  Embedding a double barrier resonant tunnelling diode (RTD) in an unipolar
InGaAlAs optical waveguide gives rise to a very low driving voltage
electroabsorption modulator (EAM) at optical wavelengths around 1550 nm. The
presence of the RTD within the waveguide core introduces high non-linearity and
negative differential resistance in the current-voltage (I-V) characteristic of
the waveguide. This makes the electric field distribution across the waveguide
core strongly dependent on the bias voltage: when the current decreases from
the peak to the valley there is an increase of the electric field across the
depleted core. The electric field enhancement in the core-depleted layer causes
the Franz-Keldysh absorption band-edge to red shift, which is responsible for
the electroabsorption effect. High frequency ac signals as low as 100 mV can
induce electric field high speed switching, producing substantial light
modulation (up to 15 dB) at photon energies slightly lower than the waveguide
core band-gap energy. The key difference between this device and conventional
p-i-n EAMs is that the tunnelling characteristics of the RTD are employed to
switch the electric field across the core depleted region; the RTDEAM has in
essence an integrated electronic amplifier and therefore requires considerably
less switching power.
