The first attached plot (H1L1DARMresidual.pdf) shows the residual DARM spectrum for H1 and L1, from a recent coincident lock stretch (9-10-2015, starting 16:15:00 UTC). I used the CAL-DELTAL_RESIDUAL channels, and undid the digital whitening to get the channels calibrated in meters at all frequencies. The residual and external DARM rms values are:
| residual DARM | external DARM | |
|---|---|---|
| H1 | 6 x 10-14 m | 0.62 micron |
| L1 | 1 x 10-14 m | 0.16 micron |
The 'external DARM' is the open loop DARM level (or DARM correction signal), integrated down to 0.05 Hz. The second attached plot (H1L1extDARMcomparison.pdf) shows the external DARM spectra; the higher rms for H1 is mainly due to a higher microseism.
Some things to note:
- L1's residual DARM is 6x smaller than H1, due to more suppression in the 2-5 Hz region. As the 2nd plot shows, there is more DARM excitation in this region on H1 than L1.
- The DARM DC offset for both interferometers is 10 pm, so the residual DARM represents a relative fluctuation of the detected power or photocurrent of 0.1% for L1, 0.6% for H1.
- The violin mode fundamentals are much lower in H1 than L1 - by around 3 orders of magnitude! This is because for H1 the violin mode active damping is left ON during low noise mode, whereas for L1 I believe it is turned OFF. However, this also means that the H1 front end calibration is not accounting for these damping paths.
The 3rd attached plot (H1L1DARMcomparison.pdf) shows the two calibrated DARM spectra (external/open loop) in the band from 20-100 Hz. This plot shows that H1 and L1 are very similar in this band where the noise is unexplained. One suspect for the unexplained noise could be some non-linearity or upconversion in the photodetection. However, since the residual rms fluctuations are 6x higher on H1 than L1, and yet their noise spectra are almost indentical in the 20-100 Hz band, this seems to be ruled out - or at least not supported by this look at the data. More direct tests could (and should) be done, by e.g. changing the DARM DC offset, or intentionally increasing the residual DARM to see if there is an effect in the excess noise band.
We briefly tried increasing the DCPD rms by decreasing the DARM gain by 6 dB below a few hertz (more specifically, it's a zero at 2.5 Hz, a pole at 5 Hz, and an ac gain of 1... it's FM5 in LSC-OMC_DC). This increased the DCPD rms by slightly less than a factor of 2. There's no clear effect on the excess noise, but it could be we have to be more aggressive in increasing the rms.
- Nominal DARM configuration: 2015-10-14 20:40:00 to 20:45:00 Z
- Reduced low-frequency gain: 2015-10-14 20:34:30 to 20:39:30 Z
interesting, but do I interpret it right that you (on the experiment reported in the comment) assume that the DARM errorpoint represents the true DARM offset/position? I thought that it is the case at least at L1 that when DARM is locked on heterodyne, and the OMC is locked onto carrier (with the usual DC offset in DARM), then the power in transmission of the OMC fluctuates by several 10%. Assuming that the TEM00-carrier coupling to the OMC would be no different when DARM is locked to OMC trans. power, then also the 'true' DARM would fluctuate this much impressing this fluctuation onto DARM. This fluctuation should show up in the heterodyne signal then. So in this case increasing the DARM gain to reduce the rms would probably not do anything. Or?