An electro-optical polymer was synthesized where a diazo dye with a dicyanovinyl group as an electron acceptor and a diethylamino group as a donor is attached to the polymer chain. The electro-optical coefficient (r) reached 30 pm/V. It was found that the edge absorption of the chromophore caused a loss increase in the near infrared region, which indicates that the increase in the r value leads to a propagation loss increase in the material. The loss is around 1.0 dB/cm in a single-mode waveguide fabricated by using oxygen reactive ion etching. The polymer waveguide is applied to two types of devices, a Mach-Zehnder optical modulator and a vertically stacked directional coupler, which both achieve electro-optical modulation. As another application, electro-optical measurement of an electric field in a high-speed circuit device is demonstrated, where the polymer is processed into a chip film probe and patched to an integrated circuit, thus enabling the electric signal to be detected.