Rib type optical waveguide and optical multiplexer / demultiplexer using same

1. A rib type optical waveguide, comprising:

a substrate; and

a multi-mode optical interference region including a rib type optical waveguide formed on an upper surface of the substrate,

wherein each of both side surfaces of the multi-mode optical interference region are formed in a stepped shape and are not formed on an input optical waveguide side and the stepped shape, and

wherein a width and a height of the stepped shape are configured such that a ratio of a first difference of the multi-mode optical interference region and a second difference of the multi-mode optical interference region are approximately equal, the first difference is a difference between an effective refractive index of a fundamental mode and an effective refractive index of a first high order mode when a main electric field of the multi-mode optical interference region is horizontal to the substrate and the second difference is a difference between an effective refractive index of the fundamental mode and an effective refractive index of the first high order mode when the main electric field of the multi-mode optical interference region is perpendicular to the substrate.

2. The rib type optical waveguide according to claim 1 , wherein a width of a stepped part is equal to or less than a propagation wavelength and a height of the stepped part is equal to or less than a half of the propagation wavelength.

3. The rib type optical waveguide according to claim 1 , wherein, on the upper surface of the substrate, an insulating film and a semiconductor layer are arranged, and a core of the rib type optical waveguide is formed by processing the semiconductor layer in a strip shape.

4. The rib type optical waveguide according to claim 1 , wherein the stepped shape is formed in a stair shape of two stages or more.

5. An optical multiplexer/demultiplexer, comprising:

M input optical waveguides and N output optical waveguides including a single mode rib type optical waveguide;

reversible tapered regions that smoothly connect the input optical waveguides and the output optical waveguides to multi-mode optical interference regions and include M×N rib type optical waveguides; and

the rib type optical waveguide according to claim 1 ,

wherein the input optical waveguides, the output optical waveguides, the multi-mode optical interference regions, and the reversible tapered regions are formed on a substrate.

6. The optical multiplexer/demultiplexer according to claim 5 , wherein, on an upper surface of the substrate, an insulating film and a semiconductor layer are arranged, and cores of the input optical waveguides, the output optical waveguides, the multi-mode optical interference regions, and the reversible tapered regions are formed by processing the semiconductor layer in a strip shape.

7. The optical multiplexer/demultiplexer according to claim 5 , wherein the input optical waveguides and the output optical waveguides include a same sectional shape.

8. The optical multiplexer/demultiplexer according to claim 5 , wherein all the M×N rib type optical waveguides of the reversible tapered regions include same length and same tapered width.

9. The optical multiplexer/demultiplexer according to claim 8 , wherein the length of the reversible tapered regions is longer than a propagation wavelength.

10. The optical multiplexer/demultiplexer according to claim 5 , wherein two input optical waveguides, two output optical waveguides, and four reversible tapered regions are provided.

11. The rib type optical waveguide according to claim 1 , wherein the stepped shape is not formed on an input optical waveguide side, and

wherein the stepped shape is not formed in the multi-mode optical interference region on an output optical waveguide side.