Jennie W, Sheila
Today we took OMC scans to help diagnose what is going on with our alignment through the OFI - that is, what is the mode-matching at our the old alignment (as of Monday 22nd) and our new alignment (as of this morning).
Sheila turned off the sidebands before the test and we had the ETMs and the ITMX mis-aligned initially for single bounce configuration.
Old alignment: SR3 M1 YAW OFFSET = -125 microradians
SR3 M1 PIT OFFSET = -437 microradians
SR2 M1 YAW OFFSET = -421 microradians
SR2 M1 PIT OFFSET = -64 microradians
Due to PEM measurements we switched from single bounce off ITMY to single bounce off ITMX.
Locked time = 1 minute from GPS 1398534847
Unlocked time = 1 minute from GPS 1398534984
Scan = 200 s starting at 1398535070 GPS
New alignment: SR3 M1 YAW OFFSET = 120.2 microradians
SR3 M1 PIT OFFSET = 437.9 microradians
SR2 M1 YAW OFFSET = 2061.7 microradians
SR2 M1 PIT OFFSET = -5.5 microradians
Locked time = 1 minute from 1398538330 GPS
Unlocked time = 1 minute from 1398538461 GPS
Scan = 200 s starting at 1398537927 GPS
Dark time with IMC offline and fast shutter closed = 1398538774 GPS
Mode mis-match measurments pending...
The loss through the OMC appears to have increased after whatever happened to the output path on April 22nd.
I use again Sheila's OMC loss calculation code as we previously used in this entry.
Power on refl diode when cavity is off resonance: 29.698 mW
Incident power on OMC breadboard (before QPD pickoff): 30.143 mW
Power on refl diode on resonance: 5.153 mW
Measured effiency (DCPD current/responsivity if QE=1)/ incident power on OMC breadboard: 56.5 %
assumed QE: 100 %
power in transmission (for this QE) 17.029 mW
HOM content infered: 14.415 %
Cavity transmission infered: 66.501 %
predicted efficiency () (R_inputBS * mode_matching * cavity_transmission * QE): 56.494 %
omc efficency for 00 mode (including pick off BS, cavity transmission, and QE): 66.009 %
round trip loss: 3495 (ppm)
Finesse: 335.598
We compare these values to that found from our scans on the 16th April and it seems like the HOM content has increased substantially, the incident power has decreased, and the measured and predicted cavity efficiency has decreased by 3%.
It would be good to cross-check these figures against the other methods of checking the losses, such as DARM offset step and the mode mis-match I still need to calculate from the mode scan taken on the same day.
I forgot to run the same analysis for the locked and unlocked measurements we got at the old (pre April 23rd) alignment of SR2 and SR3.
Power on refl diode when cavity is off resonance: 25.306 mW
Incident power on OMC breadboard (before QPD pickoff): 25.685 mW
Power on refl diode on resonance: 5.658 mW
Measured effiency (DCPD current/responsivity if QE=1)/ incident power on OMC breadboard: 54.1 %
assumed QE: 100 %
power in transmission (for this QE) 13.885 mW
HOM content infered: 19.870 %
Cavity transmission infered: 67.970 %
predicted efficiency () (R_inputBS * mode_matching * cavity_transmission * QE): 54.061 %
omc efficency for 00 mode (including pick off BS, cavity transmission, and QE): 67.467 %
round trip loss: 3289 (ppm)
Finesse: 339.266