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A model is developed that describes the power extraction in chemical oxygen-iodine lasers (COIL’s) and CO2 gasdynamic lasers with stable resonators when a large number of transverse Hermite–Gaussian eigenmodes oscillate. The extraction efficiency, mode intensities, and intensity distribution along the flow depend only on two parameters. The first is the ratio γ0 of the residence time of the gas in the resonator to the O2(1Δ) or N2(v) energy extraction time and the second is the ratio of the threshold to the small-signal gain. The efficiency is maximum for γ0 → ∞ and decreases rapidly as γ0 decreases. It is found that for a range of parameters corresponding to the highest efficiencies the intensity distribution along the flow is nonuniform and has two peaks near the upstream and downstream sections of the resonator. In this case only the highest-order modes that totally fill the resonator cross section oscillate (the so-called, experimentally observed sugar scooping bimodal intensity distribution). For the range of parameters corresponding to smaller efficiencies the intensity is uniform. In this case all the modes participate in lasing; however, the intensities of the high-order modes are larger than those of the low order. The current model is compared with the plane-mirror Fabry–Perot resonator model and with the constant intraresonator intensity and rooftop models of COIL’s with stable resonators. The extraction efficiency calculated with the last two models is close to that estimated from our model. However, the intensity distribution cannot be calculated correctly using the Fabry–Perot, the constant intraresonator intensity, or the rooftop model.
© 1998 Optical Society of America
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