[Yuta, Tega]

From our previous work (elog 17044) of shaking PR2 and seeing a signal in DCPD_A and the fact that LO beam power is far smaller than the expected nominal value, we decided to use TT1 and TT2 to realign the LO beam. This resulted in LO beam power going up by a factor of 6 and an improvement in the LO beam shape. We are still unable to find LO and AS alignment which realize BHD fringe with no clipping everywhere.

Deformed LO beam issue: Following the TT1 and TT2 alignments, used PR2 and PR3 to recover the transmission of the X and Y arms to 1. We also used LO1 and LO2 offsets to further reduce the beam deformation by eliminating the HOM concentric fringes that surounded the LO beam and to maximize the DCPD outputs. BHD optics in ITMY table was tweaked a lot to keep the LO beam centered on the BHD DCPDs and camera. The improved LO beam is still astigmatic in the yaw direction but at least now looks like a TEM00 mode. We also repositioned the DCPD_A path camera lens to remove the circular diffused fringes due to lens clipping. After the alignment, power was measured to be the following. It also reduced the coherence between DCPD outputs and suspension motions (see attached).

                       LO         ITMX
C1:HPC-DCPD_A_OUT16    +127.50    +96.24 (ITMX single bounce consistent to 40m/17040)
C1:HPC-DCPD_B_OUT16    +120.51    +141.52
Power at viewport A    504 uW (almost as expected 40m/17040)
Power at viewport B    385 uW

AP table AS beam clipping: We also noticed clipping in the AS beam in AP table which we removed by moving SR2 and AS1 in YAW and then used AS4 to keep the BHD AS beam centered in the BHD DCPDs.

BHD fringe: After overlaping the LO and AS beams, we saw diagonal fringes indicating beam tilt of LO wrt AS, so we tried to remove the AS beam tilt using AS1 and AS4 but failed to do so because the AS4 mirror seemed to completely distort the beam, so intead we decided to use SR2 and AS1 to remove the tilt between LO and AS beams, which realized BHD fringe. But the motion of SR2 and AS1 then moved the AS beam that it is no longer seen in AP table. The alignment to realize LO and AP AS beam without clipping, and that to realize BHD fringe are attached.