[Aritomi, Eleonora P&C, Yuhang]
Today we tried to improve IR alignment into filter cavity.
First we increased green power to 68mW. OPO temperature is 7.2 kOhm and p pol PLL frequency is 150 MHz. IR injected into filter cavity is around 13mW. Then we locked IR on TEM00 and maximized TEM00 by aligning two steering mirrors for IR. During IR alignment, green transmission was above 5000.
Since lock of green phase is not stable, we scanned green phase with 10Hz and 1.3Vpp. Then we scanned AOM frequency at 2 mHz. After the scan, we locked IR on TEM00 and HG10,01 and measured each power with dataviewer. We repeated alignment of IR and AOM scan twice. Attached pictures show cavity scan. IR transmission when IR is locked on TEM00 and HG10,01 is as follows.
After scan 1
| AOM frequency (MHz) | IR transmission | mode |
| 109.036 | 100 - 2000 | TEM00 |
| 109.43 | 100 - 400 | HG10 |
After scan 2
| AOM frequency (MHz) | IR transmission | mode |
| 109.03646 | 100 - 3800 | TEM00 |
| 109.43133 | 100 - 270 | HG10 |
| 109.432 | 100 - 400 | HG01 |
This scan seems strange and is not compliant with measurement when IR is locked due to green phase scan. Pic 3 shows TEM00 in scan 2. Given that linewidth of filter cavity is 100Hz and AOM scan speed is 8kHz/s, frequency of green phase scan should be higher than 80Hz. Actually we tried 1kHz once, but TEM00 peak hight became smaller.
Anyway now TEM00 is 10 times larger than highest HOM. It seems that mode mismatch is small. We'll continue the alignment tomorrow.
