Planar optical waveguide element, chromatic dispersion compensator, optical filter, optical resonator and methods for designing the element, chromatic dispersion compensator, optical filter and optical resonator

A planar optical waveguide element comprising:

• a core of an optical waveguide comprising:
• an inner side core having protruding portions that form a rib structure;
• and
• an outer side core that is provided on top of the inner side core, and covers upper and lateral surfaces of the protruding portions of the inner side core,
• wherein a refractive index of the outer side core is lower than an average refractive index of the inner side core;
• wherein the optical waveguide comprises a Bragg grating pattern provided on the core;
• wherein the Bragg grating pattern only uses discrete values of three or more different pitches;
• wherein the discrete values of three or more different pitches are each present in a plurality of locations over an entire length of the optical waveguide;
• and
• wherein, a value which has the highest distribution frequency among all of these discrete values is taken as M, a closest value to the M which is larger than the M is taken as A, and a closest value to the M which is smaller than the M is taken as B, and difference expressed as A−M is equal to a difference expressed as M−B.

A planar optical waveguide element comprising:

• a core of an optical waveguide comprising:
• an inner side core having protruding portions that form a rib structure;
• and
• an outer side core that is provided on top of the inner side core, and covers upper and lateral surfaces of the protruding portions of the inner side core,
• wherein a refractive index of the outer side core is lower than an average refractive index of the inner side core;
• wherein a gap portion that is disposed at a center of the core in a width direction and extends along the propagation direction of guided light,
• wherein the gap portion has a refractive index lower than a refractive index of the core;
• wherein the core comprises a first portion and a second portion that are separated by the gap portion;
• and
• wherein a single mode is propagated in the optical waveguide so as to span across the first area and the second area of the core.