Cheryl, Rodica

Today we concluded our thermal lens measurements of the H1 Faraday isolator for optical powers up to 140 W. In an attempt to correct for distortions in the beam shape observed with the beam scan, we repeated the measurements for several alignments and DKDP orientations. No significant diffence was found. Similar beam distortion could also be seen before the Faraday isolator, indicating vertical clipping after the EOM (possibly caused by a slightly eliptical beam coming out of the PMC?). More investigation needs to be done on the IO beam path to clarify this. For the thermal lens analysis only the data from the least distorted axis was used.

We also measured the thermal lensing of the Faraday with the eLIGO 3 mm thick DKDP piece, which has the same thickness as the aLIGO crystal, and it is considered as spare for Advanced LIGO.

The measured focal length of the FI with the aLIGO DKDP is +26 m, and with the eLIGO DKDP is +32m, the Faraday isolator being slightly undercompensated in both cases. This translates into a mode matching of 95% with the aLIGO DKDP, and 97% with the eLIGO one. Both values meet the IO requirements. M2 data and fit results are attached.

We changed the setup for isolation measurements and will do this tomorrow.