Patent ID: 8655122

Claim:
A mode converter comprising multiple single-mode waveguides, for optically coupling two different areas including an area 1 and an area 2 , wherein: when an axis that is parallel to a light propagation direction is regarded as a z axis, an axis that is perpendicular to the z axis in a direction crossing the multiple single-mode waveguides is regarded as an x axis, an axis that is perpendicular to the x axis and the z axis is regarded as a y axis, and a plane that passes through a center of the mode converter and includes the z axis is regarded as a plane 1 , the mode converter has a structure in which the multiple single-mode waveguides are arranged reflection-symmetrically with respect to the plane 1 so that light propagating in the mode converter has only one even mode with respect to the plane 1 ; the mode converter converts light entering from the area 1 into the even mode to cause the light of the even mode to propagate and couple optically with the area 2 ; and the multiple single-mode waveguides are each formed of a core part having a refractive index n 1 and a clad portion having a refractive index n 2 , and the following conditional equation is satisfied: V = k 0 2 ⁢ W · d · ( n 1 2 - n 2 2 ) < p , where V denotes a normalized frequency of the mode converter, d denotes a distance between the end faces of the core parts of the single-mode waveguides in the position most distant from the plane 1 , W denotes a total width of the core parts of the multiple single-mode waveguides included in the mode converter when a length in a direction parallel to the x axis is regarded as a width, k 0 denotes a propagation constant of incident light in a vacuum, and p denotes a ratio of a circumference of a circle to a diameter of the circle.