Polymer optical waveguide and composite optical waveguide

A polymer optical waveguide comprising a core, an under cladding that is lower in refractive index than the core and is located adjacent to the core, and an over cladding that is located adjacent to the core on the side opposite to the under cladding,

• wherein
• the polymer optical waveguide has a first section and a second section along a light propagation direction, the first section is a section on one end side in the light propagation direction where no portion of the over cladding exists and a portion of the core and a portion of the under cladding adjacent to that portion of the core are exposed, and the second section is a section on the other end side in the light propagation direction where a portion of the core is covered with the under cladding and the over cladding;
• a relative refractive index difference D (%) defined by the following Equation (i) is 0.6 or larger;
• k defined by the following Equation (ii) is 7.5 or larger and 11.5 or smaller;
• Ws/Hs is 1.50 or larger and 6.00 or smaller;
• Ws/Wf is 1.20 or larger;
• and
• Wf+Hf is 2.6 or larger:
• D(%)=100×[(core refractive index)−{(under cladding refractive index)+(over cladding refractive index)}/2]/[{(under cladding refractive index)+(over cladding refractive index)}/2] Equation (i)
• k=5D+Wf+Hf Equation (ii)
• where
• Ws (μm) is a core width in an end surface s at the one end side of the first section,
• Hs (m) is a core height in the end surface s,
• Wf (μm) is a core width in an end surface f at the other end side of the second section, and
• Hf (μm) is a core height in the end surface f.