Design considerations of all-optical A/D conversion: nonlinear fiber-optic Sagnac-loop interferometer-based optical quantizing and coding

The authors describe in detail the design considerations of our previously proposed novel optical quantizing and coding method for all-optical analog-to-digital (A/D) conversion using nonlinear optical switches based on the Sagnac interferometer. The multiperiod transfer function, which is the key to quantizing and coding, is achieved through a careful design of the Sagnac interferometer. In the experiments, the intensity of the pulse train input to our A/D converter is manually changed, and the corresponding digital signals are successfully mapped generated. Although the input-pulse trains are not the sampling of real analog signal, the principle of our proposed 3-bit A/D conversion at a 10 gigasample per second (Gsps) rate is demonstrated. The proposed optical quantizing and coding, combined with existing optical sampling techniques, will enable ultrafast photonic A/D conversion without electronics. The potential in the frequency regime of over a few hundred gigasamples per second was investigated by using an optical switch that utilizes the optical Kerr effect for fast operation. It was found out that the wavelength allocations and temporal widths of control and probe pulses have to be optimized with respect to the group-velocity dispersion of highly nonlinear fiber.