Continuing PSL work ...

The pre-modecleaner transmission photodiode (D1002929 #69 S1107860) was swapped out for
another monitoring photodiode (D1002164 #62 S1107854).  The latter having its transimpedance
gain reduced from 20k to 3.3k.  This fixed the problem with the pre-modecleaner transmission
signal that is observed on the MEDM screen.  The calibration is correct to within one or
two watts.

In getting the power stabilisation up and running again it was noted that the amount of
diffracted light from the acousto-optic modulator was at most 1%.  Far less than what is
required.  Either the alignment for the AOM is off, which is quite likely, or the modulation
signal to drive the AOM driver is insufficient.

A plot of the measured AOM driver output versus modulation input is attached.  With the
0.457V from the power stabilisation servo, 24.6 dBm (320mW) was measured.  This may be
insufficient for the AOM which has an operational RF power of 2.8W (34.5 dBm).



Jason, Peter