Propagation mode loss difference compensator

A mode loss difference compensator comprising:

• a main waveguide configured to allow propagation of N or more modes (where N is an integer of 3 or more);
• a first auxiliary waveguide whose fundamental mode is phase-matched with an LP0n mode (where n is an integer of 2 or more) propagating in the main waveguide, the first auxiliary waveguide having, at one end thereof, a first coupling portion configured to mode-convert the LP0n mode (where n is an integer of 2 or more) propagating in the main waveguide into the fundamental mode in the first auxiliary waveguide and transfer the fundamental mode from the main waveguide to the first auxiliary waveguide and having, at the other end thereof, a second coupling portion configured to mode-convert the fundamental mode propagating in the first auxiliary waveguide into the LP0n mode (where n is an integer of 2 or more) in the main waveguide and transfer the LP0n mode from the first auxiliary waveguide to the main waveguide;
• and
• at least one second auxiliary waveguide whose fundamental mode is phase-matched with a higher-order mode, other than any LP0n mode (where n is an integer of 2 or more), propagating in the main waveguide, the second auxiliary waveguide having, at one end thereof, a third coupling portion configured to convert the higher-order mode, other than any LP0n mode (where n is an integer of 2 or more), propagating in the main waveguide into the fundamental mode in the second auxiliary waveguide and transfer the fundamental mode from the main waveguide to the second auxiliary waveguide and having, at the other end thereof, a fourth coupling portion configured to mode-convert the fundamental mode propagating in the second auxiliary waveguide into the higher-order mode, other than any LP0n mode (where n is an integer of 2 or more), in the main waveguide and transfer the higher-order mode from the second auxiliary waveguide to the main waveguide,
• wherein the main waveguide includes a loss imparting portion, configured to impart a loss to a fundamental mode propagating in the main waveguide, between the first and second coupling portions and a first long-period grating, having a predetermined loss-wavelength spectrum, between the first and second coupling portions,
• the first auxiliary waveguide includes a second long-period grating, having a predetermined loss-wavelength spectrum, between the first and second coupling portions, and
• the second auxiliary waveguide includes a third long-period grating, having a predetermined loss-wavelength spectrum, between the third and fourth coupling portions.