There was strange coupling observed in elog2206, but I think it may due to the not proper excitation singal sent to END mirror.

Excitation: the excitation is a sine wave, with amplitude from 700 to 11000, sent to END mirror pitch. (An example shotscreen is shown in attached figure 2)

Measurement: the spectrum of end mirror optical lever pitch/yaw were observed. (An example shotscreen is shown in attached figure 3)

The response information is extracted by using the cursor at 6Hz on each spectrum, and read the value of cursor. The coupling is the ratio of yaw/pitch peaks.

The coupling was always around 5.2%, which is the minimum could be found now.

The method used to find minimum was by adjusting the coefficient for H1 and H3 coils (giving them same/slightly different values from 0.02 to 0.06). The minimum is around -0.05 for H1 and 0.05 for H3.