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Absorption of intracavity radiation by CO2 molecules in an unexcited region has detrimental effects on output power characteristics, especially above a certain level of output power, i.e., the threshold level. These absorption effects have been investigated experimentally in detail. The threshold level depends both on CO2 concentration of the laser gas and on the mirror transmission of the optical cavity. It is found from these dependences that the absorption effect becomes serious above ∼0.2 W/cm3, which is the density of optical power absorbed by the unexcited CO2 molecules. The influence of self-absorption on the laser efficiency and the oscillating line of 10.4-μm band can be explained well by a usual theory for laser oscillation. Furthermore, a novel method for monitoring the concentration of CO2 molecules is demonstrated.

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