Unusual reshaping of ultrashort mid-IR pulses in atmospheric carbon dioxide

Coherent pulse propagation and the resulting dramatic reshaping of picosecond IR laser pulses in the frequency region of the rovibrational absorption band due to the antisymmetric stretching vibration (v(3)) of atmospheric CO2 has been studied in detail. Measuring frequency-resolved cross correlations of the deformed small area pulses with undisturbed pulses via the optical Kerr effect, we found situations where the first recurring intensity maximum is considerably stronger than the remaining intensity of the initial pulse. This unusual reshaping is analyzed in terms of the established theoretical description, yielding the necessary conditions and parameters for this phenomenon. In particular it turns out that this effect can only occur when several, nearly equidistant and equally strong rotational lines are lying within the bandwidth of the IR pulses. Our results demonstrate an interesting new way to produce well-defined pico- or femtosecond pulse sequences in a wide range of parameters varying gas (mixture) and parameters like pressure, temperature and absorption length. (C) 2002 Elsevier Science B.V. All rights reserved.