Evan, Dan

We tested the OMC's fast PZT shutter function today.  It works!

We followed the procedure that Zach described for the OMC at L1.  On ISCT6, we installed a PDA255 (50MHz) on the OMC TRANS beam path.  Then we tee'd off the input to the 'trigger' port on the OMC PZT driver chassis.  (The trigger input to the driver chassis comes from the shutter logic controller box on top of ISCT6; the trigger PD is ASC-AS_C.)  With the OMC locked on a single-bounce, we closed the shutter by hand using the Beckhoff controls, and triggered a fast oscilloscope on the falling shutter logic signal.  Since we triggered on the output of the shutter logic box, we essentially measured how long it takes the OMC TRANS to drop to zero after the shutter logic is flipped.  (There will be additional delays due to the logic electronics inside the shutter controller; also the response of the AS_C sum output to a spike in power is finite.  We assume that these delays are less than ~1 millisecond, which is the prescribed shutter performance from T1000294.)

The OMC TRANS signal drops to 10% of the full light level zero 3.84 microseconds after the shutter logic output switched to the closed condition.  This was calculated using a sigmoid fit to the PD data, see figure.  We repeated the test three times, and the results in each case were the same to within 10 nanoseconds.  Plots and data files are attached; Channel 1 is the output of the shutter logic controller (5V is nominal/open state, 0V is triggered/closed state), Channel 2 is the PDA255 on the OMC TRANS path.  The drop from full lock to no transmission is less than a microsecond, the other 2.84 usec of the delay must come from the LV path on the PZT driver board.

One more thing we need to do before the HAM6 shutter electronics are functional for full IFO locks is swap a resistor on the AS_C transimpedance board.  We want to set the shutter threshold at 1 W into HAM6, and AS_C gets 2.5% of the light going into the chamber, so the threshold should be 25mW.  Currently the AS_C sum output calibration is 200 V/watt of power on the QPD.  With this calibration the threshold level should be 5 volts, but due to some factors of five in the Beckhoff settings and the shutter logic box that I don't understand, we only have a range of 0-2 volts for the threshold.  So we need to swap R23 on the AS_C transimpedance board again, for a 420 ohm resistor.  With this change the threshold for the trigger will be 1.6V.