NAOJ GW Elog Logbook 3.2
[Aritomi, Eleonora P]
Today we managed to align IR into filter cavity and lock both green and IR again. First we locked IR with 109.03569MHz of AOM frequency as last measurement, but we found that TEM00 is much brighter at 109.03535MHz (Pic 1).
The power of each mode is as follows. Pic 2-7 shows shape of each mode other than HG10. Since green phase lock is not stable, we scanned green phase and measured maximum and minimum of IR transmission. Green phase is scanned at 20Hz.
| AOM frequency (MHz) | Transmission power (mV) | Mode |
| 108.62391 | 55.2 - 84 | |
| 108.62430 | 55.2 - 88.2 | |
| 108.81421 | 55.2 | |
| 108.82913 | 58 - 198 | IG20 |
| 109.02021 | 60.8 | |
| 109.02083 | 60.8 | |
| 109.03535 | 64 - 608 | TEM00 |
| 109.43143 | 54 - 144 | HG10 |
| DC offset | 45.6 |
Now largest mode is TEM00 and main higher order modes are IG20 (Pic 5) and HG10.
It is expected that you don't find a TEM00 each 0.5 MHz frequency shift. As reported in entry #661, since the AOM is put on the green path, the change in the frequency which it induces is compensated by the servo with a change on the IR which is half of the frequency change in the AOM. This means that a shift of 1 MHz in the driving frequency of the AOM corresponds to a shift of 500 kHz in the frequency of the IR light.
I think that we can now start to tweak the lens position to improve the matching. It would be also good to do a cavity scan as the one done in entry #776.
You are right. We'll scan AOM frequency in the range of 1MHz.