Optical multiplexing circuit

An optical multiplexing circuit using a planar lightwave circuit to output a multiplexed light beam, the optical multiplexing circuit comprising:

• a first optical waveguide inputting a first light beam with a first wavelength in a 0-th order mode;
• a multiplexing optical waveguide inputting one or more other light beams with one or more wavelengths in the 0-th order mode, the one or more wavelengths being different from the first wavelength and different from each other;
• and
• a first multiplexing part converting a waveguide mode of the first light beam propagating through the first optical waveguide into a higher order mode to couple the converted first light beam to the multiplexing optical waveguide to thereby multiplex the coupled first light beam with the one or more other light beams propagating through the multiplexing optical waveguide,
• wherein the multiplexing optical waveguide includes a first portion arranged close to the first multiplexing part, and
• the waveguide widths of the first optical waveguide and the first portion are set such that an effective refractive index of the first optical waveguide with the first light beam in the 0-th order mode is equal to an effective refractive index of the first portion of the multiplexing optical waveguide with the first light beam in the higher order mode and that effective refractive indexes of the first portion of the multiplexing optical waveguide with the one or more other light beams in the higher order mode are not equal to effective refractive indexes of the first optical waveguide with the one or more other light beams in the 0-th order mode.

An optical multiplexing circuit comprising:

• first to fifth optical waveguides inputting first to fifth light beams with mutually different first to fifth wavelengths in a 0-th order mode, respectively;
• a first multi-mode conversion waveguide provided between the first optical waveguide and the second optical waveguide;
• a second multi-mode conversion waveguide provided between the first optical waveguide and the third optical waveguide;
• a first coupling part converting a waveguide mode of the second light beam propagating through the second optical waveguide into a higher order mode to couple the second light beam converted into the higher order mode to the first multi-mode conversion waveguide;
• a second coupling part converting a waveguide mode of the second light beam propagating through the first multi-mode conversion waveguide into the 0-th order mode to couple the second light beam converted into the 0-th order mode to the first optical waveguide to transmit the first light beam;
• a third coupling part converting a waveguide mode of the third light beam propagating through the third optical waveguide into the higher order mode to couple the third light beam converted into the higher order mode to the second multi-mode conversion waveguide;
• a fourth coupling part converting a waveguide mode of the third light beam propagating through the second multi-mode conversion waveguide into the 0-th order mode to couple the third light beam converted into the 0-th order mode to the first optical waveguide;
• a first multiplexing part converting a waveguide mode of the fourth light beam propagating through the fourth optical waveguide into a higher order mode to couple the fourth light beam converted into the higher order mode to the first optical waveguide;
• and
• a second multiplexing part converting a waveguide mode of the fifth light beam propagating through the fifth optical waveguide into the higher order mode to couple the fifth light beam converted into the higher order mode to the first optical waveguide,
• wherein the first optical waveguide includes a first portion arranged close to the first multiplexing part and a second portion arranged close to the second multiplexing part,
• in the first optical waveguide, the second light beam converted into the 0-th order mode and coupled by the second coupling part, the third light beam converted into the 0-th order mode and coupled by the fourth coupling part, and the first light beam transmitted by the second coupling part and the fourth coupling part are multiplexed and input into the first portion,
• waveguide widths of the first optical waveguide, the second optical waveguide and the first multi-mode conversion waveguide are set such that an effective refractive index of the second optical waveguide with the second light beam in the 0-th order mode is equal to an effective refractive index of the first multi-mode conversion waveguide with the second light beam in the higher order mode and that an effective refractive index of the first multi-mode conversion waveguide with the first light beam in the higher order mode is not equal to an effective refractive index of the first optical waveguide with the first light beam in the 0-th order mode,
• waveguide widths of the first optical waveguide, the third optical waveguide, and the second multi-mode conversion waveguide are set such that an effective refractive index of the third optical waveguide with the third light beam in the 0-th order mode is equal to an effective refractive index of the second multi-mode conversion waveguide with the third light beam in the higher order mode and that an effective refractive index of the second multi-mode conversion waveguide with the first light beam in the higher order mode is not equal to an effective refractive index of the first optical waveguide with the first light beam in the 0-th order mode,
• waveguide widths of the fourth optical waveguide and the first portion are set such that an effective refractive index of the fourth optical waveguide with the fourth light beam in the 0-th order mode is equal to an effective refractive index of the first portion of the first optical waveguide with the fourth light beam in the higher order mode and that effective refractive indexes of the first portion of the first optical waveguide with the first to third light beams in the higher order mode are not equal to effective refractive indexes of the fourth optical waveguide with the first to third light beams in the 0-th order mode, and
• waveguide widths of the fifth optical waveguide and the first portion are set such that an effective refractive index of the fifth optical waveguide with the fifth light beam in the 0-th order mode is equal to an effective refractive index of the second portion of the first optical waveguide with the fifth light beam in the higher order mode and that effective refractive indexes of the second portion of the first optical waveguide with the first to third light beams in the higher order mode are not equal to effective refractive indexes of the fifth optical waveguide with the first to third light beams in the 0-th order mode.