A structure comprises an inner strip waveguide (1) and an outer rib
waveguide (2) on a common substrate. The thicker inner waveguide (1) is
patterned into an inner core layer (3). The thinner outer waveguide (2)
is patterned into an outer core layer (4). The inner and outer waveguides
are separated by a gap (5) being less than 500 nm. The structure forms an
adiabatic coupler. In the method, the first (inner) waveguide (1) is
patterned into the thicker inner core layer (3) by etching trenches (8).
A thinner outer silicon layer (4) is attached on top of the inner-core
layer (3) and the first waveguide (1) to form an outer core layer (4).
The second (outer) waveguide (2) is patterned into the outer core layer
(4).