Optical multiplexer and RGB coupler

1. An optical multiplexer comprising:

a multimode waveguide to which two single mode input waveguides are connected at a distance; and

two single mode output waveguides connected at a distance to a surface opposite a surface to which the input waveguides of the multimode waveguide are connected, wherein

a width of the multimode waveguide is smaller than widths of the two input waveguides plus a distance between the input waveguides, and

the input waveguides are connected to the multimode waveguide via input tapered waveguides and the multimode waveguide is connected to the output waveguides via output tapered waveguides,

wherein the width of the multimode waveguide is set so that a change amount of the multimode waveguide with respect to a wavelength of an effective width becomes constant at a desired wavelength.

2. The optical multiplexer according to claim 1 , wherein light of a first wavelength inputted to one of the input waveguides and light of a second wavelength inputted to an other of the input waveguides generate only 0-th order and first order modes in the multimode waveguide and repeat self-imaging.

3. The optical multiplexer according to claim 1 , wherein widths of the multimode waveguide and the input and output tapered waveguides change along a light propagation direction,

wherein when a light beam having a desired input mode field is caused to propagate from an input end of the input waveguides to the input tapered waveguides in a forward direction, and a light beam having a desired output mode field is caused to propagate from the output tapered waveguides to an output end of the output waveguides in a backward direction, the width of the multimode waveguide is changed so that wave fronts of the two light beams match at any given point in the multimode waveguide.

4. The optical multiplexer according to claim 1 , wherein light of first wavelength inputted to one of the input waveguides is blue light having a wavelength of 467±10 nm and light of second wavelength inputted to another of the input waveguides is green light having a wavelength of 523.5±18.5 nm.

5. An RGB coupler comprising:

the optical multiplexer according to claim 4 ; and

a mode coupler connected to one of the output waveguides, wherein red light of 630±10 nm, which is light of a third wavelength is inputted to an input waveguide coupled with the mode coupler.

6. An RGB coupler comprising:

the optical multiplexer according to claim 4 ; and

a directional coupler connected to one of the output waveguides, wherein red light of 630±10 nm, which is light of a third wavelength is inputted to an input waveguide of the directional coupler.

7. The optical multiplexer according to claim 2 , wherein widths of the multimode waveguide and the input and output tapered waveguides change along a light propagation direction,

wherein when a light beam having a desired input mode field is caused to propagate from an input end of the input waveguides to the input tapered waveguides in a forward direction, and a light beam having a desired output mode field is caused to propagate from the output tapered waveguides to an output end of the output waveguides in a backward direction, the width of the multimode waveguide is changed so that wave fronts of the two light beams match at any given point in the multimode waveguide.

8. The optical multiplexer according to claim 2 , wherein light of first wavelength inputted to one of the input waveguides is blue light having a wavelength of 467±10 nm and light of second wavelength inputted to another of the input waveguides is green light having a wavelength of 523.5±18.5 nm.

9. The optical multiplexer according to claim 3 , wherein light of first wavelength inputted to one of the input waveguides is blue light having a wavelength of 467±10 nm and light of second wavelength inputted to another of the input waveguides is green light having a wavelength of 523.5±18.5 nm.