In order to increase the diffraction efficiency of the AOM, we need to reduce the beam size of the laser. According to the manual of my AOM, it needs to have the beam size of 220um to get more than 90% diffraction efficiency. So, I decided to do so by putting a lens in front of it and another one at the back of AOM. However, I'm not sure what is the focal length I should use for both the lenses? I remember my professor used to tell me that longer focal length is better, but I forgot why is it the case? Is 50mm considered ok?