  We report high speed optical modulation in a resonant tunneling relaxation
oscillator consisting of a resonant tunneling diode (RTD) integrated with a
unipolar optical waveguide and incorporated in a package with a coplanar
waveguide transmission line. When appropriately biased, the RTD can provide
wide-bandwidth electrical gain. For wavelengths near the material band-edge,
small changes of the applied voltage give rise to large, high-speed
electro-absorption modulation of the light. We have observed optical modulation
at frequencies up to 14 GHz, associated with sub harmonic injection locking of
the RTD oscillation at the fundamental mode of the coplanar transmission line,
as well as generation of 33 ps optical pulses due to relaxation oscillation.
