Integrated-optic focal-spot intensity modulator using electrooptic polymer waveguide

A novel integrated-optic focal-spot intensity modulator comprising an electrooptic (EO) phase-distribution modulator and a focusing grating coupler (FGC) in an EO polymer planar waveguide on a Si substrate is proposed and demonstrated. The integrated phase-distribution modulator is made up of three-finger top electrodes and a planar bottom electrode sandwiching the waveguide. The electrodes are used for both the poling process and the modulation of the phase distribution of the guided wave. The FGC couples the guided wave to a beam focused on a point in free space. The intensity distribution of the focused spot is modulated by altering the wavefront of the guided wave. The bottom metal electrode is extended to the place under the FGC and serves as a reflection film so that the output coupling occurs only into the direction of the air for increasing the efficiency. The device was designed and fabricated with an active polymer p-NAn-PVA (p-nitroaniline n-polyvinyl alcohol). An extinction ratio of the focal-spot intensity was higher than 3 dB at a modulation voltage of 30 V for 3-mm-long electrodes. The frequency response was measured to be flat over 2 MHz. The EO effect induced by poling showed no significant relaxation over 300 h in an unconditioned room (20-30°C)