On thursday 11th we continued to set up the HD Detector AA loops.
First part - Improvement of the HD Alignment using BAB
We found some strange behaviour during the alignment procedures thus Fiodor went in DET lab to see if there was something wrong in the optical setup. He found that the second mirror of the IR telescope was not well fixed. He fixed it.
One resuperposed the two beams we align the homodyne detecotor on the beams:
- we shuttered the LO
- we exploited the beat note at 4 MHz between the BAB and the CC demodulated at 4 MHz
- we add a dither at 10 Hz on HD_M6_X and a dither at 25 Hz on HD_M6_Y
- we mooved the HD_PD1_X HD_PD1_Y and HD_PD2_X and HD_PD2_Y translation stage in order to center better the two photodiodes. In this process we minimized the 10 and 25 Hz lines on the demodulated channel (figure 1)
In figure 2 the HD Audio diff sum and RF Diff Demodulated before (magenta) and after (blue) the alignment
Second part set up of dither line based AA
Once the HD detector was aligned and the two beams well superposed we un-shutterd the LO beam and we start to set up the dither based loop.
We added the following dither lines with amplitude 0.05V (/virgoData/VirgoOnline/EQB1_HD_AA_Filters.cfg for frequencies and /virgoData//VirgoOnline/EQB1_HD_AA.cfg for amplitudes):
- define HD_M4_X_FREQ 18
- define HD_M4_Y_FREQ 12
- define HD_M6_X_FREQ 8
- define HD_M6_Y_FREQ 6
We closed the Coarse CC loop in order to stibilize the phase up to 20-30Hz and we started to tune the demodulation phases (figure 3 the singla are stable and the phase are optimized in order to have the information on the I quadrature)
ACL_SINELINE_CH "HD_M4_X_line" "V" LOOP_FREQ HD_M4_X_FREQ 2.71
ACL_SINELINE_CH "HD_M4_Y_line" "V" LOOP_FREQ HD_M4_Y_FREQ 2.63
ACL_SINELINE_CH "HD_M6_X_line" "V" LOOP_FREQ HD_M6_X_FREQ 2.24
ACL_SINELINE_CH "HD_M6_Y_line" "V" LOOP_FREQ HD_M6_Y_FREQ 2.1
Then we attempted to close the loop but was impossible because when it started to align the Coarse CC loop opened.
We will retry changing the logic: instead using the analog demodulated signal we will set up the digital demodulation and we will compute the magnitude. In this way we will not need to stabilize the phase with the Coarse CC loop.
We left the dither lines off at the end of the shift
