Rick S, Michael R

After the low power transition and some first order alignment correction we had about 10.7W transmitted through the PMC and 2W at reflection. However, some peaks in the modescan indicated mode matching issues, so we decided to move the mode matching lenses L2 and L3 to improve this. We couldn't use the alignment ramp on the PMC screen so we instead ramped the PZT on the NPRO and adjusted the crystal temperature using the slow actuator to move between modes.

Our procedure was to first optimize the position of L2, then move L1 and L2 in iterative steps until the mode matching peak is minimized.

To optimize L2:

1. Scan NPRO PZT
2. Look at PMC reflection PD and ISS PDA on a scope
3. Adjust position of L2 by 1cm on rail
4. Move slow actuator to alignment peak, adjust pointing to minimize
5. Move slow actuator to mode matching peak (what faintly looked like a 02/20 on the camera) and measure voltage
6. Repeat steps 3-5 until the mode matching voltage is minimized

Once L2 position was optimized we then started adjusting L1 as well:

1. Adjust position of L1 by 1cm on rail
2. Move slow actuator to alignment peak, adjust pointing to minimize
3. Move slow actuator to mode matching peak and measure voltage
4. If the voltage decreased, then repeat steps 1-3 for L2. Otherwise adjust L1 in the opposite direction
5. Continue to iterate between moving L1 and L2 until the mode matching peak is minimized

We now have 11.6W in transmission and 0.8W in reflection. Visibility is 90% (going off of PD voltage readings). We did not fully minimize the peak so there is still more power to be gained. However, another mode appeared to grow as we minimized the 02/20 mode, which looks on a viewing card like a 02 hermite horizontal.

The attached spreadsheet shows positions of the lenses and the corresponding 02/20 mode voltages. Positions are measured at the North end (towards HAM1) of the lens mounting block.

FSS settings were checked again. UGF is at 400kHz with 55 degrees of phase margin. Common gain set to 23 dB.