Abstract: A thermo optical type variable optical attenuator suitable for application in an array type variable optical attenuator that realizes low crosstalk between neighboring waveguides in the array is provided. A thermo optical type variable optical attenuator 100 using optical material having negative optical effect provides a multimode optical propagating part 3 disposed via a tapered part 2 and a tapered part 7 between single mode optical waveguides 1 and 8. A thin-film heater 4 is disposed above the multimode optical propagating part 3 at an angle of inclination ? in relation to the direction of light propagation along a first optical waveguide 50. Moreover, a second optical waveguide 9 optically coupled with a side face 3S1 of the multimode optical propagating part 3 is provided at an angle of inclination 2? in relation to the direction of light propagation along the first optical waveguide 50.

Abstract: A planar waveguide-based variable optical attenuator having excellent mass-productiveness and suitable for miniaturization is provided at a low cost. The planar waveguide-based variable optical attenuator includes a multimode interference optical waveguide having an incoming end and an outgoing end, and a thin-film heater arranged at a predetermined position on one side of left and right sides of an optical axis of the multimode interference optical waveguide. The thin-film heater is arranged at a predetermined position including at least a part of fields, among field groups appearing on the multimode interference optical waveguide, on one side of left and right sides of the optical axis.