Things I was surprised to learn

In RF photonics, there is no difference between using a 90° hybrid optical coupler, and 180° hybrid optical optical. After taking the absolute value of the squared optical field, i.e. aftert accounting for the effect of photodection, one gets the same result. You can try it yourself for a given appliaction. This contrasts with what happens in the RF domain, where there is a big difference between a 90° hybrid RF coupler and a 180° hybrid RF coupler. The reason being that one does not have the effect of photodetection, i.e. the RF couplers deal with the filed directly and not with a quantity that is later squared.

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