what are the other ones?

I guess the rest of the experimental setup that recombines the photon amplitiudes. Like if you put 5 extra beam splitters in the bottom path, there wouldn’t be full destructive interference.

when i’m thinking about splitter with pi/4 phase shift, i’m thinking about coupled line coupler or its waveguide analogue, but i come from microwave land on this one. maybe this works in fibers?

I’m not sure how you’d actually build a symmetric beam splitter: wikipedia said you’d need to induce a particular extra phase shift on both transmission and reflection. (I’m fully theoretical physics so I’m not too familiar).