(Alexa, Pablo)
We examined the noise spectrum of REFL_DC_BIAS in several configurations. The first was in the nominal setting. We then placed an ND filter along the PSL reference path on ISCT1 after the shutter. Finally, we placed the ND filter along the x-arm green transmission path (before ALS-BS5). We saw that the beatnote signal appropraitely decreased by 3dBm (or Sqrt(2) in amplitude) with the ND filters placed. This signal was taken from the RF MON readback; I confirmed that this was consistent with an RF anaylyzer after the RF pre-amp.The 1/f noise changed by the same amount regardless of which path the ND filter was placed in. Surprisingly, the noise increased by a factor of 4. One would imagine only an increase by sqrt(2). This increase is also higher compared to what we saw when we did this measurement yesterday (alog 11221).
| REF # | ALS-C_TRX Cnts | Beatnote Signal | |
| No ND Fiilter | 4,5(RMS) | 0.85-0.93 | -14.6dBm |
| ND Filter in x-arm trans path | 6,7(RMS) | 0.35-0.48 | -18dBm |
|
NF Filter in PSL ref path |
2,3(RMS) | 0.80-0.94 | -18dBm |
While on the table, I noticed that there is clipping in the PSL reference path. The beam is clipped at the bottom of the PBS. I have not done anything to fix this clipping yet. This will affect our mode matching and our noise.
