The saturated gain coefficients (In the 10.4 μm band and 9.4 μm band fifty transition lines of $CO_2$ laser output are obtained by using the grating as a total reflector of the laser resonator.
The increasing rate of the threshold gain width of the 10P (20) transition line is 2.2 MHz/torr as the pressure of the active medium increases from 4 torr to 21 torr when the discharge current is fixed at 20 mA. And the increasing rate of the threshold gain width according to the discharge current is 0.46 MHz/mA as the current increases from 21 mA to 39 mA when the gas pressure is 7 torr.
The saturated gain coefficients ($γ_o$) and the cavity losses ($α_t$) of the 15 transition lines of 10.4 μm band are measured by varying the cavity loss which is introduced by changing the transmittance of the coupled NaCl plates placed inside the laser cavity. The results show that the transition lines which have the larger $γ_o$ have the larger $α_t$.
The passively Q-switched laser pulse of TEA $CO_2$ laser is obtained by inserting the 30 cm-long $SF_6$ gas cell inside the laser cavity. The optimum pressure of $SF_6$ gas for Q-switching of 10P (20) line is found to be about 20 torr.
The optoacoustic measurement of the absorption coefficient and lineshape of $CH_3I$ molecule is performed. Eight absorption lines are assigned to specific energy levels of the rotation-vibration spectrum in the first excited $v_6$ vibrational mode. The measurement of the pressure dependence of the homogeneous linewidth at pressures lower than 0.8 torr shows that the rate of increase of the homogeneous linewidth is not linear but that it is the larger at the lower pressure. And by fitting the numerically calculated Voigt function to the experimentally obtained absorption lineshape, it is found that two absorption lines are separated by 90 MHz.
