[ano-ther]

Looks deceptively simple. Is there a good source to learn more about how it works?

[actionfromafar]

This looks like an optical Rotman lens:

https://academy.lucedaphotonics.com/modules/awg/guide/compon...

[plasmatorch]

Used to work in photonics. The star coupler portion is a Rotman lens, the whole thing is different (an AWG). It's used to spectrally separate an input (or in this case, inputs) across different outputs.

[actionfromafar]

So the inputs are all the same spectrum and come out shifted by various amounts or something like that?

[RetroTechie]

See link "Working with Luceda AWG Designer" on top of the page you linked. It's got a nice explainer.