1.) A 532nm output port was added with an intensity of 27mW.
2.) Additional to the already existing 1064nm output for the PLL, we added another port with an intensity of 2.5mW.
3.) One translation stage with an HR Mirror was moved closer to the OPA cavity, thereby bypassing the on-board Faraday Isolator. This mirror sends the OPA output field through another new output port. Depending on the operation mode this new output port provides now either the bright alignment beam (1.3mW) or the squeezed vacuum field.
4.) The coherent control frequency was shifted from 7MHz, as used in O3, to now 4MHz. The according on-board photodiode has been replaced.
We measured squeezing with the on-board homodyne detector (Visibility=98.5%).
Due to OPA crystal degradation, the intra cavity losses are increased and the squeezing level is currently limited to approximately 5dB (7.5dB anti-squeezing).
We will prepare a replacement OPA in Hannover. At this squeezing level we could not observe a difference between using the 7MHz or 4MHz coherent control frequency.
With the settings as used in O3, we observed that the coherent control laser was running multi-mode very strongly. Both modes had roughly the same intensity.
We moved the laser crystal temperature up a little bit (31.09 -> 31.40), which solved this problem. As a consequence the main squeezer laser is pretty close to multi-mode operation. Once the external PLL is installed this needs to be further investigated to find more stable operation points.
