Optical system with optical waveguides

The present invention provides an optical system with waveguides, which comprises first, second and third optical input/output means, fourth and fifth multi-mode optical waveguides each capable of propagating light with plural propagation modes, and optical-filter mounting means for mounting an optical filter between the fourth and fifth multi-mode optical waveguides across a traveling direction of light in the fourth and fifth multi-mode optical waveguides. The first optical input/output means is connected to an end face of the fourth multi-mode optical waveguide on a side thereof opposite to the optical-filter mounting means. Each of the second and third optical input/output means is connected to an end face of the fifth multi-mode optical waveguide on a side opposite to the optical-filter mounting means. Respective widths in a direction orthogonal to the light-traveling direction of the fourth and fifth multi-mode optical waveguides are different from each other.