Just to see what the laser tube’s output looks like, I aimed a large photodiode toward the laser tube output:
That’s a venerable PIN-10AP photodiode minus its green human-eye filter, with an IR-pass / visible-block set of gel filters taped on the front to knock out everything except IR scattered from the laser’s snout. Nothing sits in the direct beamline.
The alert reader will kvetch about a CO₂ laser running at 10.6 µm, an order of magnitude off the right end of the photodiode response curve graphs, through stage filter gels not even pretending to have optical specs. Hey, stage light filters are utterly transparent to thermal IR and there’s plenty of invisible light to go around, so maybe this will work.
The coaxial cable trails off to the scope’s 1 MΩ input, so, although the photodiode does not operate in true zero-bias mode, I can at least look at its photocurrent driving a voltage into the scope input.
Surprisingly, the lashup kinda-sorta works well enough to show the laser’s light output tracking the tube’s current:
That’s a manual 20 ms pulse at 90% PWM, with the tube current at 20 mA/div. The oscillations at the start of the current pulse seem to excite the tube enough for the light output to stabilize when the real current comes along. I cannot tell if the exponential tail-off beyond the pulse is due to excited molecules cooling off in the laser tube or the poor photodiode recovering from Too. Much. Light. It. Burns.
The response is a little shakier at 50% PWM:
Dropping to 30% PWM requires more time to get up and running:
And 10% PWM looks downright awful:
Although the vertical scale for the photodiode trace doesn’t mean much, it’s obvious that the IR output matches the current input, right down to the littlest pulses. Sliding a bit of brass shimstock between the filter gels eliminates nearly all the photodiode output, so it’s not electrical noise. I think the long tail really shows the gases cooling off.
The alert reader will have noted the wee blip over there on the right, 21 ms after the start of the 20 ms long pulse and 4 ms after all those spikes shut off. Yup, the HV power supply can deliver a stray pulse when it’s not supposed to be enabled. More on that in a while.
The Smell of Molten Projects in the Morning 
Huh. That looks way better than I would have expected.
But if that exponential decay on the tail were a gas dynamic process, wouldn’t all the other PWM phase turnoff slopes be that slow also? It doesn’t look like they are?
The stray blip in the 10% trace may be the key: the photodiode voltage drops abruptly, then resumes at pretty nearly the same slope as just before the blip. I think that shows the photodiode tracks the tube’s light output, which was slowly decaying as the gas molecules relax.
The initial rising slope after each blip during the pulse seems to match the initial slope after the pulse ends, so the average light intensity seems to affect the slope.
If I knew what I was talking about, I’d be a lot more certain … [grin]
I would guess the tail stems from the 1.5nF of the PIN and the 1Mohm of the scope giving a time constant of 1.5ms.
Good point!
Now that you mention it, increasing light should make the diode act as a low-impedance source and decreasing light as a high-impedance shunt. Perhaps blinking an LED at the thing would produce crisp(er) edges and give a better idea of its overall response to both conditions.
Yet another deep rabbit hole to explore.