The authors describe the realization of a single-longitudinal-mode high-pressure CO2 laser with a plasma shutter. Single-mode operation is achieved with a three-mirror resonator with etalon. The resonator includes a subcavity, which protects the temperature-controlled etalon from destruction by high radiation intensities. In particular we have investigated two subcavity configurations, which use different etalons and grating arrangements. The maximal pulse energy of about 140 mJ was achieved with a grating that is employed in grazing incidence. A theoretical model has been developed, which illustrates the nonlinear frequency-tuning behavior of the three-mirror resonator and explains how the subcavity has to be adjusted to optimize the mode discrimination of the resonator. With the help of a plasma shutter we have produced tunable truncated 10-μm laser pulses. These pulses were employed to generate 100-ps pulses by optical free induction decay (OFID). With our system we produced for the first time OFID pulses 15 GHz off a CO2 laser line center and investigated the effects of a frequency detuning between the laser frequency and the absorption line of the spectral filter in an OFID system.