sheila.dwyer@LIGO.ORG - posted 11:37, Thursday 05 February 2026 (89046)
IMC loop gain and power checks this morning
Sheila, Matt, Masayuki
This morning Matt and I brought the SR785 back to the PSL racks to check the IMC OLG. We think that the increase in IMC gain that's needed to restore the ugf is consistent with what's expected (so the modulation depth for 24MHz is similar to before the JAC work), but there is only about 1/4 of the expected power on IM4 trans sum.
| before JAC install | today | ||
| power into HAM1 H1:IMC-PWR_IN_OUT16 | 2W | 1W | |
| H1:IMC-REFL_SERVO_IN1GAIN | 5dB | 11dB | we would need about 15dB to restore 35kHz ugf, which is consistent with what's expected for the power drop. |
| IMC ugf | 35kHz | 23.3kHz (see attachment) | |
| MC2 trans | 310 counts | 90 counts | consistent with 60% HAM1 throughput measured with a power meter (extra loss due to temporary mirror for JM3) |
| IM4 trans nsum | 1.8W | 0.1W | We should expect 0.54W here if alignment was the same as Nov 2025 |
| AS_C with single bounce power into HAM6 | 0.6mW | expect 1W *0.6 HAM1 throughput * 0.03 PRM * 0.25 BS * 0.3 SRM = 1.3mW, so we are seeing half the light we expect in HAM6 |
The misalignment onto IM4 trans might be explained by HEPI being locked, when this shift happened in December the alignment onto IM4 trans QPD shifted so that the beam was all on the upper two quadrants.
Non-image files attached to this report
