A plasma effect in injection-mode selection of TEA CO2 lasers

We present experimental evidence obtained by a laser heterodyne technique for a chirp of about 10¹⁴ Hz/s in the cavity resonance during the pulse build-up time of a TEA CO₂ laser. The frequency sweep arises from the refractive-index contribution of the time-dependent electron density in the exciting discharge plasma. The phenomenon provides a mechanism, previously unexplained, for the extended and asymmetric mode selection zone in injection experiments.