First, off I forgot a resistor on the previous diagram so here is the fixed version. Also, Anchal said that the EOM noise is probably caused by instrument noise at the low end. The phase response should be flat on both and on the PZT path the non-flat response is caused by some high pass filter caused by AC coupling or something of the sort.

I am still trying to figure out how to get a flat response on the phase. Anchal did some math that looks at the possibility of it and I am going through it.