In a star coupler, a Free Propagation Region (FPR) is bounded by a first interface
and a second opposing interface, and guides an input signal launched from the first
interface in a predetermined first plane while allowing the input signal to travel
unguided in a predetermined second plane in the FPR which is orthogonal to the
first plane. A plurality of output waveguides are formed in an array and terminate
at the second interface of the FPR. The axis of each output waveguide at the second
interface is separated from an axis of an adjacent output waveguide by a predetermined
distance "t". An input waveguide is split into a plurality of subsections which
each terminate at the first interface of the FPR. The subsections of the input
waveguide are arranged for simultaneously launching parts of the input signal into
the FPR which diffracts and produces mode patterns at the second interface having
a maximum intensity at inputs of each of the output waveguides, and a low intensity elsewhere.
