Optical functional device and laser device

An optical functional device comprising:

• a multi-mode interferometer waveguide first optical coupler that includes a multi-mode interferometer waveguide portion having a first end portion and a second end portion that oppose to each other in a longitudinal direction, two units of first input/output ports that are arranged in a row at the first end portion in a width direction, and two units of second input/output ports that are arranged in a row at the second end portion in a width direction;
• a multi-mode interferometer waveguide second optical coupler that includes a multi-mode interferometer waveguide portion having a first end portion and a second end portion in a longitudinal direction, two units of first input/output ports that are arranged in a row at the first end portion in a width direction, and two units of second input/output ports that are arranged in a row at the second end portion in a width direction;
• a first arc-shaped waveguide that optically connects one of the first input/output ports of the first optical coupler and one of the first input/output ports of the second optical coupler;
• and
• a second arc-shaped waveguide that optically connects one of the second input/output ports of the first optical coupler and one of the second input/output ports of the second optical coupler, wherein
• the first optical coupler, the second optical coupler, the first arc-shaped waveguide, and the second arc-shaped waveguide constitute a ring resonator,
• each of the multi-mode waveguide portions of the first optical coupler and the second optical coupler have a narrow portion, an average width of the narrow portion in a longitudinal direction being narrower than widths at the first end portion and the second end portion,
• each of the narrow portions includes a fixed width portion which has a substantially constant width in the longitudinal direction,
• when a length of the multi-mode interferometer waveguide portion of any one of the first optical coupler and the second optical coupler is Lmmi, a width of any one of the first end portion and the second end portion of the multi-mode interferometer waveguide portion is Wmmi, and a difference between Wmmi and the width of the fixed-width portion is A, A is decreased as Lmmi is decreased such that low excess loss is maintained,
• the width of the fixed-width portion(s) is equal to or larger than 25%, and smaller than 100%, of the width of any one of the first end portion and the second end portion,
• a part of the first end portion where the first input/output ports do not exist is located within a range of the width of each of the narrow portions, and
• a part of the second end portion where the second input/output ports do not exist is located within a range of the width of each of the narrow portions.