| A radio frequency (RF) link capable of modulating analog signals over an ultrawide bandwidth is required for phased array radar and antenna remoting applications. Low distributed loss in optical fiber makes RF-photonic links an attractive alternative to coaxial cable RF links. The critical parameters for an RF link are large spurious free dynamic range (SFDR), high signal power gain, and low noise figure. In an RF-photonic link, these performance parameters are determined by the analog intensity modulator.; By relaxing the symmetry condition in a directional coupler, we show that an asymmetric directional coupler modulator (ADCM) with distributed optical loss is capable of simultaneously providing large SFDR, high link gain, and low noise figure. An in-line ADCM consists of a polished single-mode fiber in optical contact with another dielectric electro-optic waveguide. The in-line ADCM is a robust device in which optical insertion loss is minimized because the single mode fiber is uninterrupted through the device. In an in-line ADCM, light initially launched into the fiber couples to the dielectric waveguide. Analog optical intensity modulation is achieved in the in-fine ADCM by modulating the propagation constant in the electro-optic dielectric waveguide via an RF signal.; In this dissertation, we present the fabrication and material processing techniques that allow implementation of the in-line ADCM with various RF traveling wave electrode configurations. We also present experimental results for two in-line ADCM configurations which use coplanar waveguide and microstrip RF electrodes to achieve “flat” operational bandwidths in excess of 18 GHz, large RF power handling in excess of 30 dBm, large optical power handling in excess of 150 mW, and optical insertion loss less than 1 dB. Due to the small electro-optic coefficient in the poled polymer that we used to implement the in-line ADCMs, demonstrating a low drive voltage is impossible. However, we do investigate the scaling parameters to increase modulation efficiency and show that it can be enhanced by increasing the in-fine ADCM interaction length, maximizing the spatial overlap of the transverse RF and optical fields, and incorporating a state-of-the-art electro-optic polymer into the existing in-line ADCM configurations. |
