The present invention provides an optical waveguide device having a
configuration such that the optical waveguide is folded back on an end
area of the optical waveguide substrate to widen the modulation band. The
optical waveguide device 1A includes a substrate body 2 made of an
electro-optical material, optical waveguide 6, and modulation electrodes
3, 4 and 5 for applying a voltage on the optical waveguide 6. The optical
waveguide 6 includes first primary areas 6e and 6f, a first curved area
7A, first folding-back areas 6g and 6h provided between the first curved
area 7A and a folding-back point 8, second primary areas 6m and 6n, a
second curved area 7B, and second folding-back areas 6j and 6k provided
between the second curved area 7B and the folding-back point 8. At least
a part of the signal electrode is provided in a folding-back region
extending from the first curved area 7A and the second curved area 7B to
the folding-back point 8.