An optical waveguide comprising an MgO substrate 10, and a slab waveguide
layer 24 formed on the MgO substrate 10 and including a core layer 18 of a
ferroelectric or an antiferroelectric, further comprises a stress
alleviating layer 12 which substantially lattice-matches with the MgO
substrate and the slab waveguide layer 24 and has an average thermal
expansion coefficient in the range of 7.0.times.10.sup.-6
-14.0.times.10.sup.-6 /.degree. C. at the room temperature to 700.degree.
C. Accordingly, the optical waveguide device utilizing the magnesium oxide
substrate can be formed without breaking the optical waveguide layer and
the magnesium oxide substrate itself.
