IQ-Modulation Using Phase and Amplitude Modulators and Multimode Interference

Optical amplitude and phase modulators are an integral part of modern optical communications systems. As optical data formats transitioned from encoding amplitude exclusively to both amplitude and phase, several different methods for creating the quadrature amplitude modulation (QAM) have been proposed, demonstrated and employed. This paper will provide an overview and analysis of current techniques for creating these signals and will then show how a more elegant and efficient design is possible by taking advantage of the phase properties of multimode interference devices (MMIs). Using two 2 × 2 MMIs in a Mach Zehnder configuration is a well-known technique for creating a BPSK signal using phase modulators and has also been shown to work with amplitude modulators. This paper will also show how two 4 × 4 MMIs can be used to create QAM signals using either phase or amplitude modulation.