  In this paper, we present the design, fabrication and characterization of a
traveling wave directional coupler modulator based on electro-optic polymer,
which is able to provide both high linearity and broad bandwidth. The high
linearity is realized by introducing domain-inversion technique in the
two-domain directional coupler. A travelling wave electrode is designed to
function with bandwidth-length product of 302GHz cm, by achieving low microwave
loss, excellent impedance matching and velocity matching, as well as smooth
electric field profile transformation. The 3-dB bandwidth of the device is
measured to be 10GHz. The spurious free dynamic range of about 110dB Hz^(2/3)
is measured over the modulation frequency range 2-8GHz. To the best of our
knowledge, such high linearity is first measured at the frequency up to 8GHz.
In addition, a 1-to-2 multi-mode interference 3dB-splitter, a photobleached
refractive index taper and a quasi-vertical taper are used to reduce the
optical insertion loss of the device.
