Dan, Koji (from a distance), Rana
We used an OMC mode scan to measure the modulation depth of the 9 and 45MHz sidebands, to close the loop on the recent changes to the amplification path before the EOM. Koji had done this previously, and measured Gamma1 (9MHz) = 0.198, Gamma2 (45MHz) = 0.305.
Today, we measured Gamma1 = 0.208, Gamma2 = 0.240 +/- 0.01. The error in Gamma2 is due to an asymmetry between the upper and lower sidebands; using the ratio of the 45MHz USB to the carrier returns Gamma2 = 0.251, while the LSB returns 0.233. This 10% discrepancy between the 45MHz LSB and USB is consistent across several sweeps of the cavity. The analysis code used today was thrown together a little quickly and doesn't do a sophisticated job of integrating the curve around the peaks (in fact, doesn't do any integration, just compares the peak heights to the carrier), but when applied to the same data that Koji used earlier this month it finds values similar to his to within a few percent.
The attached plot has an overlap of five cavity sweeps, with the peaks of the 9 and 45MHz sidebands used in the calculation marked with a cross. The data are here. The mode scan was performed off a single bounce from ITMX. This weekend we'll do a more careful scan for both ITMX and ITMY to calculate (more?) accurate numbers of the mode-matching for each path.
Daniel, Kiwamu, Rana
The modulation depth mystery for 45 MHz is still unsolved. Educated guesses are welcome.
In December of 2013, the people have measured that the input power to the EOM was 11 dBm and the modulation depth was 0.07 radians.
Today, we checked at the CDS electronics room, the field rack, and all around the PSL table and near the EOM, for the presence of any more un-documented amplifiers. We found none.
We measured the power at the field rack to be +12 dBm. So we think that based on the EOM calibration from December that we should be getting < 1 radian.
However, the OMC scan seems to show a modulation depth of ~0.24 radians for the 45 MHz sideband. Unless the carrier peak is saturated in the DC readout PDs with a single bounce Michelson beam, this seemsto be impossible to square with the EOM calibrations.
In December of 2012, Volker measured the modulation coefficient to be (0.33 rad / 10 Vpp) = 0.066 rad/V. 10 Vpp ~ 24 dBm (for 50 Ohms). This agrees well with Kiwamu's entry.
We have the further evidence of high modulation that the REFL and AS demod signals didn't change much (or at all) when the ZHL-1A was pulled out of the modulation chain.
What's going on here?
Measured the gain of ZHL-1A amplifier that we removed at 45.5 MHz through a 20 dB attenuator into a 50 Ohm loaded scope; gain is nominal over a wide range of input power levels.
