Using the result of the beam profile analysis, I calculated the required position and lens to put in to decrease beam width to about 300 um at the RFPD (1/3 of detector size 2mmx2mm). Attached matlab code for the same. I used a 687.5 mm Fused Silica lens for this purpose. Finally, I installed the beatnote detector after taking beam profiles with the beam profiler and finding the right position. Attached is the screenshot of the beam profile at this position shows that beam has a width of ~300 um. I have positioned the RFPD such that beam is incident at 30 degrees from normal on it.