An optical modulator modulates light propagating in a three-dimensional optical
waveguide 5 by applying a voltage on the waveguide. The modulator has a
three dimensional optical waveguide 5 having at least a pair of branched
portions 5b, 5c and a recombining portion 5f
of the branched portions 5c, 5d and radiating light
of off-mode, and a slab optical waveguide 4 guiding the light of off-mode.
The modulator also has modulating electrodes 7A, 7B, 7C for
applying a signal voltage and a direct current bias on the waveguide 5 to
modulate light propagating in the waveguide 5. The modulator further has
a photo detector 13 for detecting light radiated from the slab optical waveguide
4, and a controlling unit 15 for varying the direct current bias
based on an output from the photo detector 13 so as to control the operational
point of the modulator. According to the modulator, the operational point may be
controlled with improved efficiency and stability.
