Development of Linear and Nonlinear Components for Integrated Optical Signal Processing

Abstract

Optical processors have potentially a major advantage over electronic processors because of their tremendous bandwidth. Massive parallelism is another inherent advantage of optical processors. However, it is traditionally demonstrated with free space components and seldom used for integrated optical signal processing. In this thesis, we consider spatial domain signal processing in guided wave structures, which brings a new dimension to the existing serial signal processing architecture and takes advantage of the parallelism in optics. A novel class of devices using holograms in multimode channel waveguides is developed in this work.

Linear optical signal processing using multimode waveguide holograms (MWHs) is analyzed. We focus on discrete unitary transformations to take advantage of the discrete nature of modes in multimode waveguides. We prove that arbitrary unitary transformations can be performed using holograms in multimode waveguides. A model using the wide-angle beam propagation method (WA-BPM) is developed to simulate the devices and shows good agreement with the theory. The design principle of MWH devices is introduced. Based on the design principle, BPM models are used to design several devices including a mode-order converter, a Hadamard transformer, and an optical pattern generator/correlator. Optical pattern generators are fabricated to verify the theory and the model. Also, the bandwidth and fabrication tolerance of MWH devices are also analyzed.

Also, we examine the nonlinear optical switches which allow the integration of MWHs into modern optical communication networks. A simple optical setup using an imaged 2-D phase grating is developed for characterization of the complex third-order nonlinearity chi⁽³⁾ to identify suitable nonlinear materials for integrated optical switches. This technique provides a reliable way to characterize chi⁽³⁾ as new materials are constantly being developed. Finally, we demonstrate the concept of optical switching using XPM in segmented semiconductor optical amplifiers (SOA) based on the proven technology of semiconductor waveguides. Segmented SOA switches allow the counter-propagation of control and signal pulses in the switch and avoid the problem of parasitic oscillations encountered in high gain SOA switches. A prototype device is experimentally characterized to demonstrate the concept, and a model is developed to obtain optimal parameters for future devices.