The difficulties with H1 DRMI locking, and with getting H1 to full lock, prompt me to survey the top level configuration differences between H1 and L1.
Some other comparisons that should be made (not in the table) are:
- In loop LSC error signals in DRMI, locked using 3-f
- In loop WFS signals in DRMI
- ALS COMM and DIFF spectra
- Light levels on all PDs used during lock acquisition
At this point we don't know which of these differences, if any, are significant for the lock acquision. Please post comments to this entry if you have some ideas on this, or if there are other known differences that we should be looking at.
| parameter | L1 | H1 | comments |
|---|---|---|---|
| input power for locking | 2 W | 10 W |
|
| modulation depths, 9/45 MHz | 0.25/0.29 | 0.19/0.28 |
not sure if L1 values are current |
| ETM global feedback | hierarchical | distributed |
|
| SUS local damping | A | B |
They're different; see G1401267; Jeff K and Stuart A are working on comparison plots |
| DRMI ASC servos | 4 loops | 3 loops |
BW probably lower on H1; more complete comparison needed |
| HSTS feedback & coil drivers | increased M2 drive for PRM & SRM | increased M2 & M3 drive for all HSTS | |
| LSC servo loops |
comparison needs to be made |
||
| 3-f PD photocurrent (DRMI) | 0.15 ma | 27 mW -> 3 ma |
H1 has done limited trials with a reduced photocurrent |
| WFS centering loops |
different, but comparison needed |
||
| ALS ETM feedback | ? | Done when needed to bring frequency in range | |
| Michelson contrast defect: modeled, no arms, no TCS | 6400 ppm | 10,800 ppm |
SIS model, using as-built ITMs |
| Modeled power recycling gain: carrier, no arms, no TCS | 40 | 33 |
SIS model, using as-built ITMs |
RF spectra from the 3-f BBPD have been posted to both LHO and LLO logs recently, so here is a comparison of those.
LLO data: log 15430 , photocurrent: 0.21 ma
LHO data: log 14807 , 27 mW -> inferred photocurrent: 3.0 ma (better would be a direct measurement of photocurrent)
Comparison of 6 highest RF peaks:
Other than 9 MHz, the BBPD output RF components on H1 are all about 20 dB higher than the corresponding components for L1. This is about what is expected from the higher photocurrent used on H1 -- in fact we'd expect closer to 24 dB, if the inferred H1 photocurrent is right. The 9 MHz on H1 is another 10 dB higher (on top of the 20 dB), which is odd considering that the f1 modulation depth on H1 is smaller. This may indicate that on 3-f locking, there is more of an offset on PRCL (or MICH?) in H1 than L1, or maybe more residual motion.
In any case, L1 can hold a stable DRMI lock with the lower 3-f signal level, but H1 has not been able to so far. The LLO log entry also included demod error signal spectra for the DRMI. I'm hoping someone at LHO can post a comparison of that with the H1 situation.