Electro-optic modulator coupling loss improvement by tapering waveguides and fibers

Polymer based electro-optic (EO) modulators have the potential to provide low cost and lightweight alternative for antenna remoting and RF links because of their wide bandwidth (> 100 GHz) and low drive voltage (V-pi < IV) capabilities. Low noise figure and low power consumption systems require low insertion loss modulators. There are two major causes of loss in EO modulators: waveguide propagation loss (typically I dB/cm), and fiber-to-waveguide coupling loss. In this paper we investigate methods to minimize fiber-to-waveguide coupling loss which is mainly due to mode size mismatch. Pigtails using standard single mode fiber produce coupling loss on the order of 3 to 5 dB/connection. In order to improve mode size matching yet maintain low drive voltage, we incorporate waveguide and fibers tapers. Experimental results indicate improvement of up to 3 dB/connection (6 dB/device) using waveguide tapers, and up to 2 dB/connection (4 dB/device) using fiber tapers.