Effects of gas flow on gain of 10.6 micron CO2laser amplifiers

Small-signal gain of flowing gas CO
2
laser amplifiers at 10.6 microns has been optimized for media including pure CO
2
CO
2
: N
2
, CO
2
: He, CO
2
: CO, CO
2
: O
2
, CO
2
: N
2
: He, CO
2
: CO : He, and CO
2
: CO : N
2
. Optimum gain of all flowing gas systems studied increases monotonically with increasing gas flow rate. In the low CO
2
flow rate region, 10 < R
CO
2
: < 50 cm
³
/min, gas flow enhances the gain most for systems containing N
2
. Results provide strong evidence that the rapid increase in gain with flow rate in CO
2
: N
2
mixtures is due to removal by convection of the dissociated product CO. For 50 < R
CO
2
< 200 cm
³
/min, a slow linear increase in gain of all gas mixtures with increasing flow rate occurs and is attributed to the cooling of gas temprature by convection. A stronger dependence of gain
G
on amplifier bore
D
, viz.,
G \propto I/D
, was obtained for flowing gas media relative to that previously observed for nonflowing gas mixtures which is consistent with the proposed mechanism of gas cooling by convection. Highest gain values obtained were 7.8 and 6.2 dB/m with the flowing gas mixtures CO
2
: N
2
: He and CO
2
: CO : He, respectively, in a 12 mm bore water-cooled amplifier tube. Similarities between CO
2
: N
2
and CO
2
: CO systems suggest that pumping of the CO
2
laser by resonant transfer from CO* (
\upsilon = 1
) can be significant.