Multi-stage couplers serve to increase the working bandwidth of a coupler, be it in microstrip, stripline, slabline. Single stage microstrip, stripline, slabline, power line couplers are on the microwave tools.
Due to the distributed nature of any microwave component, so then microwave filters require the periodic nature of the design to be taken into account.
These are passive filters from 0.1Hz upto 1GHz. Typical types Butterworth, Chebychev type 1, 2, Elliptic, Bessel and others.