JustinG, PeterK, RickS

We went out to investigate issues with the PMC that might have caused it not to lock when the room temperature varied (Reported by Robert Schofield and others).  The first thing we noticed was that the resonance threshold upper limit was set above the unlocked state level, which seems to indicate that the loop would consider itself locked even when it was not.  We changed the upper level to 0.65 which was half of the unlocked level.  I suspect that this was the cause of the problems with the automatic locking.

We then adjusted the alignment into the PMC to minimize the reflected light level while the loop was locked.  We reduced the reflected light monitor level from about 1.3 to about 1.0.  However, we later noticed that the sum of the reflected and transmitted levels had dropped significantly.  We re-visited the alignment and it seemed to be correct, with a calculated visibility (using the RFPD DC out, [unlocked -locked]/unlocked) of 92%.  Something doesn't seem right with the PMC loop or the PMC itself.  I suspect contamination windows and/or cavity.  We measured the incident and transmitted light levels using the water-cooled power meter, 12 and 8 W respectively.  Thus only 67% transmission with 92% visibility.  

We also looked at the FSS.  We found that the beam was too high on the AOM and raised the AOM to significantly improve the diffraction efficiency.  Then, we adjusted the retro-retlecting curved mirror to bring the beam up at the EOM position to achieve a single-pass efficiency (sqrt of the overall double-pass efficiency) of 86%.  We then aligned into the Reference Cavity and measured a transmission efficiency of about 50%, consistent with earlier measurements.

We then proceeded to measure the Open-loop transfer function of the Frequency Stabilization Servo but found that it was too low, about 150 kHz, even with the gain slider all the way up at 30 dB.  The gain margin was only about 35 deg.  This after increasing the power directed to the reference cavity and the reference cavity alignment.  The phase of the FSS loop has a "bubble" centered around 400 kHz so the phase margin is about 25 deg. larger at 400 or 500 kHz than at 150 kHz.  I'm somewhat surprised the performance was acceptable at all with what must have been a sub-100-kHz UGF.  Don't have a simple idea for increasing the UGF.  We need about 10 dB more gain.  Increasing the light level incident on the PMC to 35 W, which we plan to do in June, will help.  Otherwise, we will have to increase the modulation depth or increase the gain in one of the OpAmps on the FSS board.

It appears that the Test2 input on the FSS is always enabled, even when disabled on the MEDM screen.

Notes from today's work included below for future reference.  

Notes:

April 29 2014
PeterK, Justin, Rick

Mac mini OS 10.7.2

"medm_f" is the command to get fixed font sitemap

PMC as found 
PowerRefl 1.3 W
RFPD DC -0.138 V
unlocked
PowerRefl 11.3 W
RFPD DC -1.25V
89% visibility
resonance threshold 1.346 changed to 1.000

Tweaked alignment into PMC.  Not much improvement 0.138 -> 0.130
Changed upper limit on resonance threshold from 1.0 to 0.65, half of the unlocked level

FSS
as-found locked state 0.028 mV at RFPD DC
measured power after the AOM and iris 18.5 mW
measured power after AOM and before iris 26 mW

adjusted AOM height up and measured power now 23.35 mW after AOM and iris
27.35 mW total power of all beams

double pass - started 14.3 mW...by adjusting beam upward return path increased to 20.35 mW
double-pass efficiency = 74%
 single-pass (sort double-pass) = 86%

measured power after EOM 19.9mW
by dropping EOM .5mm meas pwr now 19.45
better centered on apertures

Slow setting start point -0.2511
starting common gain 30.0 dB, fast gain 16.0 dB
FSS RFPD dc level locked 19 mV
FSS RFPD dc level unlocked 107 mV
visibility 82%

power incident on reference cavity ~14 mW
power transmitted by reference cavity ~7 mw

Measured power incident on PMC 12.3 - 12.5 W
Measured power transmitted by PMC 8 W

1.289 reflected when PMC unlocked
0.099 reflected when PMC locked

Note that the TEST2 enable does not appear to work for the FSS S/N S1107453, D040105