Analysis of lasing in gas-flow lasers with stable resonators

Abstract

A model is developed that describes the power extraction in chemical oxygen-iodine lasers (COIL's) and CO(2) 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 gamma(0) of the residence time of the gas in the resonator to the O(2)((1)D) or N(2)(v) energy extraction time and the second is the ratio of the threshold to the small-signal gain. The efficiency is maximum for gamma(0) ? infinity and decreases rapidly as gamma(0) 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.

Coupled simulation of chemical lasers based on intracavity partially coherent light model and 3D CFD model

“…The intensity distribution has peaks in both the upstream and downstream areas. This is in agreement with our experiments and some previous publications [13]. This effect, known as the sugar scooping phenomenon [13], can be explained by the presented model in the following way.…”

“…The predicted output power results were compared with experimental results to examine the I 2 dissociation mechanism assumptions. As shown in [13] the constant intracavity intensity model predicts accurate value of the output power from a stable resonator. However, it neglects the optical propagating mechanism and therefore cannot give reliable information about the output intensity distribution and divergence angle.…”

“…For the study of unstable cavities, numerous models faithfully preserve the cavity geometry [8][9][10]. In cases of stable resonators, however, the Fabry-Perot cavity and the roof-top cavity are often used as the approximation model [3,[12][13][14]. The Fabry-Perot model was successful in estimating the output power and the chemical efficiency with respect to the outcoupling rate and the flow condition.…”

“…However, certain difficulty arises when using the Fabry-Perot model to predict the intensity pattern. The upstream-downstream optical coupling effect in COIL with stable cavity, known as the sugar scooping phenomenon, cannot be explained by the Fabry-Perot model [3,[13][14]. In order to simulate this phenomenon, Yang adopted the roof-top cavity model which forces the optical field to flip as it is reflected by one of the mirrors [14].…”

“…Hence, both the Fabry-Perot model and the roof-top model change the geometry of cavity mirrors and obscure the optical mechanism of the device. Another model, namely the constant intracavity intensity model, assumes the intracavity intensity to be uniformly distributed over the resonator cross section [13]. Recently this model was coupled with 3D CFD model of the gas flow under various I 2 dissociation mechanism assumptions [15].…”