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We investigated the characteristics of a laser pulse width depended on an optical cavity length in a longitudinally excited CO2 laser. The CO2 laser was a simple device and had a discharge tube, an excitation circuit for a fast discharge, and an optical cavity. The discharge tube was made of a 45-cm-long alumina ceramic pipe with an inner diameter of 13 mm and metal electrodes at both ends of the tube. An excitation circuit was consisted of a low-voltage pulse power supply, a stepup transformer, a storage capacitance and a spark gap. The optical cavity was formed by a ZnSe output coupler with a reflectivity of 85% and an Au-coated mirror with a reflectivity of 99%. The length of the optical cavity was 65 cm to 185 cm. A short laser pulse with a spike pulse and a pulse tail was produced at a gas pressure of 2.6 kPa in a 1:1:2 mixture of CO2/N2/He gas. In the optical cavity length of 65 cm, the spike pulse width was 137 ns (FWHM), the pulse tail was 146μs, the energy of the whole laser pulse was 32.0 mJ, and the energy of the spike pulse part was estimated to be 0.86 mJ. The spike pulse width depended on the optical cavity length and was 137 ns, 195 ns, 274 ns, 304 ns and 332 ns at the length of 65 cm, 95 cm, 125 cm, 155 cm and 185 cm, respectively.
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