Rick S., Jason O., Matt H., Ed M., and Jeff B.
Went into the PSL today and performed a few maintenance tasks (work permit #5039).
1) Measured PSL power at several points in the beam path:
- 33w - between M1 & M2 (just outside of the front end)
- 26w - reflected from PMC
- 27.2w - incident light on PMC
- 24.4w - downstream of the EOM (in the IO section of the PSL table)
- 24.9w - PMC transmitted
- 67.5mW – upstream of the FSS AOM
- 54.5 mW - Single pass power, upstream of M25 (81%)
- 36.3mW - Double pass power, upstream of FSS EOM (66%, 54%)
- 33.7mW – At top of periscope mirror, incident on RefCav
3) Measured voltage of RefCav REFL PD (H1:PSL-FSS_RFPD_DC_OUTPUT):
- 299mV – With FSS RefCav unlocked
-
165mW – With FSS RefCav locked
- This is too high, giving a RefCav visibility of only 45%!
Once again the RefCav Trans PD (TPD) has begun to drift down. Was set to ~1.6V on 1/5/2015 (see alog 15871) and today is reading ~0.9V. Therefore we adjusted the RefCav alignment by adjusting the vertical and horizontal of the top periscope mirror. Measured voltages at the RefCav REFL PD.
- Vertical adjustment dropped RefCav to 83mV
- Horizontal adjustment dropped RefCav to 65mV
After adjustment (and locking of adjustment screws):
- RefCav REFL PD - 300mV (with FSS RefCav unlocked)
-
RefCav REFL PD - 60mV (with FSS RefCav locked)
- This gives a FSS RefCav visibility of 80%, which is better. Per Rick S. this should closer to 98%, so some more work to do here, but is good enough for now and we were running out time in our maintenance window.
- RefCav TPD - 1.63V (H1:PSL-FSS_TPD_DC_OUTPUT)
- RefCav transmitted power - 19.1mW
Will keep an eye on this as the RefCav transmitted power seems to drift down suddenly. We’re not sure what’s causing this apparent alignment drift. All the measured powers leading up to the RefCav were close to those measured during the 1/5/2015 adjustment, but the RefCav TPD was still reading ~44% less.
4) We also measured the UGF of the FSS:
-
UGF: 200kHz, phase margin 35 deg
- According to Rick S. this should be closer to 600kHz, so we have some work to do here.
- At 500kHz: -6dB, phase margin 58 deg
5) For Daniel, we adjusted power at IO PD (IO-AB-PD3-DC) to 3.3V by turning up the PD gain by 3 clicks and adding a ND filter to the PD (see Daniel's earlier alog).
6) We are now storing the 2 3IFO PMCs in the NW corner of the PSL LAE on a cart, with their lids off.
Now that have a measured power budget here at LHO for some areas of the PSL table (note for completeness that the power readings at LHO were done using 250W water cooled meter and at LLO using 50W air cooled meter) we can compare the losses through the system to (and after) the PMC.
So basically the performance of the two laser systems are similar. LLO has slightly less loss from the output of the MOPA to the input of the PMC (5.5W loss at LLO compared to 5.8W at LHO). Also LLO has slighlty less loss of power once comes out of the PMC as well (LLO ~7.35W dropped from output of MOPA to output of PMC, compared to 8.1W at LHO). But all in all the two systems in terms of loss through the PSL components is very similar
Here is a 60 day trend of the FSS RefCav transmission (H1:PSL-FSS_TPD_DC_OUT_DQ) showing the two drops in PD voltage we've seen in the last couple months. The first occurred around 12/25/2014 and was adjusted on 1/5/2015. The latest drop happened around the middle of last week and we adjusted it yesterday.