@jeremiah_rempel interesting, I just recently found some information on laser response while studying the electrical discharge characteristics of CO2 tubes.
This reference claimed the cutoff frequency is more like 3Khz which = 333 us. This is due to the time it takes molecules to collide.
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This suggests that a PWM of much longer than 200us may be necessary to get good fidelity. Like 10x, which would be in the 2000-3000us range … and your at 2500 …mmm.
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I made the assumption that the current in the tube was a good facsimile of the light output. This new information would negate that assumption. The tube exhibits negative resistance when discharging so the current is not only non-linear it is probably faster than the time it takes the tube to reach the lasing energy levels…
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See Appendix C in :