Birefringence tuning:

Yesterday morning we checked the birefringence tuning of the OMC with the DET tower in vacuum. In the first part of Fig.1 the 2 OMC are locked on the direct bounce from NI (other mirrors misaligned). One can read 10.2 mW on B1_PD1 (B1_PD2 gives a different number at that time because it was not correctly calibrated), and 0,45 mW on B1s2P. Taking into account the fact that B1_PD1 reads only half of the B1 power, this gives a polarization mismatch between OMC1 and OMC2 equal to 0,45/(2*10,2) = 2.2%.

One can conclude from the previous result that the birefringence has been slightly detuned during the transition from air to vacuum (after the tuning performed in air last week, we had only 0,7% of polarization mismatch between OMC1 and OMC2). One plausible explanation could be that the tightening of the side screw (the one thightened with 4 cNm last week) has changed.

OMC1 alignment:

The alignment of the OMC with respect to the dark fringe beam has been finely tuned with the direct bounce from NI. A scan in temperature of OMC1 performed after this tuning is shown in Fig.2. One can read the following powers: 14.95 mW on TEM00, 0.116 mW on the mode of order 1 (with about 0.045 mW of piedestal due to the tail of the TEM00), and 0.5 mW on the mode of order 2.

This gives 0.5% of alignment defect (which corresponds to very good alignment), and 3.5% of mode mismatch (which can probably be improved in the future by a careful tuning of the mode matching telescope).