[Fujimoto, Ushiba]

Abstract:

We recovered IRX lock and confirmed that initial alignment of XARM can be done.
However, we found several changes, which needs to be checked.

Detail:

Since we could not see the PDH signals, we investigated what happens.
First, we checked all circuits are ON after the blackout and confirmed it.

 Then, we checked the signals from the LO with an osilloscope and found that the output frequency is 11.248 MHz (double of the IMC FSR).
After several investigation, we noticed frequency REF setting was ON, so we turned it OFF and set the frequency to 5.624365513 MHz.
Then, we could see PDH signals.

We set the LO amplitude to 13dBm because the Harmonic Generator is designed as 13dBm input (JGW-D1808028-v5).
However, IRX could not be locked with this setting and we reduced LO amplitude to 10dBm, and then we could lock the IRX.
Figure 1 shows the OLTF of IRX feedback loop with 10dBm seed LO amplitude.
Though small bump can be seen around UGF, OLTF seems fine.
We also measured the crossover frequency between MCE and laser frequency (fig2).
Crossover frequency is slightly lower but the reason is unknown at this moment.

We also checked the IMC OLTF (fig3) and crossover frequency between PZT and EOM (fig4).
The UGF is about 70kHz, which was 120kHz during O4c (klog35394) but the reason of UGF reduction is unknown.
The crossover frequency between PZT path and EOM path is about 14kHz, which is almost same as before but the phase seems additionally delayed by 6 degrees.

According to the measured results, there seems several changes from before, so we need to check them one by one.

Note:

One possible changes is LO setting because the current setting is different from the values, which was taken during the recovery from the planned power outage (notion page).