Understanding Inter-Arm Imbalance in Mach–Zehnder Silicon Optical Modulators: Impact on OMA and Frequency Chirp

Characteristic variations caused by nonuniformities in the fabrication process are inevitable in Mach–Zehnder (MZ) silicon optical modulators. However, it is unknown how inter-arm imbalance in phase shifters that have a voltage dependence impacts device performance with respect to the optical modulation amplitude (OMA) and chirp parameter in a push-pull manner. This paper investigates this issue in carrier-depletion MZ silicon optical modulators. First, a generalized form of the large-signal modulation efficiency (ME) is used to characterize the inter-arm imbalance and quantify its impact on the OMA penalty in terms of the phase shift. This theoretical treatment is then validated experimentally by using a fully-automated wafer-level system to evaluate the extinction ratio (ER) on a 300-mm silicon-on-insulator (SOI) wafer. It is shown that the OMA penalty in terms of the absorption loss can suitably be quantified by using an evaluation methodology that treats the absorption losses in each arm separately. In addition, a generalized form of the chirp parameter is formulated that distinguishes its sources between the nonlinearities in ME and absorption loss and their inter-arm imbalance. The parameter is further evaluated over the entire wafer. The formulation is also applied to the dual-drive regime with an adjustable chirp and the increase in negative chirp at the expense of the OMA penalty is quantitatively discussed.

I. Introduction

High-volume producibility together with high-density integration in silicon photonics is an avenue for deploying low-cost optical devices in optical interconnects [1], [2], Gbps Ethernet (GbE) [3]–[7], and long-haul optical transmission systems [8], [9]. The high material index contrast in silicon waveguides, however, poses an unavoidable problem in terms of variability. Geometric variation caused by nonuniformities in the photolithography, plasma etch process, and silicon-on-insulator (SOI) thickness has been a major issue affecting silicon waveguides [10]–[14] and has stimulated a new field of research on structural variability and its impact on device performance. In particular, integration of circuits for process control monitoring (PCM) has been proposed [15], [16].