According to the spec sheet, the diffraction efficiency of AOM is 87.7% at 1060nm and 500um beam diameter with 1.50W RF power.
This morning, I tweaked the STMs to increase the diffracted beam power.
However the diffraction efficiency was about 50%.
I'm suspecting the beam diameter is so small that the efficiency is low.

Since I have another double-pass AOM, I will input larger diameter beam to another AOM and see the effect of beam diameter on efficiency.
Also I will check the RF power.