Keita Daniel
With the most recent modifications we were finally able to engage the new outer loop ISS. The gain slider was at 10 dB with 2 W input power. We increased the power to 50 W, while running the feedforward correction for the AC coupling loop. This gave us an ISS with a ugf around 300 Hz. Increasing the gain further will require more loop shaping to avoid internal saturations.
The first attached plot shows the PSD at 50 W with the servo engaged, whereas the black reference trace is with the servo off.
The second attached plot shows the power up transition—indicating that the feedforward works pretty well.
We noticed is that the power fluctuations at longer time scales become larger, when we tried to adjust the offset into the first loop. The attached plot shows that fluctuations grow, when we approach 4% diffraction power, but are fine, when we are in the 2% to 3% range. Strange.