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Entry  Tue Dec 16 21:31:21 2014, Kate, DailyProgress, , Minor changes to layout of cavity transmitted path 
    Reply  Wed Dec 17 20:30:36 2014, Aidan, Evan, DailyProgress, , Minor changes to layout of cavity transmitted path 
Message ID: 1537     Entry time: Wed Dec 17 20:30:36 2014     In reply to: 1536
Author: Aidan, Evan 
Type: DailyProgress 
Category:  
Subject: Minor changes to layout of cavity transmitted path 

Quote:

For one, we removed the QWPs which were the first optics in the transmission paths. These had been necessary for the prior cavities where the Silica Tantala mirror coatings were not birefringent. The circular polarization which was transmitted needed to be turned into linear polarization to get the beat note on the PD. Now, because the cavities with AlGaAs coatings are birefringent, the resonant and transmitted light is already linearly polarized and the QWPs unnecessary. Before removing them, the power on the main readout PD, a PD1811, was 208 mV. Afterwards, it was 194 mV.

On the south path, we have placed a HWP so that the transmitted beams can have their polarizations matched. It is on a 1" post and held down with a fork.

In the longer term, this should probably be replaced with the solid metal blocks that were used to hold the QWPs. If these blocks are reinstalled, the waveplate mount should be twisted slightly in yaw in order to reduce the amount of backscatter into the cavities.

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