With the waveplate rotated for minimum CO2X power into the IFO, we are still injecting 0.34W with no mask, see attached plots. Expect 1/2 to 1/4 of this power with masks inserted (65277). The CO2 table should be set up to only have a single polarization hitting the power control rotation stage but there must be some unpolarized light that isn't being dumped (layout T1200007). Something might need adjusting.
For CO2Y this is only 0.05W. The waveplate moving affects the laser power measured at the laser head for CO2Y but not CO2X see image... strange.
CO2Y tripped off when I moved the waveplate though maximum power with no mask ~12W into IFO. This was caused by the IR alarm (RTD_OR_IR_ALRM) which might be the correct behavior as 12W is larger than we would put into the IFO for an extended period.
This is somewhat normal.
1. The CO2X extinction ratio is low. Typically, this can be improved by adjusting the first polarizer, POL1, the alignment into the HWP and the alignment of the second and third polarizers. Procedure for this is found here: LLO aLOG 13180
2. It is possible that the CO2Y HWP is reflecting/scattering a little bit of laser light back into the laser. This light will experience some stimulated emission inside the laser cavity which will vary the output power of the laser. As the HWP rotates, the scattering will change and the output laser power will fluctuate.
This minimum power into the IFO was 0.12W though the Spring commissioning period up until 10/26/2022 where it jumped to 0.34W.
The table wasn't opened on 10/26/2022, the upper periscope mirror was aligned and the input power was adjusted twice (search for home) but these don't align with the strange CO2X jump: see blue LSRPWR trend attached.
It doesn't seem clear to us why this power would change when the power out of the laser head is stable and nothing is moved on table. TJ explained what happens on T1200007:
- Out of laser: beam should be mostly p-polarized.
- POL1: transmits p-pol and reflects and dumps any unwanted s-pol. Now beam should be 100% p-pol.
- HWP: Used for power control. Rotates beam from p to s.
- POL2, POL3: transmits and dumps p-pol, reflects s-pol, which is sent to the IFO.
Looks like the output power of the CO2X jumped by around 0.5W (from 33.6 to 34.1 in bottom left plot) after it was switched off and on. You can then see it wander on QPD-B mostly in pitch over a few hours (upper right plot) which couples with drift in the power after the HWP on the power meter (bottom middle plot) and NOT with the CO2X laser output. The relatively small jump in laser power (1.4%) doesn't explain the extinction ratio change, but might be evidence the laser switched modes which could cause the other behavior. The fact that the total sum in QPD-B tracks with the yaw signal may mean there is some clipping going on? Not sure how to explain the timescale of the Yaw wandering over time unless it is related to the laser drifting.
Mitchell and I attempted to plug in the FLIR to check that beam is pointing through the masks the same as previously (alog 61475) however we couldn't connect to CO2X FLIR (laptop connected to CO2Y FLIR via Ethernet fine). We'll check in-enclosure connections when we are laser hazard later in the week.
This issue unintentionally fixed itself on Feb 8th 2022 when the laser power jumped form 35 to 50W. See alog 67633. Maybe it was previously locked on an incorrect mode.