This paper reviews all-optical analog to digital converters (OADCs) based on the photonic crystal. They are nonlinear applications and are used in all-optical data processing systems. They are consisting of one input (i.e., continuous analog optical signal) and two or five outputs (i.e., binary code). Most of these structures have two operation steps. The first one is a nonlinear demultiplexer followed by second stage that has an optical coder. The demultiplexer creates the discrete levels for the continuous optical input signal; then the optical coder generates output bits. There are two techniques to design OADC, ring and cavity resonators. All OADCs structures use the finite difference time domain (FDTD) and plane wave expansion (PWE) analysis methods. The paper presents a comparison between the different designs in the literature for both the ring and cavity resonators using the RSOFT simulator followed by the challenges and some novel trends.

Keywords— Photonic crystal, Optical ADC, ring resonator, Demultiplexer, Optical encoder.