I. Introduction
An Electroabsorption Modulator (EAM) is a promising transmitter device for use in 40-Gbit/s very-short-reach optical link applications because of its compactness and ease of operation. In order to improve EAM performance, it is necessary to optimize various aspects of the device. The length, width, and layer structure of the device optical core are the critical design issues for achieving a wider modulation bandwidth and a higher extinction ratio [1]–[3]. If we view the EAM as an electrical device, another important issue is how to efficiently feed the microwave modulation signal into the EAM. Impedance mismatch between standard 50- terminations and the modulator core, whose characteristic impedance is typically 20–25 , inevitably causes electrical signal reflection, leading to degradation of device performance, such as the small-signal electrical-to-optical (E/O) frequency response, due to the less efficient feed of the modulation signal into the device.