In accordance with the invention, a high speed thermo-optic phase shifter comprises
a length of optical waveguide including a waveguiding core of a first material
having an index of refraction n1 and a first order temperature dependence
|dn1/dT| and, optically coupled to the core, a length of a second material
having an index n2 preferably greater than the core (n2n1)
and a first order temperature dependence |dn2/dT| than the core (|dn2/dT||dn1/dT|).
Advantageously, the length of second material is adiabatically tapered at both
ends. Upon heating, as by a resistance heater, the second material changes the
optical pathlength by an amount predominantly determined by |dn2/dT|
providing faster switching speed. In a preferred embodiment, the core comprises
silica, and the second material comprises silicon to produce switching speeds up
to a few hundred MHz.
