HIGH-ORDER POLARIZATION CONVERSION ELEMENT, OPTICAL WAVEGUIDE ELEMENT, AND DP-QPSK MODULATOR

摘要

A high-order polarization conversion device configured of a planar optical waveguide, includes: a substrate; a lower clad disposed on the substrate; a core including a lower core and an upper core, the lower core being disposed on the lower clad and having a fixed height in a rectangular sectional shape, the upper core being formed of the same material as the lower core and having a fixed height in a rectangular sectional shape that is disposed continuously on the lower core; and an upper clad that is disposed on the core and the lower clad and is formed of the same material as the lower clad. The core forms an optical waveguide through which light is configured to be guided from a start portion, at which a width of the lower core and a width of the upper core are the same, to an end portion, at which the width of the lower core and the width of the upper core are the same, at least one of the width of the upper core and the width of the lower core continuously decreases in a guiding direction of the light between the start portion and the end portion, and neither of the width of the upper core nor the width of the lower core is increased from the start portion to the end portion, at the start portion, an effective refractive index of TE₀ is larger than an effective refractive index of TE₁, and the effective refractive index of the TE₁ is larger than an effective refractive index of TM₀, at the end portion of the optical waveguide, the effective refractive index of the TE₀ is larger than the effective refractive index of the TM₀, and the effective refractive index of the TM₀ is larger than the effective refractive index of the TE₁, in a portion of the optical waveguide between the start portion and the end portion excluding the start portion and the end portion, the core has a vertically asymmetric structure in which the width of the upper core and the width of the lower core are different, and the high-order polarization conversion device performs high-order polarization conversion between TE₁ of the start portion and TM₀ of the end portion.