Study of an Electro-Optic leaky waveguide deflector for application in All-Optical Analog-to-Digital Converters

Abstract

High speed Analog-to-Digital Converters (ADCs) are key components for enhancing high-speed communication systems, such as broadband satellites at millimeter wave frequencies [1] and extremely high-throughput wireless systems [2]. Tens of gigahertz ADCs are required to cope with the increasing need of real-time digital signal processing of broad bandwidth analog RF signal in direct digital receivers. Electronic ADCs have inherent limitations for reaching high bandwidth with an appropriate resolution [3] due to the high timing jitters of electronics clocks, state of art values over 100 fs for [4]. We are studying an All-Optical Analog-to-Digital Converter (AOADC) based on an Electro-Optic (EO) polymer deflector [5] for achieving higher than 20 GHz of bandwidth with a 6 bits of resolution, taking advantage of much lower timing jitters of modes-locked lasers on one hand and very good materials qualities of EO polymers on the other hand (excellent velocity matching between the microwave signals and optical waves in polymeric materials and EO coefficient up to 350 pm/V versus 31 pm/V of the popular inorganic crystal LiNbO 3 ). The EO deflector converts magnitude variation of the analog RF voltage, applied on its Coupled Mirco-Strip (CMS) driving electrodes, into variations of deflection angle of an optical leaky beam out of the optical deflector using the linear EO effect (Pockels effect). 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. This paper reports design optimization of an EO deflector based on polymer materials. On the one hand, the leaky optical waveguide is optimized for achieving up to 6.3 bits of resolution, on the other hand, a transition from coplanar waveguide pads (CPW) to the CMS line is designed for RF characterization of optical deflectors by means of Ground-Signal-Ground (GSG) probes. Experimental validation of 2 cm long of via-free CPW-CMS transition built on a 40 µm thick Benzo Cyclo Butene (BCB) polymer shows a bandwidth higher than 25 GHz. This transition is very helpful for packaging and connectorization of photonic-microwave devices.