Y arm green transmission to the PSL table improved from ~ 20 uW to 61 uW. Improvement was done by adjusting steering mirrors before and after the faraday on the Y end table.
But 61 uW is not enough!
What I did:
1. The incident power to the faraday for the green beam on the Y end table was 1.4 mW, but the transmission was 1.2 mW. So, I adjusted the steering mirrors and the transmission increased to 1.4 mW.
2. I found that adjusting the steering mirrors to the faraday also increased alignment of the green beam to the Y arm. We always adjusted only the steering mirrors after the faraday for the alignment. I adjusted the alignment using both steering mirrors this time. Reflection of the green beam on the ETMYT camera seems more reasonable now and more frequently lock to TEM00 when closing and opening the Y end green servo loop.
3. For the adjustment, I tried to utilize PD at the reflection port, or the transmission port. However, I couldn't do that because they fluctuates too much. I don't know why.
4. Measured the green transmission to the PSL table, The transmitted power was ~20 uW, but after the aligning, it improved to 61 uW.
Current green power:
I measured the green beam power at various places using Newport power meter (Model 840) with its filter on.
Incident green power to the Y arm is ~ 1 mW (more than 1 mW because the aparture of the power meter was smaller than the beam size) and Y arm transmission is designed to be 55%. So, if the alignment and mode matching are perfect, the transmission to the PSL table should be ~ 600 uW. The measured value 61 uW seems too small. Kiwamu says it was 140 uW when he did Y arm.
I will find the beat note again tonight and check if the beat PD is working correctly and if the mode matching of the two beams at the PSL table is good.