Optical modulator depressing optical feedback and optical modulating element and channel optical waveguide used therefore

An optical intensity modulator comprising:

• an optical modulating element;
• a first quarter-wave plate disposed on the light input side of said optical modulating element;
• and
• a circular dichroic polarizer disposed on the light output side of said optical modulating element, wherein
• said optical modulating element comprises a core layer and first and second cladding layers sandwiching said core layer,
• said core layer and said first and second cladding layers constitute a waveguide for propagating light,
• said core layer is made of one or more semiconductors belonging to a point group 43m or 42m by International symbol,
• a difference between an index of refraction of said core layer and an index of refraction of said first cladding layer, a difference between the index of refraction of said core layer and an index of refraction of said second cladding layer, a thickness of said core layer and a width of said waveguide are set so that said core layer has TM.sub.00 and TE.sub.00 propagation modes,
• a direction of said waveguide is set to be parallel to the [110] or [-110] direction of crystal orientation so that the propagation direction of light of said TM.sub.00 and TE.sub.00 propagation modes is parallel to the [110] or [-110] direction of crystal orientation, and
• electrodes to apply an electric field to said core layer in the direction parallel to the [001] direction of crystal orientation,
• wherein a difference between the index of refraction of said core layer when applying the electrical field to said core layer and the index of refraction of said core layer when not applying the electric field to said core later is different for said TM.sub.00 and TE.sub.00 propagation modes.

A phase plate of channel optical waveguide type comprising:

• a core layer;
• and
• first and second cladding layers sandwiching said core layer, wherein
• said core layer and said first and second cladding layers constitute a waveguide for propagating light,
• a difference between an index of refraction of said core layer and an index of refraction of said first cladding layer, a difference between the index of refraction of said core layer and an index of refraction of said second cladding layer, a thickness of said core layer and a width of said waveguide are set so that said core layer has TM.sub.00 and TE.sub.00 propagation modes,
• a ratio of the width of said waveguide to the thickness of said core layer are set so that a propagation constant of said TM.sub.00 propagation mode is different from a propagation constant of said TE.sub.00 propagation modes, and
• a length of said waveguide is set so that the difference between a phase of TM.sub.00 light having passed through said core layer and a phase of TE.sub.00 light having passed through said core layer has a predetermined value.