Investigation of low PSL AOM diffraction efficiency

At the installation of the PSL we found that the diffraction efficiency of the FSS AOM was surprisingly low. Additionally Rana was suspecting during IMC commissioning that the beam waist was not at the center of the VCO.
A wincam measurement of the beam size showed that the waist size(170um) was in accordance to the AOM specs (80% diffraction efficiency at 1.3W drive with more than 90um beam radius). However, the waist position is 65mm upstream of the center of the AOM leading to a beam size of 213um at the single pass and 152um at the double pass.
We measured the diffraction efficiency of the old VCO (D980401, see table first column) and then looked at the rf power applied to the AOM. It was 28.57dBm and therefore too low (it is supposed to be 31dBm). In order to check by how much the diffraction efficiency is at the nominal power level we added a rf amplifier (Mini-Circuits ZHL-3A) and took another measurement (second column). It seems that without any optimization of the alignment we got close to the AOM specs. I will check whether we are missing some gain in the signal that drives the rf amplifier (modlevel) or if the amplifier itself does not act as expected (I was told this happened before).

	VCO w/o amplifier (28.5dBm)	VCO w amplifier (31dBm)	
input beam	25.5mW	28.7mW	
after single pass	10.5mW	19.57mW	
double pass	4.65mW	13mW	
diffraction efficiency	0.41	0.68