The small-signal gain coefficient and saturation intensity of cw-$CO_2$ laser are measured by using the relation which the gain coefficient is decreased as the intensity is increased. In the present work the gas pressure of the active medium is low enough, so that line broadening may be approximated as inhomogeneous broadening, and in this case the gain coefficient decreases by saturation factor $(1 + I(\nu)/I_s)^\frac{1}{2}$ and in cw laser oscillation, the saturated gain coefficient is equal to the loss coefficient.
We give losses to the reasonator and measure the output powers to obtain the small-signal gain coefficient and the saturation intensity $I_s$. The coupled NaCl plates are set symmetrically as the loss plates in the cavity. As transmittance of p-polarized laser light through the coupled NaCl plates is different according to the angle of incidence, we can give arbitrary losses by varying the angles of incidence. And from the change of output power, we calculate the small-signal gain coefficient and saturation $I_s$.
The resonator is consisted of a grating and an output mirror which has 15% transmittance. Gas mixing ratio of $CO_2:N_2$:He=1:1:9, gas pressure =1.5 torr, gas flow rate = 1.2 ℓ/min, discharge current = 10 mA and the temperature of cooling liquid is kept -10℃~0℃.
As a result of measurement and subsequent calculations for 12 lines of P-branch and 6-lines of R-branch in the transition from (001) to (100), we obtain the small-signal gain coefficient as 0.09/m - 0.55/m and saturation intensity as 4.0 W/㎠ - 7.0 W/㎠.
The wall of discharge tube is cooled by cooling liquid ($H_2O$ + glycerine, volume ratio = 1:1) to -20℃. The number of transition lines, rotational temperature, and power of a particular line are found not significantly varying in temperature range-20℃~+20℃, but the ratio of the peak intensity of R-branch relative to that of P-branch has increased approximately from 0.54 to 0.65.
