Intrinsic Flux Limits for Continuous and Q-Pulse Gain for the 10.6-μ Line of CO2

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Abstract
The limits on flux gain for the 10.6‐μ line of CO2 are discussed on the basis of saturation and spectral hole burning. Although an individual P transition is homogeneously saturable, the entire collection of rotational transitions provides a quasi‐inhomogeneous character for single‐transition operation, which is due to collisional coupling among rotational levels. The experimentally determined cross section for induced emission is shown to agree with the theoretical value upon assuming that approximately 100 rotational levels of the 10°0 vibrational state of CO2 are strongly coupled by collisions. Thus, the induced‐emission cross section for a single vib‐rotor P branch transition of the 10.6‐μ line is found to be 2×10−16 cm2. Using experimentally determined collision cross relaxation rates the cw hole‐burning flux is found to be of order 7×105 W/cm2. The hole‐burning flux for Q‐pulse amplification is estimated to be 1% of the cw value from which one can infer the saturation flux for a single vib‐rotor transition to be approx 0.25 W/cm2.