J. Oberling, P. King
Today we worked on recovering the PSL ISS, and began by re-installing the ISS AOM. The AOM mount was moved to a place where the beam was ~1mm in diameter (unfortunately I forgot to take pictures, will take some tomorrow and upload as a comment) and installed the AOM. The mirror that sits downstream of the AOM and reflects the 1st order diffracted beam into a beam dump was also re-installed and aligned. A power meter was placed in this beam path and the alignment of the AOM tweaked to maximize the power in the 1st order beam. We then adjusted the ISS offset to find the point where the AOM was diffracting ~2W; this was found at an offset of 8.0. The offset was then set to 0, and increased to 20 in steps of 1; a power reading for the 1st order beam was taken at each step. This allows for calibration of the diffracted power graph on the ISS MEDM screen. Peter has the data and is analyzing it.
The beam from the PMC to the ISS box had to be re-aligned, as we had not re-aligned this path since moving the ISS box to accommodate the damping bars for the new PMC. This complete, we then roughly aligned ISS PDB. The PD assembly was then removed from the ISS box so we could re-install ISS PDA (recall that PDA had been removed from the ISS box and installed in place of IO_AB_PD3 prior to O2). With PDA back on the PD assembly, this was installed back in the ISS box, and both PDs aligned. They both now read ~10.0 V. Next step on the ISS (for tomorrow) is to get the loop to close and measure TFs to optimize its operation.
Attached is the calculated fit to the percent diffracted power versus offset slider setting. Ihe fit is close to a parabola p(x) = 113.18 x^2 - 82.3309 x + 15.0625. The power measured in front of the AOM was 67.8 W, and after the neoVAN amplifier was 69.8 W. The power in front of the AOM was used in calculating the percentage diffraction.I went ahead and changed the coefficients in the ISS MEDM screen that calculates the percentage diffraction. Set the reference level to be -1.89 and closed the loop without any problems. The offset slider is currently set to 8.00. Please do not mess with this value. Only the reference level (labelled REFSIGNAL) and the gain slider should be adjusted. At the time of making this log entry the gain setting has not been determined as that will require a few transfer function and noise measurements.The promised pictures. The 1st is the ISS AOM installed in its new location. The 2nd is the mirror that directs the 1st order diffracted beam from the AOM into a beam dump (bottom-left of picture); the power meter shown was used for alignment of the AOM.