ANALYSIS OF LASING IN GAS-FLOW LASERS WITH STABLE RESONATORS

A model is developed that describes the power extraction in chemical o xygen-iodine lasers (COIL's) and CO2 gasdynamic lasers with stable res onators 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 i s the ratio gamma(0) of the residence time of the gas in the resonator to the O-2((1)Delta) or N-2(upsilon) energy extraction time and the s econd is the ratio of the threshold to the small-signal gain. The effi ciency is maximum for gamma(0) --> infinity and decreases rapidly as g amma(0) decreases. It is found that for a range of parameters correspo nding 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 distributio n). For the range of parameters corresponding to smaller efficiencies the intensity is uniform. In this case all the modes participate in la sing; 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 intraresonato r intensity and rooftop models of COIL's with stable resonators. The e xtraction efficiency calculated with the last two models is close to t hat estimated from our model. However, the intensity distribution cann ot be calculated correctly using the Fabry-Perot, the constant intrare sonator intensity, or the rooftop model. (C) 1998 Optical Society of A merica.