Design challenges of EO polymer based leaky waveguide deflector for 40 Gs/s all-optical analog-to-digital converters

Abstract

Design challenges and performance optimization of an all-optical analog-to-digital converter (AOADC) is presented here. The paper addresses both microwave and optical design of a leaky waveguide optical deflector using electro-optic (E–O) polymer. The optical deflector converts magnitude variation of the applied RF voltage into variation of deflection angle out of a leaky waveguide optical beam using the linear E–O effect (Pockels effect) as part of the E–O polymer based optical waveguide. This variation of deflection angle as result of the applied RF signal is then quantized using optical windows followed by an array of high-speed photodetectors. We optimized the leakage coefficient of the leaky waveguide and its physical length to achieve the best trade-off between bandwidth and the deflected optical beam resolution, by improving the phase velocity matching between lightwave and microwave on one hand and using pre-emphasis technique to compensate for the RF signal attenuation on the other hand. In addition, for ease of access from both optical and RF perspective, a via-hole less broad bandwidth transition is designed between coplanar pads and coupled microstrip (CPW-CMS) driving electrodes. With the best reported E–O coefficient of 350pm/V, the designed E–O deflector should allow an AOADC operating over 44giga-samples-per-seconds with an estimated effective resolution of 6.5bits on RF signals with Nyquist bandwidth of 22GHz. The overall DC power consumption of all components used in this AOADC is of order of 4W and is dominated by power consumption in the power amplifier to generate a 20V RF voltage in 50 Ohm system. A higher sampling rate can be achieved at similar bits of resolution by interleaving a number of this elementary AOADC at the expense of a higher power consumption.