In this thesis, polarization-independent optical modulators using the controllability of poling-induced optic axis in electro-optic polymer are proposed and fabricated. The electrodes for poling have a 45°-off poling configuration. And the device consists of the two sections, the optic axes of which are orthogonal to each other. Therefore, the intrinsic polarization dependence of electro-optic effect is effectively removed. Two types of polarization-independent optical modulators, a phase modulator and an intensity modulator, are realized. It has potential applications in optical communication systems and optical fiber sensors.
First, the dependence of the optic axis in polymeric waveguides on the gap between the poling electrodes is investigated. A PMMA based copolymer with a stilbene derivative, P2ANS, as electro-optic polymer is used. The optic axis of the poled polymer is directed along the poling field direction and therefore its direction depends on the lateral gap between the upper and the lower poling electrodes. The optic axis angle is very sensitive to the gap between the electrodes near the angle of 45°. Thus, the precise control of fabrication parameters is needed to realize the proposed device.
Second, a polarization-independent optical phase modulator is proposed and fabricated. The buried-type single-mode channel waveguides are fabricated by standard photolithography and Oxyzen reactive ion etching. The device is poled in a convection oven. The half-wave voltages $V_π$ are measured to be 70 V for both TE and TM modes when poling voltage is 1,200 V across total layers. The modulation depths are 56% and 54% for TE and TM modes, respectively, at the wavelength of 1.55 ㎛.
Third, a polarization-independent optical intensity modulator is fabricated, which consists of a Mach-Zehnder interferometric waveguide with two polarization-independent optical phase modulators. The half-wave voltages $V_π$ are measured to be 300 V for both TE and TM modes when poling voltage is 400 V. The modulation depths are 70% and 84% for TE and TM modes, respectively.
Finally, a theoretical investigation for reducing $V_π$ is made, because the fabricated devices have too large half-wave voltages to use in actual systems. The half-wave voltage per unit length is inversely proportional to the gap between modulation electrodes. But the nearer the gap, the larger the metallic loss which the guided modes experience. We approximately calculate the upper limit of the metallic loss due to the close gap by effective index method. $V_π$ can be reduced to less than 10 V by making the gap closer than that of the fabricated device and using a polymer with a large electro-optic coefficient.
