Integrated photonic 2×3 coupler

A tunable optical filter system, comprising:

• a system input;
• a 1×2 input coupler, wherein the 1×2 input coupler receives light from the system input and splits the light into a first system input and a second system input;
• a plurality of Mach-Zehnder interferometer (MZI) filter stages cascaded in series, each MZI filter stage comprising:
• a first path comprising a first path length;
• a second path comprising a second path length, wherein the second path length is greater than the first path length;
• and
• a 2×3 interferometer, wherein the 2×3 interferometer receives light from a first input and a second input, combines the light from the first input and the second input interferometrically into an in-phase combination and an out-of-phase combination, and outputs half of the out-of-phase combination to a first output, the other half of the out-of-phase combination to a second output, and the in-phase combination to a third output;
• wherein, at a first MZI filter stage, the first path receives light from the first system input and the second path receives light from the second system input, and wherein, any MZI filter stage subsequent to the first MZI filter stage, the first path receives light from the first output of a preceding MZI filter stage and the second path receives light from the second output of the preceding MZI filter stage;
• wherein the 2×3 interferometer is an adiabatic optical coupler, comprising:
• a tapered input region comprising a first waveguide core and a second waveguide core separated by a distance;
• a narrow coupling region extending from the tapered input region, wherein the first waveguide core and the second waveguide core are brought into close proximity, and wherein a third waveguide core is positioned between the first waveguide core and the second waveguide core, the third waveguide core defining a longitudinal axis;
• and
• a flared output region extending from the narrow coupling region, wherein the first waveguide core, the second waveguide core, and the third waveguide core are separated,
• wherein:
• light entering the first waveguide core and the second waveguide core at the tapered input region comprises a TE0 mode (in-phase) component and a TE1 mode (out-of-phase) component, and
• during a propagation of light, the TE0 mode (in-phase) component is routed to the third waveguide core and the TE1 mode (out-of-phase) component remains on the first waveguide core and the second waveguide core;
• and
• a system output, wherein the system output comprises the first output, the second output, and the third output of a final MZI filter stage.

An adiabatic optical coupler, comprising:

• a tapered input region comprising a first waveguide core and a second waveguide core separated by a distance;
• a narrow coupling region extending from the tapered input region, wherein the first waveguide core and the second waveguide core are brought into close proximity, and wherein a third waveguide core is positioned between the first waveguide core and the second waveguide core, the third waveguide core defining a longitudinal axis;
• and a flared output region extending from the narrow coupling region, wherein the first waveguide core, the second waveguide core, and the third waveguide core are separated, wherein the first waveguide core is a rib surrounded by a first slab portion and the second waveguide core is a rib surrounded by a second slab portion at the tapered input region;
• wherein the first waveguide core, the second waveguide core, and the third waveguide core are surrounded by a single slab portion at the narrow coupling region;
• and wherein the first waveguide core is surrounded by a first slab portion, the second waveguide core is surrounded by a second slab portion, and the third waveguide core is a rib surrounded by a third slab portion at the flared output region.