For the past week Dmass and I have been ordering parts and getting ready to construct our own modified version of EUCLID (figure). Changes to the EUCLID design could include the removal of the first lens, the replacement of the cat's eye retroreflector with a lens focusing the beam waist on a mirror in that arm of the Michelson, and the removal of the linear polarizers. A beam dump was added above the first polarizing beam splitter and the beam at Photodetector 2 was attenuated with an additional polarizing beam splitter and beam dump. Another proposed alteration is to change the non-polarizing beam splitter from 50/50 to 33/66. By changing the reflectivity to 66\%, less power coming into the non-polarizing beam splitter would be ``lost" at the reference detector (1/3 instead of 1/2), and on the return trip less power would be lost at the polarizing beam splitter (1/6 instead of 1/4). Also, here's a noise plot comparing a few displacement sensors that are used to the shot noise levels for the three designs I've been looking at.
I thought slightly harder and I think that the beamsplitter stays. We will lose too much power on the first PD if we do that:
33/66: Pwr @ PD2 = 2/3*1/3*1/2 = 1/9 Pin
Pwr @ PD3 = 2/3*2/3*1/2 = 2/9 Pin
50:50 Pwr @ PD2 = PWR @ PD3 = 1/8 Pin
balancing them is probably better.