R&D (FilterCavity)
MarcEisenmann - 23:14, Thursday 19 January 2023 (3154)
IRMC lock and effect on SHG lock

[Marc, Michael, Shalika]

We installed the power meter in reflection of the IRMC.

We scanned the IRMC and found about 90% mode-matching. By just acting on the last steering mirror before the PBS, we could recover 97% mode-matching (see fig 1 and 2 for zoom on HOM).

The remaining HOM might be due to the beam size change induced by the new EOM + longer optical path to recover the tilt induced by the EOM wedge.

Then, we tried to lock the IRMC but most of the time we locked on this HOM.

We had a look at the error signal and it was not great (fig 3) so we tuned the phase of DDS1 DAC0 ie the phase of EOM for SHG/IRMC to 120 deg.

By reducing by 90deg we got a reasonable error signal (fig 4).

On the other hand, this gave a bad error signal for SHG (fig 5) which is recovered with phase of 120 deg (fig 6)

The reason could be that all cables from DDS were disconnected so we might have reconnected cables with incorrect length.

To compensate for that we need to add a cable with length : c/fEOM*90/360 ie about 85 cm.

Images attached to this report
3154_20230119132939_irmctem00.jpeg 3154_20230119132945_irmchom.jpeg 3154_20230119133451_irmcerrbad.jpeg 3154_20230119133455_irmcerrgood.jpeg 3154_20230119133500_shgbad.jpeg 3154_20230119133504_shggood.jpeg