Greg, Nutsinee, Aidan, Elli
In the current configuration, the ITMX CO2 laser is inputting 0.2W of central heating to ITMX, just as it has been doing so for the last few months. The power put onto ITMX is still 0.2W, as measured by the power meter on the CO2X table (H1:TCS-ITMX_CO2_LSRPWR_MTR_OUTPUT). But the requested power needed to get 0.2W output in now 0.71W, up from 0.35W yesterday. The rotation stage does not appear to be returning to the same location.
Details:
The CO2 laser power is requested by a setting H1:TCS-ITMX_CO2_LASERPOWER_POWER_REQUEST to the desired value, and then a rotation stage moves a 1/2 wave plate to the required angle for that power. After the model restart this morning, we had to request a very different power (0.44W this morning compared to 0.35W yesterday, compared to 0.3W in March) to get the same output power in H1:TCS-ITMX_CO2_LSRPWR_MTR_OUTPUT. None of the gains or calibration values had changed this morning, so we suspect hysteresis in the rotation stage. I moved the rotation stage back and forth between minimum power and 0.2W output power and back again. I needed to keep requesting higher powers to bring the output power to 0.2W, although there seems to be no clear pattern to the change. Perhapse the rotation stage is sticking sometimes. See attached data set of 20 mins of data where I was moving the rotation stage around.
Other comments on ITMX CO2 laser:
We noticed a few other things while trending the output power:
The laser temperature as measured by H1:TCS-ITMX_CO2_LASERTEMPERATURE jumped from 23 degrees Celcius to ~27C on 17 March, see plot . The temperature has been fluctuating a lot more since then. This does not corelate with the enclosure temperature, the laser power has been steady at 59W, and the chiller settings have not changed. Greg and Matt were poking around the CO2 lasers that day, although according to the alog no chages were made to the ITMX CO2 laser (alogs 17303, 17302).
The laser has been mode hopping since 10 April when Greg swapped out the CO2 laser AOM (alog 17737). We hadn't seen this previously. The laser power has been fluctuating by >1% (1W fluctuations from 59W output power). This does not have a big effect on the power level going onto ITMX.
There's a correlation between spikes in output power of the laser and shifts in the rotation stage. This is quite possible if there is a small reflection from somewhere on or after the rotation stage that is coupling back into the laser and causing the laser mode to shift.
The laser temperature shift may be a glitch in the electronics box. Rich and I noticed something similar at LLO but we could never identify exactly where it came from and then it seemed to disappear. We'll look into this some more.
There definitely seems to be an electronics glitch associated with the laser interlock controller (D1200745). If we look at the attached plot from about a month ago, we see that during an event where the laser was turned off and back on again, there were jumps in the flowrate and temperature voltage monitors (both of which run through D1200745) at the same time that there was a 10% increase in the current to the laser. The laser interlock controller is connected to the laser RF driver to turn the laser on and off.
Taking a look at the laser head output, I'm not sure that those glitches are the laser mode hopping. The power doesn't look like it jumps as much as you would expect for a mode hop. I suspect Aidan is correct that it may be reflecting back into the laser a little which can cause some weird effects since the reflected beam can steal gain.