However, it is quite hard to create a flat noise in the interferometer. The usual source of flat noise is excess gas in the tubes, or any other excess differential phase noise. At the level of the beam splitter it corresponds to a optical path fluctuation of ~9e-18 m/rtHz. A noisy TCS laser shining on the compensation plate maybe could produce a flat noise, but I don't think it is hitting the CPs yet. A 1/f noise in the photodiodes could create a similar too, but it would need to be coherent between the two photodiodes, so it is not present in the subtraction of B1_PD1 and B1_PD2, which doesn't make any sense.
