[Yuta, Paul]

This afternoon we set up the hardware for the IMC sideband sweep measurement, and took a test sweep of two cavity FSRs. This is in preparation for running automated sideband sweeps while stepping up the input power to get an estimate of IMC optics absorption (see e.g. LLO alog entry 9095).

First we checked the centering of the REFL beam on the in-vac LSC diodes, adjusting PRM alignment for this purpose.

Then we re-aligned the path from the ISCT1 REFL periscope to the in-air REFL diodes, after I misaligned it earlier when doing beam size measurements. Next we connected the Agilent network analyzer source directly to the 45MHz EOM input, without using the 18.8dB RF amplifier that was used last time for this measurement (see LHO alog entry 8098), and connected the 27MHz channel from REFL_AIR_B (broadband PD on ISCT1) to the network analyzer input.

Initial observations of the transfer function from N.A. drive into the EOM to a.m. measured by REFL_AIR_B around the 45.5MHz FSR showed the expected split peak structure (see LHO alog entry 8086). We then put a razor blade beam dump partially in the beam in front of REFL_AIR_B to break HOM symmetry at the PD. We misaligned the IMC to increase the HOM content injected, and then took a sweep across 2 FSRs from 36MHz to 55MHz. At some point during this process we noticed that the TEM00 peaks got much bigger, and also ceased to exhibit the split structure (see attached plot). Discussing this with Kiwamu later it appears that the IMC length shifted somehow during this time, giving a lot of a.m. in transmission with the previously used modulation frequency. It will be interesting to see if these peaks are reduced now that he has adjusted the modulation frequency to compensate.

The attached plot shows the full 2 FSR sweep from this afternoon, overlaid with the data taken in October and the output of a Finesse model. In general, in today's data we see larger contributions from the HGx0 modes than HG0x modes compared to the previous data. We suspect that is due to the razor dump mainly breaking the horizontal symmetry on the diode. Last time we used an iris, which may have been more even in blocking vertical and horizontal axes. We might wish to go back in and use a similar technique this time. Today's data also shows the much larger TEM00 peaks. Overall the SNR looks pretty good, and we'll move on to trying the automated sweeps from the control room.

The razor dump is still partially blocking REFL_AIR_B. We disconnected the EOM 45MHz input cable from the N.A. source, and reattached it to the 45MHz port on the RF distribution panel. We reconnected REFL_AIR_B cable to the demod board.