I did some broadband intensity noise injections from about 05:04:00Z to 05:32:00Z, 2015-05-17. I also took another frequency noise coupling TF. Noise projections forthcoming.
I widened the BS violin stopband filter (FM5 in BS M2 LOCK L, 80 dB elliptic). In the course of doing MICH noise injections, I saw a wide response around the beamsplitter violin mode, as was seen at LLO (see Brett's post and the posts linked therein for a discussion). The stopband used to go from 297 Hz to 307 Hz; now it goes from 250 Hz to 350 Hz. These injections seem to indicate that the beamsplitter roll mode is also quite wide, but since this is so close to the UGF of the MICH loop I left the bandstop filter alone.
I took OLTF measurements of PRCL, MICH, and SRCL. PRCL UGF is about 45 Hz and SRCL UGF is about 25 Hz. The MICH UGF was about 12 Hz, with 12 degrees of phase (!). It seems the compensation filter from a few weeks ago is no longer necessary in the full, low-noise 23 W configuration. I'm not sure whether it's necessary at some earlier point in the locking sequence, so I've left it in the guardian for now.
We have seen for a while now that when we transition control of DARM from rf to dc readout, the fluctuations in OMC DC can be as bad as 30% pkpk. We've also seen that the interferometer sometimes loses lock either on the transition step, or on the subsequent step of switching actuators from ETMX to ETMY. For the record, one can do the final DARM stabilization steps (bringing the UGF up to 50 Hz, and engaging the boost) before transitioning to dc readout. This reduces the RIN in OMC DC to less than 10% pkpk. Then, in order to switch actuators, ramp the drive of ETMY ESD on simultaneously while ramping the drive of ETMX ESD down. I used a 30 s ramp, but I suspect we could get away with 10 s or less. I have not put this in the guardian.
I have seen the same 0.4 Hz oscillations in full lock, as Jeff reported earlier. To get around this tonight, I left the ITM oplev damping on. Removing the damping even in full lock leads to instability.
A noise budget is attached with the intensity and frequency noise projections from this lock, along with MICH and SRCL projections from the lock a few days ago. The DARM spectrum shown is from a few days ago as well.
At high frequencies, the per-optic losses in GWINC have been adjusted to give a recycling gain of 40 W/W. This lessens, but does not remove the discrepancy between the expected and observed shot noise level. At low frequencies, the ESD acutation coefficient has been adjusted to 2.8×10−10 N/V2, which is the value currently used to calibrate DARM.
Compared to the last budget, the SRCL noise is reduced above 50 Hz. MICH noise is also reduced, possibly because of the improved feedforward that was implemented last week.
This DARM spectrum was 57 Mpc. From quantum, thermal, and DAC noise alone we expect 69 Mpc. If the DAC noise is reduced according to the projection, then we expect 117 Mpc. Of course, in order to push DARM to this new DAC noise floor, we will have to make improvements to the MICH, SRCL, and frequency noise couplings, among other things.
