What is the light source that emits monochromatic light from the polarimeter?

The light source that emits monochromatic light from the polarimeter is not necessary. It depends on the state of your Michelson interferometer. If it is used in equal thickness interference, then it is the focus of the line where the image of mirror 1 and mirror 2 is located. Since 2nhcosa=kλ, a is the tilt angle. In the equal-thickness interference, this is unchanged, and h determines the interference level, so when h=0, it is level 0 interference, and the strip on the side is level 1. In the back, a few stripes are a few levels, N stripes are N-level, the highest level is the farthest distance between the two plane mirrors!