> My knowledge of optics is not sufficient to fully explain why it's easier to filter by wavelength instead of direction.
Reciprocity. For most optical devices, if x% of light coming in port A comes out port B, then x% of light coming in port B comes out port A. Non-reciprocal materials exist, but they’re unusual and may be rather inefficient and complicated to work with. Wavelength splitters are reciprocal and are relatively easy to construct. In fact they’re cheap enough that people use them for art. (The usual splitter is a piece of dichroic glass.)
Reciprocity. For most optical devices, if x% of light coming in port A comes out port B, then x% of light coming in port B comes out port A. Non-reciprocal materials exist, but they’re unusual and may be rather inefficient and complicated to work with. Wavelength splitters are reciprocal and are relatively easy to construct. In fact they’re cheap enough that people use them for art. (The usual splitter is a piece of dichroic glass.)
The Wikipedia article is so-so:
https://en.m.wikipedia.org/wiki/Reciprocity_(electromagnetis...