experimentation that the suggested two-color pumping scheme shown in Figure 1 allows the laser’s efficiency to be increased by a factor of 1.7 (Figure 2). In addition, the pump absorption coefficient was increased by more than an order of magnitude, as compared to the classical XPAL pumping scheme.

Figure 2. (Red circles) Dependence of D2 line (780 nm) ASE on E1 + E2, the sum of the energies absorbed by the first and second pump pulses. For all measurements, [Rb]=9.2*1014cm-3, [Xe]=1.8*1019cm-3, λ1=759.95 nm, and λ2=794.76 nm. E1 was fixed at 85 μJ and the time delay Δt was maintained at 8 ns; (Black Squares): Similar data were recorded when E2=0 and the laser is pumped only at 760 nm (Rb-Xe blue satellite).

As shown in Figure 1, a quartz cell filled with an alkali-noble gas mixture was pumped longitudinally with two dye lasers. The first pump pulse was used to prepare the system and populated the upper laser level to a point slightly below the lasing threshold. The second (main) pump pulse created the population inversion and triggered lasing on the Rb D2 line transition at 780 nm.

The most important feature of this pumping scheme is that the wavelength of the main pump was longer than the wavelength of the Rb D2 line laser. In other words, the energy of the emitted photons was higher than the energy of the pump photons. This fact implies the extraction of thermal energy from the gain medium to compensate for the energy difference between the ASE and excitation photons.

It has been demonstrated (Figure 2) that, in addition to the quantum efficiency being above one, the two-color pumped system exhibits higher slope efficiency as compared to the single color pumped case. It is important to note that the absolute efficiencies shown in Figure 2 describe the ASE signals due to the specific experimental arrangement and, therefore, the actual D2 line laser efficiency is significantly higher.

During the same reporting period, the kinetics of the higher energy excited states of Rb and Cs were studied in laser excitation experiments. It was previously shown that the higher excited states are significantly populated by energy pooling processes when the D1 or D2 lines are strongly excited (e.g., optical pumping conditions). For the XPAL system, these higher excited states may be photoionized, resulting in loss of both input energy and metal atom number density. In a conventional DPAL system the higher excited states may react with hydrocarbons, producing metal hydride particles and carbon deposits.

Attempts were made to observe the ionization of Rb and Cs in the presence of 500 Torr of Ar. The D 2 lines were excited using pulsed laser intensities of approximately 0.5 MW/cm2. A time-delayed, pulsed dye laser probe was used to detect metal vapor loss due to ionization. These measurements indicated that less than 1% of the neutral Rb or Cs was lost via ionization. Direct photoionization in the presence of 500 Torr of Ar, using 266 nm and 193 nm radation (P>1MW/cm2), was also found to be inefficient. Loss of ground state atoms was not detected, and there was negligible atomic line fluorescence.

Time-resolved fluorescence and laser pump-probe measurements were used to study the interactions of Rb(6p) with H2, CH4, and C2H6. At room temperature, the total removal, rate constants were found to be kH2=(7.0±0.2)×10-10, kCH4=(6.2±0.2) ×10-10, and kc2H6=(8.1±0.3)×10 -10 cm3 s-1. These values are consistent with earlier determinations. Electronic structure calculations were used to investigate the deactivation mechanisms. Quenching of Rb(6p) by H2 proceeds via a curve crossing with the potential energy curve of the Rb(6s)+H 2 collision pair, while CH4 and C2H6 quench via electronic-to-vibrational energy transfer. Measurements of ground state population recovery were used to estimate the fraction of the quenching that could be attributed to chemical reactions. For H2 the reaction channel accounted for 12% of the total removal rate constant. Production of RbH was confirmed by observing the laser induced fluorescence (LIF) spectrum of the A1Σ+-X1Σ+ transition. Reactive loss in the collisions with CH4 and C2H6 accounted for 3% and 6% of the total removal rate constants. Searches for RbH produced by these reactions yielded negative results.

This work was done by James Eden, University of Illinois Champaign, for the Air Force Research Laboratory. AFRL-0255

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New Class of Excimer-Pumped Atomic Lasers (XPALS)

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This article first appeared in the September, 2017 issue of Aerospace & Defense Technology Magazine.

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