[Yuta, Paco, Anchal]
We estimated meas diff angle for BH55 today by following this elog post. We used moku:lab Moku01 to send a 55 MHz tone to PD input port of BH55 demodulation board. Then we looked at I_ERR and Q_ERR signals. We balanced the gain on I channel to 1.16 to get the two signals to same peak to peak heights. Then we changed the mead diff angle to 91.97 to make the "bounding box" zero. Our understanding is that we just want the ellipse to be along x-axis.
We also aligned beam input to BH55 bit better. We used the single bounce beam from aligned ITMY as the reference.
We attempted to lock LO phase with just using BH55 demodulated output.
We expected that we would be able to lock to 90 degree LO phase just like DC locking. But now we understand that we can't beat the light with it's own phase modulated sidebands.
The confusion happened because it would work with Michelson at the dark port output of michelson, amplitude modulation is generated at 55 MHz. We tried to do the same thing as was done for DC locking with single bounce and then michelson, but we should have seen this beforehand. Lesson: Always write down expectation before attempting the lock.