Looking at the CO2 ISS PD trends (qty 2 on each table labeled IN and OUT of loop), as the CIT CHETA setup that uses this PD is still getting confusing results (e.g. CIT#481). Each PD as an AC and DC mon output, from the DB9 of the PD preamp board D1201111.
Inially the DC signals look as expected, dropping to zero when laser is off (CO2X plot / CO2Y plot). CO2X stays within 1% when the laser is stable (CO2X plot). Though the CO2Y plots looks worse, only staying within 5% when the laser is stable (CO2Y plot).
If you pay attention to the ~hour region once the laser is first turned on in both lasers (worse in CO2Y, see plot) the ISS PDs show power signal varying much more than the CO2 power head PD does. I don't understand why that would be, but this could explain the issues we've seen at CIT where the laser is always ina less stable state as is fan cooled rather than water cooled.
CO2Y_IN shipped to CIT in 80883.
After suggestion from Keita, I have added trends of the thermal QPD NSUMs, which also shows this 9 minute oscillation but on a much smaller scale (less than 1% wobble). Plot here
The laser head reported power is in-loop (i.e. it's output is stabilized via the laser PZT), So it makes sense that it's very flat. There seems to be not much in this feedback loop, as laser power set point a PZT set point and a 0.5Hz integrator. Also the signal seems to go straight to the chiller with a ~1 hour integrator, I'm not sure if this really makes sense, but it seems to work.
I'm still unsure why the ISS QPDs see such a big change at the start of turn on. To check if the PDs or Laser warming up cause this, I turned the CO2Y laser off then quickly (~5 seconds) on again. This seemed to decrease the time of wobbles, plot attached.
