High speed optical modulators can be made of a lateral PN diode formed in
a silicon optical waveguide, disposed on a SOI or other silicon based
substrate. A PN junction is formed at the boundary of the P and N doped
regions. The depletion region at the PN junction overlaps with the center
of a guided optical mode propagating through the waveguide. Electrically
modulating a lateral PN diode causes a phase shift in an optical wave
propagating through the waveguide. Each of the doped regions can have a
stepped or gradient doping profile within it or several doped sections
with different doping concentrations. Forming the doped regions of a PN
diode modulator with stepped or gradient doping profiles can optimize the
trade off between the series resistance of the PN diode and the optical
loss in the center of the waveguide due to the presence of dopants.