Contents I. Introduction
Vertical-cavity surface-emitting lasers (VCSELs) possess characteristics such as low power consumption, circular beam output for efficient fiber coupling, low-cost manufacturability, and scalability in two-dimensional arrays. Therefore they have become the dominant source for optical data communication links in computer server, data center, and super computer applications. There have been several reports of VCSEL modulation rate in excess of 50 Gbps [1], [2] although these experiments have been into a few 10s of meter of fiber and require high current density. Modulation rate times distance products of [3], [4], and [5] Gbpskm have been achieved using single mode or quasi-single mode VCSELs for reduced modal and chromatic dispersion. Transversely coupled dual VCSELs have recently been reported with small signal modulation bandwidth as high as 29 GHz and large signal operation of 36 Gbps, albeit in multi-mode or quasi-single mode operation [6]–[8]. The bandwidth improvement is reported to arise from photon-photon coupling [9]. To satisfy the need for optical data communications links requiring error-free transmission over long distances (>1 km) of fiber at high data rate (>20 Gbps), it is crucial to employ VCSELs with large modulation bandwidth, single mode or narrow spectral width, increased output power, and reliable operation.
