Substrate-type optical waveguide element

A substrate-type optical waveguide element comprising:

• a lower cladding whose refractive index is Nc11;
• a first core and a second core, refractive indexes of which are Nco (Nco>Nc11), the first core and the second core each being formed on the lower cladding;
• and
• an upper cladding whose refractive index is Nc12 (Nco>Nc12), the upper cladding being provided on the lower cladding so as to bury the first core and the second core, wherein:
• when (i) effective refractive indexes of a TE polarized wave and a TM polarized wave in the first core are NTE@WG1 and NTM@WG1, respectively, in a case where the second core is absent, and (ii) effective refractive indexes of a TE polarized wave and a TM polarized wave in the second core are NTE@WG2 and NTM@WG2, respectively, in a case where the first core is absent,
• (A) each of the effective refractive indexes NTE@WG1, NTM@WG1, NTE@WG2, and NTM@WG2 is continuous as a function of distance from a start position of a parallel-core section in which the first core and the second core extend in parallel to each other,
• (B) the effective refractive indexes NTE@WG1 and NTM@WG1 satisfy a relation of NTE@WG1>NTM@WG1 all along the parallel-core section, while the effective refractive indexes NTE@WG2 and NTM@WG2 satisfy a relation of NTE@WG2>NTM@WG2 all along the parallel-core section,
• (C) a magnitude relation of the effective refractive indexes NTM@WG1 and NTM@WG2 at the start position of the parallel-core section is opposite to a magnitude relation of the effective refractive indexes NTM@WG1 and NTM@WG2 at an end position of the parallel-core section, and
• (D) at least one of the first core and the second core being configured such that a cross section orthogonal to a light traveling direction is shaped in steps, the steps being n steps (n≧3) whose widths, which are parallel to a top surface of the lower cladding, decrease as a distance from the top surface of the lower cladding increases toward a top surface of the upper cladding.

A substrate-type optical waveguide element comprising:

• a lower cladding whose refractive index is Nc11;
• a first core and a second core, refractive indexes of which are Nco (Nco>Nc11), the first core and the second core each being formed on the lower cladding;
• and
• an upper cladding whose refractive index is Nc12 (Nco>Nc12), the upper cladding being provided on the lower cladding so as to bury the first core and the second core, wherein:
• when (i) effective refractive indexes of a TE polarized wave and a TM polarized wave in the first core are NTE@WG1 and NTM@WG1, respectively, in a case where the second core is absent, and (ii) effective refractive indexes of a TE polarized wave and a TM polarized wave in the second core are NTE@WG2 and NTM@WG2, respectively, in a case where the first core is absent,
• (A) each of the effective refractive indexes NTE@WG1, NTM@WG1, NTE@WG2, and NTM@WG2 is continuous as a function of distance from a start position of a parallel-core section in which the first core and the second core extend in parallel to each other,
• (B) the effective refractive indexes NTE@WG1 and NTM@WG1 satisfy a relation of NTE@WG1>NTM@WG1 all along the parallel-core section, while the effective refractive indexes NTE@WG2 and NTM@WG2 satisfy a relation of NTE@WG2>NTM@WG2 all along the parallel-core section,
• (C) a magnitude relation of the effective refractive indexes NTM@WG1 and NTM@WG2 at the start position of the parallel-core section is opposite to a magnitude relation of the effective refractive indexes NTM@WG1 and NTM@WG2 at an end position of the parallel-core section, and
• (D) the first core and the second core are provided on a first lower cladding rib and a second lower cladding rib, respectively, both of which protrude from the lower cladding so as to extend in parallel to each other and in alignment with the first core and the second core, respectively, which extend in parallel to each other on the lower cladding, wherein
• the first core and the second core each are configured such that a cross section orthogonal to a light traveling direction is shaped in steps, the steps being n steps (n≧2) whose widths, which are parallel to a top surface of the lower cladding, decrease as a distance from the top surface of the lower cladding increases in a perpendicular direction, and
• at least a space provided between (i) a laminate structure of the first lower cladding rib and the first core and (ii) a laminate structure of the second lower cladding rib and the second core is filled with a material having a refractive index smaller than the refractive index of the lower cladding.