[Ushiba, Hirose]

Abstract

REFLPZT3,4 are the DC centering loop actuator for QPD3,4 of WFSf3. As reported in klog34133, it was possible that PZT3 PIT was failed. We went into the mine to check if the PZT was working. We found PZT3 PIT was shorted. We also placed a dumper to dump the WFSf3 QPD3 optical axis.

Detail

As reported in klog29939, the PIT and YAW signals for PZT3 are connected to the two output ports of thorlab's 3-port PZT controller. I checked the YAW control of PZT3 to see if the PZT controller itself could be broken. The YAW control on PZT3 is behaving as if the error signal is going to zero in response to the feedback signal. The PIT on the PZT3 is not act to the control signal, as reported in klog34133. (FIG1) I checked if the QPD3 signal saturates on the digital system when at 10W, but it does not be saturated. (FIG2)

In the mine, we checked the PZT controller monitor. It should have been coming 75V, but it was showing about 3.0V.
After turning off the offset in the digital system, we unplugged the cable at the output of the PZT controller. When we turned on the offset in the digital system again, it displayed 75V. (FIG3)
So it is most likely that the PZT element is shorted. The PZT mirror mount and cable needs to be replaced.
Since PZT3 is BS as shown in the REFL table layout, replacing the mirror mount will take a little time including alignment adjustment. At this time, the dumper was placed downstream of the QPD3 PICO without replacing the mirror mount. (FIG4)
Currently, the cable of the QPD3PIT output of the PZT controller is unplugged. 
And, the ASC guardian's PREPARE_PZT state automatically sets the PZT offset for ASC's DCcentering loop. To avoid inadvertent Guardian changes, we left with the offset 75V on the degital system in this case.
SDF changed: klog34190